Sunlight-driven Fixation of CO2 to Cyclic Carbonates Using Carbon Dots as a Photothermal Catalyst

Sunlight-driven Fixation of CO2 to Cyclic Carbonates Using Carbon Dots as a Photothermal Catalyst | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Exploration This is a preprint and has not been peer reviewed. Data may be preliminary. 23 February 2025 V1 Latest version Share on Sunlight-driven Fixation of CO2 to Cyclic Carbonates Using Carbon Dots as a Photothermal Catalyst Authors : Ruijia Wang , Hongda Guo , Tao Zhang , Xiaoxia Chen , Min Ge , Shujun Li 0000-0002-3836-5350 , Jian Li , … Show All … , Bing Tian 0000-0003-4876-9604 [email protected] , Bernd Strehmel , Shouxin Liu , Andrey L. Rogach , Tony D. James , and Zhijun Chen 0000-0001-7203-5788 Show Fewer Authors Info & Affiliations https://doi.org/10.22541/au.174033558.83426942/v1 Published Exploration Version of record Peer review timeline 617 views 268 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Fixation of CO 2 through photocatalytic cycloaddition with epoxides to synthesize cyclic carbonates is an important but challenging process. In this work, carbon dots (CDs) synthesized from gallic acid and polyethylenimine are used for the efficient catalytic cycloaddition of CO 2 with epoxides in the absence of any solvent, additives, and halides, and importantly upon irradiation by natural sunlight. Specifically, carbon dots generated thermal energy and electrons upon solar irradiation, which together with their surface N-sites activated the inert CO 2 . Meanwhile, epoxides were activated by the surface hydroxyl and carboxylic groups of the carbon dots, which reacted with activated CO 2 at solar thermal-induced high temperatures. The CDs shows excellent stability and recyclability during the catalysis. A 1000 mmol scale reaction for cyclic carbonate synthesis performed well upon irradiation with natural sunlight in the presence of CDs, showing great potential for the industrial application due to the simple, mild, and energy-saving process. Sunlight-driven Fixation of CO 2 to Cyclic Carbonates Using Carbon Dots as a Photothermal Catalyst Ruijia Wang, Hongda Guo, Tao Zhang, Xiaoxia Chen, Min Ge, Shujun Li, Jian Li, Bing Tian,* Bernd Strehmel, Shouxin Liu,* Andrey L. Rogach,* Tony D. James,* and Zhijun Chen* R. Wang, H. Guo, T. Zhang, X. Chen, M. Ge, S. Li, J. Li, B. Tian, S. Liu, Z. Chen Key Laboratory of Bio-based Material Science and Technology of Ministry of Education Northeast Forestry University Harbin 150040, China E-mail: [email protected] ; [email protected] ; [email protected] ; Z. Chen Heilongjiang International Joint Laboratory of Advanced Bio-mass Materials Northeast Forestry University Harbin 150040, China B. Strehmel Department of Chemistry Institute for Coatings and Surface Chemistry Niederrhein University of Applied Sciences Krefeld 47798, Germany A. L. Rogach Department of Materials Science and Engineering & Centre for Functional Photonics (CFP) City University of Hong Kong Hong Kong SAR 999077, China IT4Innovations VSB–Technical University of Ostrava 17. listopadu 2172/15, 70800 Ostrava-Poruba, Czech Republic Email: [email protected] T. D. James Department of Chemistry University of Bath Bath BA2 7AY, UK Email: [email protected] T. D. James School of Chemistry and Chemical Engineering Henan Normal University, Xinxiang Henan 453007, China Keywords: carbon dots, sunlight utilization, photothermal catalysis, carbon dioxide, cyclic carbonate Abstract: Fixation of CO 2 through photocatalytic cycloaddition with epoxides to synthesize cyclic carbonates is an important but challenging process. In this work, carbon dots (CDs) synthesized from gallic acid and polyethylenimine are used for the efficient catalytic cycloaddition of CO 2 with epoxides in the absence of any solvent, additives, and halides, and importantly upon irradiation by natural sunlight. Specifically, carbon dots generated thermal energy and electrons upon solar irradiation, which together with their surface N-sites activated the inert CO 2 . Meanwhile, epoxides were activated by the surface hydroxyl and carboxylic groups of the carbon dots, which reacted with activated CO 2 at solar thermal-induced high temperatures. The CDs shows excellent stability and recyclability during the catalysis. A 1000 mmol scale reaction for cyclic carbonate synthesis performed well upon irradiation with natural sunlight in the presence of CDs, showing great potential for the industrial application due to the simple, mild, and energy-saving process. 1. Introduction not-yet-known not-yet-known not-yet-known unknown Cyclic carbonates are organic compounds with many useful properties, which exhibit widespread ap-plications as aprotic high-boiling polar solvents, electrolytes for batteries, precursors for polymeric materials, fuel additives, plastic materials, and synthetic intermediates.