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Synthesis of glycerol carbonate from glycerol and urea over tin-tungsten mixed oxide catalysts
Applied Catalysis A: General 469 (2014) 165–172
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Synthesis of glycerol carbonate from glycerol and urea over tin-tungsten mixed oxide catalysts K. Jagadeeswaraiah a , Ch. Ramesh Kumar a , P.S. Sai Prasad a , S. Loridant b , N. Lingaiah a,∗ a
Catalysis Laboratory, I & PC Division, CSIR—Indian Institute of Chemical Technology, Hyderabad 500007, India Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR 5256, CNRS, Universite Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France b
a r t i c l e
i n f o
Article history: Received 12 June 2013 Received in revised form 17 September 2013 Accepted 24 September 2013 Available online xxx Keywords: Glycerol Glycerol carbonate Tin oxide Tungsten oxide Urea
a b s t r a c t Tin-tungsten mixed oxide catalysts with varying their mole ratio were prepared by co-precipitation method. The catalysts physico-chemical properties were derived from FT-infrared, Laser Raman, X-ray diffraction, UV–Vis DRS, BET surface area and temperature-programmed desorption of NH3 . The catalysts activities were evaluated for the synthesis of glycerol carbonate from glycerol and urea. The activity results showed that Sn-W mixed oxide catalysts are highly active for selective formation of glycerol carbonate. Sn-W catalyst with 2:1 molar ratio exhibited about 52% of glycerol conversion with >95% selectivity towards glycerol carbonate. The active catalyst was subjected to calcination at different temperatures and evaluated for their activity in glycerol carbonate synthesis. The activity of the catalysts depends on mole ratio of Sn/W and treatment temperature which are influencing the surface-structural characteristics of the catalysts. Different reaction parameters such as glycerol to urea molar ratio, reaction temperature and catalyst loading were studied and optimum conditions were established. The catalysts showed consistent activity upon repeated use. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The development of biodiesel industry from biomass resource led to over plus of glycerol, produced as a byproduct in the synthesis of biodiesel. The conversion of glycerol to valuable chemicals has attracted considerable attention in recent times not only due to its availability but also for the presence of more number of functional groups [1–10]. Among different chemicals possible from glycerol, glycerol carbonate is a valuable glycerol derivative. This is being widely used in cosmetics industry, gas separation, coatings, detergents and polymers [11–15]. A number of synthetic processes are known for the preparation of glycerol carbonate from glycerol [5]. The well known methods for preparation of glycerol carbonate are the reaction from glycerol with phosgene [16] and transesterification of glycerol with a carbonate source [17]. The most interesting route to produce glycerol carbonate is from the reaction of glycerol with CO2 in the presence of a catalyst [18]. This reaction requires elevated temperature, pressure and the yield of glycerol carbonate is too low to be used for practical purposes. The alternative route is
∗ Corresponding author. Tel.: +91 40 2719 1722; fax: +91 40 2716 0921. E-mail addresses: [email protected], [email protected] (N. Lingaiah). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.09.041
glycerolysis in which glycerol is reacted with urea in the presence of a catalyst. This approach is an indirect route for utilization of CO2 as it is used in the production of urea. The synthesis of glycerol carbonate by glycerolysis is studied by different authors [17,19,20]. Mouloungui et al. patented the synthesis of glycerol carbonate by carbonylation of glycerol with urea over heterogeneous zinc catalysts such as zinc sulfate, zinc organosulfate and zinc ion exchange resins [19]. Calcined ZnSO4 gave 86% of glycerol carbonate in 2 h at a reaction temperature of 150 ◦ C and 40 mbar of pressure. The major drawback of this catalyst is that, ZnSO4 salt is soluble in glycerol. There is a strong need to develop heterogeneous catalyst for strategic and environmental benefit. Heterogeneous catalysts are particularly efficient for carbonylation of glycerol with urea [17,20] affording good yield under moderate reaction conditions. Climent et al. [17] have recently reported the carbonylation of glycerol with urea at 145 ◦ C over heterogeneous catalysts such as basic oxides (MgO and CaO), Al/Mg and Al/Li mixed oxides derived from hydrotalcites with adequate acid–base pairs. These catalysts showed about 72% of glycerol carbonate yield in 5 h of reaction time. In our previous study, we reported samarium exchanged heteropoly acid catalyst for carbonylation of glycerol with urea. The Sm0.66 TPA catalyst showed good selectivity (85.4%) towards glycerol carbonate with 49.5% glycerol conversion [21]. Recently Hutchings and co-workers reported the synthesis of glycerol carbonate using gold
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
and Pd based supported catalysts [22]. These catalysts are expensive and overall activity is not very high and requires long reaction time. The catalysts with acidic and basic sites such as Zn-Al hydrotalcite and ␥-zirconium phosphate were also reported [17,20]. The catalysts containing basic sites were reported for the selective synthesis of glycerol carbonate with high activity [23,24]. However, these base catalysts exhibited low activity during recycling. There is a need to develop highly active and stable catalysts for the synthesis of glycerol carbonate from glycerol and urea. Solid acid catalysts are promising catalysts for the carbonylation of glycerol with urea [21]. Tungsten oxide based mixed oxide catalysts are known for their role in acid catalyzed reactions. Moreover these mixed oxide catalysts are known for their stability. In the present study, a series of tin-tungsten mixed oxide catalysts were prepared with varying Sn to W molar ratio and studied for carbonylation of glycerol with urea under reduced pressure. The surface and structural properties of the catalysts were varied by treating the samples at different temperatures. The properties of the catalysts were investigated from different spectroscopic methods to establish the catalytic activities. A detailed study was undertaken to optimize the reaction parameters to achieve best results. 2. Experimental 2.1. Preparation of Sn-W mixed oxide Sn-W mixed oxide catalysts were prepared by co-precipitation method. The preparation of Sn-W oxide with a Sn/W molar ratio of 2 is given as an example. In a typical procedure, Na2 WO4 ·2H2 O (4.94 g, 15 mmol) was dissolved in deionized water, followed by the addition of SnCl4 (7.815 g, 30 mmol) in a single portion. The solution stirred for 1 h at room temperature and deionized water was added to form white slurry. This slurry kept for stirring for 24 h at room temperature and the resulting white precipitate was filtered off, washed with large amount of deionized water until it is free from chlorine. The precipitate was dried in oven at 120 ◦ C to afford Sn-W hydroxide as a white powder. SW hydroxides with different Sn/W molar ratios (Sn/W=0.5, 1, 2, 3, and 5) were successfully prepared by changing the molar ratios of the starting metal solutions. The hydroxide precursors were calcined at 500 ◦ C for 4 h under an air atmosphere to yield final catalyst. The catalysts are denoted as SW11 where the alphabets represent the metal oxide and the numerical number related to their molar ratio. 2.2. Characterization of the catalysts X-ray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku Miniflex diffractometer using CuK␣ radiation ˚ at 40 kV and 30 mA. The measurements were obtained (1.5406 A) in steps of 0.045 ◦ C with account time of 0.5 s and in the 2 range of 10◦ –80◦ . Confocal Micro-Raman spectra were recorded at room temperature in the range of 200–1200 cm−1 using a Horiba Jobin-Yvon Lab Ram HR spectrometer with a 17 mW internal He–Ne (Helium–Neon) Laser source of excitation wavelength of 632.8 nm. The catalyst samples in powder form (about 5–10 mg) were loosely spread onto a glass slide below the confocal microscope for measurements. The UV–Vis diffuse reflectance spectra (UV–Vis DRS) were recorded on a GBC UV–Visible Cintra 10e spectrometer in the range of 200–800 nm. BET Surface area was measured on Quadrasorb—SI instrument at relative pressure range of 0.05–0.3. Before analysis, the samples were degasified at 150 ◦ C for 2 h to remove moisture on the surface of the catalyst.
M-monoclinic WO3 * SnO2
* * * *
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Intensity (a.u)
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M
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2Theta ( ) Fig. 1. XRD profiles of the SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21 (c) SW31 (d) SW51 (e) SW12.
