Texture modifications of silica-alumina gels by microwave drying

The porous texture of silica-alumina hydrogel can be taylored by controlling the operating parameters of the preparation process. As the drying drastically modifies the texture it is important to minimize pore volume and radius decreases. Owing to a low drying temperature microwave heating coupled with convective drying could be a way to preserve the hydrogel texture. Its effects have been studied and it appears that the characteristics of the air flow are the main parameters of this drying mode which does not present decisive advantages as compared with other drying modes.     

Initial stages of SBA-15 synthesis: an overview

This work presents an overview of the data obtained for SBA-15 synthesis under the reaction conditions using synchrotron based small angle X-ray scattering and small angle neutron scattering. Three major stages in the synthesis of SBA-15 materials proceeding according to the cooperative self-assembly mechanism have been identified, and the structures of the intermediates species have been established. Our in situ time-resolved neutron scattering experiments demonstrate that only spherical micelles of the templating agent are present in the synthesis mixture during the first stage of the reaction. According to the neutron scattering and X-ray scattering data, in the second stage of the reaction the formation of hybrid organic-inorganic micelles is accompanied with the transformation from spherical to cylindrical micelles, which takes place before the precipitation of the ordered SBA-15 phase. During the third stage, these micelles aggregate into a two-dimensional hexagonal structure, confirming that the precipitation takes place as the result of self-assembly of the hybrid cylindrical micelles. As the synthesis proceeds, the voids between the cylinders are filled with the silicate species which undergo condensation reactions resulting in cross-linking and covalent bonding, leading to the formation of highly ordered SBA-15 mesostructure. This work demonstrates that valuable structural information can be obtained from X-ray and neutron scattering characterisation of complex systems containing periodic phases with d-spacing values up to 30 nm, and that both techniques are powerful means for in situ monitoring of the formation of nanostructured materials.     

Temperature dependence of NaA zeolite nucleation in silicoalumina gels

Conclusions For the same linear growth rate of crystals, the duration of the crystallization of NaA zeolite at 90°C exhibits a minimum dependence on the temperature at which the silicoalumina gels are held (0-90°C) prior to crystallization, which corresponds to the temperature-dependent maximum found in the dependence of the nucleation of NaA zeolite on temperature.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 727–730, April, 1988.The authors wish to express their gratitude to S. P. Zhdanov for discussing the results and for advice rendered during the writing of this paper.     

Structure-direction in zeolite synthesis

The concepts of structure-direction in the synthesis of clathrasils and high-silica molecular sieves are reviewed. The effects of size, geometry, and chemical nature of the organic structure-directing agent on the crystalline structures that are formed are discussed beginning with clathrasils (0-dimensional pore systems) and ending with 12-ring zeolites with 3-dimensional pore systems. Emphasis is focused on the energetic interactions between the organic guest and the inorganic framework. The energetic stability of porous frameworks is compared to the stability of dense pure-silica phases and the effects of trivalent (Al, B) and divalent (Zn) tetrahedral heteroatoms on the structure of zeolites formed is reviewed. The application of structure-directing concepts are described using the syntheses of ZSM-18 and SSZ-26 as examples, and the control over long-range order in zeolites by structure-directing effects is illustrated by the purposeful variation of the stacking probability of SSZ-33-CIT-1 and FAU-EMT intergrowths.     

The characterization of modified silica-alumina catalysts active in the synthesis of picolines

The SiO₂–Al₂O₃ catalyst is modified by promoters like Pb, Cr, Cu. The SiO₂–Al₂O₃ catalyst is also modified using two elements by a simultaneous or stepwise impregnation method. The catalysts are characterized by XRD, IR and ESR techniques, and are useful in the synthesis of picolines.IICT Communication No. 3514, decicated to Dr. A.V. Rama Rao on his 60th birthday.     

Hydrocarbons from synthesis gas over Pd supported on silica-alumina catalyst

We have studied the operational conditions which allow the synthesis gas to hydrocarbon transformation in only one step using Pd supported over an amorphous silica-alumina catalyst. The increase of temperature, pressure and time factor favors carbon monoxide conversion. Methanol synthesis reaction can be considered as the limiting step of the overall process. The product distribution is governed by the reaction medium (type and distribution of active sites and the presence of hydrogen).     

