D4R Units Research Articles

Evolution of D6R units in the interzeolite transformation from FAU, MFI or *BEA into AEI: transfer or reassembly?

The formation and evolution of building units for zeolites play an important role during crystallization.

A Porous POSiSil Suited for Pressure-Driven Reversible Confinement of Solutions: PSS-2.

Polyoligosiloxysilicone (POSiSil; designated PSS-2) is a copolymer of double four-ring (D4R) cyclosilicate and dimethylsiloxane. It is synthesized by linking D4R units in tetrabutylammonium cyclosilicate crystals with dimethyldichlorosilane. The structure of PSS-2 was revealed using solid state NMR spectroscopy. In this 3D copolymer D4R units are connected systematically by short siloxane chains most likely composed of 2 to 3 dimethylsiloxane monomers. Controlling the conversion of the parent material allows for tuning the porosity of PSS-2. Residual parent material is embedded inside PSS-2 polymer and can be eliminated by calcination. This leaves nanovoids inside PSS-2, which is moderately hydrophobic. Pressure-driven intrusion-extrusion cycles of aqueous solution exhibit hysteresis, thus, PSS-2 can be used as reversible confinement for liquids with a capacity of around 1000 mm3 g-1 in porosity.

Synthesis and structure determination of SCM-15: a 3D large-pore zeolite with interconnected straight 12×12×10-ring channels

A new germanosilicate zeolite named SCM-15 (Sinopec Composite Material No. 15), the first zeolite containing a 3-dimensional (3D) channel system with interconnected 12-, 12-, and 10-ring channels (pore sizes: 6.1×7.2, 6.1×7.4, and 5.2×5.9 A), has been synthesized using neutral 4-pyrrolidinopyridine as organic structure-directing agents (OSDAs). Its structure has been determined by combining single-crystal electron diffraction (SCED) and synchrotron powder X-ray diffraction (SPXD) data. The unique open framework structure of SCM-15 is related to that of FOS-5 (BEC), ITQ-7 (ISV), PKU-16 (POS), ITQ-26 (IWS), ITQ-21, Beta polymorph B, and SU-78B, since all these framework structures can be constructed from similar chains which are connected through shared 4-ring or double 4-ring (d4r) units. Based on this relation, six topologically reasonable 3D large or extra-large pore hypothetical zeolites are predicted.

Topotactic Conversion of Alkali‐Treated Intergrown Germanosilicate CIT‐13 into Single‐Crystalline ECNU‐21 Zeolite as Shape‐Selective Catalyst for Ethylene Oxide Hydration

The conversion of the alkali-treated intergrowth germanosilicate CIT-13 into the single-crystalline high-silica ECNU-21 (named after East China Normal University) zeolite, with a novel topology and a highly crystalline zeolite framework, has been realized through a creative top-down strategy involving a mild alkaline-induced multistep process consisting of structural degradation and reconstruction. Instead of acid treatment, hydrolysis in aqueous ammonia solution not only readily cleaved the chemically weak Ge(Si)-O-Ge bonds located within the interlayer double four ring (D4R) units of CIT-13, but also cleaved the metastable Si-O-Si bonds therein. This led to extensive removal of the D4R units, and also generated silanol groups on adjacent silica-rich layers, which then condensed to form a novel daughter structure upon calcination. Individual oxygen bridges in the reassembled ECNU-21 replaced the germanium-rich D4R units in CIT-13, thereby eliminating the original intergrowth phenomenon along the b axis. With an ordered crystalline structure of 10-ring (R) channels as well as suitable germanium-related Lewis acid sites, ECNU-21 serves as a stable solid Lewis acid catalyst for the shape-selective hydration of ethylene oxide (EO) to ethylene glycol (EG) at greatly reduced H2 O/EO ratios and reaction temperature in comparison with the noncatalytic industrial process.

Ionothermal Synthesis of Germanosilicate Zeolites Constructed with Double-Four-Ring Structure-Building Units in the Presence of Organic Base.

