(744g) Selective CO2 Hydrogenation to Methanol over Promoted Indium-Based Catalysts

Selective CO₂ Hydrogenation
to Methanol over Promoted Indium-based Catalysts

Chen-Yu Chou,^a Raul F. Lobo,^a,*

^aCenter for Catalytic Science and Technology, Department of
Chemical and Biomolecular Engineering, University of Delaware, Newark, DE
19716, USA

*Corresponding author: Fax: 302-831-1048, lobo@udel.edu

Keywords: Methanol, Carbon dioxide hydrogenation, Indium
oxide.

The direct CO₂
hydrogenation to methanol is potentially an important route to decrease CO₂
emissions: methanol is a versatile
compound that can be used as fuel or as a precursor to many commodity
chemicals. If a green (CO₂-free) H₂ source is
used, the process is sustainable as demonstrated by the George Olah CO₂
to Renewable Methanol Plant in Iceland. This plant uses electricity, generated
from hydro and geothermal energy, to make hydrogen. Although methanol synthesis
from CO₂ and H₂ is exothermic, CO₂ conversion
to methanol is kinetically limited at low temperatures and thermodynamically
limited at high temperatures, resulting in a low theoretical methanol yield.^1,2
Cu-ZnO-Al₂O₃ (CZA) catalysts are currently employed for
methanol synthesis from mixed syngas (CO/CO₂/H₂) in
industry. However, these catalysts have low selectivity because of the
competing reverse water–gas shift (RWGS) reaction, and limited stability, due
to the sintering of the active phase. Recent
DFT calculations have showed that the key intermediates (HCOO) involved in CH₃OH
synthesis were more stable on a defective In₂O₃ surface
than those on the Cu surface, strongly suppressing the formation of CO.³ Supported
indium oxide catalysts have been investigated as they are promising candidates
for developing effective and stable catalyst with high selectivity towards
methanol⁴. To
further enhance the selectivity and investigate the effect of reducibility in
this report, Y and La were considered as selectivity promoters due to their 3+
oxidation state and ionic sizes.

Figure 1 shows H₂-TPR
profiles displaying reduction peaks of In₂O₃/ZrO₂,
1.5Y9In/ZrO₂, 2Y8In/ZrO₂, 3Y8In/ZrO₂, and
3La10In/ZrO₂ from 350K-800K:
all dopants increase the reduction temperature (470K, 481K, 495K, and 505K,
respectively). The selectivity of the non-promoted In₂O₃/ZrO₂
at 573 K was measured at 53%; remarkably, tuning the reducibility of the
catalysts, higher selectivity (~15-20% more) toward methanol from 528K to 573K
(see Figure 2) is
demonstrated over Y and La-promoted catalysts.

Herein, a strategy to enhance the
selectivity over Indium oxide-based catalysts for direct conversion of CO₂
to methanol was investigated under mild reaction condition (528-573K, 40 bar) compared to the commercial process (513-533K, 50-100 bar). Surface reducibility, spectroscopic characteristics, and
catalytic activity were correlated to catalyst composition. Promoted catalysts,
especially Yttrium or Lanthanum-promoted indium oxide catalysts, showed 20%
higher selectivity compared to the non-promoted catalyst (In₂O₃/ZrO₂). These results show that this is a
feasible strategy for the systematic improvement of selective catalysts for
sustainable methanol economy.

Figure 1  H₂-TPR
profiles reduction peaks of In₂O₃/ZrO₂,
1.5Y9In/ZrO₂, 2Y8In/ZrO₂, 3Y8In/ZrO₂, and 3La10In/ZrO₂

Figure
2 Methanol
selectivity versus temperature over In₂O₃/ZrO₂,
1.5Y9In/ZrO₂, 2Y8In/ZrO₂, 3Y8In/ZrO₂, and
3La10In/ZrO₂.

References

1.          Porosoff,
M. D., Yan, B. &Chen, J. G. Catalytic reduction of CO₂ by H₂
for synthesis of CO, methanol and hydrocarbons: challenges and opportunities. Energy
Environ. Sci. 9, 62–73 (2016).

2.          Goeppert,
A., Czaun, M., Jones, J.-P., Surya Prakash, G. K. &Olah, G. A. Recycling of
carbon dioxide to methanol and derived products – closing the loop. Chem.
Soc. Rev. 43, 7995–8048 (2014).

3.          Ye,
J., Liu, C., Mei, D. &Ge, Q. Active oxygen vacancy site for methanol
synthesis from CO₂ hydrogenation on In₂O₃(110):
A DFT study. ACS Catal. 3, 1296–1306 (2013).

4.          Martin,
O. et al. Indium Oxide as a Superior Catalyst for Methanol Synthesis by
CO₂ Hydrogenation. Angew. Chemie Int. Ed. 55, 1–6
(2016).