Temperature programmed desorption of ammonia (TPD-NH3 ) was carried out on a laboratory-built apparatus equipped with a gas chromatograph using TCD detector. In a typical experiment about 0.05 g of oven dried sample was taken in a quartz tube. Prior to TPD run, the catalyst sample was treated at 300 ◦ C for 1 h by passing pure He gas (50 ml/min). After pretreatment, the sample was saturated with anhydrous ammonia (10% NH3 balance He gas) at 100 ◦ C with a flow rate of 50 ml/min for 1 h and was subsequently flushed with He gas at the same temperature to remove physisorbed ammonia. The process was continued until a stabilized base line was obtained in the gas chromatograph. Then the TPD analysis was carried out from ambient temperature to 700 ◦ C at a heating rate of 10 ◦ C/min. The amount of NH3 evolved was calculated from the peak area of the already calibrated TCD signal. 2.3. Reaction procedure The reactions were performed in a 25 mL two neck roundbottom (RB) flask under reduced pressures. In a typical experiment glycerol (2 g), urea (1.306 g) and catalysts (0.2 g) were taken in the round bottom flask and heated in an oil bath at 140 ◦ C with constant stirring. One neck of the RB flask was connected to vacuum line. Reaction was run under a reduced pressure in order to remove the ammonia formed during the reaction. After completion of reaction or stipulated time, methanol was added and the catalyst was separated by filtration. The products were analyzed by a gas chromatograph (Shimadzu, 2010) equipped with flame ionization detector using inno wax capillary column (diameter: 0.25 mm, length 30 m). Products were also identified by GC–MS (Shimadzu, GCMS-QP2010S) analysis. 3. Results and discussion 3.1. Catalyst characterization X-ray diffraction patterns of SW catalysts calcined at 500 ◦ C are shown in Fig. 1. The XRD patterns of SW catalysts with the molar ratio of 1:1 and 2:1 were intrinsically identical to that of amorphous Sn-W hydroxide precursor. Whereas in the case of SW12, SW31 catalysts, intense diffraction peaks related to crystalline tin oxide were observed. The peaks observed at 2 values of 26.5◦ , 33.9◦ , 37.9◦ , 43.5◦ , 51.8◦ , 54.7◦ and 61.9◦ are corresponding to tetragonal
167
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Absorbance (a.u)
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(f)
(e) (d) (c) (b) (a)
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rutile structure SnO2 crystal planes of (1 1 0), (1 0 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0) and (0 0 2), respectively [25–27]. Catalyst with 5:1 molar ratio of Sn/W showed diffractions peaks related to both tin oxide and tungsten oxide. The diffraction peaks at 2 = 23.3◦ , 24.3◦ , 33.29◦ and 34.1◦ can be indexed to (0 0 2), (2 0 0), (0 2 2) and (2 0 2) of monoclinic WO3 phase, respectively (JCPDS No. 43-1035). These peak positions and their relative intensities are in good agreement with the literature [28]. The catalysts with Sn/W molar ratio of 1:1, 2:1 and 3:1 exhibited diffraction peaks related to only SnO2 and patterns related to WO3 were absent. This might be due to (i) strong interaction between tin oxide and tungsten oxide species, (ii) poly tungstate species are highly dispersed on SnO2 and formation of homogeneous solid solution. However, SW51 catalyst showed peaks related to WO3 . This indicates that, agglomeration of tungsten oxide species took place at high Sn to W ratio. Thus, homogeneity of the mixed oxide lost beyond the Sn/W molar ratio of 3:1. These results suggest that catalysts with Sn/W molar ratio of 1:1 to 3:1 form a homogeneous solid mixed oxide. Fig. 2 shows the Raman spectra of the catalysts. The catalyst SW11 showed an intense band at 953 cm−1 corresponding to stretching mode for octahedrally coordinated poly tungstate species [29]. When the SW molar ratio varied from 1:1 to 3:1 and 1:2, the band at 953 cm−1 related to W=Ot bond shifted to higher wave number (953 to 979 cm−1 ). This suggests that different two-dimensional tungsten oxide species might be present in Sn/W catalysts and no crystalline WO3 phase was present. The SW51 catalyst showed bands at 273, 717 and 808 cm−1 corresponding to W–O–W deformation mode, W=O bending and W=O stretching mode, respectively, of crystalline WO3 [30,31]. These results indicate that, 1:1 to 3:1 ratio of S/W mixed oxide form a homogeneous mixture. The XRD patterns also supports the results observed from Raman spectra. UV–Vis diffuse reflectance spectral technique is sensitive for distinguishing the chemical nature and coordination states of tungsten oxide species. Fig. 3 shows the UV–Vis DRS of SW catalysts along with bulk WO3 . Catalyst with SW molar ratio of 1:1 clearly showed a absorption band at 303 nm which corresponds to polymeric WO6 species [32]. This band is unresolved in the case of catalysts SW21, SW31 and SW12 due to strong absorption in the range of 220–378 nm. In general bulk WO3 shows a broad band at 398 nm with a shoulder at 359 nm [33]. UV–Vis DRS of bulk WO3 sample showed a broad band centred at 390 nm. On the other hand
500
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Fig. 3. UV–Vis diffused reflectance spectra of SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21, (c) SW31, (d) SW12, (e) SW51 and (f) WO3 .
SW mixed oxide catalysts (except SW51) showed broad bands in the range of 220–378 nm and band at 398 was absent indicates that these broad bands can be assigned to isolated tungsten species or low-condensed polymeric tungsten oxide species [33]. In the case of SW51 catalyst, in addition to the broad band in the range of 220–360 nm, a shoulder peak is observed at 398 nm which corresponds to the bulk WO3 . These results indicate that SW51 catalyst exists in two phases. The results obtained from XRD and Raman results are in good agreement with these results. The TPD profiles of the SW catalysts along with pure WO3 are shown in Fig. 4. The amount of ammonia desorbed from the catalyst surface is calculated and presented in Table 1. All the samples showed broad desorption peaks indicating that the surface acid strength is widely distributed. SW11 catalyst showed two unresolved desorption peaks at 390 and 500 ◦ C corresponding to moderate and strong acidic sites. Whereas the SW21 catalyst showed two broad and intense desorption peaks in the range of 100–460 ◦ C and 450–650 ◦ C. The peak centred at 250 ◦ C is related to
(f) (e)
(d)
Intensity (a.u)
Fig. 2. Laser Raman spectra of SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21, (c) SW31, (d) SW51 and (e) SW12.
400
Wavelength (nm)
-1
Raman shift (cm )
(c)
(b) (a)
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200
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Temperature ( C) Fig. 4. Temperature programmed desorption of ammonia patterns of the SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21, (c) SW31, (d) SW12, (e) WO3 and (f) blank run of SW21.
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Table 1 Acid strength distribution and BET surface area of SW mixed oxide catalysts.
