One-step dimethyl ether (DME) synthesis from CO2/H2 was investigated using physical mixtures of Hf-promoted CuZnO catalysts and FER. Methanol synthesis, the first step of DME production, was also studied using the same Cu-based catalysts. The catalysts were characterized by XRD with Rietveld refinement, XRF, specific area analysis, CO2-TPD, NH3-TPD, DRIFTS, N2O-RFC, and H2O dissociation measurements to evaluate oxygen vacancies. DFT calculations and thermodynamic analyses were also performed. Catalytic tests for methanol and DME synthesis under different reaction conditions, together with TG analysis, were conducted to investigate catalyst deactivation. Hf promotion significantly enhances catalytic performance by increasing both specific area and oxygen vacancy concentration through the substitution of Zn2⁺ by Hf4⁺ in the ZnO lattice. Consequently, methanol and DME yields increase up to an optimum Hf loading of 4 at.%, while the metallic Cu surface area remains essentially unchanged for Hf contents below 7 at.%. DFT calculations show that Hf substitution favors the spontaneous formation of oxygen vacancies in defective ZnO nanoparticles and on ZnO crystal surfaces. At higher Hf loadings, HfO2 formation blocks oxygen vacancies, active sites, and pores, leading to lower DME yield. Methanol synthesis reversibility under reaction conditions plays a decisive role in catalytic performance, favouring methanol and DME formation while suppressing CO. The proximity between methanol synthesis and dehydration sites enhances this effect. Furthermore, the acidity of the dehydration catalyst is an important factor controlling the DME formation rate. Catalyst deactivation is mainly attributed to H2O-promoted Cu sintering. Although CO is not an intermediate in methanol or DME synthesis, at high residence times CO produced by the reverse water–gas shift reaction can be re-adsorbed and hydrogenated to methanol and DME, allowing the system to approach thermodynamic equilibrium.

This study advances the understanding of oxygen vacancies (Vo) in heterogeneous catalysis, focusing on isobutene synthesis from ethanol. Ga-doped ZrO2 was investigated and compared with Zn-doped ZrO2. Characterization techniques, including EPR, XRD, XPS, H2O dissociation, and Raman spectroscopy, revealed that Ga substitutes Zr in the m-ZrO2 lattice, generating more Vo than Zn. DFT calculations supported these findings, showing that incorporating two Ga atoms into monoclinic ZrO2 leads to spontaneous defect formation, with energies more negative than those for Zn-doped ZrO2. The isobutene synthesis from ethanol involves three steps: the acetaldehyde generation from ethanol, acetone synthesis from this aldehyde, and isobutene formation from acetone, the rate-limiting step (rls). Catalytic tests and CO2-TPD showed that the Lewis basicity of the ZrO2-based catalysts is not relevant for the isobutene formation. Experiments with Ga-doped t-ZrO2 (tetragonal) highlighted the relevance of ZrO2 ' s structure in the isobutene synthesis. It was observed that Ga-doped catalysts generate much more isobutene from acetone when compared with Zn-based catalysts. Moreover, the isobutene yield shows a linear correlation with Ga at. %, and consequently, with the Vo concentration. The superior performance of Ga-doped ZrO2 compared to Zn-doped ZrO2 might be attributed not only to Ga's greater ability to generate Vo but also to the formation of a Ga(2)-Vo atomic ensemble, proposed by the DFT studies, which likely participates in the rls of this synthesis. This study offers new insights into the role of Vo in catalytic processes, providing perspectives for improving isobutene synthesis from ethanol and advancing the design of Vo-rich catalysts.

