Organic Structure-directing Agents Research Articles

GAM-7: an organic-inorganic hybrid layered aluminophosphate crystal formed by zeolite transformation.

The interzeolite conversion (IZC) of metallo- and aluminophosphate zeolites was demonstrated using a non-cyclic secondary amine, diisopropylamine, as the organic structure-directing agent (OSDA). The IZC of AlPO4-5 with AFI topology could successfully produce an unknown highly crystalline phase, named GAM-7. In metallo-aluminophosphates (metal-AlPOs), SAPO-5 with a small amount of SiO2 was also interconverted to a single crystal GAM-7, designated as [Si]GAM-7. However, in other metal-AlPOs, the corresponding GAM-7 crystals were formed as either an intermediate or a mixture of plural crystals. Additionally, it was found that GAM-7 could also be obtained by hydrothermal synthesis (HTS), but not synthesized from a silico-aluminophosphate hydrogel, indicating that [Si]GAM-7 can be prepared by only the IZC technique. The obtained GAM-7 crystal was well-characterized to elucidate the framework structure, and this revealed that GAM-7 was a new organic-inorganic hybrid layered aluminophosphate having a terminal POH group in the corrugated layered sheets, and the OSDA molecule between the layers was robustly interacting with an acidic POH site by a hydrogen bond. Furthermore, we have performed the tracking of pH values during the transformation from AlPO4-5 into GAM-7 and have argued a reasonable consideration for the IZC process.

Embryonic zeolite-directed synthesis of ZSM-34 and SSZ-13 zeolites

The use of an organic structure-directing agent (OSDA) makes zeolite synthesis expensive and environmentally non-friendly. Seeded zeolite synthesis offers an environmentally benign alternative that avoids using an OSDA while providing a high purity, crystallinity, and yield in the product. In this study, we report using embryonic zeolites (EZs) as efficient seeds to obtain industrially important zeolites such as ZSM-34 and SSZ-13 in an OSDA-free synthesis system. Our results showed that zeolites ZSM-34, SSZ-13 and LTL could be obtained depending on the Al/Si ratio in the synthesis system. The synthesis time was considerably shorter than other ZSM-34 and SSZ-13 syntheses methods reported in the literature, achieving more than 90% crystallinity. The physicochemical analysis showed that highly crystalline zeolites with characteristics similar to the OSDA synthesized counterparts were obtained. Furthermore, the EZs seeding approach is facile, low cost, and environmentally friendly, provided the synthesis is OSDA-free, in addition, the EZs seeds can be obtained under hydrothermal synthesis conditions. The method can be potentially applied to the synthesis of other zeolite types.

Coupling Different Periodic Building Units for Intergrowth Zeolites.

Multiple-phase disordered zeolites, i.e., intergrowth zeolites, are important industrial catalysts, like single-phase ordered zeolites, but little is known about their rational synthesis and phase competition, mainly due to current poor understanding of the zeolite crystallization mechanism. Here, we theoretically demonstrated that sodalite and cancrinite cage layers, the periodic building units (PerBUs) of FAU/EMT and SBT/SBS structures, respectively, could be nondefectively connected to each other across double rings of 6 tetrahedral atoms when inverted and mirrored. We then synthesized an unprecedented family of FAU/SBT/SBS intergrowths with controllable FAU portions (named as the PST-34 family of intergrowth zeolites) using a multiple inorganic cation approach, providing clear experimental evidence for the layer-by-layer crystal growth mechanism of zeolites. This study shows that control of interactive cooperation extent between different inorganic structure-directing agents in the presence of an unselective organic structure-directing agent may enable repeated stacking of different but structurally related PerBUs in intergrowth zeolite synthesis.

Remarkably facilitated crystallization of MSE-type zeolite by using an efficient organic structure-directing agent

Abstract Very powerful organic structure-directing agent (OSDA) for the synthesis of MSE-type zeolite was designed and successfully prepared. When using conventional OSDA, more than 10 d of crystallization time was necessary, whereas the use of the new type of OSDA gave highly crystalline MSE-type zeolite in only 32 h of crystallization time.

