| Biogas,produced by anaerobic degradation of organic matter in sewage treatment plants and landfills,can be used for power and heat generation,becoming an innovative energy source to replace traditional fossil fuels and has great economic value.Biogas generally contains 35%～70%methane,15%～50%CO2,a minor amount of N2 and O2,additionally,with trace amounts of H2O,H2S,NH3 and volatile methylsiloxane（VMS）.In the energy utilization of biogas,VMS will burn with oxygen to form fine glassy silica deposits,which can limit the effective energy utilization of biogas.Therefore,the removal of VMS from biogas will be of important research value.Adsorption method is considered one of the most promising treatment methods for VMS purification in biogas due to its advantages of small equipment investment,simple operation and maintenance,and low energy consumption.At present,the solid adsorbents for adsorption of siloxane generally include activated carbon,silica gel,alumina and molecular sieve,etc.There are some problems such as small adsorption capacity,poor recycling,low selectivity caused by competitive adsorption of water and other coexisting molecules,and unclear relationship between the adsorption performance of VMS and the structure of adsorbent.Therefore,it is urgent to develop more effective adsorption materials and reveal their structure-activity relationship.Graphene-based aerogel（GOA）is an original type of three-dimensional macroscopic assembly with reticular porous structure.The pore structure and specific surface area can be improved by adjusting the structure/properties of the construction units and the assembly process,and a series of GOA materials with similar composition but different pore structures and hydrophobicity can be obtained.Therefore,it lays a material foundation for overcoming the limitation of traditional solid adsorbent for siloxane adsorption and further analyzing the structure-activity relationship.Based on this,in this paper,using industrial grade graphene oxide（IGGO）as a precursor,specific microporous/narrow mesoporous GOA was constructed under hydrothermal conditions through VC controllable reduction self assembly,β-cyclodextrin hydration-crosslinking self assembly,amine-mediated C-N graft-doping self assembly and Fe3+induced cooperative self assembly to achieve the purpose of efficient adsorption of VMS.Combined with the morphology,crystal phase,defect degree,molecular structure and specific surface analyses of GOA,the regulatory mechanism of four regulatory strategies on the pore structure was revealed.In addition,using hexamethyldisiloxane（L2）as the model,the adsorption performance of GOA on L2 was investigated,the structure-activity relationship between the structure and adsorption performance of GOA was clarified,and the adsorption mechanism was explored,in order to provide theoretical reference for the performance optimization of microporous/narrow mesoporous GOA and the removal of VMS.The main research content and innovation of this paper are as follows:（1）The microporous/narrow mesoporous reduced graphene oxide aerogel（r GOA）was constructed based on VC controllable reduction self assembly strategy.At low reaction temperature and time,the effect of the amount of VC on the step-by-step reduction of oxygen-containing groups on IGGO was investigated,and the structure of r GOA was characterized and its adsorption performance was evaluated.The results demonstrated that:The controllable preparation of r GOA was achieved by adjusting the amount of VC added.When the IGGO/VC ratio was 1/1（m/m）,the r GOA-1 obtained had the highest specific surface area(137.9 m2 g-1),total pore volume(0.879 cm3 g-1)and contact angle（143.8°）,with the average pore size of 5.08 nm.The adsorption capacity of r GOAs for L2 was positively correlated with its specific surface area,total pore volume and contact angle.In addition,it was not affected by water in the course of the adsorption process of L2,showing good water resistance.After adsorption and desorption for five times,the adsorption capacity of L2 remained almost unchanged,showing excellent cyclic stability.（2）The three-dimensional mesh microporous/narrow mesoporous r GOA was created by one-step hydrothermal method based on the self assembly strategy ofβ-cyclodextrin hydration-crosslinking.The-OH onβ-cyclodextrin could crosslink with the epoxide group on the graphene lamellar by hydration,avoiding excessive accumulation of reduced graphene oxide（r GO）lamellars and promoting r GO lamellars self assembly.When IGGO/β-CD ratio was 1/0.5（m/m）,r GOA had the highest specific surface area(163.5 m2 g-1),total pore volume(0.679 cm3 g-1)and contact angle（128.4°）,with the average pore size of 5.93 nm.The adsorption capacity of L2 was promoted to 111.8 mg g-1.It was found that low temperature was conducive to the adsorption of L2,and the adsorbed r GOA could be regenerated and reused more than once.Experiments demonstrated that the adsorption process was physical adsorption,and the main adsorption mechanisms were micropore filling,capillary condensation and hydrophobic effect.（3）Based on the amine-mediated C-N graft-doping self assembly strategy,r GO lamellar spacing and the cross-linking degree were effectively regulated,resulting in lamellar defects of r GO and increased adsorption active sites,and a novel material with high performance for L2 adsorption was obtained.The preparation of r GOAs mediated by five amine reagents（aniline,diethylenetriamine,ethylenediamine,triethylenetetramine and urea）were studied.It was found that the adsorption property of r GOA was inversely proportional to the p Ka value of amine.Urea-mediated r GOA had the highest specific surface area(167.9 m2 g-1),total pore volume(0.737 cm3 g-1)and contact angle（146.8°）,moreover,it had the best adsorption performance of L2 with 112.4 mg g-1.The used r GOA could be regenerated when heated at80℃.The adsorption mechanism was microporous filling,capillary condensation and hydrophobic effect.（4）Based on the Fe3+induced synergistic self assembly strategy,with Fe（NO3）3·9H2O and IGGO as raw materials and urea as reducing agent and providing alkaline environment,Fe3+induced r GO lamellaes self assembly,while in situ self assembly Fe3O4 microsphere deposited on the surface of r GO,effectively reducing aggregation of r GO lamellaes.The Fe3O4-r GOA with controllable pore structure was constructed.N doping and Fe3O4synergistically increased the surface roughness and adsorption active sites of Fe3O4-r GOA,and Fe3O4-r GOA showed hydrophobicity,with the adsorption capacity of L2 reaching 146.5mg g-1 at 0℃.It was found that the specific surface area,total pore volume and contact angle of Fe3O4-r GOA were the chief factors affecting the adsorption of L2.The model fit statistics showed that the adsorption performance of Fe3O4-r GOA on L2 was a good to the quasi-first-order kinetic model,which belonged to physical adsorption,with the coexistence of monolayer adsorption and multi-molecular layer adsorption.The adsorption mechanism was micropore filling,capillary condensation and hydrophobic effect.Overall,in this paper,four self assembly strategies were adopted to achieve the controllable preparation of pore structures,and four kinds of microporous/narrow mesoporous r GOAs were successfully constructed.Through the structural characterization and performance evaluation of four r GOAs,the structure-activity relationship was revealed,the factors affecting the performance were discussed,and the adsorption mechanism was clarified,providing a new material and method for the purification of VMS in biogas. |
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