Synthesis of zeolite composites with hierarchical porosity

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Synthesis of Zeolite Composites

with Hierarchical Porosity

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J. T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 20 oktober 2008 om 15:00 uur

door

Jia WANG

doctorandus in de scheikunde geboren te BaoTou, Nei Mongol, P.R.China

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Prof. Dr. sc. tech. A. Schmidt-Ott

Samenstelling promotiecommissie:

Rector Magnificus Technische Universiteit Delft, voorzitter Prof. dr. ir. M.-O. Coppens Technische Universiteit Delft, promotor Prof. dr. sc. tech. A. Schmidt-Ott Technische Universiteit Delft, promotor Prof. dr. J. A. Moulijn Technische Universiteit Delft

Prof. dr. R. Gläser Universität Leipzig Prof. dr. P. van der Voort Universiteit Gent

Dr. ing. J. J. Heiszwolf Albemarle Catalysts B.V. Dr. E. Mendes Technische Universiteit Delft

Prof. dr. Ir. H. van Bekkum Technische Universiteit Delft, reservelid

All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the author.

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To my family & To Kristina

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1

. Introduction………. 1

1.1. Zeolites……… 1

1.2. Petroleum and Oil Refining………. 3

1.3. Mesoporous Silicas and Nano-templating……….. 6

1.4. Synthesis of Zeolites with Improved Transport Properties………. 7

1.5. Aim of the Thesis……… 13

1.6. Outline of the Thesis……… 14

2

. Synthesis and Structure of Silicalite-1/SBA-15 Composites Prepared by Carbon Templating and Crystallization……...………. ... ...….. 19

2.1. Introduction………. 20

2.2. Experimental……… 21

2.3. Results and Discussion……… 24

2.4. Conclusions………. 39

3

. Synthesis of TUD-M with Hierarchical Mesoporosity: a Facile One-pot Approach……….. 43

3.4. Conclusions………. 60

4

. Single-Template Synthesis of TUD-C: Hierarchically Structured Composites with Tunable Mesoporosity……… 63

4.3. Results and Discussions……….. 67

4.3.1. The Structure of TUD-C……….. 67

4.3.2. The Hydrothermal Stability……….. 73

4.3.3. The Influence of the Si/Al Ratio and the Acidity of TUD-C……… 75

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4.3.7. Morphology of TUD-C………. 85

4.3.8. Comparison between TUD-C and TUD-M……….. 87

4.4 Conclusions……….. 88

5

. Formation Mechanism of Scaffolded Mesoporous Silica with a Controlled Pore Size………. 93

5.2. Experimental………... 96

5.3.1. Scaffolding Based on TPAOH ………. 99

5.3.2. Effects of Synthesis Parameters: Concentrations, Temperature and pH……… 110

5.3.3. The Use of Organic Scaffolds other than TPAOH……… 112

5.4. Conclusions……….. 115

6

. Reactivity Test of TUD-C………... 121

6.1. Introduction……….. 122

6.3.1. Steam Stability of TUD-C……… 123

6.3.2. Catalytic Activity Tests………. 125

6.4. Conclusions……….. 130

7

. Summary and Outlook……….. 133

Samenvatting (Summary in Dutch) ……….. 139

Acknowledgement………. 143

List of Publications……… 147

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Chapter

1 Introduction

1.1. Zeolites

Zeolites are crystalline aluminosilicates with ordered micropores (0.3-1.5 nm). Axel Fredrik Crønstedt, a Swedish mineralogist, discovered in 1756 that upon rapidly heating a natural mineral, the stones began to dance about as the water evaporated. Using the Greek words for "stone that boils," he called this material zeolite. The first synthetic zeolite was reported in 1948 by Richard Barrer. Currently, more than 170 different zeolites have been characterized and new types still appear every year.

The boiling phenomenon is the result of the microporous nature of zeolites. According to the IUPAC classification of pores, micropores are pores smaller than 2 nm, while pores in the range of 2 to 50 nm are called mesopores. Each zeolite structure type features a unique well-defined micropore system that enables the separation of molecules or preferential production of certain molecules during catalytic applications. The micropore system also results in a high internal specific surface area. The building units of zeolites are SiO4 and AlO4 tetrahedra that are linked to each other by shared oxygen atoms. When the oxygen atom is connected to a silicon atom and a trivalent aluminum atom, an exchangeable counter cation (such as H+, Na+) needs to be present to compensate the net negative charge of the tetrahedron (Figure 1). If the counter cation is a proton (H+), strong Brønsted acidic properties will be induced.

Zeolites distinguish themselves by a combination of desirable properties: well-defined micropore network, high surface area, strong acidity and high hydrothermal stability. As a result, zeolites are one of the most important families of materials currently used in industry. Zeolites are traditionally used as adsorbents, in detergents, as ion-exchangers, and as heterogeneous catalysts. Their use in emerging areas, such as membrane separation and medication, is also attracting considerable attention [1,2]. Although many types of zeolites

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have been discovered or synthesized, there are not many used in industrial catalysis – mainly zeolite Y (type FAU) and ZSM-5 (type MFI).

Figure 1. A typical zeolite framework and its primary building units.

Much of the success of zeolites in the catalytic applications can be attributed to the shape selectivity feature, which is related to the size of the micropores and the sterically confined reaction space in the vicinity of the active sites. The availability of a wide range of zeolite structures with different micropore architectures makes it possible to conduct shape-selective catalysis for a whole spectrum of reactions. This represents one of the most significant achievements in the history of catalysis [3,4]. Apart from the versatile structures of zeolites, different metal ions could be introduced into the framework of zeolites by replacing Al3+and Si4+with Fe3+, Ti4+, Ga3+, etc… This allows for even greater opportunities to tailor zeolite catalysts to achieve optimum performance in a wide range of reactions.

