Synthesis of zeolites from economic raw materials
Edgar Cardenas
Chemical Technology
Department of Civil, Environmental and Natural Resources Division of Chemical Engineering
ISSN 1402-1544 ISBN 978-91-7790-467-0 (print)
ISBN 978-91-7790-468-7 (pdf) Luleå University of Technology 2019
DOCTORAL T H E S I S
Edgar Cardenas Synthesis of zeolites from economic raw materials
Synthesis of zeolites from economic raw materials
Edgar Vladimir Cardenas
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering Division of Chemical Engineering
Chemical Technology
Printed by Luleå University of Technology, Graphic Production 2019 ISSN 1402-1544
ISBN 978-91-7790-467-0 (print) ISBN 978-91-7790-468-7 (pdf) Luleå 2019
www.ltu.se
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ABSTRACT
In the present work, synthesis methods utilizing non-expensive raw materials, such as kaolin, diatomite, fumed silica and n-butylamine have been developed for the production of zeolites. Specifically, zeolite Y and ZSM-5 have been synthetized successfully from diatomite and kaolin, respectively.
The synthesis of zeolite Y was extensively studied (Paper I) in order to obtain final products with high crystallinity and a SiO2/Al2O3 ratio suitable for application as catalyst. The influence of the alkalinity (in terms of SiO2/Na2O ratio) on the outcome of the synthesis was studied. An optimum range of the SiO2/Na2O ratio (0.6 ± 0.9) was identified. Additionally, the results showed that diatomite behaves similarly to colloidal silica, given that both silica sources are forms of highly polymerized silica.
In the next step (paper V), the zeolite was ion exchanged to lanthanum form. For comparison, a commercial powder of zeolite Y was subjected to the same ion exchange process.
The two catalysts were evaluated for catalytic cracking of cumene and the performance of the catalysts was comparable. The conversion was slightly higher for the commercial powder, which probably was an effect of smaller crystals. However, the commercial zeolite resulted in higher coke formation, which is a disadvantage.
Studies on the ZSM-5 zeolite synthesis using kaolin as raw material have also been performed. In particular, the non-homogeneous aluminum distribution in ZSM-5 crystals, a phenomenon known as aluminum zoning, was carefully studied. A thorough characterization (Paper II), of the solid, gel and liquid phases was carried out. The main finding was that the gel phase consists of a silica-alumina web. The silica is preferentially consumed to form the crystals, leaving behind an aluminum-rich nanoparticle web that is consumed later, resulting in the non-homogeneous distribution of aluminum in the zeolite crystals. In addition, the influence of the gel on the crystallization (Paper III) has been studied. When the crystal surface is in contact with the gel phase, dendritic features appear at the surface of the growing crystals, and the surface become smoother as the reaction proceeds. When only liquid phase is in contact with the crystal surface, there is no presence of dendritic features at the surface of the crystals and the growth rate is higher. It was found that the aluminum-rich nanoparticle web that remained from the gel is causing the dendritic growth. The contact with the gel phase also
ii
results in a reduced growth rate of the crystals. Consequently, the aluminum-rich nanoparticles result in formation of dendrites and aluminum zoning.
Finally, studies of the microstructure of a TPA-ZSM-5 system using fumed silica as silica source have been performed (paper IV). In this system, crystallization of ZSM-5 occurs in so- called condensed aggregates (CA), as previously reported by Ren et al. (2012). The present work showed that ZSM-5 nanocrystals are formed in the core of the CAs, which is surrounded by an aluminosilicate amorphous shell. The crystallization proceeds by competitive growth of the nanocrystals at the surface of the core to a film of ZSM-5 crystals surrounding the core, but the nanocrystals of the core remain intact. Moreover, compositional analysis showed that the silicon from the liquid phase provided most of the nutrients for growth of the ZSM-5 crystals resulting in polycrystalline ZSM-5 aggregates with an Al-rich core and a Si-rich shell.
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ACKNOWLEDGEMENTS
Foremost I would like to thank God Father for giving me the ability to conduct successfully this research. (1 Thessalonians 5:18)
I would like to express my eternal gratitude to my supervisor Prof. Jonas Hedlund for the continuous support on my Ph.D. studies and research, his extensive experience and immense knowledge are priceless. His guidance helped me in the writing of this thesis. I would also like to thank to Adjunct Prof. Johanne Mouzon for his support in the beginning of my Ph.D. studies, his patience, motivation, enthusiasm and brilliant ideas that have been so valuable. In addition, I would like to express my gratitude to Dr. Saul Cabrera, for his advice, guidance and friendship that have been invaluable. Ing. Mario Blanco is also acknowledged for all the help during this time.
The Swedish International Development Cooperation Agency (SIDA) is also acknowledged for financially supporting this work. This would have not been possible without the collaboration of SIDA.
I am also grateful to the people I met during this stage of my life, to my colleagues and friends at LTU and UMSA that have contributed to the development of this research. To all the friends I made in Luleå. Thanks to all of you.
To my beloved wife Karen, thank you for all your patience, encouraging and support along these years, thank you for your suggestions and help in my research, thanks for coming with me to Luleå. You gave me the strength to finish all of this. I love you!
To my parents Johnny ( and Esthela, without your guidance and support I would have never been reached so far. To my brother Ivan, thank you for your continuous support and taking care of mom while I was far from home.
Ahora en español, A mis padres, Johnny ( y Esthela, que sin su apoyo y guía jamás habría llegado tan lejos. A mi hermano Iván, gracias por el apoyo continuo y por cuidar a mamá mientras estuve lejos de casa.
