Selective Green Esterification and Oxidation of Glycerol over 12

Aug 30, 2014 - 12-Tungstophosphoric acid anchored to mesoporous material (MCM-48) was synthesized and characterized by thermogravimetric–differentia...
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Selective Green Esterification and Oxidation of Glycerol over 12Tungstophosphoric Acid Anchored to MCM-48 Sukriti Singh and Anjali Patel* Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science, The M. S. University of Baroda, Vadodara 390002, India S Supporting Information *

ABSTRACT: 12-Tungstophosphoric acid anchored to mesoporous material (MCM-48) was synthesized and characterized by thermogravimetric−differential thermal analysis, Brunauer−Emmett−Teller analysis, Fourier transform infrared spectroscopy, 29 Si nuclear magnetic resonance, X-ray diffraction, and transmission electron microscopy. The efficacy for the esterification of glycerol with acetic acid was investigated and 50% selectivity toward diacetyl and 30% for triaceyl glycerol was obtained under mild reaction conditions. The kinetic study was carried out, and it was found that esterification of glycerol follows first-order kinetics, and the rates are not mass transfer limited. As a cleaner alternative to traditional procedures, environmentally benign oxidation of glycerol was investigated with H2O2, and 40.5 and 33.6% selectivity for dihydroxyacetone and glyceraldehyde was obtained, respectively. The catalyst can be used up to four cycles without significant loss in conversion.

1. INTRODUCTION Wide spread commercialization of biodiesel has in turn led to a large surplus of crude glycerol being dissipated in the market as a byproduct. Since glycerol is a highly functional molecule with three hydroxyl groups, it is regarded as a potential starting molecule for numerous value added products.1−3 Among all, the esterification for glycerol with acetic acid to produce glycerol acetates as valuable bioadditives for biodiesel is one of the most interesting approaches. The products of esterification, mono- (MAG), di- (DAG), and tri- (TAG) acetyl esters have great industrial applications. MAG is used in the manufacture of dynamite, tanning leather, and as a solvent for various dyes. DAG is used as a solvent and softening agent.4 TAG is reported to function as a cosmetic biocide, plasticizer, and solvent in cosmetic formulations.4,5 It is a commonly used carrier for flavours and fragrances and was commonly recognized as a safe human food ingredient by the Food and Drug Administration. One of the major and valuable applications of TAG comes as a fuel additive, due to its very good miscibility with conventional fuel, high boiling point, and high octane/cetane numbers.6 A literature survey shows various solid acid catalysts including sulfated activated carbon,7 sulfated zirconia,8 sulfonated carbon,9 mesoporous silica with sulfonic acid groups,6 ZSM5,10 and double SO3H-functionalized ionic liquids11 have been tested for esterification of glycerol with acetic acid. Another potential transformation of glycerol is the oxidation reaction. Concerning the oxidation reaction of glycerol, the selectivity toward a specific product is difficult due to the presence of different primary and secondary hydroxyl groups.12 One of the key problems in this transformation is the potential complexity of the products as well as careful design of the catalyst to control selectivity toward the desired product. Although most of the products have practical value, the most economically important among them are dihydroxy acetone (DHA) and glyceraldehyde (GLYALD). DHA and GLYALD are versatile compounds extensively used as a cosmetic © 2014 American Chemical Society

ingredient, sunless tanning formulation, pharmaceutical precursors and chemical intermediates.12,13 Most previous studies dealing with the chemoselective catalytic oxidation of glycerol reports the use of supported noble metals (palladium, platinum, gold) as catalysts.14−18 Kimura et al. reported that on tuning the reaction conditions selectivity to products such as DHA could be increased; however, the reaction was pH sensitive.19 Much work to date has been devoted to the catalytic oxidation of glycerol with noble metals in basic medium.14,20−24 In a basic pH medium, mainly sodium glycerate is formed, and the reaction mixture needs additional neutralization and acidification in order to get the free glycerol acid. Apart from the cost of noble metals, the other disadvantage of supported Pt and Pd catalysts is the deactivation with increasing reaction time. Accordingly, in most of the reported work, DHA is not even detected in the reaction medium. It is known that changes in pH affect selectivity of the products,12,19 and selectivity for DHA is still a challenge. Only few reports are available on base free reactions (neutral conditions) for glycerol oxidation which gives good selectivity toward DHA.25−27 It was reported by Villa et al. that from an industrial point of view, working under acidic condition allows the direct formation of glyceric acid as the major product.14 Therefore, a proficient chemical process capable of generating selective products from glycerol at a low cost is desirable. Catalysis by supported heteropoly acids (HPAs) especially 12-tungstophosphoric acid (TPA) has been greatly expanded during the past few years from the viewpoint of their variety of structures and compositions. They provide the opportunities for tuning their chemical properties, such as acidities, redox properties, and reactivity28 by choice of the appropriate Received: Revised: Accepted: Published: 14592

July 6, 2014 August 28, 2014 August 30, 2014 August 30, 2014 dx.doi.org/10.1021/ie5026858 | Ind. Eng. Chem. Res. 2014, 53, 14592−14600

