Improved catalytic performance and decreased coke formation in post-treated ZSM-5 zeolites for methanol aromatization

https://doi.org/10.1016/j.micromeso.2016.11.014Get rights and content

Highlights

  • The catalytic lifetime was extended by enhancing of mesoporosity and decreasing of acid density.

  • There is an inverse correlation between mesoporosity and coke content.

  • The acid amount has a good correlation with the coke content.

Abstract

The effects of the zeolite structure and acidity on the catalyst life and coke formation in the methanol-to-aromatics (MTA) reaction were investigated using several MFI zeolites with different degrees of mesoporosity and acidity. These zeolites were prepared by post-synthesis desilication, dealumination, and combined desilication and dealumination processes. The influence of the post-synthesis on the pore characteristics, crystallinity, morphology and acidity of the ZSM-5 zeolites were studied by XRD, N2-adsorption, n-octane temperature-programmed desorption experiments, 27Al and 29Si MAS NMR, SEM, TEM and NH3-TPD. Dealumination and/or desilication leads to an increase in the mesoporosity by widening the micropores and a decrease in the acid density. The MTA catalyst lifetime is increased by several times due to the enhanced mesoporosity and decreased acid density. The deactivated catalysts were characterized by thermogravimetry and N2 adsorption/desorption experiments. More coke forms inside the micropores than on the external surfaces of the catalysts. Generalized and quantitative correlations between the mesoporosity and coke content and between the number of acid sites and coke content are given.

Introduction

Methanol-to-aromatics (MTA) conversion over microporous solid acid catalysts has drawn much industrial and academic interest in recent years because the MTA process enables coal and natural gas to be converted into chemicals [1], [2], [3] that would otherwise be derived from petroleum feedstocks. Over the past few decades, the potential of various molecular sieves for use as methanol-to-hydrocarbon (MTH) catalysts has been investigated [4], [5]. HZSM-5, which has strong Brønsted acid sites and intermediate channel dimensions, has attracted significant attention due to its good resistance to deactivation by coke and its model behaviour for studying the MTA mechanism [6], [7]. Nonetheless, the generation and deposition of carbonaceous residues often leads to severe diffusion restrictions and fast deactivation rates [8]. Hence, coke formation on the catalyst is an important cost-bearing factor not only in the MTH process but also in many other petrochemical processes that utilize zeolite catalysts, including hydrocarbon refining and fine chemical syntheses [9], [10]. It is therefore very important to investigate the zeolite characteristics that affect coke deposition and catalyst deactivation.

Research on coke formation has mainly focused on the influence of the zeolite structure, acid site strength and concentration [11]. For example, Bleken and Park [12], [13] correlated the deactivation rate with polycyclic aromatics formation. Based on the observed relation, it was suggested that coke formation occurs preferentially on the outer surfaces of zeolite crystals [14], [15]. However, it was also hypothesized that external coke formation occurs only after the catalyst is deactivated by intraporous coke formation [16], [17]. In contrast, Bjøgen et al. [18] did not find a correlation between the organic molecule concentration in the pores and zeolite deactivation at 643 K. They concluded that external coke deposition caused activity loss with time on stream. Recently, Müller et al. [19] reported that the outer surface of HZSM-5 is virtually carbon-free under the reaction conditions studied and that the catalyst deactivated rapidly at first and then at a much slower rate. During the initial phase, the deactivation rate was directly proportional to the methanol partial pressure, and the deactivation was caused by oxygen-containing surface species. These species were transformed into aromatic compounds with time on stream, and the deactivation then proceeded via aromatic compound methylation to form coke species typically produced during methanol-to-olefins processes.

The acid site concentration is also an important factor affecting the catalyst reactivity and product distribution [20], [21]. Choudhary et al. [22] proposed that the strong acid sites in the HZSM-5 zeolite were the active sites for aromatics production. However, an increase in the strong acid site concentration also promoted coke formation, leading to rapid catalyst deactivation. Aguayo et al. [23] reported that the high acidic strength of a zeolite catalyst decreased the reaction energy barrier, thus leading to an increase in the reaction rate and accelerated deactivation. McLellan et al. [24] showed that an initial significant decrease in the acid site concentration was followed by a second slower deactivation phase. The initial deactivation regime was thought to be caused by acid site blocking at the channel intersections. The second deactivation stage was ascribed to external coking and/or topological blocking of the zeolite channels.

Obviously, the zeolite structure and acidity are the two most important factors affecting the catalyst activity, product distribution and coke formation. Much effort has focused on synthesizing zeolites with controlled structures and acidities; for example, nanosized [25], [26] and hierarchical [27], [28] zeolites have been prepared, and alkali/acid post-treatments have been applied [7], [29] in an attempt to achieve the desired characteristics. To a certain extent, all of these methods improved the diffusion in the zeolites and enhanced their lifetimes in some catalytic reactions. Of these methods, mesopore generation by post-synthesis treatment with an alkaline and/or acid solution is particularly appealing, primarily due to its simplicity and reasonably broad applicability, as recently reviewed by Verboekend and Silaghi [30], [31]. This method can be used to investigate the interplay between the acidity, pore structure, catalytic activity and selectivity. However, the combined effects of the zeolite structure and acidity on the catalytic performance and coke formation have not been studied in depth, which is obviously important for the industrial development of MTA technology.

In this study, dealumination, desilication and a combination of dealumination and desilication post-treatments were used to prepare catalyst samples with different mesoporosities and acidities. The synthesized catalysts were employed in the MTA reaction. The effects of the mesoporosity and acidity on the catalytic performance and catalyst lifetime are discussed in detail. Coke formation was monitored, and its dependence on the catalyst mesoporosity and acidity was determined.