SW11 SW12 SW21 SW31 SW400 SW600 SW700
Acidity (mmol/g) Weak/moderate
Strong
Total
0.085 0.195 0.404 0.231 0.021 0.177 0.136
0.020 0.015 0.068 0.035 0.018 0.045 0.016
0.105 0.210 0.472 0.266 0.039 0.222 0.142
BET surface area (m2 /g) 12.6 27.7 37.4 47.6 14.8 39.5 51.1
moderate acidic sites and other one centred at 530 ◦ C corresponds to strong acidic sites. Catalyst with a Sn/W molar ratio of 3:1 (SW31) showed a strong desorption peak at 200 ◦ C, along with small peaks centred at 370 and 520 ◦ C. The first peak is related to weak acidic sites and two small desorption peaks corresponding to moderate and strong acidic sites. SW12 catalyst showed two desorption peaks at 380, 515 ◦ C related to moderate and strong acidic sites, respectively, along with a small hump at 180 ◦ C. The intensity of the desorption peaks related to moderate to strong acid sites increased with increase in Sn/W ratio up to 2:1. Further increase in Sn/W ratio to 3:1 intensity of the desorption peaks decreased and shifted to lower temperatures. In the case of bulk WO3 sample, there is no considerable ammonia desorption peak and desorbed ammonia was found to be negligible. Compared to all other catalysts, SW21 showed broad and strong desorption peaks. The acidity values of SW21 catalysts (Table 1) indicating that this catalyst contains high acidity compared to all other catalysts. The high acidity of this catalyst might be related to the presence of highly dispersed WO3 domains on SnO2 as evidenced from XRD and Raman analysis. In order to know the nature of the gas released, a TPD run of SW21 catalyst was performed without pre-treatment of ammonia gas and the TPD profile is shown in Fig. 4. From the TPD profile it can be noted that no considerable peak is observed above 500 ◦ C. This result indicates that the desorbed gas during TPD run is related to ammonia adsorbed on the acidic sites of the catalyst. BET surface areas of the catalysts are presented in Table 1. From the table it can be observed that as the increase in SW molar ratio from 1:1 to 3:1 the surface area of the catalysts also increased. Catalyst with SW molar ratio of 1:2 showed a surface area of 27.7 m2 /g. In the case of SW21 catalyst calcined at in the range of 400–700 ◦ C exhibited an increase in BET surface area with increased calcination temperature. This might be due to removal of hydroxyl groups on the catalysts surface in the form of water molecules with increase in calcination temperature. 3.2. Catalytic activity Catalysts performance was investigated for carbonylation of glycerol with urea and the results are presented in Table 2. The reaction was also performed without catalyst and the conversion Table 2 Glycerol conversion and glycerol carbonate selectivity over SW mixed oxide catalysts. Catalyst
Conversion of glycerol (%)
Glycerol carbonate selectivity (%)
Blank SnO2 SW12 SW11 SW21 SW31 SW51 WO3
10.5 13.6 30.3 43.6 52.1 52.6 36.0 26.2
19.3 35.1 47.7 67.5 95.3 66.4 54.1 42.0
100
90 70
Glycerol conversion (%)
Catalyst
80
80 60
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60
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50 40 40
Glycerol carbonate selectvity (%)
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30 400
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700 o
Catalyst calcination temperature ( C) Fig. 5. Effect of catalyst calcinations temperature on the glycerol conversion and glycerol carbonate selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, Reaction temperature: 140 ◦ C, reaction time: 4 h.
of glycerol was about 10%. Pure WO3 and SnO2 showed 26% and 14% of glycerol conversion with 42% and 35% selectivity towards glycerol carbonate, respectively. The main by-products formed in the glycerol carbonylation are 5-(hydroxymethyl) oxazolidin-2one and (2-oxo-1,3-dioxolan-4-yl)methyl carbamate. In the case of SW12 catalyst, 30% of glycerol conversion was observed with 48% selectivity. With increase in Sn/W molar ratio from 1:1 to 3:1, the conversion of glycerol increased up to 52% and at the same time the selectivity towards glycerol carbonate is varied significantly. The catalyst with Sn/W molar ratio of 1:1 exhibited about 67% selectivity. Although SW21 and SW31 catalysts showed same glycerol conversion, but high selectivity about 95% towards GC was observed for SW21 catalyst. SW31 catalyst showed about 66% selectivity. The high activity of SW21 catalyst can be explained based on the catalysts characteristics. This catalyst exhibited high acidity compared to other catalysts. The acidity profiles of the catalysts derived from ammonia TPD are in good agreement with the observed catalytic activity, as SW21 catalyst showed strong and intense desorption peaks of ammonia. The presence of high acidity is mainly due to the presence of nano sized WO3 domains on SnO2 . The existence of this type of WO3 species are supported by XRD and Raman analysis. These results indicate that catalyst Sn/W with 2:1 molar ratio is the most active catalyst compared to its counter parts. In order to know the thermal stability, surface and structural changes of the catalyst, the most active catalyst was subjected to calcination at different temperatures and the catalytic activity was investigated for carbonylation of glycerol. Fig. 5 shows the catalytic performance of SW21 catalyst calcined at different temperatures. With increase in catalyst calcination temperature from 400 to 500 ◦ C conversion of glycerol increased from 37% to 52% and selectivity towards glycerol carbonate also increased from 71% to 95%. Increase in calcination temperature to 600 ◦ C, conversion of glycerol was decreased to 37% and glycerol carbonate selectivity is found to be 71%. Further increase in catalyst calcination temperature to 700 ◦ C, there is no variation in the glycerol conversion compared to catalyst calcined at 600 ◦ C, however, glycerol carbonate selectivity was decreased to 62%. The SW21 catalyst calcined at different temperatures was further characterized by XRD and Raman spectroscopy to understand the difference in activity with variation in treatment temperature. XRD patterns of SW21 catalysts calcined at different temperatures are shown in Fig. 6. The catalyst calcined at 400 and 500 ◦ C
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M-monoclinic WO3 * SnO2
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2 Theta Fig. 6. XRD profiles of the SW21 catalyst calcined at (a) 400 ◦ C, (b) 500 ◦ C, (c) 600 ◦ C and. (d) 700 ◦ C.
showed similar pattern as that of amorphous Sn-W hydroxide precursor. Upon increasing the calcination temperature to 600 ◦ C, crystalline phase of tin oxide was formed. Further increase in calcination temperature to 700 ◦ C, peaks associated to monoclinic phase of WO3 phase were observed at 2 values of 23.2◦ and 24.41◦ [28]. The high temperature calcination resulted in the formation of poly tungstate aggregates. XRD results suggest that the crystallinity increased with calcination temperature. The presence of well dispersed amorphous phase of WO3 was maintained up to 600 ◦ C and formation of WO3 crystallites were noticed at 700 ◦ C. XRD results suggest that, at higher calcination temperatures (above 500 ◦ C), homogeneity of the mixed oxide was lost and leads to formation of polytungstate aggregates. Laser Raman spectra of SW21 catalyst calcined at different temperatures are shown in Fig. 7. All the catalysts showed an intense band related to W=Ot bond. Catalyst calcined at 400 ◦ C showed a band at 981 cm−1 , whereas for the catalyst calcined at higher
Intensity (a.u.)
(d) (c)
(b) (a)
200
400
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-1
Raman shift (cm ) Fig. 7. Laser Raman spectra of SW21 catalyst calcined at (a) 400 ◦ C, (b) 500 ◦ C, (c) 600 ◦ C. and (d) 700 ◦ C.
Fig. 8. Temperature programmed desorption of ammonia patterns of SW21 catalyst calcined. at (a) 400 ◦ C, (b) 500 ◦ C, (c) 600 ◦ C and (d) 700 ◦ C.
temperatures (≥500 ◦ C) this band is slightly shifted to lower wave numbers. Intensity of this band increased for the sample calcined at 500 ◦ C and further increase in calcination temperatures to 600 and 700 ◦ C, intensity of the band is decreased. This might be due to loss in homogeneity of solid mixture. XRD results are in support for observations made from Laser Raman spectra. Fig. 8 shows the TPD of ammonia patterns of SW21 catalyst calcined at different temperatures. Catalyst calcined at 400 ◦ C showed a small desorption peak at 320 ◦ C. A drastic change in the acidity profile was observed for the catalyst calcined at 500 ◦ C. It showed a strong desorption peaks at 250 and 530 ◦ C corresponding to moderate and strong acidic sites as discussed earlier. Catalyst calcined at 600 ◦ C showed three desorption peaks with low intensity. Desorption peaks centred at 150 and 270 ◦ C corresponds to weak and moderate acidic sites, respectively. Another small desorption peak observed at 580 ◦ C related to strong acidic sites. These desorption peaks intensity was further decreased with increase in calcination temperature to 700 ◦ C. The observation made from the XRD, Raman and TPD of NH3 suggests that the catalyst calcined at 500 ◦ C has high acidity due to the presence of well dispersed WO3 domains on SnO2 surface. The high activity of the catalyst is related to its surface and structural characteristics, which are directing the acidity of the catalysts. Low conversion of glycerol for the catalyst calcined at higher temperature related to decrease in acidity due to the formation of large ensembles of WO3 . Conversion of glycerol and glycerol carbonate selectivity not only depends on the nature of the catalyst but also on reaction parameters. In order to optimize the reaction conditions, different reaction parameters such as reaction time, reaction temperature, glycerol to urea molar ratio and catalyst weight were studied by using the most active SW21 catalyst calcined at 500 ◦ C. 3.3. Influence of the reaction time Reaction time is an important parameter and long reaction times were adopted usually for the carbonylation of glycerol to obtain reasonable conversion. The effect of reaction time was studied over SW21 catalyst calcined at 500 ◦ C and the results are shown in Fig. 9. The conversion of glycerol consistently increased with increase in reaction time from 1 to 4 h. The selectivity towards glycerol carbonate is varied marginally with time. The overall selectivity of the catalyst with reaction time is high suggesting that the catalyst
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Glycerol carbonate selectivity (%)
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Glycerol conversion (%)
100
Glycerol conversion (%)
100
Glycerol carbonate selectivity (%)
170
1:3
Glycerol to urea molar ratio
Fig. 9. Effect of reaction time on glycerol conversion and glycerol carbonates selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, reaction temperature: 140 ◦ C.
