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Preparation of Metal Oxide-Doped Solid Superacid Catalyst and its Catalytic Performance.

Byline: Kai Huang and Keqiang Lu

Summary: A type of SO42-/ZrO2 solid superacid catalyst, doped with aluminum nitrate and ammonium tungstate, was prepared. The flow indictor steam method was performed for acidity measurement of the catalysts, and several characterization techniques, such as XRD, FT-IR, BET, and TGA-DSC, were used for studying the catalytic mechanism. The esterification yield of the modified catalyst was up to 98.9 % for synthesis of n-butyl acetate. The influences of the aging temperature, Al2O3/ZrO2 ratio, WO3/ZrO2 ratio, impregnation concentration, and calcination temperature were studied to determine the optimal preparation conditions.

The characterization results indicat that, the lower aging temperature was the most important for forming crystal centers on catalyst surface, and getting smaller regular crystal particles; therefore, the bigger specific surface can be available. Suitable calcination temperature can improve the conversion of crystal types, which include tetragonal phase and monoclinic phase. It shows a strong orientation action for formation of useful crystal type by adding W, which also delay the crystallization of ZrO2, stable tetragonal phase, and depress the loss of SO42-. It is beneficial to increase the Lewis acid sites on catalyst surface by adding Al2O3, and the catalytic activity can be improved strongly. The stability of the modified SWZA catalyst is better than that of traditional SO42-/ZrO2 solid superacid catalyst.

Key Words: Solid superacid; Catalyst preparation; n-butyl acetate; Esterification

Introduction

Sulfated metal oxides, especially sulfated zirconia has attracted intense interest as a catalyst for acid catalyzed reactions. This is mainly due to the distinct advantage of solid superacid catalysts such as high catalytic activity, strong acid strength, non-toxicity, non-corrosiveness, ease of handling, and easy to recover and reuse 1. In view of environmental and economic reasons, there is a persistent study to replace the conventional mineral acids with solid superacid for many organic reactions like alkylation, acylation, isomerization, esterification, etc 23. Sulfated zirconia and its modified forms used for industrial reactions ranging from simple alkylation to complex decomposition of commingled waste plastics have been reviewed by Yadav and Nair 4.

It is reported that the acidity of sulfated zirconia modified by metal elements (such as W, Al, Fe, etc.) 58, non-metal elements (Si) 9, or rare earth elements (such as Gd, La, etc.) 1011 is further strengthened. The acidity of these catalysts has been improved greatly, but the total acid amount is less than liquid acid. In addition, deactivation due to coking of the catalyst shortens its service life, which causes difficulties for industrial applications. In this paper, alumina and metatungstate were added simultaneously in order to promote the acidity and stability of SO4/ZrO2 catalysts which were prepared by optimized impregnation-deposition method. The catalytic activities of composite solid superacid SO4/ZrO2-Al2O3-WO3 (abbreviated as SZWA) catalysts were evaluated by esterification reaction.

Experimental

Catalyst preparation

The ZrO2-Al2O3 was prepared by coprecipitation method. The precursor was prepared from an aqueous solution of ZrO(NO3)2*2H2O and Al(NO3)3*9H2O by dropwise addition of NH4OH (25-28 wt%) until a pH of 9.0 was reached. After stirring for 0.5 h, the suspension liquid was aged at different temperature. Then the precipitate was washed with deionized water, filtered, and dried at 110AoC for 12 h. The resulting sample was ground using a mortar and pestle to 100 mesh. The powder was impregnated with solution of ammonium metatungstate, then filtered and dried at 110 AoC for 12 h. Finally, the samples were soaked with (0.5 M, 1.0 M or 1.5 M) (NH4)2SO4 (H2SO4) solution (15 mL*g-1 catalyst) for 24 h, filtered, dried and calcined at 500 AoC for 3 h.

Catalyst characterization

N2 adsorption measurements. The BET surface area of all the catalysts was determined using Beckman Coulter SA 3100 and nitrogen was the analysis gas. Prior to analysis, each catalyst was heated to 200 AoC and outgassed to a final vacuum of 10-5 Torr.

IR measurements. Prior to each IR experiment, compressed sample placed in the IR cell was first subjected to evacuation treatment at 400 AoC for 3 h, followed by saturated adsorption of pyridine at room temperature for 1 h and subsequent removal of physiosorbed pyridine under vacuum at 150 AoC overnight. IR spectra of solid samples were recorded on NICOLET 5700 IR spectrophotometer by scanning from 4000 to 400 cm-1 (resolution of 4 cm-1); 32 repeated scans were accumulated.

