Joseph T. Grant

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.

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.

Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts

Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidations of C1-C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidations of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidations, regardless of the metal oxide, follow Mars-van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidation steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidation step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.

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.