The crystallization chemistry of silica-based zeolites in ionic liquids remains highly puzzling and interesting in the field of zeolite science. Herein, we report the successful ionothermal synthesis of germanosilicate zeolites. The ionothermal templating effect with respect to the structure, alkalinity and concentration of organic additives was comparatively studied. The results showed that a small amount of organic base could effectively aid the dissolution of silica source and facilitate the crystallization of germanosilicate zeolites with ionic liquid as template. Remarkably, STW and IRR structures constructed with double-four-ring (D4R) structure-building units were ionothermally prepared using 1-ethyl/butyl-3-methyl imidazolium and 1-benzyl-3-methyl imidazolium ionic liquids as template, respectively. Ionothermal synthesis tailored with organic base showed suitable chemistry for the formation of germanium-containing siliceous D4R units. This finding will be helpful for the rational exploration of novel extra-large-pore zeolitic structures.

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

A new germanosilicate zeolite named SCM-15 (Sinopec Composite Material No. 15), the first zeolite containing a 3-dimensional (3D) channel system with interconnected 12-, 12-, and 10-ring channels (pore sizes: 6.1×7.2, 6.1×7.4, and 5.2×5.9 Å), has been synthesized using neutral 4-pyrrolidinopyridine as organic structure-directing agents (OSDAs). Its structure has been determined by combining single-crystal electron diffraction (SCED) and synchrotron powder X-ray diffraction (SPXD) data. The unique open framework structure of SCM-15 is related to that of FOS-5 (BEC), ITQ-7 (ISV), PKU-16 (POS), ITQ-26 (IWS), ITQ-21, Beta polymorph B, and SU-78B, since all these framework structures can be constructed from similar chains which are connected through shared 4-ring or double 4-ring (d4r) units. Based on this relation, six topologically reasonable 3D large or extra-large pore hypothetical zeolites are predicted.

Cyclodextrin-Driven Formation of Double Six-Ring (D6R) Silicate Cage: NMR Spectroscopic Characterization from Solution to Crystals

Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si12O3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K+, Rb+ and Cs+), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and <SiOSi> bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).

Preparation of Siloxane-Based Microporous Crystals from Hydrogen-Bonded Molecular Crystals of Cage Siloxanes.

Controlled assembly of siloxane-based building blocks provides a rational approach toward designed architectures of silica-based porous materials. Here, a non-hydrothermal method to prepare microporous crystals from cage-type oligosiloxanes is reported. The crystals formation occurs through an ordered assembly assisted by hydrogen bonds and subsequent intermolecular connection by silylation. Cage siloxanes with a double-four ring (D4R) structure were modified with dimethylsilanol groups. Intermolecular hydrogen bonding of the dimethylsilanol groups led to the formation of a pillared-layer structure consisting of D4R units. A new crystalline microporous material retaining the original ordered arrangement was realized by bridging adjacent cages within the crystals by direct silylation of the silanol groups with trichlorosilane. The use of this silylating agent created microporous crystals containing Si-H groups, proving the advantages of the proposed concept.

Assembly of Sub‐Crystals on the Macroscale and Construction of Composite Building Units on the Microscale for SAPO‐34

The nucleation and growth of SAPO-34 crystals with triethylamine (TEA) as a single template was monitored with ex situ time-resolved characterization methods. The investigation focused on the evolution of the intermediate phases at different crystallization stages of SAPO-34. The morphology transformation of the intermediate phases at different crystallization times revealed the unique self-assembly process of the sub-crystals. The cubic SAPO-34 crystals can be constructed from eight pyramidal subunits. Additionally, the construction order of cha cages and double-six-membered ring (d6r) units in the initial crystallization stage was determined. The appearance of cha cages prior to d6r units can be attributed to the structure-directing effect of protonated TEA, which is charge balanced with the negative charge of the framework from Si incorporation. Further analysis showed that Si species were incorporated into the framework by direct participation in the initial crystallization stage and substitution for framework P atoms during the later stage.

Breaking Structural Energy Constraints: Hydrothermal Crystallization of High-Silica Germanosilicates by a Building-Unit Self-Growth Approach.