Thermogravimetric analysis (TGA). The TGA was carried out on a TA SDTQ 600 apparatus with the purge gas (nitrogen) flowrate of 30 mL*min-1 and the heating rate of 10 AoC /min from 50 to 900AoC.

Powder X-ray diffraction (XRD) measurements. Structural characterization of the powders was performed by a XD-3A X-ray diffractometer (SHIMADZU, Japan), using Cu K[alpha] radiation in the range 20-90 degree (2I,) at a scanning speed of 10 deg/min. The X-ray source was an anode operating at 35 kV and 15 mA, filtered with nickel foil (I>> = 1.5418 A).

Hammett values (H0) were measured by a self-designed experimental apparatus using flow indicator method 12.

Catalytic tests

The experiments were performed in a three-neck round bottom flask equipped with an efficient mechanical stirrer, a thermometer, a water separator and a tap water-cooled reflux condenser. The esterification with 0.4 g composite solid superacid SZWA catalyst were carried out with a acetic acid/n-butanol molar ratio of 1 : 2 at 105-110 AoC for 3 h. The liquid level of the water separator was controlled as high as the branch pipe. The reaction progress was followed by measuring the amount of collected water. After completion of reaction, the ester layer in the water separator and the reaction solution were mixed in the separatory funnel and successively washed by distilled water, 10 wt% sodium carbonate solution, distilled water. The mixture dried by anhydrous MgSO4 was distilled under 70 mmHg vacuums. Finally, the main product was obtained by collecting the distillate at 52-55AoC.

The balanced stoichiometric reaction between acetic acid and butanol was expressed as follows:

CH3COOH+CH3CH2CH2CH2OHaCH3COO CH2CH2CH2CH3+H2O

The extent of conversion for esterification of acetic acid and n-butanol was calculated by the following equation:

Esterification yield = (1-acid value/initial acid value) x 100 %

=[1-VNaOH(after reaction)/VNaOH(before reaction)] x 100 %

Where "VNaOH" is the volume of the standard NaOH solution (0.1 molL-1) consumed by titrating the reactant mixture. The acid value is measured by the GB-1668-81 standard method. The obtained values for esterification yield were in accordance with the values obtained based on collected water.

Results and Discussion

Effect of aging temperature

The SZWA catalysts were prepared at three different aging temperature (-10 AoC, 0 AoC, 20 AoC) and other conditions were the same as follows: soaked with 1.0 M (NH4)2SO4 and calcined at 550 AoC. The esterification yields and the specific surface area calculated by the BET method are presented in Table-1.

Table-1: The specific surface area and esterification yield of SZWA prepared at different aging temperature.

###Catalyst

###SZWA###SZWA###SZWA

###(-10 AoC)###(0 AoC)###(20 AoC)

###esterification yield (%)###98.9###72.6###57.3

specific surface area (m2*g-1)###246###105###34

It can be seen from Table-1, the catalytic activity of the catalyst aging at -10AoC was higher for esterification of acetic acid and n-butanol. The esterification yield was up to 98.9 %. The number of active sites of catalyst is an important factor affecting the catalytic activity, while the specific surface area is one of the determinants of active sites. The results in Table-1 show that a maximum in the specific surface area is obtained when SZWA is aged at -10 AoC. A sharp decrease in surface area was observed when the aging temperature was increased from -10 AoC to 20 AoC. For example, SZWA catalyst (-10 AoC) has a specific surface area of 246 m2*g-1 as compared to 34 m2*g-1 for SZWA catalyst (20 AoC).

From Fig. 1, The sizes of catalyst particles aging at -10 AoC are less than 100 AoC um, and the larger particle sizes can be observed in Fig. 1B and Fig.1C which the catalysts were prepared at higher aging temperatures. The good pore structure can be observed from the catalyst aging at -10 AoC, the specific surface area could be increased greatly and the catalytic activity was improved. When the catalysts were prepared at higher aging temperature, parts of the catalyst surface is smooth and fewer pores can be found. It seems that the lower aging temperature could lead to the smaller and porous crystal particles, the possible reason is that the water dehydrated from precursor could form ice cage-like crystalline compound because of the hydrogen bonds between water molecule, the metal oxide could be embedded in the ice cage, the further diffusion and aggregation of precursor could be restrained, and the smaller crystal particle could be formed finally.

