Spinel-Structured ZnCr₂O₄ with Excess Zn Is the Active ZnO/Cr₂O₃ Catalyst for High-Temperature Methanol Synthesis

Abstract

A series of ZnO/Cr 2 O 3 catalysts with different Zn:Cr ratios was prepared by coprecipitation at a constant pH of 7 and applied in methanol synthesis at 260−300 °C and 60 bar. The X-ray diffraction (XRD) results showed that the calcined catalysts with ratios from 65:35 to 55:45 consist of ZnCr 2 O 4 spinel with a low degree of crystallinity. For catalysts with Zn:Cr ratios smaller than 1, the formation of chromates was observed in agreement with temperature-programmed reduction results. Raman and XRD results did not provide evidence for the presence of segregated ZnO, indicating the existence of Zn-rich nonstoichiometric Zn−Cr spinel in the calcined catalyst. The catalyst with Zn:Cr = 65:35 exhibits the best performance in methanol synthesis. The Zn:Cr ratio of this catalyst corresponds to that of the Zn 4 Cr 2 (OH) 12 CO 3 precursor with hydrotalcitelike structure obtained by coprecipitation, which is converted during calcination into a nonstoichiometric Zn−Cr spinel with an optimum amount of oxygen vacancies resulting in high activity in methanol synthesis. Density functional theory calculations are used to examine the formation of oxygen vacancies and to measure the reducibility of the methanol synthesis catalysts. Doping Cr into bulk and the (10−10) surface of ZnO does not enhance the reducibility of ZnO, confirming that Cr:ZnO cannot be the active phase. The (100) surface of the ZnCr 2 O 4 spinel has a favorable oxygen vacancy formation energy of 1.58 eV. Doping this surface with excess Zn charge-balanced by oxygen vacancies to give a 60% Zn content yields a catalyst composed of an amorphous ZnO layer supported on the spinel with high reducibility, confirming this as the active phase for the methanol synthesis catalyst.

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

“…In general, the Cu:Zn ratio as well as the synthesis route is chosen in such a way that an intimate contact between Cu and ZnO is achieved to maximize the number of Cu-Zn interface sites. A significant influence of pure ZnO sites under the conditions of the low-temperature methanol synthesis can be neglected 31 , and the absence of acidic OH groups on the structural promoter Al 2 O 3 is also excluded due to the absence of acid-catalysed products such as dimethyl ether (Supplementary Note 3) 4 . For Al 2 O 3 , it is generally accepted that it acts as physical spacer to enhance the lifetime as well as the surface area of the Cu/ZnO catalyst maintaining economical methanol production rates over several years 4 .…”

Design of a core–shell catalyst: an effective strategy for suppressing side reactions in syngas for direct selective conversion to light olefins

“…S1. † [44][45][46][47][48] In the reaction, the selectivity of methanol is as high as 92.58% at 300 C under 2.0 MPa (Table S2 †). In detail, CO is absorbed at the oxygen vacancies then dissociated and hydrogenated by H 2 gradually to CH 3 O, followed by further hydrogenation to methanol.…”

“…Simultaneously, the ratio of O defect /O lattice was obviously enhanced from 0.79 to 12.09, which indicates that the sites supplied by oxygen vacancies favour CO adsorption and continuous activation. [44][45][46][47][48] OH increased in the H 2 -activated Zn-Cr catalyst, which resulted in the adsorption of formed H 2 O on the catalyst surface from the process of Cr 3+ reduction. In general, with increasing syngas (CO/H 2 ) pressure and reaction temperature in the STO reaction, the oxygen vacancies concentration also increased, which was conducive for the release of surface oxygen on the Zn-Cr catalyst in an anaerobic environment.…”

Ceria‐Zirconia/Zeolite Bifunctional Catalyst for Highly Selective Conversion of Syngas into Aromatics

“…Zn−Cr mixed oxides has been applied in methanol synthesis from syngas, and the oxygen vacancies can be formed on surface of Zn−Cr spinel . Besides, the nonstoichiometric ZnCr 2 O 4 spinel with excess Zn has an improved activity toward C−O bond activation . But tuning the oxygen vacancy of Zn−Cr spinel is rarely reported.…”