Synthesis Gel Research Articles

Insight into the effect of F− on acidity of H-SAPO-18 and its catalytic performance for conversion of methanol to olefins

In this study, F− was added to the synthesis gel to regulate the acid property of SAPO-18 molecular sieve and enhance its catalytic performance for the methanol-to-olefin (MTO) reactions. The results indicate that adding an appropriate amount of F− during synthesis can increase the crystallinity of SAPO-18 molecular sieve and reduce the overall strong acid density and external surface acid content. The H-SAPO-18-I-20-0.0025 molecular sieve exhibits a much higher catalytic lifetime (1000 min) and light olefins selectivity (45.2% to propene and 96.1% to C2-5=) in MTO at 400 °C than those of the traditional H-SAPO-18-1-0. The results of the isotope labeling experiments, GC-MS, and 13C MAS NMR results provided direct evidence that the aromatic-based cycle contributes more to the conversion of methanol into light olefins, and hexamethylbenzene serves as the most abundant hydrocarbon species that produces propene via the paring route.

Interzeolite conversion synthesized [Fe, Al]-FER zeolites with enhanced hydrothermal stability for NH3-SCR reaction

The [Fe, Al]-FER zeolites, which are prepared from the interzeolite conversion method (from MWW to FER topology) with high Al and Fe content (Si/Al of 8–18 and Si/Fe of 11–13), exhibit excellent catalytic performances and hydrothermal tolerance for NH3-SCR reaction. The interzeolite conversion process can be tuned by optimizing the zeolite synthesis conditions such as synthesis gel composition, crystallization time, crystallization temperature, and adding MWW seeds. The work can provide a new strategy to synthesize high Al and Fe content zeolite with high hydrothermal stability for NOx removal.

Effects of Aluminosilicate Gel Treatment and TiO2 Loading on Photocatalytic Properties of Au–TiO2/Zeolite Y

The present work reports the synthesis of efficient Ti–Au/zeolite Y photocatalysts by different processing of aluminosilicate gel and studies the effect of titania content on the structural, morphological, textural, and optical properties of the materials. The best characteristics of zeolite Y were obtained by aging the synthesis gel in static conditions and mixing the precursors under magnetic stirring. Titania (5, 10, 20%) and gold (1%) species were incorporated in zeolite Y support by the post-synthesis method. The samples were characterized by X-ray diffraction, N2-physisorption, SEM, Raman, UV–Vis and photoluminescence spectroscopy, XPS, H2-TPR, and CO2-TPD. The photocatalyst with the lowest TiO2 loading shows only metallic Au on the outermost surface layer, while a higher content favors the formation of additional species such as: cluster type Au, Au1+, and Au3+. A high TiO2 content contributes to increasing the lifetime of photogenerated charge careers, and the adsorption capacity of the pollutant. Therefore, an increase in the photocatalytic performances (evaluated in degradation of amoxicillin in water under UV and visible light) was evidenced with the titania content. The effect is more significant in visible light due to the surface plasmon resonance (SPR) effect of gold interacting with the supported titania.

Ultrafast Synthesis and Regulating Al Status of Mesoporous ZSM-48 Zeolite via a Pretreated-Seed-Solution-Assisted Strategy

Ultrafast synthesis of nanocrystal assembled mesoporous ZSM-48 zeolite spheres of highly crystalline was achieved within 1.5 h by using pretreated-seed-solution-assisted strategy. Interestingly, by altering the introducing sequence of aluminum source (in the pretreated seed solution or in the synthesis gel), the obtained ZSM-48 with similar particle size and total acid sites exhibited quite different crystallization behavior and acid property in terms of Al distribution and acid strength, because of the distinct local microenvironment (Al-rich or Al-free) in the pretreated seed solution. The HZ48-Al-pre (introduce an Al source to the pretreated seed solution) had more framework Al(IV) content and stronger acid strength than that of the HZ48-Al-post (synthesized by the Al-free pretreated seed solution). The corresponding Pt supported catalysts showed a great difference in the n-C12 hydroisomerization reation. The Pt/HZ48-Al-pre catalyst had a very low selectivity (41.1%), and the maximum iso-yield was only 35.5%. However, the ultrafast synthesizedPt/HZ48-Al-post-1.5h catalyst exhibited a superior selectivity (90.1%) with a total i-C12 yield of 82.3% at 330 °C, which was a high level in the exist literature reports.

