CHAPTER VI Synthesis of glycerol acetins from glycerol using different synthetic routes

Transcription

1 Chapter VI Synthesis of glycerol acetins by different synthetic routes 2015 CHAPTER VI Synthesis of glycerol acetins from glycerol using different synthetic routes 6.1. Acetylation and esterification of glycerol Transesterification of glycerol with methyl acetate.

2 6.1. Synthesis of glycerol acetins from acetylation and esterification of glycerol Introduction Glycerol undergoes esterification or acetylation with acetic acid or acetic anhydride respectively in presence of acid catalyst to yield acetins namely monoacetin, diacetin and triacetin (Scheme 6.1.1). Di and triacetin can be used as fuel additives which have been introduced in biodiesel formulation to improve its viscosity property as cold flow improver and it has also been used as an antiknock additive for gasoline. Triacetins are also used in cosmetics, whereas monoacetin and diacetin are used as plasticizer in cigarette filters and as raw materials for the production of biodegradable polyesters [1]. Homogeneous acids showed higher catalytic activities towards acetylation reaction with acetic anhydride [2], but faced several practical difficulties in separation of catalyst, recyclability and difficult in handling. To overcome these difficulties, a variety of Brønsted solid acid catalysts have been reported for reaction of glycerol with acetic acid or acetic anhydride. Solid acid catalysts such as AB-15, montmorillonite K-10, beta zeolite and H-USY [3, 4] were applied as catalysts for this reaction which showed high glycerol conversion but with high catalyst concentration. Supported sulfonic acid catalysts [5-7] and mixed oxides like MoO3/ TiO2 ZrO2 [8] have been reported to be active catalysts for acetylation of glycerol. Even supported heteropoly acid catalysts like PW on silica, zirconia, carbon, niobic acid with high thermal stability and high surface area have been used but showed less efficiency for this reaction [9-13]. The aim of this work is to explore a catalyst for the synthesis of glycerol acetins at mild reaction conditions (lower temperature, mole ratio and catalyst weight) and to get higher activity and selectivity to triacetins compared to other reported solid acid catalysts. In this chapter, cesium salt of phosphotungstic acid (Cs/PWA) is studied as an efficient solid acid catalyst for the esterification and acetylation of glycerol using acetic acid and acetic anhydride respectively under mild conditions. 111

3 Chemicals and Reagents Glycerol, acetic acid and acetic anhydride were purchased from Merck India Ltd. Cesium carbonate and phosphotungstic acid (PWA) were procured from SD fine chemicals. Amberlyst-15 (hereafter AB-15) was obtained from Alfa Aesar, USA. The montmorillonite K-10 clay was purchased from Sigma Aldrich, USA. H-beta (SAR- 25) was kindly donated by Süd-Chemie India Pvt Ltd. All the chemicals were of research grade and used without any further purification Catalyst preparation The acidic cesium phosphotungstate (Cs/PWA) salt was prepared by adding drop wise the required amount of aqueous cesium carbonate (0.47 M) to aqueous solution of H3PW12O40 (0.75 M) at room temperature with stirring. The precipitate obtained was evaporated under water bath at 80 C until complete dryness. Obtained powder was dried in an oven at 120 C for 4 h and activated at 150 C before use Characterization- Results and Discussion Powder X-ray diffraction pattern (PXRD) Fig XRD patterns of Cs/PWA and PWA 112

4 Powder X-ray diffraction patterns of Cs/PWA and PWA were recorded with Bruker D2 phaser X-ray diffractometer using CuKα radiation (λ = Å) with high resolution Lynxeye detector. All the samples were scanned in the 2θ range of The crystalline phase formation of Cs/PWA catalyst was confirmed by XRD with comparison of phosphotungstic acid (Fig ) Fourier transform infrared spectra (FT-IR) The characteristic presence of Keggin structure of Cs/PWA and phosphotungstic acid was confirmed by FTIR studies (Fig ). Four bands at cm -1 region corresponding to Keggin unit (HPW) structural vibrations are observed for PWA and Cs/PWA suggesting that the framework of primary Keggin Fig FT-IR spectra of Cs/PWA and PWA structure remained unaltered after modification of PWA with cesium salt. The peaks corresponding to Keggin anion vibration are as follows. The stretching frequency of P-O in the central PO4 tetrahedron is at 1084 cm -1. The peak at 991 cm -1 is due to the terminal W=O vibration in the WO6 octahedron and the peak at 890 and 794 cm -1 were assigned to W-Ob-W and W-Oc-W bridges respectively. Weaker peaks appearing at 595 cm -1 is due to bending vibrations of W-O-W bonds. 113

