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采用巨正则系综的蒙特卡罗法,由通用力场Universal、Pcff、Dreiding和Cvff模拟CO_2在LTA(4A和5A)和FAU型(NaX和NaY)分子筛上的吸附行为。计算结果显示,现有的通用力场已不适用于分子筛吸附CO_2的体系,需要优化力场。CO_2在分子筛上吸附是通过分子间非键能发生作用,而文献中优化力场常用的量子化学计算法,虽然求得的分子内键作用力较为精确,但对分子间作用力却不一定有效,特别是范德华力。而计算结果与实验数据拟合法可以更真实反映分子间作用力,故采用后一种方法优化CO_2/LTA型分子筛的力场。所得力场参数既适合CO_2与5A分子筛吸附体系又适合CO_2在4A分子筛上的吸附,但根据LTA型分子筛得到的优化力场不能正确描述FAU型分子筛的吸附行为。采用拟合实验数据的方法,同样得到1组适合NaY分子筛的优化力场,用于计算NaX分子筛的CO_2吸附等温线,结果与实验值相吻合。可见,骨架结构相同的分子筛,可以用相同的力场参数去计算其吸附性质,而不受硅铝比和骨架外阳离子的影响。但不同的骨架结构需要不同的力场参数来描述。
The Monte Carlo method with giant canonical ensemble was used to simulate the adsorption behavior of CO_2 on LTA (4A and 5A) and FAU (NaX and NaY) molecular sieves by universal force field Universal, Pcff, Dreiding and Cvff. The calculation results show that the existing universal force fields are not suitable for the system of molecular sieve adsorption of CO2, and the force field needs to be optimized. The adsorption of CO_2 on molecular sieve is through the intermolecular non-bond energy, and the quantum chemical calculation method commonly used in the literature to optimize the force field, although the intramolecular bond force obtained is more accurate, but not necessarily the intermolecular force Especially van der Waals forces. The calculation results and experimental data fitting method can more accurately reflect the intermolecular force, so the latter method is used to optimize the force field of CO 2 / LTA molecular sieve. The obtained field parameters are suitable for both CO_2 and 5A molecular sieve adsorption systems and CO_2 adsorption on 4A molecular sieve. However, the optimal force field obtained from LTA molecular sieves can not correctly describe the adsorption behavior of FAU zeolites. By fitting the experimental data, a set of optimized force field suitable for NaY zeolite was also obtained, which was used to calculate the CO_2 adsorption isotherm of NaX zeolite. The results were consistent with the experimental data. It can be seen that molecular sieves with the same framework structure can be used to calculate the adsorption properties by using the same force field parameters without being affected by the ratio of Si / Al and the cation outside the framework. However, different skeleton structures require different force field parameters to describe.