Structure–Activity Relationships in Alkane Dehydrogenation on γ-Al2O3: Site-Dependent Reactions

DFT and AIMD insights into heterogeneous dissociation of 2-chlorothiophenol on CuO(111) surface: Impact of H2O and OH

Copper oxides are vital catalysts in facilitating the formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) through heterogeneous reactions in high-temperature industrial processes. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. The initial step in this process is the metal-catalyzed production of chlorothiophenoxy radicals (CTPRs) from CTPs via dissociation reactions. This work combines density functional theory (DFT) calculations with ab initio molecular dynamics (AIMD) simulations to explore the formation mechanism of the adsorbed 2-CTPR from 2-CTP, with the assistance of CuO(111). Our study demonstrates that flat adsorption configurations of 2-CTP on the CuO(111) surface are more stable than vertical configurations. The CuO(111) surface acts as a strong catalyst, facilitating the dissociation of 2-CTP into the adsorbed 2-CTPR. Surface oxygen vacancies enhance the adsorption of 2-CTP on the CuO(111) surface, while moderately suppressing the dissociation of 2-CTP. More importantly, water molecules and surface hydroxyl groups actively promote the dissociation of 2-CTP. Specifically, water directly participates in the reaction through "water bridge", enabling a barrier-free process. This research provides molecular-level insights into the heterogeneous generation of dioxins with the catalysis of metal oxides in fly ash from static and dynamic aspects, providing novel approaches for reducing dioxin emissions and establishing dioxin control strategies.

Catalytic activity of γ-Al2O3(110) in the NO + H2 reaction: a DFT study

In this work, the interaction of the surface of γ-Al2O3(110) with NO and H2 was studied using density functional theory calculations. Free γ-Al2O3(110) adsorbs NO and binds H atoms, but repels the H2 molecule. A triplet of low-coordinated OII-AlIII-OII atoms provides the catalytic activity of γ-Al2O3(110) along the path: (i) the adsorption of NO/AlIII is followed by the binding of H2 to form a hydroxylamine derivative NHOH through an intermediate NO/AlIII + 2 × H/OII complex; (ii) recombination of NHOH with the release of N2 through an intermediate NHOH/AlIII + NHOH/AlIV or adsorption of NO followed by the release of N2O through the intermediate NHOH/AlIII + NO/AlIV; the pathway ends with the regeneration of γ-Al2O3(110). The calculated adsorption heats ensure the diffusion of H atoms from the deposited Pt to the surface (110), initiating the formation of the NH2/AlIII + H/OII complex, which releases NH3 endothermically and is stable enough to inhibit stage (ii) of the above reaction pathway. An excess of O2 in the NO + H2 mixture excludes H/Pt and eliminates inhibition. The formation of oxynitrides is suppressed, but not excluded by more exothermic surface processes. The N-doped conductivity of bulk and surface oxynitrides Al32O47N and the dependence of the heat of adsorption of H atoms on the band gap width were revealed, which suggests a relationship between the band gap width and catalytic activity.

Iterative multiscale and multi-physics computations for operando catalyst nanostructure elucidation and kinetic modeling

Modern heterogeneous catalysis has benefitted immensely from computational predictions of catalyst structure and its evolution under reaction conditions, first-principles mechanistic investigations, and detailed kinetic modeling, which are rungs on a multiscale workflow. Establishing connections across these rungs and integration with experiments have been challenging. Here, operando catalyst structure prediction techniques using density functional theory simulations and ab initio thermodynamics calculations, molecular dynamics, and machine learning techniques are presented. Surface structure characterization by computational spectroscopic and machine learning techniques is then discussed. Hierarchical approaches in kinetic parameter estimation involving semi-empirical, data-driven, and first-principles calculations and detailed kinetic modeling via mean-field microkinetic modeling and kinetic Monte Carlo simulations are discussed along with methods and the need for uncertainty quantification. With these as the background, this article proposes a bottom-up hierarchical and closed loop modeling framework incorporating consistency checks and iterative refinements at each level and across levels.

