Aqueous phase conversion of CO2 into acetic acid over thermally transformed MIL-88B

Aqueous phase conversion of CO2 into acetic acid over thermally transformed MIL-88B | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Aqueous phase conversion of CO 2 into acetic acid over thermally transformed MIL-88B Waqar Ahmad, Paramita Koley, Swarit Dwivedi, Abhijit Shrotri, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1029433/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 May, 2023 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Sustainable production of acetic acid (AA) is a high priority due to its high global manufacturing capacity and numerous applications. Currently it is predominantly synthesized via carbonylation of methanol, in which both the reactants are fossil-derived. CO 2 transformation into AA is highly desirable to achieve net zero carbon emissions, but significant challenges remain to achieve this efficiently. Herein, we report a heterogeneous catalyst, thermally transformed MIL-88B with Fe 0 and Fe 3 O 4 dual active sites, for highly selective AA formation via methanol hydrocarboxylation. This efficient catalyst showed high AA yield (590.1 mmol/g cat .L) with 81.7% selectivity at 150°C in aqueous phase using LiI as a co-catalyst. The reaction is believed to proceed via formic acid intermediate. No significant difference in AA yield and selectivity was noticed during catalyst recycling study up to five cycles. This work scalable and industrially relevant for CO 2 utilisation to reduce carbon emissions, especially if green methanol and green hydrogen are used. Chemical Engineering Energy Engineering Methanol hydrocarboxylation CO2 transformation Acetic acid Formic acid Thermally transformed MOFs Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Fixation of overabundant atmospheric carbon dioxide is an urgent and essential research area, which may lead towards climate mitigation. Several routes for carbon dioxide conversion have been investigated, but thermocatalytic CO 2 hydrogenation pathway is one of the major focus due to its fast kinetics, high productivity, scalability and selectivity 1 . Synthesis of chemicals such as methane 2 , 3 , methanol 4 , formaldehyde 5 , 6 , dimethyl ether 7 , gasoline-range hydrocarbons 8 , oxymethylene dimethyl ethers 9 , 10 , methyl formate 11 , formic acid 12 , and acetic acid 13 , 14 have been investigated in recent years. A CO 2 based chemicals industry has the potential to lower the CO 2 concentration in atmosphere, while simultaneously provide revenue for offsetting the capture costs. The production of acetic acid (AA) via CO 2 hydrogenation is one such route which is receiving attention of the researchers recently. Acetic acid is extensively used in several industrial applications, including food, chemicals, pharmaceuticals, textile, cosmetics and polymers 15 . It is a well-known food preservative and traditionally named as vinegar in food industry. Commercially, two major production processes are used for the synthesis of acetic acid – chemical and fermentative 15 , 16 . Among various chemical routes, the most common industrial processes are carbonylation of methanol (MeOH) developed by BASF, Cativa and Monsanto, in the presence of homogeneous Cobalt, Iridium and Rhodium catalysts, respectively. In Monsanto process, AA is produced from CH 3 OH and fossil fuel derived CO in the presence of CH 3 I and homogeneous rhodium-based catalyst 13 , 15 . The main reaction of acetic acid production from methanol and CO is summarized in Eq. 1 . $$C{H}_{3}OH+CO \leftrightarrows C{H}_{3}COOH$$ 1 Qian et al. recently reported AA production via hydrocarboxylation of MeOH with carbon dioxide and hydrogen in 1,3-dimethyl-2-imidazolidinone (DMI) solvent over homogeneous Rh and Ru based homogeneous co-catalysts with a combination of LiI promoter and imidazole ligand. While the authors report that imidazole played critical role in inhibiting the reverse water gas shift reaction, but the exact role of imidazole in the reaction mechanism was not clear 13 . The same group also showed AA synthesis via the above described reaction system in the presence of Rh 2 (CO) 4 Cl 2 homogeneous catalyst, LiCl as a co-catalyst, 4-methyl imidazole ligand and LiI as a promoter 17 . This reaction system is highly complex due to the presence of multiple catalysts, stabilizing ligands and organic solvents. In many cases, the authors report a black precipitate, which is not explained but is likely to be the Ru or Rh catalyst, which demonstrates that the system is not stable. Hasan et al. reported low yield of acetic acid (1.58 mmol/L) over NiO-C/Al 2 O 3 , heterogeneous catalyst at 130°C and 35 bar total pressure of CO 2 and H 2 in 1,4 dioxane solvent after 6h of reaction. Instead a higher amount of formic acid (FA, 4.08 mmol/L) was generated 18 . Therefore, there is an urgent need to develop a stable and active heterogeneous catalyst based on low cost metals for AA synthesis which can efficient for industrialisation and scaleup. He et. al. report FA and AA production via hydrothermal CO 2 reduction with Fe nanoparticles as stoichiometric reagent in which they are converted into ferrous carbonate 19 . To the best of our knowledge, Fe-based heterogeneous catalysts have not been reported CO 2 conversion in aqueous phase. Heterogeneous catalysts have advantages in scale-up, and compares favourably against homogeneous catalysts which require large downstream separation processes. Here we present a Fe-based thermally transformed metal organic framework catalyst (MIL-88B) for hydrocarboxylation of MeOH to produced AA. Recently, Metal Organic Framework (MOFs) derived carbonaceous materials have been reported for their remarkable catalytic properties 20 , 21 , 22 . Thermal transformation of MOFs results in a carbonaceous material with embedded metal or metal-oxide nanoparticles 22 . As these particles are embedded in the matrix of decomposed organic linkers, they show greater resistance to sintering at higher temperatures. Depending on the thermal treatment, the thermally transformed MOFs have features such as high surface area, porosity, and fine dispersion of metal nanoparticles that are desired in an ideal heterogeneous catalyst. Moreover, the porous carbon framework provides better mass transfer to enhance the reaction rate. In this work, thermally transformed MIL-88B, called T-MIL-88B, consisted of dual active sites – Fe 0 and Fe 3 O 4 , accelerating the conversion of CO 2 into AA, compared with other Fe-based catalysts tested which contained only Fe 3 O 4 or Fe 0 and Fe 2 O 3 . In this process, AA is produced in a series of reactions (eq. 2 - 4 ) – $$C{O}_{2 \left(aq\right)}+{H}_{2 \left(aq\right)}\underleftrightarrow{\text{Fe}} HCOOH$$ 2 $$C{H}_{3}O{H}_{\left(l\right)}+LiI\leftrightarrow C{H}_{3}I+LiOH$$ 3 $$C{H}_{3}I+HCOOH+LiOH\leftrightarrow C{H}_{3}COOH+LiI+{H}_{2}O$$ 4 Overall Reaction $$C{O}_{2 \left(aq\right)}+{H}_{2 \left(aq\right)}+ C{H}_{3}O{H}_{\left(l\right)}\underleftrightarrow{\text{Fe, LiI, 150°C}}{H}_{3}CCOOH+{H}_{2}O$$ 5 2. Methods 2.1. Materials Iodomethane (CH 3 I, 99.5%), formic acid (HCOOH, ≥ 95%), lithium Iodide (LiI, 99.9%), terephthalic acid (H 2 BDC, 98%), chromium chloride hexahydrate (CrCl 3 .6H 2 O, 98%), and iron nitrate nonahydrate (Fe(NO 3 ) 3 .9H 2 O, 98%) were purchased from the Sigma Aldrich. Commercial zeolite-beta (CBEA, SiO 2 /Al 2 O 3 = 38) was received from Zeolyst International. Methanol (HPLC grade) was obtained from the Scharlau Chemicals. Milli-Q water was used for catalysts synthesis (MIL-101 and Fe/CBEA) and acetic acid production experiments. 2.2. Catalysts synthesis 2.2.1. Fe/CBEA Wet impregnation process was used for Fe/CBEA synthesis as described in our previous publication 9 . The loading of Fe was fixed as 10 wt% in this catalyst. Typically, Fe(NO 3 ) 3 .9H 2 O (7.2 g) was dissolved in Milli-Q water (30 mL) by using 100 mL Schott bottle and stirred for 15 min at 65°C to prepare a homogeneous mixture of Fe solution. Thereafter, 9.0 g of CBEA support was immersed in this solution under stirring and maintained it for 6 h at the same temperature to achieve an even dispersion of Fe particles on CBEA support. The mixture was dried in oven at 100°C followed by calcination at 550°C with 5°C/min for 5 h in muffle furnace. The synthesised catalyst was reduced in the environment of H 2 /Ar (1:1 v/v) gas mixture at 400°C for 5 h with heating rate of 5°C/min prior to carbon dioxide conversion experiment. 2.2.2. Thermally transformed Fe/MIL-101 10 mmol of H 2 BDC and 10 mmol CrCl 3 .6H 2 O were poured into a Teflon-lined autoclave. Subsequently, Milli-Q water (72 ml) was added to it. The reaction mixture was sonicated for 30 minutes followed by stirring for another 30 minutes at 500 rpm. Thereafter, the autoclave was kept in the oven at 205°C for 24 h and allowed to cool to room temperature. The resulting solid suspension was transferred into a centrifuge tube. Initially the centrifugation was performed at 1000 rpm for 3-4 min to remove the unreacted H 2 BDC present in the reaction mixture. Thereafter, the centrifugation was carried out at 5000 rpm for 10 minutes. The solid sample was then washed with dimethylformamide (DMF) three times and then dried in an oven at 70°C for 12 h. The synthesised material was named as MIL-101. For Fe/MIL-101 synthesis, 2.7 g of MIL-101 was suspended in 70 ml ethanol in a Schott bottle and sonicated for 30 minutes. Separately, 2.17 g Fe(NO 3 ) 3 .9H 2 O was dissolved in 20 ml ethanol in a different Schott bottle and stirred for 15 minutes. The latter solution was poured into the former suspension of MIL-101 in ethanol. Then the Schott bottle which contained Fe(NO 3 ) 3 solution was washed with 10 ml ethanol three times and poured into MIL-101 suspension to ensure complete transfer of the Fe precursor. The resultant mixture was sonicated for 30 min followed by stirring at 50°C at 500 rpm for 5-6 h. Finally, the resulting reaction mixture was dried in an oven at 80°C for 2-3 days. The synthesized catalyst was named as Fe/MIL-101. The Fe loading was fixed as 10 wt% in the synthesized catalyst. Prior to catalytic activity test, this catalyst was thermally transformed under 100 ml/min H 2 /Ar (1:1) gas mixture at 500°C for 5 h with a heating rate of 5°C/min and allowed to cool in 50 ml/min Ar atmosphere and denoted as T-Fe/MIL-101. 2.2.3. Thermally transformed MIL-88B A modified hydrothermal method as described in the literature 23 was adopted for synthesis of MIL-88B. In a typical procedure, 12.12 g of Fe salt (Fe(NO 3 ) 3 .9H 2 O) was dissolved in 75 ml DMF under stirring (500 RPM) in a Schott bottle. Separately, H 2 BDC (4.98 g) and DMF (75 ml) were added in a 250 ml Teflon-liner under stirring (500 RPM). Both Fe and H 2 BDC solutions were stirred further for 15 min at room temperature. The Fe solution was then poured into H 2 BDC precursor solution. 