| In order to solve the increasingly serious exhaust pollution, modification ofmontomorillonite (MMT) in rich reserves has become a focus in the field of newmaterials. The current techniques for modifying MMT mainly depend on cationiccross-linked pillar holding and organic template assembling to synthesize nano-MMTswith abundant micro-pores and a specific surface area of over200m2/g. However, thiskind of nano-MMT usually just has a single microstructure and size and could becontaminated by toxic pillaring agents or organic templates furthermore polluteenvironment. It also cannot purify various exhausts and adsorb PM2.5particlessimultaneously. Thus, it is necessary to develop a new technique for preparing anenvironment-friendly MMT material with multi-scale pores.In the present study, according to the principle of template assembling and heat andmass transfer, the synthesis of porous MMT materials by the MMT colloidal suspensionof below5%concentration was investigated systematically. Hydrogen bonding betweenthe MMT particles and hydroxyl group of H2O molecules or PVA long-chain molecules,supramolecule-assembling induced by ice template are included. A new preparationmethod of porous MMT materials called “sol-gel-vacuum freeze drying” was developed.It was proved that automobile exhaust could be efficiently purified using porous MMTmaterials prepared by this method. The findings are described below.The physical-chemical process and characteristics of the porous MMT materialassembled by ice template were investigated. The results displayed that the mainadditive force comed from hydrogen bonding and Van der Waals force between uniquesheet-like structure in ice and surface of MMT micro-particles. The directivity andadditivity strengthened hydrogen bonding and Van der Waals force and resulted in theorientation of the MMT particles in ice crystal gaps. The force added was decisive in theassembling of porous MMT material. The macro-pores were layered, with a porosity ofover60%, a specific surface area of over150m2/g and a pore volume of over0.29cm~3/g.The sizes of the internal macro-pores and meso-pores are50μm and35~45nm,respectively. In addition, a small amount of micro-pores with sizes of less than5nmcould be found.The mechanism of the organic reaction between MMT particles and polyvinylalcohol (PVA) long-chain molecules by hydrogen bonding was revealed and pore characteristics of freeze-dried MMT/PVA composite was also disclosed. It has beenreported that the hydroxyl groups of PVA long chains and the hydroxyl groups ofsodium MMT particle surfaces can form hydrogen bonds. The hydrogen bonds played avital role in the intercalative reaction between the two substances. Osmotic pressurecaused by concentration gradients of PVA within and outside the MMT layer helped thePVA long-chain molecules to infiltrate the inter-layer domain of MMT, and reinforcedthe intercalative reaction. In the end, the two reactants generated sodium MMT/PVAcomposite colloidal suspension. By means of ice template assembly, macro-pores of thelyophilized MMT/PVA composite showed a lamellar structure, with its porosity of over60%, specific surface area of over213.8m2/g and pore volume of over0.327cm3/g. Thesizes of the internal macro-pores and meso-pores were10μm and7.0nm respectively.Some micro-pores (aperture below5nm) were also found in the mesh skeleton of thelyophilized MMT/PVA composites.The mathematical models of heat and mass transfer in the vacuum freeze-dryingprocess of MMT were built up to reveal the coupling of heat and mass transfer, thedistribution of instantaneous temperature field and concentration field of freeze-driedMMT and time variation of vacuum freeze-drying. The results showed that the thermalconductivity of the lyophilized MMT increased with the increase of its internal vaporconcentration; the water vapor diffusion flux increases with the increase of radiant heatof the system absorbed from the external environment, but the increased water vapordiffusion flux could increase heat loss of the system; the two-dimensional distribution ofthe temperature field and concentration field within the freeze-dried MMT is similar tothe parabolic curve; The sample temperature dropped gradually from its top center tothe bottom center, while the water vapor concentration was on the rise; In the samehorizontal plane, the sample temperature rise gradually from the inside to outward, andthe water vapor concentration was on the decline. The calculation results showed thatthe simulation freeze-drying time required for about39.0h when the ambienttemperature was308K, while time measured was40.0h. The calculated time wasbasically consistent with the experimental time.The inherent reason of starch to improve the structural strength of lyophilizedporous MMT material was researched. Previous studies report that the hydroxyl groupson the long-chains of starch molecules and on the surface of freeze-dried MMT particlescan generate hydrogen bonding reaction. In turn, the reaction can promote closerbonding between starch molecules and MMT particles. The process in which starch sol changes into gel helps form a three-dimensional network and makes MMT particlesform a mesh structure similar to gel matrix above. The ice templates were found toguide the MMT/starch amorphous gels to form the porous MMT materials, whichstrength was over1.0MPa and the lamellar pore structure was retained.The mechanism of the lyophilized porous MMT products used as a catalyst wasexplored. The results showed that some multi-scale pores, large number of unbalancebonds, broken bonds, and high ion-exchange capacity existed in the freeze-dried MMT.These may result in the edges, protrusions and defects on the lamellar surface of theMMT skeleton, which may become active centers for the MMT catalyst. Thelyophilized porous MMT material which we developed could be used as a strong solidcatalyst for acid because its crystal surface was capable of releasing large amount ofprotons. The catalytic mechanism of MMT is combined by adsorption, ion exchange,filtration and separation. It was found that the purifying efficiency of such porouscomposite was more than25%for CO, HC (hydrocarbons), NOx, PM2.5and CO2inautomobile exhaust, even better than the current state-of-the-art exhaust purifier ofplatinum electro-oxidation. |
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