Etherification of glycerol with benzyl alcohol catalyzed by solid acids

J. Braz. Chem. Soc., Vol. 20, No. 2, 201-204, 2009. Printed in Brazil - ©2009 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00 Camila R. B. da Silva, Válter L. C. Gonçalves, Elizabeth R. Lachter and Claudio J. A. Mota* Instituto de Química, Universidade Federal do Rio de Janeiro, Cidade Universitária, Av. Athos da Silveira Ramos 149, CT Bloco A, 21941-909 Rio de Janeiro-RJ, Brazil Neste trabalho apresentamos os resultados da eterificação do glicerol com álcool benzílico, catalisada por diferentes sólidos ácidos. O mono-éter benzílico do glicerol foi o produto principal nas reações com a zeólita β e a resina ácida Amberlyst-35. Já o éter-di-benzílico foi o produto majoritário nas reações com o ácido p-tolueno-sulfônico e a argila K-10 como catalisadores. O ácido nióbico foi inativo na reação. A estrutura porosa da zeólita impediu a formação significativa de produtos de di e tri eterificação. In this work we present the results of glycerol etherification with benzyl alcohol, catalyzed by different solid acids. The mono-benzyl-glycerol ether was the main product in the reactions catalyzed by β zeolite and Amberlyst-35 acid resin, whereas di-benzyl-ether was formed in higher yield with the use of p-toluene-sulfonic acid and K-10 montmorillonite as catalyst. Niobic acid was inactive in this reaction. The porous structure of the zeolite impaired the formation of di and tri-benzyl-glycerol ethers. Keywords: glycerol, solid acids, etherification Introduction The concern about global warming, due to the use of fossil fuels, has motivated the debate about biofuels, produced from biomass materials. Among them, biodiesel appears as one of the most promising and is normally produced through the transesterification of vegetable oils or animal fat with methanol.1 This reaction is usually carried out under alkaline catalysis conditions, affording methyl esters of fatty acids, the biodiesel themselves, and glycerol. One of the most important challenges of the biodiesel production is the destination of the glycerol formed. According to recent projections,2 the world glycerol production would reach 1.2 million tons in 2010, due to the increasing use of biodiesel. This forecast affects the price of glycerol and of the biodiesel as well, because the producers have to find a destination for the glycerol. The main use of glycerol is in personal care and cosmetics, but its use as a valuable feedstock for new products and processes is growing in importance; some reviews about it have appeared in the literature.2-4 The catalytic hydrogenation of glycerol to 1,2 and *e-mail: 1,3-propanediol5-7 might become an industrial process in a near future, as some major chemical companies announced8 the interest in building commercial plants of this process. Glycerol reforming to syn gas9 is also a potential economic process, which might be coupled with Fischer-Tropsch type catalysts,10 to yield gasoline range hydrocarbons. Yet, glycerol might be blended with vacuum gasoil for processing in catalytic cracking units.11 Many of the previous applications may require further developments or might be restricted to some particular conditions of the glycerol supply. Therefore, the search for other applications, especially involving less time demanding technologies, is still needed. Glycerol ethers have many potential uses, such as fuel additives,12 solvents,13 cryogenics,14 and anti-bacterial agents.15 Reactions of glycerol with isobutene16 or tertbutanol17 under acid catalysis conditions afford tert-butylglycerol ethers, which have potential for blending with diesel.18 Recently, Gu et al.19 reported the etherification of glycerol with different alcohols catalyzed by acidfunctionalized silica. They reported yields varying from 61 to 96% of the mono and di glycerol ethers, using batch reaction conditions. These results prompted us to report some preliminary data of glycerol benzylation with benzyl alcohol, using different types of heterogeneous Communication Etherification of Glycerol with Benzyl Alcohol Catalyzed by Solid Acids 202 Etherification of Glycerol with Benzyl Alcohol Catalyzed by Solid Acids J. Braz. Chem. Soc. Scheme 1. Etherification of glycerol with benzyl alcohol in the presence of acid catalysts. acid catalysts, aiming to produce mono, di and tri benzyl glycerol ethers (Scheme 1). Experimental Reactions were carried out in batch mode, using 5.0 g (54 mmol) of glycerol, 17.5 g (162 mmol) of benzyl alcohol and 0.5 g of the pre-treated catalyst. The system was kept at a controlled temperature and continuously purged with nitrogen, to carry away the water molecules formed in the reaction. The products were analyzed by gas chromatography and, in some cases, by gas chromatography coupled to a mass spectrometer. The catalysts used were Amberlyst 35 (Rohm and Haas), zeolite β (Zeolyst), K-10 montmorillonite (Fluka), niobic acid (CBMM) as well as p-toluene-sulfonic acid (PTSA) for comparison with a homogeneous system. Table 1 shows the pre-treatment conditions and Table 2 reports characterization and acidity data of the heterogeneous catalysts. Table 1. Pre-treatment conditions of the heterogeneous catalysts Catalyst Pre-treatment Temperature/(oC) time/conditions Amberlyst-35 105 (10 oC min-1) Overnight/vacuum Zeolite β 400 (10 oC min-1) One hour Niobic Acid 300 (5 C min ) Three hours K-10 110 (10 C min ) One hour o o -1 -1 Table 2. Surface areas and acidity of the catalysts Catalyst Área/ (m2 g-1) Si/Al Aciditya/ (mmol n-butilamine g-1) Amberlyst-35 50 - 5.2b K-10 240 6.6 0.5 Niobic Acid 187 - 0.3 H-β Zeolite 633 16 1.6 Measured21 by n-butylamine adsorption at 150 oC; bInformed by the producer. a Results and Discussion Figure 1 shows the product distribution of the acidcatalyzed reaction between glycerol and benzyl alcohol at 110 oC and 120 min of reaction time. One can see that PTSA and K-10 montmorillonite favors the formation of di-benzyl ether, arisen from the self-etherification of benzyl alcohol, relative to the benzyl-glycerol ethers. The yield of the mono-benzyl-glycerol ether was higher over the K-10 catalyst compared with PTSA. Self-etherification of benzyl alcohol may be explained by the 3:1 molar ratio of benzyl alcohol to glycerol used in this study, whereas Gu et al. 19 used a ratio of 1:1 and apparently did not find self-etherification. Except for the niobic acid, all reactions showed 100% glycerol conversion. Amberlyst-35 and zeolite β showed a superior performance than the other catalysts, producing the glycerol benzyl ethers in higher yield than the di-benzyl Vol. 20, No. 2, 2009 203 da Silva et al. ether. The zeolite showed the best selectivity to the monobenzyl-glycerol ether among the catalysts tested. In fact, the selectivity to the di and tri-benzyl-glycerol ethers was very low over the zeolite catalyst. This might be associated with the shape selectivity property20 of zeolites, which impairs the formation of bulk tran (...truncated)