Fig. 11. Influence of glycerol to urea molar ratio on glycerol conversion and glycerol carbonate selectivity. Reaction conditions: catalyst weight: 0.2 g, reaction temperature: 140 ◦ C, Reaction time: 4 h.
is selective in formation of glycerol carbonate. A decrease in the yield of glycerol carbonate was reported with reaction time over ␥-ZrP catalysts during glycerolysis of urea [20]. The present catalyst is selective in formation of glycerol carbonate and there was no increase in formation of byproducts with reaction time.
(55%). These results suggest that the optimum reaction temperature is 140 ◦ C.
3.4. Effect of reaction temperature In order to evaluate the influence of reaction temperature reaction was carried out at different reaction temperatures ranging from 100 to 160 ◦ C and the results are shown in Fig. 10. As expected, the conversion of glycerol increased with reaction temperature. Conversion of glycerol is only 13% at a reaction temperature of 100 ◦ C with 62% selectivity towards glycerol carbonate. As the temperature increased from 120 to 140 ◦ C the conversion of glycerol also increases from 29% to 52%. At the same time selectivity to glycerol carbonate is also increased from 78% to 95%. Further increase in reaction temperature selectivity towards glycerol carbonate decreased to 80% with marginal increase in glycerol conversion
100
3.5. Effect of glycerol to urea molar ratio In order to study the influence of molar ratio of reactants the reaction carried with variation in glycerol to urea molar ratio and the results are shown in Fig. 11. From the figure it can be noted that conversion of glycerol increased from 31% to 59% with increase in the concentration of urea, at the same time selectivity towards glycerol carbonate decreased from 100 to 61%. The decrease in selectivity with increase in urea concentration related to formation of a by-product which forms due to the reaction of free hydroxyl group present in the glycerol carbonate with available urea. When the glycerol to urea molar ratio is 1:1, the conversion of glycerol is 52% with 95% selectivity towards glycerol carbonate. Further increase in urea concentration (1:2 and 1:3 molar ratio of glycerol to urea) the conversion of glycerol is increased. However selectivity towards glycerol carbonate is decreased. These results suggest that 1:1 molar ratio of glycerol to urea is optimum for high glycerol carbonate selectivity.
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Glycerol carbonate selectivity (%)
Glycerol conversion (%)
3.6. Effect of catalyst weight
160 o
Reaction temperature ( c) Fig. 10. Effect of reaction temperature on glycerol conversion and glycerol carbonates selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, reaction time: 4 h.
Fig. 12 shows the effect of catalyst weight on the glycerol conversion. Increase in catalyst loading from 100 to 200 mg the conversion of glycerol increased from 26% to 52%. Selectivity also increased from 63% to 95%. Further increase in catalyst loading to 200–400 mg, only a marginal increase in glycerol conversion was observed with a decrease in selectivity towards glycerol carbonate. The catalyst present in excess might be favouring the reaction between the product glycerol carbonate and urea to yield 5-hydroxymethyloxazolidine-2-one. 3.7. Catalyst reusability The most active SW21 catalyst was studied for its reusability and results are presented in Fig. 13. Fresh catalyst showed about 52% of glycerol conversion with 95% selectivity towards glycerol carbonate. After completion of the reaction, methanol was added to reaction mixture and the catalyst was separated by filtration. The separated catalyst was washed with methanol, dried in oven and reused without any pre-treatment. In the first recycle the catalyst
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Table 3 Comparison of catalytic activity of the present catalyst with other reported solid acid catalysts. Reaction temperature (◦ C)
Catalyst
SW21 Sn-beta zeolite Zr-P* Au/Fe2 O3 2.5% Au/Nb2 O5 Sm0.66 TPA
Conversion of glycerol (%)
4 5 3 4 4 4
52.1 70 80 80 66 49.5
Glycerol carbonate (%) Yield
Selectivity
49.7 26 76 39 21 42.3
95.3 37 100 48 32 85.4
Ref.
Present work [15] [18] [28] [28] [19]
This catalysts contains both acidic and basic sites.
70 80
Glycerol conversion (%)
3.8. Comparison of SW21catalyst activity with reported solid acid catalysts
100
80
60
60
50
40 40
Glycerol carbonate selectivity (%)
*
140 145 140 150 150 140
Reaction time (h)
30
20
20 100
200
300
400
Catalyst weight (mg)
Catalytic activity of SW21 catalyst is compared with other reported solid acid catalysts and comparative results are presented in Table 3. Among the different catalysts zirconium phosphate catalysts exhibited about 80% glycerol conversion with 100% selectivity towards GC [20]. This catalyst contains both acidic and basic sites. However, its activity decreased with reaction time and showed decline in activity upon reuse. Compared to acidic catalysts base catalysts such as La2 CO3 [23] and gold supported on MgO [24] relatively showed high activity in the synthesis of glycerol carbonate. Even though these catalysts are highly active their reusability is limited. The present solid acid catalysts showed better activity and selectivity compared to other reported acid catalysts such as Sn-beta zeolite, samarium exchanged tungstophosphoric acid (Sm0.66 TPA) and gold supported on acidic supports. Moreover, the present catalysts is reusable compared other basic and acidic catalysts.
Fig. 12. Effect of catalyst weight on glycerol conversion and glycerol carbonate selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, reaction temperature: 140 ◦ C, reaction time: 4 h.
4. Conclusions
showed similar selectivity (95%) towards glycerol carbonate, with about 2% decrease in glycerol conversion. Further recycling of the catalyst there was no change in conversion. Interestingly as the catalyst is recycled, the selectivity towards glycerol carbonate is increased marginally. These results suggest that the SW21 catalyst is stable and reusable without any appreciable loss in activity.
Tin-tungsten mixed oxide catalysts with varied Sn/W molar ratio were synthesized and characterized by various spectroscopic techniques. These catalysts showed high activity towards the synthesis of glycerol carbonate from glycerol and urea. The activity of the catalysts depends on the Sn/W molar ratio and catalyst calcination temperature. The catalyst with Sn/W ratio 2:1 calcined at 500 ◦ C exhibited optimum activity. The glycerol conversion also depends on the reaction parameters and optimum reaction conditions were established. The catalyst is reusable without any pre-treatment and exhibited consistent activity.
60
100
Glycerol conversion (%)
90 50 80
40
70
60 30 50
20
Glycerol carbonate selectivity (%)
Acknowledgements
40 0
1
2
3
No. of cycles Fig. 13. Reusability of SW21 catalyst for carbonylation of glycerol. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, reaction temperature: 140 ◦ C, reaction time: 4 h.
The authors KJ and ChRK thank CSIR, India for the award of senior research fellowship. NL also thanks CSIR for the award of Raman Research Fellowship. References [1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 107 (2007) 2411–2502. [2] Y.L. Gu, A. Azzouzi, Y. Pouilloux, F. Jerome, J. Barrault, Green Chem. 10 (2008) 164–167. [3] N.C. Juben, W.H. George, A.D. James, Angew. Chem. Int. Ed. 46 (2007) 7164–7183. [4] M. Pagliaro, R. Ciriminna, H. Kimura, M. Rossi, C.D. Pina, Angew. Chem. Int. Ed. 46 (2007) 4434–4440. [5] F. Jerome, Y. Pouilloux, J. Barrault, ChemSusChem. 1 (2008) 586–613. [6] C.H. Zhou, J.N. Beltramini, Y.X. Fan, G.Q. Lu, Chem. Soc. Rev. 37 (2008) 527–549. [7] A. Behr, J. Eilting, K. Irawadi, J. Leschinski, F. Lindner, Green Chem. 10 (2008) 13–30. [8] K.M. Lee, J.X. Yu, L. Yan, Y.W. Kim, K.W. Chung, J. Ind. Eng. Chem. 13 (2007) 799–807. [9] A. Dibenedetto, A. Angelini, M. Aresta, J. Ethiraj, C. Fragale, F. Nocito, Tetrahedron 67 (2011) 1308–1313.
