Steven S.C. Chuang

Infrared study of the CO insertion reaction on reduced, oxidized, and sulfided Rh/SiO2 catalysts

The reaction of adsorbed CO with C2H4/H2 and ethylene hydroformylation over reduced, oxidized, and sulfided Rh/SiO2 catalysts has been investigated under reaction conditions by in situ infrared spectroscopy. Both reactions involve the CO insertion step resulting in the formation of propionaldehyde. The linear CO adsorbed on the Rh0 site of the reduced Rh/SiO2 and the linear CO adsorbed on the Rh+ site of the oxidized Rh/SiO2 are consumed and propionaldehyde is produced as the adsorbed CO reacts with C2H4 and H2. The results show that both single Rh0 atoms and Rh+ ion sites are active for the CO insertion reaction. In situ infrared observations of steady-state ethylene hydroformylation show that the oxidized Rh/SiO2 containing considerable amounts of Rh+ sites exhibits rate and selectivity for the formation of propionaldehyde higher than those of the reduced Rh/SiO2; the sulfided Rh/SiO2 comprising mainly single Rh sites shows rate and selectivity for the formation of propionaldehyde higher than that of the reduced Rh/SiO,. The low selectivity of the reduced Rh/SiO2 for ethylene hydroformylation is attributed to the high hydrogenation activity of the Rh crystallite surface on the reduced Rh/SiO2.

Tanshinones Inhibit Amyloid Aggregation by Amyloid-β Peptide, Disaggregate Amyloid Fibrils, and Protect Cultured Cells

The misfolding and aggregation of amyloid-β (Aβ) peptides into amyloid fibrils is regarded as one of the causative events in the pathogenesis of Alzheimers disease (AD). Tanshinones extracted from Chinese herb Danshen (Salvia Miltiorrhiza Bunge) were traditionally used as anti-inflammation and cerebrovascular drugs due to their antioxidation and antiacetylcholinesterase effects. A number of studies have suggested that tanshinones could protect neuronal cells. In this work, we examine the inhibitory activity of tanshinone I (TS1) and tanshinone IIA (TS2), the two major components in the Danshen herb, on the aggregation and toxicity of Aβ1-42 using atomic force microscopy (AFM), thioflavin-T (ThT) fluorescence assay, cell viability assay, and molecular dynamics (MD) simulations. AFM and ThT results show that both TS1 and TS2 exhibit different inhibitory abilities to prevent unseeded amyloid fibril formation and to disaggregate preformed amyloid fibrils, in which TS1 shows better inhibitory potency than TS2. Live/dead assay further confirms that introduction of a very small amount of tanshinones enables protection of cultured SH-SY5Y cells against Aβ-induced cell toxicity. Comparative MD simulation results reveal a general tanshinone binding mode to prevent Aβ peptide association, showing that both TS1 and TS2 preferentially bind to a hydrophobic β-sheet groove formed by the C-terminal residues of I31-M35 and M35-V39 and several aromatic residues. Meanwhile, the differences in binding distribution, residues, sites, population, and affinity between TS1-Aβ and TS2-Aβ systems also interpret different inhibitory effects on Aβ aggregation as observed by in vitro experiments. More importantly, due to nonspecific binding mode of tanshinones, it is expected that tanshinones would have a general inhibitory efficacy of a wide range of amyloid peptides. These findings suggest that tanshinones, particularly TS1 compound, offer promising lead compounds with dual protective role in anti-inflammation and antiaggregation for further development of Aβ inhibitors to prevent and disaggregate amyloid formation.

Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture

Oxidative degradation characteristics of silica-supported amine sorbents with varying amounts of tetraethylenepentamine (TEPA) and polyethylene glycol (PEG; P200 or P600 represents PEG with molecular weights of 200 or 600) have been studied by IR and NMR spectroscopy. Thermal treatment of the sorbents and liquid TEPA at 100 °C for 12 h changed their color from white to yellow. The CO2 capture capacity of the TEPA/SiO2 sorbents (i.e., SiO2-supported TEPA with a TEPA/SiO2 ratio of 25:75) decreased by more than 60 %. IR and NMR spectroscopy studies showed that the yellow color of the degraded sorbents resulted from the formation of imide species. The imide species, consisting of NH associated with two C—O functional groups, were produced from the oxidation of methylene groups in TEPA. Imide species on the degraded sorbent are not capable of binding CO2 due to its weak basicity. The addition of P200 and P600 to the supported amine sorbents improved both their CO2 capture capacities and oxidative degradation resistance. IR spectroscopy results also showed that TEPA was immobilized on the SiO2 surface through hydrogen bonding between amine groups and the silanol groups of SiO2. The OH groups of PEG interact with NH2/NH of TEPA through hydrogen bonding. Hydrogen bonds disperse TEPA on SiO2 and block O2 from accessing TEPA for oxidation. Oxidative degradation resistance and CO2 capture capacity of the supported amine sorbents can be optimized through adjusting the ratio of hydroxyl to amine groups in the TEPA/PEG mixture.

