D4R Units Research Articles

Facile synthesis of SSZ-16 nanoaggregates with excellent performance in NH3-SCR reaction

d6r units were proved to be crucial for the synthesis of SSZ-16 zeolite with AFX topology, and FAU zeolite owning plenty of d6r units was chosen as the essential raw material for constructing AFX frameworks. In this work, SSZ-16 zeolite was directly synthesized using colloidal silica and sodium aluminate as the raw materials in short crystallization time of 5 h. The corresponding crystallization process were carefully investigated by various characterizations, demonstrating that the key to this success was the formation of d6r units built up by a large amount of s4r units which were formed with the assistance of the appropriate organic structure directing agent (OSDA) and seeds in the synthetic media. Interestingly, the products of C-SSZ-16 zeolites presented spherical shapes consisted of relatively small particles with size range from 30 to 50 nm. Moreover, after ion-exchanged with Cu ions, the Cu-C-SSZ-16 products showed excellent performance in the selective catalytic reduction of NOx with NH3 (NH3-SCR) and better hydrothermal stability than conventional Cu-SSZ-16-con samples due to slightly higher Si/Al ratio of the Cu-C-SSZ-16. Therefore, remarkable catalytic performance and the use of low-cost raw materials as well as short period of synthesis time make AFX zeolites as potential applicants in NH3-SCR field, from an industrial viewpoint.

On the Origin of Enantioselectivity in Chiral Zeolite Asymmetric Catalyst GTM-3: Host-Guest Transfer of Chirality.

Knowledge of how extra-large-pore chiral zeolite asymmetric catalysts based on the -ITV framework imprint their chirality during a catalytic reaction is crucial in order to spread the scope for the catalytic enantioselective production of chiral compounds of interest. In this work, we have carried out a combined experimental and computational study on the catalytic activity of antipode GTM-3 catalysts during the ring-opening of trans-stilbene oxide with 1-butanol. Identification of the enantiomers of all the chiral species unraveled a surprising catalytic behavior: these chiral catalysts promote the transformation of one enantiomer of trans-stilbene oxide in the corresponding unlike product (with inversion of configuration of the attacked C) via an SN2 mechanism, and at the same time, the transformation of the other enantiomer of trans-stilbene oxide via an SN1-like mechanism into the like (with retention of configuration) and secondary products (diphenylacetaldehyde via Meinwald rearrangement and derived products). A computational study based on DFT + D methods suggested a potential explanation for this catalytic behavior, associated with a different orientation of trans-stilbene oxide enantiomers bound on the Ge(T7) positions in d4r units, which is stabilized by the development of intraframework H-bonds between the interrupted T7OH adjacent positions characteristic of this framework. Calculations suggest that each enantiomer of trans-stilbene oxide follows a different reaction pathway, one favoring the SN2 route by addition of butanol from the opposite side to form unlike-products, while the different orientation of the antipode enantiomer disfavors such SN2 route mainly by steric repulsions and at the same time favors the reaction toward the SN1 mechanism to give like- and secondary-products. Our study suggests that the strong enantioselectivity of GTM-3 catalysts for this reaction is associated with the particular orientation adopted by the chiral reactants within the chiral nanospace provided by the -ITV framework, similarly to what occurs with enzymes, and such preferential orientation is directly controlled by the asymmetric cavities where the reaction takes place, by the particular features of the Ge active sites in adjacent interrupted positions and by the presence of several framework OH groups in the nearby nanospace that interact with guest species. The experimental observations and the reaction mechanism proposed suggest that GTM-3 catalysts prepared from the (1S,2S) enantiomer of the N,N-ethyl-methyl-pseudoephedrinium organic agent should be enriched in the P4332 enantiomorphic space group of the -ITV framework and GTM-3 prepared from the (1R,2R) enantiomer in the antipode P4132. Interestingly, resolution of the absolute configuration of GTM-3 materials from 3D-electron diffraction data has been accomplished and confirms such an assignment, giving an average 82% enantio-enrichment in the corresponding chiral polymorph. Structure-solution of the location of the chiral structure-directing agents indicates that the transfer of chirality from the molecular component to the zeolite polymorph is governed by the development of strong H-bonds between the molecular hydroxyl group and the interrupted T(7)OH framework positions.

