Organic Structure-directing Agents Research Articles

A Nanostrips-Assemble Morphology of ZSM-5 Zeolite for Efficient Propylene Production from Methanol Conversion

Turning the morphology and orientated growth of zeolite crystals is essential to alter their catalytic performances. MFI-type ZSM-5 nanostripes (named as D-Z5) with a sea-urchin-like overall morphology were directly synthesized with a commercially available and small-molecular organic structure-directing agent (OSDA) of 1-butyl-1-methylpyrrolidinium hydroxide. Their nanostrip crystals exhibited a unique dioriented morphology with thin thicknesses of ∼8 nm and ∼70 nm along the a-axis and b-axis, respectively. The dealuminated D-Z5 displayed higher propylene selectivity (∼57%), longer lifetime (60 h) and much higher propylene-to-ethylene ratio (>12) in methanol-to-propylene (MTP) reaction than those conventional ZSM-5 with known crystal morphologies. Computational simulation and experimental measurements provided solid proof that the diorientated crystals decreased the bulky molecular diffusion resistance, postponed the secondary reaction and then promoted propylene to diffuse rapidly out of two sets of micropores in zeolites. This unusual diorientation phenomenon is not only beneficial for improving the MTP catalytic performance of ZSM-5, but also expected to be versatile to develop other useful zeolite catalysts.

Mild ultraviolet detemplation of SAPO-34 zeolite membranes toward pore structure control and highly selective gas separation

SAPO-34 zeolite membranes were activated by ultraviolet (UV) radiation at low temperatures to reduce grain boundary defects. The successful detemplation of SAPO-34 zeolite powders using UV radiation was confirmed by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and nitrogen adsorption-desorption. The irradiance exerted a significant impact on the quality of UV-detemplated SAPO-34 membranes. Under optimal UV detemplation conditions, SAPO-34 membranes showed a moderate H2 permeance of ∼10−7 mol Pa−1 m−2 s−1, with a H2/i-C4H10 permselectivity over 75 under room temperature. The membranes showed an exceptionally low CO2 permeance in comparison with N2 and CH4 despite the smaller molecular size of CO2, resulting in an unprecedentedly high H2/CO2 permselectivity over 50, which was ascribed mainly to the intense interaction between the CO2 molecules and confined organic structure directing agent (OSDA) residue in membrane channels. The temperature dependence of singe-gas permeance through SAPO-34 zeolite membranes showed that gases, including He, H2, CO2, CH4, C2H6, C3H8, and i-C4H10, markedly increased as the temperature increased, indicating an activated diffusion mechanism. The activation energies for H2 permeation through the SAPO-34 membranes were 17–23 kJ mol−1, which were reasonable based on the correlation between the activation energy for H2 permeation and the average pore size for reported membranes by assuming minimal grain boundary defects in the UV-detemplated SAPO-34 membranes.

Designing ABC-6 family small pore zeolites by epitaxial growth approach

Aluminosilicate small pore zeolites belonging to ABC-6 family play crucially important roles in the high methanol conversion with the high selectivity of light olefins, gas separation and storage, and selective catalytic reduction of NOx. In this work, we report a general method, called the epitaxial growth approach, for designing ABC-6 family small pore zeolites. It is mainly realized through the epitaxial growth on the nonporous SOD-type zeolite in the presence of inorganic cations (Na+ and K+) combined with a variety of organic structure directing agents (OSDAs). In this case, a series of ABC-6 family small pore zeolites such as ERI-, SWY-, LEV-, AFX-, and PTT-type zeolites have been successfully synthesized within a few hours. More importantly, the advanced focused ion beam (FIB) and the low-dose high-resolution transmission electron microscopy (HRTEM) imaging technique have been utilized for unraveling the zeolite heterojunction at the atomic level during the epitaxial growth process. It turns out (222) crystallographic planes of the SOD-type zeolite substrate provide unique pre-building units, which facilitate the growth of targeted ABC-6 family small pore zeolites along its c-axis. Moreover, the morphologies of ERI-type zeolite can also be tuned through the epitaxial growth approach, achieving a longer lifetime in the methanol conversion.