However, the presence of relatively small micropores in zeolites also imposes significant transport limitations of the molecules to and from the active sites, which are often located in the micropores. When large reactants are involved, often the outer surface of zeolite crystals is the only effective part during reactions. So there is only a very limited

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Introduction

usage of the internal porosity. Diffusion is typically orders of magnitudes slower inside zeolite micropores than in the bulk. Diffusion limitations result in lower catalytic activities, changed product distributions, and deactivation by coke formation. As a consequence, much of the on-going research focuses on improving the transport properties of zeolite catalysts. Several strategies have been developed and various aspects of these efforts have been reported.

Synthesis of zeolites is mainly based on a “templating” concept. Usually, organic molecules are used to direct the formation of micropores. For example, the synthesis of ZSM-5 zeolite (type MFI) often uses tetrapropylammonium hydroxide (TPAOH) as a template. It is observed that an adequate matching often exists between the geometries of the organic species and those of the microporous cage or channel network [5]. Though the exact formation schemes of many zeolites remain in debate, it is commonly accepted that the organic molecules play the role as template, around which the zeolite framework is built. Templating is at the basis of the synthesis of many new nanoporous materials as well.

1.2. Petroleum and oil refining

The global demand for petroleum products will grow for many years to come, despite increasing concerns about the environmental issues and the depletion of petroleum resources. There has been great progress in the development of alternative energy sources. However, no other energy source will likely be able to totally replace crude oil for the foreseeable future. Thus, the central status of petroleum as world energy source will remain. In fact, the consumption of liquid fuel is predicted to increase by 50% in about 25 years [6]. As a consequence, the capacities of the refining and petrochemical industries will expand. The purpose of oil refining is the production of fuels for transportation, power generation, heating purposes, and the production of base chemicals [6].

Crude oil consists of a wide range of hydrocarbons. These hydrocarbons can roughly be divided into different groups according to their molar mass and structure [7]. The most prominent are the groups of the paraffins (linear and branched), the naphthenes (mainly unsaturated five- and six-membered carbon rings) and aromatics (benzene derivatives and poly-aromatics). Crude oil is naturally composed of only a limited fraction of usable liquid fuel. Thus cracking processes are used to break down complex organic molecules such as

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kerogens or heavy hydrocarbons into simpler molecules (e.g., light hydrocarbons) through the breaking of carbon-carbon bonds in the precursors.

Figure 2. Scheme of a modern-day refinery. (Adapted from [6])

In Figure 2 a simplified overview of a modern refinery is given. The heart of the refinery consists of a Fluid Catalytic Cracking (FCC) unit and a hydrocracker unit. They both aim to reduce the size of the molecules and to increase the hydrogen/carbon (H/C ratio) in order to meet the market demand of transportation fuel. The main valuable product from FCC is gasoline (that possesses a relatively high-octane number, >90) and lower olefins (C3= and C4=). More than half of the global gasoline production is directly from the FCC process, while hydrocracking is the major source of diesel and jet fuel. Besides gasoline, FCC produces a large amount of propene, which is a base chemical, and butenes that can be used in the alkylation process.

FCC uses a zeolite-based catalyst (commonly zeolite Y) and moderately high temperatures (500 °C) to aid the process of breaking down large hydrocarbon molecules into smaller ones. During this process, less reactive, and therefore more stable and longer lived

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Introduction

intermediate cations accumulate on the catalysts' active sites generating deposits of carbonaceous products generally known as coke. One of the main reasons for the coke formation is the limited mobility of the intermediate cations in the catalyst, which is related to the confined micropore structure of zeolites. Such deposits need to be removed in order to restore catalyst activity. The average deactivation time of FCC catalysts is about 1 second, which is rather short as compared to other heterogeneous catalysts. Controlled burning is usually applied to constantly regenerate the catalyst as shown in Figure 3. The coked catalyst is regenerated in a separate unit and is recycled back to the reactor. By reducing the diffusion limitations in the zeolite-based catalysts, processes such as FCC could be greatly improved. That will directly increase the efficiency of the whole oil refinery process.

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1.3. Mesoporous silicas and nano-templating

Meso, the Greek prefix meaning “in between”, has been adopted by IUPAC to define the pores between 2 and 50 nm. The size of mesopores is between that of micropores (smaller than 2 nm) and macropores (larger than 50 nm). Typically mesopores are present in aerogels, and occur in nature in some diatomites. Until the early 1990’s, the known mesoporous structures were mainly disordered. There was relatively little control over the mesoporosity of synthetic mesoporous materials.

The discovery of structured mesoporous materials, such as MCM-41 [8] and SBA-15 [9] (Figure 4, left), was one of the most significant events in recent materials science history. In 1992, Kresge et al. first reported the synthesis of the M41S family. Important members of this family have different spatial arrangements of perdiodically ordered mesopores. For example, MCM-41 has a hexagonal mesoporous configuration, while MCM-48 has a cubic structure (Figure 4, right).

Figure 4. TEM images of ordered mesoporous materials: (left) SBA-15, adapted from

[9]; (right) MCM-48, adapter from Kaneda,et al., J. Phys. Chem. B 2002, 106, 1256.

The synthesis of ordered mesoporous materials is a good example of the templating concept. MCM-41 employs the cationic surfactant cetyltrimethylammonium bromide (CTAB) as a structure-directing agent, which forms micelles of a size in the meso-range. Those micelles act as template to form structured mesopores, which is why the term “supramolecular templating” is often used to describe this templating technique. Here, the

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Introduction

templating idea is an extension of the templating concept for zeolite synthesis. However, different from that of zeolite, the actual templating material is an assembly of molecules (micelles) instead of individual molecules. Many ionic mesopore templates have structural similarities to zeolite templates, though they function rather differently. Recent developments have even pushed mesopore templates into the non-ionic surfactant domain. Block co-polymer P123, perhaps the most important non-ionic template, is employed for SBA-15 synthesis. During the mesopore formation, P123 molecules also form micelles, which have larger sizes than CTAB micelles. Consequently, SBA-15 has larger mesopores than MCM-41.

