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A Mechanistic Study of Methanol-to-Aromatics Reaction over GaModified ZSM-5 Zeolites: Understanding the Dehydrogenation Process Pan Gao, Jun Xu, Guodong Qi, Chao Wang, Qiang Wang, Yanxi Zhao, Yuhua Zhang, Ningdong Feng, Xingling Zhao, Jinlin Li, and Feng Deng ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03076 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018
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A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process Pan Gao†,‡, Jun Xu*,†, Guodong Qi†, Chao Wang†, Qiang Wang†, Yanxi Zhaoζ, Yuhua Zhangζ, Ningdong Feng†, Xingling Zhao†,‡, Jinlin Liζ, Feng Deng*,†
†
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,
National Center for Magnetic Resonance in Wuhan, Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China. ‡ ζ
University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs
Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, P. R. China
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Abstract： ：Methanol-to-aromatics (MTA) reaction was investigated on Ga-modified ZSM-5 zeolites (Ga/ZSM-5). As revealed by 71Ga and 1H solid-state NMR and FT-IR of pyridine adsorption measurements, cationic Ga species are formed as Lewis sites by substitution of Brønsted acid sites on H-ZSM-5. Further experimental studies show that C5- and C6-cycloalkenes are generated during the MTA reaction, which lead to the formation of cyclic carbocations as precursors to aromatics. Isotope exchange experiments demonstrate that the reactivity of the cyclic carbocations on Ga/ZSM-5 is much higher than that on H-ZSM-5 and they play an intermediate role in the formation of aromatics. In addition to the traditional bimolecular hydrogen transfer (HT) reaction, the dehydrogenation of alkenes with the release of H2 (DeaH2 process) was identified to significantly contribute to the formation of aromatics. The transformation of cycloalkenes to aromatics is favored by promotion of the DeaH2 process and competes with the HT route on Ga/ZSM-5, while these cycloalkenes tend to crack back to alkenes, and the dominating HT route results in lower aromatic selectivity on H-ZSM-5. A DeaH2-aromatization route mediated by the cooperation of cationic Ga species and Brønsted acid sites was proposed for the enhanced formation of aromatics on Ga/ZSM-5 zeolite.
KEYWORDS: methanol-to-aromatics, dehydrogenation process, mechanism, metalmodified zeolite, solid-state NMR spectroscopy
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1. Introduction Aromatics, especially benzene, toluene and xylene (BTX), are important chemical commodities in petrochemical industry, the production of which is strongly dependent on petroleum. Over the past decades, much effort has been devoted to produce BTX via non-petroleum resources to tackle the dilemma on the up-surging demands of these chemicals against the depletion of oil reserves. Different processes using alkanes,1,
2
alkenes,3,
4
alcohols5 and biomass-derived hydrocarbons6,
7
have been
developed as alternatives to produce BTX, among which catalytic transformation of methanol to aromatics over ZSM-5 (MFI topology) zeolite has attracted extensive attention, since methanol can be readily produced from a wide range of non-oil resources such as coal, nature gas and biomass.8 Introduction of metals onto zeolites is often utilized to improve the single-pass selectivity of aromatics during the methanol conversion. Ono et al.9 first reported the significant promotion of aromatic selectivity in the MTA reaction by modifying ZSM-5 zeolite with Zn metal. Over the past decades, Zn,10, 11 Ga,12-16 Ag,17, 18 Cu,19 Ni,19 and bimetallic species such as Zn/Sn,20 Zn/Ni,21 modified ZSM-5 zeolites have been extensively explored for the MTA reaction. In particular, Zn or Ga modification was proved to produce superior catalytic performance than the other counterparts. Different methods have also been studied for the optimization of the Ga- or Zn-modified ZSM-5 catalysts and the MTA process.22-26 A surface modification of Zn/P/ZSM-5 with SiO2 layer led to a significant increase in the production of p-xylene during the MTA reaction.27 Similar catalytic performance was also achieved on a
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Zn/ZSM-5/silicalite-1 core-shell zeolite developed by Miyake et al.28 By co-feeding a small amount of n-butanol in the MTA reaction, the lifetime of Ga-modified ZSM-5 could be significantly prolonged at high aromatic selectivity.29 Understanding the reaction mechanism is the prerequisite for the rational design of efficient catalysts. The conversion of methanol over zeolites have been extensively studied experimentally and theoretically.30-32 To date, the elegant “dual-cycle” hydrocarbon-pool (HP) mechanism first proposed by Olsbye et al.33-35 is widely accepted and provides rationale for the formation of hydrocarbons on the working catalysts. In the “dual-cycle” model, the light olefins can be produced in either the alkenes-based cycle by methylation/cracking of C3+ alkenes, or the aromatics-based cycle by aromatic dealkylation. Aromatization of the olefins is responsible for the continuous formation of aromatics, which involves consecutive H-subtraction and cyclization processes of the olefinic intermediates.32 The recent work reported by Lercher et al.36,
