Daniel A. Ruddy

Recent advances in heterogeneous catalysts for bio-oil upgrading via “ex situ catalytic fast pyrolysis”: catalyst development through the study of model compounds

Advances in heterogeneous catalysis are driven by the structure–function relationships that define catalyst performance (i.e., activity, selectivity, lifetime). To understand these relationships, cooperative research is required: prediction and analysis using computational models, development of new synthetic methods to prepare specific solid-state compositions and structures, and identification of catalytically active site(s), surface-bound intermediates, and mechanistic pathways. In the application of deoxygenating and upgrading biomass pyrolysis vapors, a fundamental understanding of the factors that favor C–O bond cleavage and C–C bond formation is still needed. In this review, we focus on recent advances in heterogeneous catalysts for hydrodeoxygenation of biomass pyrolysis products. Focus is placed on studies that made use of model compounds for comparisons of catalysts and the reaction networks they promote. Applications of transition metal sulfide catalysts for deoxygenation processes are highlighted, and compared to the performances of noble metal and metal carbide, nitride, and phosphide catalysts. In general, it is found that bifunctional catalysts are required for deoxygenation in a single reactor, with bifunctionality achieved on the catalyst or in conjunction with the catalyst support. Catalysts that activate hydrogen well will be preferred for ex situ catalytic pyrolysis conditions (upgrading downstream of pyrolysis reactor prior to condensation of bio-oil, pressures near atmospheric, temperatures between 350–500 °C). Supports that limit chemisorption of large reactants (leading to blockage of catalyst sites) should be employed. Finally, the stability of the catalyst and support in high-steam and low hydrogen-to-carbon environments will be critical.

Size and Bandgap Control in the Solution-Phase Synthesis of Near-Infrared-Emitting Germanium Nanocrystals

We present a novel colloidal synthesis of alkyl-terminated Ge nanocrystals based on the reduction of GeI(4)/GeI(2) mixtures. The size of the nanocrystals (2.3-11.3 nm) was controlled by adjusting both the Ge(IV)/Ge(II) ratio and the temperature ramp rate following reductant injection. The near-infrared absorption (1.6-0.70 eV) and corresponding band-edge emission demonstrate the highly tunable quantum confinement effects in Ge nanocrystals prepared using this mixed-valence precursor method. A mechanism is proposed for the observed size control, which relies upon the difference in reduction temperatures for Ge(II) versus Ge(IV).

Control of PbSe quantum dot surface chemistry and photophysics using an alkylselenide ligand.

We have synthesized alkylselenide reagents to replace the native oleate ligand on PbSe quantum dots (QDs) in order to investigate the effect of surface modification on their stoichiometry, photophysics, and air stability. The alkylselenide reagent removes all of the oleate on the QD surface and results in Se addition; however, complete Se enrichment does not occur, achieving a 53% decrease in the amount of excess Pb for 2 nm diameter QDs and a 23% decrease for 10 nm QDs. Our analysis suggests that the Se ligand preferentially binds to the {111} faces, which are more prevalent in smaller QDs. We find that attachment of the alkylselenide ligand to the QD surface enhances oxidative resistance, likely resulting from a more stable bond between surface Pb atoms and the alkylselenide ligand compared to Pb-oleate. However, binding of the alkylselenide ligand produces a separate nonradiative relaxation route that partially quenches PL, suggesting the formation of a dark hole-trap.

Kinetics and Mechanism of Olefin Epoxidation with Aqueous H2O2 and a Highly Selective Surface-Modified TaSBA15 Heterogeneous Catalyst

