Dual-end-functionalized tin (Sn)-phyllosilicates for the esterification of oleic acid

Abstract

We designed dual-end-functionalized tin (Sn)-phyllosilicate, where the SH functional groups were subsequently oxidized for SO3H/SO4H functionalities in a Brønsted acid with/without NH2 functional groups in a Brønsted base. Dual-end-functionalized tin (Sn)-phyllosilicates [MTES:APTS=1.0:0.11, MTES:TEOS=1.0:0.11 and 1.0:0.43, v/v] were fabricated where the inorganic framework of Sn species can function as a Lewis acid. The Sn-phyllosilicates with dual acids were applied to esterify oleic acid and produce oleic acid-methyl-ester to test their feasibility as fatty-acid-conversion solid catalysts. In the absence of amine groups, the increase in ratio of tetraethyl orthosilicate (TEOS) to (3-mercaptopropyl)trimethoxysilane (MTES) for the synthesis of Sn-phyllosilicates (MTES:TEOS=1.0:0.11 and 1.0:0.43, v/v) increased the recovery of Sn-phyllosilicate in the solvent media and gradually decreased the oleic acid-methyl-ester production efficiency (%) of the oleic acid conversion. At the fixed 2.0wt% Sn-phyllosilicate (MTES:APTS=1.0:0.11, v/v) loading and oleic acid:MeOH weight ratio=1:1, an 80°C reaction temperature and 4h reaction time were determined to be the optimal conditions for the oleic acid-methyl-ester production, which had ∼90% oleic acid conversion efficiency. When the Sn-phyllosilicate (MTES:APTS=1.0:0.11, v/v) loading was increased from 0.1 to 5.0wt% at 80°C and 4h, the fatty-acid conversion efficiency (%) of oleic acid gradually increased from 39.16 to 92.23%. In contrast, Sn-phyllosilicate (MTES:TEOS=1.0:0.43) had the lowest oleic acid conversion efficiency (%) but the highest catalyst recovery. In summary, this study presents a facile method to produce oleic acid conversion catalysts on an industrial scale.