Assessing the Effect of Dopants on the C-H Activation Activity of γ-Al2 O3 using First-Principles Calculations

In recent years, the high availability of methane in the shale gas reserves has raised significant interest in its conversion to high-value chemicals but this process is still not commercially viable. Metal oxides, due to their surface heterogeneity and the presence of Lewis acidic and basic site pairs are known to facilitate the activation of C-H bonds of methane. In this work, we investigate the C-H bond activation of methane on pristine and doped γ-Al₂ O₃ clusters using density functional theory (DFT) calculations. Our results demonstrate that the polar pathway is energetically preferred over the radical pathway on these systems. We found that the metal dopants (boron and gallium) not only alter the catalytic activity of dopant sites but this effect is more pronounced on some of the adjacent sites (non-local). Among the selected dopants, gallium greatly improves the catalytic activity on most of the site pairs (including most active and least active) of pristine γ-Al₂ O₃ . Additionally, we identified a correlation between H₂ binding energies and the C-H activation free energies on Ga-doped γ-Al₂ O₃ .

Increased Light Olefin Production by Sequential Dehydrogenation and Cracking Reactions

In this study, a sequential reaction using selected metal oxides, followed by ZSM-5-based catalysts, was employed to demonstrate a promising route for enhancing light olefin production in the catalytic cracking of naphtha. The rationale for the reaction is based on the induction of alkenes into hydrocarbon feeds prior to cracking. The optimum olefin induction was achieved by carefully optimizing the dehydrogenation active sites Mo/Al2O3 catalyst. The formed alkenes have a lower activation energy for C-H/C-C bond breaking compared to alkanes. This could accelerate the formation of carbenium ions, thus promoting the conversion of n-octane to produce light olefins. Detailed product distribution and DFT calculation indicated a remarkable increase in ethylene and propylene production in the final product through a modified reaction pathway. Compared with the common metal-promoted zeolite catalysts, the new route could avoid the block of zeolite channels and corresponding decreased catalytic cracking activity. The feasibility of the proposed route was confirmed with different ratios of dehydrogenation catalyst to the reactant. The highest yields of ethylene and propylene reached 13.22% and 33.12% with ratios of Mo/Al2O3 and ZSM-5-based catalyst to n-octane both 10:1 at 600 °C. Stability tests showed that the catalytic activity of the double-bed system was stable over 10 cycles.

The carbon chain growth during the onset of CVD graphene formation on γ-Al2O3 is promoted by unsaturated CH2 ends

Chemical vapor deposition of methane onto a template of alumina (Al₂O₃) nanoparticles is a prominent synthetic strategy of graphene meso-sponge, a new class of nano porous carbon materials consisting of single-layer graphene walls. However, the elementary steps controlling the early stages of graphene growth on Al₂O₃ surfaces are still not well understood. In this study, density functional theory calculations provide insights into the initial stages of graphene growth. We have modelled the mechanism of CH₄ dissociation on the (111), (110), (100), and (001) γ-Al₂O₃ surfaces. Subsequently, we have considered the reaction pathway leading to the formation of a C6 ring. The γ-Al₂O₃(110) and γ-Al₂O₃(100) are both active for CH₄ dissociation, but the (100) surface has higher catalytic activity towards the carbon growth reaction. The overall mechanism involves the formation of the reactive intermediate CH₂* that then can couple to form C_nH_2n* (n = 2-6) intermediates with unsaturated CH₂ ends. The formation of these species, which are not bound to the surface-active sites, promotes the sustained carbon growth in a nearly barrierless process. Also, the short distance between terminal carbon atoms leads to strong interactions, which might lead to the high activity between unsaturated CH₂* of the hydrocarbon chain. Analysis of the electron localization and geometries of the carbon chains reveals the formation of C-Al-σ bonds with the chain growing towards the vacuum rather than C-Al-π bonds covering the γ-Al₂O₃(100) surface. This growth behaviour prevents catalyst poisoning during the initial stage of graphene nucleation.