The reaction kinetics of cyclohexene epoxidation using aqueous H2O2 oxidant and the highly selective epoxidation catalyst Bu(cap)TaSBA15 were studied. The reaction was determined to be first-order in Ta(V) surface coverage. The reaction rate exhibited saturation with respect to increasing concentrations of cyclohexene and H2O2. An Eley-Rideal mechanism and rate equation may be used to describe the epoxidation kinetics, which are similar to those for Ti(IV)SiO2-catalyzed epoxidations. The observed kinetics may also be modeled by a double-displacement mechanism typically associated with saturation enzyme catalysts. In addition, (1)H NMR spectroscopy was employed to investigate H2O2 decomposition by Bu(cap)TaSBA15 and the unmodified TaSBA15 catalysts. Little decomposition occurred over the surface-modified material, but the unmodified material catalyzed a 30% conversion of H2O2 after 6 h. UV-visible absorbance and diffuse reflectance UV-visible (DRUV-vis) spectroscopy were used to investigate the structure of the Ta centers on the TaSBA15 catalysts. DRUV-vis spectroscopy was also used to identify a Ta(V)-based epoxidation intermediate, proposed to be a Ta(V)(eta(2)-O2) species, which forms upon reaction of the TaSBA15 and Bu(cap)TaSBA15 materials with H2O2.

Surface Chemistry Exchange of Alloyed Germanium Nanocrystals: A Pathway Toward Conductive Group IV Nanocrystal Films.

We present an expansion of the mixed-valence iodide reduction method for the synthesis of Ge nanocrystals (NCs) to incorporate low levels (∼1 mol %) of groups III, IV, and V elements to yield main-group element-alloyed Ge NCs (Ge1-xEx NCs). Nearly every main-group element (E) that surrounds Ge on the periodic table (Al, P, Ga, As, In, Sn, and Sb) may be incorporated into Ge1-xEx NCs with remarkably high E incorporation into the product (>45% of E added to the reaction). Importantly, surface chemistry modification via ligand exchange allowed conductive films of Ge1-xEx NCs to be prepared, which exhibit conductivities over large distances (25 μm) relevant to optoelectronic device development of group IV NC thin films.

Organometallic model complexes elucidate the active gallium species in alkane dehydrogenation catalysts based on ligand effects in Ga K-edge XANES

Gallium-modified zeolites are known catalysts for the dehydrogenation of alkanes, reactivity that finds industrial application in the aromatization of light alkanes by Ga-ZSM5. While the role of gallium cations in alkane activation is well known, the oxidation state and coordination environment of gallium under reaction conditions has been the subject of debate. Edge shifts in Ga K-edge XANES spectra acquired under reaction conditions have long been interpreted as evidence for reduction of Ga(III) to Ga(I). However, a change in oxidation state is not the only factor that can give rise to a change in the XANES spectrum. In order to better understand the XANES spectra of working catalysts, we have synthesized a series of molecular model compounds and grafted surface organometallic Ga species and compared their XANES spectra to those of gallium-based catalysts acquired under reducing conditions. We demonstrate that changes in the identity and number of gallium nearest neighbors can give rise to changes in XANES spectra similar to those attributed in literature to changes in oxidation state. Specifically, spectral features previously attributed to Ga(I) may be equally well interpreted as evidence for low-coordinate Ga(III) alkyl or hydride species. These findings apply both to gallium-impregnated zeolite catalysts and to silica-supported single site gallium catalysts, the latter of which is found to be active and selective for dehydrogenation of propane and hydrogenation of propylene.

Synthesis of α‐MoC1−x Nanoparticles with a Surface‐Modified SBA‐15 Hard Template: Determination of Structure–Function Relationships in Acetic Acid Deoxygenation

Surface modification of mesoporous SBA-15 silica generated a hydrophobic environment for a molybdenum diamine (Mo-diamine) precursor solution, enabling direct growth of isolated 1.9±0.4 nm α-MoC1-x nanoparticles (NPs) inside the pores of the support. The resulting NP catalysts are bifunctional, and compared to bulk α-MoC1-x and β-Mo2 C, the NPs exhibit a greater acid-site:H-site ratio and a fraction of stronger acid sites. The greater acid-site:H-site ratio results in higher decarbonylation (DCO) selectivity during acetic acid hydrodeoxygenation (HDO) reactions, and the stronger acid sites lead to higher activity and ketonization (KET) selectivity at high temperatures. The hard-templating synthetic method could be a versatile route toward carbide NPs of varying size, composition, and phase, on a range of mesoporous oxide supports.

