Hierarchical zeolites: Synthesis and catalytic properties

Hierarchical zeolites: Synthesis and catalytic properties

Accepted Manuscript Hierarchical zeolites: Synthesis and catalytic properties Agnieszka Feliczak-Guzik PII: S1387-1811(17)30642-X DOI: 10.1016/j.mi...

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Accepted Manuscript Hierarchical zeolites: Synthesis and catalytic properties Agnieszka Feliczak-Guzik PII:

S1387-1811(17)30642-X

DOI:

10.1016/j.micromeso.2017.09.030

Reference:

MICMAT 8571

To appear in:

Microporous and Mesoporous Materials

Received Date: 5 May 2017 Revised Date:

23 September 2017

Accepted Date: 25 September 2017

Please cite this article as: A. Feliczak-Guzik, Hierarchical zeolites: Synthesis and catalytic properties, Microporous and Mesoporous Materials (2017), doi: 10.1016/j.micromeso.2017.09.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Hierarchical zeolites: synthesis and catalytic properties Agnieszka Feliczak-Guzik Nicolaus Copernicus University in Toruń, Faculty of Chemistry, 7 Gagarina Str., 87-100 Toruń, Poland 2 Adam Mickiewicz University in Poznań, Faculty of Chemistry, 89b Umultowska Str., 61-614 Poznań, Poland

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Corresponding author

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dr Agnieszka Feliczak-Guzik Nicolaus Copernicus University in Toruń Faculty of Chemistry Gagarina 7 Street 87-100 Toruń e-mail: [email protected]

Abstract

Synthesis, characterization and application of hierarchical zeolites are becoming a subject of increasing interest among scientists. Hierarchical zeolites possessing secondary porosity at meso- and macroscale, imposed on primary microporous structures are an original

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class of porous materials exhibiting both molecular sieving ability and fast mass transport. These materials, due to the combination of catalytic properties of conventional zeolites with enhanced access and transport of reagents in additional meso- or microporosity, constitute an effective solution to the problem of mass transport, which occurs while using conventional

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zeolites in catalytic reactions. The latter allows molecules of substrates to access the active sites that are located within micropores and reduce the catalysts, leading to increase reaction

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rates and catalyst life-time (slower deactivation removal time of product molecules). Besides, secondary porosity of hierarchical zeolites creates an ideal space for deposition of active catalytic phases controlling their size and allows attaining high dispersion and strong interaction between zeolite and the medium. This work focuses on the review of synthesis methods of hierarchical zeolites and their application in selected catalytic reactions.

Keywords: hierarchical zeolites, synthesis, catalytic properties 1. Introduction

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ACCEPTED MANUSCRIPT Zeolites vs hierarchical zeolites Zeolites, which are classified as microporous materials, are crystalline aluminosilicates of alkaline elements, rare earth elements or other monovalent or multivalent metals [1,2]. These materials display a number of unique properties, due to which they are applied in catalysis. The properties are as follows: presence of strong acidic centers, large specific

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surface area, high ion-exchange ability, high thermal stability, precisely defined system of micropores and channels which enable conducting shape-selective catalytic reactions, thus allowing to distinguish reagents, products and transition states in molecular scale [3, 4-6]. Despite their numerous advantages, zeolites also display diffusion limitations for branched

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molecules and transport of reagents with size similar to the size of the micropores is difficult, which is depicted in figure 1 [4,5].

Diffusion of fitting molecules to the zeolite micropores, known as „configuration

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diffusion” is frequently a stage which limits the reaction which is being catalyzed, because the size of the molecules is similar to the size of the pores in the zeolite. Molecular diffusivity rapidly drops to lower levels than e.g. in Knudsen diffusion (frequently a dominant diffusion mechanism in mesopores) or in molecular diffusion.

The diffusion limitations are usually observed for solid materials containing

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mesopores (Knudsen regime) and for macropores (molecular regime). The low diffusivities in micropores reduce the transport to various reagents to and from active sites. The delayed transport of reagents facilitates transformation of these molecules into undesired by-products, which can constitute coke precursors. This caused blocking of the micropores, which,

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consequently, leads to deactivation of the catalyst and lowering catalyst life-span [4]. Fig.1.

All this leads to a situation in which only the external part of zeolite grain takes part in

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catalytical reaction while the interior remain catalytically inactive. Therefore, works have begun on the synthesis of zeolites with hierarchical porous structure which display secondary porosity i.e. show the presence of at least one additional pore system, mainly in the mesopore range (pore size according to IUPAC from 2 to 50 nm). Such solution aims to facilitate access of larger reagent molecules to active centers of the material while simultaneously maintaining acidity and crystallinity of zeolites [4]. Shortening the length of diffusion path due to the reduction of crystal size (obtaining both nanocrystals and nanolayered zeolites) causes increase in catalyst life-span. Introducing additional porosity (meso-, macro-) also shortens the diffusion path, thus minimizing the possibility of catalyst deactivation, which is depicted in figure 2 [4,7,8]. 2

ACCEPTED MANUSCRIPT Fig.2. Hierarchical zeolites have already found application in, among other things, catalysis, in such reactions as: alkylation, isomerization, transformation of methanol to hydrocarbons (MTH), aromatization, condensation, or catalytic cracking, which is presented in figure 3

Fig. 3.

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[9-15].

However, obtaining hierarchical zeolites with secondary porosity continues to pose a challenge for scientists. New, more effective methods of synthesis of microporous and

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mesoporous materials are being developed. Moreover, such materials should display properties of zeolites, i.e. the additional porosity should not be introduced to the detriment of the microporous structure.

