利用分子動力學模擬探討二氧化碳置換甲烷水合物的機制

Abstract

甲烷水合物是一種包含水與甲烷氣體的白色結晶固體、通常會在高壓低溫的環境下生成。由於世界各地的大陸棚及海底被發現有豐富的含量，因此甲烷水合物被視為一種有潛力的新能源。除了能源開採，如何減少溫室氣體也成為近年來重要的議題。因此許多科學家建議使用置換的方法，利用二氧化碳取代水合物中的甲烷，在甲烷被置換出來當作能源的同時，又可以將二氧化碳以水合物的形式貯藏在海底中。在實驗中已經證實將液態的二氧化碳注入碎冰狀的甲烷水合物中，可以觀察到置換反應的發生，同時相關的置換機制也被提出。一般普遍的認為在水合物的晶相結構中會存在一些缺少水分子的缺陷處(我們稱此缺陷為水洞)，而水洞的存在可以幫助客體分子(甲烷、二氧化碳)在晶相中的移動以達成置換反應。然而真正的置換機制還是仍然無法確定。 在本研究中，我們運用分子動力學模擬來觀察發生在甲烷水合物晶相和純二氧化碳相之間的介面上的置換反應。結果顯示二氧化碳分子只能往內置換約一層單位晶格厚度的甲烷水合物；即使在晶相中央引入了一個水洞之後，發現水洞會快速移往介面消失、對於拓展可置換區域無明顯幫助。另外我們也研究了在有水洞和沒有水洞的情況下，甲烷和二氧化碳分子在無介面純晶相系統中的移動情形。模擬結果顯示水洞對於系統而言是高能量、不穩定的存在。當系統中只有一個水洞存在時，水洞可以自由的在晶相中移動，但是對於客體分子的移動無明顯幫助。當系統中有高濃度的水洞時，原本均勻分布的水洞會逐漸修復、並聚集成由破碎的小水籠子(512)在中心而破碎的大水籠子(51262)在四周的特殊結構，同時我們發現有移動過的客體分子都指出現在密集的水洞聚集處。為了確認本研究中模擬的可性度，我們測量了客體分子在水合物晶相中的擴散係數，並發現其與實驗值相符合。透過本研究，可以得知置換行為只會發生在像是介面等高濃度水洞聚集處，無法透過單個水洞的機制使客體分子在水合物中移動來達成置換。

Clathrate hydrates are a class of nonstoichiometric crystalline compounds forming from water and small gas molecules, such as methane and carbon dioxides, at low temperatures and high pressures. Due to its abundance in nature, methane hydrate (MH) is regarded as a potential energy resource for the future. One intriguing idea for the simultaneous recovery of energy and sequestration of global warming gas is proposed by the transformation of methane hydrate into carbon dioxide (CO2) hydrate without melting the network of hydrogen-bonded water molecules. Some experiments have shown that methane hydrate can be changed into CO2 hydrate by injecting liquid CO2 into methane hydrate powders, and some theories has been proposed that there should be some porous vacancies formed by water molecules (water vacancy) in the hydrate structure to facilitate the replacement reaction. However, the exact mechanism is still unclear. Molecular dynamics (MD) simulation has been a useful tool to unveil the molecular level details of gas hydrate. In this work, we used MD simulation to study the mechanism for the replacement of methane in MH using CO2 both with and without a hydrate interface. When a hydrate-liquid CO2 interface is present, replacement only occurred in the first few surface layers of hydrate structure. If water vacancy is introduced to the hydrate phase, the vacancy quickly diffuses to the interface and vanishes, and thus does not promote the replacement process. In the case of bulk hydrate crystal (no interface), we investigate how the concentration of gas molecules (occupancy) and water vacancy affects the diffusivity of methane and CO2 in the crystalline phase. For the system with low concentration of water vacancy, we found that the vacancy propagated within the hydrate structure; however, its propagation did not stimulate the movement of methane or CO2 molecule between cages. For the system with high concentration of water vacancy, the initially separated vacancies were found to aggregate and form larger clusters of defected structures, each of which has 5 or more water vacancies centering around a small (512) cage, and resulting in broken surrounding large cages (51262). The diffusion of methane or CO2 molecular were found to take place only in such aggregated defect structures. The diffusion coefficient of methane and carbon dioxide molecules in such systems were found to be in good agreement with experiment. The result of simulations suggest that the replacement of methane with CO2 only occurs within structures of with aggregated water vacancies, such as grain boundary or interface.