What more in Nanosized Molecular Sieves

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1 Universiti Sains Malaysia From the SelectedWorks of Eng-Poh Ng 2007 What more in Nanosized Molecular Sieves Eng-Poh Ng, University of Caen, France Gerardo Majano, University of Caen, France Louwanda Lakiss, University of Caen, France Svetlana Mintova, University of Caen, France Available at:

2 XIV Forum Zeolitowe, Kocierz, 2007 Polskie Towarzystwo Zeolitowe What more in nanosized molecular sieves Gerardo Majano, Eng-Poh Ng, Louwanda Lakiss, Svetlana Mintova Laboratoire de Matériaux à Porosité Contrôlée, UMR-7016 CNRS, ENSCMu, Université de Haute Alsace, 3 rue Alfred Werner, Mulhouse, France INTRODUCTION The importance of nanoporous materials and particularly zeolites is indisputably significant, ranging from applications in petroleum refining for fuels and petrochemical processes for various chemicals to air separation and nuclear waste management. The well-defined porous structure of zeolitic materials makes them truly shape-selective materials. Besides, the hydrophobic nature of pure silica zeolites or the hydrophilic nature of aluminosilicates and their analogues (aluminophosphates, gallophosphates, titano silicates, etc.) makes these solids useful as specific adsorbents for either organic molecules or water in the gas or liquid phase. The development of nanotechnologies and their requirements for synthesizing nanoscale multi-functional materials present new fascinating goals to the modern solid-state chemistry of nanosized porous systems. Particularly the miniaturization of advanced devices requires molecular sieve crystals in an appropriate form, namely nanocrystals with controlled size, morphology and monomodal particle size distribution. To accomplish these requirements of modern technologies new innovative approaches in the synthesis and processing of porous solids are required. Actual and new applications of nanosized molecular sieves often utilize the unique spatial structuring of zeolite channel systems for novel concepts such as the stabilization of nanoscale forms of matter, size-selective chemical sensing, separation of reaction spaces, etc. Furthermore, a number of forthcoming applications depend not only on the control of pore structure and intrazeolite chemistry, but also on the ability to control the external features and morphology of the crystals. Therefore, the new developments in zeolite science will aim most of their successful applications towards the field of nanotechnology. Moreover, nanosized zeolite-type materials are of technological importance as building blocks for fabrication of thin films and layers. Further, the nanosized zeolite crystals have to be stabilized in different solvents, such as water, ethanol or acetone with variable solid concentrations in the suspensions. It is therefore also of interest to develop efficient experimental methods for analyzing the structure of nanosized zeolite particles stabilized in colloidal suspensions. This is a rather complicated task, as the zeolites have large unit cell parameters, up to 2.5 nm, thus the examination of nanocrystals sized below 10 nm, a few unit-cells approach the intrinsic limits of the conventional X-ray diffraction methods to detect a periodic atomic ordering. Due to this fact, characterization approaches such as TEM, Raman/IR/UV spectroscopies and DLS give the opportunity to further identify nanosized zeolite particles. This paper will review some of the particularities in the preparation of nanosized zeolites, their stabilization, assembling in films and other complex structures. The applications of the nanozeolites as host for incorporation of inorganic and organic 17

3 guest compounds, sorbents for water and oxidation products from lubricants will also be discussed in detail. SYNTHESIS OF NANOSIZED MOLECULAR SIEVES (ZEOLITES) Up to date, approximately 170 conventional types of zeolites have been prepared in synthetic form and about 48 are present as natural compounds [1]. In general zeolites are synthesized under hydrothermal treatment from aluminosilicate gels under a defined time and temperature. In contrast to conventional zeolites, their nanosized counterparts are formed under hydrothermal conditions parting from amorphous reactants mixed with structure directing/stabilizing agents resulting in clear solutions. To this date several types of zeolitic materials have been synthesized in the nanoscale range; a recent review summarizes all the microporous nanomaterials in colloidal form [2]. During synthesis, the crystalline materials eventually replace between 1 to 90 % of the amorphous precursor compounds, which are separated through centrifugation or micro-filtration, washed and either freeze-dried or suspended [3]. The synthesis of nanosized zeolites can be performed in conventional air-driven ovens, through reflux of clear solutions and also under microwave heating; the later method results in remarkably short crystallization times (Figure 1). In general, precursors such as orthosilanes and metal alcoholates provide reactants in molecular form which favor the synthesis of nanoporous materials [4], while the syntheses with dry inorganic sources give precursor gels and are transformed under prolonged HT treatment into standard zeolites with sizes above 1 µm. Figure 1. Trends for different factors influencing the size of zeolites during crystal growth in colloidal systems: (a) temperature, (b) synthesis time and (c) synthesis method. There are a number of publications revealing the effect of organic additives and so called templates on the stabilization or direction of crystal growth processes of molecular sieves. Generally within a given range of molecular compositions, an increase in the structure directing agent, if organic, induces a rather narrow particle size distribution of smaller particles by hindering the agglomeration of the precursor particles. However, alkali cations used as counterbalancing ions for the crystalline structure cannot be used that liberally as their influence in the crystalline phase obtained is far greater [5] due to the different interactions during the early stages of crystallization [6]. Moreover, high amounts of alkali cations induce agglomeration, faster crystallization rate and growth of bigger crystals from gel systems. 18

