Effect of One-Pot Rehydration Process on Surface ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Effect of One-Pot Rehydration Process on Surface Basicity and Catalytic Activity of MgyAl1‑aREEaOx Catalyst for Aldol Condensation of Citral and Acetone Zheng Wang, Guanzhong Lu,* Yun Guo, Yanglong Guo, and Xue-Qing Gong Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: The liquid phase synthesis of pseudoionones (PS) by the cross-aldol condensation of citral and acetone was investigated over MgAl mixed oxides containing rare earth elements (REE = Y, La, Eu), which were obtained from corresponding REE-modified hydrotalcite materials after calcination. The results showed that the unmodified and La(Eu)-modified MgAl mixed oxide catalysts showed relatively low activity, and Y-modified MgAl mixed oxides presented an unexpected high catalytic activity. PS selectivity of ∼85% and citral conversion of 100% were achieved at 60 °C for 3 h. On the basis of the characterizations of the structural, textural, and basic properties, it was found that Mg3Al1‑aYaOx catalysts exhibited relatively well-developed small flake morphology with high surface area and pore volume, resulting in exposure of more basic sites on the catalyst surface. The formation of PS over Mg 3 Al 1‑a Y a O x may be accompanied by gradual modification of the catalyst surface to form re-Mg3Al1‑aYaOx through a rehydration process with produced water, which reconverts the O2− basic sites to OH− basic groups. Unlike La and Eu elements, the presence of Y could promote this “one-pot” or in situ rehydration process of MgAl mixed oxides during the aldol reaction. This Y-modified MgAl mixed oxides after a one-pot rehydration process with active Brønsted basic sites is responsible for the high activity in the cross-aldol condensation of citral and acetone. KEYWORDS: Aldol condensation, Pseudoionone synthesis, Yttrium modification, MgAl hydrotalcite, One-pot rehydration

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INTRODUCTION Aldol condensation is a typical base-catalyzed reaction, which will become more and more important in the development of biorefineries. Homogeneously and heterogeneously catalyzed aldol condensation have been widely studied in the last decades. Various carbohydrate-derived carbonyl compounds, such as furfural, hydroxymethylfurfural (HMF), dihydroxyacetone, acetone, and tetrahydrofurfural, were used to prepare larger water-soluble organic molecules in aqueous and organic solvents that can subsequently be converted to liquid alkanes by aqueous-phase dehydration/hydrogenation,1 which is a key reaction for biomass feedstock upgrading,2 and provide the possibility to transform the raw materials into valuable chemicals or fuel (as biodiesel).3 The cross aldol condensation of citral and acetone to pseudoionone (PS) (eq 1) is a well-known and widely used chemical process. PS is an important precursor for α- and βionones, which have been used widely to synthesize vitamins A and E, as well as carotenoids and an extended range of aroma © XXXX American Chemical Society

chemicals.4 Currently, PSs are commercially produced by aldol condensation of citral with acetone in the liquid phase catalyzed by liquid bases, such as NaOH and LiOH. In these liquid baseReceived: November 19, 2015 Revised: December 31, 2015

A

DOI: 10.1021/acssuschemeng.5b01533 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering catalyzed processes, there are normally many problems, such as high toxicity, corrosion, and disposal of spent base materials.5,6 Thus, the solid base catalysts are employed instead of liquid base catalysts to overcome these problems. One of the promising candidates for solid base catalysts is hydrotalcite (HT), and it exhibits high catalytic activity and selectivity for aldol condensation reactions. The HT structure formula can be described as [(M1−x2+Mx3+·(OH)2]x+(An−)x/n· mH2O, where M2+ and M3+ are divalent and trivalent metal cations respectively, x is the ratio of M3+/(M2+ + M3+), and A is an anion with charge n− (An− = CO32−, SO42−, NO3−, etc.), along with water molecules located in the interlayer galleries.7 In the brucite-like structure, part of the M2+ (M2+ = Mg2+) ions can be replaced by trivalent metal ions, bringing some degree of distortion to the lamellar structure.8 Thermal decomposition of HT at 723−773 K leads to the formation of Mg(Al)O mixed oxide, and then, this mixed oxide can be rehydrated in the decarbonated water vapor or aqueous solutions containing certain anions in which the layered structure can be recovered to a large extent with Brønsted base sites (OH−) incorporated in the interlayer. Among the most usual basic oxide catalysts, those containing alkaline earth or rare earth cations, particularly CaO, BaO, La2O3, and Y2O3, possess basic sites with higher strength.9 As a solid superbase, La2O3−ZnO/ZrO2 and Eu2O3/Al2O3 showed higher catalytic performance for transesterification of soybean oil to biodiesel.10,11 Yttrium-modified hydrotalcite was also used as the catalyst for styrene epoxidation with hydrogen peroxide in acetonitrile, and the presence of yttrium could induce an increase in surface basicity.12 Accordingly, it was previously attempted to improve the basicity of hydrotalcites with rare earth elements (REE). The mixed oxides made from hydrotalcite often display good reactivity because of their smaller crystal sizes, higher surface areas, tunable surface basic properties, and structure memory effects.13,14 For instance, when the MgAlOx mixed oxide catalyzed the cross-aldol condensation of citral and acetone, citral conversion of 20− 60% and pseudoionone selectivity of 60−80% were obtained.15−17 The catalytic reaction mechanisms and kinetics ́ for the aldol reactions were investigated by Abelló et al.,18 Diez et al.,19 and Climent et al.20 Although fresh rehydrated MgAl hydrotalcite shows excellent catalytic activity or high yield of pseudoionone compared with a commercial NaOH solution catalyst, the rehydrated hydrotalcite catalysts would be deactivated and need to be regenerated, which is time consuming and increases the cost, because of the extremely fast poisoning of the active OH− groups in contact with air during preparation. Thus, new strategies to design solid bases, through an optimal compromise between the composition, activity, and stability or adding a new element to form a new structure, are significant to improve the physicochemical and catalytic properties of the MgAl hydrotalcite. Therefore, we tried to find a new element to modify MgAl hydrotalcite and develop a one-pot (or in situ) rehydration method to rehydrate a modified MgAl hydrotalcite by water produced in the reaction to improve its catalytic performance. This rehydration process is unique and environmentally friendly. Herein, we adopted REE to modify MgAl hydrotalcite and investigated their physicochemical and catalytic performances for cross-aldol condensation of citral/acetone (eq 1) and acetone self-condensation (eq 2). The effects of Y modified in the Mg3AlOx sample on its particle size, chemical nature,