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Synthesis of glycerol carbonate from glycerol and urea over tin-tungsten mixed oxide catalysts K. Jagadeeswaraiah a , Ch. Ramesh Kumar a , P.S. Sai Prasad a , S. Loridant b , N. Lingaiah a,∗ a
Catalysis Laboratory, I & PC Division, CSIR—Indian Institute of Chemical Technology, Hyderabad 500007, India Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR 5256, CNRS, Universite Lyon I, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France b
a r t i c l e
i n f o
Article history: Received 12 June 2013 Received in revised form 17 September 2013 Accepted 24 September 2013 Available online xxx Keywords: Glycerol Glycerol carbonate Tin oxide Tungsten oxide Urea
a b s t r a c t Tin-tungsten mixed oxide catalysts with varying their mole ratio were prepared by co-precipitation method. The catalysts physico-chemical properties were derived from FT-infrared, Laser Raman, X-ray diffraction, UV–Vis DRS, BET surface area and temperature-programmed desorption of NH3 . The catalysts activities were evaluated for the synthesis of glycerol carbonate from glycerol and urea. The activity results showed that Sn-W mixed oxide catalysts are highly active for selective formation of glycerol carbonate. Sn-W catalyst with 2:1 molar ratio exhibited about 52% of glycerol conversion with >95% selectivity towards glycerol carbonate. The active catalyst was subjected to calcination at different temperatures and evaluated for their activity in glycerol carbonate synthesis. The activity of the catalysts depends on mole ratio of Sn/W and treatment temperature which are influencing the surface-structural characteristics of the catalysts. Different reaction parameters such as glycerol to urea molar ratio, reaction temperature and catalyst loading were studied and optimum conditions were established. The catalysts showed consistent activity upon repeated use. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The development of biodiesel industry from biomass resource led to over plus of glycerol, produced as a byproduct in the synthesis of biodiesel. The conversion of glycerol to valuable chemicals has attracted considerable attention in recent times not only due to its availability but also for the presence of more number of functional groups [1–10]. Among different chemicals possible from glycerol, glycerol carbonate is a valuable glycerol derivative. This is being widely used in cosmetics industry, gas separation, coatings, detergents and polymers [11–15]. A number of synthetic processes are known for the preparation of glycerol carbonate from glycerol [5]. The well known methods for preparation of glycerol carbonate are the reaction from glycerol with phosgene [16] and transesterification of glycerol with a carbonate source [17]. The most interesting route to produce glycerol carbonate is from the reaction of glycerol with CO2 in the presence of a catalyst [18]. This reaction requires elevated temperature, pressure and the yield of glycerol carbonate is too low to be used for practical purposes. The alternative route is
∗ Corresponding author. Tel.: +91 40 2719 1722; fax: +91 40 2716 0921. E-mail addresses: [email protected], [email protected] (N. Lingaiah). 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.09.041
glycerolysis in which glycerol is reacted with urea in the presence of a catalyst. This approach is an indirect route for utilization of CO2 as it is used in the production of urea. The synthesis of glycerol carbonate by glycerolysis is studied by different authors [17,19,20]. Mouloungui et al. patented the synthesis of glycerol carbonate by carbonylation of glycerol with urea over heterogeneous zinc catalysts such as zinc sulfate, zinc organosulfate and zinc ion exchange resins [19]. Calcined ZnSO4 gave 86% of glycerol carbonate in 2 h at a reaction temperature of 150 ◦ C and 40 mbar of pressure. The major drawback of this catalyst is that, ZnSO4 salt is soluble in glycerol. There is a strong need to develop heterogeneous catalyst for strategic and environmental benefit. Heterogeneous catalysts are particularly efficient for carbonylation of glycerol with urea [17,20] affording good yield under moderate reaction conditions. Climent et al. [17] have recently reported the carbonylation of glycerol with urea at 145 ◦ C over heterogeneous catalysts such as basic oxides (MgO and CaO), Al/Mg and Al/Li mixed oxides derived from hydrotalcites with adequate acid–base pairs. These catalysts showed about 72% of glycerol carbonate yield in 5 h of reaction time. In our previous study, we reported samarium exchanged heteropoly acid catalyst for carbonylation of glycerol with urea. The Sm0.66 TPA catalyst showed good selectivity (85.4%) towards glycerol carbonate with 49.5% glycerol conversion [21]. Recently Hutchings and co-workers reported the synthesis of glycerol carbonate using gold
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 469 (2014) 165–172
and Pd based supported catalysts [22]. These catalysts are expensive and overall activity is not very high and requires long reaction time. The catalysts with acidic and basic sites such as Zn-Al hydrotalcite and ␥-zirconium phosphate were also reported [17,20]. The catalysts containing basic sites were reported for the selective synthesis of glycerol carbonate with high activity [23,24]. However, these base catalysts exhibited low activity during recycling. There is a need to develop highly active and stable catalysts for the synthesis of glycerol carbonate from glycerol and urea. Solid acid catalysts are promising catalysts for the carbonylation of glycerol with urea [21]. Tungsten oxide based mixed oxide catalysts are known for their role in acid catalyzed reactions. Moreover these mixed oxide catalysts are known for their stability. In the present study, a series of tin-tungsten mixed oxide catalysts were prepared with varying Sn to W molar ratio and studied for carbonylation of glycerol with urea under reduced pressure. The surface and structural properties of the catalysts were varied by treating the samples at different temperatures. The properties of the catalysts were investigated from different spectroscopic methods to establish the catalytic activities. A detailed study was undertaken to optimize the reaction parameters to achieve best results. 2. Experimental 2.1. Preparation of Sn-W mixed oxide Sn-W mixed oxide catalysts were prepared by co-precipitation method. The preparation of Sn-W oxide with a Sn/W molar ratio of 2 is given as an example. In a typical procedure, Na2 WO4 ·2H2 O (4.94 g, 15 mmol) was dissolved in deionized water, followed by the addition of SnCl4 (7.815 g, 30 mmol) in a single portion. The solution stirred for 1 h at room temperature and deionized water was added to form white slurry. This slurry kept for stirring for 24 h at room temperature and the resulting white precipitate was filtered off, washed with large amount of deionized water until it is free from chlorine. The precipitate was dried in oven at 120 ◦ C to afford Sn-W hydroxide as a white powder. SW hydroxides with different Sn/W molar ratios (Sn/W=0.5, 1, 2, 3, and 5) were successfully prepared by changing the molar ratios of the starting metal solutions. The hydroxide precursors were calcined at 500 ◦ C for 4 h under an air atmosphere to yield final catalyst. The catalysts are denoted as SW11 where the alphabets represent the metal oxide and the numerical number related to their molar ratio. 2.2. Characterization of the catalysts X-ray diffraction (XRD) patterns of the catalysts were recorded on a Rigaku Miniflex diffractometer using CuK␣ radiation ˚ at 40 kV and 30 mA. The measurements were obtained (1.5406 A) in steps of 0.045 ◦ C with account time of 0.5 s and in the 2 range of 10◦ –80◦ . Confocal Micro-Raman spectra were recorded at room temperature in the range of 200–1200 cm−1 using a Horiba Jobin-Yvon Lab Ram HR spectrometer with a 17 mW internal He–Ne (Helium–Neon) Laser source of excitation wavelength of 632.8 nm. The catalyst samples in powder form (about 5–10 mg) were loosely spread onto a glass slide below the confocal microscope for measurements. The UV–Vis diffuse reflectance spectra (UV–Vis DRS) were recorded on a GBC UV–Visible Cintra 10e spectrometer in the range of 200–800 nm. BET Surface area was measured on Quadrasorb—SI instrument at relative pressure range of 0.05–0.3. Before analysis, the samples were degasified at 150 ◦ C for 2 h to remove moisture on the surface of the catalyst.
