Mechanism of the Low-Temperature Organic Removal from Imidazolium-Containing Zeolites by Ozone Treatment: Fluoride Retention in Double-4-Rings.

For zeolites synthesized using imidazolium cations, the organic matter can be extracted at very low temperatures (100 °C) using ozone. This is possible for zeolites with 12-ring or larger pores but requires higher temperatures in medium-pore zeolites. The first chemical events in this process occur fast, even at room temperature, and imply the loss of aromaticity likely by the formation of an adduct between ozone and the imidazole ring through carbons C4 and C5. Subsequent rupture of the imidazole ring provides smaller and more flexible fragments that can desorb more readily. This process has been studied experimentally, mainly through infrared spectroscopy, and theoretically by density functional theory. Amazingly, fluoride anions occluded in the small double-four-ring units (d4r) during the synthesis remain inside the cage throughout the whole process when the temperature is not too high (≤150 °C). However, fluoride in larger cages in MFI ends up bonded to silicon in penta or hexacoordinated units, likely out of the cages, after ozone treatment at 150 °C. For several germanosilicate zeolites, the process allows their subsequent degermanation to yield stable high-silica zeolites. Quaternary ammonium cations require harsher conditions that eventually also extract fluoride from zeolite cages, including the d4r unit.

Interchain-expanded extra-large-pore zeolites.

Stable aluminosilicate zeolites with extra-large pores that are open through rings of more than 12 tetrahedra could be used to process molecules larger than those currently manageable in zeolite materials. However, until very recently1-3, they proved elusive. In analogy to the interlayer expansion of layered zeolite precursors4,5, we report a strategy that yields thermally and hydrothermally stable silicates by expansion of a one-dimensional silicate chain with an intercalated silylating agent that separates and connects the chains. As a result, zeolites with extra-large pores delimited by 20, 16 and 16 Si tetrahedra along the three crystallographic directions are obtained. The as-made interchain-expanded zeolite contains dangling Si-CH3 groups that, by calcination, connect to each other, resulting in a true, fully connected (except possible defects) three-dimensional zeolite framework with a very low density. Additionally, it features triple four-ring units not seen before in any type of zeolite. The silicate expansion-condensation approach we report may be amenable to further extra-large-pore zeolite formation. Ti can be introduced in this zeolite, leading to a catalyst that is active in liquid-phase alkene oxidations involving bulky molecules, which shows promise in the industrially relevant clean production of propylene oxide using cumene hydroperoxide as an oxidant.

2-Isopropyl-1,3-dimethylimidazolium as a versatile structure-directing agent in the synthesis of zeolites.

An organic cation lacking specificity in its structure-directing action offers the possibility, through the screening of other structure-directing parameters, to synthesize a variety of zeolites. In this work we show that the organic structure-directing agent 2-isopropyl-1,3-dimethylimidazolium (2iPr13DMI) can produce up to seven different zeolite phases depending on water concentration, the presence of inorganic impurities, crystallization temperature and time, and germanium molar fraction. The obtained phases are very different in terms of pore system, connectivity of the zeolite structure and structural units. At the pure SiO2 side, ZSM-12 and SSZ-35 dominate, with ZSM-12 being favored by the presence of potassium impurities and by less concentrated conditions. The introduction of Ge at low levels favors SSZ-35 over ZSM-12 and as the Ge fraction increases it successively affords CSV, -CLO and two distinct UOS zeolites, HPM-11 and HPM-6. These two zeolites have the same topology but distinct chemical compositions and display powder X-ray diffraction patterns that are much different from each other and from that of as-synthesized IM-16 (UOS reference material). They also show different symmetry at 96 K. Rietveld refinements of the three as-made UOS materials mentioned are provided. HPM-6 and HPM-11 are produced in distinct, non-adjacent crystallization fields. The frequent cocrystallization of the chiral STW zeolite, however, did not afford its synthesis as a pure phase. Molecular mechanics simulations of the location of the organic cation and host-guest interactions fail to explain the observed trends, but also considering the intrinsic stability of the zeolites and the effect of germanium help to rationalize the results. The study is completed by DFT calculations of the NMR chemical shifts of 13C in UOS (helping to understand splittings in the spectrum) and 19F in CSV (supporting the location of fluoride inside the new [4452], which is an incomplete double 4-ring).