Section snippets

HZSM-5

The parent NaZSM-5 zeolite with a Si/Al molar ratio of 50 (Fushun Catalyst Plant, China) was calcined in air at 550 °C (heating rate of 10 °C/min) for 5 h to remove the hexanediamine template molecules. The protonated zeolite was obtained by the following procedure. Two consecutive ion exchange reactions were performed at 70 °C for 4 h using a 1.0 M NH4NO3 solution. The ratio of the NaZSM-5 mass (g) to the NH4NO3 solution volume (mL) was 1:10. After the first ion exchange reaction, the zeolite

Crystal structures and chemical compositions of the parent and post-treated HZSM-5 samples

The XRD patterns of the parent and post-treated HZSM-5 samples are shown in Fig. 1. The relative crystallinities (compared to the parent HZSM-5 zeolite) of the post-treated samples were estimated from the areas of the peaks in the 2θ range from 22.5° to 25.0°, and the relative crystallinity of parent HZSM-5 zeolite was defined as 100%. The data are given in Table 1. The XRD patterns of all the samples show that they retain good crystallinity after modification. The crystallinity slightly

Conclusions

Treating ZSM-5 with alkali and/or acid solutions changes its acidity and increases its mesoporosity considerably. The pore structure and acidity of zeolites are the two most important properties influencing methanol conversion in the MTA reaction. Strong acid sites are considered to be the catalytic active sites for converting olefins into aromatic hydrocarbons, but decreasing the number of strong acid sites is favourable for preventing coke formation. The mesoporosity can affect the conversion

Acknowledgment

This work was financially supported by Postdoctoral Fund of Heilongjiang province of China (No. LBH-Z14208) and open foundation of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, China.

D.P. Serrano, J. Aguado, G. Morales, J.M. Rodriguez, A. Peral, M. Thommes, J.D. Epping, B.F. Chmelka, Molecular and meso- and macroscopic properties of hierarchical nanocrystalline ZSM-5 zeolite prepared by seed silanization, Chem. Mater. 21 (2009)

References (63)

  • J.W. Park et al.

    Effects of cage shape and size of 8-membered ring molecular sieves on their deactivation in methanol-to-olefin (MTO) reactions

    Appl. Catal. A Gen.

    (2008)
  • B.P.C. Hereijgers et al.

    Product shape selectivity dominates the Methanol-to-Olefins (MTO) reaction over H-SAPO-34 catalysts

    J. Catal.

    (2009)
  • B.A. Sexton et al.

    An XPS study of coke distribution on ZSM-5

    J. Catal.

    (1988)
  • S. Müller et al.

    Coke formation and deactivation pathways on H-ZSM-5 in the conversion of methanol to olefins

    J. Catal.

    (2015)
  • Y. Yang et al.

    The synthesis of endurable B–Al–ZSM-5 catalysts with tunable acidity for methanol to propylene reaction

    Catal. Commun.

    (2012)
  • T.V. Choudhary et al.

    Influence of Si/Ga and Si/Al ratios on propane aromatization over highly active H-GaAlMFI

    Catal. Commun.

    (2006)
  • G.D. McLellan et al.

    Effects of coke formation on the acidity of ZSM-5

    J. Catal.

    (1986)
  • L.R. Aramburo et al.

    Interplay between nanoscale reactivity and bulk performance of H-ZSM-5 catalysts during the methanol-to-hydrocarbons reaction

    J. Catal.

    (2013)
  • Y. He et al.

    Modification of nanocrystalline HZSM-5 zeolite with tetrapropylammonium hydroxide and its catalytic performance in methanol to gasoline reaction

    Chin. J. Catal.

    (2013)
  • J. Kim et al.

    Effect of mesoporosity against the deactivation of MFI zeolite catalyst during the methanol-to-hydrocarbon conversion process

    J. Catal.

    (2010)
  • Z. Qin et al.

    A defect-based strategy for the preparation of mesoporous zeolite Y for high-performance catalytic cracking

    J. Catal.

    (2013)
  • M.-C. Silaghi et al.

    Challenges on molecular aspects of dealumination and desilication of zeolites

    Microporous Mesoporous Mater.

    (2014)
  • D.M. Bibby et al.

    Coke formation in zeolite ZSM-5

    J. Catal.

    (1986)
  • D.M. Bibby et al.

    Sorption studies of coke deposited on ZSM-5

    J. Catal.

    (1989)
  • Z. Qin et al.

    A defect-based strategy for the preparation of mesoporous zeolite Y for high-performance catalytic cracking

    J. Catal.

    (2013)
  • A.A. Rownaghi et al.

    Yield of gasoline-range hydrocarbons as a function of uniform ZSM-5 crystal size

    Catal. Commun.

    (2011)
  • M. Ogura et al.

    Alkali-treatment technique-new method for modification of structural and acid-catalytic properties of ZSM-5 zeolites

    Appl. Catal. A Gen.

    (2001)
  • T. Suzuki et al.

    Change in pore structure of MFI zeolite by treatment with NaOH aqueous solution

    Microporous Mesoporous Mater.

    (2001)
  • A. Bonilla et al.

    Desilication of ferrierite zeolite for porosity generation and improved effectiveness in polyethylene pyrolysis

    J. Catal.

    (2009)
  • C.S. Triantafillidis et al.

    Effect of the degree and type of the dealumination method on the structural, compositional and acidic characteristics of H-ZSM-5 zeolites

    Microporous Mesoporous Mater.

    (2001)
  • S.M. Campbell et al.

    Dealumination of HZSM-5 zeolites: I. Calcination and hydrothermal treatment

    J. Catal.

    (1996)
  • Cited by (0)

    View full text