Both zeolites and ordered mesoporous materials are synthesized under the same “nano-templating” frame. The concept itself is as exciting as the discovery of those new types of porous materials. Now it is even applied in areas outside sol-gel synthesis. One of the best known examples is the synthesis of ordered mesoporous carbon, in which ordered silica materials (e.g., MCM-41, SBA-15) are used as the templates [10]. A great variety of porous materials was discovered in the past decade by using nano-templating.

The discovery of structured mesoporous materials sheds light on a possible solution for the transport limitation present in zeolite materials. Compared with micropores, larger mesopores provide better accessibility. Despite this attractive feature, ordered mesoporous materials are less suitable to industrial applications because of relatively low acidity and poor hydrothermal stability [11,12]. Many researchers are trying to combine the unique properties of zeolites with mesoporous structures.

1.4. Synthesis of zeolites with improved transport properties

Diffusion limitations are a major drawback of zeolite catalysts. It is particularly significant for processes involving large molecules, such as the FCC process in oil refinery [11]. The improved performance of zeolite catalysts can be envisioned upon enhanced accessibility to the active sites in the micropores and reduction of pore blockage. Small zeolite crystals, for example, have shorter diffusion length; thus, the accessibility of the active sites is increased. To reduce pore blockage, a possible method is to introduce interconnected larger pores in the zeolite structure. Those pores can also improve the transport of large molecules. Inspired by structures in nature, such as human lungs and leaves, we expect that many catalytic processes would be more selective and efficient if the catalysts had a designed

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hierarchical pore network structure. To arrange the pores at different length scales (e.g., micro-, meso- and macro-) in a hierarchically controlled manner, instead of a random combination, is the focus of much ongoing research. This thesis focuses particularly on the hierarchical design of micro- and meso-pores.

Several approaches are reported in the literature to obtain zeolites with improved transport properties as schematically presented in Figure 5. According to the material structure, they could be summarized into three major groups: 1) Synthesis of non-hierarchically structured zeolites: wide-pore zeolites, zeolite nanocrystals, and delaminated zeolites; 2) Preparation of hierarchically structured single-phase mesoporous zeolites; 3) Synthesis of hierarchically structured zeolite composites.

Figure 5. Various approaches to obtain zeolites with improved transport properties.

1.4.1. Non-hierarchical approaches to obtain zeolites with improved transport properties

Three methods can be included in this category:

Zeolites with

improved transport

properties

Large Cavity or Wide Pore Zeolites Nano Zeolite Crystals Delamination Hard Templating Desilication Dealumination Modified supra-molecular Templating Crystallization of Mesoporous Materials Assembly of Zeolite Crystals

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Introduction

Synthesis of large cavity or wide-pore zeolites. The larger cavities or intersecting

pores will facilitate transport to the active sites. Examples are ITQ-21 [13] (with 1.18 nm cavities) and ITQ-15 [14], both of which are new types of zeolites. Nevertheless, when the application is a well-established catalytic process, new types of zeolites only have limited relevance. Use of a different type of zeolite often means going back to the beginning of the catalyst development. This is rather difficult if at all possible.

Preparation of zeolite crystals with small diameters, typically below 200 nm.

These crystals have a relatively high outer surface area and short diffusion path lengths [15]. By controlling the synthesis conditions small zeolite crystals can be obtained. This approach is a seemingly simple solution to the transport limitations. However, it was reported that the small zeolite crystals tend to sinter or grow together and the acidity is also somewhat lower than that in the larger crystals [16]. It is interesting to note that, though the small zeolite crystals do not have a hierarchical structure, they are often included in a hierarchical framework.

Synthesis of delaminated zeolites to enhance the outer surface area. Delamination

of a layered zeolite precursor leads to the formation of thin readily accessible sheets of zeolites [17].

1.4.2. Synthesis of single-phase meso-zeolites

Hierarchical porous zeolites with a single phase can be obtained by nano-templating of larger pores during zeolite synthesis or by introducing intracrystalline mesoporosity through post-synthesis treatments of zeolite crystals. Since the zeolite features are often well preserved, this approach offers great potential. Several strategies are available for incorporating mesopores into microporous zeolites:

Hard templating route. This approach pushes the nano-templating concept to a

new level. The possibility to obtain diverse porous materials is dramatically extended. Different from soft templates, such as block copolymer P123, hard templates function directly without involving micelle formation. The final product has the exact reverse structure of the hard templates. Carbon [18], polystyrene [19], and latex [20] are commonly used. Normally, the zeolite synthesis is carried out in the presence of the hard templates. Zeolite crystals grow around or in

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between the template framework to form a composite structure. Upon calcination the hard template is combusted, leaving extra voids in or around the zeolite crystals. Compared with sol-gel based templating synthesis, which is sensitive to many synthetic conditions, hard-templating methods are less affected by the sol-gel parameters. The mesopore structures of the final products can be adjusted by using different hard templates. Though the surface properties of the templates play an important role during hard templating, the growth of zeolite crystals is not strongly influenced by the presence of the templates. Some of the crystals obtained show a nearly perfect external morphology [21].

Among all the hard templates carbon materials display many special features, which make them unique for the purpose of hard templating. They are commercially available or can be easily prepared from various precursors, such as furfuryl alcohol [22] and sucrose [23]. Moreover, carbon can be easily removed by combustion.

The present hard-templating methods can be classified into two main directions. 1) Confined space templating: the hard templates restrict the growth of zeolitic crystals inside the meso voids of the template [24-28]. The mesopores of the products are mainly inter-particle space. 2) Mesopores within zeolite single crystals: hard templates are encapsulated by the growing zeolite crystals during synthesis, resulting in mesoporous “defects” after combustion of the templates [18,29]. The final product can be described as a zeolitic crystal penetrated by a mesoporous system.

