High-resolution heteronuclear correlation NMR spectroscopy; applications to heterogeneous catalysis

We describe the catalytic applications of new solid-state NMR experiments, which provide high-resolution two-dimensional heteronuclear correlation (HETCOR) spectra using through-space and through-bond interactions between nuclear spins. These methods are applied to several microporous framework aluminophosphates and to a new type of ZSM-5-based petrochemical catalysts. Solid-state nuclear magnetic resonance (NMR) spectroscopy is recognized as one of the primary spectroscopic techniques for characterization of structural and physicochemical properties of various classes of catalytic materials. For more than two decades, NMR studies relied heavily upon the use of magic angle spinning (MAS) and various decoupling schemes to obtain the spectra of H, C, Si, Al and other nuclei. The continued development of high-resolution techniques and instrumentation has resulted in remarkable new capabilities and applications of solidstate NMR in catalysis. Much of this progress stemmed from the recognition that the structural properties of complex catalytic systems can be much better characterized by using the multi-dimensional NMR techniques that involve several different types of nuclei. An important group of such methods involves heteronuclear correlation (HETCOR) spectroscopy, which can be used to identify the nuclei that remain in spatial proximity of each other. Until recently, HETCOR NMR spectra between two nuclei in solids were obtained under MAS by using cross-polarization (CP) for mixing. In the case of half-integer quadrupolar nuclei (e.g., O, Al, Na), such experiments are poorly resolved because the second-order quadrupolar interaction is not fully averaged out by MAS. However, the discovery of multiple-quantum (MQ) MAS NMR provided a tool for acquisition of isotropic NMR spectra of half-integer quadrupolar spins, which, in turn, raised the possibility of measuring the heteronuclear interactions of these nuclei with their neighbors under high resolution in both dimensions. We demonstrate two groups of high-resolution HETCOR NMR methods and their specific applications in catalysts. (1) Figure 1 shows a two-dimensional Al→P HETCOR NMR spectrum of AlPO4-14 aluminophosphate obtained using 3QMASHETCOR. The resolution in the Al dimension of this spectrum is dramatically improved when compared with the standard MAS method (not shown). The spectrum required only a few hours of the spectrometer time, and properly revealed throughspace connectivities between phosphorus and aluminum in this well crystallized sample. (2) Although most of the internuclear correlation methods used in solid-state NMR were confined to utilization of strong dipolar couplings, through-bond (J) Figure 1. Al→P spectrum of AlPO4-14 catalyst obtained with the 3QMAS-HETCOR method. All Al and P sites are resolved in the spectrum; the relative intensities of cross-peaks are in good agreement with those predicted from the structure. The individual Al and P projections are shown in the central part of the figure. correlation experiments have been also reported. These methods are based on homoand heteronuclear mixing schemes used in solutions, as well as new schemes especially dedicated to solids. Again, the J coupling based schemes reported hitherto suffer from a second order quadrupolar broadening. Herein, we report a twodimensional experiment, MQMAS-J-HETCOR, which provides a chemical bond map between quadrupolar and spin-1⁄2 nuclei in solids under isotropic resolution. High selectivity, lack of orientational dependence and insensitivity to molecular motion proved useful in exploiting of this method for studying the bond topology and local order in catalysts. The above methods can be used in the studies the structure of various molecular frameworks, their thermal transformations and their interactions with other (e.g. adsorbed or reacting) molecules. We demonstrate several examples of applications of HETCOR NMR to the studies of aluminophosphates. Most recently, these methods proved very useful in our investigations of a series of industrial ZSM-5 zeolites, which were modified by adding phosphorus in order to increase olefinicity or the octane number in catalytic cracking. The MQMAS approach combined with the use of high magnetic field allowed for detailed analysis of aluminum species in these samples. The HETCOR NMR techniques provided detailed insight into the chemical nature and location of phosphorus species with regard to the zeolite framework and led to a better understanding of their role in improving the catalytic performance. 1. G. Engelhardt and D. Michel, "High-Resolution Solid-State NMR of Silicates and Zeolites", John Wiley & Sons, Chichester, 1987. 2. L. Frydman, J.S. Harwood, J. Am. Chem. Soc., 117 (1995) 5367. 3. S.H. Wang, S.M. De Paul, L.M. Bull, J. Magn. Reson., 125 (1997) 364; C. Fernandez, C. Morais, J. Rocha, M. Pruski, Solid State Nucl. Magn. Reson., 21 (2002) 61. 10 0 -10 -20 -30 -40 60 40 20 0