27Al MAS NMR has widely been used to understand of the aluminium coordination state in zeolite catalysts ^[1,2]. As a quadrupolar nucleus, the ²⁷Al second order broadening can not be averaged by the magic angle spinning, making the spectrum not well resolved. There are still some arguments on the different coordinations of the aluminium in zeolites, especially in USY, there are several conclusions concerning the ²⁷Al NMR signal at about 30 ppm, i.e. it is a four-coordinated Al shifted upfield due to the quadrupolar broadening ^[3], or it is penta-coordinated Al ^[4], or overlap of both four and five-coordinated Al species ^[5]. By the technique of Double Rotation (DOR) NMR ^[6], it was found that there was a 47.5 ppm signal due to tetrahedral coordinated Al in a single thermally treated zeolite Y sample, while in a twice hydrothermally treated sample, the 30 ppm was attributed to the penta-coordinated Al species. Also by DOR method ^[7], it was shown that part of the tetrahedral Al species suffered from the qudrupolar effect and a 20 ppm signal in the DOR NMR spectrum was uncertainly assigned. By a spin-lattice relaxation experiment on USY ^[8], this 30 ppm hump was considered not due to a tetrahedral framework Al site, but was a mixture of at least three Al species. While on the contrary, this peak was considered as in the framework in the study of dealuminated H-Y ^[9]. This 30 ppm peak was also reported by Barras et. al ^[10] in the calcined mordenite.
    The 2D multiple quantum MAS NMR method ^[11,12] can refocus the second order anisotropic broadening of the half-integer quadrupolar nuclei by selecting multiple quantum transition and transferring it to the single quantum signal in two time domains and isotropic resolution can be obtained. Here this method is expected to enhance the resolution of the Al sites in modified zeolites. In this letter, we report the ²⁷Al MQ MAS experiments on the calcined zeolite mordenite.

1 EXPERIMENTAL
Samples: The H-form mordenite parent sample is commercially available, with the Si/Al ratio of 6.2. The samples were calcined in muffle for 2 hours at temperatures of 550, 600, 650, 700°C, respectively. All the calcined samples were put in desicator with NH₄Cl solution for fully hydration.
    NMR measurements: All the ²⁷Al NMR spectra were measured on Bruker MSL-400 spectrometer, at 104.26 MHz. The MAS spin rate is 12.5 kHz. A short pulse of 0.6 ms was used (corresponding to p/18 flip angle) for the 1D ²⁷Al MAS NMR. A "split-t1" 3Q pulse sequence ^[13] was used for the triple-quantum MAS NMR experiments to obtain a sheared spectrum without further treatment of the 2D data, with three pulse lengths of 3.5, 1.2 and 0.6 ms, and rf strength of about 100 kHz. 200 FIDs for each experiment were accumulated with the time increment of 5 ms, and 1920 transitions accumulated for each FID. TPPI method was used in the 2D data acquisition and processing, and the sheared 2D spectra can be obtained directly by the 2D Fourier transformation. ²⁷Al chemical shift was referenced to Al(NO₃)₃ solution.

2 RESULTS AND DISCUSSION
Figure 1 displays the ²⁷Al MAS NMR spectra of the parent, 550, 600, 650, 700°C calcined H-mordenite samples. In the spectrum of parent HM, the 55 ppm signal is assigned to the framework tetrahedral Al sites, and the 0 ppm is assigned to the extraframework Al sites. Although some framework octahedral coordianted Al may appear at 0 pmm in HY as reported recently ^[14], here our main interest is in the assignment of the 30 ppm signal. It should be noted that there is already trace of 30 ppm signal, which may due to the acid washing and calcination at 400°C during the preparation of the acid form sample, and some dealumination took place corresponding to the 0 ppm peak.

Fig.1 1D 27Al MAS NMR spectra of HM (a) parent sample, (b) 550°C calcined, (c) 600°C calcined, (d) 650°C calcined, (e) 700°C calcined

