(370f) Promotional Effect of Cr in Mo2C catalyst Supported on Sulfated Zirconia for Methane Dehydroaromatization

Promotional effect of Cr in Mo₂C
catalyst supported on sulfated zirconia for methane dehydroaromatization

Ashraf Abedin¹, Swarom Kanitkar¹, Srikar
Bhattar¹, James J. Spivey*¹

¹Cain Department of Chemical Engineering, Louisiana
State University, Baton Rouge, LA, USA

The need for valuable hydrocarbons in chemical engineering
sectors is now at its peak. Methane, having an abundant source of supply from
natural gas, has been well-studied as feedstock for producing heavier hydrocarbons
of high value. A number of methane activation processes have been developed. Methane
dehydroaromatization (MDHA) is a direct methane conversion process, eliminating
the need for a syngas step while producing both benzene and hydrogen.

The mechanism of MDHA consists of two reactions. The first
step is the activation of methane C-H bonds to produce CH_x species.
These free radicals react to generate C₂H_y dimers. The
dimers further oligomerize to produce C₆H₆ and related
homologues. In recent years, Mo oxide species supported on ZSM-5/MCM-22 has
been widely investigated for MDHA. Mo oxide is carburized to produce Mo₂C,
which is believed to activate the C-H bonds of methane to C₂H_y
dimers. The acidic sites on ZSM-5/MCM-22 oligomerize the dimers to produce benzene
and higher homologues. [1]

In this work different loadings of Cr were used as promoters
for the Mo oxide based bifunctional catalyst. Cr has been previously
added along with other metals in ZSM-5 in the form of extra-framework species
to facilitate the catalyst dehydrogenation function
for MDHA. [2] Cr₃C₂
species has electron configuration similar to the active Mo₂C at the
Fermi level. This property matches with that of noble metals like Pt [3], which
showed high activity for MDHA [4]. The hypothesis is that the additional active
sites would generate more dimers available for oligomerization. Sulfated zirconia
(SZ) provides acidic sites for MDHA, and was used as a novel support replacing the
conventional ZSM-5.

Three different SZ catalyst were prepared, each having 5 wt. %
of Mo with Cr loading of 0, 0.12 and 0.5 wt. % respectively. Incipient wetness
impregnation was used to dope the metal oxides onto SZ and the catalysts were
dried overnight at 110ᵒC, followed by calcination at 550ᵒC for four hours. Raman spectroscopy
was used to confirm the loading of the metal oxides. The surface acidity was
determined by FT-IR spectra of chemisorbed pyridine.

To compare the activity of the three different catalysts, each
run was made under identical reaction conditions. The catalysts were loaded in
a continuous flow reactor system and reduced under H₂ flow till the
reaction temperature of 700ᵒC was reached. Following this a 1:4 CH₄:H₂
flow was maintained for four hours to achieve the carburized form. MDHA
reaction was run afterwards for over 15 hours to investigate the activity of
the promoted catalysts. Reaction products were analyzed using GC-FID-TCD.

Primary products include Ethylene, Ethane, Propylene, Propane
and aromatics (benzene, toluene and ethylbenzene). During a 15-hr run, the 0.5
wt. % Cr promoted catalyst had the highest of methane conversion, whereas the
other two catalysts showed lower and virtually identical conversions (Figure 1).
The 0.5 wt. % Cr promoted catalyst showed the highest Benzene selectivity, while
the 0.12 wt. % Cr promoted and the unpromoted catalyst (Figure 2) had
essentially the same benzene selectivity. Based on this finding, it can be
proposed that a significant amount of Cr loading can modify the catalytic
surface of Mo₂C-SZ by increasing the number of active carbide sites
for aromatization to take place.

At 700ᵒC all of the three catalysts deactivated with time. This can
be attributed to the generation of coke on catalytic surface. This was later
confirmed by Temperature Programmed Oxidation (TPO) on the spent catalysts (not
shown).

References:      

(1) Spivey, J. J.; Hutchings, G. Chem Soc Rev 2014, 43, 792.

(2) F.
J. Maldonado-Hódar, Appl. Catal., A,
2011, 408, 156–162

(3) Levy, R. B.; Boudart, M. Science 1973, 181, 547.

(4) Tshabalala, T. E.; Coville, N. J.; Anderson, J. A.;
Scurrell, M. S. Applied Catalysis A: General 2015, 503, 218.