Carlos A. Carrero

Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts

Boron nitride catalysis Propene is one of the highest-volume organic chemicals produced. Propene has mainly been made from naphtha, but changes in the global supply chain are creating shortages. Direct conversion from propane, a component of natural gas, via reaction with oxygen is an attractive alternative, but existing approaches produce a large fraction of unwanted CO and CO2. Grant et al. report that boron nitride, normally an unreactive material, has high selectivity to catalyze the production of propene (77%) and ethene (13%). Science, this issue p. 1570 Boron nitride, often considered unreactive, can be a highly active and selective catalyst for propane oxidation to propene. The exothermic oxidative dehydrogenation of propane reaction to generate propene has the potential to be a game-changing technology in the chemical industry. However, even after decades of research, selectivity to propene remains too low to be commercially attractive because of overoxidation of propene to thermodynamically favored CO2. Here, we report that hexagonal boron nitride and boron nitride nanotubes exhibit unique and hitherto unanticipated catalytic properties, resulting in great selectivity to olefins. As an example, at 14% propane conversion, we obtain selectivity of 79% propene and 12% ethene, another desired alkene. Based on catalytic experiments, spectroscopic insights, and ab initio modeling, we put forward a mechanistic hypothesis in which oxygen-terminated armchair boron nitride edges are proposed to be the catalytic active sites.

Reverse Water–Gas Shift on Interfacial Sites Formed by Deposition of Oxidized Molybdenum Moieties onto Gold Nanoparticles

We show that MoO(x)-promoted Au/SiO2 catalysts are active for reverse water-gas shift (RWGS) at 573 K. Results from reactivity measurements, CO FTIR studies, Raman spectroscopy, and X-ray absorption spectroscopy (XAS) indicate that the deposition of Mo onto Au nanoparticles occurs preferentially on under-coordinated Au sites, forming Au/MoO(x) interfacial sites active for reverse water-gas shift (RWGS). Au and AuMo sites are quantified from FTIR spectra of adsorbed CO collected at subambient temperatures (e.g., 150-270 K). Bands at 2111 and 2122 cm(-1) are attributed to CO adsorbed on under-coordinated Au(0) and Au(δ+) species, respectively. Clausius-Clapeyron analysis of FTIR data yields a heat of CO adsorption (ΔH(ads)) of -31 kJ mol(-1) for Au(0) and -64 kJ mol(-1) for Au(δ+) at 33% surface coverage. Correlations of RWGS reactivity with changes in FTIR spectra for samples containing different amounts of Mo indicate that interfacial sites are an order of magnitude more active than Au sites for RWGS. Raman spectra of Mo/SiO2 show a feature at 975 cm(-1), attributed to a dioxo (O═)2Mo(-O-Si)2 species not observed in spectra of AuMo/SiO2 catalysts, indicating preferential deposition of Mo on Au. XAS results indicate that Mo is in a +6 oxidation state, and therefore Au and Mo exist as a metal-metal oxide combination. Catalyst calcination increases the quantity of under-coordinated Au sites, increasing RWGS activity. This strategy for catalyst synthesis and characterization enables quantification of Au active sites and interfacial sites, and this approach may be extended to describe reactivity changes observed in other reactions on supported gold catalysts.

Selective Oxidation of n-Butane and Isobutane Catalyzed by Boron Nitride

Hexagonal boron nitride (hBN) is presented as an outstanding catalyst for the selective production of C4 olefins by the oxidative dehydrogenation of n‐butane and isobutane. Unlike catalysts reported previously, hBN limits the amount of undesired COx and instead forms C2 and C3 olefins as the main side products. Kinetic experiments suggest a mechanism in which the rates of n‐butane and isobutane consumption are dependent on O2 adsorption. Kinetic and spectroscopic insights are used to formulate mechanistic hypotheses for the formation mechanisms of C2–C4 olefins.