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this, 50 mL of technical grade ethanol (0.87 mol) and 12.6 mL of aqueous ammonia (32 wt %, 0.20 mol) were added. The mixture was stirred for 10 min, and when the solution became homogeneous, 3.4 g of TEOS (16 mmol) was added. The molar composition of the gel was 1 M TEOS/12.5 M NH4OH/ 54 M EtOH/0.4 M CTAB/174 M H2O. After the mixture was stirred for 2 h at room temperature, the resulting solid material was recovered by filtration, washed with double distilled water, and dried in air at ambient temperature. The template was removed by calcination at 823 K for 6 h. The resulting material was designated as MCM-48. 2.3. Synthesis of the Catalysts (Anchoring TPA to MCM-48). A series of catalysts containing 10−40% of TPA anchored to MCM-48 was synthesized by the incipient impregnation method. One gram of MCM-48 was impregnated with an aqueous solution of TPA (0.1/10−0.4/40 g/mL of double distilled water) and dried at 100 °C for 10 h. The obtained materials were designated as TPA1/MCM-48, TPA2/ MCM-48, TPA3/MCM-48, and TPA 4/MCM-48, respectively. 2.4. Characterization. Elemental analysis was carried out using JSM 5910 LV combined with INCA instrument for EDXSEM. Thermogravimetric analysis−differential thermal analysis (TG-DTA) was carried out on the Mettler Toledo Star SW 7.01 up to 600 °C. Adsorption−desorption isotherms of samples were recorded on a Micromeritics ASAP 2010 surface area analyzer at −196 °C. From the adsorption desorption isotherms, specific surface area was calculated using the Brunauer−Emmett−Teller (BET) method. Fourier transform infrared (FT-IR) spectra of the samples were obtained using a KBr wafer on the PerkinElmer instrument. The 29Si nuclear magnetic resonance (NMR) spectra were recorded at 121.49 MHz using a 7 mm rotor probe with TMS as an external standard. The spinning rate was 5−7 kHz. The X-ray diffraction (XRD) pattern was obtained by using PHILIPS PW-1830. The conditions were Cu Kα radiation (1.5417 Å), scanning angle from 0° to 60°. Transmission electron microscopy (TEM) was done on a JEOL (JAPAN) TEM instrument (model-JEM 100CX II) with an accelerating voltage of 200 kV. The samples were dispersed in ethanol and ultrasonicated for 5−10 min. A small drop of the sample was then taken in a carbon-coated copper grid and dried before viewing. 2.5. N-Butyl Amine Acidity Determination. The total surface acidity for all the materials has been determined by nbutylamine titration.42 A 0.025 M solution of n-butylamine in toluene was used for estimation. The catalyst weighing 0.5 g was suspended in this solution for 24 h, and excess base was titrated against trichloroacetic acid using neutral red as an indicator. This gives the total acidity of the material. 2.6. Catalytic Reaction. The esterification of glycerol (0.01 mol) with acetic acid (0.06 mol) was carried out in a 100 mL batch reactor provided with a double-walled air condenser, Dean−Stark apparatus, magnetic stirrer, and a guard tube. The reaction mixture was refluxed at 100 °C for 6 h. The obtained products were analyzed on a gas chromatograph (Shimatzu2014) using a capillary column (RTX-5). Products were identified by comparison with the authentic samples and finally by gas chromatography−mass spectroscopy (GC-MS). Oxidation of glycerol was carried out in a 100 mL batch reactor provided with a double-walled condenser, glycerol (10 mmol), and 30% aq H2O2 (30 mmol) with constant stirring. After reaction completion, the catalyst was removed and the product was extracted with ethyl acetate, dried with magnesium sulfate, and analyzed by GC using a RTX-5 capillary column.

support. Recently supported HPAs have also been used for carrying out esterification of glycerol. Tungstophosphoric acid (TPA) immobilized on silica matrix,29 activated carbon,30 niobic acid,31 TPA supported on Cs-containing zirconia,32 phosphomolybdic acid encaged in USY zeolite,33 hybrid SBA15 functionalized with phosphomolybdic acid,34 and zirconia35,36 have been reported for esterification of glycerol. Despite noticeable advantages of these solid acid catalysts, pore-size limitation and reuse or low selectivity toward the preferred DAG and TAG are still unresolved issues for industrial applications. Recently, we have reported esterification of glycerol with acetic acid over TPA anchored to MCM-41 and ZrO2.36 As per our knowledge, no report is available on esterification of glycerol with acetic acid and oxidation of glycerol with H2O2 over TPA anchored to MCM-48. MCM-48 is a key member of the M41S family, as it comprises three-dimensional interwoven pore network/channels, which allows faster diffusion of reactants and products. Such a structure is expected to be less prone to pore blocking than two-dimensional channels (MCM-41).37 It is also known that thermal stability as well as surface acidity of these materials (MCM-41/48) can be improved by inserting strong acid sites such as heteropolyacids;38,39 however, the stability also depends on the synthesis conditions. Even though MCM-48 is much superior, only a few reports are available for the same. A probable reason for this may be the difficulty in synthesis conditions and extended time requirement in hydrothermal conditions. Recently, we have reported the use of TPA supported to MCM-48 for synthesis of biodiesel by esterification of oleic acid as well as transesterification of waste cooking oil and jatropha oil.40 After the end of the transesterification reaction, glycerol was obtained as a major byproduct. Hence, we thought to extend our work to value added chemical transformations of glycerol. As an extension of our work, here for the first time we are reporting esterification of glycerol with acetic acid and oxidation of glycerol with H2O2 over TPA anchored to nonhydrothermally synthesized MCM-48. The synthesized support and catalyst was characterized by different physicochemical techniques. Influence of various reaction parameters on esterification of glycerol (such as alcohol/acid molar ratio, catalyst amount, reaction time, and temperature) was studied. A detailed kinetic study for esterification reaction shows that the reaction follows a firstorder dependency on the concentration of glycerol and the catalyst. The effect of temperature on rate constant was studied, and the activation energy was also calculated. The application was also extended to the selective base free oxidation of glycerol with hydrogen peroxide under mild conditions, and different reaction parameters were optimized. Also the catalyst was regenerated and reused up to four cycles.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used were of A.R. Grade. 12Tungstophosphoric acid (Loba chemie, Mumbai), cetyltrimethylammonium bromide (CTAB), ammonia, ethanol, tetraethyl orthosilicate (TEOS), glycerol, and acetic acid were obtained from Merck. Dihydroxyacetone, glyceraldehyde, glycolic acid, glyceric acid, oxalic acid, MAG, DAG, and TAG were obtained from Sigma-Aldrich, Alfa Aesar and used as received. 2.2. Synthesis of the Support. The synthesis of MCM-48 was carried out as reported in the literature.41 Surfactant (CTAB) was dissolved in 50 mL of double distilled water. To 14593

