(425a) Selective Hydrogenation of CO2 over ZSM-5-Based Tandem Catalysts

Selective hydrogenation of CO₂
over ZSM-5-based tandem catalysts

Renjie
Liu¹ and Marc D. Porosoff^1*

¹Department
of Chemical Engineering, University of Rochester, Rochester, NY 14627 (USA)

*marc.porosoff@rochester.edu

CO₂ hydrogenation
is a reaction under investigation for production of synthetic chemicals and
fuels as an alternative to petroleum-based processes. However, the selectivity
towards hydrocarbons, and specifically desirable olefins, is generally
characterized by the highly-unselective Anderson-Shulz-Flory
(ASF) distribution. To avoid the energy-intensive, post-reaction separation
steps, researchers have recently attempted to selectively produce light olefins
(ethylene and propylene) during CO₂ hydrogenation via dual-functional,
tandem catalysts, which take advantage of secondary reactions within zeolites
(C-C bond formation and C-H bond scission), thus yielding higher selectivity to
light olefins.^[1]

The active phase of tandem
catalysts generally consists of a mixed metal oxide, which is either dispersed
on or physically mixed with an aluminosilicate zeolite. The function of the
metal oxide is to activate the CO₂ to a methanol or hydrocarbon
intermediate. The zeolite can restrict hydrocarbon products to a narrow size
distribution by sterically limiting diffusion within the micropores and
cracking larger paraffins into olefins over Brønsted acid sites (ie. H-ZSM-5) ^[4].

Tandem catalysts have
recently become the subject of intense investigations, but guidelines are
unavailable for selection of the active phase, zeolite and promoters to achieve
high selectivity towards light olefins. As such, it is necessary to develop a
fundamental understanding of the structure-property relationships for CO₂
hydrogenation. Our initial work is focused on studying the Si/Al ratio and method
of introduction of extra-framework cations in the ZSM-5 topology to control
hydrogenation of the mildly acidic CO₂ molecule. The zeolites are impregnated
with cobalt (Co), and the potassium (K) promoter is introduced either via
co-impregnation or ion exchange (IE), to demonstrate if the Si/Al ratio and
method of introducing K has a predictable effect on CO₂ hydrogenation
to olefins.

In our work, for catalysts prepared via
co-impregnation, the selectivity towards CO decreases while that of CH₄,
C₂-C₄ olefins, paraffins and C₅₊ hydrocarbons
increase with increasing Si/Al ratio. Conversely, for catalysts with K first
introduced via IE, the selectivity to all products remains relatively constant
as the Si/Al ratio increases. These distinct observations of selectivity,
combined with acid site titration, in
situ XAFS and FTIR, suggest that ion exchange can eliminate the Brønsted acid sites in ZSM-5, thereby mitigating the effect
of Si/Al ratio.

References

1.      
Jiang, N., Yang, G., Zhang, X., et al. Catal. Comm. 12, 951 (2011).

2.      
Li, Z., Wang, J., Qu, Y., et al. ACS Catal. 7, 8544 (2017).

3.      
Jiao, F.; Li, J.; Pan, X.; et al. Science 351, 1065 (2016).

4.      
Huang, Y., Meng, X., Dang, Z., et al. Chem. Soc. Chem. Comm. 10, 1025
(1995).