Solvent-free synthesis of zeolite catalysts

The most used method for preparation of zeolites is hydrothermal synthesis from silicate or aluminosilicate gels at temperatures in the range of 60–200°C.Excess water used in the industrial process results in several issues,including high autogeneous pressure,low efficiency,pollution,etc.To solve these problems,several strategies have been developed.This review describes the solvent-free synthesis of zeolites.The combination of solvent-free synthesis and organotemplate-free synthesis can open the pathway to a highly sustainable zeolite synthesis protocol in industry.     

Amphiphilic gels for peptide synthesis

A new class of “amphiphilic” copoly(styrene-acrylamide)s of potential interest for peptide synthesis is described. As examples of the new polymers, those carrying nitrophenol or piperazine functionality were used for model reactions in peptide synthesis, and were found to be substantially more efficient than similar reagents derived from polystyrene, polydimethylacrylamide or silica gel. Due to their amphiphilic structure, the new resins have general substrate compatibility and are permeated by all of the commonly used solvents, including toluene and ethyl acetate on the one hand, through dimethylformamide, to dimethylsulfoxide and water on the other. The improved performance of the new polymers is attributed to their alternating amphiphilic structure and favorable polymer–solvent–substrate interactions. © 1992 John Wiley & Sons, Inc.     

Theoretical design for zeolite synthesis

Science China Chemistry - Zeolites, an important class of microporous crystals, have been widely utilized in the fields of catalysis, ion-exchange, separation, and sorption for a long time. In...     

Rapid synthesis of MFI zeolite nanocrystals

A rapid synthesis procedure for nonagglomerated silicalite nanocrystals has been developed. This was achieved by concentrating the precursor sol before 10-12 h of aging at 80 degrees C, followed by hydrothermal synthesis at 175 degrees C for 90 min. The high silica concentration in the concentrated sol accelerated the aggregation of primary units that were present early in the system. Thus, little silica nutrients were left for growth when the secondary particles were converted to zeolite during hydrothermal reaction. As a result, fully dispersible nanocrystals were obtained within a day instead of weeks as reported previously. The aggregation of primary units during the 80 degrees C aging process as well as the conversion of these aggregates into zeolite has been followed by DLS, XRD, and FTIR. In light of the new results, the nucleation and growth mechanisms of MFI zeolite that have been under debate in the literature were reexamined.     

Subtle interactions in silica-alumina mixtures

Object of the study in this paper was the mechanical mixtures of amorphous silica and α-Al₂O₃ with different precursors (gibbsite, boehmite, and γ-Al₂O₃). The results obtained revealed that measurable interactions exist in different binary systems without previous thermal treatment. These interactions could be explained by the existence of attractive and repulsive forces which appear between the OH⁻ groups present on the surfaces of alumina and silica constituents. In thermally treated samples, the interactions are not driven by intermolecular forces but rather by polymorphic transformations of alumina and silica, which are followed by sintering.     

Zeolite synthesis from tetraalkylammonium silicate gels

Siliceous zeolite synthesis gels containing tetraalkylammonium (TAA⁺) and sodium cations were studied using X-ray diffraction, elemental analysis, ion exchange, ²⁹Si magic-angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The TAA⁺ cations are encapsulated in silicate cages, and silicalite is formed via the rearrangement of these cages by the breaking and reformation of siloxane bonds. Tetrabutylammonium (TBA⁺) cations promote silicalite growth, but not as effectively as tetrapropylammonium (TPA⁺) because the larger TBA⁺ cations do not conform as well to the silicalite lattice, thus forming an intergrowth of the silicalite-1 and silicalite-2 structures. The time to nucleate silicalite is not affected by the TBA⁺ content of the gel, but the rate of silicalite crystal growth increases with increasing TBA⁺ in the gel. The TBA⁺ occupies all the channel intersections of the silicalite formed. Tetraethylammonium (TEA⁺) cations are encapsulated in silicate cages, but not to the same extent as TPA⁺ and TBA⁺, because TEA⁺ is not as hydrophobic. No silicalite forms in the TEA⁺ silicate gel. The addition of tripropylamine (TriPA) to a TPA⁺ silicate gel has no effect on the kinetics of silicalite formation. TriPA does not incorporate into the gel because it is neutral and, therefore, does not experience a coulombic attraction to the negatively charged surface of the gel.     