Cu, Zn, and Al-based catalysts were prepared, characterized, and tested in order to describe the role of Al in the CO2 hydrogenation to methanol. This work shows that Al3+ replaces Zn2+ in the ZnO lattice, promoting the generation of oxygen vacancies (Vo) on the oxide surface. When the Al and Vo concentration increases, the rate of methanol formation also increases. Once Al3+ solubility in ZnO is reached this rate decreases. An Al-based compound is formed which occludes a portion of the catalyst’s surface, changing the behavior of the catalyst. DFT calculations highlight the role of Al in methanol formation by lowering the energy required for the formation of Vo. This species promotes the adsorption of CO2 on the interface between ZnO and Cu0. Thus, both Al and Vo show a pivotal role in the catalytic behavior of Cu/ZnO/Al.

Synthesis of isobutene from ethanol employing Zn, Zr-based catalyst.

CO2 methanation – TPSR profiles of the Ni/ZrO2, Ni/Mg(Al)O, and Ni/SiO2 catalysts.

1 ZnZr/SiO 2 is composed of Zn, Zr isolated species, ZrO 2 nanoparticles and SiO 2 . 2 Ethanol dehydrogenation occurs on pairs of Zn isolated species and neighbor O. 3 Both Zr species promote the generation of butadiene from acetaldehyde. 4 The ZrO 2 nanoparticles do not show Zn in their monoclinic lattice.

The Ni/ZrO2 catalyst doped with Ca and Ni/ZrO2 were employed in the CO2 methanation, a reaction which will possibly be used for storing intermittent energy in the future. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS, reduction in situ), X-ray diffraction (XRD, reduction in situ and Rietveld refinement), electron paramagnetic resonance (EPR), temperature-programmed surface reaction, cyclohexane dehydrogenation model reaction, temperature-programmed desorption of CO2 and chemical analysis. The catalytic behavior of these catalysts in the CO2 methanation was analyzed employing a conventional catalytic test. Adding Ca to Ni/ZrO2, the metallic surface area did not change whereas the CO2 consumption rate almost tripled. The XRD, XPS and EPR analyses showed that Ca+2 but also some Ni2+ are on the ZrO2 surface lattice of the Ni/CaZrO2 catalyst. These cations form pairs which are composed of oxygen vacancies and coordinatively unsaturated sites (cus). By increasing the number of these pairs, the CO2 methanation rate increases. Moreover, the number of active sites of the CO2 methanation rate limiting step (CO and/or formate species decomposition, rls) is enhanced as well, showing that the rls occurs on the vacancies-cus sites pairs.

A physical mixture comprising In2O3 and ZrO2 and its components were employed as catalysts in the generation of isobutene from ethanol. These solids were characterized by means of several techniques such as EPR, HRTEM, XPS, XRD, N-2 physical adsorption, isopropanol-TPD, CO2-TPD, ethanol-TPD and pyridine adsorption. The addition of 7% of In2O3 to ZrO2 employing the physical mixture procedure generates a catalyst which shows physicochemical and catalytic properties which are not a linear combination of its components properties. This catalyst is not only much more active compared with In2O3 and ZrO2, but it also generates isobutene at high selectivity. The interaction between In2O3 and ZrO2 changes the quantity and strength of the acidic and basic sites. However, these modifications do not correlate with the catalytic behavior. The HRTEM data suggest that the epitaxy alignment occurs between these two oxides which might cause stresses in the In2O3 lattice. The XRD and XPS analysis have identified distortions in the In2O3 lattice provoked by ZrO2, which facilitate the generation of O vacancies and consequently increases the concentration of these species during the reaction. This phenomenon promotes the acetone and acetaldehyde syntheses which are very relevant intermediate of this cascade reaction. Thus, it can be suggested that the catalytic behavior of the modified physical mixture is associated with the In2O3 lattice distortions promoted by ZrO2.

