Hien N. Pham

Selective Hydrogenolysis of Polyols and Cyclic Ethers over Bifunctional Surface Sites on Rhodium–Rhenium Catalysts

A ReO(x)-promoted Rh/C catalyst is shown to be selective in the hydrogenolysis of secondary C-O bonds for a broad range of cyclic ethers and polyols, these being important classes of compounds in biomass-derived feedstocks. Experimentally observed reactivity trends, NH(3) temperature-programmed desorption (TPD) profiles, and results from theoretical calculations based on density functional theory (DFT) are consistent with the hypothesis of a bifunctional catalyst that facilitates selective hydrogenolysis of C-O bonds by acid-catalyzed ring-opening and dehydration reactions coupled with metal-catalyzed hydrogenation. The presence of surface acid sites on 4 wt % Rh-ReO(x)/C (1:0.5) was confirmed by NH(3) TPD, and the estimated acid site density and standard enthalpy of NH(3) adsorption were 40 μmol g(-1) and -100 kJ mol(-1), respectively. Results from DFT calculations suggest that hydroxyl groups on rhenium atoms associated with rhodium are acidic, due to the strong binding of oxygen atoms by rhenium, and these groups are likely responsible for proton donation leading to the formation of carbenium ion transition states. Accordingly, the observed reactivity trends are consistent with the stabilization of resulting carbenium ion structures that form upon ring-opening or dehydration. The presence of hydroxyl groups that reside α to carbon in the C-O bond undergoing scission can form oxocarbenium ion intermediates that significantly stabilize the resulting transition states. The mechanistic insights from this work may be extended to provide a general description of a new class of bifunctional heterogeneous catalysts, based on the combination of a highly reducible metal with an oxophilic metal, for the selective C-O hydrogenolysis of biomass-derived feedstocks.

Thermally stable single-atom platinum-on-ceria catalysts via atom trapping

Hot single-atom catalysts For heterogeneous catalysts made from precious metal nanoparticles adsorbed on metal oxides, high temperatures are the enemy. The metal atoms become mobile and the small particles grow larger, causing a loss in surface area and hence in activity. Jones et al. turned this process to their advantage and used these mobile species to create single-atom platinum catalysts. The platinum on alumina supported transfers in air at 800°C to ceria supports to form highly active catalysts with isolated metal cations. Science, this issue p. 150 Exposure of a ceria support to mobile platinum species at high temperatures traps single atoms at the most stable sites. Catalysts based on single atoms of scarce precious metals can lead to more efficient use through enhanced reactivity and selectivity. However, single atoms on catalyst supports can be mobile and aggregate into nanoparticles when heated at elevated temperatures. High temperatures are detrimental to catalyst performance unless these mobile atoms can be trapped. We used ceria powders having similar surface areas but different exposed surface facets. When mixed with a platinum/aluminum oxide catalyst and aged in air at 800°C, the platinum transferred to the ceria and was trapped. Polyhedral ceria and nanorods were more effective than ceria cubes at anchoring the platinum. Performing synthesis at high temperatures ensures that only the most stable binding sites are occupied, yielding a sinter-resistant, atomically dispersed catalyst.

Monodisperse mesoporous silica microspheres formed by evaporation-induced self-assembly of surfactant templates in aerosoles

The present invention provides for evaporation induced self-assembly (EISA) within microdroplets produced by a vibrating orifice aerosol generator (VOAG) for the production of monodisperse mesoporous silica particles. The process of the present invention exploits the concentration of evaporating droplets to induce the organization of various amphiphilic molecules, effectively partitioning a silica precursor to the hydrophilic regions of the structure. Promotion of silica condensation, followed by removal of the surfactant, provides ordered spherical mesoporous particles.

Hydrothermally stable heterogeneous catalysts for conversion of biorenewables

The catalytic conversion of biomass-derived molecules to fuels and chemicals involves reactions carried out in the aqueous phase. The corrosive effects of the reactive environment can cause degradation of heterogeneous catalysts, but the detrimental effects depend on the state of water. For example, water vapor, superheated steam and sub- and supercritical liquid water can behave very different from each other. In this review, we focus on the hydrothermal stability of the heterogeneous catalysts in order of the increasing severity of the reaction medium: superheated steam, liquid water at temperatures below 200 °C, liquid water at temperatures above 200 °C and supercritical water. This review addresses changes in the physical structure of heterogeneous catalysts used for biomass conversion reactions. These physical changes influence the catalytic performance, but other causes for deactivation include sintering of the metal phase or coking or carbon deposition on catalysts. The latter phenomena are not the primary focus of this review. We also describe recent approaches designed to improve the hydrothermal stability of heterogeneous catalysts in biomass conversion reactions.

