Novel alkali-free of layered double hydroxides and solid base catalysts for transesterification of C4-C18 triglycerides

Abstract

This study explores the possibilities of using solid base catalysts, mainly hydrotalcites, for the transesterification of triglycerides (TAG) possessing alkyl groups in the range of C4-C18. The aim of the research is to design various types of layered double hydroxides (LDHs) formulated with different atomic ratio of M2+/Al3+ (M = Zn2+, Ni2+ and Mg2+), spanning from 1.5-4.0 via an alkali-free method. To enhance the physicochemical properties of the LDH, catalysts were calcined-rehydrated via gas-phase (GP), liquid-phase (LP), and hydrothermal (HTM) pathways. Extensive characterisation of both structural and base properties of these materials was conducted to provide a greater insight into the role of base sites for heterogeneously catalysed transesterification. Results showed HTM reconstructed method enhances the LDH reconstruction purity, surface area and basicity. A positive trend in initial rates was observed as GP < LP < HTM with reconstruction on conventional ZnAl LDH. Following this, the HTM reconstruction method was consequently applied in conventional NiAl and MgAl LDH and further underwent transesterification reaction. However, the diffusion rates of C18 was still hampered by the bulky chains and narrow pores, which the turnover frequency (TOF) in a range of 0.46 min-1 for all prepared conventional LDH materials. This was due to the steric limitation of bulkier TAG to access the Bronsted basic sites within the interlayer and edges of LDHs. To overcome this, catalysts possessing a macroporous framework have been synthesised on Zn/Al, Mg/Al and Ni/Al LDH using the polystyrene template and reconstructed under HTM method. Preliminary studies on macroporous Ni/Al LDH found that reconstruction is impossible at ratio 4:1, thus it was discontinued. Macroporous Zn/Al and Mg/Al LDH were further characterised and tested in the transesterification reaction. The TOF was increased nearly ~20 folds in both macroporous LDH, simultaneously overcoming the above issue. Additionally, a comparison on commercial solid based catalysts such as dolomite and NanoMgO were also carried out. A remarkable catalytic performance was observed on commercial catalysts compared to the conventional and macroporous LDH materials which mainly attributed from the enhanced basicity of the catalyst.