4 The influence of many factors on the early stages of crystallization such as dissolution of the precursor compounds, crystallization conditions, rate and type of heating are of significant importance for the size of the materials. Thus synthesis through microwave irradiation, reflux and hydrothermal treatment usually render particles with different sizes, morphologies, purity and stability (Figure 2). Figure 2. TEM photographs of nanosized zeolite crystals with different morphology and crystalline structures: (a) LTL, (b) FAU, (c) MFI and (d) new AlPO material. The emphasis on the nanosized and non-agglomerated nature of the particles limits the characterization of these materials mainly to be in the form of colloidal dispersions. Common parameters used for characterization of colloids are diameter of particles, molecular weight, and viscosity-average molecular weight. Additionally, the stability of the colloidal systems is of significant importance, thus no changes in the chemical composition and particle sizes with time have to be observed. Figure 3. DLS curves representing the change of the particle size distribution in precursor solutions resulting in (a) MFI and (b) AEI types molecular sieves after different times of hydrothermal treatment. Classical experimental tools for investigations of colloids include optical microscopy, electron microscopy (SEM, TEM), and surface force microscopy (STM, AFM). The optical light methods including IR/Raman spectroscopy, optical microscopy and light scattering are applied to colloids as solids, suspensions and selfassemblies. Any set of particles with a linear dimension between 10 Å and 1 µm is considered as a colloid disregarding chemical composition, type of structure, 19

5 degree and type of porosity. Examples representing the particle size distribution of MFI and AEI crystals in purified suspensions are shown in Figure 3. As can be seen, the evolution of the crystalline phase can be followed by DLS revealing the transformation from two classes of particles into one. Additionally the width of the DLS curves decreases substantially due to the formation of crystalline particles with a monomodal particle size distribution. One of the most important aspects of nanoporous materials is their stability in any given solvent. The stability is usually governed by intermolecular interactions such as electrostatic, van der Waals, London forces, etc. between the particles and the solvents [7]. The stability of colloidal suspensions is determined by measuring of the zeta potential values at constant ph and constant concentration. This refers to the electrostatic potential generated by the accumulation of ions at the surface of a colloidal particle that is organized into an electrical double-layer, consisting of a stern layer and a diffuse layer. Examples of different suspensions and their stability expressed with their characteristic zeta potential values are shown in Figure 4. Flocculates and/or aggregates will form if the coulomb interactions are lower than the van der Waals forces between the particles, and finally the sedimentation of particles also causes a substantial increase in the zeta potential value too. Figure 4. Typical zeta potential curves and photographs of suspensions with different degrees of colloidal stability. Generally, as a consequence of the small size of the crystalline particles, a decrease in the X-ray crystallinity and peak broadening are observed for all nanozeolites (Figure 5). As can be seen, the substantial decrease in the particles size is reflected on a change in the shape of the N 2 sorption isotherm of the materials. The typical porosity is evident for all nanosized zeolites, but also additional mesoporosity can be seen in the sample with low crystallinity and small particle size. This is due to the presence of the amorphous matrix which is the precursor for nanozeolites. Recently the interest towards a potentially large class of novel molecular sieves materials composed of diverse interconnected octahedral and tetrahedral metal-oxide polyhedra has significantly increased. Examples for such molecular sieves are tinsilicates, zirconium-phosphates, molybdenum-phosphates, and titanosilicates. The 20

6 Figure 5. TEM images (Scale bar: 20 nm), XRD patterns and N 2 sorption isotherms for silicalite-1 (MFI) samples with particle diameter of (a) 10 nm, (b) 50 nm and (c) > 100 nm. usual zeolitic traits, such as ion exchange, adsorption and catalytic properties can also be exploited and further enhanced by incorporation of different framework atoms. A considerable attention is paid to the synthesis of titanosilicates and aluminophosphates from clear precursor solutions by combinatorial screening of compositional parameters. The very preliminary results imply the possibilities to prepare titanosilicates and aluminophosphates from colloidal precursor solutions with size of the individual particles smaller than 1 µm. ASSEMBLY OF NANOSIZED ZEOLITES The development of new approaches for the preparation of zeolites in forms suitable for practical uses such as films, spheres, and fibers attracts considerable attention. The nanoscale microporous crystals can be assembled in two- and threedimensional structures either via controlled attachment on self-assembled monolayers (SAMs) followed by growth or via different patterning techniques such as spin and dip coating or microcontact printing. The assembly of nanoporous materials in films can be summarized in three main categories: a) direct crystallization of molecular sieve in films/coatings [8, 9], b) seeding method consisting of deposition of zeolite seeds followed by hydrothermal 21

7 treatment [10, 11, 12], and c) pattering deposition including dip- or spin-coating, micro-contact printing and physical vapour deposition (Figure 6) [13-15]. The most frequently used method is the in situ crystallization, where the crystals are directly grown on the substrates after pre-cleaning [16, 17]. This method does not always provide homogeneous and continuous coatings strongly bound to the substrates; also the highly aggressive hydrothermal treatment restricts the application of this method. The spin coating process is speedy, offers high reproducibility and consumes a very small amount of coating suspension. Additionally, very smooth layers on almost all kind of surfaces with and without surface modification can be obtained from zeolite suspensions containing nanozeolites (Figure 6). Besides, spin assembling allows effective control of film thickness by varying the concentration of coating suspensions and the spin-on rotation rates (Figure 7). Thus, in order to enhance the contact and adhesivity between zeolite coatings and the substrate, the surfaces can be modified for improving the hydrophilicity, surface irregularity and stability. Through these modifications the surface charge is altered, usually by covering with cationic polymers or silane layers. Figure 6. Self-assembled zeolite nanoparticles in three dimensional constructs (a), spin coated zeolite film (b) and grown film by seeded approach (c). Recently, the preparation of zeolite films and membranes by microwave assisted hydrothermal synthesis on alumina supports has been reported [18, 19]. The micropatterning of ZSM-5 monolayers on glass plates is also described as a very promising approach for deposition of nanoparticles in films with variable thicknesses and porosity [20]. The commonly used supports contain oxidizing layers and thus promote the adhesion of the films on stainless steel and aluminum alloy plates [8, 12, 21]. The novel applications of thin films are present in the fabrication of electronic and optoelectronic devices, electrocatalysis, sensors, etc. [10, 22-28]. Thus manufacturing and development of materials in the nanometer range becomes more crucial and critical for their further assembling and potential applications. A film is defined as a continuous intergrown or tightly deposited crystalline phase, while a layer describes a discontinuous assembly of nanoparticles with possible voids between them on porous or solid substrates [22]. In this field two types of assemblies can be distinguished, free standing films or zeolite coatings, where the zeolite materials are physically or chemically self-bound or deposited on substrates. The zeolite coating can be deposited on two- and three-dimensional supports representing microreactors with large surface area and total volume. These porous coatings present several advantages such as high heat and mass transfer rates, as well as a defined laminar flow profile inside the reactor due to the presence of regular pores/channels and a close contact of the chemical reactants to the reactor [8, 11, 29]. 22

