Bg Bruce Anderson

Nature of the sites of dissociative adsorption of dihydrogen and light paraffins in ZnHZSM-5 zeolite prepared by incipient wetness impregnation

A DRIFT study of ZnHZSM-5 zeolites with Si/Al ratios of 15 or 41 and a Zn loading of 0.8 wt% revealed a high thermal stability of bridging OH groups that was practically the same as in the pure hydrogen forms. It was concluded that the incipient wetness impregnation of NH4ZSM-5 zeolite with zinc nitrate and the subsequent high-temperature treatment results only in a minor amount of ion exchange. A considerable part of the modifying zinc forms nanometric ZnO clusters inside the channels of the zeolite. The use of the low-temperature adsorption of dihydrogen as a probe indicated the appearance, after high-temperature vacuum pretreatment, of three different Lewis acid sites connected with coordinatively-unsaturated Zn2+ ions. The strongest Lewis sites, with an H–H stretching frequency of adsorbed molecular hydrogen of 3940 cm−1, dissociatively adsorbed hydrogen, methane and propane at both room and elevated temperatures. These sites are represented either by Zn2+ ions on the walls of the main channels of the zeolite (α sites according to Mole et al.) or by Lewis-base pairs on the surface of nanometric clusters of zinc oxide.

NH3 oxidation to nitrogen and water at low temperatures using supported transition metal catalysts

Abstract The ability of several alumina-supported metal catalysts and transition-metal ion-exchanged zeolite Y catalysts to oxidize ammonia to nitrogen and water at low temperatures (between 200 and 350°C) was tested both at high and low ammonia concentrations. Copper-containing zeolite Y catalysts were comparable in activity and were more selective than noble-metal containing zeolite catalysts of similar metal loading. Cu/zeolite-Y catalysts were superior to copper/molybdenum and vanadium/alumina catalysts. Postsynthesis treatment of Cu/zeolite-Y with NaOH increased the activity for ammonia oxidation; dispersion and size of the supported copper-oxide particles were very important parameters. Co-fed steam dramatically increased the deactivation on all catalysts, especially at lower temperatures.

The Sites of Molecular and Dissociative Hydrogen Adsorption in High-Silica Zeolites Modified with Zinc Ions. III DRIFT Study of H2 Adsorption by the Zeolites with Different Zinc Content and Si/Al Ratios in the Framework

Abstractstudy of dihydrogen adsorbed at 77 K by the zinc-modified hydrogen forms of mordenite and ZSM-5 zeolites with the different Si/Al ratios in the framework indicated the existence of several adsorption sites connected with the modifying zinc ions. The fraction of the sites of the strongest perturbation of adsorbed molecular hydrogen with the H–H stretching frequency of ca. 3930–3950 cm-1 is the highest at the highest Si/Al ratios of 41 or 80. These sites dissociatively adsorb hydrogen at moderately elevated temperatures. Adsorption of hydrogen with the higher H–H stretching frequency of about 3970–4000 cm-1 predominates at the lower Si/Al ratios. It does not result in the dissociation of adsorbed molecules. It was concluded that the most active sites are most probably connected with the zinc ions, which compensate two distantly separated negatively charged [AlO4]- tetrahedra localized in the adjacent five- or six-membered rings on the walls of the large channels of the pentasils framework. The sites of the weaker perturbation of adsorbed hydrogen are most probably connected with Zn+2 ions at the conventional exchangeable cationic positions with two aluminum atoms in the same five- or six-membered ring.

An attempt to predict the optimum zeolite-based catalyst for selective cracking of naphtha-range hydrocarbons to light olefins

Abstract Theoretical calculations were performed in order to determine, a priori, which in a range of small-to-medium pore zeolites would be the most effective at suppressing hydride transfer reactions by estimating the magnitude of steric hindrance. Steric hindrance was estimated as the difference between the enthalpies of adsorption of the transition-state complex and those of the two reacting hydrocarbons, calculated using the configurational-bias Monte Carlo (CBMC) method. As the concentration of adsorbed heptane also affects reaction rates adsorption equilibria were also calculated using this same theory. Heptane cracking experiments were then performed using acidic catalysts of this same series of zeolites to test the validity of these predictions. Reaction products were sampled during the first seconds on stream and extrapolation was made to zero time on stream in order to determine the initial catalytic activities and product selectivities in the absence of deactivation. Turnover frequency (TOF) decreased with decreasing pore diameter in agreement with predicted heptane adsorption equilibria. Hydride transfer was predicted to be most restricted in FER and TON, however, lower heptane adsorption capacities suggested that MFI would be the best compromise in terms of both high activity and selectivity. Although differences in paraffin product differences were observed that were consistent with an increasing degree of protolytic cracking with decreasing pore size, no discernible pattern was observed in olefin product distributions (ethylene–butenes).