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The main objective of this overview was to characterize the most important methods of syntheses of hierarchical zeolites and their application in selected catalytic reactions. These materials have been considered as the solution for unsolved limitations of conventional microporous zeolites in heterogeneous catalytic reactions. Actually, it has been proofed that the hierarchical zeolites can substantially resolve the limitations such as low mass transfer

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problem, deactivation of catalytic activity and low activity to bulky substrates indifferent chemical reactions. Benefitting from the innovative synthesis and characterization techniques, the research on the advance of new synthesis technologies for the hierarchical zeolites almost reached the equilibrium state. This work describes the latest development of synthesis of

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hierarchical zeolites (references include 155 of items since 1990 to 2017) and examples of application these materials in catalytic tests (e.g. conversion of methanol to hydrocarbons, Friedel–Crafts alkylation of aromatics) that have been applied to show the advantages of the

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hierarchical pore system with deference to their suitability to demonstrate the positive effects of hierarchical porous zeolites. Finally the examples of different hierarchical zeolites containing various metals (e.g. Sn, Ti, Cu, Mg, Ru, B) are collected for first time in this review.

2. Methods of synthesis of hierarchical zeolites Growing interest in hierarchical zeolites in recent years has led to an increase in the number of publications on this subject. Wide variety of the methods of synthesis of hierarchical zeolites developed so far, as well as the new methods developed and published every year make their classification a challenging task. Generally, the existing methods of synthesis of hierarchical zeolites can be divided into two main groups: 'bottom-up', which 3

ACCEPTED MANUSCRIPT include: hard templating, soft templating and non-templating [4] and “top-down”, which comprise: demetallation, delamination and recrystallization [4, 16, 17] (table 1.). Table 1 In the “bottom-up” method, secondary porosity is introduced during the synthesis of zeolites with the use of templates or performing modification of synthesis conditions (non-templating

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methods). In the “top-down” method, introduction of secondary porosity is carried out through post-synthetic modifications [4].

most frequently mentioned in literature: a) removal of framework atoms: - demetallation: dealumination and desilication;

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- irradiation;

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Further this work presents a description of the methods of synthesis of hierarchical zeolites

b) surfactant-assisted recrystallization method; c) dual templating with surfactants; d) zeolitization of materials; e) nanoparticle assembly; f) template-assisted synthesis

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- hard-templating methods •

carbonaceous templates;



polymeric templates;



other solids as templates.

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- soft-templating methods

2.1.Synthesis of hierarchical zeolites – Removal of framework atoms

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One of the most common methods of synthesis of mesoporous zeolites is removing framework atoms, such as Al and Si from the structure. Removing silicon (desilication) or aluminum (dealumination) from the zeolite structure causes formation of mesoporosity. However, the major disadvantage of these methods is the damaging the zeolite structure during the processing [18]. The changes which take place in the zeolite structure in desilication and dealumination processes are shown in figures 4 and 5. Desilication is one of the most universal methods lead for generating secondary porosity in zeolites. This method is based on preferential removal of silicon from the zeolite structure in alkaline environment. The so obtained hierarchical material is characterized by the presence of secondary system of mesopores within each grain, while simultaneously maintaining its 4

ACCEPTED MANUSCRIPT microporous character and acidic properties. Introduction of an additional system of pores by desilication affects the structural and acidic properties of the obtained materials. These new properties affect the activity, selectivity, and life-span of zeolites used in the catalysis [19]. The first authors who applied this method were Ogura and co-workers. They observed, on the example of zeolite ZSM-5, that alkaline environment allows to maintain crystallinity

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with simultaneous reduction of micropores volume, from 0.177 to 0.133cm3/g which characterized the commercial ZSM-5 material. Moreover, processing with alkali caused a considerable increase in mesopore volume from 0.072 to 0.279cm3/g [20]. Continuing their research on the desilication method, these authors tested various option of the procedure using

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ZSM-5 as the original material. These modifications included, i.e., desilication in time or changing the concentration of NaOH [21]. They noticed that during desilication not only silicon but also aluminum dissolves when treated with alkali, though to a much lesser degree.

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Additionally, the amount of the dissolved silicon increases with elapsing time of desilication, however, for aluminum, this quantity was not assessed.

It was also discovered that the optimal concentration of NaOH for obtaining large quantity of mesopores (> 0.150cm3/g) without loss of crystallinity is 0.2M in the time from 30 to 120 seconds. Higher concentration of NaOH (1M) caused lowering of crystallinity.

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Suzuki and Okuhara performed desilication using a solution of NaOH of lower concentration 0.05M in time from 0.5h to 30h, obtaining supermicropores (approx. 1.8 nm) instead of mesopores [22]. These pioneering works spurred increased interest in this method as a procedure allowing generation of microporosity in zeolites. Groen and co-workers continued

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research on this method, which led to extension of knowledge of this issue. One of their first discoveries was noticing the crucial role of the atomic ratio Si/Al in the original material ZSM-5 [23]. In their research, they used various commercial zeolites ZSM-5 containing

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atomic ratios Si/Al of 15-1000. Based on chemical analysis of filtrates obtained after desilication, it was concluded that the atomic ratio Si/Al was over 1000, which confirmed that in the desilication process there is a preferential removal of silicon from zeolite structure. The specific surface of mesopores after desilication was about 200m2/g for the atomic ratio Si/Al of 25 – 50. For materials with Si/Al atomic ratio of over 25, in the process conducted in temperature below 60°C, higher specific surface area of the pores was achieved. In higher temperature of the process (85°C), a higher efficacy in mesopore generation was observed. The pore size for Si/Al atomic ratio of 25 – 50 ranged from 9 to 10 nm. The size increased to 20 -50 nm with the increase of the ratio. 5

ACCEPTED MANUSCRIPT At low Si/Al atomic ratios (<25), high aluminum content prevents removal of silicon from the material structure, which causes generation of little mesoporosity. In medium Si/Al atomic ratios (25-50), silicon is extracted in a controlled manner, which leads to generation of greater mesoporosity. High Si/Al (>50) ratio causes the desilication process to generate bigger size pores, simultaneously decreasing the mesoporous surface.

