探究三甲基鋁在空氣中的自燃反應

Abstract

為什麼三甲基鋁(Trimethylaluminum)會在空氣中自燃?”是個非常有趣且值得深入探討的現象。三甲基鋁與空氣混合自燃爆炸，此現象伴隨著熱能的釋放亦或是自由基(Radical)的產生。這些由初始反應生成的自由基可以很快的再與空氣中其他分子再次碰撞，產生連鎖反應釋放出更多的熱能，也因此三甲基鋁常被用來作為初始火箭的燃料。我們猜測此自燃反應可能是三甲基鋁與空氣中的氣體：水分子及氧氣產生作用。為了驗證這個想法，在本研究中，我們利用Gaussian氣相反應模擬軟體，計算水分子或是氧氣與三甲基鋁之反應，找出可能的反應路徑及反應中可能存在的中間產物和最終物。 從計算的結果得知，三甲基鋁無法直接跟氧氣形成穩定的中間產物。反之，此反應直接跨越一過渡能階，使得三甲基鋁可與氧氣鍵結產生(CH3)3AlO2。此反應的能障僅約10千卡，可被克服使反應持續進行。在下一個反應步驟中，經由給與少許能烼，一個甲基可直接從(CH3)3AlO2分子中被釋放出來。 另外，三甲基鋁可與水形成相對穩定的中間產物((CH3)3AlOH2)，並釋放出每莫耳約17千卡的能量。此中間產物更只需要跨越一極小的能障便可進一步反應形成甲烷及(CH3)2AlOH，並釋放出每莫耳約35千卡的能量。(CH3)2AlOH亦可能再次產生甲烷，但必須克服每莫耳約30千卡能障。雖然相對高於形成第一個甲烷的能量，但是反應仍有可能發生。反應過程中產生的甲烷，亦可參與後續的燃燒反應。 三甲基鋁亦有可能熱分解成二甲基鋁(Dimethylaluminum)。計算結果顯示，讓水與二甲基鋁結合成(CH3)2AlOH2伴隨著每莫耳約14千卡的能量被釋放。與三甲基鋁跟水反應的過程類似，在經過每莫耳3千卡的過渡態，釋放出第一個甲烷，能量相對於反應物下降了每莫耳33千卡，並生成CH3AlOH。再經過另一每莫耳10千卡的過渡態，CH3AlOH可分解出第二個甲烷及氧化鋁自由基(AlO)。此反應之產物相對能量僅僅高於反應物每莫耳2.8千卡。 若是將氧氣與三甲基鋁撞碰，氧氣不易直接與三甲基鋁鍵結不利於反應的進行。故先將水分子與三甲基鋁鍵結，利用水分子與氧氣形成物理快吸附，再讓氧氣進一步氧化三甲基鋁。但必須經過一每莫耳吸熱約24千卡的過渡態，氧氣才可直接與鋁原子鍵結並釋放出一個甲基，甲基可再與空氣中氧氣分子反應。 三甲基鋁亦可自發性的生成雙分子三甲基鋁。實驗值顯示，雙分子的鍵結能大約為20千卡。我們由比較實驗和計算結果之鍵結能，可選擇出較適當的計算方法來計算雙分子三甲基鋁與氧氣的反應。 延續在三甲基鋁自發性吸附成雙分子三甲基鋁的初步結果，雙分子三甲基鋁接著可與氧氣反應。雙分子三甲基鋁僅需與一個氧氣分子反應，就可釋放出兩個甲基。且反應不僅伴隨大量的能量釋放，反應中唯一的能障比反應物的相對能量低約2千卡，此低能障幾手無法對反應造成阻礙。故在這五種反應中，我們認為三甲基鋁在空氣中自燃最主要的反應路徑應為雙分子三甲基鋁與氧氣反應。 我們利用Gaussian氣相量子模擬軟體，對三甲基鋁與水分子和氧氣及雙分子三甲基鋁與氧氣之反應進行理論計算，發現這些反應均伴隨大量能量的釋放亦或是自由基的產生而利於連鎖反應，這樣的結果成功地的說明了三甲基鋁在空氣中可自燃爆炸的現象。因此，此模擬計算可能亦適用於其他類似的爆炸或自燃性反應。

“Why is trimethylaluminum (TMAl, (CH3)3Al) hypergolic in the air?” is a very interesting question which is worth studying. To unveil the hypergolic phenomenon, the Gaussian code (a computational chemistry software) was applied to simulate the reactions of TMAl with oxygen and/or water molecules. Possible paths of reactions and the structures and energies of the reactants, intermediates, and products in the reactions were determined by ab initio molecular orbital calculations. The results of calculations show that, a transition state with about 10 kcal/mol barrier has to be overcome in the process of TMAl binding with an O2 molecule, instead of directly forming a stable intermediate at the initial step. After passing through the transition state, (CH3)3AlO2 molecule can be formed by releasing one of the methyls from (CH3)3AlO2 with a small activation barrier. The calculational results show that TMAl can bind with a water molecule forming a stable complex, (CH3)3AlOH2, with 17 kcal/mol binding energy. By overcoming across a small barrier (0.13 kcal/mol lower than the energy of reactants), one methane is released to form a stable product, (CH3)2AlOH, dimethylaluminum hydroxide, which may decompose to give a second methane molecule, but it has to overcome an energy barrier of 30 kcal/mol. Another possible path, in which dimethylaluminum (DMAl, (CH3)2Al), formed by dissociating a methyl from TMAl, can react with another water molecule much more readily. The final products are two methane molecule and AlO radical. The reaction is endothermic by only about 2.8 kcal/mol. Because of the co-existence of O2 and H2O molecules in the air, we have also investigated the termolecular reaction of TMAl with O2 and H2O molecule. Although O2 molecule was formed to form a van der Waals complex with the TMAl:OH2 intermediate, no significant lowering of the CH3 production barrier was noted. The monomers of TMAl can spontaneously associate to form a TMAl dimer. The association energy was experimentally determined to be about 20 kcal/mol, which was found to be consistent with the value, 21.59 kcal/mol, 18.21 kcal/mol and 16.79 kcal/mol presicted by G3B3, MP2, and DFT by VASP respectively. In the reaction of dimer-TMAl with O2, two CH3 radicals can be produced with a large amount of energy released. At the DFT level of theory, the largest energy barrier in the whole complex reaction was formed to be lower than the reactants by about 2 kcal/mol, suggesting that the reaction can occur readily at low temperature. This reaction is therefore considered to be the major path to form radicals when TMAl contacts with air. By theoretical simulations, we found that the reactions of TMAl with water, and oxygen molecules are highly exothermic and in the case of O2 reaction CH3 radicals can be produced. The results are consistent with the hypergolic property of TMAl and thus may be applied to other metal alkyl reactions.