M-monoclinic WO3 * SnO2
* * * *
*
*
(e)
M
Intensity (a.u)
166
M
(d)
(c) (b) (a)
10
20
30
40
50
60
70
80
o
2Theta ( ) Fig. 1. XRD profiles of the SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21 (c) SW31 (d) SW51 (e) SW12.
Temperature programmed desorption of ammonia (TPD-NH3 ) was carried out on a laboratory-built apparatus equipped with a gas chromatograph using TCD detector. In a typical experiment about 0.05 g of oven dried sample was taken in a quartz tube. Prior to TPD run, the catalyst sample was treated at 300 ◦ C for 1 h by passing pure He gas (50 ml/min). After pretreatment, the sample was saturated with anhydrous ammonia (10% NH3 balance He gas) at 100 ◦ C with a flow rate of 50 ml/min for 1 h and was subsequently flushed with He gas at the same temperature to remove physisorbed ammonia. The process was continued until a stabilized base line was obtained in the gas chromatograph. Then the TPD analysis was carried out from ambient temperature to 700 ◦ C at a heating rate of 10 ◦ C/min. The amount of NH3 evolved was calculated from the peak area of the already calibrated TCD signal. 2.3. Reaction procedure The reactions were performed in a 25 mL two neck roundbottom (RB) flask under reduced pressures. In a typical experiment glycerol (2 g), urea (1.306 g) and catalysts (0.2 g) were taken in the round bottom flask and heated in an oil bath at 140 ◦ C with constant stirring. One neck of the RB flask was connected to vacuum line. Reaction was run under a reduced pressure in order to remove the ammonia formed during the reaction. After completion of reaction or stipulated time, methanol was added and the catalyst was separated by filtration. The products were analyzed by a gas chromatograph (Shimadzu, 2010) equipped with flame ionization detector using inno wax capillary column (diameter: 0.25 mm, length 30 m). Products were also identified by GC–MS (Shimadzu, GCMS-QP2010S) analysis. 3. Results and discussion 3.1. Catalyst characterization X-ray diffraction patterns of SW catalysts calcined at 500 ◦ C are shown in Fig. 1. The XRD patterns of SW catalysts with the molar ratio of 1:1 and 2:1 were intrinsically identical to that of amorphous Sn-W hydroxide precursor. Whereas in the case of SW12, SW31 catalysts, intense diffraction peaks related to crystalline tin oxide were observed. The peaks observed at 2 values of 26.5◦ , 33.9◦ , 37.9◦ , 43.5◦ , 51.8◦ , 54.7◦ and 61.9◦ are corresponding to tetragonal
167
(e)
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Absorbance (a.u)
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303
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(f)
(e) (d) (c) (b) (a)
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400
600
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rutile structure SnO2 crystal planes of (1 1 0), (1 0 1), (2 0 0), (2 1 0), (2 1 1), (2 2 0) and (0 0 2), respectively [25–27]. Catalyst with 5:1 molar ratio of Sn/W showed diffractions peaks related to both tin oxide and tungsten oxide. The diffraction peaks at 2 = 23.3◦ , 24.3◦ , 33.29◦ and 34.1◦ can be indexed to (0 0 2), (2 0 0), (0 2 2) and (2 0 2) of monoclinic WO3 phase, respectively (JCPDS No. 43-1035). These peak positions and their relative intensities are in good agreement with the literature [28]. The catalysts with Sn/W molar ratio of 1:1, 2:1 and 3:1 exhibited diffraction peaks related to only SnO2 and patterns related to WO3 were absent. This might be due to (i) strong interaction between tin oxide and tungsten oxide species, (ii) poly tungstate species are highly dispersed on SnO2 and formation of homogeneous solid solution. However, SW51 catalyst showed peaks related to WO3 . This indicates that, agglomeration of tungsten oxide species took place at high Sn to W ratio. Thus, homogeneity of the mixed oxide lost beyond the Sn/W molar ratio of 3:1. These results suggest that catalysts with Sn/W molar ratio of 1:1 to 3:1 form a homogeneous solid mixed oxide. Fig. 2 shows the Raman spectra of the catalysts. The catalyst SW11 showed an intense band at 953 cm−1 corresponding to stretching mode for octahedrally coordinated poly tungstate species [29]. When the SW molar ratio varied from 1:1 to 3:1 and 1:2, the band at 953 cm−1 related to W=Ot bond shifted to higher wave number (953 to 979 cm−1 ). This suggests that different two-dimensional tungsten oxide species might be present in Sn/W catalysts and no crystalline WO3 phase was present. The SW51 catalyst showed bands at 273, 717 and 808 cm−1 corresponding to W–O–W deformation mode, W=O bending and W=O stretching mode, respectively, of crystalline WO3 [30,31]. These results indicate that, 1:1 to 3:1 ratio of S/W mixed oxide form a homogeneous mixture. The XRD patterns also supports the results observed from Raman spectra. UV–Vis diffuse reflectance spectral technique is sensitive for distinguishing the chemical nature and coordination states of tungsten oxide species. Fig. 3 shows the UV–Vis DRS of SW catalysts along with bulk WO3 . Catalyst with SW molar ratio of 1:1 clearly showed a absorption band at 303 nm which corresponds to polymeric WO6 species [32]. This band is unresolved in the case of catalysts SW21, SW31 and SW12 due to strong absorption in the range of 220–378 nm. In general bulk WO3 shows a broad band at 398 nm with a shoulder at 359 nm [33]. UV–Vis DRS of bulk WO3 sample showed a broad band centred at 390 nm. On the other hand
500
600
700
800
Fig. 3. UV–Vis diffused reflectance spectra of SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21, (c) SW31, (d) SW12, (e) SW51 and (f) WO3 .
SW mixed oxide catalysts (except SW51) showed broad bands in the range of 220–378 nm and band at 398 was absent indicates that these broad bands can be assigned to isolated tungsten species or low-condensed polymeric tungsten oxide species [33]. In the case of SW51 catalyst, in addition to the broad band in the range of 220–360 nm, a shoulder peak is observed at 398 nm which corresponds to the bulk WO3 . These results indicate that SW51 catalyst exists in two phases. The results obtained from XRD and Raman results are in good agreement with these results. The TPD profiles of the SW catalysts along with pure WO3 are shown in Fig. 4. The amount of ammonia desorbed from the catalyst surface is calculated and presented in Table 1. All the samples showed broad desorption peaks indicating that the surface acid strength is widely distributed. SW11 catalyst showed two unresolved desorption peaks at 390 and 500 ◦ C corresponding to moderate and strong acidic sites. Whereas the SW21 catalyst showed two broad and intense desorption peaks in the range of 100–460 ◦ C and 450–650 ◦ C. The peak centred at 250 ◦ C is related to
(f) (e)
(d)
Intensity (a.u)
Fig. 2. Laser Raman spectra of SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21, (c) SW31, (d) SW51 and (e) SW12.
400
Wavelength (nm)
-1
Raman shift (cm )
(c)
(b) (a)
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200
300
400
o
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Temperature ( C) Fig. 4. Temperature programmed desorption of ammonia patterns of the SW mixed oxide catalysts calcined at 500 ◦ C. (a) SW11, (b) SW21, (c) SW31, (d) SW12, (e) WO3 and (f) blank run of SW21.
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Table 1 Acid strength distribution and BET surface area of SW mixed oxide catalysts.