12 ml NaOH solution (4.0 M) was slowly transferred into Fe and H 2 BDC solution mixture and stirred again for 30 min at room temperature. Thereafter, the Teflon-liner was sealed in an autoclave and heated to 100°C for 24 h. After cooling to room temperature, MIL-88B particles were collected from this mixture via centrifugation at 7000 RPM for 10 min and washed three times with DMF and methanol, respectively. Finally, the as synthesized MIL-88B was dried overnight in the oven at 80°C and denoted as MIL-88B. Thermal transformation of MIL-88B (2 g) was conducted at 500°C for 5 h with a ramp of 5°C/min under 100 ml/min H 2 /Ar (1:1 v/v) environment followed by cooling to room temperature under Ar at 50 ml/min atmosphere and denoted as T-MIL-88B. 2.3. Catalyst characterisation The crystal structure of the materials was investigated with Powder X-ray diffraction (PXRD) by using a Rigaku MiniFlex device. The powder catalysts were loaded in a zero-background sample holder and scanned between 2–80° 2θ with 4°/min scan speed at 15 mA and 40 kV. Nitrogen physisorption analysis was conducted with Micromeritics 3Flex 3500 machine to find the type of adsorption isotherm, Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore distribution. Tecani T20 was used to capture the transmission electron microscopy (TEM) images of the catalysts. All the samples were dispersed in ethanol and immobilised onto the surface of a holy carbon grid followed by drying in air prior to analysis. ThermoScientific K-Alpha machine was utilized for X-ray photoelectron spectroscopy (XPS) at 1486.6 eV Ephoton and coupled with monochromatic Al Kα radiations. The binding energy (B.E.) baseline correction was conducted by adjusting the C 1 s peaks at 284.8 eV. Thermally transformed samples were prepared ex situ prior to the XPS characterization. Shimadzu DTG-60H thermogravimetric analyser was used to check the thermal stability of Fe/MIL-101 and MIL-88B. Both samples were analysed in the temperature range of 100-800°C with a ramp of 5°C/min under Ar atmosphere. 2.4. Aqueous phase CO 2 conversion All the aqueous phase CO 2 conversion experiments were performed in a 100 mL Teflon-lined autoclave batch reactor (Amar Equipment, M4). Typically, 0.4 g of thermally transformed catalyst (T-MIL-88B) and 40 mL water was added to the reactor and CH 3 I (10 mmol) was carefully poured into it and sealed. It was purged with hydrogen three times to eliminate air from the headspace. The reactor was then pressurised with CO 2 up to 35 bar, followed by H 2 up to a total pressure of 70 bar at room temperature to achieve CO 2 :H 2 ratio of 1:1. The reactor was heated to 150°C under continuous stirring at 200 RPM for 21 h. After 21 h of reaction, the reactor was allowed to cool to room temperature and the remaining gases were carefully vented from it before dissembling it. The catalyst was recovered from the liquid product mixture by centrifugation at 8500 RPM for 1 h. The same procedure was repeated for different total pressures at equimolar CO 2 :H 2 ratio and different catalysts (T-Fe/MIL-101 and Fe/CBEA). The liquid sample was analysed at intervals for the best catalyst to check the extent of reaction against time at 150°C, equimolar CO 2 :H 2 at 70 bar with 200 RPM stirring speed. The liquid samples were analysed using an HPLC (Agilent 1220 Infinity) equipped with a C18 column and a refractive index detector (RID), using 0.5 mM H 2 SO 4 aqueous solution as the mobile phase. The product yields (mmol/g cat .L) and selectivity (%) were calculated using equations 6 and 7 , respectively. $${\text{P}\text{r}\text{o}\text{d}\text{u}\text{c}\text{t}}_{\text{i}} \text{Y}\text{i}\text{e}\text{l}\text{d}= \frac{{\text{n}}_{\text{i}}}{{\text{m}}_{\text{c}\text{a}\text{t}} . {\text{V}}_{{\text{H}}_{2}\text{O}}}$$ 6 $$\text{P}\text{r}\text{o}\text{d}\text{u}\text{c}\text{t} \text{s}\text{e}\text{l}\text{e}\text{c}\text{t}\text{i}\text{v}\text{i}\text{t}\text{y}= \frac{{n}_{i}}{{\sum }_{i}{n}_{i}}\times 100$$ 7 Where \({n}_{i}\) = moles of product, \(i\) = HCOOH or CH 3 COOH, \({\text{m}}_{\text{c}\text{a}\text{t}}\) = mass of catalyst (g) and \({\text{V}}_{{\text{H}}_{2}\text{O}}\) = volume of water (L) The best catalyst was also evaluated for aqueous phase conversion using CO 2 , H 2 and methanol (10 mmol) as reactants and lithium iodide (10 mmol) as the promoter. All other reaction conditions were identical to the above described procedure. 2.5. Catalyst recycling study The catalyst recyclability was investigated using CO 2 , H 2 and CH 3 OH (10 mmol) as reactants and lithium iodide (10 mmol) as the promoter at 150°C, equimolar H 2 /CO 2 with 70 bar pressure at room temperature and 200 RPM stirring speed. After each cycle, the catalyst was recovered from the product mixture via centrifugation at 8500 RPM for 1 h and without any intermediate treatment, resuspended into a fresh reaction mixture at the same initial conditions. After five cycles, the centrifuged catalyst was dried overnight in oven at 70°C and stored in air tight glass vial for its characterisation. 2.6. Reaction mechanism investigation Reaction mechanism was explored by designing two different experiments – (1) using FA and CH 3 I as reactants and experiment was conducted in water by using T-MIL-88B catalyst at 150°C under 35 bar hydrogen and 200 RPM stirring speed. Typically, 40 mL H 2 O, 0.4 g of T-MIL-88B, 5 mmol (312.5 mmol/g cat .L) of HCOOH and 10 mmol (625 mmol/g cat .L) of CH 3 I were added in Teflon-liner and reactor was sealed. After achieving the above described conditions, 2 mL liquid sample was withdrawn from the reactor after regular intervals (1, 2, 4, 8, 12 and 24 h) for HPLC analysis. In the 2nd reaction system, aqueous phase CO 2 hydrogenation with CH 3 OH (10 mmol) and LiI (10 mmol) was performed over MIL-88B (0.4 g) for 48 h at 150 ˚C, 40 mL H 2 O, equimolar H 2 /CO 2 under 70 bar at room temperature and 200 RPM stirring speed. After 48 h, the reactor was cooled to room temperature. Both liquid and gas samples were collected for product analysis, where, gas sample was analysed through Shimadzu 2014 GC coupled with TCD and FID detectors, respectively. 3. Results And Discussion 3.1. Characterisation Figure 1 .a-c illustrates the PXRD diffractograms of the catalysts, before and after catalytic tests. Calcined Fe/CBEA catalyst showed characteristic peaks of α-Fe 2 O 3 , most of which were not observed in the reduced catalyst. Instead, the reduced catalyst showed Fe 0 peaks at 2θ = 44.7° and 65° and residual α-Fe 2 O 3 peaks at 35.98° and 62.83°. However, there were no Fe 0 or α-Fe 2 O 3 peaks detected in the used catalyst which indicated leaching of Fe from the catalyst support. The residual reaction solution slowly turned to red colour over a period of few days, indicating presence of iron oxides in the solution. Therefore, Fe/CBEA catalyst was not considered further. Both the fresh and the used T-Fe/MIL-101 catalyst showed peaks corresponding to Fe 3 O 4 , suggesting that the catalyst was stable after the reaction. However, the α-Fe 2 O 3 peaks observed in Fe/MIL-101 (Figure S1, ESI) which did not reduce to Fe 0 in T-Fe/MIL-101. T-MIL-88B catalyst showed peaks corresponding to both Fe 3 O 4 and Fe 0 , which remained steady after a single run of 48 h reaction time and 5 cycles of 21 h each. Only Fe 3 O 4 peaks have been reported after the thermal treatment of MIL-88B at 500°C under nitrogen atmosphere 23 . However, due to the reducing atmosphere used in this study, some of iron oxide nanoparticles reduced to Fe 0 . No evidence of iron carbide was found in the PXRD results. During the thermal transformation of MOFs, first, the linkers break from the metal oxide clusters. After that, the metal oxide clusters agglomerate and reduce depending upon the chemical environment. MIL-88B consists Fe 3 O clusters coordinated by six carboxylate ligands and three adsorbed water molecules, depending on the synthesis method (Figure 2 a). Based on our earlier computational study of thermal transformations in Zr-based MOFs 24 , we expect the following physiochemical transformations in MIL-88B upon thermal treatment. First, the adsorbed water molecules desorb, and at c.a. 100°C the MOF is expected to change the morphology 25 . Near the decomposition temperature, some of the linkers start detaching from the cluster. Unlike Fe oxide nanoparticles encapsulated in MIL-101(Cr), where the movement of nanoparticles is less hindered and can easily agglomerate, in MIL-88B, the Fe 3 O metal clusters are part of the framework and hence remain less mobile. After detachment of the organic linkers, the linkers go through thermolysis, resulting in formation of small gaseous molecules such as CO and CO 2 . At high temperature, hydrogen is expected to dissociate on iron and likely to catalyse the decarboxylation of linkers, reducing the Fe-O coordination. Without the oxygen from carboxylate groups, formation of single Fe 3 O 4 phase is stoichiometrically not possible in Fe 3 O. Hence, promoted decarboxylation in H 2 environment is likely to increase the abundance of a mixed Fe/Fe 3 O 4 metal nanoparticles. Figure 2 shows the proposed mechanism of thermal evolution of MIL-88B(Fe). Figure 3 .a-f shows the TEM images of MIL-101, Fe/MIL-101, T-Fe/MIL-101, MIL-88B, T-MIL-88B, and used T-MIL-88B, respectively. MIL-101 shows the characteristic octahedral shape of ca. 200-300 nm size (Figure 3 a and Figure S2a of ESI) 26 . After impregnation of Fe over MIL-101, agglomerates of Fe nanoparticles were observed on MIL-101 (Fe/MIL-101) with approximately 50-100 nm in size (Figure 3 b), whereas after thermal transformation, T-Fe/MIL-101 exhibited approximately 5-30 nm particles (Figure 3 c). The emergence of these smaller nanoparticles is likely due to the thermal transformation of Fe/MIL-101 in reductive atmosphere, where the deconstruction of linkers leads to breakage of the Fe agglomerates. Figure 3 d and Figure S2b (ESI) show the characteristic fusiform rod shaped morphology of MIL-88B with ~360 nm length and 90 nm width 23 . After thermal transformation, T-MIL-88B shows a narrow range of Fe 0 /Fe 3 O 4 nanoparticle which are well-dispersed over the carbonaceous support (Figure 2 e). The amount of Fe on T-MIL-88B is 49.3%, with 13.7% C and negligible amount of H, N and S (Table S1, ESI), which indicates that original MOF structure is completely transformed into porous carbon. Figure 3 f shows that the T-MIL-88B catalyst retains its structure after 48h of reaction. Figure 3 .g-h illustrates the particle size distribution (PSD) for T-MIL-88B and used T-MIL-88B, respectively. 