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LIST OF PAPERS
Paper I
Synthesis of zeolite Y from diatomite as silica source
Gustavo García, Edgar Cardenas, Saúl Cabrera, Jonas Hedlund, Johanne Mouzon Microporous and Mesoporous Materials 219 (2016) 29 ± 37
Paper II
Influence of the gel internal structure on Al-zoning of NBA-ZSM-5 crystals
Edgar Cardenas, Wilson Aguilar-Mamani, Ming Zhou, Jonas Hedlund, Johanne Mouzon Submitted to Microporous and Mesoporous Materials
Paper III
Dendritic growth of NBA-ZSM-5
Wilson Aguilar-Mamani, Edgar Cardenas, Jonas Hedlund, Johanne Mouzon.
Submitted to Microporous and Mesoporous Materials
Paper IV
Microstructural evolution in condensed aggregates during the crystallization of ZSM-5
Edgar Cardenas, Wilson Aguilar, Cheuk-Wai Tai, Saul Cabrera, Jonas Hedlund, Johanne Mouzon
Manuscript
Paper V
Zeolite Y from diatomaceous earth as catalytic cracking catalyst
Edgar Cardenas, Saúl Cabrera, Jonas Hedlund Manuscript
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$XWKRU¶V&RQWULEXWLRQVWRWKHSDSHUV
Contribution to the papers:
Paper I: Participation in the experimental work. Assessment and discussion about experimental work and results, participation in writing of the manuscript.
Paper II: Performed half of the experimental work. Assessment and discussion about experimental work and results, participation in writing of the manuscript.
Paper III: Participation in the experimental work. Assessment and discussion about experimental work and results, participation in writing of the manuscript.
Paper IV: Nearly all-experimental work. Assessment of results. Writing of the manuscript.
Paper V: Lead role in experimental work. Assessment of results. Writing of the manuscript.
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CONTENTS
ABSTRACT ... i
ACKNOWLEDGEMENTS ... iii
LIST OF PAPERS ... v
CONTENTS ... vii
TABLE OF FIGURES ... xi
1. INTRODUCTION ... 1
1.1. Background ... 1
1.2. Scope of the present work ... 1
1.3. Brief description of zeolites ... 1
1.1.1. Zeolite Y ... 3
1.1.2. ZSM-5 zeolite ... 3
1.2. Main applications of zeolites ... 4
1.3. Zeolite synthesis ... 5
1.4. Crystallization of zeolites ... 6
1.4.1. Crystallization stages ... 6
1.4.2. Crystallization systems ... 7
1.4.3. Crystallization Routes ... 8
1.5. Economic Raw materials ... 8
1.5.1. Kaolin ... 8
1.5.2. Diatomaceous earth ... 9
1.5.3. n-Butylamine ... 9
2. EXPERIMENTAL ... 11
2.1. Synthesis of zeolite Y ... 11
2.1.1. Diatomite leaching ... 11
viii
2.1.2. Synthesis of Zeolite Y from leached Diatomite ... 11
2.2. Synthesis of ZSM-5 zeolites ... 11
2.2.1. Kaolin and diatomite leaching ... 11
2.2.2. Preparation of the gels ... 12
2.2.3. Hydrothermal treatment ... 13
2.3. Ion Exchange ... 14
2.4. Catalytic Evaluation ... 15
2.5. Characterization ... 15
2.5.1. X-ray Diffraction... 15
2.5.2. High-Resolution Scanning Electron Microscopy ... 16
2.5.3. High-Resolution Transmission Electron Microscopy ... 16
2.5.4. Energy Dispersive Spectrometry ... 16
2.5.5. Nitrogen Adsorption ... 17
2.5.6. Inductively coupled plasma-sector field mass spectrometry ... 17
3. RESULTS AND DISCUSSION ... 19
3.1 Zeolite Y from diatomite (Papers I and V) ... 19
3.1.1. Acid treatment and characterization of Diatomite ... 19
3.1.2. Zeolite NaY crystallization ... 20
3.1.3. Catalytic tests ... 24
3.2. Synthesis of ZSM-5 zeolite from kaolin (Papers II and III) ... 25
3.2.1. Starting materials ... 25
3.2.2. Gel formation and Crystal growth ... 28
3.2.3. Aluminum zoning ... 31
3.2.4. Dendritic growth ... 33
3.3. Synthesis of ZSM-5 zeolite from fumed silica (Paper IV) ... 37
3.3.1. Stages of crystallization ... 37
ix
4. CONCLUSIONS ... 41
5. FUTURE WORK ... 43
REFERENCES ... 45
APPENDICES ... 53
APPENDIX I ... 53
x
xi
TABLE OF FIGURES
Fig. 1 Some examples of SBUs [5]. --- 2
Fig. 2 Examples of rings in framework defining the pore size [6]. --- 3
Fig. 3 (Left) Schematic representation of the FAU structure. (Right) FAU structure viewed along [100] direction [8]. --- 3
Fig. 4 From left to right: Pentasil unit, Pentasil chain, Projection along [010] direction [9], Sinusoidal and straight channels [10]. --- 4
Fig. 5 Main applications of zeolites [3]. --- 4
Fig. 6 Example of s-shaped curve illustrating the stages of crystallization as a function of time. --- 7
Fig. 7 Sketch showing the assembly of the cross-sections of the 3 amorphous carbon wafers. --- 14
Fig. 8 Sketch of an individual crystal showing the cross-sections and distribution of the Si/Al values and different regions of interest to calculate the average values shell by shell and in the entire crystals. --- 17
Fig. 9 X-ray diffractograms of (a) pristine diatomite and (b) leached diatomite. --- 19
Fig. 10 SEM images from (a) pristine diatomite and (b) leached diatomite. --- 20
Fig. 11 X-ray diffractograms of (a) Ysynth and (b) Ycom. --- 22
Fig. 12 (a) Representative SEM image of the Ysynth sample, (b) illustration of the presence of zeolite P in the Ysynth sample, (c) representative SEM image of the Ycom sample, and (d) illustration of the presence of zeolite P in the Ycom sample. --- 23
Fig. 13 X-ray diffractograms of samples after calcination at 800°C: (a) LaYsytnh and (b) LaYcom. --- 24
Fig. 14 Product distribution from the conversion of cumene by the two catalysts LaYcom and LaYsynth. --- 25
Fig. 15 X-ray diffractograms of (a) commercial kaolin (CK), (b) metakaolin (MK) and (c) leached metakaolin (LMK). --- 26
Fig. 16 SEM images of (a) CK and (b) LMK. --- 27
Fig. 17 Solid separated from LMK after 24h of aging. --- 28
Fig. 18 Crystallinity (fc) as a function of normalized time (t*) of the products prepared from (a) LMK and (b) SRS. --- 28