5 Scanning Electron Microscopy (SEM) measurement Scanning electron microscope (SEM) images of Cs/PWA catalyst were recorded on Zeiss microscope to investigate the particle size and morphology. Cs/PWA catalyst exhibited the morphology of the spherical shaped particles with size ranging from nm as shown in Fig Fig SEM images of Cs/PWA Pyridine- FT-IR The nature of acidic sites of catalysts was investigated by pyridine adsorption study using Pyridine-FT-IR (alpha-t, Bruker) and the spectra were obtained in the range of cm 1 and cm 1. The catalyst pellets were saturated by pyridine followed by degassing at 150 C for 1 h. The FTIR spectra in absorbance mode for pyridine treated sample were subtracted with pyridine untreated sample to obtain the peaks only due to pyridine acid interaction [14]. The interaction of pyridine nitrogen with acidic sites gave two different frequency of bending vibrations. The bending vibrations around 1445 cm -1 and 1540 cm -1 are assigned as Lewis (L) and Brönsted (B) acid sites respectively and B/L ratio were measured using the peak intensities. FTIR pyridine adsorption spectra of Cs/PWA catalyst showed a strong Brönsted acidity due to the presence of protons (peak at 1540 cm -1 ) and weak Lewis acid sites (peak at 1445 cm -1 ). The Cs/PWA contained highest B/L ratio of 3.86 compared to other solid acid catalysts used in this 114

6 Table Acidity and catalytic activity with different catalysts Catalyst Amount of acidity (mmol /g) a Py- FTIR, B/L ratio Cs/PWA Amberlyst Acetylation Esterification TOF/h b * * TOF/h b H-beta Montmorillonite K-10 Sulfated zirconia (SZ) a = Potentiometric titration, b =TOF= moles of glycerol converted per mole of active sites / h, * Catalyst concentration = 1wt %. study (Table ). The B/L ratio decreased in the order; Cs/PWA > montmorillonite K-10 > H-beta > sulfated zirconia. AB-15 is a pure Bronsted acid catalyst with sulfonic acid groups on polystyrene chain. The granular form of AB-15 was not recommended to be crushed for pelletization which may give an inaccurate value and hence, pyridine adsorption study was not conducted Potentiometric titration Potentiometric acid-base titration revealed the total acidity of the catalysts (tabulated in Table ) The total acidity of Cs/PWA, H-Beta, Montmorillonite K10 and SZ was found to be 1.87, 1.49, 1.10 and 1.48 mmol/g respectively. H-beta zeolite and sulfated zirconia has same amount of acidic sites. Acidity of AB-15 was obtained from the manufacturer and it was found to be 4.7 mmol/g. 115

7 Catalytic activity studies Experimental Procedure Catalytic activity studies were performed in a liquid phase glass batch reactor. Prior to the reaction, the catalysts were activated at 120 C to remove the moisture. Acetic anhydride Acid catalyst Monoacetin Glycerol Acetic acid Diacetin Triacetin Scheme Acetylation and esterification of glycerol with acetic anhydride/ acetic acid Acetylation reaction of glycerol with acetic anhydride The reaction was performed in a 100 ml two-necked glass reactor equipped with a magnetic stirring bar, a Liebig condenser, and a thermometer. The glycerol and acetic anhydride were taken in the ratio of 1: 3 in the glass reactor and 4 wt% of catalyst (with respect to total reactants) was added into it. The reaction was performed under stirring at room temperature Esterification reaction with glycerol and acetic acid The reaction was carried out in a 100 ml two-necked glass reactor equipped with a magnetic stirring bar, a Liebig condenser, and a thermometer. The required 116

