Dry Gel Conversion Method Research Articles

A novel chemosynthetic method for the production of nitrate sodalite

This paper presents a novel method for the synthesis of nitrate sodalite from an uncalcined, dry Al 2O 3–2SiO 2 gel powder at low temperature. Nitrate sodalite particles were successfully synthesized utilizing a dry-gel conversion method (DGC) and characterized by SEM, HRTEM and SAED techniques. The mechanism of formation of the nitrate sodalite and its cation exchange capacity (CEC) were studied and the experimental results indicate that the nitrate sodalite particles were composed of numerous small grains, incompletely crystallized and arranged in a spherical cage-like structure.

Ambient Pressure Dry-Gel Conversion Method for Zeolite MFI Synthesis Using Ionic Liquid and Microwave Heating

We report an ambient pressure synthesis method for silicate zeolite. This method combines the advantages of ionothermal synthesis, dry-gel conversion, and microwave radiation, making it a promising, safe, fast, and continuous process for industrial application.

Dry gel conversion synthesis of SAPO-34 nanocrystals

SAPO-34 nanocrystals were synthesized by a dry gel conversion method using tetraethylammonium hydroxide as a structure-directing agent. The crystal growth of SAPO-34 was studied by X-ray diffraction and field-emission scanning electron microscopy. After 3 h, 45-nm SAPO-34 crystals with an amorphous phase were observed. The crystal size increased to 70 nm after 6 h, but did not increase greatly thereafter. The average crystal size of the final product was 75 nm. The nucleation density for SAPO-34 crystals in dry gel conversion appeared to be much higher than that under hydrothermal conditions, resulting in the formation of small crystals.

Transalkylation of diisopropylbenzenes with benzene over hierarchical beta zeolite

The catalytic performance of the hierarchical beta zeolite prepared via the dry gel conversion method was compared with that of the conventional beta zeolite for transalkylation of diisopropylbenzene with benzene. The results indicate that diisopropylbenzene conversion decreases from 74% to 29% after a reaction time of 32 h over the conventional beta zeolite. However, for the hierarchical beta zeolite, diisopropylbenzene conversion still remains above 60% after 32 h under the same reaction conditions. In order to learn the factors leading to the difference in catalytic activity, we studied pore structure, crystal size and acidity of the hierarchical beta zeolite and the conventional beta zeolite and we discussed their effects on catalytic activity. The results reveal that the better activity and stability of the hierarchical beta zeolite is mainly due to the presence of meso/macropores and nanocrystals for the liquid transalkylation reaction.

Synthesis of Si-MCM-41 from ternary SiO2–CTAOH–H2O system via dry gel conversion route

Steam assisted dry gel conversion method was employed for the synthesis of Si-MCM-41 from ternary SiO2:CTAOH:H2O systems wherein fumed silica was used as a source of silica. The influence of synthesis time, molar ratios of CTAOH/SiO2 in dry gel and the water content at the bottom of autoclave on the quality and formation of mesophases has been investigated. Powder XRD, N2 adsorption–desorption, TEM and hydrothermal stability test were the techniques used for sample characterization. Keeping molar ratio of CTAOH/SiO2 constant, shorter synthesis time, lower unit cell parameter and d spacing were observed when steam assisted dry gel conversion method was employed in place of conventional hydrothermal method. There exists an optimum lower limit for water content at the bottom of autoclave for reducing the synthesis period. Keeping synthesis temperature and CTAOH/SiO2 molar ratio fixed, Si-MCM-41 with improved hydrothermal stability was obtained by steam assisted dry gel conversion route.

