Acta Polytechnica CTU Proceedings doi:10.14311/APP.2017.9.0026 Acta Polytechnica CTU Proceedings 9:26–31, 2017 © Czech Technical University in Prague, 2017 available online at http://ojs.cvut.cz/ojs/index.php/app REMOVAL OF DIQUATERNARY AMMONIUM CATIONS FROM AS-SYNTHESIZED SSZ-16 ZEOLITE Tatana Supinkovaa, c, ∗, Ivan Jirkaa, Jan Drahokoupilb, d, Jan Langmaiera, Vlastimil Filac, Libor Brabeca, Milan Kocirika a J. Heyrovsky Institute of Physical Chemistry,Czech Academy of Sciences, Dolejskova 2155/3, Prague 8, Czech Republic b Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University, Trojanova 13, Prague 2, Czech Republic c Faculty of Inorganic Technologies, University of Chemistry and Technology in Prague, Technicka 5, Prague 6, Czech Republic d Institute of Physics, Czech Academy of Sciences, Na Slovance 1999/2, Prague 8, Czech Republic ∗ corresponding author: tatana.supinkova@jh-inst.cas.cz Abstract. Zeolites are stable microporous aluminosilicates with numerous applications in chemical technology such as separation of species and catalytic transformations. Our study is focused on a weakly explored zeolite SSZ-16 with pore constrictions defined by 8-membered oxygen rings. Key results are the preparation of Et6-diquat-5 dication used as a structure directing agent (SDA) and finding the optimum synthesis conditions with respect to zeolite phase purity. Stability of SDA was examined in conditions similar to those of autoclave synthesis (concentration, pH, temperature). Moreover, the content and location of SDA species in zeolite phase and conditions of SDA decomposition were investigated. Keywords: Zeolite, Hydrothermal Synthesis, SSZ-16, AFX, Et6-diquat-52+. 1. Introduction Zeolites and zeolite-like materials consist of tetrae- dral building units TO4 where T are central atoms most frequently Si and Al+. Tetraedra are linked to each other through oxygen atoms. The spaceous arrangement of tetrahedra form as a rule microporous structure where micropores take the shape of regular voids (channels and/or cavities) which are repeated through the crystals with the regularity of the crys- talline structure. In zeolites, T-atoms can be replaced by isomorphous substitution, for example Ge, Ti, P, Fe, Ga, B and thus form zeolite analogues. Zeolites and their analogues are crystalline materials with reg- ular microporous structure (with pore width < 2 nm). The intracrystalline pores are thus of molecular dimen- sions. The surface of intracrystalline pores is formed by oxygen atoms. The critical pore width is for atoms and molecules as a rule determined by oxygen rings containing 8, 10 or 12 oxygen atoms. Small cations can pass even through 6-membered oxygen rings. Mi- cropores can accommodate spherical molecules typ- ically with the diameter of 0.3-1.0 nm or cylindrical molecules with cylinder diameter in the above range. Zeolites are also called molecular sieves due to possi- ble exclusion of molecules with size exceeding some critical value. Zeolites are perspective materials for industrial applications due to their molecular sieve effect, high adsorption capacity, and frequently to the presence of catalytic sites or to their ion exchange capability. A relatively novel zeolite SSZ-16 belongs to the class of small pore zeolites characterized by 8- membered oxygen rings which may exhibit molecular sieve effect for gases with small molecules. The main field of its application is believed to be in gas separa- tion of various hydrocarbons and mixtures containing molecules such as CO2, H2, CH2 and other gases [1–3]. Furthermore, zeolite SSZ-16 can be used as microp- orous layers and particles to develop composite and mixed matrix materials [4–6]. Zeolite SSZ-16 was also tested as a catalyst. For example, Cu-SSZ-16 [7, 8] seems to be promising as a selective catalytic reduc- tion catalyst for the direct conversion of NO into N2 and O2. Zeolite H-SSZ-16 was found to be highly selective to dimethylamine in the catalytic reaction of ammonia and methanol [9]. The principal aim of this work is to optimize zeolite SSZ-16 hydrothermal synthesis and contribute to answering questions con- cerning limits to phase purity, yield and posibility to control morphology and size of crystals. Moreover, the research could clarify the role of diquaternary ammonium species in the synthesis. The principal aim of this work is to contribute to the knowledge of structure-directing agent (diquaternary ammonium dications) chemistry in relation to its (i) synthesis and purity, (ii) role in the synthesis of SSZ-16 zeolite, (iii) stability in synthesis batches and (iv) its thermal removal from the zeolite framework. 