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Product identification was done by comparison with authentic samples and finally by combined GC-MS. The conversion of products was calculated based on glycerol. 2.7. Leaching Test. Any leaching of the active species from the support makes the catalyst unattractive, and hence it is needed to study the stability as well as leaching of TPA from the support. TPA can be quantitatively characterized by the heteropoly blue color, which is observed when reacted with a mild reducing agent such as ascorbic acid.43 In the present study, this method was used for determining the leaching of TPA from the support. Standard samples containing 1−5% of TPA in water were prepared. To 10 mL of the above samples, 1 mL of 10% ascorbic acid was added. The mixture was diluted to 25 mL, and the resultant solution was scanned at maximum absorbance wavelength at 785 nm for its absorbance values. A standard calibration curve was obtained by plotting values of absorbance versus percent concentration. One gram of catalyst with 10 mL of double-distilled water was refluxed for 24 h. Then 1 mL of the supernatant solution was treated with 10% ascorbic acid. Development of blue color was not observed indicating that there was no leaching. The same procedure was repeated with alcohols and the filtrate of the reaction mixture after completion of reaction in order to check the presence of any leached TPA. The absence of blue color indicates no leaching of TPA.

Table 1. Textural and Acidity Properties of Support and Catalysts catalyst

n-butyl amine acidity (mmol/g)

surface area (m2/g)

pore volume (cm3/g)

MCM-48 TPA1/MCM-48 TPA2/MCM-48 TPA3/MCM-48 TPA4/MCM-48

1.1 1.23 1.45 1.53 1.6

755 381 339 286 262

0.56 0.43 0.43 0.38 0.35

acidity value (Table 1) indicates that MCM-48 is fairly acidic. As the TPA loadings increase, total acidity also increases as the number of acidic sites are dependent on the amount of the TPA present in the catalyst and the results are as expected. Considering this aspect, TPA3/MCM-48 was selected for further detailed characterization by FT-IR, 29Si MAS NMR, XRD, and TEM analysis. FT-IR spectra of MCM-48 show broad bands around 1100 and 1165 cm−1 corresponding to asymmetric stretching of Si− O−Si. The bands at 640 and 458 cm−1 are due to symmetric vibration of Si−O−Si and bending vibration of Si−O, respectively. The broad band around 3448 cm−1 is due to the absorption of −OH on surface from the large amount of H2O hydrogen bonded to Si−OH, which provides opportunities for forming the hydrogen bond. The obtained bands are in good agreement with the reported literature.44 The typical bands for TPA, at 982 cm−1 corresponding to Vas vibration of WOd (terminal oxygen linked to a lone tungsten atom), and 897 cm−1 for stretching vibrations of W−Ob−W (corner sharing oxygen of two different triads of tungsten atom), are clearly observed in TPA3/MCM-48. Appearance of these fingerprint bands strongly indicates that the primary structure of TPA ion is completely retained even after anchoring to the support. The absence of band at 1080 cm−1 (Vs stretching of P−O) of TPA may be due to superimposition with the bands of the support. In addition to the characteristic band for TPA, FT-IR spectra of TPA3/MCM-48 also show peaks at 1637 and 3450 cm−1 with a shoulder at 3250 cm−1, corresponding to the bending and stretching vibrations of bridging and Si−OH groups, respectively, which assists in the interaction of TPA anions with the support. 29 Si MAS NMR is the most important method to study the chemical environment around the silicon nuclei in mesoporous silica materials and concentration of the silanol groups. 29Si MAS NMR spectra of the MCM-48 and TPA3/MCM-48 can be seen in Supplementary Figure 2, Supporting Information. A broad peak between −90 and −125 ppm was observed which can be attributed to three main components with chemical shifts at −96, −102.3, and −110.5 ppm. For MCM-48 (Supplementary Figure 2a) the signals resulted from Q2 (−96 ppm), Q3 (−102.3 ppm), and Q4 (−110.5 ppm) silicon nuclei, where Qx corresponds to a silicon nuclei with x siloxane linkages, i.e., Q2 to disilanol Si-(O-Si)2(-O-X), where X is H or TPA, Q3 to silanol (X-O)-Si-(O−Si)3, and Q4 to Si-(O−Si)4 in the framework. The observed values of chemical shifts for MCM-48 are in good agreement with the reported literature.45 The spectra of TPA3/MCM-48 showed a slight shift in the chemical shift values (Supplementary Table 1, Supporting Information), and the peaks were relatively broad as compared to MCM-48, which may be due to the presence of TPA. Also the intensity of the Q2 and Q3 peaks decreases when the support (MCM-48) is loaded with TPA, i.e., TPA3/MCM-48.