Fig. 19 SEM images of the gel network forming during stage I. (a) LMK at t*=0.23 and (b) SRS at t*= 0.20. - 29 Fig. 20 BET surface area of the washed gel prepared from (a) LMK and (b) SRS. --- 29
Fig. 21 SEM images of the crystals of the LMK system at different synthesis time during: (a-c) stage IIa at t* = 0.33, 0.5, and 0.67; (d-e) stage IIb at t* = 0.83 and 0.98; (e) stage III at t* = 1.17. ROI 1 and 2 are shown at higher magnification in Fig. 26c and Fig. 26d, respectively. All solids were washed. --- 30
Fig. 22 Morphology of the final product a) LMK system and b) SRS system. --- 31
Fig. 23 Micrographs of the unwashed gel at the end of stage IIa: (a) HR-TEM for the LMK system; (b) XHR- SEM for LMK system; (c) XHR-SEM for the SRS system. The dotted rectangle in the inserted low magnification image in (a) indicates where the high magnification was recorded on the gel fragment. --- 31
Fig. 24 Evolution of the Si/Al molar ratio as a function of normalized synthesis time of samples from (a) the LMK system and (b) the SRS system. Si/Al molar ratio of crystals. (-Ɣ-): Si/Al molar ratio of unwashed gel (-Ŷ-) Si/Al molar ratio of washed gel (---). --- 32
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Fig. 25 XHR-SEM images of the early crystals at stage I of synthesis in: (a) diatomite system and (b) fumed silica system. The inserted images correspond to typical final crystals. --- 33 Fig. 26 (a) HR-TEM micrograph of a growing crystal in the diatomite system at stage II of synthesis with corresponding FFT inserted; (b-c). XHR-SEM images: (b) unwashed crystal partially fractured from the diatomite system at stage II of synthesis at 165 °C; (c) and (d) higher magnification images of ROI 1 and ROI 2 in Fig. 21c, respectively. --- 34 Fig. 27 XHR-SEM micrographs recorded at 1keV landing energy showing crystals among the gel obtained in the kaolinite system at t*=0.17 of hydrothermal synthesis (stage I). --- 35 Fig. 28 Sketch illustrating the position of the carbon wafers and the gel mass in the autoclave at the end of stage I. --- 35 Fig. 29 Low and high magnification XHR-SEM images of the films grown in (a-b) region 1 of Fig. 28 and in (c- d) region 2 of Fig. 28. --- 36 Fig. 30 SEM images of cross-sections of the films obtained by hydrothermally treating (a) Region 1 of Fig. 28;
(b) Region 2 of Fig. 28. The inserted values correspond local composition obtained by EDS in terms of Si/Al molar ratios. The dashed lines indicate the approximate original position of the seed layer. --- 37 Fig. 31 SEM images of (a) a condensed aggregate at tc = 18h surrounded by WPLs, (b) fractured condensed aggregate at tc=18h and (c) interface between the core and shell of the fractured condensed aggregate. --- 38 Fig. 32 SEM images of (a) a fractured CA; (b) TEM image of the same fractured CA and selected area of electron diffraction patterns indicated by the boxes. --- 38 Fig. 33 SEM images of (a) polycrystalline aggregate of ZSM-5 at tc = 21h, (b) fracture surface of a
polycrystalline aggregate at tc=21h. --- 39 Fig. 34 SEM images of (a) a fractured CA (tc=18 h) and (b) a fractured PA (tc=21h). The values in the squares correspond to Si/Al molar ratios determined by EDS. --- 40
1
1. INTRODUCTION
1.1. Background
Bolivia has abundant non-metallic mineral deposits such as diatomaceous earth, montmorillonite, and kaolin, which have great potential to be exploited for production of materials with added value. Numerous researchers have reported the preparation of ceramics and zeolites from high-purity reagents, which however increase the cost of production.
Most zeolites used commercially are currently produced using costly chemical reagents.
Typical synthesis procedures are usually composed of sodium silicates, aluminum salts or colloidal silica in a highly alkaline media. Some synthesis mixtures may also include quite expensive organic structure-directing agents or seeding to obtain the zeolite product of interest.
Nevertheless, previous studies have demonstrated that it is possible to synthesize high- purity products from clays or other raw economic materials rich in non-metallic deposits in Bolivia. Specifically, results suggested that is possible to transform Bolivian domestic raw materials into zeolites. Thus, it is important to develop methods of production of zeolites of industrial interest from economic Bolivian raw materials.
1.2. Scope of the present work
The scope of the present work was:
- To develop synthesis methods for the production of zeolite Y catalyst from Bolivian diatomite.
- To develop synthesis methods for the production of ZSM-5 zeolite from kaolin and n-butylamine, which is an economic structure-directing agent.
- To assess the mechanisms involved in the crystallization of ZSM-5 zeolite from gels that often are used for industrial production.
1.3. Brief description of zeolites
Zeolites, also denoted molecular sieves, are crystalline aluminosilicates, with a framework built of a three-dimensional system of tetrahedrons. Each tetrahedron is composed of a central atom of silicon or aluminum, with oxygen atoms at the four corners that acts as
2
bridges between the tetrahedrons. The structures comprise channels and cavities with molecular sizes [1]. When Al is incorporated into the framework, a negative charge in the tetrahedron results, which needs to be compensated by extra-framework cations that occupy channels and cavities. These cations are exchangeable [2]. When a proton is the charge compensating cation, it generates the Brønsted acidity of the zeolite [3]. The following empirical formula represents zeolites:
0[Q ൣሺ$O2ሻ[ሺ6L2ሻ\൧ Z+2 (1)
Where M is an extra-framework cation of valence n, y/x is the Si/Al ratio and w is the number of water molecules per unit cell.
