无碱条件下己二腈加氢制1,6-己二胺中Ni/Al_(2)O_(3)制备方法比较研究

Ni-Based Catalysts for Basic-Free Hydrogenation of Adiponitrile to 1,6-Hexamethylenediamine: A Comparative Study on Synthesis Method

采用不同方法制备了一系列Ni/Al_(2)O_(3)催化剂,在无NH3、无碱条件下对其己二腈催化加氢制1,6-己二胺的反应性能进行研究.通过XRD、 BET、 TEM、 XPS及程序升温实验等结果表明,采用沉淀剂有利于细小Ni纳米颗粒在催化剂介孔内的分散。然而较强的Ni与载体间相互作用不利于氢气的吸附活化,还原处理后催化剂中仍存在较高含量的氧化态Ni物种.采用浸渍法制备的Ni/Al_(2)O_(3)-I催化剂中颗粒的平均尺寸为18.5 nm, Ni颗粒暴露在载体表面,生成较高含量的还原态Ni^(0)物种及有利于反应物吸附的中等强度酸性位点.该Ni/Al_(2)O_(3)-I催化剂表现出优良的低温活性,在60℃时,己二腈转化频率(TOF)可达39.7 h^(-1);80℃时转化率为100%, 1,6-己二胺收率可达73.0%.然而,由于Ni/Al_(2)O_(3)-I催化剂中Ni与Al_(2)O_(3)间相互作用较弱,经过3次循环性能测试,催化剂活性明显下降.进一步以少量Cu元素修饰低含量Ni/Al_(2)O_(3)-I催化剂, Cu助剂的引入有效改善了Ni纳米颗粒的分散性,并与部分Ni组分形成双金属纳米颗粒.Ni_(15)Cu_(3)/Al_(2)O_(3)催化剂经5次循环反应后无明显失活现象,催化剂的形貌及化学结构亦无明显变化, Cu的引入在保持Ni基催化剂低温活性的同时有效提升了催化剂的循环稳定性,催化剂的TOF为52.1 h^(-1).

A series of Ni/Al_(2)O_(3) catalysts were prepared by different methods, and the reactivity for the catalytic hydrogenation of adiponitrile to 1,6-hexamethylenediamine was investigated under NH3/alkali-free conditions. The re-sults of XRD, BET, TEM, XPS and temperature-programmed experiments suggested that the precipitant helped the formation of fine-sized Ni particles dispersed in the mesoporous of support. However, the as-formed strong interaction between Ni and alumina hindered the adsorption/activation of H2, and large amounts of Ni species in oxidative states still existed in the pre-reduced catalysts. On the contrary, average size of Ni particles in Ni/Al_(2)O_(3)-I was as large as 18.5 nm. However, the Ni particles were fully-exposed on the surface of alumina, the higher content of reduced Ni^(0) species and moderate acidic sites that were conducive to adsorb reactants have been formed easily in Ni/Al_(2)O_(3)-I. The much better low-temperature activity was observed by Ni/Al_(2)O_(3)-I with complete conversion of adiponitrile with 72% selectivity of hexamethylenediamine at only 80 ℃. The TOF was calculated to be 39.7 h^(-1). Unfortunately, due to the weak interaction between Ni and Al_(2)O_(3) in the Ni/Al_(2)O_(3)-I catalyst, the catalyst activity is significantly reduced after three reaction cycles. Cu species was thus doped in the Ni/Al_(2)O_(3)-I catalyst. The results showed that the doped Cu species effectively improves the dispersion of Ni nanoparticles and Cu-Ni alloy was formed. There was not deactivation of the catalytic performance by Ni_(15)Cu_(3)/Al_(2)O_(3) catalyst during the stability test. The doping of copper species effectively improved the stability while maintaining the low-temperature activity of the Ni-based catalyst. Using a lower content of Ni-based catalyst, the adiponitrile could be completely transformed with the TOF as 52.1 h^(-1) at 70 ℃.