PtX technologies are one major building block of the future energy system based on renewables sources. Dimethyl ether (DME) is an important PtX product that can be used as intermediate the production of CO 2 -neutral base chemicals. New applications lead to an increase of the global production and the optimization of the process efficiency, especially when considering decentralized synthesis. This review article puts some spotlights on recent developments in methanol and the direct DME synthesis with a special focus on the modeling and bifunctional catalyst. This study is expected to provide a foundation for future works in the field of catalysis research based on catalysts design and kinetic modeling. the direct synthesis of Review Article: DME is a promising CO 2 -neutral energy carrier that can be efficiently produced via methanol from syngas streams derived from a variety of feedstocks such as non-fossil CO 2 resources, bio-waste and green H 2 provided through electrolysis using renewable electricity. This review considers recent developments with focus on modeling and bifunctional catalyst.

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Recent Progress in Direct DME Synthesis and Potential of Bifunctional Catalysts

Why ZnO is the Support for Cu in Methanol Synthesis? A Systematic Study of the Strong Metal Support Interactions

The synthesis of Mg‐substituted copper hydroxycarbonates by constant‐pH co‐precipitation with subsequent ageing of the precipitate has been studied in detail. This allowed the retrieval of crystalline magnesian malachite samples, which showed a “radical‐sign‐shaped” pH drop and the blue/green color change during ageing that is well‐known from the analogous Cu/Zn system. The crystallization of Cu‐rich samples has been studied elaborately by means of powder diffraction, pair distribution function (PDF), and infrared spectroscopy, thus elucidating the ageing chemistry of Cu,Mg hydroxycarbonates in general. It was found that up to 17 % Mg can be incorporated into the crystalline malachite using a synthesis route comparable to that established for Cu/ZnO catalysts, but with drastically elongated precipitate ageing times. For higher Mg contents, a step‐like pH drop was observed instead, and an amorphous variant of the hydroxycarbonate, referred to as “magnesian georgeite”, could be isolated. From such a Mg‐rich sample (Cu/Mg 70 : 30 at. %), a Cu/MgO catalyst has been prepared that shows high activity and good stability in CO hydrogenation to methanol over 800 hrs. time on stream.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cctc.202200299

CO Hydrogenation to Methanol over Cu/MgO Catalysts and Their Synthesis from Amorphous Magnesian Georgeite Precursors

This work reports initial results on the effect of low concentrations (ppm level) of a stabilizing agent (2,6-di-tert-butyl-4-methylphenol, BHT) present in an off-the-shelf solvent on the catalyst ...

Solvent Additive-Induced Deactivation of the Cu-ZnO(Al2O3)-Catalyzed γ-Butyrolactone Hydrogenolysis: A Rare Deactivation Process

The hydrogenation of CO2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.

Shape-Dependent CO2 Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO

We report on an inverse model Cu/MgO methanol catalyst modified with 5 % zinc oxide at the Cu surface to element‐specifically probe the interplay of metallic copper and zinc oxide during reductive activation. The structure of copper and zinc was unraveled by in situ X‐ray diffraction (XRD) and in situ X‐ray absorption spectroscopy (XAS) supported by theoretical modelling of the extended X‐ray absorption fine structure and X‐ray absorption near‐edge structure spectra. Temperature‐programmed reduction in H2 during in situ XAS showed that copper was reduced starting at 145 °C. With increasing reduction temperature, zinc underwent first a geometrical change in its structure, followed by reduction. The reduced zinc species were identified as surface alloy sites, which coexisted from 200 °C to 340 °C with ZnO species at the copper surface. At 400 °C Zn−Cu bulk‐alloyed particles were formed. According to in situ XRD and in situ XAS, about half of the ZnO was not fully reduced, which can be explained by a lack of contact with copper. Our experimental results were further substantiated by density functional theory calculations, which verified that ZnO with neighboring Cu atoms reduced more easily. By combining these results, the distribution, phase and oxidation state of Zn species on Cu were estimated for the activated state of this model catalyst. This insight into the interplay of Cu and Zn forms the basis for deeper understanding the active sites during methanol synthesis.

Unravelling the Zn-Cu Interaction during Activation of a Zn-promoted Cu/MgO Model Methanol Catalyst

The role of Cu:Co composition in bi-metallic Cu-Co/ZnAl2O4 catalysts on higher alcohol synthesis (HAS) was investigated at H2:CO = 4. The addition of Cu strongly facilitated Co reduction upon catalyst activation and suppressed coke deposition during HAS. Formation of predominantly hydrocarbons and higher alcohols was observed on the bi-metallic catalysts. Co/ZnAl2O4 produced mainly CH4 and Cu/ZnAl2O4 mainly CH3OH, while at Cu:Co = 0.6 the best ethanol selectivity of 4.5 % was reached. The microstructure of the spent catalysts confirmed a close interaction of Cu and Co. 

Cu‐Co/ZnAl
 2
 O
 4
 Catalysts for CO Conversion to Higher Alcohols Synthesized from Co‐Precipitated Hydrotalcite Precursors

The preparation of Al-doped ZnO via thermal decomposition of crystalline precursors, with a particular emphasis on kinetic effects on the solubility limits, was studied. The promoting effect of Al3+ on the catalyst system is discussed for methanol synthesis where ZnO:Al is employed as a support material for copper nanoparticles. The synthesis of the Al-doped zinc oxides in this study was inspired by the industrial synthesis of the methanol synthesis catalyst via a co-precipitated crystalline precursor, here: hydrozincite Zn5(OH)6(CO3)2. To determine the aluminium speciation and the solubility limit of the aluminium cation on zinc positions, a series of zinc oxides with varying aluminium contents was synthesized by calcination of the precursors. Short precipitate ageing time, low ageing temperature and aluminium contents below 3 mol% metal were advantageous to suppress crystalline side-phases in the precursor, which caused an aluminium segregation and non-uniform aluminium distribution in the solid. Even if zinc oxide was the only crystalline phase, TEM revealed such segregation in samples calcined at 320 °C. Only at very low aluminium contents, the dopant was found preferably on the zinc sites of the zinc oxide structure based on the signal dominating the 27Al NMR spectra. The solubility limit regarding this species was determined to be approximately xAl = 0.013 or 1.3% of all metal cations. Annealing experiments showed that aluminium was kinetically trapped on the site and segregated into ZnAl2O4 upon further heating. This shows that lower calcination temperatures such as applied in catalyst synthesis conserve a higher aluminium doping concentration on that specific site than is expected thermodynamically.

Phase evolution, speciation and solubility limit of aluminium doping in zinc oxide catalyst supports synthesized via co-precipitated hydrozincite precursors.

Benjamin Mockenhaupt

Papers

Unravelling the Zn-Cu Interaction during Activation of a Zn-promoted Cu/MgO Model Methanol Catalyst

CO Hydrogenation to Methanol over Cu/MgO Catalysts and Their Synthesis from Amorphous Magnesian Georgeite Precursors

Cu‐Co/ZnAl 2 O 4 Catalysts for CO Conversion to Higher Alcohols Synthesized from Co‐Precipitated Hydrotalcite Precursors

Phase evolution, speciation and solubility limit of aluminium doping in zinc oxide catalyst supports synthesized via co-precipitated hydrozincite precursors.