Synthesis Of High-silica Zeolites Research Articles

Modulation of Al Distribution in High-Silica ZSM-5 Zeolites for Enhancing Catalytic Performance.

The spatial distribution of framework Al (AlF) has been one of the important factors that affect the catalytic properties of zeolites in diverse chemical reactions; however, the synthesis of high-silica zeolites with special AlF distribution remains a challenge. In this study, we successfully synthesized high-silica ZSM-5 zeolites with a unique AlF distribution by employing pentaerythritol (PET) as an additive in the presence of a few tetrapropylammonium hydroxide (TPAOH). The results demonstrated that the introduction of PET led to a higher proportion of Al atoms located at the sinusoidal and/or straight channels. It was observed that the addition of PET prevented the interaction between TPA+ and tetrahedral [AlO4]- during the crystallization process, resulting in enhanced availability of TPA species in the form of ion-paired TPA+. This effect leads to AlF atoms dominantly distributed away from the intersection and located in narrow channels, where acidic sites more effectively inhibit hydrogen transfer and coke formation. In the reaction of dimethyl ether (DME) to olefins, the catalyst with a unique Al distribution exhibited a significant prolonged catalytic lifetime, surpassing traditional TPA-ZSM-5 by more than 2-fold and maintaining DME conversion above 90% for a maximum of 148 h. The results of multiple pulse experiments also showed that these PET-assisted ZSM-5 zeolites significantly enhanced the selectivity of propene and butene. This approach provides an effective strategy to regulate AlF distribution in high-silica ZSM-5 catalysts with the assistance of neutral alcohol. It holds great potential for application in the synthesis of other high-silica zeolites, thereby enriching the diversity of zeolite catalysis.

Investigation of the Properties of Mo/ZSM-5 Catalysts Based on Zeolites with Microporous and Micro–Mesoporous Structures

The dehydroaromatization of methane (MDA) is of great interest as a promising process for processing natural and associated petroleum gases, the main component of which is methane. The rapid loss of catalyst activity because of coke formation hinders the introduction of the DHA methane process into the industry. Therefore, the aim of this research was to find ways to improve Mo/ZSM-5 catalysts for MDA. The paper presents the results of the synthesis of high-silica zeolites of the ZSM-5 type with microporous and micro–mesoporous structures, the preparation of Mo/ZSM-5 catalysts based on them, and the study of the physicochemical and catalytic properties of the obtained samples during the non-oxidative conversion of methane into aromatic hydrocarbons. Zeolite catalysts were investigated using IR spectroscopy, X-ray diffraction, TPD-NH3, SEM, HR-TEM, and N2 adsorption. It was found that the addition of carbon black in the stage of the synthesis of zeolite type ZSM-5 did not lead to structural changes, and the obtained samples had a crystallinity degree equal to 100%. The creation of the micro–mesoporous structure in Mo/ZSM-5 catalysts led to an increase in their activity and stability in the process of methane dehydroaromatization. The highest conversion of methane was observed on a 4.0%Mo/ZSM-5 catalyst prepared based on zeolite synthesized using 1.0% carbon black and was 13.0% after 20 min of reaction, while the benzene yield reached 7.0%. It was shown using HR-TEM that a more uniform distribution of the active metal component was observed in a zeolite catalyst with a micro–mesoporous structure than in a microporous zeolite.

A Review on the Effects of Organic Structure-Directing Agents on the Hydrothermal Synthesis and Physicochemical Properties of Zeolites

The study on the synthesis of zeolites, including both the development of novel techniques of synthesis and the discovery of new zeolitic frameworks, has a background of several decades. In this context, the application of organic structure-directing agents (SDAs) is one of the key factors having an important role in the formation of porous zeolitic networks as well as the crystallization process of zeolites. There are various elements that are needed to be explored for elucidating the effects of organic SDAs on the final physicochemical properties of zeolites. Although SDAs were firstly used as pore generators in the synthesis of high-silica zeolites, further studies proved their multiple roles during the synthesis of zeolites, such as their influences on the crystallization evolution of zeolite, the size of the crystal and the chemical composition, which is beyond their porogen properties. The aim of this mini review is to present and briefly summarize these features as well as the advances in the synthesis of new SDAs during the last decades.

