(611c) Effect of Potassium Promoter in Cocu-Based Catalysts for CO and CO2 Hydrogenation

Effect of
Potassium Promoter in CoCu-based Catalysts for CO and
CO₂ Hydrogenation

By Jenny Voss, Lei
Tang, Yizhi Xiang and Norbert Kruse

Gene
and Linda Voiland School of Chemical Engineering and
Bioengineering, Washington State University, Pullman, WA

Key Words: CO₂ Hydrogenation,
reverse water gas shift, potassium-promoted CoCu-based
catalysts

Conversion of CO₂
and CO to higher alcohols and olefins is driven by incentives to produce
transportation fuels or additives on one hand and building blocks for the
polymer industry on the other. Traditionally, alcohols are made from aldehydes
which are obtained via hydroformylation of alkenes
with CO and H₂ using high-cost homogenous catalysts. Previously, our
group demonstrated ternary CoCuMn catalysts to favor
C₈-C₁₄ alcohols while, quite differently, CoCuNb catalysts turned out to favor shorter alcohols in CO
hydrogenation [1-2]. Furthermore, all CoCu-based
catalysts produced considerable amounts of olefins as well.

In the present
contribution we explore the perspectives of turning CO₂ into CO via
the reverse water gas shift reaction and using the known CO hydrogenation
patterns for CoCu-based catalysts to produce
oxygenates and olefins. Since the above CoCuMn and CoCuNb catalysts showed limited reverse water gas shift
activity, we concentrated on alkali-promoted CoCu
catalysts here.

We synthesized
such catalysts using the oxalate co-precipitation route. To do so, metal
nitrates were co-precipitated with oxalic acid in acetone to form a polymeric
oxalate structure containing double-chelating oxalate ligands with Co or Cu
within the same structure. Alkaline oxalates were precipitated at the same time
using their limited solubility in acetone. To activate the precursors, we
performed hydrogen-assisted temperature-programmed decomposition which usually
resulted in the formation of gaseous CO₂ with insignificant amounts
of concomitant CO. This observation allowed us to conclude that our CoCu catalysts are metallic, with no phase-separated oxidic structures appearing. Following previous microscopic
investigations with Atom-Probe Tomography (APT) and Transmission Electron
Microscopy (TEM), these catalysts are Co@Cu
core-shell structured [1]. Recent results by TEM and X-ray Diffraction (XRD)
seem to indicate that the presence of potassium promotes the formation of
Co-carbide while running the catalyst into steady-state CO₂
hydrogenation conditions, see Table 1 below.

All high-pressure
experiments were conducted in a fixed bed flow reactor operating at 40 bar,
with a feed ratio of H₂/CO₂= 3 and temperatures of
200-260^oC. Without a promoter, CoCu
catalysts mainly produced methane. These non-promoted catalysts also showed
unstable performance with extended times-on-stream. CoCuLi,
CoCuNa and CoCuCs catalysts
(all containing a 1:1:0.1 ratio of metals) also produced significant amounts of
methane. Most importantly, however, these catalysts (except for CoCuCs) had little reverse water gas shift activity. On the
contrary, potassium-promoted CoCu catalysts favored
formation of CO and Fischer Tropsch products such as
alkanes (RH), olefins (R=) and alcohols (ROH) as displayed in Table 1. The
importance of the reverse water gas shift reaction is therefore clearly
demonstrated here since appreciable amounts of alkanes, olefins and alcohols
are obtained when CO formation is high. However, there resides an overarching
tradeoff between high CO formation and high olefins/alcohols selectivity with
low CO₂ conversion at low temperatures and improved CO₂
conversion but lower selectivity at high temperatures.

When determining
the bulk composition of CoCu-based catalysts before
and after CO₂ hydrogenation reaction via XRD studies, all catalysts
contained a metallic cobalt phase before the reaction yet only CoCuK catalysts contained cobalt carbide (Co₂C)
post reaction. Therefore, we suggest potassium acts as a structural promoter.
Yet, we advocate the view that potassium helps establish the water gas shift
equilibrium at sufficiently low temperatures. The combined olefin /alcohol selectivities between 220^oC and 260^oC
are close to 40%; however, the CO conversion is quite low.

We are currently investigating
the electronic and structural effects of potassium promotion in CoCu-based catalysts. We are also exploring other synthesis
routes to improve catalyst activity at low temperatures where CO selectivity
and valuable product formation are favored due to the reverse water gas shift
activity.

Table 1: Results of a Co₂Cu₁K_0.1
catalyst under CO₂ hydrogenation conditions

Citations:

[1] Xiang, Y., et al., "Long-Chain
Terminal Alcohols through Catalytic CO Hydrogenation,"  Journal
of the American Chemical Society, 13 (19), pp. 7114-7117 (Apr. 2013)

[2] Xiang, Y., et al., "Ternary Cobalt-Copper-Niobium Catalysts for the
Selective CO Hydrogenation to Higher Alcohols," ACS Catalysis, 5, pp. 2929-2934 (Apr. 2015)