Organic Structure-Directing Agent Free Synthesis of Mordenite with Seeds, Used as A Support for Mo Catalysts in the Transesterification of Soybean Oil

This work prepared mordenite using seeds and without organic structure-directing agents (OSDAs). The Mo/Mordenite was prepared through wet impregnation and the catalysts’ performance was checked for transesterification of soybean oil with methanol. The mordenite zeolite was prepared through hydrothermal crystallization under static conditions with a molar composition of 6Na<sub>2</sub>O:Al<sub>2</sub>O<sub>3</sub>:30SiO<sub>2</sub>:780H<sub>2</sub>O. The catalyst samples were characterized crystallinity through X-ray diffraction, elemental composition by X-ray fluorescence spectroscopy, Surface areas by N<sub>2</sub> adsorption-desorption, surface morphology scanning electron microscopy, functional group by infrared spectroscopy and active sites by temperature programmed desorption of ammonia. The transesterification of soybean oil was carried out using the following parameters: 5% catalyst by weight, 1:12 oil to methanol molar ratio, at 200°C for either 12 h or 24 h. X-ray diffraction patterns showed the characteristic peaks of the mordenite structure. After molybdenum oxide was added, the structure of mordenite zeolite was conserved while the specific surface area was reduced. The morphology can be described as a highly crystalline material with well-defined crystalline particles having a spherical profile characteristic of the typical morphology of sodium mordenite zeolite with a low silicon/aluminum ratio. The catalyst samples exhibited sites of a weak and medium-strength nature. The higher activity of the catalyst (Mo/Mordenite) about mordenite zeolite, could be justified by the existence of molybdenum. The wet impregnation of metal (Mo) on the surface of the MOR zeolite is an effective option to increase the acidity of the solid catalysts. Mordenite with 8.84% Mo could be a promising catalyst for the biodiesel factory.

Synthesis and Characterization of Silicoaluminophosphate CIT-16P and Its Transformation to SAPO-17

A silicoaluminophosphate molecular sieve, CIT-16P, is synthesized using butane-1,4-bis(quinuclidinium) [(C7H13N)-(CH2)4-(NC7H13)]2+ dihydroxide (DiQ-C4-(OH)2) as an organic structure-directing agent (OSDA). Upon the removal of the OSDA, either by thermal treatment in air at temperatures exceeding 490 °C or by extended ozone treatment at 150 °C, CIT-16P transforms to SAPO-17 (ERI topology). The structure solution of CIT-16P in its as-synthesized form is obtained using a Rietveld refinement of the powder X-ray diffraction pattern. The primary composite building units (CBUs) of CIT-16P are highly distorted cancrinite (can) CBUs that transform into stable can units of the ERI-type framework as a result of the OSDA removal. The distortion of can CBU is maintained without transformation by the presence of tightly bound DiQ-C4 dications in the as-synthesized form of CIT-16P. The transformed material is characterized and evaluated as a catalyst in the methanol-to-olefins (MTO) reaction. The catalytic behavior of the formed SAPO-17 (Si/T-atom = 0.022) (T = Si + Al + P) at 400 °C and WHSV of 1.3 h-1 produces elevated C3+ olefin products (i.e., propylene, butenes, and pentenes) in early stages of the reaction. However, as the reaction proceeds, the C3+ fraction decreases in favor of more ethylene.

