Article pubs.acs.org/IECR
Sulfated Iron Oxide: A Proficient Catalyst for Esterification of Butanoic Acid with Glycerol Kamalpreet Kaur,† Ravinder Kumar Wanchoo,† and Amrit Pal Toor*,†,‡ †
Dr. S.S.B. University Institute of Chemical Engineering and Technology, Panjab University, Chandigarh, 160014, India Energy Research Centre, Panjab University, Chandigarh, 160014, India
‡
S Supporting Information *
ABSTRACT: Sulfated iron oxide catalysts, prepared by a simple and solvent free method, were studied in the esterification of butanoic acid with glycerol. The effect of the series of preparation parameters such as mixing time, sulfate loading, and calcination conditions, i.e., temperature and its duration, were explored. The catalyst containing 6 wt % sulfate loading with 40 min of mixing time, calcined at 500 °C for 7 h, performed optimally for the conversion of butanoic acid. The characterization of catalysts was carried out by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC)-thermogravimetric analysis (TGA), zeta potential, acidity measurements, and leaching tests. The performance of the catalyst has revealed that it is a low cost, stable, efficient heterogeneous catalyst for this reaction when compared with ion exchange resins and other heterogeneous catalysts. Furthermore, the catalyst can be easily separated by filtration and found to be reusable and active.
1. INTRODUCTION The concept of green chemistry has increased the awareness of preparation and utilization of numerous competent heterogeneous catalysts in recent times. These are acknowledged as excellent substitutes for homogeneous catalysts which have the main drawbacks of long reaction times and reactor corrosion problems; no separation of reaction products and catalyst; and environmental inconveniences.1,2 Some of the heterogeneous catalysts such as ion exchange resins and heteropolyacids have several drawbacks. Ion exchange resins have deprived thermal stability, bad regeneration capability, and less surface area which further leads to low turnover frequency. In the case of heteropolyacids, lack of accessibility and efficiency of catalyst have been reported. Thus, the concern toward the advancement of heterogeneous catalysts, especially by green processes, has led to the development of solid acid catalysts that should be economical and environmentally friendly. In this perspective, metal oxides have attracted major attention owing to their effectiveness of strong surface acidity and high activity at low temperatures. Additionally, sulfation refurbishes metal oxides into magnificent catalysts by enhancing their surface area and acidity and ensuring superior outcomes at reduced temperature and pressure conditions. Until now, various methodologies, i.e., sol gel,3 precipitation,4 mixing5 methods, etc., have been employed to prepare sulfated metal oxide catalysts. A range of sulfated metal oxides has been investigated such as sulfated zirconia, tin oxide, titanium oxide, and mixed oxides.6−11 Among these, the employment of catalysts derived from iron oxides has increased due to their availability, nontoxicity, low cost, and ability to catalyze reactions at low temperature.12 However, previous research has diplayed the obstacles of its preparation due to the lengthy procedure and resources required, use of sulfuric acid, high amount of sulfate loading, leaching of sulfate ions, and low yield of desired products.12−14 The approach for the evaluation of different iron precursors for © XXXX American Chemical Society
the preparation of sulfated iron catalyst was described by Brown et al.;12 however, some drawbacks existed including long preparation time (3−4 days), lower conversions, and the use of sulfuric acid for sulfation. Subsequently, Magnacca et al.13 employed a multistep approach in the preparation of sulfated iron oxide with varying sulfate loading from 2 to 8 wt %. The catalyst prepared with high sulfate loading was favored. Overall, there was a lack of simplicity in the proposed protocol. Recently, Shi et al.14 has presented the deactivation mechanism of sulfated iron oxide catalyst in the esterification of acetic acid and n-butanol. Nevertheless, again, the long preparation procedure, use of