1. Introduction
Today, as the world continues to grow rapidly and extensive use of mineral oil has caused fear in large fields of growth. This is due to decline in the quantity of mineral oil which would take millions of years to replenish. Also, the contamination caused by mineral oils has also led to many studies into the option of using alternative lubricants as more eco-friendly or green materials (Mahmud, 2014). It was established from the early production and uses of special application lubricants that polyol esters of fatty acid have equal or even better technical properties than mineral oils2. Consequently, vegetable oils are the perfect choice for replacing mineral oil (Lathi, 2007). In spite of the excellent lubricating properties (high viscosity index, low volatility and high shear stability), the vegetable oils have several drawbacks that cause their instability and tend to be easily damaged and have restricted their application in the lubricants industries (Wang, 2019). The poor oxidation stability is owing to the contamination of polyunsaturated fatty acids in its composition (Campanella, 2010). Many chemical and biological studies were conducted to reduce the content of polyunsaturated fatty acids and increase the monounsaturated fatty acids (Li, 2012). However, the esterification reaction of fatty acid and polyhydric alcohols as an effective industrial method to obtain desirable polyhydric esters seems to be the favored (Zhang, 2004). Of all these polyesters, in recent years TMPTO have been given considerable attention due to their high thermal and hydrolytic stability due to the absence of β-H (Miles, 1998). Meanwhile, oleic acid (OA) is chosen as a cheap monounsaturated fatty acid for synthesis the polyhydrate ester. The esterification of OA with possesses difficulties among them is the molecular inherent hindrance of the components of the reaction (Karimi, 2012). At the industrial scale, according to our knowledge, the homogenous acidic catalysts are the most employed. The advantages of the homogeneous catalysts are the high catalytic activity and high selectivity as a result of well-defined molecularly structure. This system of homogeneous catalysis requires additional steps to isolate and purify the final product (Kuzminska, 2015). Using of heterogeneous catalysts could overcome the latter problem but with low activity and selectivity.
Our work is an attempt to solve this problem. Modified catalysts, namely ”heterogenized homogeneous catalysts”, which possess both advantages of homogeneous and heterogeneous catalyst were applied in OA esterification with TMP. In the synthesis of these catalysts, the ionic liquids (as homogeneous catalyst) were electrostatically immobilized onto SBA-15 to be loosely as Brønsted acids in viscous medium of the reaction.