Highly efficient synthesis of pseudoionone from citral with acetone on Li-doped La2O3­ZnO catalysts

Highly efficient synthesis of pseudoionone from citral with acetone on Li-doped La2O3­ZnO catalysts | 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 Research Article Highly efficient synthesis of pseudoionone from citral with acetone on Li-doped La 2 O 3 ­ZnO catalysts Fang-Ling Wang, Ting Yang, Jun Zhou, Xiang-Zhou Li, Chun-Tao Kuang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8285759/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract A highly efficient Li/La 2 O 3 -ZnO catalysts was developed for aldol condensation of citral and acetone. Li/La 2 O 3 ­ZnO catalysts with different Li + loading were prepared by coprecipitation method followed by incipient wetness impregnation method, and were characterized by XRD, BET, FT-IR, SEM. The results demonstrated that Li was well dispersed on La 2 O 3 ­ZnO composite oxide, may coordinate with chemisorbed O 2− species to form active strong basic site. Therefore, Li introduced promoted the formation of active strong base sites, then significantly improved the catalytic performance. The suitable preparation conditions of catalyst were Li + loading of 4.0 wt%, calcination temperature of 300°C after Li-doped, and impregnation time of 15 h. The conversion of citral and yield of pseudoionone were 100% and 99.2% respectively under the suitable reaction conditions of catalyst dosage of 0.5 g, reaction temperature and time was 50°C and 3 h, and citral/acetone molar ratio of 1:9. Topical Heading : Reaction Engineering, Kinetics and Catalysis Li­doped La2O3­ZnO pseudoionone aldol condensation solid base catalyst base sites Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction Pseudoionone (PS) is a key chemical intermediate for synthesizing ionones, 1 irones, 2 vitamin A, 3 and related high-value compounds. In recent years, due to the significant growth of downstream products, the market demand of PS has increased markedly. Currently, PS is industrially synthesized via the aldol condensation of citral with acetone, typically catalyzed by aqueous alkali hydroxides (NaOH, KOH, or LiOH). However, these catalysts cause environmental challenges, equipment corrosion and unexpected side reactions. Therefore, it is necessary to develop a highly efficient and sustainable catalyst for the synthesis of PS. In response, there is a growing interest in solid base catalysts for the synthesis of PS. Compared to homogeneous catalysts, solid base catalysts offer several advantages —non-corrosiveness, easy separation from reaction mixtures, reusability —and high PS selectivity. A variety of materials had been explored as solid base catalysts, including powdered LiOH·H 2 O, 4 MgO 5 and Mg-Al mixed oxides, 6 Mg-Al hydrotalcites, 7 KF/Al 2 O 3 . 8 The catalytic performance of metal oxides catalysts or mixed oxides for aldol condensation of citral and acetone depends on the chemical compositions, active basic sites and strength, and surface electronic structure of catalysis, and coordination (adsorption) state of reactants required to activate the reactant of the reaction. The Na addition to MgO improves the yield of PS, while the Li addition has a poor effect. 9 However, Díez et al reported that promoters(Na, K, and Cs) of greater ionic radius than Li block the catalyst pores of MgO, and the addition to MgO enhances the PS yield. 10 Recently, lanthanum oxide and its mixed oxides have garnered significant attention for their role in the formation of C­C bonds, such as in aldol condensation, 11 transesterification, 12 esterification, 13 and coupling reaction. 14 To the best of our knowledge, Li-doped La 2 O 3 -ZnO catalyst used for synthesis of PS has not been reported. The objective of this work was to develop a new class of Li-doped La 2 O 3 -ZnO catalysts with high catalytic activity and selectivity for the aldol condensation of citral and acetone (Scheme 1 ). Zn(NO 3 ) 2 and La(NO 3 ) 3 used as initial precursors, the La 2 O 3 -ZnO composite was prepared through co-precipitation method, the Li-doped La 2 O 3 -ZnO catalysts were obtained via incipient wetness impregnation. The catalysts were characterized by means of XRD, BET, FT-IR, and SEM. The effects of Li + loading, calcination temperature, and impregnation time on catalytic activity were investigated, and the optimal reaction conditions (catalyst dosage, reaction temperature and time) were determined for the synthesis of PS. 2 Materials and Methods 2.1 Materials and reagents Citral (97%, Guangzhou baihua spice Co. Ltd., China), acetone (AR, Tianjin Damao Chemical Reagent Factory, China), La(NO 3 ) 3 ∙6H 2 O (AR, Shanghai Aladdin Bio-Chem Technology Co., LTD, China), Zn(NO 3 ) 2 ∙6H 2 O (AR, Xilong Scientific Co., Ltd., China), urea (AR, Tianjin zhiyuan chemical reagent Co. Ltd, China), all materials were used as received. 2.2 Experiment methods The synthesis of La 2 O 3 -ZnO composite oxide was carried out using the coprecipitation method. 15 An aqueous solution containing Zn(NO 3 ) 2 ∙6H 2 O and La(NO 3 ) 3 ∙6H 2 O with molar ratio of Zn/La = 4:1 was added to urea aqueous solution (molar ratio of NO 3 − /urea = 1.0). The mixture was stirred at 90°C for 7 h, precipitate was recovered by filtration, dried at 100°C for 12 h, grounded in mortar, and calcined at 500°C for 4 h to obtain the La 2 O 3 -ZnO composite oxides. Furthermore, the composite oxide was impregnated by different concentrations of LiOH aqueous solutions via the incipient wetness method 16 to obtain precursors with different Li + loading, which were subsequently dried and calcined to prepare the catalyst. The Li-doped La 2 O 3 -ZnO samples were denoted as Li/ La 2 O 3 -ZnO-x. Synthesis of PS was performed in a 100 mL three-necked flask (5 g citral). Acetone and catalyst were first charged into the flask, citral was then added dropwise with stirring under reflux condensation to afford PS. The reaction products were analyzed by the corrected area normalization method using gas chromatography (GC, Agilent-7890, FID, HP-5 column: 30 m × 320 µm × 0.25 µm). Citral conversion (X), PS selectivity (S), and PS yield (Y) were calculated as: $$\:\begin{array}{c}X\left(citral\right)=\frac{\sum\:{A}_{i}{F}_{i}-{A}_{citral}\times\:{F}_{citral}}{\sum\:{A}_{i}{F}_{I}}\times\:100\%\#\left(1\right)\end{array}$$ $$\:\begin{array}{c}S\left(\text{P}\text{S}\right)=\frac{{A}_{PS}\times\:{F}_{PS}}{\sum\:{A}_{i}{F}_{i}-{A}_{citral}\times\:{F}_{citral}}\times\:100\%\#\left(2\right)\end{array}$$ $$\:\begin{array}{c}Y\left(\text{P}\text{S}\right)=\frac{{n}_{PS}}{{n}_{citral,in}}={X}_{citral}\times\:{S}_{PS}\#\left(3\right)\end{array}$$ Where F is relative correction factor and A is the peak area of component in gas chromatogram. 2.2.1 XRD analysis X-ray diffraction patterns (XRD) of samples were recorded on a Bruker D2 Phaser diffractometer using Cu K α radiation and a Lynx Eye detector. 2.2.2 Textural properties BET surface areas ( S g ) were determined by N 2 adsorption/desorption at 77 K using Quantachrom SI. Specific surface area and pore volume were calculated using the Multi-Point-Brunauer-Emmett-Teller (MBET) and Barrett-Joyner-Hallender (BJH) methods, respectively. 2.2.3 FT-IR analysis Fourier transform infrared spectra (FT-IR) of samples were performed on a Nicolet S5 FT-IR spectrometer in the wave-number ranges of 4000 − 400 cm − 1 , and a standard KBr technique was used for the sample preparation. 2.2.4 SEM analysis The morphologies of the samples were characterized by field-emission SEM (FE-SEM, Zeiss Supra 55) with EDX mapping (Oxford) at an acceleration voltage of 15 kV. 2.2.5 Basic strength The base strength of the Li-doped La 2 O 3 -ZnO catalyst was determined by Hammett indicator method. 17 The total basicity of the catalysts was measured by the method of Hammett indicator-benzoic acid (0.01 mol/L anhydrous ethanol solution) titration. The indicators used were phenolphthalein (pKa = 9.8), 2,4-dinitroaniline (pKa = 15.0) and 4-chloro-2-nitroaniline (pKa = 17.2), all of which were prepared as 0.5 wt% indicator in anhydrous ethanol solution. 3 Results and discussion 3.1 Effect of catalyst preparation conditions 3.1.1 Effect of Li + loading The effect of Li + loading on catalytic performance of Li/La 2 O 3 -ZnO-x was performed under calcination temperature of 300°C and impregnation time of 15 h. As shown in Fig. 1 (1)-(2), no catalytic activity was observed on La 2 O 3 -ZnO. However, Li incorporation significantly enhanced the catalytic performance, and the conversion of citral achieved 100% within 2 h at Li⁺ loading of 4.0 wt%, while selectivity of PS is 99.2%. This may be attributed to well-dispersed strong base sites (Li + O − /LiOH) formed via Li + substitution for Zn 2+ /La 3+ . 18 Low loading (3.5 wt%) reduced catalytic activity (88.9% citral conversion, 95.9% PS selectivity), likely due to insufficient base sites. Conversely, high loading (4.5­5.5 wt%) promoted Li 2 CO 3 formation, potentially blocking active sites. 