Study of Hydrocracking catalysts based on modified USY zeolites

Susana Lopes Silva

Dissertação para obtenção do Grau de Mestre em

Engenharia Química

Júri Presidente: Prof. Dr. João Carlos Moura Bordado

Orientadores: Prof. Dr. Fernando Manuel Ramôa Ribeiro (IST)

Dr Laurent Simon (IFP)

Vogal: Prof. Dr. José Madeira Lopes

Setembro 2009

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Resumo

Para corresponder à procura mundial crescente de destilados médios, o processo de hydrocracking tem sido desenvolvido não só a nível processual como a nível catalítico. A maior problemática no domínio da catálise centra-se nas limitações intrínsecas do transporte molecular no sistema poroso do catalisador. A acumulação molecular resultante conduz a sucessivas reacções de condensação de hidrocarbonetos pesados, levando à formação de coque, desactivando o catalisador.

Os catalisadores de hydrocracking são bifuncionais, constituídos por uma fase metálica suportada numa fase ácida porosa. Os zeólitos são aluminossilicatos cristalinos com uma sinergia de propriedades como elevada área específica, estabilidade térmica, acidez intrínseca e capacidade de confinar espécies metálicas activas, reunindo as principqis condições para um suporte catalítico.

Assim, propusemo-nos a modificar a estrutura do zeólito USY, a fim de aumentar o tamanho dos poros para finalmente diminuir as limitações difusionais de reagentes e produtos. O tratamento alcalino aplicado – designado desilicação – utiliza uma solução de NaOH para extrair silício da estrutura cristalina.

Verificou-se então um aumento da mesoporosidade anteriormente existente no zeólito, tendo-se mantido a microporosidade praticamente na totalidade. O número de sítios ácidos aumentou ligeiramente, e a estrutura cristalina é mantida mesmo após tratamentos mais prolongados.

Os suportes foram preparados com os zeólitos manipulados, e impregnados a seco com uma solução metálica de NiMo. Os catalisadores foram testados sobre uma molécula modelo, tendo-se verificado um aumento de conversão e selectividade em destilados médios face ao catalisador industrial, assim como um aumento no rendimento de gasóleo em detrimento de jet fuel. Conclui-se portanto que a desilicação de zeólitos USY efectivamente diminui as limitações difusionais, tendo ainda acrescido a actividade do catalisador.

Palavras Chave: Desilicação, Mesoporosidade, Zeólito USY, Hydrocracking

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Abstract

To answer to the world demand on middle distillates, the hydrocracking process has been developed not only at a processual level, but also on the catalysis domain. The main difficulty is the diffusional limitations on molecular transport inside the porous system of the catalyst. The resulting molecular accumulation leads to successive condensation reactions of heavy hydrocarbons, leading to coke formation and consequent catalyst deactivation.

Hydrocracking catalysts are bifunctional, being constituted by an active metallic phase supported on a porous acid phase. Zeolites are crystalline aluminossilicates with a unique combination of properties, such as high specific surface, thermal stability, intrinsic acidity and the capacity to confine metallic active species, being therefore a good candidate to catalytic support.

Hence, we proposed ourselves to modify zeolite USY structure to improve pores size, decreasing the diffusional limitations of reactants and products. The alkaline treatment applied – named desilication – use a NaOH solution to extract silicon atoms from the zeolite crystalline structure.

Mesoporosity was indeed increased after desilication, and microporosity has been practically maintained. The number of acid sites was slightly increased, and the crystalline structure is kept even after the longest treatments.

The supports were prepared with manipulated zeolites, and impregnated with a metallic solution of NiMo performing the catalysts, tested by model molecule. An increase in conversion and MD selectivity was observed, relatively to the industrial catalyst. The yield in GO in detriment of jet fuel was also improved, meaning that desilication indeed decreased the diffusional limitations of zeolite USY, increasing also the activity of the catalyst.

Key Words: Desilication, Mesoporosity, Zeolite USY, Hydrocracking

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Acknowledgements

I feel deeply grateful for have the privilege of working in IFP, one of the most prestigious research and technologies development centre.

To Denis Guillaume, director of the Catalisys and Separation Division, and to Mr. Fabrice Bertoncini, responsible for the Catalysis by the Sulfides Department, for receiving me in this Institution.

To Professor Fernando Ramôa Ribeiro, my sincere gratitude for giving me the opportunity to do my internship in IFP, and for believing and trusting in me.

I would show great appreciation to Dr. Laurent Simon for guiding me in this study, and for all the support and encouragement during the last six months.

I would like to appreciate Mr. Stéphane Cremer not only by the technical support, but also for his daily kindness and attention.

I wish to express my gratitude also to the Sulfides Departement, for receiving me as a colleague and all sympathy.

Finally, I would like to thank my family and my friends for the support, friendship and kindness during the internship, and for trusting my capacities.