525 and 476 particles were measured from multiple images which showed most of the particles in 4-16 nm for both fresh and used T-MIL-88B, respectively. The peaks were observed at 8 nm with average particle sizes of 9.7 and 9.1 nm for fresh and used T-MIL-88B, respectively which suggested that the studied catalyst is stable and potentially reusable for this reaction. The surface oxidation state of Fe in the different catalysts was evaluated by X-Ray photoelectron spectroscopy (XPS) study, as shown in Figure 4 . For T-MIL-88B (Figure 4 .a), Fe 2p 3/2 XPS spectrum exhibited three peaks, including a peak at 706.9 eV corresponding to metallic iron 27 . Moreover, the other two peaks at 710.1 and 712.3 eV which are correlated to Fe +2 and Fe +3 oxidation state of iron and the satellite peaks for these aforementioned oxidation state appeared at 716.6 and 719.8 eV 28 . In the Fe 2p region of T-MIL-88B, Fe2p 1/2 and Fe2p 3/2 peaks are situated 710.1 and 723.8 eV, where, the spin orbital splitting is 13.7 eV that indicated the presence of Fe 3 O 4 in T-MIL-88B 29 . Fe 3 O 4 may exist as mixed FeO and Fe 2 O 3 states, which appears from Fe +2 and Fe +3 oxidation states 30 . The present XPS study shows that Fe 3 O 4 is the dominant species on the surface, where the amount of Fe +2 was 60.4% and Fe +3 was 21.0%, whereas Fe 0 was 18.6%. Therefore, the ratio of Fe 0 to Fe 3 O 4 was accounted as 1/4.38 in T-MIL-88B. The XPS spectra of Fe 2p 3/2 in T-Fe/MIL-101 exhibited two peaks at 711.7 and 712.4 eV which is related to Fe +2 and Fe +3 along with two satellite peaks at 718.1 and 722.4 eV. Furthermore, Fe2p 1/2 and Fe2p 3/2 of Fe +2 appeared at 711.7 and 725.4 eV and the spin orbital splitting is 13.7 eV which interpreted the existence of Fe 3 O 4 in T-Fe/MIL-101. Metallic Fe peak is absent in this catalyst which is in good agreement with PXRD results. For Fe/MIL-101 catalyst, Fe 2p 3/2 XPS spectra also contained both Fe +2 and Fe +3 at 711.7 and 713.4 eV, respectively. However, the spin orbit splitting for Fe2p 1/2 and Fe2p 3/2 is 14.1 eV (711.7 and 725.8 eV) which suggested the absence of Fe 3 O 4 phase. Figure 4 b represented the Cr XPS spectra of MIL-101, Fe/MIL-101 and T-Fe/MIL-101 catalysts. In MIL-101, Cr 2p XPS spectra contained only one peak at 577.6 eV which is corresponds to Cr +3 oxidation state 31 . For Fe/MIL-101, Cr XPS spectra attributed to two peaks at 577.2 and 578.8 eV which are mainly resembles with Cr +3 and CrO 3 32 . The negative binding energy shift (0.4 eV) of Cr +3 as compared to Cr +3 present in MIL-101 is most likely due to the interfacial electronic interaction (charge transfer) between Cr and Fe after the inclusion of Fe in MIL-101 28 . The Cr spectra for T-Fe/MIL-101, Cr XPS spectra mainly consisted with Cr +3 peak at 577.1 eV and the amount of CrO 3 is very less as compared to Fe/MIL-101 which may be due to the thermal transformation of Fe/MIL-101 under hydrogen atmosphere that reduces the oxidised Cr species on catalyst surface. The C 1s XPS spectra for Fe/MIL-101 (Figure 3 c) shows three different types of C peak at 285, 286.4 and 288.5 which belongs to C-C, C-O-C and O-C=O 33 . The C 1s XPS spectra of both T-Fe/MIL-101 and T-MIL-88B contains only two peaks corresponding to C-C and C-O-C, whereas, the O-C=O peak is absent, which may be due to the thermal transformation of both Fe/MIL-101 and MIL-88B under hydrogen atmosphere reducing the oxygen content in the catalyst. A thermogravimetric analysis of Fe/MIL-101 and MIL-88B has been represented in Figure 5 . For Fe/MIL-101, the weight loss in the range of 50-250°C is because of the evaporation of water and removal of free terephthalates inside the pore of MOF 34 . Thereafter, the main weight loss in the temperature range of 270 to 670°C is due to the degradation of organic ligand in the framework of MOF which is attributed to the collapse of the framework 34 . The weight loss of MIL-88B before 250°C corresponds to the removal of water and excess DMF from the framework 35 . For MIL-88B, the weight loss occurs in the temperature ranges of 300 to 500°C due to the degradation of H 2 BDC and the breakdown of the framework. The step in the TGA profile of between 550-650°C is most likely due to the carbonization of the framework and the formation of Fe 3 O 4 –carbon composites 35 . 3.2. Catalyst Activities 3.2.1. Role of Fe based zeolite and MOF catalysts Figure 6 .a-c illustrates the yield and selectivity of AA via aqueous phase CO 2 reduction with iodomethane at various pressures. All the catalysts showed some activity for AA production; however, T-MIL-88B was clearly the most active and selective catalyst with best yield of 504 mmol/g cat .L and AA selectivity of 92.4%. Based on stoichiometric calculation, it is equivalent to 80.6% conversion of CH 3 I into AA. Both Fe/CBEA and T-Fe/MIL-101 provide lower activity for CO 2 hydrogenation and >90% selectivity for FA production. With increasing pressure, the yield increased initially but the AA selectivity peaked at 60 bar for both Fe/CBEA and T-Fe/MIL-101. However, the AA yield and selectivity increases with increasing pressure for T-MIL-88B. Since Fe was present in the structural framework of T-MIL-88B, the thermally transformed catalyst consists of - embedded active metal sites dispersed evenly in a carbon matrix 23 . The high AA activity and the selectivity over T-MIL-88B catalyst is most likely due to the presence of both Fe 0 and Fe 3 O 4 which assist the hydrogenation and C-C coupling reactions, respectively 36 , 37 . 3.2.2. Extent of reaction with time Figure 7 .a illustrates the extent of reaction over T-MIL-88B to produce AA and FA via CO 2 hydrogenation with CH 3 I as the starting material in the aqueous media. The reaction proceeds via formation of FA as the initial product, whereas AA was not detected until after 8h of reaction. The AA yield and selectively sharply increased between 12 to 24 h, thereafter gradually increasing to 657.6mmol/g cat .L and 98.8%, respectively, at 48h as the reaction approached equilibrium conversion. Based on the initial CH 3 I concentration (10 mmol), 100% conversion at 100% selectivity for AA was achieved, within the range of measurement errors. However, as discussed later, CO 2 first converts into FA and after reaching the maximum yield (377.4 mmol/g cat .L) at 8h, the FA yield decreases sharply until the end of reaction at 48 h when the FA yield was measured at 8.1 mmol/g cat .L. However, since CH 3 I is consumed by this time, the residual FA cannot convert into AA. Therefore, for the CO 2 hydrogenated into carboxylic acids, the selectivity of AA is 98.8%. When CH 3 OH (10 mmol) was used as a reactant with LiI as a co-catalyst (Figure 7 b), in otherwise identical reaction conditions, the reaction generates in situ CH3I and hence the peak of FA is broader than Figure 7 a. The AA yield and selectivity increased more gradually and achieved a similar yield of 590.1 mmol/g cat .L at 81.7% selectivity after 48 h, which is equivalent to 94% conversion of CH3OH into AA. The in-situ production of CH 3 I slowed down the conversion of FA into AA, which may be due to mass transfer limitation. 3.2.3. Catalyst reusability Figure 8 shows that the catalytic activity dropped initially but after three cycles, there was no significant decline in AA yield and selectivity. The PXRD of the used catalyst after five cycles (Figure 1 .c), and the TEM image (Figure 3 .f) and PSD (Figure 3 .h) of used catalyst after 48 h confirmed that the structure is stable and there was no sintering or agglomeration of Fe and Fe 3 O 4 nanoparticles in T-MIL-88B. The initial loss in activity is likely due to the loss of small particles of the catalyst which could not be recollected in centrifuge. 3.2.4. Proposed Reaction Pathway Reaction mechanism of hydrocarboxylation of methanol in an organic solvent proceeds via reaction of CH 3 OH with LiI to produce CH 3 I and LiOH which is similar to the carbonylation of methanol (Monsanto processes) followed by formation of CH 3 Rh*I due to the insertion of CH 3 I into a Rh* complexing catalyst 13 . Further, CO 2 is inserted into CH 3 -Rh bond to produce CH 3 COORh*I. Finally, CH 3 COOH is formed via reduction of CH 3 COORh*I with H 2 molecule in the presence of Ru* to produce HI as an intermediate. Whereas, LiI is regenerated in situ via HI formation which reacts with LiOH to produce H 2 O and LiI. However, here we show aqueous phase methanol hydrocarboxylation in which the reaction pathway deviates from the published works and FA is formed as an intermediate. First, we show that FA can react with CH 3 I in water over T-MIL-88B in H 2 atmosphere (Figure 9 ). The conversion of FA closely follows AA yield and after 24 h of the reaction FA conversion of 91.5% is achieved with 100% AA selectivity. Next, we show aqueous phase hydrocarboxylation of CH 3 OH using T-MIL-88B as catalyst and LiI as co-catalyst. Here both liquid and gas samples were collected after 48 of reaction. The liquid sample showed only the presence of HCOOH and CH 3 COOH with 81.7% acetic acid selectivity (Figure 7 .b). Whereas gas analysis did not detect any carbonaceous molecules apart from CO 2 (ESI, Figure S4), which eliminates the methanol carbonylation route for AA production. Figure 10 shows the proposed reaction pathway for acetic acid production via hydrocarboxylation of CH 3 OH over T-MIL-88B. CO 2 and H 2 adsorbed over the catalyst and converted into FA, which may desorb. Subsequently, the adsorbed formate species reacts with iodomethane (CH 3 I) to allow C-C coupling reaction to take place which generates an acetate species and HI as the by-product. Finally, acetate species is converted into acetic acid, whilst LiI might be regenerated from LiOH and HI (step 8). 4. Conclusions We show that thermally transformed Fe-based metal organic framework-based catalyst (T-MIL-88B) exhibited high catalytic activity and stability for aqueous phase CO 2 transformation into acetic acid. Here, the catalytic activity and the structural property of T-MIL-88B was compared with Fe/CBEA and thermally transformed Fe deposited on MIL-101 (T-Fe/MIL-101). The T-MIL-88B consisted both Fe 0 and Fe 3 O 4 phases, which catalyse hydrogenation and C-C coupling reactions, respectively, making this catalyst superior to the others tested here. Using CH 3 OH, CO 2 and H 2 as reactants in aqueous phase, and LiI the promoter, a maximum acetic acid yield of 590.1 mmol/g cat .L, with 81.7% selectivity was achieved after 48 h at 150 ˚C. We propose that the hydrocarboxylation of methanol to make acetic acid is mediated by formate route, which is evidenced by formic acid as an intermediate. The T-MIL-88B catalyst was active for at least five cycles for acetic acid production without showing any signs of deactivation via sintering, oxidation or phase change. Declarations Acknowledgement Authors would like to thank the Faculty of Engineering, Monash University for financial support under the Researcher Accelerator Grant 2019. Authors would also acknowledge Monash Centre for Electron Microscopy (MCEM) for providing the microscopic analysis facilities. AT and AS received financial support from the Institute for Catalysis, Hokkaido University as part of their Strategic Research Fellowship grant scheme. This study was supported by the Cooperative Research Program of Institute for Catalysis, Hokkaido University (Proposal no. 19A1005). References Li W, et al. 