Significance of Ionic Character Induced by Ga-Doped γ-Al2O3 on Polyethylene Degradation to the Precursors of Gasoline and Diesel Oil with a Trace Amount of Wax

Polyethylene degradation has a significant ecological impact but is also economically beneficial because it generates fuels and useful chemical products. Our study mainly describes the cleavage of C-C and C-H bonds when polyethylene (dispersed in 1-octadecene) was low-temperature heat-treated in two steps, at 180 and 250 °C, for 24 h for each step. Finally, it was converted to a mixture of the precursors of gasoline and diesel oil with a trace amount of wax. A series of reactions resulted in cracking, dehydrogenation and oxidation, hence producing polycarboxylic acids and saturated and unsaturated hydrocarbons. ESI-MS analysis revealed that mixed oil consisted of low carbon number hydrocarbons and their derivatives of carboxylic acids, with the carbon number ranging from C-6 to C-18. In the trace amount of wax, complicated carboxylic acids and hydrocarbons with carbon number C-22 to C-58 were also identified. FT-IR analysis further confirmed the presence of carboxylic acid derivatives and double bonds in the degradation products. γ-Al₂O₃ nanorods effectively catalyzed the degradation process by enhancing the C-C chain length in the products. Lewis acid (Al) and Lewis base (oxygen) in the γ-Al₂O₃ induced ionic character of the C-C bond chain, which led to the efficient cracking of the C-C bond. Poor shielding effect, smaller atomic size and greater ionization energy made Ga a stronger Lewis acid compared to Al; hence, Ga-doped γ-Al₂O₃ catalyzed the degradation process even more effectively.

Unraveling the Mechanistic Details of Ru-Bis(pyridyl)borate Complex Catalyst for the Dehydrogenation of Ammonia Borane

Ru-Bis(pyridyl)borate complex (CAT) is an efficient catalyst for ammonia borane (AB) dehydrogenation. Although the mechanistic pathway of this catalyst has been theoretically investigated previously, the gap between the experimental findings and the computational results could not be bridged thus far. In our study, using density functional theory calculations, we elucidate the mechanism of AB dehydrogenation of CAT at a variable degree of ligand hydrogenation. Our results confirm that the acetonitrile ligands get reduced in the presence of AB and remain hydrogenated. Moreover, in line with experiments, we find that AB dehydrogenation on CAT proceeds via a concerted mechanism (with the free energy energetic span between 25.4 and 32.5 kcal/mol). We find that the ligand reduction alters the electronic structure and activity of CAT and the highest activity of the catalyst is expected at the fifth degree of hydrogenation of ligands with an energetic span of 25.4 kcal/mol. Additionally, the mechanism for the removal of molecular H₂ from the catalysts also alters with the degree of ligand hydrogenation. Furthermore, our results show that optimal H₂ binding free energy calculations can be used as a descriptor to identify the most active sites. Finally, this work demonstrates that ligand reduction improves the activity of the catalyst. These results highlight the importance of ligand hydrogenation in probing the activity and operating mechanism of the Ru-bis(pyridyl)borate complexes for AB dehydrogenation. Further, we identify a plausible dimer structure and rationalized experimental observation that the deactivation chemistry of this catalyst is different from the Shvo's catalyst.

Uncovering selective and active Ga surface sites in gallia-alumina mixed-oxide propane dehydrogenation catalysts by dynamic nuclear polarization surface enhanced NMR spectroscopy

Gallia–alumina (Ga,Al)₂O_3(x : y) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as ²⁷Al and ⁷¹Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning ¹⁵N{²⁷Al} and ¹⁵N{⁷¹Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using ¹⁵N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al)₂O₃ oxides. Notably, we demonstrate that (Ga,Al)₂O₃ catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al)₂O₃ catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance.

Coordination geometry and Lewis acidity of Ga and Al (bulk and surface) sites in mixed oxide gallia–alumina nanoparticles is correlated with the performance in propane dehydrogenation.

Hydride species on oxide catalysts

Hydride species on oxide catalysts are widely involved in oxide-catalyzed reactions, and relevant fundamental understanding is important to establish reaction mechanisms and structure-performance relations of oxide catalysts. In this topical review, recent progresses on the formation and reactivity of hydride species on the surface or in the bulk of oxides are briefly summarized. Firstly, characterization techniques for hydride species are introduced. Secondly, formation of hydride species on the surface or in the bulk of various oxides and their reactivity in oxide-catalyzed hydrogenation and dehydrogenation reactions are reviewed. Finally, short summary and outlook are given.

