D4R Cages Research Articles

Elucidating the Germanium Distribution in ITQ-13 Zeolites by Density Functional Theory.

ITQ‐13 is a medium‐pore zeolite that can be prepared in all‐silica form and as silicogermanate with Si/Ge ratios as low as 3. Usually synthesised in the presence of fluoride, ITQ‐13 is among the very few systems containing fluoride anions in two distinct cage types, cube‐like d4r units and [4 ⋅ 56] cages. Here, dispersion‐corrected density functional theory (DFT) calculations are used to investigate the energetically most favourable Ge distributions for Si/Ge ratios between 55 and 6. The calculations show Ge atoms are incorporated at both the corners of d4r cages and at the basal plane of the [4 ⋅ 56] cages, in accordance with 19F NMR spectroscopy. Two Ge atoms at adjacent corners of [4 ⋅ 56] cages are stable at the highest Ge content considered (Si/Ge=6). Such a local environment has not yet been considered in the experimental literature. A calculation of the corresponding 19F NMR resonance points to overlap with other resonances, which might preclude its clear identification. Additional calculations investigate the variation of the dynamic behaviour of the fluoride anions as a function of the local environment as well as the selective defluorination of the [4 ⋅ 56] cages.

Local Distortions in a Prototypical Zeolite Framework Containing Double Four-Ring Cages: The Role of Framework Composition and Organic Guests*.

Cube‐like double four‐ring (d4r) cages are among the most frequent building units of zeolites and zeotypes. In materials synthesised in fluoride‐containing media, the fluoride anions are preferentially incorporated in these cages. In order to study the impact of framework composition and organic structure‐directing agents (OSDAs) on the possible occurrence of local distortions of fluoride‐containing d4r cages, density functional theory (DFT) calculations and DFT‐based molecular dynamics simulations were performed for AST‐type zeotypes, considering four different compositions (SiO2, GeO2, AlPO4, GaPO4) and two different OSDA cations (tetramethylammonium [TMA] and quinuclidinium [QNU]). All systems except SiO2‐AST show significant deformations, with a pyritohedron‐like distortion of the d4r cages occurring in GeO2‐ and GaPO4‐AST, and a displacement of the fluoride anions towards one of the corners of the cage in AlPO4‐ and GaPO4‐AST. While the distortions occur at random in TMA‐containing zeotypes, they exhibit a preferential orientation in systems that incorporate QNU cations. In addition to providing detailed understanding of the local structure of a complex host‐guest system on the picosecond timescale, this work indicates the possibility to stabilise ordered distortions through a judicious choice of the OSDA, which might enable a tuning of the material's properties.

Role of Cubic Siloxane Cages in Mesostructure Formation and Photoisomerization of Azobenzene–Siloxane Hybrid

Abstract A mesostructured siloxane-based hybrid film is formed by the self-assembly of cubic, double-four-ring (D4R) siloxanes mono-functionalized with azobenzene. The azobenzene groups in the film exhibit reversible and a high degree of trans–cis isomerization upon UV–vis irradiation. The bulky D4R cages play an important role in both the formation of cylindrical assemblies and the improvement of the photoisomerization behavior as compared with conventional azobenzene–siloxane hybrid materials.

Preferential Location of Germanium in the UTL and IPC-2a Zeolites

The preferential location of Ge in the double four-membered rings (D4R) of zeolite UTL has been examined with periodic density functional theory calculations. In silicate UTL, Ge atoms are preferentially sited at the T1 and T2 atoms in the D4R cages. The preference for siting Ge atoms at the D4R sites over other T atom sites in UTL increases as more Si atoms are replaced by Ge. Our calculations show that at least 3 Ge atoms can be located in the D4R cages before any appreciable occupancy at non-D4R sites. Comparison of Ge atom siting in UTL to zeolite IPC-2a shows that the single four-membered ring (S4R) sites in the latter are not the most favorable sites.

Diol‐Linked Microporous Networks of Cubic Siloxane Cages

A new class of inorganic-organic hybrid porous materials has been synthesized by a reaction between octa(hydridosilsesquioxane) (H(8)Si(8)O(12)), which has a double-four-ring (D4R) structure, and various diols, such as 1,3-propanediol (PD), 1,4-cyclohexanediol (CHD), and 1,3-adamantanediol (AD). Solid-state (29) Si magic-angle-spinning NMR spectroscopic analysis confirmed that most of the corner Si-H groups reacted with diols to form Si-O-C bonds with retention of the D4R cage. Nitrogen adsorption-desorption studies showed that the products are microporous solids with high BET surface areas (up to ≈580 m(2) g(-1) for CHD- and AD-linked products). If n-alkanediols are used as linkers, the surface area becomes smaller as the number of carbon atoms is increased. The thermal and hydrolytic stability of the products strongly depend on the type of diol linkers. The highest stabilities are found for the AD-linked products, which are thermally stable up to around 400 °C and remain intact even after being soaked in water for 1 day. In contrast, the PD-linked product is easily hydrolyzed in water to give microporous silica. These results offer a new route toward a series of silica-based porous materials with unique structures and properties.