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Synergistic catalysis on Fe–Nx sites and Fe nanoparticles for efficient synthesis of quinolines and quinazolinones via oxidative coupling of amines and aldehydes. Chemical science 10 , 10283–10289 (2019). Koley P, et al. Leveraging Cu/CuFe2O4-Catalyzed Biomass-Derived Furfural Hydrodeoxygenation: A Nanoscale Metal–Organic-Framework Template Is the Prime Key. ACS applied materials & interfaces 12 , 21682–21700 (2020). Lee H, Lee W-J, Park Y-K, Ki SJ, Kim B-J, Jung S-C. Liquid phase plasma synthesis of iron oxide nanoparticles on nitrogen-doped activated carbon resulting in nanocomposite for supercapacitor applications. Nanomaterials 8 , 190 (2018). Madhuvilakku R, Alagar S, Mariappan R, Piraman S. Green one-pot synthesis of flowers-like Fe3O4/rGO hybrid nanocomposites for effective electrochemical detection of riboflavin and low-cost supercapacitor applications. Sensors and Actuators B: Chemical 253 , 879–892 (2017). Huo Q, et al. Preparation of a direct Z-scheme α-Fe2O3/MIL-101 (Cr) hybrid for degradation of carbamazepine under visible light irradiation. Applied Catalysis B: Environmental 255 , 117751 (2019). Guo L, Qin S, Yang B, Liang D, Qiao L. Effect of hydrogen on semiconductive properties of passive film on ferrite and austenite phases in a duplex stainless steel. Scientific reports 7 , 1–6 (2017). Fen-rong L, Wen L, Hui-qing G, Bao-qing L, Zong-qing B, Rui-sheng H. XPS study on the change of carbon-containing groups and sulfur transformation on coal surface. Journal of Fuel Chemistry and Technology 39 , 81–84 (2011). Liu Z, He W, Zhang Q, Shapour H, Bakhtari MF. Preparation of a GO/MIL-101 (Fe) Composite for the Removal of Methyl Orange from Aqueous Solution. ACS omega , (2021). Hou S, et al. Green synthesis and evaluation of an iron-based metal–organic framework MIL-88B for efficient decontamination of arsenate from water. Dalton transactions 47 , 2222–2231 (2018). Zhang L, Chen X, Peng Z, Liang C. Chemoselective hydrogenation of cinnamaldehyde over MOFs-derived M2Si@ C (M= Fe, Co, Ni) silicides catalysts. Molecular Catalysis 449 , 14–24 (2018). Zeng T, Chen W-W, Cirtiu CM, Moores A, Song G, Li C-J. Fe 3 O 4 nanoparticles: a robust and magnetically recoverable catalyst for three-component coupling of aldehyde, alkyne and amine. Green Chemistry 12 , 570–573 (2010). Additional Declarations There is NO Competing Interest. Supplementary Files 20211025AceticAcidManuscriptESI.docx Electronic Supplementary Information Cite Share Download PDF Status: Published Journal Publication published 17 May, 2023 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1029433","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":62377593,"identity":"e06b4260-2e97-46ce-a8f3-4c7679dce616","order_by":0,"name":"Waqar Ahmad","email":"","orcid":"","institution":"Monash University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Waqar","middleName":"","lastName":"Ahmad","suffix":""},{"id":62377594,"identity":"69c85b48-460d-430d-b7d7-27e794da3cb2","order_by":1,"name":"Paramita Koley","email":"","orcid":"","institution":"Monash University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Paramita","middleName":"","lastName":"Koley","suffix":""},{"id":62377595,"identity":"9c3e6a1c-cb79-4072-aede-e8f4e071fc3d","order_by":2,"name":"Swarit Dwivedi","email":"","orcid":"","institution":"Monash University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Swarit","middleName":"","lastName":"Dwivedi","suffix":""},{"id":62377596,"identity":"1dc9d081-8dd9-4c89-8322-2c957877e785","order_by":3,"name":"Abhijit Shrotri","email":"","orcid":"","institution":"Hokkaido University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Abhijit","middleName":"","lastName":"Shrotri","suffix":""},{"id":62377597,"identity":"0e01dd10-81ec-4831-9391-e45f2f552856","order_by":4,"name":"Akshat Tanksale","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABFklEQVRIiWNgGAWjYDACHgY2KAtEFzDIgdmMDUDiAD4tCQwSEC0GDMaka0lsIKSFv+f4swc/fzDU8c9uS/7ww8AmfW1778GPP3cwyPHdSMCqReJsj7lhD9AWiTvHjkn2GKTlbjtzLlma9wyDsSQOLQznedgkeEAOu5HexsBjcDh3240cA2nGNobEDTi0yJ9nfyb5B6hF/kZ688c/Bv/TzW7kGP/82cZQj0uLwdkGM2mQLQY30g5I8xgcSABqMZPgbWNIMMChxfDMGTNpmTQJyY030tKkZQySDbcBRax52yQMZ555gFWL3Jn0Z5JvbGz45W6kGX98U2Enb3a8x/jmzzYbeb7jOLwPDTgiREbBKBgFo2AUEA8AuzpgpaKGmtcAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-7087-0912","institution":"Monash University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Akshat","middleName":"","lastName":"Tanksale","suffix":""}],"badges":[],"createdAt":"2021-10-29 02:15:38","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1029433/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1029433/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-023-38506-5","type":"published","date":"2023-05-17T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":15403159,"identity":"9fdaf497-27ab-4e66-8914-79f52e3c68fa","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":109399,"visible":true,"origin":"","legend":"PXRD patterns of (a) as prepared, calcined, reduced and used Fe/CBEA, (b) as prepared and used T-Fe/MIL-101, and (c) as prepared and used T-MIL-88B catalysts after 48 h reaction in presence of CH3I and 5 cycles of 21 h each in presence of CH3OH and LiI.","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/7d7125c8b9558e5a7e8bcf52.jpg"},{"id":15403352,"identity":"33c92973-a717-4b73-b18c-f8b76c654e54","added_by":"auto","created_at":"2021-11-10 16:32:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1265005,"visible":true,"origin":"","legend":"Proposed mechanism of thermal transformation of MIL-88B(Fe) structure. (a) Shows the metal cluster topology, (b) shows MOF supercell structure, (c) desorbed water molecules within the pores of MOF, (d) shows the detachment of organic linkers, and (e) is the thermally transformed MIL-88B(Fe).","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/74d029e99b9fcc484521052d.png"},{"id":15403355,"identity":"4a0e9562-9203-4a8a-b974-70c99bca2171","added_by":"auto","created_at":"2021-11-10 16:32:14","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":132373,"visible":true,"origin":"","legend":"TEM micrographs of various studied catalysts, (a) MIL-101, (b) Fe/MIL-101, (c) T-Fe/MIL-101, (d) MIL-88B, (e) T-MIL-88B, and (f) used T-MIL-88B after 48 h of aqueous phase CO2 hydrogenation reaction in the vicinity of CH3OH and LiI additives; and particle size distribution of (g) T-MIL-88B, and (h) used T-MIL-88B.","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/b7fc7984fb2a320944b745a1.jpeg"},{"id":15403160,"identity":"b372c40f-ee40-458f-9a78-a6ffffb9108a","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":248811,"visible":true,"origin":"","legend":"Narrow scan XPS spectra of (a) Iron 2p, (b) Chromium 2p, and (c) Carbon 1s for the studied catalysts. ","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/2139135c50ad03cbbe6a68e0.jpeg"},{"id":15403793,"identity":"c60737e3-9c80-4fb2-ad75-2e2e5cc813d4","added_by":"auto","created_at":"2021-11-10 16:38:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":19732,"visible":true,"origin":"","legend":"Thermogravimetric analysis (TGA) of Fe/MIL-101 and MIL-88B under Argon atmosphere.","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/f6c4ed1713539bc59f2431c9.png"},{"id":15403169,"identity":"a49a815b-2887-48a7-a290-95009530c90d","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":85681,"visible":true,"origin":"","legend":"Activity of various Fe based catalysts during aqueous phase CO2 hydrogenation in the presence of CH3I additive at different pressure, (a) Fe/CBEA, (b) T-Fe/MIL-101, and (c) T-MIL-88B. Reaction conditions: T= 150 ˚C, H2/CO2= 1, tR= 21 h, H2O= 40 mL and stirring speed= 200 RPM.","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/98aab3cf9c8cc711300324f0.jpg"},{"id":15403162,"identity":"9988f2d4-cbbe-46e9-a659-57a7006807cd","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":60289,"visible":true,"origin":"","legend":"Effect of reaction time on carboxylic acids yield and selectivity via aqueous phase CO2 hydrogenation over T-MIL-88B in the presence of various additives, (a) CH3I, and (b) CH3OH and LiI. Reaction conditions: T= 150 ˚C, H2/CO2= 1, Ptotal= 70 bar at room temperature and stirring speed= 200 RPM.","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/5cbc3f76c102ecb82bc0ba18.png"},{"id":15403523,"identity":"c4795fc2-24e1-42b4-9a86-f6ae9cf677b8","added_by":"auto","created_at":"2021-11-10 16:35:14","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":28258,"visible":true,"origin":"","legend":"Recycling study of T-MIL-88B via aqueous phase CO2 hydrogenation in the presence of CH3OH and LiI additives. Reaction conditions: T= 150 ˚C, H2/CO2= 1, tR= 21 h, Ptotal= 70 bar at room temperature and stirring speed= 200 RPM.","description":"","filename":"Onlinefloatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/c066394300d2d18cf7537081.png"},{"id":15403166,"identity":"788ed5a2-050f-4f1b-a878-f9d28c7bbc7f","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":22598,"visible":true,"origin":"","legend":"Acetic acid production through HCOOH and CH3I reaction in water over T-MIL-88B in the presence of hydrogenation. Reaction conditions: T= 150 ˚C, nHCOOH=5 mmol, nCH3I=10 mmol, VH2O=40 mL, PH2=35 bar at room temperature and stirring speed= 200 RPM.","description":"","filename":"Onlinefloatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/c9c27a6f07e6df8dc405c3ae.png"},{"id":15403167,"identity":"307f2c54-1b0d-43fb-9e1f-376189ddbd9e","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":71571,"visible":true,"origin":"","legend":"Possible reaction route for acetic acid production via aqueous phase CO2 hydrogenation in the vicinity of methanol and LiI additives over T-MIL-88B catalyst.","description":"","filename":"Onlinefloatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/ea3e735f76e7f75cbf8f4b2a.png"},{"id":37174060,"identity":"733a5062-7074-48eb-9bc4-e6fd341ce42d","added_by":"auto","created_at":"2023-05-18 07:09:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2301805,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/289df60e-ef93-46b3-bc09-de785c46f24f.pdf"},{"id":15403165,"identity":"9684e0fa-519e-484f-bddc-e004ebba433a","added_by":"auto","created_at":"2021-11-10 16:29:14","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1318525,"visible":true,"origin":"","legend":"Electronic Supplementary Information","description":"","filename":"20211025AceticAcidManuscriptESI.docx","url":"https://assets-eu.researchsquare.com/files/rs-1029433/v1/bb4c24b881c8800e38c27d1e.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003eAqueous phase conversion of CO\u003csub\u003e2\u003c/sub\u003e into acetic acid over thermally transformed MIL-88B\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFixation of overabundant atmospheric carbon dioxide is an urgent and essential research area, which may lead towards climate mitigation. Several routes for carbon dioxide conversion have been investigated, but thermocatalytic CO\u003csub\u003e2\u003c/sub\u003e hydrogenation pathway is one of the major focus due to its fast kinetics, high productivity, scalability and selectivity \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Synthesis of chemicals such as methane\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, methanol\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e, formaldehyde\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, dimethyl ether\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, gasoline-range hydrocarbons\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, oxymethylene dimethyl ethers\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, methyl formate\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, formic acid\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e, and acetic acid\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e have been investigated in recent years. A CO\u003csub\u003e2\u003c/sub\u003e based chemicals industry has the potential to lower the CO\u003csub\u003e2\u003c/sub\u003e concentration in atmosphere, while simultaneously provide revenue for offsetting the capture costs. The production of acetic acid (AA) via CO\u003csub\u003e2\u003c/sub\u003e hydrogenation is one such route which is receiving attention of the researchers recently.