運用分子動態模擬法探討甲烷在矽沸石孔道內之運動行為

Methane Diffusion in Silicalite Zeolite From Molecular Dynamic Simulations

陳威宇(Wei-Yu-Chen)

指導教授 ： 陳立仁

共同指導教授 ： 林祥泰(Shiang-Tai-Lin)

國立臺灣大學/工學院/化學工程學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.01327

摘要

本論文利用分子動態模擬探討甲烷在矽沸石(silicalite)孔道中的運動行為，以了解甲烷在矽沸石(silicalite)孔道中之能量分佈、運行軌跡及擴散係數，以及彼此間之關係。
矽沸石內的孔道是由直孔道與鋸齒孔道相互連接之三度空間孔道網，直孔道之前進方向為平行於Y方向，鋸齒孔道之延展方向則係平行於X方向，類似正弦(Sin)函數形狀並與直孔道呈垂直關係，而直孔道與鋸齒孔道則有一交會處(intersection)。
在本研究中藉由能量最小化方法得到甲烷在直孔道與鋸齒孔道交會處(intersection)位能較高，故甲烷欲從直孔道中央處擴散至另一直孔道中央處至少需位能1.28 kcal/mol始得越過，而甲烷欲從一鋸齒孔道中央處直接擴散至另一鋸齒孔道中央處至少需位能2.24 kcal/mol方能越過。
由於鋸齒孔道不像直孔道具對稱性質，致兩端出口之能量障礙不同，一為1.15 kcal/mol，另一則為2.24 kcal/mol，使甲烷於低溫時若欲從一鋸齒孔道擴散至另一鋸齒孔道，須先往直孔道擴散才能進入下(上)一層之鋸齒孔道，其直接從一鋸齒孔道擴散至另一鋸齒孔道之情形發生機率較小。
透過分子動態模擬可得甲烷在不同溫度下之擴散係數及出現位置之分佈情形，進而計算出甲烷在孔道中之能量變化及能量障礙。藉由Arrhenius law求出甲烷在孔道中自由能障礙之值，然卻無法知曉能量分佈情形。是以，藉由沿著孔道中或然率密度(probability density)之分佈情形，進而計算出沿著孔道中自由能(Helmholtz free energy)之分佈情形。爰可得知，系統溫度高於250K時，甲烷從直孔道或鋸齒孔道越過交會處之能量障礙，其熵效應所佔之重要性降低。

關鍵字

分子動態模擬 ； 矽沸石 ； 甲烷

並列摘要

We use molecular dynamic simulation to explore the behavior of the molecular motion in silicalite pores, to understand the energy distribution, molecular motion in pore, and diffusion coefficient of the adsorbed molecule, methane, and the relationship of each other.
The silicalite structure contains a three-dimensional network of interconnecting channels. The straight channels run parallel to the y-direction, and the zigzag sinusoidal channels that run parallel to the x-direction are perpendicular to the straight channels. They meet to form large channel intersections.
We find that methane’s potential energy in the intersection is higher than other pores by using the energy minimization in the research. Methane needs at least 1.2799 kcal/mol to diffuse from straight channel center to another one, and it needs at least 2.2385 kcal/mol to diffuse from zigzag channel center to another one.
Because the zigzag channels are not symmetrical like the straight channels, the energy barriers of two entrances are different. If methane at lower temperature wants to diffuse from zigzag channel center to another one, it has to diffuse to straight channel at first, then diffuses to zigzag channel upper or lower floor. So the probability that methane diffuses from zigzag channel center to another one directly is few.
We get the diffusion coefficient of different temperatures and the probability densities of methane by MD simulations, and then calculate the energy distribution and barriers in silicalite pores. We get the free energy barriers of methane in silicalite pores, but we can’t figure out the energy distribution. If we know the probability densities along reaction coordinate in silicalite pores, we can use window sampling method to calculate the Helmholtz free energies along reaction coordinate in silicalite pores. Above 250K, the importance of entropic effect will decrease in free energy barriers.

並列關鍵字

silicalite ； methane ； MD simulation

參考文獻

1. June, R. L.; Bell, A. T.; Theodorou, D. N., Molecular-Dynamics Study of Methane and Xenon in Silicalite. Journal of Physical Chemistry 1990, 94, (21), 8232-8240.

2. Richards, R. E.; Rees, L. V. C., Sorption and Packing of Normal-Alkane Molecules in Zsm-5. Langmuir 1987, 3, (3), 335-340.

3. Demontis, P.; Yashonath, S.; Klein, M. L., Localization and Mobility of Benzene in Sodium-Y Zeolite by Molecular-Dynamics Calculations. Journal of Physical Chemistry 1989, 93, (13), 5016-5019.

4. Karger, J.; Ruthven, D. M., ON THE COMPARISON BETWEEN MACROSCOPIC AND NMR MEASUREMENTS OF INTRACRYSTALLINE DIFFUSION IN ZEOLITES. Zeolites 1989, 9, (4), 267-281.

5. Millot, B.; Methivier, A.; Jobic, H.; Moueddeb, H.; Dalmon, J. A., Permeation of linear and branched alkanes in ZSM-5 supported membranes. Microporous and Mesoporous Materials 2000, 38, (1), 85-95.