János Halász

Indium and gallium containing ZSM-5 zeolites: acidity and catalytic activity in propane transformation

Abstract Surface acidity and propane transformation on In-and Ga-containing ZSM-5 type zeolites were investigated to find relation between the acidic character of catalysts prepared by conventional or solid state ion-exchange and the catalytic activity in alkane aromatization. On the basis of the concentration of Brensted and Lewis acid sites measured by IR spectroscopy the In and Ga containing samples may be ordered into three main groups. The presence of Brensted acid sites results in cracking of the hydrocarbon molecules, whereas the aromatization activity is attributed to Lewis acidity associated with the intimate contact of the oxide/zeolite assemblies.

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.

ZSM-5 zeolites modified by solid-state ion-exchange for NO decomposition

ZSM-5 zeolites modified by conventional and solid-state ion-exchange were characterizedby X-ray diffraction, BET measurements, derivatography, IR spectroscopy in the framework vibration range and acidity measurements with pyridine as probe. NO adsorption and transformation on Cu-, Co-, Ni- and FeZSM-5 zeolites were followed by IR spectroscopy. Mono- and dinitrosyl surface species, adsorbed N2O and NO2 were detected in different concentrations on the tested catalysts. Differences in adsorption behaviour were observed for samples exchanged by the conventional and solid-state procedures.

Fischer-Tropsch synthesis: Incorporation of 14C-labeled normal and isoalcohols

Abstract 1- and 2-Propanol incorporate into the products of the Fischer—Tropsch synthesis; about 10–20% of the added 1-propanol was incorporated but only 1–3% of the 2-propanol was incorporated. The carbon skeleton that is added as alcohol remains intact during the incorporation; furthermore, the constant activity/mole in the products with carbon numbers greater than that of the alcohol indicates that this carbon fragment initiates chain growth. The n-alcohol initiates growth to produce i/n ratios that are lower than those produced from the syngas synthesis, and the n-hydrocarbons have a higher activity than the corresponding iso-hydrocarbons; the i/n ratio and labeling of i- and n-products are just the opposite for 2-propanol. 1- and 2-Propanol produce surface species that remain distinct and initiate different synthesis reaction pathways. Thus, n-alcohol initiation leads to predominately n-hydrocarbons whereas the iso-alcohol initiates chain growth that leads predominately to iso-hydrocarbons. Dehydrogenation to aldehyde or ketone occurred with both alcohols to produce the thermodynamic equilibrium composition anticipated for the particular alcohol. In addition, a significant fraction of the alcohol undergoes hydrogenolysis to the corresponding alkane. For the n-alcohol, but not the iso-alcohol, loss of the carbinol carbon to produce CO 2 occurs by a direct pathway that does not involve CO as an intermediate.

Structural consequences of mild oxidative template removal in the synthesis of modified MCM-41 silicates

Abstract Results concerning the structural consequences of template removal from MCM-41 mesoporous materials are described using ozone, N 2 O and NO 2 as oxidants in comparison with the conventional method applying oxygen. Si, TiSi-, VSi and ZrSi-MCM-41 samples were synthesized by the usual methods. For characterization of the as-synthesized and treated samples XRD, nitrogen adsorption, 29 Si MAS NMR-, IR- and UV–Vis spectroscopic methods were used. The catalytic activity of the samples was tested in the Friedel–Crafts alkylation of toluene by benzyl chloride. The comparison of template removal agents showed that ozone was the most active at low temperature (423 K), and the treatment was less destructive than burning off the template in oxygen. Nitrogen oxide treatment (NO 2 and N 2 O) resulted in template removal at relatively low temperature (573–623 K), and structure deterioration was small. Si NMR spectroscopic data and IR spectra taken in the framework vibration range revealed that more original –SiOH groups remained as hydroxyl nests, furthermore, the heteroatom remained in tetrahedral coordination after ozone and nitrogen oxide treatment compared to burning off the template by oxygen. The results proved the advantages of ozone or nitrogen oxide treatments: (i) gentler to heteroatoms situated in the framework of the materials, probably leaves them intact, (ii) does not result in the formation of secondary micropores, which would decrease the uniform arrangement of the original pore systems, (iii) by preserving the active centers in their original coordination more uniform product distribution may be expected in catalytic reactions.

