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Diglycerol synthesis via solvent-free selective glycerol etherification process over lithium-modified clay catalyst
Chemical Engineering Journal 225 (2013) 784–789
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Diglycerol synthesis via solvent-free selective glycerol etherification process over lithium-modified clay catalyst Muhammad Ayoub, Ahmad Zuhairi Abdullah ⇑ School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Diglycerol synthesis via solvent-free
glycerol etherification. LiOH modification of acid-treated
montmorillonite K-10. Elucidation of surface
physicochemical properties. High diglycerol selectivity of 53% at
98% conversion.
ab diglycerol isomer with reduced aa isomers.
Improved selectivity of
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 29 March 2013 Accepted 10 April 2013 Available online 19 April 2013 Keywords: Montmorillonite K-10 Lithium Glycerol Selective etherification Diglycerol isomer
a b s t r a c t Diglycerol was synthesized via solvent free glycerol etherification over solid-base catalyst. Acid-treated montmorillonite K-10 (Clay MK-10) was modified with LiOH (Clay Li/MK-10) through ion-exchange method and characterized using different techniques for surface and structural properties. The parent and modified clay were then used to catalyze the production of diglycerol via solvent free glycerol etherification reaction. The reaction was conducted at 240 °C for up to 12 h and the activity of the catalysts was compared with that of homogeneous catalyst (LiOH). High diglycerol selectivity of about 53% was obtained at a glycerol conversion of about 98% with Clay Li/MK-10. The selectivity to ab diglycerol isomer was also increased from 35% to 55% while the selectivity to aa isomer was decreased from 65% to 35%. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is an abundant carbon–neutral renewable resource for the production of biomaterials [1–3]. It is an attractive renewable building block for synthesis of polyglycerols which have several uses in different fields [4]. Diglycerol has numerous applications in food and pharmaceutical and cosmetic industries [5]. Potential green catalytic routes have been reported using various solid catalysts but high selectivity for diglycerol at high glycerol conversion remains a challenge. Etherification of glycerol is used in the synthesis of oxygenated components such as polyglycerols and ployglycerol ethers [6,7]. Glycerol ethers (polyglycerols) are produced from catalytic etheri⇑ Corresponding author. Tel.: +60 4 599 6411; fax: +60 4 594 1013. E-mail address: [email protected] (A.Z. Abdullah). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.04.044
fication of glycerol with the use of different solvents. Polyglycerols, especially diglycerol and triglycerol are the main products of glycerol etherification. However, the reaction is not sufficiently fast or do not selectively produce diglycerol apart from difficulties in filtration, neutralization and product purification. On the other hand, the need to eliminate the solvents from the homogeneous catalysts is a very challenging task [8]. Acid-catalyzed etherification which runs via a cationic intermediate can efficiently convert glycerol but mostly to the formation of unwanted cyclic oligomers. In addition, the catalyst deactivates quickly due to the blockage of the internal surface area and the acidic sites by the formed deposits. Another prominent drawback of this type of catalyst is that they involve the formation of acrolein due to acid-catalyzed dehydration reaction [9,10]. Base-catalyzed etherification reaction runs via an anionic intermediates to deprotonate glycerol intermediate that subsequently reacts with
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another glycerol molecule to form a dimer with the release of a hydroxyl ion [5]. Charles et al. [11] reported that 96% glycerol conversion with a corresponding selectivity to diglycerol of 24% were achieved using 2% Na2CO3 catalyst at 260 °C and 24 h. Alkaline metals impregnated onto mesoporous catalysts have been reported to achieve 80% glycerol conversion with not less than 40% diglycerol selectivity at 260 °C and 24 h [12]. Different catalysts usually show different activity while at the same time showing different capability to produce the desired product. So far, we have investigated various catalysts including homogeneous catalysts (NaOH, KOH, Na2CO3 and LiOH) [13] and mixed oxide catalyst [14] but the success was rather limited. The homogeneous catalysts led to the formation of larger molecular size products due to deep etherification reaction while the performance of mixed oxide catalyst was rather low due to low porosity of the catalyst. Clay minerals have good potential in chemical processing technologies due to their green properties. They receive considerable attention in various organic syntheses because of their environmental compatibility, low cost, operational simplicity and reusability [15]. This type of catalyst is needed especially when the product is to be used for edible purposes. In this study, the activity of montmorillonite K-10 clay intercalated with LiOH has been evaluated for solvent-free etherification of glycerol to selectively synthesize diglycerol under various reaction conditions. 2. Experimental 2.1. Preparation of Clay Li/MK-10 catalyst A 250 mL round bottom flask equipped with a reflux condenser was first charged with 10 g of montmorillonite K-10 clay, 15.8 g of LiOH and 100 mL of deionized water. This mixture was stirred and heated under reflux for 12 h. The slurry was then allowed to cool down to room temperature. The solid part was then separated from the liquid by centrifugation and washed by re-suspending it in 500 ml of deionized water and the centrifugation was repeated. This sequence was repeated for two more times to ensure complete removal of all soluble species. The prepared base modified clay was then calcined in a furnace at 450 °C for 4 h. This catalyst is denoted as Clay Li/MK-10. The parent Clay MK-10 material and the prepared Clay Li/MK-10 catalyst were subsequently characterized using XRD (Bruker; D 8 Advance), FTIR (Perkin–Elmer; 1725X), surface analyzer (Micromeritics; ASAP 2020) and TGA (Perkin Elmer STA-600). The base strength of the catalysts (H_) was determined using Hammett indicators.
product analysis after silylation according to a previously published method [16].
