Title: Understanding the atomic-scale compositions, structures, and reactive properties of bifunctional metal-zeolite catalysts

Abstract

Heterogeneous catalysts are central to countless aspects of modern life, from providing transportation fuels, pharmaceuticals, and food products to mitigating harmful environmental emissions. In particular, zeolites are a class of aluminosilicates that are commonly used in heterogeneous catalytic applications due to their ion-exchange capacity, well-defined microporous crystalline frameworks, and high surface areas. Dispersed metal species are often distributed throughout a zeolite support to form bifunctional catalysts which contain both metal and acid sites, enabling them to catalyze coupled reactions, such as hydrogenation/dehydrogenation and hydroisomerization/cracking reactions. The reactive properties of metal-containing zeolite catalysts depend strongly on their atomic-scale compositions and structures. For example, ultra-stable Y-zeolites (USY) containing dispersed platinum (Pt) are widely used industrially for hydrocracking and hydroisomerization of hydrocarbons, reactions which are essential for petroleum and bio-based fuel and chemical production. USY zeolites are formed by dealumination, which introduces mesopores and decreases framework aluminum content, leading to fewer Brønsted (H+) acid sites. Catalyst performance is influenced by both the metal and acid functions, the distributions of which depend on the framework aluminum content, mesoporosity, and the type and location of dispersed metals, all of which are modified during synthesis, processing, and subsequent operation. As a result, different metal-containing catalysts exhibit markedly different reactivities, the origins of which are not apparent from conventional techniques that are challenged by the non-stoichiometric distributions of dilute acid sites and guest metal species. Understanding such atomic-scale compositional and structural features of metal-containing zeolite catalysts is crucial to the development of strategies to improve and prolong catalyst activity, control metal dispersion, and use expensive guest metal species effectively.  

We overcome these challenges by using powerful solid-state NMR techniques, in combination with complementary X-ray diffraction, electron microscopy, infrared spectroscopy, and adsorption analyses, to resolve the local structures and interactions of metal and acid sites, as functions of treatment conditions. We show that two industrially relevant 0.5 wt% Pt/H+USY catalysts exhibit significantly different hydroisomerization reactivities and selectivities, the origins of which are not apparent from conventional techniques. By comparison, two-dimensional (2D) 27Al{29Si} and 29Si{1H} MAS NMR spectra show clear differences in the distributions and quantities of framework Al atoms, Brønsted acid sites, and silanol groups, as well as Pt atoms and clusters, all of which are correlated with macroscopic hydroisomerization and hydrocracking reactivities. These results are correlated with in situ 13C and 1H MAS NMR analyses, which provide information on the types and relative quantities of reactants and products over the Pt/H+USY catalysts for industrially significant reactions, such as n-hexadecane hydroisomerization and hydrocracking, under conditions up to 50 bar and 240 oC. In addition, we have made progress in characterizing the local chemical environments of dilute supported Pt species by using novel 195Pt NMR techniques to probe Pt interactions with adsorbed molecules or exchangeable cations, even for Pt loadings as low as 0.5 wt%. 

Similar analyses are extended to other bifunctional catalysts, such as Pt-containing H+USY zeolites in the presence of an alumina binder as well as Sn-containing H+USY for glucose isomerization. For example, we show that the addition of an alumina binder, which is commonly required to pelletize the zeolite catalyst and provide the mechanical integrity required for large-scale applications, can alter the location and distribution of Brønsted acid sites and dispersed Pt species within the Pt/H+USY zeolite. In particular, two-dimensional (2D) 27Al multiple quantum (MQ-MAS) spectra reveal the formation of four- and six-coordinate aluminum species (AlIV and AlVI, respectively) that only appear upon extrusion of H+USY zeolite with the alumina binder. In addition, 2D 27Al–29Si NMR analyses indicate that the binder-induced AlIV species form covalent 27Al–O–29Si bonds with the zeolite framework, which results in an increase in acidity. These findings demonstrate that the binder used during the zeolite catalyst pellet-forming process introduces new active sites and alters the distribution of Pt species, with implications for catalyst performance in hydroisomerization and hydrocracking reactions facilitated by Pt/H+USY zeolite catalysts. Overall, the resulting atomic-level insights on the compositions, structures, and reaction properties of bifunctional metal-containing zeolites provide a foundation for the rational design of heterogeneous zeolite catalysts with improved catalytic performance.