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. A detailed study on the characterizations of all the synthesized catalysts can be found in our earlier publication.40 However, for the reader’s convenience here we are reporting TG-DTA, FT-IR, 29Si MAS NMR, XRD and TEM analyses. Analytically calculated weight percentages of elements were comparable with the results obtained from EDS analysis: Theoretical (wt %) W-17.7, P-0.32; obtained (wt %) W-17.2, P0.30, O-54.6, Si-27.9. TG-DTA of MCM-48 (Supplementary Figure 1, Supporting Information) shows an initial weight loss of 8.4% up to 150 °C; this was further accompanied by a broad band on the DTA curve, appearing as an endothermic peak at 80 °C that may be due to desorption of physically adsorbed water molecules. The final weight loss of 3.22% above 450 °C may be due to the condensation of silanol groups to form siloxane bonds. After that absence of any weight loss indicates that support is stable up to 550 °C. The TG-DTA of TPA3/MCM-48 (Supplementary Figure 1) shows an initial weight loss of 10−13% up to 200 °C assigned to loss of adsorbed water. A second weight loss of 1−2% between 200 and 300 °C corresponds to the loss of water of crystallization of Keggin ion. A gradual weight loss from 300 to 500 °C may be due to the difficulty in removal of water contained in TPA molecules inside the channels of MCM-48. This type of inclusion can cause the stabilization of TPA molecules inside the channels of MCM-48. Absence of exothermic peak (485 °C) for TPA in catalyst indicates that TPA gets stabilized on MCM-48. The values of surface area are presented in Table 1. The drastic decrease in surface area of all the catalysts is the first indication of the chemical interaction between TPA and MCM48. The decrease in surface area is also ascribed to the fact that the TPA species enters in the mesopores channels of the support. The decrease in pore volume of the catalyst as compared to that of the support is also an indication of the presence of TPA inside the channels of MCM-48. The total 14594

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neous dispersion of TPA inside the channels of MCM-48. Transformation of glycerol to value added products by esterification and oxidation can be seen in Scheme 1 from Supporting Information. 3.2. Esterification of Glycerol. The esterification of alcohols is an equilibrium-limited reaction. The conversions can be increased by increasing the concentration of either alcohol or acid. In a practical means, to obtain maximum conversions for economic reasons, the reactant that is usually less expensive is taken in excess. In the present study, the corresponding acid is taken in excess. The effects of various reaction parameters such as % loading of TPA, alcohol/acid molar ratio, amount of catalyst, reaction time, and temperature were studied to optimize the conditions for maximum conversion. The effect of % loading of TPA on esterification reaction was carried out with 10, 20, 30, 40% TPA loadings (Figure 2). It

The decrease in the intensity of Q2 and Q3 peaks is an indication to bonding of the silanol groups with TPA inside the pore of MCM-48. This indicates that the mesoporous structure of MCM-48 is not disturbed due to the presence of TPA, and there is a strong interaction of TPA with the support. Thus, BET surface area, FT-IR, and 29Si MAS NMR data indicate that there is a strong interaction (hydrogen bonding) between terminal oxygen’s of TPA and the surface silanol hydroxyls of MCM-48. In order to study the homogeneous dispersion of TPA species inside MCM-48, materials were further characterized using XRD and TEM. The XRD pattern of MCM-48 (Supplementary Figure 3A(b), B(a), Supporting Information) exhibits a main characteristic peak corresponding to the 211 plane at 2.3° along with a broad shoulder peak from 220 at 3.0°. Several peaks in the range 4−5° diffraction angles were also observed, corresponding to the reflections of the 400, 321, and 420 planes of a typical MCM-48 mesostructure with Ia3d cubic symmetry. The broadening and decrease in the intensity of the main peak for TPA3/MCM-48 (Supplementary Figure 3A(c), B(b)) was observed. A similar observation can be found in the literature.39 Comparison of the XRD patterns of TPA3/MCM-48 reveals that the mesoporous structure is rather intact even after the TPA loading. Further, the absence of characteristic peaks of crystalline phase of TPA indicates that TPA is highly dispersed inside the three-dimensional channels of MCM-48, and there is absence of characteristic peaks of crystalline phase of TPA. This indicates high dispersion of TPA inside the three-dimensional channels of the support. The TEM image of MCM-48 shows very well ordered pore system with a pore diameter between 2.5 and 4 nm (Figure 1 a,

Figure 2. Effect of % loading of TPA; reaction conditions: mole ratio of alcohol/acid 1:6, amount of catalyst 150 mg, reaction temperature 100 °C, reaction time 6 h.

was observed that with an increase in the % loading of TPA, % conversion also increases. From Table 1 it is seen that the acidity increases with an increase in TPA loading. The value of total acidity is directly related to the concentration of active species, TPA. Hence, the increase in total acidity with an increase in % loading of TPA is obvious. It is also seen from Table 1 that the surface area decreases from 10% to 40% loading, and hence the catalytic activity should decrease but the obtained results are controversial. Hence, it is concluded that the present catalytic system is not a surface type heterogeneous catalyst in which the catalytic activity is directly proportional to surface area, but it is a pseudoliquid type heterogeneous catalyst in which catalytic activity is proportional to the total acidity. For 30 and 40% loadings, there was no significant difference in % conversion. In this work, the target products were both DAG and TAG due to their industrial importance, and maximum selectivity for DAG and TAG was observed for 30% TPA loaded catalyst, the catalyst containing 30% loading of TPA; i.e., TPA3/MCM-48 was selected for the detail study. The influence of glycerol/acetic acid molar ratio was examined using the mole ratio 1:1, 1:3, 1:5, 1:6, and 1:9 at a fixed reaction temperature of 100 °C keeping the amount of

Figure 1. TEM of (a, b) MCM-48; (c, d) TPA3/MCM-48.

