Oyama S Ted, Lee Yong-Kul
Environmental Catalysis and Materials Laboratory, Department of Chemical Engineering (0211), Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
J Phys Chem B. 2005 Feb 17;109(6):2109-19. doi: 10.1021/jp049194l.
The mechanism of hydrodenitrogenation (HDN) of 2-methylpiperidine was studied over a silica-supported nickel phosphide catalyst (Ni2P/SiO2, Ni/P = 1/2) and a commercial Ni-Mo-S/Al2O3 catalyst in a three-phase trickle-bed reactor operated at 3.1 MPa and 450-600 K. Analysis of the product distribution as a function of contact time indicated that the reaction proceeded in both cases predominantly by a substitution mechanism, with a smaller contribution of an elimination mechanism. Fourier transform infrared spectroscopy (FTIR) of the 2-methylpiperidine indicated that at reaction conditions a piperidinium ion intermediate was formed on both the sulfide and the phosphide. It is concluded that the mechanism of HDN on nickel phosphide is very similar to that on sulfides. The mechanism on the nickel phosphide was also probed by comparing the reactivity of piperidine and several of its derivatives in the presence of 3000 ppm S. The relative elimination rates depended on the structure of the molecules, and followed the sequence: 4-methylpiperidine approximately piperidine > 3-methylpiperidine > 2,6-dimethylpiperidine > 2-methylpiperidine. [Chemical structure: see text] This order of reactivity was not dependent on the number of alpha-H or beta-H atoms in the molecules, ruling out their reaction through a single, simple mechanism. It is likely that the unhindered piperidine molecules reacted by an S(N)2 substitution process and the more hindered 2,6-dimethylpiperidine reacted by an E2 elimination process.
在三相滴流床反应器中，于3.1 MPa和450 - 600 K条件下，研究了2 - 甲基哌啶在二氧化硅负载的磷化镍催化剂（Ni₂P/SiO₂，Ni/P = 1/2）和市售Ni - Mo - S/Al₂O₃催化剂上的加氢脱氮（HDN）机理。作为接触时间函数的产物分布分析表明，两种情况下反应主要通过取代机理进行，消除机理的贡献较小。2 - 甲基哌啶的傅里叶变换红外光谱（FTIR）表明，在反应条件下，硫化物和磷化物上均形成了哌啶鎓离子中间体。得出结论，磷化镍上的HDN机理与硫化物上的非常相似。还通过比较哌啶及其几种衍生物在3000 ppm S存在下的反应活性来探究磷化镍上的机理。相对消除速率取决于分子结构，顺序为：4 - 甲基哌啶≈哌啶＞3 - 甲基哌啶＞2,6 - 二甲基哌啶＞2 - 甲基哌啶。[化学结构：见原文] 这种反应活性顺序不取决于分子中α - H或β - H原子的数量，排除了它们通过单一简单机理反应的可能性。可能是未受阻的哌啶分子通过S(N)2取代过程反应，而受阻较大的2,6 - 二甲基哌啶通过E2消除过程反应。
