Combariza Aldo F, Sullivan Ethan, Auerbach Scott M, Blanco Cristian
Department of Chemistry and Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA.
J Phys Chem B. 2005 Oct 6;109(39):18439-44. doi: 10.1021/jp0524680.
We have performed equilibrium and nonequilibrium molecular dynamics simulations to study how microwave (MW)-heated zeolite systems relax to thermal equilibrium. We have simulated the relaxation of both ionic and dipolar phases in FAU-type zeolites, finding biexponential relaxation in all cases studied. Fast-decay times were uniformly below 1 ps, while slow-decay times were found to be as long as 14 ps. Fast-decay times increase with an increase in the initial temperature difference between MW-heated ions/dipoles and the equilibrium system. Slow-decay times were found to be relatively insensitive to the details of the MW-heated nonequilibrium state. Velocity, force, and orientational correlation functions, calculated at equilibrium to explore the natural dynamics of energy transfer, decay well before 1 ps and show little evidence of biexponential decay. In contrast, kinetic energy correlation functions show strong biexponential behavior with slow-decay times as long as 14 ps. We suggest a two-step mechanism involving initial, efficient energy transfer mediated by strongly anharmonic zeolite-guest forces, followed by a slower process mediated by weakly anharmonic couplings among normal modes of the zeolite framework. In addition to elucidating relaxation from MW-heated states, we expect that these studies will shed light on energy transfer in other contexts, such as adsorption and reaction in zeolites, which often involve significant heat release.
我们进行了平衡和非平衡分子动力学模拟，以研究微波（MW）加热的沸石系统如何弛豫至热平衡。我们模拟了FAU型沸石中离子相和偶极相的弛豫过程，发现在所有研究案例中均存在双指数弛豫。快速衰减时间均低于1皮秒，而慢速衰减时间长达14皮秒。快速衰减时间随着MW加热的离子/偶极与平衡系统之间初始温差的增加而增加。发现慢速衰减时间对MW加热的非平衡态细节相对不敏感。在平衡状态下计算的速度、力和取向相关函数，用于探索能量转移的自然动力学，在1皮秒之前就衰减良好，几乎没有双指数衰减的迹象。相比之下，动能相关函数显示出强烈的双指数行为，慢速衰减时间长达14皮秒。我们提出了一种两步机制，包括由强非谐沸石-客体力介导的初始高效能量转移，随后是由沸石骨架正常模式之间弱非谐耦合介导的较慢过程。除了阐明从MW加热状态的弛豫过程外，我们预计这些研究将为其他情况下的能量转移提供启示，例如沸石中的吸附和反应，这些过程通常涉及大量的热释放。
