Se sintetizaron zeolitas con fase *BEA y se determinó la relación de las condiciones de síntesis con la composición final del sólido. Los datos de acidez muestran una tendencia lineal con la cantidad de cationes alcalinos intercambiados con amonio. A partir de las zeolitas sintetizadas se prepararon catalizadores para su evaluación en la reacción hidrocraqueo de un gasóleo de vacío, donde los catalizadores más activos son aquellos que presentaron mayor acidez y mayor cantidad de aluminio incorporado en la estructura de la zeolita. Se resalta el hecho de que altos tiempos de cristalización le confiere a la zeolita propiedades fisicoquímicas especiales que permiten obtener mayor actividad catalítica comparada con aquellas zeolitas sintetizadas a bajo tiempo de cristalización. Por otro lado, el tipo de catión presente en el gel, regula de manera diferente la concentración de aluminio y por tanto modifica su comportamiento catalítico. Los materiales obtenidos se caracterizaron por diferentes técnicas analíticas como difracción de rayos X, análisis termogravimétrico y diferencial, análisis químico, área BET, desorción de amoniaco a temperatura programada, resonancia magnética nuclear 27Al MAS NMR y reducción a temperatura programada.

Eight vacuum residues and their delayed coking liquids products from Colombian crude were study by infrared spectroscopy with attenuated total reflectance (FTIR-ATR) and principal component analysis (PCA). For the samples the structural parameters of aromaticity factor (fa), alifaticity (A2500-3100cm-1), aromatic condensation degree (GCA), length of aliphatic chains (LCA) and aliphatic chain length associated with aromatic (LACAR) were determined through the development of a methodology, which includes the previous processing of spectroscopy data, identifying the regions in the IR spectra of greatest variance using PCA and molecules patterns. The parameters were compared with the results obtained from proton magnetic resonance (1H-NMR) and 13C-NMR. The results showed the influence and correlation of structural parameters with some physicochemical properties such as API gravity, weight percent sulphur (% S) and Conradson carbon content (% CCR)

Branched alkanes were adsorbed on La exchanged zeolite X and their interaction and surface chemistry were studied at 348 K. At low coverage, the CH bonds of the adsorbed alkanes were strongly polarized. This led to activation of branched alkanes by hydride abstraction, the reactivity being higher the higher the degree of branching was. Upon activation, carboxyl species were formed on the surface, which participated in isomerization and cracking reactions. The olefins, which were formed by cracking, were protonated on Bronsted acid sites or added to existing carbenium ions.

The formation of carbonaceous deposits and their effect on aging and deactivation of zeolite LaX during isobutane/2-butene alkylation at 348 K were investigated by stopping the reaction at different times on stream. Four stages of the reaction were identified: (1) stable alkylation, (2) deposit transformation, (3) slow deactivation, and (4) rapid deactivation. Deposits consist mostly of bicyclic compounds and branched carbenium ions, which are formed already at the beginning of the reaction and block Brønsted acid sites. During the deposit transformation, migration of smaller entities toward the pore mouth occurs. These cyclic compounds are further alkylated and lead to pore mouth plugging. In the final stage of rapid deactivation, the catalyst stops producing alkylate, and butene oligomerization is the main reaction leading to olefin desorption and massive deposit formation at the outside of the zeolite particles.

The influence of the activation temperature on the physicochemical properties and catalytic activity of La-X zeolites for isobutane/cis-2-butene alkylation was investigated. Under optimal activation conditions, molecularly adsorbed water was detected by in situ IR and 1H MAS NMR spectroscopy, as well as temperature programmed desorption. Bridging hydroxyl groups were not involved in hydrogen bonding of these water molecules. Isobutane/cis-2-butene adsorption together with isobutane/n-butene alkylation indicates that the water left on the catalyst after activation at 423–453K promotes the rate of hydride transfer with respect to that of alkene addition. As a consequence, the catalyst lifetime in alkylation was shorter for the sample activated at higher temperatures (553K) than for those having molecular water adsorbed in the zeolite pores.

Adsorption and surface chemistry of octane isomers on La-exchanged zeolite X was explored under near-ambient conditions. At low coverage, the sorption constants depend mainly on dispersion forces. However, very strong polarization of the C−H bonds is indicated by an unusually high extinction coefficient of the C−H vibrations. Bi- and tribranched alkanes react under these conditions, and the reactivity increases with the degree of branching. The activation proceeds via hydride abstraction forming alkoxy groups, which subsequently isomerize, and crack. Cracking products desorb primarily via hydride transfer from mobile alkanes leading predominantly to isobutane and isopentane. The surface chemistry and alkanes produced show that adsorption and desorption, cracking and alkylation, as well as hydride-transfer reactions already occur at near ambient conditions in these zeolites.