8 amounts of glycerol and acetic acid were taken in the reactor and desired catalyst weight was added into it. The reaction was performed under stirring at desired temperature. For both the reactions, same separation procedure was followed, the mixture was taken out and centrifuged for 10 min to separate the catalyst from liquid phase. The obtained product was analyzed in gas chromatography (Shimadzu, GC-2014) with flame ionization detector (FID) equipped with capillary column (0.25mm I.D and 30 m length, Stabilwax, Restek). All the products were confirmed by gas chromatography with mass spectroscopy (Shimadzu, GCMS QP 2010) Results and Discussion Acetylation and esterification of glycerol was studied over various solid acid catalysts namely, AB-15, H-beta, sulfated zirconia and montmorillonite K-10 using acetic anhydride and acetic acid respectively. The performance of the catalyst is measured by glycerol conversion and the selectivity to di and triacetins Acetylation of glycerol with acetic anhydride Acetylation of glycerol using acetic anhydride was carried out over different Brönsted solid acid catalysts at room temperature (30 C) (Fig ). Prior to the catalytic reaction, a blank run was carried out without a catalyst, which resulted in negligible glycerol conversion (2.5 %) with 100 % selectivity to monoacetins. Among the solid acid catalysts screened, the catalyst containing higher amount of acid sites viz. Cs/PWA (1.87 mmol/g) and AB-15 (4.7 mmol/g) resulted in maximum glycerol conversion (100 %) with higher DGA+TGA (glycerol diacetins and glycerol triacetins) selectivity of 99.1 % and 99.9 % respectively. The catalytic activity of Cs/PWA showed higher triacetin selectivity of 82 % at room temperature compared to all other solid acid catalysts for 2 h of reaction time. This result shows that the utilization of acetic anhydride is maximum for Cs/PWA and AB-15 with higher 117

9 Fig Catalytic activity of different solid acid catalysts with glycerol and acetic anhydride. Reaction conditions: Glycerol : Acetic anhydride = 1 : 3, Temperature = 30 C, Time= 120 min, Catalyst weight = 4 wt%. selectivity to triacetins compared with other catalysts namely K-10, H-beta and sulfated zirconia. The glycerol conversion reached to a maximum of 100 % at the initial time period, but the triacetin selectivity was found to increase with time for Cs/PWA and AB-15 with a decrease in mono and di-acetins selectivity (Fig ). Montmorillonite K10 containing B/L ratio of 2.3 gave lower triacetin selectivity of 32 %. H-beta catalyst (B/L ratio 1.92) resulted 80 % diacetin selectivity. Sulfated zirconia, the catalyst with higher Lewis acidic sites showed very low glycerol conversion of 25 %. These results clearly show that, the catalyst with higher Brönsted acidic sites gives higher glycerol conversion with high DGA+TGA selectivity. The 118

10 glycerol conversion and triacetin increased for the catalysts in the following order; SZ < H-beta < Montmorillonite-K10 < AB-15 < Cs/PWA. Since Cs/PWA and AB-15 showed complete glycerol conversion at 30 min, it was not possible to decide the best catalyst among the two. Therefore, the Fig Comparison of Cs/PWA and AB - 15 with lower catalyst concentrations Reaction conditions: Glycerol : Acetic anhydride = 1 : 3, Temperature = 30 C, Time= 120 min, Catalyst weight = 1 wt%. catalyst concentration was reduced to 1 wt % (w.r.t. total reactants) and as a result, the catalytic performance of AB-15 showed lower glycerol conversion of 25% at the 30 min. As the time increased, glycerol converted completely with increase in triacetin selectivity. But Cs/PWA catalyst showed 99.8 % glycerol conversion even at less catalyst amount for 30 min with higher triacetin selectivity compared to AB-15 (Fig ). The turn over frequency of all the catalysts increased in the following order: SZ < AB-15 < H-beta < K-10 < Cs/PWA. Higher TOF/h of 267 was observed with 119

