V. Bosáček

Reaction of ammonia with surface species formed from acetone on a HZSM-5 zeolite

Temperature-programmed conversion of preadsorbed acetone on HZSM-5 zeolite was studied using the mass spectrometric detection of the gaseous products. The effect of addition of ammonia to the surface complexes formed from acetone at mild temperatures on the composition of the products was investigated. The surface species, both with and without ammonia addition, were also characterized using 13 C cross-polarization magic angle spinning nuclear magnetic resonance. It was found that the added ammonia reacts with the ketonicm group of the surface intermediates and substantially affects the composition of the gaseous products.

ESCA study of Cu2+-Y and Cu2+-ZSM-5

The oxidation state of copper in Cu 2+ -Y and Cu 2+ -ZSM-5 after irradiation by X-rays during ESCA measurement is investigated. The Cu 2+ Cu 1+ reduction resulting from water decomposition is verified by Auger parameter analysis. The limiting step in this reaction, which can occur at ambient temperature, is the desorption of the O 2 and H 2 O produced.

Effect of heat treatment on the character of coke deposited on HZSM-5 and HY zeolites in acetone conversion

HZSM-5 and HY zeolites were coked at 350 °C during acetone conversion under on-stream conditions at atmospheric pressure, in some cases also in a static arrangement at lower pressures. The character of the carbonaceous residues deposited on both zeolites under the above conditions and the influence of pyrolysis as well as of partial oxidation were examined using infrared and 13 C cross-polarization magic angle spinning nuclear magnetic resonance spectroscopies together with the mass spectrometric detection of the volatile products. The coke on HY zeolites was found to be composed of more condensed aromatics and branched aliphatic chains than that on HZSM-5. The pyrolysis removed the aliphatic chains, and on HZSM-5, the less condensed aromatic rings. Both these coke components were also preferentially oxidized. The partial oxidation led to a substantial increase in the void volumes compared with the parent coked zeolites. Oxidized intermediates were observed on the zeolite surfaces.

Contribution of 13C Nmr Spectroscopy to the Analysis of Surface Compounds Formed in the Transformation of Acetone on Zeolites

Abstract The formation of the protonized form of acetone on HZSM-5 and HY zeolites at room temperature has been demonstrated by high resolution solid state NMR spectroscopy. As the reaction temperature increases, this form undergoes gradual dimerization followed by further transformations. At temperatures of about 180°C cyclization and the formation of undesorbable compounds with polyalkylaromatic character take place. The coke formed mostly exhibits aromatic character; the appearance of the signal at 154 ppm (TMS) indicates the presence of oxygen bonded to an aromatic ring. The presence of either an ethereal bond or of bonding of the phenoxy groups formed with the participation of lattice oxygen is suggested.

Coordination state of gallium in MFI structures prepared by direct synthesis and by postsynthetic modification of boralites

High-field 71 Ga MAS n.m.r. spectroscopy shows that gallium occupies preferentially the framework positions in Ga-MFI structures prepared by direct synthesis from a gel. However, modifications of boralite with Ga 3+ in autoclave experiments provide structures with extraframework gallium species and only partially occupied boron vacancies in framework positions. Evidence of gallium oxohydrates in the modified samples was given by X-ray analysis. Extraframework gallium species with a quadrupole coupling constant of 8.8 MHz could be detected, but part of the extraframework gallium remains n.m.r. invisible.

Interaction of acetone with ammonia and alcohols over a HZSM-5 zeolite Part 2. Ethanol

Abstract Temperature-programmed desorption accompanied by conversion (TPDC) of preadsorbed acetone was studied on a HZSM-5 zeolite with Si/Al=13.5. The reactivity of surface species created from acetone with methanol, ammonia and mixtures of these was investigated by analysing the composition of the products released (using a mass spectrometer) and the composition of the surface species (using a 13 C MAS NMR spectrometer). It was found that ammonia reacts with the carbonyl group of acetone surface species to imino carbocations in both the absence and presence of methanol, and methanol alkylates the acetone surface species whether ammonia is present or absent. The decomposition of the species created from acetone, methanol and ammonia leads to different products than the decomposition of the species formed on the zeolite either from acetone (or methanol) alone or from acetone (or methanol) and ammonia. The strong dehydrogenation function of HZSM-5 resulted in the release of acetonitrile and HCN from acetone and methanol, respectively (in the presence of ammonia), while pyridinium bases (in the presence of all 3 reactants) appeared only in trace amounts.

Transfer of Metal Ions between Metal Oxides and Zeolites. Preparation of Highly Active Cu-Zeolite Based Catalysts For Reduction of NOx at Low Temperature

Abstract A solid-solid interaction between Cu oxides and NH 4 -Y or HZSM-5 zeolite was investigated by means of IR, ESR, XPS, SEM and development of the catalytic activity. It is shown that Cu ions from the metal oxide are incorporated into the zeolite cationic sites. The process is controlled by temperature, Cu oxide content and, it is enhanced by water or ammonia presence. Moreover, it can be applied as a method for preparation of highly active Cu-zeolite based catalyst for selective reduction of NO with ammonia at low (470 K) temperature.

Changes in the vicinity of impurity Fe3+ ions in various zeolites as studied by proton n.m.r. relaxation of adsorbed molecules

Abstract In commercial zeolites, the proton relaxation time T 1 of adsorbed molecules is controlled by impurity Fe 3+ ions. A considerable number of various zeolite/adsorbate systems show an enhancement of T 1 by about one order of magnitude, corresponding to a decrease in the number of accessible Fe 3+ ions. This indicates changes in the vicinity of the Fe 3+ ions. In these zeolites, bridging hydroxyl groups were produced and water was present during or after this process.