ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
Supramolecular hosts that bind guests reversibly are investigated for potential catalysis and separations applications. Chiral Ln(3+)[15-Metallacrown-5] metallocavitands bind carboxylate guests in hydrophobic cavities generated by their ligand side chains. A thermodynamic study on Gd(3+)[15-metallacrown-5] hosts with ligands bearing phenyl side chains containing 0, 1, and 2 methylene spacers (1-pgHA, 1-pheHA, 1-hpheHA, respectively) is presented to quantitatively assess how guest affinity and chiral selectivity can be enhanced through changes to the ligand side chain. Guest binding affinity was measured with cyclic voltammetry using ferrocene carboxylate as a redox probe. K(a) values between ferrocene carboxylate and 1-pgHA and 1-pheHA were 4800 ± 400 M(-1) and 4400 ± 700 M(-1), respectively. Significantly stronger binding affinity of 12,100 ± 700 M(-1) was measured with 1-hpheHA, a result of the longer side-chains more completely encapsulating the guest. A similar trend was observed with benzoate. The side chain also influenced enantioselectivity, as K(S)/K(R) values of up to 2.2 ± 0.6 were measured. The side chain dependent guest binding supports the development of highly selective Ln(3+)[15-Metallacrown-5] hosts for use in catalysis and separations through careful ligand design.