Dry gel conversion synthesis of SAPO- and CoAPO-based molecular sieves by using structurally related preformed AlPO precursors as the starting materials

Several Si and Co substituted AlPO 4-based molecular sieves with AFI, AEL and CHA structures including SAPO- n and CoAPO- n (where n = 5, 11 and 34) have been synthesized under dry gel conversion (DGC) conditions by using the pre-prepared AlPO-based precursors which have been demonstrated previously to be structurally related to the target molecular sieves. We show that highly crystalline SAPO- n and CoAPO- n can be prepared by this approach. Unlike conventional DGC method where the Si source is mixed with P and Al sources at the very beginning to form an initial amorphous dry gel, in this work the Si is slowly introduced to the surface of the dry powder of the semi-crystalline or crystalline AlPO-based precursor via vapor phase, and subsequently incorporated into the framework gradually. The Si environments in the SAPOs prepared by this approach appear to be more diversified compared to those in the corresponding sieves made by the normal hydrothermal synthesis (HTS) method. One feature of this synthesis is that the Si substitution in the lattice may only occur at some specific positions. This is due to the fact that since certain P sites in the precursors are directly carried into the framework of the final sieves, they are not available for Si incorporation. This finding may have implication to the locations of the catalytic sites in the SAPO-based catalysts. Similarly, for CoAPO- n, the Co source was mixed with the preformed AlPO-based precursor before DGC treatment. CoAPO-5 has a higher degree of the Co incorporation than that of the CoAPO-5 made under normal HTS conditions.

The <I>β</I>-Zeolite Synthesized by Dry-Gel Conversion Method Without the Use of Sodium Hydroxide: Characterization and Catalytic Behaviors

The nano-sizedbeta-zeolites were synthesized first time by dry gel conversion (DGC) method without the use of sodium hydroxide. Resultant beta-zeolites had particle size of 35-80 nm with SiO2/Al2O3 ratios in the range of 25-400. They had BET surface areas in the range of 290-750 m2 x g(-1) and the external surface area of 41-283 m2 x g(-1): cumulative surface areas were in the range of 420-1050 m2 x g(-1). Beta-Zeolites modified with alkaline earth and rare earth metal species, such as Ca, La, and Ce, were synthesized by the introduction of these metal nitrates during the dry-gel preparation. To evaluate catalytic properties of these zeolites, they were applied for the isomerization/cracking of hexane and the phenol octylation. The influence of Ca, La, and Ce oxides on the catalytic properties was also examined.

Nanosized β-zeolites with tunable particle sizes: Synthesis by the dry gel conversion (DGC) method in the presence of surfactants, characterization and catalytic properties

Nanosized β-zeolites were synthesized using the dry gel conversion (DGC) method with tetraethylammonium hydroxide (TEAOH) as a structure-directing agent (SDA) in the presence of the surfactants: cetyltrimethylammonium bromide (CTMABr), chloride (CTMACl) and hydroxide (CTMAOH). The presence of the surfactant in the synthesis gel creates a confined environment and controls the formation of nanosized β-zeolites with SiO 2/Al 2O 3 ratios of 25–200. The β-zeolites have uniform particle sizes of 35–60 nm with large BET and external surface areas. The particle size of the β-zeolites depends on the concentration and type of counter-anion of the surfactant used. The catalytic properties of nanosized β-zeolites, mainly concerning the role of internal and external acid sites, were evaluated by the isomerization/cracking of hexane and phenol alkylation with 1-octene.

Tin-silicalite-1: Synthesis by dry gel conversion, characterization and catalytic performance in phenol hydroxylation reaction

