Fluidised cracking catalysts, which contain a form of zeolite-Y as the main catalytically active component, are widply used commercially for the conversion of crude oil into more profitable product streams. During the cracking reaction, these catalysts are contaminated with vanadium which has a marked effect on the crystallinity of the zeolite-Y component and, as a consequence, activity and selectivity for hydrocarbon processing is degraded. The purpose of this work has been to carry out a detailed investigation, on the laboratory scale, of the effect of vanadium contaminatioil on both commercial and model rare earth ion-exchanged zeolite-Y catalysts. Vanadium contamination was achieved using a standard (Mitchell) method and catalysts were subject to treatment conditions similar to those found in the regenerator part of a fluidised catalytic cracking unit using a specially constructed furnace. Investigations of the solid state chemical reactions between vanadium and rare earth compounds, both in the presence and absence of silica and alumina support materials typical of those found in commercial catalysts, extend the study. Extensive use is made of magic-angle-spinning nuclear magnetic resonance spectroscopy (5 IV and 27AI), X-ray powder diffraction and surface area measurements for sample characterisation. The apparatus for surface area measurements was constructed during the course of the work. It is suggested that the reduction in crystallinity of rare earth ion-exchanged zeolite-Y in the presence of vanadium is associated with the removal of rare earth ions from the cage structure of the zeolite. The extent of this process depends upon the details of the treatment conditions and important factors are identified. The observation of the formation of LaV04 in a range of hydrothermally treated lanthanum ion-exchanged zeolite-Y samples supports the proposed model.

Zeólitas tipo Y contendo terras raras, RE,Ca/NaY (RE = La, Ce, Nd, Eu, Gd), foram preparadas a partir da forma NaY (razão Si/Al = 2,4 ) por trocas iônicas sucessivas com soluções de cloreto de cálcio e cloreto de terras raras, -através de refluxo; e subseqüente filtragem a vácuo, secagem e tratamento térmico sob fluxo de nitrogênio. Após o tratamento térmico, os Íons aquosos trivalentes de terras raras sofrem hidrólise, formando hidroxilas catiônicas do tipo [RE. OH]2+ e prótons H+, tornando-se localizados nas cavidades e canais da zeólita. O próton reage com o oxigênio da rede cristalina, originando centros ácidos muito fortes. O tipo específico de terra rara trocado ionicamente influencia a acidez total, força e distribuição dos sítios ativos na zeólita. As composições químicas das celas unitárias das amostras foram determinadas por espectrometria de emissão atômica por plasma de argônio induzido (ICP-AES). A cristalinidade, após troca iônica e tratamento térmico, foi confirmada por espectroscopia na região do infravermelho (FT-IR) e difração de raios-X. A microssimetria do íon európio na zeólita Eu,Ca/NaY foi investigada por espectroscopia eletrônica de emissão na região do visível, sob excitação de radiação ultravioleta (396 nm). As propriedades ácidas das amostras RE, Ca/NaY foram estudadas por termogravimetria (TG) e calorimetria exploratória diferencial (DSC), usando n-butilamina como molécula sonda. Por TG, foi investigada a acidez total das amostras; enquanto que por DSC, utilizando o modelo cinético de Borchardt & Daniels, foi investigada a força ácida relativa das amostras. A presença dos centros ácidos de Brönsted e Lewis foi determinada por espectroscopia na região do infravermelho (1700-1300 cm-1) , usando piridina como molécula sonda. A atividade catalítica e seletividade das amostras foram avaliadas na reação química modelo de alquilação de benzeno com a olefina linear 1-dodeceno para formação de alquilbenzeno linear. A reação foi estudada na fase líquida, em um microrreator de batelada, à temperatura de 80°C. 0s produtos monoalquilados e os isômeros lineares de 1-dodeceno foram determinados por cromatografia em fase gasosa e espectrometria de massa (GC-MS). / The Y zeolites containing calcium and rare earth polivalent cations, RE,Ca/NaY (RE = La, Ce, Nd, Eu, Gd), were prepared by ion exchange, refluxing the NaY zeolite (Si/Al = 2,4) with calcium chloride and rare earth chloride solutions, followed by thermal treatment under nitrogen flow. The trivalent hydrated rare earth ions undergo hydrolysis, forming [RE.OH]2+ cation hydroxyls and protons H+, which react with oxygen in the lattice, generating strong acid sites. The rare earth elements influence the total acidity, strength and distribution of the active sites in the zeolite. The unit cell chemical compositions of the samples were determined by Inductively Coupled Plasma / Atomic Emission spectroscopy (ICP/AES). The crystallinity was confirmed by Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray diffraction. The europium microsimetry in the Eu,Ca/NaY zeolite was investigated by emission electronic spectroscopy in the visible region, under ultra-violet excitation (396 nm). The acid properties of the RE,Ca/NaY samples were studied by Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC), using n-butilamine as molecular probe. From TG curves, the total acidity was calculated, whereas the relative acid strength was determinated by DSC curves, using the Borchardt-Daniels kinetic model. The nature of Brönsted and Lewis acid sites was investigated by FT-IR, in the 1700 - 1300 cm-1 region, using pyridine as molecular probe. The catalytic activity and selectivity of the samples were evaluated on the alkylation of benzene with 1-dodecene model reaction, to form linear alkylbenzenes, in liquid phase, at 80°C using a bath reactor. The reactant conversion, extent of isomerization and product distribution were monitored by Gas Chromatography and Mass Spectrometry techniques (GC-MS).