37
have demonstrated that on H-ZSM-5 zeolite the H-subtraction
proceeds via the H-transfer (HT) reactions, in which hydrides are transferred from H-donors (olefins or methanol) to H-acceptors (other olefins), leading to hydrogen-deficient species such as dienes and methylbenzenes as well as concurrent formation of alkanes. Meanwhile, the cyclization process was proposed to be responsible for the formation of highly reactive cyclic intermediates such as 5-ring, 6-ring cycloalkenes/cyclocarbocations from long-chained alkenes, which were further converted into aromatics.38-40 The addition of metals onto ZSM-5 zeolite promotes the aromatization process through the dehydrogenation reaction, in which olefinic
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intermediates are converted into less saturated species with efficient release of H2. This is often accounted for by the increased dehydrogenation ability of the catalyst induced by the metal species.16,17 Most recently, the work of Pinilla-Herrero et al.41 indicated that the dehydrogenation process in the MTA reaction was favored by a higher Zn/Al ratio on Zn-modified ZSM-5. However, so far, the detailed knowledge on the dehydrogenation and aromatization processes of the MTA reaction is still lacking. In particular, the formation of key intermediates and their connections to the formation of aromatics on metal-modified zeolites remains elusive. On the other hand, the active sites on the zeolites that promote the MTA reaction are not well understood, which is complicated by the co-existence of different metal species that often act as Lewis sites. In our previous study,42 the Brønsted-Lewis synergic site was identified and quantified on Ga-modified ZSM‑5 zeolites by solid-state NMR spectroscopy, which was closely related with the aromatics selectivity, however, the detailed synergism of these sites in the aromatization process remains to be explored. Similarly, a synergic effect between the Ga species and the Brønsted acid sites was also proposed in the dehydrogenation of light alkanes43 and the aromatization of methanol. 44
In this work, the MTA reactions over Ga-modified ZSM-5 zeolites (Ga/ZSM-5) were systematically investigated. We reported a detailed structural and catalytic characterization of the Ga/ZSM-5 zeolites. The highly reactive C5- and C6cycloalkenes intermediates were identified on Ga/ZSM-5 catalysts, and their role in
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the formation of aromatics was analyzed. Two H-subtraction processes, i.e. HT and dehydrogenation, were comparatively investigated for the formation of aromatics.
2. Experimental 2.1 Catalysts preparation Ga/ZSM-5 catalysts were prepared by impregnation method with H-ZSM-5 zeolite (Si/Al=12.5, Catalyst Plant of Nankai University, China) and Ga(NO3)3·xH2O (99.99% metal basis, Aladdin Reagent Co., Ltd.). Typically, 2.5 g of H-ZSM-5 was added into 5 mL of Ga(NO3)3 solutions containing 0.2, 0.5, 1.0, 1.3 g of Ga(NO3)3·xH2O. The resulting mixtures were dried at 323 K under stirring and subsequently calcined under flowing air at 823 K for 8 h. The samples were then subjected to a reduction-oxidation treatment performed at 723 K under 50 sccm flow of hydrogen and dry air for 1 h, respectively. The obtained catalysts are denoted as 1%Ga/ZSM-5, 3%Ga/ZSM-5, 6%Ga/ZSM-5 and 8%Ga/ZSM-5 based on the ICP analysis. 2.2 Catalysts characterization The crystalline structures of the parent and Ga-modified zeolites were characterized by X-ray diffractometer (Panalytical X’Pert PRO) using a CuKα radiation with a step of 0.02° at a respective voltage of 40 kV and a current of 40 mA. The Ga content of modified catalysts was determined by an inductively coupled plasma optical emission spectrometer (ICP-OES) method using a Perkin-Elmer 3300DV instrument. The SEM images were collected on a HITACHI SU-1510 Scanning Electron Microscope.
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The N2 adsorption-desorption measurements were performed at 77 K on a Micromertics ASAP 2020 M system. Prior to nitrogen adsorption, the samples were dehydrated under vacuum for 2 h at 573 K. The total surface area and the micropore volume were determined by BET equation and t-plot method, respectively. The FT-IR of pyridine adsorption measurements were performed on a Bruker Tensor 27 spectrometer. The catalysts were first activated at 773 K under high vacuum (
