Catalytic dehydration of poly (vinyl alcohol-co-ethylene) with heterogeneous acid catalysts

Abstract

This study investigates the role of acid site density, pore size, and solvent selection in the catalytic dehydration of Poly (vinyl alcohol-co-ethylene) (EVOH) using zeolite catalysts. The first set of experiments reveals that catalysts with lower acid site densities and larger pores exhibit higher maximum rates and better activity retention after deactivation, attributed to enhanced mass transfer through additional available pores. Conversely, zeolites with higher acid site densities demonstrate consistently higher maximum rates overall, but lower acid site density zeolites outperform them when normalized, emphasizing the significance of larger pores and the mass transfer limited nature of this reaction. External active sites are found to play a crucial role in the polymer dehydration reaction, as their removal leads to reduced maximum rates and faster catalyst deactivation. The investigation of solvents shows that use of DMSO results in limited reactivity with zeolite catalysts, potentially due to competition for acid sites or inhibition of certain transition steps. In contrast, a water-propanol mixture proves effective in dissolving EVOH while reducing the degree of competition compared to DMSO. Thus, this solvent offers better activity while also enabling additional chemistry like alcohol-induced ether formation. However, the ratio of water to 1-propanol significantly impacts catalyst performance, with a 12.5% water loading resulting in optimum rates possibly due to a combination of water competing for the active sites and the degree of solvation of the polymer. However, challenges in analyzing results with NMR due to sampling inconsistencies make inviable a detailed study of the underlying mechanism and rates for this reaction under solvated environment. Cryomilling was used to improve random sampling, but heterogeneities in the final polymer sample persisted, accentuating the difficulties of characterization. DSC and FTIR analyses confirm these heterogeneities and highlight the need to find techniques that analyze representative samples to achieve accurate measurements