T. Mole

Reactions on ZSM-5-type zeolite catalysts

Catalytic reactions and sorption measurements have been carried out with ZSM-5 and silicalite catalysts which are distinguished by variation in skeletal heteroatom concentration. The catalysts were used in both the hydrogen-exchanged and sodium-exchanged forms. Sorption measurements were made with the hydrocarbons n-hexane, 3-methylpentane, and 2,2-dimethylbutane, and with the bases ammonia, n-butylamine, t-butylamine, and 4-methylquinoline. Catalytic reactions were carried out on both unpoisoned and base-poisoned catalysts using methanol, propylene, and 3-methylpentane reactants. In addition, the behavior of ethylene and ethanol reactants was also explored. The ease of base sorption and hydrocarbon sorption has been assessed in terms of effective molecular size in relation to the channel size of the catalyst, and this factor is also used as a basis for explaining the effectiveness of bases for poisoning the catalytic reactions. Temperature-programmed desorption (TPD) measurements with ammonia have been used to assess the energetic distribution of sorption sites for bases, and very strong binding sites with a TPD maximum for ammonia at about 780 °K have been identified as the probable sites used in the conversion processes. The main features of the catalytic conversion process are discussed. It is concluded that sorbed C3,C4 olefinic residues are general intermediates leading to aromatic formation. Under most circumstances, ethylene was relatively unreactive, and it is inferred that a sorbed C2 residue, which is related to ethylene by sorption and desorption, is an unlikely general intermediate. A mechanism is suggested for the formation of sorbed C3,C4 olefinic residues, and for initial carbon-carbon bond formation from methanol. Catalyst self-poisoning was observed with all catalysts except hydrogen-exchanged ZSM-5.

Mechanism of some conversions over ZSM-5 catalyst

Abstract Various carbon-carbon bond formation reactions over ZSM-5 catalyst have been studied with the help of deuterium labeling. The reactions studied were: benzene methylation (benzene/methanol), benzene ethylation (benzene/ethanol and benzene/ethylene), and the conversion of propylene and of ethylene to higher molecular weight products, together with some ancillary exchange reactions. It is shown that, in the methylation of benzene, the methyl group remains intact throughout and does not undergo exchange with surface hydrogen or aryl hydrogen. All the carbon-carbon bond formation reactions are interpreted as Bronsted acid-catalyzed electrophilic alkylations. From these data it is suggested that the ZSM-5-catalyzed conversion of methanol to hydrocarbons involves electrophilic methylation of olefinic intermediates as the main propagation reaction for carbon-carbon bond formation.

Conversion of methanol to hydrocarbons over ZSM-5 zeolite: An examination of the role of aromatic hydrocarbons using 13carbon- and deuterium-labeled feeds

A mechanism is suggested for the acceleration by aromatic hydrocarbons of zeolite-catalyzed methanol conversion. According to this mechanism, the aromatic hydrocarbon undergoes successive ring methylation, prototropic conversion to an exo-methylene-cyclohexadiene, side-chain methylation, and ring de-ethylation. The overall result is that two methanol molecules give an ethylene molecule. The mechanism is supported by various reactions observed over ZSM-5 catalyst at methanol conversion temperatures: (i) deuteration of p-xylene by D2O in the ring and methyl positions; (ii) de-alkylation of p-ethyltoluene and n-propylbenzene; and (iii) incorporation of the aromatic carbon of benzenes and alkylbenzenes into ethylene product, as revealed by 13C-labeling studies.

Aromatic co-catalysis of methanol conversion over zeolite catalysts

The rate of conversion of methanol and aqueous methanol to hydrocarbons over H-ZSM-5 zeolite is enhanced by the addition of aromatic hydrocarbons to the feed. The effect has been demonstrated by means of both continuous-feed and pulse-feed experiments, using H-ZSM-5 zeolite prepared by various methods and using zinc-exchanged dealuminized Y zeolite.

Conversion of methanol to ethylene over ZSM-5 zeolite: A reexamination of the oxonium-ylide hypothesis, using 13carbon- and deuterium-labeled feeds

13Carbon- and deuterium-labeled feeds have been used to examine the conversion of aqueous methanol to hydrocarbons over ZSM-5 zeolite in the presence of various (C3 and C4) alcohols. Particular attention has been paid to the carbon-labeled ethylene product and deuterium-labeled dimethyl ether. The isotopic composition of the ethylene requires that only part of the ethylene is formed directly from methanol or dimethyl ether. The rest of the ethylene is formed indirectly and incorporates the carbon of the other alcohol as well as the carbon of the methanol. The best explanation for the directly formed ethylene, and for hydrogen-isotope exchange in dimethyl ether, still appears to be the oxonium-ylide mechanism.

Conversion of methanol to ethylene over ZSM-5 zeolite in the presence of deuterated water

Abstract When methanol, mixed with two volumes of deuterated water, is converted to ethylene and other hydrocarbons over ZSM-5 zeolite at 300 °C, deuteration of the residual dimethyl ether accompanies ethylene formation. The ethylene is also extensively deuterated. The data are interpreted in terms of an oxonium ylide mechanism (Stevens-type rearrangement) for ethylene formation.

Ketones, carboxylic acids, and esters from conversion of aqueous methanol over H-ZSM-5 zeolite

Conversion of aqueous methanol over H-ZSM-5 zeolite in the presence of para-xylene (or toluene) has been effected in a batch, stirred autoclave under autogeneous pressure at 344 °C. Alkanes are the main products; ketones, carboxylic acids and methyl esters are also formed. A mechanism for the organic reactions is proposed.