3. Results and discussion 3.1. Catalyst characterization Through thermal analysis technique, the optimum level of heat treatment required to prepare the Clay Li/MK-10 catalyst was determined. Fig. 1 shows TGA curves recorded for the materials. The total weight losses of Clay MK-10 and Clay Li/MK-10 when heated up to 800 °C were found to be ca. 15% and 12.5%, respectively. It is clear from this figure that Clay MK-10 released physically absorbed water at an amount of 9 wt.% before 100 °C. A very strong endothermic peak in the DTA result of Clay MK-10 between 100 and 200 °C with 3% mass loss was due to the removal of structural water. In the case of the prepared Clay Li/MK-10, the amount of physically adsorbed water was recorded to be about 6% lower than that of Clay MK-10. This might be due to lower hydrophilicity of Clay Li/MK-10. There are two endothermic peaks detected in the DTA of Clay Li/MK-10. The first one occurred right before 100 °C and it was due to the removal of physically absorbed water. The second one with a rather weak intensity was noted at around 580 °C and it might be due to the dehydration of certain surface components. It could be concluded that the structure of the prepared Clay Li/MK-10 showed high stability to heat treatment at temperatures below 580 °C. A prominent background of XRD patterns (not shown here) was found for both clay samples between 20° and 30° of 2h and they correspond to amorphous phases. The diffraction peaks of these samples were not sharp enough to suggest the poor crystallinity nature of the clay materials. Clay MK-10 showed a rather weak and broad peak at 2h of 5.9° (d = 14.7 Å). This peak corresponds to montmorillonite structure to suggest the presence of clay mineral of smectite type. Moreover, some reflections that match with the diffraction patterns of albite and quartz are also observed in both Clay MK-10 and Clay Li/MK-10. These results are similar to the values previously reported for an acid-treated montmorillonite K-10 clay [16]. However, after lithium intercalation (Clay Li/MK10), these peaks were slightly shifted toward lower 2h angles to suggest a small increase in the basal spacing of the material. Some new peaks are also observed in the XRD pattern of the prepared Clay Li/MK-10 and they are attributed to lithium species as reported by previous researchers [17]. Hence, the occurrence of these new peaks was to the confirmation of lithium presence in the Clay
2.2. Catalytic activity Glycerol etherification reaction was carried out at 240 °C in a three-neck glass reactor equipped with a PID temperature controller and a magnetic stirrer. A reaction temperature of 240 °C was found to be sufficiently high to convert glycerol to diglycerol while at the same time suppressing further reaction to form larger molecules. Earlier findings showed that lower reaction was unable to give temperature sufficiently high glycerol conversion while higher temperature always led to the formation of undesired products, especially at longer reaction times. This batch reactor was operated at an atmospheric pressure under inert condition (in N2 gas) in the presence of 2 wt.% of catalyst. During this reaction, water that was formed during the reaction was eliminated and collected using a Dean–Stark apparatus. In a typical experimental run, the reactor was charged with 50 g of anhydrous glycerol followed by 1.0 g of catalyst. A gas chromatograph (GCD 7820A, Agilent Technologies) equipped with a capillary polar column DB-HT5 was used for the
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Fig. 1. TGA and DTA curves recorded for Clay MK-10 and Clay Li/MK-10.
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Li/K-10 structure. At the same time, it also indicated the successful incorporation of lithium into the parent clay material. Table 1 shows the basic strength, BET surface area and pore volume of Clay MK-10 and Clay Li/MK-10 after calcination at 450 °C. As clearly noted, the basic strength of Clay MK-10 increased from H_ < 4.6 to 9.3 < H_ < 15 after the intercalation of lithium. The acidic or basic nature of montmorillonite structure was mostly influenced by its specific character of having combined water (structurally bound OH form or loosely bound H2O). It is obvious that the base modification had neutralized most of the acid sites within the clay and replaced them with their conjugate basic sites (O-) and impregnated the clay with cations [18]. It is clear from this table that surface area and mesoporosity of the modified clay decreased after modification. On the other hand, pore diameter of this modified clay was observed to slightly increase while the pore volume was found to experience a slight decrease. The BET surface areas of Clay MK-10 and modified Clay Li/MK-10 were recorded as 194 and 123 m2/g, respectively. The surface area and mesoporosity of the modified clay significantly decreased due to partial blockage or collapse of some portion of the layered structure after the lithium intercalation treatment. The pore size and pore volume of the Clay MK-10 remained almost the same after the modification with lithium. According to Cseri and coworkers [19], the dealumination of montmorillonite K-10 clay material could result from a stronger acid treatment leading to the creation of large pores while the amount of microporosity remained virtually unchanged. Nitrogen adsorption and desorption isotherms for Clay MK-10 and Clay Li/MK-10 are shown in Fig. 2. Both samples reveal the type IV isotherm which is a typical characteristic of mesoporous materials [20]. The hysteresis loop is rather small and it possesses features which are reminiscent of both the H3 and H1 type. The adsorption path appears to be suppressed at high relative pressure P/Po to indicate that the latter classification is more applicable. In addition, H1 type hysteresis is usually associated with solids consisting of nearly cylindrical channels, agglomerates or compacts of near uniform spheres. In each case, the hysteresis loop is narrow, with almost parallel adsorption and desorption branches. This is an indication of pores with regular geometry, while the steep desorption behavior suggested that the dimensions of the pores fell in quite a narrow range. Fig. 3 shows the FTIR spectra obtained for calcined Clay MK-10 and Clay Li/MK-10 in the wave number range between 400 and 4000 cm 1. The montmorillonite K-10 clay consisted of mainly Al3+ with some Fe2+/3+ and Mg2+ as octahedral cations and Na+, K+ and Ca2+ as exchangeable interlayer cations. The results showed that the FTIR spectra of modified Clay Li/MK-10 (curve b) is slightly different from that of the parent Clay MK-10 (curve a). The most intense band at 1035 cm 1 is attributed to Si–O in-plane stretching while that at 530 cm 1 is due to Si–O bending vibrations. The band at 530 cm 1 apparently shifted towards lower frequency at 524 cm 1 after lithium modification and its intensity increase indicated the significant effect on its tetrahedral sheet. The broad band at 3440 and 1639 cm 1 are attributed to the stretching and bending vibrations of the hydroxyl groups of water molecules that presented in the clay. The intensity of these bands was found to be lower in the case of Clay Li/MK-10. The parent Clay MK-10 shows the band at 3623 cm 1 which is within OH stretching region. This band is assigned to hydroxyl groups coordinated to octahedral cat-
Fig. 2. Nitrogen adsorption and desorption isotherms for Clay MK-10 and Clay Li/ MK-10.
Fig. 3. FTIR spectra for (a) MK-10 and (b) Li/MK-10 in the wave number range between 400 and 4000 cm 1.
