Gy. Tasi

Synthesis, Characterization and Catalytic Activity of V-Zsm-5 Zeolites

Abstract A V-ZSM-5 sample with a Si/V = 42 was synthetized outgoing from VO(COO)2 and Q-brand sodium silicate using TPA-Br as template. ESR spectroscopy proved that vanadium(IV) ions in the zeolitic framework exhibit a distorted square planar symmetry. Upon heat treatment a part of the framework vanadium ions migrate to extra-framework positions. After dehydration no Bronsted acidity was found. Treatment in oxygen and hydrogen above 570 K revealed the redox character of the V-ZSM-5 sample. In oxidation of n-butane (as catalytic test reaction) the V-ZSM-5 zeolite exhibits selective dehydrogenation and aromatization activity.

Hydrogen bonding interactions of α-phenylcinnamic acid isomers in the liquid phase studied by IR and NMR spectroscopies and computational methods

Abstract Intra- and intermolecular hydrogen bonding interactions of α-phenylcinnamic acid isomers were studied in the solution phase by infrared (IR) and proton nuclear magnetic resonance ( 1 H NMR) spectroscopies and the AM1 semi-empirical method. The solvents were CDCl 3 or dimethyl sulfoxide (DMSO), the concentration of the acid isomers were varied. Spectroscopic measurements revealed that (i) intermolecular hydrogen bonds are typical for both acid isomers via their carboxylic groups (OH…O hydrogen bonding), monomer acids were only present in significant amounts at 10 −4 mol/dm 3 and (ii) CDCl 3 was not involved in the aggregated structures of any isomers, while the oxygen in DMSO seemed to interact with the olefinic proton of the E isomer. Weak intramolecular hydrogen bonds could be identified computationally in the Z isomer: (aromatic)CH…O, CO…H, but none in the E .

The thermal behaviour of sodium azide in zeolites

Abstract Upon thermal treatment, NaN3-doped zeolite transforms stepwise. After endothermic dehydration, the NaN3 decomposes exothermically in air and under N2 flow. The temperature of decomposition is higher than that of the pure azide. Part of the sodium azide remains intact up to 1000 K. The process was also monitored by IR spectroscopy. 23Na NMR measurements revealed that metallic sodium clusters were formed.

Thermogravimetric and infrared spectroscopic investigations on the interactions between mordenites and phosgene

Abstract By analogy to chlorination technologies for processing low-grade aluminium ores, an attempt was made to remove aluminium from zeolitic frameworks (dealumination) using phosgene. The more or less separated individual steps of interaction of phosgene with exchange and framework ions were followed by changes of the mass of the sample specimen using a sensitive Mettler balance. The reaction products identified permitted the setup of a reasonable sequence of steps and stoichiometry for the interactions. The rate-determining step of dealumination could be traced back to the slow release of the AlCl 3 (or decomposition of NaAlCl 4 ) product. The bulky molecules of Al 2 Cl 6 and NaAlCl 4 block the narrow pores of mordenite, thereby causing transport hindrance in both directions. “Complete” dealumination can, therefore, be achieved only by repetition of the dealumination cycles.

Investigation of the surface reactions of CCl4 on zeolites studied by IR and MAS NMR spectroscopy

Abstract The reaction of carbon tetrachloride with zeolites exchanged with alkali metal ions was investigated by means of in situ infrared and multinuclear magnetic resonance spectroscopy. Phosgene as reaction intermediate was proven; its formation is influenced by the temperature over the different ion-exchanged samples. For the infrared spectroscopic measurements, a reaction-induced Fermi resonance phenomenon was observed and is discussed. On the basis of the experimentally proven reaction intermediates, a detailed mechanism is suggested for the reaction of carbon tetrachloride with zeolites.

Investigation of surface reaction induced Fermi resonance by IR and MAS NMR spectroscopy

IR and MAS NMR data obtained on the reaction of carbon tetrachloride with faujasites proved that dealumination proceeds via formation of phosgene as an intermediate. This surface intermediate caused the Fermi resonance phenomenon on zeolites.

UV-VIS spectroscopic and theoretical studies about carbocation formation on solid surfaces

Abstract Generation and transformation of alkenyl carbenium ions formed upon adsorption of hydrocarbons in zeolites were investigated by UV-VIS spectroscopy. Bands generally observed in the ranges of 290–320, 350–370 and 430–460 nm were assigned to mono-, di- and trienylic carbenium ions. Characteristic band shifts of the alkenyl carbenium ions to the higher wavelengths were found, due to the formation of branched, i.e. more stable carbenium ions. Standard heats of formation, obtained from semiempirical methods showed that with increasing alkyl substitution the stability of carbenium ions increases. The π-π ★ MO distances computed are in accordance with the experimentally observed shifts of the band maxima.

Catalytic Activity of Modified ZSM-5 Zeolites in the Dehydrogenation and Aromatization Reactions of Propane and n-Butane

Abstract In order to enhance catalytic activity in the dehydrogenation and aromatization of light paraffins, a set of potential catalysts: (i) V(f)-ZSM-5 (originating from direct synthesis), (ii) V(ex)-ZSM-5 (“ex””: partial exchange of H+ in H-ZSM-5 by VO2+ ions), and (iii) solid exchange of H in H-ZSM-5 for Ga, In, and V by baking the mixtures of ZSM-5 with the respective oxides were investigated by physical (XRD, IR, ESR, MAS-NMR spectroscopy, and adsorption) and chemical methods. In the “as synthesized””V(f)-ZSM-5 sample V4+ ions reside in framework positions according ESR and 51V-MAS-NMR spectra. Coincidence between experimental and theoretical spectra could be attained only by supposing the existence of two different framework surroundings. The structure of V(f)-ZSM-5 is strained: heating (especially in the presence of water vapour) causes release of V from the framework. Severe conditions cause clustering of V4+ in the voids with the annihilation of hyperfine splitting in the ESR spectra. Catalytic results demonstrate that in the presence of modifyers propane and n-butane conversion turns from cracking to dehydrogenation and aromatization pathways.

Theoretical and experimental study of hydrodechlorination reactions on Pt- and Pd-containing zeolites

Chlorinated compounds emitted into the atmosphere are responsible for diminishing the ozone layer in the stratosphere. Platinum and palladium supported on different carriers play very important role in catalytic hydrodechlorination of these compounds. We have found that platinum is more selective than palladium in the hydrodechlorination of carbon tetrachloride, but not selective in the hydrodechlorination of trichloroethene and 1,1,2-thrichloroethane. Furthermore, thrichloroethene is more reactive than 1,1,2-trichloroethane. The C-Cl bond dissociation energies (BDEs) and other molecular properties, e.g. vertical electron affinities (VEAs), are important factors from which reactivity and selectivity depend. The size and the electronic structure of platinum and palladium nanoparticles also play important role in catalytic hydrodechlorination. The calculated data lend help in interpreting the experimentally found differences.

Surface intermediates in the transformation of allyl alcohol over zeolites

Abstract Interaction of allyl alcohol, acrolein and allene with zeolite HZSM-5 results in the formation of alkenyl carbenium ion oligomers as intermediates. According to the molecular electrostatic potential map and to the partial atomic charges from MNDO calculations as the preliminary step, the oxygen atom of allyl alcohol is attacked by the proton, followed by dehydration to allene, undergoing further transformation.