Abstract Selective oxidation of hydrocarbons requires highly selective catalysts toward desired reaction products. In this study, SiO2-supported Ti catalysts were synthesized featuring isolated Ti oxide species (0.2 Ti atoms/nm2) that serve as active Lewis acid sites for the selective oxidation of cyclohexene with H2O2. To investigate the effect of the surface microenvironment on catalytic performance, post-modification was performed using alcohols with different terminal groups (octyl (O), triglycol (TG), and fluorous (F)). FT-IR spectroscopy confirms that the deposited organic groups were successfully installed on the silica support. Furthermore, the change in the silanol peak (3730 cm–1) confirms that the introduced organic groups effectively minimize moisture contact on the catalyst surface, creating a hydrophobic/hydrophilic environment near the Ti active sites. In the cyclohexene epoxidation reaction (60 °C), the surface-modified catalysts increase the epoxide yield by an average of 25% compared to the unmodified catalyst. Notably, Ti-SiO2-O exhibits the highest direct pathway selectivity of 93.1%, representing a relative increase of 12% compared to the Ti-SiO2 catalyst (83.4%). Raman and XPS results confirm that the reaction proceeds via a titanium hydroperoxide (Ti-OOH) intermediate. Furthermore, density functional theory (DFT) calculations elucidate that the hydrophobic octyl groups provide an optimal balance between cyclohexene adsorption and product desorption. Conversely, the hydrophilic nature of the TG surface and the electron-rich characteristics of the fluorous chains result in the homolytic activation of H2O2. These findings demonstrate that tailoring the surface microenvironment via post-synthetic modification serves as a vital strategy for maximizing the efficiency of catalysts for selective oxidation reactions.

Three different precursors were used to synthesize isolated Ni sites as well as to stabilize them on an Al2O3 support for the dry reforming of methane (DRM) reaction. The precursors used are bis(cyclopentadienyl)nickel(II) (A cycle), trimethyl aluminum (B cycle), and water (C cycle). This so-called ABC-type atomic layer deposition (ALD) was performed for 15 cycles, and then overcoated with an Al2O3 layer via ALD to enhance the stability of the DRM catalyst. Pore morphologies were retained after this ABC-type ALD/calcination/additional Al2O3 ALD, confirming the thin film deposition occurs without compromising pore structure of the Al2O3 support. Calcination after 15 cycles of ABC-type ALD is required to recover the DRM activity as well as to improve the stability. Overcoating with 5 cycles of Al2O3 results in higher peak DRM activity versus the calcined catalyst, but that with 10 cycles does not. Additional Al2O3 overcoating changes the deactivation rate constants to a small extent, but results in induction periods indicating the presence of NiAl2O4. The catalyst with 5 cycles of Al2O3 overcoating shows a higher peak DRM rate than those with 0 and 10 cycles, and a lower deactivation rate constant. These results indicate that achieving an optimum number of ABC cycles may help resolve the trade-off between peak rate and deactivation behavior for Ni DRM catalysts.

Direct air capture (DAC) using solid sorbents is a promising negative-emissions technology, but its large-scale deployment remains constrained by coupled challenges in adsorbent performance, process efficiency, and technoeconomic viability. This review examines recent progress in solid-sorbent DAC through an integrated perspective that connects adsorbent design with process operation and system-level cost considerations. We summarize advances in physisorbent- and chemisorbent-based materials developed for ultradilute CO2 capture, with particular emphasis on amine-functionalized sorbents and porous frameworks evaluated under realistic humidity and temperature conditions. Key process configurations, including temperature swing adsorption (TSA), temperature-vacuum swing adsorption (TVSA), and steam-assisted variants, are critically compared in terms of regeneration kinetics, energy demand, and operational trade-offs. We further review recent technoeconomic analyses to identify dominant cost drivers, scale-up uncertainties, and design pathways for reducing energy consumption and capital intensity. Finally, emerging approaches for CO2 conversion using porous materials are discussed, with an emphasis on their compatibility with DAC-derived CO2 streams and their potential role in improving overall system economics.