Desilication and dealumination, i.e., the selective removal of silicon or aluminum

from the zeolite framework, which is a post-treatment to create mesoporosity into zeolites. Dealumination generally can be achieved by acid leaching, steaming, or the use of complexing agents [30-32]. This approach is applied industrially to synthesize Al-rich zeolite Y and mordenite (MOR). Compared with other approaches to reduce transport limitations, dealumination has the advantage of low cost. Desilication, however, had been little reported on, until recently. Groen et al. have done extensive research in this area, and promising results were obtained [33-35]. Both desilication and dealumination affect the acidic properties of zeolites simultaneously to modifying porosity, which is a drawback of these

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Introduction

methods. Desilication is expected to impact the acidity to a smaller extent. The Si/Al ratio is an important parameter to optimize the mesoporosity of the final materials, which certainly restricts the application of desilication and dealumination. Compared with mesoporosity created through templating, desilication and dealumination have less control over the pore size and its distribution. Moreover, recent reports show that this route does not necessarily provide the desired connectivity of the mesopores to the outside of the catalyst particles, which decreases the overall efficiency [36].

Modified supra-molecular templating route, is a very recently developed approach

that involves specially designed template molecules to create mesopores during zeolite formation. This one-step strategy was proposed soon after the discovery of the ordered mesoporous materials [37]. However, it turned out that the mesopore template and the zeolite template function in a competitive, rather than cooperative manner. The resulting material consists of separate mesoporous and bulk zeolite phases. By modifying mesoporous templating molecules this problem can be solved. Ryoo et al. have developed a silyl group (Si(OR)n) modified mesopore template [(CH3O)3SiC3H6N(CH3)2CnH2n+1]Cl, which is not expelled from the aluminosilicate sphere during zeolite crystallization [38]. The material obtained has a single crystalline phase with intra-crystalline mesopores. This method allows for very good control over the porosity. By using template molecules with different lengths, the mesopores could be systematically varied. Following the same route, silyl-group functionalized polymer templates [39] and cationic polymer templates [40] were used to synthesize mesoporous zeolites. Though this method has the drawback of employing more expensive mesoporous templates, it is a scientifically very attractive new concept.

1.4.3. Synthesis of zeolite/meso-matrix composites with a hierarchical structure

The present solutions to reduce diffusion limitations are not restricted to single-phase zeolite materials. Zeolite crystalline phase and amorphous aluminosilicate phase can be arranged in a hierarchical way to form a composite. This is also a good candidate for catalytic reactions. Usually, nanosized zeolite crystals are incorporated into a mesoporous support. The mesopores facilitate the transport of molecules and prevent the nano-crystals from sintering.

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Industrial zeolite-based catalysts are often composites, of which the FCC catalyst is a good example [6]. The mesopore structure in composites could be as ordered as that of ordered mesoporous materials, when meso-structure directing agents are used. Two major preparation routes are:

Assembly of zeolite crystals and aluminosilicate into a meso-structure. This

method usually starts with preparing zeolite precursor solutions or “seed” solutions. The conditions of the hydrothermal synthesis are such that the crystals can only grow to a limited size. In the second step, the nanoparticles are immersed in a solution containing supra-molecular templates. The conditions are similar to those of the synthesis of mesoporous material, so that nanoparticles are assembled to form meso-structures. Synthetic conditions in step one and two are usually very different. The control of the change between the two steps is therefore critical to the success of the whole synthesis. The final materials normally have an ordered mesoporous structure with partially crystalline walls. Depending on the mesopore templates used, MCM-41 analogs [41-44] and SBA-15 like materials [45,46] are obtained.

It is clear that this method cannot result in a single-phase product similar to that of the hard-templating route. This is because zeolite precursor solutions always have zeolitic species at different stages of formation at any given time [47]. Thus, the final material is a composite rather than a single-phase mesoporous zeolite [48].

Crystallization of mesoporous materials. Certain zeolites, such as ZSM-5, could

be synthesized through solid-phase crystallization of amorphous aluminosilicates in the presence of appropriate templates. So, one could first obtain structured mesoporous materials, then convert the walls into a crystalline zeolite structure. This is the reverse procedure from the assembly approach, in which a zeolite structure forms in the first step and the mesopores are obtained later. In 1997, Kloetstra et al. first reported this strategy, in which MCM-41 was partially crystallized to ZSM-5 zeolite [49]. The control over the crystallization is crucial to the whole synthesis. As zeolite crystals tend to grow large (to micron range) and become more stable, the mesoporous structure could lose its order and even collapse. It was reported that using SBA-15 as a starting material [50] is more successful than using MCM-41 [51], which is likely due to the more stable

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Introduction

structure of SBA-15. The final materials have the structure of zeolite nanocrystals embedded in a mesopore matrix, which is similar to the assembly method.

To accomplish the goal of reducing diffusion limitations in zeolite-based catalysts, scientists are facing some major challenges. Often a trade-off exists between the complexity of the synthesis and the control of the porosity. Thus, to simplify the synthesis, especially to avoid using the expensive mesopore templates, while maintaining control over the pore structure, is a research challenge. Additionally, the zeolite properties should be preserved in the new materials. It was observed that the material is less acidic and has low stability when obtained from the assembly method. Last but not least, it is a challenge to obtain truly hierarchical structures. A physical mixture of zeolite crystals and mesoporous materials cannot reduce transport limitations within zeolite, unless these crystals are small enough. The present summary serves as an introduction to the topic. Further reading is recommended for more details on the briefly discussed approaches [3,31,48,52].