Finding the Compromise between Brønsted Acidity and Mesoporosity in Hierarchical ZSM‐5 Zeolites

AbstractMesoporous ZSM‐5 catalysts were prepared from a commercial ZSM‐5 (Si/Al=15), using a 2‐step micelle‐templating procedure. By varying the NaOH/Si synthesis ratio in the range of 0.3‐0.7, ZSM‐5 catalysts with different textural and acidic properties were obtained. Increasing the NaOH/Si ratio in the synthesis gel leads to zeolites that feature reduced number of Brønsted acid sites, but higher mesopore volume, which allows for reducing the diffusion path length. In order to assess the best compromise between Brønsted acidity and mesopore volume two test reactions were run. The alkylation of phenol with tert‐butyl alcohol allowed to evidence the accessibility of bulky molecules to active sites. The esterification of benzyl alcohol with hexanoic acid allowed to probe the specificity of Brønsted acid sites in ZSM‐5. The best compromise was found for micro‐/mesoporous ZSM‐5 catalysts prepared with the ratio 0.38<NaOH/Si<0.5.

Predicting the catalytic performance of Nb-doped nickel oxide catalysts for the oxidative dehydrogenation of ethane by knowing their electrochemical properties

In the present article, a relationship between the catalytic performance in the oxidative dehydrogenation of ethane (ODHE) for Nb-doped NiO catalysts and their electrochemical properties has been proposed. To do so, highly stable and selective Nb-doped NiO catalysts for the ODHE to ethylene have been synthesized by optimizing synthesis parameters such as the amount of oxalic acid in the synthesis gel and the calcination temperature. These catalysts have been characterized by means of XRD, HRTEM, Raman and UV–vis diffuse reflectance spectroscopies, TPR and XPS. Moreover, Electrochemical Impedance Spectroscopy (EIS), capacitance measurements (Mott-Schottky analysis) and cyclic voltammetries studies were also carried out. Electrochemical characterization indicates changes in the type of semiconductivity: p-type for samples calcined at 350 °C and the sample prepared in the absence of oxalic acid and calcined at 500 °C, to n-type for samples prepared in the presence of oxalic acid in the synthesis gel and calcined at 500 °C. According to the obtained results, the most selective catalysts present a low Nb-incorporation on the NiO lattice with a large amount of tiny Nb2O5 nanoparticles covering NiO crystallites. This work presents, for the first time, a complete electrochemical characterization of Nb-doped NiO catalysts showing a correlation between electrochemical properties and catalytic performance.

Tuning the mesoscopically structured ZSM‐5 nanosheets for the alkylation between toluene and methanol

AbstractMesoscopically structured ZSM‐5 nanosheets were prepared as highly active catalysts for the alkylation between toluene with methanol. The results showed that the Brønsted acid sites of ZSM‐5 nanosheets were mainly distributed on the external surface in aluminum‐rich zeolites owing to the charge‐balance between positive quaternary ammonium cation (N+) in Cph‐10‐6‐6, Na+ ions and negative aluminosilicate species. In addition, it revealed that high alkalinity was easier to depolymerize the SiOSi species in Aluminum‐rich synthesis gel, and the high alkalinity was required for silica‐rich systems to dissolve silica species in order to finish the self‐assemble process, leading to ZSM‐5 nanosheets with high alkalinity presenting enhanced catalytic performance. Moreover, the optimized ZSM‐5‐50‐2.5 was effective for the alkylation between toluene and methanol with toluene conversion of up to 50%, and it was stably operated for 48 h under the harsh conditions of WHSV = 32.5 h−1 with a desired stability.