Surface modification via grafting of organic moieties on a Lewis acid catalyst (silica supported Ti catalyst, Ti-SiO2) alters the activation of H2O2 in vapor-phase cyclohexene epoxidation. Grafting a fluorous group (1H,1H-perfluoro-octyl) suppresses activity of Ti-SiO2. Conversely, grafting either a nonpolar group (octyl) or a polar aprotic group (triethylene glycol monomethyl ether) enhances rates and shifts selectivity toward trans-1,2-cyclohexanediol.

Selective oxidation of hydrocarbons over metal oxide catalysts frequently invokes Mars-van Krevelen (MvK) mechanisms involving lattice oxygens. At typical reaction temperatures, oxidative dehydrogenation reactions predominate and compete with total oxidation; the production of more useful oxygenates is challenging. However, because the regeneration of lattice oxygens has been previously hypothesized to involve the transient formation of surface hydroperoxides or peroxides, MvK cycles should be capable of forming products derived from both lattice oxygen and surface (hydro)peroxide species under certain conditions. In this report, we study the low-temperature reaction of cyclohexene with O-2 over CeO2-supported VOx catalysts. Below 150 degrees C and for higher VOx loading, cyclohexene converts to benzene and trans-1,2-cyclohexanediol with no appreciable CO2 formation. Control experiments show similar behavior in other supported vanadia catalysts. Conversely, using vaporized H2O2 as the oxidant exclusively gives 1,2-cyclohexanedione, suggesting that MvK reoxidation cycles do not directly involve H2O2. These findings contribute to the current understanding of selective oxidation mechanisms and demonstrate routes to the formation of industrially useful oxygenates with O-2 or vaporized H2O2 over metal oxide catalysts.

Activating the C-H bonds of alkanes without further oxidation to more thermodynamically stable products, CO and CO2, is a long-sought goal of catalytic chemistry. Inspired by the monocopper active site of methane monooxygenase, we synthesized a Cu-doped ZIF-8 metal-organic framework with 25% Cu and 75% Zn in the nodes and activated it by heating to 200 °C and dosing in a stepwise fashion with O2, methane, and steam. We found that it does oxidize methane to methanol and formaldehyde. The catalysis persists through at least five cycles, and beyond the third cycle, the selectivity improves to the extent that no CO2 can be detected. Experimental characterization and analysis were carried out by PXRD, DRUV-vis, SEM, and XAS (XANES and EXAFS). The reaction is postulated to proceed at open-coordination copper sites generated by defects, and the mechanism of methanol production was explicated by density functional calculations with the revMO6-L exchange-correlation functional. The calculations reveal a catalytic cycle of oxygen-activated CuI involving the conversion of two molecules of CH4 to two molecules of CH3OH by a sequence of hydrogen atom transfer reactions and rebound steps. For most steps in the cycle, the reaction is more favored by singlet species than by triplets.

Deposition of La2O3 and Al2O3 on Al2O3-supported Ni catalysts was performed to study their effects on heterogeneous catalysts for the dry reforming of methane (DRM) reaction. An alumina-supported Ni catalyst (Ni/Al2O3, 2 wt % of Ni), synthesized via incipient wetness impregnation, loses similar to 87% of its initial activity within 45 h under DRM conditions. While overcoating of Al2O3 on this catalyst via atomic layer deposition (ALD) helps stabilize the catalytic activity in long time-on-stream (TOS) tests, this overcoated catalyst is ca. 40 times less active than the uncoated catalyst at peak activity. This Al2O3-overcoated Ni/Al2O3 catalyst also exhibits a long induction period (similar to 20 h) due to the slow reduction of Ni2+ within the catalytically inactive nickel aluminate (NiAl2O4) phase, formed by the interaction of metallic Ni with the Al2O3 overcoat during pre-DRM treatment at the 700 degrees C reaction temperature. Here, we report that, while doping small amounts of La (similar to 0.03 wt %) into the Ni/Al2O3 catalyst does not significantly affect the catalytic activity or stability by itself, the addition of ALD-Al2O3 on top of the La2O3-promoted Ni catalysts significantly suppresses the long TOS deactivation, helps recover the peak activity of uncoated Ni/Al2O3, and eliminates the DRM induction period. This strategy obtains the stabilization benefits of Al2O3 overcoating on Ni/Al2O3 while, at the same time, avoiding the formation of undesirable NiAl2O4 species.