b). It well-known in the literature that MCM-48 has a threedimensional pore system with two non-intersecting gyroidal pores which can assist dispersion of materials having a diameter of 1−3 nm. The TEM images of TPA3/MCM-48 (Figure 1b,d) shows well-ordered 3D channels that are well arranged over a large scale. Furthermore, the absence of crystalline phase of TPA in the catalyst indicates that TPA is highly dispersed inside the channels of MCM-48. The results are in good agreement with the XRD data. The above characterization studies confirm the strong interaction between TPA and MCM-48 as well as homoge14595

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conversion as well as selectivity. The maximum conversion (99%) was obtained at 150 mg of catalyst. Effect of reaction time on glycerol conversion was studied (Table 2). Conversion increases with an increase in reaction

catalyst 150 mg (Figure 3). It was observed from Figure 3 that the glycerol conversion increases with an increase in the

Table 2. Effect of Reaction Time on Esterification % selectivity reaction time (h)

% conversion

MAG

DAG

TAG

1 2 3 4 5 6 7

53 67 79 87 99 100 100

67 61 48 40 20 12 12

33 34 42 44 50 57 57

5 10 16 30 31 31

time. It is seen that initially no TAG was observed but as the reaction time was prolonged selectivity for TAG improved. This is because with an increase in time, esterification of MAG to DAG and TAG was enhanced due to nucleophilic attack of the remaining hydroxyl groups of MAG moiety on the acid site−acetic acid complex. The glycerol conversion was 99% in 5 h over TPA3/MCM-48, and combined selectivity for DAG + TAG observed was 80% with 30% selectivity toward TAG. The influence of reaction temperature (Figure 5) was studied in the temperature range of 70−110 °C. It is very well-known

Figure 3. Effect of mole ratio glycerol/acid; reaction conditions: amount of catalyst 150 mg, reaction temperature 100 °C, reaction time 6 h.

glycerol to acid molar ratio and reaches a maximum at 1:6. The presence of excess acetic acid can promote the formation of DAG and TAG rather than MAG, which can be explained by the extra esterification agent that drives the esterification reaction toward the formation of higher glycerol acetates. Effect of amount of catalyst was investigated by varying the amount within a range of 50−250 mg (Figure 4). Increasing the

Figure 5. Effect of reaction temperature; reaction conditions: mole ratio of glycerol to acid 1:6, catalyst amount 150 mg, reaction time 5 h.

that glycerol esterification takes place at endothermic conditions. The primary advantage of higher temperatures is a shorter reaction time. Hence, at a temperature higher than 90 °C, higher conversion was achieved in a short reaction time. At a higher reaction temperature, higher selectivity toward the desired DAG and TAG products was observed. This can be explained due to the enhanced subsequent endothermic esterification at elevated temperatures. At 100 °C, maximum conversion (99%) of glycerol was observed along with highest selectivity for DAG and TAG. The optimized conditions for esterification of glycerol are molar ratio of glycerol to acid 1:6, catalyst amount 150 mg, reaction time 5 h, and temperature 100 °C.

Figure 4. Effect of amount of catalyst; reaction conditions: mole ratio of glycerol to acid 1:6, reaction temperature 100 °C, reaction time 6 h.

catalyst amount resulted in promoting the glycerol conversion. This is due to the increase in the total number of accessible active sites, which could effectively drive glycerol esterification to completion. Increasing the catalyst amount up to 150 mg resulted in 99% glycerol conversion and high selectivity for DAG and TAG. However, on increasing the catalyst amount from 200 to 250 mg, there was no significant increase in 14596

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Table 3. Optimization of the Reaction Conditions for the Conversion of Glycerol with H2O2 and Different Amounts of TPA3/ MCM-48a % selectivity entry

Gly: H2O2, catalyst amount

% conversion

DHA

GLYALD

GLYAC

GLYCAC

1 2 3 4 5 6 7 8

15 mg 20 mg 25 mg 30 mg 1:1 1:2 1:3 1:4

15.1 24.4 30.0 30.0 14.6 26.3 30.0 31.0

12.0 30.4 40.5 41.0 16.4 35.2 40.5 41.0

64.6 41.0 33.6 28.4 66.8 56.0 33.6 34.0

10.0 10.0 15.3 18.5 10.3 6.5 15.3 15.0

13.4 15.6 8.6 10.1 6.4 2.3 8.6 6.0

OA 3.0 2.0 2.0

2.0 4.0

Reaction conditions: Time 24 h, mole ratio 1:3, temperature 90 °C, where GLY - glycerol, DHA - dihydroxy acetone, GLYALD - glyceraldehyde, GLYAC -glyceric acid, GLYCAC - glycolic acid, OA - oxalic acid.