Zeolites: A Theoretical and Practical Approach with Uses in (Bio)Chemical Processes

This review provides a state-of-the-art summary of distributed zeolite technology, as well as identifying strategies to further promote the absorption of these materials in various areas of study. Zeolites are materials that can be synthesized or found in natural rock deposits a with a basic composition consisting in Al, Si, and O. Zeolite’s consideration as a future material is due to many facile synthesis methods to obtain different structures with variations in pore size, surface area, pore volume and physical properties. These methods are developed using the control of relevant synthesis parameters that influences structure formation, such as crystallization temperature, time of aging and/or crystallization, stoichiometric relationships between components of synthesis gel, pH of the medium, and in some cases the type of structure-directing agent. Each method will lead to geometric changes in the framework formation, making possible the formation of typical chemical bonds that are the fingerprint of any zeolitic structure (O-Si-O and Al-O-Si), forming typical acid sites that give specificity in zeolite and allows it to act as a nanoreactor. The specificity is a characteristic that in some cases depends on selectivity, a fundamental property derived of the porosity, mostly in processes that occur inside the zeolite. In processes outside the structure, the surface area is the main factor influencing this property. Moreover, there are many natural sources with adequate chemical composition to be used as precursors. Some of these sources are waste, minimizing the deposition of potential hazardous materials that can be recalcitrant pollutants depending on the environment. Besides its uses as a catalyst, zeolite serves as a support for many bioprocesses; therefore, this review aims to explain relevant aspects in chemical nature, physical properties, main methods of synthesis, main precursors used for synthesis, and relevant applications of zeolites in chemical catalysis and biological processes.

Alcohol‐Assisted Synthesis of Sheet‐Like ZSM‐5 Zeolites with Controllable Aspect Ratios

AbstractSheet‐like ZSM‐5 has been regarded as a promising material for catalytic applications due to its diffusion superiority. However, it still remains a challenge to obtain a desirable sheet‐like morphology because of the complex synthesis process of zeolites. Here, a facile strategy for synthesizing sheet‐like ZSM‐5 is developed by only adding ethanol as zeolite growth modifier in the synthesis gel. It is thought that ethanol might be preferentially absorbed on the {010} surface of zeolite crystals, interact with the exposed silicon hydroxyl groups on the crystal {010} facet, and suppress the growth of b axis, resulting in the sheet‐like shape. Through finely tunning synthesis parameters, sheet‐like ZSM‐5 crystals with thin b‐axis thickness of 90 nm and different aspect ratios could be obtained. Owing to its shorter diffusion path and optimized acidity, sheet‐like ZSM‐5 exhibits better catalytic performance than conventional ZSM‐5 in the alkylation of benzene with ethanol.

Effect of the preparation method on the catalytic properties of copper-containing zeolite Y applied for NH3-SCR-DeNOx

The effect of the preparation method on the catalytic properties of copper-containing zeolite Y samples applied for NH3-SCR-DeNOx was investigated.

Synthesis of hierarchical SAPO-11-based catalysts with Al-based metal-organic framework derivative as mesoporogen to improve n-decane branched isomerization

Hierarchical SAPO-11 molecular sieve (ACS-11) was successfully synthesized employing the Al2O3/carbon (Al2O3/C) composite obtained through the pyrolysis of Al-based metal-organic framework (Al-MOF-96) as mesoporogen. Unlike other carbon-based mesoporogens with strong hydrophobicity, the Al2O3/C interacts with phosphoric acid and generates the AlPO4/C structure, which promotes the Al2O3/C dispersion in the synthesis gel of SAPO-11 and avoids the phase separation between them. The Al2O3/C as mesoporogen decreases the crystallite size of SAPO-11 via preventing the aggregation of SAPO-11 crystals. Additionally, the addition of Al2O3/C improves the Si distribution in the ACS-11 framework. Consequently, ACS-11 has smaller crystallites, more mesopores, and a greater amount of medium Brønsted acid centers than the conventional microporous SAPO-11 and the SAPO-11 synthesized using activated carbon as mesoporogen. The corresponding Pt/ACS-11 catalyst exhibits the maximal selectivity to multi-branched C10 isomers (23.28%) and the minimal cracking selectivity (15.83%) in n-decane hydroisomerization among these catalysts. This research provides a new approach for preparing hierarchical silicoaluminophosphate molecular sieve-based catalysts to produce high-quality fuels.