Effect of Al2O3 doped on SO 2-/ZrO type solid superacid

Solid superacid SO 2-/ZrO (SZ) and SO 2-/ZrO -Al O (SZA) were prepared by the impregnation-deposition method. The esterification of acetic acid and n-butanol was used as a reaction probe to study the catalytic activity of SZ and SZA. Preparation conditions and the esterification yields are shown in Table-2, and H0 values measured by Hammett indicator are shown in Table-3.

Table-2: The esterification yield and specific surface area of different catalyst samples.

###Al(NO3)3*9H2O###esterification###specific surface

catalyst

###(g)###yield (%)###area (m2*g-1)

###SZ###0###91.6###53

###SZA###14.8###95.4###86

Table-3: The acidity characterization of different catalyst samples.

###p-nitrotoluene###m-nitrotoluene###1-chloro-4-nitrobenzene###1-chloro-3-nitrobenzene

###(H0=-11.35)###(H0=-11.99)###(H0=-12.70)###(H0=-13.16)

SZ (white)###+###+-###-###-

SZA (white)###+###+###+###+

As can be seen from Table-2, the esterification yield of SZA was 94.8 % as compared to 90.9 % for SZ. The specific surface area increased from 53 to 86 m2*g-1 with SZ modified by Al2O3. Thus, SZA has more acid sites and larger acidity which are consistent with H0 values included in Table-3. Apparently, both SZ and SZA are solid superacids for the Hammett acidity function H0 a$? -11.99 (corresponding to 100 % sulfuric acid). Consequently, adding Al2O3 could increase the surface area and acidity values of SZ catalysts. Hwang et al., reported that doping of alumina to SZ increased the extent of sulfur retention on the surface of zirconia.14

Influence of WO3 doped on SO42-/ZrO2 type solid superacid.

Fig. 2 shows the IR spectra of SZ modified by different metal ions. All the spectra exhibited a band at 1340-1440 cm-1, which was attributed to the stretching vibration of S=O double bond. After adding WO3 (SZW, SZWA), the S=O absorption band broaden together with the tail on the low frequency suggest the S=O stretching band has more than one component. The component of the S=O stretching band at high frequency was assigned to pyrosulfate, while the component at low frequency was attributed to monosulfates on the surface 15. The obvious absorption bands at 1038, 1090 and 1180 cm-1 were due to the S-O bond which seemed to be very sensitive to sulfate content 16. It can be seen that by adding WO3, S-O absorption peaks at 1000-1200 cm-1 was strengthened. It indicated that the introduction of WO3 enhanced the sulfate content of SZWA catalysts which was benefit to acidity. And the trend was consistent with the esterification yields of the catalysts which were shown in Table-4.

Table-4: The esterification yield by using different catalysts.

###Catalyst###SZ###SZA###SZW###SZWA

esterification yield###90.9 %###94.8 %###92.3 %###98.9 %

The results of the thermogravimetric study are shown in Fig. 3. The first weight loss occurred at approximately 50 AoC up to 200 AoC, which was due to the loss of water. The second region of weight loss occurred between 200-400 AoC by the loss of structural water of hydrous oxides. After that, a gradual weight loss was observed up to 600 AoC and was attributed to surface sulfate decomposition.

It was observed that the crystallizing temperature of SZ was around 435 AoC. After adding Al2O3, the crystallization peak moved toward high temperature. Consequently, the crystallizing temperature of SZA increased to 450 AoC. It is important to note that SZA has a significant weight loss at 100-400 AoC. It can be explained by the more water loss of catalyst surface and hydrous oxides after adding Al2O3. And surface sulfate of SZA began to intensively decompose at less than 550 AoC. The zirconia crystallization peak of SZWA emerged until 600AoC et al and no apparent weight loss of surface sulfate was observed. It is of interest to note that the 2-introduction of WO3 can mitigate the SO4 anions loss of high temperature and stabilize the active structure 17.

Therefore, SZWA exhibits strong acidity which is good agreement with the results determined by IR spectra. Arata and Hino reported the similar role by incorporating WO3 into Zr-hydroxides under certain preparation conditions 18.