Determination of Na+ Cation Locations in Nanozeolite ECR-1 Using a 3D ED Method

Until now, the comprehensive structural analysis of single crystals of zeolite ECR-1, an aluminosilicate with the EON topology, has been hindered owing to the submicron dimensions of the obtained crystals. Additionally, this zeolite, which is characterized by a topology comprising alternating periodic building units of MAZ and MOR layers, exhibits stacking faults that impede accurate refinement through the Rietveld method. In this report, we present, for the first time, the structure of ECR-1 elucidated by studying a nanocrystal with a significantly reduced number of stacking faults. The sample used was synthesized hydrothermally using trioxane as the organic structure-directing agent. The structure determination was conducted using precession electron diffraction (PED) at 103 K. Partial dehydration occurred owing to the high vacuum conditions in the TEM sample chamber. From the dynamical refinement (Robs = 0.097), 8.16 Na+ compensating cations were localized on six distinct crystallographic sites, along with approximately four water molecules per unit cell. Furthermore, a canonical Monte Carlo computational study was conducted to compare the experimental cationic distribution and location of water molecules with the simulation.

High-silica KFI zeolite: highly efficient synthesis and catalysis in methanol amination reaction

The hydrogen form of low-silica KFI zeolite, with a Si/Al ratio of 3.2 (referred to as H-KFI-3.2), has been shown to possess significant selectivity for monomethylamine (MMA) and dimethylamine (DMA) in methanol amination reactions. However, its industrial viability for MMA and DMA synthesis has been hindered by its relatively low methanol (MeOH) conversion and yield of MMA plus DMA. In this study, we synthesize high-silica KFI zeolite with an elevated framework Si/Al ratio of 5.4 (designated as KFI-5.4) using a novel K+ and 18-crown-6 complex [referred to as (K+)CCH, with a ratio of 18-crown-6 to K+ of 2.85] as an organic structure-directing agent. Control experiments have demonstrated that the presence of both K+ and OH- ions is essential for the formation of KFI-5.4 zeolite. The hydrogen form of KFI-5.4 (H-KFI-5.4) exhibits significantly enhanced MeOH conversion (95.2%) and yield (75.1%) of MMA plus DMA compared to the low-silica H-KFI-3.2 under similar reaction conditions. This represents the highest level of catalytic performance among reported small-pore zeolites to date. Furthermore, the yield and selectivity of MMA plus DMA can be further improved by modifying KFI-5.4 with an appropriate loading of Na+, which suppresses the formation of the by-product dimethyl ether. This study introduces a new high-silica KFI zeolite catalyst with exceptional MeOH conversion and yield for the production of MMA plus DMA.

Unraveling a Stable 16-Ring Aluminophosphate DNL-11 through Three-Dimensional Electron Diffraction for Atmospheric Water Harvesting.

Sorption-based atmospheric water harvesting (AWH) is a promising solution for addressing water scarcity. Developing cost-effective and stable water adsorbents with high water uptake capacity and a low-temperature regeneration requirement is a crucially important procedure. In this Communication, we present a novel and stable aluminophosphate (AlPO) molecular sieve (MS) named DNL-11 with 16-ring channels synthesized by using an affordable and commercialized organic structure directing agent (OSDA), whose crystallographic structure is elucidated by three-dimensional electron diffraction (3D ED). DNL-11 exhibits a significant water uptake capacity (189 mg/g) at a very low vapor pressure (5% relative humidity at 30 °C). In addition, most of the adsorbed water can be effortlessly removed by purging N2 at 25 °C under ambient pressure conditions. This may expand the possibility of AWH under extreme drought conditions.

Synthetic Placement of Active Sites in MFI Zeolites for Selective Toluene Methylation to para-Xylene.