11 Cs/PWA which clearly proves that Cs/PWA is highly active catalyst for acetylation reaction of glycerol Esterification of glycerol with acetic acid The esterification of glycerol was studied using acetic acid under reaction conditions; reactants mole ratio of glycerol to acetic acid of 1:6, 85 C and 7 wt% of Fig Catalytic activity of different solid acid catalysts with glycerol and acetic acid. Reaction conditions: Glycerol : Acetic acid = 1: 6, Temperature = 85 C, Time= 5 h, Catalyst weight = 7 wt% (total reactants). catalyst referred to total reactants (Fig ). As observed in the acetylation reaction, a similar catalytic performance was observed with high performance of Cs/PWA and AB-15 compared with other tested catalysts. The glycerol conversion reached to 98 % using Cs/PWA and AB-15 within 2 h with increase in diacetins and triacetin selectivity. Cs/PWA exhibited higher catalytic performance with triacetin 120

12 selectivity of 27 %, whereas AB-15 gave 22 % triacetin selectivity. Among these two catalysts, Cs/PWA exhibited much higher TOF at 30.5 h -1 compared to AB-15 (12.3 h - 1 ) (Table 6.1.1). Among lower active catalysts, large pore H-beta zeolite exhibited comparatively greater catalytic performance than montmorillonite K-10 and sulfated zirconia. Cs/PWA Cs/PWA a) Protonated acetic acid on catalyst surface and its dehydration to the acylium ion Cs/PWA Cs/PWA b) Protonated acetic anhydride on catalyst surface and its transformation to the acylium ion Scheme Formation of intermediate acylium ion Glycerol conversion increased from 28 % to 80 % with increase in time period from 1 to 5 h using H-beta zeolite and finally reached to 37 % diacetins selectivity (5 h). Triacetin did not form with H-beta catalyst. In contrast, montmorillonite K10 clay showed lower glycerol conversion (63 %) compared to H-beta zeolite but montmorillonite K10 attained triacetin selectivity of 4 % (5 h). Glycerol conversion of 70 % with 20 % diacetin selectivity was observed using sulfated zirconia catalyst. Sulfated zirconia exhibited lower activity compared to other acid catalysts which could be due to lower B/L ratio (1.46), since the esterification reactions are predominantly catalyzed by Brönsted acid sites. Thus, the catalytic activity towards esterification of glycerol with acetic acid gives a clear picture with respect to nature of acidic sites (B/L ratio) of catalyst. 121

13 The turn over frequency of the screened catalysts increased in the following order: SZ< AB-15 H-Beta < Montmorillonite K10 < Cs/PWA. The high selectivity towards triacetin using acetic anhydride as acetylating agent compared to acetic acid can be explained on the basis of formation of intermediate acylium ion [19]. Before the formation of acyl cation, the interaction of Cs/PWA catalyst surface with protonated acetic acid and acetic anhydride takes place. In case of acetic acid, the two hydrogen atoms are interacting with Cs/PWA structure through hydrogen bonds and the formation of acylium ion is difficult as it is away from the catalyst surface and not well stabilized whereas with acetic anhydride, the formation of acylium ion is more assisted on the catalyst surface favoring its formation through covalent bond interaction. Both the mechanisms are shown in Scheme Optimization of reaction conditions for esterification reaction In order to find the best reaction conditions to get high yield of acetins following conditions were studied, Effect of catalyst concentration Effect of glycerol to acetic acid mole ratio Effect of temperature Effect of catalyst concentration Effect of catalyst concentration was studied with glycerol to acetic acid mole ratio of 1 : 8 at 85 C for 2 h. The catalyst concentration was varied from 3 to 9 wt% as shown in Fig (a). The glycerol conversion was found to increase from 66 to 98% with increase in the catalyst concentration from 3 to 7 wt%. The lesser catalytic activity with 3 and 5 wt% catalyst concentration indicates the requirement of higher active sites towards the reaction. Selectivity to diacetins (31 and 34 %) was almost the same with 3 and 5 wt% catalysts, but the triacetin (7 % selectivity) was observed with 5 wt%, whereas triacetin did not form with 3 wt % catalyst. The catalytic activity was found to be almost the same with 7 and 9 wt % catalyst concentrations. The glycerol conversion increased from 84 to 98 % as the time increased from 30 minutes to 2 h. 122