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Desilication method allows to introduce mesoporosity in zeolites, however, it frequently causes loss of microporosity, e.g. for zeolite ZSM-5 with Si/Al ratio of 42 the specific surface area of mesopores was 272m2/g and the 50 % loss of porosity was recorded (pore volume dropped from 0.17cm3/g to 0.09cm3/g). Reduction of microporosity is

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undesirable due to the presence of active spots in micropores, which can cause a loss of the crystalline structure of zeolite.

Therefore, the desilication procedure should be performed in such a way that allows to

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achieving additional mesoporosity with simultaneous minimization of loss of micropore volume. To that end, the method can use organic alkali TPAOH or TBAOH instead of NaOH. [24]. In both cases, for zeolite ZSM-5, a slow desilication resulting small specific surface area of mesopores (125–180m2 /g) and formation of poresfrom 6 to 7 nm in size was observed. Moreover, a reduction of microporosity loss was achieved, from 50% for NaOH to 20% [25].

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Dealumination is a known post-synthesis method of removing aluminum from zeolite structure with the use of chemical agents or by hydrothermal treatment [6, 26]. The most frequently used method is elution of aluminum with acid in high temperature. The method was first described by Barrer in the 1960s and regarded removing aluminum from

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clinoptilolite [27].

In all the cases, the extraction of aluminum atom is accompanied by partial breakdown of zeolite structure and formation of vacancies. These vacancies constitute

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additional porosity, mainly in the range of mesopores in the microporous structure in zeolite. However, one must also take into account the fact that extraction of aluminum atoms may be also a cause of serious changes in acidic properties of the zeolite, depending on the applied method of dealumination [28]. Van Oers and co-workers [29] described a method of dealumination with the use of acid treatment based on: − preparing and ageing of a solution containing nanoparticles of Beta zeolite in temperature of 140°C; − gradual cooling down; − acidification with concentrated HCl; 6

ACCEPTED MANUSCRIPT − hydrothermal processingin temperature150°C for 75 hours. According to the authors, the key role in the final properties of zeolites was played by gradual cooling down. Slow cooling led to formation of zeolite with better properties, i.e. greater crystallinity and additional porosity with the average pore size increasing from 6 nm to 10 nm during gradual cooling down.

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Sulikowski, in turn, used acid treatment of zeolite with EDTA solution (ethylenediaminetetraacetic acid) under reflux condenser in temperature of 800°C in inert atmosphere [30]. It resulted in removing aluminum atoms located on the external surface of the material.

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Dealumination process was regarded as one of the pioneering methods in introducing porosity into zeolites. However, the poor connection and depth of the generated pores coupled with serious damage to micropores and crystalline properties in adverse processing conditions

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necessitated searching for new methods of mesoporosity generation [30]. Fig.4 &5

2.2. Irradiation This

method,

introduced

by

Valtchev

[33]

enables

preparation

of

uniform

meso/macropores oriented in zeolite crystal. The method was carried out in two stages: in the 238

U) in order to

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first one, the zeolite crystal was radiated with heavy ions (e.g. uranium ions

create hidden paths inside the zeolite crystal. In the second stage, the radiated zeolite was washed with diluted HF acid, and next with water. Zeolite ZSM-5 with large crystal size (5 um) growing in fluoride environment was used. As a result of the treatment, uniform

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macropores were obtained in the size of about 50 nm arranged parallel to the mean distance between the pores (about 23, 700 and 1400 nm) in agreement with the appropriate fluidity of

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the stream of uranium 238 ions.

2.3. Synthesis of hierarchical zeolites - surfactant-assisted recrystallization method Mesoporosity can be effectively introduced into the structure of a material with simultaneous use of cation surfactants such as: cetyltrimethylammonium bromide (CTABr) or cetyltrimethylammonium chloride (CTACl) and mild conditions of synthesis, i.e. by recrystallization of microporous zeolites to micro/mesoporous or mesoporous materials. This process prevents dissolution of crystals by providing interaction between the surfactant and zeolite and enables achieving almost total reorganization of the zeolite network around surfactant micelles [12, 34]. Ivanova with co-workers [35] used such research techniques as: X-ray diffraction (XRD), multinuclear MAS NMR, and thermogravimetric analysis to study the mechanism of the recrystallization process during hydrothermal treatment. The 7

ACCEPTED MANUSCRIPT mechanism proposed by Ivanova and co-workers is presented in figure 6 [35]. The authors show that after the initial break of Si-O-Si bonds in alkaline environment, during desilication, large inter and intra crystalline pores are formed. CTABr molecules diffuse into these spaces, CTA cations undergo exchange with Na+ and micelles are formed. In the final stage the agglomerates of the silicon source surround the micelles thus resulting in creation of single-

Fig. 6.

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phase zeolite with bimodal porous structure.

In order to obtain a material with different size of mesopores, Na and co-workers [13]

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mixed zeolite HZSM-5 with aqueous solution of sodium hydroxide, then adding aqueous solution of alkyltrimethylammonium bromide.

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In sum, synthesis of hierarchical zeolites with the use of surfactant-assisted recrystallization constitutes a simple way of preventing damage to the structure of the material in the desilication method [18].

