Zeolites conventional hydrothermal synthesis

A continuous microwave equipment (CME) has been developed to achieve a rapid and mass production for ZSM-5 and NaY zeolite. A precursor mixture for synthesis of ZSM-5 was prepared by mixing aluminosilicate gel with a nanoseed solution obtained under microwave irradiation, and pumped into the CME. Duration time in the CME was 5 min to accomplish the crystallization of ZSM-5 under microwave irradiation. For NaY zeolite, the precursor gel without nanoseeds was introduced into the CME and crystallization time was within 30 min. XRD and SEM results indicate that the structural properties of ZSM-5 and NaY zeolite obtained are similar to those obtained using batch-type microwave instrument and by conventional hydrothermal synthesis. 

Conventional hydrothermal synthesis is the most common method for zeolite membrane preparation. In the in-situ crystallization method, a porous support is immersed in a zeolite synthesis solution. A membrane... 

Zeolite-supported Re catalysts have been synthesized by chemical vapor deposition (CVD) ofMTO (CH3Re03) (3) on various zeolites such as HZSM-5, H-Beta, H-USY and H-Mordenite. HZSM-5 samples with different A1 contents were prepared by a hydrothermal synthesis method. For comparison, conventional impregnated catalysts were also prepared by an impregnation method using an aqueous solution of NH4Re04. All catalysts were pretreated at 673 K in a flow of He before use as catalyst. 

Synthesis of gallosilicate zeolites [Ga]-beta, [Ga]-ZSM-5 and [Ga]-ZSM-12 was performed by dry-gel conversion (DGC) method. The crystallization of the dry gel was performed in presence of small amount of water, without which the crystallization failed. The method was convenient and as effective as conventional hydrothermal method. The samples were pure and highly crystalline, and showed characteristic of typical gallosilicate zeolites. 

The syntheses of mordenite zeolite crystals by microwave heating revealed that there is an accelerate transformation rate of mordenite crystals and also enhanced purity and surface areas of the crystals compared to conventional heating [llLl]. Highly crystalline and pure mordenite crystals were obtained after hydrothermal synthesis of 6 h at 190 °C by microwave heating, whereas prrre mordenite crystals could not be obtained even after hydrothermal treatment of 72 h at the same temperature and conventional heating. 

Synthesis of zeolites from polysilicates has been examined for two reasons. First, the particle sizes and shapes can be very different from those formed by conventional synthesis. For instance, the solid-state transformation of kanemite gels yields distinctly smaller silicalite particles that are more cubic in shape than by using hydrothermal synthesis. A second reason is the search of zeolite systems that can be shaped without needing binding additives. A possibility would be the transformation of raw materials into a zeolite after being shaped, as illustrated by Shimizu et al. [232]. 

Huang, A. and Yang, W. (2007) Hydrothermal synthesis of NaA zeolite membrane together with microwave heating and conventional heating. Materials Letters, 61, 5129-5132. 

In order to augment the quantity and quality of final synthesis yield as obtained from conventional hydrothermal activation, another modified method has been introduced which utihzes two different steps, an initial high temperature fusion of fly ash-alkah mixture, prior to employing the final stage of hydrothermal activation of the fused product. The main variables have been fusion temperature and time, alkali type and its concentration and crystallization time in hydrothermal synthesis process, which can affect the quahty and yield of final product. As such, it has been confirmed that the final yield can be quantified to exhibit zeolitic conversion up to 62 % together with by production of alkaline waste solution which can become a threat to the environment after disposal. A flowchart of the synthesis process is depicted in Fig. 3.3 [1, 2, 9, 10, 12, 43, 44]. 

In order to synthesize zeolites from fly ash by its activation with NaOH, attempts have been made to identify a suitable fly ash out of its two disposal sites (viz., dry site at the electrostatic precipitator and wet site at the lagoons in the thermal power plants) for conventional (i.e., one step) hydrothermal activation technique [1-10]. Subsequently, the fly ash ascertained to exhibit improved zeolitization potential has been prefered to undergo novel hydrothermal treatment processes (viz., three step activation by hydrothermal technique and three step fusions) to activate the fly ash significantly for synthesis of fly ash zeolites with high cation exchange cqjadfy [11-15]. The details of both the types of alkali activations (viz., conventional with the two ashes and three step activations with the superior ash) are presented in the following. 

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