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Contents

Resumo .................................................................................................................................................................... i 

Abstract .................................................................................................................................................................. ii 

Acknowledgements ............................................................................................................................................... iii 

Contents ................................................................................................................................................................ iv 

Abbreviations ........................................................................................................................................................ v 

1.  Introduction ................................................................................................................................................. 1 

2.  Bibliographic study ..................................................................................................................................... 3 

2.1.  Hydrocracking process ......................................................................................................................... 3 2.1.1  Process configurations ..................................................................................................................... 5 2.1.2  Reactions And Mechanisms ............................................................................................................ 7 

2.2.  Hydrocracking catalysts ..................................................................................................................... 12 2.2.1.  Zeolites as hydrocracking catalysts ............................................................................................... 13 2.2.1.1.  Y zeolite ................................................................................................................................... 14 

2.3.  Modifying Treatments ........................................................................................................................ 15 2.3.1.  Dealumination ............................................................................................................................... 15 2.3.2.  Realumination ............................................................................................................................... 16 2.3.3.  Desilication ................................................................................................................................... 17 

3.  Experimental Part ..................................................................................................................................... 21 

3.1.  Zeolite Modification and Catalyst Preparation .................................................................................. 21 

3.2.  Catalytic Testing ................................................................................................................................ 21 

4.  Results and Discussion .............................................................................................................................. 22 

4.1.  Characterization ................................................................................................................................. 22 

4.2.  Catalytic Performance ........................................................................................................................ 22 

5.  Conclusions and Perspectives ................................................................................................................... 23 

Bibliographic references ..................................................................................................................................... 24 

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Abbreviations

xN/yhr Reference to the sample based on USY zeolite, treated with a NaOH solution of x N, during y hours

xN/yhrS Reference to the support respecting to the sample based on USY zeolite, treated with a NaOH solution of x N, during y hours

USY Zeolite Y ultra-stable

MOR Mordenite zeolite

NMR Nuclear Magnetic Resonance

XRD X-Ray Diffraction

ZSM-5 Zeolite of high Si/Al ratio, made by Mobil

Si/Al Silicium over aluminum molar ratio

IRS Infra-Red Spectroscopy

LOI Loss on ignition

XRF X-Ray Fluorescence

EFAl Extra framework aluminum

AAS Atomic Adsorption Spectroscopy

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1. Introduction The middle distillates supply/demand is expected to continue growing during the next

decade. A projection of global refined products points to an annual average growth rate of 1.7%. (cf. Figure 1.1).

1999 66 Mbdp

2020 105 Mbdp

Figure 1.1: Global refined products demand projection up to 2020 [1].

To face this growing demand, substantial developments for this process are required,

and successful investigations are being taken to improve the process productivity and selectivity, mainly in the catalysis domain.

The catalyst development focus to a better performance in terms of products quality and selectivity, but also in terms of a longer lifetime by keeping the catalyst stability. Hydrocracking catalysts are bifunctional catalysts containing a metal hydro/dehydrogenating function supported on an acidic cracking support. In order to achieve a high activity, the acidity of the support is provided by a modified Y zeolite mixed with alumina. The main drawback associated with this type of catalysts is that the regular micropores size of the zeolite channels limits the transportation of molecules, thus, restricting the middle distillates selectivity to a large extent and decreasing the catalyst lifetime due to deactivation [2]. The improvement of the mass transfer limitation is usually done by modification of the Y zeolite by dealumination. The dealumination allows the formation of mesopores inside the structure of the zeolite, which improves the mass transfer properties between the acidic and the metal function. The results of this modification lead to a better middle distillates selectivity but has the major drawback to simultaneously decrease the catalyst activity by decreasing the acidity of the zeolite.

The purpose of this study is the modification of Y zeolites in order to create a more accessible porous system, i.e., creates a mesoporous channel system, without losing the acidic properties of the zeolite. The modification of the zeolite should allow to keep a high activity and middle distillate selectivity. The creation of mesoporosity without losing acidity can be achieved by alkaline treatment of the zeolites. The alkaline treatment will extract framework

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silicon atoms increasing the mesoporous volume and facilitating molecules access into the active phase of the catalyst. The process, called desilication, will be carried out with different NaOH solution concentrations and for different time extents in order to evaluate those effects on the Y zeolite structure. The treatment efficiency will be evaluated by catalytic testing in a lab unit using squalane as hydrocracking model molecule.

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2. Bibliographic study

2.1. Hydrocracking process

Heavy hydrocarbons are transformed into lighter products using mainly two catalytic processes: The Fluid Catalytic Cracking process (FCC), which leads to good yields and quality of light products such as gases and gasoline, and the Hydrocracking process (HCK), which generates good yield and quality of middle distillates such as kerosene and gas oils. In fact, the two processes are complementary and can be associated to optimize the answer to the market demand (cf. Table 2.1).

Table 2.1: FCC and hydrocracking processes characteristics [1].

FCC Mild HCK HP HCK

Operating Conditions

H2 Consumption (wt./wt.feed%) - 0.5-1 2-3 Total pressure (bar) 1 40-80 120-180 Temperature (K) 773-873 623-703 623-703 Cycle extent sec's 1-3 years 1-3 years

Catalytic Performances

Conversion (%) 80 20-40 80-100 M.D. Selectivity low high high

Products Quality

Gasolines + ± ± Middle Distillates - ± + (IC ≥ 55)

Residues unused FCC Oil base; Steamcracking

While FCC’s catalyst has a rapid deactivation, the HCK process working at high hydrogen pressure prevents the fast deactivation, allowing a life time of the catalyst of about 3 years. This fact is due to the hydrogenation of some coke precursors such as olefins or aromatics.

