Susannah L. Scott

Stabilization of Copper Catalysts for Liquid‐Phase Reactions by Atomic Layer Deposition

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid-phase catalytic reactions (e.g., hydrogenation of biomass-derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid-phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X-ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ-Al2 O3 . Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under-coordinated copper atoms on the nanoparticle surface.

Recent progress in methane dehydroaromatization: From laboratory curiosities to promising technology

Direct conversion of methane to benzene or other valuable chemicals is a very promising process for the efficient application of natural gas. Compared with conversion processes that require oxidants, non-oxidative direct conversion is more attractive due to high selectivity to the target product. In this paper, an alternative route for methane dehydrogenation and selective conversion to benzene and hydrogen without the participation of oxygen is discussed. A brief review of the catalysts used in methane dehydroaromatization (MDA) is first given, followed by our current understanding of the location and the active phase of Mo species, the reaction mechanism, the mechanism of carbonaceous deposit and the deactivation of Mo/zeolite catalysts are systematically discussed. Ways to improve the catalytic activity and stability are described in detail based on catalyst and reaction as well as reactor design. Future prospects for methane dehydroaromatization process are also presented.

Surface organometallic investigation of the mechanism of ethylene polymerization by silica-supported Cr catalysts

The Phillips family of one-component catalysts, created by the interaction of a inorganic chromium compound such as CrO3 and an oxide support, typically silica, initiates ethylene polymerization in the absence of alkylaluminum activators. Despite intense scrutiny since its discovery over four decades ago, the nature of the active site has remained obscure and the mechanism of olefin polymerization ill-defined. Spectroscopic studies of the working catalyst are hampered by low active site densities. To circumvent this problem, we have prepared well-defined organochromium fragments, supported on silica, which manifest remarkably similar activity to the CrO3/SiO2 catalyst, and produce similar high-density polyethylene. The uniform nature of the active sites has made possible a kinetic investigation of the mechanism of polymerization. Our evidence points to alkylchromium(IV) active centers, bound to silica by two Si–O–Cr linkages, which coordinate ethylene by displacement of a surface siloxane. Insertion of the coordinated olefin into one of two growing alkyl chains occurs provided there is no α-substituent. The information thus obtained may be used to understand the relationship between active site structure, catalyst activity and polymer properties.

A Cu25 Nanocluster with Partial Cu(0) Character

Atomically precise copper nanoclusters (NCs) are of immense interest for a variety of applications, but have remained elusive. Herein, we report the isolation of a copper NC, [Cu25H22(PPh3)12]Cl (1), from the reaction of Cu(OAc) and CuCl with Ph2SiH2, in the presence of PPh3. Complex 1 has been fully characterized, including analysis by X-ray crystallography, XANES, and XPS. In the solid state, complex 1 is constructed around a Cu13 centered-icosahedron and formally features partial Cu(0) character. XANES of 1 reveals a Cu K-edge at 8979.6 eV, intermediate between the edge energies of Cu(0) and Cu(I), confirming our oxidation state assignment. This assignment is further corroborated by determination of the Auger parameter for 1, which also falls between those recorded for Cu(0) and Cu(I).

A Tailored Microenvironment for Catalytic Biomass Conversion in Inorganic–Organic Nanoreactors†

The efficient and selective conversion of biomass-derived renewable feedstocks into chemicals and fuels remains a major techno-economic challenge. Fructose, a simple carbohydrate that can be obtained from cellulose, can be dehydrated to a potential platform chemical, 5-hydroxymethylfurfural (HMF). The selectivity for HMF is a function of the fructose tautomer distribution, which varies with solvent polarity and temperature. Near-quantitative conversion of fructose into HMF has only been obtained in non-aqueous, polar aprotic solvents (such as, DMSO or NMP), or in ionic liquids. However, HMF separation from such high-boiling and/or costly solvents is energy-intensive and lowers the yield, even when combined with immiscible, low-boiling solvents. Herein, we describe an organic–inorganic nanocomposite catalyst that converts fructose selectively (> 80 %) into HMF in a flow reactor, while eliminating separation issues and the need for environmentally unfriendly solvents. We obtain the highest reported HMF yields to date in a monophasic, readily separable solvent, avoiding the undesirable use of salts. Our previous studies of fructose dehydration to HMF employed silicas and organosilicas with pore-directed alkylsulfonic acid groups as heterogeneous catalysts. Ordered mesoporous silica-based catalysts were found to be more selective and robust than catalysts with similar chemical compositions but non-ordered pores. Upon incorporating bifunctional organosilanes containing both alkylsulfonic acid groups and thioether/sulfone groups to promote fructose tautomerization to the desired furanose tautomers, we observed further, modest selectivity improvements relative to propylsulfonic acid-functionalized silicas. We hypothesized that in order to achieve HMF selectivities comparable to those reported with homogeneous systems, the microenvironment throughout the pore channels (rather than just localized near the active sites) should promote fructose tautomerization. Soluble organic polymers have been reported to act as pseudo-solvents, encapsulating reactants in a local microenvironment that can be favorably tailored for catalysis. Furthermore, the pores of acid-functionalized ordered mesoporous materials (both silicas and organosilicas) are large enough to accommodate such macromolecules. Poly(vinylpyrrolidone) (PVP), a polar aprotic polymer, was intercalated by incipient wetness impregnation into the pores of unmodified SBA-15 silica (Scheme 1), as well as into

