LICENTIATE T H E S I S
Department of Engineering Sciences and Mathematics Division of Materials Science
Characterization of Some Natural and Synthetic Materials With Silicate Structures
Edwin Escalera Mejia
ISSN: 1402-1757 ISBN 978-91-7439-553-2 Luleå University of Technology 2013
Edwin Escalera Mejia Characterization of Some Natural and Synthetic Materials With Silicate Structures
ISSN: 1402-1757 ISBN 978-91-7439-XXX-X Se i listan och fyll i siffror där kryssen är
Characterization of Some Natural and Synthetic Materials With Silicate Structures
Edwin Escalera Mejia
Luleå University of Technology
Department of Engineering Sciences and Mathematics Division of Materials Science
Printed by Universitetstryckeriet, Luleå 2013 ISSN: 1402-1757
ISBN 978-91-7439-553-2 Luleå 2013
www.ltu.se
Abstract
The present thesis deals with characterization of silicate structures with a determined morphology and structure such as ordered mesoporous silica and layered silicates.
Mesoporous silica groups are amorphous solids exhibiting highly ordered pore structures with narrow pore size distributions and large surface areas. Porous materials are used in various applications such as in adsorption, separation, catalysis, molds for templating, etc.
Another interesting group of layered materials are crystal silicates with minerals of natural origin. The silicates have a structure that consists of stacked layers in which planes of oxygen atoms coordinate to cations such as Si4+, Al3+, Mg2+, Fe3+ to form two dimensional sheets.
The coordination of cations in adjacent sheets typically alternates between tetrahedral and octahedral. The properties and uses of the clays vary widely due to the differences in their structure and composition. Some important applications are paints, adsorption, intercalation, removal of pollutants from water and in ceramic industry.
The thesis consists of two parts. In the first study characterization of synthesized and functionalized ordered mesoporous silica were performed. Mesoporous silica with a large surface area on which organic functional groups are grafted was used to synthesize cobalt nanoparticles. Investigation by SEM and TEM showed hexagonal particles, with a pore size about 10 nm. The functionalization of the silica was studied by FTIR and TG/DTA techniques and the obtained nanoparticles were characterized by XRD, TEM and EDX analysis.
In the second study, an extended literature review on properties of clays is presented. Samples from three different clay deposits, Ivirgarzama (IC), Entre Rios (EC) and Uspha-Uspha (U) from Bolivia were characterized by different experimental techniques in order to assess their relevant features.
The chemical and mineralogical analysis showed that the clays consist mainly of kaolinite and illite along with quartz in different amounts. Also, certain amounts of feldspar, iron and magnesium are present in the clays and with predominance in the EC clay.
Thermal analysis (DSC/TG and dilatometer) and XRD were used to study the phase transformations and their microstructural evolution at sintering. The EC clay with a high alkali and iron content influenced both the onset of liquid formation and the onset of sintering.
Mullite is a crystalline phase that strengthens the ceramics and it was formed in all the studied clays.
Based on these results, the EC and U clays provide required characteristics that enable them for use in the fabrication of products with red tonality, especially bricks, roofing tiles and rustic floor tiles. The IC clay with relatively low iron content and with relatively good refractoriness can be used for production of firebricks and also for partially replacing kaolin and silica in white firing ceramics. Thus, the clays from Ivirgarzama, Entre Rios and Uspha- Uspha are promising raw materials and they should be considered as valuable resources for the production of building ceramics.
Preface
This licentiate thesis is a part of my PhD studies carried out at the division of Materials Science at Luleå University of Technology, Sweden.
The aim of the thesis was to characterize synthetic mesoporous silicates and some natural clays from Bolivia, which can be suitable as building materials.
The thesis is divided in two parts. The first study deals with the synthesis of mesoporous silica materials used as hard templates for synthesis of cobalt nanoparticles.
The second part is about characterisation of natural clay minerals from Bolivia and their thermal behaviour.
The thesis is compiled of the following papers:
Synthesis of homogeneously dispersed cobalt nanoparticles in the pores of functionalized SBA-15 silica
E. Escalera, M. A. Ballem, J. M. Córdoba, M-L. Antti and M. Odén Powder Technology 221 (2012) 359-364.
High temperature phase transformation in Bolivian kaolinitic-illitic clays E. Escalera, R. Tegman, M-L. Antti and M. Odén
Submitted to Applied Clay Science (2013).
Acknowledgements
First of all, I would like to express my deep gratitude to my supervisor, Associate Professor Marta-Lena Antti, and my co-supervisor Professor Magnus Odén for their guidance and valuable ideas.
I wish to express my gratitude to Dr. Ragnar Tegman, for his knowledge, good suggestions, and also for all discussions concerning to the second paper.
I would like to thank Johnny Grahn for helping using the SEM instrument.
Likewise, I want to thank all my colleagues at the Division of Materials Science.
I want to thank Roberto Soto S., Coordinator of Project UMSS-ASDI-10 in Bolivia. And also I want to thank my colleagues at Chemistry-Department, FCyT-UMSS, for all good times shared.
I am deeply indebted to my parents for their unconditional love, encouragement and support.
I acknowledge the Swedish International Development Cooperation Agency, SIDA, for financial support for this project - Non Metallic Minerals as Resources for Development of Poor Bolivian Regions.