Zeolites, a class of crystalline microporous materials, have a wide range of practical applications, in particular serving as key catalysts in petrochemical and fine-chemical processes. Millions of zeolite topologies are theoretically possible. However, to date, only 235 frameworks with various tetrahedral element compositions have been discovered in nature or artificially synthesized, among which approximately 50 topologies are available in pure-silica forms. Germanosilicates are becoming an important zeolite family, with a rapidly increasing number of topological structures having unusual double four-membered ring (D4R) building units and large-pore or extra-large-pore systems. The synthesis of their high-silica analogues with higher (hydro)thermal stability remains a great challenge, because the formation of siliceous D4R units is kinetically and thermodynamically unfavorable in hydrothermal systems. Herein, it is demonstrated that such D4R-containing high-silica zeolites with unexpected crystalline topologies (ECNU-24-RC and IM-20-RC) are readily constructed by a versatile route. This strategy provides new opportunities for the synthesis of high-silica zeolite catalysts that are hardly obtainable by conventional hydrothermal synthesis and may also facilitate a breakthrough in increasing the number and types of zeolite materials with practical applications.

Two Aluminophosphate Molecular Sieves Built from Pairs of Enantiomeric Structural Building Units

AbstractHerein we report the synthesis and structures of two new small‐pore aluminophosphate molecular sieves PST‐13 and PST‐14 with mutually connected 8‐ring channels. The structure of PST‐13, synthesized using diethylamine as an organic structure‐directing agent, contains penta‐coordinated framework Al atoms bridged by hydroxy groups and thus edge‐sharing 3‐ and 5‐rings. Upon calcination, PST‐13 undergoes a transformation to PST‐14 with loss of bridging hydroxy groups and occluded organic species. The structures of both materials consist “nonjointly” of pairs of previously undiscovered 1,5‐ and 1,6‐open double 4‐rings (d4rs) which are mirror images of each other. We also present a series of novel chemically feasible hypothetical structures built from 1‐open d4r (sti) or 1,3‐open d4r (nsc) units, as well as from these two enantiomeric structural building units.

Two Aluminophosphate Molecular Sieves Built from Pairs of Enantiomeric Structural Building Units

Herein we report the synthesis and structures of two new small-pore aluminophosphate molecular sieves PST-13 and PST-14 with mutually connected 8-ring channels. The structure of PST-13, synthesized using diethylamine as an organic structure-directing agent, contains penta-coordinated framework Al atoms bridged by hydroxy groups and thus edge-sharing 3- and 5-rings. Upon calcination, PST-13 undergoes a transformation to PST-14 with loss of bridging hydroxy groups and occluded organic species. The structures of both materials consist "nonjointly" of pairs of previously undiscovered 1,5- and 1,6-open double 4-rings (d4rs) which are mirror images of each other. We also present a series of novel chemically feasible hypothetical structures built from 1-open d4r (sti) or 1,3-open d4r (nsc) units, as well as from these two enantiomeric structural building units.

New topology of levyne B under quasi-equilibrium conditions: A temperature-dependent in situ single crystal X-ray diffraction study

The dehydration behavior of natural levyne-Ca was investigated from 25 to 400 °C by in-situ single-crystal X-ray diffraction under dry conditions. The dehydration started at 50 °C. From 125 to 250 °C the structure changed to levyne B topology, characterized by statistical breaking of T1-O-T1 bonds in the original D6R units. At 275 °C, additional rupture of T2-O-T2 bonds in 6mR units started. This second rupture associated with the release of remaining water was not reported before. At 300 °C the structure is assumed to be anhydrous. The new levyne B topology, referred to levyne B′, is characterized by 37% T1B and 15% T2B that are new tetrahedral sites arising as a consequence of the T-O-T breaking process. In contrast to the T1B site, T2B is at general position, originating a system of T2B face sharing tetrahedra, mutually exclusive. Complete migration of T sites to the new positions would lead to different LEV topologies with different kind of cages and, as a consequence, of stacking sequences depending on the position occupied by the T2B tetrahedra.The different results obtained in this study compared to previous ones, confirmed that the applied experimental set-up is fundamental in tracking the transformations upon dehydration in zeolitic materials. Understanding these differences is crucial for subsequent applications of these microporous compounds.

Co-Structure-Directing Effect in Ionothermal Synthesis of Extra-Large-Pore Aluminophosphate Zeotype with -CLO Topology.