\u003c/p\u003e\n\u003cp\u003eAcetic acid is extensively used in several industrial applications, including food, chemicals, pharmaceuticals, textile, cosmetics and polymers \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. It is a well-known food preservative and traditionally named as vinegar in food industry. Commercially, two major production processes are used for the synthesis of acetic acid \u0026ndash; chemical and fermentative \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Among various chemical routes, the most common industrial processes are carbonylation of methanol (MeOH) developed by BASF, Cativa and Monsanto, in the presence of homogeneous Cobalt, Iridium and Rhodium catalysts, respectively. In Monsanto process, AA is produced from CH\u003csub\u003e3\u003c/sub\u003eOH and fossil fuel derived CO in the presence of CH\u003csub\u003e3\u003c/sub\u003eI and homogeneous rhodium-based catalyst\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. The main reaction of acetic acid production from methanol and CO is summarized in Eq.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"Equation\" id=\"Equ1\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$C{H}_{3}OH+CO \\leftrightarrows C{H}_{3}COOH$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eQian et al. recently reported AA production via hydrocarboxylation of MeOH with carbon dioxide and hydrogen in 1,3-dimethyl-2-imidazolidinone (DMI) solvent over homogeneous Rh and Ru based homogeneous co-catalysts with a combination of LiI promoter and imidazole ligand. While the authors report that imidazole played critical role in inhibiting the reverse water gas shift reaction, but the exact role of imidazole in the reaction mechanism was not clear\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. The same group also showed AA synthesis via the above described reaction system in the presence of Rh\u003csub\u003e2\u003c/sub\u003e(CO)\u003csub\u003e4\u003c/sub\u003eCl\u003csub\u003e2\u003c/sub\u003e homogeneous catalyst, LiCl as a co-catalyst, 4-methyl imidazole ligand and LiI as a promoter \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. This reaction system is highly complex due to the presence of multiple catalysts, stabilizing ligands and organic solvents. In many cases, the authors report a black precipitate, which is not explained but is likely to be the Ru or Rh catalyst, which demonstrates that the system is not stable. Hasan et al. reported low yield of acetic acid (1.58 mmol/L) over NiO-C/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, heterogeneous catalyst at 130\u0026deg;C and 35 bar total pressure of CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e in 1,4 dioxane solvent after 6h of reaction. Instead a higher amount of formic acid (FA, 4.08 mmol/L) was generated \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. Therefore, there is an urgent need to develop a stable and active heterogeneous catalyst based on low cost metals for AA synthesis which can efficient for industrialisation and scaleup.\u003c/p\u003e\n\u003cp\u003eHe et. al. report FA and AA production via hydrothermal CO\u003csub\u003e2\u003c/sub\u003e reduction with Fe nanoparticles as stoichiometric reagent in which they are converted into ferrous carbonate\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. To the best of our knowledge, Fe-based heterogeneous catalysts have not been reported CO\u003csub\u003e2\u003c/sub\u003e conversion in aqueous phase. Heterogeneous catalysts have advantages in scale-up, and compares favourably against homogeneous catalysts which require large downstream separation processes.\u003c/p\u003e\n\u003cp\u003eHere we present a Fe-based thermally transformed metal organic framework catalyst (MIL-88B) for hydrocarboxylation of MeOH to produced AA. Recently, Metal Organic Framework (MOFs) derived carbonaceous materials have been reported for their remarkable catalytic properties\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Thermal transformation of MOFs results in a carbonaceous material with embedded metal or metal-oxide nanoparticles\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. As these particles are embedded in the matrix of decomposed organic linkers, they show greater resistance to sintering at higher temperatures. Depending on the thermal treatment, the thermally transformed MOFs have features such as high surface area, porosity, and fine dispersion of metal nanoparticles that are desired in an ideal heterogeneous catalyst. Moreover, the porous carbon framework provides better mass transfer to enhance the reaction rate. In this work, thermally transformed MIL-88B, called T-MIL-88B, consisted of dual active sites \u0026ndash; Fe\u003csup\u003e0\u003c/sup\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, accelerating the conversion of CO\u003csub\u003e2\u003c/sub\u003e into AA, compared with other Fe-based catalysts tested which contained only Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e or Fe\u003csup\u003e0\u003c/sup\u003e and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. In this process, AA is produced in a series of reactions (eq. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e-\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e) \u0026ndash;\u003c/p\u003e\n\u003cdiv class=\"Equation\" id=\"Equ2\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e$$C{O}_{2 \\left(aq\\right)}+{H}_{2 \\left(aq\\right)}\\underleftrightarrow{\\text{Fe}} HCOOH$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Equation\" id=\"Equ3\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e$$C{H}_{3}O{H}_{\\left(l\\right)}+LiI\\leftrightarrow C{H}_{3}I+LiOH$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Equation\" id=\"Equ4\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e$$C{H}_{3}I+HCOOH+LiOH\\leftrightarrow C{H}_{3}COOH+LiI+{H}_{2}O$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cstrong\u003eOverall Reaction\u003c/strong\u003e\u003c/p\u003e\n\u003cdiv class=\"Equation\" id=\"Equ5\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e$$C{O}_{2 \\left(aq\\right)}+{H}_{2 \\left(aq\\right)}+ C{H}_{3}O{H}_{\\left(l\\right)}\\underleftrightarrow{\\text{Fe, LiI, 150\u0026deg;C}}{H}_{3}CCOOH+{H}_{2}O$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\n\u003c/div\u003e"},{"header":"2. Methods","content":"\u003cdiv class=\"Section2\" id=\"Sec3\"\u003e\n \u003ch2\u003e2.1. Materials\u003c/h2\u003e\n \u003cp\u003eIodomethane (CH\u003csub\u003e3\u003c/sub\u003eI, 99.5%), formic acid (HCOOH, \u0026ge; 95%), lithium Iodide (LiI, 99.9%), terephthalic acid (H\u003csub\u003e2\u003c/sub\u003eBDC, 98%), chromium chloride hexahydrate (CrCl\u003csub\u003e3\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO, 98%), and iron nitrate nonahydrate (Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e.9H\u003csub\u003e2\u003c/sub\u003eO, 98%) were purchased from the Sigma Aldrich. Commercial zeolite-beta (CBEA, SiO\u003csub\u003e2\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e= 38) was received from Zeolyst International. Methanol (HPLC grade) was obtained from the Scharlau Chemicals. Milli-Q water was used for catalysts synthesis (MIL-101 and Fe/CBEA) and acetic acid production experiments.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec4\"\u003e\n \u003ch2\u003e2.2. Catalysts synthesis\u003c/h2\u003e\n \u003cdiv class=\"Section3\" id=\"Sec5\"\u003e\n \u003ch2\u003e2.2.1. Fe/CBEA\u003c/h2\u003e\n \u003cp\u003eWet impregnation process was used for Fe/CBEA synthesis as described in our previous publication\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. The loading of Fe was fixed as 10 wt% in this catalyst. Typically, Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e.9H\u003csub\u003e2\u003c/sub\u003eO (7.2 g) was dissolved in Milli-Q water (30 mL) by using 100 mL Schott bottle and stirred for 15 min at 65\u0026deg;C to prepare a homogeneous mixture of Fe solution. Thereafter, 9.0 g of CBEA support was immersed in this solution under stirring and maintained it for 6 h at the same temperature to achieve an even dispersion of Fe particles on CBEA support. The mixture was dried in oven at 100\u0026deg;C followed by calcination at 550\u0026deg;C with 5\u0026deg;C/min for 5 h in muffle furnace. The synthesised catalyst was reduced in the environment of H\u003csub\u003e2\u003c/sub\u003e/Ar (1:1 v/v) gas mixture at 400\u0026deg;C for 5 h with heating rate of 5\u0026deg;C/min prior to carbon dioxide conversion experiment.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec6\"\u003e\n \u003ch2\u003e2.2.2. Thermally transformed Fe/MIL-101\u003c/h2\u003e\n \u003cp\u003e10 mmol of H\u003csub\u003e2\u003c/sub\u003eBDC and 10 mmol CrCl\u003csub\u003e3\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO were poured into a Teflon-lined autoclave. Subsequently, Milli-Q water (72 ml) was added to it. The reaction mixture was sonicated for 30 minutes followed by stirring for another 30 minutes at 500 rpm. Thereafter, the autoclave was kept in the oven at 205\u0026deg;C for 24 h and allowed to cool to room temperature. The resulting solid suspension was transferred into a centrifuge tube. Initially the centrifugation was performed at 1000 rpm for 3-4 min to remove the unreacted H\u003csub\u003e2\u003c/sub\u003eBDC present in the reaction mixture. Thereafter, the centrifugation was carried out at 5000 rpm for 10 minutes. The solid sample was then washed with dimethylformamide (DMF) three times and then dried in an oven at 70\u0026deg;C for 12 h. The synthesised material was named as MIL-101.\u003c/p\u003e\n \u003cp\u003eFor Fe/MIL-101 synthesis, 2.7 g of MIL-101 was suspended in 70 ml ethanol in a Schott bottle and sonicated for 30 minutes. Separately, 2.17 g Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e.9H\u003csub\u003e2\u003c/sub\u003eO was dissolved in 20 ml ethanol in a different Schott bottle and stirred for 15 minutes. The latter solution was poured into the former suspension of MIL-101 in ethanol. Then the Schott bottle which contained Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e solution was washed with 10 ml ethanol three times and poured into MIL-101 suspension to ensure complete transfer of the Fe precursor. The resultant mixture was sonicated for 30 min followed by stirring at 50\u0026deg;C at 500 rpm for 5-6 h. Finally, the resulting reaction mixture was dried in an oven at 80\u0026deg;C for 2-3 days. The synthesized catalyst was named as Fe/MIL-101. The Fe loading was fixed as 10 wt% in the synthesized catalyst. Prior to catalytic activity test, this catalyst was thermally transformed under 100 ml/min H\u003csub\u003e2\u003c/sub\u003e/Ar (1:1) gas mixture at 500\u0026deg;C for 5 h with a heating rate of 5\u0026deg;C/min and allowed to cool in 50 ml/min Ar atmosphere and denoted as T-Fe/MIL-101.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec7\"\u003e\n \u003ch2\u003e2.2.3. Thermally transformed MIL-88B\u003c/h2\u003e\n \u003cp\u003eA modified hydrothermal method as described in the literature\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e was adopted for synthesis of MIL-88B. In a typical procedure, 12.12 g of Fe salt (Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e.9H\u003csub\u003e2\u003c/sub\u003eO) was dissolved in 75 ml DMF under stirring (500 RPM) in a Schott bottle. Separately, H\u003csub\u003e2\u003c/sub\u003eBDC (4.98 g) and DMF (75 ml) were added in a 250 ml Teflon-liner under stirring (500 RPM). Both Fe and H\u003csub\u003e2\u003c/sub\u003eBDC solutions were stirred further for 15 min at room temperature. The Fe solution was then poured into H\u003csub\u003e2\u003c/sub\u003eBDC precursor solution. 