SW11 SW12 SW21 SW31 SW400 SW600 SW700
Acidity (mmol/g) Weak/moderate
Strong
Total
0.085 0.195 0.404 0.231 0.021 0.177 0.136
0.020 0.015 0.068 0.035 0.018 0.045 0.016
0.105 0.210 0.472 0.266 0.039 0.222 0.142
BET surface area (m2 /g) 12.6 27.7 37.4 47.6 14.8 39.5 51.1
moderate acidic sites and other one centred at 530 ◦ C corresponds to strong acidic sites. Catalyst with a Sn/W molar ratio of 3:1 (SW31) showed a strong desorption peak at 200 ◦ C, along with small peaks centred at 370 and 520 ◦ C. The first peak is related to weak acidic sites and two small desorption peaks corresponding to moderate and strong acidic sites. SW12 catalyst showed two desorption peaks at 380, 515 ◦ C related to moderate and strong acidic sites, respectively, along with a small hump at 180 ◦ C. The intensity of the desorption peaks related to moderate to strong acid sites increased with increase in Sn/W ratio up to 2:1. Further increase in Sn/W ratio to 3:1 intensity of the desorption peaks decreased and shifted to lower temperatures. In the case of bulk WO3 sample, there is no considerable ammonia desorption peak and desorbed ammonia was found to be negligible. Compared to all other catalysts, SW21 showed broad and strong desorption peaks. The acidity values of SW21 catalysts (Table 1) indicating that this catalyst contains high acidity compared to all other catalysts. The high acidity of this catalyst might be related to the presence of highly dispersed WO3 domains on SnO2 as evidenced from XRD and Raman analysis. In order to know the nature of the gas released, a TPD run of SW21 catalyst was performed without pre-treatment of ammonia gas and the TPD profile is shown in Fig. 4. From the TPD profile it can be noted that no considerable peak is observed above 500 ◦ C. This result indicates that the desorbed gas during TPD run is related to ammonia adsorbed on the acidic sites of the catalyst. BET surface areas of the catalysts are presented in Table 1. From the table it can be observed that as the increase in SW molar ratio from 1:1 to 3:1 the surface area of the catalysts also increased. Catalyst with SW molar ratio of 1:2 showed a surface area of 27.7 m2 /g. In the case of SW21 catalyst calcined at in the range of 400–700 ◦ C exhibited an increase in BET surface area with increased calcination temperature. This might be due to removal of hydroxyl groups on the catalysts surface in the form of water molecules with increase in calcination temperature. 3.2. Catalytic activity Catalysts performance was investigated for carbonylation of glycerol with urea and the results are presented in Table 2. The reaction was also performed without catalyst and the conversion Table 2 Glycerol conversion and glycerol carbonate selectivity over SW mixed oxide catalysts. Catalyst
Conversion of glycerol (%)
Glycerol carbonate selectivity (%)
Blank SnO2 SW12 SW11 SW21 SW31 SW51 WO3
10.5 13.6 30.3 43.6 52.1 52.6 36.0 26.2
19.3 35.1 47.7 67.5 95.3 66.4 54.1 42.0
100
90 70
Glycerol conversion (%)
Catalyst
80
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50 40 40
Glycerol carbonate selectvity (%)
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30 400
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600
700 o
Catalyst calcination temperature ( C) Fig. 5. Effect of catalyst calcinations temperature on the glycerol conversion and glycerol carbonate selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, Reaction temperature: 140 ◦ C, reaction time: 4 h.
of glycerol was about 10%. Pure WO3 and SnO2 showed 26% and 14% of glycerol conversion with 42% and 35% selectivity towards glycerol carbonate, respectively. The main by-products formed in the glycerol carbonylation are 5-(hydroxymethyl) oxazolidin-2one and (2-oxo-1,3-dioxolan-4-yl)methyl carbamate. In the case of SW12 catalyst, 30% of glycerol conversion was observed with 48% selectivity. With increase in Sn/W molar ratio from 1:1 to 3:1, the conversion of glycerol increased up to 52% and at the same time the selectivity towards glycerol carbonate is varied significantly. The catalyst with Sn/W molar ratio of 1:1 exhibited about 67% selectivity. Although SW21 and SW31 catalysts showed same glycerol conversion, but high selectivity about 95% towards GC was observed for SW21 catalyst. SW31 catalyst showed about 66% selectivity. The high activity of SW21 catalyst can be explained based on the catalysts characteristics. This catalyst exhibited high acidity compared to other catalysts. The acidity profiles of the catalysts derived from ammonia TPD are in good agreement with the observed catalytic activity, as SW21 catalyst showed strong and intense desorption peaks of ammonia. The presence of high acidity is mainly due to the presence of nano sized WO3 domains on SnO2 . The existence of this type of WO3 species are supported by XRD and Raman analysis. These results indicate that catalyst Sn/W with 2:1 molar ratio is the most active catalyst compared to its counter parts. In order to know the thermal stability, surface and structural changes of the catalyst, the most active catalyst was subjected to calcination at different temperatures and the catalytic activity was investigated for carbonylation of glycerol. Fig. 5 shows the catalytic performance of SW21 catalyst calcined at different temperatures. With increase in catalyst calcination temperature from 400 to 500 ◦ C conversion of glycerol increased from 37% to 52% and selectivity towards glycerol carbonate also increased from 71% to 95%. Increase in calcination temperature to 600 ◦ C, conversion of glycerol was decreased to 37% and glycerol carbonate selectivity is found to be 71%. Further increase in catalyst calcination temperature to 700 ◦ C, there is no variation in the glycerol conversion compared to catalyst calcined at 600 ◦ C, however, glycerol carbonate selectivity was decreased to 62%. The SW21 catalyst calcined at different temperatures was further characterized by XRD and Raman spectroscopy to understand the difference in activity with variation in treatment temperature. XRD patterns of SW21 catalysts calcined at different temperatures are shown in Fig. 6. The catalyst calcined at 400 and 500 ◦ C
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169
M-monoclinic WO3 * SnO2
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2 Theta Fig. 6. XRD profiles of the SW21 catalyst calcined at (a) 400 ◦ C, (b) 500 ◦ C, (c) 600 ◦ C and. (d) 700 ◦ C.
showed similar pattern as that of amorphous Sn-W hydroxide precursor. Upon increasing the calcination temperature to 600 ◦ C, crystalline phase of tin oxide was formed. Further increase in calcination temperature to 700 ◦ C, peaks associated to monoclinic phase of WO3 phase were observed at 2 values of 23.2◦ and 24.41◦ [28]. The high temperature calcination resulted in the formation of poly tungstate aggregates. XRD results suggest that the crystallinity increased with calcination temperature. The presence of well dispersed amorphous phase of WO3 was maintained up to 600 ◦ C and formation of WO3 crystallites were noticed at 700 ◦ C. XRD results suggest that, at higher calcination temperatures (above 500 ◦ C), homogeneity of the mixed oxide was lost and leads to formation of polytungstate aggregates. Laser Raman spectra of SW21 catalyst calcined at different temperatures are shown in Fig. 7. All the catalysts showed an intense band related to W=Ot bond. Catalyst calcined at 400 ◦ C showed a band at 981 cm−1 , whereas for the catalyst calcined at higher
Intensity (a.u.)
(d) (c)
(b) (a)
200
400
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1000
-1
Raman shift (cm ) Fig. 7. Laser Raman spectra of SW21 catalyst calcined at (a) 400 ◦ C, (b) 500 ◦ C, (c) 600 ◦ C. and (d) 700 ◦ C.
Fig. 8. Temperature programmed desorption of ammonia patterns of SW21 catalyst calcined. at (a) 400 ◦ C, (b) 500 ◦ C, (c) 600 ◦ C and (d) 700 ◦ C.
