Dry Gel Conversion Method Research Articles

Synthesis of Zeolitic Mesoporous Materials by Dry Gel Conversion under Controlled Humidity

A new variation to the dry gel conversion method for the preparation of zeolite is introduced, where the conversion to zeolite was accomplished under water vapor below the saturation condition. The dry gel was made either by vacuum concentration or by surfactant flocculation from a silica precursor sol after the preliminary hydrothermal step. Depending of the humidity under which the conversion took place, mesoporous structures with varying MFI crystallinity, BET, and crystalline external surface areas were produced. The effects of precursor preparation and surfactant flocculation conditions on mesoporosity and crystallinity of the final product, as well as the steam stability of the product, were also investigated. The approach could find general application in the synthesis of other zeolites as well as metal oxides.

Synthesis of [Al]-SSZ-31 Molecular Sieves Using [Al]-Beta Zeolite ([Al]-BEA) as Precursors

Abstract The aluminosilicate version of SSZ-31 ([Al]-SSZ-31) zeolite was synthesized by a new route including phase-transformation, wherein an aluminosilicate analog of beta zeolite ([Al]-BEA) was used as a precursor and 1,1,1,8,8,8-hexaethyl-1,8-diazoniaoctane dihydroxide was used as a structure-directing agent (SDA). [Al]-BEA precursors synthesized by two routes, viz. the hydrothermal (HTS) and dry-gel conversion (DGC) method, were employed for the synthesis. [Al]-SSZ-31 with a low output SiO2/Al2O3 ratio crystallized only when the precursor (SiO2/Al2O3 = 30), prepared by DGC method, was employed. Precursors synthesized by the HTS method produced highly crystalline samples, however, only in the high silica region. In general, [Al]-SSZ-31 samples over a wide range of SiO2/Al2O3 ratios could be synthesized using a precursor route. During crystallization, a phase transformation from [Al]-BEA to [Al]-SSZ-31 was observed, indicating dissolution of the beta phase, followed by its redirection towards the SSZ-31 phase in the presence of SDA. An increase in gel water helped to produce pure [Al]-SSZ-31. [Al]-SSZ-31 was found to crystallize in NaOH/SiO2 window of 0.040–0.056. Scanning electron microscopy (SEM) showed plate-like crystals of about 5 μm in size. 13C CP MAS NMR indicated that the SDA was intact inside the pores during crystallization. Furthermore, the samples were also characterized by 29Si and 27Al MAS NMR in addition to X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH3-TPD), SEM, etc.

Catalytic performance of silicoaluminophosphate (SAPO) molecular sieves in the isopropylation of biphenyl

Silicoaluminophosphate (SAPO) molecular sieves, SAPO-5, SAPO-11, SAPO-20 and SAPO-34, were successfully synthesized by a novel dry-gel conversion method. The isopropylation of biphenyl (BP) with propylene to selectively produce 4,4′-diisopropylbiphenyl (4,4′-DIPB) was investigated over these molecular sieves, with more emphasis on SAPO-5. SAPO-5 exhibited high selectivity for 4,4′-DIPB (65%) with high BP conversion (89%). SAPO-11 showed high selectivity for 4,4′-DIPB with low conversion, whereas SAPO-20 and SAPO-34 had only low activities for the isopropylation of BP. The activity was influenced by the acidity of the catalysts and the selectivity by their structures. The selectivity for 4,4′-DIPB was influenced by reaction temperature and by reaction period in the isopropylation of BP over SAPO-5. With the increase in temperature, the selectivity decreased; such a decrease was compensated by the increase of thermodynamically more stable 3,3′- and 3,4′-DIPBs, whereas the isomerization of 4,4′-DIPB was observed when the period was prolonged. However, the composition of 4,4′-DIPB in the encapsulated products was always higher than that in the bulk products. Propylene pressure had less effect on activity and selectivity. The isomerization of 4,4′-DIPB to 3,3′- and 3,4′-DIPBs was also studied over SAPO-5, and considerable isomerization was observed at high temperature (300 °C). Ceria modification of SAPO-5 was effective for enhancement of the selectivity for 4,4′-DIPB because of the decrease in the undesirable non-regioselective reactions on external acid sites.