A 3D extra-large-pore zeolite enabled by 1D-to-3D topotactic condensation of a chain silicate.

Zeolites are microporous silicates with a large variety of applications as catalysts, adsorbents, and cation exchangers. Stable silica-based zeolites with increased porosity are in demand to allow adsorption and processing of large molecules but challenge our synthetic ability. We report a new, highly stable pure silica zeolite called ZEO-3, which has a multidimensional, interconnected system of extra-large pores open through windows made by 16 and 14 silicate tetrahedra, the least dense polymorph of silica known so far. This zeolite was formed by an unprecedented one-dimensional to three-dimensional (1D-to-3D) topotactic condensation of a chain silicate. With a specific surface area of more than 1000 square meters per gram, ZEO-3 showed a high performance for volatile organic compound abatement and recovery compared with other zeolites and metal-organic frameworks.

A stable aluminosilicate zeolite with intersecting three-dimensional extra-large pores.

Zeolites are crystalline porous materials with important industrial applications, including uses in catalytic and adsorption-separation processes. Access into and out of their inner confined space, where adsorption and reactions occur, is limited by their pore apertures. Stable multidimensional zeolites with larger pores able to process larger molecules are in demand in the fine chemical industry and for the oil processing on which the world still relies for fuels. Currently known extra-large-pore zeolites display poor stability and/or lack pore multidimensionality, limiting their usefulness. We report ZEO-1, a robust, fully connected aluminosilicate zeolite with mutually intersecting three-dimensional extra-large plus three-dimensional large pores. ZEO-1 is stable up to 1000°C, has an extraordinary specific surface area (1000 square meters per gram), and shows potential as a catalytic cracking catalyst.

Synthesis of Extra-Large Pore, Large Pore and Medium Pore Zeolites Using a Small Imidazolium Cation as the Organic Structure-Directing Agent.

One common strategy in the search for new zeolites is the use of organic structure-directing agents (OSDA). Typically, one seeks to achieve a high specificity in the structure-directing effect of the OSDA. This study shows, however, that an OSDA lacking strong specificity towards any particular zeolite may provide opportunities for discovery when other synthesis parameters are systematically screened. Thus, 1-methyl-2-ethyl-3-n-propylimidazolium has allowed to crystallize the new large/medium pore zeolite HPM-16 as well as the recently reported extra-large pore -SYT and the medium/small pore and chiral STW. The sophisticated OSDA originally affording -SYT and the new simple OSDA have very little in common, both in terms of size, shape and flexibility, while both may still direct the synthesis of the same zeolite. In fact, molecular simulations show that the new OSDA is located in three different positions of the -SYT structure, including the discrete 8MR where the original organic could not fit.

HPM-16, a Stable Interrupted Zeolite with a Multidimensional Mixed Medium-Large Pore System Containing Supercages.

HPM-16 is a highly porous germanosilicate zeolite with an interrupted framework that contains a three-dimensional system of 12+10×10(12)×12+10-membered ring (MR) pores. The 10(12) MR pore in the b direction is a 10 MR pore with long 12 MR stretches forming 30 Šlong tubular supercages. Along one direction the 10 MR pores are fused, meaning that the separation between adjacent pores consists of a single tetrahedron that is, additionally, connected to only three additional tetrahedra (a Q3 ). These fused pores are thus decorated by T-OH groups along the whole diffusion path, creating a hydrophilic region embedded in an otherwise essentially hydrophobic environment. The structure is built from highly porous 12×12×12 MR uninterrupted layers that are connected to each other through Q3 producing a second system of 10×10×10 MR pores. This zeolite can be extensively degermanated yielding a material with high thermal stability, despite its interrupted nature.

Structure-direction towards the new large pore zeolite NUD-3.

The new zeolite NUD-3 possesses a three-dimensional system of large pore channels that is topologically identical to those of ITQ-21 and PKU-14. However, the three zeolites have distinctly different frameworks: a particular single 4-membered ring inside the denser portion of the zeolite is missing in PKU-14, disordered in ITQ-21 and fully ordered in NUD-3. We document these differences and use molecular simulations to unravel the mechanism by which a particular structure directing agent dication, 1,1'-(1,2-phenylenebis(methylene))bis(3-methylimidazolium), is able to orient this inner ring.