Controllable synthesis of mesoporous KL zeolites via a simple one-step strategy

A simple strategy with additional KF in the synthesis gel was developed to fabricate mesoporous KL zeolites with tunable morphology, hierarchical structure and catalytic selectivity. By modulating the molar ratio of KF/Al2O3 from 0 to 10, the morphology of resultant KL zeolites changed from cylinder, to gyro shape, and to loosen stacking nanocrystal aggregates. The value of BET surface and pore volume for synthesized KL zeolites increased with increasing the KF, indicating the hierarchical structure of mesoporous KL zeolites can be systematically tailored by tuning the concentrations of KF. The effect of KF on the crystallization process of KL zeolites was also investigated, it revealed that the K+ played a crucial role to produce a large number of nucleation centers as the structure directing agent, which not only significantly shortened the crystallization time but also generated the defective growth units by influencing the electric balance between K+ with negative aluminosilicate species, leading to the formation of mesopores. Besides, the introduction of mesopores in KL zeolites can greatly improve the selectivity of bulk products in the alkylation between phenol with tert-butyl alcohol owing to the opened mesoporosity.

Dual‐templating surface gel into thin SSZ‐13 zeolite membrane for fast selective hydrogen separation

AbstractHighly permeable zeolite membranes are desirable for fast gas separation in the industry. Reducing the membrane's thickness is deemed to be an optimal solution for permeability improvement. Herein, we report the synthesis route of thin SSZ‐13 zeolite membranes via the conversion of template‐contained surface gels. The synthesis gel is fully crystallized into crack‐free SSZ‐13 membranes assisted with dual templates of N,N,N‐trimethyl‐1‐adamantammonium hydroxide (TMAdaOH) and tetraethylammonium hydroxide (TEAOH). The specific functions of TMAdaOH for structure directing and TEAOH for crystallization regulating are well discussed. Thin surface gel layer is impregnated onto porous alumina with subsequent crystallization into a 500 nm thick membrane. This submicron‐thick membrane exhibits high H2 permeance with 50 × 10−8 mol s−1 m−2 Pa−1 during hydrogen separation. Meanwhile, the separation factors are retained around 23.0 and 31.5 for H2/C2H6 and H2/C3H8, respectively. This approach offers a possibility for obtaining high‐quality zeolite membranes for efficient hydrogen separation.

Structural analysis of Cu/Zeolite with controlled Si/Al ratio and the resulting thermal stability

Cu-loaded zeolite (Cu/Zeolite or Cu/CHA) has been prepared from USY (SiO2/Al2O3 =30) through hydrothermal conversion using trimethyladamantylammonium ion (TMAda) as an organic structure directing agent. Simple control over the concentration of mineralizing agent in the hydrothermal synthesis from 0.2 to 0.6 with respect to SiO2, changed the Si/Al ratio in the resulting zeolite, CHA with a decrease of overall yield. The result of 29Si NMR and 27Na spectrum of the synthesis gel and mother liquor suggested the zeolite formation through hydrothermal conversion did not occur when the monomeric Si species was increased significantly while TMAdaOH can lead to the formation of CHA similar to the synthesis of conventional silica source. Still, the textural properties and crystallographic structure of the obtained CHA have remained the same but the fraction of the strong acidity was increased significantly with the increase of the Si/Al ratio. High silica CHA shows superior hydrothermal stability in selective catalytic reduction of NOx, consistent with the result of lattice optimization using General Utility Lattice program (GULP) with the core-shell potential. Further, the results of the molecular dynamics simulation using Car-Parrinello method indicated the rapid migration of proton in CHA with a low Si/Al ratio while monovalent alkali metal ions stabilized near the double six membered rings, implying proton hopping can affect the stability of the zeolite.

