Structural investigations in pure-silica and Al-ZSM-12 with MTEA or TEA cations
Graphical abstract
Introduction
Zeolite ZSM-12 (MTW framework type) belongs to the family of 1-dimensional zeolites with 12-membered rings which run along the b-edge. Although not commonly used for industrial purposes, it shows potential for use as a catalyst at different processes in petrochemical industry, e.g. cracking, hydrocracking, different alkylations and isomerisations [1]. The fundamental properties of the framework comprise the following: unit cell parameters, a = 25.552, b = 5.256, c = 12.117 Å, β = 109.312°, V = 1535.78 Å3, space group C2/m [2,3]. The framework topology was first determined by LaPierre and co-workers [4] using a combination of electron and X-ray diffraction with model building leading to C2/m cell with parameters a = 24.88(4), b = 5.02(2), c = 12.15(3) Å, β = 107.7(1)° and seven symmetrically independent silicon atoms, confirmed by the 29Si NMR study of Trewella et al. [5]. Additional studies, especially combinations of complementary synchrotron XRPD and NMR, further confirmed the C2/c framework symmetry. In this case, the unit cell is doubled along c axis, yielding a = 24.8633(3), b = 5.01238(7), c = 24.3275(7) Å, β = 107.7215(6)° [6].
To date, ZSM-12 can be prepared in the presence of several different organic cations (organic structure directing agents, OSDA), most of which are quaternary ammonium cations. A few reports on OSDA-free synthesis of ZSM-12 are also available [7,8]. The most frequently used OSDA is the tetraethylammonium cation (TEA). We observed that the powder pattern of PS-ZSM-12 with TEA contains additional diffraction peaks which have been overlooked in the past [9]. These are not due to impurities, and additionally they disappear with the removal of the TEA from the framework. It was proven that additional diffraction peaks are due to ordering of TEA cations (tt.tt conformation) in the ZSM-12 channels along b axis and that leads to a formation of superstucture with a three times larger unit cell [9]. However, a few questions remain unanswered; firstly, does the inclusion of Al3+ in the zeolite framework affect the ordering of the TEA cations? If so, what is the highest concentration of Al3+ that enables the formation of the superstructure? On the other hand, methyltriethylammonium cations (MTEA) are of a similar size with similar physico-chemical characteristics as TEA and it also enables the formation of MTW framework [10]. The question here is, whether MTEA cations also order in the channels of ZSM-12 or not? Therefore, the aim of this study was to explore the presence of ordering of TEA and MTEA cations when trapped in the zeolite channels of pure-silica and aluminum-containing ZSM-12.
Section snippets
Synthesis
Pure-silica ZSM-12 with MTEA was prepared according to a modified double-silica source method published by Mitra et al. [11]. The molar ratio between reactants was as follows: 1 Na2SiO3: 147 H2O: 3.80 MTEAOH: 12.50 SiO2. Initially, 0.325 g (2.67 mmol) of Na2SiO3 was dissolved in distilled water (1.75 g, 0.097 mol), followed by dropwise addition of 6.67 g of MTEAOH (20%, Sigma Aldrich) to the clear solution. The mixture was allowed to stir at room temperature for 0.5 h and then, 2.00 g of SiO2
Characterization of the framework
ZSM-12 is a representative of high-silica zeolites. For almost a decade, the lowest achieved molar ratio Si:Al was ∼30 [12], i.e. the highest achievable mass per cent of Al3+ in a calcined framework was believed to be ∼1.5%. Further modification of the synthesis conditions (e.g. different source of Al, SiO2 and OSDA) led to lower Si:Al ratios were obtained: 10.4–14.6 by Kamimura et al. using an OSDA-free synthesis approach [7] and ∼12 by Paris et al. by employing of combination of organic and
Conclusions
Pure-silica as well as aluminosilicate ZSM-12 frameworks were prepared with assistance of TEA and MTEA cations. The results of standard physico-chemical characterization techniques have shown that the inclusion of Al3+ into the framework does not significantly affect the ordering of TEA cations which order also in aluminosilicate ZSM-12 framework in the same manner as in pure-silica ZSM-12. Namely, in the XRD powder patterns of both zeolites, a few additional weak superstructural peaks appear
Acknowledgements
This work was supported by the Slovenian research agency [grant numbers MR-29397, P1-0021 and P1-0175] and EPSRC [grant number EP/K007467/1]. The authors appreciate the help of prof. dr. Nataša Bukovec (thermogravimetric analyses) and prof. dr. Marjan Marinšek (SEM measurements).
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