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

Catalyst	Temperature (oC)	CO2 (wt %)	CO (wt %)	Selectivity (wt %) without CO included
		Conversion	Selectivity	CH4	RH	R=	ROH
Co2Cu1K0.1	220	5	87	31	65	27	9
	240	8	82	35	63	26	12
	260	15	74	42	64	24	13
	280	19	42	61	78	15	8

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)