Synthesis And Investigation Of MFI-type Zeolites In Acidic Medium

Zeolites as a kind of inorganic microporous materials with important uses in various areas are impacting our daily lives,namely as catalysts,absorbents,and ion exchangers.Advancements in the zeolite synthesis allowed expanding the family of aluminosilicate zeolites to new framework types and compositions,motivating innovative developments.The synthetic aluminosilicate zeolites are technical materials with a critical importance as catalysts and adsorbents in crude oil refineries and petrochemical productions,owing to their unique crystalline framework structures with defined micropore/cage systems and adjustable framework compositions.In recent years their uses have been extended to the processing of coal,and natural gas-derived methanol,environmental protection industries,and advanced applications in optics,electronics,and medicine.However,the applications in renewable energy,for example,catalytic conversion of biomass materials into useful fuels and chemicals,raise new requirements that zeolites should possess high structural integrity and fewer defects so that they survive harsh aqueous conditions.Consequently,the synthetic chemistry of zeolite should be reviewed and improved.Zeolites of MFI topology which possess a three-dimensional pore system are famous crystalline microporous aluminosilicates.The MFI is the most widely studied family of zeolitic materials from both academic and industrial researchers.Zeolites of MFI-type could be divided into all-silica（silicalite-1）,aluminum-containing（ZSM-5）,and metal-containing as the Ti-containing one（TS-1）is the most important in practical process.The reason why these materials deserve so much attention is because they have achieved practical applications in many fields.For instance,they were used to separate gases or liquids,to synthesize fine chemicals,to produce phenol by one-step method,in space research,to act as solid acid catalysts and so on.To further expand the catalytic applications,researchers need to spend more time and efforts to develop and further deeply investigate new synthesis routes of ZSM-5which can combine controlled morphology,pore architecture,and surface acidity in one material.An impressive breakthrough in zeolite synthesis occurred when the fluorides replaced hydroxyl anions,from then on,the synthesis in a neutral medium became possible.Pioneered by Flanigen,Patton and later on,Guth and Kessler,systematically studied different microporous materials,including aluminosilicates,aluminophosphates,gallophosphates via this alternative non-alkaline route.The use of the F^-as a mineralizer instead of the traditionally used OH^-presents several advantages:a.fewer undesired metastable phases appeared,which means certain ease of preparation of any goal zeolite product;b.acquisition of materials not obtainable in alkaline media like cloverite,which is a 20-T-ring gallophosphate;c.synthesis of perfect crystals with large size and few defects which are of interest as model host materials（for adsorption or diffusion studies）;d.neutral or acidic medium enables the incorporation of elements sparingly soluble in basic media,such as Co⁺,Fe³⁺,Ti⁴⁺and others;e.direct formation materials of NH₄-type,instead of Na-type avoiding repeated ion-exchange steps;thus only a calcination process is needed to remove the templates and obtain the acidic H-type materials.The development of zeolite synthesis in the acidic medium is necessary because many potential adsorption and catalysis applications require zeolites with pre-determined size and surface chemistry.Based on the above consideration,we did some research on the synthesis and investigation of MFI-type zeolites in acidic conditions:1.By determining the zeta potential of fumed silica(p H_IEP=2.0-2.2),we also addressed the formation paradigm of zeolites,i.e.,the electrostatic interaction between negative aluminosilicates precursors and the positively charged template.We first synthesized high and all-silica MFI-type zeolites in the p H range of 2-5 without adding any seeds.Thus,we successfully extended the zeolite synthesis field to an acidic medium.We also explored the influence of various silica sources and the aluminum concentration on the crystallization of MFI-type zeolites.We observed that among the 4 silica sources:TEOS,fumed silica,Ludox AS-40 and silicic acid,only silicic acid couldn’t give highly-crystallized product and the other 3 all could obtained highly-crystallized MFI-type zeolites after 160℃,13 days’hydrothermal treatment.In addition,we found that only when the precursors with Si/Al higher than 150 then we could get samples with high crystallinity.Which confirmed that excess aluminum was not suitable for our synthesis system;2.The zeolite nucleation and growth in fluoride medium is much slower than that under the conventional hydroxide-containing systems resulting in that longer crystallization period,larger crystal size and longer diffusion path problem in practical uses.Therefore,we proposed introducing highly-crystallized seeds（silicalite-1）and organic promoters（NMP）to accelerate the crystallization process and reduce the crystal size.We observed that it just needed 160℃,6 days’crystallization treatment then we could get highly-crystallized products by adding seeds of various amounts.And at the same time,the crystal size was obviously smaller and smaller with the increasing amounts of seeds.As for the experiments with different amounts of NMP,we found that when n NMP/n Si O₂=0.5,the synthesis could be finished 4 days earlier than that without NMP.In addition,we did some research about the effect of metal cation salts（Na Cl and KCl）on the crystallization of MFI-type zeolites in acidic conditions and we found that cations could change the ionic strength of reaction solution,therefore,disturbing the normal crystallization process and bringing negative impacts;3.We took advantage of 4 different synthetic aluminosilicate zeolites（H-β,NH₄-ferrierite,H-mordenite and H-Y）with different Si/Al ratios used as silica-alumina sources to perform inter-zeolite conversion（T=160℃,t=6 days）and thus to synthesize Al-containing MFI（ZSM-5）zeolites in an acidic medium.We proposed the parent zeolite materials could retain some ordered framework fragments in the process of dissolution during hydrothermal treatment.This reduces the formation of a stable fluoro-aluminate complex and favors the aluminum introduction in the zeolite framework.Thus we break the long-standing limitation that only high or all-silica zeolites could be obtained in a fluoride medium.This approach also provided an easy and efficient method for obtaining a zeolitic material containing catalytically active Br（?）nsted acid sites under acidic conditions.