Realizing Fast Synthesis of High-Silica Zeolite Y with Remarkable Catalytic Performance.

High-silica zeolite Y (FAU) plays a vital role in (petro)chemical industries. However, the slow nucleation and growth kinetics of the high-silica FAU framework limit its direct synthesis and the improvement of framework SiO2 /Al2 O3 ratio (SAR). Here, a facile strategy is developed to realize the fast crystallization of high-silica zeolite Y, which involves the combination of high crystallization temperature, ultra-stable Y (USY) seeds and efficient organic-structure directing agent (OSDA). The synthesis can be finished in 5-16 h at 160 °C and with tunable SAR up to 18.2, and the key factors affecting crystallization kinetics and phase purity are elucidated. Moreover, the crystallization process was monitored to reveal the fast crystal growth mechanism. The high-silica products possess high (hydro)thermal stability and abundant strong acid sites, which endow them excellent catalytic cracking performance, obviously superior to commercial USY.

High-silica zeolite Y: seed-assisted synthesis, characterization and catalytic properties

Seed-assisted synthesis of high-silica zeolite Y with good (hydro)thermal stability, abundant strong acid sites and excellent catalytic cracking performance.

A facile organic-free synthesis of high silica zeolite Y with small crystal in the presence of Co2+

The direct synthesis of zeolite Y with high SiO 2 /Al 2 O 3 (SAR > 6.0) and small crystal size is a great challenge. The conventional production of high silica Y usually involves organic template into the synthesis system, which takes high cost. Here, zeolite Y with SAR of 6.15 and small crystal size via introducing Co 2+ in a facile one-step hydrothermal method without organic template was achieved. The SAR increased from 5.2 to 6.15 compared to normal NaY, and the crystal size decreased to 50%. The introduction of Co 2+ in synthesis system affects the formation of 5-coordination intermediate ( M-O -Si )* around Co 2+ during crystallization process which is the critical factor to nucleation. The stability of ( Co–O –Si )* is weaker than that of ( Al–O –Si )*, which leads to the increase of SAR around Co 2+ . The microcrystals of Co(OH) 2 provide adhesive surface for the amorphous material and promote nucleation. The zeolite Y shows good performance in the cracking of 1,3,5-triisopropylbenzene because of larger surface area and more active sites being exposed. This is a new strategy for industrial synthesis of high SiO 2 /Al 2 O 3 and small crystal size zeolite Y. • Zeolite Y with SiO 2 /Al 2 O 3 of 6.15 and small crystal size was achieved by introducing Co 2+ in a facile hydrothermal method. • The nucleation free energy was reduced by the microcrystalline Co(OH) 2 species via adhering to the amorphous materials. • The ΔG of (Co-O-Si)* is higher than that of (Al-O-Si)*, Si species combine with (Al-O-Si)* to increase the SiO 2 /Al 2 O 3 .

Progress in Direct Synthesis of High Silica Zeolite Y

Progress in Direct Synthesis of High Silica Zeolite Y

Alcohol-assisted synthesis of high-silica zeolites in the absence of organic structure-directing agents

In this work, we show for the first time that high-silica zeolites (MFI, TON, MTT, and *MRE) could be synthesized from a combined strategy of both zeolite seeding and alcohol filling in the absence of organic structure-directing agents (OSDAs). High-silica ZSM-5 zeolites with Si/Al ratios ranging from 38 to 240 (TF-Al-ZSM-5) could be synthesized via this route. The key to the success of this technique was the employment of an aluminosilicate precursor with a fully 4-coordinated aluminum species as the initial source, wherein the rearrangement and condensation of the silicate species, rather than the aluminate species, occurred during zeolite crystallization. In addition, heteroatoms, such as Fe and B, could be incorporated into the zeolite frameworks. Catalytic tests for the methanol-to-propylene (MTP) reaction exhibited good catalytic performance for TF-Al-ZSM-5, which was comparable to that of the aluminosilicate ZSM-5 zeolite synthesized with OSDAs. Hence, this method offers viable opportunities for the industrial production and catalytic application of high-silica zeolites in the future.

