Synthesis Gel Research Articles

Synthesis of NiMo Catalysts Supported on Gallium-Containing Mesoporous Y Zeolites with Different Gallium Contents and Their High Activities in the Hydrodesulfurization of 4,6-Dimethyldibenzothiophene

Mesoporous MY-xGa zeolites exhibiting both crystallized pore walls and narrowly dispersed mesopores with different Ga content were successfully synthesized. The synthesized samples were characterized by XRD, N2 adsorption desorption isotherms, SEM, TEM, XPS, FTIR, 29Si MAS NMR, 71Ga MAS NMR, and Py-FTIR methods. The results show that the synthesized samples exhibit unique open channel like mesopore systems and outstanding crystallite natures; no nonframework Ga species were observed over the MY-xGa series samples, and their acidic properties can be modulated by varying the Ga/Al ratio in the initial synthesis gel. The corresponding NiMo/HMY-xGa catalysts were prepared via the incipient wetness coimpregnation method; the morphologies of the sulfide catalysts were characterized by HRTEM, and the covalent states of the active metals were characterized by XPS. The catalytic activities of the investigated catalysts for the 4,6-DMDBT HDS reaction were assessed, and the collected products were analyzed by GC and...

Seed-directed synthesis of zincoaluminosilicate MSE-type zeolites using co-precipitated gels with tetraethylammonium hydroxide as a simple organic structure directing agent

Simplified and efficient synthesis of MSE-type zeolites using tetraethylammonium hydroxide (TEAOH) is extended from aluminosilicate to zincoaluminosilicate by combining with co-precipitated gel technique. The direct crystallization of MSE-type zeolite could be realized from the synthesis gels containing co-precipitated gels which contained homogenously dispersed zinc and aluminum species with a Zn/(Zn + Al) ratio up to 0.4. The overall characterizations revealed that most of the zinc species were incorporated as a heteroatom, and created anionic charges in MSE framework. Based on the ion-exchange characteristics of those samples, it is found that the most of introduced zinc species exhibited two anionic charges per atom, which is expected to be suitable sites for divalent-cation exchange. Since the sample prepared by using Zn/(Zn + Al) ratio of 0.2 (MSE-TEAZn0.2) remained stable after thermal treatment, Ni2+ exchange was applied to that sample. The results revealed that MSE-TEAZn0.2 sample exhibited ion-exchange capacity superior to aluminosilicate MSE-TEA sample. Therefore, MSE-TEAZn0.2 sample can be shown as a potential divalent-cation exchanger. In this contribution, it is demonstrated that the simplified synthesis path is a very effective, alternative way to synthesize multipore zeolites containing medium and large pores in the frameworks with different heteroatoms.

Structure, stability and permeation properties of NaA zeolite membranes for H2O/H2 and CH3OH/H2 separations

NaA zeolite membranes were synthesised in the secondary growth hydrothermal method based on the seeding of the inner surface of a ceramic α-alumina tube. The impacts of crystallisation time and zeolite precursor concentration (in H2O) were investigated. The structure and stability of the prepared NaA zeolite membranes were also investigated with operating temperatures, times and pressures. The results indicate that the optimal synthesis gel molar composition was 3Na2O: 2SiO2: Al2O3: 200H2O. This led to cubic-shaped NaA zeolite which showed good stability. The optimal NaA zeolite membrane had H2O and CH3OH fluxes of 2.77 and 0.19kg/m2h, with H2O/H2 and CH3OH/H2 separation factors of ∞ and 0.09 at a temperature of 30°C. The NaA zeolite membrane had high thermal stability, but poor separation performance at high temperature (240°C). The results suggested that the H2 permeation flux is significantly influenced by preferential adsorption of vapour in the NaA zeolite membrane.

A simple secondary growth method for the preparation of silicalite-1 membrane on a tubular silica support via gel-free steam-assisted conversion

Silicalite-1 membranes with high pervaporation (PV) performance were prepared on a seeded tubular silica support using modified steam-assisted conversion (SAC) under gel-free conditions, called the gel-free SAC method, for the first time. The seeded support was first dipped in a TPAOH aqueous solution and dried. Subsequently, the TPAOH-coated silicalite-1 seed layer was converted into a silicalite-1 membrane layer, followed by heating in an autoclave with a small amount of water at the bottom of the autoclave. In this method, silica support was not only used as the support but also as the silica source to grow the zeolite seed layer into a continuous and dense zeolite membrane layer. In other words, no additional supply of silica source such as synthesis gel to grow the seed layer was required. The silicalite-1 membrane prepared in this study had a separation layer with a thickness of approximately 7µm and exhibited high PV performance with a separation factor of 66 and high flux of 4.47kgm−2h−1 for 10wt% ethanol/water mixtures at 323K. This membrane preparation method is quite simple and uses minimum necessary reagents. Therefore, it is economical and suitable for the preparation of membranes on an industrial scale.