Tin-silicalite-1 (Sn-MFI) has been successfully synthesized by dry gel conversion (DGC) method. The influence of various synthesis parameters such as crystallization time, temperature, water content at the bottom of autoclave, molar ratios of (TPA) 2O/SiO 2 and SiO 2/SnO 2 in dry gel on the crystallization kinetics and physico-chemical properties of Sn-MFI materials has been investigated. Powder-X-ray diffraction (XRD), XPS, AAS, DRUV–vis and SEM techniques were employed as characterization tools. Time-dependent studies revealed that, the progressive crystallization of Sn-MFI depend not only on the synthesis time but also on the other various synthesis parameters. Increase in the crystallization temperature, water content at bottom of autoclave (TPA) 2O/SiO 2 and SiO 2/SnO 2 molar ratios in dry gel resulted in the lowering of overall crystallization period. A good correspondence between the chemical composition of the surface and the bulk indicated the uniform distribution of Sn 4+ ions. The presence of these ions in tetrahedral coordination has been indicated by DRUV–vis spectral studies. Although, not better but comparable activity has been shown by Sn-MFI prepared by dry gel conversion method with Sn-MFI prepared by conventional hydrothermal crystallization method in the hydroxylation of phenol reaction. Increase in concentration of isolated Sn 4+ ions led to increase in phenol conversion and H 2O 2 efficiency.

Synthesis of silicalite-1 by dry-gel conversion method: factors affecting its crystal size and morphology

Silicalite-1 crystals were synthesized from clear and diluted solutions by the dry-gel conversion method and the hydrothermal method was used as reference to do a comparative study. The effect of the structure-directing agent, silica source, aging time and crystallization time was investigated. Zeolite crystals, about 200–500 nm and 1 micron, were obtained by the liquid phase hydrothermal (LPH) and steam-assisted crystallization (SAC), called as dry-gel conversion method, respectively. The crystals were identified by XRD and SEM. The results show that the aging time and the water content are crucial in the synthesis of zeolites. Indeed, well formed zeolite crystals are obtained by the SAC technique using a lower amount of water than in the LPH method. However, the presence of the structure directing agent (SDA) into the dry gel determines the crystallization of silicalite-1.

Novel synthesis of lace-like nanoribbons of boehmite and γ-alumina by dry gel conversion method

Pure lace-like nanoribbons of AlOOH boehmite have been fabricated by an innovative dry gel conversion without the presence of catalysts and templates. Through steam-assisted crystallization at 200 °C for 48 h, the dried powder of amorphous aluminium hydroxide gel was transformed to lace-like nanoribbons in length of 1–2 μm and width about 100 nm as well as thickness of 5–6 nm. After calcination at 600 °C for 5 h, γ-Al 2O 3 was obtained and the morphology of lacelike was preserved.

Shape-selective alkylation of biphenyl over metalloaluminophosphates with AFI topology

Metalloaluminophosphates with AFI topology (MAPO-5; M: Mg, Ca, Sr, Ba, and Zn) synthesized by the dry gel conversion (DGC) method were used as catalysts for the alkylation of biphenyl (BP). MgAPO-5 and ZnAPO-5 had high catalytic activities of the isopropylation; however, activities of CaAPO-5, SrAPO-5, and BaAPO-5 were much lower than those of MgAPO-5 and ZnAPO-5. These results suggest that the catalytic activity of MAPO-5 molecular sieves was originated by the acidity due to the isomorphous substitution of M 2+ for Al 3+. The selectivities for 4,4′-diisopropylbiphenyl (4,4′-DIPB) for all MAPO-5 molecular sieves are at the level of 70%. On the basis of various reaction behaviors, the predominant formation of 4,4′-DIBP was considered to be mainly due to the steric restriction on the transition state of the isopropylation to the least bulky isomer inside the channel of MAPO-5 molecular sieves. The isopropylation of BP over MgAPO-5 and ZnAPO-5 was accompanied by the isomerization of 4,4′-DIPB to thermodynamically more stable isomers, 3,4′-DIPB at the higher temperatures; the decrease in the selectivity for 4,4′-DIPB was observed in both bulk and encapsulated products. These results show that 4,4′-DIPB formed inside channels was isomerized inside the channels in addition to external acid sites. The channels of MAPO-5 molecular sieves are large enough to allow the isomerization of 4,4′-DIPB especially at higher temperatures: this is quite different from those of H-mordenite. The sec- and tert-butylations of BP over MgAPO-5 and ZnAPO-5 gave selectively the least bulky isomers, 4,4′-di- sec-butylbiphenyl (4,4′-DSBB) and 4,4′-di- tert-butylbiphenyl (4,4′-DTBB), respectively. The selectivity for the least bulky isomer increased with increasing the bulkiness of alkylating agents in the order: isopropylation < sec-butylation < tert-butylation. This order reflects the spatial constraints of the transition sates inside the AFI structure.