一步法合成多级孔结构ZSM-5分子筛

多级孔结构ZSM-5分子筛的合成过程复杂。利用双模板剂,通过优化晶化条件(如晶化时间与晶化温度)和Si/Al物质的量比等一步水热晶化合成了具有多级孔结构的ZSM-5分子筛,并采用XRD、N₂吸附-脱附、吡啶红外吸脱附、SEM和TEM等方法对样品的晶体结构、孔道结构、表面酸性和形貌等进行了表征。结果表明,一步法合成多级孔结构ZSM-5分子筛的适宜条件是:晶化温度160-180℃,晶化时间24-96 h,反应物组成为SiO₂/Al₂O₃/Na₂O/CTAB/TPABr/H₂O=1/x/0.4/0.05/0.12/280,(x:50-240)。其中,晶化温度160℃、晶化时间48 h和以Si/Al物质的量比50的凝胶合成的样品具有有序的介孔(平均尺寸3.60 nm)结构、较高的结晶度和较强的酸性。     

Hydrocarbons from synthesis gas; effect of Mg addition over Pd impregnated silica-alumina catalysts

The effect of Mg addition to Pd impregnated silica-alumina catalysts and the behavior of these catalysts in synthesis gas to hydrocarbons transformation have been studied. Measurements of H₂ chemisorption and temperature programmed NH₃ desorption allow us to consider that Mg addition to Pd containing catalysts increases the Pd dispersity, the support acidity not being modified. The presence of Mg enhances hydrocarbon production. Meanwhile, the hydrocarbon distribution is not modified. A large amount of light alkanes in the reaction products is obtained.     

甲胺-丝光沸石的合成及其鉴定

作者研究在不同反应混合物系统中ZSM-5沸石的自发结晶作用与非自发结晶作用时发现,含甲胺的系统比含其他有机碱的系统更容易生成丝光沸石。通过对甲胺-Na_2O-SiO_2-Al_2O_3-H_2O反应混合物系统的自发结晶的进一步研究,作出了配比-产物组成图。结果表明,在甲胺系统中,含氮的丝光沸石,即甲胺-丝光沸石(MA-Md),能在相当宽的配比范围内生成。MA-Md虽然与丝光沸石(NaM)骨架结构相同,但它的阳离子中约有一半系甲胺离子。它的某些性质也不同于NaM,例如:硅铝比提高;晶格收     

Cosynthesis, coexistence, and self-organization of alpha- and beta-cobalt hydroxide based on diffusion and reaction in organic gels

We report the cosynthesis of highly stable laminated single crystal alpha- and beta-Co(OH) 2 using the reaction and diffusion of a hydroxide solution into a gel containing Co(II). The obtained alpha-Co(OH) 2, which is known to be thermodynamically unstable and transforms in a short period of time to the beta form, has been stabilized in the gel medium for weeks. The system also exhibits Liesegang banding where complicated spatial dynamics during the formation of the two polymorphs are shown to take place.     

High yield method for nanocrystalline zeolite synthesis

Nanocrystalline zeolites, such as silicalite-1 and zeolite Y, were synthesized in high yield by periodically removing nanocrystals from the synthesis solution and recycling the unused reagents, including the template and T-atom sources.     

The synthesis and properties of zeolite CF-3

The novel zeolite CF-3, with a high ratio of SiO₂/Al₂O₃ and a characteristic X-ray powder diffraction pattern, has been synthesized hydrothermally from a TMEDA-Na₂O–SiO₂–Al₂O₃–H₂O system at 200°C. The molar composition of CF-3 is (0.4–0.6)Na₂O·(1.5–6.3)TMEDA·Al₂O₃·(80–400)SiO₂·(0–17)H₂O·CF-3 is similar to ZSM-39 and melanophlogite, which have a clathrate-type structure.