AB3. Nanoscience Advances in CBRN Agents Detection, Information and Energy Security

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1 NATO Science for Peace and Security Series - A: Chemistry and Biology Nanoscience Advances in CBRN Agents Detection, Information and Energy Security Edited by Plamen Petkov Dumitru Tsiulyanu Wilhelm Kulisch Cyril Popov AB3

2 NATO Science for Peace and Security Series This Series presents the results of scientific meetings supported under the NATO Programme: Science for Peace and Security (SPS). The NATO SPS Programme supports meetings in the following Key Priority areas: (1) Defence Against Terrorism; (2) Countering other Threats to Security and (3) NATO, Partner and Mediterranean Dialogue Country Priorities. The types of meeting supported are generally Advanced Study Institutes and Advanced Research Workshops. The NATO SPS Series collects together the results of these meetings. The meetings are coorganized by scientists from NATO countries and scientists from NATO s Partner or Mediterranean Dialogue countries. The observations and recommendations made at the meetings, as well as the contents of the volumes in the Series, reflect those of participants and contributors only; they should not necessarily be regarded as reflecting NATO views or policy. Advanced Study Institutes (ASI) are high-level tutorial courses intended to convey the latest developments in a subject to an advanced-level audience Advanced Research Workshops (ARW) are expert meetings where an intense but informal exchange of views at the frontiers of a subject aims at identifying directions for future action Following a transformation of the programme in 2006 the Series has been re-named and re-organised. Recent volumes on topics not related to security, which result from meetings supported under the programme earlier, may be found in the NATO Science Series. The Series is published by IOS Press, Amsterdam, and Springer, Dordrecht, in conjunction with the NATO Emerging Security Challenges Division. Sub-Series A. Chemistry and Biology Springer B. Physics and Biophysics Springer C. Environmental Security Springer D. Information and Communication Security IOS Press E. Human and Societal Dynamics IOS Press Series A: Chemistry and Biology

3 Chapter 45 Synthetic Micro- and Nanocrystalline Zeolites for Environmental Protection Systems Silviya Boycheva, Denitza Zgureva, Borislav Barbov, and Yuri Kalvachev Abstract Coal combustion ash with a 2.25 ratio of SiO 2 /Al 2 O 3 was used to synthesize an X-type zeolite by alkaline fusion followed by hydrothermal treatment. The reaction products were investigated for phase identification by X-ray diffraction analysis. The surface morphology and the elemental composition were determined by scanning electron microscopy with an energy-dispersive X-ray analyzer. The vibrations of the structural units in the zeolite framework were revealed by IR spectroscopy. Zeolite X obtained from pure initial materials was used as a reference sample for comparison. The results confirmed the conversion of coal ash into a zeolite possessing a dominant Na-X phase of micro- or nanocrsystalline morphology depending on the synthesis conditions. The coal ash Na-X zeolite was studied for adsorption of carbon dioxide under static conditions. Keywords Synthetic zeolites Environmental protection systems 45.1 Introduction Due to their highly developed microporous structure with a defined pore size synthetic and natural zeolites have been successfully applied as gas dryers, adsorbents of polar gaseous molecules and molecular sieves in gas cleaning systems [1]. Zeolites are also adsorptive materials and ion-exchangers for effective removal of pollutants from contaminated waters, enabled by the presence of replaceable cations in their structural network [2]. The development of nanocrystalline zeolites has been a new trend in materials sciences because of their unique surface reactivity S. Boycheva (*) D. Zgureva Department of Thermal and Nuclear Power Engineering, Technical University of Sofia, Blvd. Kl. Ohridsky 8, 1000 Sofia, Bulgaria sboycheva@tu-sofia.bg; kalvachev@clmc.bas.bg B. Barbov Y. Kalvachev Bulgarian Academy of Sciences, Institute of Mineralogy and Crystallography, Acad. G. Bonchev Str., Bl.107, 1113 Sofia, Bulgari Springer Science+Business Media Dordrecht 2015 P. Petkov et al. (eds.), Nanoscience Advances in CBRN Agents Detection, Information and Energy Security, NATO Science for Peace and Security Series A: Chemistry and Biology, DOI / _45 443