8 Prior expanding the applications of zeolite films, the optimization of the preparation techniques and modifications of the deposition technique for films are required. The crystal orientation in zeolite films is of significant importance especially in cases where diffusion limitation has to be ignored [14, 30, 31]. As an example, b- oriented silicalite-1 films are prepared either on silica substrates [30] or on chitozanmodified α-al 2 O 3 substrates [14]; the orientation of the crystals is proved by grazing incident XRD diffraction based on synchrotron irradiation. The performance of zeolites in the above applications depends on many factors such as pore size, surface area, hydrophylicity/hydrophobicity, thermal stability, chemical resistance, selectivity, etc. Some of these properties can essentially be improved by chemical modification and optimization of the porous layers, which in some cases are related with the presence of guest molecules or metal/metal oxide clusters [8, 32]. INCLUSION CHEMISTRY IN NANOSIZED ZEOLITES Metal clusters in zeolites In addition to the preparation of pure zeolite assemblies, recent trends involve modification and functionalization of the as-prepared material. One of the possibilities includes incorporation of organic and inorganic guest compounds with specific properties aiming towards an improved selectivity in sensing or sorption applications. Recent reports on the preparation of Pt/ZSM-5 films on stainless steel microreactors [8] and grafted NaA nanozeolites via alkoxysilane linkers [11] highlight the importance of post synthesis treatments. One of our focuses has been on the preparation of thin films from nanosized zeolites as a matrix for incorporation of metal clusters. It is well known that metal and semiconductor with nanometric size and shape exhibit remarkable properties with potential application for selective gas sensing. Nanosized porous materials have the ability to form thin films with enhanced transport properties. However, combining nanoscale metal clusters with nanosized porous materials in order to achieve integration into 2D sensing devices is still a great challenge. Metal clusters confined in the nanosized porous materials or strongly bound to the outer surface of the crystals can be prepared by either chemical reduction or radiolysis carried out in solutions or directly in films. These approaches have already been successfully applied in homogeneous solution to achieve the control in size and shape of Cu and Ag clusters [32]. As matrices, zeolites with GIS, BEA, and MFI type structures are used and the metal clusters are confined in the pores of the nanoparticles; firstly stabilized in the coating suspensions and then deposited on films via spin coating approach. In all experiments, films with controlled porosity, thickness, and surface smoothness are prepared (Figure 7). In order to improve the stability of the films, binders are added to the coating solutions. Three types of binders are commonly used for stabilization of the films, e.g. dense amorphous silica, crystalline non-purified nano-zeolite, and prehydrolyzed TEOS. 23

9 Figure 7. Thin films prepared from coating suspensions of Cu-GIS (a), Pt-BEA (b) and Pd-BEA (c) with different thicknesses. The thickness, smoothness, refractive index (n) and dielectric constant (k) are of significant importance for thin films and can be determined by ellipsometry. In Figure 8, a typical fitting of the experimental data is shown; the refractive indexes of films consisting of Cu-GIS and Pt-BEA are depicted. As can be seen, the refractive index is 1.14 and 1.48 for BEA and GIS, respectively. The high value for n is due to the lower porosity of the GIS film, which is determined by many factors such as the size of the pores, degree of Cu loading and higher density of the films due to the different binders used. The size and stability of Cu clusters in the films are confirmed by UV-vis spectroscopy, while the homogeneous distribution of the clusters throughout the zeolite matrix is proved by EDX SEM (Figure 9). The use of prehydrolyzed molecular precursor of silica as binder results in films with high density, smoothness, high firmness and low macroporosity. The porous MFI binder also directs the formation of films with higher porosity and prevents the agglomeration of Cu-GIS particles, consequently creating two types of micro- and mesoporosity. Figure 8. Experimental and modelled ellipsometry data of Cu-GIS film (a) and refractive index for Cu-GIS and Pt-BEA films (b). 24

10 Figure 9. (A) UV-vis absorption spectra of TEOS-Cu-GIS-1 film (a), Cu-GIS-1 coating suspension (b) and MFI-Cu-GIS-1 film (c); (B) EDX SEM of film revealing the distribution of Cu and Si throughout the film. Important envisioned applications for nanoporous films are for anti-reflection coatings, chemically sensitive coatings on optical devices and dehumidifying coatings on optical windows. In many cases the sensitive nature of the optical surface requires the spin coating method as an adequate approach for producing crystalline microporous coatings, thus avoiding the hydrothermal exposure of the substrates and reducing scattering in the film. Organic guest molecules in zeolites Inclusion chemistry in framework type materials has shown an effective tailoring of many of the physicochemical properties of molecules and compounds through orientation and/or conformation inside the porous networks. The incorporated materials usually exhibit properties, which differ of those in the pure solid or solution phases [33, 34]. For inclusion chemistry, the unique pore structure of crystalline zeolite type materials based on tetrahedrally coordinated silicon and aluminum is very attractive. One of the objectives of our work has been to apply nanosized microporous crystals with regular pore system to immobilize high density energetic materials, and to demonstrate the stabilization role of the porous host in the preparation of safe standards for diverse detection purposes. Moreover, it is important to note that all the used desensitizing procedures for energetic materials reported up to now are not aimed toward detection purposes [35, 36]. The system presented here allows not only the 25