Solid supports are crucial in heterogeneous catalysis due to their profound effects on catalytic activity and selectivity. However, elucidating the specific effects arising from such supports remains challenging. We selected a series of metal-organic frameworks (MOFs) with 8-connected Zr-6 nodes as supports to deposit molybdenum(VI) onto to study the effects of pore environment and topology on the resulting Mo-supported catalysts. As characterized by X-ray absorption spectroscopy (XAS) and single-crystal X-ray diffraction (SCXRD), we modulated the chemical environments of the deposited Mo species. For Mo-NU-1000, the Mo species monodentately bound to the Zr-6 nodes were anchored in the microporous c-pore, but for Mo-NU-1008 they were bound in the mesopore of Mo-NU-1008. Both monodentate and bidentate modes were found in the mesopore of Mo-NU-1200. Cyclohexene epoxidation with H2O2 was probed to evaluate the support effect on catalytic activity and to unveil the resulting structure-activity relationships. SCXRD and XAS studies demonstrated the atomically precise structural differences of the Mo binding motifs over the course of cyclohexene epoxidation. No apparent structural change was observed for Mo-NU-1000, whereas the monodentate mode of Mo species in MoNU-1008 and the monodentate and bidentate Mo species in Mo-NU-1200 evolved to a new bidentate mode bound between two adjacent oxygen atoms from the Zr-6 node. This work demonstrates the great advantage of using MOF supports for constructing heterogeneous catalysts with modulated chemical environments of an active species and elucidating structure-activity relationships in the resulting reactions.

Heterogeneous catalysts supported on metal-organic frameworks (MOFs), which possess uniform porosity and crystallinity, have attracted significant interest for recent years due to the ease of active-site characterization via X-ray diffraction and the subsequent relation of the active site structure to the catalytic activity. We report the syntheses, structures, and oxidation catalytic activities of single-ion iron catalysts incorporated into the zirconium MOF NU-1000. Single-ion iron catalysts with different counteranions were anchored onto the Zr node through postsynthetic solvothermal deposition. Crystallographic characterization of the resulting MOFs (NU-1000-Fe-Cl and NU-1000-Fe-NO3) revealed that, while both frameworks have similar Fe coordination, the distance between Fe and the Zr-6 node differs significantly between the two. The product rate profiles of the two catalysts for vapor-phase cyclohexene epoxidation demonstrate different initial rates and product formations, likely originating from the different Fe-O distances.

The modular structure of metal organic frameworks (MOFs) makes them promising platforms for catalyst design and for elucidating structure/performance relationships in catalysis. In this work, we systematically varied the composition of the metal nodes (Fe2M) of the MOF PCN-250 and used density functional theory (DFT) to identify promising catalysts for light alkane C-H bond activation. Oxidative dehydrogenation (ODH) of alkanes was studied using N2O as the oxidant to understand the reactivity of the oxocentered Fe2M nodes found in PCN-250, where the Fe ions are in the +3 oxidation state and M is a metal with the oxidation state of +2. We show that the N2O activation barrier is positively correlated with the oxygen-binding energy at the metal center, and the C-H activation barrier is negatively correlated with this same quantity. For clusters containing early transition metals, oxygen binds strongly, facilitating N2O activation but hindering C-H activation. To validate the DFT predictions, we synthesized and tested PCN-250(Fe2M) with M = Mn, Fe, Co, and Ni and found that PCN-250(Fe2Mn) and PCN-250(Fe-3) are more active than PCN-250(Fe2Co) and PCN-250(Fe2Ni) in agreement with the DFT predictions, demonstrating the power of DFT calculations to predict and identify promising MOF catalysts for alkane C-H bond activation in advance of experiments.