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.

Double Four Ring Units-Containing Zeolites: Synthesis, Structural Modification and Catalytic Applications.

In zeolite frameworks, double four-ring (d4r) configurations are among the most frequent composite building units. The composition variations in d4r units greatly influence the energy and structural modifiability of the zeolitic framework. The introduction of germanium, with a larger ionic radius than silicon or aluminum, not only reduces the energy constraints of d4r in the nucleation and crystal growth of zeolites, but also opens a new window for constructing novel crystalline structures, especially with large or extra-large pores and channels. Ge-enriched d4r units endow germanosilicates with structure diversity readily for post treatments. Promising catalytic materials have been gradually developed and increasingly studied by direct synthesis or post-synthetic isomorphous substitution for Ge. This review focuses on the recent progress in the synthesis, modification, and catalytic application of d4r-containing zeolites, including germanosilicates, aluminosilicates, and silicates.

ECNU-13: A High-Silica Zeolite with Three-Dimensional and High-Connectivity Multi-Pore Structures for Selective Alkene Cracking.

A high-silica zeolite ECNU-13 (Si/Al=23) with a new three-dimensional (3D) pore system and a nanosized morphology has been developed, consisting of multitudes of 10-membered ring (10-R) medium pores and one set of 8-R small pores. A phase-discrimination strategy was proposed to synthesize ECNU-13 by regulating the gel compositions and nucleation processes that were used for preparing 12-R large-pore germanosilicate IM-20 with the known UWY topology. The crystallization was directed towards forming one set of single four-ring (s4r) composite building units together with one set of double four-ring (d4r) rather than two different types of d4r units in IM-20. The electron crystallographic investigations elucidated that the ECNU-13 structure was composed of two kinds of polymorphs as a result of distinct atomic positionings in s4r units. In catalytic cracking of 1-butene, ECNU-13 exhibited high propene selectivity (55.6 %) and propene to ethylene molar ratio (>4.7) superior to well-studied conventional ZSM-5 zeolite catalyst.

ECNU‐13: A High‐Silica Zeolite with Three‐Dimensional and High‐Connectivity Multi‐Pore Structures for Selective Alkene Cracking

AbstractA high‐silica zeolite ECNU‐13 (Si/Al=23) with a new three‐dimensional (3D) pore system and a nanosized morphology has been developed, consisting of multitudes of 10‐membered ring (10‐R) medium pores and one set of 8‐R small pores. A phase‐discrimination strategy was proposed to synthesize ECNU‐13 by regulating the gel compositions and nucleation processes that were used for preparing 12‐R large‐pore germanosilicate IM‐20 with the known UWY topology. The crystallization was directed towards forming one set of single four‐ring (s4r) composite building units together with one set of double four‐ring (d4r) rather than two different types of d4r units in IM‐20. The electron crystallographic investigations elucidated that the ECNU‐13 structure was composed of two kinds of polymorphs as a result of distinct atomic positionings in s4r units. In catalytic cracking of 1‐butene, ECNU‐13 exhibited high propene selectivity (55.6 %) and propene to ethylene molar ratio (>4.7) superior to well‐studied conventional ZSM‐5 zeolite catalyst.

Insights on the seed selection criteria of SAPO-34 synthesis: structural units and their chemical microenvironment

The working mechanism of seed-assisted crystallization of SAPO-34 was systematically investigated, which reveals that both d6r units and complex Si environments are key features for a highly effective SAPO seed.

Tracking Sub-Nano-Scale Structural Evolution in Zeolite Synthesis by In Situ High-Energy X-ray Total Scattering Measurement with Pair Distribution Function Analysis.

