Preparation of a Composite of Sulfated Zirconia/Metal Organic Framework and its Application in Esterification Reaction

Abstract

A porous metal-organic framework (MOF), MIL-101, was synthesized in the presence of sulfated zirconia (SZ) to produce acidic SZ/MIL-101 composites for the first time. The composites were characterized with XRD, nitrogen adsorption, FT-IR, scanning electron microscope, chemical analysis and so on. The composites (SZ/MIL-101s) were successfully applied in a liquid-phase esterification for a high yield of ester. This catalytic result of SZ/MIL-101, compared with that of pure SZ or MIL-101 (showing a negligible yield of ester), suggests that the SZ in the composite is highly active in the acid catalysis probably because of the well-dispersed active species of SZ. Moreover, the esterification is catalyzed in heterogeneous mode as confirmed by negligible esterification after filtration of the catalyst. Finally, microwaves can be efficiently applied both in the synthesis of the composites and the esterification reaction to accelerate the two processes of synthesis and esterification by about 5 times.

Introduction

Remarkable progresses on porous materials have been achieved because of the developments in metal-organic frameworks (MOFs).1-10 The importance of MOF-type materials is due to their huge porosity, easy tunability of their pore size/shape and potential applications.1-10 Although MOF materials themselves show very promising physical and chemical properties in various aspects, their properties and applications can be improved even further by modifying the MOFs including making composites with suitable materials.10-12 Even though MOF composites are relatively new in concept, a few reports have been published recently with successful syntheses and promising applications. By composing MOFs with suitable materials, the synthesis kinetics,13-15 porosities,16-20 stabilities21,22 and potential appli-cations of MOFs can be improved to a greater extent.10-12

Several materials such as polyoxometalate (POM),21,23-26 iron oxide27-29 and graphite oxide (GO)16-20 have been used for the formation of MOF composites. POM/MOF composites have been used in various fields including acid catalyses, oxidation catalyses, adsorption/decontamination and so on. Iron oxide/MOF composites have been interesting because of the magnetic separation of the composite materials. GO/ MOF composites are also interesting because of their improved porosity; and therefore, the composites have been used in various adsorptions.16-20 So far, however, to the best of our knowledge, there is no report on the composite formation between SZ and MOF even though SZ is a very useful material in catalysis.30

SZ has been very useful for acid catalyses because of the super-acidity of the material.30 However, SZ has a well-known drawback of low porosity.30 To overcome this di-sadvantage, a few methods such as supporting on highly por-ous materials,31,32 imparting mesoporosity,33 and making nanoparticles34 have been developed. However, a new method to overcome this disadvantage is still needed.

To investigate the applicability of SZ/MOF composites, esterification (ES) of oleic acid with methanol has been carried out because ES is an important class of organic reaction to produce esters from organic acids and alcohols. Esters have been widely used as fragrances, lubricants, plasticizers, drugs, pharmaceuticals and solvents.35,36 So far, esterification reactions have been carried out by the help of various catalysts including homogeneous acid catalysts such as H2SO4, HCl, and H3PO4. Recently, heterogeneous solid acids have attracted much attention for esterification reac-tions in order to overcome various problems (such as separa-tion, environmental, corrosion, and reusability) caused by homogeneous catalysts.

Microwaves (MWs) have been widely used in organic reac-tions 37 and in the syntheses of materials including porous materials to take advantage of rapid reactions. Several ad-vantages such as rapid synthesis,38,39 phase-selectivity40,41 and decreased size42 have been reported in the synthesis of various porous materials. A rapid reaction is well-known advantage of MW irradiation in many reactions.37 Therefore, it is interesting to utilize MWs both in the synthesis of the SZ/MIL-101 composites and in the ES reaction.

Among the numerous MOFs reported so far, one of the most topical solids is the porous chromium-benzenedicarbox-ylate called MIL-10143 (MIL stands for Material of Institute Lavoisier). MIL-101, Cr3O(F/OH)(H2O)2[C6H4(CO2)2], has a cubic structure and a huge pore volume of 1.9 cm3/g.43 MIL-101 is a very important material due to its mesoporous structure and huge porosity, and is widely studied for various applications. In this study, synthesis of SZ/MIL-101 com-posites and esterification with those composites were per-formed for the first time. The synthesized composites were characterized with XRD, nitrogen adsorption, FT-IR, SEM and chemical analysis. Moreover, the composites, because of the acidic SZ, can be applied in esterification, suggesting the composites formation is a novel way to utilize the acidic SZ in heterogeneous catalyses. Finally, MWs can be applied successfully for both in the synthesis of the composites and in the esterification reactions in order to complete such reactions in a short amount of time. It is suggested that the applicability of MOF materials can be improved largely by composing MOFs with suitable materials such as SZ.