Effect of calcination temperature

In order to assess the importance of calcination temperature, SZWA catalysts were prepared by calcined at 400, 500 and 650 AoC, respectively. The results of esterification of acetic acid and n-butanol are shown in Table-5.

Table-5: The effect of calcination temperature on esterification yield.

###Catalyst###SZWA(450 AoC)###SZWA(550 AoC)###SZWA(650 AoC)

esterification yield###87.6 %###98.9 %###91.4 %

The X-ray diffraction patterns of SZWA catalysts calcined at 400, 500 and 650 AoC are given in Fig. 4. Following calcination at 450 AoC, the SZWA catalyst was mainly monoclinic phase, whereas the catalyst calcined at 550 AoC predominantly contained the tetragonal phase. When calcined at 650 AoC, a gradual increase of the monoclinic phase started and the diffraction intensity of tetragonal phase decreased.

The catalyst calcined at 550 AoC mainly exists in tetragonal phase owing to the interaction between sulfate anions and metal oxides (ZrO2, WO3 and Al2O3). The presence of SO4 needs higher thermal energy requirement for the removal of OH-ions and the incorporated metal oxides stabilize the SO4. In turn, the effect between lattice oxygen and Zr4+ is strengthened by the induction of S=O covalent bond and the conjugation of S-O bond. Thereby, the rearrangement among the lattice atoms is difficult and then the transformation of tetragonal to monoclinic phase becomes difficult 19. In contrast, the decrease of tetragonal phase for SZWA calcined at 650 AoC is due to the SO4 decomposition. Sulfur loss of the catalyst results the acidity and catalytic activity decreased, which is consistent with the esterification yield listed in Table-4.

It is seen from Table 5 and Fig. 4 that the catalyst calcined at 550 AoC has a tetragonal phase and higher acidity. Increased calcination temperature, a more complete tetragonal phase can be obtained. But too high temperature (650 AoC) causes the loss of surface sulfate. Thus the optimum temperature should be close to the critical decomposition temperature of the solid catalysts.

The IR spectra of pyridine adsorbed samples are shown in Fig. 5. All samples showed a broad peak at around 3400 cm-1 assigned to the OH-stretching mode of water associated with zirconia, the broadness of which was attributed to the hydrogen bonding effect. It was observed that SZWA calcined at 550 AoC had the strongest S=O stretching vibration (1340-1440 cm-1). A band at the frequency of 1540 cm-1 represents the Bronsted acid sites. On the other hand, the absorption bands at 1450 and 1630 cm-1 correspond to different vibrations of pyridine chemisorbed on Lewis acid sites 20. The area of Lewis acid is much larger than that of Bronsted acid, because zirconia was transformed to metastable tetragonal phase calcined at 550 AoC. While the tetragonal phase zirconia is easier to combine with SO42-, resulting in S=O stretching vibration strengthened.

The SZWA calcined at 450 AoC contained parts of inorganic chelating bidentate sulfate ions. When the calcination temperature reached 550 AoC, the absorption bands at 1360 cm-1 intensively increased and the absorption peak at 1450 cm-1 appeared, indicating that the presence of bidentate chelating sulfate ions. Thus, the enhanced covalent of S=O bond increased the acidity of SZWA catalyst. However, after calcination at 650 AoC, the absorption bands at 1180, 1090 and 1038 cm-1 was observed that inorganic chelating bidentate sulfate ions formed 21. Subsequently the surface SO42-reacted with metal oxide supports to form sulfates. Therefore, the catalytic activity of SZWA was low, which was in agreement with the esterification yield shown in Table-4. Thus, the optimum calcination temperature is 550 AoC.

Effect of impregnation solution and the concentration of the solution

The SZWA catalysts were prepared by changing the impregnation solution and its concentration. The results of probe reaction are shown in Table-6 and the acid strength determined by Hammett indicator method is given in Table-7.

Clearly, the esterification yield of catalysts promoted by (NH4)2SO4 was higher compared with the catalysts impregnated by H2SO4, of which the esterification yield was less than 95 %. The results in Table-6 show that a maximum in the esterification yield is obtained when using 1.0 M (NH4)2SO4 as impregnation solution. The corresponding acid strength of catalyst SZWA5 in Table-7 is also the largest (H0 < -16.0). When impregnated with 1.5 M (NH4)2SO4, the esterification yield and acid strength were significantly reduced. Therefore, the impregnation concentration may be an important variable of catalyst preparation. This is because using 0.5 M (NH4)2SO4 is insufficient to provide necessary SO4 2-to form superacid; while using the high concentration (1.5 M), NH3 the decomposition product of NH4 is nucleophile inhibiting the formation of Bronsted acid sites.

