Controllable Synthesis of SAPO-34 and Its Catalytic Performance in Methanol to Olefins

Author:Sai Ye De Ha Si En Syed ul Hasnain Bakht

Supervisor:zhu yu jun


Degree Year:2019





Ethylene and propylene(light olefins)are two most important petrochemical materials.Due to diminishing fossil fuel reserves copulated with growing environmental issues,it has increased demand for ethylene sources through an affordable,cheaper and benign process.The methanol-to-olefins(MTO)reaction has received more attention in the past decades and is considered as an alternative route for the production of light olefins such as ethylene and propylene,in which methanol is converted to light olefins in the presence of molecular sieves.CHA type SAPO-34 has attracted more attentions because of higher ethylene and propylene selectivity and excellent performance in the conversion of methanol to light olefins,due to its moderate acidity.The crystal size of SAPO-34 is an important parameter for the reaction of methanol to olefins,because a small crystal size is beneficial for the product diffusion,which can further slow the coke deposition.Based on this report,we have made some strategies and designed different routes to prepare a series of small sized SAPO-34 crystallites.Furthermore,their physical and chemical properties have been fully investigated,and the relationship between material structure and catalytic performance was also illustrated.Firstly,the selective synthesis of pure SAPO-5,SAPO-5/SAPO-34 mixture and pure SAPO-34 was performed by increasing Si/Al ratio and controlling pH,through hydrothermal synthesis route,in which the product was successively switched from AFI to AFI/CHA and further to pure CHA phase.These samples were characterized by XRD,XRF,SEM,and NH3-TPD techniques.It indicates that the SAPO-5/SAPO-34 mixture exhibits higher acidity than the pure SAPO-5 and SAPO-34 phases.During methanol-to-olefins reaction,the catalytic lifetime of the SAPO-5/SAPO-34 mixture was significantly longer than that of SAPO-34,which may be attributed to the moderate acidity of the SAPO-5/SAPO-34 mixture.Another sample with only physically mixed SAPO-5 and SAPO-34 were also synthesized,which presented lower acidity as well as much lower MTO lifetime than that of synthetically mixed phase.Secondly,SAPO-34 with sub-micrometer crystal size was synthesized by two-step hydrothermal treatment employing cost-effective morpholine as structure directing agent,which presented enhanced catalytic lifetime(nearly 3 times of conventional one)in the reaction of methanol to olefins with higher light olefin selectivity(total selectivity of97.1%).Detailed studies of the sample after different time interval in second crystallization with and without additional morpholine(MOR)were carried out and offered insight into crystal degradation and re-crystallization phenomena.The samples with different morpholine concentrations during second hydrothermal treatment were also prepared,in which the sample with 80%MOR aqueous solution exhibited the smallest crystal size and the longest MTO lifetime.Furthermore,the investigation about the addition amount of mother liquor(from first crystallization)required for second crystallization provided that the presence of half amount of mother liquor(nutrients)could give us the required results effectively.Finally,sub-micrometric sized SAPO-34 crystallites were also successfully synthesized by a facile surfactant-assisted hydrothermal synthesis route,in which cetyltrimethylammonium bromide(CTAB)was used as a crystal growth inhibitor(CGI).It was found that the crystal size changes in the V-shape with the increase in the amount of CTAB.The sub-micrometric sized sample(SP-0.02CTAB)was obtained when the molar ratio of CTAB to Al2O3 was 0.02 in a precursor gel.The SP-0.02CTAB catalyst presents excellent performance for the MTO reaction with 97.8%C2–C4 selectivity and 3times longer lifetime than the conventional SAPO-34 sample(SP-Conv),which is attributed to its much higher surface area and enhanced acidity derived from the formation of the sub-micrometric sized crystallites.Further study about the appropriate time of CTAB addition demonstrates that CTAB can play its role as a crystal growth inhibitor effectively,only when it is added at the very initial stage of the crystallization process.