11 stabilization of high energetic compounds, but also the creation of standards for possible identification based on spectroscopic techniques. The successful immobilization of a high density energetic material (FOX-7) inside the MFI type nanosized zeolites is demonstrated in Figure 10. Stabilization of FOX-7 leads to a non-explosive decomposition about hundred degrees above the explosive temperature for the pure substance [37, 38]. Aside from the thermal and mechanical stability that the nanosized porous host provides, they also possess low spectral profiles in the techniques such as Raman, 13 C NMR spectroscopies and mass spectrometry, which comprise actually used techniques for detection of explosives in post-explosion debris. These features make nanosized porous materials exceptional candidates for host systems aimed at detection of highly sensitive guest molecules. Temperature ( C) Figure 10. Immobilization of high-energy density material FOX-7 in silicalite-1 (MFI) host. Another work demonstrates that colloidal zeolite solutions can be used for developing host/guest systems with medium-size organic molecules [39, 40]. Samples are prepared by in situ incorporation of hydroxyl 2-(2 -hydroxyphenyl)benzothiazole (HBT) in the precursor colloidal solutions resulting in the formation of nanosized zeolites under hydrothermal treatment. The diameter of the zeolite particles formed in the crystalline suspensions is in the range of nm. It is shown that the HBT loading does not influence the degree of the zeolite crystallinity but does change the size and the morphology of the individual zeolite nanoparticles. It is important to notice that colloidal suspensions containing crystalline nanoparticles are well suited for optical investigations since they are sufficiently transparent and clear. The photochemical properties of the HBT-guest in the zeolite-host systems demonstrate that depending on the acid-base properties either the enol or the keto-tautomer of HBT is found. Upon UV excitation, the HBT-keto tautomer is converted to the enol form in the zeolite host (Figure 11). UV-vis spectroscopic data reveal the stabilization of the keto-tautomer or enol tautomer of HBT inside the molecular sieves, depending on the sample preparation. 26

12 Wavelength (nm) Figure 11. Normalized absorption (A) and emission (B) spectra of HBT in ethanol; normalized absorption (C) and emission (D) spectra of HBT immobilized in colloidal zeolite (emission, λ ex = 380 nm, excitation, λ em = 465 nm). NANOSIZED MOLECULAR SIEVES AS SORBENTS There is a particular interest in using zeo-type materials as gas storage media due to the fact that by changing the size and charge of exchangeable cations, the ionmolecule interactions can be adjusted and the diameter of the channels can be controlled, thus enabling an effective trapping of different amount of gases. In addition, the large internal surface area of micro- and mesoporous materials leads to a relatively high sorption capacity of the materials toward guest compounds. In order to fully benefit from the unique sorption and shape-selectivity effects of the micropores without suffering from diffusional limitations, the diffusional path length between the micropores should be very short. In zeolite nanocrystals, the diffusion paths are a priori shorter; nonetheless these are difficult to handle in practical applications. Thus, in order to help to improve their thermal and mechanical stability, they have to be embedded into a matrix, for example alumina or mesoporous compounds. The ideal pore architecture for molecular transport is one where short micropores are connected by meso- or macropores going throughout the whole structure. Self-assembly techniques can be used to arrange zeolite nanocrystals into regular supra-structures fulfilling the above stated requirements. Also by controlling the self-assembly, the pore-architecture of this supra-structure can be adjusted to the needs of the storage applications. Nanosized zolites with high capacity for water Nanozeolites show a remarkable hydrophilicity at low relative pressure which is based on the fact that the zeolite nanocrystals have a higher amount of accessible active sites at the external surface which increase the hydrophilic character of molecular sieves. Therefore, in the processes involving water adsorption such as thermochemical 27

13 storage, dehumidification, cooling and sensing, the nanosized zeolites can bring a better performance to the systems due to their intrinsic higher hydrophilicity and water capacity. In order to improve their performance as superior water sorbents, the hydrophilicity can be enhanced by several methods. The common method involves ionexchange of zeolites with small alkaline earth cations (e.g. Mg 2+ and Ca 2+ ). Moreover, similar to micron-sized zeolites, the hydrophilicity of their nanosized counterparts can also be easily adjusted by changing the amount of Al in the framework and/or tuning the amount of surface silanol groups. Also, the hydrophilicity and water capacity of a molecular sieve is affected by the presence of coke deposits through pore blocking especially for those prepared using organic structural directing agents. Thus, it is important to completely and effectively remove the organic residues, which will help to obtain zeolitic water sorbent with more hydrophilic character. In comparison with other adsorbents, conventional zeolites are more hydrophilic and show higher capacity for water retention. However, there are drawbacks in using micron-sized crystals for applications requiring regeneration, mainly in connection to their dehydration at rather high temperatures (> 200 o C), which is due to strong electrostatic interactions between the framework and water molecules in the pore channels [41]. This limitation, however, can be overcome by reducing the particle size, thus compelling most of the water molecules to be adsorbed on the active sites located at the external surface of the crystallites. As a result, water molecules are more easily released from the external surface than those adsorbed in the pore channel, thus avoiding diffusion problems. Such unique properties open new possibilities for extending the application of zeolites to new areas which need lower regeneration temperatures. Currently, several water sorbents such as zeolites, clays, aluminas and organic polymers are used. Almost all of the zeolites, especially high Al containing, show Type I water sorption isotherms which indicate the high affinity to water at low relative pressure. Table 1 summarizes the water capacity of commonly used molecular sieves. As expected, large pore and high Al containing aluminosilicates such as zeolites X and Y have high capacity for water, while pure silicalites show the lowest water uptake owing to the hydrophobic character of the Si-O-Si surface. 28 Table 1. Water capacity of various zeolites and zeotype materials. Adsorbent Ring size Pore diameter (nm) Water capacity (g /g) VPI Na-Y EMT AlPO Na-LSX Li-X 12 n.s Na-X K-X 12 n.s U.S. Rb-X U.S. Cs-X Silicalite AlPO Na-A n.s. = not specified

