Increasing energy demand and dwindling natural resources have led to wide interest in developing synthetic fuels and chemicals that can supplement and replace the fuels and chemicals obtained from ...conventional petroleum based feedstocks. Alternative fuels and chemicals can be derived from non petroleum based feedstocks such as coal and biomass. Biomass is considered particularly attractive because of the potential sustainability and carbon neutrality associated with its use. Thus great effort is being dedicated to the use and conversion of biomass to fuels and chemicals. Cellulose is the most abundant renewable carbon source present in the lignocellulosic (non edible biomass) realm, and has a rigid structure consisting of a number of glucose units linked together. Consequently effort has focused on the catalytic conversion of glucose and also cellulose directly to 5-hydroxymethylfurfural (HMF), a versatile intermediate for synthesis of fuels and chemicals. A number of studies have claimed that Cr catalysts in ionic liquid solvents convert glucose and fructose to HMF in unprecedentedly high yields. However, ambiguity over the oxidation state of Cr has remained. Knowledge of the specific metal oxidation state is essential to gain an understanding of the reaction mechanism and to design still more active catalysts. Herein we demonstrate Cr(III) to be the active species, rather than Cr(II) which had previously been implicated and subsequently assumed by others.
► Cr(III) and not Cr(II) is the active catalyst for the conversion of sugars to HMF. ► The substitution inertness of Cr(III) is largely responsible for its activity. ► Cr(III) complexes with weak ligands are very active catalysts for making HMF. ► Other substitution inert ions also yield active catalysts for making HMF.
Nanosize-controllable Cu, Ag, Pd, Ni, CuPd alloy and Cu-Ag bimetal encapsulated in inert carbon shells can be synthesized by carbonization of metal-cyclodextrin complexes with a remarkable capability ...of experimentally determining the size-dependent melting temperature depression of many metal or alloy nanoparticles.
An effective copper-based oxygen carrier for use in chemical looping with oxygen uncoupling (CLOU) has been developed, and its physical and reactive properties have been evaluated. The preparation ...method involves coating β-SiC support material with CuO and then baking the coated material at 980 °C which causes the β-SiC to convert to SiO2, thus enveloping the CuO. Variations of the preparation technique, including different forms of SiC, methods of CuO addition, and the order of CuO addition and baking, were tested. It was determined that preparation by rotary evaporation CuO deposition and final sintering produced superior carrier particles. Loadings as high as 60 wt % CuO were achieved. The carrier particles fluidized well, and for loadings to 40 wt % CuO, no agglomeration was observed at temperatures as high as 1000 °C. The particles retained reactivity over many oxidation and reduction cycles. The coat-then-bake preparation method using β-SiC is a viable candidate to be used as oxygen carrying material in CLOU.
A systematic study on the adsorption of xenon on silver clusters in the gas phase and on the (001) surface of silver-exchanged chabazite is reported. Density functional theory at the B3LYP level with ...the cluster model was employed. The results indicate that the dominant part of the binding is the σ donation, which is the charge transfer from the 5p orbital of Xe to the 5s orbital of Ag and is not the previously suggested dπ−dπ back-donation. A correlation between the binding energy and the degree of σ donation is found. Xenon was found to bind strongly to silver cluster cations and not to neutral ones. The binding strength decreases as the cluster size increases for both cases, clusters in the gas-phase and on the chabazite surface. The Ag+ cation is the strongest binding site for xenon both in gas phase and on the chabazite surface with the binding energies of 73.9 and 14.5 kJ/mol, respectively. The results also suggest that the smaller silver clusters contribute to the negative chemical shifts observed in the 129Xe NMR spectra in experiments.
Xenon Adsorption on Modified ETS-10 Kuznicki, Steven M; Ansón, Alejandro; Koenig, Andrée ...
Journal of physical chemistry. C,
02/2007, Letnik:
111, Številka:
4
Journal Article
Recenzirano
The interaction of xenon with silver ETS-10 is found to be unusually strong. Xenon adsorption was studied on Na-ETS-10 and its silver exchanged counterpart, Ag-ETS-10, by gas chromatography and ...gravimetric adsorption. High adsorption capacities were observed even at low pressure (6 wt % Xe at 0.5 Torr and 25 °C). High isosteric heats of adsorption for xenon on Ag-ETS-10 were observed, higher than on any other adsorbent reported to date. High selectivity of xenon over nitrogen and oxygen is also observed, especially at low xenon partial pressures. The great affinity of this adsorbent for xenon is attributed to the presence of silver nanoparticles, which grow on the surface of the molecular sieve after heat treatment of Ag exchanged material.
Cobalt catalysts supported on silica aerogel have been prepared using sol–gel chemistry followed by drying under supercritical ethanol conditions. Three different loadings of cobalt were synthesized: ...2, 6, and 10% by weight. Transmission electron micrographs indicate that the metallic cobalt exists as discrete particles 50–70
nm in diameter for the 2 and 6% loadings. The 10% catalyst shows long needles of cobalt. BET and BJH measurements indicate that the catalysts retain the silica aerogel properties of high surface area (∼800
m
2/g), large pore volume (∼5
cm
3/g), and an average pore diameter in the mesoporous regime (∼25
nm). The catalysts were evaluated for Fischer–Tropsch activity in a laboratory-scale packed bed reactor. All three catalysts were active with the 10% Co catalyst achieving more than 20% CO conversion which corresponds to a rate of 1.53
g CO per g-cat per hour. The catalysts were selective for the C
10+ hydrocarbons with more than 50% of the carbon contained within this fraction. A significant portion of the C
9–C
15 hydrocarbon product was observed as 1-olefins which reflects the enhanced mass transport within the very porous aerogel support.
Iron aerogels, potassium-doped iron aerogels, and potassium-doped iron xerogels have been synthesized and characterized and their catalytic activity in the Fischer−Tropsch (F-T) reaction has been ...studied. Iron aerogels and xerogels were synthesized by polycondensation of an ethanolic solution of iron(III) chloride hexahydrate with propylene oxide which acts as a proton scavenger for the initiation of hydrolysis and polycondensation. Potassium was incorporated in the iron aerogel and iron xerogel by adding aqueous K2CO3 to the ethanolic solutions of the Fe(III) precursor prior to addition of propylene oxide. Fischer−Tropsch activities of the catalysts were tested in a fixed bed reactor at a pressure of 100 psi with a H2:CO ratio of 2:1. Iron aerogels were found to be active for F-T synthesis, and their F-T activities increased on addition of a K containing promoter. Mössbauer spectroscopic data are consistent with an open, nonrigid iron(III) aerogel structure progressing to an iron carbide/metallic iron catalyst via agglomeration as the F-T synthesis proceeds in the course of a 35 h fixed bed reaction test.
The silylation of SBA-15 enhances the reducibility of cobalt oxides on a SBA-15 supported cobalt catalyst, and consequently increases the catalytic activity for Fischer-Tropsch synthesis of ...hydrocarbons from syngas and selectivity for longer chain products.
Ferrate(VI) Oxidation of Aqueous Phenol: Kinetics and Mechanism Huang, Hua; Sommerfeld, David; Dunn, Brian C ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
04/2001, Letnik:
105, Številka:
14
Journal Article
Recenzirano
Kinetic and thermodynamic parameters for ferrate(VI) oxidation of phenol have been measured in isotopic solvents, H2O and D2O, using ambient and high-pressure stopped-flow UV−visible spectroscopy. An ...increase (fast stage) and then a decrease (slow stage) in absorbance at 400 nm are observed when potassium ferrate (K2FeO4) and aqueous phenol solutions are mixed rapidly. This suggests that small amounts of unstable intermediate 4,4‘-biphenoquinone are produced during this redox process. An electron paramagnetic resonance signal for the reaction mixture of ferrate and phenol trapped by spin-trap α-(4-pyridyl-1-oxide)-N-tert-butylnitrone indicates a radical reaction pathway. Gas chromatographic/mass spectrometric measurements show p-benzoquinone is a major organic product, and the red ferric thiocyanate complex formed from addition of potassium thiocyanate to the spent reaction solution indicates that Fe(VI) is reduced to Fe(III). Activation enthalpy, entropy, and volume changes have been determined. There is a primary isotope effect for the formation of the intermediate (fast stage), k fast(H2O)/k fast(D2O) = 2.4 ± 0.6. Because the phenol hydroxylic hydrogen is deuterated in D2O, this isotope effect suggests that a hydrogen bond is formed in the transition state.
Diethyl carbonate (DEC) has been produced by the oxidative carbonylation of ethanol in the gas phase over a heterogeneous CuCl2/PdCl2 catalyst supported on activated carbon. Yields of DEC with this ...catalyst are approximately 10 wt % with the byproducts diethoxymethane, ethyl formate, and acetaldehyde also formed in significantly lower yields. Treatment of the catalyst immediately after preparation with potassium hydroxide enhances the production of DEC almost 2-fold without increasing the amount of byproducts formed. The reactions that form DEC and the byproducts occur in a parallel, rather than a sequential, manner indicating that it should be possible to identify a catalyst which is more selective for DEC.