Positron emission imaging in catalysis

In situ imaging technology developed for nuclear medicine is now being applied to study the kinetics of heterogeneous catalytic reactions under actual process conditions. Minute quantities of molecules (ca. 10 -~ tool), radio-labeled with positron-emitting isotopes such as ltC;13 N; or t50 are injected as pulses into the feed streams of chemical reactors. Subsequent coincident detection of pairs of gamma photons produced via positron--electron annihilation allows the concentration of reactants, intermediates and products to be mapped as a function of both time and position within the reactor bed. As well as providing qualitative information regarding the mechanism of the reaction under investigation the data obtained can be compared with mathematical models based on the reaction kinetics in order to refine parameters such as activation energies and pre-exponential factors for elementary reaction steps. Since the technique is capable of imaging transient phenomena, information is provided that is not accessible to steady-state techniques.

On the nature of the sites of dihydrogen molecular and dissociative adsorption in ZnHZSM-5. II. Effects of sulfidation

The nature of zinc ions in ZnHZSM-5 zeolite prepared by incipient wetness impregnation was studied by DRIFT spectroscopy using dihydrogen adsorbed at low temperature as a molecular probe. The results obtained indicate the appearance of low-coordinated zinc ions following dehydroxylation at high temperature. It is concluded that the ions that most strongly perturb adsorbed hydrogen are localized on the surface of nanometric ZnO clusters that are formed in the channels of the zeolite framework upon high temperature pretreatment. This was demonstrated by the inhibition of dihydrogen adsorption by high temperature sulfidation of the zeolite with H2S. The latter presumably converts ZnO particles into ZnS clusters. This was also confirmed by UV diffuse reflectance spectra of the sulfided samples. By contrast, sulfidation did not influence DRIFT spectra of hydroxyl groups in either HZSM-5 or HY zeolites, indicating that substitution of framework oxygen by sulfur does not occur in these materials.

Influence of zeolite pore structure on catalytic reactivity

The influence of zeolite pore-size and shape on the hydro-isomerization of n-hexane was studied. Computer simulations and kinetic measurements, in combination with Positron Emission Profiling (PEP) experiments have been performed. Results showed that, for zeolites with a Al/Si molar ratio of less than 0.1 acidic protons had the same activity. Differences in reactivity are due to differences in the heat of adsorption. Smaller pores result in a higher steady state concentration of adsorbed substrate and adsorbed alkoxy species. Based on a kinetic model, and using experimental heats of adsorption and theoretical protonation energies, the activation energy of hydro-isomerization of n-hexane was found to be approximately 127 kJ/mol. This is consistent with results of a recent quantum chemical model of reactions catalysed by a zeolite proton.

Fourier transform Raman spectral measurements of powdered quaternary mixtures of organic compounds: Exceptional pure component spectral reconstruction using band-target entropy minimization (BTEM)

Fourier transform Raman spectra of eight mixtures of four organic solids, namely dicyandiamide, melamine, acetamide and urea were measured. Matrices formed from these spectra were first subjected to singular value decomposition to obtain the right singular vectors. The right singular vectors were then subjected to blind source separation using band-target entropy minimization (BTEM), thus no a priori information (i.e. involving the nature of the components present, their spectra, nor their concentrations) was included in the analysis. The recovered pure component spectra are of exceptionally high quality and are consistent with pure reference spectra. Various empirical and statistical tests, such as the Euclidean norm and target transform factor analysis, were employed to assess the quality of the recovered spectra. The present results indicate the applicability of combined Raman and BTEM analysis for solid mixtures.

Imaging of hydrocarbon reactions in zeolite packed-bed reactors using positron emission profiling

The ability of positron emission profiling (PEP) to measure concentration profiles of molecules labelled with positron-emitting nuclei, such as 11C, 13N, and 15O, inside chemical reactors has been demonstrated for the system n-hexane–Pt/H-zeolites under conditions typical of the hydroisomerization reaction. Data obtained in the absence of reaction were first measured and used to model mass transport processes in these biporous, packed-bed reactors. Images obtained under conditions where injected, labelled pulses underwent reaction revealed that the products did not exit the reactor and thus demonstrated the need for in situ measurement. Such experiments should provide a valuable new tool in the study of transient, initial phenomena so often of importance in heterogeneous catalysis, such as “preconditioning” and deactivation.

Imaging of n-hexane in zeolites by Positron Emission Profiling (PEP)

Positron Emission Profiling (PEP) has been used for in-situ measurement of the surface coverage of H-mordenite by n-hexane, as a function of hexane partial pressure, at the elevated temperatures typically used for hydroisomerization by monitoring the retention time of an injected radio-labelled pulse of n-hexane. The labelled molecules ( 11 CH 3 C 5 H 11 ) were produced via a two-step alkene homologation reaction in which 11 C, produced using a cyclotron, was added to 1-pentene. The PEP method described is similar to the “tracer pulse technique” however it has the significant advantage of in-situ imaging of the pulse.