HPM-14: A New Germanosilicate Zeolite with Interconnected Extra-Large Pores Plus Odd-Membered and Small Pores*.

HPM-14 is a new extra-large pore zeolite synthesized using imidazolium-based organic structure-directing agents (SDAs), fluoride anions, and germanium and silicon as tetrahedral components of the framework. Owing to the presence of stacking disorder, the structure elucidation of HPM-14 was challenging, and different techniques were necessary to clarify the details of the structure and to understand the nature of the disorder. The structure has been solved by three-dimensional electron-diffraction technique (3D ED) and consists of an intergrowth of two polymorphs possessing a three-dimensional channel system, including an extra-large pore opened through windows made up of sixteen tetrahedral atoms (16-membered ring, 16MR) as well as two additional sets of odd-membered (9MR) and small (8MR) pores. The intergrowth has been studied by scanning transmission electron microscopy (Cs -STEM) and powder X-ray diffraction simulations (DIFFaX), which show a large predominance of the monoclinic polymorph A.

Structural characterization of HPM-7, a more ordered than expected germanosilicate zeolite.

HPM-7, a germanosilicate synthesized using long imidazolium-based dications with two different linkers, is shown to possess the POS topology, although disorder may exist but it is very difficult to discern. First, three simple ordered polymorphs (POS-A to POS-C) with very similar energies and structural motifs could give rise to intergrowths that would be very difficult to recognize by powder X-ray diffraction, according to DIFFaX simulations. Another four structures (POS-D to POS-G) can be derived from POS by changing the orientation of two single four rings within the structure, possibly providing an additional source of disorder. While 3D EDT strongly suggests that HPM-7 basically possesses the POS-A (i.e. POS) topology, a detailed HR-STEM study demonstrates the rare existence of some disorder compatible with the polymorph POS-D. The general avoidance of polymorphs with very similar structural motifs and comparable energies points to a rather specific structure-direction by the organic dications used.

IDM-1: A Zeolite with Intersecting Medium and Extra-Large Pores Built as an Expansion of Zeolite MFI.

IDM-1 is a new silica zeolite with an ordered and well-defined framework constructed by alternating pentasil layers and interrupted layers, giving rise to an intersecting system of straight medium pores and undulating extra-large lobed pores. This unique structure was solved by rotation electron diffraction and refined against synchrotron powder X-ray diffraction data. Despite the presence of both Si(OSi)3 (OH) and Si(OSi)2 (OH)2 sites, this new zeolite presents high thermal stability, withstanding calcination even to 1000 °C. The location of defects at specific sites of the structure results in alternating hydrophobic SiO2 and hydrophilic SiO(2-x) (OH)2x intracrystalline regions. This peculiar combination of intersecting medium and extra-large pores and alternating regions of different chemical character may provide this zeolite with unique catalytic properties.

Synthesis of a germanosilicate zeolite HPM-12 using a short imidazolium-based dication: structure-direction by charge-to-charge distance matching.

A short imidazolium based dication, with only three methylene units in the spacer linker, selectively directs the crystallization of zeolite HPM-12 (*UOE) as long as there is enough germanium present in the synthesis gel. The integrity of the dication is proved by dissolution of the zeolite and 1H and 13C NMR spectroscopy, where significant effects of organic dication concentration and the presence and concentration of HF need to be taken into account. For the as-made HPM-12 zeolite, a large shift of 9 ppm of one resonance in the 13C MAS NMR spectrum is due to the particular conformation of the dication imposed by confinement in the zeolite framework, as found by DFT calculations. The structure-directing ability of this kind of dication with varying length of the linker suggests that matching of the distance between positive charges (imidazolium moieties) and negative charges (double four rings in which fluoride resides) plays a crucial role during crystallization.

Synthesis of Pure Silica MWW Zeolite in Fluoride Medium by Using an Imidazolium-Based Long Dication.