[1-3] In industry, one main route for producing cyclic carbonates is the cycloaddition reaction of CO2 with epoxides. Importantly, the conversion of CO2 to high value-added chemicals is of vital importance in terms of CO2 fixation from the atmosphere.[4-7] To avoid the harsh reaction conditions required, such as high temperature, high pressure, etc., and to enhance the conversion yield of the reaction between CO2 and epoxides, several efficient catalysts have been suggested, which include both homogeneous catalysts (Schiff bases, ionic liquids, metal complexes)[8-11] and heterogeneous catalysts (covalent-organic frameworks, metal-organic frame-works, mesoporous oxides, porous polymers) (Figure 1A).[12-18] Amongst these catalysts, heterogeneous photocatalysts received particularly attention, since they can catalyze the reaction in an energy-efficient and environmentally friendly manner.[19-30] However, most of these photocatalysts suffer from several challenges, hindering their practical applications. They exhibit low intrinsic catalytic reactivity, leading to necessity of co-catalysts such as active Lewis-acidic metal complexes or environmentally harmful halide ions (see comparison in Supporting Information); require a complicated/expensive synthesis and/or external thermal heating/high pressure; and can only work well under laboratory conditions, and could not be efficiently activated using natural sunlight. Recently, carbon dots (CDs) have appeared as novel environmentally friendly and easy to produce carbon nanomaterials,[31-33] which enable electron photo-transfer and photothermal conversion, rendering them as promising photocatalysts (Figure 1B).[34-36] In particular, CDs which are small colloidal nanoparticles with a large variety of surface moieties have been shown to exhibit high photothermal catalytic efficiency when they were coupled with conjugated organic structures or metal-incorporating materials.[37-41] However, using CDs as photothermal catalysts without such a modification has rarely been reported.[42] Herein, we employ CDs synthesized from gallic acid (GA) and polyethylenimine (PEI) via a simple hydrothermal process, as photothermal catalysts for the reaction between CO2 and epoxides under sunlight irradiation (Figure 1C). These CDs exhibited highly efficient solar-to-thermal conversion with an efficiency of 54%, providing thermal energy for activating the reaction, and at the same time the photo-induced electron activation of CO2. Meanwhile, the N-sites and the surface polar groups (hydroxyl and carboxylic acid) of the CDs were able to efficiently activate CO2 and epoxides, respectively. Our study offers an efficient and sustainable approach using CDs for CO2 conversion in a metal-free, additive-free, and solvent-free manner under irradiation by sunlight. Figure 1. Background and catalytic synthesis of cyclic carbonates. A) Catalytic synthesis of cyclic carbonates using epoxides and CO2. B) Structure of CDs. C) CDs-catalyzed reaction of CO2 and epoxides for cyclic carbonates synthetic procedure used in this research under sunlight via a photothermal process. 2. Results 2.1. Reaction Design GA and PEI which contain abundant polar groups (hydroxyl, carboxylic, and amino) were selected as CDs precursors. The CDs were synthesized by a hydrothermal process (180°C, 8 h) using a 1:1 molar ratio of GA/PEI (Figure 2A). As shown by the transmission electron microscopic (TEM) images, the as-prepared CDs have an average size of 2.3 ± 0.2 nm (Figure 2B), and are spherical in shape. The high-resolution TEM image (inset in Figure 2B) revealed a lattice spacing of 0.21 nm, attributed to the (100) lattice fringes of graphene. Their X-ray diffraction (XRD) pattern exhibits a broad peak around 20° (Figure S1), a typical signal of carbon dots.[43] The Raman spectrum of the CDs exhibits two peaks at 1323 and 1523 cm-1 that correspond to the disordered (D band) and graphite (G band) of carbon materials (Figure 2C). An IG/ID ratio of 1.37 indicated a high extent of graphitization in the CDs, which should be beneficial for photothermal conversion.[44,45] The FT-IR spectrum of the CDs displayed an enhanced absorption band at 3110-3631 cm-1, belonging to the stretching vibrations from N–H and O–H groups (Figure S2). The peaks at 1658 cm-1 and 1578 cm-1 belong to C=O/C=N stretching vibrations and N–H bending vibrations, respectively, and the peaks at 1450 cm-1, 1348 cm-1, and 1125 cm-1 can be assigned to aromatic C–N, C–O, and alkyl C–N stretching, respectively. The hydroxyl, carboxylic acid, and amino groups present at the surface of CDs are beneficial for activating the epoxides via hydrogen bonding interaction.