PART ONE
Mesoporous materials SBA-15 and its application in the synthesis of cobalt nanoparticles
Contents
1. Introduction ... 2
1.1 Mesoporous silica ... 2
1.2 Uses and applications ... 3
2. Theoretical Background ... 4
2.1 Mesoporous silica SBA-15 ... 4
2.1.1 Surfactants and silica precursors ... 4
2.2 Synthesis of mesoporous silica ... 5
2.2.1 Formation ... 5
2.2.2 Hydrothermal treatment ... 6
2.2.3 Removal of surfactants ... 6
2.3 Functionalization of mesoporous silica ... 7
2.4 Metal incorporation in functionalized mesoporous silica ... 8
3. Materials and Methods ... 9
3.1 Materials ... 9
3.2 Experimental procedure ... 9
3.3 Characterization methods ... 10
3.3.1 Scanning Electron Microscopy (SEM) ... 10
3.3.2 Fourier Transform Infrared Spectroscopy (FTIR) ... 10
3.3.3 Termogravimetry and Differential Thermal Analysis (TG/DTA) ... 10
3.3.4 Nitrogen adsorption/desorption isotherms... 10
3.3.5 Transmission Electron Microscopy (TEM) ... 10
3.3.6 X-ray diffraction (XRD) ... 11
4. Summary of results ... 12
5. Conclusions ... 14
References ... 15
PART TWO
Characterization of Bolivian clay minerals Contents
1. Introduction ... 20
1.1 Ceramics in Bolivia ... 21
2. Theoretical background ... 23
2.1 Clay minerals ... 23
2.1.1 Main clay minerals groups ... 23
2.1.2 Associated minerals of clays ... 25
2.2 Decomposition and phase transformations ... 27
2.2.1 Metakaolinite and spinel phases ... 27
2.2.2 Mullite phase ... 28
2.3 Sintering ... 28
3. Materials and methods ... 30
3.1 Raw materials ... 30
3.2 Characterization methods ... 31
3.2.1 Mineralogical and phase analysis by XRD ... 31
3.2.2 Chemical analysis by ICP-AES ... 31
3.2.3 Thermogravimetry and Differential Scanning Calorimetry analysis (TG/DSC) ... 31
3.2.4 Dilatometry analysis (DIL) ... 31
3.2.5 Bulk density and open porosity ... 31
3.2.6 Scanning Electron Microscopy (SEM) ... 32
4. Results and discussions ... 33
4.1 Mineralogical analysis of raw clays (XRD) ... 34
4.2 Chemical analysis of raw clays (ICP-AES) ... 35
4.3 Morphology of clays (SEM) ... 37
4.4 Thermal analysis ... 38
4.4.1 Thermogravimetry - Differential thermal analysis (TG/DSC) ... 38
4.4.2 Dilatometry analysis (DIL) ... 40
4.4.3 Bulk density and open porosity ... 40
4.5 Microstructural evolution analysis of fired samples (XRD) ... 41
4.6 Microstructure of fired samples (SEM) ... 43
5. Conclusions ... 45
Future work ... 46
References ... 47
PART ONE
MESOPOROUS MATERIALS SBA-15 AND ITS APPLICATION IN THE SYNTHESIS OF COBALT NANOPARTICLES
Part one of the thesis contains the synthesis, surface modification and characterization of mesoporous silica, SBA-15, with two-dimensional hexagonal arrangements. The obtained mesoporous material was then used as hard template for synthesizing cobalt nanoparticles.
Scope and objectives of part one
In this study, the synthesis of the mesoporous silica SBA-15 and the subsequent surface modifications are presented. Both external and internal walls of the synthesized silica were modified through incorporation of organosilane groups, in order to enhance the synthesis of cobalt nanoparticles.
The objectives are to:
- Synthesize mesoporous silica SBA-15.
- Modify internal and external surfaces of mesoporous silica.
- Synthesize cobalt nanoparticles.
The strategy used here is to synthesize nanoparticles with narrow size distribution using functionalized mesoporous silica as hard template.
1. Introduction
In the past few decades, the increasing routine use of advanced structural materials with defined and controlled porosity has led to deeper knowledge in the field of porous solids.
Nowadays, porous materials such as porous carbon, synthetic silicate zeolites, mesoporous silicates and ordered porous metal oxides are currently being studied for a large range of applications. Examples of applications are catalysts (Liu et al., 2005), new media for pollutant removal in air and water as well as in fuel production, and gas storage materials for energy technologies (Takagi et al., 2004) (Lee et al., 1999).
Porous materials are defined as solids containing pores i.e. voids, channels or interstices. Pore architectures such as size, shape, connectivity and the nature of the pore distribution, in combination with the chemical characteristics of the pore walls determine the properties and hence the possible applications for such materials.
The pores can be classified in closed and open pores, according to their accessibility to surroundings. Materials containing closed pores are mainly used for thermal and sonic insulation due to that they are completely isolated from their surroundings (Zdravkov et al., 2007). In contrast, the open pores have connectivity in between them which makes materials with open porosity suitable for adsorption, filters, catalysis, etc. Another classification of pores is based on the pore geometry. Pores can have different shapes such as spherical or cylindrical and they can be arranged in varying structures.
Materials with high open porosity normally have a large available surface area compared to materials with no or closed porosity. The porosity is the ratio of the pore volume to the total volume of the material.
The size of the pores in inorganic materials may range from the nano-scale to the macro-scale.
According to the International Union of Pure and Applied Chemistry (IUPAC) porous materials can be classified into three classes based on their pore diameter (d), microporous d <
2 nm, mesoporous 2 ≤ d ≤ 50 nm and macroporous d > 50 nm (Sing et al., 1985) (Rouquerol et al., 1994).
1.1 Mesoporous silica
Inorganic mesoporous materials such as mesoporous silica is one of the most investigated materials due to many applications in industrial fields such as filters and catalyst supports, as hard template for nanocasting of oxide nanoparticles (Yang and Zhao, 2005) and also in biochemical applications such as drug delivery system (Giri et al., 2007).
Ordered mesoporous silica may be readily synthesized under a wide range of pH from acidic to basic conditions, and also using cationic, anionic and neutral surfactants as well as a variety of commercially available copolymers (Muth et al., 2001).
An important type of ordered mesoporous materials (M41S) were discovered by scientists at Mobil Oil Corporation, who demonstrated remarkable features of this novel type of silica, and this opened up a new field of research (Beck et al., 1992). The M41S mesoporous family are often referred to as MCM materials. The most common MCM material is MCM-41which stands for Mobil Composition of Matter No. 41. MCM-41shows a highly ordered hexagonal array with a very narrow pore size distribution.