Catalytic oxidation of hydrocarbons with hydrogen peroxide (H2O2) has been of the utmost importance for several decades. The vast majority of studies have been performed in the condensed phase, even though condensed phases introduce complex solvent effects and can promote the leaching of active sites. In response, we have built a custom reactor system to understand H2O2 activation and selective oxidation in the vapor-phase. In this report, we study the epoxidation of cyclohexene with H2O2 over four Lewis-acidic metal oxide catalysts: Ti and Nb grafted on SiO2 and on the Zr based metal-organic framework, NU-1000. The M-SiO2 materials are highly selective to the formation of epoxides and diols, as they can be in the condensed phase, while the NU-1000 based materials are far more prone to overoxidation to CO2, which appears to be connected to their strong reactant adsorption. Apparent activation energies are calculated for all materials when operating in the same kinetic regime, and the heats of cyclohexene adsorption into their pores are then used to directly compare intrinsic enthalpies of activation in the vapor vs condensed phase for the M-SiO2 catalysts. Nb-SiO2 catalysts exhibit similar intrinsic enthalpies of activation in the vapor and condensed phases, whereas the condensed phase transition state in Ti-SiO2 is 24 kJ/mol lower in energy than that of the same material in the vapor phase. These experiments establish another methodology for understanding the various roles of solvent in selective oxidation reactions and studying these reactions under conditions that differ significantly from the thousands of prior studies in the condensed phase.

A V-containing metal–organic framework exhibits significantly different catalytic mechanisms between liquid-phase and gas-phase cyclohexene oxidations.

Synthesis of single-site catalysts, whereby the local structure and surrounding chemical environments are identical, has been challenging, particularly in heterogeneous catalysis, as the support often presents spectrum of chemically distinct binding sites. Yet, the above criteria are crucial in attributing the apparent catalytic performance to the structural motif. The presented work augments on our previous work using monometallic molybdenum sulfide tethered within a zirconium-based metal-organic framework (MOF), NU-1000; the monometallic nature enables all presented sites to be catalytically addressable. As the molybdenum sulfide species resided within two distinct pores (micro- and mesopores) of the MOF support, we have imparted uniformity in the local chemical environment by reducing the pore heterogeneity down to a single mesopore. Single-site and single-atom nature of the candidate catalyst was established via X-ray diffraction measurements. Redox mediators were implemented, which, under reductive potentials, provide reduced species; they can effectively deliver the necessary reducing equivalences to the catalytic units that can otherwise not be addressed electrochemically due to the low electron mobility within the framework. Our results indicate the micropore-allocated molybdenum sulfide is approximately four times more active as that in mesopores, whereas its catalytic mechanism is identical, underscoring the importance of controlling chemical environment beyond the active site.

MIL‐100(Fe), a metal−organic framework containing coordinatively unsaturated iron sites, was prepared by a solvothermal method and utilized in cyclohexane oxidation. Interestingly, the composition and fraction of redox‐active Fe(II/III) sites in MIL‐100(Fe) could be tuned by using different pretreatment temperatures. The obtained materials were characterized by means of field‐emission scanning electron microscopy (FE‐SEM), powder X‐ray diffraction (PXRD), N2 adsorption‐desorption isotherms, and X‐ray photoelectron spectroscopy (XPS). Due to the presence of redox‐active coordinatively unsaturated sites (CUS) at the iron atoms, MIL‐100(Fe) exhibited good performance for cyclohexane oxidation under mild reaction conditions. Oxidation rates were significantly enhanced when reduced FeII sites were generated by high‐temperature pretreatments. In either high‐temperature or low‐temperature pretreatment, the isolated Fe(II/III) sites in MIL‐100 were far more reactive in cyclohexane oxidation than were bulk iron oxide materials. Finally, possible reaction pathways were proposed based on radical, Haber‐Weiss routes.