1.5. Aim of the thesis

The work presented in this thesis focuses on the synthesis of hierarchically structured zeolite/meso-structure composites through different routes in order to reduce the transport limitations in zeolite-based catalysts. The goal is to obtain new types of materials with a designed hierarchical architecture. To achieve this goal it is particularly important to address some of the above challenges in the present research. The highlight of the thesis is the single-template approaches to obtain the new composite materials TUD-C and TUD-M. In order to understand the crucial variables in the synthesis, different characterization techniques were combined to provide a comprehensive understanding of the new materials. An equally exciting discovery in this thesis is the formation mechanism of the disordered mesopores through scaffolding, which has not been extensively investigated before. Finally, some of the new composite materials were tested catalytically to demonstrate the effects of the new architecture on catalysis.

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1.6. Outline of the thesis

In Chapter 1 the background of the Ph.D. research and its potential areas of impact were discussed. A summary of existing approaches to address transport limitations in zeolites was given to introduce the state-of-the-art and some major challenges.

Chapter 2 describes a method that combines carbon templating and crystallization to obtain a zeolite/meso-structure composite. Ideally, such a combination of hard-templating and crystallization could make up for the shortcomings of each individual method. This approach is compared with two similar methods. The results also show that the final material is always a hierarchical composite instead of an ordered mesoporous material with fully crystalline walls.

Chapter 3 introduces a synthesis method to obtain the novel TUD-M material. This route aims to reduce the complexity of the synthesis, while maintaining control over the porosity. Only one structure-directing agent was used in the entire synthesis, which makes this method remarkably simple. TUD-M has a truly hierarchical structure with zeolite nanocrystals embedded in a disordered mesoporous matrix. The mesopore size distribution could easily be tuned, as shown by N2adsorption. The zeolite properties were characterized by TEM and XRD.

Chapter 4 presents the synthesis of TUD-C, which has a similar structure to TUD-M but improved properties. TUD-C is more acid and hydrothermally stable. The mesopore size could also be tuned within a wider range than TUD-M. The synthesis of TUD-C involves solid-state crystallization, while TUD-M is fully obtained through sol-gel synthesis. Both TUDs are closely related and they were developed simultaneously. Nevertheless, depending on the application either material could be chosen because they both have pros and cons.

Chapter 5 attempts to answer the fundamental question: how do mesopores form during TUD-M and TUD-C synthesis? By carrying out in-situ studies, such as ATR-IR, the formation mechanism is discussed and summarized as “scaffolding” of TPAOH. The investigation shows that many organic molecules could function in a similar manner as scaffolding agents. Though the concept is not new, many scaffolding agents have previously been overlooked. Redefining the scaffolding concept is another important outcome of this thesis.

Chapter 6 shows the catalytic testing results of TUD-C. It is an important step towards industrial applications of the novel materials. Cracking of decalin and dealkylation of

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Introduction

tri-iso-propyl benzene are chosen. Both reactions are catalyzed by zeolite acid sites. The information obtained is valuable to optimize the synthesis of TUD-C.

Chapter 7concludes the thesis. Research in materials synthesis, catalysis, and sol-gel chemistry could benefit from the key ideas of this Ph.D. work. Ideas for future research are presented.

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Chapter

2 Synthesis and Structure of Silicalite-1/SBA-15 Composites

Prepared by Carbon Templating and Crystallization

In many catalytic applications of zeolites, slow diffusion of large molecules and pore blockage are serious limitations toward their efficient usage. Composites containing zeolites and a controlled mesoporosity aim to address this problem. We explore three different approaches based on solid phase crystallization and carbon templating to synthesize silicalite-1/SBA-15 composites. Of these three, the method involving a fully developed mesoporous carbon structure offers clear advantages over the other two. The final products are composites, rather than a single phase of fully crystalline mesoporous zeolite.

This chapter is based on the following publications:

Jia Wang, Ajayan Vinu, Marc-Olivier Coppens, “Synthesis and Structure of Silicalite-1/SBA-15 Composites Prepared by Carbon Templating and Crystallization”, J. Mater. Chem.., 17 (2007) 4265-4273.

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2.1. Introduction

Zeolites are crystalline, microporous aluminosilicates with important applications in heterogeneous catalysis, separation, and purification.[1] They are widely used in oil refining and petrochemical processes, as well as in fine chemical and environmental processes. The micropore system in zeolites provides excellent shape selectivity and chemical functionality.[2] However, catalytic applications of zeolites may be transport limited due to slow diffusion of reactants and desired products in the micropores (0.3-1.5 nm). Another major problem is catalyst deactivation by pore blockage, which is often caused by the formation of heavy components within the micropores [3,4]. To address these issues, industry employs various types of post-synthesis treatments of zeolites, such as steaming, which creates mesopores but also affects the intrinsic catalytic activity. Whether this improves the overall diffusion, however, has recently been debated [5-8]. The post-synthesis treatments create mesopores of rather arbitrary shape and size, which are not necessarily well connected to the outside of the particles –– a key factor influencing diffusion, and overall catalytic yield. Reducing the zeolite crystals to sub-micrometer size could also enhance diffusion. This might not always be easy since it has been reported that the micropore volume decreases when the zeolite particle size is below 100 nm, due to a less perfect crystallization [9].

The discovery of structured mesoporous materials, such as MCM-41 [10] and SBA-15 [11], presents opportunities to increase the accessibility of the zeolite active sites to large molecules, and reduce deactivation by micropore blocking. Moreover, the possibility to vary the mesopore sizes [12] should allow an optimization of diffusion, and thus catalyst performance [7]. Despite their attractive features, ordered mesoporous materials have the drawback of having relatively low acidity and poor hydrothermal stability [13,14]. If mesoporosity is incorporated in zeolites in a controlled way, the resulting materials could combine the unique acidity of zeolites with fast diffusion. But simply mixing ordered mesoporous materials with zeolites will not improve the hydrothermal stability, and diffusion will not be faster. The first attempt to combine zeolites with a mesostructure was in 1996, when Kloetstra et al. reported an overgrowth of faujasite on MCM-41.[15] Many researchers aiming for such “meso-structured” zeolites attempt the synthesis of a mesoporous material with fully crystalline walls. Despite some claims of success, are those materials really hierarchical, meso-structured zeolites?

