A Stable Mo@HZSM-5 Catalyst for Direct Conversion of Methane to Aromatics
Yi Liu, 1 Tianyun Wang, 1 Franklin Feng Tao,2 Yi Zhang1*
1State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, 15 Beisanhuan East Road, Beijing 100029, PR China.
2Department of Chemical and Petroleum Engineering, University of Kansas, KS 66045, USA
Email: yizhang@mail.buct.edu.cn
Introduction
Methane (CH4), the principal component of natural gas and shale gas,
is considered to be an alternative feedstock for the
production of commodity chemicals and fuels currently produced from crude oil.
Thus, there are strong drivers for direct methane conversion. One of such
pathways, non-oxidative methane dehydroaromatization
(MDA), is particularly promising for the direct conversion of methane into
aromatics using catalysts with Mo nanostructures supported on shape-selective
zeolites such as Mo/HZSM-5, the most adequate catalysts. However, from the
beginning in 1993 (1), application of this technology is still limited by rapid
catalyst deactivation (severe carbon deposition) (2). It is
proposed that the state and location of Mo species in zeolite matrix are
crucial in determining the catalytic performance of MDA, especially for the
formation of carbonaceous deposits, and the ensuing catalyst deactivation.
Herein, we report the imbedment of Mo sites into
ZSM-5 crystal using an in situ synthesis strategy via hydrothermal method. Thus
obtained Mo@HZSM-5 catalyst with imbedded structure, highly dispersed
stabilizing Mo species and confined reaction environment, showed a remarkably
stable catalytic performance for MDA, and no any deactivation was observed during a 150 h test. Meanwhile, via careful
characterization of the active sites, the unique and simple structure of this
catalyst would contribute to discover the truth of MDA catalyst.
Materials and Methods
The precursor catalyst MoO3/SiO2 was prepared by
incipient wetness impregnation using commercial silica support with 6.7 nm pore
diameter. The molybdenum-imbedded microporous zeolite catalyst (Mo@HZSM-5) with
nominal 5 wt% molybdenum loading was then obtained by
using MoO3/SiO2 as the only Si source for the crystallization of HZSM-5;
meanwhile the Al species added to the synthetic solution were incorporated into
the zeolite framework, according to our previously reported method (3). By
comparison, a conventional zeolite-supported 5 wt% Mo/HZSM-5 catalyst was also prepared by
incipient wetness impregnation using HZSM-5 support. MDA reactions were conducted with 0.3 g of catalyst in a fixed bed,
tubular quartz reactor (8 mm id) at atmospheric pressure and 973 K.
Results and Discussion
Figure 1 shows that the reaction performance of Mo@HZSM-5 was extremely stable,
and no deactivation was observed during a 150-hour
test. Methane conversion remained at ~12% throughout this long run. Meanwhile, selectivities to benzene (69.3%), toluene (13.1%) and
naphthalene (17.2%) were constant with TOS, and the total selectivity to these
products remained >99%. In contrast, CH4 conversion decreased sharply with
TOS and dropped to less than 6% after 10 h because of the rapid carbon deposits
on the catalyst. On the basis of HR-TEM, SRPES, NMR, in-situ XANES and EXAFS
observations before and after reaction, the MoOC(O) active phase were
formed and maintained the initial structure as the reaction proceeded in 150
h-consistent with the stability in catalytic performance. The suitable
interaction between Mo and Bronsted acid sites and
the encapsulating system prevent reduction of highly active molybdenum oxycarbide (MoOC(O)) species to less active molybdenum carbide (Mo2C) large
particles, resulting in stable activity of such catalysts. Moreover, the
modified BAS nature and shaped channels of zeolite suppress the formation of
the PAHs and preferentially produce the single ring aromatics and naphthalene,
contributing to the stability of MDA catalyst. Extensive characterization work
will be discussed in this presentation together with more recent results on a much improved catalyst.
Figure 1. Long-term stability test of Mo@HZSM-5 catalyst at atmospheric pressure, 973 K and 2000 mL (CH4)/gcat h for MDA reaction.
References
1. L. Wang, L. Tao, M. Xie, G. Xu, J. Huang and Y.
Xu, Catal. Lett. 1993, 21,
35-41.
2. Y. Liu, D. Li, T. Wang, Y. Liu, T. Xu, and Y. Zhang, ACS Catal.
2016, 6, 5366−5370
3. J. Y. Liu, J. F. Chen, Y. Zhang, Catal. Sci.
Technol. 2013, 3, 2559-2564.
See more of this Group/Topical: General Submissions