R.F. Howe

Effects of coke formation on the acidity of ZSM-5

The catalytic activity of ZSM-5 for the conversion of methanol to gasoline has been attributed to the combination of two properties of the zeolite: its intrinsic acidity and its unique pore structure. The slow deactivation of ZSM-5 during methanol conversion is attributed to the buildup of nonvolatile hydrocarbon residues termed coke which block access of reactant molecules to the active sites. The authors have undertaken a detailed investigation of the acidity of ZSM-5 as a function of the amount of coke deposited during methanol conversion, with the objective of understanding the role played by acid sites in the formation of coke. The acidity was monitored by measuring the adsorption of two different bases, NH/sub 3/ and pyridine, using the techniques of FTIR spectroscopy and temperature-programmed desorption (TPD). 20 references.

Characterization of MoY zeolites prepared by aqueous ion exchange

The characterization of MoY zeolites prepared by aqueous ion exchange is described. Zeolites containing up to 5 Mo per unit cell have been obtained. The ion exchange produces a zeolite containing Mo/sup +6/, acidic protons, and adsorbed pyridine. Activation by heating in vacuo causes loss of pyridine and reduction of up to 18% of the molybdenum to Mo/sup +5/. Further reduction cannot be achieved by heating in H/sub 2/ up to 700/sup 0/C. EPS spectra of Mo/sup +5/ species indicate that molybdenum cations occupy sites both in the supercages and the ..beta.. cages of the zeolite. 34 references.

Preparation of molybdenum mordenite from MoCl5

Abstract Molybdenum loaded mordenites are prepared by vapour phase adsorption of MoCl 5 into H-mordenite. The adsorption and subsequent decomposition of MoCl 5 is monitored by e.p.r. spectroscopy and by measurement of HCl evolution. It is shown that the vapour phase adsorption method produces Mo-mordenites containing up to 6 wt% Mo, of which 30% is present as paramagnetic Mo +5 , with no significant loss of zeolite crystallinity or adsorption capacity of the linear channels.

The variation of xenon-129 n.m.r. chemical shifts with aluminum content in ZSM-5 and ZSM-11 zeolites

The technique of 129 Xe n.m.r. has been applied to the study of electric field effects in ZSM zeolites. For ZSM zeolites in the acid form, the chemical shift at zero coverage varies with aluminum content. This variation is explained by changes in the effective electric field experienced by xenon within the zeolite pores as the aluminium content and, hence, the proton concentration vary

ESR studies of O−2 adsorbed on silica gel: Photoformation and rotational dynamics

An ESR investigation is reported of oxygen radicals formed by UV radiation of silica gel surfaces in the presence of O2. The species O−2 and O−3 are identified. The O−2 radical shows a temperature dependent signal shape between −196 and 25u2009°C which is interpreted in terms of Brownian rotational surface diffusion about an axis perpendicular to the O–O bond and to the surface. Correlation times for surface diffusion are estimated from comparison of observed and computer simulated spectra, and an activation energy for surface diffusion of 2.4±0.8u2009kJu2009mol−1 calculated. Comparison with previously reported spectra for W/SiO2 and Ti/SiO2 surfaces indicates that the mobile O−2 species formed in all three systems must be located on the SiO2 support, and that the mobility is due to the absence of strong interaction with transition metal sites.

Comparison of zeolites and aluminophosphates as hosts for transition metal complexes

Abstract The adsorption and decomposition of the volatile complexes Mo(CO) 6 , Fe(CO) 5 , Co(CO) 3 NO, and MoCl 5 in various zeolites and the aluminophosphate molecular sieve AlPO 4 -5 is described. These complexes are used to generate highly dispersed metal clusters or oxycations which show novel catalytic properties.

Oxidation of o-xylene over vanadium-porous titania glass catalysts

Abstract Porous titania glass (PTG) has been loaded with vanadia by several different methods. Nitrogen adsorption-desorption isotherms and X-ray powder diffraction measurements indicate that impregnation methods produce oxovanadium species within the mesopores of the glass. The vanadia-PTG catalysts exhibit similar levels of selectivity and conversion for the oxidation of o-xylene to phthalic anhydride as conventional anatase or rutile supported catalysts, but at significantly lower temperatures. The activity for phthalic anhydride production increases with vanadium loading; the best performance of 50% selectivity to phthalic anhydride at 100% o-xylene conversion was achieved at 280°C with a catalyst containing 5.6% V (w/w).

Catalysis on Mo(CO)6-derived supported molybdenum catalysts: CO oxidation with N2O

The catalytic nature of Mo(CO)6 supported on γ-Al2O3, KOH-doped γ-Al2O3, and HY-zeolite was investigated in CO oxidation with N2O in comparison with that of a conventional partially reduced MoO3γ-Al2O3 catalyst. Kinetic parameters of this reaction were obtained in the range 0 to 100 °C; the rate law r = kP1N2OP0CO was found on all catalysts, and the activation energy was estimated to be 9.1 kcal/mol on the Mo(CO)6-derived catalysts and 7.1 kcal/mol on the partially reduced MoO3γ-Al2O3 catalyst. Maximum catalytic activities were obtained by activating the Mo(CO)6-derived catalysts at 400 °C. To obtain similar activity on the MoO3γ-Al2O3 catalyst, it was necessary to reduce at 600 °C. The former catalysts were deactivated on repeating the reaction. On the basis of these results and those of ESR studies through the activation or deactivation process, an active site on the Mo(CO)6-derived catalysts has been proposed. Also, clear IR absorption bands due to chemisorbed CO and N2O species were observed on the HY-zeolite-supported catalysts. A reaction mechanism is proposed from the kinetic and IR spectroscopic results.

Effects of Coke Formation and Removal on the Acidity Of ZSM-5

Abstract Previous work(ref.5) used the techniques of chemisorption of NH 3 and pyridine, TPD/MS(ref.7) and IR spectroscopy to study the effect of coking on the acid sites in the zeolite ZSM-5. This work has been extended to include two other ZSM-5 samples and to study regeneration of the sites by oxidation and pyrolysis of the coke. Correlations between the loss of acid sites (as indicated by NH 3 chemisorption) and the changes in IR bands associated with silanol groups and Bronsted acid sites can be interpreted in terms of the relative Al contents and distributions in the three samples. However, pyridine chemisorption can give lower values for acid site concentrations, because of steric effects. This work confirms the deduction(ref.2) that coking is initially in the channels of ZSM-5 and occurs at a later stage at the surface of the crystals. Lengthy heat treatment to regenerate the activity of the ZSM-5 can result in some permanent loss of active sites.

Natural abundance 17O NMR spectra of molybdenyl, uranyl and zirconyl complexes

Etude de complexes oxo de molybdene, zirconium et uranium par RMN de loxygene 17. La sensibilite est telle que la methode peut etre utilisee sans marquage isotopique de loxygene, labondance naturelle de 17 O etant suffisante