| The light olefins such as ethylene and propylene have been the important raw materials for organic chemical and petrochemical industry.However,due to the rising price of petroleum caused by the consumption of resources,the development of the olefins industry is highly restricted for China.In recent years,along with the MTO(Methanol To Olefins)/MTP(Methanol To Propylene)technology unceasing development and mature,this technology has been able to large-scale industrial production.Because methanol is easily obtained from cheap raw material such as coal or gas following the modern technology,the cost of MTO/MTP is far lower than that of traditional methods.ZSM-5 has the highest propylene selectivity compared with other catalysts in the methanol to propylene process.The catalytic performance of zeolite is closely related with its skeleton structures;therefore the catalyst performance of ZSM-5 in MTO/MTP can be modified through using different synthesis methods.Even though it is the main means of synthesis Zeolite for the scientific research and industrial production,hydrothermal synthesis method still has some shortcomings.For example,the high-cost of hydrothermal synthesis come from the expensive template agent in the process of synthesis.Therefore it is important to find a suitable alternative method to synthesize zeolite.Recently,we adopted a new synthesis route-the solvent-free synthesis route which has been developed by professor Xiao Feng-shou in Zhejiang university-to prepare zeolites.In this paper,other than applying sol-gel synthesis method,a newly developed "solvent-free" synthesis route was chosen to prepare ZSM-5 samples.Under the solvent-free synthesis condition,the crystallization time is quite short(ca.24 h)and the solid raw materials are mechanically mixed and heated at 180 ?.In this synthesis route,the key factor is the utilization of NH4HF2 because its dissociation products are HF and NH3 at high temperature and NH3 molecules react with H2O to produce the OH-mineral agents.The mineralizing component(OH-)and F-in zeolite synthesis converts the reactants into mobile and reactive silicate species,thus enabling the synthesis to proceed,and it facilitates the transformation process from totally amorphous material to ordered zeolite crystalline.Through the whole high temperature heating period the solid raw materials were not melted.The short crystallization time and the solid state of as-synthesized materials let in situ solid-state NMR studies of the zeolite crystallization process be possible.In a zeolite synthesis gel these interactions between the organic and inorganic components are not clear and the key condensation steps in its crystallization process are hard to be identified.Consequently,the crystallization mechanism of zeolite is hard to be understood,and no a universal mechanism can describe all zeolite crystallization processes.ZSM-5(MFI topology),as one of the most commercially important zeolites,is very important to figure out its crystallization mechanism.In the structure of pure silica ZSM-5(Si-ZSM-5)the parallel,linear channels are intersected by sinusoidal channels and the TPA molecules are located at these channel intersections with the propyl chains extending into both the linear and sinusoidal channels.Through a series of advanced techniques,especially through the nuclear magnetic resonance(NMR)techniques,the crystallization mechanism of ZSM-5 has been analyzed and we suggested that the Coulombic interactions between the positive charges of TPA and the negative charges in the framework such as the F anions and Si-O...OSi hydrogen bonds oriented the crystallization process.The two-dimensional 1H DQ-SQ MAS NMR method is a powerful tool for detecting nuclear proton-proton spatial proximity.The technique utilizes the same-core dipole-dipole coupling interaction to correlate the multi-spin dipole coupling network,and the two spin correlations dominate.The 1H DQ-SQ MAS NMR NMR spectra of ZSM-5 provide a wealth of information on proton proximity in ZSM-5.The interaction between the methyl group and the silicate backbone of TPA has been clearly demonstrated on the 1H DQ-SQ MAS nuclear magnetic resonance spectrum.Our synthesis method is more dependent on the position of fluorine in the zeolite skeleton,and the acquisition of a series of solid nuclear magnetic resonance spectra will help us to better understand the role of fluoride ions.The fluoride ions were directly bonded to one of the silicon atoms of the silicon tetracyte ring by the 19F MAS and 19F-29Si CP/MAS NMR to form the five-coordinated silicon SiO4/2F-.The SSNMR technique was used to confirm the distance between the TPA of the template and the negative charge center on the skeleton,and the template TPA was identified by Columbic interaction to synthesize the molecular sieve ZSM-5 with special pore... |
PDF Full Text Request