2.4. Dual templating with surfactants

The most common representative of family of silicate mesoporous materials with ordered structure is the MCM-41 material discovered by researchers from Mobile Oil company in

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1992 [37]. The material is characterized by the presence of uniform mesopores in its structure. Thanks to the X-ray diffraction in the small angle range, the Mobil Oil researchers were able to achieve successful results. The results of the structural research proved that the obtained materials are having an entirely new, previously unknown structure with long-range ordering

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and hexagonal arrangement of mesopores. However, these materials cannot be regarded as zeolites, as their walls are totally amorphous.

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Fig.7.

One of the key factors affecting the synthesis of MCM-41 is the use of different types of surfactants whose molecules tend to form micelles in aqueous solutions. These micelles are a pore-generating factor in synthesis of materials of regular structure. After removing the surface active agent as a result of calcination or extraction we obtain a material with uniform system of mesopores, which is depicted in figure 7. Based on earlier research, it was observed that it is possible to introduce mesoporosity into zeolites by treatment with surfactants according to a procedure known as double templating. The strategy of double templating includes using a structure directing factor in zeolite synthesis and a surface active agent as a template for obtaining mesophase at the beginning of 8

ACCEPTED MANUSCRIPT the synthesis in the reaction medium. The main idea behind the method is for the surfactant to cause formation of micelles, which serves template of the mesostructure, while the agent directing the structure in zeolite leads to its crystallization. First research showed that the presence of both surfactant and the agent directing the structure in the starting reaction mixture did not result in formation of hierarchical zeolites. Instead, segregation of phases was

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observed carried out towards obtaining zeolite, mesoporous material or their physical mixture [39].

Pinnavaia and co-workers [40] proposed an alternative approach consisting of initially formed protozeolytic units which are then templated by the surfactant. These authors

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conducted an initial concentration of zeolite nanoclusters called „zeolite seeds” in the presence of surfactants (e.g. hexadecyltrimethylammonium bromide). In this way, they

stability and catalytic activity [41,42].

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succeeded in generating mesoporous aluminosilicates. These materials display increased

Xiao and co-workers [43] described a method of hydrothermal synthesis of hierarchical zeolites with the use of a mixture of small organic ammonium salts which serve as a structure-generating agent of the zeolite and of mesoscopic cationic polymers. According this

procedure,

hierarchical

Beta

(tetraethylammonium

hydroxide)

as

zeolite

an

was

agent

synthesized

directing

the

using

TEAOH

structure

and

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to

polydiallyldimethylammonium chloride as mesoscale cationic polymer. The material obtained in this way consists of molecules which are 600 nm in size, with a clear presence of mesopores in the range of 5-40 nm.

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Moller et al. modified the previous method by using nanoparticles to amass Beta nanozeolite, with the use of the same cationic polymer (polydiallyldimethylammonium chloride) [44]. The addition of the cationic polymer causes flocculations of the nanoparticles,

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which lead to formation of the hierarchical Beta zeolite with 100% efficiency. The obtained materials show the presence of micro/mesopores whose size depends on the amount of the polymer added. Depending on the concentration of the polymer and the source of silicon we can receive meso/macropores in the range of 40-400 nm. The reason for increased size of mesopores was the expansion of the original molecules with the rising concentration of the polymer. Concentration of above 0.3g of the polymer per gram of SiO2 resulted in loss of crystallinity and formation of amorphous material. According to this procedure, it is possible to obtain large volume of mesopores (0.66-0.9cm3/g) while maintaining considerable porosity (0.19-0.23cm3/g) and specific surface area in the range of 600 – 680m2 /g. 9

ACCEPTED MANUSCRIPT The group of Ryoo and et al. [45] described a synthesis of hierarchical zeolites with the use of polyquaternary ammonium surfactant, which led to generation of both micropores and mesopores. A typical example of the applied templates was the compound C18H37– N+(CH3)2–C6H12–N+(CH3)2–C6H12–N+(CH3)2–C18H37(Br-)3, which contains two hydrophobic groupings which aggregate with other templates to micellar forms necessary for generation of

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mesostructure. Besides, this compound contains ammonium groupings which are necessary for crystallization of the zeolite structure inside mesoporous walls.

2.5. Synthesis of hierarchical zeolites - zeolitization of materials.

Research conducted by Lysenko and Yue [46,47] enabled to transform mesoporous

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structures with amorphous walls into structures of crystalline zeolites. Conversion of dry gel in the presence of organic, microporous SDA led to formation of ordered mesoporous materials such as: SBA-15 or MCM-41, subjecting their walls to partial crystallization, which

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led to obtaining zeolite structure. This method leads to formation of intercrystalline mesopores.

The morphology, topology, distribution, size, shape and location of mesopores are strictly dependent on the type of applied template [48]. However, during a traditional hydrothermal synthesis, zeolite and mesoporous substrates simultaneously crystalline, which results in

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creation of a physical mixture which consists of both components [49]. In 1990, Xu with co-workers [50], introduced an alternative way into the conventional, hydrothermal synthesis of zeolite based on crystallization of dry aluminosilicate gel as a result of treatment in water and amine vapors, which causes transformation of ZSM-5 Zeolite. Since

described [51]: 1)

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then, the method has been widely used and frequently modified. Two main procedures were

Vapor Phase Transport - VPT, VPT, in which the mixture of water and structure-

2)

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directing agent is vaporized which leads to contact with dry zeolite;

Steam-Assisted Conversion - SAC, where only water is vaporized and the structure-

directing agent is contained in solid gel. In this case all the forms creating the zeolite structure are contained in the solid phase. Compared with traditional ways of zeolite crystallization, this method has a few advantages: the zeolite material can have the same ratio of SiO2/MxOy as the gel precursor, the crystallization process takes less time, and the use of the structure-directing agent is minimal. Moreover, in the SteamAssisted Conversion the waste products created after the gaseous phase contain only water.