strength, and distribution of the basic sites were investigated by various techniques. Especially, the effect of a one-pot rehydration process on its surface basicity and catalytic activity was investigated in detail. The results show that a Y-modified Mg3AlOx catalyst presented the highest catalytic activity, that is, the citral conversion and selectivity to PS reached 100% and 90% after 3 h of reaction, respectively. The catalytic reaction mechanism for the aldol condensation of citral/acetone over the MgyAl1‑aYaOx catalyst is proposed, and the reasons for the unexpected high catalytic activity of the Mg3Al1‑aYaOx sample are discussed.

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EXPERIMENTAL SECTION

Preparation of Catalysts. MgAl-HT and La-, Y-, and Eu-modified MgAl-HT were prepared by the coprecipitation method at a controlled pH value (10.5) and different atomic ratios of Mg/Al in the synthesis solution. In a typical procedure, an aqueous solution A was prepared by dissolving weighed Al(NO3)3·6H2O and Mg(NO3)2·6H2O in deionized water. An alkaline solution B containing 0.404 mol KOH and 0.1 mol K2CO3 was prepared. Both solutions A and B were simultaneously dropped under vigorous stirring to the recipient containing 200 mL of water at 25 °C. The pH value of the mixed solution was maintained at 10.5 by controlling the pomp speed of solution B. After precipitation, the suspension was continually vigorously stirred at room temperature for 16 h. The precipitate after filtration was washed with deionized water several times to remove the K cation and then dried at 100 °C overnight. For the HT samples containing REE, a proper amount of REE(NO3)3·6H2O (REE = La, Y, Eu) was added into solution A according to the composition of REE in the sample. The as-synthesized REE-modified MgAl hydrotalcite samples were denoted as REE-MgAl-HT. After the REE-MgAl-HT precursor was calcined at 500 °C for 8 h in air flow, the mixed oxide was obtained. These mixed oxides were denoted as MgyAlOx and Mg3Al1‑aREEaOx for the unmodified and REE-modified MgAl-HT, respectively. After the mixed oxide was saturated with water (partial pressure = 20 Torr) in a N2 flow (40 mL/ min) at ambient temperature for 16 h,21 the reactivated hydrated sample was obtained and denoted as re-Mg3Al1‑aREEaOx. Characterization of Catalysts. The chemical composition of solid catalysts was analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES) on a Varian 710-ES instrument after being dissolved in an aqueous acid solution. The surface areas of the samples were determined by N2 adsorption at −196 °C on a Micrometrics ASAP 2020 apparatus and calculated by the Brumauer− Emmett−Teller (BET) method. Before measurement, the samples were first outgassed overnight at 500 °C. X-ray powder diffraction patterns of the samples were recorded on a Bruker AXS D8 Focus diffractometer instrument with CuKα radiation. Scanning electron microscopy (SEM) images of the samples were taken on a FEI-Quanta scanning electron microscope. The sample was coated with a thin layer of gold before testing. Thermogravimetric/differential thermal analysis (TG-DTA) of the samples was performed on a PerkinElmer Pyris Diamond with a WCT-2 thermal analyzer at a heating rate of 10 °C/ min from room temperature to 800 °C in an air atmosphere. Temperature-programmed desorption (CO2-TPD) of CO2 adsorbed on the sample was performed in a quartz microreactor. A 200 B


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ACS Sustainable Chemistry & Engineering mg sample was treated in He at 500 °C for 1 h to remove absorbed CO2, exposed to 5% CO2/He (40 mL/min) at 50 °C until saturation adsorption of CO2, and flushed with He flow at the same temperature for 1 h to remove weakly adsorbed CO2. The sample was heated from 50 to 650 °C at a heating rate of 10 °C/min. The outlet gas was analyzed by a quadrupole mass spectrometer (INFICON Transpecter 2). The signal of CO2 was recorded at m/z = 44. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO2 adsorbed on the catalyst was measured on a Nicolet Nexus 670 spectrometer equipped with a MCT detector. A total of 40 mg of catalyst was placed firmly in the diffuse reflectance cell fitted with a ZnSe window and a heating cartridge. The DRIFTS spectra were obtained with a resolution of 4 cm−1 and 64 scans. The catalyst was pretreated in He flow (30 mL/min) at 500 °C for 1 h and cooled to room temperature for 15 min; then, the background spectrum was recorded. After admission of 5 kPa of CO2 into the cell at room temperature, the sample was evacuated consecutively at 20, 100, 200, 300, and 400 °C, respectively, and the resulting spectra were recorded at room temperature. The IR spectra of the rehydrated samples after adsorbing CO2 were recorded only at room temperature. After subtracting the background spectrum, the IR spectra of the adsorbed species were obtained. Self-Condensation of Acetone. To overcome the thermodynamic equilibrium limitation in the self-condensation of acetone, the self-condensation reaction was carried out in a Soxhlet extractor, which can remove the product through cycling the reaction solution (about 7 min per cycle). Soxhlet extraction was accomplished in cellulose thimbles containing 0.5 g of catalyst mixed with ∼36 g of porcelain heads. A total of 100 mL of acetone and internal standard (benzyl alcohol) were added to the Soxhelt vessel (250 mL), and the reaction was carried out for 24 h. The products were analyzed by a PerkinElmer Clarus 500 gas chromatograph with a SE-54 column and a flame ionization detector (FID). The main reaction products of acetone selfcondensation were diacetone alcohol (DAA) and mesityl oxide (MO) (eq 2), and their yields and selectivities were calculated by the area normalization method. Aldol Condensation of Citral and Acetone. The aldol condensation of citral with acetone (eq 1) was carried out in a batch reactor with a magnetic stirrer and heated in an oil bath equipped with an automatic temperature control system. The catalyst was treated at 500 °C for 2 h under N2 before the reaction. After thermal treatment, the catalyst was rapidly transferred to the reactor with the less contamination by CO2 and H2O from air as little as possible during the transfer. Then, He was introduced into the reactor instead of air to keep a static He atmosphere for prohibiting contact of the catalyst with air. The typical reaction conditions are as follows: weight ratio of catalyst/citral, 23 wt % (0.6 g catalyst, WCat/WCitral = 23 wt %); molar ratio of acetone/citral, 5/1; internal standards benzyl alcohol, ∼1.5 g. The aldol condensation of citral with acetone was carried out at 40, 60, and 80 °C and under autogenous pressure. After the end of the reaction, the reactor was cooled to room temperature and the catalyst was separated by centrifugation. The reaction solution was analyzed by ICP to determine the solid catalyst leaching; the results showed that there are no rare earth elements or Mg or Al leaching in the reaction solution, which eliminates the contribution of a homogeneous reaction. The reaction solution was simultaneously analyzed by a PerkinElmer Clarus 500 gas chromatograph with a SE54 column and FID. The conversion of citral and yield and selectivity of pseudoionones were calculated as follows: Ccit =