Zeolites are classified according to the topology of their framework and more than 200 zeolite framework types are included in the International Zeolite Association database. The classification is given by a three-letter code, for instance, LTA for zeolite A, FAU for zeolites X and Y, or MFI for silicalite -1 and ZSM-5 zeolites.
The physicochemical properties of zeolites are governed by several factors including the framework structure, the Si/Al ratio and extra-framework cations. The framework structure results from the particular arrangement of the tetrahedrons in the structure. The tetrahedrons are denoted as primary building units, and they arrange by sharing oxygen atoms to form the secondary building units (SBU) [4] as illustrated in Fig. 1.
Fig. 1 Some examples of SBUs [5].
The special arrangement of SBUs defines the crystalline structure [4] with cavities and channels, that varies from 2 to 13 Å. Zeolites can be classified as zeolites of small, medium, large and extra-large pores according to the number of oxygen atoms in the ring 8, 10, 12 or
>12, respectively, as illustrated in Fig. 2. The zeolites studied in this work were zeolite Y and ZSM-5, with 12 and 10 rings, respectively.
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Fig. 2 Examples of rings in framework defining the pore size [6].
1.1.1. Zeolite Y
Zeolite Y has a FAU type framework with 12-ring and consequently, it is a zeolite with a relatively large pore size of 7.4 Å. Zeolite Y has per definition a Si/Al ratio higher than 1.5, meanwhile its polymorph denoted as zeolite X has a Si/Al ratio between 1 ± 1.5 [7]. This zeolite is composed of the combination of two SBUs, the sodalite cage (also known as ȕ-cage) that links to the double six ring (D6R), forming the super cage with a 13 Å diameter (Į-cage). The structure of FAU zeolite is illustrated in Fig. 3.
Fig. 3 (Left) Schematic representation of the FAU structure. (Right) FAU structure viewed along [100] direction [8].
1.1.2. ZSM-5 zeolite
ZSM-5 zeolite belongs to the MFI framework and it is a 10-ring zeolite of medium pore size. ZSM-5 zeolite has a Si/Al ratio ranging between 10 and 200, and it has a polymorph of pure silica called silicalite-1. Pentasil units build the structure of ZSM-5. The combination of such units results in the ZSM-5 structure with two types of channels, sinusoidal and straight, with pore apertures of 5.1 x 5.5 Å and 5.3 x 5.6 Å, respectively. The structure of ZSM-5 zeolite is shown in Fig. 4.
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Fig. 4 From left to right: Pentasil unit, Pentasil chain, Projection along [010] direction [9], Sinusoidal and straight channels [10].
1.2. Main applications of zeolites
Fig. 5 illustrates the three main applications of zeolites, catalysis, gas adsorption and ion exchange.
Fig. 5 Main applications of zeolites [3].
Catalysis
Catalysis is the most important application of zeolites in industry. It takes advantage of the zeolite framework with well-defined pore and its acidity. The largest application is the Fluid Catalytic Cracking (FCC) process, which is used for production of gasoline from heavy oil through cracking [11]. Zeolite Y is the key component in the FCC catalysts, due to it pore size,
5
and high activity provided by acid sites when is ion exchanged with rare earth or hydrogen cations. In addition, ZSM-5 zeolite is used extensively as catalyst in the petrochemical industry e.g. in xylene isomerization and methanol conversion [12].
Gas Separation.
Adsorption is a general phenomenon, which occurs when a gas or liquid (fluid) is in contact with a solid. A fraction of the fluid is retained and concentrated at the surface of the solid [13]. Zeolites are widely used in adsorption-related applications. Gas separation through adsorption may be achieved via the strength of adsorption (selective adsorption) or rate of adsorption (molecular sieving by size and shape) [14]. Examples of gas separation by selective adsorption using zeolites are the selective uptake of CO2 over N2 [15,16] and separation of CO2 from natural gas or synthesis gas [17]. An example of gas separation by molecular sieving using zeolites is the separation of xylene isomers [18].
Ion exchange
The ions held in channels and cavities of zeolites are exchangeable and zeolites can take up cations from mixtures with other cations [19]. For instance, zeolite A is applied industrially as detergent builder to replace phosphate ions for water softening. The zeolite takes up calcium ions from water, and exchanges them with sodium ions, which makes the water soft. The success of zeolite A as detergent builder is due to several properties, which include a high and fast calcium uptake, good dispersibility, low sedimentation tendency and low abrasiveness. In addition, the zeolite A when used as a detergent builder must have high brightness and should be free of impurities (appearance of other phases such as sodalite); it should have a small particle size (around 4 μ m) and narrow particle size distribution. Additionally, zeolite crystals should not have sharp edges [20].
1.3. Zeolite synthesis
In general, zeolites synthesis involves hydrothermal crystallization from an aqueous aluminosilicate synthesis gel at high pH. For preparation of the gel, suitable sources of silica and alumina are first dispersed in a solvent (usually water), mixed with a mineralizing agent and a structure-directing agent (SDA) if needed, forming a homogeneous gel. The gel is then heated, usually at temperatures below 200 °C. The reaction time can vary from few hours to several days. Finally, the solid products are separated, often by filtration [2,3].
The role of each component in the synthesis gel is as follows:
6
Silica and alumina provide the SiO44- and AlO45- tetrahedrons that are theprimary building units for the zeolite.
The role of the mineralizing agent is to increase the solubility of the silica and alumina species [21]. Zeolite synthesis is usually performed at high pH values using OH- ions as mineralizing agent. In some cases, F- ions were also used as mineralizing agent [22,23].
