X-ray Imaging of Zeolite Particles at the Nanoscale: Influence of Steaming on the State of Aluminum and the Methanol-To-Olefin Reaction

Abstract

In view of the limited oil reserves the methanol-to-olefin (MTO) process is an interesting catalytic route to provide raw materials for chemical industries. In the last decades, a vast number of studies have been devoted to increase our understanding of this important catalytic reaction leading to a consensus concerning the mechanism.[1–4] Accordingly, MTO is thought to proceed through the so-called “hydrocarbon pool” (HCP) mechanism,[5, 6] in which methanol is added to an organic scaffold present within the zeolite framework. This is followed by elimination of olefinic species in a closed catalytic cycle. Microporous silicoaluminophosphates and aluminosilicates, such as SAPO-34 and ZSM-5, are often used as MTO catalysts because of their unique acidic and structural properties. In the case of ZSM-5 the formation of ethene and propene is governed by two different catalytic routes,[7,8] allowing in principle to control the ethene/propene ratio. Unfortunately, throughout the MTO reaction undesired carbon deposits are formed in the narrow micropore system of ZSM-5, leading to severely restricted diffusion and therefore limited catalytic activity.[9] To overcome these limitations efforts have been made to improve the pore accessibility during synthesis,[10–12] and/or in post-synthetic steps,[13, 14] resulting in significant improvements in the diffusion properties of ZSM-5. In this work, two commercial ZSM-5 zeolites with dimensions of approximately 200–800 nm have been studied by scanning transmission X-ray microscopy (STXM). The first sample, denoted as ZSM-5-C, was calcined for 6 h at 5508C, whereas the second sample, further labeled as ZSM-5-S, was steamed for 3 h at 7008C. Details on the preparation and characteristics of ZSM-5-C and ZSM-5-S can be found in the Supporting Information (Figures S1–S13, Tables S1–S6). We will show how STXM, in combination with bulk characterization techniques, allows investigating the physicochemical properties of ZSM-5 zeolites in a novel way at the nanoscale.[ 15, 16] More specifically, detailed chemical maps, with a spatial resolution of 70 nm, have been obtained of aluminum, oxygen, and carbon, even under realistic reaction conditions.[17–19] In this manner, the influence of steaming on the state of aluminum, that is, the coordination and spatial distribution, as well as on the MTO performance, has been unraveled.