The performances of In2O3/ZrO2, ZrO2 and In2O3 were evaluated in the synthesis of isobutene from ethanol in one‐step. These catalysts were characterized by XRD, EPR, TPD‐H2O, TPD‐NH3, TPD‐ethanol and pyridine adsorption. The In2O3/ZrO2 catalyst is a promising catalytic system for the synthesis of isobutene from ethanol. The catalytic tests showed that acetone, hydrogen and carbon dioxide are the main co‐products of this synthesis. The In2O3/ZrO2 catalyst can promote the synthesis of isobutene due to the inter‐diffusion process between In2O3 and ZrO2. This phenomenon generates, among others, the Zr insertion and a high concentration of anionic vacancies in the In2O3 lattice. This increases the In2O3/ZrO2 redox properties leading to a high activity of this catalyst for the acetone generation, a key intermediate of the isobutene synthesis. These changes also promote the acid and basic properties of In2O3/ZrO2, which are in charge of the acetaldehyde synthesis, acetone condensation and, finally, dehydration of the intermediates of the isobutene synthesis.

The t-ZrO2 doped with Zn catalyst and t-ZrO2 as reference were employed in the butadiene synthesis from ethanol. Both catalysts were characterized by NH3-TPD, CO2-TPD, TPSR, the MPV model reaction, ICP, BET and EPR. Adding 0.2 wt% Zn to t-ZrO2, the selectivity to butadiene increases three fold whereas the one to ethylene decreases. When ZrO2 is doped, the number of basic sites increases and the number of acid sites decreases. The TPSR spectra indicate that the acetaldehyde generation is the rate limiting step of the butadiene synthesis. The slowest step of the acetaldehyde generation is the H abstraction by a strong basic site. The EPR spectra show the replacement of Zr4+ by Zn2+ in the lattice of the t-ZrO2 oxide(.) This phenomenon forms pairs of oxygen vacancies and coordinatively unsaturated Zr4+ ions (cus), which are strong basic sites and acid sites, respectively. Doping ZrO2 with Zn, the ethanol dehydrogenation and the butadiene synthesis are promoted not only due to the changes in the acidity and basicity of the catalyst but mainly because of the generation of oxygen vacancies and cus pairs during the reaction. These oxygen vacancies seem to behave as strong Bronsted basic sites.

Two catalysts, Cu/ZnO/Al2O3 and Cu/ZrO2/Al2O3, were employed for acetic acid synthesis from ethanol and water. They were characterized by X-ray fluorescence, N-2 physisorption, N2O titration, in situ X-ray diffraction refined by the Rietveld method, in situ X-ray photoelectron spectroscopy, temperature-programmed reduction followed by X-ray absorption near-edge structure, and temperature-programmed desorption of ethanol, H2O, CO2, and NH3. The Zn-based catalyst is composed of ZnO nanoparticles, Cu-0, CuZn alloy, and Al2O3, whereas the Cu/ZrO2/Al2O3 is composed of Cu-0, ZrO2, and Al2O3. Both catalysts show the same Cu-0 metallic surface area. On one hand, the spectra of the H20 temperature-programmed desorption show that the ZnO nanoparticles of Cu/ZnO/Al2O3 promote water dissociation and, consequently, the redox properties of this catalyst. On the other hand, the Zr-based catalyst shows low activity for H2O dissociation and higher acidity and basicity than for the Zn-based catalyst. These different properties lead to different mechanisms for acetic acid synthesis from ethanol. Taking the redox mechanism of the WGS reaction into account, the steps of the acetic acid synthesis promoted by the Zn-based catalyst can be described as follows: first, ethanol is dehydrogenated alloy to acetaldehyde on Cu-0 and CuZn; then, this aldehyde is oxidized on the ZnO-Cu-0 interface to acetate, which desorbs, forming acetic acid; finally, H2O dissociates on the O vacancies of the ZnO-Cu-0 interface and reoxidizes the oxide. The ZnO-CuZn alloy interface should also be considered in this mechanism. The second mechanism, which occurs mainly on the Zr-based catalyst, is related to the acetaldehyde and ethanol condensation forming ethyl acetate. This ester is hydrolyzed, synthesizing acetic acid and ethanol. (C) 2019 Elsevier Inc. All rights reserved.

