Clear Synthesis Solutions Research Articles

Temperature and time controlled crystallization in Na2O–SiO2–TiO2–H2O system

Phase formation upon hydrothermal crystallization of sodium titanosilicates has been studied in detail. It was shown that an excessive amount of Na and Si with respect to Ti affects the crystallization. It demonstrates that in a clear starting solution, high crystalline microporous titanosilicate (CST, Na 2 Ti 2 SiO 7 ·2H 2 O) with sitinakite structure can be formed in 6 h and the crystallization is controlled by nucleation. With less amount of water, CST can be formed in 2 h. At low NaOH content, ETS-4 forms in 2 h at 230 °C. By simply increasing NaOH content in starting mixture, the crystallinity of CST improved. With increasing synthesis time CST re-crystallized to AM-4 with unusual hierarchal morphology. The synthesis temperature has a clear influence on the phase formation. At low synthesis temperature, a material with ivanyukite structure (GTS-1) was obtained in 4 h at 150 °C. Additionally, we discussed evidence for CST/GTS-1 intergrowths. The rapid crystallization of the microporous titanosilicates CST (TAM-5), GTS-1, ETS-4, and layered AM-4 can be achieved by adjusting NaOH content, synthesis temperature and time. • The rapid crystallization of the microporous titanosilicates CST (TAM-5), GTS-1, ETS-4, and layered AM-4 has been achieved. • The rapid formation of high crystalline CST from clear synthesis solution is controlled by nucleation. • The first observation of CST to AM-4 phase transformation is revealed. • The evidences of temperature controlled CST/GTS-1 intergrowth are provided.

Silicalite-1 synthesized with geothermal and Ludox colloidal silica and corresponding TiO2/silicalite-1 hybrid photocatalysts for VOC oxidation

Abstract Silicalite-1 samples of uniform sub-micron size with two distinct morphologies were prepared using colloidal silica extracted from geothermal fluids. The colloidal silica used was provided by a New Zealand Company, GEO40 Ltd, and was produced by them using their patented process for recovering colloidal silica from geothermal brine. Spherical crystals with an average size of ca. 250 nm (S1N) as well as typical MFI-type coffin-shaped crystals with a size of ca. 900 nm (S1M) were prepared from clear synthesis solutions made from colloidal silica and aqueous tetrapropylammonium hydroxide of different concentration. The silicalite-1 samples prepared showed very similar characteristics to samples prepared using a conventional silica sol, Ludox SM, as a silica source. Silicalite-1 samples prepared with geothermal and conventional silica sols were used as supports for TiO2 nanoparticles. The hybrid TiO2/silicalite-1 materials were tested as photocatalysts for the photocatalytic degradation of trichloroethylene for air pollution control. The hybrid samples reduced the formation rates of non-desirable reaction products, promoting the mineralization process in comparison with a reference TiO2 xerogel and high Si/Al ratio ZSM-5-based hybrid materials.

Preparation of high‐performance zeolite NaA membranes in clear solution by adding SiO2 into Al2O3 hollow‐fiber precursor

Zeolite NaA membranes were prepared in a clear synthesis solution without the aid of nanoseeds. To improve the properties of the membranes formed in a clear solution, alumina hollow fibers were fabricated by adding silica powder to the conventional spinning slurry, resulting in hollow fibers with a mullite phase. Prior to the membrane synthesis, the hollow fibers were pretreated by dipping in an aged synthesis solution diluted with isopropanol. Dense zeolite NaA membranes on mullite‐containing alumina hollow fibers were successfully obtained at 100°C for 2 h without the aid of nanoseeds. The membranes have a good pervaporation performance with a high flux of 10.8 kg m−2 h−1 and a separation factor of over 10,000. The abundant mullite‐phase hydroxyl groups on the support surface promote the nucleation and growth of zeolite crystals on the support, resulting in dense membranes. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2679–2688, 2018

