Synthesis and characterization of mesoporic materials containing highly dispersed cobalt

Highly dispersed Co particles in MCM-41 were prepared by direct addition of CoCl2 to the synthesis gel. The small clusters of Co did not sinter during reduction and sulfidation. Incorporation of Co into the MCM-41 lattice was not observed. The addition of Co to the synthesis gel did not alter the structural characteristics of the MCM-41 samples

Di- and Tetrameric Molybdenum Sulfide Clusters Activate and Stabilize Dihydrogen as Hydrides

NaY zeolite-encapsulated dimeric (Mo2S4) and tetrameric (Mo4S4) molybdenum sulfide clusters stabilize hydrogen as hydride binding to Mo atoms. Density functional theory (DFT) calculations and adsorption measurements suggest that stabilization of hydrogen as sulfhydryl (SH) groups, as typical for layered MoS2, is thermodynamically disfavored. Competitive adsorption of H2 and ethene on Mo was probed by quantifying adsorbed CO on partly hydrogen and/or ethene covered samples with IR spectroscopy. During hydrogenation, experiment and theory suggest that Mo is covered predominately with ethene and sparsely with hydride. DFT calculations further predict that, under reaction conditions, each MoxSy cluster can activate only one H2, suggesting that the entire cluster (irrespective of its nuclearity) acts as one active site for hydrogenation. The nearly identical turnover frequencies (24.7 ± 3.3 molethane·h–1·molcluster–1), apparent activation energies (31–32 kJ·mol–1), and reaction orders (∼0.5 in ethene and ∼1.0 in H2) show that the active sites in both clusters are catalytically indistinguishable

Non-neutral processes drive the nucleotide composition of non-coding sequences in Drosophila

The nature of the forces affecting base composition is a key question in genome evolution. There is uncertainty as to whether differences in the GC contents of non-coding sequences reflect differences in mutational bias, or in the intensity of selection or biased gene conversion. We have used a polymorphism dataset for non-coding sequences on the X chromosome of Drosophila simulans to examine this question. The proportion of GC→AT versus AT→GC polymorphic mutations in a locus is correlated with its GC content. This implies the action of forces that favour GC over AT base pairs, which are apparently strongest in GC-rich sequences

Labile sulfate species as key active components in sulfated zirconia for activating n-butane

Sulfated zirconia and other sulfated metal oxides have been studied for over 2 decades owing to their high catalytic activity for activation of short alkanes at low temperature. The surface structure of sulfated zirconia has been studied widely in order to elucidate the nature of active sites since the discovery of its catalytic property for alkanes conversion at low temperature. Nevertheless, no consensus has been reached so far. Here, we report that the labile sulfate, which can be removed from sulfated zirconia by water washing, acts as crucial component for the active site of sulfated zirconia. Experimental Sulfate-doped zirconium hydroxide was obtained from Magnesium Electron, Inc. (XZO 1077/01), which was heated up to 873 K with a ramp rate of 10 K/min in static air and kept at 873 K for 3 h, denoted as SZ. Water washing technique was applied to the above calcined sulfated zirconia. 20 g of SZ were suspended in 400 mL bi-distilled water and then filtered. Repeated the washing procedure for 3 times, then the cake was dried at room temperature. The resulting powder is denoted as SZ-WW. The materials were characterized by IR spectroscopy, (including the sorption of probe molecules such as pyridine and CO2), TAP measurements, XRD and the sulfate content was determined. n-Butane isomerization reactions were carried out in a quartz micro tube reactor under atmospheric pressure. Prior to the reaction, the catalyst was activated at 473 K for 2 h in He flow (10 ml/min). Results and Discussion The calcined sulfated zirconia, SZ, showed a catalytic activity of 0.015 mol/g/s for n-butane skeletal isomerization with an initial iso-butane selectivity of 96 % at 373 K. It is interesting to note that the removal of water soluble sulfate by water washing treatment of the parent sample resulted in an inactive sample (SZ-WW). The content of sulfate of the water washed sample (SZ-WW) is 0.25 mmol/g, which is much lower than that of the original calcined sulfated zirconia, 0.44 mmol/g. Thus, 43 % of the total sulfate of sulfated zirconia was removed by water washing. The water washing treatment not only removed the water soluble sulfate of SZ, but also the Brønsted acid sites leading to an increase of Lewis acid concentration. The IR spectra of water washed sulfated zirconia (SZ-WW) and sulfated zirconia (SZ) samples showed pronounced difference in the region OH and S=O vibrations. In the IR region of OH group above 3600 cm-1, water washing increased the intensity of the OH band at 3634 cm-1 and shifted it to higher frequency, 3661 cm-1. In addition, water washing reduced a fraction of sulfate groups at high frequency leading to sulfate stretching vibrations of water washed sample (SZ-WW) at 1391 cm-1 compared to the parent sample (SZ) at 1404 cm-1. Note that the wavenumber of the S=O stretching vibration is related to the SO bond order [ , ], indicating that fractions of highly covalent sulfate were removed. IR spectra recorded during adsorption of CO2 showed the formation of bicarbonate on the surface of the washed sample but not on the original sample. SZ-WW featured an about equal number of two different types of Lewis acid sites, while for SZ one type of Lewis acid sites was predominant. The data indicate that water washing produces domains of bare zirconia surface, free of sulfate. The results show for the first time that the water soluble sulfate species are responsible for the formation of highly covalent sulfates as well as the Brønsted acid sites, which are essential for the alkane isomerization reaction on sulfated zirconia at low temperature. Elementary steps are discussed based on steady state and transient kinetic measurements