Synthesis of all-silica zeolites from highly concentrated gels containing hexamethonium cations

A pure and highly crystalline all-silica EU-1 zeolite has been obtained from the crystallization of gels containing very low water contents in the presence of hexamethonium cations. Decreasing the water content in the gel down to H2O/Si<1 inhibited the formation of ZSM-48, which is usually observed under more diluted standard crystallization conditions. Moreover, addition of NH4F to the synthesis led to the formation of “half-fluorinated” ITQ-13 in which fluoride anions occupied only the center of D4R cages. In larger cages, the charge of the template was compensated by framework connectivity defects, clearly demonstrating once more the essential role of F− in the formation of D4R units. The formation of such hybrid (F,OH) is particularly interesting from a synthesis point of view, particularly for understanding the respective roles of fluoride and hydroxide anions in the crystallization process.

Fluoride Removal from Double Four-Membered Ring (D4R) Units in As-Synthesized Ge-Containing Zeolites

Fluoride anions can be removed from the framework of as-prepared Ge-containing zeolites ITQ-13 and ITQ-17 without modification of the crystallinity and crystal habit. By contrast to all-silica ITQ-13 for which fluoride could not get out from D4R units, F is completely removed from Ge rich zeolites, even from D4R cages. This has been explained by the relaxing effect of germanium, making F less necessary for the stabilization of the small D4R units. Si/Ge ratios are similar in as-prepared and treated zeolites, indicating that the framework composition is not affected by the removal of anions. The fluoride-free zeolites possess XRD patterns similar to those of the as-made solids but their 29Si NMR spectra are significantly different, revealing the sensitivity of the method to the environment of silicon atoms in the framework. The extent of fluoride that can be removed from D4R units depends not only on the framework Ge content but also on the zeolite topology: for similar contents, F is much more easily eliminated from ITQ-17 than from ITQ-13.

Microporous Hybrid Polymer with a Certain Crystallinity Built from Functionalized Cubic Siloxane Cages as a Singular Building Unit

Microporous inorganic−organic hybrid with high surface area and large pore volume has been synthesized from bromophenylethenyl-terminated double four-ring (D4R) siloxane cages as a single precursor. Unprecedentedly, the resulting hybrid possesses preferential long-range order and uniform micropore size distribution. In addition, D4R cages have been proven to be retained in the networks after the reaction.

Mechanism of F− Elimination from Zeolitic D4R Units: A Periodic B3LYP Study on the Octadecasil Zeolite

The mechanism of elimination of F− trapped inside the double four-membered ring (D4R) cages of octadecasil has been determined by means of periodic calculations at the B3LYP/VDZP//VTZP level of theory with the CRYSTAL06 program. Finite temperature and entropic effects are taken into account to study the efficiency of the elimination reaction, which is exothermic, requires acid media, and is catalyzed by cationic polar molecules with accessible protons, from which a hydronium ion has been considered in the present study. The mechanism consists of three steps that schematically correspond to hydrolysis of one SiOSi bridge in the D4R cage, extraction of the F− from the unit, and recondensation of the bridge with formation of slightly adsorbed HF·H2O in the large cavity that can be easily eliminated by diffusion. The limiting step is the first one and involves a quite low Helmholtz free energy barrier even at 100 °C (51.6 kJ mol−1) that slowly decreases with increasing temperature.

Synthesis, characterization, and framework heteroatom localization in ITQ-21.

ITQ-21 has been synthesized in a wide range of compositions. By rationally modifying the synthesis variables and zeolite composition, it is possible to fine-tune the crystallite size from nanocrystals (<80 nm) up to microns and to avoid the competition of other phases such as CIT-5, SSZ-24, or a laminar phase that can also be synthesized with the same organic structure directing agent. By means of XRD and (19)F MAS NMR, Ge and Si have been localized among the different crystallographic positions, and it is shown that Ge preferentially occupies T1 positions at the D4R cages, avoiding formation of Ge-O-Ge pairs. However, at high Ge loadings (Si/Ge = 1.7), a new (19)F MAS NMR signal at -14 ppm has been observed and assigned to the presence of Ge-O-Ge in Ge-rich D4R cages. Energetic configurations obtained by theoretical calculations fully agree with experimental observations, with the following increasing order in energy for Ge substitution: T1 < T2 < Ge-O-Ge in T1 < T3.