In Situ ATR and DRIFTS Studies of the Nature of Adsorbed CO2 on Tetraethylenepentamine Films

CO2 adsorption/desorption onto/from tetraethylenepentamine (TEPA) films of 4, 10, and 20 μm thicknesses were studied by in situ attenuated total reflectance (ATR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques under transient conditions. Molar absorption coefficients for adsorbed CO2 were used to determine the CO2 capture capacities and amine efficiencies (CO2/N) of the films in the DRIFTS system. Adsorption of CO2 onto surface and bulk NH2 groups of the 4 μm film produced weakly adsorbed CO2, which can be desorbed at 50 °C by reducing the CO2 partial pressure. These weakly adsorbed CO2 exhibit low ammonium ion intensities and could be in the form of ammonium-carbamate ion pairs and zwitterions. Increasing the film thickness enhanced the surface amine-amine interactions, resulting in strongly adsorbed ion pairs and zwitterions associated with NH and NH2 groups of neighboring amines. These adsorbed species may form an interconnected surface network, which slowed CO2 gas diffusion into and diminished access of the bulk amine groups (or amine efficiency) of the 20 μm film by a minimum of 65%. Desorption of strongly adsorbed CO2 comprising the surface network could occur via dissociation of NH3(+)/NH2(+)···NH2/NH ionic hydrogen bonds beginning from 60 to 80 °C, followed by decomposition of NHCOO(-)/NCOO(-) at 100 °C. These results suggest that faster CO2 diffusion and adsorption/desorption kinetics could be achieved by thinner layers of liquid or immobilized amines.

Role of silver promoter in carbon monoxide hydrogenation and ethylene hydroformylation over Rh/SiO2 catalysts

The effect of silver promotion on CO hydrogenation and ethylene hydroformylation over Rh/SiO2 has been studied. Catalyst characterization before reaction reveals that Ag and Rh form separate crystallites; part of the Ag is atomically spread on the surface of Rh crystallite and blocks the Rh ensembles required for the bridge CO. Although both reactions restructure the catalyst surface and increase the ratio of bridge to linear CO sites on AgRh/SiO2 catalysts, the ratio of the bridge to the linear CO sites remains lower on Ag-Rh/SiO2 than on Rh/SiO2 under reaction conditions. Ag is found to increase the selectivity and the rate of formation for acetaldehyde (C2 oxygenate) during CO hydrogenation and for propionaldehyde (C3 oxygenate) during ethylene hydroformylation on Rh/SiO2. The increased activity and selectivity for C2 and C3 oxygenates is attributed to a high ratio of the linear CO to bridge CO sites and a possible presence of isolated Rh+ sites under reaction conditions. The results are consistent with a previous report that the single Rh atom site that chemisorbs linear CO is active for CO insertion.

CATALYTIC HYDRODECHLORINATION OF 1,2-DICHLOROETHANE AND TRICHLOROETHYLENE OVER Rh/Si0 2 CATALYSTS

Abstract Reactions of hydrogen with 1,2-dichloroethane and trichloroethylene have been investigated over Rh/Si0 2 catalysts at 365-553K and 0.1 MPa. The major products of the reactions are HCl and C 2 H 6. The selectivity for C 2 H 6 increases with increasing temperature. C 2 HC 5 Cl is a minor product for the dichloroethane reaction; partially dechlorinated products, i.e. C 2 H 5 Cl, C 2 H 4 CI 2, C 2 H 3 Cl, and C 2 H 2 CI 2, constitute the minor products for the trichloroethylene reaction. The selectivity for the partially dechlorinated C 2 hydrocarbons decreases with increasing temperature. Cl dissociated from chlorinated hydrocarbons poisons the catalyst. Regeneration of catalyst by hydrogen treatment restores the catalyst to more than 70% of the initial activity for the trichloroethylene reaction. Probing the hydrogenation activity of the used Rh/Si0 2 and RhCl 3 /Si0 2 with ethylene hydrogenation shows that the presence of CI on Rh/Si0 2 decreases hydrogenation but increases ethylene hydrogenolysis...