M(mol of consumption of citral) M(mol of citral fed)

YPS =

152 × M(mol of PS) × 100% 192 × M(mol of citral fed)

SPS =

YPS × 100% Ccit

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RESULTS AND DISCUSSION Textural, Structural, and Surface Properties. Figure 1 presents the XRD patterns of as-synthesized modified and

Figure 1. XRD patterns of as-synthesized (a) Mg3Al-HT, (b) Mg3Al0.9Y0.1-HT, (c) Mg3Al0.8Y0.2-HT, and (d) Mg3Al0.7Y0.3-HT.

unmodified hydrotalcite samples. The results show that the Mg3Al-HT sample exhibits the typical features of a wellcrystallized hydrotalcite material (JCPDS 70-2151) with sharp and symmetric diffraction peaks of (003), (006), (110), and (113) facets and broader/asymmetric peaks of (012), (015), and (018) facets. Note that there is no detectable peak of impurity in the XRD patterns of the MgAl-HT and Y-MgAl-HT samples, which shows that the Y element can be completely introduced into the MgAl-HT structure. The XRD patterns of the Y-MgAl-HT samples show that their diffraction peaks become less intense and broader with an increase in Y content, that is, their crystallinities are decreased. This situation results probably from the distortions introduced by adding Y3+ with larger ionic radii than Al3+ (Y3+, 1.04 Å; Al3+, 0.675 Å).22 In addition, the slight increase in basal spacing with an enhancing Y content in hydrotalcite has a good agreement with the lower polarizing ability of Y3+ (compared with Al3+), which decreases the Coulombic attractive force between the positively charged brucite-like layers and the negatively charged interlayer anions.23 A similar phenomenon was observed in other hydrotalcite-like compounds upon substitution with different rare earth elements in the layers.21 After the thermal decomposition of as-synthesized hydrotalcite at 500 °C, the mixed oxide catalysts were obtained. Figure 2 presents the XRD patterns of the MgAl-HT and YMgAl-HT precursors after being calcined. The layered structure has collapsed, and only two diffraction peaks can be observed, corresponding to the (200) and (220) facets from a solid solution like a MgO-periclase type. This structure is actually a defective structure with the cationic vacancies generated by the introduction of Al3+ and Y3+ in the octahedral sites.24 For the Ymodified sample, no diffraction peak of yttrium oxides can be detected. TG profiles of as-synthesized Mg3Al-HT and Mg3Al0.9Y0.1HT are shown in Figure 3. There is a gradual weight loss at 50−600 °C, with two main endothermic effects at ∼200 and 400 °C. This decomposition profiles are in good agreement with those of hydrotalcite-like compounds with a total weight loss (Δm) of 34−45% and two distinct weight-loss processes.20,25 The first weight loss at below 200 °C is C


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hydrotalcite, suggesting that the presence of Y3+ in the lattice somewhat favors water removal. Thus, the presence of Y in Mg3AlOx might facilitate reactant access or removal of water in the interlayer. The chemical composition, BET surface area, and pore volume of Mg3AlOx and Mg3Al1‑aREEaOx samples are listed in Table 1. The ratios of Mg/Al in oxides analyzed by ICP are close to the compositions in the synthesis solutions, although there is a slight loss of Mg2+. Since water and carbon dioxide are released by dehydroxylation and carbonate decomposition in the interlayer, a great deal of micropores are formed, resulting in higher BET surface areas of the mixed oxides after calcination. Y-modified mixed oxides present higher surface areas and larger pore volumes than unmodified and other rare earth-modified samples. The Mg3Al0.8Y0.2Ox sample (Y/(Y+Al) = 0.2, mol) has the highest BET surface area and pore volume; further increasing its molar ratio to 0.3, the surface area and pore volume of the sample are declined. This may be related to the poor crystalline and less-ordered structure of Mg3Al0.7Y0.3Ox, which is also confirmed by the XRD pattern (Figure 1d). The SEM images of the Mg3AlOx and Mg3Al1‑aYaOx samples are shown in Figure 4. After calcination at 500 °C for 8 h, the

Figure 2. XRD patterns of (a) Mg3AlOx, (b) Mg3Al0.9Y0.1Ox, (c) Mg3Al0.8Y0.2Ox, and (d) Mg3Al0.7Y0.3Ox after being calcined at 500 °C for 8 h in air flow.