4 444 S. Boycheva et al. and ability to functionalization which make them advanced materials for catalytic and photocatalitic reduction processes [3]. Zeolite synthesis is usually performed by hydrothermal crystallization of silica gel, alkaline activators and aluminium salts at elevated temperatures in strong alkaline media. Recently, low-cost sorbents have been produced by conversion of mineral aluminosilicate wastes that can replace successfully their natural analogues in the systems for environment protection. Coal ash is mainly composed of silica and alumina oxides obtained by thermal decomposition of fuel mineral constituents. The chemical composition of this abundant resource makes it suitable for processing of synthetic zeolites [4]. The fusion of the alkaline ash mixture facilitates the formation of soluble Na-aluminate and silicate, what enhances the zeolite formation and leads to a full utilization of the raw material [5]. X-type zeolite, which is an analog on the natural faujasite, is characterized by an extremely developed surface and a structure with large size pores. Therefore it is extensively used for gas adsorption. Fusion followed by hydrothermal treatment has been reported to be the most reliable method to selectively obtain the Na-X type zeolite from coal ash of different compositions [1]. The present study reports on the conversion of coal combustion aluminosilicate by-products into synthetic zeolites of the Na-X type and on initial studies of their potential to act as adsorbents for carbon dioxide Materials and Methods Ash residues, containing wt% SiO 2 and wt% Al 2 O 3, were subjected to a double stage fusion-hydrothermal conversion. Coal ash and NaOH mixtures with different ratios were sintered at 550 C; after dilution in water under continuous stirring, the reaction mixture was charged into an autoclave for hydrothermal reaction. The product morphology was observed by scanning electron microscopy (SEM) with a JEOL JSM6390 microscope coupled with an energy-dispersive X-ray (EDX) analyser (Oxford Instruments). Phase identification was performed by X-ray diffraction (XRD) with a Brucker D2 Phaser diffractometer with CuK α -radiation and a Ni filter. Na-X zeolite was exposed for carbon dioxide adsorption under static conditions. The experiment was performed in an autoclave at 0.1 MPa. The framework structure of the synthesized materials and the possibility of CO 2 adsorption were studied by Fourier Transform Infrared Spectroscopy (FT-IR) with a Brucker Tensor 37. The referent zeolite X was synthesized by hydrothermal treatment of pure starting materials with a stoichiometric ratio Results and Discussions The conditions under which the coal ash zeolitization experiments were performed are listed in Table 45.1.

5 45 Synthetic Micro- and Nanocrystalline Zeolites 445 Table 45.1 Experimental conditions of coal ash zeolitization Sample NaOH/coal ash ratio Fusion [ C] Hydrothermal synthesis Temperature [ C] Duration [h] Sample 18 Sample 13 Intenaity, a.u. Sample 12 Zeolite X Theta Fig XRD pattern of the reference and synthesized zeolites XRD patterns of the synthesized samples and the reference zeolite X are plotted in Fig It is evident that the coal ash zeolites are X type, as the characteristic diffraction reflexes are present in the patterns of all samples. Sample 18 is characterized by low crystallinity, while samples 12 and 13 are dominantly crystalline, but their peaks are less intensive in comparison to those of the referent zeolite. The coal ash zeolites consist of octahedral crystals typical for zeolite X, as revealed by SEM analysis (Fig. 45.2). The reference zeolite X and sample 13 consists of particles with micrometer sizes, while nano-sized crystallites are predominant for the structure of samples 12 and 18. The elemental composition of coal ash zeolites, studied by integral EDX analyses, is presented in Table It can be seen, that the content of Si and Al in the coal ash zeolites is very similar to that of the reference composition of zeolite X, while it has a deficit of Na + -ions. In general, there are three stages in alkaline hydrothermal conversion of coal ash into zeolites: (1) dissolution of Si 4+ and Al 3+ ions from the raw aluminosilicates into

6 446 S. Boycheva et al. Fig SEM micrographs of the reference sample (a) and coal ash zeolite samples 12 (b), 13 (c) and 18 (d) Table 45.2 Elemental composition of zeolite samples (in wt. %) Sample O Na Mg Al Si Fe Ca X the alkaline solution; (2) condensation of the aluminosilicate hydrogel; (3) crystallization of the zeolite phase from the aluminosilicate gel. The OH ions contribute to the dissolution stage, while Na + ions play a role in the zeolite crystallization. Na + ions are stabilizers of zeolite structure as they compensate the negative charges of AlO 4/2 -tetrahedra. It could be expected that cations from the raw ash acting as charge compensators in coal ash zeolites also contribute to keep their overall framework neutral. Hence, coal ash zeolites could not be obtained in a defined ionic form corresponding to the alkaline activator due to drawback ions present in the starting composition. A typical IR spectrum of a coal ash zeolite is presented in Fig The IR spectra of zeolites possess common features as their framework is composed of TO 4 tetrahedra (T ¼ Si/Al). However, each zeolite type is characterized by its own specifics. The framework type and the basic structural units of

7 45 Synthetic Micro- and Nanocrystalline Zeolites 447 Transmittance, % Fig FT-IR spectrum of the synthesized coal ash zeolite Wavenumber, cm Fig Schematic presentation and structural units of zeolite X. SOD sodalite cage, SR-singleand DR-double rings composed of the indicated number of atoms zeolite X are presented in Fig. 45.4, while the vibration modes and their assignments are summarized in Table The framework of zeolite X is built of sodalite cages connected by double six rings (DR6). The characteristic internal and external sodalite cages vibrations, which are typical for zeolite X, are shown in the experimental spectrum of the coal ash zeolite (Fig. 45.3).