Interzeolite transformation from H+-type FAU zeolite into pure CHA zeolite via OSDA/seeds-free and steam-assisted conversion process

Pure CHA zeolite was transformed from FAU zeolite via organic structure-directing agent (OSDA)/seeds-free steam-assisted conversion (SAC). CHA zeolite was transformed using H+-type FAU zeolite with a low Si/Al ratio (Si/Al = 2.8) and a small amount of potassium hydroxide (KOH) and H2O. The Si/Al ratio of the CHA zeolite of the final product was the same as that of FAU-type zeolite as raw material because OH− ions derived from KOH reacted with H+ ions in FAU zeolite. Then, Si constituting FAU zeolite was little dissolved in a small amount of the basic solvent during the interzeolite transformation process. In general, in the interzeolite transformation process, the Si/Al ratio of the final product is lower than that of zeolite as raw material due to the dissolution of Si in the framework in the basic solvent. However, the interzeolite transformation process in this study can reduce the dissolving Si ratio using H+-type FAU zeolite. In addition, the PHI framework with low CO2 adsorption capacity arose with CHA zeolite transformed from Na+-type FAU zeolite via OSDA-free interzeolite SAC in a previous study. However, in this study, the PHI framework did not arise in OSDA-free steam-assisted interzeolite transformation process due to the use of H+-type FAU zeolite. Furthermore, the second particle morphology of CHA zeolite transformed from H+-type FAU zeolite was spherical, which has not been seen in CHA zeolite particles synthesized in previous reports. Finally, the CO2 adsorption capacity and performance of FAU and CHA zeolites were measured. CHA zeolite showed higher CO2 adsorption capacity than H+-type FAU zeolite as the starting material and high CO2 loadings at low CO2 equilibrium pressure. This study investigated that the synthesis condition, chemical properties, and CO2 adsorption performance of CHA zeolite transformed from H+-type FAU zeolite via the OSDA/seeds-free steam-assisted interzeolite transformation process.

Synthesis and Structure of RUB-58: An Almost Ordered, Highly Crystalline Member of the ZSM-48 Family of Zeolites

ZSM-48 describes a family of disordered high-silica zeolites with a one-dimensional pore system of straight 10-ring channels. Although many different synthesis procedures and various organic structure-directing agents have been tried, all ZSM-48-type materials produced to date have proven to be highly disordered. A few structural models were proposed by Schlenker et al. ( Zeolites 1985, 5, 355−358) and by Lobo and van Koningsveld ( J. Am. Chem. Soc. 2002, 124, 13222−12230) to describe the framework topology and the disorder of the real structure, but it has not been possible to refine the structure of a ZSM-48-type material using experimental diffraction data. RUB-58 was synthesized under hydrothermal conditions at 160 to 200 °C in the system SiO2/H2O/ethylenediamine/pyridine. The product consists of spherelike aggregates of small, needlelike, colorless crystals and has an X-ray powder diffraction (XPD) pattern similar to those published for ZSM-48-type materials. The pattern, however, contains a much larger number of sharp reflections indicative of a more ordered, crystalline material. The reflections could be indexed with the unit cell parameters a = 14.2095(3) Å, b = 20.0874(5) Å, c = 8.3752(2) Å in the space group Pna21, a sub-subgroup of Imma, which is the highest possible symmetry of the ordered ZSM-48 polymorph A (see the Database of Disordered Zeolite Structures). The structure analysis was performed in successive steps: (I) analysis of the average structure, which showed that the framework structure of RUB-58 can be described in terms of polymorph A and that the pyridine molecules occupy defined positions in the elliptical 10-ring channels; (II) refinement of a faulted structure using two separate, but identical polymorph A-type framework structures, with one shifted by exactly 0.5 of the unit cell along the channel direction (volume ratio: 92/8); (III) analysis of the structure of guest-free RUB-58 to examine the geometry of the channels in the empty framework (elliptical cross section nearly identical to that of the as-made material); and (IV) development of convincing disordered models to describe the real structure of RUB-58.

Pure Silica with Ordered Silanols for Propylene/Propane Adsorptive Separation Unraveled by Three-Dimensional Electron Diffraction.