14 Following the discovery of zeotypes in the 1980s, the aluminophosphates (AlPO s ) have also been studied extensively for water sorption. Several AlPO s molecular sieves such as AlPO-5, AlPO-18 and VPI-5 were found to have comparable water capacity as high as Al containing zeolites (see Table 1). Investigations on the hydrophilicity of AlPO s molecular sieves have been of fundamental interest since their hydrophilic character has not been fully understood yet. Unlike the aluminosilicate zeolites which have extra-framework cations and negative framework charge, the AlPO s frameworks are electrically neutral without the presence of ions due to the replacement of framework Si 4+ by Al 3+. Thus, there are no free ions present in the structure of these materials, fact which accounts for their low ion exchange properties [42]. As might be anticipated, AlPO s molecular sieves have only weak catalytic and sorption properties, which can be due to the presence of a low concentration of surface hydroxyl groups. The substitution of heteroelements into these structures introduces possibilities for many types of catalysis. However, recent studies show that some of the AlPO s materials such as AlPO-5, AlPO-18, AlPO-37, AlPO-40 and VPI-5 are remarkably suited as water adsorbents due to their high pore volume. The hydrophilicity of AlPO s materials has also been explained based on the existence of defect sites (Al-OH and P-OH) [42, 43], in conjunction with the nature of aluminium in the framework [44]. The influence of the difference in the Pauling electronegativity between Al (1.5) and P (2.1) atoms in the framework is the other very important issue to be considered as a factor with a great importance for the high hydrophilicity of AlPO s materials [45]. Temperature ( o C) Figure 12. TG curves of (a) micronsized AlPO-18, (b) micronsized zeolite X, (c) nanosized zeolite X, and (d) nanosized AlPO-18, treated at high humidity for 48 h. Recently, AlPO s molecular sieves such as AlPO-18, AlPO-5 and AlPO-11 with nanosized dimensions were synthesized [46-48]. However, only AlPO-18 nanocrystals have been extensively studied in terms of hydrophilicity relative to other AlPO s 29

15 nanomaterials. Figure 12 depicts TG curves of zeolite X and AlPO-18 (in both micron and nano sizes) after treatment at high humidity. The curves show that micronsized AlPO-18 adsorbs a similar amount of water compared to those of zeolite X. However, it releases water at a lower temperature which can be explained by the weak interactions between its neutral framework and the water molecules [49]. Additionally, an interesting fact is that the nanosized AlPO-18 shows the highest water weight loss and lower temperature of release in comparison to the other three candidates. PURIFICATION OF LUBRICANTS BASED ON NANOSIZED ZEOLITES Lubrication technology has been one of the frontline researches during several decades. Currently, the demand for lubricants with high performance, low cost and with stringent environmental requirements is increasing. Our research is also aimed at the preparation of environmentally friendly and effective sorbents based on nanozeolites for capturing the resulting compounds produced during the oil oxidation reactions. In petroleum and lubricant based industries, efficient control of humidity levels is among the issues with the highest priority since water acts as an interfering impurity. Inefficient control of humidity affects the performance of the operating systems. It is known that water vapour speeds up rust formation, which corrodes and destroys the engine components. Also, moisture, when mixed with oil, causes oil-water emulsions formation leading to degradation of the oil s ability to lubricate and hence, decreases the service life of the lubricants. Due to the great impact of the presence of water in oil even at very low concentrations (ppm), its control and removal are factors that have to be taken care for an appropriate performance of the lubricant. In the face of this requirement, an adsorptive process using nanosized hydrophilic molecular sieves may be the best choice to solve this problem. Although this method has been used in other systems using micrometer size molecular sieves as sorbents [50], more research is needed to discover and develop suitable hydrophilic nanozeolites. In order to improve the selectivity of the sorbent for water in the lubricants not only the high capacity of water is of importance, but also the knowledge about the surface chemistry. The hydrophilicity of molecular sieves can be adjusted by varying the crystallite size or introducing of heteroatoms in their framework structure, whereas the selectivity towards water adsorption in mixtures can be controlled by the surface hydrophilicity and pore channel system. Hence, with the proper choice of the nanoporous material, the moisture in lubricants can effectively be adsorbed stronger and faster than with other available methods. On the other side, lubricants suffer from the presence of a large amount of oxidation products which cause increase in the viscosity, acidity, etc. Molecular sieves can also be considered as a more environmentally friendly and cost effective compounds to capture the oxidation products produced during the first oil oxidation reactions. Nanosized powder microporous and mesoporous nanocrystals are, however, difficult to handle in the applications such as preparation of trapping column for adsorption of oxidant molecules coming from used oil. The use of nanoporous materials and selective sorbents is a fine complementation to the above mentioned process of water separation. If for water separation, hydrophilic materials are used, the other spectrum is to be applied for the adsorption of organics, hydrophobic molecular sieves. Nonetheless, the separation of detrimental oil by-products requires adequate tailoring of the material 30

16 in terms of channel network and chemical composition. While the correct pore size and channel network provide size selectivity and diffusivity discretion, the chemical composition regarding hydrophobicity gives the much needed hydrocarbon affinity and further improve the sorption process. Figure 13. Extrudates based on nanosized molecular sieves before (a) and after (b) treatment of oil samples. An oil purification process may not be exclusively carried out by selective sorption of oxidation products, but also by breakage of the oxidation processes. These processes usually include a polymerization initialized by the presence of oxygen radicals, and may be stopped by ion-exchange of the counterbalancing cations present in the nanoporous materials by other metal cations, or inclusion of metal cations in the framework structure itself. Figure 14. N 2 sorption isotherm of nanosized zeolite. Insert: photographs of oxidized and purified oil with nanosized zeolites. Although a coupling of both materials may appear difficult at first sight, dynamic environments found in systems with intense oil flow might provide a mixture heterogeneous enough so that a separation of both water and oxidation 31

