Locating Organic Guests in Inorganic Host Materials from X-ray Powder Diffraction Data

Abstract

Can the location of the organic structure-directing agent (SDA) inside the channel system of a zeolite be determined experimentally in a systematic manner? In an attempt to answer this question, we investigated six borosilicate zeolites of known framework structure (SSZ-53, SSZ-55, SSZ-56, SSZ-58, SSZ-59, and SSZ-60), where the location of the SDA had only been simulated using molecular modeling techniques in previous studies. From synchrotron powder diffraction data, we were able to retrieve reliable experimental positions for the SDA by using a combination of simulated annealing (global optimization) and Rietveld refinement. In this way, problems arising from data quality and only partially compatible framework and SDA symmetries, which can lead to indecipherable electron density maps, can be overcome. Rietveld refinement using geometric restraints were then performed to optimize the positions and conformations of the SDAs. With these improved models, it was possible to go on to determine the location of the B atoms in the framework structure. That is, two pieces of information that are key to the understanding of zeolite synthesis-the location of the organic SDA in the channel system and of the positions adopted by heteroatoms in the silicate framework-can be extracted from experimental data using a systematic strategy. In most cases, the locations of the SDAs determined experimentally compare well with those simulated with molecular modeling, but there are also some clear differences, and the reason for these differences can be understood. The approach is generally applicable, and has also been used to locate organic guests, linkers, and ligands in metal-organic compounds.

Synthesis and Structure Determination of Large‐Pore Zeolite SCM‐14

“…The chemical composition of the as-made SCM-14 sample can be roughlyc alculateda s j (C 9 N 2 H 13 F) 1 Figure S6). The approximate location and occupancy of OSDA and H 2 Oi n the channel were found using the simulated-annealing algorithm in software To pas, followingt he procedure described by Smeets et al [33] The final refinement converged with agreement values R B = 0.020, R wp = 0.164, and R exp = 0.132 (Table 1, FigureS7). To find the location of OSDA, an idealized model of OSDA was generated using ChemBio3 D, [32] and then added to the SCM-14 structure model as ar igid body.…”

“…The approximate location and occupancy of OSDA and H 2 Oi n the channel were found using the simulated-annealing algorithm in software To pas, followingt he procedure described by Smeets et al [33] The final refinement converged with agreement values R B = 0.020, R wp = 0.164, and R exp = 0.132 (Table 1, FigureS7). [33][34][35] SCM-14 zeolite possesses at hree-dimensional channel system with straight 12-ring and 8-ring channels along the caxis which are intersected by 8-ring channels running along the a-a nd b-axes (Figure 4), giving it af ramework density of 17.5 Tatoms per 1000 3 .T he effective pore diameters of the 12-ring and 8-ring channels alongt he c-axis are 7.2 6.3 and 5.2 1.5 ,r espectively,w hile the 8-ring channels along the aand b-axes have openings of 4.9 2.6 and 4.9 1.7 ,r espectively,c onsidering an oxygen radius of 1.35 .I tc ould be seen that the projectiono f1 2-ringa nd 8-ring channels along the caxis in the framework structure of SCM-14 is the same as those of zeolitesG US-1 andm ordenite ( Figure S8 a-c), and that they all possess quite similar pore sizes. There are severalr easonst hat may lead to the disordered guest molecules:1 )the determinedc ell is as ub-cell of the real structure;2 )the arrangement of OSDAs is ordered but their symmetry is lower than that of the framework structure;3 )the OSDAs are distributedr andomly and without long range order in the channels.…”

Synthesis and Structure Determination of SCM‐15: A 3D Large Pore Zeolite with Interconnected Straight 12×12×10‐Ring Channels

“…For the Rietveld refinement of as‐made SCM‐15, an impurity phase that was identified as as‐made SCM‐14 was included with fixed atomic parameters . The locations of the guest species (F − , OSDAs, and H 2 O) in the framework structure of SCM‐15 were elucidated using the simulated annealing algorithm as described previously . The final agreement residuals for the Rietveld refinement were R I =0.029, R wp =0.232, with R exp =0.147 (Table , Figure ).…”

Preferential Siting of Aluminum Heteroatoms in the Zeolite Catalyst Al‐SSZ‐70

“…[42] Such control over heteroatom locations is achievable in part for borosilicate zeolites by changing the OSDAa nd synthesis conditions. [13] However,s imilar capabilities have been lacking for aluminosilicate zeolites,i n part because of the absence of experimental techniques that are capable of resolving Al occupancies at different Ts ites. Ther esults reported here demonstrate that solid-state 2D NMR analyses,i nc ombination with synchrotron XRPD,c an establish Al locations and occupancies in complicated aluminosilicate zeolite frameworks like SSZ-70.…”

“…While the similar electron densities of Al and Si atoms make them challenging to distinguish using scattering techniques, [9] Al occupancies in different tetrahedral (T) sites have been determined for large single crystals by synchrotron X-ray standing-wave diffraction. [11][12][13] Heteroatom distributions in zeolites can be influenced in favorable cases by the substitution of Al for B [14] or the careful selection of OSDAs and synthesis conditions. [11][12][13] Heteroatom distributions in zeolites can be influenced in favorable cases by the substitution of Al for B [14] or the careful selection of OSDAs and synthesis conditions.…”