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Effects of Support for Vanadium Phosphorus Oxide Catalysts on Vapor-Phase Aldol Condensation of Methyl Acetate with Formaldehyde Hui Zhao, Cuncun Zuo, Dan Yang, Chunshan Li,* and Suojiang Zhang* Beijing Key Laboratory of Ionic Liquids Clean Process, Key Laboratory of Green Process and Engineering, Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *

ABSTRACT: Vapor-phase aldol condensation of methyl acetate with formaldehyde over vanadium phosphorus oxide (VPO) deposited on SiO2, TiO2, ZrO2, Nb2O5, Sb2O3, γ-Al2O3, and n-γ-Al2O3 (γ-Al2O3 treated with H3PO4) for methyl acrylate production was developed. The VPO/n-γ-Al2O3 was found to have the best performance based on the evaluated experiments in a fixed-bed reactor. The physicochemical properties of the catalysts were investigated by different characterization methods such as the Brunauer−Emmett−Teller method, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, H2 temperature programmed reduction, Fourier transfrom infrared spectroscopy, and NH3- and CO2-temperature programmed desorption. The influence of P−OH intensity and acid strength on activity was obtained through systematic studies over the formation process of VPO phases and the acidity distribution. Preparation and reaction parameters were investigated and optimized.

1. INTRODUCTION As a kind of intermediate in the organic synthesis industry, acrylic ester is widely used in the production of adhesives, synthetic resin, and coatings.1,2 Methyl acrylate (MA) is known as an important monomer of macromolecular synthesis,3,4 and its copolymeric emulsion with butyl acrylate is an effective method for improving the quality of leather. The traditional route of MA production is a two-step oxidation of propylene.5,6 However, because of the rising price and under production of propylene, the development of a new alternative route is desperately needed. Recently, one-step aldol condensation of low-cost methyl acetate (MeOAc) with formaldehyde (FA) has attracted attention,7−9 and the design of catalysts with high yield of MA is necessary. Aldol condensation is an important organic reaction because of forming new C−C bonds; however, the mechanism has not been established. In 1966, Vitcha and Sims reported the mechanism of the aldol condensation of acetic acid and formaldehyde.10 They proposed that the acetic acid absorbed on the surface of decalso catalyst was attacked by the base to form a carbanion, which is followed by addition, proton transfer, and dehydration. Therefore, much attention has been focused on solid base catalysts and alkali supported catalysts.11−13 The loading of alkali metal elements on the surface of metal oxides can diminish the acidity of supports or create their own basicity or Lewis acidity if alkali metal occupies the cationic position.14 However, according to current reports, both the acid and base properties of catalysts play an important © XXXX American Chemical Society

role in promoting aldol condensation; therefore, modified acid−base bifunctionalized catalysts have been developed. Zeidan and Davis found that the catalyst which contains primary amine groups and carboxylic acid groups with a number of different electrophilic components could increase the reaction conversion to 99%.15 Li et al. obtained a 32.9% yield of MMA over Zr−Mg−Cs/SiO2 catalyst via aldol condensation of methyl propionate with formaldehyde.16 Feng et al. reported a 84.2% conversion of methyl acetate over VPO catalyst that used poly ethylene glycol (PEG) in the fabrication process. 7 Ai et al. have focused on aldol condensation over V2O5−P2O5 binary oxide catalyst for many years and found the reaction activity could be influenced by the acid sites.17−19 The surface P−OH groups and VO double bonds are considered to contribute the acidity of the VPO catalysts.20 According to previous research, supported catalysts have excellent characteristics including better heat transfer, better mechanical strength and better dispersity.21 The use of supported catalysts can solve many industrial problems such as high attrition resistance for moving in fluid-bed reactors, controllable texture leading to decreased internal and external mass-transfer limitation, and reduction of active component Received: Revised: Accepted: Published: A

