Zdeněk Dolejšek

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.

A comment on the oxidation of coke deposited on zeolites

Abstract Temperature-programmed oxidation of coke formed during acetone conversion on HZSM-5 and HY zeolites with continuous detection of all the products is described using dioxygen as the oxidation agent. The effects of the following factors were studied: (i) the coke composition, which was changed by pyrolysis prior to oxidation, (ii) the amount of coke formed during acetone conversion on HZSM-5, and (iii) the different reactants, also using diethylketone and isobutene for conversion over HZSM-5. It appears that the zeolite structure is the predominant factor in the oxidation of coke, affecting the oxidation temperature as well as the CO CO 2 ratio in the products.

Catalytic activity of dealuminated Y and HZSM-5 zeolites measured by the temperature-programmed desorption of small amounts of preadsorbed methanol and by the low-pressure flow reaction of methanol

The activity of HZSM-5, NH4Y and Y zeolites, dealuminated using SiCl4, in methanol transformation has been compared and correlated with the acid properties and the Si:Al ratio of the respective zeolites. The temperature-programmed desorption of small amounts of methanol preadsorbed at ambient temperature together with measurements in a flow reactor under low pressure allowed the primary reaction steps to be observed. HZSM-5 and Y zeolites which had been moderately dealuminated showed the highest activity; however, the product distribution was different above 630 K:HZSM-5 gave predominantly methane and formaldehyde (aromatics appearing at higher temperatures), while C3–5 olefins and aromatics were formed above the Y zeolite. HY and highly dealuminated Y zeolite with Si/Al 20 were less active than moderately dealuminated Y and HZSM-5 zeolites. The high activity of the latter zeolites is caused by the action of strong proton-donor sites and electron-acceptor centres (extralattice aluminium in dealuminated zeolites). The difference between HZSM-5 and dealuminated Y zeolites is caused by their different structures, which lead to different reaction mechanisms.For comparison, the temperature-programmed desorption of preadsorbed ethanol was also studied. This resulted predominantly in ethene formation via intramolecular dehydration of ethanol.

Properties of different Fe (III) species introduced into NH4-Y zeolites

Depending on preparation conditions, two types of Fe(III) species were found (a cationic and a hydrated oxide form) and characterized in Fe(III)NH4-Y zeolites with iron content in the range 0.3–5.8 wt %. These two forms were found to differ mutually, as well as from separate Fe2O3 phase, as far as self-reduction and reduction with hydrogen are concerned. Up to 23% exchange was found to yield only Fe(III) ions in the zeolite, whereas the other species only appears at higher levels of exchange. These processes were studied by means of X,p.s., e.p.r., t.p.r. and CO adsorption measured by i.r. spectrometry.

TPD/MS studies of the interaction of simple ketones with ZSM-5 zeolites

Abstract Temperature-programmed desorption (TPD) of products formed by the interaction of simple ketones with HZSM-5 zeolites has been studied mass spectrometrically. Acetone (MMK), methyl ethyl ketone (MEK) and diethyl ketone (EEK) were preadsorbed on HZSM-5 and NaZSM-5 zeolites at room temperature in amounts ranging from 0.04 to 2 mmol g−1. It was shown that the qualitative composition of the products did not depend on the amount of preadsorbed ketone; the reactivity of the ketones increased from MMK to EEK. The conversion of MMK on HZSM-5 proceeded via acid intermolecular condensation, which could not take place on NaZSM-5. In the transformation of MEK, intramolecular dehydration played a role in the interaction, along with the above condensation mechanism; in this case NaZSM-5 also exhibited some activity. The mechanism of intramolecular dehydration prevailed in the reaction with EEK where, unlike NaZSM-5, HZSM-5 caused further conversion of the dehydrated products. The experiments with deuterated ketones, MMK-d6 and MEK-d5, have shown involvement of zeolitic hydrogen in the reaction complexes.

Intermediates in methanol and ethanol transformation on HZSM-5 zeolite studied by chemical trapping and deuterium labeling

Abstract Surface species created by the interaction of methanol and ethanol with HZSM-5 zeolite were studied using mass spectrometry with chemical traps at the temperature of the initial formation of olefins. C 1 species formed by the interaction of the zeolite with methanol (surface methoxyls and formaldehyde-like species) were found to react readily with benzene, ethane, propane and ethylene yielding primarily [ n + (CH 2 ) 1 ] homologues of these scavengers ( n ). Surface ethanol species exhibited lower activity towards these chemical traps, reacting only with benzene to give ethylbenzene. Isotopic exchange experiments confirmed a weakening of the β-hydrogen bonds of ethanol during interaction with the zeolite.

Deuterium-labeled methanol in reactions with HZSM-5 zeolite

Methanol-d3 or a mixture of methanolt-d0 and methanol-d3 were adsorbed at room temperature on HZSM-5 in amounts ranging from 0.2 to 2 equivalents of the amount of zeolite structural hydroxyls. The isotopic composition of products released at various temperatures allowed us to discuss the mobility of the hydrogen of the surface species and the participation of zeolitic hydrogens in the formation of hydrocarbons.

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.

Reactions of Acetone, Methanol and Ammonia on Zsm-5 Zeolites

Abstract The competition of 3 bases, acetone, methanol and ammonia, for reaction with the active acid centers of HZSM-5 zeolites was investigated and compared with that at the ZnHZSM-5 zeolite. These studies were carried out under low reactant-to-active center ratios using mass spectrometric detection of the gaseous products released during the decomposition of the surface species, which were identified using 13 C MAS NMR spectroscopy. Possible mechanisms of these interactions are discussed.