As shown in Figure 1.1, a variety of pore structures of this type of mesoporous silica can be synthesized, such as MCM-41with two-dimensional hexagonal alignment of mesoporous channels, MCM-48 with three-dimensional cubic order, and the layered material MCM-50 (Kresge et al., 1992).
Another common mesoporous silica (SBA) was discovered by Zhao et al., (1998).
Mesoporous SBA which stands for Santa Barbara amorphous were first synthesized using triblock copolymers as surfactant. Since then, a variety of mesoporous SBA have been synthesized, such as SBA-15 which represents a two-dimensional hexagonal structure (Figure 1.1a) and SBA-16 with three-dimensional cubic structure (Figure 1.1b), etc.
Other families of mesoporous silica have also been reported in the literature, such as TDU-1 (Technische Universiteit Delft), first reported in 2001 by Maschmeyer et al., (2001), KIT, FDU and AMS (Fan et al., 2003), where the surfactants and synthesis conditions are variable.
1.2 Uses and applications
Since the discovery of M41S and later SBA ordered mesoporous materials, there has been an increasing interest in the tailoring of this materials for many potential applications such as molecular sieves, drug delivery systems (Song et al., 2005), catalysis and for use as meso- reactors, adsorption and separation of biomolecules, host-guest chemistry, templates and as electrodes in solid-state ionic devices (Ishizaki et al., 1998). Morphology, pore size and surface area of the mesoporous silica can be tuned in many ways. This can be achieved by addition of salts (Qiao et al., 2006), co-surfactants (Li et al., 2006), oil (Lettow et al., 2000), solvents such as heptane as swelling agents and also by changing the reaction conditions (Johansson et al., 2010).
This wide range of applications are due to the unique structure of the materials which exhibits a regular array of uniform pore openings, uniform pores with narrow pore size distribution, high surface area and large pore volumes.
The application of mesoporous silica has been extended for use as hard templates for the synthesizing of metal nanoparticles, nanowires, nanorods, etc. with various applied approaches of synthesizing (Wu et al., 2006). By functionalization, the properties of mesoporous silica can be finely tuned by changing the organic groups on the surface making them suitable material for instance for metal adsorption (Aguado et al., 2009).
a) Hexagonal b) Cubic c) Lamellar
Figure 1.1 Structures of mesoporous silica materials.
2. Theoretical Background
This chapter presents a literature review about mesoporous silica, functionalization of the external and internal walls and the incorporation of transition metal into the silica pores, which is presented in paper 1.
2.1 Mesoporous silica SBA-15
Since the discovery of family SBA mesoporous silica, there has been an increasing interest of these materials for use in many potential applications. This is due to their uniform pores, large surface area and high pore volume, which ensure their application in separation and adsorption processes, catalysis and for use as templates for synthesizing nanoparticles, nanowires, etc. There is a number of different types of mesoporous silica SBA reported, such as SBA-12 with a 3-dimensional hexagonal structure, SBA-11 with a cubic structure, and SBA-1, SBA-2, SBA-3, among others. The structure of mesoporous silica depends on the surfactant used (Kim and Ryoo, 1999).
One of the most studied mesoporous silica is SBA-15 (Santa Barbara Amorphous No. 15), with 2-dimentional hexagonal structure, space group (p6mm), which can be synthesized in large quantities from tetraethyl ortosilicate (TEOS) in the presence of tri-block copolymer and strong acidic media (Zhao et al., 1998). Synthesis of mesoporous silica is based on the well- known sol-gel process (Hench et al., 1990). Therefore surfactants, silica precursors, hydrothermal treatment, and removal of the surfactants are needed to form the final mesoporous material.
2.1.1 Surfactants and silica precursors
Surfactants are known as structure directing agents. Surfactants are amphiphilic molecules that are composed by hydrophilic and hydrophobic parts. The hydrophobic part is often a hydrocarbon chain (Fröba et al., 2006).
The surfactants are classified by their head group. They can be anionic, cationic, non-ionic and amphoteric surfactants. Syntheses of M41S mesoporous material employ cationic alkylammonium surfactants and cethyltrimethyl ammonium bromide (CTA+Br-). It is an example of such a cationic surfactant commonly used to synthesize MCM-48 (Kresge et al., 1992).
The most common non-ionic surfactant used to synthesize several SBA mesoporous materials such as SBA-15, SBA-16 and SBA-12, is the triblock copolymer family which is commonly called Pluronics. There exist several different pluronics (EOxPOyEOx) with different molecular weights such as F108 (EO133PO50EO133), F127 (EO106PO70EO106), and P123 (EO20PO70EO20). The non-ionic surfactants consist of hydrophilic part of poly-ethylene oxide chains (EO) and hydrophobic part of poly-propylene oxide chains (PO). The initial letter P refers to paste and F refers to flakes.
The silica precursor is another essential component in the synthesis of mesoporous silica.
Several types of silica precursors can be used for synthesis of mesoporous silica. The most common used are alkoxides, which are hydrophobic molecules such as tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS). Another alternative silica precursor could be sodium silicate which is cheaper and it is often used in combination with other alkoxides (Matos et al., 2002).
2.2 Synthesis of mesoporous silica
The synthesis of mesoporous silica involves three steps. The first step is the formation of the mesoporous structure using surfactants and silica precursors. The second step is the hydrothermal treatment at moderate temperatures. The final step is removal of surfactants from the mesoporous silica in which various techniques can be applied.
2.2.1 Formation
The micelle formation begins when the surfactant is dispersed in an aqueous solution due to the interactions between themselves. The micelles consist of a hydrophobic (PO) core surrounded by hydrophilic (EO) chains which form a corona around the core (see Figure 2.1b).
The formation of micelles is determined by the nature of the surfactant and conditions in the solution such as concentration of surfactants and temperature of the solution. When the silica precursor is added to the solution containing micelles, it hydrolyses and the silica network is formed. The transition from micelles to gel is gradually making the micelles become elongated, and this is known as polymerization of the silica (Fröba et al., 2006).
