Determination of the Apparent Crystal Structure of a Highly Faulted UPRM-5 Type Flexible Porous Titanium Silicate via a Polymorph Based Superposition Model, a Rietveld Refinement and a Pair Distribution Function

Abstract

The crystal structure of a UPRM-5 titanium silicate prepared using tetraethylammonium (TEA + ) has been approximated using a superposition model product of a polymorph stacking and a dual-phase Rietveld refinement method. Na + -UPRM-5 polymorphs were employed to elucidate the level of polymorphism or faulting in the crystal structure. DIFFaX simulations revealed that it was impossible to match the experimental diffraction data based solely on "pure" polymorphs. Instead, an intergrowth of combinations of polymorphs in the a and c directions resulted in the best faulting simulation scenario. The most suitable model combined two (2) orthorhombic polymorphs with faulting of 90 and 10% in the a and c directions, respectively. A refinement using this model did not yield a reliable structure, but an approximation was possible after employing a combination of orthorhombic and faulted triclinic phases. The superposition model, however, was not able to predict the final configuration of the TiO 5 plausibly due to the unprecedented level of faulting in the structure. Upon convergence (χ 2 = 13.68), the triclinic phase accounted for ca. 14% (molar basis) of the overall phase, being this further evidence of the level of faulting present in UPRM-5. The refined structure also revealed Si−O and Ti−O distances and angles that contrast with those reported for a titanium silicate known as ETS-4, and related to structural distortion. These changes are plausibly attributed to the presence of the TEA + cations and the strong interaction of the framework oxygen with sodium cations, which were also exposed to the 8MR pore channel as described by a pair distribution function (PDF) refinement. In general, the UPRM-5 structural features appear to commensurate well with the gas adsorption and thermal stability properties previously reported, which differ considerably from those exhibited by Zorite type titanium silicates prepared in the absence of a quaternary ammonium cation.

Long- and Local-Range Structural Changes in Flexible Titanium Silicates with Variable Faulting upon Thermal Treatment and Corresponding Adsorption and Particle Size Polydispersity-Corrected Diffusion Parameters for CO2/CH4 Separation

“…A set of diffraction patterns gathered from 30 to 300 °C is shown in Figure . Unit cell parameters for both Sr 2+ -UPRM-5 variants are shown in Table S1 in Supporting Information and corresponded to an orthorhombic symmetry, which correlates well with the results recently reported for the crystal refinement of Na + -UPRM-5 prepared with TEA + . The structure and faulting in this variant were described via stacking of orthorhombic and triclinic polymorph phases, but the former is the predominant one (90%).…”

“…As reported recently for Sr 2+ -UPRM-5 prepared using TEA + , these materials have a set of two accessible pore channels each with different dimensions . Adsorption studies made for ETS-4, which also contains a bimodal pore system, have shown that this feature results in poor prediction of adsorption–desorption diffusivities when traditional phenomenological transport models are employed. , The narrower channels in UPRM-5 correspond to the 8-MR pore system previously mentioned, which is characteristic of flexible titanium silicates. ,, The other dominant pore system is a result of the structural faulting, which is controlled by the choice of SDA and effectively results in larger pore dimensions . Per the discussion of the XRD results, the 8-MR pores or channel system in Sr 2+ -UPRM-5 (TPA) and Sr 2+ -UPRM-5 (TBA) seems to be the most affected by thermal contraction during thermal activation.…”

“…Lattice parameters b and c exhibited the largest reduction at high temperatures, especially for Sr 2+ -UPRM-5 (TPA). Lattice parameter c is plausibly associated with the 8-member ring (8-MR) channels or pores . It is also believed that these pores should dominate gas adsorption kinetics because they can be size tailored in Sr 2+ -ETS-4 to separate similar-sized gas molecules .…”

“…Inspection of the CH 4 adsorption isotherms show an increase in loading upon activation of the Sr 2+ -UPRM-5 (TBA) adsorbent at 180 °C, probably due to, once again, an enhanced adsorbate–adsorbent interaction brought about by a surface potential overlapping. However, because this phenomenon appears to be absent during the adsorption of CO 2 onto the Sr 2+ -UPRM-5 (TPA) adsorbent variant, a possible explanation can be found by considering the bimodal pore system displayed by these materials …”

“…The ARE and RRMSE values calculated for fits of fractional uptakes of CH 4 onto Sr 2+ -UPRM-5 (TPA) are significantly smaller than those calculated for CO 2 , suggesting in a way that the adsorption of CH 4 in the 8-MR pore system is negligible. CO 2 molecules also seem to diffuse through both pore systems, but with some restrictions probably associated with the presence of residual Na + and linked structural water and possible blockage of the pore channels of other CO 2 adsorbed molecules. ,, …”