Hydrothermal synthesis procedure

Some interesting devices, both on classical (46) and miniamrized scale (47, 48) have been developed for the combinatorial hydrothermal synthesis of zeolites (49,50). Solutions of suitable precursors were dispensed into the wells of the multireactor autoclaves, which were then sealed and heated following appropriate protocols. X-ray characterization of the individuals allowed the determination of the properties of the library individuals. Both protocols and instruments allowed the synthesis of hundreds of composites and their automated (47) or semiautomated (46, 48) work-up and characterization. Structural information was derived from both reports, proving the applicability of combinatorial technologies to hydrothermal synthesis procedures. 

Basically, the hydrothermal synthesis procedures used to prepare zeolite membranes can be classified in two general groups ex situ and in situ methods, that is with and without a previous seeding step, which are briefly discussed below. 

Detailed description of the synthesis procedures as weh as thorough characterisation of the materials whl be described elsewhere. For both materials, hydrothermal synthesis procedures... 

The synthesis procedure led to a highly crystalline MCM-22 zeolite, as indicated by XRD data (Fig. 1A, a). The introduction of platinum by ion exchange procedure does not modify the crystallinity of the zeolite (Fig. 1A, b). SEM micrographs show that the sample obtained in static hydrothermal conditions is characterized by the presence of homogeneous round-shaped particles of ca. 10 pm in diameter formed by aggregation of lamellar particles [7,8], The morphology of the zeolite is not modified after platinum introduction (Fig. IB). 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

Zeolite synthesis around a metal complex was introduced by Balkus [40]. The metal complex is added to the zeolite synthesis mixture and is incorporated into the zeolite structure during the zeolite synthesis. Of course, this procedure is only applicable when the metal complex is soluble in the synthesis mixture and can withstand the hydrothermal synthesis conditions. Another requirement is that the zeolite structure-directing agent added must be removable by a milder... 

The synthesis of MCM-41 was carried out according to the following molar compositions of Si02 NaOH CTMABr Acid H20= 1 x y z 100, where Acid stands for HF or HC1, and molar ratios x, y, and z can be changed accordingly. A detailed synthesis procedure has been reported elsewhere [12]. For the hydrothermal stability studies, 0.5 g of the solid calcined at 550 °C for 10 h in air was tested in 60 g of boiling water in a plastic bottle for several days. 

Synthesis of MCM-41 with Additives. The hydrothermal crystallization procedure as described earlier [10] was modified by adding additional salts like tetraalkylammonium (TAA+) bromide or alkali bromides to the synthesis gel [11]. Sodium silicate solution ( 14% NaOH, 27% Si02) was used as the silicon source. Cetyltrimethylammonium (CTA) bromide was used as the surfactant (Cl6). Other surfactants like octadecylltrimethylammonium (ODA) bromide (C,8), myristyltrimethylammonium (MTA) bromide (C,4) were also used to get MCM-41 structures with different pore diameter. Different tetralkylammonium or alkali halide salts were dissolved in little water and added to the gel before addition of the silica source. The final gel mixture was stirred for 2 h at room temperature and then transferred into polypropylene bottles and statically heated at 100°C for 4 days under autogeneous pressure. The final solid material obtained was washed with plenty of water, dried and calcined (heating rate l°C/min) at 560°C for 6 h. 

Research efforts have thus been made to improve the hydrothermal stability of PSM by either changing the synthesis procedure or post synthesis modification. The addition of various sodium salts into the gel mixtures was reported to form MCM-41 having disordered structures, making MCM-41 relatively stable in boiling water for 12 h [5]. The hydrothermal stability of MCM-41 was also found to be improved by adding cations such as tetraalkylammonium or sodium to the synthesis gel [6] or by modification with organic molecules in the synthesis [7], MCM-41 could be stabilized... 

The current work is focused on application of thermogravimetry in qualitative characterization of structure of MCM-41 samples prepared using the hydrothermal restructuring method [22-25] and direct synthesis procedure involving hexadecyldimethylamine as an expander [25,26]. It is demonstrated that the weight-change patterns of as-synthesized samples provide qualitative information about phase purity, stability upon calcination and the resulting structure of the calcined samples. 

