【作者】 刘蓓；

【导师】 汪文川；

【作者基本信息】 北京化工大学 ， 化学工程， 2004， 硕士

【摘要】 本论文由两部分内容组成,即MCM-41中混合势模型及简单流体吸附的巨正则Monte Carlo模拟,以及MCM-41中吸附现象的混合势模型密度泛函研究。 多孔材料的研究与开发在材料领域占有重要的地位,被广泛用于吸附,分离,催化等领域。被限制在微孔中的流体,随着限定空间尺寸的变小,孔壁对流体的作用不可忽略,其与流体间的作用相互竞争,从而使微孔中的流体表现出一系列不同于体相流体的物理性质。因此,研究多孔材料的表征方法及其吸附行为具有重要的科学与应用价值。本文主要选择在化学工业中被广泛应用的分子筛MCM-41作为研究对象,研究其在不同条件下对内部流体的限制作用,以及由此而引起的吸附现象。 MCM-41具有规则的圆柱状孔结构,在利用分子模拟及密度泛函研究其吸附性质时需要圆柱孔的墙势模型。前人的研究结果表明,若只采用均匀势模型而不考虑MCM-41表面的非均匀性,则无法很好地拟合实验数据。因此本文采用了一个混合势模型来考虑MCM-41表面的非均匀性,即采用本组提出的完全解析的势函数描述孔壁中氧原子对MCM-41中流体分子的作用,而采用Tjatjopoulos等提出的势函数来近似表征MCM-41表面硅醇基团和一些未知因素对流体分子的作用。为了验证这一模型,我们首先利用GCMC方法研究了MCM-41中N2在77 K下的吸附,并将计算的结果与实验值及Maddox等的计算结果进行了比照。Maddox等将MCM-41的孔墙沿圆周方向均匀分成八部分,每部分中墙壁原子与流体分子的相互作用参数不同以考虑MCM-41表面的非均匀性,用GCMC方法研究了MCM-41中N2在77 <WP=5>K下的吸附,可以同时较好地拟和高压和低压实验数据。结果表明两种方法的结果接近,均与实验结果吻合很好。但本工作的势模型所采用的参数少,且计算量明显小于Maddox等的工作。利用本工作提出的混合势模型,我们用GCMC方法进而预测了甲烷和乙烷在MCM-41孔中的等温吸附行为。计算结果亦与实验数据吻合较好。研究表明本工作提出的混合势模型可以很好地表征MCM-41与孔中流体的相互作用,较好地预测简单流体在MCM-41中吸附行为。其将材料表面非均匀性与壁内粒子对流体的作用分开考虑的想法是可取的,能更好地表征多孔材料的结构。本文进一步将提出的混合势模型扩展应用于密度泛函的研究中,考察混合势模型在利用密度泛函理论研究MCM-41中流体吸附性质的可行性。本工作首先比较了采用混合势模型、本组提出的完全解析的势能函数及Tjatjopoulos 的势函数在同样条件下氮气的计算吸附等温线。研究表明混合势模型吸收了两个势模型的优点,较单独使用两个势模型能在整个压力范围内更好地拟合吸附等温线实验数据,具有一定的改进。前人的研究成果表明毛细凝聚压力与孔径之间的关系对势模型非常敏感。为了进一步检验本工作提出的混合势模型的优越性,本文采用混合势模型做出了孔径D-毛细凝聚压力Pc图,同时给出了Maddox等确定的毛细凝聚压力与孔径之间关系的数据作为比较对象,研究表明两种方法结果接近。利用本工作提出的混合势模型,我们用DFT方法亦预测了甲烷、乙烷在MCM-41孔中的吸附。计算结果表明混合势模型可以提高密度泛函方法用于描述气体在MCM-41中吸附等行为的精度,更大限度地发挥密度泛函方法的优点。更多还原

【Abstract】 This thesis consists of two parts, i.e. a hybrid potential model for characterization of MCM-41 and the adsorption of simple fluids in it by the Grand Canonical Monte Carlo simulation (GCMC) and density functional theory (DFT).Porous materials have been applied in many fields, such as gas separation, purification, and reaction processes, etc. In the meantime, when a fluid is confined to a region of a molecular scale, its phase behavior can be strongly affected, and a rich variety of new types of phase transitions can occur. Therefore, investigations on characterization and the adsorption behavior of porous materials are of great importance from both scientific and practical points of view. In this thesis, MCM-41 zeolites are chosen to study for their special characteristics and widespread use. The phase behavior and the adsorption isotherms of simple fluids in it at different conditions are investigated. In both the DFT and MC methods, a potential model for the wall-fluid interactions is required, which plays a key role in the accuracy of the calculated properties. Previous works show that the experimental isotherms can not be reproduced very well by using <WP=7>the existing potential models in the whole pressure range if the surface heterogeneity of MCM-41 is not taken into account. Therefore, a hybrid potential model was proposed in this work to consider the surface heterogeneity of MCM-41, which consists of two parts: (1) the potential energies from the oxygen atoms inside the wall, represented by the complete analytical model developed by our group recently;(2) the potential energies from the silanol coverage and/or other unknown factors in the surface of the channels of MCM-41, represented by the cylindrical surface potential function suggested by Tjatjopoulos et al. To test the new model, the GCMC method was used to calculate the adsorption isotherm of nitrogen in MCM-41 at 77 K first .The isotherm calculated is compared with the experimental data as well as the calculated results of Maddox et al., which divided the surface of MCM-41 into eight sectors and adopted different parameters for each sectors to consider the heterogeneity of the surface. Our model shows good agreement with the experimental data and comparable results with the work of Maddox et al. with much less parameters and computation efforts. In addition, adsorption and phase behavior of methane and ethane are studied by this potential, and the calculation results also coincide well with the experimental data. As a result, this work demonstrates that the hybrid potential model can consider the surface heterogeneity of MCM-41, and the partition of the interactions between the fluid molecules and the wall into two parts is reasonable and useful.Furthermore, this idea is extended to the DFT method to study the possibility of incorporating the hybrid model into the DFT method for characterization of MCM-41. First we compare the adsorption isotherms of nitrogen at the same condition using the hybrid potential model, the complete analytical model and the model of Tjatjopolous. It is found that the hybrid model can incorporate the advantages of the two constituent models, leading to a better description of the experimental adsorption isotherm in the pressure ranges. Then, the relationship between the reduced pressure at which capillary condensation takes place and the pore diameter is determined with our model. As is shown the results are in good agreement with that obtained by the method of Maddox et al. using Monte Carlo method. To further test the new model, we use DFT method to <WP=8>predict the isotherms of methane and ethane in MCM-41 pores. This again proves to be that the hybrid potential model incorporating into the DFT method provides a useful tool for characterization of MCM-41.更多还原