(19b) Propane Aromatization Using Sulfated Zirconia Supported Mo Catalysts

Aromatization of propane into benzene and other aromatics
has been a very well-studied reaction over Ga- and Zn- doped HZSM5 catalysts.
In aromatization, both metal and acidic sites are required. Although Mo-doped
HZSM5 is known to have both of these sites, and has
been extensively studied for CH₄ aromatization, it has not been
widely reported for propane aromatization. 
In addition, we are not aware of the use of Sulfated Zirconia (SZ) as a
solid acid for propane aromatization, despite its well-known superacid
properties. Like HZSM-5, SZ also possesses strong Brønsted acid sites that can catalyze
olefinic intermediates into aromatic products. 
To our knowledge, no other reports have tested SZ supported Mo catalysts
for the aromatization of propane, despite the fact that
this catalyst contains both the Mo metal and acid sites required for this
reaction.

In the present study, Mo was doped on SZ and these catalysts
were characterized using DRIFTS, XRD. Using pyridine adsorption, DRIFTS showed small
effects on acidity when Mo is added to SZ in that the peaks corresponding to
Lewis (L) and Brønsted (B) acid sites are shifted to lower wavenumbers as shown
in Figure 1.   XRD showed no peaks for MoO_x
species indicating high dispersion of Mo, and SZ crystallinity is not affected
by the addition of Mo. This catalyst was then evaluated for propane
aromatization around 650 ⁰C. Primary products included ethylene,
ethane, and propylene along with aromatics including benzene, toluene and
xylenes. However, catalyst deactivated over time, likely due to coking on the
surface of the catalysts that was confirmed by TPO. The effect of temperature
and Mo content on acidity, activity, and selectivity was also investigated. Temperature
plays a key role as higher temperatures although showed high activity, it also
leads to faster deactivation. Whereas, lower temperatures showed lower activity
but lesser rate of deactivation. Higher Mo content also increased the activity
as more number of active sites are available for the intermediates to form but
too high Mo can lead to non-uniform dispersion and can affect the acidity of
the catalyst significantly.

Figure  SEQ Figure \* ARABIC 1:
Comparison of acid sites for SZ and Mo doped SZ catalysts (after pyridine
desorption at 100 ⁰C, (a) Sulfated Zirconia (SZ); (b) 1%Mo doped SZ;
(c) 5%Mo doped SZ; (d) 10%Mo doped SZ), L= Lewis acid site, B= Brønsted acid
site.