August 12, 2016 November 28, 2016 November 29, 2016 November 29, 2016 DOI: 10.1021/acs.iecr.6b03079 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research dosage.22 However, the study of VPO supported on Al2O3, SiO2, SiC, and TiO2 discovers that the support−oxide interaction may hinder the formation of (VO)2P2O7 phase and promote the formation of V5+-containing phases (α1VOPO4, γ-VOPO4).22 Mikolajska et al. found that alumina support stabilized V5+ and that V4+ predominated in bulk vanadyl pyrophosphate phase.23 It has been proven that large pore size enabled good dispersion of VPO species in crystalline form.24 In addition, Feng et al. reported that H3PO4-treatment conducted on ZrO2 affected the nature of supports, as well as the state and structure of the VPO component.25 Hence, it could be determined that the modification of supports could improve the performance of supported VPO catalysts. Sun et al. prepared Al2O3 with controllable structure and adjusted the surface property by the addition of phosphor, which advanced the isomerization of 1-butene and the hydrogenation activity of butadiene.26 A study of Morales et al. showed that the adsorption of phosphorus followed a Langmuir-type process and formed a phosphate ion monolayer.27 Therefore, H3PO4-treatment was adopted to modify the support features and then to improve the properties of the VPO component. In this research, supported VPO catalysts fabricated in an organic medium were tested for aldol condensation of methyl acetate with formaldehyde. The focus of this study is the influence of modified supports on the activity of VPO catalysts. H2 temperature-programmed reduction (H2-TPR) profiles revealed that the interaction between modified supports and the VPO phase was strengthened. Both the acidity of catalysts and the physical properties of γ-Al2O3 were enhanced by the introduction of phosphorus. The enhancement of P−OH vibration characterized by Fourier transform infrared spectroscopy (FTIR) and the change of acidity distribution characterized by NH3 temperature-programmed desorption (NH3-TPD) explained the increase of MA yield. These obtained results are helpful for further catalyst development and intensive study of the mechanism of aldol condensation.

furnace, and held for another 15 h. For comparison, the loading amount on different supports including treated γ-Al2O3 was uniformly 15 wt %, the final catalysts are denoted as VPO/ MxOy (M = Si, Ti, Zr, Nb, Sb; x,y = atomic number of metal oxides). For the catalysts pretreated with phosphoric acid, they are denoted as VPO/n-γ-Al2O3, where n is the concentration of H3PO4 in moles per liter. To investigate the influence of loading amount, 5, 10, 15, 20, and 30 wt % VPO/γ-Al2O3, catalysts were prepared and designated as m% VPO/γ-Al2O3 (m = 5, 10, 15, 20, 30) for convenience. 2.2. Catalyst Evaluation. The catalytic performance of supported VPO catalysts for the synthesis of methyl acrylate by aldol condensation was evaluated in a fixed-bed reactor. The reactor was made of a stainless steel tube mounted vertically in the furnace. A 10 mL sample of catalyst was loaded in the middle of the reactor with quartz wool placed on both sides, and the extra space was filled with stainless pipes. Methyl acetate and formaldehyde (sourced from trioxane) with a molar ratio of 5/1 were mixed intensively before being injected into the reactor from the top with the feed rate of 0.1 mL/min. The reaction was carried out under atmospheric pressure with the temperature range of 573−663 K, and the carrier gas was air. To collect the product taken away by the exhaust, deep-cooling equipment was installed in the outlet setting 253 K. The product was collected every 2 h and then analyzed by a gas chromatography system with iso-butyl alcohol as internal standard. The methyl acetate in the feedstock was in excess, so the reactions involving formaldehyde were taken into consideration. The products obtained at the end of experiment were MA, methyl propionate (MP), and methyl methacrylate (MMA). The reaction equations are as follows; R2, R3, and R4 are side reactions. FA + MeOAc → MA + H 2O

(R1: 1 mol MA = 1 mol FA)