26 http://dx.doi.org/10.14311/APP.2017.9.0026 http://ojs.cvut.cz/ojs/index.php/app vol. 9/2017 Removal of diquaternary ammonium cations from as-synthesized SSZ-16 zeolite Figure 1. Left AFX framework type representa- tion using CBU and their arrangement (adapted from www.iza-online.org), right - AFX framework type rep- resentation using 6-6 SBU (adapted from Ilyushin et al. 2015). Figure 2. View of the AFX structure along the c- axis; yellow spheres - T-atoms, red spheres - oxygen atoms, blue patterns - visualization of electron density distribution. 2. Theory 2.1. Zeolite SSZ-16 and its structure Zeolite SSZ-16 is isotypic with its zeolite analogue SAPO-56 [7, 10, 11]. Both these microporous materi- als exhibit the AFX framework type. This framework shows the hexagonal symmetry, the corresponding space group of symmetry is P63/mmc and the unit cell idealized parameters are a = b = 13.7 Å, c = 19.7 Å. Fundamental articles on SSZ-16 present the AFX framework type in the terms of composite build- ing units (CBU): gmelinite (gme) and aft cavities and d6r (double six-ring) cages [10, 12, 13], see Fig- ure 1 (left). This method of AFX framework type representation is widely used to visualize spaces capa- ble of accommodating sorbing species. However, this method is less suitable for the analysis of various zeo- lite framework topology features due to the existence of common T-atoms for the neighbouring CBU. An al- ternative way of AFX framework representation is by secondary building units (SBU) using either an array of 6-membered rings (6 code) or double 6-membered rings (6-6 code), see Figure 1 (right). Figure 2 repre- sents the view of the AFX structure along the c-axis using the visualization of electron density distribution (computed by DFT calculation implemented in Castep program). Zeolites SSZ-16 and SAPO-56 belong to small-pore zeolites with 3D pore system. The narrowest passages through these pores (also called bottlenecks or con- Figure 3. 8-membered oxygen ring in SAPO-56 viewed normal to [001], red spheres - oxygen atoms, yellow spheres - T-atoms Figure 4. Structure of SDA - hexaethylpentane di- ammonium dication (blue spheres - nitrogen N+, grey - carbon, white - hydrogen). strictions) are formed by 8-membered oxygen rings. Dimensions of these rings were published for SAPO- 56 [11], see Figure 3. 2.2. Role of SDA species in synthesis of SSZ-16 Essential components for zeolite synthesis are silica, alumina, source of alkali, SDA and water. Theoreti- cal approaches offer a great number of hypothetical zeolite structures. However, only a small fraction of them can be synthesized. Besides, a consider- able number of zeolite phases can be formed only in the presence of particular organic species termed as structure-directing agents (SDA). A mechanism of SDA action in synthesis mixtures is still an open question. Attempts were made to explain SDA ef- fect on the basis of SDA species size and shape, its rigidity, hydrophobicity/hydrophilicity and SDA species interactions with solution components. Di- quaternary ammonium species were proposed in the nineties as a novel group of SDA species for the synthesis of zeolites. The research in our labora- tory showed that hexaethylpentane diammonium di- cation (C2H5)3N+(CH2)5N+(C2H5)3 (also called as Et6-diquat-52+), appears to be, among diquaternary ammonium species, the most efficient SDA for the synthesis of zeolite SSZ-16 [14]. The model of this SDA dication is shown in Figure 4. 