Time- and space-resolved study of the methanol to hydrocarbons (MTH) reaction – influence of zeolite topology on axial deactivation patterns

Abstract

Zeolites representing seven different topologies were subjected to life-time assessment studies as methanol to hydrocarbons (MTH) catalysts at 400 °C, P(MeOH) = 13 kPa and P(tot) = 100 kPa. The following topologies were studied: ZSM-22 (TON), ZSM-23 (MTT), IM-5 (IMF), ITQ-13 (ITH), ZSM-5 (MFI), mordenite (MOR) and beta (BEA). Two experimental approaches were used. In the first approach, each catalyst was tested at three different contact times, all giving 100% initial conversion. The life-time before conversion decreased to 50% at each contact time was measured and used to calculate critical contact times (i.e. the contact time needed to launch the autocatalytic MTH reaction) and deactivation rates. It was found that the critical contact time is strongly correlated with pore size: the smaller the pore size, the longer the critical contact time. The second experimental approach consisted of testing the catalysts in a double tube reactor with 100% initial conversion, and quenching the reaction after 4 consecutive times on stream, representing full, partial, and zero conversion. After quenching, the catalyst bed was divided into four segments, which were individually characterised for coke content (temperature-programmed oxidation) and specific surface area (N adsorption). The axial deactivation pattern was found to depend on pore size. With increasing pore size, the main source of coke formation changed from methanol conversion (1D 10-ring structures), to partly methanol, partly product conversion (3D 10-ring structures) and finally mainly product conversion (3D 12-ring structure). As a result, the methanol conversion capacity changed little with contact time for ZSM-5, while it increased with increasing contact time for the catalysts with smaller pore sizes, and decreased with increasing contact time for pore sizes larger than ZSM-5.

Impact of post-synthetic treatments on unidirectional H-ZSM-22 zeolite catalyst: Towards improved clean MTG catalytic process

“…The fact that similar type and amount of coke is found for the microporous and micro-/mesoporous spent catalysts combined with the observed slower coking rate/lower coke selectivity for the hierarchical catalyst crystals, supports the idea that the diffusion of coke precursors out of the micropores is slower for the microporous catalysts owing to the longer diffusion paths, which ultimately result in faster coking [20,25]. In agreement with the work by Rojo-Gama et al [37], we suggest that coke species formed from methanol are accumulated in the micropores.…”

“…The observation that coke is formed mainly from methanol has been previously evidenced by structure-activity correlations derived from time-and space-resolved high energy XRD operando studies over c-ZSM-22 and hierarchical c-ZSM-22-ats1-HCl catalysts [34]. Apart from the origin of coke; its location, type and amount may also affect the mode of catalyst deactivation [21,22,25,36,37]. The total amount of oxidable coke in the spent catalysts was determined by TGA, and the data is presented in Fig.…”

“…1b). The surfactant treated catalyst (m-ZSM-22-ats1-HCl) showed a progressive reduction of methanol conversion with time on stream as the product concentration decreases, which may be indicative of a different deactivation mechanism, in which the coke is formed mainly from the reaction products [36,37]. The m-ZSM-22-tba1-HCl catalyst showed an intermediate behaviour, which suggest a contribution of both methanol and reaction products to the coke formation.…”

Improving HSAPO‐34 Methanol‐to‐Olefin Turnover Capacity by Seeding the Hydrocarbon Pool

“…The dual-cycle schematic, used widely in the mechanistic interpretation of MTO data, describes the interconversion between hydrocarbon chain carriers and their role in ethenea nd propene formation, but does not address the transformation of activec hain carriers to inactive ones. Recent work [11][12][13][14][15][16][17][18] has helped elucidate the role of formaldehyde, formed by the transfer dehydrogenationofm ethanol, as an accelerant for catalyst deactivation mediated by the transformation of monocyclic to polycyclic aromatic hydrocarbons. Rates of chain termi-nation relative to propagation necessarily increase with methanol pressure, and hence, decreasing local methanolp ressures by lowering inlet methanolp ressures, [11] operating in ac ontinuous stirred tank configuration (CSTR) insteado faplug flow reactor configuration( PFR), [15] or using dimethyl ether (DME) as feed instead of methanol [17,19] have all been shown to increase catalystl ifetime.…”

Complex relationship between SAPO framework topology, content and distribution of Si and catalytic behaviour in the MTO reaction

“…In a further step, the reaction between methanol and the hydrocarbon intermediates or the combination of such intermediates would lead to the formation of large molecules that are not able to diffuse out of the catalyst and keep retained in the porous structure forming carbonaceous deposits (coke) that may block the access of methanol to the active centres or hinder the diffusion of reaction products, resulting in the deactivation of the catalysts. [17][18][19][20] Coke formation has also been attributed to the presence of formaldehyde molecules, formed by hydrogen transfer reactions between methanol molecules, and an increase of the catalyst time life has been found when using dimethyl ether as reactant instead of methanol. 13,21 Since the fast deactivation is the main drawback of SAPO materials as catalysts for the MTO transformation, great efforts have been devoted to the optimization of the physicochemical properties of SAPO-34 in order to improve its catalytic performance.…”