Zeólitas tipo Y contendo terras raras, RE,Ca/NaY (RE = La, Ce, Nd, Eu, Gd), foram preparadas a partir da forma NaY (razão Si/Al = 2,4 ) por trocas iônicas sucessivas com soluções de cloreto de cálcio e cloreto de terras raras, -através de refluxo; e subseqüente filtragem a vácuo, secagem e tratamento térmico sob fluxo de nitrogênio. Após o tratamento térmico, os Íons aquosos trivalentes de terras raras sofrem hidrólise, formando hidroxilas catiônicas do tipo [RE. OH]2+ e prótons H+, tornando-se localizados nas cavidades e canais da zeólita. O próton reage com o oxigênio da rede cristalina, originando centros ácidos muito fortes. O tipo específico de terra rara trocado ionicamente influencia a acidez total, força e distribuição dos sítios ativos na zeólita. As composições químicas das celas unitárias das amostras foram determinadas por espectrometria de emissão atômica por plasma de argônio induzido (ICP-AES). A cristalinidade, após troca iônica e tratamento térmico, foi confirmada por espectroscopia na região do infravermelho (FT-IR) e difração de raios-X. A microssimetria do íon európio na zeólita Eu,Ca/NaY foi investigada por espectroscopia eletrônica de emissão na região do visível, sob excitação de radiação ultravioleta (396 nm). As propriedades ácidas das amostras RE, Ca/NaY foram estudadas por termogravimetria (TG) e calorimetria exploratória diferencial (DSC), usando n-butilamina como molécula sonda. Por TG, foi investigada a acidez total das amostras; enquanto que por DSC, utilizando o modelo cinético de Borchardt & Daniels, foi investigada a força ácida relativa das amostras. A presença dos centros ácidos de Brönsted e Lewis foi determinada por espectroscopia na região do infravermelho (1700-1300 cm-1) , usando piridina como molécula sonda. A atividade catalítica e seletividade das amostras foram avaliadas na reação química modelo de alquilação de benzeno com a olefina linear 1-dodeceno para formação de alquilbenzeno linear. A reação foi estudada na fase líquida, em um microrreator de batelada, à temperatura de 80°C. 0s produtos monoalquilados e os isômeros lineares de 1-dodeceno foram determinados por cromatografia em fase gasosa e espectrometria de massa (GC-MS). / The Y zeolites containing calcium and rare earth polivalent cations, RE,Ca/NaY (RE = La, Ce, Nd, Eu, Gd), were prepared by ion exchange, refluxing the NaY zeolite (Si/Al = 2,4) with calcium chloride and rare earth chloride solutions, followed by thermal treatment under nitrogen flow. The trivalent hydrated rare earth ions undergo hydrolysis, forming [RE.OH]2+ cation hydroxyls and protons H+, which react with oxygen in the lattice, generating strong acid sites. The rare earth elements influence the total acidity, strength and distribution of the active sites in the zeolite. The unit cell chemical compositions of the samples were determined by Inductively Coupled Plasma / Atomic Emission spectroscopy (ICP/AES). The crystallinity was confirmed by Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray diffraction. The europium microsimetry in the Eu,Ca/NaY zeolite was investigated by emission electronic spectroscopy in the visible region, under ultra-violet excitation (396 nm). The acid properties of the RE,Ca/NaY samples were studied by Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC), using n-butilamine as molecular probe. From TG curves, the total acidity was calculated, whereas the relative acid strength was determinated by DSC curves, using the Borchardt-Daniels kinetic model. The nature of Brönsted and Lewis acid sites was investigated by FT-IR, in the 1700 - 1300 cm-1 region, using pyridine as molecular probe. The catalytic activity and selectivity of the samples were evaluated on the alkylation of benzene with 1-dodecene model reaction, to form linear alkylbenzenes, in liquid phase, at 80°C using a bath reactor. The reactant conversion, extent of isomerization and product distribution were monitored by Gas Chromatography and Mass Spectrometry techniques (GC-MS).