[8] X-ray photoelectron spectra (XPS) of CDs (Figure S3) are consistent with the FT-IR analysis. Specifically, the high-resolution N 1s XPS spectrum revealed strong signals from the pyridinic (399.1 eV), pyrrolic (400.1 eV), and graphitic (402.1 eV) nitrogen, respectively. These N-containing moieties will be useful as efficient CO2 activation sites.[46-48] The UV-vis-IR absorption spectrum (Figure 2D) indicates that the CDs have a broad absorption extending all the way from the UV-vis to the IR region (250-2500 nm), with a significant absorption over the visible light region, which means that they can efficiently absorb and utilize the whole spectrum of sunlight. Importantly, an excellent photothermal conversion efficiency of these CDs equal to 54% was observed, as given in Figure S7. Stability of photothermal catalysts against photobleaching is an important aspect. Figure 2E shows the temperature curves for the CDs after 10 light on/off cycles, which confirm the excellent photothermal stability of the CDs under simulated sunlight irradiation. We subsequently evaluated the photothermal effect of CDs with epoxide 1 in a solvent-free system for reaction with CO2. As shown in Figure 2F, a mixture of 5 wt% of CDs (41 mg) with 5 mmol of epoxide 1 (0.8 mL) was irradiated under 0.1, 0.3, 0.5, 0.7 and 0.9 W/cm2 simulated sunlight, resulting in a change of temperature from initially 26 °C to 43, 61, 71, 94, and 104 °C within 20 min, respectively. Given that temperatures in the range of 80-120 °C are usually applied for the reaction of CO2 with epoxides in catalytic systems,[8] we employed 0.7 W/cm2 of irradiation to evaluate the CO2 conversion using our CDs. The reaction of CO2 with benzyl glycidyl ether (Figure 2G) was conducted using 5 wt% of CDs under the irradiation of 0.7 W/cm2 simulated sunlight (100 °C was achieved), yielding cyclic carbonate 1 with an excellent reaction yield of 90% (Entry 2). The yield decreased from 90% to 75% when 2.5 wt% of CDs was used under the same reaction conditions (Entry 1), which was attributed to a reduced system temperature (90 °C) caused by a decrease in amounts of CDs. Notably, no reaction occurred in the absence of CDs, either upon solar irradiation or under heating conditions. While the screening of different ratios of GA/PEI and nitrogen sources in the synthesis of photothermal CDs did not afford better results (Table S2). Catalysts consisting of GA, PEI, GA/PEI, and CDs synthesized using either GA or PEI as a sole precursor all exhibited low photothermal conversion and poor catalytic reactivity (Entries 3-7). Notably, when the reaction was carried out with CDs using heating-induced thermal conditions (100 °C, oil bath) instead of photothermal conditions, the reaction between CO2 and epoxides happened but with a decreased yield (74%, Table S2). Additionally, the time course of the reaction showed a much faster generation of cyclic carbonate 1 under photothermal rather than thermal conditions, indicating a significant photochemical activation effect by CDs. Figure 2. Basic characteristics of CDs and photocatalytic transformation of CO 2 with epoxides to cyclic carbonates. A) Hydrothermal reaction leading to the formation of CDs. B) TEM image of CDs; insets show lattice spacing and size distribution of CDs. C) Raman spectrum of CDs. D) Vis-NIR absorption spectrum (in blue) of CDs; the solar spectrum is also provided in orange. E) Temperature change of CDs under ten on/off cycles of simulated sunlight irradiation (0.7 W/cm 2 ). F) Temperature changes of CDs in epoxide 1 under simulated sunlight irradiation ranging from 0.1 to 0.9 W/cm 2 . G) Optimization of the reaction of CO 2 with epoxide 1. The reaction yield was determined by 1 H NMR using CF 3 -DMA as an internal standard. H) Time course of CDs-catalyzed reaction of epoxide 1 with CO 2 using photothermal (simulated sunlight) and thermal conditions (oil bath). 2.2. Mechanistic Considerations The photothermal catalytic mechanism of the CDs-catalyzed CO 2 cycloaddition to epoxides was further investigated. Figure 2H shows that that CDs can catalyze this reaction in a dual mode, including both thermal and photothermal catalysis, which means that both photo-induced activation and chemical activation of CDs are involved in the catalytic process. As the polar groups such as -OH, -CO 2 H, and -NH 2 play key roles to chemically activating the epoxide through hydrogen-bonding in the metal-free catalytic systems, the catalytic effect of these polar groups on the surface of CDs was evaluated using a functional group blocking strategy. As shown in Figure 3A, the carboxylic acid, amino, and hydroxyl groups on the surface of CDs were selectively protected by methylation, aldimine condensation and acetylation reactions (Figures S11-17), respectively, and the photothermal effect of these modified CDs was evaluated (the resulting samples were denoted as Me-CDs, Ar-CDs, and Ac-CDs, respectively). These functional group-blocked CDs were then employed for the photothermal and thermal catalysis