The hydrocracking process is very versatile, not only in terms of feed, but also in terms of products (cf. Figure 2.1). The feed varies from heavy gasoline or vacuum residues to gas oils from FCC unit recycling. The production, according to the geographic demanding and seasonal market, can be directed to the production of middle distillates or gasoline. The most frequent feed for HCK process is the Vacuum Gas Oil (VGO) in order to increase the yield in middle distillates, which has the typical composition presented in Table 2.2.

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Figure 2.1: Typical Refinery scheme representing the HCK unit position [5].

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Table 2.2: Hydrocracking feed characteristics [10].

Compounds Amount Sulfur (wt.%) 0,1 - 5 Nitrogen (ppm) 200 - 3000 Paraffins (wt.%) 10 - 30 Naphtenes (wt.%) 20 - 50 Aromatics (wt.%) 30 - 70 Polar compounds: resins (wt.%) 1 - 15 Asphaltenes (ppm) 20 - 200 Metals (ppm) 0 - 10

HCK catalysts are bifunctional catalysts, having a hydro/dehydrogenation function, and an acidic function (§ 2.2). They can be deactivated by certain species present in the feed, leading to a decrease in the product yield and in the catalyst lifetime.

Sulfur compounds affect the hydro/dehydrogenating function, forming hydrogen sulfide in the hydrotreating step (§ 2.1.1), having a negative effect on the catalyst cracking function. Nitrogen compounds neutralize the acid sites, causing their progressive deactivation.

Aromatic species also inhibits the catalysts, once they are extremely stable and have a great adsorption capacity, apart from being coke precursors, by Dials-Alder reactions.

The asphaltenes and metals contents are also very critical, due to the irreversible poisoning of the catalyst, being limited to 200 and 10 ppm, respectively. According to the feed, different products can be obtained as shown in Table 2.3.

Table 2.3: Typical hydrocracking feeds and respective products [7].

Feeds Products Kerosene Naphtha Straight-run diesel Naphtha and/or jet fuel Atmospheric Gas Oil Naphtha, jet fuel, and/or diesel Vacuum Gas Oil Naphtha, jet fuel, diesel, lube oil FCC LCO Naphtha

FCC HCO Coker LCGO Coker HCGO

Naphtha and/or distillates

2.1.1 Process configurations

Hydrocrackers are usually classified as single-stage or two-stage units. However, other hybrid configurations are possible.

In the single-stage process (cf. Figure 2.2), the feed is initially treated to convert the organic sulfur and nitrogen containing compounds to lighter products, such as H2S and NH3. Nevertheless, given that there are no intermediate separation of this products between steps, this gases, mainly NH3, will inhibit the acid catalyst activity, being therefore necessary severe conditions of operation. However, the presence of small quantities of nitrogenated compounds

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is needed, to prevent extensive cracking of feed to gasoline. Once pre-treated, the mixture follows to a second reactor, where the cracking reactions take place under a high hydrogen partial pressure. Due to the fact that there is no intermediate separation, high H2S and NH3 partial pressures are present too, which increase the difficulty of the feed processing.

Figure 2.2: Hydrocracking process – one-step configuration [4].

This configuration can be disposed without any recycle from the fractionator to the

hydrocracking reactor, the once-through process, or with a recycle from the distillation column to the second reactor allowing the total conversion of the feed. In the first case, the feed conversion is between 50 and 90% and the residue can be used as FCC feed. A continuous removal before the recycling to the second reactor is applied, (dashed line in Figure 2.2), in order to prevent the accumulation of heavy polyaromatics in the process. Otherwise, the catalyst can be rapidly deactivated by the coke formation. This straightforward approach is widely used in hydrocrackers especially designed for middle distillate stocks such as diesel and jet fuel [3].

In the two-stage process (cf. Figure 2.3), an intermediate separation of the products is employed, where H2S and NH3 produced in the pre-treating stage are removed from the effluent of the first stage. The two-stage includes both a HDT and HCK reactors like in single stage configuration. After the removal of gases and products, the non converted mixture gets in the third reactor of HCK. In this configuration, H2S and NH3 are totally removed. The conversion of the first stage is about 50% and for the second stage between 50 and 70% with a residue recycle. There are still other possible configurations, where two consecutive beds can be disposed in the same reactor, with different catalysts.

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Figure 2.3: Hydrocracking process – two-stage configuration [4].

2.1.2 Reactions And Mechanisms

The hydrocracking process converts the heavy feed such as vacuum gas oil to lower weight products. The process also allows to remove organic sulfur and nitrogen compounds, and to saturate olefins and aromatics. Organic sulfur compounds are transformed into H2S, while organic nitrogen compounds are transformed in NH3.

Inside the hydrocrackers, many reactions take place, some desired such as hydrotreating and hydrotreating reactions, others not such as coking.

The main reactions taking place in a hydrocracking unit can be displaced in two types:

Hydrotreating reactions

Cracking reactions

Hydrotreating reactions are desired reactions for sulfur removal (HDS), nitrogen removal (HDN), aromatic removal (HDA) and halides removal (HDM). Depending on the source of the feed, these compounds are presents, though in variable amounts.

Kinetic Aspects

The rate of reaction has an extreme importance in the determination of the key properties of a hydrocracking catalyst: activity, selectivity, stability, etc. The rate of reaction is typically accepted as shown in equation 2.1.