14 The maximum of 98% glycerol conversion was attained at 1 h using 7 wt % catalyst concentrations, but the selectivity to diacetin increased from 57 to 59 % and triacetin selectivity, 16 % at 2 h. No major variation in catalytic performance was observed in Fig (a). Effect of conditions on performance: Catalyst amount (wt %) Conditions : Glycerol : Acetic acid = 1 : 8, Temperature = 85 C, Time = 2 h. increased catalyst concentration of 9 wt %. Moreover, the selectivity to all the acetins remained the same as in the case of 7 wt % catalyst concentration. This indicates that the amount of active acidic sites in 7 wt % catalyst concentration is sufficient to get the maximum activity and selectivity to the desired product Effect of glycerol to acetic acid mole ratio The effect of reactants mole ratio of glycerol to acetic acid was studied at 85 C for 2 h. The mole ratio of glycerol to acetic acid was varied from 1:4 to 1:10 as shown in Fig (b). The conversion of glycerol increased with increase in mole ratio from 1:4 to 1:8 due to increase in availability of accessible acetic acid with 123

15 glycerol. The glycerol conversion and selectivity to acetins remained almost the same with further increase in mole ratio of reactants from 1:8 to 1:10. The reaction condition with 1:8 mole ratio was found to be the best compared with other mole ratios. A gradual increase in glycerol conversion from 45 to 69 % with increase in reaction time was observed for 1:4. Formation of triacetin was found to be nil at this. Fig (b). Effect of conditions on performance: Mole ratio = Glycerol : Acetic acid : Conditions: Temperature = 85 C, Time = 120 min ; Catalyst weight = 0.3 g. mole ratio. This indicates that at 1:4 mole ratio, the amount of accessible acetic acid was not sufficient for the maximum conversion of glycerol to yield higher amount of diacetin and triacetin. For mole ratio 1:6, the glycerol conversion increased from 67 to 92 % with increase in time from 30 to 120 min. The catalytic activity with 1: 8 and 1:10 mole ratio was found to be almost the same. The glycerol reached a maximum conversion of 98 % with negligible changes in the acetins selectivity (diacetin and triacetin was 59 and 16 % respectively). Therefore, 1:8 reactants mole ratio was found to be the best mole ratio for the further studies. 124

16 Effect of temperature The effect of temperature was studied at four different temperatures using glycerol to acetic acid mole ratio of 1:8 with the temperature ranging from 65 to 95 C for 2 h. From the Fig (c), it is observed that the glycerol conversions were Fig. 7 (c). Effect of conditions on performance: Temperature ( C) Conditions: Glycerol : Acetic acid = 1: 8, Catalyst weight = 7 wt %, Time = 120 min. low and slowly increased with time at temperatures 65 and 75 C, which could be attributed due to lesser formation of acylium ion from acetic acid at lower temperatures. At higher temperatures of 85 and 95 C, glycerol conversion reached to a maximum of 98 % and remains almost the same, indicating that the formation of acylium ion is faster at these temperatures. It is also observed that di and tri acetins increased with increase in reaction time. 125

17 The catalytic activity at 85 C was found to be best temperature for esterification reaction since the glycerol reached a maximum conversion of 98 % with the selectivity to diacetin and triacetin of 59 % and 16 % respectively Reusability test Catalyst recyclability test was performed for Cs/PWA catalyst under optimized reaction conditions for both acetylation and esterification reactions. The catalyst showed good recyclability with marginal decrease in activity after 3 recycles (Fig (a)). XRD of fresh and 3 times recycled catalyst showed no change in the phase purity of the catalyst for acetylation (Fig (b)). XRD patterns also matched well for fresh and reused catalyst for esterification of glycerol. Fig (a) Reusability test (b) XRD patterns fresh 3 time recycled catalyst (for acetylation) Conditions: Glycerol : acetic anhydride = 1:3, Catalyst = 3 wt %, Temperature = 30 C, time = 2h. ; Glycerol : Acetic acid = 1: 8, Catalyst weight = 7 wt %, Temperature = 85 C Time = 120 min Proposed reaction mechanism Based on activity of the catalyst and previous reports, two possible mechanisms for acetylation are proposed. The first one is the normal acetylation mechanism 1, involving protonation of the carbonyl oxygen atom and nucleophilic 126