19 Consequently, Li/La 2 O 3 -ZnO-4.0 was selected for further investigation. 3.1.2 Effect of calcination temperature after Li-doped The effect of calcination temperature on catalytic performance of Li/La 2 O 3 -ZnO-4.0 was performed under impregnation time of 15 h. In Fig. 1 (3), citral conversion and PS yield increased gradually below 300°C, while they decreased significantly over 300°C, and citral conversion dropped to 75.5% at 400°C. Which may be either incomplete decomposition of the catalyst precursor at low temperatures or weakening of strong basic sites at high temperatures, such as the loss of surface CO 3 2− from La 2 O 2 CO 3 . 20 Thus, an optimal calcination temperature of 300°C was determined for the Li/La 2 O 3 -ZnO-4.0 catalysts. 3.1.3 Effect of impregnation time The effect of impregnation time on catalytic performance of Li/La 2 O 3 -ZnO-4.0 was performed under calcination temperature of 300°C. From Fig. 1 (4), citral conversion and PS yield increased gradually at impregnation time below 15 h. However, basic groups of catalyst exposed to air with the increase of impregnation times (> 15 h), causing carbonate formation and diminishing catalytic activity. 21 Therefore, the optimal impregnation time was 15 h. Consequently, the optimal preparation conditions were 4.0 wt% Li + loading, 300°C calcination after Li-doped, and 15 h impregnation. 3.2 XRD The XRD patterns of La 2 O 3 -ZnO and Li/La 2 O 3 -ZnO-x samples were given in Fig. 2 . The La 2 O 3 -ZnO composite oxide shown in XRD pattern (Fig. 2 a) was a mixture of ZnO, La(OH) 3 , La 2 O 2 CO 3 and La 2 O 3 phases. 22 The La 2 O 2 CO 3 diffraction peaks at 22.6°, 26.1°, 30.6° and 42.8° (JCPDS No.48-1113) were occurred, which may be explained that La(OH) 3 exposed to the air may became La 2 O 2 CO 3 , and then gradually decomposed into La 2 O 3 during calcination process. 21 The XRD patterns of Li/La 2 O 3 -ZnO-x samples shown that characteristic diffraction peaks of hexagonal wurtzite ZnO (2 θ = 31.9°, 34.6°, 36.5°, 47.7°, 56.8° ,63.0° and 68.2°), La(OH) 3 phase at 27.5°, 28.2°, 55.5° and 69.2°, La 2 O 2 CO 3 phase at 26.1°, 30.8° and 42.4°, and La 2 O 3 phase at 30.8° and 50.5° were observed (Fig. 2 b-f). Furthermore, after addition of Li, there were no remarkable shift of the diffraction peaks belonging to ZnO, La(OH) 3 , La 2 O 2 CO 3 and La 2 O 3 . However, compared with the La 2 O 3 -ZnO composite oxide, the intensities of the diffraction peaks for Li/La 2 O 3 -ZnO-x catalysts increased, and diffraction peaks became sharp with the addition of Li, indicating the increase of crystallinity after impregnation and subsequent calcination. Meanwhile, after La 2 O 3 -ZnO doped with different Li loading, the diffraction peaks of Li 2 CO 3 at 21.5°, 30.2° and 37.1° (JCPDS No.831454), 23 Li 2 O 2 at 33.8° and 39.6° (JCPDS No.9-0355), 24 LiOH at 48.8° were observed, 25, 26 and there was no significant difference for the intensity of diffraction peaks in different Li/La 2 O 3 -ZnO-x samples, which demonstrated that Li-containing phase was well dispersed on La 2 O 3 -ZnO composite oxide. The ionic radius of Li + ( r Li + =0.73 Å) is smaller than that of Zn 2+ ( r Zn 2+ =0.74 Å) and La 3+ ( r La 3+ =1.16 Å), 27 substitution of Li + for Zn 2+ and La 3+ in composite oxide may occur, furthermore, Li + may coordinate with chemisorbed O 2− species on surface of catalyst to form active basic site. 28 Therefore, it could be inferred that the addition of Li was beneficial for the formation of strong Li + O − (Li 2 O 2 ) and LiOH basic site in Li/La 2 O 3 -ZnO-x catalysts, which probably improved catalytic activity of Li/La 2 O 3 -ZnO-x catalysts for aldol condensation of citral and acetone. The unit cell parameter a for the La 2 O 3 -ZnO and Li/La 2 O 3 -ZnO-x was calculated from the diffractograms in Fig. 2 , and the values obtained were given in Table 1 . Li loading on the surface of La 2 O 3 -ZnO composite oxides, causing contraction of the La 2 O 3 -ZnO lattice and formation of strongly basic an ionic vacancy. 10 The crystallite sizes of the ZnO phase in the La 2 O 3 -ZnO and Li/La 2 O 3 -ZnO-x were determined from XRD patterns of Fig. 2 using the Scherrer equation for the reflection at 2 θ = 36.5° and were given in Table 1 . The results showed that doping of lithium increased the lattice size of ZnO phase. Table 1 Chemical, textural and structural characterization of Li/La 2 O 3 -ZnO-x and La 2 O 3 -ZnO catalysts catalysts Li + loading (wt%) Surface area, S g (m 2 /g) Pore diameter Dv Pore volume (ml/g) XRD analysis Lattice parameter a (Å) Crystallite size (Å) La 2 O 3 -ZnO 0 34.029 33.81 0.295 4.947 176 Li/La 2 O 3 -ZnO-3.5 3.5 9.614 3.98 0.078 5.345 502 Li/La 2 O 3 -ZnO-4.0 4.0 8.135 3.98 0.078 5.350 431 Li/La 2 O 3 -ZnO-4.5 4.5 8.342 3.99 0.071 5.354 462 Li/La 2 O 3 -ZnO-5.0 5.0 9.975 3.99 0.073 5.336 476 Li/La 2 O 3 -ZnO-5.5 5.5 10.212 3.98 0.061 5.336 450 3.3 Textural properties The nitrogen adsorption/desorption experiment results showed that the specific surface area of La 2 O 3 -ZnO was 34.029 m 2 /g, but after doping with lithium, the Li/La 2 O 3 -ZnO-x specific surface area was significantly reduced, ranged from 8.135 to 10.212 m 2 /g. Pore diameter and pore volume were also affected by addition of Li. La 2 O 3 -ZnO had a pore diameter of 33.807 nm and a pore volume of 0.295 mL/g. However, pore diameter and pore volume decreased in the Li/La 2 O 3 -ZnO-x samples. This behavior was due to particle agglomeration of lithium during La 2 O 3 -ZnO impregnation. 9 , 29 3.4 FT- IR analysis The FT-IR spectra of La 2 O 3 -ZnO and Li/La 2 O 3 -ZnO-x were shown in Fig. 3 . The broad bands at 3426.9 and 1633.0 cm − 1 for La 2 O 3 -ZnO were assigned to associated O­H or H-O-H stretching and bending respectively due to the adsorption of moisture on the surface of La 2 O 3 -ZnO or the existence of M­OH (M = Zn or La). 30 The sharp peak at 1466.0 cm − 1 could be attributed to the characteristic absorption peaks of La 2 O 3 . The absorption bands at 1402.1 and 1086.6 cm − 1 were associated with stretching and bending vibration of C-O bond, indicating the existence of La 2 O 2 CO 3 , 31 which was agree with XRD result and previous reports. 21 The strong absorption peak appearing at 855.7 cm − 1 was ascribed to stretching vibration of La-O bond. The stretching vibration of Zn-O bond at 422.7 cm − 1 was observed. 32 As shown in Fig. 3 b-f, there were obvious difference for Li/La 2 O 3 ­ZnO­x compared to La 2 O 3 -ZnO. After addition of Li, the characteristic absorption of Zn­O bond in Li/La 2 O 3 -ZnO-x shifted toward high wavenumber at about 486.0 cm − 1 , which was in accordance with previous work reported by Yogamala et al. 32 Compared with FT-IR spectrum of La 2 O 3 -ZnO, the sharp and narrow band assigned to O­H at about 3443, 3565 and 3610 cm − 1 were observed for Li/La 2 O 3 -ZnO-x samples, indicating the presence of generous isolated O­H. After impregnated in LiOH solution, the hydrogen bond of associated O­H was broken to form isolated O­H, furthermore, Li + may be exchanged with proton of M­OH (M = Zn or La) to form M ­ O ­ Li bond. 33 The broad adsorption bands at about 1500 and 1433.0 cm − 1 were assigned to C ­ O overlapping peaks of M­CO 3 .(M = Li or La), which was significantly different from FT-IR spectrum of La 2 O 3 -ZnO between 1400­1500cm − 1 . Meanwhile, symmetric C ­ O stretching vibration at about 1087 cm − 1 was observed. In addition, a few weak adsorption bands at about 640, 742 and 998 cm − 1 were arose from Li 2 O 2 , 26 and intensity of these peak increases with the increased of doping amount of Lithium. These results further demonstrated that Li/La 2 O 3 -ZnO-x catalysts possess active Li + O − (Li 2 O 2 ) and LiOH basic sites. 3.5 SEM For morphology of La 2 O 3 -ZnO (Fig. 4 a-b), a large number of rod-shaped and fluffy shape particles were observed, and fluffy shape particles were uniformly and loosely dispersed on the surface of rod-shaped materials. However, the morphology of Li/La 2 O 3 -ZnO catalyst was dominated with irregular particles that tended to agglomerate, and presented with a flat, smoother appearance. These characteristics became more evident with the increase of Li-doped amount from 3.5 wt% to 5.0 wt%, these results were agreed with previous report. 10 3.6 Basic strength The basic strength of La 2 O 3 -ZnO and Li/La 2 O 3 -ZnO-x samples was determined by Hammett indicator method. 