A representative metal-organic framework, NU-1000, was functionalized with MoSx. The previously determined crystal structure of the material, named MoSx-SIM, consists of monometallic Mo(IV) ions with two sulfhydryl ligands. The metal ions are anchored to the framework by displacing protons presented by the -OH/-OH2 groups on the Zr-6 node. As shown previously, the MOF-supported complexes are electrocatalytic for hydrogen evolution from acidified water. The earlier electrocatalysis results, together with the nearly ideal formal potential of the Mo(IV/II) couple (i.e., nearly coincident with that of the hydrogen couple), and the physical proximity of UV-absorbing MOF linkers to the complexes, suggested to us that the linkers might behave photosensitizers for catalyst reduction, and subsequently, for H-2 evolution from water. To our surprise, MoSx-SIM, when UV-illuminated in an aqueous buffer at near-neutral pH, displays a biphasic photocatalytic response: an initially slow rate of reaction, i.e. 0.56 mmol g(-1) h(-1), followed by an increase to 4 mmol g(-1) h(-1). Ex-situ catalyst examination revealed that nanoparticulate MoSx suspended within the reaction mixture is the actual catalyst. Thus, photo-assisted restructuring and detachment of the catalyst or pre-catalyst from the MOF node appears to be necessary for the catalyst to reduce water at neutral (C) The Author(s) 2019. Published by ECS.

The postmodification of metal organic frameworks (MOFs) affords exceedingly high surface area materials with precisely installed chemical features, which provide new opportunities for detailed structure-function correlation in the field of catalysis. Here, we significantly expand upon the number of vapor-phase postmodification processes reported to date through screening a library of atomic layer deposition (ALD) precursors, which span metals across the periodic table and which include ligands from four distinct precursor classes. With a large library of precursors and synthesis conditions, we discern trends in the compatibility of precursor classes for well-behaved ALD in MOFs (AIM) and identify challenges and solutions to more precise postsynthetic modification.

Controlling metal nanoparticle size and preserving metal dispersion at elevated temperature remain key challenges in designing new supported metal catalysts. Many methods have been proposed to stabilize metal nanoparticles for catalysis, but the use of specialized equipment or metal precursors can limit the application of these methods for scalable production. Here, we demonstrate a synthesis strategy to improve the dispersion and thermal stability of Pt nanoparticles on an oxide support. A thin SiO2 coat (< 2 nm) was deposited on TiO2 through repeated condensation cycles of tetraethyl orthosilicate (TEOS) with or without an organic template on the surface. H2PtC16 was deposited using wetness impregnation, and the samples were dried, calcined, and reduced. The as-synthesized Pt nanoparticles are 1-2 nm by TEM and maintain dispersion > 45% by CO chemisorption even after prolonged heating at 500 degrees C, whereas Pt nanoparticles on unmodified TiO2 are less dispersed (-33%) and their dispersion falls further upon prolonged heating. Ethylene hydrogenation demonstrates that the Pt nanoparticles on modified TiO2 preserve the catalytic activities of Pt on unmodified TiO2. The use of wet chemistry-based oxide modification and wetness impregnation makes this strategy a scalable and generalizable synthesis method to prepare other supported metal nanoparticles for catalysis applications.

Acid-catalyzed skeletal C-C bond isomerizations are important benchmark reactions for the petrochemical industries. Among those, o-xylene isomerization/disproportionation is a probe reaction for strong Brønsted acid catalysis, and it is also sensitive to the local acid site density and pore topology. Here, we report on the use of phosphotungstic acid (PTA) encapsulated within NU-1000, a Zr-based metal-organic framework (MOF), as a catalyst for o-xylene isomerization at 523 K. Extended X-ray absorption fine structure (EXAFS), 31P NMR, N2 physisorption, and X-ray diffraction (XRD) show that the catalyst is structurally stable with time-on-stream and that WO x clusters are necessary for detectable rates, consistent with conventional catalysts for the reaction. PTA and framework stability under these aggressive conditions requires maximal loading of PTA within the NU-1000 framework; materials with lower PTA loading lost structural integrity under the reaction conditions. Initial reaction rates over the NU-1000-supported catalyst were comparable to a control WO x-ZrO2, but the NU-1000 composite material was unusually active toward the transmethylation pathway that requires two adjacent active sites in a confined pore, as created when PTA is confined in NU-1000. This work shows the promise of metal-organic framework topologies in giving access to unique reactivity, even for aggressive reactions such as hydrocarbon isomerization.

A Ni(ii) catalyst incorporated into a new porous organic polymer, Ni(ii)-POP-1, is prepared via a click reaction followed by metalation with NiCl2. It shows good catalytic activity for ethylene dimerization.