The ionothermal synthesis of extra-large-pore aluminophosphate zeotype with -CLO topological structure, DNL-1, was studied by using 1,6-hexanediamine and several quaternary ammonium compounds with different types of cations and anions as co-structure-directing agents (co-SDAs). The phase selectivity and crystallization process were investigated, and results showed that the crystallization of the -CLO and LTA structures correlated with the alkalinity and concentration, rather than with the structure of the co-SDA. By combining solid/solution 13 C NMR spectroscopy and other characterizations on DNL-1 obtained with different co-SDAs, it was demonstrated that the ionic-liquid cation was unalterably occluded in the structure, whereas occlusion of the co-SDA cation was not imperative. A plausible crystallization pathway was accordingly deduced, in which alkaline co-SDA, which acted as an acidity regulator in the ionothermal system, could facilitate the formation of the double-four-ring (d4r) unit (Al4 P4 O20 ), and influence their subsequent assembly mode around ionic-liquid cations, and thus, direct the crystallization of -CLO and LTA structures at different concentrations of co-SDA.

Synthesis of a Single-Crystalline Macroporous Layered Silicate from a Macroporous UTL-Type Zeolite and Its Accelerated Intercalation.

A single-crystalline macroporous layered silicate was obtained for the first time. Firstly, UTL-type zeolite with macropores was prepared hydrothermally under the presence of acetylene black as a macropore template and the subsequent calcination to remove the template. Double four-membered ring (d4r) units in the UTL framework were selectively dissolved to yield a layered silicate with macropores. Intercalation of tetrabutylammonium ions into the macroporous layered silicate is accelerated if compared with that into the same silicate without macropores, indicating the effectiveness of macropores due to easy diffusion. The layered silicate with macropores was converted into PCR-type zeolite with macropores, a hierarchically micro- and macroporous material, through interlayer condensation.

Double-Four-Ring [Si8O12][OH]8 Cyclosilicate and Functionalized Spherosilicate Synthesis from [N(n-C4H9)4]H7[Si8O20]·5.33H2O Cyclosilicate Hydrate Crystals

A new synthesis method for preparing spherosilicates with a double-four-ring (D4R) silicate core (Si8O12) and functional siloxy substituents such as (OSi(CH3)2Cl) on the vertices was discovered. Key to this achievement was the synthesis of [N(n-C4H9)4]H7[Si8O20]·5.33H2O D4R cyclosilicate hydrate precursor crystals having only 5.33 water molecules per D4R unit, which is much smaller than the number in the state-of-the-art tetramethylammonium- and choline-containing cyclosilicate hydrate crystals that have up to 65 water molecules per D4R. The low water content allows the use of bifunctional silylating agents such as dichlorodimethylsilane that lose their chloride functionality upon reaction with water molecules. Dissolution of [N(n-C4H9)4]H7[Si8O20]·5.33H2O crystals in acidified tetrahydrofuran liberates uncharged D4R [Si8O12][OH]8 cyclosilicate molecules, as shown by small angle X-ray scattering and 29Si nuclear magnetic resonance. A concentrated stable solution of [Si8O12][OSi(CH3)2Cl]8 spherosilicate co...

Identification of double four-ring units in germanosilicate ITQ-13 zeolite by solid-state NMR spectroscopy

Well-crystallized Ge-free and Ge-ITQ-13 were successfully obtained by solid state synthesis method. The Ge/Si ratio and the water content that are the two important factors in the synthesis of germanosilicate zeolites were explored for the formation of ITQ-13. The effect of the mineralizing agents (NH4F and NH4Cl) on the ITQ-13 synthesis was investigated as well. The obtained pure silica ITQ-13 and Ge-ITQ-13 were characterized by one- and two-dimensional solid- state NMR techniques. One-dimensional (1D) 19F MAS, 1H→29Si CP/MAS and 19F→29Si CP/MAS NMR spectroscopy evidenced the formation of pure Si-D4R (double four ring) and Ge-D4R units, with the latter being generated by substitution of Si atom from the former units. The detailed configurations of the Ge-D4R units in ITQ-13 was revealed by two-dimensional (2D) 29Si{19F} HETCOR NMR spectroscopy. With the help of theoretical calculations on the 19F and 29Si NMR chemical shifts, six types of D4R units were determined. The formation of the specific D4R unit confirms the structural directing roles of Ge atom and F ions in the formation of the D4R units in zeolite framework. The identification of the SiOSi bonds in the D4R units that connects the layers of ITQ-13 framework provided rationale for the high stability of the ITQ-13 in the degermanation treatment.