Synthesis of AlPO4 Molecular Sieves with AFI and AEL Structures by Dry-Gel Conversion Method and Catalytic Application of Their SAPO Counterparts on Isopropylation of Biphenyl

AlPO4-5 and AlPO4-11 were synthesized by dry-gel conversion (DGC) method. Steam-assisted conversion (SAC) and vapor-phase transport (VPT) techniques were applied for this purpose. The synthesis was successful in presence of a certain minimum amount of external bulk water, without which the crystallization failed. Crystallization by VPT method was slower than corresponding SAC and HTS method. SAPO analogs of the samples, SAPO-5 and SAPO-11 were also synthesized by DGC method. Samples made by DGC methods had higher yield than the conventional hydrothermal synthesis (HTS); otherwise the samples showed similar characteristics as that made by HTS. XRD, SEM and N2-adsorption results showed high crystallinity and purity of the samples made by DGC, and 27Al MAS NMR spectra indicated the tetrahedral framework nature of Al. SAPO-5 and SAPO-11 were tested for their catalytic activity in isopropylation of biphenyl, and in terms of conversion and selectivity, SAPO-5 was found to be suitable for this application.

Synthesis of high-silica [Al]-SSZ-31 by a steam-assisted conversion method and its catalytic performance in the isopropylation of biphenyl

Synthesis of high-silica [Al]-SSZ-31 zeolite by a dry-gel conversion method was performed and the catalyst was tested for the isopropylation of biphenyl with propylene. In the case of the small-scale synthesis, the product phase was found to be dependent on the alkali concentration of the initial gel, and a phase change from BEA to SSZ-31 to MFI was observed with time at the optimum alkali concentration. In the case of the large-scale synthesis, temperature had a profound effect on the crystallization of SSZ-31. A phase transformation from BEA to SSZ-31 was observed when the crystallization temperature was changed from 150°C to 175°C. [Al]-SSZ-31 was active in the isopropylation of biphenyl and selectively catalyzed the production of the least bulky isomers. Biphenyl conversion decreased with increasing silica to alumina ratio while selectivity remained nearly constant. Triisopropyl isomer yields were high in the case of lower silica to alumina ratios.

Acidity of β zeolite with different Si/Al 2 ratio as measured by temperature programmed desorption of ammonia

High quality β zeolite (BEA) with a Si/Al 2 ratio of 30:70 was readily prepared by a dry gel conversion method. Acidity of the thus prepared β zeolite was measured by an improved technique of temperature programmed desorption of ammonia. Concentration of acid site, measured from the desorbed ammonia, was nearly equal to that of aluminum in the zeolite, and the enthalpy change of ammonia desorption, i.e., the strength of acidity, was 124–127 kJ mol −1, and independent of the concentration of acid site. The long tailing desorption of ammonia was distinct at higher temperature, and this was characteristic of BEA. The tail-like desorption spectrum may be correlated with the presence of strong acid site due to the defect or the tetrahedral site with different structural environments; the conclusion was supported by the characterization data using NMR, IR, and test reaction. Thus found solid acidity was compared with that of the commercially available β zeolite; the observed small difference was explained due to the presence of extra-framework Al.

Synthesis of AlPO4-5 and AlPO4-11 Molecular Sieves by Dry-Gel Conversion Method

Abstract Synthesis of aluminophosphate molecular sieves AlPO4-5 and AlPO4-11 has been studied by dry-gel conversion method using triethylamine and dipropylamine as structure-directing agents (SDAs), respectively. Amount of SDAs and the bulk water as the source of steam played vital roles during the crystallization. SAPO counterparts of the AlPO4’s were also synthesized by this method.

Synthesis of [Al]-SSZ-31 by Dry-Gel Conversion (DGC) Method

Abstract Synthesis of large-pore zeolite [Al]-SSZ-31 has been studied by dry-gel conversion method using 1, 8-diazoniaoctane type of structure-directing agent. Pure SSZ-31 was obtained with NaOH/SiO2 = 0.05-0.12, while the product was contaminated with MFI below or above this range. Snowtex-40 (colloidal), as the silica source yielded SSZ-31, while Cab-O-Sil M-5 and Ludox AS-40 yielded mostly MFI phase. A phase change: BEA→SSZ-31→MFI was observed with time course at optimum NaOH/SiO2.

Pure silica NU-1 and Na- and Al-free Ti-NU-1 synthesized by the dry gel conversion method

Pure silica NU-1 and its titanium silicate analog Ti-NU-1, free from Na and Al, have been synthesized for the first time using the dry gel conversion technique, which was not previously possible under the conventional hydrothermal synthesis procedure. These materials were characterized by XRD, FT IR, UV-vis, TG/DTA, SEM and N 2 adsorption measurements. The presence of a strong framework IR band at ∼960 cm −1 and an UV-vis diffuse reflectance band at 220–240 nm in Ti-NU-1 is an indication of highly dispersed tetrahedral titanium species in the silicate matrix.

