High-Productivity Continuous Conversion of Glucose to α-Hydroxy Esters over Postsynthetic and Hydrothermal Sn-Beta Catalysts.

The retro-aldol fragmentation of glucose is a complex reaction of industrial relevance, which provides a potentially sustainable route for the production of α-hydroxyester compounds of relevance to the green polymer industry, such as methyl lactate and methyl vinyl glycolate. Although the zeolite catalyst, Sn-Beta, has shown itself to be a promising catalyst for this process, important information concerning the stability of the catalyst during continuous operation is not yet known, and improvements to its yield of retro-aldol products are also essential. Here, we perform detailed spectroscopic studies of a selection of Sn-Beta catalysts and evaluate their performances for the retro-aldol fragmentation of glucose under continuous processing conditions, with the dual aims of developing new structure-activity-lifetime relationships for the reaction and maximizing the productivity and selectivity of the process. Kinetic studies are performed under both established reaction conditions and in the presence of additional promoters, including water and alkali salts. Generally, this study demonstrates that the reaction conditions and choice of catalyst cannot be optimized in isolation, since each catalyst explored in this study responds differently to each particular process perturbation. However, by evaluating each type of the Sn-Beta catalyst under each set of reaction conditions, we reveal that postsynthetic Sn-Beta catalysts exhibit the best levels of performance when activity, selectivity, and stability are taken into account. Specifically, the best levels of performance are obtained with a postsynthetic Sn-Beta catalyst that is preactivated in a flow of methanol prior to reaction, which provides α-hydroxyester yields over 90% at the early stages of continuous operation and operates at high yield and selectivity for over 60 h on stream. Space-time-yields over two orders of magnitude higher than any previously reported for this reaction are achieved, setting a new benchmark in terms of the retro-aldol fragmentation of glucose.