Adsorptive separation of propylene (C3H6) from propane (C3H8), which could deal with energy-intensive cryogenic distillation technologies, remains challenging due to their similar physiochemical properties. Herein, we present a pure silica zeolite with ordered silanols (OSs), whose crystallographic structure was unraveled by the advanced three-dimensional electron diffraction (3D ED), displaying the highly efficient separation of propylene from propane under ambient conditions. The 3D ED technique enables us to investigate its 8-ring pore opening transformation from the round one to the elliptical one during the removal of organic structure-directing agents. Such unique elliptical 8-ring pore openings can exclude larger-size propane and only adsorb propylene. Its C3H6/C3H8 separation performance is also confirmed by column breakthrough experiments, showing a high dynamic adsorption capacity of 53.36 cm3 g-1 for C3H6 but negligible C3H8 under ambient conditions. The dynamic capacity for C3H6 is superior to that of the well-known pure silica DDR-type zeolite (31.07 cm3 g-1). The density functional theory calculation demonstrates that the adsorbed propylene is distributed in the heart-shaped cavity and has a weak interaction with the OSs.

Facile one-pot synthesis of Fe-UZM-35 catalysts for ammonia selective catalytic reduction

Owing to the narrow crystallization field, the synthesis of the large-pore zeolite UZM-35 with MSE topology starting from amorphous reagents in the presence of dimethyldipropylammonium ions as an organic structure-directing agent is often non-reproducible. Here we report the synthesis of UZM-35 via an interzeolite conversion route using USY zeolites, as the sole source of Al and Si, and a small amount of previously synthesized UZM-35 as seed crystals. We also present the one-pot synthesis of Fe-UZM-35 with the best low-temperature NH3-SCR activity among the iron-exchanged zeolites reported so far. These synthesis routes showed high reproducibility, together with considerably faster crystallization kinetics (≤1.5 vs 7 days) compared to conventional UZM-35 synthesis. When the Fe loading level was properly adjusted, the NH3-SCR activity of one-pot-synthesized Fe-UZM-35 was found to be essentially the same as that of post-synthetically exchanged Fe-UZM-35 due to the similarity in the nature of their iron active sites.

Accelerated Crystallization of LTA Zeolite Using a Combination of High-Silica FAU Zeolite with Low Crystallinity and Additional Al Sources

Synthesis methods and crystallization mechanisms of LTA zeolites have been studied by various approaches such as using alkali metal hydroxides, organic structure-directing agent cations, or fluoride anions, so that the understanding of the crystallization pathway is one of particular interest. As a fundamental research, we succeeded in synthesizing a highly crystalline LTA zeolite by combining dealuminated FAU with low crystallinity and an additional aluminum source (Al(OH)3). The crystallization rate of the LTA zeolite was higher than that from conventional colloidal silica (Ludox HS-40) and Al(OH)3. In addition, there was a large difference in the particle size distribution of the as-made LTA zeolites. Furthermore, when using dealuminated FAU and Al(OH)3, Al-rich aluminosilicate species with four-ring structures were observed in the initial induction period, after which the gradual incorporation of TMA+ cations into sod cages of the LTA zeolite was found. Based on the obtained results, a crystallization scheme for LTA zeolites synthesized using zeolitic sources of dealuminated FAU as starting materials is proposed, which deepens the understanding of the crystallization pathway of LTA zeolite.

Synthesis of PHI-type zeolite harmotome using barium as an inorganic structure-directing agent inspired from natural mineral compositions

