Acetalization of Symmetric Ketones with 1,2-Propanediol over Amorphous Solid Lewis Acids

Glycerol is a sustainable carbon source that can serve as a feedstock for functionalized molecules with desirable properties for a range of applications. Previous work has established the oxidation of glycerol-derived diether secondary alcohols (e.g., 1,3-dimethoxy-propan-2-ol) to symmetric glycerol-derived diether ketones (e.g., 1,3-dimethoxy-propan-2-one) via Swern oxidation and aqueous-phase oxidative dehydrogenation. Here, we report the acetalization of symmetric ketones with 1,2-propanediol to produce acetals, which are useful as green solvents and fuel additives. Metallosilicate catalysts (M = Zr, Hf, and Ga) were synthesized via a fast and simple one-pot method such that metal atoms were incorporated into a porous silica matrix (M-xerogels) as dispersed Lewis acid sites. The Lewis and Brønsted acid site densities of these materials were quantified at multiple M loadings via pyridine chemisorption and transmission infrared spectroscopy. Per Lewis acid site time yields (STYs) for acetalization of 1,2-propanediol with 3-pentanone, used as a model acetalization reaction, were highest for Ga-xerogel and lowest for Hf-xerogel. Apparent activation energies for this model reaction are reported over representative Zr-, Hf-, and Ga-xerogels, and apparent reaction orders are reported over Ga-xerogels. The impact of solvent identity, equilibrium limitations, and water concentration on STY over the Ga-xerogel are reported and demonstrate that in addition to the thermodynamic impacts of water on acetalization, water also inhibits the kinetics of acetalization. Acetalization of 1,3-dimethoxy-propan-2-one with 1,2-propanediol is reported over Ga-xerogel (tert-butanol solvent, T = 313–343 K, autogenous pressure), resulting in the formation of 2,2-bis(methoxymethyl)-4-methyl-1,3-dioxolane. The reusabilities of the catalysts are reported, demonstrating deleterious effects of intermediate washing with acetone and similar impacts of either intermediate washing with tert-butanol or intermediate calcination prior to reuse. These findings provide insight toward the formation of functionalized (and tunable) glycerol-derived acetals over amorphous solid Lewis acid catalysts.