Ultrathin zeolite X membranes for pervaporation dehydration of ethanol

Discrete Faujasite zeolite nanocrystals with an average size of about 60nm were attached as a well-defined monolayer on the surface of porous graded alumina supports. Ultrathin zeolite X films with a total thickness of about 1μm were grown from the seed monolayers by hydrothermal treatment in clear synthesis solutions. One of the membranes showed a total flux of 3.37±0.08kgm−2h−1 and a separation factor of 296±4 for dehydration of a 90/10wt.% ethanol/water mixture by pervaporation at 65°C. Moreover, the membranes displayed stable performance during pervaporation for 5.5h operation. A mathematical model indicated that the flux and selectivity of the membranes were limited by pressure drop in the supports. Therefore, in order to obtain higher flux, the permeability of the support must be improved.

Structured zeolite NaX coatings on ceramic cordierite monolith supports for PSA applications

Novel structured adsorbents in the form of thin zeolite films grown on substrates designed for low pressure drop have a great potential to improve pressure swing adsorption (PSA) processes. In the present work, template free films of NaX zeolite were grown on the walls of ceramic cordierite supports using a seeding technique. The supports had 400 parallel channels per square inch. Films were grown both from a gel and a clear synthesis solution. The materials were analyzed by scanning electron microscopy, X-ray diffraction, N 2 adsorption/desorption measurements, Hg-porosimetry as well as CO 2 breakthrough experiments. When a gel was used for film growth, a film consisting of well intergrown crystals with a thickness of about 1 μm was obtained. However, a large amount of sediments were deposited on top of the film, which resulted in a dispersed CO 2 adsorption breakthrough front. Zeolite films grown in one longer hydrothermal treatment in a clear solution were less intergrown and consisted of both NaX and hydroxysodalite crystals and, in addition, some sediments were deposited on top of the film, which again resulted in a dispersed breakthrough front. By using a multiple-step synthesis procedure and a clear synthesis solution, well intergrown NaX films, free from sediments and with only a very small fraction of hydroxysodalite crystals could be prepared. The CO 2 breakthrough front for the latter adsorbent was sharper than the front for an empty adsorption column and only shifted in time. This indicates that the flow distribution in the adsorbent is even and that the mass transfer resistance in the film is very low due to the small film thickness and high effective diffusivity for CO 2 in the NaX film and still, the adsorption capacity is considerable. The even flow distribution, very low mass transfer resistance and low pressure drop in combination with considerable adsorption capacity in this adsorbent indicates that it is a promising adsorbent for PSA applications. The findings from the present work will be important for the development of structured adsorbents to use as a competitive alternative to traditionally used adsorbents in PSA.

Crystallization of silicalite‐1 from clear synthesis solutions: Effect of template concentration on crystallization rate and crystal size

AbstractSynthesis of silicalite‐1 powders and membranes from initially clear solutions with different tetrapropylammonium hydroxide or bromide concentrations was studied. While tetrapropylammonium bromide acts only as template, tetrapropylammonium hydroxide provides both the template and hydroxyl ions to the synthesis medium. The effects of template and hydroxyl ion concentration on the product yield, crystallization rate and crystal size were investigated. Pure and highly crystalline silicalite‐1 was obtained with all compositions. The nucleation time decreases from 100 h to 20 h and the crystal size decreases from 3.5 μm to 0.35 μm as the template amount x is increased from 5 to 30 moles at a batch composition of 80SiO2.xTPAOH.1500H2O at 95 °C. Yield of silicalite‐1 passes through a maximum at intermediate TPA concentration. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancement of NaA zeolite membrane properties through organic cation addition