2FA → HCOOCH3 + H 2O → CH3OH + HCOOH HCOOH → CO2 + H 2

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Alumina material was purchased from Juteng Chemical Co., Ltd. of Zibo. Other supports (SiO2, TiO2, ZrO2, Nb2O5, Sb2O3) were provided by Sinopharm Chemical Reagent Co., Ltd. To improve the property of supported VPO, γ-Al2O3 was pretreated with H3PO4 of different concentrations (e.g., 1 g of γ-Al2O3/20 mL of H3PO4 solution of 1 mol/L). γ-Al2O3 and acid solution were mixed together and stood at room temperature for about 12 h. The treated γ-Al2O3 was filtered and washed with ethanol, then dried at 393 K for 12 h in the oven, and finally calcined at 773 K for 8 h under atmospheric conditions. The supported VPO catalysts were prepared by means of impregnation−deposition in an organic medium as described previously.24,25 V2O5 was first refluxed in a mixture of iso-butyl and benzyl alcohols (volume ratio of 1:1) at 413−423 K for 5− 7 h. Subsequently, PEG 6000 and a particular support were introduced in a suitable amount with the temperature lowering to 343−353 K. After 10 h, phosphoric acid (85 wt %) was added dropwise to reach a P/V atomic ratio of 1:1 with increasing the temperature to 393−403 K. After being refluxed for another 5−7 h, the suspension was filtered and the obtained filter cake was washed with ethyl alcohol and dried at 393 K for 12 h. The precursor was sieved with 30−60 mesh, then heated from room temperature to 773 K at a rate of 2 K/min in muffle

MA + H 2 → MP

(R2: 1 mol H 2 = 2 mol FA)

(R3: 1 mol MP = 3 mol FA)

FA + MP → MMA + H 2O

(R4: 1 mol MMA = 4 mol FA)

The yield and selectivity were calculated based on the amount of FA. The equation in parentheses represents the consumption of FA to form the target product. The yield was the ratio between the amount of methyl acrylate over the total amount of products. The selectivity was the ratio between the amount of formaldehyde to form methyl acrylate over the consumption of formaldehyde to form all products. 2.3. Catalyst Characterization. The N2 adsorption− desorption isotherms of samples were measured by a surface area analyzer (Quanta Chrome Instrument NOVA 2000) at 77 K. All the samples were first degassed at 623 K for 6 h. The surface area and pore size distribution were determined by Brunauer−Emmett−Teller (BET) method and Barrett−Joyner−Halenda (BJH) model, respectively. Powder X-ray diffraction (XRD) was conducted on a diffractometer (Smartlab 9) with Cu Kα radiation at 40 kV and 2θ region from 5° to 90°. The morphology of catalysts was observed in an SU8020 scanning electron microscopy (SEM) unit. Catalysts that were ground into powder were stuck onto the sample stage through conductive adhesive. The samples were scanned with a very B

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From the results of Table 2, the surface area of modified catalysts is smaller than that of 15 wt % VPO/γ-Al2O3 because

narrow electron beam, and the surface morphology was displayed by the two-electron signal of the samples. To distinguish VPO from support through inner structure, transmission electron microscopy (TEM) was performed with the JEOL JEM-2100 system. Samples were dispersed in ethanol by ultrasound; then the supernatant was dropped onto copper net while waiting for testing. Temperature-programmed reduction (TPR) was carried out in the temperature range of 303−1173 K. The samples were reduced in 10% H2/Ar (50 mL/min) at a rate of 10 K/min. FTIR spectra of the supported VPO catalysts were tested on the IR (Nicolet 6700) spectrometer under ambient conditions. The sample powder was mixed in KBr with the volume ratio of 1:100 and then pressed into center transparent sheet. The acidity and basicity of the catalysts were measured by temperature-programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO2-TPD) on an Autochem II 2920 apparatus from Micromeritics. The deposition profile was measured by a thermal conductivity detector in helium flow at a heating rate of 10 K/min.