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ACCEPTED MANUSCRIPT Simultaneously, these methods were used in the synthesis of hierarchical zeolites. In this case different types of solid precursors were used, such as: dry gels [52], silica-based nanoparticles [53] mesostructural solids [54] or amorphous hierarchical solids [55]. In 2010 Zhou [56] described a method in which gel containing precursor TS-1 triethanolamine and tetrapropylammonium hydroxide was crystallized by treatment with

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vapors, washed dries and calcinated. This led to obtainingTS-1 zeolite which had mesopores in the size of about 11.2 nm.

Li [53] crystallized silica and packed nanoforms of silica containing aluminum using SAC method. In this way he obtained nanocrystals MFI containing both micro and

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mesopores.

Xue described a synthesis of mesoporous zeolite MFI by partial crystallization of mesoporous silica – carbon composite using vapor/steam-assisted treatment [54]. The

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obtained porous material had acidic centers similar to those found in the conventional MFI zeolite, however, diffractograms showed that the pores in the material walls are not fully crystalline.

2.6. Synthesis of hierarchical zeolites - nanoparticle assembly. Mesopores in hierarchical zeolites may come in three forms: intercrystalline;

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intracrystalline, and both between inside crystals. Hierarchical zeolites containing intercrystalline mesoporous structures can be obtained by controlling the conditions facilitating crystallization. These materials have better properties than traditional zeolites because nanometric zeolite crystals are characterized by larger external surface area and the

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presence of large pores between nanocrystals accelerating the transport of the mass of reagents and products [14].

2.7. Synthesis of hierarchical zeolites - template-assisted synthesis.

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Synthesis of hierarchical zeolites can be conducted using „hard” and „soft” templating. Among „hard” templates are those containing carbon [57-64]; aerogels [65,66], mesoporous silicates [67-74] and others [75-79]. Hierarchical zeolites can be synthesized with the use of various types of carbon materials such as: carbon nanotubes, carbon nanofibers, ordered mesoporous carbons etc. [57-64]. Jacobsen and co-workers described synthesis of mesoporous zeolites using carbon nanoparticles in acidic or alkaline solution as mesoporous templates [80]. They noticed that application of excessive amount of zeolite gel caused growth the zeolite around carbon particles. Obtaining mesoporous zeolite ZSM-5 was possible thanks to removing the carbonaceous matrix significantly limiting the transfer of mass by calcination. In order to 11

ACCEPTED MANUSCRIPT overcome this limitation, carbon nanoparticles were replaced with nanofibers and carbon nanotubes as mesoporous templates. It led to obtaining simple and uniform mesoporous channels open to the external surface in zeolite crystals [63,81]. Janssen [82] also noticed that carbon nanofibers are promising candidates for acting as secondary templates during the synthesis of mesoporous crystalline zeolites.

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The application of mesoporous silicates as ordered templates during the synthesis of ordered mesoporous carbons (CMKs) suggests the possibility of CMKs as a „hard” template for obtaining zeolites which copies of ordered mesoporous carbon (CMKs). For example, Yang described a synthesis of ZSM-5 material with unique supermicropores obtained by

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synthesis of zeolite with the use of ordered mesoporous carbon CMK-3 as „hard template” [83]. Fan and co-workers [58] described a synthesis of cubic ordered mesoporous silicate-1 using ordered cubic mesoporous carbons as „hard” templates.

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The synthesis of hierarchical zeolites obtained with the use of carbonaceous matrices is presented in figure 8.

Fig.8.

Polymers are an alternative for carbonaceous materials used as solid templates in the synthesis of hierarchical zeolites. When the zeolite structure has been formed, the polymer

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template is easily removed by calcination in air atmosphere. The main disadvantage of using polymers is the fact that hydrothermal temperature of zeolite crystallization is limited by the temperature of polymer glassification [17].

Polystyrene spheres (PS) were used as hard templates by Holland and co-workers.

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They mixed silicate-1 zeolite precursors with polystyrene spheres of 526 nm in size in polyethylene bottle, shaked for 10 minutes and left to harden for an entire night. Next, the

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obtained solid body was heated under pressure in the temperature of 130°C for 40h in order to crystallize the amorphous silicate in silicate-1 [84]. Xu et al. [85] used the centrifugal force stage in the production of highly packed

composite from a precursor which, in this case, was ZSM-5 gel and PS spheres of 580 nm size, which was then subjected to hydrothermal crystallization in 100°C. After calcination in air atmosphere in 550°C, protonation by ion exchange with NH4NO3 and the final calcination in temperature of 500°C, zeolite ZSM-5 displayed macroporous structure (pore size from 0.3 to 0.5 nm) with walls formed by nanodimensional crystals ZSM-5 and aluminosilicate amorphous phases.

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ACCEPTED MANUSCRIPT Tao and co-workers [86] synthesized NaA zeolite by replica of resorcinol-formaldehyde aerogel. The hierarchical zeolite NaA displayed almost the same microporosity as that which occurred in conventional zeolite (Vmicro = 0.2cm3/g), and additional mesoporosity with the mean pore size of 15 nm and 3 times bigger total volume of pores. Apart from carbonaceous materials and polymers used as hard templates in synthesis

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of hierarchical zeolites, also other structure were studied, for example, hierarchical crystals silicate-1 were obtained by crystallization of precursor of zeolite gel containing nanoparticles CaCO3 sized 50 do 100 nm [87]. Nanoparticles CaCO3 trapped inside zeolite crystals during hydrothermal treatment are removed using acid. This solution caused generation of

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intraparticle pores. Besides, the researchers observed, characteristic for zeolites, micropores and secondary porosity with a wide distribution of pore size in the range of 50-100 nm, depending on the size of particles of the original template.

