(119e) Organic-Functionalized Molecular Sieves As Direct Acylation Catalysts

Acylation is a key carbon-carbon bond formation reaction in several synthetic chemistry applications. While anhydrides, formed via carboxylic acid dehydration, are facile acylating agents, the direct use of carboxylic acids as the acylating agents is of interest from a process design perspective. The direct route is difficult to achieve in practice, however, because the water generated as a by-product inhibits the acid sites in the zeolite catalysts. Using the acylation of toluene as a model reaction, we found that a zeolite beta with Si/Al= 37.5 exhibited good activity (85% conversion after 24 hr) towards the acylated aromatic product, tolyl hexanone, when the anhydride of hexanoic acid was used; direct acylation via the acid itself gave <10% conversion over the same time period. Specific zeolite synthesis techniques can provide a hydrophobic intracrystalline pore space that assists in the removal of water from areas around active sites to prevent this inhibition, but the nature of the active site and the hydrophobicity are related to the Si/Al ratio—with higher values giving more hydrophobic environments, but lower active site densities. Thus, optimization of conventional Bronsted acid zeolite catalysts for both high activity and robustness to water is challenging.

Here, we investigate the use of organic-functionalized molecular sieve (OFMS) catalysts, that incorporate a different type of acid site within a hydrophobic micropore. Specifically, we synthesized pure silica zeolite beta-type molecular sieves that contain intracrystalline sulfonic acid sites. In this way, we can independently vary the acidity and the hydrophobicity to understand the activity of a site that is locally hydrophilic, but in an environment free of a H-bonded network that contributes to extended site inhibition. Through measurements of site strength, water uptake, and direct vs. indirect acylation reactivity, we compare these materials to conventional Bronsted acid zeolites across a range of Si/Al and sulfonated mesoporous silica. These experiments systematically evaluate the impact of the nature of the active site, hydrophobicity, and confinement on the catalytic performance in the indirect and direct acylation reactions and offer promising results for the optimized design of a direct acylation catalyst.