[ ] [ ]nna0 XkX

RTE

expkr =⎟⎠⎞

⎜⎝⎛ −= Eq. 2.1

where k0 is the rate constant, Ea is the activation energy, R the gas constant, T the absolute temperature and n the reaction order. Representing the activity (at 588- K in this example) vs.

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1/LHSV as shown in Figure 2.4, is possible to conclude that vacuum gas oil typically follows a first-order kinetics (n = 1).

Figure 2.4: VGO hydrocracking first-order kinetics.

In this case, the reaction rate equation is expressed as in equation 2.2.

[ ]Xkr = Eq. 2.2

Hydrogen is consumed in all hydrotreating reactions, as in the hydrodesulfurization and

hydrodenitrogenation typical reactions (cf. Figure 2.5 to Figure 2.8).

Figure 2.5: Generally accepted desulfurization mechanism.

In the hydrodesulfurization process, the sulfur is removed from the initial molecule, and is formed an intermediate olefin and H2S. Then, the olefin is hydrogenated to form a paraffin, or other molecules depending of the organic sulfur compound as shown as examples in Figure 2.5 and Figure 2.6. Once the hydrocracking process runs under high H2 pressure, the feed desulfurization is almost complete [5].

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Figure 2.6: Typical reactions of desulfurization.

Hydrodenitrogenation removes nitrogen from the feed of the process, proceeding

through a different path. The aromatic hydrogenation occurs first followed by hydrogenolysis and, finally, denitrogenation releasing NH3 (cf. Figure 2.6). Hydrodenitrogenation typical reactions are shown in Figure 2.7.

Figure 2.7: Typical mechanisms of hydrodenitrogenation.

Respecting to aromatics compounds, they may have substantial inhibiting effects over the catalyst as coke precursors and their levels have to be controlled. Their cracking reaction is very complex, requiring hydrogenation prior to cracking [6], but is necessary for cracking cyclic hydrocarbons.

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Figure 2.8: Typical reactions of hydrodenitrogenation.

The hydrocracking reactions proceed through a bifunctional mechanism, requiring both types of catalytic sites – acidic and metal sites – to catalyze separated steps in the reaction sequence – cracking and hydro/dehydrogenation. There are plenty of simultaneous reactions occurring in the hydrocracking, but in general, the mechanism of hydrocracking is that of catalytic cracking with hydrogenation superimposed [7]. Moreover, cracking and hydrogenation are complementary, once cracking provides olefins for hydrogenation, while hydrogenation in turn provides heat for cracking. The overall reaction is exothermic, as the heat by the exothermic hydrogenation reactions is much higher than the amount of heat consumed by the endothermic cracking reactions [7]. The postulated mechanism of hydrocracking reactions starts with the generation of an olefin or cycleolefin on a metal site of the catalyst. Then, an acid site adds a proton to the formed olefin, to lead to the formation of a carbenium ion, which tends to cracks, to give a smaller carbenium and a new olefin, also smaller than the initial – these are the primary cracking products. These products can react further to produce smaller secondary products, and so on, until the abstraction of a proton from the carbenium ion to form an olefin at an acid site; and its saturation at a metal site. Figure 2.9 shows the detailed mechanism involved in the hydrocracking of paraffins.

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Figure 2.9: Hydrocracking mechanism of n-paraffins.

Besides hydrotreating and hydrocracking reactions, there are several other possible reactions occurring simultaneously, such as aromatic saturation, coke precursors formation or even coke formation. These reactions can take place either in the hydrotreating or in the hydrocracking sections. In fact, the aromatic saturation is the only reaction, which is limited at higher temperatures, reached by the catalyst deactivation. This leads to an incomplete aromatic saturation that will react from different ways to form coke. The coke molecules will at first place deactivate the acid sites in the supercages of the catalyst, due either to the poisoning of the sites or to the blocked access to the reagents. However, even if there is no coke molecules inside a pore, there is the possibility to an acid site being blocked by the incrustation of coke molecules from the outside, blocking in the same way the access of reagent molecules. Normally, the hydrocracking unit feed contains polynuclear aromatics (PNA), and due to the catalyst deactivation will be restraint to saturate, leading to important, though undesirable, large PNA formation.

This multiring formation may proceed via two different pathways (cf. Figure 2.10).

it can occur starting with saturation of the ring on a metal site, followed by cracking reactions on an acid site;

or it can start with a ring condensation on an acid site, and form a large aromatic compound, and subsequently a large PNA.

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Figure 2.10: Possible ways to form coke precursors.

The result of operating HydroCracking Units (HCU) with recycle of the unconverted feed is the creation of PNA, which may deactivate the catalyst. The coking process also needs the retention of the coke precursors under the catalyst. That's the reason why the coke formation occurs essentially inside the zeolite pores – the retention is more favorable on the FAU supercages than on the external surface [8]. Besides, the chemical reactions involved in the coke formation are catalyzed by the Brønsted acid sites: the greater the concentration of these sites, the faster is the coke formation, the larger the influence on the activity.

2.2. Hydrocracking catalysts A wild range of catalysts are industrially used in HCU according to the feed used and

desired products. A synergetic effect between hydrotreating and hydrocracking catalyst is achieved balancing both functions differently inside the catalyst beds (cf. Figure 2.11).

Figure 2.11: Bifunctional catalyst mechanism [1].

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The hydrogen-transfer functionality is promoted by a metal phase. The promoters of hydro-dehydrogenation function can be noble metals, but with high nitrogen or sulfur contents they are rapidly deactivated. The catalysts formulated with metal sulfides from group VI and VIII or associations of both groups are less expensive and have a better resistance to N and S containing feedstocks. The association between nickel and molybdenum or tungsten is the most active to promote hydrogenation reactions [9]. This function also protects the catalyst acid sites to prevent the rapid coking by hydrogenation of heavy polyaromatics and allows a continuous hydrogenation reaction to avoid nitrogen accumulation and hydrogenation of aromatics. The cracking function is provided by an acidic support that may be amorphous oxides, crystalline zeolite or a combination between both. The metal active phase is dispersed over the acid support by dry impregnation.

The ratio between the catalyst's hydrogenation function and the cracking function is manipulated in order to optimize activity and selectivity. The most common combined catalysts used in HCU are presented in Figure 2.12. In HCU, the first reactor contains a hydrotreating catalyst that is preferably supported on amorphous alumina or ASA, containing a strong hydrogenating agent (NiMo or NiW). The main aim of this catalyst is to promote the DHS, DHN and DHA reactions of heavy feeds, protecting the catalyst in the succeeding reactor, where hydrocracking occurs. For this second reactor, two options are available to optimize the M.D. yield: amorphous SiO2/Al2O3, or Y zeolites. The first one is less active than Y zeolite; however has a better selectivity to gas oils, which represent the most part of D.M.. Y zeolites have a kerosene directed selectivity, given that is more active than amorphous SiO2/Al2O3. Regarding the hydrogenating function, NiMO or NiW are the most commonly used.

Note: The size of the dots is proportional to the frequency of use.

Figure 2.12: Hydrocracking Catalysts [9].

2.2.1. Zeolites as hydrocracking catalysts

Zeolites are crystalline aluminosilicates, possessing a unique combination of properties such as high surface area and thermal stability, intrinsic acidity, shape selectivity, and the ability to confine active metal species. The Brønsted acid sites properties are a due to the presence of tetrahedral coordinated Al in the zeolite framework, and enable the

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replacement of environmentally mineral acids as catalysts. The high surface area is a result of purely microporous network of pores, with molecular dimensions offering an ideal matrix for shape selectivity. There are numerous zeolites macrostructures, as mordenite (MOR), MFI or faujasite (FAU), defined by the International Zeolite Association (IZA), differing on the porous system. The FAU structure includes zeolites X and Y, that differs in the chemical composition, with a Si/Al ratio between 1-1.5 for X zeolites and between 1.5-3 for Y zeolites.

2.2.1.1. Y zeolite

The Y zeolite structure is constituted by three secondary building units (SBU): a hexagonal prism (double hexagon with 12 tetrahedra), a square (4 tetrahedra) or a hexagon (6 tetrahedra) (cf. Figure 2.13).

Figure 2.13: FAU secondary building units.

The main channel is constituted by 12 sides polygon (dodecagonal), which has a maximum free dimension in the undeformed polygonal window of 8 Å. The SBU present in this zeolite form assemblies, forming the sodalite cages (or β−cages) connected by hexagonal prisms.

The successive connection of these polygons leaves a large free volume, named α supercage (cf. Figure 2.14). A molecule moves from one α-cage, also named supercage, to another through the dodecagonal window, in a three-dimensional porous system. Once the structure is cubic, the pores run in three perpendicular directions, intersecting at each α supercage. Regardless of the importance of the channel system, the zeolite's pore system is not advantageous for high catalytic performances, once the microporous structure imposes diffusional limitations to the larger molecules. Post synthesis treatment can be carried out to increase accessibility to the active sites by creating mesoporosity within the zeolite crystals.

Figure 2.14: Zeolite Y structure.

Regarding Figure 2.14, each line intersection is occupied by a T atom (T = Si or Al). The four crystallographic oxygen atom positions within the framework are labeled o1 to o4. Generally accepted nomenclature is used to label extra framework sites: IV and U sites in the

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middle of α− and β− cages, V at the surface between a 12-ring window and I to II* sites along the threefold [1 1 1] axis. The catalytic activity is directly related to the Brønsted acid sites. This kind of acid site is generated by the presence of a trivalent aluminum atom, compensated by a proton fixed in framework oxygen atom. The Brønsted acid sites density is proportional to the aluminum content in the structure: the greater is the Si/Al ratio, the bigger is the acid sites concentration [10]. Concerning to the acid strength, the natural trend to donate a proton is directly related with the affinity with the framework. When the Si-(OH)-Al bond angle is higher, the O-H bond is less covalent and has then more ability to donate the proton. This angle depends on the structure type and the resonance energy which will determine the deformation degree that the structure can support [10]. Also the number of consequent aluminum atoms separated only by one silicium tetrahedra has an important effect on the strength of the Brønsted acid sites. When the number of these neighbor atoms increase, the lower is the electronegativity of the framework [11]-[13] and therefore the lower the acid strength, once hydrogen carries a lower positive charge. Respecting to Lewis acid sites thought they have no catalytic activity, they can influence it [14].

