Origins of Unusual Alcohol Selectivities over Mixed MgAl Oxide-Supported K/MoS₂ Catalysts for Higher Alcohol Synthesis from Syngas

Abstract

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.

Tuning of higher alcohol selectivity and productivity in CO hydrogenation reactions over K/MoS2 domains supported on mesoporous activated carbon and mixed MgAl oxide

“…These studies showed that catalytic activity is enhanced by dispersing the MoS 2 over a support and that Mo-support interactions can affect the reactivity and product distribution of the catalyst by facilitating alcohol reaction pathways. Recently, Morrill et al [37,38] developed new potassium promoted MoS 2 catalysts supported on a mixed Mg/Al oxide (MMO) support, derived from a hydrotalcite, that strongly perturbed the product distribution toward higher alcohols. The intrinsically basic properties of the support were suggested to promote alcohol-forming reactions, unlike common acidic c-alumina [39][40][41][42] known to promote alcohol dehydration and subsequent hydrocarbon formation.…”

“…The studies indicated that MMO supports yielded high C 3+ OH selectivity influenced by Mo-MMO interactions, but generally also yielded low catalytic productivity. It has further been shown that the Mo:MMO ratio greatly affected the reaction selectivity, while catalyst preparation methods had little impact on this parameter [38].…”

Catalytic oxidation of styrene over Cu-doped hydrotalcites

“…The detailed procedure was described in our previous publications [16,29]. In brief, a 150mL-mixed Table 1S in Supplementary Materials).…”

“…Indeed, the binding energy of C 1s is presumably composed of two overlapping peaks at 284.5 and 285.8 eV. The first peak is assigned from the carbons in contaminated organic product of spent samples while the higher binding value is characteristically assigned to carbonate anions (C = O, C -O bond) in the interlayered spaces [29,36,47,49,50].…”

“…Following this trend; we are in an endeavor to design an efficient catalyst based on hydrotalcites for the liquid oxidation of styrene. It is well known that hydrotalcite-like compounds are a two-dimensional material and sometimes show good ability to the oxidation reactions [21][22][23][24][25][26][27][28][29]. In essence, isomorphous substitution of transition metal ions in a brucite-like lattice leads to the appearance of oxidation-reduction centers in the sheets which may acts as active sites for the catalytic oxidation reaction [16,[25][26][27][28].…”