of the reaction of CO 2 with epoxide 1 (Figure 3B). Notably, under the standard conditions of photothermal catalysis, the temperature of the reaction system could be kept at 97-100 °C by irradiation of the modified CDs, which indicates that the protection of surface groups has a negligible effect on photothermal conversion ability of CDs (Figure 3C). The Ar-CDs with protected amino groups exhibited almost identical catalytic reactivity as the original (non-protected) CDs in both photothermal and thermal catalytic systems, indicating that the amino groups on the surface were not essential for activation of both epoxides and CO 2 . Moreover, this observation together with the observed absence of catalytic reactivity of the N atom-free GA-CDs indicates that the activation of CO 2 should be attributed to the presence of pyridinic, pyrrolic and/or graphitic-nitrogen in the CDs core, rather than their surface amino groups. The reactions using the Me-CDs and Ac-CDs with carboxyl acids and hydroxyl groups blocked led to an obvious decrease of the yield of cycloaddition product 1 under both photothermal conditions (76 and 72% yields) and thermal conditions (69 and 59% yields), respectively. These observations suggest that the catalytic ability of surface polar groups on the CDs decreases in the order of -OH > -CO 2 H > -NH 2 (Figure 3D). Figure 3. Mechanistic study of epoxide activation. A) Functional group blocking strategy for CDs. Me-CDs, Ar-CDs, and Ac-CDs correspond to carboxylic acid, amino, and hydroxyl groups blocked CDs, respectively. B) Effect of functional groups on the catalytic reactivity of CDs in the reaction of CO 2 with epoxide 1 using photothermal conditions (simulated sunlight) and thermal conditions (oil bath). C) Effect of selectively protected CDs on the photothermal conversion. D) Effect of polar groups on the activation of epoxides. Given that photo-induced electron transfer in photothermal catalysis to substrates under irradiation can improve the reactivity of the substrate as compared to the corresponding heating-induced thermal catalytic reactions (Figure 2H), we evaluated the importance of photogenerated electrons in this transformation using CDs. Firstly, the radical and hole quenching experiments were conducted to explore the underlying mechanisms. As shown in Figure 4A, the yield of cyclic carbonate 1 under standard reaction conditions was 46% over 8 h, while no reaction occurred in the absence of CDs. When AgNO 3 and Na 2 S 2 O 3 were used as the radical scavengers to capture photogenerated electrons, the reaction yield decreased from 46% to 7% and 37%, respectively, indicating the crucial role of photogenerated electrons. In another experiment, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) and isopropanol were added to the reaction system containing CDs as hole scavengers. An increase in the reaction yield was observed when EDTA-2Na and isopropanol were used (55% and 71%, respectively), indicating that the photogenerated holes do not contribute to the transformation reaction, but rather increase the recombination of photogenerated electrons, leading to lower degree of conversion. To further explore the possible pathway for activation of photogenerated electrons, electron paramagnetic resonance (EPR) measurements were conducted on CDs, using 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as the spin-labeling agent which could be reduced by photogenerated electrons, leading to a decrease in intensity of the EPR signal. As shown in Figures 4B and 4D, the EPR signal intensity decreased by 36% under N 2 and 39% in epoxide 1 /N 2 after 3 min irradiation, indicating that the electrons could hardly be transferred to epoxide 1 . A 28% decrease of the signal intensity was observed in the presence of CO 2 , indicating the transfer of electrons from the CDs to CO 2 (Figure 4C). When epoxide 1 was added to the CDs under a CO 2 atmosphere, the EPR signal intensity exhibited similar decrease (29%), further confirming the transfer of electrons to CO 2 rather than to epoxide (Figure 4E). This is because the conduction band (CB) of the CDs is located at -1.9 eV (Figures S18-20), which is enough to activate the CO 2 to CO 2 •− . Based on these observations, an activation pathway by single electron transfer to CO 2 could be proposed for the photo-enhanced transformation, which is illustrated in Figure 4F. To further understand if UV or the visible part of the solar spectrum drives the transfer of the electrons from CDs to CO 2 , simulated sunlight with or without the UV light band has been checked for the photothermal catalysis. The absence of UV light band had almost no influence on the conversion efficiency for the transformation reaction, which proceeded with 87% yield (Figure 4G, entry 2). Thermal conditions with additional UV irradiation had no effect on the