Synthesis, optical, and photocatalytic properties of cobalt mixed-metal spinel oxides Co(Al1−xGax)2O4

Cobalt mixed-metal spinel oxides, Co(Al1−xGax)2O4, have been predicted to exhibit promising properties as photocatalysts for solar energy conversion. In this work, Co(Al1−xGax)2O4 were synthesized with a range of 0 ≤ x ≤ 1 via both single-source and multi-source routes. Single-source molecular precursors, [Co{M(OtBu)4}2] (M = Al or Ga), were decomposed at 300 °C to form amorphous oxides. Multi-source precursors, stoichiometric mixtures of metal acetylacetonate (acac) complexes, were used to form nanocrystalline spinel materials. Both were subsequently converted to bulk spinel products by annealing at 1000 °C. The properties of materials fabricated from the single-source and multi-source synthetic routes were compared by analysing data from X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis spectrophotometry, inductively coupled plasma-optical emission spectroscopy, and gas sorption measurements. The X-ray diffraction data of the materials showed ideal solid solution behavior that followed Vegards law for both routes, with the multi-source route giving more crystalline bulk material than the single-source route. UV-vis absorbance data revealed that the absorption onset energies of Co(Al1−xGax)2O4 decreased monotonically with increasing x (from 1.84 eV for x = 0 to 1.76 eV for x = 1 from the single-source method; 1.75 eV for x = 0 to 1.70 eV for x = 1 from the multi-source method). The photocatalytic activities of the spinel oxides were evaluated via the photodegradation of methyl orange and phenol, which showed that the photoactivity of Co(Al0.5Ga0.5)2O4 was dependent on both pH and substrate. Remarkably, under appropriate substrate binding conditions (pH 3 with methyl orange), low energy (<2.5 eV) ligand–field transitions contributed between 46 and 72% of the photoactivity of Co(Al0.5Ga0.5)2O4 prepared from the multi-source route.

Acylative Dimerization of Tetrahydrofuran Catalyzed by Rare‐Earth Triflates

Abstract Ytterbium, scandium, and lanthanum triflates catalyze the cleavage reactions of cyclic ethers to give various products. Most notable was the acylative cleavage of tetrahydrofuran with acetic anhydride catalyzed by Yb(triflate)3 to give the dimeric compound CH3CO2(CH2)4O(CH2)4OCOCH3 in 69% yield.

An investigation into support cooperativity for the deoxygenation of guaiacol over nanoparticle Ni and Rh2P

The production of hydrocarbon fuels from biomass pyrolysis requires the development of effective deoxygenation catalysts, and insight into how the properties of the support influence performance is critical for catalyst design. In this report, nanoparticles of Ni and Rh2P were synthesized using solution-phase techniques and dispersed on high surface area supports. The supports included a relatively inert material (C), an acidic reducible metal-oxide (TiO2), an acidic irreducible metal-oxide (Al2O3), and a basic irreducible metal-oxide (MgO). The eight active phase/support combinations were investigated for the deoxygenation of guaiacol, a pyrolysis vapor model compound, under ex situ catalytic fast pyrolysis conditions (350 °C, 0.44 MPa H2). Compared to the baseline performance of the C-supported catalysts, Ni/TiO2 and Rh2P/TiO2 exhibited higher guaiacol conversion and lower O : C ratios for C5+ products, highlighting the enhanced activity and greater selectivity to deoxygenated products derived from the use of an acidic reducible metal-oxide support. The Al2O3-supported catalysts also exhibited higher conversion than the C-supported catalysts and promoted alkylation reactions, which improve carbon efficiency and increase the carbon number of the C5+ products. However, Ni/Al2O3 and Rh2P/Al2O3 were less selective towards deoxygenated products than the C-supported catalysts. The MgO-supported catalyst exhibited lower conversion and decreased yield of deoxygenated products compared to the C-supported catalysts. The results reported here suggest that basic metal-oxide supports may inhibit deoxygenation of phenolics under CFP conditions. Contrastingly, support acidity and reducibility were demonstrated to promote conversion and selectivity to deoxygenated products, respectively.