量化计算在预测NMR参数和固体催化剂酸性中的应用

Dissertation for the Degree of Doctor of Science Application of Quantum Chemical Calculation in Predicting NMR Parameters and Acidity of Solid Catalysts

本论文将量化计算和NMR实验方法相结合,研究了有机分子在分子筛孔道中的吸附以及分子筛H-MCM-22和重要烷基化催化剂BF3/-γAl2O3的酸结构和酸强度,同时我们成功将组合量子化学ONIOM-GIAO方法用到大体系化学位移的计算.(1)将ONIOM-GIAO方法用来预测有机固体化合物和吸附在分子筛孔道中有机小分子的13C化学位移屏蔽张量.以前该方法仅被用来研究小的体系,我们成功将其用于预测氨基酸分子和一系列被吸附有机分子的13C的化学位移.ONIOM-GIAO方法充分将氨基酸晶体(或分子筛骨架)的空间结构和静电势的影响考虑到活性中心化学位移的计算中,计算得到的13C化学位移与实验值符合得非常好.(2)分子筛中铝原子和Bro¨nsted酸性质子的分布将直接影响到分子筛的催化反应活性.我们利用ONIOM方法研究了铝原子在分子筛MCM-22骨架中的分布状态,结合探针分子(丙酮和三甲基膦氧)的固体NMR实验,确定了MCM-22分子筛的酸强度和酸分布,合理地解释该分子筛对某些催化反应所表现出的独特选择性.(3)结合固体NMR实验结果,用量化计算方法从原子分子水平揭示了重要烷基化催化剂BF3/γ-Al2O3酸位形成机理,提出来了BF3表面修饰γ-Al2O3后所产生Br¨onsted和Lewis酸性位的结构.理论计算提出的酸性位的结构和酸强度特征完全符合NMR的实验结果.(4)量化计算表明当异丙醇与萘分子共吸附在丝光沸石孔道中,异丙醇呈现被分子筛Bro¨nsted酸质子化的趋势.这是由于萘分子大的共轭体系所表现出的强溶剂诱导效应,分散了异丙醇碳正离子的电荷,这是一种新的非极性溶剂所产生的诱导效应.

In this dissertation, quantum chemical calculations and NMR experimental methods were combined to study in detail the acidities of zeolite H-MCM-22 and commercialized catalyst BF3/γ-Al2O3. In addition, a new computational scheme ONIOM-GIAO was applied to calculate the NMR parameters of large systems. （1） We applied the new embedding NMR calculational method ONIOM-GIAO to accurately predict ^13C NMR chemical shifts of amino acids as well as a series of organic species absorbed on zeolites. Since the effects of framework environments around the part interested or active center have been taken into account by the ONIOM method, the ^13C chemical shifts predicted by the ONIOM-GIAO are in well agreement with the NMR experimental values. （2） The distribution of Bronsted acidic protons and their acid strengths in zeolite H-MCM-22 have been characterized by density functional theory （DFT） calculations as well as probe molecule magic angle spinning （MAS） NMR experiments. Our theoretical and experimental results may partially explain the special selectivity of acid-catalyzed reactions occurring inside the channels of H-MCM-22 zeolite. （3） Based on the multinuclear solid-state NMR results, DFT calculations have revealed the detailed structures of various acid sites formed on the BF3/γ-Al2O3 catalyst. The detailed mechanism of interaction between the BF3 and γ-Al2O3 surface as well as the newly formed Bronsted and Lewis acid structures were proposed. The DFT computational results are well consistent with the NMR observations. （4） Our theoretical calculations have revealed a new solvent-assisted proton transfer effect in zeolite catalysis. We found that when isopropanol was co-adsorbed with the naphthalene molecule in the channels of zeolite mordenite, acid strength of the Broensted acidic proton of the zeolite has been considerably increased, leading to the protonation of adsorbed isopropanol molecule. The big conjugate system of naphthalene molecule is likely to stabilize the resultant isopropanol carbenium cations by dispersing the charges around them.