Brønsted acidic zeolites are ubiquitous catalysts in fuel and chemical production. Broadening the catalytic diversity of a given zeolite requires strategies to manipulate the acid site placement at framework positions within distinct microporous locations. Here, we combine experiment and theory to elucidate how intermolecular interactions between organic structure-directing agents (OSDAs) and framework Al centers influence the placement of H+ sites in distinct void environments of MFI zeolites and demonstrate the catalytic consequences of active site location on kinetically controlled (403 K) toluene methylation to xylene regioisomers. Kinetic measurements, interpreted using mechanism-derived rate expressions and transition state theory, alongside density functional theory (DFT) calculations show that larger intersection environments similarly stabilize all three xylene isomer transition states without altering well-established aromatic substitution patterns (ortho/para/meta ∼ 60%:30%:10%), while smaller channel environments preferentially destabilize transition states that form bulkier ortho- and meta-isomers, thereby resulting in high intrinsic para-xylene selectivity (∼80%). DFT calculations reveal that the flexibility of nonconventional OSDAs (e.g., 1,4-diazabicyclo[2.2.2]octane) to reorient within MFI intersections and their ability to hydrogen-bond to form protonated complexes favor the placement of Al in smaller channel environments compared to conventional quaternary OSDAs (e.g., tetra-n-propylammonium). These molecular-level insights establish a mechanistic link between OSDA structure, active site placement, and transition state stability in MFI zeolites and provide active site design strategies that are orthogonal to crystallite design approaches harnessing complex reaction-diffusion phenomena to enhance regioisomer selectivity in the industrial production of valuable polymer precursors.

Organic template residues confined in silicalite-1 zeolite membranes for tuning pore structures and boosting H2/CO2 separation

Silicalite-1 zeolite membranes were successfully activated at 300–350 °C in a hydrogen atmosphere. FTIR, TGA, and N2 adsorption-desorption characterizations confirmed that the amount of the organic structure directing agent (OSDA) residue in silicalite-1 zeolite channels was tunable. The H2-calcined membrane showed a lower gas permeance than that of the air-calcined membrane due to the partial pore blockage by the OSDA residue. The permeance of H2 and CO2 increased with increasing temperature, showing an activated diffusion-like mechanism. The activation energies for the permeation of H2 and CO2 were in the range of 1.63–4.15 and 0.76–5.90 kJ mol−1, respectively, which confirmed the reduced average membrane pore size and different pore surface chemistry due to the presence of the OSDA residue in the membrane. Significant interactions between CO2 molecules and OSDA residues confined in the membrane resulted in a notably lower CO2 permeance compared to N2 and CH4, despite CO2 having a smaller kinetic diameter. Enhanced H2/CO2 selectivity in the silicalite-1 zeolite membrane, featuring confined OSDA residue, was attributed to the reduction in membrane pore size and the fine-tuning of pore surface chemistry.

Synthesis, characterization, and evaluation of the CO2 adsorption capacities of beta, ZSM-12, ZSM-48, and silicalite-1 zeolites obtained in the presence of tetraethylammonium hydroxide and 1,6-diaminohexane as organic structure-directing agents

Synthesis, characterization, and evaluation of the CO2 adsorption capacities of beta, ZSM-12, ZSM-48, and silicalite-1 zeolites obtained in the presence of tetraethylammonium hydroxide and 1,6-diaminohexane as organic structure-directing agents

Confining carbon dots in extra-large pore zeolite SYSU-3 assembled in a mixed-template system

The 24-membered ring -SYT type extra-large pore zeolite, denoted as DESO-SYSU-3, has been synthesized by employing N,N′-diethylsophoridine and trimethylsulfanium in a mixed-template system, where the N,N′-diethylsophoridine and trimethylsulfanium played different roles in the crystallization process. A single usage of N,N′-diethylsophoridine led to the formation of an unknown phase (donated as SYSU-5). These results reveal that the utilization of alkaloid-derived organic structure-directing agents (OSDAs) is an efficient synthetic strategy to produce specific extra-large pore zeolites and related structures. Compared to the prototype SYSU-3, the addition of TMSF into the system not only improved the silicon content in the framework of DESO-SYSU-3 but also facilitated the generation of carbon dots (CDs) confined in extra-large pore structure. The structure and the proposed formation mechanism of CDs were investigated based on HRTEM, XPS, FI-IR, UV–Vis, Raman and photoluminescence spectrum. The CDs@zeolite composite exhibited intriguing optical properties of both fluorescence and room temperature phosphorescence, respectively. This work extends extra-large pore zeolite as the matrix for the development of CDs@zeolite composites with tunable optical properties.