A study on the synthesis of [Fe,B]-MFI zeolites using hydrothermal method and investigation of their properties

Synthesis of MFI-type zeolites with boron and iron in their framework using conventional hydrothermal method was investigated. The effect of several parameters including gel composition and pH on the crystallization of zeolites was systematically studied. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and N2 physical adsorption analysis. The XRD patterns showed that the crystallization of boroferrisilicate zeolite takes place at pH values higher than 9 with molar ratio of Si/(Fe+B)≥8.3. The crystallinity and BET surface area of zeolites was decreased by increasing the amount of Fe3+ ions in the synthesis gel. The SEM images revealed that Fe/B ratio of synthesis gel could also influence the morphology of zeolite particles. The drug loading capacity of boroferrisilicates with different Fe/B ratios was evaluated by loading of doxorubicin (DOX) as a model drug. Among zeolites with Fe/B ratios of 0.5, 1 and 2, higher loading efficiency was achieved for sample with lower iron content. However, for sample with Fe/B=∞ which the formation of iron oxide particles on the external surface of sample was confirmed by XRD and SEM analysis, the DOX loading efficiency was remarkably high. Since the desorption of DOX molecules from zeolites surface was prevented by the strong interaction between drug molecules and the surface of heteroatom-substituted zeolites, the release efficiency for all boroferrisilicate zeolites was low.

Fluoride-mediated synthesis of TON and MFI zeolites using 1-butyl-3-methylimidazolium as structure-directing agent

In this work, 1-butyl-3-methylimidazolium (1B3MI) cation has been employed as a structure-directing agent for the synthesis of pure silica and Ti-containing zeolites in fluoride media. The 1B3MI OSDA exhibited distinct phase selectivity depending on the synthesis conditions, and particularly the concentration of the synthesis gel. Thus, under more diluted conditions (H2O/SiO2 = 14), the 1B3MI cation yielded pure silica TON zeolite with high crystallinity. Attempts to obtain Ti-containing zeolites, however, failed at this diluted condition even after 15 days of synthesis. In contrast, highly crystalline Ti-MFI was the only phase obtained upon reducing the H2O/SiO2 molar ratio from 14 to 7. The presence of Ti in the MFI zeolite framework was confirmed by both infrared and DRS UV-vis spectroscopies. The presence of intact OSDA molecules within the micropores was inferred from elemental analysis and 13C CP-MAS NMR spectroscopy. On the other hand, thermogravimetric analyses showed a different packing of the 1B3MI cation in the pure silica TON and Ti-MFI samples. Moreover, the environments for Si and F species in the as-made materials were investigated by means of 29Si and 19F MAS NMR, respectively. Finally, molecular simulations showed that the most stable arrangement of the 1B3MI cations involves the location of the imidazolium rings on the channel intersections and of the butyl chains on the sinusoidal channels.

Low-temperature methanol dehydration to dimethyl ether over various small-pore zeolites

Eight-membered ring small-pore zeolites Rho and KFI have been synthesized, characterized and tested for dehydration of methanol to dimethyl ether at low-temperature, and compared with other zeolites and three different samples of γ-Al2O3. Both the zeolites were mainly crystallized from the synthesis gels with the Si/Al ratio of 5.0 by a conventional hydrothermal method without any agitation. The amount of total solid-acid sites was 1.65mmolg−1 and 2.53mmolg−1 for zeolite KFI and Rho, respectively. In addition, zeolites SSZ-13, RUB-13, and ZSM-5 were also employed for the reaction. Reaction conditions were optimized for a low-temperature catalytic dehydration of methanol selectively to dimethyl ether. Methanol dehydration efficiency of various zeolitic frameworks is discussed against the strength of solid-acidity, type of channel structure, specific surface area, and particle size. At temperatures≤200°C, the overall catalytic efficiency of the small-pore zeolites with appropriate medium-strong acidity and 3-D channels was superior to that of the reference γ-Al2O3 materials.