conversion efficiency (Figure 4G, entry 4), resulting in almost the same yield (Figure 4G, entry 3). These results indicate that the CDs-mediated activation process under visible light irradiation can enhance the conversion of CO 2 into cyclic carbonates. We also noticed that both light irradiation and heating failed to initiate the transformation reaction in the absence of CDs (Figure 4G, entries 5 and 6). Figure 4. Mechanistic study of CO 2 activation. A) Photoactivity of CDs in the CO 2 cycloaddition reactions using electron and hole quenchers (reaction time: 8 h, simulated sunlight). B-E) EPR spectra of CDs measured under N 2 (B), CO 2 (C), N 2 /epoxide 1 (D), and CO 2 /epoxide 1 (E), IL = intensity of signal under light irradiation, ID = intensity of signal under dark. F) Photo-activation of CO 2 by electron transfer from CDs rather than photo-activation of epoxides. G) Table showing the outcome of the control experiments for photochemical effect. H) Proposed mechanism of CDs-catalyzed fixation of CO 2 with epoxide. Based on these results, a proposed mechanism for the reaction is depicted in Figure 4H. CDs generated thermal energy and electrons upon solar irradiation, and then together with N-sites activated the inert CO 2 . Meanwhile, epoxides were efficiently activated by surface hydroxyl and carboxylic groups of CDs through hydrogen-bonding interactions. Thus, activated CO 2 and epoxides could efficiently react at the high temperature induced by solar irradiation of the photothermal CDs. Moreover, according to the control experiments in Figure 2H, thermal conditions afforded cyclic carbonate 1 in 74% yield while photothermal conditions could achieve 90% yield, indicating that the thermal catalysis for this cycloaddition reaction is still the main pathway and the photo-activation for CO 2 could significantly enhance this process. Considering together the experimental evidences and the relevant reported mechanism for this metal- and halogen-free transformation, [46-48] the plausible reaction route is shown in Figure 4H: a nucleophilic attack of activated CO 2 (CO 2 − and CO 2 •− ) to epoxide selectively occurred at the terminal C–O bond, leading to ring-opening intermediate. An intramolecular cyclization further produced cyclic carbonate and simultaneously regenerated CDs. 2.3. Reaction Development Using Sunlight Using natural sunlight directly in the catalytic reactions represents a significant challenge, since unstable and non-continuous irradiation by sunlight requires catalysts exhibiting an extremely high catalytic reactivity, especially for reactions with prolonged irradiation. As such, a home-made solar reactor was constructed as shown in Figure 5A and Figure S26, where Fresnel lenses were employed as an optical concentrator to increase solar flux, and optical fibers were used as a light conduit for sunlight. With this device, we investigated the photothermal catalytic performance of the CDs operated under natural sunlight. As shown in Figure 5D, the reaction of epoxide 1 with CO 2 irradiated for 8 h using natural sunlight exhibited almost the same efficiency (48% yield) to that of under simulated sunlight (46% yield). Remarkably, although the reaction was interrupted during the night, the catalytic reactivity of the system could be reactivated by sunlight irradiation during the following day (Figure 5C). The reaction also provided a high yield (87%) under intermittent irradiation for totally 24 h (8 h during the daytime for 3 days). These results demonstrate the practicability of CDs used as photothermal catalysts for CO 2 conversion using abundant but discontinuous natural sunlight. Given that this system represents a simple and efficient photocatalyst for the reaction of CO 2 with epoxides, the scope of epoxides was investigated using natural sunlight assisted by the device in Figure 5A at 1 atm of CO 2 (Figure 5F). Epoxides bearing a variety of functional groups could successfully react with CO 2 , which resulted in formation of the corresponding cyclic carbonates in excellent yields and selectivity under irradiation of natural sunlight within 24 h. For sterically hindered epoxides containing aromatic ether groups, the reaction generated cyclic carbonates 1-4 in high yields (87%-96%). Active functional groups such as alkyls and alkenes were also well tolerated in this photothermal catalytic process and provided the corresponding products 5 and 6 in high yields (93% and 89%), respectively. For the challenging low reactivity and sterically bulky substrate styrene oxide, CDs also exhibited high catalytic reactivity, resulting in the cyclic carbonate 7 with 70% yield. In addition, enantiopure epoxide could be smoothly transformed to the corresponding cyclic carbonate ( S )- 1 without any loss of enantiomeric excess (> 99% e.e.), indicating a selective ring opening at the terminal