Covalent organic framework (COF) represents an emerging class of porous materials that have exhibited great potential in various applications, particularly in catalysis. In this work, we report a newly designed 2D COF with incorporated Re complex, which exhibits intrinsic light absorption and charge separation (CS) properties. We show that this hybrid catalyst can efficiently reduce CO 2 to form CO under visible light illumination with high electivity (98%) and better activity than its homogeneous Re counterpart. More importantly, using advanced transient optical and X-ray absorption spectroscopy and in situ diffuse reflectance spectroscopy, we unraveled three key intermediates that are responsible for CS, the induction period, and rate limiting step in catalysis. This work not only demonstrates the potential of COFs as next generation photocatalysts for solar fuel conversion but also provide unprecedented insight into the mechanistic origins for light-driven CO 2 reduction.

Hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNT) were recently reported as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins in the gas phase. Previous studies revealed a substantial increase in surface oxygen content after exposure to ODH conditions (heating to ca. 500 °C under a flow of alkane and oxygen); however, the complexity of these materials has thus far precluded an in-depth understanding of the oxygenated surface species. In this contribution, we combine advanced NMR spectroscopy experiments with scanning electron microscopy and soft X-ray absorption spectroscopy to characterize the molecular structure of the oxygen functionalized phase that arises on h-BN and BNNT following catalytic testing for ODH of propane. The pristine BN materials are readily oxidized and hydrolyzed under ODH reaction conditions to yield a phase consisting of three-coordinate boron sites with variable numbers of hydroxyl and bridging oxide groups which is denoted B(OH)xO3-x (where x = 0-3). Evidence for this robust oxide phase revises previous literature hypotheses of hydroxylated BN edges as the active component on h-BN.

In this contribution we report on the oxidative dehydrogenation (ODH) activity of silica-supported boron oxide prepared via incipient wetness impregnation. Characterization of pristine and spent catalysts with infrared, Raman, and solid-state NMR spectroscopy reveals the presence of both isolated and aggregated oxidized boron sites. The results of these investigations, in combination with our earlier work on bulk boron-containing ODH catalysts (e.g., h-BN, metal borides, and elemental boron), bolster the hypothesis that oxidized boron species in situ formed on the surface of these materials are responsible for the exceptional catalytic behavior. We anticipate that investigation of supported boron materials can provide insight into the structural characteristics required for selective boron-containing ODH catalysts. Disciplines DisciplinesPhysical Chemistry Comments CommentsAbstract. In this contribution we report on the oxidative dehydrogenation (ODH) activity of silica-supported boron oxide prepared via incipient wetness impregnation. Characterization of pristine and spent catalysts with infrared, Raman, and solid-state NMR spectroscopy reveals the presence of both isolated and aggregated oxidized boron sites. The results of these investigations, in combination with our earlier work on bulk boron-containing ODH catalysts (e.g., h-BN, metal borides, and elemental boron), give direct evidence that oxidized boron species formed in situ on the surface of these materials are responsible for the exceptional catalytic behavior. We anticipate that investigation of supported boron materials can provide insight into the structural characteristics required for selective boron-containing ODH catalysts.

, respectively. Her graduate research focused on the fundamental understanding of deoxygenation mechanisms for biomass probe molecules on single crystals and thin films. During her graduate career, she received the DOE Science Graduate Student Research (SCGSR) award (2019) and conducted surface science research at Brookhaven National Lab. She now is a postdoctoral researcher at the University of Wisconsin-Madison. Her research focuses on surface spectroscopy and controlled design of heterogeneous catalysts. Melissa C. Cendejas received her B.A. (2016) in Chemistry and English from Williams College. There, she worked on the synthesis of conjugated organic molecules and phosphorusbased surfactants. She then started her PhD in Chemistry at the University of Wisconsin-Madison under the supervision of Prof. Ive Hermans. She studies active site formation on boron-based catalysts. She received the DOE Office of Science Graduate Student Research (SCGSR) award (2020) to perfrom in situ x-ray spectroscopy of catalysts at Stanford Synchrotron Radiation Lightsourse. Prof. Dr. Ive Hermans obtained his Ph.D. from KU Leuven University in Belgium in 2006. In addition to his scientific education, he also holds a postgraduate degree in Business Administration (KU Leuven, 2006). After postdoctoral research on in situ spectroscopy and reaction engineering with Prof. Alfons Baiker, he became assistant professor for heterogeneous catalysis (spring 2008) at ETH Zurich in Switzerland. January 2014, Prof. Hermans moved to the University of Wisconsin-Madison, holding a dual appointment in the Department of Chemistry and the Department of Chemical and Biological Engineering. His group focuses on the mechanistic understanding of catalytic technology using a variety of techniques.

Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B2(OH)xO(3−x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using 11B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO3 units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO3 units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle.

Boron-based heterogeneous catalysts, such as hexagonal boron nitride (h-BN) as well as supported boron oxides, are highly selective catalysts for the oxidative dehydrogenation (ODH) of light alkanes to olefins. Previous catalytic measurements and molecular characterization of boron-based catalysts by 11 B solid-state NMR spectroscopy and other techniques suggest that oxidized/ hydrolyzed boron clusters are the catalytically active sites for ODH. However, 11 B solid-state NMR spectroscopy often suffers from limited resolution because boron-11 is an I = 3/2 half-integer quadrupolar nucleus. Here, ultrahigh magnetic field (B 0 = 35.2 T) is used to enhance the resolution of 11 B solid-state NMR spectra and unambiguously determine the local structure and connectivity of boron species in h-BN nanotubes used as an ODH catalyst (spent h-BNNT), boron-substituted MCM-22 zeolite (B-MWW), and silica-supported boron oxide (B/SiO 2 ) before and after use as an ODH catalyst. One-dimensional direct excitation 11 B NMR spectra recorded at B 0 = 35.2 T are near isotropic in nature, allowing for the easy identification of all boron species. Two-dimensional (2D) 1 H-11 B heteronuclear correlation NMR spectra aid in the identification of boron species with B−OH functionality. Most importantly, 2D 11 B dipolar double-quantum single-quantum homonuclear correlation NMR experiments were used to unambiguously probe boron−boron connectivity within all heterogeneous catalysts. These experiments are practically infeasible at lower, more conventional magnetic fields due to a lack of resolution and reduced NMR sensitivity. The detailed molecular structures determined for the amorphous oxidized/hydrolyzed boron layers on these heterogeneous catalysts will aid in the future development of nextgeneration ODH catalysts.

We present the controlled grafting synthesis of pinacolborane on amorphous silica. 11B solid-state NMR and IR spectroscopies reveal that the precursor molecule anchors monopodally to the surface and can form hydrogen-bonding interactions with neighboring unreacted silanol groups. The extent of hydrogen bonding can be controlled by the silica pretreatment dehydration temperature. Thermal treatment of the grafted boron materials under vacuum generates clusters of oxidized/hydrolyzed boron regardless of boron weight loading, illustrating that boron is highly mobile on the silica surface at elevated temperatures. The materials exhibit propane oxidative dehydrogenation activity expected for silica-supported boron catalysts. Interestingly, the kinetic behavior of these supported catalysts deviates from that of previously reported bulk boron materials, prompting further studies into the reaction kinetics over these materials. The synthetic and catalytic insights gained here can inform future studies of improved synthesis routes and reaction kinetics.

Bulk boron materials, such as hexagonal boron nitride (h‐BN), are highly selective catalysts for the oxidative dehydrogenation of propane (ODHP). Previous attempts to improve the productivity of these systems involved the immobilization of boron on silica and resulted in less selective catalysts. Here, we report that acid‐treated, activated carbon‐supported boron prepared via incipient wetness impregnation with boric acid (B/OAC) exhibits equal propylene selectivity and improved productivity (kgpropylene kgcat−1 hr−1) as compared to h‐BN. Characterization of the fresh and spent catalysts with infrared, Raman, X‐ray photoelectron, and solid‐state NMR spectroscopies reveals the presence of oxidized/hydrolyzed boron that is clustered on the surface of the support.