Preparation of composite zeolites in polymer hydrogels and their catalytic performances in the methanol-to-olefin reaction

ZSM-5/AlPO4-5 composite zeolites were prepared in polyacrylamide (PAM) hydrogels and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), N2 isothermal adsorption-desorption, nuclear magnetic resonance (NMR) and temperature-programmed desorption of ammonia (NH3-TPD). In addition, the catalytic performances of the composite zeolites in the methanol-to-olefin (MTO) reaction were examined. The results indicate that introducing PAM into the synthesis gel of ZSM-5/AlPO4-5 facilitates both the formation of the ZSM-5/AlPO4-5 composite zeolite and the improvement of the catalyst performance. Moreover, the catalyst acidities are closely related to their selectivities for the MTO reaction. The composite zeolite obtained in the PAM hydrogel has a lower density of total acid sites and a higher selectivity for light olefins compared with the composite zeolite prepared without the PAM.

Self-assembly of fibrous ZSM-5 zeolites in the presence of sodium alginate

ZSM-5 zeolites with a fibrous morphology were successfully self-assembled in the presence of sodium alginate. The effect of the sodium alginate concentration, Si/Al molar ratio in the synthesis gel, crystallization temperature and time, and the addition order of the sodium alginate on the morphology of the ZSM-5 zeolites was investigated. The possible formation mechanism of fibrous ZSM-5 zeolite crystals was also proposed. The results indicated that the carboxyl and hydroxyl groups of sodium alginate synergistically induced the self-stacking of ZSM-5 nanocrystals and thus the formation of the fibrous ZSM-5 zeolites. The Si/Al molar ratio of the fibrous ZSM-5 zeolites could be adjusted by controlling the amount of the NaAlO2 additive; however, high Si/Al molar ratios also hindered the self-stacking of ZSM-5 nanocrystals. A high crystallization temperature (180°C), a moderate sodium alginate concentration (8.33g/L), and the addition of sodium alginate prior to tetraethoxysilane were necessary for the formation of fibrous ZSM-5 zeolites.

Effect of synthesis parameters on the crystallinity of EU-1 zeolite for the m-xylene isomerization reaction

The EU-1 zeolite with Si/Al = 25 was synthesized by hexamethonium bromide as a template. The effects of crystallization time, synthesis temperature, and the pH of the synthesis gel on the structure and crystallinity of EU-1 zeolite were investigated. The synthesized EU-1 zeolite was characterized by X-ray powder diffraction, BET, Fourier transforms infrared spectroscopy, scanning electron microscope, and inductively coupled plasma. The most effective conditions for synthesis of EU-1 were identified as follow: synthesis time 72 h, temperature 200°C, pH of gel ≃ 13, and aging time of 12 h. The catalytic performance of this catalyst was investigated for meta-xylene isomerization reaction in a fixed bed reactor under the following conditions: pressure of 10 bars, the temperature of 380°C, feed space velocity of 3.35 h–1 and 4 g of the EU-1 catalyst. In comparison to other reported catalysts our synthetic EU-1 showed good m-xylene conversion and high PX/OX ratio.

A new soft template-oriented method for the preparation of hollow analcime microspheres with nanosheets-assembled shells

This paper highlights a controlled synthesis of two-dimensional analcime nanosheets templated by organic additives and an impressive strategy that hollow hierarchical analcime microspheres with layered shells can be assembled by taking advantage of the intrinsic growth law of material. Specifically, ultrathin analcime nanosheets were initially obtained by precisely manipulating the amounts of cetyltrimethylammonium cation (CTA+) and ethylenediaminetetraacetate (EDTA4−) in the synthesis system. As building blocks, these nanosheets then self-assembled layer by layer from outside to inside driven by the reversed crystal growth mechanism of analcime, resulting in a hollow structure with lamellar shells and enhanced specific surface area of 722.3 m2 g−1. Series of experiments were carried out in order to explore the influence of CTA+ and EDTA4− on the formation of analcime nanosheets. The results indicated that CTA+ was the micro-mesoporogen of hierarchical analcime and synergistically collaborated with EDTA4− in directing analcime nanosheets. The effect of hydrothermal temperature was discussed and a surfactant packing parameter (g = V/a 0 l) was cited to explain the behavior of organics. In addition, the investigation of hydrothermal process clearly revealed the crystallization and self-assembly process of hollow structure. And the UV Raman results unraveled that four-membered rings (4MRs) as the active building units for analcime framework were firstly formed in the synthesis gel, followed by reconstruction and self-assembly which lead to the formation of 6MRs and 8MRs.