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A very interesting issue in the synthesis of hierarchical zeolites is using macrotemplates, which allow for obtaining an interesting architecture of the material with three levels of porosity (micro-, meso- and macroporosity). Obtaining such materials depends on the origin of the templates i.e. whether they are synthetic or biological). As a result of hydrothermal treatment of mesoporous silicate previously implanted with zeolite

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nanocrystals, Dong obtained mechanically stabile silicate-1 monoliths [88]. Biological templates such as: bacterial threads [89] natural sponges [90] leaves of plants [91] and wood tissue [92] enabled to obtain macroscopic constructions. Despite the fact that different strategies of synthesis using biological templates were developed, due to their

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own individual characteristics, hard templating comprises mainly: creating a mixture of zeolite precursor template, hydrothermal crystallization and, in the final stage, calcination in order to remove the template.

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The group of soft templates includes i.e., surfactants [12] or block co-polymers [93,

94]. Block co-polymers are quite commonly used because they are inexpensive and easily available. For the synthesis of hierarchical zeolites, J. Zhou with co-workers used such copolymers as Pluronic F127, Pluronic P123 and Brij [94]. For the synthesis of ZSM-5, H. Zhou used co-polymer F127 and cetyltrimethylammonium bromide as co-template. Thanks to such solution, aggregation of crystallites occurred, which created larger mesoporous channels in the zeolite structure [95, 96]. Zhao and co-workers performed a synthesis of mesoporous zeolites from a hollow spherical/ellipsoidal capsule structure using TPAOH and CTABr as micro- and mesoporous templates. These materials are characterized by the presence of microstructure, high specific 13

ACCEPTED MANUSCRIPT surface area, small size of mesopores (about 3 nm) and a high concentration of strong acidic centers which contribute to achieving higher catalytic activity towards aldol condensation of benzaldehyde in comparison with amorphous mesoporous aluminosilicates or conventional ZSM-5 [97]. For the increase of the surfactant and a silica-based species, it is necessary to introduce additives enlarging micelles in the system [98]. However, obtaining hierarchical

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zeolites using soft templating continues to pose a great challenge for researchers. The key factor is the coordination of interaction between mesoscale organic templates and silica-based species at the crystallization stage. Due to the fact that organosilanes more easily interact with silica-based species, they can be commonly used in preparation of hierarchical mesoporous

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zeolites as soft templates [99-106].

Ryoo et al. presented a series of examples of obtaining mesoporous zeolites using organosilanes as „soft” templates. They developed a series of amphiphilic organosilanes with

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the positive charge as mesoporous agents directing the structure in the synthesis of mesoporous zeolites [99-101, 107,108]. Xiao and et al. [109] prepared a series of materials with the use of cationic polymers as mesoporous templates. They presented a universal way of synthesis of mesoporous zeolite Beta (Beat-H) templated by a mixture of microporous structure directing agent and a mesoscale cationic polymer polydiallyldimethylammonium

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chloride [PDADMAC] in hydrothermal synthesis [109]. Mesoporous zeolites Beta and ZSM11 templated by polyvinylbutyral (PVB) gel were obtained by Zhu and co-workers [110]. These zeolites displayed higher resistance of the material to deactivation in cracking of trimethylbenzene and 1,2,4-trimethylbenzene in comparison with conventional zeolites. It is

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worth mentioning that the diversity of the available and inexpensive cationic polymers creates the possibility of obtaining, on industrial scale, of commercially interesting hierarchical mesoporous zeolites.

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2.8. Other hierarchical zeolites based on zeolite seeds Hierarchical zeolites like as MRE or MTW were obtained in the form of nanostructures using cationic surfactants containing three or more ammonium ions. The hydrophobic tails in the surfactant play two important roles: first - they can be used for generating the mesoporous structure by assembling themselves to a micellar structure; the second function is that they inhibited further zeolite growth beyond the hydrophilic head group [96]. SAPO-34 mesoporous zeolite was synthesized by application of soft templating with organosilane. This material was applied e.g. in transformation of methanol to hydrocarbons [111].

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ACCEPTED MANUSCRIPT MWW hierarchical zeolite was described by Chu et al. [112]. This material was obtained by aggregation of small crystallites and then was applied in Friedel–Crafts alkylation of aromatics with benzylalcohol. Table 2 shows the examples of various hierarchical zeolites containing different metals, such as: Sn, Ti, Mg, Cu, Ag, B, V, Cu, Pd, Ni, Ru.

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Table 2 3. Examples of hierarchical zeolite applications in catalysis

Similar results as for reactions conducted using conventional zeolites were obtained for catalytic reactions carried out using hierarchical zeolites and including diffusion of small

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particles in micropores. However, for reactions conducted with the use of larger substrates it was more advantageous to use multimodal porosities of hierarchical zeolites [136]. In table 3

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are presents examples of hierarchical zeolites application in selected catalytic reactions. Table 3

Below is a broader description of the selected catalytic reactions conducted with the use of hierarchical zeolites.

3.1. Conversion of methanol to hydrocarbons

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Transformation of methanol to hydrocarbons constitutes an attractive alternative for obtaining, on a large scale, such hydrocarbons as: gasoline or light olefins. Methanol can be produced from coal, natural gas or biomass. Transformation of methanol to gasoline was introduced for the first time by Mobil Oil Company using ZSM-5. However, when using

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ZSM-5 material, the obtained gasoline contained large quantities of aromatic compounds, which is undesirable from the ecological point of view.