27 T. Supinkova, I. Jirka, J. Drahokoupil et al. Acta Polytechnica CTU Proceedings Figure 5. Computer simulated location of SDA molecules in UC of SSZ-16 A theoretical picture of SDA species in SSZ-16 struc- ture was derived using the following assumptions: • A non-decomposed SDA dication can be accom- modated only in the aft cavity and thus oriented with its maximum dimension along the c-axis (see Figure 5). • Any aft cavity accomodates just a single SDA dica- tion. • There are two aft cavities per UC. Therefore, the UC contains two SDA dications non-decomposed during the synthesis procedure. • In principle, the gme cavities can accomodate only some products of SDA decomposition; this phe- nomenon is not considered in the theoretical esti- mate of SDA content. Based on these assumptions, the following formula for SDA dication mass fraction (gSDA)th in molar mass of UC for non-calcined zeolite SSZ-16 can be formulated as (gSDA)th = NSDA MSDA (MU C )0 + NSDA MSDA , where NSDA stands for the number of SDA per UC and is equal to 2, MSDA denotes the molar mass of SDA and equals to 272.32 g mol−1. (MU C)0 represents the molar mass of zeolite SSZ-16 free of SDA and can be expressed depending on the atomic ratio Si/Al = z as (MU C )0 = 48 MN a 1 + z + 48 MSi z 1 + z + 48 MAl 1 + z + 96 MO, for z = 6 and (MU C)0 = 3029.23 g mol−1. Based on these equations, the SDA dication mass fraction (gSDA)th can be calculated and it equals to 0.152. The experimentally measured SDA content (gSDA)th in the zeolite synthesis product will be com- pared to this value. 3. Experimental 3.1. Synthesis of SDA In view of the fact that the price of species containing Et6-diquat-52+ dication appeared to be prohibitive, a protocol for its synthesis was developed. The reac- tants used for this purpose were 1,5-dibromopentane and triethylamine. The reaction proceeded in ethanol solvent. The crystallization was performed in diethyl ether solvent. Because the synthesis proceeds in two- stages, the purity of the synthesis product were exam- ined by X-ray diffraction and by thermogravimetry. The heating program for SDA thermogravimetry was the same as for zeolite samples calcination and zeolite thermogravimetry, see below. SDA with significant content of the first stage synthesis product can thus be rejected or recycled. 3.2. Synthesis of SSZ-16 particles The research was directed to the development of a static in-situ hydrothermal crystallization proce- dure of SSZ-16 zeolite [14, 15]. The synthesis was performed in three steps: Preparation of starting synthesis mixture, ageing of synthesis mixture (for- mation of viable nuclei), and crystallization of aged mixture. The most suitable silica source appeared to be colloidal silica. The starting synthesis mix- ture was composed of colloidal silica LUDOX AS 30 (30% Sigma-Aldrich), aluminum nitrate nonahydrate, sodium hydroxide, organic template Et6-diquat-5 Br2 and deionized water [14, 16]. The synthesis process was adapted by changing the synthesis solution com- position, ageing period, temperature and duration of crystallization [15, 16]. The composition of synthe- sis solution was varied and optimized to obtain pure phase of SSZ-16 particles. The ageing period was performed for seven days to promote nucleation of precursor particles (formation of viable nuclei). The ageing process of SSZ-16 was performed at elevated temperature with the synthesis solutions heated up to 80 °C inside an oil bath, continuous stirring was ensured by a magnetic stirrer inside Teflon vessels. The crystallization process was performed for seven days, temperature adjusted to 160 °C. The hydrother- mal in-situ synthesis of SSZ-16 crystalline particles performed inside Teflon-lined stainless steel stationary autoclaves from the pre-aged solutions under autoge- nous pressure. The crystalline SSZ-16 products were purified inside an ultrasonic bath, washed and dried in a pre-heated oven overnight. After the synthesis, the SDA was removed by the two-cycle thermal calci- nation process under nitrogen/air atmosphere to open pores inside the aft cage. The temperature program for both the cycles was as follows: heating with the rate 0.5 °C/min to 120 °C, keeping at 120 °C for two hours, heating to the temperature 550 °C with the rate 0.5 °C/min, then keeping at 550 °C for 8 hours and cooling to the room temperature with the