more resources, use of high catalyst amount, and leaching of catalyst have made it unfeasible to utilize extensively. Therefore, it is important to establish the relationship among catalyst preparation parameters and its performance. Additionally, efforts should be made to produce catalyst by a solvent free synthesis from the point of view of green chemistry. Hence, to accomplish the requirements, we have prepared low cost, efficient, stable heterogeneous catalysts for esterification of butanoic acid with glycerol. Nowadays, glycerol is among the abundantly produced raw materials by numerous biodiesel industries. However, the selective processes to convert accessible glycerol to a high value-added chemical or product are less developed.15−18 Among selective oxidation, etherification, dehydration, esterification, pyrolysis, etc., esterification is one of the widely used processes to produce various industrially important esters through glycerol over different series of catalysts.19−22 Received: December 20, 2014 Revised: March 10, 2015 Accepted: March 11, 2015
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resolution of 4 cm−1. TGA-DSC (thermogravimetric analysis and differential scanning calorimetry) was done at the heating rate of 10 °C min−1 from 25 to 393 °C with 10 mg of the sample in a platinum crucible. Surface charge analysis and particle size of the catalyst were evaluated with a Malvern Nanozetasizer series ZS90. The number of acid sites was determined by using acid−base potentiometric titrations. The leaching tests of the catalyst were also carried out. Gas chromatography with mass spectra (GC-MS) analysis of the end product was done on a Trace 1300 GC, TSQ 8000, Triple Quadrupole MS HP (Thermofisher Scientific, USA) equipped with an autosampler and capillary column TG 5MS (30m × 0.25 mm, 0.25 μm). Helium was used as carrier gas with a flow rate of 0.7 mL/min. The injection volume of the sample taken was 1 μL dissolved in 10 mL of acetone. The oven temperature was sustained at 60 °C for the period of 3 min and then increased at 60 °C/min to 120 °C, which was maintained for a further 3 min. After achieving this, the oven temperature was increased at 80 °C/min to 200 °C for 7 min. Before being reduced to 60 °C for the injection of the next sample, the oven temperature was increased to 250 °C and maintained for the next 2 min to restart the column. Column effluent was examined with the mass spectrometer using electron impact ionization with an ion source temperature of 200 °C and interface temperature of 250 °C. 2.4. Performance of the Catalyst for the Esterification Reaction. The esterification reaction was chosen as a model reaction to evaluate the performance of different catalysts. In a 250 mL three necked round-bottom flask (attached with reflux condenser and stirrer), a desired amount of butanoic acid along with catalyst (weight of catalyst/total volume of reactants) was taken and heated in an oil bath up to the reaction temperature of 368.15 K within an accuracy of ±0.3 °C. Further, glycerol was heated separately up to the same temperature and added to the reactor. The total volume of the reaction mixture was taken up to 150 mL at the specified molar ratio of butanoic acid and glycerol. Then, the reaction mixture was stirred vigorously at the speed of 400 rpm. One milliliter of reaction mixture was withdrawn at regular intervals for analysis. The progress of the reaction was observed by the potentiometric titration of the collected samples by using 0.2 N NaOH and phenolphthalein as an indicator.
The main objective of the present work is to investigate the effects of preparation variables such as mixing time, sulfate loading, and calcination temperature and its duration on the catalytic performance of prepared sulfated iron oxide catalysts. The catalysts were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC)-thermogravimetric analysis (TGA), scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDX), zeta potential, and acidity and leaching tests. In addition, the prepared catalyst was compared with ion exchange resins and other heterogeneous catalysts for their catalytic performance toward esterification of butanoic acid with glycerol.