 

Experimental

Chemicals and Synthesis of the Adsorbents. All of the chemicals such as zirconium (IV) hydroxide (Zr(OH)4, Sigma Aldrich, 97%), terephthalic acid (TPA, Sigma Aldrich, 99%), chromium nitrate nonahydrate (Samchun chemicals, Cr(NO3)3·9H2O, 99%), methanol (OCI chemicals, 99.9%), sulfuric acid (OCI chemicals, 95%) and oleic acid (Junsei chemicals, EP) utilized in this study were commercial pro-ducts and used without any further purification.

MIL-101 was synthesized from Cr(NO3)3·9H2O, TPA, and deionized water similar to a previously described method,43 and the molar composition of the reactants was 1.0 Cr(NO3)3·9H2O:1.0 TPA: 300 H2O. Thirty grams of precursor were loaded in a Teflon-lined autoclave, put in a preheated (at 210 °C) electric oven, and maintained for 10 h. After the reaction, the autoclave was cooled to room temper-ature and solid green-colored products were recovered by filtration. To remove the unreacted TPA, the as-synthesized MIL-101 was further purified by treatment with hot water, ethanol and NH4F solution following a previously described method.44 The purified MIL-101 was dried overnight at 100 °C and stored in a desiccator after cooling to room temper-ature.

Sulfated zirconia was prepared as previously described.45 Briefly, 1.0 g of zirconium (IV) hydroxide with 20.0 mL of aqueous 0.1 M H2SO4 was vigorously stirred at room temper-ature for 5 h. The solid product was separated by centrifu-gation and dried at 100 °C overnight. The obtained solids were ground into fine powder and calcined at 600 °C for 5 h.

To synthesize the SZ/MIL-101 composites, the so-called method of ‘bottle-around-ship’ or ‘templated synthesis’11 was utilized. The as-prepared SZ was introduced in a specific amount in the precursor mixtures of the MIL-101. The rest of the synthesis procedure is the same as the synthesis of the pure MIL-101. SZ/MIL-101 composites were named as n%SZ-MIL-101 where n is the weight percentage of the SZ compared with the weight of the MIL-101 that can be pro-duced (based on the maximum yield) through hydrothermal synthesis at 210 °C. Preparation of the SZ/MIL-101 com-posites under microwave irradiation was done similarly with a Teflon reactor in a microwave oven (MARS-5, CEM).

Characterization. The X-ray powder diffraction patterns were obtained with a diffractometer D2 Phaser (Bruker, with CuKα radiation). The crystallization curves of the SZ/MIL-101s were obtained by the relative XRD intensity of a SZ/MIL-101 with a fully crystallized SZ/MIL-101 as a standard material. The nitrogen adsorptions of the composites were obtained at −196 °C with a surface area and porosity analy-zer (Micromeritics, Tristar II 3020) after evacuation at 150 °C for 12 h. FT-IR spectra were recorded with a resolution of 2.0 cm−1 by using a Jasco FT-IR-4100 equipped with an ATR accessory. SEM images of the SZ/MIL-101s, SZ and MIL-101 were obtained with a scanning electron microscope (Hitachi, S-4300). Analyses of the sulfur content of the cata-lysts were done using an elemental analyzer (Thermo Fisher, Flash-2000) equipped with a TCD detector. The acid density of the prepared samples was measured through acid base back titration using phenolphthalein as an indicator. In brief, an excess amount of (20 mL) 0.01 M sodium hydroxide aqueous solution was added to a catalyst (0.1 g) and the mixture was stirred for 60 min at room temperature. After centrifugal separation, the supernatant solution was titrated by 0.01 M hydrochloric acid aqueous solution.46

General Procedures for the Esterification. The catalytic esterification of oleic acid with methanol was carried out under microwave irradiation using a Teflon reactor in a microwave oven (MARS-5, CEM). Oleic acid (1 mL), 10 mL of methanol and 0.10 g of catalyst were added to a 100 mL Teflon reactor, and maintained at 100 °C for a predeter-mined time. The reactions were also conducted further after the filtration of the catalyst at the reaction temperature. After the reaction, the catalyst was separated and the product was analyzed by a GC (IGC 7200 (DS science, Korea) equipped with a FID detector. Esterification with conventional electric (CE) heating was done similarly using a round bottomed flask (50 mL) under continuous stirring.