ions which is due to hydroxyl group bonded with Al3+ cations [21]. The intensity of this band in clay after intercalation of lithium was slightly reduced. It might be due to the removal of octahedral cations causing the loss of water and hydroxyl groups coordinated to them. In addition, two new bands that appeared at 1480 cm 1 and 1440 cm 1 in the lithium modified clay are characteristics of the Bronsted and the Lewis acidities. The presence of these two bands indicated the formation of Li complexes with montmorillonite K10 [18]. The band at 915 cm 1 is associated with Al–Al–OH vibration of montmorillonite. The bands at 790 and 692 cm 1 suggest the presence of quartz admixtures in the sample while the strong bands at 790 cm 1 and 692 cm 1 are assigned to platy form of disordered tridymite and for quartz content, respectively [22]. The band at 790 cm 1 is entirely changed in the case of lithiummodified clay and strong disorder of tridymite is demonstrated. Meanwhile, the band at 692 cm 1 (quartz) shows the increase in intensity after lithium treatment to indicate an increase in SiO2 content in the clay structure. The band at 460 cm 1 is attributed
Table 1 Surface characteristics and basic strength of Clay MK-10 and Clay Li/MK-10. Sample
Basic strength (H_)
SBET (m2/g)
Smeso (m2/g)
dpore (nm)
Vpore (cm3/g)
Clay MK-10 Clay Li/MK-10
H_ < 4.6 9.3 < H_ < 15
194 123
143 97
5.9 6.1
0.37 0.35
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to the vibration of the double six-member ring, and bending vibration of O–Si–O and Si–O–M where M is Al, Li or any other alkaline/ alkaline earth metal [23]. These results suggested that the structure of Clay MK-10 was not significantly changed by the modification. However, a little difference between Clay MK-10 and modified Clay Li/MK-10 was observed due to presence of lithium in interlayer of parent clay after the successful intercalation.
3.2. Catalytic activity Table 2 summarizes some previously reported results and that were obtained in the present study for glycerol etherification to diglycerol under various conditions. Our previous study suggested that homogeneous LiOH catalyst gave high activity in selective glycerol conversion to diglycerol in 4 h at 240 °C. However, the selectivity started to decrease after 4 h as no more unconverted glycerol was available in reaction system while at the same time diglycerol started to convert to other products [24]. It is clear from this table that zeolite type catalysts were more active but less selective for diglycerol with short reaction time as compare to the other types and this was associated with their microporous structure. Due to small pore size of smaller than that of the glycerol molecule (0.52 nm), bulk reaction was deemed to mainly occur on the external surface of the catalyst. Thus, it showed lower selectivity to diglycerol with significant amount of some other higher molecules formed during the reaction [25]. On the other hand, mesoporous MCM-41 showed rather high selectivity but with poor glycerol conversion at a slightly high temperature of 260 °C. This type of mesoporous basic catalysts has high surface area with pore diameters of larger than 2 nm. Thus, most reaction can occur inside the pores so that enhanced selectivity to diglycerol was observed. However, the formation of larger molecules could block the mesopores channels causing a general decrease in the activity [27]. Due to their basic and surface properties, mixed oxides could also show significant conversion and selectivity at slightly low temperature but only after a long reaction time of 24 h [2]. Montmorillonite saponite clay (Mg-saponite) was also used by some researchers for this application but results showed that it was less active as well as less selective due to acidic behavior of this clay [5]. The prepared lithium-intercalated montmorillonite K-10 (Clay Li/MK-10) showed high glycerol conversion and selectivity to diglycerol under mild reaction conditions as compared to other catalysts. Theoretically, montmorillonite clay has a layer structure with basal spacing greater than glycerol molecule. The clay became basic but it sustained its layered structure after lithium intercalation. The activity and selectivity to diglycerol of this catalyst were enhanced due to high basic strength with suitable basal spacing to allow the reaction to occur in the internal pores to selectively form diglycerol while the formation of higher molecules was retarded. Fig. 4 shows glycerol conversion and selectivity to diglycerol versus time at 240 °C by the homogeneous LiOH, Clay MK-10 and Clay Li/MK-10. The activity and selectivity of homogenous LiOH
Fig. 4. Glycerol conversion and diglycerol synthesis over catalysts via solvent free etherification reaction at 240 °C.
catalyst under the same reaction conditions was obtained from our previous study [24]. The acid-treated Clay MK-10 was found to be inactive for glycerol etherification reaction with less than 20% conversion after 12 h. The low amount of diglycerol formed was noticed throughout the reaction with this clay sample. This might be due to presence of high acidity and insufficient basicity in the material. After the acid treatment, montmorillonite became strongly acidic in nature and its structure was also partially destroyed as confirmed from its surface properties and XRD results. For Clay Li/MK-10, an activation period was observed in the first 2 h of reaction with considerably low glycerol conversions (about 30%) as compared to that of homogenous LiOH. The conversion gradually increased with increasing reaction time and reached almost 100% after 12 h over Clay Li/MK-10 as shown in Fig. 4. On the other hand, the activation period of homogenous LiOH was found to be very fast and glycerol conversion achieved about 96% after only in 4 h while Clay Li/MK-10 showed about 40% of glycerol conversion under the same condition. It can also be seen in Fig. 4 that homogenous LiOH brought about reasonable formation of diglycerol (33% selectivity) after 2 h but further increase in reaction time was detrimental and it reached a low level of 8% after 12 h. On the other hand, Clay Li/ MK-10 showed a steady increase in the selectivity to reach its maximum value of 53% after 12 h. The corresponding glycerol conversion was also at its maximum level of 98%. It could be deduced from the plot that the yield of homogenous LiOH catalyst was very high initially but gradually decreased with increasing reaction time. On this respect, Clay Li/MK-10 clearly showed more superior results. Actually, montmorillonite K10 clay is Brønsted acid, but it was made basic by treating it with LiOH. Obviously, due to increasing basic strength of modified lithium clay as shown in Table 1 and improvement in its basal spacing confirmed by XRD results, the
Table 2 Activity of different catalysts in glycerol conversion to diglycerol. Catalyst type
Reaction temp. (°C)
Reaction time (h)
Glycerol conversion (%)
Diglycerol selectivity (%)
Reference
Homogenous Na2CO3 Homogenous LiOH Zeolite (NaX) Mesoporous (Cs-MCM-41) Mixed oxides (MgAl–Na) Alkaline earth oxide (BaO) Montmorillonite clay (Mg-saponite) Montmorillonite clay (CLAY Li/MK-10)
240 240 245 260 220 220 250 240
9 4 12 8 25 20 24 12
76 98 80 25 50 80 24 98
46 32 20 100 85 40 17 53
[24] [13] [24] [25] [2] [26] [5] Present study
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activity and diglycerol selectivity of Clay Li/MK-10 in the glycerol etherification reaction were improved. This modified clay also had an edge over homogenous LiOH in the sense that the lithium quantity used in Clay Li/MK-10 was 5 times lower. However, the use of this basic material resulted in the adsorption of glycerol over its surface which could reduce its activity after 12 h. As a conclusion, the overall results for the selective glycerol conversion to diglycerol via solvent free etherification reaction over Clay Li/ MK-10 were satisfactory and comparable with previously reported results as summarized in Table 2. Fig. 5 shows a detail view of glycerol conversion and diglycerol selectivity over the prepared lithium modified clay. The diglycerol selectivity increased from 37% to about 53% as glycerol conversion gradually increased with time. Triglycerol was also detected during the reaction but its selectivities were generally lower than 25%. The formations of undesired products other than diglycerol and triglycerol were significant at the beginning of the reaction but exhibited a decreasing trend with increasing reaction time. Higher glycerol conversion to diglycerol by the Clay Li/MK-10 catalyst as compared to Clay MK-10 or homogenous LiOH suggested that the pore size coupled with the basic strength of the catalyst could significantly affect the product selectivity [27]. Fig. 6 shows the presence of at least three different peaks in GC chromatogram which were identified and attributed to the three linear and branched isomers of diglycerol i.e. bb, ab and aa. The reaction mechanism leading to the formation of different isomers of di- and triglycerol through the multiple etherification reaction steps has been well understood and widely reported in earlier publications [5,8,12] and it is not deemed necessary to be repeated here. Some significant differences were observed in the diglycerol isomers distributions when homogenous LiOH and heterogeneous Clay Li/MK-10 catalyst were used (Fig. 7). This figure reflects the reactivity of the secondary OH group at position 2 of glycerol molecule which has some importance in the case of heterogeneous catalyst (having mesopores) for the synthesis of the desired diglycerol isomer. It can be seen that bb and ab isomers selectivity in Clay Li/ MK-10 increased while that of aa isomer decreased when shifting from homogenous LiOH to Clay Li/MK-10 catalyst. In this case, the selectivity of ab isomer increased from 35% to 55% while that of aa isomer decreased from 65% to 35%. In this study, the formations of bb and ab diglycerol isomers were clearly more favored when Clay Li/MK-10 was used as the catalyst as compared to those of the homogeneous LiOH catalyst. In the homogeneous reaction system, shape selectivity catalysis did not occur so that the formation of products was mainly influenced by the reactivity of OH groups as well as the stearic effect of the reactant molecules. However, the use of the clay based cat-
Fig. 5. Glycerol conversion and selectivity to diglycerol and triglycerol over Clay Li/ MK-10 catalyst at 240 °C.