The replacement of OH- by F- allows the zeolite synthesis at acidic or near neutral pH values [24].
The structure-directing agents have several distinct roles. The most important is the structuring effect, i.e. the ability to direct the synthesis to a certain zeolite structure.
Moreover, these molecules can contribute to the high pH of the reaction mixture and finally, the SDA can function as a charge-compensating ion. The SDAs are usually organic molecules such as tetrapropylammonium bromide (TPABr), tetrapropylammonium hydroxide (TPAOH) or crown ethers that may be very expensive.
However, alkali cations may also serve as SDA in some cases.
The nature and purity of the components can have an important impact on the products obtained after synthesis [25]. Therefore, researchers have investigated several sources of silica and alumina. Numerous sources of silica for the synthesis of zeolites have been investigated, and the most common reported are water glass [26], fumed silica [27], and colloidal silica [28].
In the same way, some common sources of alumina used in zeolite synthesis are sodium aluminate or aluminum sulfate [29].
1.4. Crystallization of zeolites
1.4.1. Crystallization stages
Zeolite crystallization processes are generally illustrated by a characteristic S-shaped curve [30], as shown in Fig. 6. The crystallinity (wt. % of crystalline material), determined by distinct methods e.g. X-ray diffraction, plotted as a function of reaction time gives a representation of the evolution of zeolite crystallization. Several important stages of crystallization such as induction time, nucleation, crystal growth and ripening have been deduced from the curve. Crystallization curves have been reported by a numerous researchers, some examples are given in the literature list [31±33].
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Fig. 6 Example of s-shaped curve illustrating the stages of crystallization as a function of time.
For instance, a remarkable study was recently reported by Ren and coworkers [27]. In this work, fumed silica and sodium aluminate as silica and alumina sources, respectively, and TPABr was used as SDA. The authors identified three main stages during the crystallization of ZSM-5 in this heterogeneous system. During Stage I, the temperature rises to 170°C causing the dissolution of fumed silica (primary amorphous phase), which together with aluminate ions result in the formation of the gel phase (secondary amorphous phase). During Stage II, further dissolution of the gel phase leads to the formation of individual entities denoted by the authors as worm-like particles (WLPs), which remain without any substantial change until the beginning of Stage III. During Stage III, the WLPs start to aggregate and form a third amorphous phase, referred to as condensed aggregates (CAs). Nuclei form in the CAs, which leads to the crystallization of polycrystalline aggregates (PAs) of ZSM-5 zeolite. The stages I and II correspond to the induction period and the stage III corresponds to crystal growth as illustrated in Fig. 6.
1.4.2. Crystallization systems
Generally, there are two types of crystallization systems used for zeolite synthesis.
8
(i) Homogeneous system DOVR UHIHUUHG DV ³FOHDU VROXWLRQ V\VWHP´ LQ ZKLFK WKH
particles in solution are in the colloidal or sub-colloidal dimensions, thus not visible by the naked eye.
(ii) Heterogeneous system, in which a gel phase is formed upon mixing of the reactants [34]. In the present work, zeolite synthesis from heterogeneous systems has been investigated, because such systems are usually used for industrial preparation of zeolites [20,35].
1.4.3. Crystallization Routes
As mentioned earlier, zeolites are crystallized from aluminosilicate gels and several crystallization mechanisms have been proposed for these systems. Two extremes for the zeolite crystallization mechanisms have been proposed by Davis and Lobo [24]. The first mechanism is liquid phase ion transportation, which involves the diffusion of aluminate, silicate, and/or aluminosilicate species from the liquid phase to the nucleation site for crystal growth. The second mechanism is the solid phase transformation, suggesting that the crystallization occurs via a reorganization of the gel phase.
1.5. Economic Raw materials
More cost-effective zeolite synthesis has attracted the interest of researchers during the last decades. Consequently, the number of research papers reporting economic raw materials for the production of zeolites has increased. These economic raw materials includes but are not limited to kaolin [36], diatomite [37] and fly ash [38]. In the present work, kaolin and diatomite have been studied extensively for the production of ZSM-5 and zeolite Y, respectively. Also, the organic SDA may be quite expensive, but an economic alternative is n-butylamine, which may be used for the synthesis of ZSM-5 zeolite [39].
1.5.1. Kaolin
Kaolin is a clay material with low concentration of iron impurities, and consequently with a white color. Kaolinite is the main component of kaolin with an approximate composition of
$O6L2 2+ , and other compounds if present represent impurities or adsorbed compounds [40]. Kaolinite in its natural form is not reactive for zeolite crystallization, thus activation of kaolinite is needed prior to synthesis. The activation of kaolinite can be achieved by calcination above 550°C, which results in elimination of structural water and formation of metakaolin [41].
9
$O6L2 2+ &ሱۛۛሮ $O2 6L2+2 (2)
Metakaolin has a Si/Al ratio of approximately 1, which is suitable for the synthesis of zeolites with a low Si/Al ratio such as LTA [42]. However, there are zeolites that require higher Si/Al ratios in the reaction mixture, such as zeolite Y or ZSM-5 zeolite. Therefore, the introduction of additional silica [43] or dealumination of the metakaolin may be needed [44].
Kaolin has been reported as silica source in zeolite preparation in several research papers, and some examples are given in the literature list [36,45,46].
1.5.2. Diatomaceous earth
Diatomaceous earth, or simply diatomite, is a siliceous rock with sedimentary origin, mainly formed by fragments of skeletons of diatom algae. Diatomite originates from deposition of fossilized diatoms in seas or lakes [47]. The main components of the siliceous armor is amorphous silica in a polymeric state [48]. Diatomite is abundant and exploited at low cost.