Table 2. Textural Properties of VPO Supported on n-γ-Al2O3

Table 1. Textural Properties of Supported VPO Catalysts with Different Loading Capacity surface area (BET m2 g−1)

av Dp (BJH nm)

total Vp (BJH cm3 g−1)

γ-Al2O3 5% VPO/γ-Al2O3 10% VPO/γ-Al2O3 15% VPO/γ-Al2O3 20% VPO/γ-Al2O3 30% VPO/γ-Al2O3

362.02 242.01 217.23 210.32 210.17 190.68

6.17 7.37 7.54 7.75 6.87 6.79

0.47 0.49 0.46 0.43 0.37 0.33

surface area (BET m2 g−1)

av Dp (BJH nm)

total Vp (BJH cm3 g−1)

VPO/γ-Al2O3 VPO/1.0-γ-Al2O3 VPO/1.5-γ-Al2O3 VPO/2.0-γ-Al2O3 VPO/3.0-γ-Al2O3

210.32 178.36 139.56 128.68 104.64

7.75 9.42 9.74 9.93 11.84

0.43 0.45 0.35 0.32 0.30

of the pores broken after H3PO4 treatment. While with the increase of H3PO4 concentration, the total pore volume decreased, and the loading amount in theory was all 15 wt %, indicating that H3PO4 treatment increased the real content on the catalysts. The atomic ratio of vanadium in XPS analysis in Table S2 shows the increase of real VPO content. Accompanied with the profile of pore size distribution in Figure S1c, the incremental pore volume of VPO/2.0-γ-Al2O3 is smaller than that of VPO/γ-Al2O3, and the incremental pore volume of 2.0γ-Al2O3 is bigger than that of γ-Al2O3, indicating that H3PO4 treatment enlarged the pore size of γ-Al2O3 and increased the real amount of VPO phase. 3.1.2. TEM. From the figure of N2 adsorption−desorption, the slit-shaped pores had been identified by H3 type hysteresis loop. The porous channel can be seen clearly in Figure S2a as a cracklike feature. After the sample is treated with acid, acicular crystals were distributed over the interface, which was supposed to be AlPO4. Because of the dispersion of active phase on the support, the supported VPO could not be observed easily; TEM can help with the deficiency. Figure 1a is the TEM image of γ-Al2O3 pretreated with 2.0 mol/L H3PO4, corresponding to Figure S2b. The slender rod is supposed to be the acicular crystal in Figure S2b, that is, AlPO4. In Figure 1c, the acicular crystal existed as well, because AlPO4 also formed on 15 wt % VPO/γ-Al2O3 catalyst which have been detected by XRD. For comparison, the TEM image of pure VPO is shown in Figure 1b. Interestingly, the similar substance was observed on the top of the acicular crystal, shown in Figure 1d, indicating that AlPO4 provided the place for VPO growth. 3.1.3. XRD. The wide-angle X-ray diffraction patterns of supported VPO catalysts are shown in Figure 2. All the catalysts showed poor presence of (VO)2P2O7 phase, which is similar to the case of VPO supported on Al-MCM-41.29 In Figure 2a, (VO)2P2O7 was detected only on γ-Al2O3, and the VOPO4 phase was detected on every support. While differences exist in the peak positions of the VOPO4 component, this may because of the difference in the crystalline phase. The vanadium phosphorus oxides are a complex catalytic system, even VOPO4 itself can be found in many crystalline state, e.g. αI-, αII-, β-, γ-, and ε-VOPO4. The state of VOPO4 can be influenced by addition of metals, preparation conditions, etc.,30 revealing that supports also play an important role in the formation of the active phase.22 The patterns of VPO loaded on γ-Al2O3 (Figure 2b) show that not only the formation of crystalline varied with VPO amount, but also the intensity of diffraction peaks enhanced with the increased amount [V5+-containing phases (VOPO4 and V2O5) and V4+-containing phase ((VO)2P2O7)].31 From the XRD pattern shown in Figure 2c, catalysts whose support is treated with phosphoric acid have fewer impurities, and the greater the concentration of