Fig. 6. Typical GC chromatogram showing three different peaks of diglycerol isomers.
Fig. 7. Diglycerol isomers distribution with homogenous LiOH and Clay Li/MK-10 catalysts.
alyst would allow some degree of shape selectivity to take effect as the formation of linear molecules (aa diglycerol isomer) is generally not favored due to geometrical constraint post mainly by the interlayer spacing of the clay catalyst. Branched bb and ab isomers have relatively more compact molecules to allow relatively easy formation and diffusion within the internal interlayer spacing of the catalyst. However, the formation aa diglycerol isomer would still be allowed by the large external pores of the material to explain the presence of this isomer but at lower concentration in the product mixture. Data in Fig. 7 give a strong indication that the reaction could have taken place inside the internal pores of the catalyst (interlayer spacing) which were in meso size range (Table 1). Barrault et al. [25] also observed similar behaviors during glycerol etherification reaction over mesoporous catalyst and they correlated these changes to the occurrence of reaction within the porous area of catalyst. Thus, this property of the prepared clay catalyst was mainly responsible for the enhanced selectivity to diglycerol during the glycerol etherification reaction. Higher glycerol conversion to diglycerol by Clay Li/MK-10 in 12 h suggested that the basic strength and pore size could significantly affect the product selectivity. This result also suggested that the reaction might have taken place mostly in the internal pores rather than on the external surface of the catalyst. Therefore, with increasing reaction time, the synthesis of diglycerol steadily increased in the presence of active basic lithium component over porous layer structure of Clay Li/MK-10. This result indicated that pore size of the prepared Clay Li/MK-10 catalyst was an important factor to achieve higher selectivity to diglycerol.
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4. Conclusions Montmorillonite K-10 with intercalated LiOH between its interlayer was successfully prepared using ion-exchange method. The results of XRD and FTIR analyses revealed considerable changes in the parent clay after lithium intercalation by increasing its basal spacing and improving its partially damage interlayer structure. Surface properties of Clay MK-10 were also found to be improved after this modification. Increasing basic strength and pore size of modified clay was found to enhance the catalytic activity in selective glycerol etherification to diglycerol. It was concluded from diglycerol isomers distribution that the reaction predominately occurred in the internal pores of the clay catalyst. Compared to homogeneous LiOH catalyst, the Clay Li/MK-10 was found to be highly active for glycerol conversion and selective to diglycerol synthesis in the solvent-free glycerol etherification process performed under various reaction conditions. Acknowledgements A USM Fellowship and a Research University (RU) Grant from Universiti Sains Malaysia as well as an e-Sciencefund from MOSTI are gratefully acknowledged. References [1] Y. Shi, W. Dayoub, G.R. Chen, M. Lemaire, Selective synthesis of 1-O-alkyl glycerol and diglycerol ethers by reductive alkylation of alcohols, Green Chem. 12 (2010) 2189–2195. [2] C. García-Sancho, R. Moreno-Tost, J.M. Mérida-Robles, J. Santamaría-González, A. Jiménez-López, P.M. Torres, Etherification of glycerol to polyglycerols over Mg Al mixed oxides, Catal. Today 167 (2011) 84–90. [3] L. Shen, H. Yin, A. Wang, Y. Feng, Y. Shen, Z. Wu, T. Jiang, Liquid phase dehydration of glycerol to acrolein catalyzed by silicotungstic, phosphotungstic and phosphomolybdic acids, Chem. Eng. J. 180 (15) (2012) 277–283. [4] M. Ayoub, A.Z. Abdullah, Critical review on the current scenario and significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable energy industry, Renew. Sust. Energy Rev. 16 (2012) 2671–2686. [5] A. Martin, M. Richter, Oligomerization of glycerol – a critical review, Eur. J. Lipid Sci. Technol. 113 (2011) 100–117. [6] Z. Yuan, S. Xia, P. Chen, Z. Hou, X. Zheng, Etherification of biodiesel-based glycerol with bioethanol over tungstophosphoric acid to synthesize glyceryl ethers, Energy Fuels 25 (2011) 3186–3191. [7] J.A. Melero, G. Vicente, M. Paniagua, G. Morales, P. Muñoz, Etherification of biodiesel-derived glycerol with ethanol for fuel formulation over sulfonic modified catalysts, Biores. Technol. 103 (2012) 142–151. [8] J.M. Clacens, Y. Pouilloux, J. Barrault, C. Linares, M.G. Wasser, Mesoporous basic catalysts: comparison with alkaline exchange zeolites (basicity and porosity). Application to the selective etherification of glycerol to polyglycerols, Stud. Surf. Sci. Catal. 118 (1998) 895–902.