Thus, it has been used in several applications such as catalytic or biological support, functional filler, filter aid and adsorbent [49]. Consequently, diatomite raises as a suitable and low-cost source of silica for zeolite synthesis. Diatomite used as silica source for zeolite synthesis, has been reported by several research papers, some examples are given in the literature list [50±52].
However, impurities such as iron, potassium or calcium might be present in the diatomite.
Therefore, purification methods [37] may be needed prior to utilization of diatomite as silica source in zeolite synthesis.
1.5.3. n-Butylamine
The organic compound n-butylamine (NBA) is a colorless liquid, which however turns yellow upon storage in air. It is one of the four butane amine isomers. It is known to have the ammonia-like odor common to amines [53]. NBA is an important chemical raw material and intermediate in organic synthesis, widely used in industry, agriculture and medicine [54], and it also has reported used as a low cost SDA in zeolite synthesis [39,55]. According to Sun et al.
[39] the cost of NBA is only around 10 to 30% of the cost of more common SDAs, such as TPAOH or TPABr, often used in the synthesis of ZSM-5 zeolite [27,29].
10
11
2. EXPERIMENTAL
2.1. Synthesis of zeolite Y
2.1.1. Diatomite leaching
Samples of diatomite collected from the Llica zone (Potosi, Bolivia) were used as alumina and silica sources for the synthesis of zeolite Y. The diatomite was leached in H2SO4 due to the presence of impurities by the following procedure.
Crushed diatomite was leached with a 6M H2SO4 solution (solid to liquid ratio 1:10) in a Teflon lined autoclave, in a pre-heated oven at 100 °C during 24 h. Then, the solid was separated by filtration and rinsed with water until the washing water reached a pH close to 7. The leached diatomite was then dried in air at 100 °C overnight.
2.1.2. Synthesis of Zeolite Y from leached Diatomite
Leached diatomite was used as aluminosilicate source for zeolite Y synthesis. In addition, sodium hydroxide (NaOH, Merck, p.a. 99%) was used as mineralizing agent, and aluminum sulfate octadecahydrate (Al2(SO4)3*18H2O, Riedel-de Häen, p.a. 99%) was used to adjust the SiO2/Al2O3 ratio of the synthesis mixture. Synthesis suspensions with molar compositions of XNa2O: Al2O3: YSiO2: WH2O with X varying in the range 4.4 - 22, Y varying in the range 11 - 25 and W varying in the range 176-880 were prepared by the following procedure. A clear solution was prepared by dissolving NaOH and Al2(SO4)3 in water. In the next step, leached diatomite was added to the solution under stirring. Finally, the suspension was aged 24 h under stirring at room temperature.
2.2. Synthesis of ZSM-5 zeolites
2.2.1. Kaolin and diatomite leaching
Kaolin (Riedel-de Haën, proanalysi) and diatomite were used as silica source for the synthesis of ZSM-5 crystals. In order to achieve a suitable SiO2/Al2O3 ratio for the preparation of zeolites, the kaolin was calcined and leached in HCl as described below.
Kaolin was calcined in air in a crucible at 750 °C during 2 hours with a heating rate of 10
°C/min to produce metakaolin. Metakaolin and a 3M HCl solution were mixed (solid to liquid
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ratio 1:17) in a round bottom flask during 0.2 h to homogenize the dispersion, then it was heated under stirring in reflux at 115 °C in a pre-heated oil bath during 2.5 h. Finally, the solid was recovered by filtration and rinsed with water until the washing water pH was close to 7.
Diatomite was not calcined, yet it was subjected to the same acid leaching treatment.
2.2.2. Preparation of the gels
ZSM-5 from leached diatomite
Synthesis suspensions for growth of ZSM-5 crystals were prepared using leached diatomite (LD) as silica source, NaOH as mineralizing agent and n-butylamine (NBA, Sigma Aldrich, 99.5 %) as structure-directing agent as follows. A clear solution was prepared by dissolving NaOH in water and adding NBA dropwise under stirring. In the next step, LD was added to the NBA-NaOH solution. Finally, the suspension was aged 24 h at room temperature, with a final gel molar composition of 5.9Na2O: 4.7NBA: Al2O3: 44SiO2: 990H2O.
ZSM-5 from Leached Metakaolin
Synthesis suspensions to obtain ZSM-5 crystals were prepared using leached metakaolin (LMK) as silica source. A clear solution was prepared by dissolving NaOH in water and adding NBA dropwise under stirring. In the next step, LMK was added to the NBA-NaOH solution.
Finally, the suspension was aged 24 h at room temperature, with a final gel molar composition of 5.9Na2O: 4.7NBA: Al2O3: 33SiO2: 990H2O.
ZSM-5 from Silicon Rich Solution
To reduce the impurities of the LMK, a solution rich in silicon was prepared from LMK as follows. A suspension was formed by mixing the LMK with distilled water, NaOH and NBA.
The molar composition of the suspension was 5.9Na2O: Al2O3: 4.7NBA: 303iO2: 990H2O.
After 24h of stirring at room temperature, the suspension was separated by centrifugation and filtration using a 0.1μm pore filter. The liquid fraction, a solution rich in silicon, is hereinafter abbreviated SRS. This solution does not contain the quartz, orthoclase and muscovite impurities that are present in the LMK.
A new synthesis suspension was formed by aging the SRS with NaAlO2 (Sigma Aldrich, technical grade, anhydrous powder), during 24 h at room temperature, resulting in a gel with a molar composition of 15. 3Na2O: 4.7NBA: Al2O3: 60SiO2: 2400H2O.
13 ZSM-5 from fumed silica
Synthesis suspensions to obtain ZSM-5 crystals were also produced using fumed silica (Aldrich, powder 0.007 μm) as silica source. Two kind of suspensions were prepared either using NBA or tetraSURS\ODPPRQLXPEURPLGH 73$%U0HUFN DV6'$
Suspension 1: A clear solution was prepared by dissolving NaOH, NaAlO2 and NBA in water. In the next step, fumed silica was added under stirring. Finally, the suspension was aged 24h at room temperature with a molar composition of 5.9Na2O: Al2O3: 4.7NBA: 33SiO2: 990H2O.
