Distribution of Tetrahedral Aluminium Sites in ZSM-5 Type Zeolites: An 27 Al (Multiquantum) Magic Angle Spinning NMR Study Priit Sarv* Institute of Chemical Physics and Biophysics, Akadeemia 23, EE0026 Tallinn, Estonia Christian Fernandez and Jean-Paul Amoureux Laboratoire de Dynamique et Structure des Mate ´ riaux Mole ´ culaires, CNRS URA801, UniVersite ´ des Sciences et Technologies de Lille, F-59655 VilleneuVe d’Ascq Cedex, France Kari Keskinen Neste Oil, Catalysis Research, Technology Centre, P.O. Box 310, FIN-06101 PorVoo, Finland ReceiVed: August 15, 1996; In Final Form: October 16, 1996 X Up to now the distribution of Al atoms in the zeolite lattice could be monitored mainly by 29 Si MAS NMR. The data about the nonequivalent aluminium T sites, present in the 27 Al MAS NMR spectra, was obscured by second-order effects of quadrupolar interaction. By applying the five-quantum 27 Al MQMAS NMR method to ZSM-5 type zeolites, we were able to distinguish at least two nonequivalent aluminium T sites in the H-ZSM-5 and establish the relation between 27 Al, 29 Si, and 1 H NMR data. Comparison with 29 Si MAS NMR spectra gives information about the distribution and siting of aluminium in the zeolite framework. Introduction Zeolites are porous aluminosilicates made up from corner and edge-sharing SiO 4 and AlO 4 tetrahedra. In commercial zeolites, the ratio of Si/Al is usually in the range of 1-100 and distribution of Al and Si over the tetrahedral sites (T sites) is disordered. Over the past decade high-resolution solid-state NMR spectroscopy has established itself as a powerful technique for characterization of zeolites and related materials with respect to structure elucidation, catalytic behavior, and mobility properties. 1-3 In the case of low Si/Al ratio zeolites, the 29 Si magic angle spinning (MAS) NMR spectra of the simplest systems show five resonances corresponding to the five local silicon environments: Si[4Al], Si[3Al, Si], Si[2Al, 2Si], Si[Al, 3Si], and Si[4Si]. 4 In the more complex systems (ZSM-5, mordenite, ferrierite), resonances belonging to crystallographi- cally nonequivalent T sites can be resolved, but because of the lack of Si/Al ordering the resonances are relatively broad, about 3 ppm. Removal of all the aluminium from zeolites produces completely siliceous structures, which are now perfectly ordered, and the 29 Si MAS NMR spectra show very sharp resonances (hwhh < 0.5 ppm), whose number correspond to crystallo- graphically nonequivalent sites in the unit cell and whose relative intensities reflect the population of these sites. 5 The chemical shift (CS) of Si[4Si] sites and the average T-O-T bond angle of these sites have a linear relationship. 6 The primary informa- tion one can get from 27 Al MAS NMR spectra is related to coordination of Al atoms. 1 Usually the tetrahedral lattice aluminium in zeolites gives only one line with the CS in the range of 50-65 ppm. Lippmaa et al. have shown that in analogy to silicon T-O-T sites there is a linear relationship between the CS and the mean Al-O-Si bond angle of tetrahedral Al atoms in aluminosilicates. 7 Accordingly, one should be able to resolve crystallographically nonequivalent sites also in 27 Al MAS NMR spectra of zeolites. For example, in the 29 Si MAS NMR spectrum of ZSM-5 we can see a -113 ppm peak and a shoulder at -115 ppm, but in the 27 Al MAS NMR spectrum there is only one peak, assigned to framework aluminium. 1 So far, only in zeolite omega 8 and lately also in MCM-22 9 the crystallographically nonequivalent framework sites of aluminium could be detected. The reason for this is the overlap of lines caused by the second-order broadening and line shift of 27 Al resonances due to quadrupolar interaction. 7 Recently a two-dimensional (2D) multiquantum MAS (MQ- MAS) NMR experiment of quadrupolar nuclei was proposed by Frydman and Harwood 10 and further developed by Fernandez and Amoureux. 11,12 In this experiment it is possible to separate the contributions from the two interactionssCS and quadrupole interaction (QI), and one can get information about the distribu- tion of CS of 27 Al (nonequivalent sites) and electric-field gradient (EFG) on these sites respectively. In this Letter we study what new knowledge can we get by applying this MQMAS method to various ZSM-5 type zeolites. Experimental Section The MAS NMR spectra were measured on Bruker AMX500 spectrometer using a homemade (3.5 mm o.d. rotor) probe head. The spinning speed was approximately 15 kHz. AMX500 was equipped with an additional 250 W power amplifier to ensure the radio-frequency (rf) field strength of 120 kHz. KAl- (SO 4 ) 2 ·12H 2 O was used to measure the rf power and to set the magic angle. 29 Si MAS NMR spectra were measured with a 30° flip angle and with an 8 s repetition delay. 1 H MAS NMR spectra were measured with a 30° flip angle and with a 10 s repetition delay. 29 Si spectra were deconvoluted using Gaussian lines, and 1 H spectra were deconvoluted using Lorentzian lines. 13 Deconvolution was done with a Bruker 1D WIN-NMR program. To perform five-quantum MQMAS experiments on zeolite samples, a precise setting of experimental parameters is needed. We give a brief description of the experiment, but more details can be found elsewhere. 11,12,14-17 The MQMAS experiments were performed with a two-pulse sequence. The first pulse has the length optimized to obtain the best efficiency for the multiquantum ((5Q) coherence creation. The second pulse is X Abstract published in AdVance ACS Abstracts, November 15, 1996. 19223 J. Phys. Chem. 1996, 100, 19223-19226 S0022-3654(96)02519-1 CCC: $12.00 © 1996 American Chemical Society