The objective of this work was the development of structured zeolite catalysts by growing of ZSM-5 and Y zeolites layers on the pre-treated FeCrAlloy wires, which could now offer technical advantage in catalytic application. The advantages of implementation of zeolitic coatings in industrial applications are that they have; lower pressure drop, high heat and mass transfer rates compared to standard pelleted or extruded catalysts. The key focus of this research was the generation of thin films of zeolite ZSM–5 and Y zeolite catalysts on the surface of a FeCrAlloy metal substrate. Using in-situ hydrothermal synthesis, the influence of the synthesis parameters such as substrate oxidation and crystallisation time on the zeolite crystallisation process in both the bulk phase (powder) and on the structured zeolite was studied and optimised. Then powder and structured Na-ZSM-5 and Na-Y were treated by calcination and ion exchange in post-synthesis treatment. Further post-synthesis modification was required in the zeolite Y case to improve the catalytic properties. The post synthetic modification of zeolite Y was carried out using acidified ammonium nitrate which was optimised to produce dealuminated zeolite Y with good crystallinity and a Si/Al = 8. Characterisation was performed after each stage of this work to optimise catalyst development using XRD, SEM, EDAX, BET, MAS-NMR, and TGA. Once the optimised zeolite Y and ZSM-5 structured catalysts prepared, cracking of n-heptane was carried out to assess the in catalytic performance compared with Y and ZSM-5 pellets in a fixed-bed reactor under the same operation conditions. The cracking of n–heptane over the pellets and structured catalysts for both ZSM–5 and Y zeolite showed very similar product selectivities for similar amounts of catalyst with apparent activation energy of around 60 kJ mol-1. This research demonstrates that structured catalysts can be manufactured with excellent zeolite adherence and when suitably activated/modified give comparable cracking results to the pelleted powder forms. These structured catalysts will improve temperature distribution in highly exothermic and endothermic catalysed processes.

Mankind makes extensive use of crude oil to fuel its insatiable demands for energy and hydrocarbon derivatives. The refining of crude oil is based on a process known as cracking, where long-chain hydrocarbons are systematically broken into smaller chain hydrocarbons known as fractions with each fraction allowing for the production of a specific material. The maximum efficiency of cracking can be achieved in the petroleum refining processes by controlling the operating parameters of the units, and over the years many studies have attempted to optimize the cracking conditions such as temperatures, pressures and the use of a variety of catalysts to reach maximum productivity. Catalysts such as the Y-type zeolite catalysts are often used because their acidity and thermal stability makes them an ideal cracking catalyst; however the developments of enhanced catalytic properties for zeolite-Y catalysts are essential to increase the production yields. Optimization of the Y-type zeolite catalyst is the focus of this research and accordingly the synthesis, characterization, modifications and catalysis have been studied in depth. A review of the literature has shown that there are three main techniques used to improve the zeolite properties following the synthesis process; (Cation exchange, Dealumination and Desilication), since the crystalline structure of a Y-type zeolite is prepared from an alkaline aluminosilicates gel. However, the literature focuses mainly on the reaction variables used in the modifications. As such this study focuses on the effects of treatment processes on the composition, behaviour and catalytic properties of the synthesized Y-zeolite framework. Laboratory experimental data has confirmed that a synthesis process using 24 h aging for crystal nuclei at 25 °C and 18 h crystallization time for crystal growth at 100 °C produced the desired zeolite NaY morphology, and NaNH4Y zeolite forms with various cation contents (3, 1.5 and 0.5 wt% Na+) were obtained by subjecting the NaY form to a multi-stage ion exchange using 0.5 M NH4NO3 at 80 °C, while the HY form was obtained by the calcination of NH4Y form under high temperature. Calcination temperatures above 450 °C were shown to indicate a removal of the framework hydroxyl groups via dehydroxylation, which led to a collapse of zeolite-Y structure, whereas raising the level of Na+ inside the zeolite lattice throughout the calcination was led to a delay in the starting point of the dehydroxylation region as confirmed via TG and DSC-analyses. This finding was also used in the preparation of the USY form by steaming the HY form, as the former is a traditional zeolite-Y form utilized in the refining units. It was found that Na-ions hindered the extraction of Al-atoms from the Y-lattice thus reducing the rate of dehydroxylation, and minimising rapid contraction of the unit cells and Y-structure collapse, which helped make a rigid structure and a more