C2 oxygenate synthesis from CO hydrogenation on AgRh/SiO2

The effect of silver on carbon monoxide hydrogenation over Rh/SiO2 has been studied. Silver is found to decrease the rates of formation for methane and C2+ hydrocarbons more than those for C2 oxygenates resulting in a marked increase in C2 oxygenate selectivity. Infrared spectroscopic studies reveal that Ag blocks the bridge-CO sites. Ethylene addition studies show that Ag promotes carbon monoxide insertion and suppresses hydrogenation. The results suggest that the number of Rh atoms required for carbon monoxide insertion may be less than that for hydrogenation and methanation.

Ethylene addition to CO hydrogenation over Sslfided Ni, Rh, and Ru

Abstract The effect of H2S on ethylene addition to CO hydrogenation has been studied over Ni/SiO2, Rh/ SiO2, and Ru/SiO2 catalysts at 300°C and 10 atm. The major products of the ethylene addition on the unsulfided catalysts are ethane and propionaldehyde which result from ethylene hydrogenation and the insertion of CO into adsorbed ethylene species, respectively. Sulfidation of the catalysts led to the suppression of ethylene hydrogenation and CO hydrogenation. CO insertion over Ni/SiO2 and Rh/SiO2 catalysts was insensitive to sulfidation, whereas CO insertion on Ru/SiO2 was inhibited by sulfidation. Infrared studies of CO adsorption reveal that sulfidation of the catalysts led to the disruption of neighboring surface atoms for bridge CO resulting in the formation of isolated atom sites which maybe responsible for CO insertion on the sulfided Ni and Rh catalysts.

Mechanistic study in catalysis using dynamic and isotopic transient infrared spectroscopy: CO/H2C2H4 reaction on MnRh/SiO2

Abstract In situ infrared (IR) spectroscopy coupled with dynamic and steady-state isotopic transient kinetic analysis permits observation of the transient response (i.e., dynamics) of IR-observable adsorbates as well as gaseous reactants and products. This technique was used to examine the reaction pathway, reactivity of adsorbates, and nature of sites for the CO/H2/C2H4 reaction on Mn Rh/SiO2. Dynamic infrared study reveals that Rh0 sites which chemisorb linear CO actively catalyze CO insertion, a key step for the formation of propionaldehyde from the CO/H2/C2H4 reaction. Tilted CO on Mn Rh/SiO2 is not active for the reaction, demonstrating a lack of connection in reactivity between the low wavenumber CO on metal surfaces and in metal complexes. Steady-state isotopic transient IR studies by pulse injection of D2 and13CO produce transient responses which carry mechanistic information for elucidation of catalysis of the CO/H2/C2H4 reaction. The broad response of D-containing propionaldehyde reveals that multiple pathways for hydrogenation take place simultaneously with a wide distribution of the reactivity of adsorbed deuterium or hydrogen; the sharp response of C2H513CHO unravels the narrow distribution of reactivity of adsorbed CO for CO insertion.

Combined infrared and mass spectrometric study of reactions of adsorbed NO and CO on 0.5 wt% Rh/SiO2 catalyst

Abstract NO adsorption, CO adsorption, NO temperature-programmed desorption and decomposition (TPD), and temperature-programmed reaction (TPR) of NOCO have been studied over 0.5 wt% Rh/SiO2 catalysts by a combined infrared and mass spectrometric technique. Infrared study reveals that the high wavenumber RhNOδ− at 1723–1740 cm−1 is the dominant adsorbate during TPD and TPR with NO:CO = 1:1. During TPR, CO reduces part of Rh surface resulting in the formation of the low wavenumber Rh-NOδ−1 at 1634–1680 cm−. Increasing CO partial pressure (i) promotes the formation of Rh0 sites, producing linear CO, (ii) increases the selectivity to N2O below the light-off temperature, (iii) raises the light-off temperature, and (iv) promotes the formation of SiNCO and RhNCO. Comparison of results of the present study with those of previous studies on 4 wt% Rh/SiO2 shows that different dispersion of Rh crystallites on SiO2 support results in significant variation in chemisorptive and reactive properties of Rh metal for the NOCO reaction.