The Guerbet coupling of ethanol into butanol was investigated using multiproduct steady-state isotopic transient kinetic analysis (SSITKA) in a comparative study between stoichiometric hydroxyapatite (HAP) and magnesia (MgO) catalysts at 613 and 653 K, respectively. The steady-state catalytic reactions were conducted in a gas-phase, fixed-bed, differential reactor at 1.3 atm total system pressure. Multiproduct SSITKA results showed that the mean surface residence time of reactive intermediates leading to acetaldehyde was significantly shorter than that of intermediates leading to butanol on both HAP and MgO. This finding may suggest that the dehydrogenation of ethanol to acetaldehyde is fast on these surfaces compared with C–C bond formation. If adsorbed acetaldehyde is a key reaction intermediate in the Guerbet coupling of ethanol into butanol, then SSITKA revealed that the majority of adsorbed acetaldehyde produced on the surface of MgO desorbs into the gas-phase, whereas the majority of adsorbed acetaldehyde on HAP likely undergoes sequential aldol-type reactions required for butanol formation. Adsorption microcalorimetry of triethylamine and CO2 showed a significantly higher number of acid and base sites on the surface of HAP compared with those on MgO. Diffuse reflectance infrared Fourier transform spectroscopy of adsorbed ethanol followed by stepwise temperature-programmed desorption revealed that ethoxide is more weakly bound to the HAP surface compared with MgO. A high surface density of acid–base site pairs along with a weak binding affinity for ethanol on HAP may provide a possible explanation for the increased activity and high butanol selectivity observed with HAP compared with MgO catalysts in the ethanol coupling reaction.

A series of MoS2 catalysts supported on Mg/Al hydrotalcite-derived mixed-metal oxide (MMO) supports promoted with K2CO3 is used for alcohol synthesis via CO hydrogenation. Alcohol selectivities are found to vary greatly when the Mo is loaded on the support at 5 wt % compared with 15 wt % Mo samples, all with a Mo/K atomic ratio of 1:1. The most striking difference between the catalysts is the comparatively low methanol and high C3+ alcohol selectivities and productivities achieved with the 5% Mo catalyst. This catalyst also produces more ethane than the 15% Mo catalyst, which is shown to be associated with ethanol dehydration and hydrogenation over residual acid sites on this catalyst with lower K content. A series of catalysts with common composition (5% Mo and 3% K supported on MMO) prepared in different manners all yield similar catalytic selectivities, thus showing that selectivity is predominately controlled by the MMO-to-Mo ratio rather than the synthesis method. When the Mo loading is the same, catalytic higher alcohol productivity shows some correlation to the degree of stacking of the MoS2 layers, as assessed via X-ray diffraction and scanning transmission electron microscopy. Control reactions in which K loading is increased or the positioning of the MMO in the catalyst bed is changed via creation of multiple or mixed catalyst beds show that Mo/K/MMO domains play a significant role in alcohol-forming reactions. Higher alcohol-forming pathways are proposed to occur via CO insertion pathways or via coupling of adsorbed reaction intermediates at or near MoS2 domains. No evidence is observed for significant alcohol-coupling pathways by adsorption of alcohols over downstream, bare MMO supports. Nitrogen physisorption, XRD, Raman, UV–vis DRS, STEM, and XANES are used to characterize the catalysts, demonstrating that the degree of stacking of the MoS2 domains differs significantly between the low (5% Mo) and high (15% Mo) loading catalysts.

A Mg/Al mixed metal oxide material (MMO) is introduced as a support for K 2 CO 3 promoted MoS 2 in CO hydrogenation reactions at 310°C and 1,500 psig. The catalyst is shown to be more selective for C 2 -C 4 linear alcohols (substantially so for C 3 -C 4 linear alcohols) than for methanol and offers good alcohol to hydrocarbon selectivity. Methanol selectivity of the MMO supported catalyst deviates greatly from the Anderson-Shultz-Flory distribution.

Unpromoted and Rb-promoted Mo2C nanoparticles (∼1 nm in size) were synthesized on α-alumina and used for CO hydrogenation. The reaction was performed in a fixed-bed reactor system operating at 573 K and 30 bar with a gas flow rate of 24,000 cm3 gMo –1 h–1 and H2/CO ratio of 1:1. Unpromoted Mo2C functioned as a Fischer–Tropsch catalyst producing mostly hydrocarbons at the standard conditions. Passivation of freshly synthesized Mo2C nanoparticles in 1% O2 prior to CO hydrogenation decreased the Fischer–Tropsch activity of the catalyst by about 40% compared with a nonpassivated sample. Addition of Rb promoter to Mo2C shifted the selectivity of the catalyst by inhibiting the formation of hydrocarbons while preserving the formation of alcohols. The presence of Rb with Mo2C also reduced substantially the sensitivity of the catalyst to passivation by 1% O2 compared with unpromoted Mo2C. The partial oxidation of the Mo2C nanoparticles by passivation and partial reduction by syngas was confirmed by Mo K edge X-ray absorption spectroscopy.

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