The synthesis of 1,3-butadiene (BD) from ethanol is a complex reaction system in which several steps are involved. The use of zirconia-based catalysts as well as physical mixtures containing Cu/ZnO/Al2O3 (CZA) and t-ZrO2 were evaluated in order to better understand this reaction. The catalysts employed were characterized by the following techniques: TPSR-IR, NH3-TPD, CO2-TPD, XRD, SEM/EDS and BET. Moreover, the isopropanol conversion and the acetone and ethanol reaction (Meerwein-Ponndorf-Verley mechanism (MPV)) were used as model reactions to describe the catalysts behavior. It was verified that doping oxides with metals is a very simple procedure in order to change the properties of the catalysts so as to reach the requirements of the butadiene synthesis. Doped t-ZrO2-based catalysts are promising systems for the BD generation from ethanol. The addition of Na and Ag to t-ZrO2 promotes the selectivity towards BD and decreases the selectivity to ethylene. The catalyst with Ag is more active than the one with Na. It was observed that employing physical mixtures, the dehydrogenation rate of ethanol should be consistent with the activity of the catalyst toward the aldol condensation and the MPV step. The physical mixture with low concentration of CZA generates high BD yield. The ZrO2 catalytic behavior in the MPV step, and also in the BD synthesis changes according to the crystalline structure of this oxide.

The main purpose of this work is to contribute to the description of the acetone synthesis from ethanol employing ZnxZr1-xO2-(y) based catalysts. The catalytic behavior of these solids was evaluated (isoconversion) in the acetone synthesis. The most active catalyst and m-ZrO2 (used as a reference) were characterized by the following techniques: pyridine adsorption, TPD of NH3, TPD of CO2, TPD of ethanol followed by IR (DRIFTS)/MS, TPD of ethanol followed by XANES at the Zr K-edge and Zn K-edge and XRD in situ. The present study suggests that the main steps of the acetone generation from ethanol are the following: firstly, ethoxide species are generated and, then, they are dehydrogenated to acetaldehyde. Both steps are related to strong basic and acid sites. Acetaldehyde reacts with the O of the solid solution generating acetate species and vacancies on the catalyst surface. These carboxylate species condensate (strong basic sites) and generate acetone and CO2. Water dissociates on the vacancies of the catalyst and reoxidizes the its surface, closing the catalytic cycle. All these steps might occur on Zn+2 and on the species in its vicinity (XANES). (C) 2016 Elsevier B.V. All rights reserved.

Acetone is an important solvent and widely used in the synthesis of drugs and polymers. Currently, acetone is mainly generated by the Cumene Process, which employs benzene and propylene as fossil raw materials. Phenol is a co-product of this synthesis. However, this ketone can be generated from ethanol (a renewable feedstock) in one-step. The aim of this work is to describe the influence of physical-chemical properties of three different catalysts on each step of this reaction. Furthermore, contribute to improve the description of the mechanism of this synthesis. The acetone synthesis from ethanol was studied employing Cu/ZnO/Al2O3, Ce0.75Zr0.25O2 and ZrO2. It was verified that the acidity of the catalysts needs fine-tuning in order to promote the oxygenate species adsorption and avoid the dehydration of ethanol. The higher the reducibility and the H2O dissociation activity of the catalysts are, the higher the selectivity to acetone is. In relation to the oxides, these properties are associated with the presence of O vacancies. The H2 generation, which occurs during the TPSR, indicates the redox character of this synthesis. The main steps of the acetone synthesis from ethanol are the generation of acetaldehyde, the oxidation of this aldehyde to acetate species (which reduces the catalyst), the H2O dissociation, the oxidation of the catalyst producing H2, and, finally, the ketonization reaction. These pieces of information will support the development of active catalysts for not only the acetone synthesis from ethanol, but also the isobutene and propylene syntheses in which this ketone is an intermediate. Graphical abstract Acetone from ethanol.