Destruction of chlorobenzenes by catalytic oxidation over transition metal containing ZSM-5 and Y(FAU) zeolites

A potential method for oxidative destruction of environmentally harmful chlorobenzenes has been studied in a fixed-bed laboratory reactor using several tran- sition metal containing ZSM-5 and Y(FAU) zeolite catalysts prepared by conventional (denoted L) and solid state (denoted S) ion-exchange procedures. The catalysts used for 10 hours in chlorobenzene (or dichlorobenzenes) oxidation have been destroyed in different degrees. The ZSM-5 structure proved to be more stable than that of Y(FAU). Considering the conversion and destruction efficiency of chlorobenzene oxidation at 450 °C as test reaction, activities of the ZSM-5 samples can be listed as follows: Na < CoL < CoS < MnS < MnL < FeL < FeS < CuS < H < CrL < CrS. In the side reactions polychlorinated derivatives (i.e. dioxines and PCB-s as undesired environmental pollution compounds) may be formed by the participation of elemental chlorine produced by the Deacon process. The activity series for the “coupling” reactions is as follows: CrS, CrL < H < FeL, FeS < CoS, CoL < MnS < MnL < CuS. Using CrZSM-5(S) catalyst at 550–600 °C with relatively high residence time and stoichiometric air, the conversion of chlorobenzene exceeds 90% with a decomposition efficiency (formation of HC1 and Cl2 from organic chlorine) of about 85–90%.

Selective reduction of no over copper-containing modified zeolites

Abstract The most efficient method for NO removal from stationary and mobile sources is catalytic reduction with ammonia, hydrocarbons, CO or H 2 . Modified zeolites are active catalysts in these processes. For Cu-ZSM-5 especially high activity and stability have been reported. In this work the properties of copper-containing ZSM-5 zeolites prepared by wet or solid state ion-exchange have been investigated. The Bronsted acidity of the Cu 2+ -exchanged samples was much lower than that of the parent zeolites, and they had high activity in selective reduction with ammonia, propene or propane. A comparison of Cu-ZSM-5 activity in the decomposition of NO and in the reaction of NO with propene or propane revealed that the hydrocarbons as well as the nitrogen oxides play important role in the performance of NO reduction catalysis.

Infrared spectroscopic studies on the surface chemistry of bimetallic zeolite systems. Acidity of Pt,Co- and Pt,CuZSM-5 zeolites

Abstract Acid sites on Co-, Cu- and PtZSM-5 as well as on Pt,Co- and Pt,CuZSM-5 samples were investigated by means of pyridine adsorption monitored by IR spectroscopy. After reduction in hydrogen a significant amount of acidity was observed on each zeolite sample. The concentration of the Broensted acid centers was substantially diminished after treating these samples in NaCl solution. When NaBH 4 was used as the reducing agent much less, in some cases a negligible amount of Broensted acid sites was detected, which did not significantly increase after reduction with hydrogen. Creation of the Lewis acid sites proved to be more complex indicated by several IR bands of coordinatively bound pyridine. As a result of this study, an applicable procedure is suggested for preparation of bimetallic zeolites of predetermined acidity.

The mechanisms of the ammoxidations of propene and isobutene to pyridines over mixed SnSbTe oxide catalysts

Abstract The ammoxidations of propene and isobutene over binary mixtures of SbTe and SnTe oxides and ternary mixtures of SnSbTe oxides were studied at 623–773 K in a pulse reactor and in a recirculatory batch reactor. The main products of the ammoxidation were acrylonitrile and methacrylonitrile, respectively, but doping of the tin-antimony oxide system with tellurium dioxide was found to influence the selectivity towards the formation of pyridines (pyridine, picolines and dimethylpyridines), while the formation of nitriles was suppressed. With regard to pyridine production, the catalyst with composition Te:Sb:Sn = 1:3:6 behaved most appropriately at the optimum reaction temperature of 700–730K. The formation of pyridines can be interpreted in terms of a possible mechanism which includes the coupling of aldehydes (acetaldehyde and acrolein) formed on surface Sb or Te centres and [NH]−2 ions on specific SnSbO surface complexes.

Selective ammoxidation of propene over promoted tin-antimony mixed oxide catalysts

Abstract The selective ammoxidation of propene was studied over SnO2-Sb2O4 and promoted (Bi2O3, Ga2O3, In2O3 and Tl2O3) tin-antimony mixed oxide catalysts. Over binary tin-antimony oxides, the main products of the allyl type oxidation are acrylonitrile and acrolein, but doping with oxide promoters was found to influence the selectivity towards formation of special products. Over catalysts promoted with In2O3, 1,5-hexadiene and benzene are produced from two propene molecules chemisorbed on the same In3+ center residing in the surface layer of the catalyst. The formation of 4-methylpent-2-enonitnle over catalysts doped with Bi2O3, Ga2O3 or Tl2O3 is explained by the ammoxidation of 4-methylpent-2-enyl carbenium ion produced by the interaction of a π-allyl intermediate and an isopropyl carbenium ion formed on surface Bronsted acidic centers.