Zeolite Catalysis for Mandelic Acid Conversion and Cyclic Monomer Synthesis

Abstract

Petroleum-based plastics are ubiquitous, but their persistence in the environment due to slow (or even the absence of) degradation pathways has resulted in a desire to seek alternative materials with a lower environmental impact. Polyesters of alpha hydroxy acids (AHAs) are one such alternative, as they can be biodegradable and, in some cases, bioderived. Mandelic acid, an AHA bearing a phenyl substituent, can be synthesised into biodegradable polymers with high glass transition temperatures. Poly(mandelic acid) (PMA) has been described as a biodegradable alternative to polystyrene, as a result of the similarities in their structure and their thermal properties. Poly(AHAs) are often best synthesised by ring opening polymerisation (ROP) of a cyclic monomer. In the case of PMA, this has posed some challenges to researchers, requiring the investigation of various novel synthetic strategies, with a particular focus on the nature of the cyclic monomer. The role of catalysis in the synthesis of cyclic monomers from mandelic acid is comparably under-explored, with all reports thus far using the homogeneous catalyst, p-TSA. In this thesis, the use of heterogeneous zeolite catalysts in the synthesis of cyclic monomers of mandelic acid is investigated.

The synthesis of the cyclic dimer of mandelic acid, mandelide, was investigated over a range of Brønsted acidic zeolites. The conversion of mandelic acid over these catalysts was found to give rise to a wider variety of products than the previously reported conversion of lactic acid, under similar conditions. The effects of zeolite properties such as framework type, silica:alumina ratio and porosity were investigated.

The use of zeolites in the synthesis of 5-membered 1,3-dioxolan-4-ones of mandelic acid was also investigated. Zeolites performed comparably to p-TSA in batch reactions. After an initial catalyst screening, the best catalyst (H-Y-30) was taken forward for testing in a liquid phase flow reactor. Results varied depending on the monomer synthesised, with the best results achieved for 2-methyl-5-phenyl-1,3-dioxolan-4-one (MePhDOX).

Following the successful synthesis of MePhDOX over USY zeolites in the liquid phase flow reactor, ROP of this monomer using catalysts previously reported for the related monomers PhDOX and Me2PhDOX was investigated but a successful synthesis of PMA was not achieved.