Interaction between Sulfated Zirconia and Alkanes: Prerequisites for Active Sites – Formation and Stability of Reaction Intermediates

Two sulfated zirconia catalysts were prepared via sulfation and calcination at 873 K of zirconium hydroxide aged at room temperature for 1 h (SZ-1) or aged at 373 K for 24 h (SZ-2). SZ-1 was active for n-butane isomerisation at 373 K; SZ-2 reached similar performance only at 473 K. Both materials contained about 9 wt% sulfate and were tetragonal. Due to a BET lower surface area (105 m2/g vs. 148 m2/g) SZ-1 featured a higher sulfate density, and XRD and EXAFS analysis showed larger (ca. 10 nm) and more well ordered crystals than for SZ-2. n-Butane TPD on SZ-1 showed a butene desorption peak at low temperature, whereas, no obvious butene desorption was observed with SZ-2, suggesting that SZ-1 has a higher oxidizing power at low temperature than SZ-2. The number of sites capable of dehydrogenation are less than 5 µmol/g, because the differential heats of n-butane adsorption as measured by microcalorimetry were 45–60 kJ/mol for higher coverages, indicating weak and reversible interaction. TAP experiments describe the adsorption and desorption behavior of n-butane at different activity states and are the basis for a simple adsorption model. Reactant pulses and purge experiments show that the active species, presumably formed in an oxidative dehydrogenation step, are stable at the surface under reaction conditions

Structural and Active Site Characterization of Sulfated Zirconia Catalysts for Light Alkane Isomerization

Two different sulfated zirconia catalysts were produced through precipitation from zirconyl nitrate solutions, followed by aging of the precipitate either at 298 K for 1 h (SZ-1) or 373 K for 24 h (SZ-2). After drying, the samples were sulfated with ammonium sulfate and calcined for 3 h at 873 K. SZ-1 had a smaller surface area (90 m2 g-1) than SZ-2 (140 m2 g-1) but displayed a one order of magnitude higher maximum n-butane isomerization rate (373–423 K, 1–5 kPa n-butane at 101.3 kPa total pressure). Both materials consisted predominantly of tetragonal ZrO2, contained 9 wt% of sulfate, and adsorbed about 0.5 mmol g-1 NH3. Measurements of adsorption isotherms and differential heats for propane and iso-butane at 313 K reveal a larger number of adsorption sites on SZ-1 than on SZ-2, extrapolated to 1 kPa, 42 vs. 20 µmol g-1 (propane) and 120 vs. 44 µmol g-1 (iso-butane). At coverages > 2 µmol g-1 the heats were similar for both samples with both probes and decreased from 60 to 40 kJ mol-1. Temporal analysis of products measurements indicated shorter residence times for n-butane than for iso-butane, and SZ-1 retained both of these molecules longer than SZ-2. The activation energy for n-butane desorption was 45 kJ mol-1 for both samples. Interaction with pulses of CO2 suggested that non-sulfated, basic ZrO2 surface is exposed on SZ-2, consistent with the larger surface area at the same sulfate content as SZ-1. The results suggest that only a fraction of the sulfate groups participates in adsorption and that product desorption may be of importance

Structural and Active Site Characterization of Sulfated Zirconia Catalysts for Light Alkane Isomerization

Sulfated zirconia (SZ) is active for light alkane isomerization at temperatures as low as 373 K [1]. The material has been investigated extensively in the past 2 decades [2] but so far no convincing structure-activity relationship has been established. Here, we report on the investigation of two different SZ materials with an interesting combination of properties. Both materials have a sulfate content of 9 wt.%; however, the material with lower specific surface area (SZ-1, 90 m2og-1) displays a maximum n-butane isomerization rate (373-423 K, 1-5 kPa n-butane at 101.3 kPa total pressure) that is about one order of magnitude higher than that of the material with higher specific surface area (SZ-2, 140 m2og-1). Both materials were produced through precipitation from zirconyl nitrate solution, followed by aging of the precipitate either at 298 K for 1 h (SZ-1) or 373 K for 24 h (SZ-2). After drying, the samples were sulfated with ammonium sulfate and calcined for 3 h at 873 K. Scanning electron microscopy showed typical particle sizes of 5 to 20 µm for SZ-1, and of 1 to 5 µm for SZ-2. X-ray diffraction and Zr K-edge X-ray absorption spectra identified both materials as predominantly tetragonal ZrO2, but SZ-2 exhibited smaller crystalline domains than SZ-1 (7.5 vs. 10 nm). Diffuse reflectance IR spectra taken during catalyst activation (523 K, inert gas) suggest that the sulfate structures on the two materials rearrange in a slightly different way during dehydration. This is tentatively attributed to different sulfate group densities that result from the ratios of sulfate content to surface area. By ammonia adsorption/desorption, the concentration of acid sites was determined to be 0.52 and 0.48 mmolog-1 for SZ-1 and SZ-2, respectively; this result is not reflected by the catalytic activities. Temporal analysis of products measurements indicated that the residence times for n-butane were shorter than for i-butane, and SZ-1 retained both these molecules longer than SZ-2. The activation energy for n-butane desorption was equivalent for both samples, i.e., 40-41 kJomol-1. Calorimetric measurements of the adsorption of reactant and product at 313 K produced the following results. At 0.3 kPa alkane pressure, SZ-1 and SZ-2 adsorbed similar amounts of n-butane (20 and 25 µmol), but very different amounts of i-butane (80 and 25 µmol). At coverages below 2 µmol the differential heats of adsorption of n-butane were much higher on SZ-2 than on SZ-1, while at higher coverages the heats were nearly identical for both samples and decreased from 60 to 40 kJomol-1. The samples did not differ with respect to the strength of interaction with i-butane, the heats decreased with increasing coverage from 60 to 40 kJomol-1. The results demonstrate that (i) typical SZ catalysts have fewer than 100 µmolog-1 sites, rendering identification by spectroscopic techniques difficult, and (ii) product desorption is a critical factor for the catalytic performance. References: [1] M. Hino, K. Arata, J. Chem. Soc. Chem. Comm. (1980) 851. [2] X. Song, A. Sayari, Catal. Rev. Sci. Eng., 38 (1996) 32

The role of mutation rate variation and genetic diversity in the architecture of human disease

Background We have investigated the role that the mutation rate and the structure of genetic variation at a locus play in determining whether a gene is involved in disease. We predict that the mutation rate and its genetic diversity should be higher in genes associated with disease, unless all genes that could cause disease have already been identified. Results Consistent with our predictions we find that genes associated with Mendelian and complex disease are substantially longer than non-disease genes. However, we find that both Mendelian and complex disease genes are found in regions of the genome with relatively low mutation rates, as inferred from intron divergence between humans and chimpanzees, and they are predicted to have similar rates of non-synonymous mutation as other genes. Finally, we find that disease genes are in regions of significantly elevated genetic diversity, even when variation in the rate of mutation is controlled for. The effect is small nevertheless. Conclusions Our results suggest that gene length contributes to whether a gene is associated with disease. However, the mutation rate and the genetic architecture of the locus appear to play only a minor role in determining whether a gene is associated with disease