12 ml NaOH solution (4.0 M) was slowly transferred into Fe and H\u003csub\u003e2\u003c/sub\u003eBDC solution mixture and stirred again for 30 min at room temperature. Thereafter, the Teflon-liner was sealed in an autoclave and heated to 100\u0026deg;C for 24 h. After cooling to room temperature, MIL-88B particles were collected from this mixture via centrifugation at 7000 RPM for 10 min and washed three times with DMF and methanol, respectively. Finally, the as synthesized MIL-88B was dried overnight in the oven at 80\u0026deg;C and denoted as MIL-88B. Thermal transformation of MIL-88B (2 g) was conducted at 500\u0026deg;C for 5 h with a ramp of 5\u0026deg;C/min under 100 ml/min H\u003csub\u003e2\u003c/sub\u003e/Ar (1:1 v/v) environment followed by cooling to room temperature under Ar at 50 ml/min atmosphere and denoted as T-MIL-88B.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec8\"\u003e\n \u003ch2\u003e2.3. Catalyst characterisation\u003c/h2\u003e\n \u003cp\u003eThe crystal structure of the materials was investigated with Powder X-ray diffraction (PXRD) by using a Rigaku MiniFlex device. The powder catalysts were loaded in a zero-background sample holder and scanned between 2\u0026ndash;80\u0026deg; 2\u0026theta; with 4\u0026deg;/min scan speed at 15 mA and 40 kV. Nitrogen physisorption analysis was conducted with Micromeritics 3Flex 3500 machine to find the type of adsorption isotherm, Brunauer-Emmett-Teller (BET) surface area and Barrett-Joyner-Halenda (BJH) pore distribution. Tecani T20 was used to capture the transmission electron microscopy (TEM) images of the catalysts. All the samples were dispersed in ethanol and immobilised onto the surface of a holy carbon grid followed by drying in air prior to analysis. ThermoScientific K-Alpha machine was utilized for X-ray photoelectron spectroscopy (XPS) at 1486.6 eV Ephoton and coupled with monochromatic Al K\u0026alpha; radiations. The binding energy (B.E.) baseline correction was conducted by adjusting the C 1 s peaks at 284.8 eV. Thermally transformed samples were prepared \u003cem\u003eex situ\u003c/em\u003e prior to the XPS characterization. Shimadzu DTG-60H thermogravimetric analyser was used to check the thermal stability of Fe/MIL-101 and MIL-88B. Both samples were analysed in the temperature range of 100-800\u0026deg;C with a ramp of 5\u0026deg;C/min under Ar atmosphere.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec9\"\u003e\n \u003ch2\u003e2.4. Aqueous phase CO\u003csub\u003e2\u003c/sub\u003e conversion\u003c/h2\u003e\n \u003cp\u003eAll the aqueous phase CO\u003csub\u003e2\u003c/sub\u003e conversion experiments were performed in a 100 mL Teflon-lined autoclave batch reactor (Amar Equipment, M4). Typically, 0.4 g of thermally transformed catalyst (T-MIL-88B) and 40 mL water was added to the reactor and CH\u003csub\u003e3\u003c/sub\u003eI (10 mmol) was carefully poured into it and sealed. It was purged with hydrogen three times to eliminate air from the headspace. The reactor was then pressurised with CO\u003csub\u003e2\u003c/sub\u003e up to 35 bar, followed by H\u003csub\u003e2\u003c/sub\u003e up to a total pressure of 70 bar at room temperature to achieve CO\u003csub\u003e2\u003c/sub\u003e:H\u003csub\u003e2\u003c/sub\u003e ratio of 1:1. The reactor was heated to 150\u0026deg;C under continuous stirring at 200 RPM for 21 h. After 21 h of reaction, the reactor was allowed to cool to room temperature and the remaining gases were carefully vented from it before dissembling it. The catalyst was recovered from the liquid product mixture by centrifugation at 8500 RPM for 1 h. The same procedure was repeated for different total pressures at equimolar CO\u003csub\u003e2\u003c/sub\u003e:H\u003csub\u003e2\u003c/sub\u003e ratio and different catalysts (T-Fe/MIL-101 and Fe/CBEA). The liquid sample was analysed at intervals for the best catalyst to check the extent of reaction against time at 150\u0026deg;C, equimolar CO\u003csub\u003e2\u003c/sub\u003e:H\u003csub\u003e2\u003c/sub\u003e at 70 bar with 200 RPM stirring speed. The liquid samples were analysed using an HPLC (Agilent 1220 Infinity) equipped with a C18 column and a refractive index detector (RID), using 0.5 mM H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e aqueous solution as the mobile phase. The product yields (mmol/g\u003csub\u003ecat\u003c/sub\u003e.L) and selectivity (%) were calculated using equations \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e and\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e, respectively.\u003c/p\u003e\n \u003cdiv class=\"Equation\" id=\"Equ6\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e$${\\text{P}\\text{r}\\text{o}\\text{d}\\text{u}\\text{c}\\text{t}}_{\\text{i}} \\text{Y}\\text{i}\\text{e}\\text{l}\\text{d}= \\frac{{\\text{n}}_{\\text{i}}}{{\\text{m}}_{\\text{c}\\text{a}\\text{t}} . {\\text{V}}_{{\\text{H}}_{2}\\text{O}}}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Equation\" id=\"Equ7\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e$$\\text{P}\\text{r}\\text{o}\\text{d}\\text{u}\\text{c}\\text{t} \\text{s}\\text{e}\\text{l}\\text{e}\\text{c}\\text{t}\\text{i}\\text{v}\\text{i}\\text{t}\\text{y}= \\frac{{n}_{i}}{{\\sum }_{i}{n}_{i}}\\times 100$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({n}_{i}\\)\u003c/span\u003e\u003c/span\u003e= moles of product, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(i\\)\u003c/span\u003e\u003c/span\u003e = HCOOH or CH\u003csub\u003e3\u003c/sub\u003eCOOH, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{m}}_{\\text{c}\\text{a}\\text{t}}\\)\u003c/span\u003e\u003c/span\u003e = mass of catalyst (g) and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\text{V}}_{{\\text{H}}_{2}\\text{O}}\\)\u003c/span\u003e\u003c/span\u003e = volume of water (L)\u003c/p\u003e\n \u003cp\u003eThe best catalyst was also evaluated for aqueous phase conversion using CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003e and methanol (10 mmol) as reactants and lithium iodide (10 mmol) as the promoter. All other reaction conditions were identical to the above described procedure.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec10\"\u003e\n \u003ch2\u003e2.5. Catalyst recycling study\u003c/h2\u003e\n \u003cp\u003eThe catalyst recyclability was investigated using CO\u003csub\u003e2\u003c/sub\u003e, H\u003csub\u003e2\u003c/sub\u003e and CH\u003csub\u003e3\u003c/sub\u003eOH (10 mmol) as reactants and lithium iodide (10 mmol) as the promoter at 150\u0026deg;C, equimolar H\u003csub\u003e2\u003c/sub\u003e/CO\u003csub\u003e2\u003c/sub\u003e with 70 bar pressure at room temperature and 200 RPM stirring speed. After each cycle, the catalyst was recovered from the product mixture via centrifugation at 8500 RPM for 1 h and without any intermediate treatment, resuspended into a fresh reaction mixture at the same initial conditions. After five cycles, the centrifuged catalyst was dried overnight in oven at 70\u0026deg;C and stored in air tight glass vial for its characterisation.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec11\"\u003e\n \u003ch2\u003e2.6. Reaction mechanism investigation\u003c/h2\u003e\n \u003cp\u003eReaction mechanism was explored by designing two different experiments \u0026ndash; (1) using FA and CH\u003csub\u003e3\u003c/sub\u003eI as reactants and experiment was conducted in water by using T-MIL-88B catalyst at 150\u0026deg;C under 35 bar hydrogen and 200 RPM stirring speed. Typically, 40 mL H\u003csub\u003e2\u003c/sub\u003eO, 0.4 g of T-MIL-88B, 5 mmol (312.5 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L) of HCOOH and 10 mmol (625 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L) of CH\u003csub\u003e3\u003c/sub\u003eI were added in Teflon-liner and reactor was sealed. After achieving the above described conditions, 2 mL liquid sample was withdrawn from the reactor after regular intervals (1, 2, 4, 8, 12 and 24 h) for HPLC analysis. In the 2nd reaction system, aqueous phase CO\u003csub\u003e2\u003c/sub\u003e hydrogenation with CH\u003csub\u003e3\u003c/sub\u003eOH (10 mmol) and LiI (10 mmol) was performed over MIL-88B (0.4 g) for 48 h at 150 ˚C, 40 mL H\u003csub\u003e2\u003c/sub\u003eO, equimolar H\u003csub\u003e2\u003c/sub\u003e/CO\u003csub\u003e2\u003c/sub\u003e under 70 bar at room temperature and 200 RPM stirring speed. After 48 h, the reactor was cooled to room temperature. Both liquid and gas samples were collected for product analysis, where, gas sample was analysed through Shimadzu 2014 GC coupled with TCD and FID detectors, respectively.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results And Discussion","content":"\u003cdiv class=\"Section2\" id=\"Sec13\"\u003e\n \u003ch2\u003e3.1. Characterisation\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.a-c illustrates the PXRD diffractograms of the catalysts, before and after catalytic tests. Calcined Fe/CBEA catalyst showed characteristic peaks of \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, most of which were not observed in the reduced catalyst. Instead, the reduced catalyst showed Fe\u003csup\u003e0\u003c/sup\u003e peaks at 2\u0026theta;\u0026thinsp;=\u0026thinsp;44.7\u0026deg; and 65\u0026deg; and residual \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e peaks at 35.98\u0026deg; and 62.83\u0026deg;. However, there were no Fe\u003csup\u003e0\u003c/sup\u003e or \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e peaks detected in the used catalyst which indicated leaching of Fe from the catalyst support. The residual reaction solution slowly turned to red colour over a period of few days, indicating presence of iron oxides in the solution. Therefore, Fe/CBEA catalyst was not considered further.\u003c/p\u003e\n \u003cp\u003eBoth the fresh and the used T-Fe/MIL-101 catalyst showed peaks corresponding to Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, suggesting that the catalyst was stable after the reaction. However, the \u0026alpha;-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e peaks observed in Fe/MIL-101 (Figure S1, ESI) which did not reduce to Fe\u003csup\u003e0\u003c/sup\u003e in T-Fe/MIL-101.\u003c/p\u003e\n \u003cp\u003eT-MIL-88B catalyst showed peaks corresponding to both Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe\u003csup\u003e0\u003c/sup\u003e, which remained steady after a single run of 48 h reaction time and 5 cycles of 21 h each. Only Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e peaks have been reported after the thermal treatment of MIL-88B at 500\u0026deg;C under nitrogen atmosphere\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. However, due to the reducing atmosphere used in this study, some of iron oxide nanoparticles reduced to Fe\u003csup\u003e0\u003c/sup\u003e. No evidence of iron carbide was found in the PXRD results.