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Silicalite-1/SBA-15 Composites by Carbon Templating and Crystallization

It has been proposed to assemble zeolite nano-particles or seeds into a meso-structured material by using supra-molecular templates.[16-21] This is a very attractive route. However, it is debatable whether the walls of the mesopores are truly crystalline zeolite, as some have suggested. Recently, Davis et al. made an important discovery concerning the mechanism of zeolite growth, which reconciles the contradicting views [22]. They proposed that the zeolite precursor solutions (MFI type in their report) are not homogeneous. Several intermediate states can contribute to the growth of zeolite crystals. The concentration of the important species remains very low during the entire growth period. Without an effective method to isolate the specific species from the precursor solution, it remains an elusive goal to achieve a meso-structured zeolitic material with fully crystalline walls by using the nano-particle assembly method. Composites, rather than hierarchically embedded structures, are very likely formed. This is not necessarily bad for practical purposes since they are equally good candidates to combine the unique zeolite acidity and improved diffusion. This explains the increasing interest in composites that contain zeolite and meso/macroporous material [23-27].

An alternative route to try obtaining uniform meso-structured zeolites is to use carbon particles or mesoporous carbons as templates, e.g., mesostructured carbons (CMKs [28]), carbon black [29], or carbon nanotubes [30]. This is one of the most reliable methods to achieve mesoporous zeolites with fully crystalline walls. Other researchers have used solid phase transformations to partially crystallize the walls of mesoporous materials [31], though it appears that the deterioration of mesoporosity is hard to avoid during zeolite crystal growth.

Here, the possibility is examined to synthesize meso-structured silicalite-1 by carrying out a combination of carbon templating and solid crystallization methods. This method may overcome some of the drawbacks appearing in each individual method. The novel route is compared with two alternatives.

2.2. Experimental

SBA-15 was synthesized as described in reference 32. Briefly, 4 g block co-polymer Pluronics P123, 15 ml hydrochloric acid (HCl, 35%), and 85 ml water were first mixed and homogenized. Then 8.5 g tetraethoxysilane (TEOS, 98%, Aldrich) was slowly added into the solution under continuous stirring. Hydrolysis was subsequently performed at 40 C for 4 h, followed by ageing in an autoclave at 110 C for 48 h. The wet solid samples were washed and dried at 60 C without calcination.

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Starting with this uncalcined SBA-15, still containing P123, three different routes were followed, as illustrated in an idealized cartoon (Scheme 1):

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1. Formation of a carbon structure in SBA-15 followed by crystallization. First, SBA-15 was calcined at 550 C for 6 h. Then, furfuryl alcohol (98%, Aldrich) with a small amount of oxalic acid (polymerization catalyst) as the carbon precursor was impregnated into the pores. The subsequent carbonization procedure was similar to the one reported by Lu et al. [28] The amount of precursor was carefully controlled according to the pore volume of the SBA-15. The solid SBA-15 and liquid carbon precursor were well mixed to ensure that the carbon precursor enters the inside of the pores. Samples were then polymerized at 50 !"#$%"&'" !"()*"+"%#,"-#./0"!#*1)$23#42)$" was performed under Ar gasflow at 850 !" ()*" 5" /" 4)" )14#2$" 62 -C composites. The following carbonization process is similar in the three different routes, i.e., this route and the two methods described below. 0.18 ml tetrapropylammonium hydroxide solution (TPAOH, 1M) was introduced into 0.15 g carbonized SBA-15/carbon composite. After 16 h at amb2-$4".)$%242)$78"9'":;"<#4-*"<#7"#%%-%"4)"4/-"7#=>;-70"?/-" crystallization was performed in an autoclave at 130 !"()*"@"4)"AB"/.

2. Crystallization of 15 with P123 still inside the structure. The calcination of SBA-15 was not performed before crystallization, in order to keep P123 inside the mesopores. 0.18 ml 1M TPAOH solution was introduced into 0.15 g of a dried sample of SBA-15/P123 composite. As in route 1, the crystallization process was performed at 130 !8"1C4"$)<"()*"5"4)"+9"/.

3. Carbonization of P123 followed by crystallization. The block co-polymer was carbonized by treating the dry SBA-15/P123 samples at 850 !"C$%-*"#$"D*"#4=)7>/-*-" for 3 h. Grey SBA-15/carbon composites were obtained. Then, the crystallization was performed as in method 1, for a time varying between 6 and 24 h.

In all samples, organic components were finally removed by combustion in air at 550 !"()*"E"/)C*70"

Powder X-ray diffraction measurements were performed on a Bruker-AXS D5005 diffractometer equipped with a Huber incident-beam CuK!1 FG" H" '0+IA" $=J" =)$)./*)=#4)*" and a Braun Position Sensitive Detector PSD-50M. X-ray scattering results were obtained using a Bruker-D8 Discover set-up. High-resolution transmission electron microscopy (HRTEM) images (Fig. 7, 8, 10) were obtained using an ultra-high-resolution high voltage TEM, Hitachi-1500, operated at an accelerating voltage of 820 kV. Images in Fig. 9 were obtained with an advanced field emission electron microscope JEOL JEM-2100F featuring

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ultrahigh resolution (0.1 nm) and rapid data acquisition. The accelerating voltage of the electron beam was 200 kV. The preparation of samples for HRTEM analysis involved sonication in ethanol for 5 min and deposition on a copper grid. Figure 11 was recorded using a Philips CM30T electron microscope with a LaB6 filament as the electron source, operated at 300 kV. Nitrogen adsorption-desorption isotherms were measured using a Quantachrome Autosorb-6B sorption analyzer. A Bruker Avance400 instrument recorded the 29Si MAS NMR spectra.