2. EXPERIMENTAL SECTION 2.1. Materials. Amberlyst 15 (wet) and Amberlyst 35 (wet) have been procured from Rom and Hass. Purolite D5081 and D5082 were obtained from Purolite International Ltd., UK. Dowex 50Wx2 was procured from Dow Ltd. Tungstophosphoric acid, zirconium oxychloride octahydrate, butanoic acid (>99%), and glycerol (>99%) were procured from SD Fine Chem. Ltd., Mumbai. Raw clay was procured from the native clay market. Hydrochloric acid (purity 35%), sodium hydroxide, tin dioxide, iron(III) oxide, zinc oxide, sodium chloride, and ammonium sulfate were procured from Merck, Mumbai. Titanium dioxide P-25 was procured from Evonik, Mumbai. All the procured chemicals were of analytical grade and not further purified. 2.2. Catalyst Preparation. Zirconia supported tungstophosphoric acid (Zr-TPA) was synthesized as published by Sharma and Patel,23 and acid activated clay (AAC) was prepared by the method provided by Igbokwe and Olebunne.24 Sulfated iron oxide (SIO) was prepared through dry synthesis as follows: (i) by mixing ammonium sulfate, i.e., (NH4)2SO4, with iron(III) oxide in different molar ratios for different intervals, (ii) the resulting mixture was air-dried at room temperature for 18 h for maturation, and (iii) then calcining at different temperatures for varying time intervals in a muffle furnace.5 The same procedure was followed for the preparation of sulfated zirconia (SZ), sulfated zinc oxide (SZO), and sulfated titanium dioxide (STD) by mixing ammonium sulfate with the corresponding metal oxides. The optimum mixing time of precursor, sulfate loading, calcination temperature, and calcination duration was determined by testing the catalysts performance in esterification of butanoic acid with glycerol. 2.3. Characterization. To systemically identify the structure and morphologies of the catalysts, characterization was performed on the sulfated iron catalysts after their calcination. X-ray powder diffraction (XRD) studies were performed using a X’Pert Pro diffractometer, D/max rA using Cu Kα radiation of wavelength 1.54 Å from a sealed X-ray tube in the range of 2θ = 2−90° at the rate of 1°/min. Crystallite size was determined from the most intense peak using the Scherrer equation. The images of surface morphology were captured by scanning electron microscopy (SEM) (Zeiss EV050) to determine the particle size of agglomerates. The phase composition and weight % of Fe, S, and O were obtained from energy dispersive X-ray spectroscopy (EDX) (Bruker AXS, QuanTax 200). Meanwhile, for structure analysis, Fourier transform infrared spectroscopy (PerkinElmer-Spectrum RXIFTIR) was obtained for pure iron(III) oxide and sulfated iron oxide catalysts calcined for 5 and 7 h. The pattern was observed in the range of 4000−400 cm−1 using KBr pellets with
3. RESULTS AND DISCUSSION 3.1. Catalytic Tests. Preliminarily, the reaction was carried out in the absence of catalyst and the conversion of butanoic acid was found to be only 6% after the reaction time of 6 h. This suggests the use of an appropriate heterogeneous catalyst that would stimulate the reaction. Thus, the esterification reaction was studied over different heterogeneous catalysts and their behaviors were observed in terms of conversion of butanoic acid. The reaction was carried out at the reaction temperature of 368.15 K with butanoic acid and glycerol for the corresponding molar ratio of 3:5 at a stirring speed of 400 rpm. The reaction was continued for a time period of 6 h, and the progress was monitored by using potentiometric titration with 0.2 N NaOH. It was found in the literature that the final reaction products were only glyceryl esters.25 Catalytic performance of the catalysts at their different amounts is shown in Table 1. In the case of organic resins, Dowex50Wx2 has shown the maximum fractional conversion of 0.853 as compared to others. As the cost of Dowex series is high, efforts were made to develop some green catalysts (solvent free) that B
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Industrial & Engineering Chemistry Research Table 1. Fractional Conversion of Different Heterogeneous Catalysts at the Reaction Temperature of 368.15 K, RPM of 400, and Molar Ratio of (Butanoic Acid and Glycerol) 3:5 after 6 h catalyst
catalyst loading, kg/m3
Table 2. Fractional Conversion (FC) after 6 h (a) for Varying Mixing Time (M.T.), (b) Variation in Sulfate Loading, (c) Variation of Calcination Temperature (C.T.), and (d) Variation of Calcination Time (C. t.) for Sulfated Iron Oxidea
fractional conversion after 6h
(a)
Organic Resins Amberlyst 15 (wet) Amberlyst 35 (wet) Dowex50WX2 Purolite D5081 Purolite D5082
28.8 28.8 28.8 28.8 28.8 Sulfated Metal Oxides sulfated ZnO 4.0 sulfated TiO2 4.0 sulfated ZrO2 4.0 sulfated Fe2O3 4.0 sulfated SnO2 4.0 Others acid activated clay 28.8 zirconia supported tungstophosphoric 4.0 acid (Zr-TPA uncalcined) Zr-TPA (calcined at 300 °C) 4.0 Zr-TPA (calcined at 400 °C) 4.0 Zr-TPA (calcined at 500 °C) 4.0 Zr-TPA (calcined at 600 °C) 4.0
0.790 0.739 0.853 0.717 0.712
M.T. (min) 20 30 40 50 60
(b)
(c)
FC
S.L. (wt %)
FC
0.839 0.855 0.883 0.866 0.824
2 4 6 8 10
0.876 0.879 0.883 0.883 0.887
(d)
C.T. (°C)
FC
C. t. (h)
FC
300 400 500 600
0.774 0.833 0.883 0.585
4 5 6 7 8
0.800 0.883 0.840 0.828 0.780
a
At the esterification temperature of 368.15 K, catalyst loading of 4.0 kg/m3, and RPM of 400; molar ratio of butanoic acid and glycerol was 3:5.
