Jp Jillus Wolthuizen

Infrared, Microcalorimetric, and Electron Spin Resonance Investigations of the Acidic Properties of the H-ZSM-5 Zeolite

The infrared spectra of the H-ZSM-5 zeolite calcined at temperatures up to 1173 K, and the corresponding electron spin resonance and microcalorimetric data are discussed. Two types of hydroxyl groups are characterized by absorption bands at 3720 and 3605 cm −1 , a small shoulder being present at 3665 cm −1 . Infrared spectra were also recorded after pyridine adsorption, showing the presence of Bronsted acid sites and Lewis acid sites. After calcination at increasing temperatures, dehydroxylation of the zeolite is observed: above 675 K, the number of Bronsted acid sites decreases, while that of strong Lewis acid sites increases; however, a small dealumination occurs as shown by chemical analysis measurements and XPS data. Moreover, dehydroxylation enhances the constraint character of this zeolite, as observed by pyridine inability to titrate the total Lewis sites. Electron spin resonance studies of trapped hydrogen atoms, of adsorbed NO, and of adsorbed benzene radical cations formed on H-ZSM-5 at various calcination temperatures are discussed in terms of the number and strength of the acid sites. It is suggested that the acid sites which are present are very similar to those of H-mordenite although slightly stronger. A microcalorimetric study of ammonia adsorption confirms the very strong acidic character of the acid sites and shows their dependence in strength and heterogeneity upon calcination temperature.

Low-temperature oligomerization of small olefins on zeolite H-ZSM-5 : an investigation with high-resolution solid-state 13C-NMR

Oligomerization reactions of ethene, propene, isobutene, and 2-methyl-butene-1 on zeolite H-ZSM-5 at 300 and 373K were investigated using high-resolution solid-state 13C-NMR spectroscopy. It is shown that at 300K only linear oligomers are formed, while at 373K some branching is observed. Thus at 300K on zeolite H-ZSM-5 initially formed branched oligomers are stretched in consecutive isomerization reactions. Differential scanning calorimetry experiments on the adsorption of n-hexane and isohexane show that the specific pore dimensions of zeolite ZSM-5 are responsible for an increased heat of adsorption of linear paraffins compared to branched paraffins. A reaction scheme is presented to explain the experimental data.

Reaction of small olefins on zeolite H-ZSM-5. A thermogravimetric study at low and intermediate temperatures

Oligomerization and cracking reactions of ethene, propene, and isobutene on zeolite H-ZSM-5 (300 ≤ T < 600K) were investigated using temperature-programmed adsorption and desorption experiments, high-resolution 13C-NMR spectroscopy, and gas Chromatographic product analysis. Evidence is gained that at 300K only the stronger part of the Bronsted-acid sites are active in ethene oligomerization, while at increased temperatures more sites become active. On the contrary, in propene and isobutene oligomerization all sites are already active at 300K. This results in completely analogous products formed upon oligomerization of ethene, propene, and isobutene above 373K. The rate of oligomerization increases sharply with increasing reaction temperatures, resulting in a hindered transport of reactant molecules through the pores due to pore mouth blocking. The reactions on the outer surface become more important, which results in an increased branching of the oligomers formed at higher reaction temperatures. At 400K cracking of the oligomers starts, and at 490K the rate of cracking equals the rate of oligomerization. At this temperature desorption products show considerable branching, while at 573K only highly branched products are desorbed. At temperatures above 500K zeolite H-ZSM-5 becomes a dynamically operating catalytic system in the conversion of small olefins.

Reactions of propane over a bifunctional Pt/H-ZSM-5 catalyst

Abstract Propane can be converted to products with more than 3 carbon atoms by using a combination of platinum and H-ZSM-5. Zeolites with Pt incorporated in the zeolite pores lead to better results than physical mixtures of zeolite and supported Pt; the conversion is higher and almost no deactivation occurs. At least two steps can be distinguished: formation of propene over Pt and subsequent oligomerization over acid sites of the zeolite. Moreover a number of inevitable side reactions occur: Pt catalyzed hydrogenation and hydrogenolysis and acid catalyzed cracking. The main higher hydrocarbons formed are normal and iso butane. The high nC 4 /iC 4 ratio suggests that at reaction temperatures the propene oligomerization is controlled by the shape selective properties of the H-ZSM-5 zeolite.

