Font Size: a A A

Seeded Syntheses Of Zeolites In The Absence Of Organic Templates

Posted on:2011-04-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:B XieFull Text:PDF
GTID:1101360305453545Subject:Inorganic Chemistry
Abstract/Summary:PDF Full Text Request
Zeolites with intricate micropores as an important class of industrial materials in different areas of the chemical industries such as gas adsorption, ion exchange, shape selective catalysis have been extensively studied for a long time. The early works for synthesis of zeolites such as LTA and FAU zeolites are carried out in aluminosilicate gels containing inorganic cations, and mordern synthesis methodologies typically involve the use of organic structure-directing agents (SDA) that direct the assembly pathway and ultimately fill the pore space. For example, ZSM-5 and Beta zeolites are generally templated from tetrapropylammonium (TPA+) and tetraethylammonium (TEA+) cations, respectively. Notably, the use of organic templates not only increases the cost of zeolites but also results in consumption of energy as well as produces the harmful gas during the removal of the templates normally carried out by high temperature combustion. In some cases, the high temperature is also detrimental to the inorganic framework of zeolites.To reduce the use of organic templates, the scientists are to recycle organic templates in the syntheses of zeolites. For examples, Takewaki et al. reported that partial tetraethylammonium cations (TEA+) in the synthesis of BEA-type zeolite can be recycled by ion-exchange procedure, but the organic cations interacted strongly with the zeoltie framework still cannot be removed. Jones et al summarized SDA removal in BEA-type zeolites containing various heteroatoms via solvent extraction, and it was found that organic cations strongly bonded to the framework through charge-balancing interactions at the heteroatom sites can only removed with concomitant hydrolysis of the Si-O-M (where M=B, Zn, Al) bonds in acidic media. Furthermore, Lee et al. showed a complete recycle of organic template in the synthesis of ZSM-5. They use a SDA that can be disassembled within the zeolite pore space to allow removal of their fragments for use again by reassembly. This SDA contains a cyclic ketal group that could be removed inside the zeolite without destruction of the inorganic framework.The alternative route for solving the problems brought with organic templates is to avoid the use of organic templates for synthesizing zeolites normally requried organic templates (organotemplate-free synthesis). The first example for organotemplate-free synthesis of ZSM-5 was discovered by Li et al., and they found that ZSM-5 with good crystallinity was successfully prepared in Na2O-SiO2-Al2O3-H2O system. Later, Clearfield et al. reported that the factors of Si/Al and Na/Al ratios are key for the organotemplate-free synthesis of ZSM-5 zeolite. Recently, Song et al. showed a successful organotemplate-free synthesis of large-pore aluminosilicate zeolite of ECR-1 by carefully adjusting the molar ratio of Na2O/SiO2 in the synthesis, and Wu et al. reported an organotemplate-free synthesis of ZSM-34 zeolite from an assistance of zeolite L seeds solution. ZSM-34 and ECR-1normally synthesized in the presence of organic templates of choline [(CH3)3NCH2CH2OH] and bis-(2-hydroxyethyl)dimethylammonium chloride.Beta Zeolite with unique three-dimensional network of large pores (12-ring) 19 exhibits excellent properties in a series of catalytic reactions such as the petroleum refining and fine chemical synthesis, but its wide applications are still challenging by the use of organic templates such as TEAOH in the synthesis, compared with Y zeolite usually synthesized in the absence of organic templates. Notably, Beta structure as one of natural zeolites has been found in the mineral in the last decade, which offers a possibility to synthesize Beta zeolite in the absence of organic templates because natural zeolites do not contain organic compounds such as TEA cations.In chapter two, we reported primary results for organotemplate-free route for synthesizing Beta zeolite by addition of about 8.8 % seeding crystals (weight ratio of the seeds with aluminosilicate solid) in the starting aluminosilicate gels at 140°C. And the whole crystallization process of Beta-OTF samples based on XRD and SEM technique. According to the XRD intensity points of Beta-OTF140 samples crystallized at 140°C for certain time on the amount of Beta seeds (2.8-16.1%) stood on a line, the line almost pass through coordinate origin (0,0), suggesting that the Beta seeds are necessary for the synthesis of Beta-OTF. And the experiment proved that also. In the chapter three, we show a careful investigation on organotemplate-free synthesis of Beta zeolite (Beta-OTF), served as a model for the seeded synthesis of zeolites in the absence of organic templates. The results show that Beta-OTF crystals with core-shell structure grow up from Beta seeds (core) in amorphous aluminosilicate gels, which proved by XRD, HRTEM, EDS, SEM, EDX and XPS technique. According to the ICP analysis, the Si/Al in Beta-OTF was near to 5.4, which was very similar with the natral Beta zeolite in Tschernichite. Compared with conventional Beta templated by TEAOH, Beta-OTF exhibits large BET micropore surface area and stable 4-coordinated Al sites, which would be potentially important for design and preparation of stable and active zeolite catalysts.In chapter four, we used the proposed core-shell growth mechanism successfully synthesized EON and HEU zeolites by organotemplate-free method. The EON and HEU zeolites 1st synthesized were also can be recycled as seeds for the 2nd run, which supplied good evidence on avoiding organic template for zeolites synthesis.In chapter five, we synthesized four new micropous materials and two titanosilicat micropous materials by IEZ (Interlayer Expanded Zeolites) method, which named COE-1, COE-2, COE-3, COE-4, Ti-COE-3 and Ti-COE-4 (BASF-International Network of Centers Of Excellence). The nitrogen isotherm shows the COE samples have characteristic of Langmuir adsorption, which due to the filling of micropores. Further more, Ti-COE-4 was very active for oxidation of cyclohexene reaction compared with Ti-MWW. Finally, because of the relative position of the silane fixed, the framework of COE-1 was showed by Prof. Gies H in the use of XRD refinement method, which was the first example framework of zeolites gotten by IEZ method.
Keywords/Search Tags:Aluminosilicate Zeolites, Organotemplate-free synthesis, Seeds, Mechanism, Oxidation of cyclohexene reaction
PDF Full Text Request
Related items