Synthesis of oxygenated fuel additives via the solventless etherification of glycerol Muhammad Ayoub, M.S. Khayoon ⇑ , Ahmad Zuhairi Abdullah School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia article info Article history: Received 9 January 2012 Received in revised form 20 February 2012 Accepted 21 February 2012 Available online 3 March 2012 Keywords: Glycerol Fuel additives Diglycerol Lithium hydroxide Glycerol ethers abstract The synthesis of oxygenated fuel additives via solvent freebase-catalyzed etherification of glycerol is reported. The products of glycerol etherification arediglycerol (DG) and triglycerol (TG) with DG being the favorable one. The catalytic activity of different homogeneous alkali catalysts (LiOH, NaOH, KOH and Na 2 CO 3 ) was investigated during the glycerol etherification process. LiOH exhibited an excellent cat- alytic activity during this reaction, indicated by the complete glycerol conversion with a corresponding selectivity of 33% toward DG. The best reaction conditions were a reaction temperature of 240 °C, a cat- alyst/glycerol mass ratio of 0.02 and a reaction time of 6 h. The influences of various reaction variables such as nature of the catalyst, catalyst loading, reaction time and reaction temperature on glycerol ether- ification were elucidated. Industrially, the findings attained in this study might contribute towards pro- moting the biodiesel industry through utilization of its by-products. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The forecasted decline in the production of petroleum fuels and the growing concern about atmospheric greenhouse gas concen- trations have necessitated the search for clean, renewable (sustain- able), efficient and affordable alternative fuel. Biodiesel is becoming a key fuel in motor engines if blended in certain portions with petrodiesel (Hasheminejad et al., 2011). It can be readily pro- duced via the transesterification of vegetable oils (edible, non-edi- ble or reused) with low alcohols (methanol or ethanol). Indeed, the inevitable formation of glycerol that accompanies the biodiesel production process is affecting the process economy (Olutoye and Hameed, 2011; Yuan et al., 2010). Moreover, the growth of the biodiesel industry will result in overproduction of glycerol and create a superfluity of this impure product as its production is equivalent to 10% of the total biodiesel produced (Cardona et al., 2007; Khayoon and Hameed, 2011). Glycerol is an abundant carbon-neutral renewable resource for the production of biomaterials as well as source for a variety of chemical intermediates (Rahmat et al., 2010; García-Sancho et al., 2011). Unfortunately, biodiesel-derived glycerol is not bio- compatible due to its contamination with toxic alcohol (methanol or ethanol). Therefore, global research is focused on the effective conversion of glycerol to valuable chemicals to ameliorate the economy of the whole biodiesel production process. Recently, many studies have been dedicated to the transformation of this renewable polyol by various catalytic processes (Rahmat et al., 2010; Melero et al., 2012). This encompasses oxidation process to obtain dihydroxyacetone, glyceraldehyde, glyceric acid, glycolic acid and hydroxyl pyruvic acid (Liebminger et al., 2009; Augugliaro et al., 2010); fermentation process towards 1,3-propanediolpro- duction (Tokumoto and Tanaka, 2011); acetylation process with acetic acid to obtain polyglycerol esters (Gonçalves et al., 2008; Balaraju et al., 2010; Dosuna-Rodríguez et al., 2011; Khayoon and Hameed, 2011) and acetalisation process with ketones to obtain oxygenated acetals and ketals (Umbarkar et al., 2009; Vicente et al., 2010; da Silva and Mota, 2011; Silva et al., 2010). Glycerol is also an efficient platform for the synthesis of oxy- genated components such as polyglycerols and ployglycerol ethers by means of etherification process (Melero et al., 2010, 2012; Yuan et al., 2011). Glycerol ethers (polyglycerols) are produced from cat- alytic etherification of glycerol with the use of different solvents. Particularly, these components have found colossal potential appli- cations as fuel additives (Rahmat et al., 2010; Martin and Richter, 2011). Polyglycerols, especially diglycerol and triglycerol (called herein later as DG and TG, respectively) are the main products of glycerol etherification. The use of solvent could create some prob- lems in the production process leading to a more complex overall process. In this respect, solventless etherification process could promise several advantages but limited information is currently available on this mode of glycerol etherification process. Venturing into the possibility of such process is a worthwhile research effort. Glycerol etherification has been extensively investigated with or without the use of organic solvents. For both cases, different homogeneous alkali catalysts like carbonates and hydroxides or heterogeneous catalysts like zeolite, mesporous silica and metal oxides have been applied (Clacens et al., 2002; Jerome et al., 2008; Martin and Richter, 2011). DG and TG are produced from the consecutive condensation of two or three glycerol molecules, 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.02.103 ⇑ Corresponding author. Fax: +60 4 594 1013. E-mail address: muatazshakir@gmail.com (M.S. Khayoon). Bioresource Technology 112 (2012) 308–312 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech