Density functional theory study of the ethylene epoxidation over Ti-substituted silicalite (TS-1)
Density functional theory study of the ethylene epoxidation over Ti-substituted silicalite (TS-1)
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DOI：
10.1016/s1381-1169(03)00473-4
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发表时间：
2004-01
影响因子：
--
通讯作者：
J. Limtrakul;Chan Inntam;T. Truong
中科院分区：
文献类型：
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作者：
J. Limtrakul;Chan Inntam;T. Truong
在B3 LYP/6- 31 G（d）水平上，采用团簇和包埋团簇方法研究了Ti取代硅分子筛（TS-1）催化乙烯环氧化反应机理.确定了完整的催化循环。乙烯的环氧化由三个步骤组成。首先，H2 O2在Ti活性中心的化学吸附形成氧供体Ti OOH物种，然后氧原子从Ti OOH物种转移到吸附的乙烯。最后一步是Ti的OH物种的脱水以再生为活性中心。在B3 LYP/6- 31 G（d）水平上，用嵌入团簇模型计算了反应的速率限制步骤，发现氧原子转移步骤的零点能修正势垒为17.0kcal/mol，与实验估计的16.7kcal/mol一致.再生的活性中心通过脱水的Ti xOOH物种被发现有一个相当小的障碍，整个过程是放热的。我们的研究结果还表明，列入沸石晶体骨架的影响是至关重要的，以获得定量的能量信息。例如，马德隆势使氧原子转移步骤的势垒增加了5.0kcal/mol。
The mechanism of ethylene epoxidation with hydrogen peroxide over Ti-substituted silicalite (TS-1) catalyst was investigated by using both the cluster and embedded cluster approaches at the B3LYP/6-31G(d) level of theory. The complete catalytic cycle was determined. The epoxidation of ethylene consists of three steps. First, the chemisorption of H2O2at the Ti active site forms the oxygen donating TiOOH species and then the transfer of an oxygen atom from the TiOOH species to the adsorbed ethylene. The final step is the dehydration of the TiOH species to regenerate to active center. The oxygen atom transfer step was found to be the rate-limiting step with the zero-point energy corrected barrier of 17.0kcal/mol using the embedded cluster model at B3LYP/6-31G(d) level of theory, which is in agreement with the experimental estimate of about 16.7kcal/mol. Regeneration of the active center by dehydration of the TiOOH species was found to have a rather small barrier and the overall process is exothermic. Our results also show that inclusion of the effects of the zeolite crystal framework is crucial for obtaining quantitative energetic information. For instance, the Madelung potential increases the barrier of the oxygen atom transfer step by 5.0kcal/mol.