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ZIF-90/GO复合吸附剂的制备及其对正己烷的吸附性能研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/10/8 8:55:07  

摘要:石油及其产品释放出来的有机组分(Volatile OrganicCompounds,VOCs)是一个主要的污染源。正己烷是VOCs的最重要代表物之一,同时也是汽油的关键组成成分,其极易挥发的特性受到了人们的广泛关注。目前采用较多的回收方法为吸附法,而常用的吸附剂都存在一些问题。近年来,金属-有机骨架材料(Metal Organic Frameworks,MOFs)发展迅猛,由于其大BET比表面积、表面易修饰、孔径可调和结构多样性等特点,使得其在气体吸附方面有着良好的发展前景。作为MOFs材料的一个分支,类沸石咪唑酯骨架材料(Zeolitic ImidazolateFrameworks,ZIFs)结合了天然沸石和金属骨架材料的优点,不仅具有MOFs的优良性质,而且还具有更好的热稳定性性和化学稳定性,成为近些年的研究热点。氧化石墨烯(Graphene Oxide,GO)是石墨烯的氧化物,具有超高的BET比表面积,同时表面丰富的含氧官能团为其与MOFs材料的复合提供了大量的结合位点,从而使其成为MOFs材料生长的良好平台。因此,制备MOFs/GO复合材料有望得到性能优越的复合吸附剂,进而用于油气回收领域。主要取得的研究成果如下:

(1)在溶剂热法的基础上,通过研究ZIF-90制备时不同搅拌时间和搅拌速度处理合成液对ZIF-90吸附剂的结构参数及正己烷静态吸附性能的影响,优选出最佳的制备条件是搅拌速度为640r/min,搅拌时间为6 h,此时ZIF-90吸附剂的BET比表面积和总孔容分别为1406.67 m2/g和0.64 cm3/g,相比于文献的报道都有较大的提升。同时,ZIF-90吸附正己烷实验表明,其吸附容量最高可达211 mg/g,且10次循环吸附-脱附后吸附量仍可维持原样的77%,具有较好的吸附稳定性。

(2)通过回流法合成了粒径小、孔径大的ZIF-90吸附剂,研究了合成液在不同TEA添加量下对ZIF-90吸附剂的孔隙结构参数的影响,得出在摩尔比Zn: H-ICA: TEA=1: 2: 24时合成的材料最优,此时ZIF-90吸附剂的BET比表面积和总孔容分别为932.65 m2/g和0.85 cm3/g。同时中孔孔容占比高达65%,相较于溶剂热法有大幅度的提升,但其对正己烷的吸附量仅为108 mg/g。相反,其吸附速率得到了提升,具有更好的可再生性能(99%),更高的吸附稳定性,为工业应用奠定了基础。

(3)在回流法的基础上合成了孔径更小的新型ZIF-90/GO复合吸附剂,通过研究GO的添加量对ZIF-90/GO复合吸附材料的孔隙结构参数的影响,得出GO添加量为的1 wt%时所合成的材料最优,此时ZIF-90/GO的BET比表面积和总孔容分别为1174.04 m2/g和0.76 cm3/g。同时,对正己烷的吸附量为186 mg/g,相较于本征吸附剂提高了72%,吸附速率也有一定的提高,且在循环吸附-脱附后吸附量略有提高,主要是由于掺杂的石墨烯不仅提供了更多的吸附位点而且促进了热量的传递,为新型复合吸附剂的制备提供了支撑。

(4)通过分子动力学模拟研究了不同温度和不同压力下正己烷在ZIF-90内的吸附规律,对其吸附密度场、吸附位点和均方位移进行了分析。吸附等温线表明,随着温度的升高,吸附量逐渐降低,而随着压力的增大,吸附量呈增大趋势。通过密度分布图发现,正己烷主要吸附在ZIF-90吸附剂的侧链醛基、配体的五元环和金属位点附近。均方位移图表明,环境温度的升高,扩散加快,扩散系数也随之增大,而随着环境压强的升高,扩散就减弱,导致扩散系数的降低。

Recent years, Metal Organic Frameworks(MOFs) have been considered as one of important porous solid. Due to theirlarge BET specific surface area, surface modifiability, tunable pore size anddiverse structure, MOFs is deem to a promising prospects for gas adsorption. Inparticular, as a subfamily of MOFs, Zeolitic Imidazolate Frameworks (ZIFs)combine the advantages of both the zeolites and MOFs, such as ultrahigh surfaceareas, uniform pore size, high crystallinities, abundant functionalities,exceptional thermal and chemical stabilities. Graphene Oxide (GO) is an oxideof graphene with an ultrahigh BET specific surface area and abundant surfaceoxygen functional groups, which provides efficient nucleation nods for theformation of MOFs. Therefore, the synthesis of MOFs/GO composites provides away in the field of oil vapour recovery. The major research results obtainedare as follows:

(1) Based on solvothermal method, theinfluences from the various stirring time and speed in the acceleration of theprecursor dissolution are revealed. The study shows that a moderate stirringspeed (640 r/min) and reaction time (6 h) are the optimal conditions tosynthesize the ZIF-90 with a high BET specific surface area (1406.67 m2/g) anda large total pore volume (0.64 cm3/g). Meanwhile, the maximum adsorptionamount of n-hexane is up to 211 mg/g by using this as-prepared sample. Moreimportantly, the adsorption capacity is still maintain 77% after 10 cycles use,indicating the stable adsorbability.

(2) The ZIF-90 with smaller partical sizeand lager pore diameter are synthesized by reflux method. Pore structureparameters of ZIF-90 adsorbents under different TEA additions are studied. Thematerial synthesized with the molar ratio of Zn: H-ICA: TEA = 1: 2: 24 is theoptimal choice. Meanwhile, the BET specific surface area and total pore volumeof ZIF-90 adsorbent reach 932.65 m2/g and 0.85 cm3/g, respectively.Surprisingly, the volume ratio of mesoporous can reach 65%, which is muchbetter than the former method. Though the adsorption amount of n-hexane is only108 mg/g, the adsorption rate is increased dramatically. Moreover, the improvedadsorbability (99%) maintains the potential prospect industrial applications.

(3) A novel ZIF-90/GO composite adsorbentswith smaller pore diameter are synthesized by reflux method. The effect of theaddition amount of GO is studied. The ZIF-90/GO composite show a higherspecific surface area (1174.04 m2/g) as the mass fraction of the GO is 1 wt%.Compared with the intrinsic adsorbent, the adsorption amount of n-hexane is 186mg/g, which increased by 72%. Moreover, the adsorption rate increased in a way.And the adsorption amount is slightly increased after 10 cycle reuse due to thehigh thermal conductivity and abundant oxygen functional groups of grapheneoxide. These functional groups provide more adsorption sites and promote heattransfer of adsorption process, which will provide a support for thepreparation of novel composite adsorbent.

(4) Molecular dynamics simulation isadopted to study the adsorption of n-hexane on the ZIF-90 at differenttemperatures and pressures, including adsorption density field, adsorption siteand mean square displacement. The simulation results show that the adsorptionamount decreases gradually with the increase of temperature and the decrease ofpressure. The density field show that n-hexane was mainly adsorbed on aldehydegroup, five-membered ring of H-ICA and the metal site of ZIF-90.The mean squaredisplacement diagram reveal that the diffusion coefficient increases with theincreased temperature. However,the diffusion decreaseswith the increase of pressure, resulting in a decrease in the diffusioncoefficient.

关键词:ZIF-90;GO;复合吸附剂;正己烷;吸附容量

ZIF-90;GO;composite adsorbent;n-hexane;adsorption capacity

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