A DFT Study of CO2 Hydrogenation on Faujasite-Supported Ir4 Clusters: on the Role of Water for Selectivity Control

Advanced zeolite and ordered mesoporous silica-based catalysts for the conversion of CO2 to chemicals and fuels

For many years, capturing, storing or sequestering CO₂ from concentrated emission sources or from air has been a powerful technique for reducing atmospheric CO₂. Moreover, the use of CO₂ as a C1 building block to mitigate CO₂ emissions and, at the same time, produce sustainable chemicals or fuels is a challenging and promising alternative to meet global demand for chemicals and energy. Hence, the chemical incorporation and conversion of CO₂ into valuable chemicals has received much attention in the last decade, since CO₂ is an abundant, inexpensive, nontoxic, nonflammable, and renewable one-carbon building block. Nevertheless, CO₂ is the most oxidized form of carbon, thermodynamically the most stable form and kinetically inert. Consequently, the chemical conversion of CO₂ requires highly reactive, rich-energy substrates, highly stable products to be formed or harder reaction conditions. The use of catalysts constitutes an important tool in the development of sustainable chemistry, since catalysts increase the rate of the reaction without modifying the overall standard Gibbs energy in the reaction. Therefore, special attention has been paid to catalysis, and in particular to heterogeneous catalysis because of its environmentally friendly and recyclable nature attributed to simple separation and recovery, as well as its applicability to continuous reactor operations. Focusing on heterogeneous catalysts, we decided to center on zeolite and ordered mesoporous materials due to their high thermal and chemical stability and versatility, which make them good candidates for the design and development of catalysts for CO₂ conversion. In the present review, we analyze the state of the art in the last 25 years and the potential opportunities for using zeolite and OMS (ordered mesoporous silica) based materials to convert CO₂ into valuable chemicals essential for our daily lives and fuels, and to pave the way towards reducing carbon footprint. In this review, we have compiled, to the best of our knowledge, the different reactions involving catalysts based on zeolites and OMS to convert CO₂ into cyclic and dialkyl carbonates, acyclic carbamates, 2-oxazolidones, carboxylic acids, methanol, dimethylether, methane, higher alcohols (C₂₊OH), C₂₊ (gasoline, olefins and aromatics), syngas (RWGS, dry reforming of methane and alcohols), olefins (oxidative dehydrogenation of alkanes) and simple fuels by photoreduction. The use of advanced zeolite and OMS-based materials, and the development of new processes and technologies should provide a new impulse to boost the conversion of CO₂ into chemicals and fuels.

Computational Study of the Adsorption of Phosphates as Wastewater Pollutant Molecules on Faujasites

The adsorption of sodium dihydrogen phosphate (NaH2PO4) onto X- and Y-type faujasite zeolites was computationally studied using the Density Functional Theory (DFT) method. The structures were modeled using the Materials Studio software. The Si/Al ratios for the X- and Y-type zeolite models were 1.2 and 2.5, respectively. The central pore of the zeolite provided a more favorable coordination for adsorbing NaH2PO4. Full molecular optimization and adsorption energy calculations were performed using the VASP code. The adsorption was more effective on zeolite Y, with an adsorption energy of 161 kJ/mol, compared to the zeolite X system, with an adsorption energy of 31.64 kJ/mol. This calculated value for X-type faujasite was found in the interval of the adsorption energy of H2PO4− on hydrated Fe oxide (94.4 kJ/mol) and modified polyether sulfone (22.5 kJ/mol), and the calculated adsorption energy of the molecule on Y-type faujasite coincides with the reported value for this adsorbate on Mg/Ca-modified biochar structures. The molecular conformations of the adsorbate on the two studied models are very different, so the difference between the adsorption energy values of each type of zeolite model is comprehensible. On the one hand, the oxygen atoms of the molecule formed a bidentate complex with the hydrogen atoms of the pore in the X-type faujasite model, and the O-H distance was 1.5 Ǻ. On the other hand, an adsorbed oxygen atom of the phosphate was placed on a hydrogen atom at site II of the Y-type faujasite zeolite, and two of the hydrogen atoms of the phosphate were placed on the oxygen atoms. The Bader analysis results indicated that the negative charge of the phosphate anions was delocalized on the zeolites protons. The hydroxy groups of the phosphate form bonds between their hydrogen atoms and the oxygen atoms of the zeolite porous structure; therefore, we concluded that these sites have an alkaline character. The aim of this study was to include a computational analysis of possible phosphate adsorption mechanisms in faujasite zeolites that can be confirmed by experimental tests, and hence contribute to the generation of new technologies for capturing pollutant molecules in wastewater. The results are in agreement with the experimental information concerning the influence of pH on the adsorption activity of phosphate adsorption on zeolites.