As the spacer length in 1,2-dimethylimidazolium-based dications increases beyond a specific point (six methylene units), they fail in structure-directing towards STW zeolites in any synthetic conditions. These dications can instead produce, under fluoride concentrated conditions, either *BEA [in the case of the eight-methylene-unit structure-directing agent (SDA)] or MWW (ten methylene units) zeolites. For any length of the dication, the default zeolite (MTW) is a relatively dense zeolite containing a unidimensional channel, whereas the zeolite demanding most specificity (STW, *BEA or MWW) is more porous, affording a larger concentration of the dication to be occluded. This work provides the first reported fluoride synthesis of pure silica MWW zeolites. Charge balance of the organic dications in this zeolite was achieved by combining "structural" silanolates, regular "connectivity defects" and occluded fluoride. Molecular mechanics calculations showed a perfect fit of the decamethylenebis(dimethylimidazolium) dication in the sinusoidal intralayer pore system of MWW. The calculations showed also that the dication is able to stabilize the interlayer space without disturbing the hydrogen-bonding system that holds the layers together in the as-made material. The 19 F magic-angle spinning (MAS) NMR presented two distinct resonances at -71 and -83 ppm, which, on the basis of DFT calculations, we tentatively assigned to fluoride occluded in [46 62 ] and [41 52 62 ] cages of the MWW structure, respectively. The same DFT study determines a different chemical shift of one methyl 13 C nuclear magnetic resonance according to the imidazolium ring residing in the sinusoidal channels or in the large cup cavities, thus explaining an experimentally observed splitting of that resonance.

Synthesis of pure silica MFI zeolites using imidazolium-based long dications. A comparative study of structure-directing effects derived from a further spacer length increase.

Length-dependent structure direction of linear methylimidazolium-based dications towards MFI zeolite, previously known only for the tetramethylene spacer, has also been found for octamethylene and decamethylene spacers. This works only under highly concentrated conditions, whereas dilution always tends to favor TON, a default structure that is the only zeolite obtained with the other reported dications (with tri-, penta-, and hexamethylene spacers). The locations and conformations of the dications have been studied by molecular mechanics simulations. As longer dications introduce lower density of positive charges in the zeolite, the density of connectivity defects also decreases. Finally, these long dications cannot easily place each charged imidazolium ring in the two possible orientations (either parallel or perpendicular to 4MR close to F- sites) found for the tetramethylene case. Hence, although the three MFI materials display two 19F NMR resonances at similar chemical shifts, their relative intensities strongly vary as a function of the spacer length.

Stepped Propane Adsorption in Pure-Silica ITW Zeolite.

Gas adsorption over zeolites is at the basis of important applications of this class of microporous crystalline solids, notably as separation media and catalysts, but it may also be complex and not straightforward to understand. Here we report that for temperature below 323 K propane adsorption on the small-pore pure-silica zeolite ITW exhibits a clear step (pseudosaturation). This is absent in the case of propene and the other small linear alkanes. An intermediate plateau, clearly observed in the 293 K isotherm, always occurs when one molecule of propane is loaded in every other cage, i.e., at half-saturation. The simulation results show a swelling of the ITW structure upon propane adsorption. The strong dependence of available pore volume on the adsorbate loading level implies that adsorption cannot occur on the void structure while saturation can only be reached on highly loaded structures. To account for this unprecedented adsorption phenomenon, we propose the term "guest-modulated effect".

Influence of Flexibility on the Separation of Chiral Isomers in STW-Type Zeolite.

Molecular simulation, through the computation of adsorption isotherms, is a useful predictive tool for the selective capacity of nanoporous materials. Generally, adsorbents are modelled as rigid frameworks, as opposed to allowing for vibrations of the lattice, and this approximation is assumed to have negligible impact on adsorption. In this work, this approach was tested in an especially challenging system by computing the adsorption of the chiral molecules 2-pentanol, 2-methylbutanol and 3-methyl-2-butanol in the all-silica and germanosilicate chiral zeolites STW and studying their lattice vibrations upon adsorption. The analysis of single- and multicomponent adsorption isotherms showed the suitability of STW-type zeolites as molecular sieves for chiral separation processes, which pose a challenging task in the chemical and pharmaceutical industries. Moreover, new experimental adsorption data validate the force field employed. The results reveal that the lattice vibrations of the all-silica framework are sorbate-independent, while those of germanosilicate STW show host-guest coupling modulated by uptake and sorbate type that disrupts the chiral recognition sites. This study indicates that the effects of intrinsic flexibility on the selective capacity of nanoporous materials may range from low to high impact, and some of them could not have been foreseen even after examination of the structural dynamics of an empty framework.

Microporous aluminophosphates synthesized with 1,2,3-trimethylimidazolium and fluoride.

