Rational Design And Appilcaton Of Structure-Directing Agents For Zeolite Synthesis

Zeolites with large and extra-large pores have been paid great attention because of their importance in industrial applications such as in hydrocracking,catalysis,separation,drug delivery,etc.The past decade has witnessed the miraculous development and prosperity of the large and extra-large pore zeolites.However,large and extra-large pore silicate zeolites are still limited in the numbers,and their synthesis remains a challenge.The silicate zeolites are the most promising materials for industrial applications because of their good stability and chemical properties.Up to date,there are only 13 reported siliceous zeolites with pores more than 12MR channels amongst the 213.structurally-characterized zeolites.It is known that structure directing agents(SDAs)have play important roles in zeolite synthesis.The stability,rigidity,shape,size,hydrophilicity and some other properties of the SDAs all make great impact on directing the formation of zeolites with specific sturctue topology.Rigid aromatic imidazolium derivatives,which are highly anisotropic in shape and electronic density distribution,have been proved to be a class of efficient SDAs for zeolite synthesis.However,because of the limitation of their molecular size,most of the synthesized zeolites have small or medium pore structures.This thesis was aimed to synthesize new zeolites with specific pore channels,particularly those with large-and extra-larege pores,by using pre-designed novel organic imidazolium cations of certain size,geometry and hydrophobicity as structure-directing agents.The roles of the SDAs in the formation of zeolites will be studied,which would provide new insights into the crystallization of zeolites and guide us to design and synthesize novel extra-large pore zeolites with specific structures and properties.The main results are as follows:1.The recent research developments on the zeolite synthesis,particularly on the synthesis of large-and extra-large pores zeolites and the synthetic methods,were summarized.This led to the research proposal of this thesis that is based on the rational design of organic imidazolium cations of certain size,geometry and hydrophobicity as structure-directing agents for synthesis of large-and extra-large pores zeolites.2.By using imidazole-substituted quaternary boron compounds NaB(Im)4 and B(ImMe2)4(OH)3(Im=imidazole,ImMe2=2,3-dimethylimidazole)as SDAs in the zeolite synthesis,we successfully obtained single crystals of aluminophosphate AlPO-11 and germanosilicate ITQ-12,respectively.The crystal structure of ITQ-12 was for the first time solved from the single crystal diffraction data to give a more accurate and unambiguous structure solution in comparison with those of the previously reported powder diffraction data.Ex situ electrospray ionization mass spectrometry(ESI MS),19F and 11B NMR studies showed that these quaternary boron compounds acted as SDA buffers,undergoing the decomposition process to slowly release a low concentration of imidazole molecules under solvothermal conditions.The latter served as SDAs to form a limited number of zeolite nuclei,thus leading to the growth into large single crystals.Under similar conditions,the direct use of corresponding free imidazole molecules in the synthesis produced only fine powders.This approach could be potentially applied to synthesize other zeolites,given the availability of various quaternary boron compounds.3.A generalized synthesis of large and extra-large pore zeolites is presented based on supramolecular assemblies of semirigid aryl organic cations as SDAs.A series of aryl imidazolium cations with various substitutes,charges,sizes and geometries were prepared and used as SDAs for zeolite synthesis in the presence of Ge and F.They were shown to be efficient SDAs,producing a variety of zeolites including extra-large pore zeolites ITQ-37(30×3O×30)and ITQ-43(28×12×12),large pore zeolites ITQ-24(12×12×12),ITQ-21(12×lO×1O)and IM-20(12×lO×10),and small pore zeolites ITQ-29(8×8x8 but with extra-large cavities)and ITQ-12(8×8)as well as several unknown phases.The extra-large pore zeolites ITQ-37 and ITQ-43 as well as extra-large cavity ITQ-29 were obtained with high SDA concentrations in the gels and low crystallization temperatures,while the SDA-dilute gels and high crystallization temperatures resulted in the formation of other zeolites with smaller pores.XRD,elemental analysis,TGA,NMR and photoluminent studies showed that the formation of supramolecular assemblies via ?-? interactions between the aromatic rings contributed to the formation of extra-large pore zeolites.The presented supramolecular assemble SDA approach provides scope for synthesizing a range of large and extra-large pore zeolites in a cost effective and highly flexible manner.4.By using subsitituted benzyl-imidazolium cations as SDAs,we have successfully synthesized a new extra-large pore zeolite NUD-1 with interconnecting 18 x 12/10 x 12/10-membered ring channels.NUD-1 consists of isolated 3-membered ring,double-3-membered ring(D3R)and double-4-membered ring(D4R)units,which has not been observed in any other known zeolites.In addition,there is a preferential germanium occupancy in the D3Rs and D4Rs.The framework density of NUD-1 is 11.8 T atoms per 1000 A3,and its BET surface area and the t-Plot micropore volume are 646 m2/g and 0.26 cm3/g,respectively as measured by N2 adsorption.Other zeolites with a low framework density including ITQ-33,ITQ-44,ITQ-17 and ITQ-29 were also obtained by using these SDAs under different conditons.Photoluminescence studies confirmed that the formation of supramolecular assemblies via ?-? interactions between the aromatic rings contributed to the formation of extra-large pore zeolites.This study demonstrated that the use of aromatic-containing cations that self-assemble into supramolecular aggregates as SDAs can be employed to synthesize new extra-large pore zeolites.