Thin NaA zeolite membranes, with uniform and small crystals, were prepared on the tubular α-Al 2O 3 support by adding a small amount of tetramethylammonium hydroxide (TMAOH) in the clear synthesis solution. The as-synthesized NaA zeolite membranes were characterized by XRD and SEM. The permeation properties of the membranes were evaluated by pervaporation and gas permeation. The effects of TMAOH amount on membrane formation and permeation properties were investigated. By addition of suitable amount of TMAOH in the clear synthesis solution, the crystals size of NaA zeolite could be remarkably reduced from about 10 μm to 3–4 μm, and the membrane thickness correspondingly reduced from about 16 μm to 5 μm. The thinner membrane prepared by adding TMAOH in the clear synthesis solution, with uniform and small crystal, displayed higher perm-selective properties than that without adding TMAOH. For the as-synthesized NaA zeolite membrane prepared with adding suitable amount of TMAOH ( x = 1), the separation factor (water/isopropanol) was 4700 and the flux was 1.67 kg/(m 2 h), which were higher than that without adding TMAOH of 339 and 1.08 kg/(m 2 h), respectively. The ideal separation factor of H 2/N 2 was 6.60, higher than that without adding TMAOH of 3.41.

Silicalite-1 crystallization on glass fiber filter discs

Silicalite-1 was in situ crystallized from clear synthesis solutions onto commercial millipore glass fiber filters. The filters with diameter 2.5 cm were soaked in the precursor mixtures and hydrothermally treated at 80 °C or 150 °C for time up to 150 h. After the synthesis, the filters were treated in an ultrasonic bath for 15 min with a 0.1 M ammonia solution. The disc composites prepared at 80 °C swelled during synthesis, were easy to disintegrate upon subsequent manipulations and wrinkled when dried. The calcined materials were brittle with a porcelain-like appearance. The filters synthesized at 150 °C, both as synthesized and calcined, seemed identical in shape and mechanical characteristics to the initial glass fiber filters. Prolongation of the treatment at 150 °C up to 100 h led to densified fiber discs according to the SEM study but without visual changes in their mechanical strength. The difference in crystallization at the two different temperatures was attributed to the slower nucleation process at 80 °C, which resulted in greater dissolution of the fiber units and a certain inhibition of the crystallization process caused by the changed chemical environment at the fiber–synthesis solution interface. The silicalite-1/filter composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering (DLS), chemical analysis and N 2 adsorption measurements.

