Applied Catalysis A: General 527 (2016) 9–18 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Understanding the role of Keggin type heteropolyacid catalysts for glycerol acetylation using toluene as an entrainer S.S. Kale a , U. Armbruster a , R. Eckelt a , U. Bentrup a , S.B. Umbarkar b , M.K. Dongare b,c , A. Martin a,∗ a Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany b Catalysis Division, CSIR-National Chemical Laboratory, Pune 411008, India c Mojj Engineering Systems Ltd., 15-81/B, MIDC, Bhosari, Pune 411026, India article info Article history: Received 27 April 2016 Received in revised form 28 July 2016 Accepted 17 August 2016 Available online 18 August 2016 Keywords: Glycerol Acetylation Azeotropes Heteropolyacids Triacetin Heterogeneous catalyst abstract The heterogeneously catalyzed esterification (acetylation) of glycerol toward triacetin in batch mode in presence of toluene as entrainer was studied. Silicotungstic acid, tungstophosphoric acid and phospho- molybdic acid as heteropolyacids (HPAs) supported on silica, alumina or silica-alumina were used as catalysts. The course of the reaction was found to be very sensitive to the nature of the HPA as well as the support. Solid characterization by Raman spectroscopy, XRD, and pyridine-FTIR revealed that only combinations of tungsten-based HPAs and silica support were able to preserve the structure of active com- ponent throughout the preparation process, which was essential to obtain active and selective catalysts. The interaction between HPA and support was decisive for stability and dispersion of the catalytically active species. With the best performing catalyst H 4 SiW 12 O 40 /SiO 2 , selectivity to triacetin reached 71% at complete conversion within 24 h. The high selectivity to triacetin is attributed the Brønsted acidic sites originated from stabilized Keggin structure and continuous removal of water during course of reaction. Toluene is able to form azeotropic mixtures with water and acetic acid and keeps the reaction tem- perature below the boiling point of acetic acid. Thus, water-free reaction conditions can be established. The catalyst was reusable; however, the activity and selectivity towards triacetin slightly decreased in a repetition run due to loss of active sites. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Glycerol is inevitably produced in the biodiesel production from transesterification of vegetable oil or animal fats and has led to a drastic surplus in chemical market. Global production of biodiesel market is estimated to reach 36.9 million metric tons in 2020, which will give approximately 3.7 million metric tons of crude glycerol [1–3]. The current use of glycerol in pharmaceuticals, food and cos- metics consumes only a small part and hence, demand is somehow limited. To sustain chemical market and industry, it is important to convert glycerol into value added chemicals by several catalytic processes, such as selective oxidation to glyceric acid or hydroxy- acetone [4], dehydration to acrolein [5–7], hydrogenolysis to 1,2- or 1,3-propanediol [8], etherification to alkyl ether [9,10], conden- sation to dimers or higher oligomers [11] and many others [3,12]. Another interesting approach to convert glycerol into monoacetyl glycerol (MAG, monoacetin), diacetyl glycerol (DAG, diacetin) and ∗ Corresponding author. E-mail address: andreas.martin@catalysis.de (A. Martin). triacetyl glycerol (TAG, triacetin) by means of esterification with acetic acid (acetylation) as shown in Fig. 1. These products have application in food and leather industry, as plasticizers and also they may serve as solvent and fuel additive [13–15]. Glycerol acetylation is an acid catalyzed reaction with equimolar formation of water as by-product at every consecutive step and the chemical equilibrium limits the extent of the esterification. In addi- tion, Gibbs free energies of the first two acetylation steps (MAG and DAG) are 19.15 and 17.80 kJ/mol, respectively, whereas this value for the third step (TAG) is relatively high (55.58 kJ/mol) and thus, the third step should be the most difficult one [16]. Beyond that, elevated temperatures are necessary to obtain high reaction rates. Generally the reaction is performed using homogeneous catalysts like sulfuric acid and paratoluene sulfonic acid [17] or heteroge- neous catalysts such as sulphated mesoporous silica [18], sulphated zirconia [19,20], sulphated activated carbon [21,22], double SO 3 H- functionalized ionic liquids [23] and acid ion exchange resins like Amberlyst-15 or Amberlyst-36 [24–26]. On the other side, use of acetic anhydride as acetylation agent instead of acetic acid boosts the selectivity to TAG close to 100% as no water is formed, and the reaction rates are high even in the absence of catalysts. However, http://dx.doi.org/10.1016/j.apcata.2016.08.016 0926-860X/© 2016 Elsevier B.V. All rights reserved.