Assembled pentasil nanosheet zeolites: Adjusting the sheet thickness through a simple and direct dual templating approach

A simple one-step synthesis method is demonstrated, enabling the thickness of assembled silicalite nanosheets to be tuned from 2 to 30 nm. The adjustement is achieved by mixing two common organic structure-directing agents (OSDAs): tetrapropylammonium (TPA) and tetrabutylammonium (TBA). During the hydrothermal synthesis, there is a preferential uptake of TPA, and the proportion of incorporated TBA can be deduced accordingly. The proportion of TBA, known to favour silicalite nanosheets, determines the nanosheets’ thickness and the morphological variations observed.

Efficient degradation of naproxen in wastewater using Ag-deposited ZnO nanoparticles anchored on a house-of-cards-like MFI-type zeolite: Preparation and physicochemical evaluations of the photocatalyst

We utilized Pr6-diquat-5 as an organic structure-directing agent (OSDA) to synthesize hierarchically intergrown ZSM-5 (HI-ZSM-5) as a catalytic support. ZnO particles were deposited onto HI-ZSM-5 via precipitation to remediate naproxen (NPX) in wastewater, while Ag was photo-deposited onto ZnO to prevent electron-hole pair recombination. XRD analysis revealed crystalline structures and particle sizes influenced by synthesis temperatures, and FT-IR confirmed ZnO and Ag deposition onto HI-ZSM-5 surfaces. Based on N2 adsorption-desorption isotherms, all samples exhibited Type I and IV isotherms with H4-type hysteresis loops, indicating the presence of both micro- and mesopores within HI-ZSM-5. XPS analysis confirmed the presence of peaks indicating Zn in ZnO and metallic Ag interacting with ZnO. TEM analysis identified aggregated Ag/ZnO nanoparticles in the photocatalysts, while EDX confirmed uniform element distribution. FE-SEM images revealed a uniform distribution of ZnO and Ag on HI-ZSM-5 with “house-of-cards” morphologies. PL analysis indicated reduced electron-hole recombination and an extended lifespan. NPX photodegradation experimental results showed that the reaction rate constant of the 3 %AZ composite was 2.8 times higher than that of ZnO/HI-ZSM-5 (0.0163 vs. 0.059 min−1, respectively). Evaluation of influential operating parameters demonstrated 90.8 % elimination of NPX under optimum conditions: a catalyst dosage of 1.5 g L−1 at pH 5 within 120 min. The impact of common anions on NPX decontamination and the catalyst's capability for COD and TOC removal (up to 62.6 and 53.4 %, respectively) was also discussed. The 3 %AZ optimum catalyst could be recycled for 5 cycles with remarkable reusability, maintaining 80.1 % removal efficiency after five regeneration cycles.

Insights into the Synthesis of Spiral Beta Zeolite with Enhanced Catalytic Performance in VOC Abatement.

The rational synthesis of zeolites with designed morphology is a highly challenging task. In this study, we propose 1,5-bis(methylpiperidine)pentylammonium hydroxide (BMPPAOH) as an organic structure-directing agent (OSDA) based on theoretical calculations. The morphology of zeolite samples is characterized by XRD, SEM, TEM, N2 sorption isotherms, and UV Raman spectroscopy. This simple bis-quaternary ammonium salt favored the formation of spiral morphology in Beta zeolite spheres (S-Beta). The crystallization of zeolite in the presence of BMMPAOH is a two-stage process, where nanoparticles agglomerate into spheres in the early stages followed by the emergence of S-Beta crystals with spiral morphology. The synthesized Pt-S-Beta catalysts show higher catalytic activity in VOC abatement compared with other Pt-Beta samples.