Screening of transition metal doped copper clusters for CO2 activation

Activation of CO₂ is the first step towards its reduction to more useful chemicals. Here we systematically investigate the CO₂ activation mechanism on Cu₃X (X is a first-row transition metal atom) using density functional theory computations. The CO₂ adsorption energies and the activation mechanisms depend strongly on the selected dopant. The dopant electronegativity, the HOMO-LUMO gap and the overlap of the frontier molecular orbitals control the CO₂ dissociation efficiency. Our calculations reveal that early transition metal-doped (Sc, Ti, V) clusters exhibit a high CO₂ adsorption energy, a low activation barrier for its dissociation, and a facile regeneration of the clusters. Thus, early transition metal-doped copper clusters, particularly Cu₃Sc, may be efficient catalysts for the carbon capture and utilization process.

The pathways of the CO2 hydrogenation by NiCu/ZnO from DFT molecular dynamics simulations

Metal nanoparticles supported on semiconductor surfaces have been proposed as promising nanocatalyst candidates of CO₂ conversion to energy carrier molecules such as formic acid or carbon monoxide, which can be used as a feedstock for fuels synthesis. This study is focused on the bimetallic Cu/Ni nanoparticles supported on the ZnO. The respective reaction mechanisms have been studied by means of the Molecular Dynamics with the DFT methodology. The results suggest that on CuNi/ZnO CO₂ hydrogenation to formate pathway is more favorable than carboxyl route. These pathways are competitive with the CO₂ reduction to CO.

Montmorillonite-catalyzed conversions of carbon dioxide to formic acid: Active site, competitive mechanisms, influence factors and origin of high catalytic efficiency

Design of heterogeneous catalysts for CO₂ conversions to value-added chemicals is highly desirable. Montmorillonite and other clay minerals have been used widely in catalytic reactions including CO₂ hydrogenation, while a molecular-level understanding remains lacking. In this study, periodic density functional theory calculations are employed and a comprehensive understanding about montmorillonite-catalyzed CO₂ hydrogenation to formic acid is given, including active site, mechanism, influence factors, competitive reaction paths, and origin of superior catalysis. Catechol that is readily available and can also be considered as a fragment of abundantly distributed humic substances is an effective hydrogen source. The penta-coordinated M³⁺ (M²⁺) sites of edge surfaces are active sites, and reactions occur preferentially at M²⁺ rather than M³⁺ sites. The catalytic activities depend strongly on the identity of M²⁺ (M³⁺) cations, and all reaction paths follow the concerted mechanisms transferring two hydrogen atoms in one step, with those producing formate being highly preferred. M²⁺/Al³⁺ substitutions and substituent effects are two critical factors to affect catalytic activities, and with synergy of Mg²⁺/Al³⁺ substitutions and -NMe₂ substituent, reactions are exergonic (-0.09 eV) and activation barriers are so low (0.48 eV) that formate can be facilely produced at ambient conditions. Edge surfaces of clay minerals are bifunctional catalysts, with M²⁺ cations showing Lewis acids and MOH groups playing similar effects as basic additives. Results provide new insights about heterogeneous catalysis of CO₂ hydrogenation and other reactions.

New trends in tailoring active sites in zeolite-based catalysts

This review discusses approaches for tailoring active sites in extra-large pore, nanocrystalline, and hierarchical zeolites and their performance in emerging catalytic applications.

A DFT Study of CO2 Hydrogenation on Faujasite-Supported Ir4 Clusters: on the Role of Water for Selectivity Control

Reaction mechanisms for the catalytic hydrogenation of CO₂ by faujasite‐supported Ir₄ clusters were studied by periodic DFT calculations. The reaction can proceed through two alternative paths. The thermodynamically favoured path results in the reduction of CO₂ to CO, whereas the other, kinetically preferred channel involves CO₂ hydrogenation to formic acid under water‐free conditions. Both paths are promoted by catalytic amounts of water confined inside the zeolite micropores with a stronger promotion effect for the reduction path. Co‐adsorbed water facilitates the cooperation between the zeolite Brønsted acid sites and Ir₄ cluster by opening low‐energy reaction channels for CO₂ conversion.