3. RESULTS AND DISCUSSION 3.1. Physical Characterization of Catalysts. 3.1.1. BET. Different supports exhibit different physical structure. Most materials of high surface area correspond to large pore volume and smaller pore size according to Table S1 in the Supporting Information. For gas−solid reactions, the catalytic efficiency could be affected by textural properties. From the data, the pore diameter (av Dp) of VPO/SiO2 and VPO/γ-Al2O3 are smaller than others, but the BET surface and total pore volume (Vp) are larger, which could provide more places for reaction. As shown in Table 1, the surface area decreases as a function of VPO amount; the average pore diameter first increased and

catalyst

catalyst

then decreased after 15 wt %. One reason for the increase in pore size may be the corrosion of phosphoric acid, because AlPO4 as the product of γ-Al2O3 and H3PO4 had been detected by XRD; the other reason may be the block of small pores by component.21,24 While the decrease in pore size may be due to the additional small porosity built by VPO loading and component deposition in big pores. The conjecture could be validated by the pore size distribution curve in Figure S1b, especially the enlarged view of the graph in the red box. To study the textural properties of catalysts, the N2 adsorption− desorption profiles of VPO supported catalysts are presentated in Figure S1a. As shown, the supported catalysts exhibited type IV isotherm,28 with a clear H3 type hysteresis loop at the relative pressure of 0.5−1.0. The H3 type loop, which does not present any adsorption limit at high P/Po, is regarded as aggregate of platelike particles providing slit-shaped pores. C

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Figure 1. Representative TEM images of (a) 2.0 mol/L γ-Al2O3, (b) pure VPO, (c) 15 wt % VPO/γ-Al2O3, and (d) 15 wt % VPO loaded on 2.0 mol/L H3PO4.

VPO/γ-Al2O3 catalyst indicates its perfect behavior in reaction activity. The activity of catalysts is also affected by the interaction between VPO and supports. Temperature-programmed reduction provides the information about VPO stability. The peaks (Figure 5) below 700 °C and above 700 °C are attributed to the removal of lattice oxygen related to V5+ and V4+ species, respectively.32,33 Only the removal of lattice oxygen related to V5+ can be observed clearly; the results are in good agreement with XRD measurements. When the six catalysts are compared, the reduction temperature of VPO/γ-Al2O3 is the lowest and even lower than that of pure VPO (529 and 752 °C; the profile is presented in Figure S4). That is to say the VPO on γ-Al2O3 is dispersed well and more active. In conclusion, for supported catalysts, the role of supports is more than a support for the structure; they also affect the state of VPO phase and support for catalytic performance. 3.2.2. Effect of VPO Amount. The catalytic performance obtained from a series of VPO/γ-Al2O3 catalysts is shown in Figure 6; 15 wt % VPO/γ-Al2O3 exhibited the optimal catalytic activity. The different performances in conversion and selectivity indicate that the gas−solid reaction could be affected by the amount of VPO greatly. There are a few reasons for this finding. First, the VPO was loaded on γ-Al2O3 in the form of deposition, and the amount of active phase changed the pore nature; av Dp (Table 1) reached the top value at 15 wt %, and this benefited the diffusion of gaseous substances in pores. Second, the stability of VPO varied with amount. The reduction temperature (Figure 7) decreased with the increase of VPO amount except 30 wt %, resulting from the location of VPO in small size pores and dispersed in small dimensions at low load, which could be easily reduced. At 30 wt %, VPO components were located mostly on the external surface of γAl2O3, and the particles were larger, as in the bulk phase; hence, the reduction temperature increased.24 Moreover, the conversion is related to the ability to adsorb HCHO. According to Singh et al, some aldehyde is adsorbed on TiO2 through Hbonding with surface hydroxyl groups.34 FTIR is mainly used as