0.349 0.487 0.822 0.839 0.245
oxide catalysts with varying sulfate loadings of 2, 6, and 10 wt % used to study their surface morphology and agglomerate size distribution in atomic detail. All the catalyst samples taken were in powder form for testing. SEM images of catalysts are shown in Figure 1. As seen from the images, sulfation has enhanced the surface morphology of iron oxide. The micrograph obtained for the catalysts depicts the agglomeration of particles ranging in length from 2 to 5 μm in the form of a regular sheet like arrangement in cylindrical contour. The images also demonstrated that the particle size and morphological features of the catalysts have changed significantly as compared to the precursor and also with variation in sulfate loading. EDX was done to determine the composition of the catalyst which has confirmed the even distribution of iron (Fe), sulfur (S), and oxygen (O) in the material. The practical weight percentages of the Fe, S, and O are compared and found to be in good agreement with the theoretical data as explained in Table 3. 3.1.2. Effect of Mixing Time. In this study, we have also investigated the effect of the mixing time of the precursors used for sulfated iron oxide preparation on its catalytic performance in the esterification reaction. Therefore, the consequence of variation in mixing time of ammonium sulfate and iron oxide was explored from 20 to 60 min (denoted as SIO30, SIO40, SIO50, and SIO60) under this protocol and screened in the esterification reaction of butanoic acid with glycerol. Sulfate loading was 6 wt %, and catalyst was calcined at the temperature of 500 °C for 5 h. There is considerable variation in the catalytic performance with respect to mixing time, and the catalyst mixed for 40 min gave the optimal conversion. As seen from Table 2, the increase in conversion with an increase in mixing time, i.e., from 20 to 40 min, was observed (maximum of 0.883 for SIO40); this may be due to the proper embedment of particles into each other up to this point. Further, it started declining from SIO50 to SIO60 which can be due to the distortion created in the catalyst structure with overmixing, as was also observed from its XRD pattern. XRD of catalysts with varied mixing time from 30 to 60 min was performed to determine the variation in their structures and crystallinity. The diffraction patterns are shown in Figure 2. The observed patterns are assigned as characteristics of orthorhombic structure with primitive bravais lattice.26 There was no significant change in the number of diffraction peaks for SIO30, SIO40, and SIO50, specifying that no crystal transformation occurred during the variation in mixing time up to 50
0.682 0.708 0.720 0.766 0.827 0.715
should be of low cost and provide high conversions toward desirable products. In contrast, we prepared some catalysts and evaluated them in experiments. As sulfation leads to an increase in the surface area of metal oxides, they work effectively at low temperature and enhance conversion with an increase in temperature. Consequently, sulfation of several metal oxides (zinc oxide, titanium dioxide, zirconia, and iron oxide) was done by ammonium sulfate and tested in esterification reactions (mixing time was 20 min) as summarized in Table 1. It can be observed from Table 1 that sulfated iron oxide gave the maximum fractional conversion of 0.839 which is relatively comparable to Dowex50Wx2 owing to its strong activity. Due to high cost of Dowex50Wx2, sulfated iron oxide was further modified to attain a maximum conversion in the esterification reactions. The effect of different variables, on morphology and crystalline structure, for the preparation of sulfated iron oxide catalyst has been investigated. Further, the successive influence of these variables on the performance of the optimum catalyst, i.e., sulfated iron oxide, has been explored with respect to the conversion of butanoic acid to find out the optimal preparation conditions. 3.1.1. Effect of Sulfate Loading. Sulfated iron oxide catalysts were prepared by varying the sulfate loading from 2 to 10 wt % (mixing time: 40 min; calcined at 500 °C for 5 h and termed as SIOa, SIOb, and SIOc, respectively). As shown in Table 2, the catalytic performance varied markedly with different sulfate loadings. The overall optimal catalyst for conversion of acid under the reaction conditions had a sulfate loading of 10 wt % whose performance is comparable to 6 wt % (Table 2). The former catalyst was considered as the most efficient, due to its good performance even at low sulfate composition, which overall reduces the final cost of the catalyst. These results were supported by scanning electron microscopy (SEM) of pure iron(III) oxide and sulfated iron C
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Figure 1. SEM-EDX of sulfated iron oxide catalysts with varying sulfate loadings.
