STRUCTURAL STUDY OF NAY AND NAX ZEOLITES WITH CHEMISORBED METHYLIUM IONS BY POWDER NEUTRON DIFFRACTION
S.Vratislav1 , M.Dlouhá1 and V.Bosácek2
1Faculty
of Nuclear Physics and Physical Engineering, 180 00 Prague 8,
Czech Republic
vratisla@troja.fjfi.cvut.cz
2Heyrovský Institute of Physical Chemistry, 182 23
Prague 8, Czech Republic
Keywords: neutron
diffraction, 13C MAS NMR, crystal structure, NaY and
NaX zeolites, surface methyl groups, chemisorbed methylium ion
Continued interest in the structure investigation of zeolites (mainly of the synthetic origin) is stimulated by their potential practical use in the chemical technology. Zeolites exhibiting regular structure which can be easily modified, are important candidates for very effective catalysts. Problems of adsorbed and chemisorbed molecules in zeolitic structures are therefore permanently topics of discussion [1].
The crystal structure of NaY and NaX zeolite with chemisorbed methylium ion have been investigated by powder neutron diffraction and 13C MAS NMR method. Neutron powder diffraction patterns were collected at 7 K on the KSN-2 diffractometer which is placed at the LVR-15 research reactor in Rez near Prague. The complete structural parameters were determined by Rietveld analysis of powder neutron diffraction data using the GSAS package. Difference Fourier maps were calculated to localize the nonframework cations and the CD3 groups. Experimental evidence of the created chemisorbed methyl groups after reaction of methyliodide with Na+ cations of the zeolite was obtained from 13C MAS NMR spectra as described earlier [2]. Proton decoupled 13C MAS NMR spectra were measured on BRUKER DSX 200 spectrometer equipped with magic angle spinning (MAS) probehead. This method allowed to controle the conversion of methyliodide to surface methoxy species on a quantitative level. Chemisorbed methyls exhibit 13C NMR signal at 56.5 ppm TMS which is typical for surface methoxy groups on NaY and two signals at 55.3 and a shoulder at 58 ppm on NaX.
A powder diffraction and 13C MAS NMR study of the structures of NaY zeolite with chemisorbed CD3+ species created by a reaction of Na+ cations with d-methyliodide show that chemisorbed methyl groups are preferentially located in alfa cages at O(l) oxygen sites. A complete set of the structural parameters in the frame of Fd3m space group for unperturbed NaY (a = 2.4851(7) nm, Rwp = 0.0692, Rp = 0.0514) and for NaY after the formation of surface methoxy groups (a = 2.4897(8) nm, Rwp = 0.0712, Rp = 0.0568) were given and an influence of chemisorbed species on the distribution of Na+ cations in the lattice was detected by neutron diffraction. Our results show that the population of cationic sites has been changed significantly after the chemisorption of methyl iodide. While the occupation of SII in NaY without adsorbate was 32 Na+ per unit cell (i.e. 100%), after the chemisorption of CH3I was found 19.6 (61%) and in case of CD3I 21.4 (67%). On the same samples also a significant decrease of population in SI´ was detected accompanied with a slight increase of population in SI sites.
Rietveld analysis of the neutron diffraction data of NaX samples led to the complete set of the structural parameters for both the origin NaX sample and that with chemisorbed methyl species. The structural analysis was treated in frame of Fd3 space group with a = 2.4976(7) nm, Rwp = 0.0516, Rp = 0.0468 (origin NaX sample) and with a = 2.4895(6) nm, Rwp = 0.0523, Rw = 0.0487 (chemisorbed NaX sample). Our structural parameters for the origin NaX sample are in fully agreement with results presented by Olson [3]. For chemisorbed NaX sample we have determined the occupation numbers of cations and the location of CD3 groups with the center in the position 96g (0.387, 0.387, 0.119). The occupation numbers of Na cations in chemisorbed NaX were decreased for SI´ and SI´´ and increased for SIII in comparison with the initial NaX. SII is practically fully occupied in both cases. The treatement of our diffraction data is in progress.