phosphoric acid, the better the crystallinity of VOPO4. However, there are no peaks of V4+ species, which have been proven to exist over catalysts by H2-TPR, suggesting that the active phase has been highly dispersed. The different crystal behavior of the two VPO phases on the support illustrates that γ-Al2O3 stabilizes V5+ and that V4+ predominates in bulk vanadyl pyrophosphate.23 In addition, H3PO4 treatment improves the crystallization and dispersion of the VPO component on γ-Al2O3. 3.2. Catalytic Performance. 3.2.1. Effect of Supports. In general, the structure and strength of supported catalysts was provided by supports. While according to the results of XRD, the state of the active phase also is affected by supports. Selection of appropriate support affects not only the physical property of catalysts but also the excellent reaction activity. The aldol condensation of methyl acetate with formaldehyde to synthesize methyl acrylate was carried out over a series of supported catalysts. As shown in Figure 3, the selectivity of VPO loaded on different supports did not present a significant difference; the value fluctuated around 95%, except 100% for VPO/Sb2O3. The conversion over these catalysts was as follows (in the form of said support): γ-Al2O3 > Nb2O5 > SiO2 > TiO2 > ZrO2 > Sb2O3. To determine whether supports contribute to the yield of MA, the aldol condensation reaction was carried out on clean supports (Figure S3). The result shows that supports participated in the reaction as well, and the yield on supports performed in a sequence similar to that of supported catalysts. Therefore, the best catalyst VPO/γ-Al2O3 with excellent catalytic performance could be found. Figure 4 shows the NH3 and CO2 desorption profiles of catalysts on different supports; the peak area represents the amount of acidity or basicity. It is clear to see that VPO/γAl2O3 and VPO/SiO2 catalysts show strong NH3 desorption peaks in Figure 4a. Weak acid predominates on VPO/SiO2; medium and strong acids predominate on VPO/γ-Al2O3. While in Figure 4b, only VPO/γ-Al 2 O 3 shows obvious CO 2 desorption pattern. The excellent acid−base property of D

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Figure 3. Catalytic performance of VPO loaded on different supports.

The IR spectra of samples with different VPO amounts are shown in Figure 8. The bands in the region 2600−3600 cm−1 exist on every IR spectra and are hydrogen-bonded OH;20,21 the intensity of the peak increased with loading capacity until 15 wt % then decreased at higher amounts (20 and 30 wt %). The peak at 1642 cm−1 was assigned to P−OH groups. According to the literature,35 the formation of P−OH was due to topotactic transformation of VOHPO4·0.5H2O to the active phase (VO)2P2O7. At higher capacity (above 20 wt %), the catalyst gave the following bands: P−OH stretching, 1642 cm−1; PO3 bending, 1160 and 1055 cm−1; VO stretching, 1003 cm−1; V−OH stretching, 938 cm−1. However, the chemical bands on the supported catalysts have some displacements compared with that on pure VPO,36 which indicates that the stability of supported VPO is changed. The wide absorption band in the region 400−1000 cm−1 is the characteristic peak of γ-Al2O3,37 and it also changes with VPO amount. For vapor phase reaction, the diffusion and adsorption are key factors for reaction. The biggest pore diameter and a large number of OH groups on 15 wt % VPO/γ-Al2O3 contributed to its excellent performance. 3.2.3. Effect of H3PO4 Treatment. Referring to BET data, the pore size of H3PO4-treated catalysts was enlarged; therefore, the gas reactant could spread into channels easily. The activity evaluation results of treated catalysts are shown in Figure 9.The yield of treated catalysts was increased with H3PO4 concentration, and the best yield was obtained over catalyst VPO/2.0-γ-Al2O3. The selectivity stabilized around 93% and did not show a large difference. Therefore, H3PO4 treatment mainly affected the yield of MA.38 Figure 10a shows the CO2 desorption profile of treated catalysts. The amount of weak strength base sites decreased after treatment; therefore, the adsorption competition between different base sites on treated catalysts weakened compared with that on untreated ones. To separate the contributions of different acid sites, deconvolution of TPD peaks (Figure 10b) has been performed using a professional software, Peak Fit v4.12, supposing Gaussian-type curves. The results of acidity are shown in Figure 10c. The desorbed ammonia appearing at ≤150 °C should correspond to the physically adsorbed and hydrogenbonded NH3.39 The NH3 molecules desorbed at higher temperature were attributed to acid-bonded NH3, and the acid sites could be defined as weak (≤150 °C), medium (150−

Figure 2. XRD patterns of (a) 15 wt % VPO loaded on different supports, (b) VPO loaded on γ-Al2O3 from 5 to 30 wt %, and (c) VPO loaded on γ-Al2O3 treated with different H3PO4 concentration.

a means of identification of molecular structure and chemical bonds due to the highly characteristic features. E

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Figure 4. TPD profiles of VPO loaded on different supports: (a) NH3-TPD and (b) CO2-TPD.

