The most abundant, active, and selective catalysts i n nature are enzymes. Their operational domain is. however, relatively narrow as far as temperature and solvent are concerned. Enzyme mimicking is the building of the active center of enzymes (or an analogy of it) into a matrix which allows a larger operational temperature domain and a broader spectrum of solvents. In the case of inorganic matrices. zeolites are the obvious choice because the pore diameters and pore geometry introduce shape selectivity in the reactions. Up to now, the active center of the enzyme has been mimicked by construction of robust, rigid. and stable transition metal ion complexes in zeolite cavities and channels.". 21 The most prominent examples are phthalocyanine and bipyridine complexes.[" 41 However. these complexes can only be synthesized in situ and this procedure is time-consuming. Moreover, the turnover numbers in catalytic oxidations are relatively low.Here, we report for the first time on the immobilization of Cu(histidine) complexes in zeolites by a simple ion exchange procedure. Amino acids. like histidine, are the key building units of natural enzymes; therefore, the obtained inorganic enzyme has an active center, very close to that of its natural enzyme counterparts. In addition, the complexes are built in by a simple ion exchange procedure. Cu2+ was chosen as the transition metal ion. because its complexes are stable ['] and easily characterized by electron spin resonance (ESR) and diffuse reflectance spectroscopy (DRS) in the UV/VTS/NIR region.161 Furthermore, copper proteins play a key role in both plant and animal physiology. for example hemocyanin is the oxygen-carrying protein in the hemolymph of molluscs and arthropods; ascorbate oxidase catalyzes the oxidation of L-ascorbate and galactose oxidase. which catalyzes the oxidation of galactose and many other substrates, like aliphatic and aromatic alcohol~.['-"~ Cu(histidine) complexes are typically prepared in bidistilled water with a histidine:Cu2 + ratio of 5 : 1 . By using an increasing amount of preformed [Cu(His),]+ complex at pH 7.3 in bidistilled water['21 together with a zeolite Na-Y, we have measured the amount of released Na'. together with the amount of Cu" taken up by the zeolite material. The results are presented in Figure 1. Although there is a considerable spread of points, the data show unambiguously that ion exchange is operative. The slope of the straight line (7Na'iCu '') indicates that besides [Cu(His)J+, His' is exchanged too. Chemical analysis indeed confirms that the histidine:Cu2+ ratio on the solid is 6 : l . Thus at the low exchange levels investigated.'' 31 for each [Cu(His),]' exchanged, four His+ cations are co-exchanged. The fact that on the average seven N a + ions are released and not six is indicative for charge compensation by H + ; thus some residual acidity is expected on the solids.
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, N 2 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, Cr 3 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, Cr 3 is almost completely oxidized to Cr 6 , which is anchored onto the surface as dichromate, some chro-mate and traces of small amorphous Cr 2 O 3 clusters and square pyramidal Cr 5 ions. These materials are active in the gas-phase and slurry-phase polymer-ization of ethylene at 100 8C. The poly-merization activity is dependent on the Cr loading, precalcination temperature and the support characteristics; a 1 wt % [Cr(acac) 3 ]-AlMCM-41 catalyst pre-treated at high temperatures was found to be the most active material with a polymerization rate of 14 000 g poly-ethylene 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 nano-fibres 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 1.38 g per 10 min; this indicates the very high molecular weight of the polymer .
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Transition metal ion containing aluminophosphate molecular sieves with AlPO 4 -16 (AST) structure have been synthesized using the classic template, quinuclidine, and the corresponding salts of V 4+ , Co 2+ , Cr 3+ , and Mn 2+ . The cubic AST form was the main product, but in the presence of fluoride ions, the tetragonal AST form could also be obtained. The AST materials were characterized with X-ray diffraction, thermal analysis, scanning electron microscopy, electron spin resonance, infrared spectroscopy, and diffuse reflectance spectroscopy in the UV-vis-NIR region. Spectroscopic measurements showed that small amounts of Co 2+ and Mn 2+ can be incorporated in the framework. The diffuse reflectance spectra are indicative for the presence of two types of Co 2+ , most probably taking the two framework positions T 1 and T 2 with a T 1 :T 2 occupation ratio of 3-8. In contrast, V 4+ is present in defect sites by coordination to two (or three) framework oxygen atoms. The incorporation of Cr 3+ in the AST framework is also uncertain and two surface Cr 3+ species were observed: clusters similar to the bulk Cr 2 O 3 oxide and isolated Cr 3+ ions in strongly distorted octahedral coordination. These spectroscopic observations were confirmed by thermal analysis results, which showed that the degree of metal ion incorporation is inversely proportional with the phase transition temperature of AST to berlinite. Furthermore, Co 2+ and V 4+ are very stable in the AST structure and difficult to oxidize upon heating.
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