resilient lattice for steaming at high temperature. In addition, analyses data confirmed that the extraction of EFAl-species from the USY-structures using an EDTA chemical treatment led to an increase in the acidity of treated catalyst and the introduction of mesopores. Lower lattice Si/Al ratio and larger porosity were also found using the dealuminated-desilicated rather than the desilicated-dealuminated leaching method in the treatment of both Y and USY structures via dislodgement of both Si and Al-atoms in NaOH and HCl solution. Cracking was performed on deactivated catalysts (450 °C) in a PFTR using nC7 in N2 at 325 - 425 °C and W/F = 22 - 44 g.h.mol-1, and confirmed that the tuned steaming/leaching conditions succeeded in modifying the catalytic properties of the in house made catalysts, as they possess superior performance when compared to the industrial catalysts typically used.

Gas detection is vital in different fields including environmental applications, clinical analysis, and homeland security. To perform these tasks the sensors need to be stable, sensitive, selective, operating at room temperature, rapidly responding, and easy to regenerate. On the other hand, most chemical sensors often suffer from a lack of selectivity, i.e., reacting more or less similarly to a collection of substances. As a result, these sensors may lead to false alerts. Even worse, the molecules to be detected could be masked by some interfering compounds which may result in failure to detect the targets. The goal of this research is to develop a portable gas-sensing device that integrates a zeolite/dye unit with an optoelectronic detector. At nano-scale the sensor is expected to be more accurate, more sensitive, and can better differentiate and detect one chemical component in a mixture of different gases. This could be achieved by incorporating fluorescent dyes into the zeolites' cavities, measuring gas absorption, desorption and photo-chromic interaction of dye and gases, interfacing the zeolite/dye sensor arrays with light source and electronic detectors and fully integrating the sensor arrays into a portable unit. This research addresses many of the above-sated threads. The highly fluorescent organic dye, nile red, was successfully included in the supercages of different zeolites Y (ammonium Y, hydrogen Y, and sodium Y) via chemical reaction. The research also developed an effective method to clean the synthesized inclusions, which is a combination of ultrasound and centrifuge. The cleaned inclusions were baked to remove any gases and/or moisture trapped inside the zeolites' structure. The spectra of the baked inclusions were used as references. The cleaned inclusions were optically characterized in terms of light absorption and fluorescence emission. When exposed to acetone, ethanol, methanol, and de-ionized water, the fluorescence emission spectra of zeolite-sodium-Y/nile-red inclusion showed a similar spectral shift compared to the reference spectrum. On the other hand, the fluorescence emission spectra of zeolite-hydrogen-Y/nile-red inclusion and zeolite-ammonium-Y/nile-red inclusion showed different spectral shifts compared to the reference spectra. This shows the successful proof of encapsulating the nile red dye in zeolites Y's cages, cleaning the zeolite/nile-red combinations, and measuring the desorption and fluorescence emission of the combinations. The optical characteristics of the nile red adsorbing to the external surface of the zeolites Y were studied as well. The research also included the design of the optical system to excite the sensing elements (zeolite/nile-red inclusions), and to collect the fluorescence response, the design and simulation of electronic circuits to condition and process electrical signal, and overall design of an integrated gas detector onto a pressed ceramic optical bench. / Electrical and Computer Engineering

The efficacy of using zeolitic materials for the removal of nickel (II) and vanadium (IV) ions from solution has been evaluated in order to provide a method for the removal of the metal ions during hydroprocessing of crude oil. Batches of sodium based zeolites with a variety of pore sizes and Si/Al ratios were prepared using standard methods (high causticity solutions and templating agent). Characterisation of the products was carried out using powder X-ray diffraction, infrared spectroscopy, Raman spectroscopy and thermogravimetric analysis to confirm the presence of single zeolitic phases (zeolite A, zeolite X, zeolite Y, sodalite Na8 [AlSiO4]6Cl2 and hydrosodalite Na6 [AlSiO4]6. 