Dehydrogenation of light alkanes to mono-olefins

In the past several decades, light alkane dehydrogenation to mono-olefins, especially propane dehydrogenation to propylene has gained widespread attention and much development in the field of research and commercial application. Under suitable conditions, the supported Pt-Sn and CrO_x catalysts widely used in industry exhibit satisfactory dehydrogenation activity and selectivity. However, the high cost of Pt and the potential environmental problems of CrO_x have driven researchers to improve the coking and sintering resistance of Pt catalysts, and to find new non-noble metal and environment-friendly catalysts. As for the development of the reactor, it should be noted that low operation pressure is beneficial for improving the single-pass conversion, decreasing the amount of unconverted alkane recycled back to the reactor, and reducing the energy consumption of the whole process. Therefore, the research direction of reactor improvement is towards reducing the pressure drop. This review is aimed at introducing the characteristics of the dehydrogenation reaction, the progress made in the development of catalysts and reactors, and a new understanding of reaction mechanism as well as its guiding role in the development of catalyst and reactor.

Nb2O5 as a radical modulator during oxidative dehydrogenation and as a Lewis acid promoter in CO2 assisted dehydrogenation of octane over confined 2D engineered NiO–Nb2O5–Al2O3

Ordered mesoporous 2D NiO–Nb₂O₅–Al₂O₃ nano-composites were used for CO₂ assisted dehydrogenation of n-octane; and the close proximity of Ni and Nb₂O₅ in the optimised catalyst promoted CO₂ dissociation and substantially prolonged alkane activation.

Revealing fundamentals affecting activity and product selectivity in non-oxidative propane dehydrogenation over bare Al2O3

A detailed study was carried out to elucidate the factors affecting the activity and, particularly, selectivity of bare Al₂O₃ in the non-oxidative propane dehydrogenation (PDH) to propene under industrially relevant conditions.

Mechanistic understanding of methane-to-methanol conversion on graphene-stabilized single-atom iron centers

DFT calculations and kinetic modeling elucidate solvent effects and complex mechanisms for the room-temperature methane-to-methanol conversion on an FeN4/graphene catalyst.

Current status and perspectives in oxidative, non-oxidative and CO2-mediated dehydrogenation of propane and isobutane over metal oxide catalysts

Conversion of propane or isobutane from natural/shale gas into propene or isobutene, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes.

Predicting the Energetics of Hydrogen Intercalation in Metal Oxides Using Acid-Base Properties

The ability to predict intercalation energetics from first principles is attractive for identifying candidate materials for energy storage, chemical sensing, and catalysis. In this work, we introduce a computational framework that can be used to predict the thermodynamics of hydrogen intercalation in tungsten trioxide (WO₃). Specifically, using density functional theory (DFT), we investigated intercalation energetics as a function of adsorption site and hydrogen stoichiometry. Site-specific acid-base properties determined using DFT were used to develop linear structure screening models that informed a kernel ridge energy prediction model. These regressions provided a series of hydrogen binding energy predictions across stoichiometries ranging from WO₃ to H_0.625WO₃, which were then converted to equilibrium potentials for hydrogen intercalation. Experimental validation using cyclic voltammetry measurements yielded good agreement with the predicted intercalation potentials. This methodology enables fast exploration of a large geometric configuration space and reveals an intuitive physical relationship between acidity, basicity, and the thermodynamics of hydrogen intercalation. Furthermore, the combination of theoretical and experimental results suggests H_0.500WO₃ as a maximum stable stoichiometry for the bronzes that arises from competition with hydrogen evolution rather than the inability of WO₃ to accommodate additional hydrogen. Our experimental results further indicate hydrogen insertion in WO₃ is highly irreversible for low H-stoichiometries, which we propose to be a consequence of the semiconductor-to-metal transition that occurs upon initial H-intercalation. Overall, the agreement between theory and experiment suggests that local acid-base characteristics govern hydrogen intercalation in tungsten trioxide, and this insight can aid the accelerated discovery of redox-active metal oxides for catalytic hydrogenations.