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Di and co-workers developed a direct synthesis of composite ZSM-5/MCM-48 in order to minimize the amount of aromatic compounds obtained in the reaction of transformation of methanol [150]. The composite consists of ZSM-5 phase and internally connected phase of mesoporous material MCM-48, which causes higher activity and stabilityin the reaction of conversion of methanol to hydrocarbons. The contents of aromatic compounds in liquid fraction decreased from the range of 65-84 wt. % to 35 do 46 wt. % in comparison to the commercial zeolite HZSM-5 [151]. Qian et al. [152] proposed using a composite containing 70 wt. % HZSM‐5 core and 30 wt. % mesoporous SBA‐15 shell in comparison to the original material HZSM‐5 (100 wt. %) and a mechanical mixture containing 70 wt. % of HZSM‐5 and 30 wt. % of SBA‐15 15

ACCEPTED MANUSCRIPT in the reaction of transformation of methanol to olefins. 98% conversion of methanol using composite and 39% selectivity to propene was obtained [152]. 3.2. Friedel–Crafts alkylation of aromatics Friedel-Crafts alkylation reactions are an important class of reactions used in petrochemical industry. Among these reactions, of particular significance is benzylation of

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aromatic compounds with benzyl alcohol, in which diphenylmethane and substituted diphenylmethanes are produced, which are key compounds quite commonly used as components in pharmaceutical and chemical industries. Therefore, Friedel-Crafts alkylation of aromatic compounds such as benzene, toluene, mesitylene with benzyl alcohol is frequently

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used as test reaction in liquid phase [111, 112, 153-155]. Due to the presence of large substrates or intermediate products, the additional porosity is necessary in order to increase the porosity and efficiency of the catalyst.

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Sun et al. [111] noticed that the hierarchical mesoporous zeolite MFI displayed nine times higher catalytic activity than the conventional zeolite ZSM-5, while maintaining the same Si/Al ratio in reactions conducted in liquid phase. The higher catalytic activity was connected with the reduction of diffusion limitations in mesoporous material MFI [111]. Wang [153] observed that in the reaction of benzene benzylation using BEA as

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catalyst (material obtained by desilication and desilication with subsequent acid treatment) no significant differences between desilicated catalyst and standard materials. After the subsequent treatment with acid, the catalytic activity was improved. Desilication caused a partial loss of acidity, which can be recovered again by additional washing with acid. In the

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reaction of mesitylene benzylation, a significant increase of catalytic activity with the use of modified catalysts was observed [154]. Leng [155] and co-workers, by demetallation, acid-treatment, and acid-base treatment,

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obtained a hierarchical zeolite with MOR structure, thanks to which it was possible to achieve higher catalytic activity in comparison with a conventional zeolite of MOR structure. For hierarchical materials, this activity was as follows:acid–base–acid leached > acid-leached > acid–base leached, which was adequate to the degree of mesoporosity, which decreased in the same order as the obtained catalytic activity [155]. SUMMARY The synthesis, characteristics and application of hierarchical zeolites are becoming the subject of research by an increasing number of scientists. Hierarchical zeolites possess, besides micropores, secondary porosity which contains pores of different sizes, from supermicropores, through mesopores, to macropores. The presence and properties of 16

ACCEPTED MANUSCRIPT secondary porosity i.e. the specific surface area, size and distribution of pores and volume of pores depend on the method of synthesis of hierarchical zeolites. Introduction of secondary porosity improves availability of pores for larger reagent particles and increases the speed of diffusion. Apart from that, this porosity constitutes an ideal space for deposition of active

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catalytic phases (metals, metal oxides, sulfides, nitrides, etc.) controlling their size, enables achieving high dispersion and strong interactions between zeolite and the medium.

Despite the large number of hierarchical zeolites currently available, they can be divided, depending on the origin of the secondary porosity, into pure zeolite phases and

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composites. Pure zeolite phases or genuine hierarchical zeolites are obtained when the additional porosity is located within the zeolite phase. In such case, the secondary porosity is present either in zeolite crystals or in the intercrystalline voids. As regards the composites,

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consisting of zeolite phase and non-zeolite phase, the hierarchy in zeolites results from the additional phases. In the situation when this phase is a medium, the secondary porosity stems from the media.

The wide variety of the methods of synthesis of hierarchical zeolites developed to date is generally based on the processes of aggregation, extraction and crystallization. Some

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methods are based only on the performance of one synthesis process, while others combine two or even more different syntheses (fig.9).

Fig.9.

Hierarchical zeolites can be used in numerous catalytic reactions such as Friedel–

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Crafts-alkylation of aromatics and conversion of methanol to hydrocarbons. The application of hierarchical zeolites allows to: eliminate diffusion limits in material pores, reduce elution of the active phase into the solution, and enables multiple regeneration of the catalyst.

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Besides, in comparison with the conventional microporous zeolites, these materials display higher catalytic activity and smaller vulnerability of the catalyst for deactivation.

ACKNOWLEDGEMENTS This work was supported by the National Science Centre (project FUGA-5 no:

2016/20/S/ST4/00547). I thank Prof. Boguslaw Buszewski, and Prof. Myroslav Sprynskyy (Nicolaus Copernicus University in Toruń, Faculty of Chemistry) and Prof. Izabela Nowak (Adam Mickiewicz University in Poznań, Faculty of Chemistry) for comments that greatly improved the manuscript. 17

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Desilication

Assembly of nanosized zeolite

Irradiation

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HIERARCHICAL, MESOPOROUS ZEOLITES Routes for secondary porosity generation BOTTOM-UP TOP-DOWN Hard-templating methods Dealumination

Zeolitization of materials

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Table 2. Examples of hierarchical zeolites containing various metals

Sn Sn Sn Sn Sn Sn Sn

Type of hierarchical zeolite H-ZSM-5 Zeolite Beta Zeolite Beta MWW, MFI, MOR, Zeolite Beta Zeolite Y

MFI MFI Mordenite

V V V Cu

ZSM-5

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Sn Sn Ti Cu, Sn, Mg, Ni Ag B B

ZSM-5

ZSM-5 Zeolite Y ZSM-5

Silicate-1 zeolites ZSM-5 Zeolite Y

Ref.