2.3. Modifying Treatments

An efficient way to enhance the zeolite performance used in the hydrocracking catalyst preparation can be envisioned upon enhanced accessibility to the active species and/or shortening of the diffusion path length in the micropores. Several approaches are proposed to achieve these features: synthesis of large cavity or wide-pore zeolites, synthesis of zeolite crystals with small diameters in the lower nanometer size range (<200 nm), preparation of mesoporous composite material, preparation of delaminated zeolite crystals to enhance the outer surface area, or the induction of mesopores in zeolite crystals.

2.3.1. Dealumination

This treatment aims preferably the stabilization of the support structure, leading or not to the mesopores formation, as well as EFAl species.

Dealumination is usually achieved by steam treatment at relatively high temperatures – between 778 and 873 K, or by acid leaching with some strong acid solution, leading to the selective removal of Al atoms from the framework. It can also be achieved with or without Si introduction.

By acid leaching, the treatment leads to the formation of structural defects. However, if the treatment is well done, there is no EFAl formation, allowing to obtain a large Si/Al global ratio.

By steaming, the zeolite is maintained in contact with 773 – 1123 K steam. The Al atoms are extracted, leading to the formation of EFAl, being afterward substituted or not by Si atoms. Although this EFAl species stay in the mesoporous channels.

Dealumination allows structure stabilization. This stability is due to the Al elimination and consequent isomorphic substitution by Si atoms that migrate from the OH rich sites. This migration leads to a secondary porosity – mesoporosity – with >20 Å pore sizes. The EFAl

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species inside the channels can be removed by acid leaching. Generally, the dealumination process includes one steaming step and consequent acid leaching.

As referred before, the dealumination can also be achieved with Si introduction The support can be treated with a fluorosilicate solution, leading to structure stabilization with no secondary mesoporous channels nor EFAl formation. This technique its exclusively applied in zeolites with sufficient large pores to allow the Al isomorphic substitution. Zeolites can also be treated by silicium tetrachloride vapors, forming AlCl3, eliminated by the vapor phase.

Dealumination treatments have frequently shown to result in encapsulated mesoporosity, which has been proven to speed up mass transport to a moderate extent. The beneficial effect of the moderate mesoporosity development by dealumination is often exceeded by a loss in acidity, resulting in poorer catalytic activity [15].

Y zeolite cannot be used for hydrocracking in their synthesis states as their Si/Al ratio is very low. The Si/Al is about of 2.6 after synthesis, which means that there is too many charges in the structure to resist in any kind of treatment. The Y zeolites are therefore dealuminated in order to stabilize the framework. The dealumination treatment removes framework aluminum atoms, decreasing the number of charge in the framework and creating mesopores in the microporous system. The cracking activity of the catalyst results from a good compromise between mesoporosity and acidity. After the dealumination process, Y zeolites are called Ultra-Stabilized-Y zeolites (USY).

2.3.2. Realumination

This technique aims to increase the global acidity of zeolites by insertion of Al atoms inside the structure.

If there is EFAl species inside the zeolite, realumination can be achieved by alkali-treatment using NaOH of low concentrations or KOH solutions. Si/Al ratio decreases without losing significant crystallinity. Although, a slight desilication may also occurs simultaneously according to Zang et al. [16].

Realumination may also be achieved using NaAlO2 solutions inserting Al atoms in the framework from an outlet source. This treatment should be carried out using diluted solutions to preserve the structure crystallinity. As a result, the quantity of aluminum inserted is very low and the resulting Si/Al ratio does not decrease much.

Honna et al. [17],[18] proposed a similar realumination method, which integrates an initial step of ethanol impregnation before the treatment by NaAlO2, filling the pores and thus inducing a selective incorporation of aluminum atoms mainly in mesopores sites. The reinsertion of Al atoms is performed during zeolites calcination, according to the authors. The ethanol impregnation step allows to perform the treatment using higher NaAlO2 concentrations without destroying the zeolitic structure. The BET surface area is also maintained, and both Lewis and Brønsted acid sites increased, indicating a large insertion of Al inside the framework. Indeed, the hydrocracking conversion of Arabian heavy-atmosphere residue, using a catalyst prepared with the realuminated zeolite presents a higher value than that for the conventional catalyst.

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2.3.3. Desilication

Y zeolites have a three-dimensional pore system. However, the micropore structure imposes diffusional limitations to bulky molecules inducing pore blocking, and consequently the active acid sites inside the porous system remain inaccessible. Creation of mesoporosity in zeolite crystals has proven to be an effective approach to minimize this diffusional limitations [19][20]. Recently, alkaline treatment has been explored as an efficient methodology to create extra-pores in high silica zeolites such as ZSM-5, generally using NaOH as alkaline source [21][22][25].

Groen et al. [24] showed the important role of aluminum as a pore-directing agent to control desilication process. Framework Al atoms makes the alkaline treatment selective towards intracrystalline mesopore formation. Nevertheless, the presence of non-framework aluminum species inhibits mesopore formation due to their reinsertion into the zeolite framework [26]. At low Si/Al ratios, the relatively high Al contents inhibit framework silicon extraction. In contrast, in high siliceous materials unselective desilication induces large pores formation, leading occasionally to the structure collapse (cf. Figure 2.15).