rate 0.5 °C/min. 28 vol. 9/2017 Removal of diquaternary ammonium cations from as-synthesized SSZ-16 zeolite 3.3. Characterization Zeolite SSZ-16 particles were characterized by Scan- ning Electron Microscopy (SEM) technique using JEOL JSM 5500LV to examine crystal morphology and particle size. Powder X-ray diffraction (XRD) technique was used to determine crystallinity and phase purity of zeolite SSZ-16 particles. X-ray diffrac- tion was performed on PANalytical X’Pert PRO diffractometer with Co anode (wavelength λ = 0.1789 nm) in Bragg-Brentano geometry. The fixed divergent slits in primary beam and 1D detector X’Celerator in diffracted beam were used. The spectra were collected in the 2θ range of 5-30°. The measurements and analy- ses of collected spectra were performed in the Institute of Physics, CAS. The micropore volume of calcined zeolite SSZ-16 particles was evaluated from nitrogen adsorption-desorption isotherms. The isotherms were measured on ASAP 2020 (Micromeritics, USA) vol- umetric instrument at -196 °C. The sample was de- gassed under a vacuum at 350 °C for 8 h prior to the analysis. The micropore volume was evaluated by the t-plot method. Particular attention was paid to SDA. It concerned SDA purity (XRD analysis), thermal sta- bility and its decomposition both in synthesis solutions and products. Thermal stability of SDA in alkaline solutions were analyzed after the heat treatment in an autoclave. SDA concentration was measured using cyclic voltammetry (CV) [17] with the home-made apparatus in J. Heyrovsky Institute of Physical Chem- istry, CAS. SDA content in zeolite samples and SDA residua after their calcination were measured using el- emental analysis performed on Elementar Vario EL III in CHNS operation mode, and further on with X-ray Photoelectron spectroscopy XPS using ESCA 3 Mk II spectrometer (VG) in fixed transmission mode. The kinetics of SDA removal/decomposition and related heat effects were investigated with thermogravime- try and differential scanning calorimetry using SetSys Evolution TGA, Setaram. 4. Results and Discussion Among silica sources for synthesis mixtures, LUDOX- based synthesis mixtures had the best properties both from the point of view of hydrogel density and ca- pacity of crystallization. Optimum synthesis batch composition to eliminate unwanted phases in product was determined as 3 SDA: 15 Na2O: 0.5 Al2O3: 30 SiO2: 1200 H2O. Deviation from this composition causes considerable changes in phase composition and formation of different zeolitic phases. The introduc- tion of LUDOX AS 30 as silica source had two con- sequences: (i) reducing the size of amorphous core of zeolite particles which was observed in the early stages of synthesis procedure development (see Figure 6A) and (ii) dramatic reducing the size of particles. The size of particles decreased to approximately 2 µm and well-developed polycrystalline particles formed (see Figure 6B). This favourable development of product quality manifests itself also (i) in X-ray spectra (see Figure 6. A - amorphous core and crystalline shell of a particle (2 000×), B - well developed polycrystalline particles (10 000×). Figure 7. XRD spectra of selected samples and theoretical spectrum of SSZ-16 Figure 7) and (ii) in SDA content measured gravimet- rically from mass difference between non-calcined and calcined samples, showing on selected zeolite samples the development of (gSDA)exp. In contrast to ear- lier preparations where amorphous cores were visible on disrupted particles and the values of (gSDA)exp ranged between 0.160 and 0.180, in more recent exper- iments the amorphous cores were not observed and (gSDA)exp ranged between between 0.140 and 0.150 approximately. It is suspected that amorphous cores exhibited a higher content of SDA as compared with zeolite phase. Figure 7 represents an example of the X-ray diffrac- tion spectra of selected calcined samples compared to the theoretical diffraction spectrum for the zeolite SSZ-16. The samples were confirmed to be crystalline and pure phase SSZ-16 particles. Regarding the content of organic species in the synthesized zeolite, a question arises whether some