17 products can take place at a highly effective rate. Preliminary experiments have shown that the oxidized oil can be purified by microporous materials pre-shaped as extrudates (Figures 13 and 14). As can be seen the very dark oxidized oil was purified by molecular sieves with extremely high porosity and high degree of hydrophobicity. Moreover, the extrudates are very stable after oil treatment and can be regenerated under specified conditions and reused for purification of oil. SUMMARY AND OUTLOOK This paper presents some of the new trends in processing of nanosized microporous molecular sieves and new applications related to their new properties. Depending on the conditions of synthesis and the initial sources for silicon, aluminium and the organic templates, the discrete particles can be tuned to be either single or polycrystalline with variable morphology and particle sizes. Significant advantages of these materials are the controlled monomodal particle size distribution and the stability of the crystals in the purified and non-purified solutions. Functionalization of the surface of these particles is a challenge for preventing coagulation and agglomeration during storage for extended time. Decreasing the crystal size of the colloidal molecular sieves is an ongoing task, and the foreseen target is about 10 nm in diameter, but the stability of these extremely small particles is expected to be low. Some of the developments in advanced applications of nanocrystalline molecular sieves are presented. It is evident that the zeolite pore systems with their extremely well-defined crystalline cages and channels, their ion-exchange capability, and their tuneable properties with respect to acid/base behaviour, hydrophilicity/hydrophobicity can offer a solid platform for the development of many novel functional concepts. In addition, the growing ability to control the size and morphology of either zeolite single crystals or zeolite intergrowths has opened up entirely new applications such as zeolite membrane separations, chemical sensors or spatial confinement. Microporous materials or molecular sieves with well-defined pore sizes between about 0.2 and 2 nm can be used for adsorption of many molecules having dimensions in the same range; moreover the molecular sieves can separate certain molecules from a mixture based on their selectivity. Zeolites can be rendered hydrophobic when they are completely aluminium-free, or otherwise they are more or less hydrophilic. ACKNOWLEDGEMENT The financial support from SOILCY IP and PNANO projects is gratefully acknowledged. REFERENCES 1. Ch. Baerlocher, W. M. Meier, D. H. Olson, Atlas of Zeolite Framework Types, Elsevier, L. Tosheva, V. Valtchev, Chem. Mater. 17 (2005) C. S. Cundy, P. A. Cox, Microporous Mesoporous Mater. 82 (2005) 1 4. S. Mintova, V. Valtchev, Microporous Mesoporous Mater. 55 (2002) R. Mostowicz, F. Testa, F. Crea, R. Aiello, A. Fonseca, J. B. Nagy, Zeolites 18 (1997) Valtchev, V. P., Bozhilov, K. N., J. Am. Chem. Soc. 127 (2005)

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20 XIV Forum Zeolitowe, Kocierz, 2007 Polskie Towarzystwo Zeolitowe From nanoconstituents toward zeolite macrostructures: crystallization mechanism and controlled assembly Feifei Gao, Valentin Valtchev Laboratoire de Matériaux à Porosité Contrôlée, UMR-7016 CNRS, ENSCMu, Université de Haute Alsace, 3 rue Alfred Werner, Mulhouse, France Valentin.Valtchev@univ-mulhouse.fr ABSTRACT An overview is given about the current status of nanosized zeolitic materials, in particular their synthesis and processing to integrated structures. First, the formation of nanozeolite from clear solutions, hydrogels and within inert matrixes (confined space) is revised. Next, attention is paid to the new insights into zeolite crystallization mechanism gained by using nanocrystals yielding systems. The emphasis is put on the room temperature crystallization as a convenient way to study zeolite nucleation/growth process. The paper is completed with a short review of the approaches used in the processing of the zeolite nanocrystals to structured materials with hierarchical organization. INTRODUCTION The great interest in the nanosized microporous materials at the beginning of the new millennium is a part of the revolution in the nanotechnology, which is expected to make most products lighter, stronger, cleaner, less expensive and more precise. The significance of these materials for the modern technology is exemplified by the burst of scientific publications devoted to nanozeolites. Having in mind the diversity of possible areas of application, which are far beyond the traditional uses, it is not risky to predict that this is the beginning of a long-lasting and fruitful period in the era of zeolite nanotechnology. A brief look at the developments of synthetic porous materials shows that each decade is marked by different, sometimes opposite trends. For instance, in the seventies the Si/Al ratio in zeolite frameworks was expanded up to all-silica zeolitic materials [1-3], while the eighties were marked by the discovery of the aluminophosphate molecular sieves [4, 5]. In the nineties the ordered mesoporous materials focused the attention of the scientists from zeolite and collaborating communities [6-9]. In the dawn of new century the interest is turned to the metal organic frameworks (MOFs) [10-12]. This period was also marked by the introduction of Ge as a co-structuring agent together with the organic template that resulted in many new microporous tectosilicates [13]. Although, new families of microporous solids were discover the tectosilicate molecular sieves continue to be in the main focus of the community since they are indispensable for nowadays industry. Consequently, the fine tuning of the properties of the microporous aluminosilicates has always been in the focal point of academic and applied researchers. Thus, the fluorite route pointed out by Flanigen [14] and further developed by Guth and Kessler allowed the synthesis of large almost defects-free zeolite crystals [15-19]. After that the interest of zeolite chemists turned to the very small crystals. If we consider the permanently increasing number of 35