Suspension 2: A clear solution was prepared by dissolving NaOH, NaAlO2 and TPABr in water. In the next step, fumed silica was added under stirring. Finally, the suspension was aged 1h at room temperature with a molar composition of 12.5Na2O: 8TPABr: Al2O3: 60SiO2: 4000H2O.
ZSM-5 films
Amorphous carbon wafers (HTW Hochtemperatur-Werkstoffe GmbH, Tierhaupten, Germany) of 20 u 10 u 1 mm3 were used for seeded growth of zeolite films. The method for the preparation of the silicalite-1 nanocrystals and seeding developed by Hedlund et al. [56]
was followed. To summarize, an aqueous solution of a cationic polymer was used in order to attach the seeds to the carbon wafers. The wafers were rinsed with a 0.1 M ammonia solution to remove the excess of polymer and seeds after the first and second immersion steps, respectively. In the next step, films were grown by hydrothermal treatment as described below.
The same synthesis suspension used for LMK system was used.
2.2.3. Hydrothermal treatment
After aging the synthesis suspensions, hydrothermal treatment was carried out in Teflon lined stainless steel autoclaves for different times in an oil bath kept at 100 °C for zeolite Y (90°C in selected experiments) and 165°C for ZSM-5 (170 °C in selected experiments). After different synthesis times, the crystallization process was stopped by quenching the autoclave with cold water. Products obtained from the zeolite Y and ZSM-5 zeolite from fumed silica systems were separated by filtration and washed until the wash water reached a pH close to 7.
In the case of the ZSM-5 films, growth of films was performed by introducing the seeded wafers held by a Teflon stand into the synthesis suspension inside the autoclave. Three seeded
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wafers were assembled in the manner described in Fig. 7 and held in the upright position by a special PTFE holder placed in the autoclave.
Fig. 7 Sketch showing the assembly of the cross-sections of the 3 amorphous carbon wafers.
Washing effect on the gel phase
The supernatant liquid from the samples of the LMK and SRS systems were carefully separated and filtered through a syringe filter with 0.2 μm pore size for further analysis. In addition, a set of solid products obtained from the LMK and SRS systems were dried at room temperature during 48 h after hydrothermal synthesis, without washing. Furthermore, to study the effect of washing on the solid phase, another set of samples were directly washed after hydrothermal treatment. Finally, the unwashed samples were subjected to a post-washing treatment in water to compare with washed samples.
Partially fractured surfaces of crystals
To obtain partially fractured surfaces of crystals, tablets with a diameter of 10 mm were first prepared by pressing the powder in a die using a manual hydraulic press under a pressure of 5 tons. In the next step, fracture surfaces were obtained by breaking the tablets.
2.3. Ion Exchange
Selected samples from the zeolite Y system were ion exchanged by partial modification of two methods [57,58]. To summarize, the NaY sample was first converted to the NH4Y form by ion exchange. The following procedure was used; the powder sample was mixed with 1.5 M (NH4)2SO4 solution (liquid to solid ratio 7.5:1) under reflux at 100 °C during 1h. The powder was separated from the solution by filtration. After a second ion exchange, the suspension was filtered and washed until the assay for sulfate ions was negative. Finally, the sample was dried overnight at 100 °C. The LaNH4Y form was obtained by ion exchange of the NH4Y sample
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with a 0.6 M La (NO3)3 solution (liquid to solid ratio of 10:1), under reflux at 100 °C during 1 h. The powder was separated by filtration. After second ion exchange, the suspension was filtered and washed until the assay for nitrate ions was negative. The sample was then filtered and dried overnight at 100 °C. For comparison, commercial zeolite Y (Akzo Nobel CBV100- LT) was subjected to the same ion exchange process.
2.4. Catalytic Evaluation
Cumene (C6H5CH(CH3)2, Merck, 99%) was used as feedstock for the catalytic experiments. The catalysts were evaluated using a Zeton Fluid Catalytic Cracking Micro Activity Test (FCC MAT) unit. The catalysts were first activated in dry air during 6h at 650°C.
Then, cumene was injected by a syringe pump with a catalyst/oil ratio of 2 and the catalytic reaction was performed at 460 °C. The liquid products were condensed and analyzed by GC MS/FID in a Shimadzu GCMS ± QP2010 ultra instrument, with an Agilent DP Petro column and the gas products collected were analyzed by GC-FID in a Varian CP-3800 instrument. The amount of coke on the catalyst was determined in accordance with the ASTM D-3907 standard, where regeneration gases are passed through a reactor containing CuO to catalyze the oxidation of any CO to CO2, after which the amount of CO2 is analyzed by an IR cell. Conversion is defined as moles of cumene reacted/moles of cumene fed. The molar yield is defined by moles of product/moles of cumene reacted.
2.5. Characterization
2.5.1. X-ray Diffraction
The mineralogical composition of the raw materials as well as for the zeolite products was determined by X-ray diffraction (XRD) using a PANalytical Empyrean X-ray Diffractometer equipped with a PIXcel3D detector and Cu LFF HR X-ray tube and a graphite PRQRFKURPDWRU7KHLQYHVWLJDWHGșUDQJHZDVWRDWDVFDQQLQJVSHHGRIV7KH
X-ray tube was operated at 45 kV and 40 mA.
The degree of crystallinity of zeolites synthetized was calculated by comparison of the area under characteristic peaks in a selected 2ș region after background removal following the Van Hoff method [59]. For zeolite Y products, a commercial zeolite Y sample (Akzo Nobel CBV100-LT) was used as a standard and the peaks used for estimation of crystallinity were in the 2ș region between 30 and 32°. The standard for ZSM-5 products was a ZSM-5 zeolite
16
synthetized from silicic acid and TPAOH following the method reported by Lechert and Kleinwort [60] and the peaks used for estimation of the crystallinity were in the 2șUHJLRQ between 22 and 25°.