Synthesis of novel aluminosilicate YNU-5 and enhancement of the framework thermal stability by post-synthesis treatment

Abstract An unconventional hydrothermal synthesis of high-silica zeolite starting from a highly concentrated silicate solution was examined. As a result, the novel zeolite YNU-5 (YFI), which contains a 12−12−8-ring pore system and isolated 8-ring straight channels, was synthesized using dimethyldipropylammonium, one of the simplest tetraalkylammonium compounds, as the organic structure-directing agent. Although several competing phases such as MCM-68 (MSE), ZSM-5 (MFI) and beta (*BEA) were observed during investigation, YNU-5 (YFI) was obtained as a pure phase within a narrow synthesis window. The dealumination behavior by post-synthesis treatment using nitric acid was investigated and the Si/Al molar ratios of YNU-5 were controlled in the range from 9 to >200 by tuning acid concentration and/or temperature. Thermal stability of the framework was enhanced under severer acid-treatment conditions, and the stabilized material exhibited better catalytic performance.

Synthesis of high silica zeolites using a combination of pyrrolidine and tetramethylammonium as template

High silica ZSM-39 (MTN structure), ZSM-35 (FER structure) and ZSM-5(MFI structure) zeolites were successfully synthesized using pyrrolidine and tetramethylammonium as structure directing agents (SDAs), in absence of alkaline cation and fluoride medium. The effect of the relative amount of tetramethylammonium and pyrrolidine, and Si/Al molar ratio on the crystalline phases was investigated. All structures could be synthesized using pyrrolidine as solely SDA. On the other hand, when a mixed template system being used, the crystallization was accelerated by a factor of 2 times, TMA+ would then play a generally beneficial but structurally non-specific role in the crystallization spicily for FER and MFI zeolites. The obtained products were characterized by XRD, 13C solid-state CP MAS NMR, TGA and SEM techniques. The XRD patterns confirmed the formation of pure zeolites with high crystallinity.13C CP MAS NMR spectroscopy confirmed the incorporation of pyrrolidine and tetramethylammonium in the structure of Al-ZSM-5 and Al-ZSM-35 zeolites. These two kinds of SDAs played a cooperative role in the crystallization of these zeolites. The role of pyrrolidine was to provide the initial nucleation and tetramethylammonium to provide both space-filling and basicity capacities.

Eco-friendly synthesis of high silica zeolite Y with choline as green and innocent structure-directing agent

Zeolite synthesis in contemporary chemical industries is predominantly conducted using organic structure-directing agents (OSDAs), which are chronically hazardous to humans and the environment. It is a growing trend to develop an eco-friendly and nuisanceless OSDA for zeolite synthesis. Herein, choline is employed as a non-toxic and green OSDA to synthesize high silica Y zeolite with SiO2/Al2O3 ratios of 6.5–6.8. The prepared Y zeolite samples exhibited outstanding (hydro)thermal stability at ultrahigh temperature owing to the higher SiO2/Al2O3 ratio. The XRF, SEM, 29Si-NMR and 13Na+ results suggested that choline plays a structure-directing role in the synthesis of Y zeolite, while the feed molar fraction of Na+ is a crucial determinant for the framework SiO2/Al2O3 ratio and the crystal morphology.

Accelerated Construction of High-Silica RHO and CHA Zeolites via Interzeolite Transformation and Their NH3–SCR Performances after Copper Exchange

Rapid and selective synthesis of high-silica zeolites, including topologies of RHO and CHA, has been realized by interzeolite transformation process. The XRD patterns and morphology images collecte...

