Morphology-induced shape selectivity in zeolite catalysis

Abstract

a b s t r a c tZeolites are crystalline aluminosilicates that are widely used as shape-selective catalysts in the refinery and petrochemical industries. By synthesizing a series of structurally related zeolite catalyst and applying them in the industrially relevant conversion of methanol to hydrocarbons, new and extended understanding of the concept of shape selectivity has been reached. We have analyzed the relationship between particle morphology and pore system for catalysts having both small-and medium-sized pores using microscopy and diffraction, as well as performing extensive standard characterization. Two instances of preferential exposure of certain crystal facets, favoring access to one pore system over the other, have been found. Zeolites ZSM-57 (MFS) and SUZ-4 (SZR) both have both 8-and 10-ring channels. MFS prefers a plate-like or nanosheet-type morphology, where the 10-ring channels are perpendicular to the plates/sheets. This leads to favorable diffusion properties and a pronounced deactivation resistance. For SZR, needle-like crystals with high aspect ratio are preferred. In this instance, the sides of the needles are covered by the 8-ring pore openings, whereas the 10-ring pore openings are only exposed at the ends of the needles. This leads to unexpected product selectivity, as the material behaves essentially as an 8-ring catalyst. We argue that it should be possible to tune the shape selectivity of zeolite catalysts having pore systems of different dimensions by controlling the crystal morphology.

The Role of Non-Framework Lewis Acidic Al Species of Alkali-Treated HZSM-5 in Methanol Aromatization

“…Table 2 presents the framework Si/Al ratios ((Si/Al) F ) estimated through the equation proposed by Thomas [27] and the bulk Si/Al composition ((Si/Al) B ) acquired by ICP analysis. The (Si/Al) F ratio declined monotonically in the following order: HZ (40) > HZ-D (39) > HZ-DA (38), which was in agreement with the above-mentioned trend of decreasing framework Si concentration. The close (Si/Al) F ratios of tested catalysts implies that the employed alkaline and acid treatments remove mainly amorphous Si and Al species, and the ZSM-5 framework could be preserved.…”

“…C3 + hydrocarbons and ethylene profiles were quite similar; their desorption temperatures were quite close to each other in the temperature range (250-400 °C) of an autocatalysis system [34][35][36]. Ethylene is a tracer of the arene cycle, while C3 + hydrocarbons are produced through the alkene cycle [36][37][38]. The closely-neighboring desorption temperatures of C3 + hydrocarbons and ethylene emphasize that arene and alkene cycled proceed concurrently and are highly interwined [39].…”

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

“…1 Zeolites containing Brønsted acidity are the catalysts of choice, 2 with ZSM-5 and SAPO-34 being the two materials industrially applied. 3 Zeolite topology is an important 4 yet not the sole parameter in defining catalyst performance in MTO. 5 Accessibility, 6 strength, distribution, 7 amount 8 and nature 9 of acid sites are also of high importance.…”

Metal oxide–zeolite composites in transformation of methanol to hydrocarbons: do iron oxide and nickel oxide matter?

“…Moreover, dramatic changes in the product distribution (a much higher fraction of C1-C4 hydrocarbons after four consecutive reaction/regeneration cycles) reveal that the microporous network is filled with coke while there is no coke in the close vicinity of Fe3O4 NPs. There are several avenues to minimize coke formation including decreasing the size of zeolite particles, 18,19 creating hierarchical porosity, [8][9][10][11][12][13] etc. However, the Fe3O4-ZSM-5 catalysts already display hierarchical porosity and comparatively small particle size (see Figure S6, ESI and the discussion below).…”

“…7 In recent years several new avenues were developed to prevent coke formation and/or increase the catalyst activity. These avenues include the development of hierarchical zeolites, i.e., zeolites containing both micro-and mesopores, [8][9][10][11][12][13][14][15][16] minimizing internal framework defects in the zeolite structure, 17 formation of nanosized ZSM particles, 18,19 etc. The other approach to improve catalyst stability of ZSM-5 is its modification with various metals.…”