a

3.2.1. Kinetics. A detailed study on the kinetic behavior was carried out. In all the experiments, reaction mixtures were analyzed at fixed interval of time using gas chromatography. The esterification of glycerol with acetic acid was carried out with a 1:6 molar ratio, and since acetic acid was taken in large excess, the rate law is expected to follow first-order dependence. The plot of log Co/C versus time (Supplementary Figure 4, Supporting Information) shows a linear relationship of glycerol consumption with respect to time. With an increase in reaction time, there is a gradual and linear decrease in the glycerol concentration over the catalyst. These observations indicate the esterification of glycerol follows first-order dependence with respect to time. The catalyst concentration was varied from 4 × 10−3 to 20 × 10−3 mmol at fixed substrate concentration of 10 mmol and at 100 °C. It can be observed from the Supplementary Figure 5, Supporting Information that the rate of reaction increases with an increase in the catalyst concentration. The plot of reaction rate versus catalyst concentration (Supplementary Figure 5) also shows a linear relationship with respect to catalyst concentration. 3.2.2. Estimation of Activation Energy. The graph of ln k versus 1/T was plotted (Supplementary Figure 6, Supporting Information), and the value of activation energy (Ea) was determined from the plot. The value of activation energy (Ea), the pre-exponential factor (A) was determined using Arrhenius equation. It is important to recognize whether the reaction rate is diffusion limited/mass transfer limited or it is truly governed by the chemical step where the catalyst is being used to its maximum capacity. It is reported that Ea for diffusion limited reactions is 10−15 kJ/mol and for the chemical step it is higher than 25 kJ/mol.46 In the present case, the activation energy was 46.8 kJ/mol, and hence the rate is truly governed by the chemical step. 3.3. Oxidation of Glycerol with Hydrogen Peroxide. A detailed study on oxidation of glycerol was carried out by varying different parameters to optimize the conditions for maximum conversion and selectivity for desired DHA and GLYALD. To ensure the catalytic activity, all reactions were carried out without a catalyst. It was found that no oxidation takes place. In order to determine the optimum temperature, the reaction was investigated at four different temperatures 70, 80, 90, and 100 °C, keeping other parameters fixed (25 mg catalyst amount, reaction time 24 h). The results show that conversion increased on increasing the temperature from 70 to 90 °C

(10.7−30.0% conversion and 30−40% selectivity for DHA, 44− 33% selectivity for GLYALD). On increasing the temperature from 90 to 100 °C, there is no significant increase in conversion, but a decrease in selectivity for DHA and GLYALD (at 90 °C - 40.5, 33.6%, at 100 °C - 35, 25%, respectively) was observed due to further oxidation of these products to acid. So, 90 °C was optimized for further studies. The pH plays a crucial role in the interconversion of DHA to GLYALD.12 In acidic conditions, the secondary hydroxyl group of glycerol oxidizes to give DHA, and it is reported in the literature that DHA is not stable under alkaline conditions.19 In basic conditions, GLYALD is formed and the reaction mechanism is based on the oxidation of a primary hydroxyl group of glycerol. Hence, adjusting the solution pH of reaction medium seems to be a reasonably adequate way to stabilize the target compound, i.e., DHA/GLYALD. In the present work on monitoring the pH of the reaction medium after addition of substrate, H2O2, and catalyst, the pH was acidic (around 4). It is seen from Table 3 that, with an increase in the mole ratio of Gly to H2O2 from 1:1 to 1:3, there is an increase in the conversion, which may be due to the increase in concentration of H2O2, which assists in further oxidation of the primary and secondary hydroxyl group of glycerol. On further increase in the mole ratio from 1:3 to 1:4, no significant increase in conversion was observed. Therefore, a 1:3 mole ratio was found to be optimum in terms of conversion. It is also observed from Table 3 that % conversion increases as the active species (TPA) increases; i.e., the catalyst amount increases, which is as expected (acidic pH assists oxidation of the secondary hydroxyl group, giving selectivity toward DHA). Under the optimized conditions at a 1:3 molar ratio, 25 mg of catalyst, conversion was 30%, and the product selectivity for DHA, GLYALD was 40.5, 33.6%, respectively. The effect of reaction time was studied, and results are presented in Figure 6. It was observed from Figure 6 that with an increase in the reaction time, % conversion also increases. This may be due to the reason that more time is required for the formation of a reactive intermediate species (substrate + catalyst), which is finally converted into the products. Highest conversion (30%) and selectivity of DHA (46.8%) is obtained at 20 h. No significant change in % conversion as well as % selectivity was observed after 20 h. The optimized conditions for oxidation of glycerol over TPA3/MCM-48 are molar ratio of glycerol to acid 1:3, catalyst amount 25 mg, reaction time 20 h, and temperature 90 °C. 3.3.2. Effect of the Support in Selectivity of Products. The selectivity of the products obtained in glycerol oxidation could 14597

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species from the support MCM-48, and the present catalysis is truly heterogeneous in nature. 3.6. Recycling of the Catalyst. The catalyst was recycled in order to test its activity as well as its stability. The catalyst, TPA3/MCM-48, was separated from the reaction mixture only by simple filtration, first washed with methanol (unreacted substrates if any), ethyl acetate in the case of the oxidation product, followed by distilled water, and then dried at 100 °C, and the recovered catalyst was charged for the further run. There is no appreciable change in the % conversion of glycerol using regenerated catalyst up to four cycles (Table 4); however Table 4. Recycling of the Catalyst % selectivity

Figure 6. Glycerol oxidation; reaction conditions: mole ratio 1:3, catalyst amount 25 mg, temperature 90 °C.