Electronic structure of alumina-supported monometallic Pt and bimetallic PtSn catalysts under hydrogen and carbon monoxide environment

The structure of supported platinum and platinum-tin nanoparticles was investigated by Pt L(3) high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) and resonant inelastic X-ray scattering (RIXS). The incorporation of tin decreased the ability of particles to adsorb both hydrogen and carbon monoxide due to tin enrichment on the surface. The platinum d band of platinum-tin particles was narrower and was shifted down relative to the Fermi level in comparison to platinum particles. The difference in electronic structure between pure and alloyed particles persisted after adsorption of hydrogen. The Pt-H antibonding state was clearly identified for the pure platinum particles. The strong adsorption of carbon monoxide changed the geometric structure of the PtSn particles. After carbon monoxide adsorption, the geometric structures of both systems were very similar. Room temperature adsorption of carbon monoxide affects the structure of platinum catalysts.

Catalytic ring expansion, contraction, and metathesis-polymerization of cycloalkanes

Tandem dehydrogenation-olefin-metathesis catalyst systems, comprising a pincer-ligated iridium-based alkane dehydrogenation catalyst and a molybdenum-based olefin-metathesis catalyst, are reported to effect the metathesis-cyclooligomerization of cyclooctane and cyclodecane to give cycloalkanes with various carbon numbers, predominantly multiples of the substrate carbon number, and polymers.

Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides

Abstract The reaction of some organometallic complexes with the surfaces of inorganic oxides leads to the formation of surface organometallic complexes, chemically bound to the surface yet retaining many features of their molecular structure. These surface organometallic complexes can therefore be considered to belong to both the molecular and solid states. In cases where such complexes have been structurally characterised, their reactivity can be interpreted with molecular concepts. In this review article, the stoichiometric and catalytic reactivity of some relatively well-defined surface organometallic fragments is surveyed. Many elementary steps which have precedent in molecular organometallic chemistry and homogeneous catalysis have now been demonstrated with surface organometallic fragments, including reversible ligand binding, oxidative addition, reductive elimination, protonation, heterolytic metal—carbon bond cleavage, electrophilic CH bond activation and insertion into metal—carbon bonds. In some cases, the supported organometallic complexes are highly effective low temperature catalysts, a phenomenon which is not always observed with molecular analogues nor with conventionally prepared heterogeneous catalysts. Applications of surface organometallic chemistry to catalytic alkane hydrogenolysis, olefin isomerisation and hydrogenation, the Fischer—Tropsch synthesis and the water—gas shift reaction are discussed. Proposed mechanisms for several representative catalytic cycles are presented.

A Biomimetic Pathway for Vanadium‐Catalyzed Aerobic Oxidation of Alcohols: Evidence for a Base‐Assisted Dehydrogenation Mechanism

The first step in the catalytic oxidation of alcohols by molecular O(2), mediated by homogeneous vanadium(V) complexes [LV(V)(O)(OR)], is ligand exchange. The unusual mechanism of the subsequent intramolecular oxidation of benzyl alcoholate ligands in the 8-hydroxyquinolinato (HQ) complexes [(HQ)(2)V(V)(O)(OCH(2)C(6)H(4)-p-X)] involves intermolecular deprotonation. In the presence of triethylamine, complex 3 (X = H) reacts within an hour at room temperature to generate, quantitatively, [(HQ)(2)V(IV)(O)], benzaldehyde (0.5 equivalents), and benzyl alcohol (0.5 equivalents). The base plays a key role in the reaction: in its absence, less than 12% conversion was observed after 72 hours. The reaction is first order in both 3 and NEt(3), with activation parameters ΔH(≠)=(28±4) kJ mol(-1) and ΔS(≠)=(-169±4) J K(-1)  mol(-1). A large kinetic isotope effect, 10.2±0.6, was observed when the benzylic hydrogen atoms were replaced by deuterium atoms. The effect of the para substituent of the benzyl alcoholate ligand on the reaction rate was investigated using a Hammett plot, which was constructed using σ(p). From the slope of the Hammett plot, ρ=+(1.34±0.18), a significant buildup of negative charge on the benzylic carbon atom in the transition state is inferred. These experimental findings, in combination with computational studies, support an unusual bimolecular pathway for the intramolecular redox reaction, in which the rate-limiting step is deprotonation at the benzylic position. This mechanism, that is, base-assisted dehydrogenation (BAD), represents a biomimetic pathway for transition-metal-mediated alcohol oxidations, differing from the previously identified hydride-transfer and radical pathways. It suggests a new way to enhance the activity and selectivity of vanadium catalysts in a wide range of redox reactions, through control of the outer coordination sphere.

Phillips Cr/Silica Catalyst for Ethylene Polymerization

The Phillips Cr/silica catalyst, discovered by Hogan and Banks at the Phillips Petroleum Company in the early 1950s, is one of the most important industrial catalysts for polyethylene production. In contrast to its great commercial success during the past half-century, academic progress regarding a basic understanding of the nature of the active sites and polymerization mechanisms is lagging far behind. During the last decade, increasing research efforts have been performed on the Phillips catalyst through various approaches, including spectroscopic methods, polymerization kinetics, heterogeneous model catalysts, homogeneous model catalysts, and molecular modeling. Much deeper mechanistic understanding, together with successive catalyst innovations through modifications of the Phillips catalyst, has been achieved.