6H2O). In a batch exchange process, divalent nickel and tetravalent vanadium ion solutions of concentration range 0.01M - 0.1M were placed in contact with the zeolite samples at 110°C for a period of 24h. Nickel (II) exchange was found to occur for all the zeolites at concentrations considered. Zeolite X was found to be most efficient at removing nickel from the solutions while zeolite Y was least efficient. Characterisation of zeolite X after ion exchange using powder X-ray diffraction and scanning electron microscopy showed that the structure of the zeolite had been maintained. Simplistic modelling of powder X-ray diffraction data have shown that the nickel ions are preferentially substituted on one of the four sodium sites. Vanadium (IV) exchange was also found to occur for all the zeolites at the concentrations considered. Zeolite A was found to be most efficient for the vanadium uptake. Characterisation with PXRD, FTIR and SEM-EDS however, shows that in addition to exchange at the zeolite s normal cation exchange sites, a significant amount of framework silicon species were also exchanged by the vanadium ions thus having a destructive effect on the zeolite framework leading to structural collapse. Ion exchange of the sodium-based zeolites with potassium and lithium showed that the uptake of nickel and vanadium of the zeolites significantly increased compared to the as- synthesised zeolites. Zeolite Y was surface-modified with the APTES ligand and showed a similar trend to that observed for alkali metal-zeolites; showing significantly greater nickel uptake at lower concentrations. Nickel-tetraphenylporphrin was synthesised as a mimic for the nickel-asphaltenes found in crude oil and an α-hydrogen donor solvent used to remove the nickel in the presence of zeolite ion exchangers. A similar trend was observed to that seen in aqueous solution, implying the process would be transferrable to a live medium. Analysis to determine the metal ions present in ashed Nigerian crude samples before and after solvent and/or complexing agent extraction was carried out using inductively coupled plasma mass spectroscopy (ICPMS) and energy fluorescence analysis by X-rays (XRF). The process showed varying amounts of nickel was extracted by the different media along with iron. For nickel, the extent of extraction in the order of increasing % extraction is H2O<H3PO4<EDTA<IPA. For iron the order of increasing % extraction was H2O=EDTA<H3PO4<CH3OH while zinc extraction was in the order H2O<H3PO4 <CH3OH=EDTA.

Zeolites and zeolite membranes are two important advanced chemical materials which are widely used in chemical processes. The manufacture of these materials usually involves the use of expensive chemicals. This study involves the use of Ahoko Nigerian kaolin (ANK) as precursor material for the development of zeolites and zeolite membranes. The synthesis of zeolite A, Y and ZSM-5 was successfully obtained following a sequence, collection of the raw clay from Nigeria, metakaolinization, dealumination and actual hydrothermal synthesis of the zeolites. Raw ANK was refined using sedimentation technique and about 97% kaolin was recovered from the raw sample. A novel metakaolinization technique was developed to convert kaolin into a reactive metastable phase. Amorphous metakaolin was obtained at a temperature of 600°C and exposure time of 10 min. This is a significant result because previous studies use higher temperatures and longer exposure times for the metakaolinization step. The metakaolin was used to prepare a number of different zeolites under various conditions. Highly crystalline zeolite A was obtained at an ageing time of 12 h, crystallization time of 6 h and crystallization temperature of 100oC. Zeolite Y was obtained at an ageing time of 3 h, crystallization time of 9 h and crystallization temperature of 100oC. Zeolite Y was also synthesised by using a dealuminated kaolin and highly crystallized zeolite Y with Si/Al ratio of 1.56 and BET surface area was obtained of 630 m2/g. ZSM-5 was synthesised using an ageing period of 36 h, crystallization time of 48 h and temperature of 140oC. The results obtained from zeolite powder synthesis from ANK were then used as guide to prepare supported zeolite films and membranes by a hydrothermal method. The effect of the support surface (stainless steel) was investigated using two synthesis methods namely modified in-situ and secondary (seeded) growth. Zeolite A, Y and ZSM-5 films were successfully prepared from ANK for the first time and on two modified supports, etched and oxidised. The zeolite films and membranes developed showed complete coverage on the two supports with the oxidised showing better adhesion and intergrowth. The separation performance of the three developed zeolite membrane was tested by pervaporation of water/ethanol mixture. The results of pervaporation of ethanol/water mixture showed that zeolite A membrane is highly selective towards water mainly because of hydrophilic properties occasioned by the high aluminium content. Zeolite Y membrane show a similar response when their separation performance was evaluated but with less selectivity because of reduced aluminium content. ZSM-5 showed selectivity towards ethanol because of it hydrophobicity allowing only ethanol to permeate. In all the zeolite membranes, the flux is lower in comparison to commercial zeolite membranes due mainly to the thickness of the zeolite layer. Oxidised support membranes showed better performance because of their better interaction between the oxide surface and the aluminosilicate gel. The results show that ANK can successfully be used to prepare zeolites and zeolite membrane.

