Organic-Free Synthesis of CHA-Type Zeolite Catalysts for the Methanol-to-Olefins Reaction

Abstract

Chabazite (CHA)-type zeolites are prepared from the hydrothermal conversion of faujasite (FAU)-type zeolites, dealuminated by high-temperature steam treatments (500−700°C), and evaluated as catalysts for the methanol-to-olefins (MTO) reaction. The effects of temperature and partial pressure of water vapor during steaming are investigated. Powder X-ray diffraction (XRD) and Ar physisorption data show that the steam treatments cause partial structural collapse of the zeolite with the extent of degradation increasing with steaming temperature. 27 Al MAS NMR spectra of the steamed materials reveal the presence of tetrahedral, pentacoordinate, and octahedral aluminum. NH 3 and i-propylamine temperature-programmed desorption (TPD) demonstrate that steaming removes Brønsted acid sites, while simultaneously introducing larger pores into the CHA materials that make the remaining acid sites more accessible. Acid washing the steamed CHA-type zeolites removes a significant portion of the extra-framework aluminum, producing an increase in the bulk Si/Al ratio as well as the adsorption volume. The proton form of the as-synthesized CHA (Si/Al = 2.4) rapidly deactivates when tested for MTO at a reaction temperature of 400°C and atmospheric pressure. CHA samples steamed at 600°C performed the best among the samples tested, showing increased olefin selectivities as well as catalyst lifetime compared to the unsteamed CHA. Both lifetime and C 2 −C 3 olefin selectivities are found to increase with increasing reaction temperature. At 450°C, CHA steamed at 600°C reached a combined C 2 −C 3 olefin selectivity of 74.2% at 100% methanol conversion, with conversion remaining above 80% for more than 130 min of time-on-stream (TOS) before deactivating. More stable time-on-stream behavior is observed for 600°C-steamed CHA that underwent acid washing: conversion above 90% for more than 200 min of TOS at 450°C with a maximum total C 2 −C 3 olefin selectivity of 71.4% at 100% conversion.

Steam-dealuminated, OSDA-free RHO and KFI-type zeolites as catalysts for the methanol-to-olefins reaction

“…From fraction of tetrahedral Al (Al T /Al Total ) estimated from the 27 Al NMR and the bulk P/Al ratio, the ratio of phosphorous to tetrahedral Al (P/Al T ) is estimated to be 0.38 for CHA-S600-P. From our previous work [13], the ratio of acid sites measured by i-propylamine TPD to that measured by NH 3 TPD for CHA steamed at 600°C was 0.32. This ratio is equivalent to the ratio of accessible mesoporous and external surface acid sites M A N U S C R I P T…”

“…The presence of mesopores after steam treatment of these materials, as indicated by Ar physisorption measurements, likely also contribute to the increased lifetimes of the steamed zeolites by allowing for greater accessibility of reactants to the acid sites in the remaining microporous regions. We have previously observed using NH 3 and i-propylamine TPD that steam treatment of high-aluminum CHA-type zeolites lead to decreased total Brønsted acid site densities while simultaneously increasing accessibility of the remaining acid sites via mesopores[13]. An optimal steaming temperature of 600°C for CHA was found that gave the best balance of intact acid sites and accessibility.…”

“…CHA steamed at 600°C was prepared following the OSDA-free method that we previously reported [13] and compared to 800°C steamed RHO and 700°C steamed KFI (the samples that showed the most stable C 4= /C 2= ratio and lowest DME production). The trimethylphosphite treatment consists of a liquid phase exchange followed by a calcination to permanently attach phosphate groups onto the acid sites in the zeolites.…”

“…We have recently explored a post-synthetic dealumination strategy to convert high aluminum 8MR zeolites that are prepared without the use of OSDAs into useful catalysts for MTO and demonstrated the preparation method with CHA-type zeolites [13]. CHA was synthesized in the absence of OSDAs at a Si/Al ratio of 2.4, steam-treated to extract a portion of the framework aluminum and evaluated for MTO.…”

Conversion of Y into SSZ-13 zeolites and ethylene-to-propylene reactions over the obtained SSZ-13 zeolites

“…SSZ-13 zeolites have been synthesized via various methods, including direct synthesis (from precursors of silica and alumina) and conversion of zeolite Y. For the synthesis of SSZ-13s, various templates, such as N,N,Ntrimethyl-1-adamantanamine iodide (TMAda-I) [5,7], TMAda-OH [8][9][10][11][12], choline chloride [13,14], copper-tetraethylenepentamine (Cu-TEPA) [15][16][17], tetraethylammonium hydroxide (TEA-OH) [18], benzyltrimethylammonium hydroxide [19], and N,N,Ndimethylethylcyclohexylammonium bromide [20] have been used despite several reports of its template-free [21][22][23][24] synthesis.…”