Improved Supported Metal Oxides for the Oxidative Dehydrogenation of Propane

The oxidative dehydrogenation of propane (ODHP) is an attractive reaction for the on-purpose production of propylene. Unfortunately, rapid consecutive over-oxidation of the desired olefin limits the selectivity and hampers the industrial feasibility. Supported metal oxides, and in particular dispersed vanadium-containing materials, have shown promising results. Yet one has to improve both the selectivity and activity (space–time–yield) to make this reaction attractive. In this contribution we build upon our previous work that allowed us to increase the dispersion of group V metal oxides on silica using a sodium promoter. Using Raman spectroscopy and 51V MAS NMR, we postulate that the minor decrease in our observed turnover frequency (TOF) for ODHP using sodium-promoted materials may be due to Na+ ions weakly interacting with the V=O site, responsible for the initial H-atom abstraction. While our observed TOF is well within the range of literature reported TOF for these materials, such a large deviation in reported TOF (varying almost three orders of magnitude) may be due to various impurities used in the silica of these previously reported studies. Subsequently, we prepared a ternary metal oxide catalyst based on vanadium and tantalum that shows superior selectivity and productivity. Indeed, productivity of a combined V- and Ta-oxide catalyst supported on silica doubles the productivity of catalysts with low loadings of vanadium oxide supported on silica. The reasons for the significant improvement are currently under investigation.

Thick and freestanding MXene/PANI pseudocapacitive electrodes with ultrahigh specific capacitance

Two-dimensional (2D) titanium carbide MXene (Ti3C2Tx) has shown great promise as a high-performance electrode material for electrochemical capacitors (ECs). However, similar to other 2D materials, processing MXenes into freestanding films results in their restacking, thus decreasing the ion transport inside the electrodes. This problem significantly hinders the specific capacitance and rate capability of freestanding electrodes, particularly for those with thicknesses higher than a few microns. Here, we demonstrate a strategy based on surface modification of MXene sheets to fabricate electrodes with highly accessible structure and improved electrochemical performance even at very high electrode thicknesses. 2D Ti3C2Tx and polyaniline (PANI) hybrid materials were synthesized through oxidant-free in situ polymerization of PANI on the surface of MXene sheets and were assembled into freestanding films with various thicknesses. Thin MXene/PANI hybrid electrodes delivered outstanding gravimetric and volumetric capacitances as high as 503 F g−1 and 1682 F cm−3, respectively. As the electrode thicknesses and mass loadings were increased, the hybrid electrodes still showed high electrochemical performance. For example, an electrode with a thickness of 90 μm and a mass loading of 23.82 mg cm−2 could deliver a specific capacitance of about 336 F g−1 (∼888 F cm−3 volumetric capacitance). The hybrid electrodes also showed a high cycle lifetime with a capacitance retention of 98.3% after 10 000 cycles. This paper explains a simple and fast approach for the fabrication of MXenes/conducting polymer hybrid electrodes with superior electrochemical performance.

Correction: Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC

Correction for ‘Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC’ by Carlos A. Carrero et al., Catal. Sci. Technol., 2017, DOI: 10.1039/c7cy01036b.

Supported two- and three-dimensional vanadium oxide species on the surface of β-SiC

A series of supported two- and three-dimensional vanadium oxide surface species on β-SiC with various V coverages are prepared via incipient wetness impregnation and characterized by a variety of ex and in situ techniques. The oxidative dehydrogenation of propane (ODHP) is also used as a probe reaction to complementarily distinguish between two- and three-dimensional VOx surface species. Herein, we show that treating pristine β-SiC with oxygen transforms the existing amorphous SiOxCy surface layer into a more SiO2-type layer, though with a negligible formation of Si–OH sites, which initially were expected to be the anchor sites for VOx species. In its place, the C–OH functional groups identified by X-ray photoelectron spectroscopy (XPS) act as anchor sites for the VOx species during the impregnation process, and are consumed as a function of V coverage. Our experimental observations all corroborate the formation of two- and three-dimensional VOx species on the surface of β-SiC.