decomposition products of SDA cannot be located in gme cavities. The evaluation of SDA thermal stability in alkaline solutions performed by cyclic voltammetry showed a relatively rapid SDA decomposition. The time dependence of SDA concetration during its ther- mal treatment in alkaline solution at temperature, pH, and SDA concentration close to those in synthesis mix- tures is shown in Figure 8. It is obvious that during the reaction time 6 to 8 hours the SDA concentration decreases to a value lower than 50 % of its initial value. This may generate a considerable number of SDA frag- ments and enhance the mass of organics incorporated in SSZ-16 crystals. Nevertheless, for optimized synthe- sis procedure the values of (gSDA)exp remain in the range 0.140 to 0.150 which cast doubt on additional places of organics accommodation in SSZ-16 crystals. Accessibility of microporous crystal cavities after 29 T. Supinkova, I. Jirka, J. Drahokoupil et al. Acta Polytechnica CTU Proceedings Figure 8. Kinetics of Et6-diquat-52+ decomposi- tion at 160 °C. Initial concentration of Et6- diquat-5 Br2 c0SDA = 0.168 mol dm−3, initial concentration of NaOH c0SDA = 1.66 mol dm−3, solvent deionized water. Figure 9. N2 adsorption-desorption isotherm. Full circles - adsorption, empty circles - desorption. template removal was confirmed by measurements of adsorption and desorption isotherms of N2 at -196 °C. This result is exemplified by isotherms measured on calcined sample (Figure 9). The isotherm shows a sharp rise in N2 adsorption at low relative pressure and negligible hysteresis loop. Its shape shows that the material is microporous with negligible effect of mesopores. The sample showed the micropore volume of 0.24 cm3 g−1. At the same time, the elemental anal- ysis excluded presence of organic residua in crystal bulk after SSZ-16 calcination. This is a contrast to the situation in crystal subsurface region where carbona- ceous deposits were detected after zeolite calcination by XPS. The thermogravimetric curves monitored dur- ing SDA removal from zeolite samples using the above temperature program are exemplified on Fig- ure 10. Temperature calibration was performed using CuSO4.5H2O and Sn. On the picture is the compar- Figure 10. Examples of TG curves - percentage of the sample mass based on initial mass vs. temperature; red - silicalite-1 templated with TPA+; blue - SSZ-16 templated with Et6-diquat-52+. ison of SDA removal from silicalite-1 (TPA+) with that from SSZ-16 (Et6-diquat-52+). There is bigger amount of organics in SSZ-16 and steeper decay in the first portion of the TG curve. We suspect that it may be related to more rapid thermal decomposi- tion of Et6-diquat-52+ in synthesis batch as compared with TPA+ in silicalite-1 which is considerably more stable. We also suspect that SDA fragments may be incorporated into gme cavities. 5. Conclusions The principal contribution of this work is the feasi- bility of zeolite SSZ-16 synthesis. The selected SDA hexaethylpentane diammonium cation turned out to be appropriate and the synthesis route to this species was elaborated. By variation of synthesis parame- ters of aging and crystallization, the particle size was reduced from cca 50 µm to approximately 2 µm. More- over, the conditions to attain purity of SSZ-16 higher than 98 % were found and polycrystalline samples with well developed morphology were obtained. Using N2 adsorption and desorption at -196 °C, volume of micropores Vmicro = 0.24 cm3 g−1 was evaluated. An insight into the chemistry of SDA was presented. In particular, in its synthesis and conditions of its pu- rity. Further on, a novel technique was introduced to measure concentration variation of cationic SDAs under conditions close to those at synthesis conditions. Particular attention was also paid to the investigation of template removal kinetics. Acknowledgements The financial support by Czech Science Foundation via grant GA16-02681S is gratefully acknowledged. References [1] M. Pera-Titus et al. Porous inorganic membranes for co2 capture: Present and prospects. Chem Rev 114(2):1413–1492, 2014. doi:10.1021/cr400237k. [2] J. Caro et al. Zeolite membranes - recent developments and progress. Microporous Mesoporous Mater 15(3):215– 233, 2008. doi:10.1016/j.micromeso.2008.03.008. [3] S. G. Li et al. Improved sapo-34 membranes for co2/ch4 separations. Adv Mater 18(19):2601–2603, 2006. doi:10.1002/adma.200601147. 