Introduction Acidic zeolite catalysts have a wide range of applications for reactions like catalytic cracking, alkylation, hydrocracking and isomerization. Lanthanum exchange is among the most important methods for increasing the hydrothermal stability of large pore zeolites. Additionally, it has been suggested that the presence of lanthanum cations increases the strength of the Brønsted acid sites in the zeolite [1]. Alkane activation on zeolites is usually triggered by protolytic cracking [2]. Alternatively, olefin impurities or hydride abstraction by soft Lewis acid sites have been reported [3]. The protolytic route is only observed at high temperatures [4]. Recently, the activation of alkanes over zeolites at ambient conditions has been demonstrated for the first time [5].

The acid sites generated at different steps during the preparation of La-H-X zeolites were characterized by physicochemical methods. The resulting materials were tested in isobutane/cis-2-butene alkylation in a continuously operated stirred tank reactor, under industrially relevant conditions.The concentration and strength of acid sites depend subtly on the ion exchange procedure. Especially, the rehydration of materials calcined for the first time after ion exchange changes the distribution of hydroxyl groups, the Bronsted acidic bridging hydroxyl groups (3640 cm(-1)) being strongly affected. Rehydration leads to dealumination and, as consequence, the concentration of silanol groups (3740 cm(-1)) and of Lewis acid sites increases. This in turn results in an enhanced stability towards the subsequent thermal treatments and rehydration processes of the rare earth zeolite in the next steps of catalyst preparation. The strength of the Bronsted acid sites was shown to be a function of the hydrolysis of hydrated lanthanum cations and removal of sodium cations.The catalytic activity in isobutane/cis-2-butene alkylation and the fraction of strong Bronsted to total Bronsted acid sites are directly related. Catalysts with similar concentration of strong Bronsted acid sites and higher concentration of weak Bronsted sites showed shorter lifetime. (c) 2005 Elsevier Inc. All rights reserved.

Well-characterized examples of the large-pore zeolites X and Y in their acidic form were explored as catalysts for isobutane/butene alkylation in order to understand the principal requirements for successful solid acid catalysts. The materials were tested in a continuously operated stirred tank reactor under industrially relevant conditions. A high ratio of Brønsted to Lewis acid sites and a high concentration of strong Brønsted acid sites are seen to cause high hydride transfer activity and are mandatory for long catalyst life. Isobutane “self-alkylation” activity is higher in catalysts with a high hydride transfer activity. The catalyst lifetime is very sensitive with respect to the reaction temperature reaching the optimum between 70 and 80°C concurrent with a maximum in self-alkylation activity. The lifetimes were found to be correlated linearly with the reciprocal of the olefin space velocity, while it hardly influenced the total productivity of the catalysts. The selectivities on the other hand strongly depended on the butene feed rate per active site.

The catalytic chemistry of a new process for alkylation of iso-butane with n-butene with zeolite based solid acid catalysts is described. The Bronsted acid site concentration in the zeolite catalysts is high, while the Lewis acid site concentration is low. The catalysts show high stability against deactivation by coking. In order to retard coking further and to extend the catalyst lifetime, operation conditions are adjusted in a way to periodically after the feed concentration and add hydrogen to the reactant mixture. The process uses a tray distillation column with the catalysts cycling between the reactor and a regeneration unit. Thorough backmixing of reactants at each tray and a staged introduction of butene is realized with this design.

The deactivation of rare-earth-exchanged zeolite X as catalysts in isobutane/2-butene alkylation between 40 and 130°C was studied. The deactivated samples and the isolated deposits were analyzed by a range of techniques, including for the first time (matrix assisted) laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS, LDI-TOF MS). The compounds found in the deactivated zeolites are, in addition to large alkanes and alkenes, highly unsaturated and highly branched species containing cyclic structures, which are increasingly aromatic as the reaction temperature increased. The deposits in part interact strongly with the acid sites and block the sites for further alkylation reactions. Their structure and route of formation resemble those of conjunct polymers formed in liquid acid-catalyzed alkylation.

Recently developed mechanistic and technological concepts concerning the isobutane/butene alkylation reaction on liquid and solid acids are addressed. Differences and similarities between the reactions of alkenes and alkanes with these acids are emphasized. Hydride transfer is shown to be the important step for high catalyst productivity and product quality. High concentrations of strong Brønsted acid sites and low concentrations of Lewis acid sites are mandatory for long catalyst life. Deactivation in liquid and solid acids proceeds through the formation of highly unsaturated compounds, which form strong complexes with the acid sites. Implications of the most important process parameters on the alkylation performance are discussed.