Recent trends in the synthesis of high-silica zeolites

ABSTRACTNovel zeolites continue to be synthesized using organic structure directing agents in combination with unconventional choices of inorganic conditions. The latter include synthesis in concentrated fluoride media, synthesis in concentrated hydroxide media, synthesis in the presence of germanium, and synthesis with high concentrations of boron. Post-synthetic treatments of germanosilicates with the ADOR method have resulted in fully intact zeolites that cannot be obtained through direct synthesis. Unusual compositions of known frameworks have been obtained through topotactic condensation of layered precursors. Coupled with the appropriate choice of inorganic conditions, the use of new SDA molecules has also produced materials with known frameworks but with amazingly small crystal dimensions that are less than 10 nm. The preparations of once “exotic” materials are now being performed with commercially available or easily prepared organic structure directing agents. In some instances methods have been found to prepare high-silica zeolites from seeded preparations in the absence of an organocation. Methods to target zeolite catalysts for a particular reaction have been realized by identifying structure directing agent molecules that are a mimic of the transition state of the reaction.

Positron annihilation lifetime spectroscopy (PALS) study of the as prepared and calcined MFI zeolites

The synthesis of high silica zeolites in many cases implies the usage of organic structural direction agents (SDA). However, to manifest their functionalities, the SDA occluded inside the channels of the as-synthesized structure should be removed, usually by a high temperature treatment (calcination). In this paper, the positron annihilation lifetime spectroscopy (PALS) was used to monitor the development of accessible spaces, their sizes and distributions in MFI zeolites, ZSM-5 and silicalite-1 in order to give an additional insight in the process of the SDA removal. For that purpose, a conventional PALS setup with 22Na positron source was applied. It was established that there is a pronounced difference between positron annihilation data for these two zeolites of the same structural type. The samples were additionally analysed by X-ray powder diffraction at room temperature with a crystal structure refinement and thermogravimetry.

Hydrothermal synthesis of high silica zeolite Y using tetraethylammonium hydroxide as a structure-directing agent.

High silica zeolite Y with a SiO2/Al2O3 ratio of 7.76 is successfully synthesized with tetraethylammonium hydroxide (TEAOH) as a structure-directing agent. The high silica zeolite Y shows outstanding high temperature thermal stability and hydrothermal stability.

Synthesis of zeolite Y from diatomite as silica source

Bolivian diatomite was successfully used as a silica source for the synthesis of zeolite Y. Prior to synthesis, the diatomite was leached with sulfuric acid to remove impurities and aluminum sulfate was used as an aluminum source. The raw materials were reacted hydrothermally at 100 °C in water with sodium hydroxide and different Na2O/SiO2 ratios were investigated. The final products were characterized by scanning electron microscopy, X-ray diffraction, gas adsorption and inductively coupled plasma-atomic emission spectroscopy. Diatomites originating from different locations and therefore containing different types and amounts of minerals and clays as impurities were investigated. After optimization of synthesis time, zeolite Y with low SiO2/Al2O3 ratio (3.0–3.9) was obtained at a high yield for high alkalinity conditions (Na2O/SiO2 = 0.85–2.0). Lower Na2O/SiO2 ratios resulted in incomplete dissolution of diatomite and lower yield. Nevertheless, decreasing alkalinity resulted in a steady increase of the SiO2/Al2O3 ratio in zeolite Y. Consequently, it was possible to synthesize almost pure zeolite Y with a SiO2/Al2O3 ratio of 5.3 for a Na2O/SiO2 ratio of 0.6, albeit at a low yield. In this respect, diatomite enables the synthesis of high silica zeolite Y and behaves similarly to colloidal silica in traditional syntheses, with both sources of silica having in common a high degree of polymerization. Interestingly, the presence of minerals and clays in the starting diatomite had marginal effects on the outcome of the synthesis. However, their dissolution resulted in presence of calcium and magnesium in the zeolite Y crystals. Finally, overrun of all investigated compositions resulted in the formation of zeolite P nucleating and growing onto dissolving zeolite Y crystals, which was shown to be triggered when aluminum was completely depleted at high alkalinity.

High temperature synthesis of high silica zeolite Y with good crystallinity in the presence of N-methylpyridinium iodide

A novel synthetic route is designed, employing both high temperature and a nontoxic organic structure-directing agent (SDA), for the synthesis of high silica zeolite Y. The N-methylpyridinium used as an organic SDA is stable during the synthesis, and the high silica zeolite Y shows high hydrothermal stability and good catalytic performance, as well as excellent adsorptive properties.

