Peter A. Jacobs

Isomerization and hydrocracking of C9 through C16 n-alkanes on Pt/HZSM-5 zeolite

Abstract The pure n-alkanes with 9 to 16 carbon atoms were converted on a Pt/HZSM-5 zeolite in the presence of hydrogen. The products formed by isomerization and hydrocracking were analyzed by high-resolution capillary GLC. The product distributions differ substantially from those observed earlier on Y type zeolites. Isomerization on Pt/HZSM-5 gives mainly monomethyl isomers. The cracked products consist of n- and i-alkanes with a single methyl branching. Neither methane nor ethane are formed which is indicative of an ionic mechanism of cleavage. The carbon number and isomer distributions of the cracked products are best interpreted in terms of a so-called type C β-scission mechanism combined with secondary isomerization and competitive adsorption at the acidic sites inside the Pentasil channel system.

Properties of the end members in the Pentasil-family of zeolites: characterization as adsorbents

Domestically synthesized HZSM-5 and HSZM-11 zeolites were shown to have the same crystal morphology and unit cell dimensions as the Mobil materials. The zeolites were 100% crystalline and crystallographically pure. Sorption of N 2 , O 2 , H 2 S, CO 2 and of C 3 –C 10 n-paraffins, of i -C 4 , i -C 5 , neopentane, the monomethylnonanes and the xylene isomers was followed by thermogravimetric or volumetric techniques. Generally, the sorption capacity of HZSM-5 was always higher than HSZM-11. The sorption behaviour could be explained by simple models. When pore intersections of both zeolites are considered, the maximum sorption capacity corresponds to the closest possible packing at these intersections. Paraffin sorption occurs according to an end-to-end adsorption model. The concept of total adsorbate length was useful to rationalize the adsorption.

Crystallization mechanism of zeolite beta from (TEA)2O, Na2O and K2O containing aluminosilicate gels.

Abstract Zeolite BETA is prepared in hydrothermal conditions from tetraethyl orthosilicate, sodium aluminate, tetraethylammonium hydroxide, sodium and potassium hydroxide and water. Detailed chemical analysis of intermediate solid and liquid phases obtained from a large number of gel compositions at different synthesis times as well as SEM and XRD investigations of the solids allow to determine the crystallization mechanism. After nucleation in solution zeolite BETA crystals grow by incorporating TEA-aluminosilicate species from solution. The influence on this process of sodium, potassium, tetraethylammonium, silica, alumina and alkalinity in the gel are discussed.

Acid Zeolites: An Attempt to Develop Unifying Concepts (P. H. Emmett Award Address, 1981)

Abstract The recent developments of zeolite chemistry are characterized by the appearance of attempts to explain certain properties of these porous crystalline solids in terms of general chemical properties developed for other solids or even for other states of matter. Transition metal complexes in zeolites sometimes exhibit the same properties as in more conventional solvents [1]. Thus the zeolite behaves as a mono to polydentate macroligand of the transition metal ion [2]. Although numerous observations [2] underline the role of the zeolite as a solid solvent or as a common solid matrix, its unique properties in terms of structure or cage geometry should not be overlooked. Indeed, the appropriate activation of Ru(III) hexammine in faujasite-type zeolites gives a very active water gas-shift catalyst [3]. A cationic Ru-complex seemed to be formed under these conditions, having no homogeneous equivalent, and its stabilization requires the specific geometry of a faujasite supercage.

Catalytic activity and Si, Al, P ordering in microporous silicoaluminophosphates of the SAPO-5, SAPO-11, and SAPO-37 type

The catalytic activity in the conversion of decane on SAPO-5, SAPO-11, and SAPO-37 samples with Si T-atom fractions ranging from 1 to 42% were determined. The relationship of catalytic activity with compositional parameters is discussed. Due to the nonuniform distribution of silicon, aluminium, and phosphorus over individual crystals, the overall chemical composition of a SAPO-n sample is unable to rationalize its catalytic activity. In order to explain the catalytic activity of SAPO-5, SAPO-1 l, or SAPO-37 materials, the contributions of aluminosilicate (SA) and silicoaluminophosphate (SAPO) crystal domains have been determined using 29Si MAS NMR. Acid sites located in the SA and SAPO domains and at their interphases contribute to the catalytic activity. The contribution of the different types of acid sites to the catalytic activity is strongly dependent on the SAPO-n structure type.

