| Hierarchically porous silica monolith are a kind of porous materials with gradiently distributing interconnected macropores, mesopores or/and micropores, owning more superior properties of high specific surface area, adsorption performance and porosity derived from three-dimensional co-continuous macropores and fine mesoporous structure, and have wider applications, such as HPLC separation, adsorbents, filters, catalysis and so on. Sol-gel process accompanied by phase separation is an effective technique to prepare hierarchically porous silica monolith. Yet there are still some challenging tasks need to be solved including that the mesopores usually display disordered, the post-heat treatment in the existed procedure to introduce ordered mesopores into the macroporous skeleton are complicated, the process for modification and embedment are traditionally complex and the modification solvent and reductant are generally hazardous.Aiming at solving these problems, on the foundation of comprehensive review about preparation of hierarchically porous monolith via sol-gel process accompanied by phase separation, this paper study on the synthesis of macroporous and hierarchically porous silica monolith, a green way of its modification and silver nanoparticles embedment, and amino-modified hierarchically porous silica monolith for CO2 capture. The mechanism of co-continuous macroporous structure controlled by the competitive process of sol-gel transition and phase separation, ordered mesopores introduced into macroporous skeleton by the rod-like micelles, the process of APTES modifying the silica monolith and the detail reduction mechanism of silver ions by ethylene glycol are analyzed, which are favorable for their application in HPLC separation, catalysis reaction and CO2 adsorbents.(1)Macroporous silica monoliths with three dimensional co-continuous macropores and surface area of 418 m2·g-1 were prepared via a sol-gel process accompanied by phase separation using a tetramethoxysilane (TMOS) precursor,0.01 mol·L-1 HCl catalyst, propylene oxide (PO) gelation agent, and poly(ethylene oxide) phase separation inducer; On the basis of TMOS-HCL-PEO-PO silica system, we adopted twelve sodium dodecyl sulfate (SDS) as mesopore forming agent to prepare hierarchically porous silica monolith. Monoliths had a narrow macropore size distribution of 1-3 μ m, mesopore size distribution of 4-5 nm and the surface area were raised to 650 m2·g-1.(2)We explored the silica system of TMOS-HCL-P123-PO-TMB, using a tetramethoxysilane (TMOS) precursor,0.01 mol·L-1 HCl catalyst, propylene oxide (PO) gelation agent, and employing the poly(ethyleneoxide)-block-poly (propyleneoxide)-block-poly(ethyleneoxide) (P123) as the phase separation inducer as well as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as the micelle-swelling agent for the spontaneous preparation of hierarchically porous silica monoliths with periodic/ordered mesopores cofined in well-defined macroporous framework, and the as-prepared silica monolith possesses surface area as high as 848 m2·g-1, a narrow macropore size distribution of 1-2μm, mesopore size distribution of 10-11 nm. The highly ordered mesoporous and co-continuous macroporous structure remains intact after heat treatment at 800℃.(3)A facile and "green" method was proposed to introduce Ag nanoparticles (Ag NPs) into the hierarchically monolithic silica uniformly in the presence of (3-aminopropyl)-triethoxysilane (APTES) as modifier, ethanol as the modification solvent and ethylene glycol as the productive reductant for the first time. The embedment amount of Ag NPs can reach 15.44 wt.%. The mechanism of modification and reduction and factors influencing loading amount of Ag NPs were investigated, and then we proposed a method to effectively raise the area surface of Ag NPs embedded silica monolith by heat treated at properly temperature of 300℃.(4)The CO2 adsorption performance of macroporous and hierarchically porous silica monoliths after being amino-modified was investigated. The results show that: The adsorption capability were affected by pore structure of matrix silica monolith, modification method and relative amount of modifier. The greatest adsorption capacity of amino-functionalized hierarchically porous silica monolith(H-SiO2) via physical impregnation method by polyethylenimine (PEI) could be up to 1.37mmol/g; The greatest adsorption capacity of amino-functionalized macroporous silica monolith (T-SiO2) and hierarchically porous silica monolith (H-SiO2) via chemical grafting method by (3-aminopropyl)-triethoxysilane (APTES) could be up to 1.45mmol/g and 1.30mmol/g. The amino-modified silica monoliths possess good CO2 adsorption performance and are suitable for long time adsorption. |
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