8 448 S. Boycheva et al. Table 45.3 IR modes for zeolite X and their assignments No. IR vibrations Wavenumber, cm 1 1 Water region, H-O-H 3,467, 1,643 Internal tetrahedral vibrations 2 Asymmetric stretching mode in TO 4/2 tetrahedra 1,048, Symmetric stretching mode in TO 4/2 tetrahedra 744,666 4 T-O bending mode in TO 4/2 tetrahedra 460 External linkages vibrations 5 Double rings (DR6) Asymmetric T-O stretching mode 1,070 Recently, the greenhouse effect has provoked greater attention to carbon dioxide capture and storage technologies (CCS) and their compatibility with thermal power plants (TPPs) [6]. Three approaches for carbon dioxide (CO 2 ) removal during energy generation have been outlined: pre-combustion capture, oxy-fuel combustion and post-combustion capture. The last technologies are the most compatible to the existing TPPs; theirs cost- and energy-efficiency depends mainly on the capturing mechanism. A variety of methods exists for the post-combustion capture, such as chemical or physical sorption as well as membrane or cryogenic separation. Adsorption on solids as a CO 2 capture technology has a number of advantages compared to others, due to the reusable adsorbents, simple reactors, low capital investments, and adsorption/desorption enhancement by pressure and temperature variation. The highest adsorption potential toward CO 2 has been reported for zeolite 13X which is a commercial artificial faujasite prototype [7]. In this context, the synthesized coal ash Na-X zeolites have been investigated with respect to their adsorption ability toward carbon dioxide. Tests were performed under static conditions in a stainless steel autoclave filled with СО 2 through a gas transportation line to reach pressure of 0.1 MPa. The required pressure was adjusted using a booster pump and а channel for inert gas (N 2 ). The experimental set-up is presented in Fig A powder zeolite sample was packed in a gas permeable bag, dried at 105 C and exposed immediately to a CO 2 atmosphere for 2 h. The adsorption of CO 2 onto the zeolite was confirmed by IR spectroscopy. The IR spectra of samples exposed to CO 2 are plotted in Fig The IR active modes of physically adsorbed CO 2 which are O-C-O bending and C-O asymmetric stretching, are found at 667 and 2,350 cm 1, respectively. The IR vibration of free CO 2 can be observed clearly in the spectrum of the zeolite after CO 2 capture. This result is indicative for physical surface adsorption of CO 2 onto the zeolite tested. It has been reported that chemisorptions of CO 2 onto zeolites yield carbonate species, whose signals appear as a pair of bands at 1,711 and 1,365 cm 1, assigned to the bicoordinated CO 2 (type I) with both a T atom and a cation (Fig. 45.7). The bicoordinated species can be converted into a true carbonate structure (type II) characterized by bands at 1,488 and 1,431 cm 1. The chemisorption characteristic lines fall into the vibration regions of the zeolite groups. Such a chemisorption

9 45 Synthetic Micro- and Nanocrystalline Zeolites 449 Fig Picture (a) and a schematic diagram (b) of the adsorption test set-up Transmittance, % Physisorption CO 2 vibrations dash line sample 18 solid line- sample Wavenumber, cm -1 Fig FT-IR spectra of coal ash zeolites after exposition to CO 2 Fig Species yielded from the chemisorptions of CO 2 onto zeolite X [8]

10 450 S. Boycheva et al. mechanism can not be confirmed by IR spectroscopy in the case of weak appearance. Thus, it could be supposed that CO 2 physisorption is the dominant adsorption process at a pressure of 0.1 MPa Conclusions Coal ash was converted into zeolite X by a double stage fusion-hydrothermal synthesis, which is confirmed by structural and morphological studies. Coal ash zeolites can be obtained as micro- or nanocrystalline materials by varying the synthesis conditions. These zeolites are CO 2 adsorptive materials which will be further studied under dynamic conditions and at higher pressures to elucidate their morphology. The utilization of coal ash after its zeolitization for carbon capture is an environment-friendly method for zero emissions and end of pipe technologies in energy generation plants. References 1. Majchrzak-Kuceba I, Nowak W (2005) Thermochim Acta 437:67 2. Moreno N, Auerol X, Ayora C, Pereira CF, Janssen-Jurkovicova M (2001) Environ Sci Technol 35: Larsen SC (2007) J Phys Chem C 111: Querol X, Moreno N, Umana JC, Alastuey A, Hernandez E, Lopez-Soler A, Plana F (2002) Int J Coal Geol 50: Purnomo C, Salim C, Hinode H (2012) Microporous Mesoporous Mater 162:6 6. Davidson R (2010) Woodhead Publ Ser Energy 1: Ruen-ngam D, Rungsuk D, Apriratikul R, Pavasant P (2009) J Air Waste Manag Assoc 59: Auerbach S, Carrado K, Dutta P (2003) Handbook of zeolite science and technology. Marcel Dekker, Inc., New York