Two possible mechanisms have been proposed for the formation of these materials: a) True liquid-crystal templating (TLCT) and b) Cooperative self-assembly of the P123 and TEOS that together can develop a liquid-crystal templating phase with hexagonal arrangement, (see Figure 2.2) (Kresge et al., 1992).
a)
Pluronic: P123 EO PO EO
Silicon source: TEOS
b)
Figure 2.1 Chemical structures of (a) surfactant P123 and the silica precursor TEOS. (b) Micelle formation showing hydrophobic core (PO) and hydrophilic corona (EO).
Figure 2.2 Mechanism of the formation of mesoporous silica by surfactans. a) True liquid crystal templating, b) cooperative self-assembly.
In the TLCT mechanism it is observed that the concentration of the P123 is relatively high and that under the right conditions of temperature and pH a lyotropic liquid-crystalline phase is formed without requiring the presence of TEOS (Attard et al., 1997). In this case the polymerization process begins in the core/corona region interface. In addition the polymerization is simultaneous to the elongation of the micelles.
2.2.2 Hydrothermal treatment
The hydrothermal treatment is a good way of tuning the properties in terms of pore size, micropore volume and surface area of the synthesized mesoporous silica. These properties are dependent of time and temperature of the hydrothermal treatment (Liu et al., 2008).
The hydrothermal treatment begins when the formation step of the mesoporous silica is finished. By increasing the hydrothermal treatment temperature for instance, there is an increased pore size, reduced microporosity and as consequence also reduced surface area.
Similar effect, but not as pronounced, is obtained by increasing the hydrothermal treatment time. It was also pointed out that the hydrothermal treatment decreases the shrinkage of the silica walls upon calcination. This is an advantage in order to get large pores for determined applications such as functionalization and metal incorporation into the formed channels.
2.2.3 Removal of surfactants
The final step of the synthesis is the removal of surfactants. Surfactants are often removed by calcination, but there are alternatives such as chemical removal or decomposition by microwave activation and digestion using acids (Gallis et al., 2001).
Normally the calcination is carried out in oxidizing conditions by increasing the temperature from room temperature to 500 °C, and holding time approximately 6 hours to decompose the surfactant P123 completely (Yamada et al., 2002). The removal of surfactants by calcination produces mesoporous silica with narrow pore size distributions and highly ordered mesostructures.
In addition, the surfactants can also be removed by wet chemical oxidation using hydrogen peroxide (Yang et al., 2005), sulphuric acid, hydrochloric acid or perchlorates under acidic conditions (Cai and Zhao, 2009). Also, mixtures can be used such as hydrochloric acid and ethanol to remove surfactants. Thereby only the surfactants are removed and the pore size distribution of the mesoporous silica structure remains intact.
2.3 Functionalization of mesoporous silica
Functionalization is the addition of functional groups onto the surface of a material by chemical methods. Organic functionalization of mesoporous silica permits precise control over the surface properties and pore size for specific applications, such as chromatography, catalysis and adsorption.
Functionalization is employed for surface modification of mesoporous silica in order to achieve desired surface properties such as water repellent coatings, (hydrophobicity) (Rao et al., 2007). On the other hand, the surface modification can be used to generate a monolayer of charged groups into the pore surface and it can facilitate a uniform distribution of ion- exchanged metal precursors into the channels of mesoporous silica SBA-15 (Yang et al., 2003).
The physical properties of functionalized silica can vary within a wide range depending on the nature of the silylating agent used. There are a large number of functional groups that can be used for different applications in various materials. The most common groups of silanes to functionalize mesoporous silica are alkoxysilanes and alkylsilanes families. The incorporation of the functional groups into the mesoporous silica can be obtained either during the synthesis (co-condensation) or after the synthesis (grafting) (Zhu et al., 2002).
The functionalization process by grafting is a method based on slow hydrolysis of organic functional groups and condensation of the compounds with free OH-binding sites at the silica surface, thus forming new covalent -Si-O-Si- bonds (Rao et al., 2007).
Equation (1) shows the reaction between silanol group and organosilane agent.
4-n(≡Si–OH) + (R’O)4-nSiRn → (≡Si–O)4-n-Si-Rn + (4-n)R’OH (1)
silica surface organosilane agent surface modified silica alcohol
The reaction shows the substitution of the hydrogen in OH groups by replacement of the organic functional group of type (R’O)4-nSiRn where (R’O) is a hydrolyzable group, such as methoxy, ethoxy or acetoxy, and R is an organic functional group such as alkyl, amino, etc.
The co-condensation method is an alternative that directly incorporates functional groups into the mesoporous silica simultaneously with the synthesis of mesoporous silica. It is carried out by the co-condensation of tetraalkoxysilanes such as TEOS and TMOS with trialkoxyorganosilanes of the type (R’O)3SiR in the presence of surfactants, leading to mesoporous silica with organic anchored covalently to the pore walls. Typically, by grafting method the hydrophilic silica surface can be modified to hydrophobic surface. This process is carried out by reaction of chlorosilanes ClSiR3 with the free silanol groups on the surface of the silica (Hitzky and de Juan, 2000). However, various silylating agents can be used such as mono-, di- alkyl and tri alkyl. The most conveniently used are those with tri alkyl silylating agents, due to high surface modification. Although, a more hydrophobic silica surface has been achieved using tri alkyl agents, compared with those achieved after use of mono and di alkyl agents (Rao et al., 2007).
Both grafting and co-condensation methods can be used to functionalize the internal pores of mesoporous silica. However, the co-condensation method has a number of disadvantages. In general, the degree of mesoscopic order of the mesostructure decreases with increasing concentration of organosilane groups in the reaction mixture, which leads to totally disordered mesostructure. However, after applying the grafting method the surface silica remains intact when organosilane groups are grafted to the silica walls. Another disadvantage of the co- condensation method is that care must be taken not to destroy the organic functionality during removal of the surfactant, thus only extractive methods should be used instead such as calcination at elevated temperatures (Fröba et al., 2006).