Zeolites are nanoporous aluminosilicates widely used as commercial adsorbents, heterogeneous catalysts, and ion-exchange materials due to their unique porosity, acidity, and thermal stability. The vast majority of synthetically-realized zeolite structures are prepared under hydrothermal conditions in alkaline media, and often in the presence of an organic structure-directing agent that facilitates the crystallization of diverse porous networks. Due to economic and environmental disadvantages of organic-based syntheses, it is often desirable to produce zeolites in organic-free media using alkali metal ions as the most commonly employed inorganic structure-directing agents (ISDAs). In this study, we were inspired by the elemental compositions of natural zeolite minerals to examine the use of an alkaline earth metal ion as the ISDA to prepare zeolites with the PHI framework type. Two natural minerals exhibiting this crystal structure are phillipsite and harmotome. Both structures contain sodium and potassium as extra-framework cations, but only harmotome contains barium cations. Here we examined how the combination of alkali and alkaline earth ISDAs promote the formation of PHI-type zeolites. Our findings reveal that each isostructure is prepared under different synthesis conditions where simply adding barium to a growth mixture does not always lead to harmotome formation. Harmotome is formed in more dilute and alkaline growth media; however, time-resolved analyses of zeolite crystallization reveal that harmotome can be synthesized at low temperatures (ca. 65 °C) with rates that are 5-times faster than syntheses of phillipsite. This seemingly indicates that barium reduces the energetic barrier for PHI-type zeolite nucleation in ways that are not fully understood. Moreover, harmotome crystals exhibit an average particle size of ca. 60 nm, which is nearly 40-fold smaller than phillipsite crystals prepared under similar conditions. Collectively, these findings identify routes to control crystallization of PHI-type zeolites and may provide a broader strategy for using the composition of natural zeolite minerals to optimize the preparation of synthetic analogues.

Solid-State Synthesis of Aluminophosphate Zeotypes by Calcination of Amorphous Precursors

Because of the growing interest in the applications of zeolitic materials and the various challenges associated with traditional synthesis methods, the development of novel synthesis approaches remains of fundamental importance. Herein, we report a general route for the synthesis of aluminophosphate (AlPO) zeotypes by simple calcination of amorphous precursors at moderate temperatures (250-450 °C) for short reaction times (3-60 min). Accordingly, highly crystalline AlPO zeotypes with various topologies of AST, SOD, LTA, AEL, AFI, and -CLO, ranging from ultra-small to extra-large pores, have been successfully synthesized. Multinuclear multidimensional solid-state NMR techniques combined with complementary operando mass spectrometry (MS), powder X-ray diffraction, high-resolution transmission electron microscopy, and Raman characterizations reveal that covalently bonded fluoride in the intermediates catalyze the bond breaking and remaking processes. The confined organic structure-directing agents with high thermal stability direct the ordered rearrangement. This novel synthesis strategy not only shows excellent synthesis efficiency in terms of a simple synthesis procedure, a fast crystallization rate, and a high product yield, but also sheds new light on the crystallization mechanism of zeolitic materials.

Conquering the solubility barrier of di-n-octylamine as structure-directing agent in hierarchical silicoaluminophosphate synthesis by using phase-transfer synthesis

Silicoaluminophosphates (SAPOs) are a class of microporous solids employed as solid acid catalysts or adsorbents in petrochemical and coal chemical processes, which are often generated in hydrothermal synthesis using water-soluble organic structure-directing agents (OSDA), resulting in the formation of micron sized, rod-like crystals running along pore-channel direction for unidimensional SAPOs. The use of a low-cost, barely soluble di-n-octylamine (DOA) as OSDA in a water/toluene biphasic media to crystallize SAPO-31 is reported. The product is disclosed to be a hierarchically porous material built up of self-assembled nanocrystals (∼110 nm) with enhanced diffusion property and preserved acidity comparable to that of a standard sample derived using di-n-hexylamine as OSDA. The formation is mediated by the formation of a Pickering emulsion structure that favors crystal nucleation over growth, thus overcoming the solubility barrier of candidate OSDA. The obtained material exhibited higher isomer yield in catalytic hydroisomerization of n-dodecane compared to the control sample. Product distribution analysis discloses that mono-branched isomers are generated mainly inside the micropores, while di-branched isomers are produced via key-lock catalysis on pore-mouth sites. The increase in isomerization selectivity is attributed to the formation of more mono-branched isomers and their lowered cracking probability associated with enhanced diffusion property. This new synthetic protocol expands the toolkit of candidate OSDAs towards hydrophobic molecules and exemplifies porogen-free fabrication of hierarchical SAPOs. The uncovered origin of shape-selectivity offers a guidance towards manipulating hydroisomerization product slates.