be due to the combination of the nature of the catalyst (particle dimensions, pore structure of support) and experimental conditions. It has been reported by Rodrigues et al. that gold supported on multiwalled nanotubes and on activated carbon showed a significant difference in the selectivity for DHA and GLYAD.21,24 They showed that the high DHA selectivity observed with Au/MWCNT might be explained by the peculiar porous characteristics of the support (large mesopores), which favors the binding of glycerol through the primary −OH leaving the secondary −OH to get oxidized. Whereas in Au/AC the glycerol molecule is likely in a more confined form due to pore network and confined space, glycerol binds to Au via secondary −OH, which leaves the two primary −OH group to react. In the present work, the selectivity toward the product can be explained on the same basis. 3.4. Control Experiment. The control experiments with MCM-48 and TPA were also carried out under optimized conditions for both the reactions. It is seen that MCM-48 (Supplementary Table 2, Supporting Information) is not very active toward the esterification of glycerol indicating the catalytic activity is mainly due to TPA. The same reaction was carried out by taking the active amount of TPA (35 mg). It was found that the active species gives 89% conversion and 50% combined selectivity for DAG + TAG. In oxidation, the reaction support is inactive, and the observed conversion is due to TPA. High catalytic activity was obtained for catalyst in both reactions, which indicates that TPA is the real active species, and the dispersion of TPA in MCM-48 causes a significant change in the selectivity of the products. Thus, we were successful in supporting TPA on MCM-48 without any significant loss in activity and hence in overcoming the traditional problems of homogeneous catalysis. 3.5. Heterogeneity Test. For the rigorous proof of heterogeneity, a test (esterification reaction) was carried out by filtering the catalyst from the reaction mixture at 100 °C after 2 h, and the filtrate was allowed to react up to 5 h. The reaction mixture of 2 h and the filtrate were analyzed by gas chromatography. A similar experiment was also carried out for glycerol oxidation by filtering the reaction mixture at 90 °C after 10 h and filtrate was allowed to react for 20 h. No change in the % conversion (between reaction mixture and filtrate) of glycerol indicates that the present catalyst falls into category C.47 Category C means the metal does not leach and observed catalysis is truly heterogeneous in nature. On the basis of these results, it can be concluded that there is no leaching of TPA

catalyst

% conversiona,b

TONa,b

MAG

DAG

TAG

fresh first run second run third run fourth run

99/30 98/29 97/28 97/28 96/27

825/1500 817/1450 808/1400 808/1400 800/1350

20 23 23 24 24

50 53 53 53 53

30 24 24 23 23

a Reaction conditions for esterification of glycerol: molar ratio 1:6, amount of catalyst 150 mg, reaction time 5 h, reaction temperature 100 °C. bReaction conditions for oxidation of glycerol: molar ratio 1:3, amount of catalyst 25 mg, reaction time 20 h, reaction temperature 90 °C. Turnover number (TON) = moles of product obtained/mols of catalyst (active species).

a decrease in selectivity of TAG is observed to a slight extent. Recycling studies of the heterogeneous catalyst clearly confirmed that the loaded TPA was firmly bound to the MCM-48 surface. 3.7. Characterization of Regenerated Catalyst. The regenerated catalyst was characterized by EDS, FT-IR, Raman, XRD, and total acidity measurement in order to confirm the retention of the catalyst structure, after the completion of the reaction. The EDS analysis (in wt %) of the reused catalyst RTPA3/MCM-48 was carried out, and values are close to the fresh catalyst (fresh: O-54.6, Si-27.9, W-17.7, P-0.30; recycled: O-55, Si-28, W-17.4, P-0.30). The FT-IR data for the fresh as well as the regenerated catalyst are represented in Supplementary Figure 7A, Supporting Information. No appreciable shift in the FT-IR band position of the regenerated catalyst compared to TPA3/MCM-48 indicates the retention of Keggintype TPA into MCM-48. XRD also shows no change in the property of recycled catalyst (Supplementary Figure 7B, Supporting Information. Further, from the n-butyl amine acidity value, it is clear that fresh (1.53 mmol/g) and reused catalyst (1.50 mmol/g) did not show any appreciable change in acidity. Hence there is no deactivation of catalyst. This also confirms the truly heterogeneous mode of action. 3.8. Comparison with the Reported Catalysts. It is observed from Supplementary Table 3, Supporting Information that zirconia supported HSiW, HPW, and HPMo35 showed very good conversion and 32−26% selectivity toward TAG in 4 h, but the reaction was carried out at a higher temperature (120 °C), molar ratio of glycerol to acetic acid (1:10), and catalyst amount (0.3 g). The present catalyst shows a higher glycerol conversion under mild reaction conditions as well as good selectivity toward the glycerol esters, especially DAG and TAG. On comparing the oxidation reaction with reported catalyst in H2O2, it was seen from Supplementary Table 3 that silicalite containing Ti, Fe, V catalysts48 in base free conditions gives 14598

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financial support. One of the authors (S.S.) is thankful to the same for a fellowship.

34.8, 31.9, and 33.9% conversion, respectively, with only selectivity toward formate ester, hydroxyethanoic acid, and acetals. It was reported by the same group48 that Ti containing catalysts TS-1, Ti-MCM-41, EP50od, and EP50fd give 10−35% conversion but with a higher catalyst amount and selectivity was obtained for glyceraldehydes, formate ester. Dihydroxy acetone as well as other acid products were obtained in trace amounts. Sulphonato-salen-chromium(II) intercalated LDH27 in base free oxidation with 3% H2O2 shows 71.3% conversion with 43.5% and 20.9% selectivity for DHA and glyceraldehyde, respectively. Remaining major selectivity was obtained for formic acid and oxalic acid. The present catalyst was also compared with reactions conducted by O2 where gold supported MWCNT and activated carbon showed high conversion (72, 93% respectively) with 60%, 18% selectivity toward DHA, respectively.24 Pd−Ag on carbon49 in neutral condition showed 24.5% conversion and very high selectivity for DHA. A similar report16 was obtained for Pd−Ag supported to other supports (SiO2, TiO2, Al2O3, and ZrO2), and in all the cases low conversion (1−10%) was observed but with 80−90% selectivity for DHA. All the attempts pertaining to Au and Pd have been focused on reactions with molecular oxygen in autoclave reactors but fewer reports were found on reactions with hydrogen peroxide. The major limitation for noble metal supported catalysts would be the cost as well as deactivation of the metal. From all the comparison data with the reported catalysts, it was clear that the present catalyst has significant potential for oxidative conversion of glycerol to DHA and GLYALD.