Rational design on hierarchically porous Cu/ZSM-5 zeolite for promoting low-temperature NH3-SCR performance based on protectively alkali-etching strategy

In this study, organic structure-directing agents (OSDA) of TPA+ are used as templating agents to synthesize zeolites with different topologies, and are also served as protectants for alkali treatment to inhibit the severe silicon extraction from zeolite skeleton. The TPA+ cations could be bound to the negatively charged Al sites in the framework, so we prepared a series of Cu-based ZSM-5 catalysts with mesoporous/microporous composite structure by using TPA+ cation as a protectant for alkali treatment and evaluated the pore structures by using the hierarchy factor (HF). The results show the hierarchically mesoporous/microporous structure of the alkali-etched catalyst with the maximal HF (0.115) effectively improved the low-temperature activity on selective catalytic reduction (SCR) of NO by ammonia, achieving a complete NOx to N2 conversion at 225–400 ºC. It was shown that the TPA+ cation played an important role on protecting the crystallinity, long-range order, and pore connectivity of zeolites, and substantially widened the mesopore volume with the loss of a small portion of micropores. In addition, the alkali treatment strategy involving protectant was systematically studied, including the corresponding effect on the pore structure, redox ability, acidity, and copper ion dispersion. The N2 adsorption-desorption experiments revealed that the maximization of the HF can be obtained by adjusting the ratio of protectant to etchant (TPA+/OH-). Among them, the Cu/ZSM-5 catalyst associated with the ratio of TPA+/OH-=0.4 and the best activity has a maximum surface area of 515 m2/g, which enhances the mesopore volume (0.325 cm3/g) by approximately three times simultaneously with only a 16.9% loss of the micropore volume (0.154 cm3/g). The hierarchically porous zeolites not only enhance the diffusion of mobile hydrated copper ions and ammonia-solvated copper complexes, but also facilitate the dispersion of copper ions. This work provides a feasible strategy to rapidly enhance the low-temperature deNOx efficiency on the zeolite catalysts.

One-Pot Synthesis of CHA/ERI-Type Zeolite Intergrowth from a Single Multiselective Organic Structure-Directing Agent.

We report the one-pot synthesis of a chabazite (CHA)/erionite (ERI)-type zeolite intergrowth structure characterized by adjustable extents of intergrowth enrichment and Si/Al molar ratios. This method utilizes readily synthesizable 6-azaspiro[5.6]dodecan-6-ium as the exclusive organic structure-directing agent (OSDA) within a potassium-dominant environment. High-throughput simulations were used to accurately determine the templating energy and molecular shape, facilitating the selection of an optimally biselective OSDA from among thousands of prospective candidates. The coexistence of the crystal phases, forming a distinct structure comprising disk-like CHA regions bridged by ERI-rich pillars, was corroborated via rigorous powder X-ray diffraction and integrated differential-phase contrast scanning transmission electron microscopy (iDPC S/TEM) analyses. iDPC S/TEM imaging further revealed the presence of single offretite layers dispersed within the ERI phase. The ratio of crystal phases between CHA and ERI in this type of intergrowth could be varied systematically by changing both the OSDA/Si and K/Si ratios. Two intergrown zeolite samples with different Si/Al molar ratios were tested for the selective catalytic reduction (SCR) of NOx with NH3, showing competitive catalytic performance and hydrothermal stability compared to that of the industry-standard commercial NH3-SCR catalyst, Cu-SSZ-13, prevalent in automotive applications. Collectively, this work underscores the potential of our approach for the synthesis and optimization of adjustable intergrown zeolite structures, offering competitive alternatives for key industrial processes.

Green and stable synthesis of MCM-56 zeolite by using choline chloride as template

It is desirable to develop eco-friendly structure-directing agents as an alternative to the highly toxic templates during the synthesis process of many zeolites. In this work, a MWW type zeolite (denoted as CC-MCM-56) was stably synthesized with ITQ-1 seed assistance by using a green and nuisance-less choline chloride (CC) as organic structure-directing agents instead of the conventional toxic (HMI) template. The CC-MCM-56 zeolite exhibited similar physicochemical properties to the conventional MCM-56 (HMI), including acid properties, high specific surface area, especially high external specific surface area and mesoporous volume, which performed an excellent result in the liquid-phase alkylation reaction of benzene with long-chain olefins. Our findings reveal that the synergistic effect of ITQ-1 seed and CC molecule enable a longer and more stable crystallization period during the synthesis process of MCM-56 zeolite. The SAXS result coupled with other techniques indicate that ITQ-1 seed is firstly dissolved to form more structural units which could interact with CC molecules, making more CC molecules attached to the surface half cup of the MWW lamellae and promoting the stable synthesis of CC-MCM-56 zeolite with thinner-layer structure. Finally, the synthesized CC-MCM-56 zeolite showed high reactivity and the conversion of 1-hexane and 1-dodecene achieved 90.5% and 90.6% respectively.

