UC Santa Barbara

Obtaining molecular-level structural information about heterogeneous catalysts and catalyst supports such as silicas and all-silica zeolites (zeosils) is critical for the rational design of new catalysts. Magnetic resonance techniques, including solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) and electron paramagnetic resonance (EPR) spectroscopies, are powerful and non-invasive tools, which were used to acquire detailed structural and dynamic information about these silica-based systems. Thermal dehydroxylation reduces the number of silanol sites on silica surfaces and is typically used to increase the fraction of non-interacting silanols for well-defined active sites, however, several studies have suggested that pairs or clusters of silanols persist even upon extreme heat treatment. The spatial distribution of these silanols was investigated using VCl4 as a paramagnetic probe molecule. EPR signals for the grafted V(IV) were absent at room temperature, but a Lorentzian lineshape characteristic of spin-spin coupled centers was observed below 20 K. The latter finding indicates strong electron spin-spin coupling and implies that the silanols are clustered. 1H double-quantum single-quantum MAS NMR also suggests that many silanols are closely spaced. Varying the V-loading combined with an analysis of the contribution of the coupled V(IV) component to the total EPR spectrum led to the finding that silanols are clustered in groups of 7 or more, and consequently cannot be described as isolated. Zeosils modified with H3PO4, referred to as P-zeosils, selectively catalyze the dehydration of biomass-derived alcohols, but are unstable in the presence of water. The nature of the active P-sites in these catalysts, and how they are impacted by water is not known. The P-site distribution in a P-modified self-pillared pentasil (P-SPP) was probed using solid-state 31P MAS NMR with frequency-selective detection, dynamic nuclear polarization-enhanced 29Si-filtered 31P detection and 31P-31P correlation experiments. The P-sites in the dehydrated material are surface-bound via hydrolytically sensitive P-O-Si linkages, while some are also oligomers containing hydrolytically robust P-O-P linkages. The P-sites evolve rapidly when exposed to water, even at room temperature. Initial cleavage of some P–O–Si linkages results in an evolving mixture of surface-bound mono- and oligonuclear P-sites with acidity due to the generation of POH groups. Eventually all are converted to H3PO4. The effect of the zeosil framework on the stability of the P-sites was determined by comparing the solid-state 31P and ultrafast 1H MAS-NMR of P-SPP to that of a hydrophobic P-modified BEA zeosil (P-BEA). P-BEA contains a higher fraction of hydrolytically-stable P-O-P bonds and a lower accessibility to water compared to P-SPP. Thus, both the water content and the framework play a role in the P-site distribution, which in turn impact the acidity and hence, catalytic activity of P-zeosils. However, elucidating the precise nature of the active P-sites under reaction conditions requires the use of operando NMR. At elevated temperatures (140 °C) and in the presence of 2-propanol, 31P and 13C operando MAS NMR spectra suggest the POH sites are converted to phosphate esters. The ability to identify acidic sites in P-zeosils, and to describe their structure and stability, by combining insights using conventional and operando NMR, will play an important role in controlling the activity of microporous catalysts.