\u003c/p\u003e\n \u003cp\u003eDuring the thermal transformation of MOFs, first, the linkers break from the metal oxide clusters. After that, the metal oxide clusters agglomerate and reduce depending upon the chemical environment. MIL-88B consists Fe\u003csub\u003e3\u003c/sub\u003eO clusters coordinated by six carboxylate ligands and three adsorbed water molecules, depending on the synthesis method (Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea). Based on our earlier computational study of thermal transformations in Zr-based MOFs\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, we expect the following physiochemical transformations in MIL-88B upon thermal treatment. First, the adsorbed water molecules desorb, and at c.a. 100\u0026deg;C the MOF is expected to change the morphology\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Near the decomposition temperature, some of the linkers start detaching from the cluster. Unlike Fe oxide nanoparticles encapsulated in MIL-101(Cr), where the movement of nanoparticles is less hindered and can easily agglomerate, in MIL-88B, the Fe\u003csub\u003e3\u003c/sub\u003eO metal clusters are part of the framework and hence remain less mobile. After detachment of the organic linkers, the linkers go through thermolysis, resulting in formation of small gaseous molecules such as CO and CO\u003csub\u003e2\u003c/sub\u003e. At high temperature, hydrogen is expected to dissociate on iron and likely to catalyse the decarboxylation of linkers, reducing the Fe-O coordination. Without the oxygen from carboxylate groups, formation of single Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase is stoichiometrically not possible in Fe\u003csub\u003e3\u003c/sub\u003eO. Hence, promoted decarboxylation in H\u003csub\u003e2\u003c/sub\u003e environment is likely to increase the abundance of a mixed Fe/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e metal nanoparticles. Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e shows the proposed mechanism of thermal evolution of MIL-88B(Fe).\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.a-f shows the TEM images of MIL-101, Fe/MIL-101, T-Fe/MIL-101, MIL-88B, T-MIL-88B, and used T-MIL-88B, respectively. MIL-101 shows the characteristic octahedral shape of ca. 200-300 nm size (Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea and Figure S2a of ESI)\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. After impregnation of Fe over MIL-101, agglomerates of Fe nanoparticles were observed on MIL-101 (Fe/MIL-101) with approximately 50-100 nm in size (Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb), whereas after thermal transformation, T-Fe/MIL-101 exhibited approximately 5-30 nm particles (Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec). The emergence of these smaller nanoparticles is likely due to the thermal transformation of Fe/MIL-101 in reductive atmosphere, where the deconstruction of linkers leads to breakage of the Fe agglomerates. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed and Figure S2b (ESI) show the characteristic fusiform rod shaped morphology of MIL-88B with ~360 nm length and 90 nm width\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. After thermal transformation, T-MIL-88B shows a narrow range of Fe\u003csup\u003e0\u003c/sup\u003e/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticle which are well-dispersed over the carbonaceous support (Figure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee). The amount of Fe on T-MIL-88B is 49.3%, with 13.7% C and negligible amount of H, N and S (Table S1, ESI), which indicates that original MOF structure is completely transformed into porous carbon. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ef shows that the T-MIL-88B catalyst retains its structure after 48h of reaction. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.g-h illustrates the particle size distribution (PSD) for T-MIL-88B and used T-MIL-88B, respectively. 525 and 476 particles were measured from multiple images which showed most of the particles in 4-16 nm for both fresh and used T-MIL-88B, respectively. The peaks were observed at 8 nm with average particle sizes of 9.7 and 9.1 nm for fresh and used T-MIL-88B, respectively which suggested that the studied catalyst is stable and potentially reusable for this reaction.\u003c/p\u003e\n \u003cp\u003eThe surface oxidation state of Fe in the different catalysts was evaluated by X-Ray photoelectron spectroscopy (XPS) study, as shown in Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e. For T-MIL-88B (Figure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.a), Fe 2p\u003csub\u003e3/2\u003c/sub\u003e XPS spectrum exhibited three peaks, including a peak at 706.9 eV corresponding to metallic iron \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Moreover, the other two peaks at 710.1 and 712.3 eV which are correlated to Fe\u003csup\u003e+2\u003c/sup\u003e and Fe\u003csup\u003e+3\u003c/sup\u003e oxidation state of iron and the satellite peaks for these aforementioned oxidation state appeared at 716.6 and 719.8 eV \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. In the Fe 2p region of T-MIL-88B, Fe2p\u003csub\u003e1/2\u003c/sub\u003e and Fe2p\u003csub\u003e3/2\u003c/sub\u003e peaks are situated 710.1 and 723.8 eV, where, the spin orbital splitting is 13.7 eV that indicated the presence of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in T-MIL-88B \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e may exist as mixed FeO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e states, which appears from Fe\u003csup\u003e+2\u003c/sup\u003e and Fe\u003csup\u003e+3\u003c/sup\u003e oxidation states \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The present XPS study shows that Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is the dominant species on the surface, where the amount of Fe\u003csup\u003e+2\u003c/sup\u003e was 60.4% and Fe\u003csup\u003e+3\u003c/sup\u003e was 21.0%, whereas Fe\u003csup\u003e0\u003c/sup\u003e was 18.6%. Therefore, the ratio of Fe\u003csup\u003e0\u003c/sup\u003e to Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was accounted as 1/4.38 in T-MIL-88B.\u003c/p\u003e\n \u003cp\u003eThe XPS spectra of Fe 2p\u003csub\u003e3/2\u003c/sub\u003e in T-Fe/MIL-101 exhibited two peaks at 711.7 and 712.4 eV which is related to Fe\u003csup\u003e+2\u003c/sup\u003e and Fe\u003csup\u003e+3\u003c/sup\u003e along with two satellite peaks at 718.1 and 722.4 eV. Furthermore, Fe2p\u003csub\u003e1/2\u003c/sub\u003e and Fe2p\u003csub\u003e3/2\u003c/sub\u003e of Fe\u003csup\u003e+2\u003c/sup\u003e appeared at 711.7 and 725.4 eV and the spin orbital splitting is 13.7 eV which interpreted the existence of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in T-Fe/MIL-101. Metallic Fe peak is absent in this catalyst which is in good agreement with PXRD results. For Fe/MIL-101 catalyst, Fe 2p\u003csub\u003e3/2\u003c/sub\u003e XPS spectra also contained both Fe\u003csup\u003e+2\u003c/sup\u003e and Fe\u003csup\u003e+3\u003c/sup\u003e at 711.7 and 713.4 eV, respectively. However, the spin orbit splitting for Fe2p\u003csub\u003e1/2\u003c/sub\u003e and Fe2p\u003csub\u003e3/2\u003c/sub\u003e is 14.1 eV (711.7 and 725.8 eV) which suggested the absence of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb represented the Cr XPS spectra of MIL-101, Fe/MIL-101 and T-Fe/MIL-101 catalysts. In MIL-101, Cr 2p XPS spectra contained only one peak at 577.6 eV which is corresponds to Cr\u003csup\u003e+3\u003c/sup\u003e oxidation state\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. For Fe/MIL-101, Cr XPS spectra attributed to two peaks at 577.2 and 578.8 eV which are mainly resembles with Cr\u003csup\u003e+3\u003c/sup\u003e and CrO\u003csub\u003e3\u003c/sub\u003e \u003csup\u003e32\u003c/sup\u003e. The negative binding energy shift (0.4 eV) of Cr\u003csup\u003e+3\u003c/sup\u003e as compared to Cr\u003csup\u003e+3\u003c/sup\u003e present in MIL-101 is most likely due to the interfacial electronic interaction (charge transfer) between Cr and Fe after the inclusion of Fe in MIL-101\u003csup\u003e28\u003c/sup\u003e. The Cr spectra for T-Fe/MIL-101, Cr XPS spectra mainly consisted with Cr\u003csup\u003e+3\u003c/sup\u003e peak at 577.1 eV and the amount of CrO\u003csub\u003e3\u003c/sub\u003e is very less as compared to Fe/MIL-101 which may be due to the thermal transformation of Fe/MIL-101 under hydrogen atmosphere that reduces the oxidised Cr species on catalyst surface.\u003c/p\u003e\n \u003cp\u003eThe C 1s XPS spectra for Fe/MIL-101 (Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec) shows three different types of C peak at 285, 286.4 and 288.5 which belongs to C-C, C-O-C and O-C=O\u003csup\u003e33\u003c/sup\u003e. The C 1s XPS spectra of both T-Fe/MIL-101 and T-MIL-88B contains only two peaks corresponding to C-C and C-O-C, whereas, the O-C=O peak is absent, which may be due to the thermal transformation of both Fe/MIL-101 and MIL-88B under hydrogen atmosphere reducing the oxygen content in the catalyst.\u003c/p\u003e\n \u003cp\u003eA thermogravimetric analysis of Fe/MIL-101 and MIL-88B has been represented in Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e. For Fe/MIL-101, the weight loss in the range of 50-250\u0026deg;C is because of the evaporation of water and removal of free terephthalates inside the pore of MOF\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. Thereafter, the main weight loss in the temperature range of 270 to 670\u0026deg;C is due to the degradation of organic ligand in the framework of MOF which is attributed to the collapse of the framework\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. The weight loss of MIL-88B before 250\u0026deg;C corresponds to the removal of water and excess DMF from the framework\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. For MIL-88B, the weight loss occurs in the temperature ranges of 300 to 500\u0026deg;C due to the degradation of H\u003csub\u003e2\u003c/sub\u003eBDC and the breakdown of the framework. The step in the TGA profile of between 550-650\u0026deg;C is most likely due to the carbonization of the framework and the formation of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u0026ndash;carbon composites\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec14\"\u003e\n \u003ch2\u003e3.2. Catalyst Activities\u003c/h2\u003e\n \u003cdiv class=\"Section3\" id=\"Sec15\"\u003e\n \u003ch2\u003e3.2.1. Role of Fe based zeolite and MOF catalysts\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e.a-c illustrates the yield and selectivity of AA via aqueous phase CO\u003csub\u003e2\u003c/sub\u003e reduction with iodomethane at various pressures. All the catalysts showed some activity for AA production; however, T-MIL-88B was clearly the most active and selective catalyst with best yield of 504 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L and AA selectivity of 92.4%. Based on stoichiometric calculation, it is equivalent to 80.6% conversion of CH\u003csub\u003e3\u003c/sub\u003eI into AA. Both Fe/CBEA and T-Fe/MIL-101 provide lower activity for CO\u003csub\u003e2\u003c/sub\u003e hydrogenation and \u0026gt;90% selectivity for FA production. With increasing pressure, the yield increased initially but the AA selectivity peaked at 60 bar for both Fe/CBEA and T-Fe/MIL-101. However, the AA yield and selectivity increases with increasing pressure for T-MIL-88B. Since Fe was present in the structural framework of T-MIL-88B, the thermally transformed catalyst consists of - embedded active metal sites dispersed evenly in a carbon matrix\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. The high AA activity and the selectivity over T-MIL-88B catalyst is most likely due to the presence of both Fe\u003csup\u003e0\u003c/sup\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e which assist the hydrogenation and C-C coupling reactions, respectively\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec16\"\u003e\n \u003ch2\u003e3.2.2. Extent of reaction with time\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e.a illustrates the extent of reaction over T-MIL-88B to produce AA and FA via CO\u003csub\u003e2\u003c/sub\u003e hydrogenation with CH\u003csub\u003e3\u003c/sub\u003eI as the starting material in the aqueous media. The reaction proceeds via formation of FA as the initial product, whereas AA was not detected until after 8h of reaction. The AA yield and selectively sharply increased between 12 to 24 h, thereafter gradually increasing to 657.6mmol/g\u003csub\u003ecat\u003c/sub\u003e.L and 98.8%, respectively, at 48h as the reaction approached equilibrium conversion. Based on the initial CH\u003csub\u003e3\u003c/sub\u003eI concentration (10 mmol), 100% conversion at 100% selectivity for AA was achieved, within the range of measurement errors. However, as discussed later, CO\u003csub\u003e2\u003c/sub\u003e first converts into FA and after reaching the maximum yield (377.4 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L) at 8h, the FA yield decreases sharply until the end of reaction at 48 h when the FA yield was measured at 8.1 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L. However, since CH\u003csub\u003e3\u003c/sub\u003eI is consumed by this time, the residual FA cannot convert into AA. Therefore, for the CO\u003csub\u003e2\u003c/sub\u003e hydrogenated into carboxylic acids, the selectivity of AA is 98.8%.