2.4 Metal incorporation in functionalized mesoporous silica
Nanostructured materials represent a transition between the individual molecules and bulk solids. The synthesis of metals and metal oxide nanoparticles has received increased attention due to their unique physiochemical properties which make them desirable in many technological applications such as optics, magnets, electronics, catalysis, sensors (Belkacem et al., 2008), and hydrogenation processes (Takagi et al., 2004).
Within the past years, several techniques have been developed for synthesizing of nanoparticles. One of the most common routes used is based on introducing a suitable metal or metal oxide precursor into a selected template. Different kinds of mesoporous silica can be used as templates to synthesized nanoparticles. The most frequently used mesoporous silica is SBA-15 due to pore size larger than other mesoporous materials such as MCM-41and FDU-1.
For instance, metal nanoparticles of Ag, Pt, Au, Pd, nanowires of Cu, Ni (Lin et al., 2008), and also alloys of Pt-Au and Au-Ag have been synthesized in the channels of the mesoporous silica (Liu et al., 2008). Also by using SBA-15 silica as template different oxides such as Fe2O3, Co3O4 and In2O3 have been synthesized (Fuertes, 2004).
3. Materials and Methods
3.1 Materials
Pluronic P123 (EO20PO70EO20, Aldrich), tetraethyl orthosilicate (TEOS) (98%, Aldrich) and hydrochloric acid (37%, p.a., Fluka) were used for the synthesis of mesoporous silica SBA- 15. Trimethyl-chlorosilane (TMCS ≥ 99%, Aldrich), 3-aminopropyl-trimethoxysilane (APTMS 97%, Aldrich) and toluene (anhydrous 99.9%) were used for external and internal functionalization of the silica walls. Cobalt (II) sulfate heptahydrate (CoSO4·7H2O, 99%, Aldrich) was used as source of cobalt and sodium borohydride (NaBH4, 99%, Aldrich) for the chemical reduction process. Sodium hydroxide (purity ≥ 97%, p.a., Fluka), was used to dissolve mesoporous silica.
3.2 Experimental procedure
For the synthesis of mesoporous silica SBA-15 using surfactant P123 as a structure directing agent and TEOS as a silica source a detailed procedure was used developed by Sayari et al., (2004).
The schematic illustration of the synthesis of mesoporous silica SBA-15 and the sequence step followed in this work to obtain cobalt nanoparticles is shown in Figure 3.1 (I) and (II), respectively.
(I)
∞
∞ HCl sol.
P123 + HCl
Filtration and drying Aging 100˚C 24h Pluronic (P123)
TEOS
Mesoporous silica (as-SBA-15)
(II)
(b) External surface functionalization (TMCS)
(c) Removal template (P123)
(d) Internal surface functionalization (APTMS)
(e) Metal incorporation and chemical deposition (a) Mesoporous silica template
(as-SBA-15)
Figure 3.1 Schematic illustrations: (I) Synthesis of mesoporous silica SBA-15 and (II), gradual synthesis procedure of the cobalt nanoparticles.
The synthesized mesoporous silica (as-SBA-15) (a), has been used to follow the sequence of surface functionalization of the silica walls as illustrated in Figure 3.1(II). These sequence steps include the grafting of alkyl hydrophobic (-Si(CH3)3) groups on the external silica surface (b), extraction of the surfactant P123 by calcination at 300 °C for 5 hours (c), and grafting of amine (-Si(CH2)3-NH2) groups inside the pores (d).
The obtained functionalized mesoporous silica was impregnated with cobalt sulphate aqueous solution and subsequent chemical reduction by sodium borohydride aqueous solution (e).
Detailed information about the functionalization and metal incorporation procedures is addressed in paper 1.
3.3 Characterization methods
Several methods were used to characterize the structure and textural properties for the samples at different synthesis steps using SEM, FTIR, TG/DTA, N2-physisorption, TEM and XRD.
3.3.1 Scanning Electron Microscopy (SEM)
Scanning electron microscopy was used to examine the morphology and estimate the size of the synthesized mesoporous silica. The sample was placed on a carbon tape and coated with a thin layer of gold before being inserted into the microscope. The SEM images were recorded with FEI Magellan 400 field emission XHR-SEM.
3.3.2 Fourier Transform Infrared Spectroscopy (FTIR)
FTIR spectroscopy is a technique that provides information about the chemical bonds between molecules such as -OH, Si-OH, Si-O-Si, C-Si and C-H. The surface chemical modification of the mesoporous silica was studied by Fourier transform infrared spectroscopy (FTIR) with a Shimadzu FTIR-8400S spectrometer, using KBr pellets. To prepare the pellets, 0.8 mg of sample and 120 mg of KBr powder were ground and mixed to remove scattering effects, and finally the powder mixture was pressed to a pellet size, suitable for the instrument.
3.3.3 Termogravimetry and Differential Thermal Analysis (TG/DTA)
TG/DTA was used to study the decomposition of polymers of mesoporous silica before and after functionalization. The analyses were performed using a Netzsch STA 449C Jupiter instrument. The measurements were carried out in air. Approximately 20 mg of material was placed in a sintered alumina crucible and the temperature was increased from room temperature to 700 ºC at a heating rate of 10 ºC min-1.
3.3.4 Nitrogen adsorption/desorption isotherms
The specific surface area, pore size and pore volume of the samples at different synthesis steps were measured using physisorption with N2 gas. Nitrogen adsorption-desorption measurements were performed at 77 K using a Micromeritics ASAP 2020 surface area and porosity analyzer. The samples were degassed at 373 K for 9 hours before the measurement.
The specific surface area was determined by BET model (Brunauer et at., 1938), at a relative pressure (P/Po) range of 0.08-0.2; the pore size distribution was derived from the adsorption isotherm branch using KJS method (Kruk et al., 1997). Finally the total pore volume was calculated from the amount of adsorbed N2 at P/Po = 0.975.