position of the epoxide. A functionalized bisepoxide substrate was amenable to the CDs catalyst as well, affording bis-cyclic carbonate 8 in 92% yield, which can be used as an important building block for further synthesis of polymers such as polyurethanes and polycarbonates. Remarkably, photo-induced activation was observed for all these cases. Using the thermal conditions (oil bath) at the same system temperature as for the photothermal conditions, the yields of products 1-6 reduced by 6-22%. Significant enhancement of photo-induced activation was observed for the formation of cyclic carbonates 7 and 8 since using thermal conditions resulted in only 14% and 7% of the target products being obtained in the absence of light irradiation, respectively. This shows that different extent of photo-induced activation can occur in reactions with different substrates. To explore the recyclability of the CDs catalyst, the reaction for the synthesis of the useful bis-cyclic carbonate 8 was investigated under natural sunlight using 1 atm of CO 2 . The yield slightly decreased from 92% to 84% after 5 cycles of recycling, but still exhibited high catalytic activity (Figure 5E). Only 1.8 wt% loss of CDs was observed after 5 cycles of reaction, showcasing excellent stability and recyclability of this system (Table S3). FT-IR, XRD, XPS, and 1 H NMR spectra of the recovered CDs also proved their stability in the recycling catalytic experiments (Figure S22-25). Moreover, the photothermal effect and catalytic activity of the CDs were also evaluated on a large scale to determine the practicability of this photocatalyst. The reaction could be carried out on a 1000 mmol scale to yield 176.98 g of cyclic carbonate 1 with a high yield of 85% (Figure 5B), confirming that this method has great potential for the industrial scale synthesis of cyclic carbonates from epoxides and CO 2 due to the simple, mild, and energy-saving process. not-yet-known not-yet-known not-yet-known unknown Figure 5. Applicability and practicability of CDs. A) Schematic of the sunlight-catalysis device, including a Fresnel condenser, a sunlight tracking sensor, and a guidance system (optical fibers). B) 1000 mmol scale experiment. The experiment under natural sunlight lasting for 40 h was carried during 5 days (8 h × 5). C) Representative irradiation situation of standard reaction under natural sunlight. D) Reaction yields for 8 and 24 h under conditions of heating, simulated sunlight, and natural sunlight. The experiment under natural sunlight lasting for 24 h was carried during 3 days (8 h × 3) in July, 2023, in Harbin, China. E) Catalyst recycling experiments of CDs for CO2 conversion to epoxide 8. F) Substrate scope of epoxides in photothermal (sunlight) and thermal catalysis (oil bath, the reaction temperature is consistent with that of photothermal reaction). All the photothermal reactions were conducted using home-made device under natural sunlight, and the system temperature for different substrates varied from 97 to 103 °C. We notice that only few examples of photothermal catalysts were reported for this transformation such as metal carbides,[20] metal/phthalocyanine hybrids,[49] metal/covalent-organic frameworks,[50,51] metal-organic frameworks,[52] metal/carbon nanosheets,[53] metallic oxides/organic ligands,[54] and porous biochar.[55] However, all these catalysts themselves showed inert or poor catalytic activity in the absence of the co-catalyst, metal, or halogen; they also had to be prepared using multiple and complex procedures (Table S4). In contrast, the CDs catalyst studied here exhibited excellent catalytic activity for both CO2 and epoxide activation, catalyzing the cycloaddition reaction without any solvent, metal, halogen, and co-catalyst (see Table S5 for comparison with references). Notably, the simple and green preparation of CDs by a one-pot hydrothermal synthesis in H2O shows great superiority in both fabrication process and catalytic applications comparing to previous carbon-based catalysts (Table S6). 3. Conclusion In summary, we have developed CDs as a photothermal catalyst for the fixation of CO2 to cyclic carbonates. The carbon cores of the CDs generated thermal energy and electrons upon solar irradiation. The as-generated electrons and N-sites activated the inert CO2. Meanwhile, epoxides were efficiently activated by surface hydroxyl and carboxylic groups of the CDs through hydrogen-bonding interactions. Activated CO2 and epoxides efficiently reacted at solar thermal-induced high temperatures. Conquering the problems associated with previous photocatalysts, CDs were able to efficiently catalyze the reaction without any co-catalyst under natural solar irradiation, and were suitable for producing 8 different cyclic carbonates from different epoxides. Considering its low energy consumption and low cost, CDs photocatalysts are promising candidates for the practical industrial scale production of cyclic carbonates. 