Synthesis of SBA-15 mesoporous silica via dry-gel conversion route

Dry-gel conversion method has been successfully applied for the synthesis of mesostructured silica SBA-15 materials with the particle size in submicron range. The effects of duration of self-assembly and dry-gel conversion on the structure of SBA-15 are investigated. Submicron particles with uniform mesoporous structure were formed during the first 2 h of static hydrolysis and self-assembly. The particle morphology and pore properties were not obviously influenced by the duration of self-assembly. Without dry-gel conversion, the mesoporous structure significantly shrank after calcination due to its weak stability of mesostructure. After steam-assisted dry-gel conversion at 100 °C, the pore wall is consolidated without damaging the mesostructure because of the support of surfactant micelles inside the pore channels. The larger surface area and pore volume of mesostructured silica were well preserved after calcination. In addition, the surface morphology and particle size were not affected by dry-gel conversion, and submicron particles of mesostructured silica SBA-15 materials were obtained after calcination at 550 °C.

Dry-Gel Synthesis of Mesoporous MCM-41 Materials with Modified Pore Structure

Abstract For the first time, the dry-gel conversion method was applied for the preparation of MCM-41 materials with partially crystalline framework. The X-ray diffraction pattern of the material obtained after a dry-gel conversion time of 36 h consists of reflections of both long-range ordered MCM-41 material and zeolite ZSM-5. The nitrogen adsorption/desorption isotherm confirms the presence of micropores and uniform mesopores with a narrow pore size distribution. By 29Si MAS NMR spectroscopy it was evidenced that a significant portion of the amorphous MCM-41 framework was transformed into a partially crystalline framework. TEM and SEM images indicate the occurrence of mesopores with disturbed arrangements, but no formation of separate zeolite ZSM-5 crystals occurred. By 1H MAS NMR spectroscopy of the above-mentioned material, the formation of hydroxyl groups with a chemical shift of ca. 4 ppm and a concentration of ca. 0.1 mmol/g was observed. The acid strength of these hydroxyl groups, as studied by adsorption of the 13C MAS NMR probe molecule 13C-2-acetone, was found to agree with that of bridging OH groups in zeolite H-ZSM-5. After conversion times of 48 h and 72 h, an extensive to total transformation of the dry gel into zeolite ZSM-5 occurred.

Magnesioaluminophosphate molecular sieves with ATS topology: Synthesis by dry-gel conversion method and catalytic properties in the isopropylation of biphenyl

MgAPO-36, a magnesioaluminophosphate molecular sieves with ATS topology, was successfully synthesized by means of dry-gel conversion (DGC) method. Heating protocol during the synthesis was a key factor for the crystallization of pure MgAPO-36. The best results were obtained by heating at 140 °C for 24 h after aging of the gel at 105 °C for 48 h. However, ATS phase without magnesium was not obtained under our synthesis conditions because of co-crystallization of AFI phase. MgAPO-36 with high surface area was obtained by the calcination of as-synthesized samples in nitrogen stream or in vacuo; however, the calcinations in air stream resulted in decrease of surface area due to collapse of microporous structure. Characterization was performed by XRD, NH 3-TPD, TG-DTA, SEM, N 2 adsorption, and ICP analyses. Brønsted acidic character appeared in MgAPO-36 because of the replacement of a part of Al 3+ with Mg 2+ in the framework. The selectivity for 4,4′-diisopropylbiphenyl (4,4′-DIPB) was about 30–35% among DIPB isomers in the isopropylation of biphenyl (BP) over MgAPO-36: this value is lower than MOR and other zeolites with straight channels. Because the selectivity of 4,4′-DIPB is higher than the composition in equilibrium, the catalysis is controlled by the pores of MgAPO-36. The relative low selectivity for MgAPO-36 is not due to their pore entrance, but to differences in their pore structure. ATS channel with annular side pocket is too loose for selective formation of the transition state to 4,4′-DIPB, and the formation of bulkier transition states for the other isomers, such as 3,3′- and 3,4′-DIPB, are also allowed inside the channel.

Seeding on the Synthesis of MCM-22 (MWW) Zeolite by Dry-Gel Conversion Method and its Catalytic Properties on the Skeleton Isomerization and the Cracking of Hexane

The effects of seeding on synthesis of MCM-22 (MWW) have been studied by the dry gel conversion (DGC) method. Highly reproducible crystalline MWW zeolite could be obtained by DGC method, using seeds of calcined sodium MWW with SiO2/Al2O3=31. Crystallization of MWW phase was completed in the presence of seeds (2 mass% against SiO2) at 150°C within two days. The small amount of seed (0.25 mass% against SiO2) was sufficient for the required phase formation of MWW. The MWW with SiO2/Al2O3 ratio from 35 to 66 was obtained as pure phase; however, the crystallinity of MWW decreased with further increase in the ratio: products were contaminated with impurities. The catalytic properties of MWW were examined in the skeleton isomerization and the cracking of hexane, and compared to zeolites such as BEA, MFI, and MOR. The catalytic activities were in the order: MFI>BEA>MWW>>MOR; however, the selectivity for the isomerization were in the order: BEA>MWW>MFI. These catalytic properties were due to the differences of structures and acid strength of zeolites.

Zincoaluminophosphate Molecular Sieves with AFI and ATS Topologies: Synthesis by Dry-Gel Conversion Methods and Their Catalytic Properties in the Isopropylation of Biphenyl

Zincoaluminophosphates, ZnAPO-5 and ZnAPO-36 with AFI and ATS topologies, were successfully synthesized by dry-gel conversion (DGC) method: vapor-phase transport (VPT) and steam-assisted conversion (SAC) methods. ZnAPO-5 was synthesized successfully at 175°C by VPT and SAC methods using Et 3 N as structure directing agent (SDA). ZnAPO-36 was also obtained by VPT and SAC methods using Pr 3 N as SDA. The heating protocol was a key factor for the crystallization in the synthesis of ZnAPO-36: the best result was obtained by heating at 140°C for 1 d after aging of the gel at 105°C for 2 d. Calcination procedure is important to get microporous molecular sieves. ZnAPO-5 by VPT method calcined at 550°C gave clear XRD with high microporosity. Highly microporous ZnAPO-36 by VPT method was obtained by careful calcination. Characterization of these zincoaluminophosphates was performed by XRD, NH 3 -TPD, TG, SEM, N 2 adsorption, and ICP analysis. ZnAPO-5 and ZnAPO-36 have the Bronsted acidic characters by the substitution of a part of aluminum with zinc. The isopropylation of biphenyl (BP) over these molecular sieves gave different level of the selectivities for 4,4'-DIPB: 60-75% for ZnAPO-5 and 35-45% for ZnAPO-36: these selectivities are in the similar level to corresponding magnesioaluminophosphates corresponding magnesioaluminophosphates. These differences of ZnAPO-5 and ZnAPO-36 in the selectivity are due to the pore structure: ZnAPO-5 has one-dimensional straight channel; however, ZnAPO-36 has one-dimensional channel with side pocket. The steric restriction by ZnAPO-5 effectively controls the transition state to differentiate the slim isomers from others; however, pores of ZnAPO-36 cannot differentiate effectively the isomers because of their side pockets.