3.3.5 Transmission Electron Microscopy (TEM)
TEM micrographs were used to investigate the mesoporous silica with cobalt nanoparticles inside the pores and the particle size distribution of the nanoparticles. TEM was performed with an FEI Tecnai G2 microscope operated at 200 kV, and the chemical composition
determined by energy dispersive X-ray spectroscopy (EDX) in the TEM. For preparing TEM samples, the material of interest was dispersed in acetone and then deposited onto carbon copper grids and allowed to dry before analysis.
3.3.6 X-ray diffraction (XRD)
Powder X-ray diffractometer (Siemens D 5000) was used for determination of the phase of crystalline cobalt structure, using Cu Kα radiation over 30° ≤ 2θ ≤ 70°.
4. Summary of results
In this study, a procedure via wet chemical process was successfully carried out to synthesize cobalt nanoparticles at room temperature by reducing cobalt sulphate heptahydrate with sodium borohydride in aqueous solution using functionalized SBA-15 mesoporous silica as a hard template. Silica template dissolution by NaOH aqueous solution resulted in well dispersed Co nanoparticles ranging in size from 2 to 4 nm.
It was shown that the synthesized mesoporous silica SBA-15 has hexagonally ordered mesoporous channels running along the length of the particles, with a pore size of about 10 nm, see Figure 4.1a. The SEM micrograph in Figure 4.1b shows particle as rod-like hexagonal shaped agglomerate with a diameter of 0.4-0.5 µm and length 1-1.5 µm.
Figure 4.1 Micrographs of mesoporous SBA-15 silica: (a) TEM micrograph showing the channel structures and pore arranged in a hexagonal order (inset). (b) SEM micrograph showing the particle
morphology size by agglomeration of various porous spheres.
Likewise, it was found that both external and internal functionalization of silica walls play a crucial role on the infiltration and reaction of the reagents in the silica framework. This process was performed step by step, as described in the experimental part of this thesis.
The external silanol groups (-OH) of the silica was then first modified with TMCS (Cl- Si(CH3)3) groups, as can be seen in Equation (2).
≡Si–OH + Cl–Si(CH3)3 → ≡Si–O–Si(CH3)3 + HCl (2) external surface hydrophobic agent silica external functionalization
Hence, a highly hydrophobic surface was achieved which proved to be sufficient to avoid formation of large cobalt particles on the outside of the silica particles. The incorporation of these groups on the SBA-15 surface has been qualitatively confirmed by FTIR analysis.
The results have shown that the absorption intensity of the Si-OH groups decreases due to replacement of H from silanols (-OH) with Si to form a new O-Si bond. On the other hand, the absorption intensity of the C-H and Si-C groups was increased due to presence of -Si- (CH3)3 groups attached on the silica surface.
(a) (b)
The surfactant P123 was efficiently removed from the pores by calcination at 300 °C without affecting the alkyl groups recently anchored to the external silica surface. It was also demonstrated by FTIR and TG/DTA results.
The internal functionalization of the mesoporous silica was successfully achieved with APTMS molecules, as can be seen in Equation (3).
3(≡Si–OH) + (CH3O)3–Si(CH2)3–NH2 → (≡Si–O)3–Si(CH2)3–NH2 + 3CH3OH (3)
internal surface silica hydrophilic agent internal silica functionalized
By anchoring amino (-Si-(CH2)3-NH2) groups inside the pore silica a stronger negative charge than silanol groups was achieved thereby enhancing the attraction of cobalt ions into the silica pores. By using aqueous solution of NaBH4 as a strong reducing agent (Özkar et al., 2005) the cobalt ions inside the pores was reduced to cobalt nanoparticles as shown in the TEM- micrographs in Figure 4.2.
Figure 4.2 TEM micrographs: (a) Mesoporous silica with cobalt nanoparticles inside the pores (b) cobalt nanoparticles on a Cu/carbon grid after silica removal, and (c) the inset, particles size
distribution histogram of the obtained nanoparticles and their Gaussian fit.
It was found that after anchoring amino groups with silanol groups on the internal pore surface, the specific surface area and pore volume decreased. Also FTIR results showed that absorption intensity of -OH is decreased even more and also a new absorption band attributed to the -C-N of the primary amine (-CH2-NH2) was observed in the FTIR spectra.
The cobalt metal nanoparticles seen as dark spots in Figure 4.2 a) are dispersed in the silica matrix. No bulk aggregation of the cobalt on the outer surface could be observed, which indicates that the cobalt is confined to the pores. The presence of ultra-fine Co nanoparticles was further supported by EDX spectra. (See Figure 6 in paper 1).
10 nm 50 nm
(a) (b)
5. Conclusions
In conclusion, the results show that highly dispersed cobalt nanoparticles was obtained via wet chemical process using NaBH4 as the reducing agent and silica SBA-15 with both external and internal functionalized surfaces as the template. The functionalization of the silica walls plays a crucial role on the infiltration and reaction of the reagents inside the silica pores. It is believed that it is possible to use the same procedure for deposition of other metals inside the mesoporous silica.
References
Aguado J., Arsuaga J., Arencibia A., Lindo M., Gascón V., Aqueous heavy metals removal by adsorption on amine-functionalized mesoporous silica, J. Hazard. Mater., 163 (2009) 213- 221.
Attard G., Goltner C., Corker J., Henke S., Templer R., Liquid-Crystal templates for nanostructured metals, Angew. Chem. Inter. Ed., 36 (1997) 1315-1317.
Beck J., Vartuli J., Roth W., Leonowicz M., Kresge C., Schmitt K. D., Chu C. T., Olson D.
H., Sheppard E. W., A new family of mesoporous molecular sieves prepared with liquid crystal templates, J. Am. Chem. Soc., 114 (27) (1992) 10834-10843.