calculated by the Scherrer equation.3 The crystallite size increased with increasing mixing time from 30 to 40 min. The less intense peak of SIO50 as compared to SIO40 gave an indication of low crystallinity. This low crystallinity further reduced the crystallite size of the catalyst. Distortion of the crystalline structure, which occurred during the overmixing of particles, could probably decrease the crystallinity and
min. Thus, the orthorhombic structure remained intact up to the catalyst SIO50 due to the proper embedment of particles into each other. Concomitantly, SIO60 confirms the distortion of crystalline structure due to overmixing of particles. All catalysts have shown the predominant peak at a higher diffraction angle of 2θ = 24.9, ascribing to the crystallite size of 60 nm for SIO30, 70 nm for SIO40, and 67 nm for SIO50 as D
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crystallite size. These results are found to be in good agreement with experimental data as given in Table 1. 3.1.3. Effect of Calcination Temperature and Duration. The performance of the catalysts can be affected by its calcination temperature which is one of the main aspects in the activation of the active sites. The effect of calcination temperature on the catalytic activity was evaluated by varying it from 300 to 600 °C for the catalyst with optimized mixing time of 40 min and sulfate loading of 6 wt %. Table 2 shows that the maximum fractional conversion of 0.883 at the calcination temperature of 500 °C follows the decrease in catalytic activity with an increase in temperature. This is due to the depreciation of the sulfur concentration and active sites present on the surface of the catalyst after a temperature of 500 °C which leads to the decomposition of the acidic quality.27,28 After calcination temperature, its duration needed to be optimized, as this has a significant influence on the active sites of the catalyst. In order to investigate the effect of calcination time on performance of the catalysts, a series of catalysts were prepared by calcining at 500 °C for different times between 4 and 8 h. All these catalysts were tested for the esterification reaction under the same optimal conditions, and it was found that the catalyst calcined for 5 h gave an optimal fractional conversion of 0.883 followed by 6 and 7 h (Table 2). With the increase in calcination time, the catalyst was constantly in contact with a high temperature for a long time; so, active sites got depleted, which is a reason behind the low conversion of catalysts. However, owing to the leaching problem of catalyst samples calcined for 5 and 6 h, the optimum calcination time of the catalyst was 7 h.
Table 3. Elemental Analysis of Catalysts Determined by EDX Measurement catalyst
theoretical calculated (wt %)
wt % obtained from EDX
pure Fe2O3
O, 48.00 Fe, 52.00 O, 50.00 Fe, 31.80 S, 18.00 O, 58.60 Fe, 15.20 S, 26.00 O, 61.40 Fe, 10.00 S, 28.50
O, 40.39 Fe, 59.61 O, 58.88 Fe, 27.80 S, 13.33 O, 56.54 Fe, 25.09 S, 18.37 O, 65.92 Fe, 16.87 S, 17.21
SIOa
SIOb
SIOc
Figure 2. X-ray diffraction patterns of sulfated iron oxide SIO30, SIO40, SIO50, and SIO60.