Catalytic cracking of n-dodecane over hierarchical HZSM-5 zeolites synthesized by double-template method

Hierarchical HZSM-5 zeolites have been prepared by a double-template method with tetrapropylammonium hydroxide and polydiallydimethylammonium chloride (PDDA) as micro- and meso-scale template, respectively, and Al(NO3)3 as aluminum source. The as-prepared zeolites were characterized by XRD, SEM, TEM, N2-adsorption–desorption, TGA, ICP, in situ FT-IR and NH3-TPD. The hierarchical HZSM-5 zeolite prepared at the PDDA/TEOS weight ratio of 0.2 in the synthesis gel (HZ-0.2P) had uniform mesopores (11–13 nm) and high acid site accessibility. An increase of the PDDA/TEOS resulted in enlargement of mesopore size and broadening of size distribution, together with appearance of some non-uniform accumulation pores. Moreover, the accessibility of acid sites in hierarchical HZSM-5 decreased. Catalytic cracking of n-dodecane carried out at 500 °C and 4 MPa in a wall-coated tubular reactor with HZSM-5 zeolites showed that HZ-0.2P exhibited the superior catalytic activity and stability, with the increase higher than 20% in the conversion of n-dodecane and reduction to ca. 1/2 in the deactivation rate compared with micro-porous HZSM-5 zeolite. This could be ascribed to its highest acid site accessibility, which improves the diffusion of n-dodecane and its cracking product molecules inside pore channels of catalyst.

On the synthesis and performance of hierarchical nanoporous TS-1 catalysts

Hierarchical TS-1 zeolite was successfully prepared using chitosan as a sacrificial template. The X-ray diffraction showed that the presence of chitosan with the synthesis precursor had no deleterious effect on the crystallinity and phase purity of this zeolite. X-ray absorption spectroscopy at the Ti K-edge, FTIR and Raman spectroscopies revealed the titanium ions in the zeolite structure have predominantly tetrahedral coordination. However, it appears that the higher chitosan content in the synthesis gel imparted some hydrophilic character to the TS-1 system. Furthermore, the technique adopted for the preparation of the synthesis gel – e.g partially dried or fully dried – appears to affect the amount of framework titanium in the zeolite structure. The calcined form of the chitosan templated TS-1 zeolites exhibited higher cyclohexene conversion compared to the TS-1 material synthesised without this template, but these catalysts showed lower selectivity for cyclohexene epoxide.

Synthesis and characterization of Al-magadiite and its catalytic behavior in 1,4-pentanediol dehydration

Two layered aluminosilicate samples having magadiite structure and different Al-contents were prepared by conventional hydrothermal synthesis method. The samples were characterized by XRD, N2 adsorption isotherms, temperature-programmed ammonia evolution (TPAE), and 27Al MAS NMR spectroscopy. The acidity of the samples was characterized by the FT-IR spectra of adsorbed pyridine (Py). The specific surface area (SSA) of the crystalline samples was found to depend on the Al-content of the synthesis gel. The sample of low and high Si to Al ratio (Si/Al = 19 and 35) had SSA of 61 and 260 m2/g, respectively. 27Al MAS NMR spectra of the samples indicated tetra-coordinated Al, incorporated into Q3 and/or Q4 framework sites of the aluminosilicate layer. Aluminum in Q4 sites generates bridged hydroxyls (Si–OH–Al). These hydroxyls are strong Bronsted acid sites. Bases, like Py and ammonia, are bound to these sites strongly in protonated form. There are three-coordinated Al atoms in the Q3 sites. In dehydrated samples, these sites have Lewis-acidity. These acid sites, however, are only partially accessible to Py. Acid sites were also characterized in the reaction of 1,4-pentanediol (1,4-PD) dehydration. The acidity and catalytic behavior of the magadiite samples were compared to the corresponding properties of zeolite H-ZSM-5 (Si/Al = 20) and commercial γ-Al2O3.

Boron-containing MFI-type zeolites with a hierarchical nanosheet assembly for lipase immobilization.

Hierarchical nanosheet assemblies of boron-containing MFI-type zeolites were successfully synthesized using bifunctional surfactant molecules under static conditions. With the optimized Si/B = 50 in the synthesis gel, a material with high specific surface area (483 m2 g-1) and a maximum pore size of 6.5 nm suitable for lipase encapsulation was obtained.

Effect of ZSM-5 crystal size on its catalytic properties for conversion of methanol to gasoline

Four-sized monodispersed ZSM-5 crystals, being 70, 200, 400 and 650 nm, were hydrothermally synthesized by changing the H2O/Si molar ratio in the synthesis gel, and characterized with XRD, TEM, BET and NH3-TPD techniques. The crystal size effect of ZSM-5 on its catalytic performance for conversion of methanol to gasoline (MTG) was investigated. It was shown that the external surface area of the sample decreased with its crystal size, while the acid site amount firstly increased, and then kept almost constant. Nevertheless, 650 nm ZSM-5 crystals attaching small particles exhibit large external surface area and strong acidity. The catalytic stability and the liquid hydrocarbon yield decreased with increasing crystal size. The sample with a crystal size of 70 nm shows a catalytic lifetime of 96 h and a gasoline yield of 30.8%. The large external surface area and relatively strong acidity endow the sample with a crystal size of 650 nm also has a catalytic lifetime of 91 h, indicating that synthesis of large ZSM-5 crystals with small crystallites adhered to their surface could be a potential way to improve the catalytic performance.

