(296g) Hydrolysis of Phosphoric Acid Supported on Silica Bilayers/Ru(0001)
AIChE Annual Meeting
Tuesday, November 9, 2021 - 2:20pm to 2:40pm
Phosphorus-modified siliceous zeolites, or P-zeosils, catalyze the selective dehydration of biomass derivatives to platform chemicals such as 1,3-butadiene and pentadienes. While these materials are not active in the absence of phosphorus, their enhanced activity and selectivity were attributed to surface-bound P-sites. However, the characterization of such sites is challenging due to their dynamic and heterogeneity at reaction conditions. Surface confinement could lead to phosphoric acid condensation leading to the formation of P-O-Si or P-O-P bonds. In our efforts to describe these P-sites' speciation, we synthesized a 2D zeolite model that consists of a silica bilayer supported on Ru/(0001) to investigate the anchoring points of P-species on the zeosil. The strategy implemented involves a two-step process consisting of a hydroxylation step and an impregnation step. The first promotes the formation of defects (silanol groups) on the surface of the silica bilayer. We hypothesize that these silanol groups work as anchoring points for the phosphate molecules. The impregnation step proceeds by anchoring phosphoric acid to the bilayer. We used X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), X-ray reflectivity, and Atomic-force microscopy to characterize the surface, which confirms homogeneous incorporation of phosphate to the bilayer with a Si/P ratio of 20. We further investigated the nature of the silicon-phosphorus interaction by in situ water dosing by both XPS and IRRAS. The results suggest that as temperature and pressure increase, both Si-OD and P-OD increase. The ratio of Si-OD/P-OD is also found constant at different conditions of temperature and pressure, which is an indication that the number of moles formed of each species is the same, supporting the hypothesis of Si-O-P hydrolysis. Kinetic data suggest that the energy barrier to hydrolyze Si-O-P bonds is about 3 kcal/mol, which is in the same order of magnitude as what was found by DFT.