Impact of raw materials for TS-1 zeolite on its crystallization process, Ti species and catalytic properties

To obtain a highly active TS-1 zeolite, we focused on the manipulation of the crystallization process of TS-1 zeolite; the incorporation of Ti species into the MFI-framework along with the hydrolysis and/or dissolution processes was investigated with various Si sources including TEOS, amorphous silica and silicalite-1. We also investigated the effect of the TPAOH content in the synthesis gel on the formation of TS-1. Based on the catalytic evaluations for 1-hexene epoxidation reaction, it was found out that the TS-1 zeolite synthesized using amorphous SiO2 was more efficient than that with TEOS and silicalite-1. Finally, we have found that the framework Ti species experiencing the dissolution and re-construction processes are highly active for 1-hexene epoxidation.

Regulating the distribution of acid sites in ZSM-11 zeolite with different halogen anions to enhance its catalytic performance in the conversion of methanol to olefins

Purposive regulation of the spatial location and distribution of acid sites in a zeolite catalyst, though still rather challenging, has proved to be an effective measure to enhance its performance in the conversion of methanol to olefins (MTO). Herein, a series of ZSM-11 zeolites were synthesized using different sodium halides (with tetrabutylammonium hydroxide (TBAOH) as the structure-directing agent); the effect of halogen anions (Cl-, Br- and I-) in the synthesis gel on the Al siting and thereon the spatial location and distribution of acid sites in the ZSM-11 zeolites as well as their relation to the catalytic performance in MTO was elaborately investigated. The results indicate that for the synthesis of high Si/Al ratio ZSM-11 zeolites (Si/Al > 60), the Cl- anions present in the synthesis gel can induce more Al atoms siting on the external surface of ZSM-11 crystals, thereby leading to a short catalytic lifetime in MTO due to the rapid coke deposition. In contrast, larger Br- or I- anions in the synthesis gel, owing to their stronger covalent interaction with the TBA+ cations, may moderate the formation of crystal silicate nuclei and lead to a uniform distribution of Al species in the inner straight channels, which can effectively suppress the surface coke deposition and inhibit the aromatic-based cycle for MTO. As a result, the high Si/Al ratio ZSM-11 zeolites obtained with Br- or I- in the synthesis gel exhibit much better catalytic stability in MTO and higher selectivity to propene and butene, in comparison with the ZSM-11 counterpart synthesized using Cl-. These results help to clarify the relation between the catalytic performance of ZSM-11 in MTO and the location of acid sites and thus bring forward a facile strategy to enhance the catalytic performance of zeolites through purposively regulating the spatial distribution of acid sites.

Catalytic Decomposition of H2O2 for NO Oxidation‐Removal over Hierarchical Fe‐ZSM‐5: Effect of Ethanol on Zeolite Performance

AbstractHierarchical Fe‐ZSM‐5 zeolite catalysts hydrothermally prepared using tetrapropylammonium hydroxide (TPAOH) and [3‐(trimethoxysilyl)propyl]octadecyldimethylammonium chloride (TPOAC) as micropore structure‐directing agent and meso‐porogen, respectively, were investigated for oxidation‐removal of NO via catalytic decomposition of gas‐phase H2O2 into hydroxyl free radicals (⋅OH). The effects of ethanol in synthesis gel on the structure, porosity, Fe species state, and catalytic performance of zeolite catalysts were explored. And the function mechanism of ethanol was speculated. Appropriate ethanol dosage in synthesis gel probably stabilized TPOAC micelles during MFI crystallization, facilitated mesopores integration in zeolite crystals, benefited oxygen vacancy formation, accelerated ⋅OH radicals generation, and thereby enhanced NO oxidation‐removal efficiency. The hierarchical Fe‐ZSM‐5 prepared with ethanol/H2O ratio of 1 in synthesis gel displayed an excellent catalytic activity with the optimal NO conversion up to 35.9 % enhancement compared to the microporous Fe‐ZSM‐5. The SO2 resistance and ⋅OH scavenger test during catalytic reaction, as well as the reaction condition influence on the catalytic performance of hierarchical Fe‐ZSM‐5 were explored, and then the oxidation‐removal mechanism of NO over hierarchical Fe‐ZSM‐5 was preliminarily proposed.

