Benco L, Bucko T, Hafner J, Toulhoat H
Institut für Materialphysik and Center for Computational Materials Science, Universität Wien, Sensengasse 8, A-1090 Vienna, Austria.
J Phys Chem B. 2005 Nov 3;109(43):20361-9. doi: 10.1021/jp0530597.
The local stability of Al atoms replacing Si in the zeolite framework is compared for all inequivalent tetrahedral (T) sites in mordenite. For Al/Si substitutions in two T sites the stable location of the compensating extraframework Zn(2+) cation forming a Lewis acid site is determined. In the most stable Zn-MOR structures Zn(2+) is located in a small ring (5MR, 6MR) containing two Al/Si substitutions. In less stable structures the Al atoms are placed at larger distances from each other and Zn(2+) interacts with only one Al site. The simulated adsorption of H(2) and CH(4) shows that adsorption strength decreases with increasing stability of the Zn(2+) Lewis site. A higher adsorption strength is observed for Zn(2+) deposited in the 5MR than for the 6MR. The reactivity of a series of stable Zn(2+) Lewis sites is tested via the dissociative adsorption of H(2) and CH(4). The heterolytic dissociation of the adsorbed molecule on the extraframework Zn(2+) cation produces a proton and an anion. The anion binds to Zn(2+) and proton goes to the zeolite framework, restoring a Brønsted acid site. Because bonding of the anion to Zn(2+) is almost energetically equivalent for Zn(2+) in any of the extraframework positions the dissociation is governed by stabilizing bonding of the proton to the framework. Those structures which can exothermically accommodate the proton represent reaction pathways. Due to the repulsion between the proton and Zn(2+) the most favorable proton-accepting O sites are not those of the ring where Zn(2+) is deposited, but O sites close to the ring. Large differences are observed for neighboring positions in a- and b-directions and those oriented along the c-vector. Finally, among the stable Zn(2+) Lewis sites not all represent reaction pathways for dehydrogenation. For all of them the dissociation of H(2) is an exothermic process. In structures exhibiting the highest reactivity the Al/Si substitutions are placed at a large distance and the Zn(2+) cation interacts with O-atoms next to Al in the T4 site of the 5MR. This Lewis site is strong enough to break the C-H bond in the CH(4) molecule.
比较了丝光沸石中所有不等价四面体（T）位上铝原子取代硅在沸石骨架中的局部稳定性。对于两个T位上的铝/硅取代，确定了形成路易斯酸位的补偿性骨架外锌（2+）阳离子的稳定位置。在最稳定的锌-丝光沸石结构中，锌（2+）位于含有两个铝/硅取代的小环（5元环、6元环）中。在稳定性较差的结构中，铝原子彼此间距离较大，锌（2+）仅与一个铝位相互作用。氢气和甲烷的模拟吸附表明，吸附强度随锌（2+）路易斯位稳定性的增加而降低。沉积在5元环中的锌（2+）比沉积在6元环中的锌（2+）具有更高的吸附强度。通过氢气和甲烷的解离吸附测试了一系列稳定的锌（2+）路易斯位的反应活性。吸附分子在骨架外锌（2+）阳离子上的异裂解离产生一个质子和一个阴离子。阴离子与锌（2+）结合，质子进入沸石骨架，恢复一个布朗斯特酸位。由于阴离子与锌（2+）的键合在任何骨架外位置的锌（2+）上几乎在能量上是等效的，所以解离由质子与骨架的稳定键合控制。那些能够放热容纳质子的结构代表反应途径。由于质子与锌（2+）之间的排斥作用，最有利的质子接受氧位不是锌（2+）沉积所在环的氧位，而是靠近环的氧位。在a方向和b方向以及沿c向量取向的相邻位置观察到很大差异。最后，在稳定的锌（2+）路易斯位中，并非所有都代表脱氢反应途径。对于所有这些位，氢气的解离都是放热过程。在反应活性最高的结构中，铝/硅取代彼此距离较大，锌（2+）阳离子与5元环T4位中铝旁边的氧原子相互作用。这个路易斯位足够强，能够断裂甲烷分子中的碳氢键。