Figure 3. TG profiles of as-synthesized Mg3Al-HT and Mg3Al0.9Y0.1HT.

attributed to the loss of the physically adsorbed interlayer water, and the second weight loss at 200−400 °C originates from the dehydroxylation of brucite-like sheets and decomposition of carbonates in the interlayer. The total weight loss of MgAl-HT is slightly higher (2%) than Y-modified MgAl-HT, which indicates an increase in dehydroxylation of brucite-like sheets and decomposition of carbonates in the interlayers. Fernández et al.26 reported similar results, such that for the Ymodified hydrotalcite sample its weight losses and DTA minima are recorded at lower temperatures than those of

Figure 4. SEM images of (a) Mg3AlOx, (b) Mg3Al0.9Y0.1Ox, (c) Mg3Al0.8Y0.2Ox, and (d) Mg3Al0.7Y0.3Ox samples.

MgAl-HT and Y-MgAl-HT compounds were transformed to MgAl(Y) mixed oxides, and the morphologies of their nanoscale platelets were still retained. The Mg3AlOx and Mg3Al1‑aYaOx samples show similar flake morphology, but their particle sizes are different. Mg3AlOx presents the largest particle size. After introducing Y in Mg3AlOx, the particle sizes of Mg3Al1‑aYaOx diminish with an increase in Y content. When the

Table 1. Composition, BET Surface Area (SBET), and Catalytic Activities of Mg3AlOx and Mg3Al1‑aREEaOx Catalysts for Acetone Self-Condensation at Reflux Temperatures of Acetone (50−55 °C) for 24 h (100 mL of acetone and 0.5 g of catalyst) ICP analysis catalyst

Mg2+/Al3+

REE3+/Al3+

SBET (m2/g)

pore volume (mL/g)

acetone conversion (%)

DAA selectivity (%)

Mg3AlOx Mg3Al0.9Eu0.1Ox Mg3Al0.9La0.1Ox Mg3Al0.9Y0.1Ox Mg3Al0.8Y0.2Ox Mg3Al0.7Y0.3Ox

3.30 2.92 2.91 2.97 2.96 2.97

− 0.093 0.094 0.096 0.193 0.289

117 133 121 187 215 122

0.77 0.58 0.55 1.09 1.26 0.63

14.8 16.8 12.2 18.2 27.2 13.9

82.1 89.6 88.3 96.1 85.4 97.0

D


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Figure 5. CO2-TPD profiles of MgyAlOx and Mg3Al1‑aYaOx samples.

Y content was increased to a = 0.2, smaller platelets were still retained; for the Mg3Al0.7Y0.3Ox (a = 0.3) sample, the flake morphology was hardly observed, that is, Mg3Al0.7Y0.3Ox seems to lose most of its layered structure. To determine the density and strength of the basic sites on the MgyAlOx and Mg3Al1‑aYaOx samples, TPD of CO2 adsorbed on the sample was tested, and the results are shown in Figure 5. In the CO2-TPD curve of the Mg2AlOx sample, there are three main CO2 desorption peaks. One is located at 83 °C (top temperature), and the second is at 110−150 °C. The third is a broad peak at 150−300 °C. Unlike the Mg2AlOx sample with three adsorption peaks of CO2, Mg3AlOx and Mg4AlOx have two peaks at 113−118 °C and 150−300 °C, respectively, and the peak at 83 °C cannot be observed, which indicates that the surface basicity of the Mg3AlOx is very similar to that of Mg4AlOx and is stronger than that of Mg2AlOx. After using Y0.1 to replace Al0.1, the CO2-TPD curve of Mg3Al0.9Y0.1Ox is similar to that of Mg3AlOx. With an increase in the Y amount, the adsorption peak at 80 °C appeared gradually; when the Y amount reached a = 0.3, the Mg3Al0.7Y0.3Ox sample displayed three desorption peaks in its CO2-TPD curve at 75, 140, and 240 °C, respectively, which is similar to the CO2-TPD curve of Mg2AlOx. The FT-IR spectra of Mg3AlOx and Mg3Al1‑aYaOx after CO2 adsorption at room temperature and sequential evacuation at 100, 200, 300, and 400 °C are shown in Figure 6. At least three IR absorption bands of CO2 adsorbed on the different surface basic sites can be detected. The bicarbonates species involving the surface hydroxyl groups correspond to the symmetric and asymmetric O−C−O stretching bands at 1480 and 1690 cm−1, respectively,16,27 as well as a C−OH bending mode at 1285 cm−1. The bidentate carbonate can form on Lewis acid− Brønsted base pairs (Mn+−O2− pair site, where Mn+ is the metal cation Mg2+, Al3+, or Y3+), which presents a symmetric O−C− O stretching at 1410 cm−1 and an asymmetric O−C−O stretching at 1663 cm−1.28 The third adsorbed species is an unidentate carbonate, which requires isolated surface O2− ions (low-coordination anions, such as those located at corners or edges) and exhibits a symmetric O−C−O stretching at 1540 cm−1 and an asymmetric O−C−O stretching at 1630 cm−1.19 Compared with the results reported,16,29 the bands of all carbonate species shift to a higher wavenumber, probably due to the introduction of rare earth elements in the lattice of

Figure 6. IR spectra of CO2 adsorbed on Mg3AlOx and Mg3Al1‑aYaOx samples at room temperature after evacuation at (a) 25 °C, (b) 100 °C, (c) 200 °C, (d) 300 °C, and (e) 400 °C.

hydrotalcite. The following base strength order for these surface oxygen species were also determined: low coordination O2− anions > oxygen in Mn+−O2− pairs > OH groups.29 In the IR spectrum of the Mg3AlOx catalyst at 25 °C, its band at 1686 cm−1 is mainly attributed to the bicarbonate, and the band at 1663 cm−1 is attributed to the bidentate carbonate species. After evacuation at 200 °C, the band at 1686 cm−1 disappeared rapidly, that is, the decomposition of bicarbonate species, and the band at 1630 cm−1 is relatively strong compared with other bands and can be observed even after evacuation at 400 °C. E