18 attack on the carbonyl to form a tetrahedral intermediate (Scheme 6.1.3). In acetylation mechanism 2, the protonation takes place on the oxygen attached to the carbonyl group, followed by formation of an acylium ion (Scheme 6.1.3). The first pathway is normally less energetic, because of the higher stability of the intermediate formed upon protonation on the carbonyl oxygen atom. On the other hand, formation of the tetrahedral intermediate is space demanding, and the second mechanism, involving the acylium ion, prevails in situations of steric constraints. H + Tetrahedral intermediate (TI) Acetylation Mechanism 1: Formation of TI H + H 3 C C H 3 C Acylium ion (AI) C Acetylation Mechanism 2: Formation of AI Scheme Plausible reaction mechanisms for acetylation reaction of glycerol with acetic anhydride The plausible reaction mechanism for esterification of glycerol with acetic acid proceeds by the activation of acetic acid carbonyl group by the Cs/PWA catalyst where by increases the electrophilicity of carbonyl carbon (Scheme ). Then the carbonyl carbon is attacked by the oxygen of glycerol. The transfer of proton from the intermediate to the second hydroxyl molecule of glycerol gives an activated complex with the formation of water molecule. Later, with the loss of water molecule gives 127

19 monoacetyl ester. Subsequently, the above said mechanism continues further by reacting with acetic acid to form diacetin and triacetin. Scheme Plausible reaction mechanism for esterification reaction of glycerol with acetic acid 128

20 6.2. Synthesis of glycerol acetins from transesterifictaion of glycerol with methyl acetate Introduction Recently, glycerol transesterification with methyl acetate has been introduced as an alternative pathway for the synthesis of glycerol acetates (acetins). Methyl acetate with low boiling point and non toxic nature has led to the new strategy for the synthesis of glycerol acetates. Compared to the esterification reaction [5] that generates water as a byproduct which may harm the catalyst and affect the product quality, the transesterification reaction produces methanol as a byproduct, which can be recycled to the biodiesel production process. In addition, it provides an environmental friendly pathway for the synthesis of glycerol acetins. In 2011, the first research paper for the stated reaction has been reported [15]. The authors studied the reaction using sulfonic acid-functionalized mesostructured SBA-15 catalyst, using large amount of methyl acetate (methyl acetate to glycerol molar ratio of 50 : 1), high reaction temperature of 170 C with 4 h of reaction time. The process utilized the conversion of 99 % of glycerol with corresponding selectivity of 74.2% towards both DGA and TGA with no more than 6 % of TGA formed. Later in 2013, Hameed and co workers carried the same reaction using yttrium grafted SBA-3 catalyst with glycerol to methyl acetate molar ratio (1 : 12), temperature of 120 C with 3 h of reaction time [16]. Thus, it seems encouraging for the research community to explore this interesting reaction toward more economized process. For the first time, in the present work, the transesterification of glycerol with methyl acetate was studied using solid base catalyst Chemicals and Reagents Glycerol, methyl acetate, calcium chloride, zinc chloride, strontium chloride, tin (IV) chloride, potassium fluoride, calcium oxide (CaO), calcium hydroxide (Ca(OH)2, magnesium oxide (MgO), magnesium nitrate, aluminum nitrate, sodium 129

