Ammonia structural transformation

Figure 3.8 Monitoring the structural transformation with temperature and ammonia gas atmosphere starting from gallium nitrate reaching a spinel-type structured gallium...

ZeoUte-supported Re clusters prepared by chemical vapor deposition (CVD) of CH ReO onto zeohte HZSM-5 catalyzed efficiently the gas-phase oxidation of benzene with in the presence of ammonia [68]. Phenol selectivities of 91.6-93.9% at 1.7-9.9% conversions and 82.4— 87.7% at 0.8-5.8% conversions were achieved in pulse reactions and steady-state reactions, respectively. The process involves structural transformation between Re ji clusters and Re monomers (Scheme 14.11). EXAFS confirmed the formation of the Re clusters after catalyst treatment with NH, [68]. 

Zhang, Y., Meisel, M., Martin, A., Liicke, B., Witke, K., and Brzezinka, K.W. In situ Raman investigation on the structural transformation of oxovanadium hydrogenphosphate hemihydrate in the presence of ammonia. Chem. Mater. 1997, P, 1086-1091. 

Aluminum oxide is found to block the iron surface which could otherwise dissociate ammonia. This blocking of the restructuring process by Alj,Oy is in sharp contrast to the case where aluminum oxide catalyzes the restructuring of iron in the presence of water vapor prior to ammonia synthesis (Section 4.6). In this circumstance iron oxide is found to migrate on top of the aluminum oxide overlayer as a result of the oxidizing environment (water vapor). The major driving force for this structural transformation is most likely compound formation between iron oxide and aluminum oxide. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

The proposed Re6 cluster (8) with terminal and bridged-oxygen atoms acts as a catalytic site for selective propene oxidation under a mixture of propene, Oz and NH3. When the Re6 catalyst is treated with propene and Oz at 673 K, the cluster is transformed back to the inactive [Re04] monomers (7), reversibly. This is the reason why the catalytic activity is lost in the absence of ammonia (Table 8.5). Note that NH3, which is not involved in the reaction equation for the acrolein formation (C3H6+02->CH2=CHCH0+H20) is a prerequisite for the catalytic reaction as it produces the active cluster structure under the catalytic reaction conditions. 

Using various amines added to the ammonia bath (in most cases with added hydrazine), sphalerite ZnS fihns were obtained with a crystal size of ca. 3 nm [ 118]. Rutherford Backscattering Spectroscopy (RBS) analyses showed that there was about twice as much Zn in the fihns as S. (More basic solution and more hydrazine gave more stoichiometric films). Extended X-ray Absorption Fine Structure (EX-AFS) and Fourier Transform Infra-red (EUR) spectroscopy showed that the fihns did not have Zn-0 groups but rather Zn-OH ones [122] and that there is probably a mixture of ZnS and unreacted Zn(OH)2, quite likely as a ZnS shell around a Zn(OH)2 core. Optical spectra gave a bandgap of ca. 3.85 eV, considerably blue-shifted from the bulk value of 3.6 eV, as expected from such small crystals. 

The tetrametaphosphate anion is transformed into the amidotetraphos-phate, (P4O12NH2)5 , by concentrated ammonia solution just as the anion ring of trimetaphosphate is cleaved by ammonia to amidotriphosphate. Acidification of the solution results in partial reconversion to the tetrametaphosphate anion, but there is also partial hydrolysis to NH4+ and tetraphosphate ions as well as to amidomonophosphate and triphosphate ions, with simultaneous formation of trimetaphosphate (800). A complete structure determination has been made for ammonium tetrametaphosphate (1) from which the cyclic structure of the anion is evident (245). The diacid salt Na2H2(P40i2) (108) (see Section V,A) is also undoubtedly a tetrametaphosphate (150) (for structural data, see Section IV,E,3). 

The adsorption of ammonia and amines has been studied many times as a method of estimation of the acidity of solid surfaces. Some of the results are pertinent to the mechanism of amine transformation on these catalysts. Depending on the structure of the catalyst surface, several types of adsorbed species have been observed. 

The final molecule of this series is methane, the tetrahedral structure of which follows if a fourth unit positive charge is removed from the nucleus in the ammonia lone-pair direction. There are now four equivalent bonding orbitals, which may be represented approximately as linear combinations of carbon s-p hybrid and hydrogen Is functions. The transformation from molecular orbitals into equivalent orbitals or vice versa is exactly the same as for the neon atom. 

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