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Aldol Condensation of Citral and Acetone. The aldol condensation of citral and acetone can form pseudoionone and ionones and seems to be one of the most attractive reactions of aldolization involving acetone, in which the self-condensation of acetone might occur to produce DAA as the main byproduct and α- and β-ionone could be produced in the side reactions. The effects of catalyst composition on catalytic performance for the aldol condensation of citral and acetone are shown in Table 2. The results show that the MgyAlOx catalysts exhibited

For Y-modified MgAl mixed oxides, there are the similar distributions of IR absorption bands as the Mg3AlOx catalyst, but the intensities of the bicarbonates bands are lower than those over the Mg3AlOx catalyst at 20 °C. In addition, the Mg3Al1‑aYaOx samples exhibited the band at 1540 cm−1, which can be ascribed to the symmetric O−C−O stretching of unidentate carbonate. The results above show that the surfaces of the Mg3AlOx and Mg3Al1‑aYaOx samples are nonuniform and contain several basic sites with different basic strengths; in other words, the sample surfaces contain oxygen atoms with different chemical natures, which can bond CO2 with different coordination and binding energies.16 The higher the CO2 desorption temperature is, the higher the basic strength of the sites is. The desorption peak at 70−90 °C is attributed to bicarbonates formed on weak basic sites (Brønsted OH groups). The desorption peak at 110−150 °C is related to the bidentate carbonates desorbed on the medium basic sites (metal−oxygen pairs). The desorption peak at 150−220 °C should be assigned to unidentate carbonates released from the strong basic sites (low coordination oxygen anions). The mixed oxides and reconstructed samples were monitored by IR spectroscopy, and their IR spectra are shown in Figure S1. As shown in Figure S1A, the mixed oxides showed very weak absorption peaks of the OH− group at 3000 cm−1 and a typical absorption peak at ∼1500 cm−1 from different carbonate species on the surface. Note that after the mixed oxide samples were rehydrated for 1 h their bands at 3000 and 1640 cm−1 appeared (Figure S1B), showing an amount of water uptake during rehydration. The band at 1640 cm−1 was increased with increasing the rehydration time to 1 h. For the rehydrated samples, two absorption bands of carbonate at 1520 and 1426 cm−1 disappeared, and the band at 1330 cm−1 (mainly interlayer bidentate carbonates) was recovered as a symmetric peak together with a small peak at about 1485 cm−1, which is assigned to the symmetric O−C−O stretching vibration mode of bicarbonate anions. These results above indicate that a significant amount of carbonates are still present after rehydration treatment, indicating the presence of strong basic sites. Catalytic Performances. Acetone Self-Condensation. The catalytic performance of the hydrotalcite-derived Mg−Al and Mg−Al−Y mixed oxides was first investigated for acetone self-condensation, in which diacetone alcohol (DAA) was formed as the main product and mesityl oxide (MO) was the byproduct formed by the dehydration of DAA over the acid sites. The reaction was conducted in a Soxhlet extractor to overcome the limitation of thermodynamic equilibrium at 50− 55 °C (temperature of the catalyst bed) for 24 h. The conversion of acetone and selectivity of DAA are shown in Table 1. Over the unmodified Mg3AlOx catalyst, 14.8% conversion of acetone and 82.1% selectivity to DAA were achieved. After adding Eu (or La) to Mg3AlOx, 12.2% (or 16.8%) conversion of acetone and 89.6% (or 88.3%) selectivity to DAA was obtained. It is interesting that Y-modified Mg3AlOx is a more active catalyst for acetone aldol condensation. Using the Mg3Al0.8Y0.2Ox catalyst, the conversion of acetone reached 27.2%, much higher than the results over other catalysts. The Mg3Al1‑aYaOx catalyst also exhibited higher selectivity of DAA than Mg3AlOx without Y. Therefore, it is possible that the Mg3Al1‑aYaOx catalyst for aldolization of acetone can be used as an excellent catalyst for other aldolization reactions.

Table 2. Catalytic Activities of MgyAl1‑aREEaOx Samples for Condensation of Citral and Acetonea selectivity (%) catalyst

reaction time (h)

citral conversion (%)

PS

DAA

Mg2AlOx Mg3AlOx Mg4AlOx Mg3Al0.9Eu0.1Ox Mg3Al0.9La0.1Ox Mg3Al0.9Y0.1Ox Mg3Al0.8Y0.2Ox Mg3Al0.7Y0.3Ox

8 8 8 6 6 6 6 6

10.0 34.3 19.7 38.7 25.6 100 100 95.6

90.2 97.9 96.0 84.2 86.2 76.3 70.2 78.1

2.0 1.1 1.5 2.0 1.9 4.2 4.3 3.6

a

Reaction conditions: acetone/citral = 5/1 (mol), reaction temp. = 60 °C, Wcat = 0.6 g, and WCat/WCitral = 23 wt %.

low catalytic activities for the cross-aldol condensation of citral and acetone. When the molar ratio of Mg/Al was 3 (Mg3AlOx), the highest citral conversion of 34% was obtained after 8 h of reaction at 60 °C for the MgyAlOx catalysts. Table 2 also gives the results of citral and acetone condensation over Mg3AlOx modified by different rare earth elements. Under the same reaction conditions, the significant differences of citral conversion can be observed between various rare earth-modified Mg3AlOx catalysts. Over Y-modified Mg3AlOx, 100% citral conversion was achieved at 60 °C for 6 h, which is much higher than that over Eu- or La-modified Mg3AlOx, 38.7% and 25.6%, respectively, although the PS selectivity over Y-modified Mg3AlOx was slightly lower than Euor La-modified catalysts due to slightly higher selectivity to byproducts such as DAA. Therefore, Mg3Al1‑aYaOx may be a highly efficient catalyst for the condensation of citral and acetone. The influence of the Y amount in Mg3Al1‑aYaOx on its catalytic performance was also studied. Under the reaction conditions of 60 °C/6 h and 0.6 g of catalyst, 100% conversion of citral was obtained over the Mg3Al1‑aYaOx catalyst (a = 0.1− 0.2), and the conversion was 95.6% when a was 0.3. The selectivity to PS was somewhat lower (70−76%) than 90−98% over MgyAlOx, probably due to the formation of byproducts and an increased adsorption of citral and products on the surface of the catalyst, which is confirmed by the color change of the catalyst (brownish in contrast with the white color of the fresh catalyst) after being used. The adsorption of reactant (as pure citral or citral−acetone mixture) and products was also reported by Abelló et al.,18 which might influence the catalytic activity and product selectivity. Figure 7 shows the effect of the reaction time on the citral conversion and PS selectivity over the Mg3Al1‑aYaOx samples at 40 °C because the lower reaction temperature or rate can better describe the dynamic features of the catalyst. The results show that the citral conversion was enhanced and the PS selectivity F