The MCM-41 sample DS-BC was prepared via direct synthesis procedure at 353 K [22] using the synthesis gel composition reported earlier [27], The HR-Bx and HR-Cx series of samples were prepared via the postsynthesis hydrothermal restructuring of materials prepared under the same conditions as DS-BC in the mother liquor at 433 and 443 K, respectively, for... 

Simple silica matrices To further illustrate the limit of this chapter, we briefly discuss one type of materials that will not be systematically reviewed layered lanthanide silicates obtained by hydrothermal synthesis. An alkaline solution of sodium silicate and lanthanide chlorides was stirred to produce a gel which was subsequently put into an autoclave under pressure at 230 °C during seven days, a procedure during which lamellar photoluminescent silicates... 

The synthesis procedure for DMS silicas was described elsewhere [3.4]. The postsynthesis hydrothermal treatment was carried out by replacing the mother liquor with pure water after the normal synthesis. Typically, the as-synthesized DMS silica was mixed with a certain amount of pure water, which was equivalent to about 20ml of water per gram of dried uncalcined samples, transferred into an autoclave, and kept in the oven maintained at 373K and 423K, respectively, for different periods of time. Finally, the solid product was filtered, washed, dried, and calcined as in the normal preparation. 

In this work the primary pore structure of the beads was varied according to a hydrothermal treatment procedure (step 2). In order to control the secondary pore structure of the agglomerates, the primary particle size has to be adjusted. This can be carried out varying synthesis parameters (step 1) like the concentration of ammonia and water [4], the chain length of the alcohol [5] and the synthesis temperature [6],... 

A new type of mesoporous silica has been prepared via a dual-template, thiee-step hydrothermal-flocculation-steaming (HFS) synthesis procedure. This material showed 780 mVg of BET surface area and 0.6 ml/g of primaiy mesopores narrowly distributed around 4.2 nm. More importantly however, is that it showed short-range MFI zeolite crystallinity as demonstrated by FTIR and XRD analysis, and hydrophobicity as demonstrated by water and n-hexane adsorption. 

From our earlier experiences, we have learned that the zeolite seed hydrothermal approach is rather difficult to reproduce. Consequently, a dvee-step-synthesis procedure involving the preparation of zeolite nanoprecursors (NPs) by a short hydrothermal step, the flocculation of these NPs using a sui ctant, and the steaming of the NPs/surfactant composite to produce the final material was developed. We have recently demonstrated that aggregates of less than 30 nm silicalite nanocrystals can be prepared from this procedure. We further discovered that the nature of the as-collected NPs was very much dependent on the stirring time of NPs/CTAMeBr flocculants. Under identical steaming condition, the 3 h-stirred NPs were converted into nanocrystals of silicalite-1, whereas. 

Various redox metals, including Ti, V, Cr, Mn, Fe, Co, Cu, Zn, As, Zr and Sn, have been incorporated into microporous materials such as silicalites through hydrothermal synthesis by the addition of the respective cations to the synthesis gel. The disadvantages of this method include the time-consuming optimization of synthesis procedure for each metal-zeolite combination and the necessity of A1 for crystallization of certain structures. The presence of A1 leads to Bronsted acidity... 

Layered double hydroxides are typically synthesized from coprecipitation of mixed metal cations by base titration either with or withont hydrothermal treatment which nsnally enhances crystalline properties." " Becanse contamination with airborne carbonate ions freqnently occurs during synthesis procedures, special caution is needed to prepare carbonate-free LDHs. On the other hand. 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

The layered silicates hectorite and magadiite were synthesized using hydrothermal procedures based on those described in 111. For hectorite which contains lithium and magnesium atoms in octahedral sites between the silicate sheets, this synthesis procedure was modified. The crystallizations were carried out at temperatures ranging from 100°C to 200°C. The compositions of the reaction mixtures (expressed as molar ratios of the oxides) were ... 

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