30 http://dx.doi.org/10.1021/cr400237k http://dx.doi.org/10.1016/j.micromeso.2008.03.008 http://dx.doi.org/10.1002/adma.200601147 vol. 9/2017 Removal of diquaternary ammonium cations from as-synthesized SSZ-16 zeolite [4] M. Moliner et al. Synthesis strategies for preparing useful small pore zeolites and zeotypes for gas separations and catalysis. Chem Mater 26(1):246–258, 2014. doi:10.1021/cm4015095. [5] R. Krishna et al. Separating n-alkane mixtures by exploiting differences in the adsorption capacity within cages of cha, afx and eri zeolites. Sep Purif Tech 60(3):315–320, 2008. doi:10.1016/j.seppur.2007.09.008. [6] N. Kosinov et al. High flux high-silica ssz-13 membrane for co2 separation. J Mater Chem A 2(32):13083–13092, 2014. doi:10.1039/c4ta02744b. [7] D. W. Fickel et al. The ammonia selective catalytic reduction activity of copper-exchanged small-pore zeolites. Appl Catal B 102(3-4):441–448, 2011. doi:10.1016/j.apcatb.2010.12.022. [8] M. J. Wulfers et al. Conversion of methane to methanol on copper-containing small-pore zeolites and zeotypes. Chem Comm 51(21):4447–4450, 2015. doi:10.1039/C4CC09645B. [9] H. Y. Jeon et al. Catalytic evaluation of small-pore molecular sieves with different framework topologies for the synthesis of methylamines. Appl Catal A 305(1):70–78, 2006. doi:10.1016/j.apcata.2006.02.044. [10] R. F. Lobo et al. Synthesis and rietveld refinement of the small-pore zeolite ssz-16. Chem Mater 8(10):2409–2411, 1996. doi:10.1021/cm960289c. [11] S. T. Wilson et al. Synthesis, characterization and structure of sapo-56, a member of the abc double-six- ring family of materials with stacking sequence aabbccbb. Microporous Mesoporous Mater 28(1):125– 137, 1999. doi:10.1016/S1387-1811(98)00293-5. [12] Y. Bhawe et al. Effect of cage size on the selective conversion of methanol to light olefins. ACS Catal 2(12):2490–2495, 2012. doi:10.1021/cs300558x. [13] P. Y. Feng et al. Synthesis and single crystal structure of an afx-type magnesium aluminophosphate. Microporous Mesoporous Mater 50(2-3):145–149, 2001. doi:10.1016/S1387-1811(01)00441-3. [14] S. H. Lee et al. Zeolite synthesis in the presence of flexible diquaternary alkylammonium ions (c(2)h(5))(3)n(+)(ch(2))(n)n(+)(c(2)h(5))(3) with n=3-10 as structure-directing agents. Microporous Mesoporous Mater 60(1-3):237–249, 2003. doi:10.1016/S1387-1811(03)00381-0. [15] P. Hrabanek et al. Static in-situ hydrothermal synthesis of small pore zeolite ssz-16 (afx) using heated and pre-aged synthesis mixtures. Microporous Mesoporous Mater 228:107–115, 2016. doi:10.1016/j.micromeso.2016.03.033. [16] P. Hrabanek et al. Combined silica sources to prepare preferentially oriented silicalite-1 layers on various supports. Microporous Mesoporous Mater 174(1-3):154– 162, 2013. doi:10.1016/j.micromeso.2013.03.007. [17] J. Langmaier et al. Extreme basicity of biguanide drugs in aqueous solutions: Ion transfer voltammetry and dft calculations. J Phys Chem A 120:7334–7350, 2016. doi:10.1021/acs.jpca.6b04786. 31 http://dx.doi.org/10.1021/cm4015095 http://dx.doi.org/10.1016/j.seppur.2007.09.008 http://dx.doi.org/10.1039/c4ta02744b http://dx.doi.org/10.1016/j.apcatb.2010.12.022 http://dx.doi.org/10.1039/C4CC09645B http://dx.doi.org/10.1016/j.apcata.2006.02.044 http://dx.doi.org/10.1021/cm960289c http://dx.doi.org/10.1016/S1387-1811(98)00293-5 http://dx.doi.org/10.1021/cs300558x http://dx.doi.org/10.1016/S1387-1811(01)00441-3 http://dx.doi.org/10.1016/S1387-1811(03)00381-0 http://dx.doi.org/10.1016/j.micromeso.2016.03.033 http://dx.doi.org/10.1016/j.micromeso.2013.03.007 http://dx.doi.org/10.1021/acs.jpca.6b04786 Acta Polytechnica CTU Proceedings 9:26–31, 2017 1 Introduction 2 Theory 2.1 Zeolite SSZ-16 and its structure 2.2 Role of SDA species in synthesis of SSZ-16 3 Experimental 3.1 Synthesis of SDA 3.2 Synthesis of SSZ-16 particles 3.3 Characterization 4 Results and Discussion 5 Conclusions Acknowledgements References