An attempt to rationalize stretching frequencies of lattice hydroxyl groups in hydrogen-zeolites

Hydrogen zeolites of different structure and chemical composition have been prepared and their lattice OH vibrations determined by high-resolution infrared spectroscopy. The frequency of all the lattice OH groups in these zeolites has been rationalized in terms of two concepts: (i) the frequency of the unperturbed OH groups — vibrating in structural elements larger than eight-membered rings — is linearly correlated with the Sanderson intermediate electronegativity of the zeolites, which in its turn varies with the chemical composition; (ii) hydroxyl groups vibrating either in 6- or 8-membered rings undergo a bathochromic shift with respect to their unperturbed positions. This shift is caused by electrostatic interactions with the nearest oxygen atoms and therefore inversely proportional to the average of the squared distances.

Synthesis and shape-selective properties of ZSM-22

Abstract Zeolite ZSM-22 was synthesized in the presence of diaminohexane. From the characterization of the synthesized material, it could be deduced that the organic compound acts as a pore-filling agent during synthesis. The H-form of the zeolite contains surface hydroxyl groups, which are identical with those found in H-ZSM-5 and other high-silica zeolites. The intracrystalline voids of ZSM-22 were characterized with catalytic test reactions. The constraint index was determined and the n-decane test applied. ZSM-22 catalytically behaves as a ten-membered ring zeolite, with an effective pore size slightly smaller than that of ZSM-5. As a result of shape-selective effects, n-decane isomerization products are mainly branced at the ends of the hydrocarbon chain.

The potential and limitations of the n-decane hydroconversion as a test reaction for characterization of the void space of molecular sieve zeolites

Abstract The bifunctional conversion of long-chain n-paraffins is a very attractive reaction for the characterization of the pore structure of zeolites. Based on the distribution of the isomerization and hydrocracking products, independent criteria are developed for the determination of the zeolite intracrystalline void space. This selection of reliable and sensitive criteria is supported by classical and non-classical concepts used in ideal bifunctional catalysis. The chemistry of long-chain carbenium ions in acid zeolites is reviewed and illustrated with new data. It is confirmed that the classical model of bifunctional catalysis is valid for Pt-loaded USY and H-ZSM-5. Effects of crystal size, crystal morphology and Si Al ratio can be explained in terms of matching and mismatching of the catalytic functions. At many stages of the complex reaction network the distribution of the products is a unique function of the conversion of the feed and is rather insensitive to changes in reaction temperature and pressure. This allows one to compare zeolite structures based on product distributions obtained in different conditions. The five criteria proposed earlier to characterize the structure and dimensions of the zeolite void volume are extended with three new criteria. Finally, the way to proceed to the characterization of a zeolite void volume with the n-decane test is explained.

Factors affecting the synthesis efficiency of zeolite BETA from aluminosilicate gels containing alkali and tetraethylammonium ions

The efficiency of the synthesis of zeolite BETA and the chemical composition of the crystals depend very much on the concentration of aluminium in the liquid phase, the SiO 2 /Al 2 O 3 ratio of the gel particles, the presence of ethanol in the system, and the synthesis temperature. Characterization of intermediate crystallization products with various techniques further indicates that the mechanism of nucleation and crystal growth of zeolite BETA does not vary if these synthesis parameters are changed.

Shape-selectivity changes in high-silica zeolites

Y-type, ZSM-5 and ZSM-11 zeolites with the same chemical composition have been prepared. Peak indexation and microscopic examination were used to identify the phases and to examine the samples for degree of crystallinity. The acidity was measured by temperature-programmed desorption of NH3 and infrared examination of OH groups upon interaction with hydrogen-bond-acceptor molecules. These data were consistent with the results derived from total activity and selectivity measurements of the isomerization and hydrocracking of n-decane on the Pt-loaded zeolites.The distribution of the feed isomors served to distinguish between HZSM-5 and HZSM-11 type structures, while the cracked products could be predicted using simple reaction schemes. 2,2,4-Trimethylpentane was not able to enter the pores of the Pentasil zeolites, reacting near the pore mouth or entering the pores after dehydrocyclisation on Pt clusters external to the zeolite.