Methane C-H bond heterolysis versus homolysis on Cu-MFI and Au-MFI

The methane activation reaction is of fundamental importance for its transformation into high-value chemicals. Despite the fact that both homolysis and heterolysis are competitive mechanisms of C-H scission, experimental and DFT studies have revealed heterolytic scission of the C-H bond over metal-exchange zeolites. To rationalize the new catalysts, work must be done on the homolytic versus heterolytic scission of the C-H bond mechanism on these catalysts. We have performed the quantum mechanical calculations for the C-H bond homolysis versus heterolysis over Au-MFI and Cu-MFI catalysts. Calculations results showed that homolysis of the C-H bond is favorable both thermodynamically as well as kinetically over Au-MFI catalysts. However, over Cu-MFI, heterolytic scission is favorable. Both Cu(I) and Au(I) activate the CH₄ via electronic density back donation from filled nd¹⁰ orbitals, according to NBO calculations. Cu(I) cation has a higher electronic density back donation than Au(I) cation. This is also supported by the charge on the C-atom of Methane. Additionally, a greater negative charge on the O-atom in the active site in case of Cu(I), where proton transfer occurs, promotes heterolytic scission. Because of the larger size of the Au-atom and the smaller negative charge of the O-atom in the active site, where proton transfer occurs, C-H bond homolytic fission is preferable over Au-MFI.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

Zeolite chemistry and catalysis are expected to play a decisive role in the next decade(s) to build a more decentralized renewable feedstock-dependent sustainable society owing to the increased scrutiny over carbon emissions. Therefore, the lack of fundamental and mechanistic understanding of these processes is a critical "technical bottleneck" that must be eliminated to maximize economic value and minimize waste. We have identified, considering this objective, that the chemistry related to the first-generation reaction intermediates (i.e., carbocations, radicals, carbenes, ketenes, and carbanions) in zeolite chemistry and catalysis is highly underdeveloped or undervalued compared to other catalysis streams (e.g., homogeneous catalysis). This limitation can often be attributed to the technological restrictions to detect such "short-lived and highly reactive" intermediates at the interface (gas-solid/solid-liquid); however, the recent rise of sophisticated spectroscopic/analytical techniques (including under in situ/operando conditions) and modern data analysis methods collectively compete to unravel the impact of these organic intermediates. This comprehensive review summarizes the state-of-the-art first-generation organic reaction intermediates in zeolite chemistry and catalysis and evaluates their existing challenges and future prospects, to contribute significantly to the "circular carbon economy" initiatives.

Excess CO2 Reductions during CH3COOH Formation from CH4 and CO2 under Periodic Operation: Downhill Side Reactions in an Uphill Target Reaction under Unsteady Conditions

Acetic acid (CH3COOH) formation from methane (CH4) and carbon dioxide (CO2) is an ideal reaction for chemical production, whereas this reaction possesses a severe thermodynamic limitation. To address this issue, it has been reported that periodic operation allowing a non-equilibrium condition can overcome the thermodynamic limitation. However, although an intrinsic issue of uphill reactions in non-equilibrium conditions generally is occurrence of unfavorable downhill reactions, this issue has seldom been discussed for the CH3COOH formation under periodic operation. Herein, excess CO2 reductions were found to be the unfavorable downhill reactions possibly occurring in the reaction aiming at CH3COOH formation under periodically operated CH4 and CO2 feeds. The reaction using an isotopic reactant (i.e., 13CH4 ) unveiled that excess CO2 reductions to CO and even to CH3 moiety could occur, indicating importance of catalyst development. Furthermore, it was proposed that H2 O vapor introduction into the CO2 feed, which increased the CH3COOH product, most likely facilitated the reverse reaction of the excess CO2 reductions and thereby is effective to hamper the unfavorable side reaction.

Property-activity relations of multifunctional reactive ensembles in cation-exchanged zeolites: a case study of methane activation on Zn2+-modified zeolite BEA