Preparation of composite carbon–zeolite membranes using a simple method

Zeolite thin films and composites supported on different materials have been widely used as highly selective membrane separators, membrane reactors, chemical sensors, and microdevices [1–11]. In particular, zeolite membranes supported on polymers and ceramics as supports are universally studied. However, polymers are vulnerable to organic solvents and could not tolerate high temperatures. Ceramics provide better mechanical strength and thermal stability, but are more difficult to fabricate and therefore are more expensive. Moreover, ceramics also have a little etching of aluminum by a strong alkaline solution [1]. Porous carbon materials are promising candidates as supports for zeolite membranes because they not only possess comparable mechanical strength, thermal characteristics of metallic supports, and excellent resistance to chemical attack, but also exhibit organic substrate’s flexibility of forms and lower capital cost [10, 11, 12]. Zeolite films grown on porous carbon materials can modify the nature of the porous carbon itself and present different potential advantages in a number of applications. On the other hand, the pores of porous carbon materials can be narrowed down to a desired uniform subnanometer size by the zeolite growth on the porous carbon surface, which is one of the most difficult but a fascinating task that carbon researchers have to attempt [12]. The main difficulty is that there are few nucleation centers on the hydrophobic carbon supports, which make it difficult for zeolites to nucleate and grow on them from the hydrophilic zeolite synthesis solution. Up to now, there are few studies on the growth of zeolites on the activated carbon and hollow fibers as supports by using the oxidizing pretreatment of the carbon materials to increase the oxygen functional groups and the surface change in the carbon materials with a cationic polymer for the zeolite growth [11, 13–15]. Recently, continuous zeolite/carbon composites are obtained by electrophoretic deposition of zeolite nanocrystals on porous carbon disks, then followed by hydrothermal synthesis [16]. In this communication, we report a simple and effective method for preparation of continuous carbonsilicalite-1 membranes on porous carbon tubes. A 1%wt zeolite silicalite-1 colloidal suspension in ethanol prepared from the recipes described by Yeung and coworkers [9]. Porous carbon tubes prepared at Dalian University of Technology have 9 mm OD, 5 mm ID, 75 mm long, and a nominal pore size of 0.3 μm. Prior to use, the tubes were rinsed with DDI water and then dried at 393 K for 10 hr. A thin layer of zeolite seeds was coated onto the inner surface of the tubes using a slip-casting technique in 1wt% seed suspension for 30 s contact time. The seeded supports were dried at room temperature overnight and calcined in air at 523 K for 6 hr. After calcination, the seeded tubes were immersed vertically in the clear synthesis solution with a molar composition of 20SiO2:2.5 (TPA)2O:10 000H2O at 403 K for 36 hr. After the synthesis, the sample was characterized by scanning electron microscopy (SEM, JEOL JSM-6300F) and X-ray powder diffraction (XRD, Philips PW 1030). Fig. 1 is the SEM images of the prepared carbon– zeolite composites. Without seeding, no zeolite film is formed on the untreated carbon support except for the very few crystal deposits (Fig. 1a). This reflects that the carbon surface is extremely inert and zeolites are unable to grow on the untreated carbon support. It is clear from Fig. 1b that after the support is seeded by using slip-casting in seed ethanol suspension, the seeds are successfully introduced onto the carbon surface. The carbon surface is fully covered with 5 μm thick layer of silicalite-1 seeds. This not only makes the carbon surface even and smooth, but also provides anchoring centers for zeolite growth, which both favors forming a continuous and dense membrane on the carbon support. After hydrothermal synthesis, a continuous zeolite membrane of 5–6 μm thickness is formed on this seeded support as shown in Fig. 1c. The zeolite crystals grow outward from the surface of the seed layer. From the surface (Fig. 1c inset), the zeolite membrane is well intergrown and possesses a smooth and uniform surface. Under the conditions of 1 bar and room temperature, the membrane shows no N2 permeation. This fully proves that the carbon–zeolite membrane is dense and perfect. XRD patterns of the samples in Fig. 2 show that both the seeded carbon support and the membrane have strong characteristic peaks with a silicalite-1 structure [17], but with different relative peak intensities. This confirms the existence of a seed layer on the support and the formation of the silicalite-1 zeolite layer on the seeded support.

Synthesis of ZSM-5 zeolite membranes with large area on porous, tubular α-Al2O3 supports

Large-area ZSM-5 zeolite membranes were successfully prepared on porous α-Al 2 O 3 supports with optimized synthesis solution composition and synthesis conditions. The membranes were achieved by repeated hydrothermal synthesis with cheap NBA ( n -butylamine) as template and from clear synthesis solution. Not only the membranes were prepared on tubular supports, but also the membrane area is up to 260 cm 2 . All the membranes were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM) and single-gas permeation. As a result of these measures, the yield of high-quality membranes with a minimum permselectivity of H 2 to n -C 4 H 10 of 27 (which is five times the Knudsen factor) as a quality criterion was increased to 50% of all preparations.

Formation of Silicalite-1 Hollow Spheres by the Self-assembly of Nanocrystals

A simple approach is reported for the preparation of hollow spheres with a 10−20-nm-thick shell of silicalite-1 nanocrystals. The nanocrystals were produced by the steaming of silicalite-1 nanoprecursors (NPs), collected with the help of surfactant from a clear synthesis solution immediately following the induction period. The nanocrystals produced were ≈10−20 nm with a BET surface area of 440−470 m2/g and an external surface area >112 m2/g. A water adsorption isotherm confirmed that the nanocrystals were hydrophobic in nature. These nanocrystals self-assembled into hollow spheres of 100−300-nm diameter when ultrasonicated in ammoniac ethanol.

