Observation of Pt Migration on Pt/ZSM-5 Catalyst for Ethane Aromatization

Observation of Pt Migration on Pt/ZSM-5 Catalyst for Ethane Aromatization

Hui Wang^{a, *}, Junjun Shan^a, Lisa Nguyen^a, Joshua Miles^a, Hua Liu^b, Fu-Kuo Chiang^b, Jihong Cheng^a

^a NICE America Research Inc, 2091 Stierlin Ct, Mountain View, California USA 94043

^b National Institute of Clean-and-Low-Carbon Energy (NICE), Future Science & Technology City, Changping District, Beijing China 102211

*Corresponding author: wanghui@nicenergy.com

Owing to the low cost and increased supply of natural gas in the United States, the direct conversion of natural gas liquids (C2-C4) into aromatics is of great industrial importance.¹ In order to convert light alkanes such as ethane into aromatic products in a single step, a bi-functional catalyst is required where ethane is first activated on metal sites to ethylene, which is then selectively converted into aromatics on zeolite, as illustrated in Figure 1. Therefore, a catalyst with acid-metal intimacy could have synergy that allows for optimal performance in ethane aromatization reaction.

Figure  SEQ Figure \* ARABIC 1 Simplified mechanism for ethane aromatization

Such a composition can be easily achieved by impregnating or ion-exchanging ZSM-5 with Pt²⁺ during catalyst preparation. In the presence of binder such as alumina, Pt can be distributed between binder and zeolite, leading to an inferior catalyst. We demonstrated experimentally that under certain conditions, Pt located on alumina binder can migrate onto zeolite crystal. Through advanced characterizations such as dynamic CO adsorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), STEM, X-ray absorption near edge structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS), it is evident that Pt was almost atomically dispersed in zeolite micropore in the form of nanoclusters. The stabilization effect of micropore on Pt nanocluster is the primary driving force for Pt migration from alumina binder to zeolite that is almost irreversible.

The synthesis of atomically dispersed metal nanoclusters encapsulated within the zeolite micropores have attracted much attentions in recent decades due to many unique benefits offered by these confined nanoclusters. For example, such high metal dispersion can minimize the metal loading while maximizing metal activity. Furthermore, the encapsulation of such metal clusters within the zeolite pores allows one to take full advantage of shape selectivity of zeolite pore channels. Often time, this is achieved by carefully designed synthesis² or inter-structure transformation^{3, 4}. Our work presents another way to achieve metal nanoclusters encapsulated within zeolite micopores . 

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