Aromatization of glicerol Conversion of sugars to alkyl lactates Conversion of glucose to alkyl lactate Conversion of carbohydrate

[113] [114] [115] [116]

Baeyer-Villiger oxidation Catalytic conversion of bulky ketone substrates Conversion of dihydroxyacetone to methyl lactate Isomerization of cellulosic sugars Isomerisation of sugars Oxidative desulfurization of dibenzothiophene Deoxygenation of Bio-Oil

[117] [118] [119]

Conversion of glycerol to allyl alcohol Cracking of hydrocarbon MTP process – reaction of methanol to propylene reaction Epoxidation of cyclohexene Styrene epoxidation with organic hydroperoxide Glycerol conversion to acrolein and acrylic acid Degradation of Congo red dye

[124] [125] [126]

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Application

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Metal

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[120] [121] [122] [123]

[127] [128] [129] [130]

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Ni, Ru

Zeolite Beta

Oxidation of cyclohexane Oxidation of aromatic amines Suzuki reaction in water Catalytic removal of trichloroethylene from water Hydroreforming of the oils from LDPE thermal cracking

Table 3. Application of hierarchical zeolites in catalytic reactions.

Method of synthesis

Reaction

SC

Zeolite structure

[131] [132] [133] [134] [135]

RI PT

Cu Cu Pd Pd

Ref.

Conversion of furfural to GVL, Glucose conversion to 5-hydroemthyl-furfural, Triose conversion of ethyl lactate

[137]

Dealumination, desilicication and metal incorporation

Zeolite β

Desilication

Conversion of ethanol into 1,3-butadiene

[138]

Hierarchical ferrierite zeolite

Synthesis by using pyrrolidine as sole organic structuredirecting agent

Skeleton isomerization of 1-butene to isobutene

[139]

Alkylation of benzene with propan-2-ol

[109]

TE D

One-step hydrothermal reaction using a small organic ammonium salt and a mesoscale cationic polymer as templates Desilication by NaOH and TPA+, carbon templating and seed-silanization Desilication; dry-gel conversion with nanocarbon templates Soft templating with two different surfactants(TPAOAC and CTAB) Desilication; soft or hard templating with starch Soft templating with organosilane Conversion of silica gel in afluoride medium - Nontemplating

AC C

EP

B Zeolite (Beta-H)

M AN U

Meso-Zr-Al-beta

MFI

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[140] [141, 142] Conversion of methanol to hydrocarbons

[143] [144] [145] [146]

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Carbon templating

MFI

[147] Conversion of ethanol to hydrocarbons Benzenealkylation with ethene

[148] [7]

[149]

RI PT

MFI

Soft templating with diquartenary ammonium surfactants Dry-gel conversion Fluoride leaching, alkaline leaching, nano-sized crystals

CAPTIONS TO FIGURES

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Figure 1.Diffusion of large, small molecules within macropores, mesopores and micropores [4]. Reproduced with permission from ref. [4], Royal Society of Chemistry 2016.

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Figure 2. Influence of the reduced diffusion path length and its effect in catalysis [4]. Reproduced with permission from ref. [4], Royal Society of Chemistry 2016. Figure 3. Application of hierarchical zeolites in catalytic reactions. Figure 4. Desilication and Dealumination method [31].

Figure 5. Desilication method [32]. Reproduced with permission from ref. [32], Royal Society of Chemistry 2013.

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Figure 6. Mechanism of mordenite recrystallization to micro/mesoporous structure [35]. Reproduced from ref. [35], Elsevier, Microporous and Mesoporous Materials. Figure 7. Synthesis of MCM-41 [38]. Adapted with permission from ref. [38]. Copyright (1992) American Chemical Society.

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Figure 8. Synthesis of hierarchical, mesoporous zeolites using carbonaceous matrices as hard templates [18]. Reproduced with permission from ref. [18], Copyright Royal Society of Chemistry 2012.

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Figure 9. Classification of hierarchical zeolites – origin of the secondary porosity [4]. Reproduced with permission from ref. [4], Royal Society of Chemistry 2016.

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Fig.1. Diffusion of large, small molecules within macropores, mesopores and micropores [4]. Reproduced with permission from ref. [4], Royal Society of Chemistry 2016. .

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Fig.2. Influence of the reduced diffusion path length and its effect in catalysis [4]. Reproduced with permission from ref. [4], Royal Society of Chemistry 2016.

Fig.3. Application of hierarchical zeolites in catalytic reactions. 26

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Fig.4. Desilication and Dealumination method [31].

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Fig.5. Desilication method [32]. Reproduced with permission from ref. [32], Royal Society of Chemistry 2013.

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Fig.6. Mechanism of mordenite recrystallization to micro/mesoporous structure [35]. Reproduced from ref. [35], Elsevier, Microporous and Mesoporous Materials.

Fig. 7. Synthesis of MCM-41 [38]. Adapted with permission from ref. [38]. Copyright (1992) American Chemical Society.

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Fig. 8. Synthesis of hierarchical, mesoporous zeolites using carbonaceous matrices as hard templates [18]. Reproduced with permission from ref. [18], Copyright Royal Society of Chemistry 2012.

Fig. 9. Classification of hierarchical zeolites – origin of the secondary porosity [4]. Reproduced with permission from ref. [4], Royal Society of Chemistry 2016.

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Comparison of zeolites with hierarchical zeolites Variation of synthesis methods of hierarchical zeolites Application of hierarchical zeolites in catalytic reactions