Direct observation of carbon nanotube formation in Pd/H-ZSM-5 and MoO 3 /H-ZSM-5 based methane activation catalysts
- PDF / 428,627 Bytes
- 6 Pages / 595.276 x 790.866 pts Page_size
- 52 Downloads / 199 Views
Catalysis Letters, Vol. 116, Nos. 3–4, August 2007 (Ó 2007) DOI: 10.1007/s10562-007-9125-6
Direct observation of carbon nanotube formation in Pd/H-ZSM-5 and MoO3/H-ZSM-5 based methane activation catalysts S. Burns,a J. G. Gallagher,a J. S. J. Hargreaves,a,* and P. J. F. Harrisb a WestCHEM, Department of Chemistry, University of Glasgow, Joseph Black Building, Glasgow G12 8QQ, UK Centre for Advanced Microscopy, J. J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 6AF, UK
b
Received 19 March 2007; accepted 12 April 2007
The nature of carbonaceous species deposited upon MoO3/H-ZSM-5 and Pd/H-ZSM-5 based catalysts during methane activation at 700 °C has been studied. TEM evidences the formation of open-ended multi-walled carbon nanotubes on MoO3/HZSM-5 based dehydroaromatisation catalysts. Pd/H-ZSM-5 is more active, exclusively towards methane cracking and postreaction analysis reveals the distribution of different carbonaceous species is more homogeneous which TEM demonstrates to be in the form of closed-end multi-walled carbon nanotubes. KEY WORDS: carbon nanotubes; methane; dehydroaromatisation; zeolite; TEM; TGA.
1. Introduction In recent years the production of benzene by dehydroaromatisation has been the subject of much interest [1,2]. The most well studied catalytic system has been that based upon ZSM-5 supported MoO3. In this system, it has been proposed that in the active state molybdenum is in the form of molybdenum carbide or molybdenum oxycarbide [2–5]. The reaction mechanism is generally considered to occur via the bifunctional activation of methane on the molybdenum containing phase with cyclisation being promoted by the Bronsted acid sites on the zeolite. It is often proposed that ethylene is the primary product of reaction, which is then cyclised to form benzene [6]. However, alternative proposals suggest the occurrence of acetylene as an intermediate [7], or indeed the involvement of partially dehydrogenated C1 entities [8]. C2 hydrocarbons, particularly ethylene, are reported to be co-products of the reaction and a distribution of aromatic products is observed. Whilst benzene is the major aromatic product, significant selectivities to other aromatic products such as naphthalene, methylnaphthalenes and toluene are frequently reported, e.g. [8,9]. In terms of the reaction mechanism, the latter two products comprising molecules with an odd number of carbon atoms, 11 and 7, respectively, demonstrate that the reaction cannot simply be considered in terms of exclusive cyclisation of C2 intermediates. The reaction is equilibrium limited and is generally run at 700 °C or above. Major obstacles to its applica*To whom correspondence should be addressed. E-mail: [email protected]
tion are low activity and limited lifetime. To date, the activity of the best catalysts is such that conversion approaching equilibrium is only achieved when low space velocities (typically 1000 h)1) are applied. It is common that MoO3 based catalysts will display an induction period, wherein the
Data Loading...