Patching NaA zeolite membrane by adding methylcellulose into the synthesis gel

Uniform NaA zeolite membranes, with good permeation and separation performance were successfully synthesized with the assistance of methylcellulose (MC) via microwave heating (MH). The results of X-ray diffraction spectroscopy (XRD) and Scanning electron microscopy (SEM) showed that this strategy could not only reduce the crystal size, but also promote the patching of the defects in the substrate membrane with a slight increase of the membrane thickness. The gas permeation performance demonstrated that the NaA zeolite membranes with MC modification exhibited excellent H2 permeation, and their permselectivities of H2 over other small molecule gases were obviously larger than those corresponding Knudsen diffusion coefficients. The effects of temperature, pressure and binary mixtures of gases on gas permeation behavior were systemically studied. At 298K and 0.1MPa, the H2 permeation value of M6 membrane achieved as high as 4.23×10−6molm−2s−1Pa−1, meanwhile its permselectivity for H2 over CO2, N2 CO, CH4 and C3H8 was 6.31, 5.87, 7.98, 8.46 and 15.67, respectively.

Influence of crystal size on the catalytic performance of H-ZSM-5 and Zn/H-ZSM-5 in the conversion of methanol to aromatics

H-ZSM-5 zeolites with a uniform crystal size from 0.25 to 2μm were obtained by adding colloidal silicalite-1 seed in the synthesis gel; with H-ZSM-5 as the support, Zn/H-ZSM-5 was prepared by incipient wet impregnation. The influence of crystal size on the state of Zn species and its relation to the catalytic performance of Zn/H-ZSM-5 in the conversion of methanol to aromatics (MTA) was then investigated. The results illustrated that the state of Zn species and catalytic performance of Zn/H-ZSM-5 are closely related to the crystal size, though the crystal size has little influence on the overall acidity. There exist mainly two types of zinc species, viz., ZnO and ZnOH+; Zn/H-ZSM-5 with smaller crystal size is provided with more ZnOH+ species. The selectivity to aromatics and catalyst stability can be improved greatly by using small crystal Zn/H-ZSM-5. A good linear correlation is observed between the amount of ZnOH+ species and the selectivity to aromatics, suggesting that ZnOH+ species plays an important role in enhancing the dehydrogenation of alkanes and aromatization of alkenes to aromatics. As a result, small crystal Zn/H-ZSM-5 with large portion of ZnOH+ species exhibits high selectivity to aromatics and long lifetime in MTA.

Propane dehydrogenation catalyzed by gallosilicate MFI zeolites with perturbed acidity

The propane dehydrogenation (PDH) behavior of gallosilicate MFI catalysts synthesized in the presence of 3-mercaptopropyl-trimethoxysilane (MPS) is investigated. MPS addition to the zeolite synthesis gel results in gallosilicate molecular sieves with reduced Brønsted acidity and enhanced Lewis acidity, as shown by combined NH3 temperature-programmed desorption (TPD), isopropylamine (IPA) TPD and IR analysis of pyridine-loaded catalysts. In particular, the gallosilicate MFI catalysts prepared using MPS provide a significant concentration of strong Lewis acid sites, which are important in controlling the selectivity of PDH. Enhanced PDH performance with higher propane conversion rates and improved propylene selectivity with limited cracking and aromatization products are obtained from the catalysts synthesized with MPS. The gallosilicate MFI materials prepared using MPS are compared with a benchmark chromia-alumina catalyst, and the gallosilicate materials have a lower rate of deactivation (>50% lower) but slightly inferior propylene selectivity (Ga-MFI: 75%; Chromia-Alumina: 85%).

Optimization of MCM-48 synthesis using factorial design

Abstract MCM-48 mesoporous materials were hydrothermally synthesized according to the 22 factorial design by varying the crystallization time and temperature of the synthesis gel, and characterized by means of X-ray diffraction analysis and adsorption of N2. In the crystallization temperature and time conditions used, specific areas between 924 to 1102 m2.g-1, pore volumes between 0.015 to 0.087 cm3.g-1 and pore diameters between 3.2 to 4.0 nm were obtained. It was observed that for the syntheses performed at high temperature, the crystallization time should be reduced so that the material structure is formed.