Direct Acylation of 2-Methylfuran with Carboxylic Acids Over Zeolite Catalysts

Abstract

Zeolites have garnered considerable attention in recent years as highly promising catalysts for direct Friedel-Crafts acylation reactions, employing renewable carboxylic acids, leading to the production of a versatile range of high-value fuels and essential commodity chemicals. In particular, the acylation of furanic compounds using carboxylic acids offers an enticing avenue for the sustainable synthesis of surfactants. Despite significant progress in recent years in enhancing our understanding of acylation reactions involving sustainable biomass-derived compounds via heterogeneous zeolite catalysis, there remains a notable knowledge gap concerning certain aspects of catalyst behavior, including activity, stability, and the factors influencing product selectivity. In chapter 2, the catalyst deactivation mechanisms during acylation are investigated using commercial ZSM-5 zeolites with different Si/Al ratios. The role of 2-MF self-coupling, side reactions with ketenes, and sequential reactions with formed products are decoupled by feeding each species independently and comparing activity loss rates with those of varying surface coverage. The specific reactants employed in this study are 2-methylfuran (2-MF) and acetic acid (AA) to yield the product was 2-acetyl-5-methylfuran (Ac-MF). We reveal that the deactivation constant associated with direct 2-MF adsorption on a clean catalyst surface is approximately twice that of product self-coupling under identical conditions. Co-feeding carboxylic acids improves catalyst stability by diminishing the deactivation rate by several orders of magnitude via inhibiting adsorption of 2-MF and the formed products. Furthermore, evaluating the prominent route toward catalyst deactivation at high acid coverage was determined by systematic modification of surface coverage and the diffusion path of produced products. These results uncover the critical role of acetic acid to limit direct interaction between highly reactive acyl acceptors with surface Bronsted sites. Moreover, the addition of varying partial pressures of non-reactive adsorbent molecules with significant adsorption constants allows manipulation of surface coverage while maintaining the gas phase chemical potential. These experiments show that the product (Ac-MF) reacting with gas phase 2-MF is the primary mechanism of deactivation at high acid coverage, not sequential reactions along the diffusion path or direct interaction of 2MF with the zeolite surface. Based on the aforementioned results, we proposed that attenuating the acid strength could accelerate the product (Ac-MF) desorption which in return decreases the probability of a sequential reaction between the product and the gas phase 2-MF. A method was developed in chapter 3 involving incorporation of phosphorus in MFI zeolites which leads to decrease the acidity of Brønsted acid site in the catalyst. Here we report the acylation of 2-methylfuran with acetic acid over phosphorus-modified MFI zeolites with various phosphorus loadings. We show that at lower reaction temperatures (160°C), only the sites that are not titrated by phosphorus exhibit activity for acylation. However, when increasing the reaction temperature to 240°C the weaker phosphorus-modified sites begin to exhibit activity for acylation. Interestingly, the phosphorus-modified samples exhibited slower coke formation, and therefore deactivation, due to diminished side reactions. The active sites in aluminosilicate zeolite have distinct activity that is subjective to several factors associated with the local environment. For example, the proximity of extra-framework aluminum (EFAL) to Brønsted acid sites (BAS) gives rise to distinct active sites exhibiting notably enhanced activity. In chapter 4, we undertake a comprehensive investigation into the acylation of 2-methylfuran (2-MF) using acetic acid (AA) over EFAL-incorporating MFI (Si/Al=40) and BEA (Si/Al=150) zeolites, then contrasting their behaviors after EFAL removal via ammonium hexafluorosilicate (AHFS) washing. EFAL concentrations were measured via solid-state Al27-NMR analysis. Acylation under mild conditions, specifically within the 160-180°C range, remarkably uncovers higher activity over zeolites possessing EFAL, whereas reaction rates experience a pronounced decline post-AHFS treatment. Interestingly, the removal of EFAL leads to a twofold increase in measured activation barriers, aligning them with those observed for all-framework aluminum MFI (Si/Al=140) zeolites. DFT calculations are presented that agree with the experimental activation energies, highlight modifications to adsorbed intermediates and kinetically relevant transition states. Understanding this complex interplay between extra-lattice alumina and Brønsted acid sites is significant for comprehending the underlying factors that govern the kinetics of the reaction and holds key implications for optimizing catalytic performance. This study substantially contributes to understanding of zeolite-catalyzed acylation, offering a pivotal framework to enhance catalytic efficiency within the territory of renewable chemical synthesis. The properties of acyl donor in acylation reactions play a significant role in determining the behavior of resulting products and their industrial applications in the commodity chemicals. In Chapter 5, we investigate the role of carbon chain length in direct acylation of 2-MF over HZSM-5 zeolite under varied conditions. Our research reveals that reaction rates increase with larger acids, despite the accompanying rise in activation barriers, due to the formation of more stable surface acyl intermediates. This increased stability can be attributed to a stronger adsorption, which was obtained from fitting Eley-Rideal kinetic model. Additionally, our experiments show varying deactivation rates of the catalyst during acylation with different carboxylic acid mixtures, underscoring the importance of co-feeding acetic acid alongside larger carboxylic acids. The addition of acetic acid not only mitigates catalyst deactivation but also maintains product selectivity towards acylation with larger carboxylic acids.