Figure 5. TPR profiles of 15 wt % VPO loaded on different supports. Figure 7. TPR profiles of VPO loaded on γ-Al2O3 from 5 to 30 wt %.

300 °C), strong (300−450 °C), and very strong (≥450 °C). Compared with the untreated catalysts, the medium strength acid sites dominated in amount and increased with H3PO4 concentration. It could be concluded that the distribution of surface acidity had a great influence on the yield of MA, especially the effect of medium acid. In the IR spectra (Figure 11) of VPO loaded on treated γAl2O3, the peak in the region 2600−3600 cm−1 increases in width compared with that of the untreated sample, indicating that the OH groups assigned to hydrogen-bonded OH or free hydroxyl increased in amount. Moreover, the intensity of the bands at 1642 and 1130 cm−1 is rising with the concentration of H3PO4. The two absorption peaks are both the vibration of P− OH groups, and the peak at 1130 cm−1 covers other vibrations of VPO groups to become the biggest one. In conclusion, the VPO species on supports were improved because of H3PO4 treatment, which is consistent with XRD and TPR results. According to Busca et al.,20 the acidity of the VPO catalysts could be attributed to the presence of surface P−OH and

Figure 6. Catalytic performance of VPO loaded on γ-Al2O3 from 5 to 30 wt %. F

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Figure 8. FTIR spectra of VPO loaded on γ-Al2O3 from 5 to 30 wt %.

Figure 9. Catalytic performance of VPO loaded on γ-Al2O3 treated with H3PO4.

unsaturated VO double bonds. The unsaturated cations or oxygen vacancies, as well as structural defects, are assigned to Lewis acidity,40 while acid hydroxyl groups connected to P or V atoms are divided into Brønsted acidity.41,42 Therefore, it could be concluded that H3PO4 treatment strengthens the Brønsted acidity. In view of the formation of AlPO4 on catalyst, the improvement of reactive activity may be due to the new phase. Aldol condensation of methyl acetate with trioxane was carried out on a series of AlPO4/γ-Al2O3 catalysts. From 0.1 to 1.0 wt %, the conversions and selectivities both showed a downward trend; the best conversion and selectivity was 18% and 73%, respectively, indicating that AlPO4 was not catalytically active for the synthesis of MA; it just provided the place for VPO growth (Figure 1c). 3.2.4. Effect of Operation Conditions. To realize the industrial application of catalysts and make the reaction mechanism clear, the study of operation conditions is of importance. Figure 12 shows the effect of feed ratio evaluated on VPO/γ-Al2O3. With the increase of the proportion of

Figure 10. (a) CO2-TPD and (b) NH3-TPD profiles of VPO loaded on treated γ-Al2O3; (c) acidity distribution of VPO loaded on treated γ-Al2O3.

methyl acetate, the yield of MA increased and remained constant at high ratio. Vitcha ea al. thought the drop of yield and conversion as the ratio of acetic acid to FA decreased was because the reaction of FA with itself to form polymers predominated.10 G

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results are shown in Figure 13. The VPO active phase supported on γ-Al2O3 was evaluated at a temperature range of 300−370 °C. All figures show an optimal point at 350 °C. This was not in accordance with linear growth trend, because hydrolysis reaction is endothermic; water as a side product accelerated the decomposition of ester at high temperature. In the meantime, high reaction temperature led to more byproducts, which could be explained by the decrease of selectivity shown in Table 3. Table 3. Seletivity at Different Temperatures selectivity (%) catalyst loading capacity (wt %) 10 15 20 H3PO4 concentration (mol/L) 1.0 2.0 3.0

Figure 11. FTIR spectra of VPO loaded on γ-Al2O3 treated with H3PO4.

