Sr(ii)-UPRM-5 titanium silicate framework thermally induced contraction: in situ high temperature XRD and 29Si MAS NMR

Abstract

The thermally induced contraction process of a titanosilicate prepared with tetraethylammonium hydroxide and ion exchanged with Sr(II) (Sr-UPRM-5) after detemplation has been characterized via in situ high temperature X-ray diffraction (XRD) and (29)Si magic angle spinning nuclear magnetic resonance (MAS NMR). The as-synthesized material was prepared via conventional and microwave-assisted hydrothermal methods, the latter resulting in a reaction time an order of magnitude shorter than the former case. In situ high temperature XRD tests performed on Sr-UPRM-5 indicate that at 120 °C, water coordinated to the structure is released initiating the collapse of the framework. At much higher temperatures, the material eventually becomes an amorphous phase. Indexing of the XRD patterns indicates that lattice constant a was more affected by the heat treatment, probably related to the material's pore system, while the unit cell volume experienced a 44% reduction. (29)Si MAS NMR analyses for as-synthesized UPRM-5 confirmed two different silicon environments: Si(2Si, 2Ti(octa)) and Si(3Si, 1Ti(semi-octa)), which are similar to those exhibited by titanosilicate ETS-4. On the other hand, in situ high temperature (29)Si MAS NMR analyses for Sr-UPRM-5 demonstrated that changes in the silicon environment due to the presence of titanium centers possessing additional multiple coordination states, which arise from elimination of framework coordinated water molecules, are responsible for the structure collapsing. In general, these results underline the importance of avoiding complete removal of tenacious water molecules in order to preserve the Sr-UPRM-5 properties suitable for adsorption and catalysis applications.

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

“…Although the simulations made for UPRM-5 covered faulting probabilities near the 100% mark, none of the peaks in the 20–24° 2θ range were resolved since the faulting scenario represented by the stacking transitions employed in both a and c directions do not exhibit peaks within that range. Furthermore, upon further consideration of the crystal systems of the polymorphs, we developed a new hypothetical “pure” polymorph that contained a triclinic intergrowth to account for the missing diffraction peaks or that at least contained a contribution from a secondary phase based on results from indexing routines previously reported for UPRM-5. , It is important to note at this stage of the discussion that the reflections corresponding to angles less than 20° 2θ may commensurate with a (TEA + , Na + )-UPRM-5 material that has a highly faulted crystal structure in the a and c directions, this probably induced by the TEA + cations. In general, none of the simulations based on faulted UPRM-5 single “pure” polymorphs and along a single direction were capable of matching the complete experimental XRD pattern in a suitable, qualitative fashion.…”

“…This could in turn result in additional operation cycles during applications like pressure swing adsorption. In an attempt to provide a solution to this quandary, Hernández-Maldonado and co-workers developed a soft titanium silicate named UPRM-5, which is prepared via templated hydrothermal reactions using quaternary ammonium (NR 4 + ) cations. ,,, Apparently these cations act not only as templates, but also as molecular structure-directing agents (SDAs), producing different levels of faulting depending on the size and alkyl chain length of cation employed. , This serendipitous finding is apparently linked to the UPRM-5 exceptional adsorption working capacities upon detemplation, inclusion of an alkaline earth extraframework cation, and upon activation at moderate temperatures.…”

“…11,15 Although the existence of five-coordinated titanium in the framework is concomitant with the flexible structure and adjustable pores dimensions under thermalvacuum treatment, 2,15,16 Hernandez-Maldonado and co-workers used in situ high temperature XRD and 29 Si MAS NMR data to show that the Sr 2+ -UPRM-5 framework contraction process is related to the appearance of additional titanium coordination levels. 15 In the case of Sr 2+ -ETS-4, Tsapatsis and co-workers found that five-coordinated titanium also contributes to the faults in the structure along a and c directions in the unit cell. 21 On the basis of the similarities between the frameworks of ETS-4 and UPRM-5 and aiming at further understanding the role of a quaternary ammonium cation (tetraethylammonium, TEA + ) in the synthesis of UPRM-5, here we report an extensive structural study based on XRD pattern simulations with DIFFaX, 22,23 a superposition model, a Rietveld refinement and a pair distribution function (PDF) analysis.…”

“…According to standard X-ray diffraction (XRD) measurements made for UPRM-5, ,,, the material should possess a structure substantially different from that of ETS-4, , but still related to the Zorite mineral. , Comparisons made between Zorite and ETS-4 have led to several reports spanning nearly a decade now. ,− Although UPRM-5 and ETS-4 are probably related to Zorite in many similar ways, UPRM-5 exhibits surface areas nearly an order of magnitude larger than that of ETS-4 when thermally dehydrated at the same conditions. Remarkably, both titanium silicates share similar local chemical environments as corroborated by 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy measurements.…”

“…Remarkably, both titanium silicates share similar local chemical environments as corroborated by 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy measurements. These correspond to Si(2Si,2Ti octa ) and Si(3Si,1Ti semiocta ) silicon environments, respectively. , Although the existence of five-coordinated titanium in the framework is concomitant with the flexible structure and adjustable pores dimensions under thermal-vacuum treatment, ,, Hernandez-Maldonado and co-workers used in situ high temperature XRD and 29 Si MAS NMR data to show that the Sr 2+ -UPRM-5 framework contraction process is related to the appearance of additional titanium coordination levels . In the case of Sr 2+ -ETS-4, Tsapatsis and co-workers found that five-coordinated titanium also contributes to the faults in the structure along a and c directions in the unit cell …”