Solid State NMR Method for the Determination of 3D Zeolite Framework/Sorbate Structures:  ¹H/²⁹Si CP MAS NMR Study of the High-Loaded Form of p-Xylene in ZSM-5 and Determination of the Unknown Structure of the Low-Loaded Form

Abstract

A general protocol is described for structure determinations of organic sorbate-zeolite complexes based on the selective, through-space, distance-dependent transfer of magnetization from protons in selectively deuterated organics to framework silicon nuclei. The method was developed using the known structure of the high-loaded ZSM-5/p-xylene complex containing p-xylene-d(6) or p-xylene-d(4). It was then applied to determine the unknown structure of the low-loaded ZSM-5/p-xylene complex using NMR alone. For the high-loaded complex improved data were obtained below 273 K, where slow motions and exchange processes of the p-xylene are eliminated. The general approach was validated by the exact agreement of the experimental (1)H-(29)Si CPMAS spectra obtained at a specific contact time and the complete 24-line spectra simulated using 1/T(CP) vs M(2) correlations from only the six clearly resolved resonances. For the low-loaded complex the (29)Si resonances were assigned at 267 K, and variable contact time CP experiments were carried out between 243 and 173 K using the same specifically deuterated p-xylenes. All possible locations and orientations of the p-xylene guests were sampled, and those solutions that gave acceptable linear 1/T(CP) vs M(2) correlations were selected. The optimum p-xylene location in this temperature range was determined to be in the channel intersection with the long molecular axis parallel to [0,1,0] (ring center fractional coordinates {-0.009, 0.250, 0.541}) with the ring plane oriented at an angle of 30 +/- 3 degrees about the crystallographic b axis. A subsequent single-crystal X-ray study confirmed this predicted structure.

NMR spectroscopy of citrate in solids: cross‐polarization kinetics in weakly coupled systems

“…The role of homonuclear dipolar couplings is discussed further below. A similar correlation curve was obtained for 29 Si{ 1 H} CP/MAS by Fyfe et al [45] in a study of ZSM-5/p-xylene. 2 )].…”

“…These differences can be attributed in part to the differences of 1 H homonuclear dipolar coupling between these systems, but the different ν 1 used in Ref. [45] could also have an effect.…”

A three dimensional integral equation approach for fluids under confinement: Argon in zeolites

“…In fact, zeolite design for catalytic applications has also been inspired by the processes of molecular recognition characteristic of enzymes, 14 where solvation is an essential element. Experimental techniques such as FTIR and NMR spectroscopy, 15,16 or neutron scattering 17,18 provide very valuable information in this context, but if one desires a detailed three dimensional map of the confined fluid structure, the use of computer simulation is unavoidable. The calculation of density maps, particularly in regions where the sampling is poor due to problems of accessibility might turn into a computationally lengthy process.…”

A structure refinement strategy for NMR crystallography: An improved crystal structure of silica-ZSM-12 zeolite from 29Si chemical shift tensors

“…The areas of present research in NMR crystallography can be roughly categorized according to the classes of materials being studied: inorganic ''network" materials such as zeolites and related materials [1][2][3][4][5][6][7][8], organic molecular crystals [9][10][11][12][13][14][15][16][17], and bio-molecules [18][19][20][21][22][23]. The challenges for each class of materials are somewhat different.…”

“…Two-dimensional (2D) correlation experiments, in which proximities and connectivities between atoms can be established, may also be employed to identify possible space groups [29], to aid in assignment of the NMR resonances [13,[30][31][32][33][34][35][36][37][38][39], or to derive structural models [7,8]. The distance information provided by solid-state NMR can be used to derive or constrain structural models [5,6,[40][41][42] in combination with other methods. In some cases, 2D correlation experiments have even been extended to an additional dimension in order to provide short and long-range distance information that has been used to actually solve crystal structures directly from the NMR data alone [1,2] or in combination with molecular modeling [9,10].…”