2.5.2. High-Resolution Scanning Electron Microscopy
The morphology of the raw materials and products was investigated with extreme high resolution ± scanning electron microscopy (XHR-SEM) using a Magellan 400 instrument (FEI Company). The electrons in the beam had a landing energy of 1 or 3 keV. A bias voltage of -2 kV was applied on the sample stage when a landing energy of 1keV was used.
2.5.3. High-Resolution Transmission Electron Microscopy
High-resolution transmission electron microscopy (HR-TEM) characterization was carried out using a JEOL JEM-2100F TEM instrument operating at an accelerating voltage of 200 kV. The samples were prepared by crushing the solid in a mortar followed by dispersion in propanol for 2 min by ultra-sonication. Finally, one drop of the suspension was placed and dried on a holey carbon film supported by a TEM copper grid.
2.5.4. Energy Dispersive Spectrometry
For zeolite Y products, the local composition was determined by energy dispersive spectroscopy (EDS) using the Magellan SEM instrument, equipped with an X-Max 80mm2 X- ray detector (Oxford Instruments).
For ZSM-5 zeolite products, the local composition was determined by EDS using a Carl Zeiss Merlin SEM instrument, equipped with an X-Max 50mm2 X-ray detector (Oxford Instruments). The Si/Al ratio of cross-sections of films was measured using an acceleration voltage of 3.5 kV.
EDS measurements of crystals grown in the LMK system were performed at locations distributed throughout the (010) face of crystals as illustrated by the sketch shown in Fig. 8. As illustrated in the Fig. 8, the distribution of the Si/Al ratio along the average semi-axis of the flat tablet-shaped crystals with elliptical base was first determined as a function of distance from the crystal center. Subsequently, this data was converted to Si/Al ratio as a function of synthesis time by using the growth rates for the length and thickness directions determined in our previous work [61]. EDS measurements in the SRS system were performed in the center of the (010)
17
face of the crystals grown for determined synthesis times. EDS measurements on the gel walls were performed as detailed elsewhere [62].
Fig. 8 Sketch of an individual crystal showing the cross-sections and distribution of the Si/Al values and different regions of interest to calculate the average values shell by shell and in the entire crystals.
2.5.5. Nitrogen Adsorption
Nitrogen adsorption data was recorded at -196 °C (77 K) using a Micromeritics ASAP 2010 instrument. The surface area of the sample powders was estimated by using the Brunauer- Emmett-Teller (BET) method.
2.5.6. Inductively coupled plasma-sector field mass spectrometry
The chemical composition of the raw materials was obtained by the use of inductively coupled plasma-sector field mass spectrometry (ICP-SFMS) analysis. For this purpose, 0.1 g sample was melted with 0.375 g of LiBO2 and dissolved in HNO3. Loss on ignition (LOI) was determined by heating the sample to 1000 °C.
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3. RESULTS AND DISCUSSION
3.1 Zeolite Y from diatomite (Papers I and V)
3.1.1. Acid treatment and characterization of Diatomite
The synthesis of zeolite Y was performed using purified diatomite as silica source. Table 1 shows the chemical composition of the pristine and leached diatomite, respectively.
Table 1 Chemical Composition determined by ICP-MS of pristine and leached diatomite.
Weight % Molar ratio
SiO2 Al2O3 CaO Fe2O3 K2O MgO SO4 Na2O SiO2/Al2O3
Pristine Diatomite 76.4 5.8 0.8 1.0 1.6 2.9 1.4 10.0 22.5 Leached Diatomite 95.6 3.1 0.3 0.1 0.3 0.1 0.1 0.3 53.2
The ICP results show that pristine diatomite is mainly composed of silicon, sodium and aluminum in that order of abundance. However, impurities such as potassium, iron, magnesium, and sulfur are also detected. The high content of sodium is expected since the sample was taken from the surroundings of the Uyuni Salt Lake. Acid leaching not only reduced the amount of impurities but also reduced the amount of aluminum.
Fig. 9 X-ray diffractograms of (a) pristine diatomite and (b) leached diatomite.
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X-ray diffractograms of pristine and leached diatomite are shown in Fig. 9. The data shows that the main phases in the pristine diatomite are NaCl and amorphous silica, with gypsum, plagioclase, quartz and muscovite as impurities. The diffractogram of the leached diatomite shows that the NaCl and gypsum were removed by the acid leaching and that the main phase is amorphous silica, accompanied by traces of quartz, plagioclase and muscovite as impurities. Thus, the acid leaching decreased the amount of impurities in the diatomite.
SEM images of typical diatomite skeletons of pristine and leached diatomite are shown in Fig. 10. No significant differences are observed for the two samples.
Fig. 10 SEM images from (a) pristine diatomite and (b) leached diatomite.
3.1.2. Zeolite NaY crystallization
Extensive studies of crystallization of zeolite Y from leached diatomite were performed, in order to obtain a zeolite Y product suitable as catalyst in the catalytic cracking reaction.
Optimization of the synthesis parameters was performed.
Alkalinity optimization
Since the leached diatomite was mainly composed of amorphous silica, dissolution of silica at sufficient alkalinity was required for the subsequent crystallization of zeolite Y.
Experiments keeping the synthesis conditions in terms of silica/alumina ratio (SiO2/Al2O3 = 11), aging time (24h), aging temperature (room temperature), reaction time (48h), and reaction temperature (100°C) were carried out, while varying only the alkalinity (in terms of SiO2/Na2O ratio) from 0.4 to 2.0. These experiments showed that a SiO2/Na2O ratio lower than 0.6 was not enough to dissolve the amorphous silica to produce zeolite Y, giving amorphous products and a SiO2/Na2O ratio higher than 0.9 produced zeolite P as main product. Therefore, an optimum