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Diglycerol synthesis via solvent-free selective glycerol etherification process over lithium-modified clay catalyst Muhammad Ayoub, Ahmad Zuhairi Abdullah ⇑ School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Diglycerol synthesis via solvent-free
glycerol etherification. LiOH modification of acid-treated
montmorillonite K-10. Elucidation of surface
physicochemical properties. High diglycerol selectivity of 53% at
98% conversion.
ab diglycerol isomer with reduced aa isomers.
Improved selectivity of
a r t i c l e
i n f o
Article history: Received 12 January 2013 Received in revised form 29 March 2013 Accepted 10 April 2013 Available online 19 April 2013 Keywords: Montmorillonite K-10 Lithium Glycerol Selective etherification Diglycerol isomer
a b s t r a c t Diglycerol was synthesized via solvent free glycerol etherification over solid-base catalyst. Acid-treated montmorillonite K-10 (Clay MK-10) was modified with LiOH (Clay Li/MK-10) through ion-exchange method and characterized using different techniques for surface and structural properties. The parent and modified clay were then used to catalyze the production of diglycerol via solvent free glycerol etherification reaction. The reaction was conducted at 240 °C for up to 12 h and the activity of the catalysts was compared with that of homogeneous catalyst (LiOH). High diglycerol selectivity of about 53% was obtained at a glycerol conversion of about 98% with Clay Li/MK-10. The selectivity to ab diglycerol isomer was also increased from 35% to 55% while the selectivity to aa isomer was decreased from 65% to 35%. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Glycerol is an abundant carbon–neutral renewable resource for the production of biomaterials [1–3]. It is an attractive renewable building block for synthesis of polyglycerols which have several uses in different fields [4]. Diglycerol has numerous applications in food and pharmaceutical and cosmetic industries [5]. Potential green catalytic routes have been reported using various solid catalysts but high selectivity for diglycerol at high glycerol conversion remains a challenge. Etherification of glycerol is used in the synthesis of oxygenated components such as polyglycerols and ployglycerol ethers [6,7]. Glycerol ethers (polyglycerols) are produced from catalytic etheri⇑ Corresponding author. Tel.: +60 4 599 6411; fax: +60 4 594 1013. E-mail address: [email protected] (A.Z. Abdullah). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.04.044
fication of glycerol with the use of different solvents. Polyglycerols, especially diglycerol and triglycerol are the main products of glycerol etherification. However, the reaction is not sufficiently fast or do not selectively produce diglycerol apart from difficulties in filtration, neutralization and product purification. On the other hand, the need to eliminate the solvents from the homogeneous catalysts is a very challenging task [8]. Acid-catalyzed etherification which runs via a cationic intermediate can efficiently convert glycerol but mostly to the formation of unwanted cyclic oligomers. In addition, the catalyst deactivates quickly due to the blockage of the internal surface area and the acidic sites by the formed deposits. Another prominent drawback of this type of catalyst is that they involve the formation of acrolein due to acid-catalyzed dehydration reaction [9,10]. Base-catalyzed etherification reaction runs via an anionic intermediates to deprotonate glycerol intermediate that subsequently reacts with
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another glycerol molecule to form a dimer with the release of a hydroxyl ion [5]. Charles et al. [11] reported that 96% glycerol conversion with a corresponding selectivity to diglycerol of 24% were achieved using 2% Na2CO3 catalyst at 260 °C and 24 h. Alkaline metals impregnated onto mesoporous catalysts have been reported to achieve 80% glycerol conversion with not less than 40% diglycerol selectivity at 260 °C and 24 h [12]. Different catalysts usually show different activity while at the same time showing different capability to produce the desired product. So far, we have investigated various catalysts including homogeneous catalysts (NaOH, KOH, Na2CO3 and LiOH) [13] and mixed oxide catalyst [14] but the success was rather limited. The homogeneous catalysts led to the formation of larger molecular size products due to deep etherification reaction while the performance of mixed oxide catalyst was rather low due to low porosity of the catalyst. Clay minerals have good potential in chemical processing technologies due to their green properties. They receive considerable attention in various organic syntheses because of their environmental compatibility, low cost, operational simplicity and reusability [15]. This type of catalyst is needed especially when the product is to be used for edible purposes. In this study, the activity of montmorillonite K-10 clay intercalated with LiOH has been evaluated for solvent-free etherification of glycerol to selectively synthesize diglycerol under various reaction conditions. 2. Experimental 2.1. Preparation of Clay Li/MK-10 catalyst A 250 mL round bottom flask equipped with a reflux condenser was first charged with 10 g of montmorillonite K-10 clay, 15.8 g of LiOH and 100 mL of deionized water. This mixture was stirred and heated under reflux for 12 h. The slurry was then allowed to cool down to room temperature. The solid part was then separated from the liquid by centrifugation and washed by re-suspending it in 500 ml of deionized water and the centrifugation was repeated. This sequence was repeated for two more times to ensure complete removal of all soluble species. The prepared base modified clay was then calcined in a furnace at 450 °C for 4 h. This catalyst is denoted as Clay Li/MK-10. The parent Clay MK-10 material and the prepared Clay Li/MK-10 catalyst were subsequently characterized using XRD (Bruker; D 8 Advance), FTIR (Perkin–Elmer; 1725X), surface analyzer (Micromeritics; ASAP 2020) and TGA (Perkin Elmer STA-600). The base strength of the catalysts (H_) was determined using Hammett indicators.
product analysis after silylation according to a previously published method [16].