Direct synthesis of Al-rich ZSM-5 nanocrystals with improved catalytic performance in aromatics formation from methane and methanol

Nanosized ZSM-5 (<100 nm) zeolites with high acidity (Si/Al < 15) can bring distinct advantages as catalysts for obtaining aromatics in C1 chemistry. Synthesis of such nanosized zeolites in a cheap and scalable manner remains a challenge. Herein, nanosized ZSM-5 (20–50 nm) with high acidity (Si/Al = 11) was hydrothermally synthesized in high yield using p-phenylenedimethylene-bis(tripropylammonium) dichloride (Pr3N-benzyl-NPr3) as the sole organic structure-directing agent (OSDA). By investigating the solid products formed during zeolite synthesis, it was found that crystallization follows a solid-state transformation mechanism. An in-depth NMR study combined with TGA measurements reveals that, after early electrostatic interaction between condensed aluminosilicate and the ammonium groups of OSDA, ZSM-5 crystallizes around the OSDA. Owing to its high acidity and shortened diffusion paths, such nanosized ZSM-5 demonstrates, compared to bulk ZSM-5, enhanced aromatics formation in methanol dehydration and non-oxidative dehydroaromatization of methane reactions.

High-throughput phase elucidation of polycrystalline materials using serial rotation electron diffraction

Rapid phase elucidation of polycrystalline materials is essential for developing new materials of chemical, pharmaceutical and industrial interest. Yet, the size and quantity of many crystalline phases are too small for routine X-ray diffraction analysis. This has become a workflow bottleneck in materials development, especially in high-throughput synthesis screening. Here we demonstrate the application of serial rotation electron diffraction (SerialRED) for high-throughput phase identification of complex polycrystalline zeolite products. The products were prepared from a combination of multiple framework T atoms ([Si,Ge,Al] or [Si,Ge,B]) and a simple organic structure-directing agent. We show that using SerialRED, five zeolite phases can be identified from a highly complex mixture. This includes phases with ultra-low contents undetectable using X-ray diffraction and phases with identical crystal morphology and similar unit cell parameters. By automatically and rapidly examining hundreds of crystals, SerialRED enables high-throughput phase analysis and allows the exploration of complex synthesis systems. It provides new opportunities for rapid development of polycrystalline materials.

Synthesis and Crystal Structure of a New RTH-Type Precursor and Its Interlayer Expanded Zeolite.

Two dimensional zeolites have drawn a lot of attention due to their structural diversity and chemical composition, which can be used to obtain 3D zeolites, for which there is no direct synthesis. Here, a new layer silicate zeolite L was synthesized using the N, N-dimethyl-(2-methyl)-benzimidazolium as the organic structure-directing agent (OSDA) in the presence of fluoride. Structure determination by single-crystal X-ray diffraction reveals that the pure silica precursor with five-ring pores in the crystalline sheets is composed of the rth layer stacking along the (001) direction in an …AAAA… sequence with SDA+ cations and F- residing within the interlayer spaces. Variable temperature powder X-ray diffraction (PXRD) results showed that the new layer could transform into a 3D RTH topology structure at 350 °C via 2D-3D topotactic transformation. Furthermore, a new 3D zeolite material is obtained by treating the original layer with a diethoxydimethylsilane agent under hydrochloric acid condition (HCl-DEDMS). Based on the PXRD results and the original layer structure, the new 3D zeolite structure expanding the rth layer with another Si atom is constructed, which possesses a 10×8×6 channel system. It displays a high BET surface area of 188 cm3 /g with an external surface area of 130 cm3 /g. The structure and textural properties pave a way for potential catalytic applications. The research not only provides a new layered zeolite, broadening the 2D zeolite framework types, but also allows for the discovery of a new stable 3D zeolite expanding the RTH structure with Si atom, which hasn't been reported yet.