2.3. Results and Discussion

Method 1 is designed to overcome the problem of meso-structural deterioration during the solid crystallization process. A well-developed carbon structure inside the mesopores of SBA-15 should be rigid enough to sustain the force of the zeolite crystals’ growth. Thus, the growth of zeolite crystals in the walls of SBA-15 is restricted. Since there is no preparation of separate mesoporous carbons, our design requires less time as compared to using carbon replicas of SBA-15 (CMKs) as templates to synthesize meso-structured zeolites. The amorphous SBA-15 directly serves as the silica source to form zeolite crystals. A fine control of the water amount in all three methods is necessary, since during a solid-phase crystallization process water plays an important role [33]. We observed that water would significantly increase the rate of crystallization. The meso-structure of SBA-15 may deteriorate in a short time when the crystallization process goes too fast.

An important question related to the crystallization process is whether TPAOH molecules can reach the inside of the SBA-15/carbon composite? To answer this question, we checked the pore structures of the composites after the carbonization step. There are mesopores left in the structure, which are about one third of the total pore volume in the original SBA-15. The volume shrinking of the carbon materials during the carbonization process is the cause for those remaining mesopores. In other words, the complete filling of mesopores with carbon precursor solution will not result in a complete filling with carbon after carbonization. By repeating the impregnation of carbon precursor solutions a composite without mesopores could be obtained. However, we consider the mesopores left in the composite to be important, since it is these mesopores that provide the transport channels for TPAOH solution to reach the inside of the composite particles. Thus, we carefully controlled the amount of carbon precursor

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solution by using just the right amount to fill the pore volume of SBA-15 samples. It is very likely that our carbon matrix in the composite has a CMK-5 structure, which is an interconnected assembly of carbon hollow tubes [34], although this was difficult to ascertain, given the presence of silica.

Figure 1. N2adsorption isotherms of the samples obtained from method 1. Samples 24 h, 7 h and 0 h

were vertically offset by 100, 200, 300 cm3 STP g-1, respectively. The insert shows the pore size distributions corresponding to the adsorption branch of the isotherms.

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N2adsorption and desorption isotherms of the samples obtained by method 1 are shown in Figure 1. Sample 0 h (calcined, but without crystallization) has very similar isotherms to SBA-15, demonstrating that there is no significant change in mesopore structure during the carbonization and the final combustion. Because the samples were treated under high temperature it is reasonable to observe a decrease in mesopore size and BET surface area compared to an SBA-15 sample (Figure 2, 0 h sample). In the other samples, the uptake around

Figure 2. N2 adsorption isotherms of the samples obtained from method 2. The insert shows the

pore size distributions corresponding to the adsorption branch of the isotherms.

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a relative pressure of 0.6 reveals that a SBA-15 like mesopore structure was preserved after crystallization. It becomes less pronounced over longer crystallization times, indicating that the volume occupied by mesopores decreases. However, even after 48 h crystallization a significant amount of mesopores remain (0.26 cc/g, Table 1). Interestingly, some small mesopores seem to appear (around 2.5 nm according to the BJH method). As discussed by Groen and Pérez-Ramírez, this is very likely related to the fluid-to-crystalline-like phase transition of nitrogen in MFI-type zeolite pores [35], which means that they are artifacts rather than real pores. The mesopore size remains fairly stable in all the samples (Table 1, between 5 and 6 nm), though a small increase can be observed over longer crystallization times. This is an important feature, as the mesopore size is independent of the crystallization treatment. Samples obtained from methods 2 and 3 (isotherms shown in Figure 2 and 3) show a similar decrease in mesopore volume with increasing crystallization time. Compared to method 1, the meso-structures deteriorate more quickly as the crystallization proceeds. There are very few mesopores left after 6 h of crystallization in method 2 or after 12 h in method 3. This confirms that a well-developed carbon structure stabilizes the mesopores during crystallization. On the other hand, a partially developed carbon structure generated from P123 in method 3 also leads to an improved stability of the meso-structure during crystallization, when compared to method 2, though it is less stable than that of method 1.

Powder X-ray diffraction (XRD) of the samples obtained via method 1 (Figure 4a) shows the appearance of clear diffraction patterns of MFI-type zeolite over time. The increased intensity reveals that the amorphous phase (featureless diffractogram for the 0 h sample, not shown here) is gradually converted to zeolite crystals during the crystallization. Only a small amount of crystals can be detected after 24 h crystallization. The strong diffraction intensity of the 48 h sample suggests that there is substantial amount of crystalline material present. A detailed analysis of N2adsorption data (Table 1) shows the appearance of micropore volume in sample 48 h, which is a typical feature of zeolite crystals, and, thus, in agreement with XRD results. Compared with method 1, zeolite crystals appear faster in methods 2 and 3. As shown in Figures 5a and 6a, clear peaks of silicalite-1 already appear after crystallization for 6 hours in both methods. Therefore, it is very likely that zeolite crystals grow more slowly in method 1 because of the restrictions imposed by the carbon matrix.

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Figure 3. N2adsorption isotherms of the samples obtained from method 3. Samples 0 h and 12 h were

vertically offset by 50, 30 cm3STP g-1. The insert shows the pore size distributions corresponding to the adsorption branch of the isotherms.

Table 1. Textural properties of the samples obtained from method 1 corresponding to Figure 1.

Crystallization time S_BET (m2g-1) V_micro (cm3g-1) D_meso (adsorption, nm) V_meso (cm3g-1) 0 h 447 0.03 6.0 0.58 7 h 333 - 5.0 0.48 24 h 382 - 5.5 0.46 48 h 327 0.07 6.0 0.26

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Figure 4. XRD patterns (a, wide angle) and X-ray scattering patterns (b, small angle) of the samples obtained from method 1.

