Department of Chemistry, Visva-Bharati University, Santiniketan-731235, India.
Nanoscale. 2016 May 21;8(19):10205-18. doi: 10.1039/c6nr00841k. Epub 2016 Apr 28.
The role of alumina nanoclusters as a catalyst on the reactivity of alkyl halides has been explored. The thermochemical data obtained from Density Functional Theory (DFT) calculations and the analyses of the transition structures reveal that, between the two competing reactions, elimination (via E2) versus dissociative addition (via SN2), elimination is the kinetically controlled one and thus at room temperature, olefin is the major product. The results are in excellent agreement with the recent experimental observation where more than 97% of ethylene is formed at room temperature with the reaction of ethyl fluoride over an alumina surface, although the dissociative addition product is being thermodynamically more stable. We have tried to rationalize the fact by using alumina clusters of different sizes as well as different alkyl halides having β-H for elimination. It has been shown that, during the elimination (E2) pathway, the transition structure is oriented in such a way that the eliminating halogen and the β-H are in the interacting position with the three-centered Al and two-centered O atoms, respectively, where the Lewis acid/base interaction is the main guiding factor. We have also shown a possible pathway for regenerating the catalyst. Finally, the possibility of the reactions has been tested in the presence of H2O to mimic the same on the hydrated alumina surface.
氧化铝纳米团簇作为一种催化剂对卤代烷烃反应性的作用已被探索。从密度泛函理论（DFT）计算获得的热化学数据和过渡态结构的分析表明，在两种竞争反应——消除（E2）与解离加成（SN2）之间，消除是动力学控制的，因此在室温下，烯烃是主要产物。这些结果与最近的实验观察结果非常吻合，即在氧化铝表面上氟化乙基反应时，室温下形成的乙烯超过 97%，尽管解离加成产物在热力学上更稳定。我们尝试通过使用不同尺寸的氧化铝簇以及具有β-H 的不同卤代烷烃来合理化这一事实。结果表明，在消除（E2）途径中，过渡态的取向使得消除的卤素和β-H 分别与三中心 Al 和二中心 O 原子处于相互作用位置，其中路易斯酸碱相互作用是主要的导向因素。我们还展示了一种可能的催化剂再生途径。最后，在存在 H2O 的情况下测试了反应的可能性，以模拟水合氧化铝表面上的相同反应。
