Homogeneous Catalysis and Heterogeneous Recycling: A Simple Zn(II) Catalyst for Green Fatty Acid Esterification

Massimo Melchiorre,Roberto Esposito,Francesco Ruffo,Maria Elena Cucciolito,Martino Di Serio,Oreste Tarallo,Marco Trifuoggi

doi:10.1021/acssuschemeng.1c01140

Abstract

This work describes the use of simple zinc(II) salts (ZnCl2, ZnCO3, Zn(OAc)2, ZnO, Zn(ClO4)2, Zn(TfO)2, and Zn(BF4)2) as effective catalysts for the esterification of fatty acids with long-chain alcohols and simple polyols through a homogeneous system that allows the gradual and selective removal of water. The results show that the catalytic activity depends on the nature of the counterion: the most effective are the salts with poorly coordinating anions (perchlorate and triflate) or containing basic Brønsted anions (oxide, acetate, and carbonate). However, only with the latter is it possible to fully recover the catalyst at the end of each run, which is easily filtered in the form of zinc carboxylate, given its insolubility in the ester produced. In this way, it is possible to recycle the catalyst numerous times, without any loss of activity. This beneficial prerogative couples the efficiency of the homogeneous catalysis with the advantage of the heterogeneous catalysis. The process is, therefore, truly sustainable, given its high efficiency, low energy consumption, ease of purification, and the absence of auxiliary substances and byproducts.

Highlights

The growing concern about global warming and fossil resource consumption has led to the development of sustainable technologies, processes, and products that exploit renewable energies and feedstocks.[1,2]Among renewable resources, fatty acids (FAs) from vegetable oils occupy a relevant position due to their similarity to the aliphatic fractions of fossil materials.[3,4] For a full application of the principles of the circular economy, it is necessary that the vegetable raw material comes from waste or nonfood oils, a condition that creates a virtuous route for the sustainable production of many commodities.[5]
A first catalytic screening was performed on a series of Zn(II) salts to evaluate the effect of the counterion on both activity and leaching
The comparison was carried out considering coordinating properties and Brønsted basicity of the anion, which allowed identifying the best catalyst for subsequent optimizations

Summary

■ INTRODUCTION

The growing concern about global warming and fossil resource consumption has led to the development of sustainable technologies, processes, and products that exploit renewable energies and feedstocks.[1,2]. At the end of each run, the reaction mixture was allowed to cool overnight at room temperature; the resulting precipitated zinc(II) carboxylate was filtered, washed with ethyl acetate and n-hexane, dried under vacuum, weighed, and reintroduced quantitatively into the flask as a catalyst for the run. The efficiency of the catalytic system was verified in the esterification of polyols, such as pentaerythritol and glycerol, whose esters find applications in different industrial sectors, such as in cosmetics and engine lubricants.[55] The optimized molar ratio condition cannot be transferred to systems containing polyols to obtain complete conversion of the alcoholic groups. The Lewis contribution (right circle) becomes relevant by zinc carboxylate species obtained through exchange reactions (as I′ from step iv): the acid carbonyl is activated against the nucleophilic attack of the alcohol (v) through coordination to the metal ion (I′), and rearrangement of intermediate II′ forms the ester (vi). Proposed Pathway of the Esterification Reaction pubs.acs.org/journal/ascecg

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES