Effect of Pore and Cage Size on the Formation of Aromatic Intermediates During the Methanol-to-Olefins Reaction

Abstract

Six eight-membered-ring (8MR), microporous materials are synthesized and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. The molecular sieves SSZ-13, SAPO-34, SAPO-39, MCM-35, ERS-7 and RUB-37 are investigated since they have 8MR access to the crystal interior but have differences in pore structure and cage size. The polymethylbenzene species that are the proposed reaction intermediates of the MTO reaction should only be able to form in materials with intra-molecular sieve void spaces of sufficient size to accommodate them. Thus, it is hypothesized that 8MR materials without adequately large pores or cages will be inactive for the MTO reaction. SSZ-13 and SAPO-34 (both with CHA framework topology) have interconnected 3-dimensional pore-and-cage systems sufficiently large for formation of the proposed reaction intermediates, while the other 8MR materials have intra-molecular sieve void spaces that are too small to allow formation of these species. The molecular sieves are tested as MTO catalysts at 400°C, and only the molecular sieves with the CHA topology show MTO activity. Post-reaction analysis of the organic content of each solid material is accomplished by HF acid digestion with subsequent 1 H NMR analysis of the extracted hydrocarbon products to confirm the presence of aromatics in the 8MR materials with sufficiently large cages (those with CHA topology), and absence with materials that have smaller void spaces. These data provide further support for the necessity of polymethylbenzene species in the hydrocarbon pool for MTO activity.

Further Studies on How the Nature of Zeolite Cavities That Are Bounded by Small Pores Influences the Conversion of Methanol to Light Olefins

“…[10] Structural features of the catalysts for the MTO and deNO x reactions are knownt os ignificantly contribute to their success. For both reactions, the small pores are important.F or the MTO reaction, there appears to be ar equirement of ac age with sufficient size to accommodate and confine aromatic intermediates in order for the reaction to proceed with high selectivity to light olefins, [1,11] and the product distributions can dependo nt he cage geometries. [7,12,13] Additionally,t he Si/Al ratios of the zeolite catalysts can be critical to their performance, as Bronsteda cid sites that are in close proximity to one another (at lower Si/Al ratios and quantified by divalentc ation exchange capacity) have been shown to be detrimental to the lifetimes andolefin selectivities in the MTO reaction( with SAPOs, the Bronsted acid sites are typicallyi solated).…”

Effect of framework topology of SAPO catalysts on selectivity and deactivation profile in the methanol-to-olefins reaction

“…Although SAPO-34 is one of the most studied silicoaluminophosphate catalysts in the MTO process, there are other related materials that could act as acid catalysts in this process [32][33][34][35][36][37][38][39][40][41]. It is known that the catalytic performance is strongly dependent on the physicochemical properties of the catalyst employed [15,35,[42][43][44], but also small differences in the framework topology of the zeotypes may result in different catalytic behavior [45].…”

Methanol-to-olefin conversion over H-MCM-22 catalyst

“…Furthermore, in view of the depletion of world oil reserves, it is urgent to develop new routes to produce light olefins using non-oil feedstocks [1]. During the past decades, methanol-to-olefin (MTO) conversion, as an alternative route for the production of light olefins, has attracted much attention, because methanol can be conveniently manufactured from any carbon-containing resources such as coal, natural gas and biomass [2][3][4][5]. In the process of MTO, the dual-cycle mechanism was widely accepted, the initial step is the formation of dimethyl ether (DME) through dehydration of methanol, and then reacts to produce light olefins.…”