Effects of Al distribution in the Cu-exchanged AEI zeolites on the reaction performance of continuous direct conversion of methane to methanol

The effects of the organic structure-directing agent (OSDA) with or without Na cations for the synthesis of AEI zeolite on the location and content of the Al atoms in the framework as well as the Cu speciation and acidic features of the exchanged Cu/AEI zeolite catalysts and their catalytic properties in the continuous direct conversion of methane to methanol (cDMTM) were investigated. The AEI zeolite synthesized with Na cations led to more Al and a higher percentage of Al pairs than the one prepared without Na cations. Consequently, the Cu/AEI zeolite catalysts synthesized with Na cations exhibited lower apparent activation energy of methane conversion, higher methanol formation rate, and less stable ability in the cDMTM reaction due to the more active dicopper species and higher acid amount than those prepared without Na cations. Adjustment of the close Al content in the framework of the two AEI zeolites by calcination, the Cu/AEI zeolite synthesized with Na cations evidenced a higher methanol selectivity and more stable reaction performance because of the different relationship between Cu species and acid sites, such as the distance, caused by the Al distribution. The methanol formation rate of 27.3 µmol·g−1·min−1 with about 50% selectivity and stable performance in the cDMTM reaction at 350 °C were obtained. Moreover, the stability of both 5Cu/AEI zeolite catalysts could be upgraded by further calcination to diminish the acid amount, the high methanol formation rate of the 5Cu/AEI zeolite synthesized with Na cations was maintained and the reaction stability was newly attained. The 5Cu/AEI zeolite synthesized without Na cations, merely the stable performance was reached, while the methanol formation rate and selectivity seriously declined. This work contributed to the development of AEI zeolite with different Al distributions and its significant impacts on the Cu speciation and acidic properties, thus providing a valuable strategy to obtain a robust Cu/AEI zeolite catalytic system for efficient and stable methanol production from methane.

Solvent-free synthesis of zeolites: New insights into the mechanism and non-mechanochemical route

The solvent-free (SF) synthesis is a green alternative to the conventional method of synthesis for zeolites. In this study, we show for the first time the feasibility of carrying out the SF synthesis of silicalite-1 (F− route) without any grinding. We also monitor the changes during the synthesis thanks to multiple characterization techniques: XRD, TGA, SEM, EDX, and multinuclear NMR (13C, 19F, 29Si). We show how the duration of the thermal treatment impacts the zeolite crystal size and morphology, as well as the local degree of order probed by solid-state NMR. The obtained results provide new insights on the SF formation mechanism based on solid phase transformations. Our findings also show that once the organic structure directing agent (OSDA), here tetrapropylammonium, is incorporated, the crystals are formed and no other intermediate phases incorporating the OSDA exist. Further, we study the role of water in the synthesis using 17O NMR. For this purpose, we develop a simple and efficient 17O enrichment method of zeolite frameworks. The efficiency of this enrichment demonstrates the role of water in the hydrolysis and condensation reactions and its need for a successful synthesis.

Analysis of Al site-directing ability of organic structure-directing agents in FER and CHA zeolites: a computational exploration of energetic preferences.

This study explores the control of Al location in zeolites by organic structure-directing agents (OSDAs) using atomistic simulations. We examine several zeolite-OSDA complexes to quantify the Al site-directing ability. The results show that OSDAs induce different energetic preferences to direct Al at certain locations. In particular, these effects can be enhanced by OSDAs with N-H moieties. Our findings will be useful for the development of novel OSDAs that can modulate Al site-directing properties.

Multiscale structural control of MFI zeolite using poly-quaternary ammonium cation

Polycation [–N+(CH2CH2CH3)2–(CH2)6–]n[OH−]n, conceptually equivalent to bond-linked tetrapropyl ammonium cation, is designed to generate reduced a-axis and 36° rotational MFI nanosheet intergrowth with enhanced catalytic n-heptane cracking property.