34 The results were shown in the Table 2 . After the introduction of Li, the basic strength of Li/La 2 O 3 -ZnO-x catalysts was in the range of 15 ≤ H-<17.2. The catalytic performance for aldol condensation of citral and acetone was related to basic strength of catalyst. In this work, Li/La 2 O 3 -ZnO-x catalysts possessed excellent catalytic activity for synthesis of PS, and no catalytic activity was observed for La 2 O 3 -ZnO composite oxide, therefore, Li/La 2 O 3 -ZnO-x catalysts were used in the following experiments. Table 2 Surface basic strength of La 2 O 3 -ZnO and Li/La 2 O 3 -ZnO-x samples Catalyst base strength(H-) La 2 O 3 -ZnO 9.3­15 Li/La 2 O 3 -ZnO-3.5 15­17.2 Li/La 2 O 3 -ZnO-4.0 15­17.2 Li/La 2 O 3 -ZnO-4.5 15­17.2 Li/La 2 O 3 -ZnO-5.0 15­17.2 Li/La 2 O 3 -ZnO-5.5 15­17.2 3.7 Effect of reaction conditions The effect of reaction conditions on aldol condensation of citral with acetone catalyzed by Li/La 2 O 3 -ZnO-4.0 was investigated. The amount of citral used in the following experiments was 5 g. 3.7.1 Effect of Li/La 2 O 3 -ZnO-4.0 catalyst dosage The effect of catalyst dosage was investigated under the conditions (50°C, 3h, and molar ratio citral/acetone of 1:9). Figure 5 a indicates that the increase of the catalyst dosage from 0.2 g to 0.5 g improved the aldol condensation reaction, increasing citral conversion from 68.1% to 100% and PS selectivity from 94.1% to 99.2%. However, dosages beyond 0.5 g caused a slight reduction in both conversion and selectivity, possibly due to side reactions promoted by excess alkali. 4 Therefore, 0.5 g was selected as the optimal catalyst dosage. 3.7.2 Effect of reaction time The effect of reaction time was investigated under the following conditions (50°C, 0.5 g catalyst dosage, and molar ratio citral/acetone of 1:9). Citral conversion increased sharply within the 1 hour and reached 100% after three hours. Meanwhile, the maximum PS selectivity of 99.2% was achieved at three hours and then gradually declined (Fig. 5 b). Which suggested that by-products were formed and reduced PS selectivity. 35 The optimal reaction time was 3 h. 3.7.3 Effect of reaction temperature The effect of reaction temperature was investigated under the following conditions (0.5 g catalyst dosage, 3 h, and molar ratio of citral/acetone 1:9). From Fig. 5 c, citral conversion and PS selectivity increased gradually below 50°C, and decreased over 50°C. Therefore, 50°C was chosen as the reaction temperature for the synthesis of PS. 3.7.4 Effect of material ratio of citral to acetone The effect of the molar ratio of citral/acetone (1:3 to 1:11) was evaluated under the following conditions (0.5 g catalyst dosage, 3 h, and 50°C). Figure 5 d presented that the citral conversion and PS selectivity were initially increased, but declined slightly with excess acetone, the reaction performed optimally at a molar ratio of citral to acetone of 1:9. This trend may be attributed to the decline in citral concentration and competitive acetone self-condensation. 36 The optimal citral/acetone molar ratio was found to be 1:9. 3.8 Evaluation of the Li/La 2 O 3 -ZnO-4.0 catalyst reusability stability To evaluate the reusability and stability of the Li/La 2 O 3 -ZnO-4.0 catalyst, the used catalyst was washed several times with acetone, dried and subsequently reused in the synthesis of PS. The results were presented in Table 3 . Table 3 Recyclability test of Li/La 2 O 3 -ZnO-4.0 catalyst Times X (Citral)/% S (PS)/% Y (PS) /% 1 100 99.2 99.2 2 89.4 93.8 83.9 3 72.2 90.4 65.3 The catalyst showed acceptable stability during recycling. With the catalyst, PS selectivity remained above 90% throughout three reaction cycles, however, a gradual decrease in the conversion of citral was observed. This decline in performance may be attributed to the loss of active components from the catalyst surface as well as the surface pores were blocked by organic residues during recovery, 37 which could hinder the contact between the catalyst and the reactants, thereby reducing its catalytic efficiency. 4 Conclusion A highly efficient Li-doped La 2 O 3 -ZnO catalyst was employed for the aldol condensation of citral and acetone. The introduction of Li into the La 2 O 3 -ZnO composite notably enhanced catalytic performance, with citral conversion and PS selectivity dramatically increasing, especially for Li-loading amount of 4.0 wt%. The formation of strong Li + O − and LiOH basic sites on the catalyst's surface was crucial for the aldol condensation activity. Optimal preparation conditions for the Li/La 2 O 3 -ZnO catalyst were Li-loading amount of 4.0 wt%, calcination temperature of 300°C after Li-doped, and impregnation time of 15 h. The conversion of citral and yield of PS was 100% and 99.2% respectively under the suitable reaction conditions of catalyst dosage of 0.5 g, reaction temperature of 50°C, reaction time of 3 h and citral/acetone molar ratio of 1:9. From a practical standpoint, the proposed Li/La 2 O 3 -ZnO catalysts offer high catalytic activity and selectivity, along with a straightforward product purification process. To address the loss in catalytic activity after repeated cycles, future efforts will be directed toward implementing structural modifications to prevent active component leaching and developing a robust regeneration protocol for the removal of organic residues. Abbreviations Abbreviation Definition PS Pseudoionone Declarations Ethics and Consent to Participate This article does not contain any studies with human participants performed by any of the authors. Consent for Publication The results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration (from you or one of your Contributing Authors) by another publisher. Competing Interest I declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper. Author Contribution FL.W. and T.Y. wrote the main manuscript text and prepared all figures and tables; All authors made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data; or the creation of new software used in the work; reviewed the manuscript; CT.K. drafted the work or revised it critically for important intellectual content; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. Funding National Key Research and Development Program of China (No. 2017YFD0600704); Scientific Research Fund of Hunan Provincial Education Department(NO. 15A196). Availability of data and materials The numerical data from Figures 1, 5 are tabulated in the Supplementary Material. Numerical data for the XRD spectra from Figure 2 and the Numerical data for the FT-IR spectra from Figure 3 are available as a .zip file in the Supplementary Material. Error bars (where shown) in Figures 1, 5 show the spread of data observed in quintuplicate measurements, where independent samples were tested for each measurement. Acknowledgments This work was supported by the National Key Research and Development Program of China (No. 2017YFD0600704) and Scientific Research Fund of Hunan Provincial Education Department (NO. 15A196). References Díez VK., Apesteguía CR., Cosimo JID. Kinetic and mechanistic study of the synthesis of ionone isomers from pseudoionone on Brønsted acid solids. Catal Today. 2017;296:127-134. Hu T, Pi SF, Wang Y, et al. Synthesis of Irone by Cyclization Reaction. Chinese Journal of Applied Chemistry. 2014;31(11):1297-1301. Parker GL., Smith LK., Baxendale IR. Development of the industrial synthesis of vitamin A. Tetrahedron. 2016;72(13):1645-1652. 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Supplementary Files Supplementarymaterial.docx GraphicalAbstract.jpg XRDandFTIR.zip SCHEME1.docx Cite Share Download PDF Status: Posted 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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16:28:26","extension":"html","order_by":35,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":116214,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/2f15ce0862befabf80fe0794.html"},{"id":97911283,"identity":"1ef97db0-f0be-4943-a77e-6769914b07c1","added_by":"auto","created_at":"2025-12-10 16:28:26","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":537872,"visible":true,"origin":"","legend":"\u003cp\u003e(1) Effect of the different Li\u003csup\u003e+\u003c/sup\u003e loading of catalysts on conversion of citral.\u003c/p\u003e\n\u003cp\u003e(2) Effect of Li\u003csup\u003e+\u003c/sup\u003e loading on the catalytic performance of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts.\u003c/p\u003e\n\u003cp\u003e(3) Effect of calcination temperature after Li-doped on the catalytic performance of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalysts.\u003c/p\u003e\n\u003cp\u003e(4) Effect of impregnation time on the catalytic performance of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalysts\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/ff86086099ef51839810af10.png"},{"id":98421309,"identity":"b0acf759-a47c-4cea-b820-749f89d37208","added_by":"auto","created_at":"2025-12-17 16:26:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":330158,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO(a), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-3.5(b), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0(c), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.5(d); Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.0(e) and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.5(f) (● corresponds to Li-containing phase, □ corresponds to La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, ■ corresponds to La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, ◇corresponds to La(OH)\u003csub\u003e3\u003c/sub\u003e, ▼corresponds to ZnO).