Acetylation of glycerol to synthesize bioadditives over niobic acid supported tungstophosphoric acid catalysts

Abstract

Acetylation of glycerol to yield mono, di and triacetin (acetylated glycerol) was carried over niobic acid supported tungstophosphoric acid (TPA) catalysts. The catalysts with varying TPA content were prepared and characterized by different techniques. The characterization results reveal the presence of well-dispersed Keggin ion on support. The results suggest that the glycerol conversion and selectivities depend on the acidity of the catalysts, which in turn is related to the content of TPA on niobic acid. The change in conversion and selectivities during the acetylation also is attributed to the reaction time, catalyst concentration and glycerol to acetic acid molar ratio.

Introduction

The usage of biofuels for transportation in place of petroleum-derived fuels will increase throughout the world in coming decades. The demand for biofuels will rise to 5.75% in Europe on the basis of energy content for all the petrol fuels by the end of the year 2010 [1]. Other countries also have adopted policies that will result in much higher biofuels use over the next decade. Among the biofuels, bioethanol and biodiesel are produced on the large scale and the demand for biodiesel is increasing with time.

Biodiesel is produced by the transesterification of vegetable oils with methanol over alkali-based catalysts. Approximately 10% glycerol byproduct is generated during biodiesel synthesis [2]. The increase of biodiesel production will result in the accumulation of glycerol. This not only creates a glut in the market but also affects the overall economics of biodiesel production [3]. This situation has prompted a search for new glycerol uses. In the future, glycerol will be a cost-effective raw material for the preparation of valuable chemicals and fuel additives. The utilization of glycerol to produce different value added chemicals has been studied and details are reported in recent reviews [2], [3], [4]. Among different value added chemicals, hydrogenolysis to propane diols [5], [6], [7], [8], oxidation to different products [9], [10], [11] has been studied extensively. The use of glycerol-based additives to improve properties of biodiesel is also being explored [12], [13], [14]. Among different alternatives, the acetylation of glycerol with acetic acid to yield glycerin acetates as valuable transportation fuel additives is such an option [15].The products mono, di and tri acetyl esters of glycerol acetylation have great industrial applications [16], [17]. The triacetylated derivative also known as triacetin has applications ranging from cosmetics to fuel additives [18], [19].

Acid catalysts generally facilitate the acetylation reactions. The acetylation of glycerol using acidic sulfonic acid functionalized mesostructured materials was studied by Melero et al. [14]. Recent reports describe the acetylation of glycerol catalyzed by different commercially available solid acids such as Amberlyst-15, K-10 montmorillonite, niobic acid and zeolites [15]. The functionalized mesostructured catalysts showed considerable activity and selectivity. However, the preparation of these materials is complex. The commercially available solid acids showed poor selectivity towards the desired triacetin within reasonable conversion of glycerol [15]. Zeolite supported heteropoly acids have been studied for the esterification of glycerol with acetic acid [20].

Heteropolyacids (HPAs) are typical strong Bronsted acids and catalyze a wide variety of reactions in both homogeneous and heterogeneous phases offering efficient and cleaner processes [21], [22]. The major disadvantages of HPAs as catalysts lie in their low thermal stability, low surface area (1–10 m²/g) and solubility in polar media. HPAs can be made into eco-friendly insoluble solid acid catalysts with high thermal stability and surface area by supporting them on suitable supports. The support provides an opportunity for HPAs to be dispersed over a large surface area, which facilitates the increase of catalytic activity. Supported HPAs also were studied for simple esterification reactions and were found to be highly active [23]. The support plays not only an important role in dispersion of HPA but also on the acidic nature of the final catalyst. Niobic acid is known for its high acidity [24] and is commonly used for acid catalyzed reactions. It is of interest to study the catalytic properties of HPA supported on niobic acid.

In the present study, niobic acid supported tungstophosphoric acid catalysts are prepared and evaluated for the acetylation of glycerol with acetic acid. This reaction is tested under different reaction parameters to yield the desired product. The catalyst performance is discussed with the observed physico-chemical properties derived from different characterization methods. Different reaction parameters are studied to optimize the reaction conditions to obtain maximum activity and selectivity.