Alumina-supported potassium iodide as a heterogeneous catalyst for biodiesel production from soybean oil

Abstract

Biodiesel fuel, a promising alternative diesel fuel produced by a catalytic transesterification of vegetable oils, has become more attractive recently because of its environmental concerns and the fact that it is made from renewable resources. In this work, the transesterification of soybean oil with methanol has been studied in a heterogeneous system, using alumina loaded with potassium iodide as a solid base catalyst. After loading KI of 35 wt.% on alumina followed by calcination at 773 K for 3 h, the catalyst gave the highest basicity and the best catalytic activity for this reaction. The catalysts were characterized by means of XRD, IR, SEM and the Hammett indicator method. Moreover, the dependence of the conversion of soybean oil on the reaction variables such as the catalyst loading, the molar ratio of methanol to oil and the reaction time was studied. The conversion of 96% was achieved under the optimum reaction conditions. Besides, a correlation of the catalyst activity for the transesterification reaction with its basicity was proposed.

Introduction

Due to the predicted shortness of conventional fuels and environmental concerns, a search for alternative fuels has gained recent significant attention. As the calorific value of vegetable oils is comparable to that of diesel, they could be used as fuels in compression ignition engines. However, their direct use in injection diesel engines is problematic mainly because of their high viscosity and low volatility. Indeed, the viscosity of vegetable oils is about ten times higher than that of diesel. As a result, the vegetable oils could cause poor fuel atomization, incomplete combustion and carbon deposition on the injector and valve seats resulting in serious engine fouling. The commonly employed methods for reducing the viscosity of vegetable oils are blending with diesel, emulsification, pyrolysis, cracking and transesterification. Among these, transesterification of vegetable oils to methyl esters appears to be the best method [1].

The transesterification of vegetable oils is a catalytic transesterification reaction where a triglyceride reacts with methanol producing glycerine and a mixture of fatty acid esters. The result is that triglyceride molecules, which are long and branched, are transformed into smaller esters whose size and properties are similar to those of diesel oils. These esters have significant potential as an alternative diesel fuel (better known as biodiesel) both in neat forms or blended with conventional diesel fuels. Biodiesel is characterized by excellent properties as diesel engine fuels and thus can be used in compression-ignition (diesel) engines with little or no modifications. Besides, it also effects a decrease in emissions of SO_x, CO, unburnt hydrocarbons and particulate matter during the combustion process when compared with fossil fuels [2], [3].

Biodiesel is usually prepared in the presence of homogeneous base or acid catalysts. The acid-catalyzed process often uses sulfonic acid and hydrochloric acid as catalysts; however, the reaction time is very long (48–96 h) even at reflux of methanol, and a high molar ratio of methanol to oil is needed (30–150:1, by mol) [4], [5], [6]. Potassium hydroxide, sodium hydroxide and their carbonates, as well as potassium and sodium alkoxides such as NaOCH₃, are usually used as base catalysts for this reaction [7], [8], [9]. As the catalytic activity of a base is higher than that of an acid and acid catalysts are more corrosive, the base catalysis is preferred to acid catalyzed routes, and is thus most often used commercially.

However, in this conventional homogeneous method removal of these catalysts after reaction is technically difficult and a large amount of wastewater was produced to separate and clean the catalyst and the products. Therefore, conventional homogeneous catalysts are expected to be replaced in the near future by environmentally friendly heterogeneous catalysts. The replacement of homogeneous catalysts by heterogeneous catalysts would have various advantages, most important being the application of easier working up procedures, the easy catalyst separation from the reaction mixture and the reduction of environment pollutants. At the laboratory scale, many different heterogeneous catalysts have been developed to catalyze the transesterification of vegetable oils with methanol. For example, Beynese et al. [10] reported the use of EST-4 and EST-10 catalysts to provide conversions of 86% and 53%, respectively, at 493 K and 1.5 h reaction time. Besides, Suppes et al. [11] achieved conversions of 78% at 513 K and >95% at 533 K for the transesterification of vegetable oils using calcium carbonate rock as a catalyst. Both the above studies required temperatures higher than 473 K to achieve high conversions within the time scales of the experiments. More recently, Kim et al. [12] prepared a solid superbase of Na/NaOH/γ-Al₂O₃, which showed almost the same catalytic activity under the optimized reaction conditions as that of the conventional homogeneous NaOH catalyst. Vegetable oils and methanol are not miscible and form two liquid layers upon their initial introduction into reactor. In most of the experiments using heterogeneous catalysts, the reaction proceeded at a relatively slow rate compared to those conducted with homogeneous base catalysts such as KOH and NaOH. These slow reaction rates are due to diffusion problems since these heterogeneous media behave as a three-phase system (oil/methanol/catalyst). In spite of this, heterogeneous catalysts could improve the synthesis methods for the development of an environmentally benign process and the reduction of production cost.

Yamaguchi et al. [13] reported that the catalysts, which were prepared by loading alkali metal compounds such as KNO₃ on alumina, followed by heating at 773–873 K, were very strongly basic catalysts and active for the isomerization of cis-but-2-ene at 273 K and, the activity of the catalyst was also confirmed by other researchers [14]. These materials did not show any strong basicity before activation, which made them distinguished from other solid bases for industrial applications, since the contamination of CO₂ from atmosphere could be avoided. In the in situ activation prior to reaction, unusually strong basic sites such as K₂O [14] were formed on the composites resulting from the decomposition of loaded KNO₃ and had retained high efficiency in the catalytic processes. By drawing on this, it can then be supposed that loading KI on alumina followed by calcination at a high temperature could also generate strong basic sites and this material can be used for base-catalyzed reactions because of easy thermal decomposition of KI to K₂O in air. In this work, the KI/Al₂O₃ sample was tested as a heterogeneous catalyst for the transesterification of soybean oil with methanol. The catalytic efficiency in this reaction was estimated regarding the conversion of soybean oil to methyl esters. XRD, IR, SEM, DTA-TG and the Hammett indicator method were employed for the catalyst characterizations. Besides, the effect of the reaction variables such as the ratio of methanol to oil, the reaction time and the catalyst amount on the conversion to methyl esters was investigated. Further, a correlation between the basicity of the catalysts and their catalytic activity towards the transesterification of soybean oil was also proposed.