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Weckhuysen BM Ramachandra Rao R Pelgrims J Schoonheydt RA Bodart P Debras G Collart O Van Der Voort P Vansant EF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(16):2960-2970
Chromium acetyl acetonate [Cr(acac)3] complexes have been grafted onto the surface of two mesoporous crystalline materials; pure silica MCM-41 (SiMCM-41) and Al-containing silica MCM-41 with an Si:Al ratio of 27 (AlMCM-41). The materials were characterized with X-ray diffraction, N2 adsorption, thermogravimetrical analysis, diffuse reflectance spectroscopy in the UV-Vis-NIR region (DRS), electron spin resonance (ESR) and Fourier transform infrared spectroscopy. Hydrogen bonding between surface hydroxyls and the acetylacetonate (acac) ligands is the only type of interaction between [Cr(acac)3] complexes and SiMCM-41, while the deposition of [Cr(acac)3] onto the surface of AlMCM-41 takes place through either a ligand exchange reaction or a hydrogen-bonding mechanism. In the as-synthesized materials, Cr3+ is present as a surface species in pseudo-octahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a strong distortion from O(h), symmetry. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface as dichromate, some chromate and traces of small amorphous Cr2O3 clusters and square pyramidal Cr5+ ions. These materials are active in the gas-phase and slurry-phase polymerization of ethylene at 100 degrees C. The polymerization activity is dependent on the Cr loading, precalcination temperature and the support characteristics: a 1 wt % [Cr(acac)3]-AlMCM-41 catalyst pretreated at high temperatures was found to be the most active material with a polymerization rate of 14000 g polyethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr(6+/5+) and the oxidation and coordination environment of Cr(n+) species during catalytic action. It will be shown that the polymer chains initially produced within the mesopores of the Cr-MCM-41 material form nanofibres of polyethylene with a length of several microns and a diameter of 50 to 100 nanometers. These nanofibres (partially) cover the outer surface of the MCM-41 material. The catalyst particles also gradually break up during ethylene polymerization resulting in the formation of crystalline and amorphous polyethylene with a low bulk density and a melt flow index between 0.56 and 1.38g per 10 min; this indicates the very high molecular weight of the polymer. 相似文献
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Iva Jovanovic Jon K. Magnuson Frank Collart Barbara Robbertse William S. Adney Michael E. Himmel Scott E. Baker 《Cellulose (London, England)》2009,16(4):687-697
Genome sequencing of a variety of fungi is a major initiative currently supported by the Department of Energy’s Joint Genome
Institute. Encoded within the genomes of many fungi are upwards of 200+ enzymes called glycoside hydrolases (GHs). GHs are
known for their ability to hydrolyze the polysaccharide components of lignocellulosic biomass. Production of ethanol and “next
generation” biofuels from lignocellulosic biomass represents a sustainable route to biofuels production. However, this process
has to become more economical before large scale operations are put into place. Identifying and characterizing GHs with improved
properties for biomass degradation is a key factor for the development of cost effective processes to convert biomass to fuels
and chemicals. With the recent explosion in the number of GH encoding genes discovered by fungal genome sequencing projects,
it has become apparent that improvements in GH gene annotation processes have to be developed. This will enable more informed
and efficient decision making with regard to selection and utilization of these important enzymes in bioprocess that produce
fuels and chemicals from lignocellulosic feedstocks. 相似文献
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