The synthesis of microporous aluminophosphates using 1,2,3-trimethylimidazolium (123TMI) and fluoride produces three phases (HPM-3, PST-27 and triclinic AlPO4-34) depending on the amount of water and organic structure-directing agents in the synthesis mixture. Fluoride occluded in double 4-ring units was not detected by (19)F MAS NMR spectroscopy in any product. While the structure of HPM-3 remains unknown, PST-27 has been determined to be a monoclinic version of AlPO4-5 with a distorted and likely complex structure. Rietveld analysis using synchrotron diffraction data for as-made triclinic AlPO4-34 reveals that each of its cha-cages holds two 123TMI cations, forming a displaced anti-parallel dimer with a short distance between aromatic rings (3.78 Å from center to center, 3.63 Å from plane to plane). This suggests that π-π interactions may play a role in the synthesis of this phase and related CHA-type systems. A study of the optical properties of PST-27, AlPO4-34 and other materials containing 123TMI cations shows their complex fluorescence behavior, sometimes displaying a red-edge effect, i.e., a red shift of the fluorescence as the excitation wavelength is shifted toward the red edge of the absorption band. An absorption band at 291 nm appears only in the dimer-containing triclinic AlPO4-34 and is sensitive to the introduction of Si into the framework.

Intraframework migration of tetrahedral atoms in a zeolite.

The transformation from a disordered into an ordered version of the zeolite natrolite occurs on prolonged heating of this material in the crystallizing medium, but not if the mother liquor is replaced by water or an alkaline solution. This process occurs for both aluminosilicate and gallosilicate analogues of natrolite. In cross experiments, the disordered Al-containing (or Ga-containing) analogue is heated while in contact with the mother liquor of the opposite analogue, that is, the Ga-containing (or Al-containing) liquor. Therefore, strong evidence for the mechanism of the ordering process was obtained, which was thus proposed to proceed by intraframework migration of tetrahedral atoms without diffusion along the pores. Migration is first triggered, then fuelled by surface rearrangement through reactions with the mother liquor, and stops when an almost fully ordered state is attained. Classical dissolution-recrystallization and Ostwald ripening processes do not appear to be relevant for this phase transformation.

Structure-direction in the crystallization of ITW zeolites using 2-ethyl-1,3,4-trimethylimidazolium.

2-Ethyl-1,3,4-trimethylimidazolium is so far the fifth and largest imidazolium cation able to produce the pure silica zeolite ITW. The crystallization is not direct, but occurs as an in situ transformation from either the chiral HPM-1 (STW) zeolite or the new layered zeolite precursor HPM-2 (preMTF) and results in large interpenetrated crystals with a habit so far unseen in this zeolite. The results of chemical, physicochemical and structural characterization allow us to discuss structure-direction issues. The large cation is occluded intact in the zeolite, causing an expansion of the zeolite volume. The degree of polarization of the Si-O bond is similar to that observed in other as-made ITW zeolites.

Synthesis, structure, and optical activity of HPM-1, a pure silica chiral zeolite.

2-Ethyl-1,3,4-trimethylimidazolium is a poor organic structure-directing agent in the synthesis of pure silica zeolites using fluoride as a mineralizer at 150 °C. Under these conditions only ill-crystallized solids are obtained after long hydrothermal treatments (several weeks). It disappoints despite its relatively large size, conformational rigidity, and intermediate hydrophilic/hydrophobic character, attributes which would qualify it as a promising structure-directing agent, according to prior investigations. By raising the crystallization temperature to 175 °C under otherwise identical conditions, crystallization is dramatically accelerated. Depending on the water/silica ratio and crystallization time, two different materials are obtained: the recently reported pure silica polymorph of the chiral STW-type zeolite, HPM-1, and the new layered organosilicate, HPM-2. Prolonged heating transforms these phases into the small-pore ITW-type zeolite, while no signs of the SOF-type zeolite (formally built from the same layers as STW) was found. A complete physicochemical and structural characterization of the as-made chiral HPM-1 zeolite is provided, and the proposed stabilization of this zeolite by polarization of the Si-O bond is supported by the observed deviation from tetrahedrality. HPM-1 is optically active, and a study of several crystallites by Mueller matrix microscopy shows that their optical activity can be individually measured and that this technique could be useful for the assessment of the enantiomeric purity of a microcrystalline powder.