Diquaternary Ammonium Compounds in Zeolite Synthesis: Cyclic and Polycyclic N-Heterocycles Connected by Methylene Chains

An additional dimension has been added to our long-standing studies in high silica zeolite synthesis via a guest/host synergism. We have created and studied the impact of making symmetric diquaternary ammonium compounds, by varying the chain length between nitrogen charge centers, and the heterocycle size and geometry containing the nitrogen. This allows the introduction of a second spatial parameter in the use of the charged organo-cation guest in the zeolite synthesis. The series of 15 diquaternary ammonium compounds (5 heterocycles synthesized onto chain lengths of C4-C6) were tested in a total of 135 zeolite syntheses reactions. Nine screening reactions were employed for each guest molecule, and the conditions built upon past successes in finding novel high silica zeolites via introduction of boron, aluminum, or germanium as substituting tetrahedral framework atoms for silicon. Eighteen different zeolite structures emerged from the studies. The use of specific chain lengths for derivatives of the pyrrolidine ring system produced novel zeolite materials SSZ-74 and 75.

High temperature synthesis of silicalite-1 in cationic microemulsions

The current investigation reports on the synthesis and characterization of silicalite-1 made in cationic microemulsions using conventional synthesis conditions (polymeric silica sources, alkaline media, high temperatures (433 K)). Silicalite-1 materials formed in microemulsion-mediated syntheses possess different morphological properties as compared to samples grown using the same synthesis mixture in the absence of the microemulsion. Syntheses are also performed to ascertain how the microemulsion composition, silica precursor, alkali content, presence of salt, and the surfactant identity influence material properties, most notably crystal morphology. The morphology of the crystals obtained is fairly insensitive to surfactant structure and salt, but highly sensitive to the silica source. Polymeric silica sources lead to silicalite-1, whereas monomeric silica sources (e.g. TEOS) lead to amorphous solids. The results also indicate that a key parameter is the ratio of surfactant to structure-directing agent, suggesting that electrostatic forces are important in determining the synthesis outcome. Control experiments show all components of the microemulsion are necessary to obtain the observed crystal morphology. Based on the results presented it is concluded that the surfactant-silicate electrostatic forces are primarily responsible for the modulation of crystal morphology observed. The results indicate that surfactant adsorption on the growing crystal surface, not the confined space afforded by the microemulsion, is essential. The results suggest that this may be a useful approach to controlling zeolite crystal morphology and growth of crystals obtained from conventional high-silica zeolite synthesis procedures.

Two new silicate hydrates (C 20H 30N 2) 8 · [Si 8O 20] 2 · 110H 2O and (C 20H 30N 2) 4 · [Si 8O 20] · 42H 2O, and their implications for the role of non-covalent interactions in high-silica zeolite synthesis

Two new silicate hydrates, (C 20H 30N 2) 8 · [Si 8O 20] 2 · 110H 2O ( 1) and (C 20H 30N 2) 4 · [Si 8O 20] · 42H 2O ( 2) have been prepared and their structures determined using single-crystal X-ray diffraction. The structures of 1 and 2 are formed from silicate octameric cubes bridged by hydrogen-bonded water molecules. Crystal data for 1: P-1, Z=1, a=18.508(1) Å, b=20.393(1) Å, c=20.784(1) Å, α=97.747(4)°, β=92.249(5)°, γ=116.396(3)°; 2: P-1, Z=1, a=14.345(1) Å, b=14.855(1) Å, c=16.162(1) Å, α=80.339(5)°, β=84.916(5)°, γ=67.805(5)°. Both 1 and 2 have apertures formed by hydrogen-bonded water molecules which are approximately 9.5×5.5 and 12.0×9.0 Å in 1 and 2, respectively. The structures of both hydrates have numerous weak C–H ⋯ O hydrogen bonds between the organic cations and water molecules. The importance of these and organic–inorganic electrostatic forces are discussed in the context of zeolite synthesis.