A green approach for template free synthesis of Beta zeolite incorporated in ZSM-5 zeolite to enhance catalytic activity in MTG reaction: Effect of seed nature and temperature

A new composite zeolite, ZSM-5/Beta, was developed by incorporating Beta zeolite into the synthesis gel of ZSM-5 zeolite. Beta and ZSM-5 zeolite have been prepared from rice husk ash without using an organic template. First, the effect of synthesis temperature and seeding agent on structural properties of Beta zeolite was investigated. Then, prepared Beta zeolite was added to the ZSM-5 structure with the hydrothermal method. The prepared samples were characterized by X-ray diffraction, field emission scanning electron microscopy, N2 physical adsorption, and temperature-programmed ammonia desorption techniques. Results indicated that the synthesis at a temperature of 150 °C and using Na-Beta seeds showed suitable crystallinity as well as textural properties. Furthermore, the crystalline structure of ZSM-5 wasn't damaged by the addition of Beta zeolite. The incorporation of Beta zeolite into the ZSM-5 structure increased the total pore volume by 45%. The micropore volume and the mesopore volume also increased by 2% and 300%, respectively. Using composite zeolite led to tuning the catalyst acidity. The catalytic activity of this new ZSM-5/Beta composite structure was evaluated in the MTG process at 400 °C, WHSV of 20 h−1 and atmospheric pressure. The results illustrated that over this composite catalyst, the selectivity to alkylated aromatics increased by almost 13% compared to the initial ZSM-5 zeolite. Furthermore, gasoline production yield and catalyst lifetime also increased by 9% and 61% respectively compared to the initial ZSM-5 zeolite. The better performance of the composite zeolite can be ascribed to its higher mesoporous volume and lower acidity. The composite catalyst has a lower number of acid sites, especially the number of strong acid sites that are one of the main factors in coke deposition.

Te-doped MoV-Oxide (M1 phase) for ethane ODH. The role of tellurium on morphology, thermal stability and catalytic behaviour

Undoped and Te-doped MoV-Oxide (M1 phase) catalysts have been prepared hydrothermally (Te/Mo ratio in the synthesis gel from 0 to 0.17; and heat-treated at 400 or 600°C in N2 atmosphere), characterized by several physicochemical techniques and tested in the oxidative dehydrogenation (ODH) of ethane. The morphology and microporosity of the catalysts, the nature of V-species on the catalyst surface and the catalytic performance strongly depend on the composition and the heat-treatment temperature. When calcined at 400°C, the selectivity to ethylene decreases when the amount of tellurium increases, whereas when heat treated at 600°C, the selectivity to ethylene increases when the Te-loading increases. These trends have been explained on the basis of the good correlation between selectivity to ethylene and the concentration of V4+ species on the surface of catalysts, in which the most selective catalyst is that prepared with a Te/Mo ratio of 0.17 and heat-treated at 600°C.

Formation process of organic template-free chabazite zeolite membrane and its separation performance of water-rich mixtures

The growth process of chabazite seed crystal transformed into chabazite zeolite membrane in an organic template-free and fluoride-containing synthesis gel was investigated in details. The characterization results showed that the formation process of organic template-free chabazite zeolite membrane included the processes of seed crystal dissolution and chabazite crystal reconstruction. In the first process of seed dissolution into amorphous, the secondary building units (D6Rs) of CHA framework were completely destroyed except for the presence of a small number of 4Rs. While the amorphous contained many primary building units (TO4) as the nucleation sites in the second process of chabazite crystal reconstruction, which nucleated rapidly again under the promotion of fluorine anion. With the further prolongation of crystallization time, the regenerated secondary structural units connected together to reconstruct the framework structure of chabazite zeolite, and gradually grew into a continuous and dense membrane layer with the cross-linking growth of chabazite zeolite. In addition, chabazite zeolite membrane exhibited high permeation flux and excellent long-term hydrothermal stability for the separation of water-rich mixtures. For 50 wt% EtOH/H2O and 50 wt% IPA/H2O binary mixtures at 348 K, the stable permeation fluxes of the membranes were as high as 13.50 kg m−2 h−1 and 14.80 kg m−2 h−1, respectively.