Fabrication Of Core-shell Structured AC@ZSM-5Catalysts And Their Application In Fischer-Tropsch Synthesis

To develop the catalytic materials with core-shell structure (denoted as "core@shell") can avoid the catalysts deactivation due to the coalescence, poison or loss of active species. It also provides a new way to couple different catalytic processes. Owing to the catalytic and separate properties of zeolite membrane, encapsulating solid catalyst particles by zeolite membrane can realize the integration of catalysis and separation process and improve their overall performance. Activated carbon (AC) is a typical catalyst or supports. And with the further understanding of its pore structure and surface physicochemical properties, AC will obtain increasing attention in chemical industry. As mentioned above, coating a zeolite membrane shell on AC can be expected to maintain the stabilities of AC-based catalysts. But for the roughness and hydrophobic property of AC surface, it is difficult to coat zeolite membranes on AC particles with direct hydrothermal synthesis. The objectives of this work are to develop a method to coat continuous and dense MFI-type zeolite membrane on the surface of irregular AC particles. Based on successfully fabricated composite material AC@ZSM-5, active species of Co and Zr are introduced into AC and the prepared catalysts (CoZr/AC@ZSM-5) are investigated in Fischer-Tropsch synthesis (FTS). The effects of the zeolite membrane shell on the reaction will be discussed. The main contents of the dissertation are as follows.After the direct hydrothermal synthesis, the zeolite crystals were loosely spread on AC particles with much of the surface uncoated. To increase the functional groups of AC surface, AC particles were pre-treated by air oxidation, concentrated nitric acid, urea solution and cationic polymer. Although the pretreatment enhanced the crystals coverage on AC, the attached zeolite crystals were still not sufficient to form a membrane. Secondary growth with the help of organics coking and intermediate alumina layer could obtain a better zeolite encapsulated AC particle. An important discovery was zeolite membrane fully covered AC particle could be prepared on the surface of boehmite gel/alumina coated AC particles, even no seeds deposited.Continuous and dense ZSM-5membranes encapsulated AC particles (AC@ZSM-5) were obtained by using intermediate boehmite gel layer with dynamic hydrothermal treatment. No powder mixed with final products was observed. Zeta potential tests indicated that there were more TPA-silicate precursors exist on the surface of boehmite gel coated AC (AC/Gel) than that on AC. In order to study the growth of membrane, a series of samples under different hydrothermal durations were synthesized and then examined with SEM. Results show that zeolite crystals nucleate at the support/solution interface rather than in solution with the help of the intermediate layer. Optimum conditions for synthesis of AC@ZSM-5were identified through synthesis and characterization of these core-shell structured particles prepared with varying the mass ratio of gel to AC and the times of coating.Template removal process of as-synthesized AC@ZSM-5was investigated. Serious cracking and peelings occurred during calcinations. Calcined tests indicated some cracks of membrane shell appeared with the temperatures in the range of200～250℃. By coating a secondary beohmite gel layer onto AC@ZSM-5, crack free samples were obtained. Combined the thermal gravity analyses (TG) with the temperature-programmed desorption/mass spectrum (TPD/MS) techniques, it was indicated that template contained in AC core and membrane shell decomposed in different way. For as-synthesized AC@ZSM-5, AC particles are wrapped by a compact membrane shells. As the temperatures increase, the template adsorbed in AC pyrolyzed first and the decomposed products like tripropylamine would accumulate in AC core. Subsequently, the inner pressure of the composite is strong enough to break through the membrane shell. An additional outer gel layer could increase the strength of the zeolite shell significantly and made crack free after calcination.1,3,5-Triisopropylbenzene (T/PB) adsorption tests suggested that defects like inter-crystalline gaps exist in shell membrane due to the shrink of crystals after template removal. Part of defects could be modified by using tetraethylorthosilicate (TEOS) or template-free secondary growth.In order to test the potential use of core-shell structured AC@ZSM-5in Fischer-Tropsch synthesis, the active metals of cobalt and zirconium were introduced by incipient impregnating AC particles with Co(NO3)2-6H2O and solution. The obtained particles were denoted as CoZr/AC. Following the membrane synthesis using the intermediate layer as mentioned previously, the particles, named as CoZr/AC@ZSM-5, were obtained. For comparison, ZSM-5powders with the same Si/Al atomic ratio as the membrane shell were shaped to20～40mesh and used to mix with the CoZr/AC catalysts. The weight ratio of ZSM-5to CoZr/AC was1:10, which was identical with the mass ratio of membrane contained in CoZr/AC@ZSM-5. The FTS results of bare CoZr/AC, mixed CoZr/AC-ZSM-5and encapsuled CoZr/AC@ZSM-5were summarized. On the basis of the reaction results, the effects of the zeolite membrane shell on the reaction were illustrated.