21 hydroxide pellets and sodium carbonate were purchased from Merck India Ltd. All the chemicals were of research grade and used without any further purification Catalyst preparation Synthesis of MSn(OH)6 was discussed in Chapter III, section Characterization- Results and Discussion Characterization of CaSn(OH)6 catalyst by XRD, FT-IR, SEM and TGA were discussed in Chapter III, section Catalytic activity studies Experimental Procedure Catalyst Glycerol Methyl acetate Monoacetin Diacetin Scheme Transesterification of glycerol with methyl acetate Transesterification reaction of methyl acetate with glycerol was carried out in a 100 ml two necked glass reactor equipped with a magnetic stirring bar, a Liebig condenser, and a thermometer. The required amount of glycerol and methyl acetate were taken in the glass reactor and catalyst was added into it. The reaction was performed under stirring at desired temperature. After the reaction, the mixture was taken out and centrifuged for 10 min to separate the catalyst from liquid phase. The obtained product was analyzed in gas chromatography (Shimadzu, GC 2014) with flame ionization detector (FID) equipped capillary column (0.25 mm I.D and 30 m length, Stabilwax, Restek). All the products were confirmed by gas chromatography 130

22 with mass spectroscopy (Shimadzu, GCMS QP 2010). The product yield was calculated by the GC analysis using the formula, Product yield (mol %) = [Conversion (mol %) X Selectivity (mol %)] / Results and Discussion Transestrification of glycerol with methyl acetate Transesterification of glycerol with methyl acetate to form mono and diacetin was carried out using synthesized solid base catalysts. Among all, CaSn(OH)6 catalyst Table Basicity and catalytic activity of different solid acid catalysts for reaction of glycerol and methyl acetate Catalysts Calcined Basicity Glycerol Selectivity Selectivity temperature (HI) conversion to to ( C) µmol/g (mol %) Diacetin Monoacetin (mol%) (mol%) CaSn(OH) a /22 b MgSn(OH) a /23 b CaSn(OH)6- calcined a /20 b ZnSn(OH) a SrSn(OH) a /15 b KF/CaO b HTc (Mg/Al) a /15 b Ca(OH) a /24 b CaO b MgO b Reaction conditions: Glycerol : Methyl acetate = 1:10, Catalyst amount = 7 wt%, Temperature = 30 C. a Nile blue, b Phenolphthalein 131

23 showed the maximum glycerol conversion of 78 % and other hydroxy stannates MSn(OH)6(M = Mg, Zn, Sr) gave glycerol conversion of 65.4, 40, 25 % respectively. The other solid base catalysts like KF/CaO, Mg/Al HTc, CaO and MgO exhibited glycerol conversion of 69, 68.5, 47.2 and 25.2 respectively (Table ). The catalysts with higher basicity showed high glycerol conversion with high selectivity towards monoacetin. The selectivity for diacetin was high for metal hydroxy stannates compared with conventional solid base catalysts except for KF/CaO. High activity and selectivity for diacetin for metal hydroxy stannate, in particular, CaSn(OH)6 could be attributed to the presence of stronger basicity (Hammett basicity for nile blue). CaSn(OH)6 upon calcination to 800 C lost the basicity by converting into oxide form and hence activity decreased. CaSn(OH)6 was selected as the best catalyst among the catalysts tested and taken for further study Optimization of reaction conditions In order to find the best reaction conditions to get high yield of acetins following conditions were studied at room temperature (30 C), Effect of glycerol to methyl acetate mole ratio Effect of catalyst concentration Effect of time Effect of glycerol to methyl acetate mole ratio The effect of glycerol to methyl acetate mole ratio was studied for CaSn(OH)6 catalyst to get better yield of acetins. Glycerol to methyl acetate mole ratio was varied from 1 : 2 to 1 : 20 to get high yield of acetins. The conversion increased with increase in mole ratio from 1 : 2 to 1 : 10. The maximum glycerol conversion achieved was around 78 % for 1 : 10 mole ratio. Further increase in the mole ratio gave high monoacetin selectivity but the conversion remained same. So 1 : 10 mole ratio was optimum to get high glycerol conversion (Fig ). 132

24 Fig Effect of Glycerol to methyl acetate mole ratio Reaction conditions: Catalyst amount = 7 wt%, Temperature = 30 C Effect of catalyst concentration Fig Effect of catalyst concentration Reaction conditions: Gly. : methyl acetate mole ratio = 1:10, Temperature = 30 C. 133