The reactivity theories and characterization studies for metal-containing zeolites are often focused on probing the metal sites. We present a detailed computational study of the reactivity of Zn-modified BEA zeolite towards C-H bond activation of the methane molecule as a model system that highlights the importance of representing the active site as the whole reactive ensemble integrating the extra-framework Zn_EF²⁺ cations, framework oxygens (O_F^2-), and the confined space of the zeolite pores. We demonstrate that for our model system the relationship between the Lewis acidity, defined by the probe molecule adsorption energy, and the activation energy for methane C-H bond cleavage performs with a determination coefficient R² = 0.55. This suggests that the acid properties of the localized extra-framework cations can be used only for a rough assessment of the reactivity of the cations in the metal-containing zeolites. In turn, studying the relationship between the activation energy and pyrrole adsorption energy revealed a correlation, with R² = 0.80. This observation was accounted for by the similarity between the local geometries of the pyrrole adsorption complexes and the transition states for methane C-H bond cleavage. The inclusion of a simple descriptor for zeolite local confinement allows transferability of the obtained property-activity relations to other zeolite topologies. Our results demonstrate that the representation of the metal cationic species as a synergistically cooperating active site ensembles allows reliable detection of the relationship between the acid properties and reactivity of the metal cation in zeolite materials.

Molecular insight into the role of zeolite lattice constraints on methane activation over the Cu-O-Cu active site

Understanding the factors that influence the activity of a catalyst toward CH₄ activation is of high importance for tuning the catalyst performance or designing new, better catalysts. Here, we performed a set of density functional theory (DFT) calculations on the H-CH₃ bond cleavage over the Cu-O-Cu active site in the MOR zeolite with various Al-pair arrangements to obtain molecular insight into the structure-activity relation and clarify key parameters that define the Cu-O-Cu reactivity toward CH₄. We found that weakening of the Cu-O-Cu bond during CH₄ activation is crucial for determining the O-H bond strength and thus the Cu-O-Cu reactivity. In this regard, the zeolite lattice constraints are found to play a significant role as, on the one hand, it strengthens the Cu⋯Cu interaction and consequently weakens the Cu-O-Cu bonds and, on the other hand, it forces the Cu-O-Cu bond elongation process to destabilize the active site structure. The non-planar Cu-O-Cu geometry, due to lattice constraints, is also found to make the CH₄ adsorption site, whether positioned closer to the μ-O or the Cu atom, crucial in determining the C-H activation product, i.e., a ˙CH₃ radical or a Cu₂-CH₃^- ligand.

Material Evolution with Nanotechnology, Nanoarchitectonics, and Materials Informatics: What will be the Next Paradigm Shift in Nanoporous Materials?

Materials science and chemistry have played a central and significant role in advancing society. With the shift toward sustainable living, it is anticipated that the development of functional materials will continue to be vital for sustaining life on our planet. In the recent decades, rapid progress has been made in materials science and chemistry owing to the advances in experimental, analytical, and computational methods, thereby producing several novel and useful materials. However, most problems in material development are highly complex. Here, the best strategy for the development of functional materials via the implementation of three key concepts is discussed: nanotechnology as a game changer, nanoarchitectonics as an integrator, and materials informatics as a super-accelerator. Discussions from conceptual viewpoints and example recent developments, chiefly focused on nanoporous materials, are presented. It is anticipated that coupling these three strategies together will open advanced routes for the swift design and exploratory search of functional materials truly useful for solving real-world problems. These novel strategies will result in the evolution of nanoporous functional materials.

Synthesis and Photocatalytic Activity of Hierarchical Zn-ZSM-5 Structures

Hierarchical Zn-ZSM-5 photocatalyst structures were synthesized via a hydrothermal one-pot synthesis route using a double template. Activated attapulgite (Si-ATP) and zinc nitrate (Zn(NO3)2) precursors were used as silicon and zinc sources, respectively. The structural properties, morphology, photocatalytic activity and the texture properties of the synthesized Zn-ZSM-5 photocatalysts were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), diffracted ultraviolet–visible (UV–Vis) spectrometry (DRUV–Vis) and N2 adsorption/desorption, respectively. It was found that the composites exhibit a typical MFI framework structure, a hexahedral twin structure and typical UV absorption peaks at 292 nm and 246 nm, when the Zn/Si mole ratio reaches its optimum value of 1:100. The hierarchical nanocrystals exhibit a similar Brunauer–Emmett–Teller surface area (309 m2 g−1) and a high mesopore ratio (37.47%) as compared to commercial zeolites. Sub-nano-sized zinc oxide (ZnO) particles with small size moieties were implanted and isolated in the silica matrices of micro-mesoporous zeolite, which had a significant photocatalytic activity and reusability of degrading methylene blue (MB) dyeing wastewater. Using a 500 W mercury lamp with the wavelength range from 185–500 nm operating during an illumination time of 30 min, the concentration of MB decreases significantly in the presence of Zn-ZSM-5 photocatalyst leading to a 95.56% of degradation, where the ratio still remained at 94.32% after six times of reuse.