Figure 3. FT-IR spectra of catalyst (a) Fe2O3 and (b) SIOb calcined for 7 h and (c) SIOb calcined for 5 h at 500 °C. E
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Figure 4. TGA-DSC profile of catalyst SIOb calcined for 7 h.
Figure 5. (a) Hydrolysis of fresh SIOb catalyst (calcined for 5, 6, and 7 h) and (b) recovered SIOb catalyst (pH of solution vs time) at room temperature for 0.33 g of catalyst immersed in water.
Hence, the sulfated iron oxide catalyst with sulfate loading of 6 wt %, mixing time of 40 min, and calcined for the duration of 7 h was found to be optimum. The esterification reaction of butanoic acid with glycerol over optimum sulfated iron oxide was performed at a temperature of 368.15 K for the reaction time of 6 h and a conversion of 82.8% was obtained. The selectivity of the final products obtained during esterification and analyzed by gas chromatography with mass spectra was 67.57% tributyrin, 24.3% (−)-O-acetylmalic anhydride, 7.4% monobutyrin, and 0.64% dibutyrin. 3.2. Characterization of Optimum Sulfated Iron Oxide Catalyst. 3.2.1. Thermal Stability. TGA of optimum sulfated iron oxide catalyst was carried out to measure the weight loss as a result of the increase in sample temperature as shown in Figure 4. The weight losses found from TGA graphs agree fairly well with those from DSC. The catalyst has shown main two thermal losses. The first weight loss of 1.6% occurred with an onset temperature of around 0 to 180 °C, which corresponds to the removal of free and adsorbed water. The second weight loss of 1% was observed at 180−300 °C, which is considered to be due to dehydroxylation. Overall, TGA of the catalyst has revealed a total weight loss of up to 2.6% with a sample temperature of 330 °C. DSC evaluates the thermal behavior of the catalyst. The endothermic peak at the temperature of 180 °C indicates the removal of physically adsorbed water from catalyst material, and the second endothermic peak at the higher temperature represents dehydroxylation which is also revealed by TGA.14 DSC and TGA curves became stable after
In order to know the structural properties of iron(III) oxide and sulfated iron oxide catalysts SIOb calcined for 5 and 7 h, FTIR analysis was carried out in the range of 4500 to 400 cm−1. The spectra of the fresh iron(III) oxide and sulfated iron oxide are shown in Figure 2a−c. From Figure 3, no significant band position shift was observed relative to pure iron oxide, which confirmed the proper embedment of sulfate groups in the Fe2O3 structure. Referring to Figure 3b,c, catalyst prepared at different calcination durations of 5 and 7 h exhibited almost similar patterns and bands. As observed, the presence of a band at 3367 cm−1 was attributed to the O−H stretching vibrating absorption and the band at 1633 cm−1 corresponded to coordinated water, i.e., δ HOH mode. As seen from the comparison of spectra of iron oxide and sulfated systems, the addition of sulfate groups has modified the OH stretching of Fe2O3.13 Meanwhile, Fe−O and Fe2O3 stretching has been observed between the wavenumber range of 580−680 cm−1. Catalyst calcined for 5 h (Figure 3c) has shown the bands at 1212, 1126, and 1031 cm−1, confirming the SO and S−O symmetric and asymmetric vibrations, which are assigned for the chelating sulfate ions that are coordinated to the iron cation (Fe3+).14 These asymmetrical bands are due to the presence of free sulfate ions on the surface of the catalyst which has further resulted in the leaching of these ions in the reaction mixture. On the contrary, catalyst calcined for 7 h (Figure 3b) has shown the only explainable characteristic band at a subsequent wavenumber of 1128 cm−1 which indicates the presence of significant sulfate ions with Td symmetry. F
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Industrial & Engineering Chemistry Research the temperature of 330 °C which shows the stability of the catalyst at high temperatures. 3.2.2. Particle Size Distribution. Particle size distribution (PSD) and surface charge analysis was performed with the Malvern Nanozetasizer series ZS90 in the particle size range of 0.1−1000 μm. The agglomerate size of the above catalyst is found to be 3.81 μm, which falls in the same range as that specified by the SEM micrographs. As is evident from the value shown by the zeta potential, i.e., −0.158 mV (Figure S1, Supporting Information), the catalyst does not follow the stable range of zeta potential higher than +30 mV and less than −30 mV, hence showing the instability of the catalyst in dispersion medium and immiscibility with the reaction mixture. This is due to the agglomeration of particles, which also prevents the leaching of catalyst in the reaction mixture. As a result, sulfated iron oxide is found to exhibit a heterogeneous nature in the reaction mixture. 