Thermal Detrapping Rates of Atomic Hydrogen and Atomic Deuterium Trapped in Double Four-Ring Si8O12 Cages Measured Simultaneously Using EPR

Does atomic hydrogen or atomic deuterium escape faster from the D4R cage of Q8M8 in thermal detrapping? An isotope effect was clearly observed by a simultaneous measurement using EPR. Arrhenius parameters were obtained and allowed the application of the absolute rate theory to explain the process.

Simple simultaneous encapsulation of both single H atoms and single D atoms in octasilsesquioxane, double four-ring cages using a deuterated organic solvent.

We investigated the encapsulation behavior of hydrogen and deuterium atoms in double four-ring (D4R) cages of octasilsesquioxane ([(CH3)3SiO]8Si8O12, Q8M8) under variable experimental conditions when dissolved in n-hexane and n-hexane-d14. Encapsulated atomic H and D were detected simultaneously upon γ-ray irradiation in room temperature ESR spectra for samples of Q8M8 suspended in n-hexane-d14. Although the encapsulation of H, generated from D4R cage substituents, decreased with increasing n-hexane-d14, encapsulation of D atoms increased remarkably with increasing n-hexane-d14.

Studies on the Trapping and Detrapping Transition States of Atomic Hydrogen in Octasilsesquioxane Using the Density Functional Theory B3LYP Method

B3LYP level optimizations were performed on the structures of the octasilsesquioxane (Si8O12H8, HT8) double four-ring (D4R) cage and single hydrogen atom-trapped HT8 (H@HT8). Moreover, the transition state in the detrapping process of the hydrogen atom from the D4R cage was examined. The basis sets used were 6-31G** for HT8 and (3 1 1/1*1*/1*) for the trapped hydrogen atom. Both HT8 and H@HT8 were structure-optimized with Oh molecular symmetry and the resulting cage conformations were similar. The trapped H atom was located at the center of the D4R cage. The weak interaction between the D4R cage and the trapped H atom in H@HT8 was determined by examining the singly occupied molecular orbital (SOMO) [8a1g] of H@HT8. The SOMO was constructed from an antibonding interaction between the lowest unoccupied molecular orbital (LUMO) [8a1g] of HT8 and the 1s orbital of the trapped H atom. For the transition state, the structure was optimized with C4v molecular symmetry. As a result, the position of the Si8 cube framework was unchanged, and four O atoms in a silicon single four-ring were displaced, thereby opening one of the oxygen windows of the D4R cage. The detrapping H atom was located near the center of the oxygen window and the MO illustrations showed a change in shape from spherical to ellipsoid. Consequently, it is clear that the detrapping process is not due to the formation of chemical bonding. The calculated activation and reaction energies of this detrapping process were +98.6 and −26.1 kJ/mol, respectively. In addition, single-point calculations at the MP2 level were done for each optimized structure, and the obtained activation and reaction energies were +128.7 and −9.3 kJ/mol, respectively. Both calculated activation energies were comparable to Stösser's experimental data (+109.6 ± 3.1 kJ/mol) for H•:Si8O12(OSi(CH3)3)8 (Q8M8). Furthermore, additional explanations are given on the IR vibrational frequencies of HT8 and H@HT8 and the hyperfine coupling constant for caged atomic hydrogen by ESR.

Calculation of the Structural, Energetic, and Spectral Properties of Alumoxane and Aluminosilicate "Drum" Molecules.