No description available.

L’objectif de cette thèse est d’étudier l’influence de l’architecture poreuse de zéolithes à porosité multimodale sur la sélectivité de catalyseurs dans l’hydrocraquage de charges lourdes, et aussi d’établir des relations prédictives propriétés/performances des matériaux catalytiques.Plusieurs séries de matériaux modèles possédant des réseaux mésoporeux secondaires différents ont été préparés par traitement alcalin de zéolithes de référence de type faujasite, par dessilication et par recristallisation en présence d’agent structurant organique. Les différentes méthodes de structuration de la mésoporosité ont été comparées et discutées en termes de rendements de synthèse des matériaux micro-mésoporeux, de volumes mésoporeux créés, de distribution de tailles des mésopores et de leur co-localisation avec les micropores. Des catalyseurs bifonctionnels ont ensuite été préparés par introduction d'une fonction hydrogénante/déshydrogénante, et utilisés pour l’hydroconversion du n-hexadecane et du squalane.L'activité des catalyseurs, exprimée par la vitesse de réaction à 230°C dans la conversion dun-hexadecane, et à 210°C dans celle du squalane, est linéairement corrélée au nombre de sites acides forts de la zéolithe.Quel que soit le mode de restructuration du réseau mésoporeux et l'architecture de celui-ci, une augmentation du volume mésoporeux conduit à un gain de sélectivité en isomères, produits primaires de réaction, et de symétrie de la distribution de produits de craquage. La sélectivité des catalyseurs, caractérisée par le rendement maximum en isomères, est gouvernée par le transfert de matière au sein des cristaux zéolithiques. Une corrélation directe entre le coefficient de diffusion effectif du n-hexane et la sélectivité des catalyseurs a été établie. / The objective of this thesis is to study the influence of the porous architecture of zeolites with multimodal porosity on the selectivity of catalysts in the hydrocracking of heavy feedstock and also to establish predictive relations between properties and performances of catalytic materials.Several series of model materials possessing different secondary mesoporous networks have been prepared by treating in alkaline medium faujasite reference zeolites either by desilication and or by recrystallization in the presence of an organic structuring agent. The different mesoporosity structuring methods were compared and discussed in terms of yields, created mesoporous volumes, mesopore size distribution and their co-localization with micropores. Bifunctional catalysts were then prepared by introducing a hydrogenating/dehydrogenating function and used for the hydroconversion of n-hexadecane and squalane.The activity of the catalysts, expressed as the reaction rate at 230°C in the conversion ofn-hexadecane and at 210°C in the case of squalane, is linearly correlated with the number of strong acid sites of the zeolite. Whatever the restructuring process of the mesoporous network and its architecture, an increase in the mesoporous volume leads to a higher selectivity into isomers, primary reaction products, and a higher symmetry of the distribution of cracking products. The selectivity of the catalysts, characterized by the maximum yield of isomers, is governed by the transfer of matter within the zeolite crystals. A direct correlation between the effective diffusion coefficient of n-hexane and the selectivity of the catalysts was established.