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/92fbfd84f0c17474974a811e.png"},{"id":97911285,"identity":"fc0ef5b7-c4bf-4381-a847-c2c3ad9d6a2a","added_by":"auto","created_at":"2025-12-10 16:28:26","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":386046,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectra of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO(a), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-3.5(b), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0(c), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.5(d), Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.0(e) and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.5(f)\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/289dd49c11520937c52defee.png"},{"id":97911289,"identity":"5993cfa6-a6d2-4acb-80a1-5aee2c2371a5","added_by":"auto","created_at":"2025-12-10 16:28:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1612563,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrographs of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO ((a)×5.0k, (b)×20.0k); Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-3.5 ((c)×5.0k, (d)×20.0k) ; Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 ((e)×5.0k, (f)×20.0k); Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.5 ((g)×5.0k, (h)×20.0k)\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/e981aa06caf446e98c8bde5a.png"},{"id":98420994,"identity":"f88f4713-f98f-4332-ba28-ea002dc7084a","added_by":"auto","created_at":"2025-12-17 16:21:57","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":203651,"visible":true,"origin":"","legend":"\u003cp\u003ea). Effect of catalyst dosage on the aldol condensation of citral with acetone.\u003c/p\u003e\n\u003cp\u003eb). Effect of reaction time on the aldol condensation of citral with acetone.\u003c/p\u003e\n\u003cp\u003ec). Effect of reaction temperature on the aldol condensation of citral with acetone.\u003c/p\u003e\n\u003cp\u003ed). Effect of citral/acetone molar ratio on the aldol condensation of citral with acetone.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/79b9f596f07ed11ecd84235c.png"},{"id":99789646,"identity":"e87a78aa-61df-4801-9f4f-49e2469ec0a9","added_by":"auto","created_at":"2026-01-08 12:50:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4138247,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/48194183-2a68-47b6-b07c-2d090c558b7d.pdf"},{"id":98421848,"identity":"8b515486-b6eb-4554-8011-1130a20bfb60","added_by":"auto","created_at":"2025-12-17 16:29:40","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":130770,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/37f80e40d93d60480bbe6f9a.docx"},{"id":98421315,"identity":"daef38d9-5e93-4f21-ad15-2e43575c1466","added_by":"auto","created_at":"2025-12-17 16:26:30","extension":"jpg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":221345,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/189ce67a8e1b159b35dbc00e.jpg"},{"id":97911292,"identity":"4f7e7a04-db64-4d66-81bb-f09fd8fe2ac0","added_by":"auto","created_at":"2025-12-10 16:28:26","extension":"zip","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":310371,"visible":true,"origin":"","legend":"","description":"","filename":"XRDandFTIR.zip","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/7b633a42f7298abc68792eb4.zip"},{"id":98421023,"identity":"bccaf655-4ef4-46e2-8d5d-dd8baf5d1b8c","added_by":"auto","created_at":"2025-12-17 16:22:21","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":29393,"visible":true,"origin":"","legend":"","description":"","filename":"SCHEME1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8285759/v1/2eb77a35e95d7d90f6e5006c.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eHighly efficient synthesis of pseudoionone from citral with acetone on Li-doped La\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e­ZnO catalysts\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003ePseudoionone (PS) is a key chemical intermediate for synthesizing ionones,\u003csup\u003e\u003cb\u003e1\u003c/b\u003e\u003c/sup\u003e irones,\u003csup\u003e2\u003c/sup\u003e vitamin A,\u003csup\u003e3\u003c/sup\u003e and related high-value compounds. In recent years, due to the significant growth of downstream products, the market demand of PS has increased markedly. Currently, PS is industrially synthesized via the aldol condensation of citral with acetone, typically catalyzed by aqueous alkali hydroxides (NaOH, KOH, or LiOH). However, these catalysts cause environmental challenges, equipment corrosion and unexpected side reactions. Therefore, it is necessary to develop a highly efficient and sustainable catalyst for the synthesis of PS. In response, there is a growing interest in solid base catalysts for the synthesis of PS. Compared to homogeneous catalysts, solid base catalysts offer several advantages \u0026mdash;non-corrosiveness, easy separation from reaction mixtures, reusability \u0026mdash;and high PS selectivity.\u003c/p\u003e\u003cp\u003eA variety of materials had been explored as solid base catalysts, including powdered LiOH\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO,\u003csup\u003e4\u003c/sup\u003e MgO\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e and Mg-Al mixed oxides,\u003csup\u003e6\u003c/sup\u003e Mg-Al hydrotalcites,\u003csup\u003e7\u003c/sup\u003e KF/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e.\u003csup\u003e8\u003c/sup\u003e The catalytic performance of metal oxides catalysts or mixed oxides for aldol condensation of citral and acetone depends on the chemical compositions, active basic sites and strength, and surface electronic structure of catalysis, and coordination (adsorption) state of reactants required to activate the reactant of the reaction. The Na addition to MgO improves the yield of PS, while the Li addition has a poor effect.\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e However, D\u0026iacute;ez et al reported that promoters(Na, K, and Cs) of greater ionic radius than Li block the catalyst pores of MgO, and the addition to MgO enhances the PS yield.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eRecently, lanthanum oxide and its mixed oxides have garnered significant attention for their role in the formation of C\u0026shy;C bonds, such as in aldol condensation,\u003csup\u003e11\u003c/sup\u003e transesterification,\u003csup\u003e12\u003c/sup\u003e esterification,\u003csup\u003e13\u003c/sup\u003e and coupling reaction.\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e To the best of our knowledge, Li-doped La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalyst used for synthesis of PS has not been reported.\u003c/p\u003e\u003cp\u003eThe objective of this work was to develop a new class of Li-doped La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalysts with high catalytic activity and selectivity for the aldol condensation of citral and acetone (Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and La(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e used as initial precursors, the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite was prepared through co-precipitation method, the Li-doped La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalysts were obtained via incipient wetness impregnation. The catalysts were characterized by means of XRD, BET, FT-IR, and SEM. The effects of Li\u003csup\u003e+\u003c/sup\u003e loading, calcination temperature, and impregnation time on catalytic activity were investigated, and the optimal reaction conditions (catalyst dosage, reaction temperature and time) were determined for the synthesis of PS.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials and reagents\u003c/h2\u003e\u003cp\u003eCitral (97%, Guangzhou baihua spice Co. Ltd., China), acetone (AR, Tianjin Damao Chemical Reagent Factory, China), La(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO (AR, Shanghai Aladdin Bio-Chem Technology Co., LTD, China), Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO (AR, Xilong Scientific Co., Ltd., China), urea (AR, Tianjin zhiyuan chemical reagent Co. Ltd, China), all materials were used as received.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Experiment methods\u003c/h2\u003e\u003cp\u003eThe synthesis of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxide was carried out using the coprecipitation method.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e An aqueous solution containing Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO and La(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e∙6H\u003csub\u003e2\u003c/sub\u003eO with molar ratio of Zn/La\u0026thinsp;=\u0026thinsp;4:1 was added to urea aqueous solution (molar ratio of NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e/urea\u0026thinsp;=\u0026thinsp;1.0). The mixture was stirred at 90\u0026deg;C for 7 h, precipitate was recovered by filtration, dried at 100\u0026deg;C for 12 h, grounded in mortar, and calcined at 500\u0026deg;C for 4 h to obtain the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxides. Furthermore, the composite oxide was impregnated by different concentrations of LiOH aqueous solutions via the incipient wetness method\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e to obtain precursors with different Li\u003csup\u003e+\u003c/sup\u003e loading, which were subsequently dried and calcined to prepare the catalyst. The Li-doped La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO samples were denoted as Li/ La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x.