The function of Cu(II) ions in the Mo/CuH-ZSM-5 catalyst for methane conversion under non-oxidative condition

doi:10.1016/S0926-860X(99)00231-8

Applied Catalysis A: General

Volume 187, Issue 2, 25 October 1999, Pages 199-206

https://doi.org/10.1016/S0926-860X(99)00231-8 Get rights and content

Abstract

Non-oxidative aromatization of methane was carried out over Mo/CuH-ZSM-5 and compared with that over Mo/H-ZSM-5. Cu(II) ions act as promoter in this reaction and improve the activity of the Mo/H-ZSM-5 catalyst notably. The effect of Cu species on the chemical state of Mo species and the changes of Cu species itself after reaction were studied by ESR and XPS methods. The introduction of Cu ions by ion-exchange method altered the reduction of Mo species, suppressed the dealumination of ZSM-5 framework and decreased coke formation over the catalyst. The results showed improved catalytic performance of Mo/H-ZSM-5 catalyst. The amount and nature of the coke which were influenced by the introduction of Cu ions were also investigated.

Introduction

Dehydrogenation and conversion of methane directly to high value and easily separated liquid products aromatics under non-oxidative condition has drawn much attention [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. Bragin et al. [1] reported methane conversion to benzene for the first time at 1023 K in a pulse reactor over Pt–Cr/H-ZSM-5 catalyst, and obtained 18% of methane conversion with 14% of benzene yield. Ga/H-ZSM-5 and Zn/H-ZSM-5 were later found by Clyson [2] to be active for methane activation, and a 4.9% of methane conversion with 51.6% of benzene selectivity was achieved over Ga–Re/H-ZSM-5 catalyst. Moreover, Marczewski [3], [4] carried out methane aromatization reaction over MnO_x–Na/SiO₂ and H-ZSM-5 catalysts in a two-step way and an aromatics yield of 6.9% was achieved. In addition, Murata [5] realized methane aromatization utilizing amorphous carbon as catalyst, which was prepared by organic precursor (such as triphenylphosphine) thermal decomposition directly at high temperature, and reached 40.5% of methane conversion with 55.8% of benzene selectivity at 1323 K. Recently, Wang et al. [6], [7], [8], [9] reported about 7.0% conversion of methane with benzene selectivity higher than 95% on Mo/H-ZSM-5 bifunctional catalyst at 973 K with 1440 ml/g h space velocity at 200 kPa, which demonstrates an attractive future for utilization of natural gas. Chen et al. [10], [11] carried out detailed studies on the aromatization performance of Mo/H-ZSM-5 catalyst and found that Pt promoter improved the stability of catalyst notably, while it seemed not to increase the activity of catalyst. Solymosi [12], [13] and Lunsford et al. [14] also observed that MoC₂/H-ZSM-5 exhibited high activity for methane aromatization; a benzene selectivity higher than 80% at a methane conversion of 5–7% was achieved at 973 K.

All the studies mentioned above show that Mo species is the most active component for methane non-oxidative aromatization so far, but its activity and stability need to be improved. Recently, we found that the introduction of Cu(II) ions by an ion-exchange method can remarkably increase the activity of Mo/H-ZSM-5 for methane aromatization and can improve its stability to some extent [15]. Ichikawa et al. [16] also reported the considerable enhancement of aromatic products by the addition of Fe, Co to Mo/H-ZSM-5 catalyst. In the present paper, the function of Cu(II) ions in improving the catalytic performance of Mo/H-ZSM-5 catalyst for non-oxidative aromatization of methane is investigated.

Section snippets

Preparation of catalysts

H-ZSM-5 (supplied by Nankai University, SiO₂/Al₂O₃ = 25) was firstly calcined at 773 K for 4 h in air and then boiled in water for 4 h. Afterwards, ion-exchange with a 1 mol/l NH₄NO₃ aqueous solution at about 368 K was carried out. This was followed by drying at 393 K and calcining at 773 K for 4 h. Cu(II) ion-exchanged H-ZSM-5 (CuH-ZSM-5) zeolite was prepared by a conventional ion-exchange method using an aqueous solution of a prescribed concentration of Cu(CH₃COO)₂. About 37.8% of H⁺ was exchanged by Cu

Effect of Cu(II) ions on the activity and stability of the Mo/H-ZSM-5 catalysts

Fig. 1 illustrates the variation of methane conversion and benzene selectivity with time on stream over H-ZSM-5, CuH-ZSM-5, Mo/H-ZSM-5 and Mo/CuH-ZSM-5, respectively. It is observed that H-ZSM-5 shows very low activity for methane conversion. The introduction of Cu(II) ions, however, increases methane conversion from 0.15% to 2.4% with the increment of benzene selectivity from 15.4% to 70.4%.

Similar results are also obtained in the case of Mo-supported catalysts. Mo/CuH-ZSM-5 shows higher

Conclusions

1.
The Cu(II) ions introduced to the exchanged sites of H-ZSM-5 act as promoter. It enhances the activity of Mo/H-ZSM-5 catalyst markedly and improves the stability of the catalyst to some extent.
2.
The introduced Cu(II) ions suppress the reduction of Mo species, and the Cu(II) ions itself in Mo/CuH-ZSM-5 are readily reduced under reaction conditions. It is proposed that the Mo(V) species is most likely to be responsible for the methane conversion.
3.
The Cu (II) ions suppress the dealumination of ZSM-5

Acknowledgements

The authors thank D.Z. Jiang for helpful discussion and gratefully acknowledge financial support from National ‘JiuWu’ PanDeng Project.

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The function of Cu(II) ions in the Mo/CuH-ZSM-5 catalyst for methane conversion under non-oxidative condition

Abstract

Introduction

Section snippets

Preparation of catalysts

Effect of Cu(II) ions on the activity and stability of the Mo/H-ZSM-5 catalysts

Conclusions

Acknowledgements

Catal. Lett.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Topics Catal.

J. Mol. Catal.

Appl. Catal.

Izv. Ser. Khim.

React. Kinet. Catal. Lett.