van Jhc Jan Hooff

Hydroxylation of phenol with hydrogen peroxide on EUROTS-1 catalyst

Abstract The catalytic performance of a standard titanium molecular sieve catalyst, EUROTS-1, has been evaluated at four European universities. The hydroxylation of phenol into catechol and hydroquinone with aqueous H 2 O 2 solution was selected as catalytic test reaction. The use of standard EUROTS-1 catalyst permitted to establish a reproducible standard reaction procedure. The calcination conditions of the catalyst, the use of internal standards, and the nature and the amount of the solvent added to the reaction mixture were found to be major factors determining phenol conversion, product selectivity and hydrogen peroxide product yield.

Aromatization of propane over MFI-gallosilicates

The results of propane aromatization over gallium containing HZSM-5 zeolites are described. Highly dispersed gallium in the zeolite (framework or extra-framework) was found to possess dehydrogenation activity, especially in the presence of strong Bronsted acid sites. The extra-framework gallium was introduced by mildly steaming a H-(Ga) ZSM-5 zeolite or by physically mixing Ga2O3 or a GAPSO-11 molecular sieve with H-(Al) ZSM-5. The higher dehydrogenation rates in the presence of gallium facilitated the formation of carbonaceous products, resulting in higher deactivation rates. A mechanism for hydrogen formation is discussed.

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.

A new method for the preparation of titanium-silicalite (TS-1)

Titanium-silicalite can be prepared by subsequent dealumination of ZSM-5 with hydrochloric acid and reaction with titaniumtetrachloride vapour at elevated temperatures. The titanium atoms are probably inserted into lattice vacancies which were formed upon acid leaching.

Aluminum incorporation in MCM-41 mesoporous molecular sieves

Twelve MCM-41 samples with a SiAl ratio varying from 12 to 200 were prepared by using sodium aluminate in a tetramethylammonium hydroxide solution as aluminum source and an unexchanged CTMACl template solution. The thermogravimetric analysis (TGA) weight losses caused by thermal removal of the template cations, which are associated with the siloxy groups on the one hand and the aluminum sites on the other hand, correspond with these different Al contents. X-Ray diffraction (XRD) revealed that at SiAl = 5, in addition to MCM-41, a lamellar phase and a dense crystalline tridymite structure are formed. 27Al Magic-angle spinning nuclear magnetic resonance (MAS NMR) indicates that upon calcination the tetrahedral aluminum sites are altered. Fourier transform infrared (FTIR) analysis of proton-exchanged MCM-41 with acetonitrile as probe molecule showed a very weak Bronsted and a rather strong Lewis acidity. The latter may be the result of the distortion of the aluminum sites by thermal treatment.

Acidity and activity of H-ZSM—5 measured with NH3-t.p.d. and n-hexane cracking

NH3-t.p.d. and n-hexane cracking were used to characterize the acidity and activity of zeolite H-ZSM—5. From the NH3-t.p.d. experiments it could be concluded that two types of acid sites are present in H-ZSM—5: Weak acid sites corresponding with desorption at low temperature and small ΔHdes, and strong acid sites corresponding with desorption at high temperature and large ΔHdes. Especially the desorption at high temperature could be explained satisfactorily by theoretical models as presented in literature. From the n-hexane cracking experiments information about the initial activity and the deactivation were obtained. Combination of the results of both methods led to a relation between the initial activity and the amount of strong acid sites. However, it was impossible to relate the deactivation with one of the acidic properties.

Parameters affecting the synthesis of titanium silicalite 1

Abstract The influence of the SiO2 source, crystallization time, stirred and static crystallization conditions, TPAOH source, OH−/Si ratio, Si/Ti ratio and sol concentration were studied. Crystallizations were carried out at 175°C in teflon-lined autoclaves. We were able to synthesize well shaped, homogeneous, highly crystalline batches of TS-1 zeolites with Si/Ti ratio ranging from 24 to infinity (0 to 4 mol-% Ti) and a crystallite size ranging from 0.09 to 12 μm. In order to form small crystallites, seeds are added and the reaction mixture is agitated during crystallization. The best conditions to form small crystallites are; high silicon concentration, low OH− concentration and low water concentration.

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.

Aromatization of propane over gallium-containing H-ZSM-5 zeolites Influence of the preparation method on the product selectivity and the catalytic stability

Abstract Three types of gallium-containing H-ZSM-5 zeolites were prepared in order to compare the catalytic stabilities during propane aromatization. The gallium was introduced into the zeolite in three different ways: isomorphous substitution of aluminum by gallium, impregnation of H-(Al)ZSM-5 with a GaCl 3 or a Ga(N0 3 ) 3 solution, and physically mixing Ga 2 O 3 powder with a H-(Al)ZSM-5 zeolite. In all cases, addition of 0.6 Ga/unit cell or more enlarged the aromatics selectivity from about 35 C-% to 60 C-%, while the activity remained almost unaffected. A negative side effect was an enhanced rate of deactivation by coke formation, which was most severe for the Ga/H (A1)ZSM-5 catalyst prepared via impregnation or physical mixing, as the gallium is located on the outer surface of the zeolite particles in these samples.