Title: Correlating the atomic-level compositions, structures, and reaction properties of Pt-zeolite catalysts

Abstract:

Heterogeneous catalysts are used for approximately 80% of the chemical catalysts used in the world, among which metal supported on zeolite catalysts are particularly versatile and widely used in chemical industries. The reaction properties of these catalysts depend strongly on the compositions and nanoscale architectures of the zeolite support, as well as the types and locations of metal species within the zeolite pores, which are influenced by the catalyst synthesis and treatment conditions. Understanding the atomic-scale structures of metal-zeolite systems is crucial to the development of strategies to control metal dispersion and thereby improve catalyst performance.Results and analyses of three examples of platinum supported on zeolite systems will be discussed in this talk:

1.     Pt on F-KL zeolite: Sub-nanometer Pt particles supported on KL zeolite converts low-value alkanes into high-value aromatic compounds with significantly higher selectivity compared to Pt supported on other types of zeolites. Interestingly, the addition of a small amount (<1 wt%)  of fluorine further improved the catalytic activities of the Pt/KL catalyst without increasing the dispersion of the Pt particles. Distinct types of fluorine species are identified and quantified in fresh and spent Pt/F-KL catalysts by using solid-state nuclear magnetic resonance spectroscopy analyses. Specifically, interactions between fluorine species that are directly bonded or strongly interacting with the zeolite framework are unambiguously revealed by two-dimensional correlation NMR experiments. The detailed compositional and structural information provides new insights on the roles of such species, in particular the beneficial effects of dilute fluorine species, on the macroscopic reaction properties of Pt/F-KL catalysts.

2.     Pt on H⁺USY zeolite: Two types of bifunctional Pt- H⁺USY catalysts (with Si/Al molar ratios of 30 and 54) were used for n-hexadecane hydroisomerization to identify reaction pathways and correlate atomic-scale structures to their catalytic activities. Though the catalysts have similar Pt loadings (~0.5 wt%) and comparable dispersions, they exhibit significantly different hydroisomerization reaction pathways Atomic-scale origins of such differences in macroscopic behaviors are investigated by a combination of characterization techniques such as X-ray diffraction, electron microscopy, infrared spectroscopy, and solid-state nuclear magnetic resonance (ssNMR) spectroscopy Moreover, recent advancements in NMR instrumentation enabled to conduct in situ variable temperature and pressure (up to 200 bar & 240 ^oC) ¹³C MAS NMR measurements, which provide opportunities to identify types of reactants and products under reaction conditions.

3.     Pt on NaY zeolite: Calcination temperature-dependent types and distributions of Pt in calcined Pt-NaY are revealed, i.e., platinum oxide particles form in supercages of zeolite NaY when calcined at 400 ^oC, while 600 ^oC calcined Pt-NaY yields monoatomically dispersed Pt²⁺ cations stabilized by zeolite framework oxygen atoms in hexagonal prisms. Unprecedented detailed understandings of atomic structures of calcined Pt-NaY are obtained by advanced two-dimensional (2D) solid-state NMR techniques. Notably, 2D through-bond ²⁷Al{²⁹Si} measurement and through-space ²³Na{²⁹Si} NMR correlation spectra of the parent NaY and Pt-NaY materials reveal calcination temperature-dependent differences in their local framework structures that provide information on the locations of the supported Pt species. Moreover, resolved 2D NMR results enabled the identification and quantification of different framework tetrahedral (T) sites and their local changes due to proximate Pt species.

The methods and analyses presented for three types of Pt-zeolite systems are expected to be of broad importance in understanding correlations between reaction properties and atomic-scale structures of industrially significant catalysts.