The crystallization mechanism of zeolites remains unclarified to date because of lack of effective techniques in characterizing the local structures of amorphous precursors under synthetic conditions. Herein, in situ high-energy X-ray total scattering measurement with pair distribution function analysis is performed throughout the hydrothermal synthesis of SSZ-13 zeolite to investigate the amorphous-to-crystalline transformation at the sub-nano level in real time. Ordered four-membered rings (4Rs) are dominantly formed during the induction period, prior to the significant increase in the number of symmetric six- and eight-membered rings (6Rs and 8Rs) in the crystal growth stage. These preformed ordered 4Rs contribute to the formation of d6r and cha composite building units containing 6Rs and 8Rs with the assistance of the organic structure-directing agent, leading to the construction of embryonic zeolite crystallites, which facilitate the crystal growth through a particle attachment pathway. This work enriches the toolbox for better understanding the crystallization pathway of zeolites.

Tailored alkyl-imidazolium templating synthesis of extra-large-pore germanosilicate zeolite ITT as Baeyer-Villiger oxidation catalyst

Extra-large-pore germanosilicate zeolite ITT was prepared with several structure-tailored alkyl-substituted imidazolium templates. The effect of alkyl chain length and substitution number of template on zeolite topology structure was studied with the variation of synthesis Si/Ge ratio. 1-hexyl-3-methyl-imidazaolium (1H3M) exhibited superior templating effect on the formation of ITT structure within the widest range of synthesis Si/Ge molar ratio of about 2–6. 1H→13C CP/MAS NMR, TGA and molecular simulation results revealed that two 1H3M was occluded in the 18-ring channels per unit cell, presenting the lowest interaction energy with framework. With the combination of XRD, SEM, EDS, ICP, 29Si, 19F MAS NMR, FTIR, N2 physisorption results, it was found that the increase of synthesis Si/Ge ratio improved framework Si/Ge ratio within d4r unit to a certain degree, and in the meantime resulted in the decrease of zeolite crystallinity with the formation of some amorphous phase. ITT showed better Lewis-acid catalytic activity and lactone selectivity for B–V oxidation of 2-adamantanone than BEC, the best germanosilicate zeolite ever reported. The excellent catalytic performance was ascribed to the presence of 18-ring channels in ITT, which facilitated the diffusion of bulky ketone substrate and lactone product. The structure stability of ITT during the reaction was evaluated with the use of either aqueous H2O2 or non-aqueous tert-butyl hydroperoxide (TBHP) as oxidant.

A facile strategy of preparing nanosized NaY zeolite by stimulating negative crystals to accelerate the formation of D6R units

Nanosized Y zeolite has excellent potential as a catalyst or adsorbent to boost the chemical industry. However, the synthesis of nanosized Y zeolite under traditional industrial conditions still faces many huge challenges. Herein, we report a facile strategy of preparing nanosized NaY zeolite by applying sodium halides (NaCl or NaBr) to regulate amorphous gel. Sodium halides regulate the micro-structural transformation of the initial amorphous gel during the induction period and promote the polymerization between silicate ions and aluminosilicate. Thus, many more negative crystals are stimulated to form and the emergence of double six-ring (D6R) building units is accelerated. In a short 12 h, nanosized NaY zeolite with small crystal size (about 100 nm), large pore volume (0.476 cm3 g−1), and rich mesoporous volume (0.165 cm3 g−1) is synthesized. These synthesized nanosized NaY zeolites are expected to be better precursors to exert their advantages.

Synthesis of silica-rich chabazite using conventional Si and Al sources and the low-cost tetraethylammonium (TEA+) cation as structure directing molecule

High-silica zeolites with the CHA topology have been synthesized from conventional Si and Al sources in the presence of zeolitic seeds, and using the non-expensive tetraethylammonium (TEA+) cation as structure directing agent. Seeds with the CHA topology effectively enabled the crystallization of chabazite but the solid was difficult to obtain pure and almost systematically contaminated by zeolites with the GME framework topology. Better results were obtained when high-silica seeds with the FAU topology were added. Under those conditions, pure chabazite zeolites (CHA) with SiO2/Al2O3 > 13 could be obtained within a few days at 130 °C. Kinetic studies revealed that the zeolite chabazite was not formed directly from the D6R units of the faujasite seeds, as generally reported in the literature, but from the dissolution of an intermediate [TEA+]FAU zeolite that completely crystallized within less than 12 h. Moreover, pure chabazite zeolites (CHA) could be obtained with amounts of seeds as low as 2.5 wt %, that is, an order of magnitude lower than amounts previously reported in the literature.