Role of the water content of clear synthesis solutions on the thickness of silicalite layers grown on porous α-alumina supports

In situ hydrothermal synthesis of silicalite layers on macroporous α-Al 2O 3 disks were performed from a batch composition of 6.5Na 2O:25SiO 2: xH 2O:6.9TPABr by varying the water content of a clear crystallization solution from 500 to 2000 moles at 200 °C. As the water content of the clear solution was increased, continuous and thinner silicalite layers formed from smaller crystals were obtained and the layer quality was improved. The silicalite layer which was synthesized from the most concentrated batch was formed from crystals with an average size of 100 μm and had a thickness of 36 μm. The layer thickness and average crystal size decreased to about 8 and 7 μm, respectively, when the batch containing 1400 moles of water was used for synthesis. The results showed that the layer thickness on porous alumina supports could be adjusted by varying the water content of the synthesis solution. As-synthesized membranes were impermeable to N 2 after a single crystallization step. The membranes were tested for possible pinholes and microcracks by measuring single gas permeations of N 2 and SF 6 after calcination at 500 °C by a slow and step-wise calcination procedure. The calcined membranes showed an N 2 permeance of 16.3 mmol/(bar m 2 s) and an ideal selectivity of 1630 for N 2/SF 6 at room temperature. The high selectivity showed that the silicalite layer is free of large interzeolitic pores and the applied calcination procedure is suitable for TPA removal without forming microcracks.

Synthesis and characterisation of MFI-type zeolites supported on carbon materials

A wide variety of silicalite/carbon and ZSM-5/carbon composites have been prepared by conventional hydrothermal synthesis. The influence of the surface chemistry and porosity of the carbon supports on the growth of the zeolite has been studied. The presence of oxygen surface groups on the carbon supports has been found to be necessary to allow the anchorage of the zeolite layer on the carbon. In the cases of carbons with few surface groups an oxidising pretreatment favours the interaction between the synthesis solution and the carbon surface. Different synthesis conditions have been tried to evaluate their influence on the final properties of the composites. The use of clear synthesis solutions and the ageing time before the hydrothermal synthesis are key steps for the successful synthesis. The burn-off of the zeolite/carbon fibres and the zeolite/carbon cloths composites yields pure zeolite hollow tubes.

Investigations of secondary nucleation by initial breeding in clear solution zeolite NaA systems

An investigation of the effects of adding seed crystals to a clear synthesis solution, which produced molecular sieve zeolite NaA, was carried out. The initial breeding secondary nucleation mechanism, whereby microcrystalline fragments are washed off seed crystal surfaces to catalyze zeolite crystal nucleation, was studied using SEM, powder X-ray diffraction, and quasi-electric light scattering spectroscopy (QELSS). It was determined that small seed crystals (about 1–3 μm) did not promote nucleation of a new population of zeolite crystals when added to a clear solution synthesis batch. However, much larger zeolite NaA crystals (about 40 μm) were demonstrated to promote zeolite crystal nucleation by an initial breeding mechanism. Small particulates were demonstrated to exist only on the larger crystal surfaces, could be washed off, and were observed in a filtrate solution by QELSS. Lastly, it has been shown that the source of these particulates is residual aluminosilicate material in the bulk solution phase which was not incorporated into the crystal mass at the time the seed crystals were synthesized. As such, stopping the syntheses of smaller seed crystals prematurely was sufficient to cause the formation of initial-bred nuclei in those samples.

Preparation of Zeolite ZSM-5 Membranes by In-Situ Crystallization on Porous .alpha.-Al2O3

Zeolite ZSM-5 membranes were grown on porous α-Al_2O_3 disks by in-situ hydrothermal synthesis at 175 C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. Extensive experimentation was carried out to find compositions that gave continuous polycrystalline films. The films grown with the optimized composition were about 10 µm thick and consisted of well-intergrown crystals of about 2 µm size. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185 °C, respectively.

Formation of bimodal zeolite crystal populations from clear synthesis solutions

Several recent reports on molecular-sieve zeolite synthesis from clear aluminosilicate solutions have indicated that bimodal crystal-size distributions were produced. This curious result was attributed to agitation or heterogeneous nucleation in one case. It is suggested by results presented here that bimodal crystal-size distributions may be the result of ageing the clear solutions at room temperature.