Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts

Abstract

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al₂O₃ has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu⁰ particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al₂O₃ catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO₂-containing syngas mixtures when pulsing NH₃ or methylamines as basic probe molecules.

Fig. 1. High-pressure pulse experiments and the determined inhibition strengths.

a Recorded methanol (black curves) and NH₃ (blue curves) mole fractions during the injection of pulses with different NH₃ partial pressures over the Cu/ZnO/Al₂O₃ catalyst (conditions: 210 °C, 60 bar, 13.5% CO, 3.5% CO₂, 73.5% H₂, 9.5% N₂). b Correlation of the determined Δn_MeOH (non-produced amount of methanol over a defined period of time) values (Supplementary Note 1) with the injected mole fractions of the investigated N compounds (NH₃ red squares and line, NO black points and line, monomethylamine (MMA) blue triangles and line, dimethylamine (DMA) violet stars and line, trimethylamine (TMA) orange diamonds and line). The corresponding pulse experiments are shown in Supplementary Figs. 1–5. a, the slope of the corresponding linear interpolation, is defined as the inhibition strength.

Fig. 2. Continuous dosing experiments coupled with HPPEs.

a Continuous dosing of 1% of ethylene (blue curve), which is hydrogenated to ethane (green curve) under methanol (black curve) synthesis conditions, coupled with HPPEs with NH₃. b Comparison of the continuous dosing of 0.05% NH₃ with the continuous dosing of 0.2% NO. The black bars describe the initial methanol mole fraction in pure syngas (y_{MeOH, initial}) and in the presence of the corresponding impurity (y_{MeOH, poisoned}). The red bars describe the dosed mole fraction of NH₃ (y_{NH3, dosed}) and mole fraction in the product gas stream (y_{NH3, effluent}). The blue bar (y_{NO, dosed}) stands for the dosed mole fraction of NO. X_NH3 describes the corresponding degree of NH₃ conversion to the three methylamines: monomethylamine (orange bar), dimethylamine (grey bar) and trimethylamine (violet bar). c Syngas switching experiments from CO/CO₂/H₂ to CO/H₂ and back to CO/CO₂/H₂ at 210 °C and 60 bar coupled with HPPEs with NH₃. Here, the resulting methanol (black curve) and NH₃ (green curve) mole fractions in the product gas stream are shown. The images visualize the oxidative effect of CO₂ in the syngas mixture and the corresponding interaction of NH₃ with the catalyst surface. Orange area = Cu⁰ sites, blue area = Zn^δ+/ZnO sites, grey area = Zn⁰ sites.

Fig. 3. Influence of CO₂ on the methanol productivity and on the oxidation state of the catalyst surface.

Correlation of the methanol productivity (black points and line) and of the normalized Δn_MeOH values (Supplementary Note 2) of NH₃ (blue points and line) and TMA (green points and line) as a function of the CO₂ content in the syngas from pure CO hydrogenation (0%) to pure CO₂ hydrogenation (100%). Illustrations: Metallic Cu⁰ particles (orange balls), metallic Zn⁰ species (white balls), positively charged Zn species (blue balls) and formate as adsorbates. The error bars were determined by measuring every point five times.

Fig. 4. Long-term methanol synthesis over the industrial Cu/ZnO/Al₂O₃ catalyst at 210 °C and 60 bar.

Recorded degrees of conversion (black points) under differential controlled conditions. The dashed red curve describes the intra- and extrapolation of the experimental data, which was calculated with the MATLAB^® software according to the studies of Fichtl et al.. Illustrations from right to left: Cu⁰–Zn⁰ surface alloy according to Nakamura et al.. Zn^δ+ species at the defective Cu⁰ surface according to Behrens et al., graphitic-like ZnO_x layer on Cu⁰ according to Lunkenbein et al.. ZnO layer on Cu⁰ according to Lunkenbein et al. as well as Fichtl et al. and ZnO on the top layer of Cu⁰ according to Kattel et al..

Fig. 5. Scheme of the high-pressure pulse unit integrated in the flow set-up.

PI is defined as pressure sensor in the corresponding line. The area in blue describes the flow direction of the syngas mixture, the red area of the injected impurities and the orange area of the reference gas (Ar) for pressure regulation.

Fig. 6. Validation of the self-built high-pressure pulse unit with inert CH₄ pulses over α-Al₂O₃.

a Recorded methane pulses (green curves) injected over α-Al₂O₃ as inert bed at different pressures from 5 to 60 bar. τ is defined as the retention time of the CH₄ pulses. b Correlation of the determined retention times τ (red points and line) and pulse areas A_pulse (blue points and line) against the set pressure levels. The error bars were determined by measuring every point 5 times.

Fig. 7. Experimental protocol of the long-term measurement adapted from the studies by Fichtl et al..

The violet arrow describes the period of time to record the kinetic data point, the red arrow describes the heating rate, the black arrow describes the aging of the catalyst bed under equilibrium conditions and the blue arrow describes the cooling rate.