temperatures (≥500 ◦ C) this band is slightly shifted to lower wave numbers. Intensity of this band increased for the sample calcined at 500 ◦ C and further increase in calcination temperatures to 600 and 700 ◦ C, intensity of the band is decreased. This might be due to loss in homogeneity of solid mixture. XRD results are in support for observations made from Laser Raman spectra. Fig. 8 shows the TPD of ammonia patterns of SW21 catalyst calcined at different temperatures. Catalyst calcined at 400 ◦ C showed a small desorption peak at 320 ◦ C. A drastic change in the acidity profile was observed for the catalyst calcined at 500 ◦ C. It showed a strong desorption peaks at 250 and 530 ◦ C corresponding to moderate and strong acidic sites as discussed earlier. Catalyst calcined at 600 ◦ C showed three desorption peaks with low intensity. Desorption peaks centred at 150 and 270 ◦ C corresponds to weak and moderate acidic sites, respectively. Another small desorption peak observed at 580 ◦ C related to strong acidic sites. These desorption peaks intensity was further decreased with increase in calcination temperature to 700 ◦ C. The observation made from the XRD, Raman and TPD of NH3 suggests that the catalyst calcined at 500 ◦ C has high acidity due to the presence of well dispersed WO3 domains on SnO2 surface. The high activity of the catalyst is related to its surface and structural characteristics, which are directing the acidity of the catalysts. Low conversion of glycerol for the catalyst calcined at higher temperature related to decrease in acidity due to the formation of large ensembles of WO3 . Conversion of glycerol and glycerol carbonate selectivity not only depends on the nature of the catalyst but also on reaction parameters. In order to optimize the reaction conditions, different reaction parameters such as reaction time, reaction temperature, glycerol to urea molar ratio and catalyst weight were studied by using the most active SW21 catalyst calcined at 500 ◦ C. 3.3. Influence of the reaction time Reaction time is an important parameter and long reaction times were adopted usually for the carbonylation of glycerol to obtain reasonable conversion. The effect of reaction time was studied over SW21 catalyst calcined at 500 ◦ C and the results are shown in Fig. 9. The conversion of glycerol consistently increased with increase in reaction time from 1 to 4 h. The selectivity towards glycerol carbonate is varied marginally with time. The overall selectivity of the catalyst with reaction time is high suggesting that the catalyst
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60
60
40
40
20
20
0
0 1
2
3
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60
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40
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20
1:0.5
4
Reaction time (h)
1:1
1:2
Glycerol carbonate selectivity (%)
80
100
Glycerol conversion (%)
100
Glycerol conversion (%)
100
Glycerol carbonate selectivity (%)
170
1:3
Glycerol to urea molar ratio
Fig. 9. Effect of reaction time on glycerol conversion and glycerol carbonates selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, reaction temperature: 140 ◦ C.
Fig. 11. Influence of glycerol to urea molar ratio on glycerol conversion and glycerol carbonate selectivity. Reaction conditions: catalyst weight: 0.2 g, reaction temperature: 140 ◦ C, Reaction time: 4 h.
is selective in formation of glycerol carbonate. A decrease in the yield of glycerol carbonate was reported with reaction time over ␥-ZrP catalysts during glycerolysis of urea [20]. The present catalyst is selective in formation of glycerol carbonate and there was no increase in formation of byproducts with reaction time.
(55%). These results suggest that the optimum reaction temperature is 140 ◦ C.
3.4. Effect of reaction temperature In order to evaluate the influence of reaction temperature reaction was carried out at different reaction temperatures ranging from 100 to 160 ◦ C and the results are shown in Fig. 10. As expected, the conversion of glycerol increased with reaction temperature. Conversion of glycerol is only 13% at a reaction temperature of 100 ◦ C with 62% selectivity towards glycerol carbonate. As the temperature increased from 120 to 140 ◦ C the conversion of glycerol also increases from 29% to 52%. At the same time selectivity to glycerol carbonate is also increased from 78% to 95%. Further increase in reaction temperature selectivity towards glycerol carbonate decreased to 80% with marginal increase in glycerol conversion
100
3.5. Effect of glycerol to urea molar ratio In order to study the influence of molar ratio of reactants the reaction carried with variation in glycerol to urea molar ratio and the results are shown in Fig. 11. From the figure it can be noted that conversion of glycerol increased from 31% to 59% with increase in the concentration of urea, at the same time selectivity towards glycerol carbonate decreased from 100 to 61%. The decrease in selectivity with increase in urea concentration related to formation of a by-product which forms due to the reaction of free hydroxyl group present in the glycerol carbonate with available urea. When the glycerol to urea molar ratio is 1:1, the conversion of glycerol is 52% with 95% selectivity towards glycerol carbonate. Further increase in urea concentration (1:2 and 1:3 molar ratio of glycerol to urea) the conversion of glycerol is increased. However selectivity towards glycerol carbonate is decreased. These results suggest that 1:1 molar ratio of glycerol to urea is optimum for high glycerol carbonate selectivity.
100
80
80
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40
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20
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Glycerol carbonate selectivity (%)
Glycerol conversion (%)
3.6. Effect of catalyst weight
160 o
Reaction temperature ( c) Fig. 10. Effect of reaction temperature on glycerol conversion and glycerol carbonates selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, reaction time: 4 h.
Fig. 12 shows the effect of catalyst weight on the glycerol conversion. Increase in catalyst loading from 100 to 200 mg the conversion of glycerol increased from 26% to 52%. Selectivity also increased from 63% to 95%. Further increase in catalyst loading to 200–400 mg, only a marginal increase in glycerol conversion was observed with a decrease in selectivity towards glycerol carbonate. The catalyst present in excess might be favouring the reaction between the product glycerol carbonate and urea to yield 5-hydroxymethyloxazolidine-2-one. 3.7. Catalyst reusability The most active SW21 catalyst was studied for its reusability and results are presented in Fig. 13. Fresh catalyst showed about 52% of glycerol conversion with 95% selectivity towards glycerol carbonate. After completion of the reaction, methanol was added to reaction mixture and the catalyst was separated by filtration. The separated catalyst was washed with methanol, dried in oven and reused without any pre-treatment. In the first recycle the catalyst
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Table 3 Comparison of catalytic activity of the present catalyst with other reported solid acid catalysts. Reaction temperature (◦ C)
Catalyst
SW21 Sn-beta zeolite Zr-P* Au/Fe2 O3 2.5% Au/Nb2 O5 Sm0.66 TPA
Conversion of glycerol (%)
4 5 3 4 4 4
52.1 70 80 80 66 49.5
Glycerol carbonate (%) Yield
Selectivity
49.7 26 76 39 21 42.3
95.3 37 100 48 32 85.4
Ref.
Present work [15] [18] [28] [28] [19]
This catalysts contains both acidic and basic sites.
70 80
Glycerol conversion (%)
3.8. Comparison of SW21catalyst activity with reported solid acid catalysts
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80
60
60
50
40 40
Glycerol carbonate selectivity (%)
*
140 145 140 150 150 140
Reaction time (h)
30
20
20 100
200
300
400
Catalyst weight (mg)
Catalytic activity of SW21 catalyst is compared with other reported solid acid catalysts and comparative results are presented in Table 3. Among the different catalysts zirconium phosphate catalysts exhibited about 80% glycerol conversion with 100% selectivity towards GC [20]. This catalyst contains both acidic and basic sites. However, its activity decreased with reaction time and showed decline in activity upon reuse. Compared to acidic catalysts base catalysts such as La2 CO3 [23] and gold supported on MgO [24] relatively showed high activity in the synthesis of glycerol carbonate. Even though these catalysts are highly active their reusability is limited. The present solid acid catalysts showed better activity and selectivity compared to other reported acid catalysts such as Sn-beta zeolite, samarium exchanged tungstophosphoric acid (Sm0.66 TPA) and gold supported on acidic supports. Moreover, the present catalysts is reusable compared other basic and acidic catalysts.
Fig. 12. Effect of catalyst weight on glycerol conversion and glycerol carbonate selectivity. Reaction conditions: glycerol: 2 g, urea: 1.306 g, reaction temperature: 140 ◦ C, reaction time: 4 h.
4. Conclusions
showed similar selectivity (95%) towards glycerol carbonate, with about 2% decrease in glycerol conversion. Further recycling of the catalyst there was no change in conversion. Interestingly as the catalyst is recycled, the selectivity towards glycerol carbonate is increased marginally. These results suggest that the SW21 catalyst is stable and reusable without any appreciable loss in activity.
Tin-tungsten mixed oxide catalysts with varied Sn/W molar ratio were synthesized and characterized by various spectroscopic techniques. These catalysts showed high activity towards the synthesis of glycerol carbonate from glycerol and urea. The activity of the catalysts depends on the Sn/W molar ratio and catalyst calcination temperature. The catalyst with Sn/W ratio 2:1 calcined at 500 ◦ C exhibited optimum activity. The glycerol conversion also depends on the reaction parameters and optimum reaction conditions were established. The catalyst is reusable without any pre-treatment and exhibited consistent activity.
60
100
Glycerol conversion (%)
90 50 80
40
70
60 30 50
20
Glycerol carbonate selectivity (%)
Acknowledgements
40 0
1
2
3
No. of cycles Fig. 13. Reusability of SW21 catalyst for carbonylation of glycerol. Reaction conditions: glycerol: 2 g, urea: 1.306 g, catalyst weight: 0.2 g, reaction temperature: 140 ◦ C, reaction time: 4 h.
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