Photocatalytically‐mediated disinfection of water using tio2 as a catalyst and spore‐forming Bacillus pumilus as a model

Abstract The photocatalytic oxidation process has been evaluated as a basis for disinfecting large volumes of water. A 95 percent reduction in viability of B. pumilus spores, suspended with TiO2 in water, was observed after exposing the model organism to ultraviolet (UV) light (λ 365 nm). Kinetic plots showed that the viability of B. pumilus spores decreased exponentially. No reduction in viability occurred if B. pumilus spores were incubated in the dark (control) or if illumination occurred in the absence of a TiO2 catalyst (TiO2 condition). Intermittent illumination reduced viable B. pumilus spores more effectively than continuous exposure to UV. For various TiO2 concentrations suspended in water from 1 mg/ml to 4 mg/ml, maximum losses of viability were observed at TiO2 concentrations of 2 mg/ml of water and spore densities ≥ 109 CFU/ml. On the basis of the data presented here, the TiO2‐based photocatalytic process shows significant potential as a basis for both stationary and portable water disinfectin...

Improved Hydrothermal Stability of Mesoporous Oxides for Reactions in the Aqueous Phase

A simple and inexpensive approach is used to coat metal oxide surfaces (SBA-15) with thin films of carbon. These carbon films provide improved hydrothermal stability to oxides, such as silica and alumina, which are not otherwise stable at elevated temperatures in the presence of liquid water. Furthermore, the carbon film changes the surface chemistry of the support.

Improving the attrition resistance of slurry phase heterogeneous catalysts

Abstract Slurry phase heterogeneous catalysts for processes such as Fischer–Tropsch (F–T) synthesis must exhibit a high degree of attrition resistance. The precipitated Fe–Cu catalyst used for F–T synthesis is quite weak in its as-prepared state. Spray-drying yields spherical particles which show some improvement in attrition resistance. However, the formation of fines (

Measuring the strength of slurry phase heterogeneous catalysts

Abstract Fischer–Tropsch (F–T) synthesis is a process used to convert coal-derived syngas to hydrocarbon liquids and waxes. A slurry phase bubble-column reactor (SBCR) is the preferred reactor type due to improved heat and mass transfer and operational simplicity in terms of catalyst loading and discharge. A potential disadvantage in the SBCR vs. a fixed-bed reactor is the attrition of the catalyst which can cause difficulty with the catalyst/wax separation, resulting in gradual loss of catalyst from the reactor. In this work, we have evaluated two approaches to measure the strength and attrition resistance of heterogeneous catalysts: uniaxial compaction and ultrasonic fragmentation. A commercial catalyst developed for F–T synthesis was tested along with a sample of an alumina support having a similar particle size distribution. It was found that the cumulative particle size mass distribution plots after ultrasonic fragmentation show significant differences in strength, whereas the same powders show small differences in strength as measured by the uniaxial compaction method. Erosion was found to be the dominant fragmentation mechanism for the alumina support whereas fracture was the dominant mechanism for the F–T catalyst. Ultrasonic fragmentation also was applied to Fe F–T catalysts containing a kaolin binder. The catalyst with the binder was very weak, comparable to the binderless catalyst. Further analysis using transmission electron microscopy (TEM) showed that kaolin and the Fe F–T catalyst occurred as two distinct phases, with plate-like structures which did not help create strong interlocking forces between them. These results provide clues for the design of attrition resistant catalysts.

Selective Aerobic Oxidation of Alcohols over Atomically-Dispersed Non-Precious Metal Catalysts

Catalytic oxidation of alcohols often requires the presence of expensive transition metals. Herein, it is shown that earth-abundant Fe atoms dispersed throughout a nitrogen-containing carbon matrix catalyze the oxidation of benzyl alcohol and 5-hydroxymethylfurfural by O2 in the aqueous phase. The activity of the catalyst can be regenerated by a mild treatment in H2 . An observed kinetic isotope effect indicates that β-H elimination from the alcohol is the kinetically relevant step in the mechanism, which can be accelerated by substituting Fe with Cu. Dispersed Cr, Co, and Ni also convert alcohols, demonstrating the general utility of metal-nitrogen-carbon materials for alcohol oxidation catalysis. Oxidation of aliphatic alcohols is substantially slower than that of aromatic alcohols, but addition of 2,2,6,6-tetramethyl-1-piperidinyloxy as a co-catalyst with Fe can significantly improve the reaction rate.

Role of pore curvature on the thermal stability of gold nanoparticles in mesoporous silica

Pores arranged in a two-dimensional hexagonal structure inside spherical mesoporous silica particles help to prevent the thermal sintering of gold nanoparticles compared to straight pores in MCM-41.