Belkacem W., Labidi A., Guérin J., Mliki N., Aguir K., Cobalt nanograins effect on the ozone detection by WO3, Sensors Sens. Actuators, B. 132 (2008) 196-201.
Brunauer S., Emmett PH., Teller E., Adsorption of gases in multimolecular layers, J. Am.
Chem. Soc., 60 (1938) 309-319.
Cai H. and Zhao D., A mild method to remove organic templates in periodic mesoporous organosilicas by the oxidation of perchlorates, Microporous Mesoporous Mater., 118 (2009) 513-517.
Fan J., Yu C., Gao F., Cubic mesoporous silica with large controllable entrance sizes and advanced adsorption properties, Angew. Chem. Int. Ed., 42 (2003) 146-150.
Fröba M., Hoffmann F., Cornelius M., Morell J., Silica-Based Mesoporous Organic-Inorganic Hybrid Materials, Angew. Chem. Int., 45 (2006) 3216-3251.
Fuertes A.B., A general and low-cost synthetic route to high surface area metal oxides through asilica xerogel template, J. Phys. Chem. Sol., 66 (2005) 741-747.
Gallis, K.W., Landry C.C., Rapid Calcination of Nanostructured Silicate Composites by Microwave Irradiation, Adv. Mater., 13 (2001) 23-26.
Giri S., Trewyn B., Lin V.Y., Mesoporous silica nanomaterial-based biotechnological and biomedical delivery systems, Nanomedicine, 2 (2007) 99-111.
Hench LL., West J.K., The sol-gel process. Chemical Review, 90 (1990) 33-72.
Hitzky R. and de Juan F., Selective functionalization of mesoporous silica, Adv. Mater., 12 (2000) 430-432.
Ishizaki K., Komarnemi S., Nanko M., Porous materials: process technology and applications, Kluver Academic Publishers, (1998).
Jansen J.C., Shan Z., Marchese L., Zhou W., Puil N., Maschmeyer Th., A new templating method for three-dimensional mesopore networks, Chem. Commun., (2001) 713–714.
Johansson E., Córdoba J., Odén M., The effects on pore size and particle morphology of heptane additions to the synthesis of mesoporous silica SBA-15, Microporous Mesoporous Mater.,133 (2010) 66-74.
Kim M.J., Ryoo R., Synthesis and pore size control of cubic mesoporous silica SBA-1, Chemistry of Materials, 138 (1999) 487-491.
Kresge C.T., Leonowicz M.E., Roth W.J., Vartuli J.C., Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism, Nature, 359 (1992) 710-712.
Lee J., Yoon S., Hyeon T., Oh S.M., Kim K.B., Synthesis of a new mesoporous carbon and its application to electrochemical double-layer capacitors, Chem. Commun., (1999) 2177- 2178.
Lettow J.S., Han Y.J., Schmidt P., Yang P., Zhao D., Stucky G.D., Ying J.Y., Hexagonal to mesocellular foam phase transition in polymer-templated mesoporous silicas, Langmuir, 16 (2000) 8291-8295.
Li D., Guan X., Song J., Di Y., Zhang D., Ge X., Zhao L., Xiao F.S., Highly efficient synthesis of ordered mesoporous silica materials with controllable microporosity using surfactant mixtures as templates, J. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 272 (2006) 194-202.
Lin D.H., Wang Y., Sun S.G., Silver Nanoparticles Confined in SBA-15 Mesoporous Silica and the Application as a Sensor for Detecting Hydrogen Peroxide, J. NANOMATER. 2008 (2008) 1-10.
Liu J., Yang Q., Kapoor M.P., Setoyama N., Inagaki S., Yang J., Zhang L., Structural relation properties of hydrothermally stable functionalized mesoporous organosilicas and catalysis, J.
of Physical Chemistry B, 109 (2005) 12250-12256.
Liu X., Wang A., Yang X., Zhang T., Mou C-Y., Su D-S., Synthesis of Thermally Stable and Highly Active Bimetallic Au-Ag Nanoparticles on Inert Supports, Chem. Mater. 21 (2) (2008) 410-418.
Matos J.R., Mercuri L.P., Kruk M., Jaroniec M., Synthesis of large-pore silica with cage-like structure using sodium silicate and triblock copolymer template, Langmuir, 18 (3) (2002) 884-890.
Muth O., Schellbach C., Fröba M., Triblock copolymer assisted synthesis of periodic mesoporous organosilicas (PMOs) with large pores, Chem. Commun., 19 (2001) 2032-2033.
Özkar S., ZahmakIran M., Hydrogen generation from hydrolysis of sodium borohydride using Ru nanoclusters as catalyst, J. Alloys Compd. (2005) 728-731.
Qiao S.Z., Yu C.Z., Hu Q.H., Jin Y.G., Zhou X.F., Zhao X.S., Lu G.Q., Control of ordered structure and morphology of large-pore periodic mesoporous organosilicas by inorganic salt, Microporous Mesoporous Mater., 91 (2006) 59-69.
Rao A.P., Rao A.V., Pajonk G.M., Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents, Appl. Surf. Sci. 253(14) (2007) 6032-6040.
Rouquerol J., Avnir D., Fairbridge C.W., Recommendations for the characterization of Porous Solids, Pure and Applied Chemistry, 66 (1994) 1739-1758.
Sayari A., Han B., and, Yang Y., Simple Synthesis Route to Monodispersed SBA-15 Silica Rods, J. Am. Chem. Soc., 126 (2004) 14348-14349.
Song S.W., Hidajat K., Kawi S., Functionalized SBA-15 Materials as Carriers for Controlled Drug Delivery: Influence of Surface Properties on Matrix-Drug Interactions, Langmuir, 21 (2005) 9568-9575.
Takagi H., Hatori H., Soneda Y., Yoshizawa N., Yamada Y., Adsorptive hydrogen storage in carbon and porous materials, Materials Science and Engineering: B, 108 (2004) 143-147.