21 publications per year the activity in the area still not reached the highest point. The interest in these materials is mainly related with the emerging applications that go far beyond the traditional separation and catalytic processes. The possibility to obtain stable colloidal suspensions of nanoporous particle able to be processed on different supports by rapid techniques like spin or deep coating is also of great importance, especially for the emerging advanced application. The objective of present paper is to revise the last developments in the synthesis of nanosized zeolite crystals. Different synthetic approaches and their effect on the physicochemical properties of the product will be first discussed. The emphasis will be put on the systems providing colloidal products. The utility of nanocrystals yielding systems in the efforts for better understanding of the zeolite nucleation/crystal growth process will be then revised. The processing of the colloidal zeolite suspensions to integrated structures with different level of organization will be highlighted in the final part of the paper. HISTORICAL REMARKS First example of nanosized zeolite crystals can be found in D. Breck s book [20]. In a TEM micrograph well shaped zeolite A crystals with narrow particle size distribution and average size of about 50 nm can be seen. Although the first synthesis of colloidal zeolite particles can be found in the early seventies, almost twenty years were needed for these materials to attract the interest of academic and applied scientists. In the early nineties the synthesis of nanosized zeolite L was reported by Meng et al [21]. However, the major merit to turn the interest of the zeolite community to the tiny crystals should be attributed to the group from Lulea. In a series of papers Schoeman et al. demonstrated the possibility to obtain zeolite crystals with narrow particle size distribution [22-25]. The colloidal nature of these materials and first studies on the sub-colloidal precursor particles formed prior to the nucleation of the MFI-type material were also reported by this group [26-28]. In the late nineties and beginning of new century many other groups focused their attention to the nanozeolite yielding systems in the efforts to shed more light on the mechanism of zeolite formation. Most of the studies were based on silicalite-1 yielding systems with slight variation in the synthesis formulations [29-31]. The interest in microporous nanocrystals resulted in an increase of the number of aluminosilicates synthesized in colloidal form. Nanocrystals of aluminophosphate molecular sieves were also obtained [32-35]. Attempts to synthesize zeolite nanoparticules with relatively narrow particle sized distribution from hydrogel systems were reported as well. New synthetic approaches as confined space synthesis and confined media synthesis were also introduced. All these developments are revised hereafter. SYNTHESIS OF ZEOLITE NANOCRYSTALS Factors controlling the synthesis of nanozeolites The zeolite syntheses are performed in close systems, where the supersaturation leads to spontaneous nucleation. Upon such conditions the control on the nucleation allows directing the crystal size. In other words, the nutrient pool is limited and thus after the exhausting of a building component the growth stops. The relationship nucleation-crystal size is presented on Figure 1. As can be seen the increase of the number of the nuclei leads to decreased of the ultimate crystals size. Hence, the 36

22 formation of zeolite nanocrystals requires conditions that favor the nucleation over crystal growth. Crystal size Number of nuclei Figure 1. Number of viable nuclei - crystal size relationship in a closed zeolite-yielding system. Besides the number of viable nuclei per unit gel the homogeneity of initial system determines the uniformity of the crystalline product. The latter is of paramount importance when the goal is monomodal particle size distribution. Nanocrystals with narrow particle size distribution are generally obtained from so called clear solutions. Syntheses from clear solutions Transparent initial systems containing colloidal or sub-colloidal particles are usually denoted clear solutions. The homogeneity of such systems is much higher in respect to the hydrogels conventionally employed in zeolite syntheses. The homogeneity of the starting system and simultaneity of the events leading to the formation of precursor gel particles and their transformation into crystalline zeolitic material determine the uniformity of crystallites in the product. In order to obtain such homogeneous starting systems abundant amounts of tetraalkylammonium hydroxides and water are employed [24, 36]. On the other hand the content of alkali cations is very limited. These factors together with the careful choice of the reactants allow the stabilization of a clear starting mixture, where only discrete gel particles are present [37-40]. Amongst different reactants the impact of the silica source on the evolution of the system is probably most important [41]. Usually the lowest possible temperature for a particular zeolite is employed in order to favor the nucleation over the growth and thus smaller crystals to be obtained. The syntheses at relative low temperature (<90 C) and atmospheric pressure allows the application of in situ techniques without using sophisticated equipment. For instance, dynamic light scattering (DLS) is often employed in the investigation of clear solution, thus the transformation of different populations of particles can be followed in real time. Figure 2 represents such a study on a silicalite-1 yielding system, where the evolution of the system at 88 C in the time interval 8-10 h is monitored. It can clearly be seen that the population of 50 nm particles grows at the expense of 4-5 nm primary units. The above example illustrated the utility of the clear solutions in the investigation of different population of precursor particles involved in the crystallization process. Such systems were also found very useful to track down the atomic scale reorganization of the precursor species [42]. Contrary to the conventional 37

23 gel systems, where a large diversity of (alumino)silicates species is usually present, the initially clear solutions contain a limited number of well defined discrete amorphous precursor particles. Hence, the investigations based on such systems facilitate the interpretation of the results and thus the ambiguous conclusions could be avoided. (c) Scattering intensity (b) (a) Diameter (nm) Figure 2. DLS data of the evolution of a system with composition 4.5(TPA) 2 O:13Na 2 O:25SiO 2 :420H 2 O (silica source TEOS) subjected to hydrothermal treatment at 88 C for 8 h (a), 9 h (b) and 10 h (c). Confined-space syntheses of nanocrystals Syntheses within an inert matrix providing a steric hindered space for zeolite crystal growth have been developed for preparation of zeolite nanocrystals. The first example of such a synthesis was reported by Madsen and Jacobsen [43] for the preparation of nanosized ZSM-5 crystals. A schematic illustration of the confinedspace procedures is given in Figure 3. The synthesis procedure consisted of incipient wetness impregnation of mesoporous carbon black with clear solutions containing TPAOH, distilled water, ethanol and alumina, subsequent impregnation with TEOS, transferring the impregnated matrix into a porcelain cup and treatment in an autoclave with sufficient water to provide saturated steam at 180 C. Detailed studies on the properties of the ZSM-5 crystals as well as of features of other zeolites such as Beta, X and A prepared using this procedure have been performed [44, 45]. Tang et al. employed carbon nanotubes as an inert matrix in order to synthesize NaY nanocrystals [46]. Similar approach was employed Ivanova et al. using silicon carbide supports [47]. In latter case the support plays a dual role, as hindered space for growth and as a source emanating the silicon that takes place in the reaction. Crucial steps in these syntheses were (i) to restrict the crystallization of the zeolite gel within the pore system of the matrix, which was achieved by the incipient wetness impregnation method employed to load the mesopores with a synthesis gel; and (ii) to prevent diffusion of the zeolite gel species from the mesopores, which was ensured by avoiding direct contact between the impregnated carbon black matrix and the water at the bottom of the autoclave. Drawbacks of the method are the requirements imposed to the matrix to be used as a confine space media, namely inertness and stability under the experimental conditions and sharp mesopore size distribution to yield uniform zeolite crystals. Another approach to zeolite nanocrystal preparation that could be denoted confined-media synthesis was also developed. The synthesis was performed in the absence of an organic structure directing agent (SDA) and yielded smaller crystals compared to one performed without space confinement additives. Wang et al. reported 38