25 The catalyst weight was optimized from 3 to 10 wt % w.r.t total reactant weight. From 3 to 7 wt %, the conversion increased from 52 to 78 % for 2 h but with further increase to 10 wt %, only marginal increase of conversion was observed (Figure ) Effect of time Effect of reaction time was studied for transesterification of glycerol with methyl acetate under the optimized reaction conditions. The glycerol conversion was 70 % for 30 min and increased upto 78 % for 2 h reaction time and then the reaction was further continued to another 5 h for which the conversion was almost remained same. This could be due to the product saturation in the reaction mixture which may hinder the active site accessibility to the less polar reactant, methyl acetate (acetins and byproduct methanol are more polar than methyl acetate). Fig Effect of time on transesterification of glycerol with methyl acetate Reaction conditions: Glycerol : methyl acetate mole ratio = 1:10, Catalyst amount = 7 wt%, Temperature = 30 C. 134

26 Reusability test The reusability study was conducted with CaSn(OH)6 catalyst glycerol. Up to successive 3 cycles, the test was performed and the catalyst showed the consistent activity with marginal decrease of 3 % glycerol conversion (Fig (a)). Interestingly, the catalyst without regeneration at high temperatures was able to retain the activity after successive reuse. Furthermore, XRD patterns of both fresh and recycled catalysts indicated that there was no change in phase purity after the two recycles (Fig (b)). Fig (a) Catalyst Reusability (b) XRD pattern of recycled catalyst Reaction conditions: Glycerol : methyl acetate mole ratio = 1:10, Catalyst amount = 7 wt%, Temperature = 30 C Proposed reaction mechanism A plausible mechanism was suggested for transesterification of glycerol using CaSn(OH)6 as shown in Scheme The basic site of the catalyst removes the proton from secondary hydroxyl group of glycerol, which in turn attacks the carbonyl group of methyl acetate. With the removal of one mole of methanol forms the monoacetin. Subsequently, the above said mechanism continues with the removal of another mole of methanol to form diacetin. 135

27 Scheme Proposed reaction mechanism of transesterification of glycerol with methyl acetate using CaSn(OH)6 catalyst. 136

28 6.3. References [1] M. J. Climent, A. Corma, S. Iborra, Green Chem. 16 (2014) [2] M. S. Khayoon, B.H. Hameed, Biores. Technol. 102 (2011) [3] L. N. Silva, V. L. C. Goncalves, C. J. A. Mota, Catal. Commun. 11 (2010) [4] V. L. C. Goncalves, B. P. Pinto, J. C. Silva, C. J.A. Mota, Catal. Today 133 (2008) [5] L. Zhou, E. Al-Zaini, A. A. Adesina, Fuel 103 (2013) [6] I. D. Rodriguez, C. Adriany, E.M. Gaigneaux, Catal. Today 67 (2011) [7] J. A. Melero, R.V. Grieken, G. Morales, M. Paniagua, Energy & Fuels 21 (2007) [8] P. S. Reddy, P. Sudarsanam, G. Raju, B.M. Reddy, Catal. Commun. 11 (2010) [9] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Appl. Catal. B: Environ. 91 (2009) [10] K. Jagadeeswaraiah, M. Balaraju, P.S. Sai Prasad, N. Lingaiah, Appl. Catal. A: Gen. 386 (2010) [11] P. Ferreira, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Catal. Commun. 12 (2011) [12] M. Balaraju, P. Nikhitha, K. Jagadeeswaraiah, K. Srilatha, P.S.S. Prasad, N. Lingaiah, Fuel Process. Technol. 91 (2010) [13] A. Alsalme, E. F. Kozhevnikova, I. V. Kozhevnikov, Appl. Catal. A: Gen. 349 (2008) [14] V. S. Marakatti, G. V. Shanbhag, A. B. Halgeri, Appl. Catal A: Gen. 451 (2013) [15] G. Morales, M. Paniagua, J. A. Melero, G. Vicente, C. Ochoa, Ind. Eng. Chem. Res., 50 (2011) [16] M. S. Khayoon, B. H. Hameed, Appl Catal A: Gen. 460 (2013)

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