Solvent-Free Thermal Synthesis of Extra-Large-Pore Aluminophosphate Zeotype via Self-Assembly of Double-Four-Ring Unit.

The use of Ge as framework atom to direct the formation of double-four-ring (d4r) unit is an effective way to prepare zeolite with extra-large pores, which however also brings about serious problems with regard to production cost and structure stability for their practical application. A new solvent-free thermal synthesis strategy is presented here for facile preparation and molecular-level mechanism study of extra-large-pore aluminophosphate zeotype DNL-1 with -CLO structure. For the first time, the formation of intermediate d4r in the induction period was successfully confirmed, and was correlated with the synthesis condition of low water content and quaternary ammonium template with suitable alkyl chain length. The self-assembly pathway via d4r to form lta, afterwards clo and super cages was further revealed. This work shed light on the rational synthesis of Ge-free extra-large-pore zeotypes based on d4r route.

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.

A Facile Strategy of Preparing Nanosized Nay Zeolite by Stimulating Negative Crystals to Accelerate the Formation of D6r Units

A Facile Strategy of Preparing Nanosized Nay Zeolite by Stimulating Negative Crystals to Accelerate the Formation of D6r Units

Stabilitas Zeolit HY Hidrofobik terhadap Uji Hot Liquid Water (HLW)

Initially, zeolite catalysts cannot be used for reactions involving hot liquid water, so it is necessary to modify the zeolite to be stable under these conditions. The synthesis of HY and hydrophobic zeolites aimed to determine the stability of zeolite in hot liquid water (HLW). The stability of zeolite in HLW is related to zeolite hydrophobicity. In this study, the main raw materials for the synthesis of HY zeolite were sodium aluminate (NaAlO2) and Ludox HS-40 by calcining the product at 350 oC for 1 hour. Furthermore, increasing the hydrophobicity of HY zeolite was carried out by modifying the external zeolite surface using organosilanes (aminopropyltriethoxysilane). The stability of two zeolites in HLW was set at 200 oC. Zeolite analysis used physical test and Fourier Transform-Infra Red (FTIR) Spectrometer. The results of the zeolite distribution in two phases confirmed that HY zeolite was completely dispersed in the water phase, while the zeolite one was dispersed in the organic phase. Moreover, the IR spectra of HY zeolite showed that the wide peak detection at wave number 720 cm-1 for beginning to disappear. This means that the HLW condition could be damaged to the d6r unit. Another result was shown by hydrophobic zeolite spectra with a stable peak for 72 hours. This indicates that the hydrophobic zeolite has thermal stability in HLW so that it can be used in chemical reaction catalytic applications that use on phase.

Synthesis and Structure of a 22 × 12 × 12 Extra-Large Pore Zeolite ITQ-56 Determined by 3D Electron Diffraction.

A multidimensional extra-large pore germanosilicate, denoted ITQ-56, has been synthesized by using modified memantine as an organic structure-directing agent. ITQ-56 crystallizes as plate-like nanocrystals. Its structure was determined by 3D electron diffraction/MicroED. The structure of ITQ-56 contains extra-large 22-ring channels intersecting with straight 12-ring channels. ITQ-56 is the first zeolite with 22-ring pores, which is a result of ordered vacancies of double 4-ring (d4r) units in a fully connected zeolite framework. The framework density is as low as 12.4 T atoms/1000 Å3. The discovery of the ITQ-56 structure not only fills the missing member of extra-large pore zeolite with 22-ring channels but also creates a new approach of making extra-large pore zeolites by introducing ordered vacancies in zeolite frameworks.