MOR nanosheets with tunable c-axis thickness and their catalytic performance in DME carbonylation

Controlling the anisotropic growth of crystals to achieve desired morphology is a challenge in zeolite crystallization, which often requires the use of bulky organic surfactants or additional crystal growth modifiers. Here, MOR crystals with tunable c-axis thickness (12-MR channel direction) were readily synthesized with commercial tetraethylammonium hydroxide (TEAOH) as an organic-structure directing agent (OSDA). It is revealed that the relative growth rates of different crystal faces of MOR zeolite can be tuned by the crystallization temperature and TEAOH amount in the developed gel system. Higher temperature and lower TEAOH concentration can effectively retard the growth of MOR crystals along the c-axis, yielding MOR nanosheets with short c-axis thickness (ca. 40 nm). Investigation on the crystallization process demonstrates that the crystal growth follows a particle attachment mechanism. Theoretical calculation was further employed to elucidate the c-axis thickness variation of MOR zeolite under different conditions. A kilogram-scale synthesis of MOR nanosheets has been realized with a 10 L autoclave, achieving high solid yield of 92 %. The MOR nanosheets, offering short 12-MR diffusion path, exhibit substantially enhanced catalytic activity and stability in the dimethyl ether (DME) carbonylation to methyl acetate reaction.

Enhancing nucleation to synthesize Al-rich CHA zeolites under ultra-low OSDA contents: Application for heavy-duty NH3-SCR of NOx

The crystallization of Al-rich CHA zeolites with an ultra-low organic structure directing agent (OSDA) content and without using pre-crystallized FAU zeolites as Si and/or Al sources has been accomplished by controlling the nucleation step. Indeed, the use of “gel-type” seeds containing discrete nanosized CHA nanoparticles (i.e. below 100 nm) combined with the understanding of the seed-assisted crystallization mechanism has allowed the efficient preparation of Al-rich CHA materials using ultra-low methyltriethylammonium (MTEA) contents (MTEA/Si molar ratios ∼ 0.03–0.04), making this synthetic route very cost-competitive compared to recent synthesis improvements for CHA using substoichiometric contents of expensive OSDAs (i.e. N,N,N-trimethyladamantammonium, TMAda). The optimized Al-rich CHA materials, after incorporating the catalytically active copper cations, perform as ideal catalysts for the selective catalytic reduction (SCR) of NOx with ammonia relevant for heavy-duty applications.

ZeoSyn: A Comprehensive Zeolite Synthesis Dataset Enabling Machine-Learning Rationalization of Hydrothermal Parameters.

Zeolites, nanoporous aluminosilicates with well-defined porous structures, are versatile materials with applications in catalysis, gas separation, and ion exchange. Hydrothermal synthesis is widely used for zeolite production, offering control over composition, crystallinity, and pore size. However, the intricate interplay of synthesis parameters necessitates a comprehensive understanding of synthesis-structure relationships to optimize the synthesis process. Hitherto, public zeolite synthesis databases only contain a subset of parameters and are small in scale, comprising up to a few thousand synthesis routes. We present ZeoSyn, a dataset of 23,961 zeolite hydrothermal synthesis routes, encompassing 233 zeolite topologies and 921 organic structure-directing agents (OSDAs). Each synthesis route comprises comprehensive synthesis parameters: 1) gel composition, 2) reaction conditions, 3) OSDAs, and 4) zeolite products. Using ZeoSyn, we develop a machine learning classifier to predict the resultant zeolite given a synthesis route with >70% accuracy. We employ SHapley Additive exPlanations (SHAP) to uncover key synthesis parameters for >200 zeolite frameworks. We introduce an aggregation approach to extend SHAP to all building units. We demonstrate applications of this approach to phase-selective and intergrowth synthesis. This comprehensive analysis illuminates the synthesis parameters pivotal in driving zeolite crystallization, offering the potential to guide the synthesis of desired zeolites. The dataset is available at https://github.com/eltonpan/zeosyn_dataset.