    In the spectra of the several calcined samples (Fig.1, b-e), besides the 55 and 0 ppm peak, there gradually emerges a broad line at around 30 ppm with the increase of the calcination temperature and the 0 ppm signal gradually became broadened. When the intensities of the spectra were compared, it is shown that there is a gradual intensity loss and for the 700°C calcined sample, a 15-20% intensity loss was found compared with the parent sample. This intensity loss is obviously due to the increas of quadrupolar constant of some Al species.
    An enhanced resolution can be obtained by the 2D 3Q MAS NMR method, as shown in Fig.2. In the spectrum of parent HM (Fig.2a), besides the normal framework tetrahedral coordinated Al (signal A, some artificial peaks along F1 dimension are due to the cut-tail effect in F1 dimension) and octahedral coordinated Al (signal B), a small signal C appears near the four-coordinated framework site (signal A) and is corresponding to the small hump at about 30 ppm in the 1D spectra (Fig.1a). By its position, this signal should also be regarded as four-coordinated and we assign it to some distorted Al formed in the preparation of the H-from sample. In the spectrum of the 550°C calcined sample (Fig.2b), there still are signals A and B, while only the distorted 4-coordinated Al signal C's intensity increased, and it stretched along the F2 dimension implying that its broadening is due to the quadrupolar interaction, which is stronger than that of the framework Al (signal A). When the calcination temperature increased to 600°C (Fig.2c), the intensity of signal C became stronger, aligning along the F2 axis, and could no doubt be assigned to the distorted four-coordinated Al species. The broad 30 ppm hump in it corresponding 1D spectrum (Fig.1d) results from this kind of Al species.

Fig.2 2D 27Al MQ MAS NMR spectra of (a) parent sample, (b) 550°C calcined, (c) 600°C calcined, (d) 650°C calcined, (e) 700°C calcined, ** denote spining side bands.

    It should be noticed that in the MQMAS spectrum of the 650°C calcined samples (Fig.2d), besides the signals A, B, C, there appears another weak signal D. By its position on the 2D MQMAS spectra, the Al species giving rise to signal D should be penta-coordinated. In the MQMAS spectrum of the 700°C calcined sample (Fig.2e), the signal D becomes more pronounced. In the one-dimensional spectrum, this signal should also give rise to the 30 ppm broad hump.
    It can also clearly be seen from the MQMAS spectra that signal B gradually became broad in both F2 and F1 dimensions, implying that its Cq as well as other environmental parameters became more and more dispersed.

Fig. 3 Simulation of the slice of signal C in figure 2d with calculated quadrupolar powder pattern, with Cq is 5.8 MHz, h is 0.4, and dcs iso is 59 ppm.

    By the ²⁷Al quadrupolar nutation NMR experiments in USY ^[5], the quadrupolar coupling constant (Cq) of the 30 ppm signal species was determined to be 4.5-5 MHz or > 6MHz for the different Si/Al ratio of the samples, and also by the DOR method ^[6], a Cq of 6.2 MHz was obtained for the distorted tetrahedral coordinated Al species in the dealuminated HY. Here in Fig.3 the simulation of the slice of the signal C in the 2D spectrum of 650°C calcined sample (Fig.2d) is shown. A calculated powder pattern ^[15] with quadrupolar constant (Cq) of 5.8 MHz and electric gradient asymmetry parameter (h) of 0.4 was used in the simulation and a Gaussan line broadening width of 50 Hz was used. The isotropic chemical shift of the calculated pattern was at 59 ppm, giving evidence it is four-coordinated Al species. There should be some distortion of line shape due to the multiple quantum transition excitation, so the the simulation parameters are only estimated values.
    Based on the fact that the intensity of the central transition of quadrupolar nuclei is related with the flip pulse length ^[16], we performed the 1D ²⁷Al MAS NMR with different pulse lengths on the 650°C calcined sample and the results were displayed in figure 4. Compared with the spectrum with a very short pulse (0.61 ms), when the pulse length increased to 2 ms, all the components of signals (55, 30, 0 ppm) increased. At pulse length of 3.5 ms, the 30 ppm signal began to decrease, and totally come to nearly zero at 5.5 ms pulse length. With 6 ms pulse length that signal became negative. This gives another evidence that in the 1D ²⁷Al MAS NMR spectrum of 650°C calcined sample (Fig.1d) the 30 ppm hump was dominated by the quadrupolar broadening effect.

Fig.4 1D 27Al MAS NMR spectra of 650°C calcined HM, with different pulse-length indicated in the figure.

3 CONCLUSION  
In the 1D ²⁷Al MAS spectra of the parent, 550, 600, 650, 700°C calcined zeolite mordenite, three signals of Al species can be resolved, in which the 55 ppm and 0 ppm are due to four-coordinated and six-coordinated Al, respectively. With the increasing of the calcination temperature, the 30 ppm broad signal gradually appears, which is difficult to be assigned based on the 1D experiments. By the ²⁷Al MQ MAS NMR method, it can be concluded that during the calcination of mordenite, some framework Al gradually become distorted to give rise to the 30 ppm shoulder due to the second order quadrupolar broadening. With the increasing of the temperature to 650°C, penta-coordinated Al starts to appear. The MQMAS NMR method is expected to be a powerful method in the signal assignment of Al sites in other types of thermal treated zeolites.

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