Effect of addition of a second metal in Mo/ZSM-5 catalyst for methane aromatization reaction under elevated pressures

doi:10.1016/j.cattod.2015.02.035

Catalysis Today

Volume 256, Part 2, 1 November 2015, Pages 269-275

https://doi.org/10.1016/j.cattod.2015.02.035 Get rights and content

Highlights

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Long-term methane aromatization experiments on (Co, Ce, Mn)-Mo/ZSM-5 catalysts.
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Elevated reaction pressure improved catalyst stability.
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Optimal Co loading in Mo/ZSM-5 catalyst was determined.
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Ce-Mo/ZSM-5 catalyst performance tested.

Abstract

Non-oxidative methane dehydro-aromatization (MDA) over transitional metal-doped zeolites (i.e. ZSM-5) has a potential to be an alternative for methane transformation processes to higher hydrocarbons. Despite of systematic studies of the effects of the catalyst composition, preparation procedure, pretreatment, and the reaction conditions systematically followed since the pioneering works of the Chinese group in 1993, the suppression of fast deactivation of catalyst by coke deposition was still not solved successfully.

In the present work the long-term MDA experiments at 973 K and elevated pressures up to 600 kPa were carried out with metal-doped Mo/ZSM-5 catalysts. Several Mo-based catalysts doped with various amount of Co, Mn, and Ce were prepared and tested in the MDA reaction. Experimental results are presented in terms of profiles of methane conversion as well as ethylene, benzene, and toluene concentrations as a function of time on stream, accompanied by the TGA analysis of the coke residue accumulated on the spent catalysts. The amount and nature of coke deposits which were influenced by introduction of different metals were also investigated. The Co content of 0.8 wt% in Co-Mo/ZSM-5 catalyst was evaluated as optimal for total benzene production. Prolonged experiments showed the evident positive effect of higher reaction pressure on catalyst stability for all tested catalysts. Moreover, the Co–Mo/ZSM-5 catalyst showed better resistance to the deactivation by coke formation in comparison to Mo/ZSM-5 and M–Mo/ZSM-5 (where M = Mn, Ce) catalysts. In addition, the Ce-modified catalyst showed comparable higher production of toluene at elevated pressures.

Graphical abstract

Introduction

Methane dehydro-aromatization over transitional metal-doped zeolites (i.e. ZSM-5) is a non-oxidative process of methane transformation to higher hydrocarbons [1]. This process has a potential to be an alternative to other technologies for natural gas transformation, such as methane oxidative coupling, steam reforming, or methane partial oxidation.

During last two decades several types of catalysts has been tested aiming an increase of catalyst activity, stability, and selectivity [2], [3], [4], [5]. The most studied catalyst in MDA reaction is based on ZSM-5 zeolite modified by Mo. The Mo/ZSM-5 catalyst is generally accepted as a bifunctional catalyst. It is believed that under the reaction conditions, the active phase based on carburized Mo activates the methane and the C₂H_x intermediates species are formed followed, and the cyclisation reaction is associated with the acid sites of HZSM-5 [6]. The aromatic compounds as benzene, toluene, and naphthalene together with C₂ hydrocarbons and hydrogen are the main products of this reaction. There are also present in reaction gases the C₃–C₅ hydrocarbons and other aromatic compounds (e.g. xylenes, anthracene and others C₉, and higher aromatics) in minor concentrations [7].

The commercial viability of this process is hindered by the relatively fast deactivation of the catalyst, mostly by coking at reaction conditions (700 °C, 101 kPa). Accordingly, most of the research was focused on finding the ways to suppress coke formation. The different approaches has been applied such as using different zeolite frameworks [8], [9], or catalyst modification [10], [11], or addition of co-reactant [12], [13] or increasing the reaction pressure [14], modification of the reaction conditions [15], and addition of promoters.

The different promoters have been tested in literature. The addition of noble metals such as Pt, Pd, Ru, Ir were tested by Tan et al. [16]. The addition of Pt was found as enhancing the selectivity of aromatics. The same conclusions were given by Kojima et al. [17] who also included Rh. The W as promoter was also reported [18]. The improvement of the catalyst activity was observed when using Fe, Co, Zn as promotors [19], [20].

The present study aims to examine the effect of combination of elevated reaction pressure and promotion of Mo/ZSM-5 catalyst by Co, Mn, and Ce doping.

Section snippets

Catalyst preparation

The ZSM-5 zeolite with SiO₂/Al₂O₃ mole ratio of 30 was used for Mo/ZSM-5 catalyst preparation. Two different specimens were used, either supplied by Zeolyst International (CBV 3024E, batch No. 1822-56-02) or by Research Institute of Petroleum and Hydrocarbon Gases, Bratislava (batch PS48). Ammonium heptamolybdate tetrahydrate and a metal nitrates (Co(NO₃)₂ 6H₂O, Ce(NO₃)₃ 6H₂O, Mn(NO₃)₂ 4H₂O) of reagent grade were purchased from LACHEMA and were used without further purification.

The protonated

Results and discussion

The BET surface and microporous volume and total pore volume of some fresh catalysts are summarized in Table 2. The results showed the microporosity typical for ZSM-5 zeolite.

Data from experiment carried out with the Co–Mo catalyst at 400 kPa are shown in Fig. 2 to demonstrate the typical time courses of concentrations of analyzed reaction products, i.e. C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₆H₆, and C₇H₈. The relative concentrations defined as ratio of actual molar fraction and maximal molar fraction

Conclusions

Doping of the Mo/ZSM-5 catalysts with Co has been shown to be positive for the stability of benzene production both at standard experimental conditions, i.e. using atmospheric pressure, as well as after reaction pressure was increased up to 600 kPa. The total production of benzene (g C₆H₆/g_cat) during the active period of the Co–Mo/ZSM-5 catalyst increased with increasing pressure from 0.724 g C₆H₆/g_cat at 150 kPa, to 1.05 and 1.20 g C₆H₆/g_cat at 400 and 600 kPa, respectively.

Acknowledgement

Part of the presented work was supported by the European Community through the NEXT-GTL project (NMP3-LA-2009-229183).

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Effect of addition of a second metal in Mo/ZSM-5 catalyst for methane aromatization reaction under elevated pressures

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

Results and discussion

Conclusions

Acknowledgement

J. Energy Chem.

J. Catal.

Energy

Appl. Surf. Sci.

J. Catal.

Stud. Surf. Sci. Catal.

Catal. J.

Appl. Catal. A

Appl. Catal. A: Gen.

Catal. Today

Catal. Today