\u003c/p\u003e\u003cp\u003eSynthesis of PS was performed in a 100 mL three-necked flask (5 g citral). Acetone and catalyst were first charged into the flask, citral was then added dropwise with stirring under reflux condensation to afford PS. The reaction products were analyzed by the corrected area normalization method using gas chromatography (GC, Agilent-7890, FID, HP-5 column: 30 m \u0026times; 320 \u0026micro;m \u0026times; 0.25 \u0026micro;m). Citral conversion (X), PS selectivity (S), and PS yield (Y) were calculated as:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}X\\left(citral\\right)=\\frac{\\sum\\:{A}_{i}{F}_{i}-{A}_{citral}\\times\\:{F}_{citral}}{\\sum\\:{A}_{i}{F}_{I}}\\times\\:100\\%\\#\\left(1\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}S\\left(\\text{P}\\text{S}\\right)=\\frac{{A}_{PS}\\times\\:{F}_{PS}}{\\sum\\:{A}_{i}{F}_{i}-{A}_{citral}\\times\\:{F}_{citral}}\\times\\:100\\%\\#\\left(2\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:\\begin{array}{c}Y\\left(\\text{P}\\text{S}\\right)=\\frac{{n}_{PS}}{{n}_{citral,in}}={X}_{citral}\\times\\:{S}_{PS}\\#\\left(3\\right)\\end{array}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eWhere \u003cem\u003eF\u003c/em\u003e is relative correction factor and \u003cem\u003eA\u003c/em\u003e is the peak area of component in gas chromatogram.\u003c/p\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1 XRD analysis\u003c/h2\u003e\u003cp\u003eX-ray diffraction patterns (XRD) of samples were recorded on a Bruker D2 Phaser diffractometer using Cu \u003cem\u003eK\u003c/em\u003e\u003csub\u003e\u003cem\u003eα\u003c/em\u003e\u003c/sub\u003e radiation and a Lynx Eye detector.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2 Textural properties\u003c/h2\u003e\u003cp\u003eBET surface areas (\u003cem\u003eS\u003c/em\u003e\u003csub\u003e\u003cem\u003eg\u003c/em\u003e\u003c/sub\u003e) were determined by N\u003csub\u003e2\u003c/sub\u003e adsorption/desorption at 77 K using Quantachrom SI. Specific surface area and pore volume were calculated using the Multi-Point-Brunauer-Emmett-Teller (MBET) and Barrett-Joyner-Hallender (BJH) methods, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\u003ch2\u003e2.2.3 FT-IR analysis\u003c/h2\u003e\u003cp\u003eFourier transform infrared spectra (FT-IR) of samples were performed on a Nicolet S5 FT-IR spectrometer in the wave-number ranges of 4000\u0026thinsp;\u0026minus;\u0026thinsp;400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and a standard KBr technique was used for the sample preparation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section3\"\u003e\u003ch2\u003e2.2.4 SEM analysis\u003c/h2\u003e\u003cp\u003eThe morphologies of the samples were characterized by field-emission SEM (FE-SEM, Zeiss Supra 55) with EDX mapping (Oxford) at an acceleration voltage of 15 kV.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003ch2\u003e2.2.5 Basic strength\u003c/h2\u003e\u003cp\u003eThe base strength of the Li-doped La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalyst was determined by Hammett indicator method.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e The total basicity of the catalysts was measured by the method of Hammett indicator-benzoic acid (0.01 mol/L anhydrous ethanol solution) titration. The indicators used were phenolphthalein (pKa\u0026thinsp;=\u0026thinsp;9.8), 2,4-dinitroaniline (pKa\u0026thinsp;=\u0026thinsp;15.0) and 4-chloro-2-nitroaniline (pKa\u0026thinsp;=\u0026thinsp;17.2), all of which were prepared as 0.5 wt% indicator in anhydrous ethanol solution.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1 Effect of catalyst preparation conditions\u003c/h2\u003e\n \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\n \u003ch2\u003e3.1.1 Effect of Li\u003csup\u003e+\u003c/sup\u003e loading\u003c/h2\u003e\n \u003cp\u003eThe effect of Li\u003csup\u003e+\u003c/sup\u003e loading on catalytic performance of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x was performed under calcination temperature of 300\u0026deg;C and impregnation time of 15 h. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(1)-(2), no catalytic activity was observed on La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO. However, Li incorporation significantly enhanced the catalytic performance, and the conversion of citral achieved 100% within 2 h at Li⁺ loading of 4.0 wt%, while selectivity of PS is 99.2%. This may be attributed to well-dispersed strong base sites (Li\u003csup\u003e+\u003c/sup\u003eO\u003csup\u003e\u0026minus;\u003c/sup\u003e/LiOH) formed via Li\u003csup\u003e+\u003c/sup\u003e substitution for Zn\u003csup\u003e2+\u003c/sup\u003e/La\u003csup\u003e3+\u003c/sup\u003e.\u003csup\u003e18\u003c/sup\u003e Low loading (3.5 wt%) reduced catalytic activity (88.9% citral conversion, 95.9% PS selectivity), likely due to insufficient base sites. Conversely, high loading (4.5\u0026shy;5.5 wt%) promoted Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e formation, potentially blocking active sites.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Consequently, Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 was selected for further investigation.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\n \u003ch2\u003e3.1.2 Effect of calcination temperature after Li-doped\u003c/h2\u003e\n \u003cp\u003eThe effect of calcination temperature on catalytic performance of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 was performed under impregnation time of 15 h. In Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(3), citral conversion and PS yield increased gradually below 300\u0026deg;C, while they decreased significantly over 300\u0026deg;C, and citral conversion dropped to 75.5% at 400\u0026deg;C. Which may be either incomplete decomposition of the catalyst precursor at low temperatures or weakening of strong basic sites at high temperatures, such as the loss of surface CO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e from La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e Thus, an optimal calcination temperature of 300\u0026deg;C was determined for the Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalysts.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e\n \u003ch2\u003e3.1.3 Effect of impregnation time\u003c/h2\u003e\n \u003cp\u003eThe effect of impregnation time on catalytic performance of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 was performed under calcination temperature of 300\u0026deg;C. From Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(4), citral conversion and PS yield increased gradually at impregnation time below 15 h. However, basic groups of catalyst exposed to air with the increase of impregnation times (\u0026gt;\u0026thinsp;15 h), causing carbonate formation and diminishing catalytic activity.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Therefore, the optimal impregnation time was 15 h.\u003c/p\u003e\n \u003cp\u003eConsequently, the optimal preparation conditions were 4.0 wt% Li\u003csup\u003e+\u003c/sup\u003e loading, 300\u0026deg;C calcination after Li-doped, and 15 h impregnation.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 XRD\u003c/h2\u003e\n \u003cp\u003eThe XRD patterns of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples were given in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. The La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxide shown in XRD pattern (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea) was a mixture of ZnO, La(OH)\u003csub\u003e3\u003c/sub\u003e, La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phases.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e The La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e diffraction peaks at 22.6\u0026deg;, 26.1\u0026deg;, 30.6\u0026deg; and 42.8\u0026deg; (JCPDS No.48-1113) were occurred, which may be explained that La(OH)\u003csub\u003e3\u003c/sub\u003e exposed to the air may became La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, and then gradually decomposed into La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e during calcination process.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eThe XRD patterns of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples shown that characteristic diffraction peaks of hexagonal wurtzite ZnO (2\u003cem\u003e\u0026theta;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;31.9\u0026deg;, 34.6\u0026deg;, 36.5\u0026deg;, 47.7\u0026deg;, 56.8\u0026deg; ,63.0\u0026deg; and 68.2\u0026deg;), La(OH)\u003csub\u003e3\u003c/sub\u003e phase at 27.5\u0026deg;, 28.2\u0026deg;, 55.5\u0026deg; and 69.2\u0026deg;, La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e phase at 26.1\u0026deg;, 30.8\u0026deg; and 42.4\u0026deg;, and La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase at 30.8\u0026deg; and 50.5\u0026deg; were observed (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb-f). Furthermore, after addition of Li, there were no remarkable shift of the diffraction peaks belonging to ZnO, La(OH)\u003csub\u003e3\u003c/sub\u003e, La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. However, compared with the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxide, the intensities of the diffraction peaks for Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts increased, and diffraction peaks became sharp with the addition of Li, indicating the increase of crystallinity after impregnation and subsequent calcination. Meanwhile, after La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO doped with different Li loading, the diffraction peaks of Li\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e at 21.5\u0026deg;, 30.2\u0026deg; and 37.1\u0026deg; (JCPDS No.831454),\u003csup\u003e23\u003c/sup\u003e Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e at 33.8\u0026deg; and 39.6\u0026deg; (JCPDS No.9-0355),\u003csup\u003e24\u003c/sup\u003e LiOH at 48.8\u0026deg; were observed,\u003csup\u003e25, 26\u003c/sup\u003e and there was no significant difference for the intensity of diffraction peaks in different Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples, which demonstrated that Li-containing phase was well dispersed on La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxide. The ionic radius of Li\u003csup\u003e+\u003c/sup\u003e (\u003cem\u003er\u003c/em\u003e\u003csub\u003eLi\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e=0.73 \u0026Aring;) is smaller than that of Zn\u003csup\u003e2+\u003c/sup\u003e (\u003cem\u003er\u003c/em\u003e\u003csub\u003eZn\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e=0.74 \u0026Aring;) and La\u003csup\u003e3+\u003c/sup\u003e (\u003cem\u003er\u003c/em\u003e\u003csub\u003eLa\u003c/sub\u003e\u003csup\u003e3+\u003c/sup\u003e=1.16 \u0026Aring;),\u003csup\u003e27\u003c/sup\u003e substitution of Li\u003csup\u003e+\u003c/sup\u003e for Zn\u003csup\u003e2+\u003c/sup\u003e and La\u003csup\u003e3+\u003c/sup\u003e in composite oxide may occur, furthermore, Li\u003csup\u003e+\u003c/sup\u003e may coordinate with chemisorbed O\u003csup\u003e2\u0026minus;\u003c/sup\u003e species on surface of catalyst to form active basic site.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Therefore, it could be inferred that the addition of Li was beneficial for the formation of strong Li\u003csup\u003e+\u003c/sup\u003eO\u003csup\u003e\u0026minus;\u003c/sup\u003e (Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) and LiOH basic site in Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts, which probably improved catalytic activity of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts for aldol condensation of citral and acetone.\u003c/p\u003e\n \u003cp\u003eThe unit cell parameter \u003cem\u003ea\u003c/em\u003e for the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x was calculated from the diffractograms in Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, and the values obtained were given in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. Li loading on the surface of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxides, causing contraction of the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO lattice and formation of strongly basic an ionic vacancy.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e The crystallite sizes of the ZnO phase in the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x were determined from XRD patterns of Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e using the Scherrer equation for the reflection at 2\u003cem\u003e\u0026theta;\u003c/em\u003e\u0026thinsp;=\u0026thinsp;36.5\u0026deg; and were given in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. The results showed that doping of lithium increased the lattice size of ZnO phase.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eChemical, textural and structural characterization of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x and La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalysts\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003ecatalysts\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eLi\u003csup\u003e+\u003c/sup\u003e loading\u003c/p\u003e\n \u003cp\u003e(wt%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003eSurface area, S\u003csub\u003eg\u003c/sub\u003e (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003ePore diameter\u003c/p\u003e\n \u003cp\u003eDv\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003ePore volume\u003c/p\u003e\n \u003cp\u003e(ml/g)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colspan=\"2\"\u003e\n \u003cp\u003eXRD analysis\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eLattice parameter\u003c/p\u003e\n \u003cp\u003e\u003cem\u003ea\u003c/em\u003e (\u0026Aring;)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCrystallite size\u003c/p\u003e\n \u003cp\u003e(\u0026Aring;)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLa\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e34.029\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e33.81\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.295\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.947\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e176\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9.614\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.078\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.345\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e502\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.135\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.078\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.350\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e431\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e4.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.342\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.071\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.354\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e462\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9.975\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.073\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.336\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e476\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10.212\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.061\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.336\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e450\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 Textural properties\u003c/h2\u003e\n \u003cp\u003eThe nitrogen adsorption/desorption experiment results showed that the specific surface area of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO was 34.029 m\u003csup\u003e2\u003c/sup\u003e/g, but after doping with lithium, the Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x specific surface area was significantly reduced, ranged from 8.135 to 10.212 m\u003csup\u003e2\u003c/sup\u003e/g. Pore diameter and pore volume were also affected by addition of Li. La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO had a pore diameter of 33.807 nm and a pore volume of 0.295 mL/g. However, pore diameter and pore volume decreased in the Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples. This behavior was due to particle agglomeration of lithium during La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO impregnation.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 FT- IR analysis\u003c/h2\u003e\n \u003cp\u003eThe FT-IR spectra of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x were shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. The broad bands at 3426.9 and 1633.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO were assigned to associated O\u0026shy;H or H-O-H stretching and bending respectively due to the adsorption of moisture on the surface of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO or the existence of M\u0026shy;OH (M\u0026thinsp;=\u0026thinsp;Zn or La).\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The sharp peak at 1466.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e could be attributed to the characteristic absorption peaks of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The absorption bands at 1402.1 and 1086.6 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were associated with stretching and bending vibration of C-O bond, indicating the existence of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e,\u003csup\u003e31\u003c/sup\u003e which was agree with XRD result and previous reports.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e The strong absorption peak appearing at 855.7 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was ascribed to stretching vibration of La-O bond. The stretching vibration of Zn-O bond at 422.7 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was observed.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n \u003cp\u003eAs shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb-f, there were obvious difference for Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026shy;ZnO\u0026shy;x compared to La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO. After addition of Li, the characteristic absorption of Zn\u0026shy;O bond in Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x shifted toward high wavenumber at about 486.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which was in accordance with previous work reported by Yogamala et al.\u003csup\u003e32\u003c/sup\u003e Compared with FT-IR spectrum of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO, the sharp and narrow band assigned to O\u0026shy;H at about 3443, 3565 and 3610 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were observed for Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples, indicating the presence of generous isolated O\u0026shy;H. After impregnated in LiOH solution, the hydrogen bond of associated O\u0026shy;H was broken to form isolated O\u0026shy;H, furthermore, Li\u003csup\u003e+\u003c/sup\u003e may be exchanged with proton of M\u0026shy;OH (M\u0026thinsp;=\u0026thinsp;Zn or La) to form M\u003cem\u003e\u0026shy;\u003c/em\u003eO\u003cem\u003e\u0026shy;\u003c/em\u003eLi bond.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e The broad adsorption bands at about 1500 and 1433.0 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were assigned to C\u003cem\u003e\u0026shy;\u003c/em\u003eO overlapping peaks of M\u0026shy;CO\u003csub\u003e3\u003c/sub\u003e.(M\u0026thinsp;=\u0026thinsp;Li or La), which was significantly different from FT-IR spectrum of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO between 1400\u0026shy;1500cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Meanwhile, symmetric C\u003cem\u003e\u0026shy;\u003c/em\u003eO stretching vibration at about 1087 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was observed. In addition, a few weak adsorption bands at about 640, 742 and 998 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were arose from Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e,\u003csup\u003e26\u003c/sup\u003e and intensity of these peak increases with the increased of doping amount of Lithium. These results further demonstrated that Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts possess active Li\u003csup\u003e+\u003c/sup\u003eO\u003csup\u003e\u0026minus;\u003c/sup\u003e (Li\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) and LiOH basic sites.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 SEM\u003c/h2\u003e\n \u003cp\u003eFor morphology of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea-b), a large number of rod-shaped and fluffy shape particles were observed, and fluffy shape particles were uniformly and loosely dispersed on the surface of rod-shaped materials. However, the morphology of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalyst was dominated with irregular particles that tended to agglomerate, and presented with a flat, smoother appearance. These characteristics became more evident with the increase of Li-doped amount from 3.5 wt% to 5.0 wt%, these results were agreed with previous report.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec19\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6 Basic strength\u003c/h2\u003e\n \u003cp\u003eThe basic strength of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples was determined by Hammett indicator method.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e The results were shown in the Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. After the introduction of Li, the basic strength of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts was in the range of 15\u0026thinsp;\u0026le;\u0026thinsp;H-\u0026lt;17.2. The catalytic performance for aldol condensation of citral and acetone was related to basic strength of catalyst. In this work, Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts possessed excellent catalytic activity for synthesis of PS, and no catalytic activity was observed for La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite oxide, therefore, Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x catalysts were used in the following experiments.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eSurface basic strength of La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO and Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-x samples\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCatalyst\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ebase strength(H-)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLa\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9.3\u0026shy;15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15\u0026shy;17.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15\u0026shy;17.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15\u0026shy;17.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15\u0026shy;17.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eLi/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-5.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e15\u0026shy;17.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec20\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7 Effect of reaction conditions\u003c/h2\u003e\n \u003cp\u003eThe effect of reaction conditions on aldol condensation of citral with acetone catalyzed by Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 was investigated. The amount of citral used in the following experiments was 5 g.\u003c/p\u003e\n \u003cdiv id=\"Sec21\" class=\"Section3\"\u003e\n \u003ch2\u003e3.7.1 Effect of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalyst dosage\u003c/h2\u003e\n \u003cp\u003eThe effect of catalyst dosage was investigated under the conditions (50\u0026deg;C, 3h, and molar ratio citral/acetone of 1:9). Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea indicates that the increase of the catalyst dosage from 0.2 g to 0.5 g improved the aldol condensation reaction, increasing citral conversion from 68.1% to 100% and PS selectivity from 94.1% to 99.2%. However, dosages beyond 0.5 g caused a slight reduction in both conversion and selectivity, possibly due to side reactions promoted by excess alkali.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e Therefore, 0.5 g was selected as the optimal catalyst dosage.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec22\" class=\"Section3\"\u003e\n \u003ch2\u003e3.7.2 Effect of reaction time\u003c/h2\u003e\n \u003cp\u003eThe effect of reaction time was investigated under the following conditions (50\u0026deg;C, 0.5 g catalyst dosage, and molar ratio citral/acetone of 1:9). Citral conversion increased sharply within the 1 hour and reached 100% after three hours. Meanwhile, the maximum PS selectivity of 99.2% was achieved at three hours and then gradually declined (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb). Which suggested that by-products were formed and reduced PS selectivity.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e The optimal reaction time was 3 h.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e\n \u003ch2\u003e3.7.3 Effect of reaction temperature\u003c/h2\u003e\n \u003cp\u003eThe effect of reaction temperature was investigated under the following conditions (0.5 g catalyst dosage, 3 h, and molar ratio of citral/acetone 1:9). From Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec, citral conversion and PS selectivity increased gradually below 50\u0026deg;C, and decreased over 50\u0026deg;C. Therefore, 50\u0026deg;C was chosen as the reaction temperature for the synthesis of PS.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec24\" class=\"Section3\"\u003e\n \u003ch2\u003e3.7.4 Effect of material ratio of citral to acetone\u003c/h2\u003e\n \u003cp\u003eThe effect of the molar ratio of citral/acetone (1:3 to 1:11) was evaluated under the following conditions (0.5 g catalyst dosage, 3 h, and 50\u0026deg;C). Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed presented that the citral conversion and PS selectivity were initially increased, but declined slightly with excess acetone, the reaction performed optimally at a molar ratio of citral to acetone of 1:9. This trend may be attributed to the decline in citral concentration and competitive acetone self-condensation.\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e The optimal citral/acetone molar ratio was found to be 1:9.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec25\" class=\"Section2\"\u003e\n \u003ch2\u003e3.8 Evaluation of the Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalyst reusability stability\u003c/h2\u003e\n \u003cp\u003eTo evaluate the reusability and stability of the Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalyst, the used catalyst was washed several times with acetone, dried and subsequently reused in the synthesis of PS. The results were presented in Table \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eRecyclability test of Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO-4.0 catalyst\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTimes\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eX\u003c/em\u003e(Citral)/%\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eS\u003c/em\u003e(PS)/%\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eY\u003c/em\u003e(PS) /%\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e100\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e99.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e99.2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e89.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e93.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e83.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e72.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e90.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e65.3\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003e\u003c/p\u003e\n \u003cp\u003eThe catalyst showed acceptable stability during recycling. With the catalyst, PS selectivity remained above 90% throughout three reaction cycles, however, a gradual decrease in the conversion of citral was observed. This decline in performance may be attributed to the loss of active components from the catalyst surface as well as the surface pores were blocked by organic residues during recovery,\u003csup\u003e37\u003c/sup\u003e which could hinder the contact between the catalyst and the reactants, thereby reducing its catalytic efficiency.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eA highly efficient Li-doped La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalyst was employed for the aldol condensation of citral and acetone. The introduction of Li into the La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO composite notably enhanced catalytic performance, with citral conversion and PS selectivity dramatically increasing, especially for Li-loading amount of 4.0 wt%. The formation of strong Li\u003csup\u003e+\u003c/sup\u003eO\u003csup\u003e\u0026minus;\u003c/sup\u003e and LiOH basic sites on the catalyst's surface was crucial for the aldol condensation activity. Optimal preparation conditions for the Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalyst were Li-loading amount of 4.0 wt%, calcination temperature of 300\u0026deg;C after Li-doped, and impregnation time of 15 h. The conversion of citral and yield of PS was 100% and 99.2% respectively under the suitable reaction conditions of catalyst dosage of 0.5 g, reaction temperature of 50\u0026deg;C, reaction time of 3 h and citral/acetone molar ratio of 1:9. From a practical standpoint, the proposed Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalysts offer high catalytic activity and selectivity, along with a straightforward product purification process. To address the loss in catalytic activity after repeated cycles, future efforts will be directed toward implementing structural modifications to prevent active component leaching and developing a robust regeneration protocol for the removal of organic residues.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv align=\"\"\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 277px;\"\u003e\n \u003cp\u003eAbbreviation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 277px;\"\u003e\n \u003cp\u003eDefinition\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 277px;\"\u003e\n \u003cp\u003ePS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 277px;\"\u003e\n \u003cp\u003ePseudoionone\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003eEthics and Consent to Participate\u003c/p\u003e\n\u003cp\u003eThis article does not contain any studies with human participants performed by any of the authors.\u003c/p\u003e\n\u003cp\u003eConsent for Publication\u003c/p\u003e\n\u003cp\u003eThe results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration (from you or one of your Contributing Authors) by another publisher.\u003c/p\u003e\n\u003cp\u003eCompeting Interest\u003c/p\u003e\n\u003cp\u003eI declare that the authors have no competing interests as defined by Springer, or other interests that might be perceived to influence the results and/or discussion reported in this paper.\u003c/p\u003e\n\u003cp\u003eAuthor Contribution\u003c/p\u003e\n\u003cp\u003eFL.W. and T.Y. wrote the main manuscript text and prepared all figures and tables;\u003c/p\u003e\n\u003cp\u003eAll authors made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data; or the creation of new software used in the work; reviewed the manuscript;\u003c/p\u003e\n\u003cp\u003eCT.K. drafted the work or revised it critically for important intellectual content; agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.\u003c/p\u003e\n\u003cp\u003eFunding\u003c/p\u003e\n\u003cp\u003eNational Key Research and Development Program of China (No. 2017YFD0600704);\u003c/p\u003e\n\u003cp\u003eScientific Research Fund of Hunan Provincial Education Department(NO. 15A196).\u003c/p\u003e\n\u003cp\u003eAvailability of data and materials\u003c/p\u003e\n\u003cp\u003eThe numerical data from Figures 1, 5 are tabulated in the Supplementary Material. Numerical data for the XRD spectra from Figure 2 and the Numerical data for the FT-IR spectra from Figure 3 are available as a .zip file in the Supplementary Material.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eError bars (where shown) in Figures 1, 5 show the spread of data observed in quintuplicate measurements, where independent samples were tested for each measurement.\u003c/p\u003e\n\u003cp\u003eAcknowledgments\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Key Research and Development Program of China (No. 2017YFD0600704) and Scientific Research Fund of Hunan Provincial Education Department (NO. 15A196).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eD\u0026iacute;ez VK., Apestegu\u0026iacute;a CR., Cosimo JID. 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Synthesis of Ionones by Cyclization of Pseudoionone on Solid Acid Catalysts. \u003cem\u003eCatal Lett.\u003c/em\u003e 2008;123(3-4):213-219.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Scheme ","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Li­doped La2O3­ZnO, pseudoionone, aldol condensation, solid base catalyst, base sites","lastPublishedDoi":"10.21203/rs.3.rs-8285759/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8285759/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA highly efficient Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO catalysts was developed for aldol condensation of citral and acetone. Li/La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026shy;ZnO catalysts with different Li\u003csup\u003e+\u003c/sup\u003e loading were prepared by coprecipitation method followed by incipient wetness impregnation method, and were characterized by XRD, BET, FT-IR, SEM. The results demonstrated that Li was well dispersed on La\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u0026shy;ZnO composite oxide, may coordinate with chemisorbed O\u003csup\u003e2\u0026minus;\u003c/sup\u003e species to form active strong basic site. Therefore, Li introduced promoted the formation of active strong base sites, then significantly improved the catalytic performance. The suitable preparation conditions of catalyst were Li\u003csup\u003e+\u003c/sup\u003e loading of 4.0 wt%, calcination temperature of 300\u0026deg;C after Li-doped, and impregnation time of 15 h. The conversion of citral and yield of pseudoionone were 100% and 99.2% respectively under the suitable reaction conditions of catalyst dosage of 0.5 g, reaction temperature and time was 50\u0026deg;C and 3 h, and citral/acetone molar ratio of 1:9.\u003c/p\u003e\u003cp\u003e\u003cb\u003eTopical Heading\u003c/b\u003e: Reaction Engineering, Kinetics and Catalysis\u003c/p\u003e","manuscriptTitle":"Highly efficient synthesis of pseudoionone from citral with acetone on Li-doped La2O3­ZnO catalysts","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-10 16:28:19","doi":"10.21203/rs.3.rs-8285759/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"38f9b33a-5105-43eb-a6c9-798ba58b9e9e","owner":[],"postedDate":"December 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-01-02T16:09:44+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-10 16:28:19","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8285759","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8285759","identity":"rs-8285759","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}