The PdO and m-ZrO2 oxides show different catalytic properties which are not only complementary but also relevant for the ethanol selective oxidation. Due to the spillover of the oxygenated intermediates from PdO to m-ZrO2 and vice versa these compounds are able to generate a promising system for ethanol oxidation to acetic acid.

The performance of Ni supported on Mg(Al)O catalysts was evaluated in the RWGS reaction. The catalysts were characterized by TPR, EPR, TPSR, XRD, XPS, cyclohexane dehydrogenation reaction, atomic absorption and N-2 physisorption. Two different catalytic systems were prepared by impregnation using water or ethanol as solvents of the Ni precursor. When ethanol was employed, Ni diffuses into the Mg(Al)O particles generating a homogeneous distribution of this metal through the catalyst particle leading to a Mg(Al,Ni)O solid solution, a very small amount of Ni and NiAl2O4. On the other hand, hydrotalcite is generated on the external layers of Mg(Al)O when water is the solvent. Nickel is trapped in the hydrotalcite structure. After calcination and reduction its concentration on the catalyst surface is higher than the one in the bulk. Both Mg(Al,Ni)O and Ni are observed on the catalyst surface. Oxygen vacancies were also observed for the two catalysts via EPR analyses due to the replacement of Mg by Al in the MgO lattice. The Mg(Al,Ni)O vacancies are active species for the reduction of CO2 to CO. These species and Ni are the catalytic sites of the RWGS reaction. Increasing the Ni concentration on the catalysts surface the activity of the catalysts increases as well. As far as we are concerned, this is the first time that Mg(Al)O or Mg(Al,Ni)O vacancies are associated with the performance of a catalytic system. This work exhibits that Ni/Mg(Al,Ni)O is a promising catalytic system for the RWGS reaction.

In order to improve the redox properties of m-ZrO2, three different amounts of Zn were added to this oxide using a simple and effective technique. The XRD, XPS and Raman techniques showed that Zn did not form a ZnO phase but instead it diffused inside the first layers of m-ZrO2 lattice. The XRD, EPR and also TPD of H2O results indicated the formation of oxygen vacancies when Zn was added to m-ZrO2. These results showed that Zn replaces Zr in the m-ZrO2 lattice forming a surface solid solution (ZnxZr1−xO2−y). The TPR-H2 experiments exhibited that as the Zn concentration increased, the H2 consumption raised as well. The TPD of ethanol depicted that Zn catalysts were able to oxidize ethanol whereas m-ZrO2 was not. These results are associated with the higher oxygen mobility and reducibility of these samples compared with m-ZrO2. Finally, it was verified that increasing the Zn concentration in the catalysts, increases the CO2 conversion rate in the RWGS reaction, as well. Thus, the formation of the ZnxZr1−xO2−y surface solid solution generates O vacancies and improves the reducibility and O mobility of m-ZrO2. Adding Zn to m-ZrO2 changes the redox properties of the oxide

The catalytic behavior of CeO2 and this oxide doped with Ag (AgCeO2) were evaluated for the ethanol reaction to acetone in order to assess the mechanism proposed by Rodrigues et al. (2013). The oxides were characterized by BET, ICP-OES,TPD-NH3, isopropanol dehydration/dehydrogenation model reaction, TPR-H-2, TPD-H2O, WGS reaction and TPD-ethanol followed by DRIFTS-MS. The main effect of doping CeO2 with Ag is the increase of the reducibility of this oxide which leads to a higher selectivity to acetone and a lower towards ethylene. The acetone synthesis reduces the catalyst. Ceria dissociates H2O, which provides oxidant species for its own re-oxidation. Acetaldehyde is generated by the ODH reaction. The WGS reaction is a very interesting model for the redox reactions when H2O is employed as an oxidant agent. The mechanism proposed by Rodrigues et al. (2013) describes the behavior of CeO2 in the acetone synthesis from ethanol. (C) 2016 Elsevier B.V. All rights reserved.