Importance of hydrophobicity in the liquid phase oxidation catalyzed by titanosilicates

In the oxidation of alkanes, Ti-beta catalysts synthesized by the dry gel conversion method show higher turnover number and H2O2 selectivity than the conventionally synthesized ones, which is ascribed to higher hydrophobicity of the former. Trimethylsilylation of mesoporous materials is effective in enhancing the activity through increasing hydrophobicity. It has been revealed that the hydrophobicity of the catalysts whether microporous or mesoporous is a crucial factor in their activity in the liquid-phase oxidation of hydrophobic reactants such as alkanes and simple alkenes using aqueous H2O2 solutions as oxidant.

Synthesis of borosilicate zeolites by the dry gel conversion method and their characterization

Borosilicate zeolites, [B]-BEA, [B]-MFI and [B]-MTW, were synthesized by the dry gel conversion technique. Phase selection of the crystallization product depended on the gel composition, mainly upon the SiO 2/B 2O 3 and TEAOH/SiO 2 ratio. Nearly complete conversion of gel to zeolites was achieved by this synthesis method, and there was no phase change during the crystallization. It was observed from the 13C CP MAS NMR of the as-synthesized samples that TEA + was fully intact in [B]-BEA, whereas partial decomposition of the template occurred in [B]-MFI and [B]-MTW. FTIR and 11B MAS NMR analyses of the samples indicated that boron was located mainly in the tetrahedral framework position. Most of the tetrahedral boron could be removed and substituted by repeatedly refluxing the calcined sample with aluminum nitrate solution.

Conversion of dry gel to microporous crystals in gas phase

Recently the dry gel conversion (DGC) technique, where a hydrogel is dried and the resultant dry gel is converted into microporous crystals in steam or in a mixed vapor of steam and organic structure-directing agents (SDAs), has been developed. It has been shown that a wide variety of microporous crystals, pure silica microporous crystals, aluminosilicates, metallosilicates, and aluminophosphates, can be synthesized using the DGC method. Remarkable results have been reported in the synthesis of BEA types zeolites, namely aluminosilicate, titaniumsilicate, zincosilicate, and borosilicate with BEA topology, using tetraethylammonium hydroxide, a commercially available SDA. It has also been found that zeolite OU-1, probably analogous to SSZ-31 and NCL-1, is formed via phase transformation from BEA. Dense zeolite coatings like membranes are possible using this method. Characteristics of the DGC method are discussed in detail.

Oxidation Activity of Ti-Beta Synthesized by a Dry-Gel Conversion Method

Ti-beta zeolite was synthesized in the presence of sodium cations by a dry-gel conversion method using tetraethylammonium hydroxide as a structure-directing agent. As evidenced by diffuse reflectance UV spectroscopy, incorporated titianium species were tetrahedrally coordinated in the zeolites and neither anatase-like phase nor hexacoordinated titianium species belonging to an amorphous titanosilicate phase was detected at the sodium content of lower than 1.0 mol%. It was found that the Ti-beta catalysts gave very high H2O2conversion and selectivity in the oxidation of cyclohexene in methanol used as solvent. The C6-C8cyclic alkenes and alkanes reacted faster than corresponding linear alkenes and alkanes, showing the lack of steric impediments in the pores of Ti-beta. Dimethylcyclohexanes were more reactive when tertiary H atoms predominantly occupy the equatorial position than when they occupy the axial position. Ti-beta samples exhibited high activity also for the oxidation of C6-C8cyclic alcohols.

Properties of Ti-Beta Zeolites Synthesized by Dry-Gel Conversion and Hydrothermal Methods

The large-pore beta zeolite containing Ti has been synthesized in the presence of alkali cations (Na+) by a dry-gel conversion (DGC) technique using tetraethylammonium hydroxide (TEAOH) as a structure-directing agent. The obtained Ti-beta zeolites were compared with hydrothermally synthesized (HTS) Ti-beta. It was found that Ti-beta-DGC samples adsorbed less water (12−13 wt %) than Ti-beta-HTS (19 wt %), indicating that Ti-beta-DGC is more hydrophobic than Ti-beta-HTS. We propose that this leads to a higher activity for the former in the oxidation of cyclohexane. Upon being washed with H2SO4, the turnover number (TON) of DGC samples slightly increased for the oxidation of cyclohexane. DGC and HTS samples demonstrated no significant difference in the activity for oxidation reactions of unsaturated alcohols and C6−C8 cyclic alcohols.

Synthesis of Borosilicate Zeolites by Dry Gel Conversion (DGC) Method

Abstract Synthesis of borosilicate zeolites, namely [B]-BEA, [B]-MFI and [B]-MTW by dry gel conversion (DGC) method was described for the first time. Phase selection and thereby the outcome of the product was dependent upon the gel composition, mainly on the SiO2/B2O3 ratio and the TEAOH content.