3. Results and discussion 3.1. Catalyst characterization Through thermal analysis technique, the optimum level of heat treatment required to prepare the Clay Li/MK-10 catalyst was determined. Fig. 1 shows TGA curves recorded for the materials. The total weight losses of Clay MK-10 and Clay Li/MK-10 when heated up to 800 °C were found to be ca. 15% and 12.5%, respectively. It is clear from this figure that Clay MK-10 released physically absorbed water at an amount of 9 wt.% before 100 °C. A very strong endothermic peak in the DTA result of Clay MK-10 between 100 and 200 °C with 3% mass loss was due to the removal of structural water. In the case of the prepared Clay Li/MK-10, the amount of physically adsorbed water was recorded to be about 6% lower than that of Clay MK-10. This might be due to lower hydrophilicity of Clay Li/MK-10. There are two endothermic peaks detected in the DTA of Clay Li/MK-10. The first one occurred right before 100 °C and it was due to the removal of physically absorbed water. The second one with a rather weak intensity was noted at around 580 °C and it might be due to the dehydration of certain surface components. It could be concluded that the structure of the prepared Clay Li/MK-10 showed high stability to heat treatment at temperatures below 580 °C. A prominent background of XRD patterns (not shown here) was found for both clay samples between 20° and 30° of 2h and they correspond to amorphous phases. The diffraction peaks of these samples were not sharp enough to suggest the poor crystallinity nature of the clay materials. Clay MK-10 showed a rather weak and broad peak at 2h of 5.9° (d = 14.7 Å). This peak corresponds to montmorillonite structure to suggest the presence of clay mineral of smectite type. Moreover, some reflections that match with the diffraction patterns of albite and quartz are also observed in both Clay MK-10 and Clay Li/MK-10. These results are similar to the values previously reported for an acid-treated montmorillonite K-10 clay [16]. However, after lithium intercalation (Clay Li/MK10), these peaks were slightly shifted toward lower 2h angles to suggest a small increase in the basal spacing of the material. Some new peaks are also observed in the XRD pattern of the prepared Clay Li/MK-10 and they are attributed to lithium species as reported by previous researchers [17]. Hence, the occurrence of these new peaks was to the confirmation of lithium presence in the Clay
2.2. Catalytic activity Glycerol etherification reaction was carried out at 240 °C in a three-neck glass reactor equipped with a PID temperature controller and a magnetic stirrer. A reaction temperature of 240 °C was found to be sufficiently high to convert glycerol to diglycerol while at the same time suppressing further reaction to form larger molecules. Earlier findings showed that lower reaction was unable to give temperature sufficiently high glycerol conversion while higher temperature always led to the formation of undesired products, especially at longer reaction times. This batch reactor was operated at an atmospheric pressure under inert condition (in N2 gas) in the presence of 2 wt.% of catalyst. During this reaction, water that was formed during the reaction was eliminated and collected using a Dean–Stark apparatus. In a typical experimental run, the reactor was charged with 50 g of anhydrous glycerol followed by 1.0 g of catalyst. A gas chromatograph (GCD 7820A, Agilent Technologies) equipped with a capillary polar column DB-HT5 was used for the
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Fig. 1. TGA and DTA curves recorded for Clay MK-10 and Clay Li/MK-10.
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Li/K-10 structure. At the same time, it also indicated the successful incorporation of lithium into the parent clay material. Table 1 shows the basic strength, BET surface area and pore volume of Clay MK-10 and Clay Li/MK-10 after calcination at 450 °C. As clearly noted, the basic strength of Clay MK-10 increased from H_ < 4.6 to 9.3 < H_ < 15 after the intercalation of lithium. The acidic or basic nature of montmorillonite structure was mostly influenced by its specific character of having combined water (structurally bound OH form or loosely bound H2O). It is obvious that the base modification had neutralized most of the acid sites within the clay and replaced them with their conjugate basic sites (O-) and impregnated the clay with cations [18]. It is clear from this table that surface area and mesoporosity of the modified clay decreased after modification. On the other hand, pore diameter of this modified clay was observed to slightly increase while the pore volume was found to experience a slight decrease. The BET surface areas of Clay MK-10 and modified Clay Li/MK-10 were recorded as 194 and 123 m2/g, respectively. The surface area and mesoporosity of the modified clay significantly decreased due to partial blockage or collapse of some portion of the layered structure after the lithium intercalation treatment. The pore size and pore volume of the Clay MK-10 remained almost the same after the modification with lithium. According to Cseri and coworkers [19], the dealumination of montmorillonite K-10 clay material could result from a stronger acid treatment leading to the creation of large pores while the amount of microporosity remained virtually unchanged. Nitrogen adsorption and desorption isotherms for Clay MK-10 and Clay Li/MK-10 are shown in Fig. 2. Both samples reveal the type IV isotherm which is a typical characteristic of mesoporous materials [20]. The hysteresis loop is rather small and it possesses features which are reminiscent of both the H3 and H1 type. The adsorption path appears to be suppressed at high relative pressure P/Po to indicate that the latter classification is more applicable. In addition, H1 type hysteresis is usually associated with solids consisting of nearly cylindrical channels, agglomerates or compacts of near uniform spheres. In each case, the hysteresis loop is narrow, with almost parallel adsorption and desorption branches. This is an indication of pores with regular geometry, while the steep desorption behavior suggested that the dimensions of the pores fell in quite a narrow range. Fig. 3 shows the FTIR spectra obtained for calcined Clay MK-10 and Clay Li/MK-10 in the wave number range between 400 and 4000 cm 1. The montmorillonite K-10 clay consisted of mainly Al3+ with some Fe2+/3+ and Mg2+ as octahedral cations and Na+, K+ and Ca2+ as exchangeable interlayer cations. The results showed that the FTIR spectra of modified Clay Li/MK-10 (curve b) is slightly different from that of the parent Clay MK-10 (curve a). The most intense band at 1035 cm 1 is attributed to Si–O in-plane stretching while that at 530 cm 1 is due to Si–O bending vibrations. The band at 530 cm 1 apparently shifted towards lower frequency at 524 cm 1 after lithium modification and its intensity increase indicated the significant effect on its tetrahedral sheet. The broad band at 3440 and 1639 cm 1 are attributed to the stretching and bending vibrations of the hydroxyl groups of water molecules that presented in the clay. The intensity of these bands was found to be lower in the case of Clay Li/MK-10. The parent Clay MK-10 shows the band at 3623 cm 1 which is within OH stretching region. This band is assigned to hydroxyl groups coordinated to octahedral cat-
Fig. 2. Nitrogen adsorption and desorption isotherms for Clay MK-10 and Clay Li/ MK-10.
Fig. 3. FTIR spectra for (a) MK-10 and (b) Li/MK-10 in the wave number range between 400 and 4000 cm 1.
ions which is due to hydroxyl group bonded with Al3+ cations [21]. The intensity of this band in clay after intercalation of lithium was slightly reduced. It might be due to the removal of octahedral cations causing the loss of water and hydroxyl groups coordinated to them. In addition, two new bands that appeared at 1480 cm 1 and 1440 cm 1 in the lithium modified clay are characteristics of the Bronsted and the Lewis acidities. The presence of these two bands indicated the formation of Li complexes with montmorillonite K10 [18]. The band at 915 cm 1 is associated with Al–Al–OH vibration of montmorillonite. The bands at 790 and 692 cm 1 suggest the presence of quartz admixtures in the sample while the strong bands at 790 cm 1 and 692 cm 1 are assigned to platy form of disordered tridymite and for quartz content, respectively [22]. The band at 790 cm 1 is entirely changed in the case of lithiummodified clay and strong disorder of tridymite is demonstrated. Meanwhile, the band at 692 cm 1 (quartz) shows the increase in intensity after lithium treatment to indicate an increase in SiO2 content in the clay structure. The band at 460 cm 1 is attributed
Table 1 Surface characteristics and basic strength of Clay MK-10 and Clay Li/MK-10. Sample
Basic strength (H_)
SBET (m2/g)
Smeso (m2/g)
dpore (nm)
Vpore (cm3/g)
Clay MK-10 Clay Li/MK-10
H_ < 4.6 9.3 < H_ < 15
194 123
143 97
5.9 6.1
0.37 0.35
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to the vibration of the double six-member ring, and bending vibration of O–Si–O and Si–O–M where M is Al, Li or any other alkaline/ alkaline earth metal [23]. These results suggested that the structure of Clay MK-10 was not significantly changed by the modification. However, a little difference between Clay MK-10 and modified Clay Li/MK-10 was observed due to presence of lithium in interlayer of parent clay after the successful intercalation.