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Figure 7. Effect of reaction time on (A) conversion of citral and (B) selectivity to pseudoionone over Mg3Al1‑aYaOx catalysts at 40 °C (acetone/citral = 5 (mol), Wcat = 0.6 g, Wcat/Wcitral = 23%).

Figure 8. (A) Conversion of citral and (B) selectivity to pseudoionone over Mg3Al0.8Y0.2Ox catalysts with different catalyst amounts at 40−80 °C for 4 h (acetone/citral = 5/1, mol).

catalyst was used, PS selectivity was 85% at 40 °C, and PS selectivity only was ∼73% at 60−80 °C. On the basis of the results in Table 2, the change in catalytic performance of Mg3Al0.9Y0.1Ox was tested with the reaction time, and the conversion of citral and PS selectivity as a function of the reaction time over Mg3Al0.9Y0.1Ox at 60 °C are shown in Figure 9. It is observed that citral conversion reached 100% after 180 min of reaction. PS selectivity reached a maximum value of ∼90% at 60 °C, and stable PS selectivity was

was decreased slowly with an increase in reaction time, which suggests that prolonging the reaction time would result in a decrease in PS selectivity. Among three catalysts, the Mg3Al0.7Y0.3Ox catalyst performed the highest citral conversion, and the Mg3Al0.9Y0.1Ox catalyst exhibited slightly higher selectivity to PS. That is to say, a higher Y amount in the Mg3Al1‑aYaOx catalyst can help to enhance catalytic activity, and a lower Y amount in the Mg3Al1‑aYaOx catalyst can improve the selectivity to PS. The effects of reaction temperature and catalyst amount on citral conversion and PS selectivity over the Mg3Al0.8Y0.2Ox catalyst are shown in Figure 8. It is shown that the conversion of citral increased gradually with increasing the catalyst amount, and increasing the reaction temperature can enhance obviously the conversion of citral. When the reaction temperature was controlled at 40 °C, the conversion only reached ∼80% over the 0.8 g of Mg3Al0.8Y0.2Ox catalyst for 4 h. After raising the reaction temperature to 60 or 80 °C for 4 h, 100% conversion could be achieved over 0.7 g of catalyst. Being different from the citral conversion, the selectivity to pseudoionone was changed a little by the catalyst amount and reaction temperature. As shown in Figure 8B, the effect of reaction temperature on PS selectivity is hardly obvious, and a lower catalyst amount is favorable to an increase in PS selectivity. For instance, when 0.3 g of catalyst was used, the highest PS selectivity was ∼97%, and when using 0.6 g of catalyst, PS selectivity was decreased to ∼84%. As 0.8 g of

Figure 9. Conversion of citral and selectivity to pseudoionone vs reaction time over Mg3Al0.9Y0.1Ox catalysts at 60 °C (acetone/citral = 5 (mol), Wcat = 0.6 g, Wcat/Wcitral = 23%). G


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and 400 °C and a total weight loss of 34−45% at 60−600 °C. The differences in weight loss between the four rehydrated samples gave expression to the variation of rehydration degree and water amount in the HT structure interlayer. The total weight loss of re-Mg3Al0.9Y0.1Ox is slightly higher (4%) than the three other rehydrated catalysts. This indicates an increase in dehydroxylation degree of the brucite-like sheets and decomposition of carbonates in the interlayer, which may induce a higher rehydration degree and larger amount of water in the interlayer of the re-Mg3Al0.9Y0.1Ox catalysts. Figure S4 shows the IR spectra of the rehydrated samples after adsorbing CO2 at room temperature. Two main bands appeared at ∼3500 and 1600 cm−1. The absorption bands at ∼3500 cm−1 should be attributed to the stretching vibration mode of the hydrogen-bonded hydroxyl groups in the brucitelike layers and interlayer water, which indicates that the sample was rehydrated and there is additional water in the interlayer brought in the rehydration process. Compared with other samples, the re-Mg3Al0.9Y0.1Ox sample has a larger amount of water in the interlayer. The second absorption band located at 1500−2000 cm−1 is assigned to the carbonate species adsorbed on the surface of the sample. Note that the rehydrated Mg3Al0.9Y0.1Ox sample after adsorption of CO2 also showed a larger area of the IR absorption peak from the carbonate species adsorbed on the surface, which indicates that the Mg3Al0.9Y0.1Ox sample has more basic sites on the surface. The results above indicate that the unmodified and modified MgyAlOx mixed oxide, after being used in the reaction, can recover partly to the hydrotalcite structure. The Y−MgyAlOx mixed oxide showed the highest degree of rehydration, but its crystallite size was relatively smaller than that of the assynthesized sample. It was reported that the rehydrated mixed oxides catalysts were more active for aldol condensation than the mixed oxide catalysts, such as acetone self-condensation,9 cross-aldol condensation of benzaldehyde/citral with acetone,30 and the Claisen−Schmidt condensation reaction.31 The water effect on the aldol condensation reaction was studied also. Climent et al. found that the high yield of pseudoionones was obtained after a short reaction time over the calcined MgAl hydrotalcite catalyst with 36 wt % water added to the fresh catalysts20 or using the calcined MgAl mixed oxides after rehydration in decarbonated water.25 Herein, the catalytic performances of the mixed oxide catalysts after rehydration (re-MgyAl1‑aREEaOx) were investigated, and the results are shown in Figure 11. The results show that 100% citral conversion was achieved after 2 h of reaction for all rehydrated catalysts, and their selectivities to PS were 70−75% and hardly varied with reaction time. Compared with the results in Table 2, after the rehydration of Mg3AlOx and Mg3Al0.9La0.1Ox catalysts, their activities were increased obviously. These different reaction results between MgAl mixed oxides and corresponding rehydrated catalysts indicate that the one-pot rehydration process has barely or not occurred on MgAl and Mg3Al0.9La0.1Ox mixed oxides. However, for the Mg3Al0.9Y0.1Ox catalyst, its activity was hardly changed after its rehydration, which indicates that the high catalytic activity of the Mg3Al0.9Y0.1Ox mixed oxides might result from the rehydrated form of catalysts. However, has the rehydration of Mg3Al0.9Y0.1Ox been completed in the process of the reaction? The results of TGA in Figure 3 show that the total weight loss of as-synthesized hydrotalcite is ∼40%, which is the maximum absorption water amount when the oxide catalyst is rehydrated. When the citral conversion reached 100%, the