3.2.3. Leaching Studies. Leaching of the catalyst makes it incompetent to reuse in successive reactions. Thus, a leaching test of the catalyst was done to check the presence of sulfate ions in the reaction mixture as they mix with water and hydrolyze to give the homogeneous behavior of H2SO4. The test was performed on both fresh and recovered catalyst from the reaction mixture by following the procedure given in the literature.29 When the freshly prepared sulfated iron oxide was mixed with water, the pH of the suspension decreased very slowly in case of catalyst calcined for 7 h as compared to 5 and 6 h, which shows the catalyst was not easily hydrolyzed. For the recovered catalyst, the decrease in pH was also found to be very slow as shown in Figure 5a,b. This may be due to the presence of hydrophobic compounds, i.e., acid, alcohol, or ester, on the surface of the catalyst during the reaction which resists the change in pH.29 Further, the reaction mixtures obtained at the temperature of 368.15 K were investigated for sulfate ions by treating them with BaCl2. A very minute quantity of sulfate ions was found in the mixtures; i.e., very low amounts of precipitates were formed by adding BaCl2 in a very large amount for the catalyst sample calcined for 7 h. However, huge numbers of sulfate ions were observed in the case of catalyst samples calcined for 5 and 6 h. Thus, sulfated iron oxide calcined for 7 h was selected for further studies. Acid−base back potentiometric titration was used to determine the total number of acid sites present in the catalyst by the procedure followed by Pérez et al.30 The acid density sites of sulfated iron oxide were calculated to be 14.4 mmol H+/ g catalyst, which confirms its highly acidic nature. 3.3. Catalyst Reusability. To analyze the strength and reusability of the catalyst, after the completion of the reaction, the catalyst was separated from the reaction mixture by filtration using Whatman filter paper 42, washed with methanol, and then dried at 100 °C overnight for its reuse. It can be pointed out from Figure 6 that the total conversion for fresh catalyst is very close to the first run of the recovered catalyst. In these successive runs, it was found that the efficiency decreased after every reuse of the catalyst. This decrease is due to the deactivation of some acid sites of the catalyst because of blockage of the fine pores during the reaction.31,32
Figure 6. Reuse of catalyst sulfated iron oxide in esterification of glycerol with butanoic acid at the temperature of 368.15 K, catalyst loading of 4.0 kg/m3, RPM of 400, and molar ratio of butanoic acid and glycerol of 3:5.
In this work, sulfated iron oxide with variable preparation parameters was synthesized by a solvent free method, and its catalytic performance was compared with ion exchange resins and other heterogeneous catalysts over esterification reactions. The mixing time of precursors, sulfate loading, and calcination temperature and its duration were varied and found to be important factors influencing the synthesis of the catalyst. The optimum catalyst has a mixing time of 40 min, sulfate loading of 6 wt %, and calcination at 500 °C for 7 h which resulted in 82.8% acid conversion. The characterization using XRD, SEMEDX, FTIR, DSC-TGA, zeta potential, acidity, and leaching tests showed that the catalyst precursors are sensitive to preparation conditions. In addition, sulfated iron oxide reveals itself as a low cost, effective, stable, efficient, and reusable heterogeneous catalyst in the esterification process even when used in a low amount.
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ASSOCIATED CONTENT
S Supporting Information *
Particle size distribution (PSD) and zeta potential data of sulfated iron oxide. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +919814173832. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS K.K. acknowledges UGC-MANF (Government of India) for providing a scholarship for her Ph.D. research work and Energy Research Centre (ERC) for providing the facilities to carry out the work.
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REFERENCES
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DOI: 10.1021/ie504916k Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