3.2. Catalytic activity Table 2 summarizes some previously reported results and that were obtained in the present study for glycerol etherification to diglycerol under various conditions. Our previous study suggested that homogeneous LiOH catalyst gave high activity in selective glycerol conversion to diglycerol in 4 h at 240 °C. However, the selectivity started to decrease after 4 h as no more unconverted glycerol was available in reaction system while at the same time diglycerol started to convert to other products [24]. It is clear from this table that zeolite type catalysts were more active but less selective for diglycerol with short reaction time as compare to the other types and this was associated with their microporous structure. Due to small pore size of smaller than that of the glycerol molecule (0.52 nm), bulk reaction was deemed to mainly occur on the external surface of the catalyst. Thus, it showed lower selectivity to diglycerol with significant amount of some other higher molecules formed during the reaction [25]. On the other hand, mesoporous MCM-41 showed rather high selectivity but with poor glycerol conversion at a slightly high temperature of 260 °C. This type of mesoporous basic catalysts has high surface area with pore diameters of larger than 2 nm. Thus, most reaction can occur inside the pores so that enhanced selectivity to diglycerol was observed. However, the formation of larger molecules could block the mesopores channels causing a general decrease in the activity [27]. Due to their basic and surface properties, mixed oxides could also show significant conversion and selectivity at slightly low temperature but only after a long reaction time of 24 h [2]. Montmorillonite saponite clay (Mg-saponite) was also used by some researchers for this application but results showed that it was less active as well as less selective due to acidic behavior of this clay [5]. The prepared lithium-intercalated montmorillonite K-10 (Clay Li/MK-10) showed high glycerol conversion and selectivity to diglycerol under mild reaction conditions as compared to other catalysts. Theoretically, montmorillonite clay has a layer structure with basal spacing greater than glycerol molecule. The clay became basic but it sustained its layered structure after lithium intercalation. The activity and selectivity to diglycerol of this catalyst were enhanced due to high basic strength with suitable basal spacing to allow the reaction to occur in the internal pores to selectively form diglycerol while the formation of higher molecules was retarded. Fig. 4 shows glycerol conversion and selectivity to diglycerol versus time at 240 °C by the homogeneous LiOH, Clay MK-10 and Clay Li/MK-10. The activity and selectivity of homogenous LiOH
Fig. 4. Glycerol conversion and diglycerol synthesis over catalysts via solvent free etherification reaction at 240 °C.
catalyst under the same reaction conditions was obtained from our previous study [24]. The acid-treated Clay MK-10 was found to be inactive for glycerol etherification reaction with less than 20% conversion after 12 h. The low amount of diglycerol formed was noticed throughout the reaction with this clay sample. This might be due to presence of high acidity and insufficient basicity in the material. After the acid treatment, montmorillonite became strongly acidic in nature and its structure was also partially destroyed as confirmed from its surface properties and XRD results. For Clay Li/MK-10, an activation period was observed in the first 2 h of reaction with considerably low glycerol conversions (about 30%) as compared to that of homogenous LiOH. The conversion gradually increased with increasing reaction time and reached almost 100% after 12 h over Clay Li/MK-10 as shown in Fig. 4. On the other hand, the activation period of homogenous LiOH was found to be very fast and glycerol conversion achieved about 96% after only in 4 h while Clay Li/MK-10 showed about 40% of glycerol conversion under the same condition. It can also be seen in Fig. 4 that homogenous LiOH brought about reasonable formation of diglycerol (33% selectivity) after 2 h but further increase in reaction time was detrimental and it reached a low level of 8% after 12 h. On the other hand, Clay Li/ MK-10 showed a steady increase in the selectivity to reach its maximum value of 53% after 12 h. The corresponding glycerol conversion was also at its maximum level of 98%. It could be deduced from the plot that the yield of homogenous LiOH catalyst was very high initially but gradually decreased with increasing reaction time. On this respect, Clay Li/MK-10 clearly showed more superior results. Actually, montmorillonite K10 clay is Brønsted acid, but it was made basic by treating it with LiOH. Obviously, due to increasing basic strength of modified lithium clay as shown in Table 1 and improvement in its basal spacing confirmed by XRD results, the
Table 2 Activity of different catalysts in glycerol conversion to diglycerol. Catalyst type
Reaction temp. (°C)
Reaction time (h)
Glycerol conversion (%)
Diglycerol selectivity (%)
Reference
Homogenous Na2CO3 Homogenous LiOH Zeolite (NaX) Mesoporous (Cs-MCM-41) Mixed oxides (MgAl–Na) Alkaline earth oxide (BaO) Montmorillonite clay (Mg-saponite) Montmorillonite clay (CLAY Li/MK-10)
240 240 245 260 220 220 250 240
9 4 12 8 25 20 24 12
76 98 80 25 50 80 24 98
46 32 20 100 85 40 17 53
[24] [13] [24] [25] [2] [26] [5] Present study
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activity and diglycerol selectivity of Clay Li/MK-10 in the glycerol etherification reaction were improved. This modified clay also had an edge over homogenous LiOH in the sense that the lithium quantity used in Clay Li/MK-10 was 5 times lower. However, the use of this basic material resulted in the adsorption of glycerol over its surface which could reduce its activity after 12 h. As a conclusion, the overall results for the selective glycerol conversion to diglycerol via solvent free etherification reaction over Clay Li/ MK-10 were satisfactory and comparable with previously reported results as summarized in Table 2. Fig. 5 shows a detail view of glycerol conversion and diglycerol selectivity over the prepared lithium modified clay. The diglycerol selectivity increased from 37% to about 53% as glycerol conversion gradually increased with time. Triglycerol was also detected during the reaction but its selectivities were generally lower than 25%. The formations of undesired products other than diglycerol and triglycerol were significant at the beginning of the reaction but exhibited a decreasing trend with increasing reaction time. Higher glycerol conversion to diglycerol by the Clay Li/MK-10 catalyst as compared to Clay MK-10 or homogenous LiOH suggested that the pore size coupled with the basic strength of the catalyst could significantly affect the product selectivity [27]. Fig. 6 shows the presence of at least three different peaks in GC chromatogram which were identified and attributed to the three linear and branched isomers of diglycerol i.e. bb, ab and aa. The reaction mechanism leading to the formation of different isomers of di- and triglycerol through the multiple etherification reaction steps has been well understood and widely reported in earlier publications [5,8,12] and it is not deemed necessary to be repeated here. Some significant differences were observed in the diglycerol isomers distributions when homogenous LiOH and heterogeneous Clay Li/MK-10 catalyst were used (Fig. 7). This figure reflects the reactivity of the secondary OH group at position 2 of glycerol molecule which has some importance in the case of heterogeneous catalyst (having mesopores) for the synthesis of the desired diglycerol isomer. It can be seen that bb and ab isomers selectivity in Clay Li/ MK-10 increased while that of aa isomer decreased when shifting from homogenous LiOH to Clay Li/MK-10 catalyst. In this case, the selectivity of ab isomer increased from 35% to 55% while that of aa isomer decreased from 65% to 35%. In this study, the formations of bb and ab diglycerol isomers were clearly more favored when Clay Li/MK-10 was used as the catalyst as compared to those of the homogeneous LiOH catalyst. In the homogeneous reaction system, shape selectivity catalysis did not occur so that the formation of products was mainly influenced by the reactivity of OH groups as well as the stearic effect of the reactant molecules. However, the use of the clay based cat-
Fig. 5. Glycerol conversion and selectivity to diglycerol and triglycerol over Clay Li/ MK-10 catalyst at 240 °C.