Solid State NMR Techniques Study the Structural Characteristics of As-Synthesized ITQ-33

Multinuclear solid state NMR techniques have been applied to study the structural characteristics of extra-large-pore zeolite ITQ-33. Through analysis of 2D 29Si{19F} HETCOR NMR spectra, the configurations of Ge-D4R units in ITQ-33 can be confirmed to have the most separation between Si and Ge atoms. Because F anions are not in the center of D4R units for shorter Ge–F bond lengths and Ge-D4R units with Ge–F bonds in the crystals are related by mirror symmetries, 29Si NMR signals of D4R units are magnetically inequivalent, therefore two 29Si peaks are observed in 19F–29Si CP/MAS and 19F–29Si HETCOR spectra. The formation of specific D4R configurations proves that Ge atoms and F– anions play important structural directing roles in the formation process of zeolite ITQ-33. 27Al 5QMAS experimental results confirm that a major amount of Al atoms is incorporated into the 3-ring in the framework. 1D 13C–27Al S-RESPDOR experimental results show that Al atoms in the framework are spatially close to the methyl group...

Synthesis and Characterization of CIT-13, a Germanosilicate Molecular Sieve with Extra-Large Pore Openings

The synthesis of the germanosilicate CIT-13, a molecular sieve that is the first to have a two-dimensional (2D) pore system possessing pores that are bounded by 14- and 10-rings, is accomplished using a family of monoquaternary, benzyl-imidazolium organic structure-directing agents (OSDAs) in aqueous media containing fluoride. CIT-13 is prepared using either hydrogen fluoride (HF) or ammonium fluoride (NH4F). The structure refinement suggests that most of the Ge atoms are located in the d4r (double-4-rings) units, and that there are framework disorders in the arrangement of those d4r units. Other characterizations of CIT-13 such as 29Si MAS NMR spectra, Ar-adsorption isotherms, and so forth are presented and compared to those of IM-12 (UTL), a previously reported germanosilicate with 14- and 12-ring pores.

Interaction of water with (silico)aluminophosphate zeotypes: a comparative investigation using dispersion-corrected DFT

Porous aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs) with zeolite-like structures have received considerable attention as potential adsorbents for heat transformation applications using water adsorption/desorption cycles. Since a detailed experimental characterisation of the water adsorption properties has only been performed for some of these materials, such as AlPO-18 (AEI topology) and SAPO-34 (CHA topology), more systematic insights regarding the influence of the pore topology and (for SAPOs) the arrangement of the framework protons on the affinity towards water are lacking. To study the relationships between structure and properties in more detail, the interaction of water with six structurally different AlPOs (with AEI, AFX, CHA, ERI, GIS, RHO topologies) and their SAPO analogues was investigated using dispersion-corrected density-functional theory (DFT-D) calculations. Different possible locations of silicon atoms and charge-balancing protons were considered for the SAPO systems. The calculations for SAPOs at low water loadings (one H2O molecule per framework proton) revealed that the interaction energies exhibit a considerable variation, ranging from −75 to −100 kJ mol−1 (per water molecule). The differences in interaction energy were rationalised with the different structural environment of the framework protons at which the water molecules are adsorbed. At high water uptakes (near saturation), interaction energies in the range of −65 kJ mol−1 were obtained for all AlPOs, and there was no evidence for a marked influence of pore size and/or topology on the interaction strength. The interaction of water with SAPOs was found to be approximately 5 kJ mol−1 stronger than for AlPOs due to an increased contribution of electrostatic interactions. An analysis of the structural changes upon water adsorption revealed striking differences between the distinct topologies, with the materials with GIS and RHO topologies being distorted much more drastically than the systems based on double six-ring (d6r) units. Moreover, the direct coordination of water molecules to framework aluminium atoms occurs more frequently in these materials, an observation that points towards a reduced structural stability upon hydration.