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Surprisingly, we observed in method 2 and 3 that the characteristic XRD pattern of silicalite-1 disappears after a certain crystallization time. In both cases, this happened after 6 to 12 h. This seems difficult to understand, since it implies that the crystals become amorphous again. Verhoef et al. reported that small crystals or nuclei of zeolites in a meso-structure easily loose their integrity during activation of the materials through calcination at a high temperature. [36] They could transfer back to amorphous silica alumina during a catalytic reaction. In our case, though the crystallization process is not a catalytic reaction, it seems possible that a similar phenomenon takes place. Another possible explanation is that TPAOH gradually decomposes at the crystallization temperature. Some of the samples were slightly yellow after the reaction, which supports this speculation. It is likely that the crystallization process goes into a direction other than the growth of zeolite crystals, because of the formation of other species from TPAOH molecules. Figure 2 and 3 indicate that the mesoporous structure has nearly completely disappeared in the last samples of methods 2 and 3 (i.e., after 12 h and 24 h respectively). Therefore, prolonged heating in an autoclave did not form zeolite crystals, but damaged the ordered meso-structure. More experiments and 13C NMR analysis [37] are necessary to elucidate this process.

In order to follow the meso-structure over time, X-ray scattering was employed at relatively low angles (Figures 4b, 5b, and 6b). The peak around 1 degree is the (100) peak related to the hexagonal ordering of the mesopores in SBA-15. Sample 0 h in method 2 also shows (110) and (200) peaks. However, after severe thermal treatment (sample 0 h of method 1) these two peaks are not easily recognized. It is clear that all three methods show a decreased ordering at the meso-scale upon increasing the crystallization time. The (100) peak of materials synthesized using method 2 decreases much faster than when method 1 or 3 is used. This reveals that the carbon structures not only help to preserve the mesopore volume, but also the mesoporous ordering. The decrease of the (100) peak over crystallization time is in a good agreement with the adsorption results.

From the N2adsorption/desorption, and the X-ray diffraction and scattering data we can conclude that method 1 provides the best control over mesoporosity and crystallization, even though this method still does not allow to independently control the growth of silicalite-1 crystals and the mesoporosity. Some samples obtained from method 2 and 3 also combine mesoporosity with a crystalline zeolite structure. However, the fast deterioration of the mesoporosity and the disappearance of the small zeolite crystals after prolonged crystallization make those methods less attractive than method 1.

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Figure 7. TEM image of the 7 h sample obtained from method 1.

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To understand the local pore structure, a few selected samples were viewed under a high-resolution transmission electron microscope (HRTEM) (Figures 7-11). Both the 7 h and 24 h samples synthesized using method 1 show ordered mesopores, which confirms the observation from X-ray scattering measurements. The 24 h sample is a composite of zeolite crystals and a mesoporous matrix; separated crystalline phases and mesoporous regions can be clearly recognized. No large crystals are observed, and the different phases are well mixed at sub-micrometer scales. The crystalline domains appear over a very short range (less than 100 nm). Figure 9a shows small crystallites, which are embedded in the mesoporous matrix. Other areas show larger zeolite crystallites, giving rise to the sharp peaks in the X-ray diffractogram. The electron diffractogram (insert) was taken over a large area of the material, and confirms the orthorhombic MFI structure, seen by X-ray diffraction. The amount of carbon in the material is less than 2 wt%, as shown by elemental analysis over a composite particle (supplementary information), demonstrating that most of the carbon was removed during combustion, although here and there some graphite could be found. Interestingly, zooming in on some of the nanocrystallites by using an ultra-high resolution TEM that is capable of achieving atomic resolution, micropores inside and near the rim of the nanocrystallites can be distinguished (Figure 9b). The diameter of the particle shown is 12 nm; this particle is representative of many more nanoparticles that are seen throughout the structure, several of which are only a few nanometers in diameter. Diagonal lines with a spacing of about 0.5 nm near the lower left rim can be observed. These suggest straight micropores, which we attribute to the straight pores along the b-axis of MFI. We stress that the interpretation of such ultra-high magnification images should be done with caution, but the image appears to point to MFI nanocrystals. Some of these nanocrystals are not so much larger than the wall thickness of SBA-15. This further suggests that parts of the walls of SBA-15 were successfully transformed into a crystalline structure, and that the growth of the crystals was restricted by the carbon framework. Despite doubts about the possibility to obtain MFI-type zeolite nano-crystals, 4-6 nm in size, under solution conditions [38,39], it therefore appears possible to obtain zeolite nanoparticles of such small size by solid-phase transformation.

A longer crystallization period (48 h), however, results in worm-like mesopores (Figure 10). Though the mesopores are retained during the crystallization process, the ordered meso-structure of SBA-15 is transformed into a disordered pore system. The TEM images are in a good agreement with X-ray scattering results. A similar transformation was reported by Trong On and Kaliaguine [31,33] in a synthesis route that resembles our method 2. We suspect that the transformation to a worm-like structure occurred during the final combustion of the carbon

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framework, since the possibility of crushing the carbon structure during crystallization is small. In both the 24 h and 48 h samples zeolitic features can be recognized by EDX. Crystalline

Figure 9. a) TEM image of the composite material (sample 24 h from method 1), showing individually embedded nanoparticles, and electron diffractogram (insert) taken over a large area, confirming the MFI particle structure. b) Embedded MFI nanoparticle, visualized by ultra-high resolution TEM. Individual zeolite atoms can be distinguished, as well as straight pores along the b-axis (parallel to the two lines).

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material appears to be homogeneously embedded in the mesoporous structure. No large individual zeolite crystals are found. It is often hard to distinguish the amorphous and crystalline phases. Method 1 produces more uniform composites than method 2 or 3. TEM images of the 3 h sample using method 2 (Figure 11) show a composite of a mesopore phase and large crystals. The hexagonal ordering of mesopores is clearly seen. An enlarged image shows that the zeolite crystalline region may be over 150 nm long. Because this is too large to fit into the SBA-15 mesopore wall, the crystals are likely growing over the mesoporous phase instead of being part of the walls. This is essentially different from the materials obtained from method 1.

Synthesis of zeolite composites with hierarchical porosity