4. Experimental Section Synthesis of CDs : All the carbon dots (CDs) were synthesized according to the following procedure: GA (0.5 mmol) and PEI (0.5 mmol) were dissolved in ultrapure water (30 mL) and mixed uniformly. A reactor containing the mixed solution was placed in an oven at 180 °C for 8 h and then allowed to cool to room temperature naturally. The mixture was filtered through a 0.22 μm syringe filter after centrifugation followed by a dialysis through a 1000 Da dialysis bag. The purified solution was dried in an oven to give the CDs (310 mg, 64 wt% yield). The CDs prepared from GA and PEI in the ratios of 4:1, 2:1, 1:1 and 1:2 were named as 4-CDs, 2-CDs, CDs and 0.5-CDs, respectively. The CDs prepared from GA and urea (1:1), and from GA and melamine (1:1) were named as U-CDs and M-CDs, respectively. The PEI-CDs and GA-CDs were prepared using sole PEI and GA, respectively. CDs-catalyzed reaction of CO2 with epoxides : For reaction conditions screening, epoxide 1 (5 mmol, 1 equiv.) and CDs (41 mg, 5 wt%) were weighted into a 25 mL reaction tube, followed by charging with CO2. The reaction mixture was stirred under 1 atm of CO2 atmosphere (balloon) and irradiation with 0.7 W/cm2 simulated sunlight. The reaction was monitored by TLC. After the reaction was completed, the residue was purified by column chromatography on silica gel with a gradient eluent of petroleum ether/ethyl acetate to afford the desired product. For substrate screening, the same procedure was conducted using sunlight. CO2 was provided by a balloon filled with pure CO2 gas and the distance between light outlet and reaction tube was ~1 cm. Large-scale reaction of CO2 with epoxide : Epoxide 1 (164.2 g, 1000 mmol) and CDs (8.2 g, 5 wt%) were weighted into a 250 mL flask, followed by charging with CO2. The reaction mixture was stirred for 5 days (8 h × 5) under 1 atm of CO2 atmosphere (balloon) and irradiation of sunlight. After the reaction was completed, ethyl acetate (500 mL) was added and the mixture filtrated through a short pad of silica gel. The residue was purified by column chromatography on silica gel with a gradient eluent of petroleum ether/ethyl acetate to afford the desired product 1 (183.2 g, 88% yield). Recycling experiments of CDs for CO2 conversion : Epoxide 8 (5 mmol, 1 equiv.) and CDs (82 mg) were weighted into a 25 mL reaction tube, followed by charging with CO2. The reaction mixture was stirred under 1 atm of CO2 atmosphere (balloon) and irradiation of sunlight. After the reaction was completed, ethyl acetate (10 mL) was added into the mixture. The CDs sediment was formed after a 30 min standing and the supernatant liquid (cyclic carbonate) was collected with a dropper carefully. The CDs sediment was washed with EA for 3 times and the dried and reused for the next cycle. The EA solution was collected and concentrated under vacuum to afford the cyclic carbonate 8 . Procedure for the protection of CDs’ surface groups : For protection of carboxyl acid, to a 10 mL oven-dried sealed tube containing a magnetic stir bar, CDs (200 mg) was added in the mixture of dichloromethane (4 ml) and methanol (4 ml). After that, (trimethylsilyl)diazomethane (1.5 equiv. corresponding to the carboxyl acid groups on the surface of CDs calculated according to the XPS analysis) was added to the mixture dropwise, and the mixture was stirred at room temperature for 12 h. After the reaction was completed, the solvent was removed under vacuum and the residue was washed with EA 3 times to remove the residual small molecules. The residue was dried to provide the methylated Me-CDs (202 mg). For protection of amino group: To a 50 mL oven-dried sealed tube containing a magnetic stir bar, CDs (200 mg) and 4-fluorobenzaldehyde (1 equiv. corresponding to the amino groups on the surface of CDs calculated according to the XPS analysis) were added in anhydrous ethanol (10 ml) and stirred at room temperature for 72 h. After the reaction was completed, the solvent was removed under vacuum and the residue was washed with EA 3 times to remove the residual 4-fluorobenzaldehyde. The residue was dried to provide the protected Ar-CDs (218 mg). For protection of hydroxyl group: To a 50 mL oven-dried sealed tube containing a magnetic stir bar, CDs (200 mg) and acetic anhydride (1 equiv. corresponding to the hydroxyl groups on the surface of CDs calculated according to the XPS analysis) were added in the anhydrous dichloromethane (8 ml) and stirred at room temperature for 12 h. After the reaction was completed, the solvent was removed under vacuum and the residue was washed with EA 3 times to remove the residual small molecules. The residue was dried to provide the acetylated Ac-CDs (196 mg). Calculation of Photothermal Conversion Efficiency (η) : Under the simulated sunlight (0.1 W/cm-2), photothermal conversion efficiency was determined for an aqueous solution of CDs (1 mg mL-1, 1 mL) in a vessel with an insulating layer (Fig. S7). It was surrounded by foam to create an adiabatic environment that minimizes heat exchange with the air. The temperature of the solution was recorded using a digital thermometer, and the photothermal conversion efficiency (η) was estimated using the following equation: \(\eta=\frac{Q}{E}=\frac{\text{CmΔT}}{\text{PSt}}=\frac{\text{CρVΔT}}{\text{PSt}}\) (1) where, Q is the thermal energy generated and E is the total energy of incident light. Q is determined by the heat capacity (C), density (ρ), volume (V) and ΔT over 700 s irradiation of the solution. E is determined by the power of the incident light (P), irradiation area (S) and irradiation time (t). The values of C (4.2 J g-1 °C-1), S (2.3 cm2), and ρ (1 g cm-3) for water were used in calculations. Thus, the photothermal efficiency was calculated as 54%. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. not-yet-known not-yet-known not-yet-known unknown Author Contributions Z.C. B.T., S.L., and T.D.J. conceived the work and designed the experiments. R.W., H.G., T.Z., X.C. and M.G. were primarily responsible for the experiments. R.W., S.L., S.L., J.L. B.T., B.S., A.L.R., T.D.J., and Z.C. prepared the paper. All the authors contributed to the data analysis. Acknowledgements We thank the National Natural Science Foundation of China (22378056 and 32171716), the Natural Science Funding of Heilong Jiang province for Excellent Young Scholar (YQ2022C004), and the Fundamental Research Funds for the Central Universities (2572022BB01, 2572022CG02 and 2572023CT06) for financial support. Tony D. James thanks the University of Bath and the Open Research Fund of the School of Chemistry and Chemical Engineering, Henan Normal University (2020ZD01) for support. Andrey L. Rogach acknowledges financial support from the Global Experts project funded by the Moravian-Silesian Region and VSB-TUO (contract 00734/2023/RRC). Conflict of Interest Statement The authors declare no competing interests. Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. 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Table of Contents entry The first example using carbon dots (CDs) for coupling CO 2 with epoxides to generate cyclic carbonates in a metal-, additive, solvent- and electricity input-free manner under sunlight via photothermal catalytic route is developed. The CDs exhibit highly efficient solar-to-thermal conversion with an efficiency of 54%, providing thermal energy for activating the reaction. Additionally, a photo-induced electron transfer between CDs and CO 2 happens, which can significantly enhance the catalytic process. Ruijia Wang, Hongda Guo, Tao Zhang, Xiaoxia Chen, Min Ge, Shujun Li, Jian Li, Bing Tian,* Bernd Strehmel, Shouxin Liu,* Andrey L. Rogach,* Tony D. James,* and Zhijun Chen* Sunlight-driven Fixation of CO 2 to Cyclic Carbonates Using Carbon Dots as a Photothermal Catalyst ToC figure Information & Authors Information Version history V1 Version 1 23 February 2025 Peer review timeline Published Exploration Version of Record 11 Feb 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection Exploration Keywords carbon dots photothermal catalysis sunlight utilization Authors Affiliations Ruijia Wang Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Hongda Guo Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Tao Zhang Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Xiaoxia Chen Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Min Ge Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Shujun Li 0000-0002-3836-5350 Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Jian Li Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Bing Tian 0000-0003-4876-9604 [email protected] Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Bernd Strehmel Hochschule Niederrhein View all articles by this author Shouxin Liu Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Andrey L. Rogach Vysoka skola banska-Technicka univerzita Ostrava View all articles by this author Tony D. James University of Bath Department of Chemistry View all articles by this author Zhijun Chen 0000-0001-7203-5788 Northeast Forestry University Key Laboratory of Bio-based Material Science and Technology Ministry of Education View all articles by this author Metrics & Citations Metrics Article Usage 617 views 268 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Ruijia Wang, Hongda Guo, Tao Zhang, et al. Sunlight-driven Fixation of CO2 to Cyclic Carbonates Using Carbon Dots as a Photothermal Catalyst. Authorea . 23 February 2025. DOI: https://doi.org/10.22541/au.174033558.83426942/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. 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