Wu C.W., Yamauchi Y., Ohsuna T., Kuroda K., Structural study of highly ordered mesoporous silica thin films and replicated Pt nanowires by high-resolution scanning electron microscopy (HRSEM), J. of Mater. Chem., 16 (2006) 3091-3098.
Yamada T., Zhou H., Asai K., Honma I., Pore size controlled mesoporous silicate powder prepared by triblock copolymer templates, Materials Letters, 56 (2002) 93.
Yang Ch., Liu P., Ho Y., Chiu Ch., Chao K., Higly dispersed Metal nanoparticles in Functionalized SBA-15, Chem. Mater., 15 (2003) 275-280.
Yang H. and Zhao D., Synthesis of replica mesostructures by the nanocasting strategy, J.
Mater. Chem., 15 (2005)1217-1231.
Zdravkov B., Čermák J., Šefara M., Janků J., Pore classification in the characterization of porous materials: A perspective. Central European Journal of Chemistry, 5 (2007) 385-395.
Zhao D., Feng J., Huo Q., Melosh N., Fredrickson G. H., Chmelka B.F., Stucky G. D., Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores, Science, 279 (1998) 548-552.
Zhu H., Jones D.J., Zajac J., Dutartre R., Rhomari M., Roziare J., Synthesis of periodic large mesoporous organosilicas and functionalization by incorporation of ligands into the framework wall, Chemistry of Materials,14 (2002) 4886-4894.
PART TWO
CHARACTERIZATION OF BOLIVIAN CLAY MINERALS
Part two of the thesis is about characterization of natural clay minerals from Bolivia and their thermal behaviour.
Scope and objectives of part two
Three main kaolinitic-illitic clay deposits are investigated in the present work, in order to assess their application in the ceramic industry as valuable building materials. Two of the deposits are located in the tropical region (IC and EC) and one is located in the valley (U) of Cochabamba-Bolivia.
In this study, emphasis is given to the clay characteristics and phase-microstructural changes of the clay materials taking place during the firing step.
The objectives are to:
- Assess the clay characteristics by chemical and mineralogical analysis on representative samples from each deposit.
- Study the phase transformation and microstructural evolution during firing.
- Investigate microstructural characteristics of the sintered clays.
The results will determine the feasibility of these clays for application in the ceramic industry as useful building materials for this tropical region.
1. Introduction
The general term “ceramic” includes most of the inorganic materials with ionic or covalent bonding. This broad definition of ceramic materials involves most of the elements in the crust of the earth. Clays are one of the most abundant resources that serve as a main raw material to produce traditional ceramics, pottery, white-wares, refractories and technical-engineering including ceramic matrix composites (Murray, 1999).
Traditional ceramics refers to the products commonly used as building materials or internally used in home and industry. Products such as bricks, tiles, sewer pipes, etc., are made mainly of mineral clays that can contain quartz, carbonates, feldspars, iron oxides, etc. (Grim, 2006).
Raw materials used in the traditional ceramic products are varied as well as the wide span of ceramic products available in the market. Some of these clay-based products are shown in Figure 1.1.
The phases and microstructural composition developed of fired products vary significantly according to the composing minerals in the clay materials. In addition, the physical and mechanical properties of the resulting ceramics will be determined by the microstructure and phase composition developed in the fired products (Lee and Yeh, 2008).
Furthermore, from an application point of view the final product properties also depend on the applied processing techniques, such as shaping technique, firing temperature, thermal cycle, type of kiln, etc. (Baccour et al., 2009).
Figure 1.1 A variety of white and red clay-based products.
Figure 1.2 shows the relationship between properties of the final products and the features of raw material and the applied manufacturing processes.
The nature and abundance of other minerals in the clays have significant effect on the thermal behaviour. Some of these common components, that play a fundamental role for optimum processing and hence performances of the structural final products, are feldspars for fluxing, and silica as filler material (McConville and Lee, 2005).
In order to use clay materials in a variety of applications as construction materials, often a proper mixture is prepared by mixing several types of clays to a determined application, such as wall tiles, roof tiles, bricks, etc. Thus, a detailed knowledge of their compositions and thermal behaviour is of fundamental importance to maximize an efficient use of such materials mix to be used for a given application.
It is also very important to understand the mineralogical transformations in the clay minerals, which take place during the firing process. As a result the sintered material might be crystalline or partly crystalline, porous or highly densified.
1.1 Ceramics in Bolivia
In Bolivia the ceramic industry has experienced a fast growth during the last 15 years.
Especially in those directly related to the construction such as bricks, roof tiles and glazed- tiles.
Brick production in Bolivia is about 645 million pieces per year. Cochabamba department with a production of 226 million of solid bricks is equivalent to 440 458 tons of clay processed by the second largest producer in Bolivia (EELA, 2011).
Nowadays, the internal demand of these building materials is increasing constantly due to the growth rate of the population in some tropical regions such as Ivirgarzama and Entre Rios, municipalities of Cochabamba department.
Actually, some small primitive factories are operating in these regions, producing solid (adobe) bricks from red clay for economic habitat, (Figure 1.3). However, the quality of these products is often poor. They show low mechanical resistance and durability. This is essentially due to the lack of knowledge on the chemical and mineralogical features, the processing like mixing, shaping and drying, and also the use of an adequate firing cycle.
Figure 1.2 Relationship between features of raw materials, processing and final properties.
RAW MATERIALS (Clay minerals and oxides)
FINAL PROPERTIES (Physical and mechanical) PROCESSING
(Shaping and firing)
It is quite often that the exploitations of the clays are located in the vicinity of the factories as the low value of the raw material does not allow for lengthy transport.
In this context, is important to localize the industries near to the clay deposits. The exploring and characterization of new deposits of clays found in these tropical regions is being considered an important aspect to promote the production of traditional ceramics of good qualities in an efficient way.
Figure 1.3 Old style artisanal production of red-bricks (left) and furnace (right) in Cochabamba-Bolivia.