 

Results and Discussion

Synthesis of the Composites. Figure 1 shows the XRD patterns of the MIL-101 and SZ-MIL-101s synthesized with different amounts of SZ along with pure SZ. The XRD peaks of all the composites were very similar to the patterns of the pure MIL-10143,44 although the intensity was low for the composites with a high SZ content (5-10%) in the syn-thesis. The low intensity might be due to the relatively low concentration of MIL-101 or decreased synthesis rate in the presence of high SZ. The XRD patterns of the crystalline SZ having a tetragonal phase of zirconia45 were also observed in the SZ/MIL-101s which is probably due to the preservation of the zirconia structure in the MIL-101 even after the pre-paration of the composites.

Several attempts have been tried to accelerate the syn-thesis of porous materials including MOFs.10 because rapid syntheses have various advantages. Microwave38,39 or ultra-sound-assisted syntheses42 and dry gel conversion methods47 are the most prominent of them. Figure 2 shows the relative crystallinity of 0.5% SZ/MIL-101 with different times in the syntheses using conventional electric and microwave (MW) methods. Compared with CE heating, MW accelerated the synthesis of SZ/MIL-101 by around 5 times similar to the accelerations in the syntheses of several MOF materials.39 Therefore, MW syntheses have the advantage of fast crystal-lization for both the virgin MIL-10139 and SZ/MIL-101 composite.

Figure 1.XRD patterns of SZ/MIL-101s.

Figure 2.Crystallization curves of 0.5%SZ/MIL-101 with micro-wave and conventional electric heating.

Characterization of the SZ/MIL-101 Composites. Figure 3(a) shows the changes in sulfur content and acid density of the SZ/MIL-101s depending on the content of the SZ that was used in the synthesis of the SZ/MIL-101. By increasing the SZ in the synthesis up to 1 wt %, the sulfur content and acid density increase monotonously with the introduced SZ content; however, further increases in SZ lead to only a slight increase in these two properties, showing that the SZ and MIL-101 efficiently form the composite with only a low concentration of SZ. As shown in Figure 3b, the acid density depends linearly on the sulfur content of the SZ/MIL-101, showing that the SZ has acidic properties, similar to the virgin SZ, which are useful for acid catalyses. Therefore, it can be assumed that the SZ of the SZ/MIL-101 may be similar to the virgin SZ in acidity which is suitable for acid catalysis.

Figure 3.(a) Sulfur content and acid density of SZ/MIL-101s depending on the amount of SZ used in the synthesis of SZ/MIL-101s; (b) Change of acid density on the sulfur content of SZ/MIL-101s.

Figure 4(a) shows the nitrogen adsorption isotherms of the MIL-101 and composites (SZ/MIL-101s) synthesized from the reactants with different amounts of SZ. Figure 4(b) shows the BET surface area of the different composites with differ-ent sulfur contents (or amounts of SZ) obtained by elemental analysis of the SZ/MIL-101 composites. The surface area decreased monotonously as the sulfur content increased in the composites, suggesting the SZ is composed with MIL-101 to decrease the porosity of the virgin MIL-101. Even though the amount of adsorbed nitrogen or surface area is decreased linearly with the sulfur content of the SZ/MIL-101, the shape of the nitrogen isotherms does not change appreciably with the increase in SZ content in the SZ/MIL-101s. This result and the linear decrease of the surface area with the sulfur content might be due to the partial blocking of the porous structure of MIL-101 with SZ through the preparation of the composites. However, the surface areas of the SZ/MIL-101s are appreciable even after the com-posites preparation, suggesting possible applications of the composites in various fields including catalyses and adsorptions. Typical textural properties (such as the BET surface areas and total pore volumes), along with the sulfur contents and acid densities, of the prepared SZ/MIL-101s are summarized in Table 1.

Figure 4.(a) Nitrogen adsorption isotherms of SZ/MIL-101s; (b) Change of surface area of SZ/MIL-101s on the sulfur content of SZ/MIL-101s.