In aluminosilicate cage compounds Al-O-Al linkages are usually not found, in accord with the "Al avoidance" rule. In some cases Al-O-Al linkages can be stabilized by forming additional bonds to the bridging O atom. This can occur through direct coordination of cations or by the fusing of aluminosilicate rings to create "drum"-like molecules. We have calculated the structures, energetics, and NMR and vibrational spectra for several such aluminosilicate drum-like molecules and for related alumoxanes, as well as for analogous silicate molecules. Calculations on Si(2)Al(4)O(6)H(8), Al(6)O(6)H(6), Si(4)Al(4)O(8)H(12), Si(6)O(9)H(6), Si(8)O(12)H(8), Si(4)Al(4)O(12)H(8)(4)(-), and Si(4)Al(4)O(12)H(8)Na(4) reproduce the experimental structures (where available) and the observed trends in Si NMR shieldings. In the double 3-ring and double 4-ring (D3R and D4R) silicate cages Si(6)O(9)H(6) and Si(8)O(12)H(8) the Si atoms are shielded compared to their monomeric units, with a significantly greater shielding for the D4R compared to the D3R. The Si(4),Al(4) D4R bare anion has about the same Si shielding as Si(8)O(12)H(8), while the neutral Si(4)Al(4)O(12)H(8)Na(4) shows a 13 ppm deshielding. These trends in Si NMR shieldings are in accord with those observed in both the molecules and in aluminosilicate minerals. On the other hand, the fused or "drum"-like D3R and D4R rings show Si atoms and Al atoms which are calculated to be deshielded with respect to their corresponding monomers, in accord with experiment. We also reproduce the difference in frequency of the most intense IR absorptions for the various double-ring molecules. While silicate D4R cages are considerably more stable than corresponding D3R, the stabilities of the D3R molecule Al(6)O(6)H(6) and the hypothetical D4R symmetry Al(8)O(8)H(8) are quite similar. Calculated gas-phase proton affinities for Si(6)O(9)H(6), Si(2)Al(4)O(6)H(8), and Al(6)O(6)H(6) are similar, although protonation causes cage breaking for Si(2)Al(4)O(6)H(8) and Al(6)O(6)H(6). For a drum-like D4R of composition Si(4)Al(4)O(8)H(12), which is calculated to be stable with respect to Si(2)Al(4)O(6)H(8) and Si(4)O(4)H(8), we present a calculated structure as well as NMR properties.

Synthesis and Crystal Structure of Ulm-16, A New Open Framework Fluorinated Gallium Phosphate with 16-Ring Channels

AbstractThe synthesis and characterization of ULM-16, a new fluorinated gallophosphate Ga4(PO4)F2• 1.5 NC6H14• 0.5 H2O • 0.5 H3O, is described. This material is hydrothermally prepared at 180°C using cyclohexylamine as the structuring agent. It cristallizes in the orthorhombic space group Pbcn (n° 60) with a = 27.329(4) Å, b = 17.377(2) Å, c = 10.2124(5) Å, V = 4849.6 (9) Å3and Z = 8. The structure is built up from vertex-linked GaO4and PO4tetrahedra, GaO4F trigonal bipyramids and GaO4F2octahedra forming hexameric units and infinite double crankshaft chains (calledccor UUDD). The hexameric unit is similar to that found in ULM-3, ULM-4, ULM-5 and ULM-8. The two types of building units are fully connected by vertex-sharing with respect to a strict Ga-P alternation. The 3-D framework consists of cylindrical tunnels delimited by sixteen and six polyhedra running along [001]. The cyclohexylamine molecules are trapped in the wider pores and interact with the bridging fluorine atoms and hydration water through hydrogen bonds. The ULM-16 topology is closely related to that of ULM-5 where, in ULM-16, the infinite crankshaft chains replace the isolated D4R cages of ULM-5. TGA curve (under argon) indicates that the ULM-16 structure is intact up to 800°C. Structural relationships will be stated explicitly for the different structures of the ULM-n series containing the hexameric units.

Inorganic-organic polymers derived from functional silicic acid derivatives by additive reaction

Inorganic-organic polymers were synthesized by additive reaction of vinyl-, allyl-, and H-silylated double four-ring (D4R) silicic acids and polymeric silicic acids. The structure and properties of the hybrid polymers were investigated by means of 29Si NMR spectroscopy, thermoanalysis and BET nitrogen adsorption measurements. Using the defined vinylsilylated D4R silicicacid [(CH 2CH)(CH 3) 2Si] 8Si 8O 20 and the corresponding H-silylated compound [(CH 3) 2HSi] 8Si 8O 20 as precursors, the additive reaction results in a microporous polymer with a ordered Si 8O 20 substructure. The structural units are connected by six-membered bridges. Shorter (four-membered) or longer (seven-membered) bridges between D4R cages lead to non-porous polymeric materials. The connection of water-glass-derived silicic acid units by six-membered bridges similarly leads to porous polymers with specific surface areas of 500 m 2/g. For the preparation of the porous hybrid polymers a new, simple two-step reaction route is described.

Stabilization of Atomic Hydrogen in Both Solution and Crystal at Room Temperature

Atomic hydrogen has been stably encapsulated in both solution and crystal at room temperature. Upon gamma-ray irradiation of [(CH(3))(3)Si](8)Si(8)O(20), which is the trimethylsilylated derivative of the silicate anion with a double four-ring (D4R) cage, electron spin resonance (ESR) spectra revealed that a single hydrogen atom is encapsulated in the center of the D4R cage and is stable for periods of many months. Attack by chemically reactive species such as oxygen was prevented by the D4R cage, but the ESR signal of the hydrogen atom was sensitive to the magnetic interaction caused by the presence of the O(2) molecule near the cage.