\u003c/p\u003e\n \u003cp\u003eWhen CH\u003csub\u003e3\u003c/sub\u003eOH (10 mmol) was used as a reactant with LiI as a co-catalyst (Figure \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003eb), in otherwise identical reaction conditions, the reaction generates \u003cem\u003ein situ\u003c/em\u003e CH3I and hence the peak of FA is broader than Figure \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ea. The AA yield and selectivity increased more gradually and achieved a similar yield of 590.1 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L at 81.7% selectivity after 48 h, which is equivalent to 94% conversion of CH3OH into AA. The \u003cem\u003ein-situ\u003c/em\u003e production of CH\u003csub\u003e3\u003c/sub\u003eI slowed down the conversion of FA into AA, which may be due to mass transfer limitation.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec17\"\u003e\n \u003ch2\u003e3.2.3. Catalyst reusability\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e shows that the catalytic activity dropped initially but after three cycles, there was no significant decline in AA yield and selectivity. The PXRD of the used catalyst after five cycles (Figure \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.c), and the TEM image (Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.f) and PSD (Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.h) of used catalyst after 48 h confirmed that the structure is stable and there was no sintering or agglomeration of Fe and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanoparticles in T-MIL-88B. The initial loss in activity is likely due to the loss of small particles of the catalyst which could not be recollected in centrifuge.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv class=\"Section3\" id=\"Sec18\"\u003e\n \u003ch2\u003e3.2.4. Proposed Reaction Pathway\u003c/h2\u003e\n \u003cp\u003eReaction mechanism of hydrocarboxylation of methanol in an organic solvent proceeds via reaction of CH\u003csub\u003e3\u003c/sub\u003eOH with LiI to produce CH\u003csub\u003e3\u003c/sub\u003eI and LiOH which is similar to the carbonylation of methanol (Monsanto processes) followed by formation of CH\u003csub\u003e3\u003c/sub\u003eRh*I due to the insertion of CH\u003csub\u003e3\u003c/sub\u003eI into a Rh* complexing catalyst \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Further, CO\u003csub\u003e2\u003c/sub\u003e is inserted into CH\u003csub\u003e3\u003c/sub\u003e-Rh bond to produce CH\u003csub\u003e3\u003c/sub\u003eCOORh*I. Finally, CH\u003csub\u003e3\u003c/sub\u003eCOOH is formed via reduction of CH\u003csub\u003e3\u003c/sub\u003eCOORh*I with H\u003csub\u003e2\u003c/sub\u003e molecule in the presence of Ru* to produce HI as an intermediate. Whereas, LiI is regenerated in situ via HI formation which reacts with LiOH to produce H\u003csub\u003e2\u003c/sub\u003eO and LiI. However, here we show aqueous phase methanol hydrocarboxylation in which the reaction pathway deviates from the published works and FA is formed as an intermediate.\u003c/p\u003e\n \u003cp\u003eFirst, we show that FA can react with CH\u003csub\u003e3\u003c/sub\u003eI in water over T-MIL-88B in H\u003csub\u003e2\u003c/sub\u003e atmosphere (Figure \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e). The conversion of FA closely follows AA yield and after 24 h of the reaction FA conversion of 91.5% is achieved with 100% AA selectivity.\u003c/p\u003e\n \u003cp\u003eNext, we show aqueous phase hydrocarboxylation of CH\u003csub\u003e3\u003c/sub\u003eOH using T-MIL-88B as catalyst and LiI as co-catalyst. Here both liquid and gas samples were collected after 48 of reaction. The liquid sample showed only the presence of HCOOH and CH\u003csub\u003e3\u003c/sub\u003eCOOH with 81.7% acetic acid selectivity (Figure \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e.b). Whereas gas analysis did not detect any carbonaceous molecules apart from CO\u003csub\u003e2\u003c/sub\u003e (ESI, Figure S4), which eliminates the methanol carbonylation route for AA production.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e shows the proposed reaction pathway for acetic acid production via hydrocarboxylation of CH\u003csub\u003e3\u003c/sub\u003eOH over T-MIL-88B. CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e adsorbed over the catalyst and converted into FA, which may desorb. Subsequently, the adsorbed formate species reacts with iodomethane (CH\u003csub\u003e3\u003c/sub\u003eI) to allow C-C coupling reaction to take place which generates an acetate species and HI as the by-product. Finally, acetate species is converted into acetic acid, whilst LiI might be regenerated from LiOH and HI (step 8).\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eWe show that thermally transformed Fe-based metal organic framework-based catalyst (T-MIL-88B) exhibited high catalytic activity and stability for aqueous phase CO\u003csub\u003e2\u003c/sub\u003e transformation into acetic acid. Here, the catalytic activity and the structural property of T-MIL-88B was compared with Fe/CBEA and thermally transformed Fe deposited on MIL-101 (T-Fe/MIL-101). The T-MIL-88B consisted both Fe\u003csup\u003e0\u003c/sup\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phases, which catalyse hydrogenation and C-C coupling reactions, respectively, making this catalyst superior to the others tested here. Using CH\u003csub\u003e3\u003c/sub\u003eOH, CO\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003e as reactants in aqueous phase, and LiI the promoter, a maximum acetic acid yield of 590.1 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L, with 81.7% selectivity was achieved after 48 h at 150 ˚C. We propose that the hydrocarboxylation of methanol to make acetic acid is mediated by formate route, which is evidenced by formic acid as an intermediate. The T-MIL-88B catalyst was active for at least five cycles for acetic acid production without showing any signs of deactivation via sintering, oxidation or phase change.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgement\u003c/h2\u003e \u003cp\u003eAuthors would like to thank the Faculty of Engineering, Monash University for financial support under the Researcher Accelerator Grant 2019. Authors would also acknowledge Monash Centre for Electron Microscopy (MCEM) for providing the microscopic analysis facilities. AT and AS received financial support from the Institute for Catalysis, Hokkaido University as part of their Strategic Research Fellowship grant scheme. This study was supported by the Cooperative Research Program of Institute for Catalysis, Hokkaido University (Proposal no. 19A1005).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLi W, \u003cem\u003eet al.\u003c/em\u003e A short review of recent advances in CO 2 hydrogenation to hydrocarbons over heterogeneous catalysts. \u003cem\u003eRSC advances\u003c/em\u003e \u003cb\u003e8\u003c/b\u003e, 7651\u0026ndash;7669 (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAhmad W, Younis MN, Shawabkeh R, Ahmed S. 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Fe 3 O 4 nanoparticles: a robust and magnetically recoverable catalyst for three-component coupling of aldehyde, alkyne and amine. \u003cem\u003eGreen Chemistry\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e, 570\u0026ndash;573 (2010).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Methanol hydrocarboxylation, CO2 transformation, Acetic acid, Formic acid, Thermally transformed MOFs ","lastPublishedDoi":"10.21203/rs.3.rs-1029433/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1029433/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSustainable production of acetic acid (AA) is a high priority due to its high global manufacturing capacity and numerous applications. Currently it is predominantly synthesized via carbonylation of methanol, in which both the reactants are fossil-derived. CO\u003csub\u003e2\u003c/sub\u003e transformation into AA is highly desirable to achieve net zero carbon emissions, but significant challenges remain to achieve this efficiently. Herein, we report a heterogeneous catalyst, thermally transformed MIL-88B with Fe\u003csup\u003e0\u003c/sup\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e dual active sites, for highly selective AA formation via methanol hydrocarboxylation. This efficient catalyst showed high AA yield (590.1 mmol/g\u003csub\u003ecat\u003c/sub\u003e.L) with 81.7% selectivity at 150\u0026deg;C in aqueous phase using LiI as a co-catalyst. The reaction is believed to proceed via formic acid intermediate. No significant difference in AA yield and selectivity was noticed during catalyst recycling study up to five cycles. This work scalable and industrially relevant for CO\u003csub\u003e2\u003c/sub\u003e utilisation to reduce carbon emissions, especially if green methanol and green hydrogen are used.\u003c/p\u003e","manuscriptTitle":"Aqueous phase conversion of CO2 into acetic acid over thermally transformed MIL-88B","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-11-10 16:29:12","doi":"10.21203/rs.3.rs-1029433/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"ee02fb01-8849-43d2-a369-e057d31217af","owner":[],"postedDate":"November 10th, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":8412703,"name":"Chemical Engineering"},{"id":8412704,"name":"Energy Engineering"}],"tags":[],"updatedAt":"2023-05-18T07:09:14+00:00","versionOfRecord":{"articleIdentity":"rs-1029433","link":"https://doi.org/10.1038/s41467-023-38506-5","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2023-05-17 04:00:00","publishedOnDateReadable":"May 17th, 2023"},"versionCreatedAt":"2021-11-10 16:29:12","video":"","vorDoi":"10.1038/s41467-023-38506-5","vorDoiUrl":"https://doi.org/10.1038/s41467-023-38506-5","workflowStages":[]},"version":"v1","identity":"rs-1029433","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1029433","identity":"rs-1029433","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}