Enhanced hydrothermal stability of Cu/SSZ-39 with increasing Cu contents, and the mechanism of selective catalytic reduction of NO

Cu/SSZ-39 is an alternative highly effective catalyst for selective catalytic reduction of NOx compounds in Diesel exhaust besides Cu/SSZ-13. The hydrothermal stability of Cu/SSZ-39 exhibits an obvious dependency on Cu contents. Further experiment evidence reveals a mutual protective effect between Cu2+ ion species and framework tetrahedral Al atoms upon hydrothermal treatments. Rietveld refinement has revealed that ionic Cu species mainly locate on the face of double 6 ring (d6r) units of AEI framework. With increasing Cu contents, the amount of highly stable isolated Cu2+ ions increases. In consequence, the stability of framework Al atoms under high-temperature hydrothermal conditions is enhanced. Furthermore, in situ DRIFTS suggests a mixed Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanism for SCR reactions over this catalyst at low temperatures, where NH4NOx (x = 2 or 3) molecules are identified as key intermediates.

Extra-Large Pore Titanosilicate Synthesized via Reversible 3D–2D–3D Structural Transformation as Highly Active Catalyst for Cycloalkene Epoxidation

Titanosilicates with extra-large pores or cages are expected to effectively release the diffusion constraints suffered by the bulky substrates in the hydrogen peroxide-involved liquid-phase selective oxidation reactions. A reversible 3D–2D–3D structural transformation was developed to fabricate a highly active IWV-type titanosilicate (Ti-IWV) with a two-dimensional intersecting 12-membered ring (MR) channel system and extra-large 14-MR supercages. The IWV germanosilicate was readily disassembled into a layered 2D material (Hydro-IWV) in HNO3 aqueous solution, which was reconstructed to Ti-IWV with various Ti contents (Si/Ti ratio of 40–∞) through the (NH4)2TiF6-assisted isomorphous substitution of Ti and structure repair. The fluoride anions were critical to recover the interlayer double-four-ring (d4r) units, which were destroyed in the hydrolysis process. Ti-IWV was extremely active in the liquid-phase epoxidation reaction of cycloalkenes, especially showing a much higher conversion (99%) for cyclooctene than conventional titanosilicates. Rather than diffusion rate, the high capacity for the adsorption of bulky alkene molecules of extra-large 14-MR cages contributed to the outstanding activity of Ti-IWV.

Ab initio investigation of the relative stability of silicogermanates and their (Alumino)Silicates counterparts

Abstract The diversity of synthetic zeolites with silicogermanate composition has grown significantly. Many of these zeolites have large pores, thanks to the double 4-ring (d4r) structural subunits occupied by Ge atoms. The wide pores make them potentially interesting for catalytic transformation of bulky molecules, but the thermal and hydrothermal stability and the acidity are insufficient for practical applications. In the present work, the stability of known silicogermanate zeolite structures and their silicate and aluminosilicate analogues is evaluated using periodic density functional theory calculations. The thermodynamics of isomorphic substitution of Ge atoms for Si and Al via chemical processes are investigated. The study reveals that thermodynamically, (alumino)silicate counterparts of all known silicogermanates are intrinsically stable, and that the energetics of isomorphic substitution reactions of Ge atoms are almost independent of the distribution of d4r units in the different framework topologies. Chlorides are found better isomorphic substitution reagent candidates, at least theoretically. This work opens perspectives for the catalytic use of stable derivatives of silicogermanate zeolites.

Importance of controllable Al sites in CHA framework by crystallization pathways for NH3-SCR reaction

As a commerical catalyst of high silica CHA zeolite for NH3-SCR reaction, controllable Al sites in CHA framework for improving catalytic performance are rarely reported. In this work, two organic templates (TMAda+ and DMCHA+) were employed to prepare aluminosilicate CHA zeolite with similar Si/Al ratios, donated as CHA-T and CHA-D. Detailed characterizations demonstrate that two completely distinguishable crystallization pathways can be identified as the preferential formation of smaller D6R units for CHA-D with DMCHA+ and larger 8MRs for CHA-T with TMAda+, resulting in spatially closer Al sites in CHA-D than CHA-T. As an interpretation, CHA-D with more paired Al atoms in 6MR generating more hydrothermally stable Cu2+-2Z sites contributes to higher activity than CHA-T at 400−550 °C. These results suggest that the crystallization pathways directed by rational choice of organic templates might be responsible for the controllable Al sites in the framework, which is important for designing efficient NH3-SCR catalysts.