Mechanistic Aspects of the Role of K Promotion on Cu–Fe-Based Catalysts for Higher Alcohol Synthesis from CO₂ Hydrogenation

Abstract

Direct CO2 hydrogenation to higher alcohols (HAs) is a promising way to achieve the conversion of CO2 to high-value chemicals. Alkali metals as promoters are generally crucial for Cu–Fe-based catalysts, but their critical role in higher alcohol synthesis (HAS) is still far from clear. Here, we report the regulating effect of a potassium (K) promoter from a reactant activation perspective on Cu–Fe-based catalysts for HAS from CO2 hydrogenation using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and chemisorption methods. The optimized catalyst denoted as 4.6K-CMZF with a moderate K content exhibits the highest HA space time yield (STY) in a fixed-bed reactor. It is found that the K can promote reverse water gas shift (RWGS) reaction and tailor the ratio of nondissociated CO to dissociated CO by strengthening linear CO adsorption and weakening bridging CO adsorption. A proper amount of K can balance the nondissociated and dissociated activation of CO, thus providing an adequate *CH x and *CO species to take part in *CH x –*CO coupling reaction. The K promoter can also suppress H2 activation, thereby inhibiting alkylation reaction. The promoting effect of K can be attributed to the balance of surface *CH x , *CO, and *H species by regulating CO activation and H2 activation, thus favoring HA synthesis via *CH x –*CO coupling and hydrogenation reactions.

Selective C2+ alcohol synthesis by CO2 hydrogenation via a reaction-coupling strategy

“…These results indicate that K plays an important role in HAS and the moderate content of K is beneficial to HAS. Our previous report 19 has proved that the K promoter in CuFe catalysts can regulate the activation behaviors of CO intermediates and H 2 and the balanced relative concentrations of *CH x , *CO, and *H species, making 4.7KCFZ the best primary catalyst for HAS in this work. After catalyst screening, 4.7KCFZ and CuZnAlZr were chosen as the two components for further activity optimization of multifunctional catalysts.…”

“…Notably, the HA selectivity and STY increase first and then drop, peaking at 33.0% and 42.0 mg g cat −1 h −1 (300 °C) at a 4.7KCFZ/CuZnAlZr mass ratio of 1/1. Such high HA selectivity has ranked as top values in the literature 17–20,22,23,32,33,37–42 (Table S1†). Increasing the reaction temperature (from 300 to 320 °C) can promote the CO 2 conversion and HAS activity, but the HA selectivity drops to some extent.…”

“…7a and S12a†), 2930 cm −1 (C 2 H 5 O*), 2868 and 2969 cm −1 (HCOO*), 2826 cm −1 (CH 3 O*) and 2110 and 2181 cm −1 (CO) were observed. 19,33 We also recorded the dynamic change of the IR signals as a function of time (Fig. 7b and S13†).…”

“…CO 2 hydrogenation to HAs has attracted considerable attention because of the urgency to relieve pressure from excessive CO 2 emission and the demand for clean fuels. The catalysts for CO 2 hydrogenation to HAs include noble-metal-based (Pd, Pt, Rh), 11–14 modified FTS, 15–18 Cu-based MS, 19,20 and Mo-based 21–23 catalysts. However, a low CO partial pressure, the existence of many parallel and consecutive reactions, 24 and a higher energy barrier for CO insertion than that for CH x hydrogenation 25 result in relatively low selectivity and activity for HAS by CO 2 hydrogenation over a sole catalyst.…”

Direct Conversion Syngas to Isobutanol over Ce/ZC Catalysts: Effect of Ce Promoter on the Catalytic Performance

“…A similar conclusion was also confirmed over K modified CuLaZrO 2 sample for isobutanol synthesis. [8] Xu et al [9] comprehensively explored the effect of varied K content on the C 2 + OH synthesis over CuFe based catalyst. They proposed that K promoter played a pivotal role in tuning balance of *CH x , *CO, and *H species via adjusting adsorption strength and adsorption form of CO, as well as H 2 activation.…”