∼85%. This high conversion and selectivity can continue up after 6 h of reaction. During the recyclability testing of the catalyst, it was found that the Mg3Al0.9Y0.1Ox catalyst reclaimed from solution by centrifugation before reuse without any treatment performed relatively low catalytic activity. However, after the spent catalyst was treated in air flow at 500 °C for 2 h, its catalytic activity could be recovered because the used catalysts were poisoned by H2O and atmosphere CO2 after being reclaimed from the reaction solution and exposed to air. Figure S2 shows the citral conversions as a function of recycling times over the Mg3Al0.9Y0.1Ox catalyst (reaction time of 4 h). The results show that with an increase in recycling times the citral conversion decreased gradually, and the citral conversion dropped to 82% after four times of recycling use. The selectivity to pseudoionone was hardly changed and could stay around 80%. Note that the catalyst lost about 5 wt % after centrifugation separation and calcination at 500 °C. Therefore, the slight reduction of the citral conversion with the recycling times should be ascribed to the weight loss of the reclaimed catalyst rather than degradation of the catalyst.

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DISCUSSION The catalysts, after being used at 60 °C for 6 h, were characterized by XRD, and the results are shown in Figure 10.

Figure 10. XRD patterns of the Mg3AlOx, Mg3Al0.9La0.1Ox, and Mg3Al0.9Y0.1Ox catalysts after being used in the reaction at 60 °C for 6 h.

Compared with the XRD pattern of the fresh Mg3Al0.9REE0.1Ox sample (Figure 2), there are broad diffraction peaks of the (003), (006), (012), (015), (018), and (110) facets of hydrotalcite and (200) and (220) facets of MgO periclase in the XRD patterns of all used catalysts, which show that after the mixed oxide catalysts were used in the reaction for 6 h, the hydrotalcite structure in the catalyst was partly rebuilt. However, the intensities of the diffraction peaks for the rebuilt hydrotalcite structure, which indicate the degree of rehydration of the mixed oxides, are different for different catalysts. As shown in Figure 10, the used Mg3AlOx sample showed the weakest hydrotalcite phase and the strongest MgO phase, and the used Mg3Al0.9Y0.1Ox sample exhibited the strongest hydrotalcite phase and the weakest MgO phase. The rehydrated samples were also characterized by TG, and the results are shown in Figure S3. The weight losses of the rehydrated HT samples showed a similar tendency as the HT sample in Figure 2. The four samples have similar TG profiles, in which there are two main endothermic effects at about 200 H


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relatively clean surface and small particle size might accelerate the rehydration process, which make it possible to rehydrate by water produced in the reaction. Thus, it can be concluded that the excellent catalytic activity of MgyAl1‑aREEaOx is related to its composition, and Y is one of the rare earth elements in the MgAl mixed oxides to conduct the highly catalytic activity by in situ rehydration in the reaction process. On the basis of the research results above, we can see, in the aldol condensation of citral and acetone over the MgyAl1‑aYaOx catalyst, that the catalytic behavior of Mg3Al1‑aREEaOx was tightly correlated to its one-pot rehydration process and surface acid−base property. The reaction mechanism is presented in Scheme 1. First, the reaction was initially catalyzed by the mixed oxide catalyst, and the α-proton of acetone was abstracted on the surface O2− sites to form a carbanion. The carbonyl group of citral chemically adsorbed on the sites of Mg2+ (or M3+) to form an intermediate, and the carbanion consecutively attacks the carbonyl group of citral adsorbed on Mg2+ (or M3+) to form an intermediate, while this unstable intermediate was rapidly dehydrated to form the products PS and water. It is interesting that the active sites on the catalyst surface were regenerated for the Mg-substrate oxide,13,16 but for the Mg3Al1‑aYaOx catalyst under this reaction condition, the formed water during PS formation reconverted Mg3Al1‑aYaOx to a rehydrated catalyst with the OH− species as the compensating anions. Once the rehydrated samples were formed, the more active sites (Brønsted basic sites) would be introduced to the reaction pathway. The α-proton of acetone was abstracted by the surface OH− groups, and whole reaction rate was improved. As is well known, aldol condensation is a reversible reaction, and water is one of the products. Thus, the consumption of water by the rehydration process of the oxide catalysts can promote the shift of the reaction equilibrium toward the formation of products until complete conversion of citral. Therefore, it is important to understand that the formation of PS over Mg3Al1‑aYaOx may be accompanied by the gradual modification of the catalyst surface