C-H activation of light alkanes on MXenes predicted by hydrogen affinity

C-H activation of light alkanes is one of the most important reactions for a plethora of applications but requires catalysts to operate at feasible conditions. MXenes, a new group of two-dimensional materials, have shown great promise as heterogeneous catalysts for several applications. However, the catalytic activity of MXenes depends on the type and distribution of termination groups. Theoretically, it is desired to search for a relation between the catalytic activity and the termination configuration by employing a simple descriptor in order to avoid tedious activation energy calculations. Here, we show that MXenes are promising for splitting C-H bonds of light alkanes. Furthermore, we present how a quantitative descriptor - the hydrogen affinity - can be used to characterize the termination configuration of Ti₂CT_z (T = O, OH) MXenes, as well as the catalytic activity towards dehydrogenation reactions, using propane as model system. First-principles calculations reveal that the hydrogen affinity can be considered as an intrinsic property of O and OH terminated Ti₂C MXenes, in which the mean hydrogen affinity for the terminated Ti₂C MXenes is linearly correlated to the statistical average of their OH fraction. In addition, the C-H activation energies exhibit a strong scaling relationship to the hydrogen affinity. This quantity can therefore yield quick predictions of catalytic activity of terminated Ti₂C MXenes towards C-H activations, and even predict their chemical selectivity toward scissoring different C-H bonds. We believe that the hydrogen affinity will accelerate the discovery of further applications of the broad family of MXenes in heterogeneous catalysis.

Iron doping boosts the reactivity and stability of the γ-Al2O3 nanosheet supported cobalt catalyst for propane dehydrogenation

This study describes a new iron-doping strategy to improve both the reactivity and stability of a cobalt catalyst in propane dehydrogenation, meanwhile, the defective γ-Al₂O₃ nanosheet synergistically boosted the dehydrogenating activity of that.

Unmodified bulk alumina as an efficient catalyst for propane dehydrogenation

A series of unmodified bulk Al₂O₃ materials show an intrinsic activity for selectively catalyzing propane dehydrogenation.

Improving alkane dehydrogenation activity on γ-Al2O3 through Ga doping

Doping the surface of γ-Al₂O₃ with gallium enhances the alkane dehydrogenation catalytic activity.

Cyclic redox scheme towards shale gas reforming: a review and perspectives

Alkanes are potential precursors to many value-added chemicals such as olefins and other petrochemicals.

Facile synthesis of ordered mesoporous zinc alumina catalysts and their dehydrogenation behavior

Ordered mesoporous Zn/Al2O3 materials with varying Zn content were simply prepared via an evaporation-induced self-assembly (EISA) method. Dehydrogenation of isobutane to isobutene was carried out on these materials; an isobutane conversion of 45.0% and isobutene yield of 39.0% were obtained over the 10%Zn/Al2O3 catalyst at 580 °C with 300 h-1 GHSV. The obtained materials with Zn content up to 10% possess large specific surface area and big pore volume and zinc species can be highly dispersed on the surface or incorporated into the framework. The acidity of these catalysts was changed by the introduction of Zn, the decrease of strong acid sites is conducive to the promotion of isobutene selectivity and the weak and medium acidic sites played an important role in isobutane conversion. In addition, these catalysts exhibited good catalytic stability, due to the effective inhibition of coke formation by the ordered mesoporous structure.

Computational Study of Methane Activation on γ-Al2O3

The C-H activation of methane remains a longstanding challenge in the chemical industry. Metal oxides are attractive catalysts for the C-H activation of methane due to their surface Lewis acid-base properties. In this work, we applied density functional theory calculations to investigate the C-H activation mechanism of methane on various sites of low-index facets of γ-Al₂O₃. The feasibility of C-H activation on different metal-oxygen (acid-base) site pairs was assessed through two potential mechanisms, namely, the radical and polar. The effect of surface hydroxylation on C-H activation was also investigated to examine the activity of γ-Al₂O₃ under realistic catalytic surface conditions (hydration). On the basis of our calculations, it was demonstrated that the C-H activation barriers for polar pathways are significantly lower than those of the radical pathways on γ-Al₂O₃. We showed that the electronic structure (s- and p-band center) for unoccupied and occupied bands can be used to probe site-dependent Lewis acidity and basicity and the associated catalytic behavior. We identified the dissociated H₂ binding and final state energy as C-H activation energy descriptors for the preferred polar pathway. Finally, we developed structure-activity relationships for the C-H activation of methane on γ-Al₂O₃ that account for surface Lewis acid-base properties and can be utilized to accelerate the discovery of catalysts for methane (and shale gas) upgrade.