A multiscale investigation of polypropylene glycol polymer upcycling to propionaldehyde via catalytic cracking on acid sites of mesoporous Y zeolites†

We investigate acid-catalyzed upcycling of PPG polymer, emphasizing crucial features on multiple length scales that span reaction engineering on macroscopic length scales down to zeolite catalyst design on the nanoscale. We modified a previously described semi-batch reactor configuration to minimize coking and enhance recovered selectivities by incorporating rapid quenching of reaction products (instead of slower quenching with a condenser, which facilitates sequential coupling reactions), and decreased the initial carrier-gas residence time in the bed consisting of mixed catalyst and PPG polymer, further reducing the deposition of solid residues in the used catalyst. Our results highlight the importance of tight interfacial contact between the catalyst surface and the initial PPG polymer reactant, which is achieved via a pretreatment that removes adsorbed water, for drastically increasing the propionaldehyde selectivity, particularly for the large surface-area mesoporous catalysts. Our best catalyst consisted of mesoporous Y zeolite synthesized at an alkalinity of 0.16 M and exhibited nearly the same high propionaldehyde selectivity of approximately 95% (86% propionaldehyde yield) for a PPG polymer with molecular weights of 425 and 2000 Daltons (Da), suggesting the absence of mass transport restrictions. We also deconvolute the catalyst attribute between extra-framework aluminum (Al_EF) content and mesopore external surface area that most sensitively controlled propionaldehyde selectivity. This was performed by synthetically incorporating Al_EF content into our optimum catalyst, at a high and low alumina dispersion. The high dispersion alumina catalyst consisted of a uniform 10 nm-thick alumina layer covering the interior pores of the mesoporous Y catalyst, whereas the low dispersion alumina catalyst had a completely phase-separated alumina phase, commensurate in size to the zeolite particles. Our results demonstrate that Al_EF content in the catalyst decreases propionaldehyde yield by increasing the amount of solid residues in the catalyst post reaction, and had a minor effect on the propionaldehyde selectivity. These results point to a Brønsted rather than Lewis acid-catalyzed mechanism of catalysis for PPG polymer upcycling to propionaldehyde. In summary, our study demonstrates the most sensitive controlling attribute of the zeolite catalyst for selective propionaldehyde synthesis is its mesoporosity (as reflected in the mesopore volume and surface area) and that the multiscale details of the catalyst and reactor design also have profound consequences in achieving high propionaldehyde selectivity and yield.