Figure 2.15: Si/Al ratio influence on desilication rate.

Besides Si/Al ratio, there are other factors that may further tune the mesoporosity identify as temperature, concentration of the alkali solution, or time of the treatment [22], [23].

Groen et al. [26] showed that for ZSM-5 zeolite, a higher temperature leads to a higher degree of mesopore formation. Below 318 K almost no changes occurs in porous properties, whereas above this temperature, extra porosity starts to develop, and while the pore volume increases, the mesopore size distribution broadens and shifts to a larger average of pore size. The most significant changes were observed between 328 and 338 K in the ZSM-5 zeolite, where formation of mesopores with diameters centered on 10 nm occurs.

A similar evolution is observed relatively to time of alkaline treatment. A longer treatment time leads to higher mesopore volume and size. For the ZSM-5 zeolite, the most significant changes occurs between 15 and 30 minutes of treatment. However, a too high temperature or too long time of treatment conduct to a superfluous extraction of silica,

18

without generating mesopore surface area, and eventually leading to structure collapse, or in less scale, to a loss of mechanical stability.

Ogura et al. [22] also studied the concentration, temperature and time of treatment conditions over ZSM-5 zeolite. The alkali-treatment was performed with 0.05, 0.1 and 0.2M NaOH solutions with a dilution of 75 mLSolution/gzeolite, during 5, 30, 90,120 or 300 min, at 338 K and 353 K under reflux and using a polyethylene flask to avoid dissolution. After treatment, the slurry was cooled down by quenching in dry ice. The quenching allows to maintain the zeolite crystallinity and stabilizes the silicon atoms, even after long time of treatment [27]. Ogura et al. showed that the micropore volume decreased and that the mesopore volume increased significantly with concentration and time of treatment. Besides, BET surface decreased with less severe treatments, but increased with the highest concentration solution. The zeolite crystallinity decreased with higher temperatures and higher concentrations, and a structural change of ZSM-5 on a micrometer scale was observed, but not at an atomic or a molecular level. The acidic properties of ZSM-5 were maintained as the ZSM-5 non-treated, which associated to the mesopore formation, implies re-alumination of extracted Al species [24].

Though, Jin et al. [28] observed that the alkaline treatment dislodged framework aluminum, leading to a decrease in Brønsted acid sites in the zeolite framework and the formed EFAl and amorphous Al may act as Lewis sites [29][30]. The treatment was performed using 0.2M NaOH solution at 338 K for 30 min over ZSM-5 zeolite, with a dilution of 30 mLSolution/gzeolite. In a later study at the same operating conditions, Jin et al. [31] concluded the total number of acid sites increased. While the number of strong sites decreased and the number of weak sites increased. Thus, Jin proposes that part of FAl transforms to amorphous and EFAl during the treatment and the detached species transformed from a tetrahedral coordination to an octahedral coordination. However, for 353 K Gopalakrishnan et al. [32] found that no Al have been leached from the framework.

Mei et al. [33] also studied the alkaline desilication treatment in HZSM-5 zeolites, but using different conditions: 0.45M Na2CO3 solution, with a dilution of 15 mLSolution/gzeolite, at 348 K, for 30 hours. The study leads to the conclusion that the treatment is similar to the reported ones using NaOH solutions, although a milder and more controllable alkaline treatment is achieved. After the alkaline treatment, the external zeolite surface becomes rough and rugged: there are many nanoscale open holes on the exterior surface, without penetrating into the crystals, i.e., the desilication takes place mainly on the surface and the interior of the zeolite crystals is almost unaffected upon alkaline treatment. However, the dissolved siliceous species are easy to precipitate in the crystals surface, forming a layer of amorphous silica [25][34], which may cause pore blocking.

Another treatment is proposed by Bjørgen et al. [35]: 0.05M and 0.2M NaOH solutions, at 348 K for 4 hours (about 20 mLSolution/gzeolite). The samples were filtered and, then, submitted to another 4 hours of treatment with fresh NaOH solution of the same concentration. The alkali-treated zeolites presented more aluminum than the parent zeolite, and this effect is much more pronounced for the most concentrated treated sample. Holm et al. [36] thoroughly examined this samples using FTIR, applying CO and collidine (2,4,6-trimethylpyridine) as molecular probes. CO was employed to study the acidic properties of the material, and collidine, which is too bulky to diffuse into the H-ZSM-5 micropores, was used to study the mesopore formation and accessibility to active sites.

CO and collidine adsorption both reveals incipient dealumination, leading to strong Lewis acidity. The major part of those created Lewis sites can be accessed by collidine, implying that the sites are located on the external surface of the zeolites crystals, as found by Mei et al. [33]. The acid strength of Brønsted sites is unaltered by the treatment. Silanols

19

become more homogeneous and to a much larger extent unperturbed, meaning that the defects either are healed (i.e., dissolved Si or Al species are re-inserted into the vacancies originally containing SiOH nests) or that the defects simply are eroded by NaOH, thus, creating mesopores. Furthermore, the results point to a selective mechanism for formation of mesopores as the framework dissolution preferentially takes place at defective sites in the crystallites.