Figure 5 shows the FT-IR spectra of the SZ/MIL-101s and the virgin MIL-101. Different to the virgin MIL-101, the SZ/MIL-101s show absorbance bands at 1166 and 1018 cm−1. Moreover, the absorbance of the two bands increases with the increasing SZ content of the SZ/MIL-101s (up to 5%SZMIL-101). It has been reported that the bands between 1200 and 870 cm−1 are attributed to the stretching vibrations of S-O.48 Therefore, based on the FT-IR, it can be confirmed that SZ exists in the MIL-101s which is in agreement with the results of the elemental analysis for sulfur content and acid site density. The SEM images (Figure 6) of the SZ/MIL-101s show that the morphologies of the SZ/MIL-101s are homogeneous even with the composing of MIL-101 with SZ. Moreover, the size of SZ is very small which cannot be detected by SEM under a magnification of X50,000.

Table 1.Textural property, sulfur content and acid density of SZ/MIL-101s

Figure 5.FT-IR spectra of SZ/MIL-101s.

Esterification Experiments. The prepared SZ/MIL-101 catalysts were applied in the esterification (ES) of oleic acid with methanol. As shown in Figure 7, the yield of methyl oleate (MO) increased with an increasing reaction time at 100 °C under MW irradiation, and after a 30 min reaction, the yield was around 87%. So far, the MO yields over conv-entional solid acid catalysts have been 67-95%,49-52 suggest-ing the competitiveness of the SZ/MIL-101 catalyst in the ES for MO. Interestingly, the MO yield over pure SZ was negligible, confirming the necessity of dispersion or sup-porting of SZ for catalysis similar to reported results.30-34 Moreover, the virgin MIL-101 does not have any activity because of a lack of acidity. The ready conversion of oleic acid into MO over SZ/MIL-101 suggests the beneficial effect of composing of SZ in a MOF such as MIL-101. This remarkable activity of SZ/MIL-101, compared with that of the pure SZ or MIL-101, might be due to the dispersed SZ which is suitable for catalysis. Similar beneficial effects of confined active catalytic materials in nanoporous materials have been reported previously a few times.53-56

The esterification reaction was further done (without any solid catalyst) after filtration under the same reaction condi-tion in order to understand the heterogeneity of the catalysis. As shown in Figure 7, there was a negligible increase in the yield of MO after the filtration of the catalyst, confirming the heterogeneous catalysis with the SZ/MIL-101 catalyst and the stability of the catalyst under the ES reaction condi-tion.

Figure 8 shows the MO yield with the reaction time with microwave irradiation and conventional electric heating under the same reaction condition. As shown in the Figure, the esterification with MW is quite fast compared to the reaction with conventional electric heating. About 5 times of acceleration were observed with MW heating as a yield of around 95% was obtained at 60 and 300 min with the MW and CE heating, respectively. Therefore, MWs were found very effective in the ES reaction with the SZ/MIL-101 catalyst, similar to the synthesis of SZ/MIL-101 (see above), the syntheses of porous materials 39 and general organic syntheses.37

Figure 6.SEM images of MIL-101, SZ/MIL-101s and SZ: (a) MIL-101; (b) 0.5%SZ/MIL-101; (c) 1%SZ/MIL-101; (d) 5%SZ/MIL-101; (e) 10%SZ/MIL-101; and (f) SZ.

Figure 7.Change of yield of methyl oleate in the esterification of oleic acid with methanol with 0.5%SZ/MIL-101, MIL-101 and SZ catalysts (at 100 °C under microwave heating).

 

Conclusion

In this study, acidic composites of SZ/MIL-101 have been successfully prepared for the first time, and applied in the esterification (ES) of oleic acid with methanol. The com-posites are highly porous, compared to the virgin sulfated zirconia, even though the surface area of the composites was decreased with the SZ content of the composites. We have shown that the SZ/MIL-101 composite is highly active in ES, different from the inactive pure SZ and MIL-101. The catalysis with the SZ/MIL-101 is heterogeneous in nature, suggesting the reusability of the catalyst in ES. MWs have been successfully used both in the synthesis of the com-posite and in the ES reaction. The MW method was very beneficial to complete the two processes in a reduced time (around 20% of the processing time compared to electric heating) because of rapid reactions under microwaves. Finally, it is suggested that composing MOFs with func-tional materials such as acidic SZ is a competitive way to widen the applications of highly porous MOFs.

Figure 8.Change of yield of methyl oleate in the esterification of oleic acid with methanol with 0.5%SZ/MIL-101 catalyst under microwave and conventional electric heating.