CoAPO molecular sieve acidity investigated by adsorption calorimetry and IR spectroscopy

The acidic properties of cobalt-and silicon-substituted AlPO-5, –11 and –44 have been characterized by adsorption calorimetry and IR measurements. Adsorption calorimetric measurements indicate that the adsorption potential of the samples for acetonitrile is enhanced upon cobalt incorporation. The calorimetrically measured heats of adsorption indicate the formation of strong acid sites, due to this cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P—OH groups. IR measurements of adsorbed ammonia and acetonitrile indicate that some Bronsted-bonded ammonia and acetonitrile are present, in combination with Lewis-bonded species. The Bronsted-bonded species presumably originate from interaction of the base with the weakly acidic P—OH groups. The Lewis-bonded species are due to the coordinative interaction of the lone pair of the base with framework cobalt ions.

Low Temperature Reactions of Olefins on Partially Hydrated Zeolite H-ZSM-5

Summary Thermogravimetry is used to study the adsorption and reaction of ethene, propene and i-butene on H-ZSM-5. It is shown that already at roomtemperature ethene, although much slower than propene and i-butene, reacts on dehydrated H-ZSM-5 to form strongly adsorbed products. The inhibiting effect of preadsorbed water on this reaction has been investigated. By solid-state 13C-NMR the reaction product of ethene could be identified as an aliphatic hydrocarbon with 8–10 carbon atoms.

Preparation of bifunctional platinum/H-ZSM5 catalysts and their application for propane conversion

By ionexchange the Pt(NH3)42 complex can be introduced m the poresystem of ZSM5. Provided a gentle calcination under He/O2 is applied, this complex can be converted to small metal oxide particles that remain in the poresystem. After reduction the obtained bifunctional Pt/H-ZSM5 catalyst with internal Pt proofs to be appropriate for the conversion of propane. The formation of coke on the Pt particles is inhibited by the shape selective properties of the surrounding poresystem.

Acidic properties of metal substituted aluminophosphates studied by adsorption calorimetry and IR spectroscopy

The heats of adsorption of acetonitrile on metal substituted AlPO 4 -5 and AlPO 4 -11 compared to the pure AlPOs indicate the formation of strong acidic sites. Besides these strong sites some weaker acidic centres, probably P-OH groups have been found. Accordingly, IR measurements on the loaded MeAPOs show some weakly Bransted bonded acetonitrile in combination with a strong Lewis complex. The Lewis bonded acetonitrile in the MeAPOs originate from coordinative interaction of the lone pair with the bivalent framework metal ions such as Mg 2+ , Mn 2+ , Co 2+ or Zn 2+ .

Commensurate freezing of hydrocarbons in silicalite

Normal hexane and normal heptane exhibit a two step desorption profile from silicalite while shorter and longer normal alkanes desorb in a single step. The two step desorption profiles are explained by, and give evidence for, commensurate freezing of the hydrocarbon molecules in the zig-zag channels of silicalite. The freezing results in an additional loss of entropy upon adsorption in silicalite. Also linear alkenes with appropriate chainlength display a two step, but less pronounced, desorption profile. The less pronounced desorption profiles are the result of the lower energetic interaction of the alkenes with the silicalite lattice.

The conversion of dimethyl ether over Pt/H-ZSM5. A bifunctional catalyzed reaction

At low temperatures dimethylether mixed with hydrogen reacts over a platinum loaded H-ZSM5 catalyst selectivity to methane. Two successive steps can be distinguished; first the acid-catalyzed formation of a trimethyloxoniumion, followed by a metal-catalyzed hydrogenation to methane. Experiments with other zeolites show that the first step is rate determining; on stronger acid sites the activation energy is lower thus the reaction is faster. Reversely the experimental determined rate of methane formation can be used to characterize the acid strength.