The thermal and hydrothermal stabilities of the alkali-treated ZSM-5 zeolites are inferior to their parent zeolite, but are still considered as intriguing materials possessing hierarchical porosity and satisfactory thermal and hydrothermal stabilities simultaneously. These results indicate the promising outlook of applying the alkali-treated ZSM-5 zeolite in the actual catalytic reactions [37].

Other studies have been carried out using other zeolitic structures. Groen et al. [15] have established an alkaline treatment over mordenite zeolite crystals with 0.2M NaOH solution at 338 K for 30 minutes. The treatment has promoted an increase in the mesopore surface area and a decrease in micropore volume of ca. 10%. That means that besides the mesoporosity establishment, the treatment mostly preserved the microporosity in the crystals.

These studies suggest that desilication treatment can be universally applied over different zeolite families, provided that the Si/Al ratio dependency is adequately taken into account [15].

In 1994, Mao et al. [38] have investigated the mild treatment of Y type zeolites. A treatment with a 0.8M Na2CO3 solution and a dilution of 30 mLSolution/gzeolite was performed, heating the slurry in a Teflon beaker up to 353 K for 4 hours. Afterward, the solids were filtered, and a Na2CO3 fresh solution was added. This procedure was repeated for 3 times, so that the entire operation lasted 12 hours. Mao found that although Si/Al ratio decreased almost 50%, the Al component remained in the tetrahedral configuration and that there was narrower micropores formation. Later, Mao et al. [39], verified that using 0.8M Na2CO3 solutions with different amounts of NaOH – meaning a higher pH treatment, the Si/Al ratio decreases as well as crystallinity. The mesopores were significantly enlarged by desilication process, while textural properties do not change noticeably.

In 2007, A. de Oliveira [27] studied desilication treatment over Y zeolites at IFP. This treatment allowed to verify the feasibility of the desilication treatment on Y zeolites and that the desilication treatment lead to silicon preferential extraction in this zeolitic structure, being the Al neighbored silicon atoms stabilized. The quenching bath carried out after treatment allows stopping the alkaline attack immediately, and keep the crystallinity and stability of framework silicon atoms. Mesopore formation was verified, and the quenching allowed keeping microporous volume in the zeolite structure.

The purpose of this study is the optimization of the desilication treatment on Y zeolites in order to create a mesoporous channel system without losing the acidic properties of the zeolite. The treatment efficiency will be evaluated by catalytic testing in a lab unit using squalane as a hydrocracking model molecule.

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Bibliographic study brief conclusions

Creation of mesoporosity in zeolite crystals has proven to be an effective approach to

minimize this diffusional limitations [19][20]. Groen et al. [24] found that framework Al atoms leads to a selective silicon extraction. So, the global Si/Al molar ratio is a crucial property. USY with 6.2 (Si/Al)global ratio has shown to have good perspectives for desilication treatment.

The most significant changes in mesopore structure were observed after an alkaline treatment at between 328 and 338 K, over ZSM-5 zeolite. So, a temperature of 338 K will be applied, in order to evaluate only the behave of the zeolite mesopore system with different NaOH concentrations and with treatment period.

In MFI structures, with ZSM-5 zeolites, the most significant changes occurred after 15-30 minutes of treatment. However, USY zeolites have a different framework (FAU structure), so it is expected that a longer period is necessary to take effect on the porous system. Besides, according to Aleixo de Oliveira et al. [27], 15 to 30 minutes does not make significant changes in the porous structure of USY zeolites.

The mesopore volume increases with the alkaline solution concentration. However, a too strong base leads to structure collapse by excessive silicon extraction. The BET surface area is also expected to decrease, as well as crystallinity.

As found by different authors [22][28][37][27], a quenching bath after treatment helps to stabilize the zeolite structure after treatment, and stops the alkaline attack instantly, allowing to control exactly the treatment extent.

In the related experiments, the total number of acid sites increases with alkaline treatment.

The desilication takes place mainly on the surface. However, the dissolved siliceous species are easy to precipitate, forming an amorphous silica layer [25][34], and eventual pore block may occur.

Given the experiments results, USY zeolite has good perspectives in terms of mesopores improvement with desilication method. The present work aims to study the effect of concentration and time of alkaline treatment with NaOH on the porous system of a USY zeolite. This work aims as well the improvement of molecular transport in hydrocracking reactions with the developed porous system, which will be verified with the most promoting catalysts.

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3. Experimental Part

The study aimed to modify the zeolite and prepare modified hydrocracking catalysts to test them in a model reaction, i.e., the squalane hydrocracking reaction.

3.1. Zeolite Modification and Catalyst Preparation

3.2. Catalytic Testing

After the catalysts preparation, the performance of the catalysts were tested for the hydrogenating function (metallic phase) and the cracking function (acid sites).

22

4. Results and Discussion

4.1. Characterization

Zeolite properties hardly depend on the operating conditions applied during desilication treatments, which are going to alter the physical and chemical base properties. So, it is important to determine the most relevant characteristics of the catalyst relatively of the main objective for what he has been designed.

4.2. Catalytic Performance

The main aim for using model-reactions in the characterization of catalysts is to

evaluate their performances in terms of activity and selectivity. In order to analyze the performance of the desilicated zeolites, three zeolites were catalytically tested with a model molecule.

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5. Conclusions and Perspectives

Appendix C: Catalysts Data

24

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