Recrystallization Of Layered Silicate Magadiite Into Zeolites And Insights Into The Mechanism

Zeolites have wide applications in adsorption,catalysis and ion-exchange because of their unique structures and properties.Recrystallization of two-dimensional layered silicates,magadiite,into zeolites has attracted much attention from researchers,and relative research is developing and innovating.It is meaningful to exploit the green and efficient synthesis route of zeolites.At present there is great blindness in the synthesis mechanism of zeolites,caused some waste of manpower and materials.Understanding the formation mechanism of zeolites is very important for exploiting efficient synthesis of zeolites.In this dissertation,several types of zeolites have been synthesized in different system by using methods of solid-state transformation or hydrothermal conversion of magadiite.The characterization of as-synthesized products is investigated and the influences of various parameters are discussed.Furthermore,the insights into the changes of mediun-range structure during the crystallization have been studied by using a series of methods,such as X-ray diffraction,scanning electron micrograph,vibrational spectroscopy,solid state nuclear magnetic resonance and mass spectrometry,and we propose the reasonable formation pathways according to experimental results.The main results are concluded as follows:1.Zeolite omega,Si-Nu-1 and chabazite or zeolite omega have been synthesized by hydrothermal recrystallization of magadiite in the presence of TMABr.TMAOH and choline,respectively.The characterization of as-synthesized zeolites is investigated and the effects of synthesis conditions on the crystallization of magadiite have been analyzed.In addition,the differences of the three systems are investigated.2.Zeolite omega,Si-Nu-1 and ferrierite have been obtained by solid-state transformation of magadiite in TMABr.TMAOH and ethylenediamine(EDA)system,respectively.The characterization of as-synthesized zeolites is discussed and the effects of synthesis conditions on the crystallization of magadiite have been studied.Pure zeolites are also obtained by recycling the waste mother liquids which were separated from the synthesis of precursor.Comparing with hydrothermal synthesis,the method of solid-state transformation of magadiite into zeolites has several obvious features,such as the high yields of zeolites,expanding the scope of synthesis conditions,better utilization of autoclave and a significant reduction of waste liquid.3.On the basis of hydrothermal synthesis of zeolite omega,Si-Nu-1 and chabazite,the insights into the formation mechanism are investigated.It is found that part of secondary building units(SBUs)in magadiite could selectively retained in system and participate in the growth of zeolite.In TMABr system,the fraction of four-membered rings(4Rs)and six-membered rings(6Rs)increases with the reaction time goes on.And the Al atoms prefer to occupy the 6Rs in gmelinite cages.During the crystallization of Si-Nu-1,the formation of double four-membered rings(D4Rs)plays vital effects.Furthermore,in choline system,parts of 6Rs in magadiite are preserved as SBUs and help the formation of chabazite at low Si/Al ratio condition.While at high Si/Al ratio condition,parts of 5Rs and 6Rs could be retained in system and participated in the formation of zeolite omega.4.On the basis of solid-state synthesis of zeolite omega,Si-Nu-l and ferrierite,the possible mechanisms are proposed.And we discussed the reason why the solid-state transformation is faster than hydrothermal method in the process of heating.We proposed that the composition building units(CBUs)form during the preparation of precursors.The CBUs may provide the growth surface or structure units for the synthesis of zeolites.So they could help accelerate the the nucleation and growth of zeolites.Furthermore,whether it is solid-state or hydrothermal synthesis of zeolites,parts of SBUs in magadiite may preserve in system and participate the formation of target zeolites.