Synthesis And Performance Of Y@nanoY Zeolites With High Silica To Alumina Molar Ratio

Zeolite Y?FAU?holding many prominent features,such as larger pore size,higher specific surface areas and surpassing thermal stability,is frequently employd as an important component of fluid catalytic cracking?FCC?catalyst during petroleum refining process,it is one of the most extensively used catalysts in petrochemical industry.However,with the aggravating tendency of heavier and inferior of crude oil,many large molecules cannot effectively contact with active centers lying in the pores of zeolite Y,which results that the cracking efficiency of zeolite Y to large molecules is reduced largely.On the other hand,the micropores of zeolite Y also bring strong diffusion resistance to reaction and product molecules.There are usually two strategies to solve the defects of conventional microporous zeolite Y.First,reducing mass transfer paths,nano-sized zeolite Y with reduced particle was prepared;Second,altering pore structure,introducing mesopore and macropore based on microporous zeolite Y to prepare hierarchical zeolite Y.However,nano-sized zeolite Y with smaller particles,larger external surface areas and higher surface energy lead to poor instability of single nanoparticle and easy agglomeration by attracting each other.In addition,it is difficult to realize separation of solid and liquid to monodisperse nanoparticle.The preparation of hierarchical zeolite Y often requires the introduction of organic templates,not only improving the cost of preparation but bringing pollution at some extent during calcination process of organic templates in the late.Based on analysis above,the target of this paper is to prepare high silicon aluminum ratio composite zeolite named Y@nanoY with a core-shell structure,the industrial NaY zeolite was used as the core during this process,by means of adding silicon source,aluminum source,inorganic alkali,distilled water,seed appropriately to synthesis system,and then the gel system was crystallized rapidly using hydrothermal method at 90?.The prepared Y@nanoY zeolite had following advantages compared to core NaY zeolite,?1?possessing higher silicon aluminum ratio?Si/Al?than core NaY zeolite,which will leads to better thermal and hydrothermal stability;?2?combining characteristics of core NaY zeolite and shell Y zeolite,its sizes could reach micron level so that it was easy to precipitate and separate;?3?the channels between shell Y zeolite and core NaY zeolite were connected,which will be conducive to pre-cracking of large molecules firstly on shell Y zeolite,and then deeper cracking occurs on core NaY zeolite by entering of pre-cracking products through connected channels;?4?larger specific surface areas,smaller sizes of shell Y zeolite,the accessibility of active centers will be improved largely and diffusion paths lying in shell Y zeolite will also be shortened obviously,which bring about less mass transfer resistance and significantly improve the conversion of reactants and prolong the life of catalyst;?5?the raw materials are cheap very much and the prepared method is simple and fast,which are beneficial to industrial application.The synthesized method of Y@nanoY zeolite will be introduced firstly in this paper,and then affecting factors including addition of seed?S?,silicon source,water content?H₂O/Al₂O₃?,silicon aluminum ratio of crude materials?Si₂O/Al₂O₃?,basicity?Na₂O/Al₂O₃?will be investigated in detail.Based on that,the synthesis of Y@nanoY zeolite under the condition of open system was investigated.Conventional characterization methods such as X-ray diffraction?XRD?,scanning electron microscopy?SEM?,Fourier transform infrared?FT-IR?,N₂ adsorption-desorption,thermogravimetric analysis?TGA?,acidity determination?NH₃-TPD?,transmission electron microscopy?TEM?,nuclear magnetic resonance?NMR?,UV-Raman spectroscopy?UV-Raman?were used to reveal characterizations of Y@nanoY zeolite detailedly.The results showed that optimal prepared condition was that alkalinity?Na₂O/Al₂O₃?=12.85,silicon aluminum ratio?Si₂O/Al₂O₃?=40,water volume?H₂O/Al₂O₃?=829,seed volume?S?=4 mL,mass of core NaY zeolite?M=3.0?.Subsequently,the function of core NaY zeolite,addition of core NaY zeolite,and crystallization time were explored separately.The result showed that the main role of core NaY zeolite during synthesis of Y@nanoY zeolite was carrier.The sizes of shell Y zeolite could be effectively controled by altering addition of core NaY zeolite.For example,when addition of core NaY zeolite was little?M=0.5².0?,the shell Y zeolite crystals?50¹00 nm?were small.The possible explain for that phenomenon was that less core NaY zeolite was not enough to offer space needed by the growth of shell Y zeolite due to spatial confinement,so the silicon and aluminum species could not be carried effectly by core NaY zeolite and became amorphous eventually.With the addition of core NaY zeolite largening?NaY=4.0⁶.0?,small spatial confinement could provide adequate space for the growth of shell Y zeolite,so the final shell Y zeolite crystals were larger?100³00 nm?.Therefore,the most suitable addition for core NaY zeolite was the“M”between 2.5 and 3.5,the crystals of shell Y zeolite were between50 and 200 nanometers under this circumstances.Finally,the crystallization time was discussed in detail,and the growth mechanism of shell zeolite on the surfaces of core NaY zeolite was proposed.It was thought that the growth processes of shell zeolite mainly held three steps,?1?nucleation period of zeolite;?2?primary growth period of crystals;?3?rapid growth period of crystals,which showed a typical“S”curve.Shell zeolite having preferred growth orientation during this process,which showed that shell zeolite preferred growing on edges and vertices of NaY zeolite with eight surfaces morphology,and then grew on surfaces of NaY zeolite.Prolonging crystallization time,shell zeolite grew gradually,so the specific surface areas(S_BET)and microporous areas(S_mic)increased gradually,and pore structure was better,order degree further improved and pore sizes distribution was more uniform.After complete crystallization of shell zeolite,channels between core NaY zeolite and shell zeolite were connected.1,3,5-Triisopropylbenzene?TIPB?was used as probe molecule to evaluate the catalytic performance of HY@nanoY zeolite and single HY zeolite.The results showed that HY@nanoY zeolite possessed a better catalytic performance than single HY zeolite under the conditions of different catalyst/oil and different running numbers.