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(1) Pagliaro, M.; Rossi, M. The Future of Glycerol, 2nd ed.; RSC Green Chemistry Series-8; Royal Society of Chemistry: Cambridge, 2010. (2) Sakthivel, A.; Nakamura, R.; Komura, K.; Sugi, Y. Esterification of glycerol by lauric acid over aluminium and zirconium containing mesoporous molecular sieves in supercritical carbon dioxide medium. J. Supercritical Fluids 2007, 42, 219−225. (3) Rahmat, N.; Abdullah, A. Z.; Mohamed, A. R. Recent progress on innovative and potential technologies for glycerol transformation into fuel additives: a critical review. Renew. Sustainable Energy Rev. 2010, 14, 987−1000. (4) Yin, A.-Y.; Guo, X.-Y.; Dai, W.-L.; Fan, K.-N. The synthesis of propylene glycol and ethylene glycol from glycerol using Raney Ni as a versatile catalyst. Green Chem. 2009, 11, 1514−1516. (5) Wessendorf, R. Glycerol derivatives as fuel components. Petrochemia 1995, 48, 138−143. (6) Melero, J. A.; Grieken, R.; Morales, G.; Paniagua, M. Acidic Mesoporous Silica for the Acetylation of Glycerol: Synthesis of Bioadditives to Petrol Fuel. Energy Fuels 2007, 21, 1782−1791. (7) Luque, R.; Budarin, V.; Clark, J. H.; Macquarrie, D. J. Glycerol transformations on polysaccharide derived mesoporous materials. Appl. Catal. B: Environ. 2008, 82, 157−162. (8) Rodriguez, I. D.; Adriany, C.; Gaigneaux, E. M. Glycerol acetylation on sulphated zirconia in mild conditions. Catal. Today 2011, 167, 56−63. (9) Sanchez, J. A.; Hernandez, D. L.; Moreno, J. A.; Mondragon, F.; Fernandez, J. J. Alternative carbon based acid catalyst for selective esterification of glycerol to acetylglycerols. Appl. Catal. A: Gen. 2011, 405, 55−60. (10) Goncalves, V. L. C.; Pinto, B. P.; Silva, J. C.; Mota, C. J. A. Acetylation of glycerol catalyzed by different solid acids. Catal. Today 2008, 133, 673−677. (11) Liu, X.; Ma, H.; Wu, Y.; Wang, C.; Yang, M.; Yan, P.; WelzBiermann, U. Esterification of glycerol with acetic acid using double SO3H-functionalized ionic liquids as recoverable catalysts. Green Chem. 2011, 13, 697−701. (12) Katryniok, B.; Kimura, H.; Skrzynska, E.; Girardon, J. S.; Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Selective catalytic oxidation of glycerol: perspectives for high value chemicals. Green Chem. 2011, 13, 1960−1979. (13) Namiki, M. Chemistry of Maillard Reactions: Recent Studies on the Browning Reaction Mechanism and the Development of Antioxidants and Mutagens. Adv. Food Res. 1998, 32, 115−184. (14) Gil, S.; Marchena, M.; Fernandez, C. M.; Silva, L. S.; Romero, A.; Valverde, J. L. Catalytic oxidation of crude glycerol using catalysts based on Au supported on carbonaceous materials. Appl. Catal. A: Gen. 2013, 450, 189−203. (15) Villa, A.; Veith, G. M.; Prati, L. Selective Oxidation of Glycerol under Acidic Conditions Using Gold Catalysts. Angew. Chem., Int. Ed. 2010, 49, 4499−4502. (16) Hirasawa, S.; Watanabe, H.; Kizuka, T.; Nakagawa, Y.; Tomishige, K. Performance, structure and mechanism of Pd-Ag alloy catalyst for selective oxidation of glycerol to dihydroxyacetone. J. Catal. 2013, 300, 205−216. (17) Hu, W.; Lowry, B.; Varma, A. Kinetic study of glycerol oxidation network over Pt−Bi/C catalyst. Appl. Catal. B: Environ. 2011, 106, 123−132. (18) Brett, G. L.; He, Q.; Hammond, C.; Miedziak, P. J.; Dimitratos, N.; Sankar, M.; Herzing, A. A.; Conte, M.; Sanchez, J. A. L.; Kiely, C. J.; Knight, D. W.; Taylor, S. H.; Hutchings, G. J. Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base-Free Conditions. Angew. Chem., Int. Ed. 2011, 50, 10136−10139. (19) Kimura, H.; Tsuto, K. Selective oxidation of glycerol on a platinum-bismuth catalyst. Appl. Catal. A: Gen. 1993, 96, 217−228.

4. CONCLUSIONS TPA anchored to MCM-48 proved to be an excellent solid catalyst for glycerol transformations. The results of this study show that the product distribution for the esterification and oxidation of glycerol is clearly dependent on the reaction conditions as well as the catalyst nature. The advantages of the present catalyst include operational simplicity, lower reaction temperature (90−100 °C), and high TON. In addition, environmental and economic benefits are achieved as the glycerol which is a byproduct of biodiesel has been utilized to give value added products such as DAG, TAG, DHA, and GLYALD. The catalysts can be reused up to four cycles without any significant loss in conversion as well selectivity. FT-IR and XRD of regenerated catalysts indicated that the catalysts were undegraded and stable after the reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Graphs for TG-DTA, 29Si MAS NMR, powder XRD, kinetics data, FT-IR, and XRD graphs for recycled catalyst. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Department of Science and technology (DST), Project. No. SR/S5/GC-01/2009, New Delhi, for the 14599

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