Infrared spectroscopic studies of zeotype and zeolite catalyzed hydrocarbon/oxygenate conversion reactions

Abstract

The methanol to hydrocarbons (MTH) reaction is an important reaction involved in the chain of processes for the conversion of alternative carbon sources into liquid fuel and olefins. One key reaction in the complex MTH mechanism was studied using benzene methylation with methanol over H-ZSM-5 and H-Beta and monitored in-situ using FT-IR spectroscopy at comparable conditions with kinetic measurements. At steady state conditions, the Brønsted acid band for H-Beta is significantly reduced, whereas the band is nearly unaffected for H-ZSM-5. This trend is correlated with the lower methylation rate for H-Beta as measured by the kinetic studies. During the MTH reaction, the reactive intermediates for product formation may undergo further reaction to form coke precursors and finally coke. The MTH reaction at a fairly low temperature and short reaction time over H-Beta and H-Mor was carried out to follow this transformation. Ex-situ analyses of the zeolites after the reaction suggest that hydrocarbon compounds detected by GCMS after HF dissolution are to some extent seen also with FT-IR spectroscopy. FT-IR spectra reveal the presence of other species unseen from the ex-situ analyses namely methoxy species. The MTH reaction over H-ZSM-5, H-Beta, and H-Y at comparable conditions with the regular MTH reaction was carried out in-situ with the aim of elucidating the deactivation mechanism. The MTH reaction occurs via different pathways as revealed by FT-IR spectroscopy. Minor amounts of retained hydrocarbons are produced over H-ZSM-5; and significant amounts of hydrogen deficient hydrocarbons are produced over H-Beta; whereas both hydrogen rich and hydrogen deficient hydrocarbons are produced over H-Y. The Brønsted acid band reduction is mainly due to formation of cationic species for all zeolites. For H-Y, the reduction is also contributed by the formation of methoxy species. Ex-situ analyses show that coked H-ZSM-5 contains mainly polymethylbenzenes (PMBs); whereas coked H-Beta contains PMBs and large amounts of polyaromatic hydrocarbons (PAHs). Coked H-Y contains lower amounts of PMBs and PAHs as compared to coked H-Beta which is consistent with the trend seen in the infrared spectra. H-ITQ-7 is a zeotype relevant to the MTH chemistry. This material was synthesized and characterized in order to get deeper insight into its acid properties. The incorporation of aluminum into the framework is proven challenging and samples with high aluminum content tend to be less stable. The calcination procedure is found to be important, where in-situ calcination is the preferred method. H-ITQ-7 consistently display three discernible Brønsted acid bands which are all accessible by CO. H-ITQ-7 possess a wide range of acidic strength, from the strongest (BA-I) comparable to H-Beta/H-ZSM-5 to the weaker (BA-II and BA-III). The use of N2 in combination with CO as probe molecules is beneficial for determination of the acidic strength of H-ITQ-7; while the use of N2 as single probe molecule appears not suitable due to the broad range of acidic strength.