In addition, SO4 ions easily combine with metal ions to form sulfates covering the surface of the solid catalyst. So that, the acid strength and the catalytic activity of the catalyst are lower. Therefore, choosing 1.0 M (NH4)2SO4 as impregnation solution is beneficial to improve the acidity and catalytic activity of the catalyst.

Catalytic performance and reusability of composite solid superacid SZWA

The comparative experiment of SWZA catalyst and the concentrated sulfuric acid catalyst was carried out to study the catalytic performance of SWZA catalyst for the n-butyl acetate esterification reaction at the same experimental condition. The results showed that the esterification yield of SWZA catalyst was 98.9 %, and that of the sulfuric acid catalyst was 98 %. It can be found that the catalytic performance of SWZA catalyst is better than that of the sulfuric acid catalyst, and the SWAZ catalyst is easier to separate from the reaction product.

The n-butyl acetate esterification yield in consecutive five experiments with recycled SWZA catalyst was described in Fig. 6. It was found that the esterification yield in Fig. 6 was 98.9 %, 91.3 %, 82.1%, 73.6 % and 65.4 % for five consecutive runs. In the successive runs, its activity dropped at every reuse of the catalyst. In order to investigate the reusability of the catalyst, the content of sulfate groups in the reaction product mixture was detected by high temperature calcination-coulomb titration method for every recycle tests. According to the results, a small quantity of the sulfate was detected in the product mixture; these indicate that the leaching of the sulfate is the important reason for the catalyst deactivation.

Table-6: The effect of impregnation concentration on esterification yield.

###catalyst###SZWA1###SZWA2###SZWA3###SZWA4###SZWA5###SZWA6

Impregnation concentration (molL-1)###0.5 M H2SO4###1.0 M H2SO4###1.5 M H2SO4###0.5 M (NH4)2SO4###1.0 M (NH4)2SO4###1.5 M (NH4)2SO4

###esterification yield (%)###92.6###93.9###93.1###97.4###98.9###90.3

Table-7: The acidity characterization of different SZWA samples.

###indicatora

catalyst###1-chloro-4-nitrobenzene###1-chloro-3-nitrobenzene###2,4-dinitrotoluene###1-fluoro-2,4-dinitrobenzene###1,3,5-trinitrobenzene

###(H0=-12.70)###(H0=-13.16)###(H0=-13.76)###(H0=-14.52)###(H0=-16.04)

SZWA1###+###+-###-###-###-

SZWA2###+###+-###-###-###-

SZWA3###+###+###+-###-###-

SZWA4###+###+###+###+-###-

SZWA5###+###+###+###+###+

SZWA6###+###+###+###-###-

Conclusions

The results show that the optimal conditions for preparation of composite solid superacid SZWA are as follows: the mass ratio of WO3/ZrO2 is 1 : 1, aging temperature is -10AoC, the mass ratio of Al2O3/ZrO2 is 3.5: 1, impregnation concentration is 1.0 M (NH4)2SO4 and the calcination temperature is 550AoC. The characterization results indicate that, the lower aging temperature is the most important for forming crystal centers on catalyst surface, and getting smaller regular crystal particles; therefore, the bigger specific surface can be available. Suitable calcination temperature can improve the conversion of crystal types, which include tetragonal phase and monoclinic phase. It shows a strong orientation action for formation of useful crystal type by adding W, which also delay the crystallization of ZrO2, stable tetragonal phase, and depress the loss of SO4.

It is beneficial to increase the Lewis acid sites on catalyst surface by adding Al2O3, and the catalytic activity can be improved strongly. When the molar ratio of acetic acid to n-butanol is 1 : 2, reaction time is 3 h, the n-butyl acetate esterification yield is 98.9 % by using 0.4 g catalyst. The stability of the modified SWZA catalyst is better than that of traditional SO4/ZrO2 solid superacid catalyst.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21576049), the Fundamental Research Funds for the Central Universities (No. 2242016K40082).

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Author:Huang, Kai; Lu, Keqiang
Publication:Journal of the Chemical Society of Pakistan
Article Type:Technical report
Date:Jun 30, 2018
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