Figure 11. Conversion of citral and selectivity to pseudoionone vs reaction time over re-Mg 3 AlO x , re-Mg 3 Al 0.9 La 0.1 O x , and reMg3Al0.9Y0.1Ox catalysts at 60 °C (acetone/citral = 5 (mol), Wcat = 0.6 g, Wcat/Wcitral = 23%).

formed water was ∼0.3 g. If the catalyst (0.6 g) used in the reaction was completely rehydrated, a maximum 0.24 g of water must be required; thus, 0.3 g of water produced in the process of the reaction might be a feasibly decarbonated water source for the complete rehydration of the mixed oxide catalysts. Compared with MgyAlOx and MgyAl(La, Eu)Ox catalysts, the Y-modified MgAl oxide may possess unique structural natures, resulting in the one-pot rehydration of Mg3Al0.9Y0.1Ox in the process of the reaction. We have prepared the hydrotalcite doped with REE (La, Ce, and Y),21 and since La and Ce have larger ionic radii than Y, these REE ions can not be completely introduced into the lattice structure and easily form corresponding carbonate or hydroxyl−carbonate species deposits on the catalyst surface. In Y-modified MgAlO catalysts, the Y ions have been relatively completely introduced into the lattices and have smaller particle sizes than Mg3AlOx. The

Scheme 1. Proposed Mechanism for Citral/Acetone Aldol Condensation over MgyAl1‑aYaOx Catalyst through Two Different Reaction Pathways

I


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ACS Sustainable Chemistry & Engineering to form re-Mg3Al1‑aYaOx, which reconverts the O2− basic sites to OH− basic groups. Compared with a high citral conversion, the selectivity to PS over Mg3Al1‑aYaOx is relatively lower than other catalysts, which is possibly because Y ions introduced in the MgAl-HT structure varied the properties of the surface Lewis basic sites (O2−) and Brønsted basic sites (HO−) after rehydration. The Brønsted basic sites resulted from Lewis basic sites after the oxide catalyst was rehydrated, resulting in an increase in the surface Brønsted acidic property, which would enhance adsorption of the products16,25,32 and stronger adsorption of citral.18,33 In fact, it was reported that the citral/acetone cross-aldolization on the Mg−Al catalyst is a negative order with respect to citral, which reflects the stronger adsorption of citral in comparison to acetone.32 The former would promote a deep reaction of PS to ionone catalyzed by acidic sites,16 and the latter (stronger adsorption of citral) makes the occurrence of side reactions on the Brønsted basic sites, such as the self/cross-condensation of citral or acetone and possibly the oligomerization of reactants. Therefore, the adsorption and deep reaction of PS and formation of byproducts could be the main reasons for the decrease in PS selectivity. Although selecting the citral/acetone aldol condensation as the model reaction was not the intent to ascertain the changes in the active site nature during the reaction, it is noteworthy that eventual changes on the catalyst surface actually improve the catalytic activity without further pretreating the catalyst. The normal way to obtain rehydrated hydrotalcite with the Brønsted basic sites is through reconstruction of mixed oxides in decarbonated liquid or gas phase water circumstances.9 These rehydration pathways are easily affected by CO2 from liquid or atmosphere to form the inactive carbonate species adsorbed on the catalyst surface. This direct rehydration process during the reaction opens the possibility to conduct aldolization and dehydration reactions over REE-MgAl mixed oxides by one-pot rehydration in a pure water resource. Above all, we concluded that incorporating the REE (especially yttrium element) to MgAl mixed oxides not only brought high catalytic activity for the adole condensation reaction of citral and acetone as a corresponding rehydrated catalyst, but also proposed a novel method to generate rehydrated catalysts without a time-consuming and vulnerable to contamination rehydration process.

and then consecutively attacked the carbonyl group of citral to form the product PS and water. It is interesting that the formation of PS over Mg3Al1‑aYaOx may be accompanied by the gradual modification of the catalyst surface to form reMg3Al1‑aYaOx through a rehydration process by produced water, which makes the O2− basic sites reconvert to OH− basic groups. The interlayer hydroxyl groups, as the more active sites, promoted the cross-aldol condensation of citral and acetone, and 100% citral conversion and ∼85% PS selectivity were achieved at 60 °C for 3 h. This one-pot rehydration process, which was successfully carried out in this cross-aldol condensation of citral and acetone over Mg3Al1‑aYaOx catalysts, might become a feasible strategy to overcome technical barriers and provide fresh and active OH− groups for the aldol condensation reaction in future studies of biomass transformation to biorefineries.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01533. FT-IR spectra of REE-Mg3AlOx and re-REE-Mg3AlOx (Figure S1). Citral conversion vs recycling times over Mg3Al0.9Y0.1Ox (Figure S2). TG profiles of re-REEMg3AlOx (Figure S3). FT-IR spectra of CO2 adsorbed on re-REE-Mg3AlOx (Figure S4). (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-21-64252923. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Basic Research Program of China (2010CB732300) and 111 Project (B08021).

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REFERENCES

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CONCLUSIONS In summary, unmodified and REE (Y, La, and Eu)-modified MgAl mixed oxides are prepared by the coprecipitation method, and their catalytic activity for the aldol condensation reaction of citral and acetone was studied. It was found that unmodified and La- and Eu-modified MgAl mixed oxides showed relatively low catalytic activities for aldol self-condensation. However, Ymodified MgAl mixed oxides exhibited unexpectedly high catalytic activity. Further characterization showed that the particle size of Mg3Al1‑aYaOx decreased with an increase in Y content. When the Y3+/M3+ molar ratio is 0.1−0.2, welldeveloped small flake morphology samples with high surface area and pore volume could be obtained, which results in an increasing number of exposed basic sites on the surface of catalysts to further improve the catalyst activity for the aldol reaction. In the PS synthesis by citral/acetone aldol condensation over the MgyAl1‑aYaOx catalyst, an α-proton of acetone molecule on the basic sites was abstracted to form a carbanion intermediate J


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