24 the synthesis of zeolite NaY with a size in the range nm by applying starch as a space limit agent [48]. However, calcination was necessary to remove the starch template. An elegant calcination-free approach has been used by Yan and co-workers for the synthesis of NaA ( nm) and NaX ( nm) nanocrystals employing thermo- reversible polymer hydrogels to space limit the crystal growth [49]. The zeolite crystals obtained were recovered from the polymer matrix by simple cooling of the mixture and washing away the water-soluble polymer. Zeolite precursor Calcination Heating Inert matrix Matrix embedded zeolite nanocrystals Zeolite nanocrystals Figure 3. Schematic presentation of the syntheses of zeolite nanocrystals in confined space. Syntheses from template-free hydrogels at room temperature A serious drawback, that has important economic and environmental impact, is the large amount of organic structure directing agents used in the syntheses of zeolite nanoparticles. In addition, the organic SDA requires high temperature calcination of the zeolite in order to open zeolite porosity, which leads to aggregation of the individual particles. A calcination procedure, where an organic polymer network is used as a temporary barrier during the high temperature treatment in order to prevent zeolite nanocrystals aggregation has been developed [50]. Nevertheless, the synthesis of discrete zeolite crystals from organic SDA-free systems is highly desired since it would open an alternative route for preparation of nanozeolites. This approach could be applied only in the synthesis of low silica zeolites crystallizing from highly reactive gels without employing organic structure directing agents. The crystallization temperature has a pronounced effect on the ultimate zeolite crystal size. As a rule, lower temperatures lead to smaller particle sizes, however at the expense of a substantial decrease of the crystallization rate. Besides the temperature, the crystallization kinetics of any particular zeolite is affected by the alkalinity and the composition of the reaction mixture. In order to obtain a zeolite for a reasonable period of time under ambient conditions, a highly alkaline, very reactive initial system was employed. It was also found that the zeolite formation is very sensitive to the nature of the reactants, in particular that of the silica source. Thus, a great variety of silica sources, which differ in their specific surface area, impurities and ability to dissolve in alkaline mixtures have been employed so far in zeolite syntheses. The silica source can influence different aspects of the zeolite crystallization, including the kinetics of crystal growth and the properties of the final product [51, 52]. Since the crystallization temperature was as low as 25 C in order to reduce crystallization time special attention was paid to all other factors controlling the kinetics of zeolite growth. A silica source containing low-mass species was employed and further depolymerized by adding sodium hydroxide so as to obtain a completely transparent initial solution. The state of the silica in the solution was studied by 29 Si liquid NMR spectroscopy. Under the conditions used, only a single peak at ppm characteristic of silicate monomer was 39

25 detected. The latter was mixed with a clear sodium aluminate solution where Al(OH) -4 are the dominant anionic species [53]. The vigorous mixing of the alkaline silicate and aluminate solutions produced a precursor gel where all components were expected to be homogeneously distributed. Applying the above principles nanocrystals of zeolites A and X were successfully synthesized at room temperature from hydrogels with the following composition: LTA: 6.0Na 2 O:0.55Al 2 O 3 :1.0SiO 2 :120.0H 2 O FAU: 4.0Na 2 O:0.2Al 2 O 3 :1.0SiO 2 :200.0H 2 O Highly crystalline zeolite A and X were obtained within 3 and 21 days, respectively [54, 55]. Representative TEM micrographs of obtained nanoparticules are shown in Figure 3. As can be seen both materials crystallize in the nanosized range. However, their morphological features differ substantially. Thus, namely single nm zeolite A crystals where obtained, whereas zeolite X crystallized in the form of spherical nm aggregates built of very small nm crystallites. The relatively rapid transformation of the amorphous aluminosilicate species into zeolite a b 100 nm Figure 3. TEM micrographs of LTA-type (a) and FAU-type (b) crystallites synthesized at room temperature. under room temperature conditions was achieved by a fine tuning of all parameters affecting the crystallization kinetics. The post-synthesis ultrasonic treatment broke free the loosely attached particles and provided a product with a relatively narrower particle size distribution. The fraction of the silica converted into zeolite A and X was 75 and 83 %, respectively. Thus, the room temperature synthesis of zeolite nanoparticles from organic-template-free precursors offers an attractive alternative for the preparation of colloidal zeolite crystals. Furthermore the utilization of systems yielding zeolite nanocrystals under ambient conditions might provide important insights into fundamental aspects of the mechanism of zeolite crystallization. This section revised different methods of nanozeolite syntheses. The most interesting examples in the period are summarized in Table 1 [56-74]. Earlier syntheses were properly revised in Ref 75. Mechanism of zeolite formation The reaction media in zeolite yielding systems is inhomogeneous and zeolite nucleation and crystal growth involve numerous simultaneous equilibria and condensation steps. The efforts for a detailed understanding of the phenomena 40