Simultaneous coking and dealumination of zeolite H-ZSM-5 during the transformation of chloromethane into olefins

Abstract

ARTICLE This journal isThe deactivation pathways of a zeolite H-ZSM-5 catalyst, containing bentonite and α-Al2O3 as binder material, have been studied during the transformation of chloromethane into light olefins, which is considered as a possible step to valorize methane from natural gas. The reactions have been carried out in a fixed bed reactor, feeding pure chloromethane at 400, 425 and 450 °C; 1.5 bar and with a space-time of 5.4 (gcatalyst) h (molCH2) -1 during 255 min. The properties of the fresh and spent catalysts have been assessed by several techniques, such as N2 physisorption, adsorption/desorption of NH3, XPS and 29 Si NMR. Additional measurements of the spent catalysts have been performed to study the nature of the deactivating coke species: TG-TPO, SEM, FT-IR and UV-vis spectroscopies. With the results in hand, two deactivation mechanisms were proposed: irreversible dealumination at temperatures higher than 450 °C by HCl and reversible coke fouling, while coke formation results from the condensation of poly-alkylbenzenes, which are also intermediates for olefin production. The coke deposits grow in size with the addition of Cl to the carbonaceous structure.

Kinetic and Deactivation Differences Among Methanol, Dimethyl Ether and Chloromethane as Stock for Hydrocarbons

“…The two main players behind deactivation are coke deposition and dealumination of the zeolite due to the presence of H 2 O or HCl in the reaction medium. The reaction temperature employed in this work (400 °C) enable to exclude the dealumination pathway for all reactions, and point to coke as the main cause of catalyst deactivation, as previously determined . Based on the fact that the main cause of deactivation is coke formation, then the results of Figure a suggest that chloromethane is more prone to form coke, which should be verified later.…”

“…However, the observed reactivity of chloromethane is slower due the thermodynamically harder formation of methoxy species from this reactant . Other significant difference of CTH is the inexistence of water in the reaction medium, which is regarded as one key factor governing the faster deactivation rate of this reaction as compared with MTH and DTH …”

Discerning the Location and Nature of Coke Deposition from Surface to Bulk of Spent Zeolite Catalysts

“…This result suggests that the structural modification of the zeolite by dealumination process due to the coke formation during catalyst process. Such dealumination of zeolite H-ZSM-5 has been observed during the catalytic process of different oxygenates (i.e., methanol, ethanol and bio-oil) to hydrocarbons, which is attributed to steaming at high reaction temperatures38. In addition, the dominant tetrahedral alumina peak also shifts towards higher field (i.e.…”

“…The first one is leaching of alumina (dealumination) from the unit structure of the zeolite in the presence of steam; this mechanism causes the irreversible deactivation of the catalyst121314. The alumina leached from the zeolite unit cell structure remain in the micropores but affect the catalytic activity for the chemistry of interest1.…”

Nature and Location of Carbonaceous Species in a Composite HZSM-5 Zeolite Catalyst during the Conversion of Dimethyl Ether into Light Olefins

“…The intensity of the first peak decreases when the reaction temperature is increased, which suggests that the H/C ratio of coke decreases at higher temperatures [43]. Otherwise, TPO profiles (Figure 3b) show a main combustion peak at 550 • C and a shoulder (at higher DTO temperatures) at 450 • C. The TPO peak with maximum at 450 • C can be associated with aliphatic coke deposited on the mesoand macropores of the matrix [36,44], whereas the peak with a maximum at 550 • C can be attributed to the combustion of condensed aromatics (polyaromatics) of coke in the micropores or heavy structures in the meso-and macropores. In a previous work, we proved that the deposition of aliphatic coke is less selective, and also occurs in the matrix material [31].…”