Small Organic Molecule Assisted Synthesis Of Inorganic Porous Materials And Their Studies On Performances

Zeolites, with the well-known physicochemical characteristics (e. g. individualmicropores, large internal surface areas, tunable acidity and high hydrothermalstability), have been successfully applied in separation, ion-exchange and catalysis. Inparticular, the uniform micropore channels throughout the whole zeolites frameworkplay an irreplaceable role in the shape-selective catalysis, but simultaneously, alsoseriously limit their further applications in bulky molecules-involved procedures.Mesoporous materials with adjustable pore sizes of2~50nm (e. g. MCM-41,MCM-48, SBA-15and SBA-1) may offer more accessible active sites in junction withfaster mass transfer for effective conversion of large molecules. Regrettably, thesematerials suffer from weak acidity and poor stability as a result of their intrinsicamorphous nature of pore walls. Despite tremendous efforts, the optimized frameworkcrystallinity is still far from those of crystalline zeolites. More recently, mesoporouszeolites, with positive features of native microporous zeolites as well as mesoporousmaterials, have become a hot point in both scientific researches and industrialapplications. Several strategies, involving amphiphilic organosilanes, cationicpolymers, inorganic nanoparticles and post-treatments, have been successfullydeveloped for the synthesis of mesoporous zeolites. For instance, Ryoo’s groupdeveloped a complicated amphiphilic organosilane featuring a long-chainalkylammonium moiety and a hydrolysable methoxysilyl group, with which theysynthesized highly mesoporous MFI zeolites with tunable mesopore structure. Besides, Verboekend et al. successfully created large amounts of intracrystalline mesoporosityin bulk MFI zeolites by desilication treatment in alkaline medium followed by acidwashing. With respect to these methods, the procedural complexity and/or economicfeasibility may conflict with the modern industry expectations.To begin with, we present our detailed work on an organic template-freesynthesis of ZSM-5zeolites by a one-step hydrothermal method. Organic templatefree method (OTFM) is considered as the best choice for actual synthesis of ZSM-5zeolites due to the low cost and environment friendness. And much effort has beenmade in further developing this kind of synthetic route over the past few years.Regrettably, there still remain some issues to be solved. Typically, OTFM-basedZSM-5zeolites usually suffer from poor crystallinity and/or presence of impurityphases. Besides, available range of silica-alumina ratio is much narrower than that ofzeolites products achieved by organic template methods. It is believed that intensivestudy on the synthetic regular pattern of OTFM would be of great importance for thecontrollable synthesis, performance investigations and also commercial production ofZSM-5zeolites. To this end, we systematically studied the influence of synthesisconditions involving hydrothermal parameters, alkalinity, silica-alumina ratio andconcentration on the ZSM-5products under organic template free synthetic system.The experimental results clearly reveal that all these mentioned factors have greatimpacts on the final hydrothermal products, and we need to synthetically adjust theseconditions to yield desired ZSM-5zeolites.Secondly, we report on a novel synthesis of hierarchical porous ZSM-5zeolite byaddition of N-methyl-2-pyrrolidone (NMP) organic micromolecule into abovetemplate-free ZSM-5synthesis system. The resulting hierarchical ZSM-5presents aregular house-of-cards-like (HCL) morphology (~10μm), which consists of someorthogonal sheet-like crystals (~100nm thichness). To the best of our knowledge, thisis the first observation of such a complex morphology for ZSM-5zeolites synthesizedwithout presence of quaternary ammonium salts. The morphologic evolution wascarefully studied by time-consumed experiments, which shows that the formation ofHCL structure is a typical step-by-step epitaxial intergrowth process. Although the detailed developing mechanism is still unclear, the formation of house-of-cards-likeZSM-5(HCL-ZSM-5) may be related with the defect intergrowth. With large externalsurface and strong acidity, HCL-ZSM-5exhibits much better catalytic performancesin cracking of cumene and1,3,5-triisopropylbenzene (TIPB) contrast thanconventional porous catalysts (ZSM-5, Y zeolite and Al-MCM-41).Finally, inspired by the finding of above HCL-ZSM-5, we sought to make furtherefforts to bring the NMP additive into common TEAOH-based alkaline Beta zeolitesynthesis mixtures, and study the impact of such kind of organics. Expectly, theresultant Beta zeolite features a typical micro-meso hierarchical porous texturalcharacteristic wherein highly crystalline Beta famework and large mesopore of9.2nmcoexist. To our surprised, the mesoprous Beta (M-Beta) is complete a single crystalrather than random aggregation of nanocrystals. Throughout the whole crystallizationprocess, there is no sign of an ordered assembly of preformed nanocrystals, instead,classical―nucleation and dissolution-precipitation growth‖model of hydrothermalcrystallization is followed. Unlike to above study about HCL-ZSM-5, the evolution ofM-Beta, typically in the mid-and late stage (after6h), behaves similar as an alkalineleaching process. On the basis of the theoretical model proposed by Verboekend et al.,we think that the strongly dipolar NMP molecules or their complex cations (e. g.Na+(NMP)nor TEA+(NMP)n) may serve as so-called efficient pore-directing agents(PDAs) that have great effects on the dissolution and recrystallization ofaluminosilicates, allowing formation of highly crystalline Beta zeolite frameworkcoupled with large intracrystalline mesopores. Also worth a mention is the fact thatM-Beta zeolite exhibits excellent catalytic properties in both weak (α-pineneisomerisation) and medium-strong (aldol condensation of benzaldehyde with n-butylalcohol) acid-catalyzed reactions involving large molecules.