Synthesis of Ti-MWW by a dry-gel conversion method

MWW type titanosilicate, Ti-MWW, has been synthesized by the dry-gel conversion (DGC) method, and its physicochemical properties and catalytic performance in the liquid-phase epoxidation of alkene have been compared with that of hydrothermally synthesized (HTS) Ti-MWW. The roles in the crystallization of silica source, alkali cation, cyclic amine as a structure-directing agent (SDA), and boric acid structure-supporting agent have been investigated. The crystallization of Ti-MWW did not occur for the dry gels free of boric acid, but was feasible at a Si/B molar ratio as high as 12 in marked contrast to the ratio of 0.75 required in the hydrothermal synthesis. The sodium as a mineralization agent was not necessary and on the contrary inhibited the crystallization particularly at a high content. The seeding technique using deboronated MWW effectively accelerated the crystallization speed and reduced the amount of boric acid required. As-synthesized Ti-MWW-DGC lamellar precursors contained both tetrahedral and octahedral species but the latter was selectively removed by acid treatment. Ti-MWW-DGC catalysts showed lower intrinsic activity than Ti-MWW-HTS in the epoxidation of hex-1-ene with hydrogen peroxide probably because the crystal size of the former was 10–20 times as large as that of the latter and then imposed significant diffusion problems for both the substrates and the products.

Synthesis of AFI aluminophosphate molecular sieves partly substituted with magnesium by dry-gel conversion method

Aluminophosphate molecular sieves with AFI topology, in which framework Al 3+ is partly substituted with Mg 2+ (MgAPO-5), were synthesized by dry-gel conversion (DGC, including vapor-phase transport [VPT] and steam-assisted conversion [SAC]) method using triethylamine (Et 3N) as a structure-directing agent (SDA). Systematic studies were carried out in DGC method using different compositions of gel (concentration of Mg and SDA), amount of external bulk water and period of synthesis. MgO/Al 2O 3 ratios in the range of 0.025<MgO/Al 2O 3<0.10 gave pure MgAPO-5, and further increase in Mg-content caused contamination of MgAPO-34 with CHA topology. The amount of SDA played an important role, synthesized with a SDA/Al 2O 3 ratio of 0.38–0.76. Very minute amount of external bulk water was necessary for the crystallization. Appropriate crystallization time was 12–18 h. Incorporation of Mg 2+ into the framework was suggested by generation of acid sites and the change of unit cell parameters.

Dry-gel synthesis of zeolites [Al]EU-1 and [Ga]EU-1

The dry-gel conversion method using hexamethonium bromide (HMBr) as template allows to prepare zeolites [Al]EU-1 and [Ga]EU-1 (framework type EUO) with n Si/ n Al ratios of 18–142 and n Si/ n Ga ratios of 12–86, respectively. The most critical parameters of the dry-gel synthesis of zeolites EU-1 are the amount of water present in the autoclave during the crystallization process and the contents of sodium cations and template molecules in the dry gel. Zeolites EU-1 with a high crystallinity were obtained with a water content in the autoclave of m H 2O / m gel⩾0.67 and in the presence of a liquid water phase in the pores of the solid material during the conversion process occurring at T=453 K. For a typical dry gel with an n Si/ n Al ratio of 41, contents of sodium cations n Na/ n Si and template molecules n HMBr/ n Si of 0.3 and >0.11, respectively, are necessary to obtain zeolite [Al]EU-1 without impurities such as zeolite EU-2, amorphous phases, or α-quartz. The concentration of Brønsted acid sites as determined by 1H MAS NMR spectroscopy of dehydrated zeolites H-EU-1 shows a linear dependence on the contents of aluminum or gallium. The strength of the Brønsted acid sites in the obtained zeolites H-EU-1 was investigated by adsorption of 13C-2-acetone as probe molecule and application of 13C MAS NMR spectroscopy. The experimentally obtained chemical shifts of 13C-2-acetone adsorbed on zeolites H-[Al]EU-1 and H-[Ga]EU-1 of 214.9 and 213.3 ppm, respectively, indicate a higher acid strength of the aluminum-containing zeolite in comparison with the gallium-containing material.