Fig. 6. Typical GC chromatogram showing three different peaks of diglycerol isomers.
Fig. 7. Diglycerol isomers distribution with homogenous LiOH and Clay Li/MK-10 catalysts.
alyst would allow some degree of shape selectivity to take effect as the formation of linear molecules (aa diglycerol isomer) is generally not favored due to geometrical constraint post mainly by the interlayer spacing of the clay catalyst. Branched bb and ab isomers have relatively more compact molecules to allow relatively easy formation and diffusion within the internal interlayer spacing of the catalyst. However, the formation aa diglycerol isomer would still be allowed by the large external pores of the material to explain the presence of this isomer but at lower concentration in the product mixture. Data in Fig. 7 give a strong indication that the reaction could have taken place inside the internal pores of the catalyst (interlayer spacing) which were in meso size range (Table 1). Barrault et al. [25] also observed similar behaviors during glycerol etherification reaction over mesoporous catalyst and they correlated these changes to the occurrence of reaction within the porous area of catalyst. Thus, this property of the prepared clay catalyst was mainly responsible for the enhanced selectivity to diglycerol during the glycerol etherification reaction. Higher glycerol conversion to diglycerol by Clay Li/MK-10 in 12 h suggested that the basic strength and pore size could significantly affect the product selectivity. This result also suggested that the reaction might have taken place mostly in the internal pores rather than on the external surface of the catalyst. Therefore, with increasing reaction time, the synthesis of diglycerol steadily increased in the presence of active basic lithium component over porous layer structure of Clay Li/MK-10. This result indicated that pore size of the prepared Clay Li/MK-10 catalyst was an important factor to achieve higher selectivity to diglycerol.
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4. Conclusions Montmorillonite K-10 with intercalated LiOH between its interlayer was successfully prepared using ion-exchange method. The results of XRD and FTIR analyses revealed considerable changes in the parent clay after lithium intercalation by increasing its basal spacing and improving its partially damage interlayer structure. Surface properties of Clay MK-10 were also found to be improved after this modification. Increasing basic strength and pore size of modified clay was found to enhance the catalytic activity in selective glycerol etherification to diglycerol. It was concluded from diglycerol isomers distribution that the reaction predominately occurred in the internal pores of the clay catalyst. Compared to homogeneous LiOH catalyst, the Clay Li/MK-10 was found to be highly active for glycerol conversion and selective to diglycerol synthesis in the solvent-free glycerol etherification process performed under various reaction conditions. Acknowledgements A USM Fellowship and a Research University (RU) Grant from Universiti Sains Malaysia as well as an e-Sciencefund from MOSTI are gratefully acknowledged. References [1] Y. Shi, W. Dayoub, G.R. Chen, M. Lemaire, Selective synthesis of 1-O-alkyl glycerol and diglycerol ethers by reductive alkylation of alcohols, Green Chem. 12 (2010) 2189–2195. [2] C. García-Sancho, R. Moreno-Tost, J.M. Mérida-Robles, J. Santamaría-González, A. Jiménez-López, P.M. Torres, Etherification of glycerol to polyglycerols over Mg Al mixed oxides, Catal. Today 167 (2011) 84–90. [3] L. Shen, H. Yin, A. Wang, Y. Feng, Y. Shen, Z. Wu, T. Jiang, Liquid phase dehydration of glycerol to acrolein catalyzed by silicotungstic, phosphotungstic and phosphomolybdic acids, Chem. Eng. J. 180 (15) (2012) 277–283. [4] M. Ayoub, A.Z. Abdullah, Critical review on the current scenario and significance of crude glycerol resulting from biodiesel industry towards more sustainable renewable energy industry, Renew. Sust. Energy Rev. 16 (2012) 2671–2686. [5] A. Martin, M. Richter, Oligomerization of glycerol – a critical review, Eur. J. Lipid Sci. Technol. 113 (2011) 100–117. [6] Z. Yuan, S. Xia, P. Chen, Z. Hou, X. Zheng, Etherification of biodiesel-based glycerol with bioethanol over tungstophosphoric acid to synthesize glyceryl ethers, Energy Fuels 25 (2011) 3186–3191. [7] J.A. Melero, G. Vicente, M. Paniagua, G. Morales, P. Muñoz, Etherification of biodiesel-derived glycerol with ethanol for fuel formulation over sulfonic modified catalysts, Biores. Technol. 103 (2012) 142–151. [8] J.M. Clacens, Y. Pouilloux, J. Barrault, C. Linares, M.G. Wasser, Mesoporous basic catalysts: comparison with alkaline exchange zeolites (basicity and porosity). Application to the selective etherification of glycerol to polyglycerols, Stud. Surf. Sci. Catal. 118 (1998) 895–902.
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