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.