Impact of Adsorption Configurations on Alcohol Dehydration over Alumina Catalysts

Abstract

The dehydration of alcohols to value-added chemicals using alumina catalysts has been industrialized decades ago; however, despite years of fundamental research, the molecular-level understanding of the reaction process is still lacking. Herein, the dehydration reactions of ethanol on two representative aluminas, i.e., γ-Al2O3 and five-coordinated Al-rich Al2O3-nanosheet (Al2O3-NS), are comparatively investigated by a combination of solid-state NMR spectroscopy and reaction evaluations. NMR results reveal that the presence of hydroxyl groups nearby the Lewis acid sites (LASs) leads to different ethanol adsorption modes. The surface hydroxyls nearby LASs interfere with ethanol adsorption, which causes the methyl group to move away from the alumina surface and hinders the elimination of β-H, resulting in low reaction activity. In addition, the desorption property of surface hydroxyls will affect the dehydration reaction as they participate in the catalytic reaction cycles. Such effects may also explain the lower reaction activities for diethyl ether to ethylene compared with ethanol. Although both four- and five-coordinated Als are the possible active sites for ethanol dehydration, the reactivity of alcohol dehydration may also be substantially affected by the local environment of undercoordinated Als besides coordination numbers. These new insights demonstrate that the host–guest interaction regulated by the local environment of active sites plays an important role in the catalytic reaction.

Spectroscopic Characterizations of Porous Mixed Metal Oxides Derived from Metal–Organic Framework MIL-53(Ga, Al) for Propane Dehydrogenation

“…Solid-state NMR has been extensively utilized to characterize the structure and property of porous materials and metal oxides. − 13 C CP/MAS NMR spectra of bimetallic MIL-53­(Ga x , Al 2– x ) ( x = 0.1, 0.5, 1.0, 1.5) precursors confirm the synthesized samples are in large pore (lp) and/or narrow pore (np) forms (Figure S3). Herein, 1 H, 27 Al, and 71 Ga solid-state NMR were used to extract the structural information and coordination environments of the MIL-53­(Ga, Al)-derived porous metal oxides.…”

A perspective on catalytic production of olefinic compounds from biomass

“…The vicinal C-O and C-H bonds are dissociated simultaneously, with the acid and base sites, respectively. This is a variant of the E2 mechanism; one difference from the general E2 reaction is that the two bonds are dissociated with syn conguration, 27,28 while the general E2 reaction with a strong homogeneous base is the dissociation with anti conguration. An important feature of the dehydration over bifunctional catalysts is the suppression of cyclization of diols.…”

Sustainable routes to alkenes: applications of homogeneous catalysis to the dehydration of alcohols to alkenes

“…Alumina γ-Alumina is a very important industrial catalyst and catalyst support, and yet its structure is still under discussion. 70 Alumina has long been known to catalyse the dehydration of alkenes to alcohols, 71 such as 1-hexanol to hexene at 450 °C, 72 but the product and extent of isomerisation is highly dependent on the nature of the alumina catalyst. 73 Older ethanol to ethylene industrial processes successfully utilised alumina catalysts, 32 but unmodified γ-alumina required temperatures of 450 °C and the yield of ethylene is relatively low (80%).…”