300 °C

330 °C

350 °C

370 °C

94.56 93.16 93.53

93.44 89.98 92.40

93.19 92.92 90.74

84.62 76.51 78.68

100 95.51 95.73

100 96.45 95.57

89.69 91.89 92.45

82.00 91.23 83.68

3.3. Catalyst Stability. The experiment related to catalyst stability was carried out on 15 wt % VPO/γ-Al2O3 at 350 °C. The mixture of methyl acetate and formaldehyde was fed at the same molar ratio of 5/1 at 0.1 mL/min. In 60 h, the yield of methyl acrylate decreased by 50% (Figure 14). The mechanism of deactivation may be due to the formation of carbon deposition according to the black color of used catalyst43 and the thermogravimetric (TG) curve in Figure S5. The main weight loss was detected between 300 and 600 °C, which is attributed to the decomposition of deposited carbon. The mass the catalyst gained after 60 h is about 0.12 mg/mg. To measure the catalyst stability, the inactive catalyst was regenerated by calcination in air at 400 °C for 24 h. The performance of refreshed catalyst reached the same level with the fresh catalyst and was able to operate for another 60 h, indicating the good reusability of supported VPO catalyst.

Figure 12. Effect of feed ratio evaluated on VPO/γ-Al2O3.

The effect of reaction temperature on the synthesis of methyl acrylate was investigated over a series of catalysts, and the

Figure 13. Effect of reaction temperature carried out on VPO supported on (a) γ-Al2O3 and (b) H3PO4-treated γ-Al2O3. H

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financed by the National Key Projects for Fundamental Research and Development of China (2016YFB0601303), the National Science Fund for Excellent Young Scholars (21422607), the International Cooperation and Exchange of the National Natural Science Foundation of China (51561145020), and the National Natural Science Fund for Distinguished Young Scholars (21425625).

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Figure 14. Stability of 15 wt % VPO/γ-Al2O3.

4. CONCLUSIONS The catalytic performance of supported catalysts was influenced by supports. The appropriate support can not only provide proper catalyst structure and disperse active phase efficiently, but also affect the composition and crystallization of active phase and the acid−base properties of catalysts. In addition, supports can contribute to the activity of catalysts. VPO loaded on γ-Al2O3 exhibited an excellent catalytic activity for synthesis of MA. The optimal loading amount was 15 wt %. According to BET and FTIR results, catalysts with larger pore size and more hydroxyl groups behaved better in the reaction. Therefore, the catalyst of VPO loaded on n-γ-Al2O3 with enlarged pore size strengthened P−OH vibration and increased medium acidity, which enhanced catalytic performance. A 42% conversion for HCHO and 92% selectivity for MA was obtained on VPO/2.0γ-Al2O3 catalyst. The effect of reaction temperature on production yield and the stability of 15 wt % VPO/γ-Al2O3 were also examined. The best activity was obtained at 350 °C, and the catalyst could be continuously used for 60 h. After calcination in air at 400 °C for 24 h, the catalyst was able to be fully activated.

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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/acs.iecr.6b03079. Textural properties of VPO on different supports, XPS results for supported VPO catalysts, N2 adsorption− desorption isotherms and pore size distribution of VPO/ γ-Al2O3, SEM images of γ-Al2O3, yield of MA over clean supports, TPR profiles of pure VPO, and TG/DTA curves of used 15% VPO/γ-Al2O3 catalyst (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-82544800. Fax: +86-10-82544800. E-mail: csli@ home.ipe.ac.cn. *Tel.: +86-10-82627080. Fax: +86-10-82627080. E-mail: [email protected]. ORCID

Chunshan Li: 0000-0003-2460-8697 I

DOI: 10.1021/acs.iecr.6b03079 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.iecr.6b03079 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX