Zn改性的Pt/SAPO-11催化剂及其提高二甲基烷烃选择性

doi:10.1016/S1872-2067(17)62755-4

摘要/Abstract

摘要：

随着环保法规的日益苛刻，世界各国对发动机燃料指标的要求也越来越严格，汽油组分中的烯烃含量逐渐被限制.作为提高汽油辛烷值的有效手段，正构烷烃的临氢异构化越来越引起人们的重视.烷烃碳数相同时，具有多支链的异构体往往具有更高的辛烷值.因此，在正构烷烃临氢异构化过程中，提高多支链异构体的选择性具有重要意义.Pt/SAPO-11作为双功能催化剂具有适宜的孔结构、温和的酸性和适度的加氢脱氢活性，在烷烃临氢异构化反应中表现出色，得到了广泛研究.已有文献报道中，通过添加第二金属改性可以调节双功能催化剂的物理化学性质，进而影响其催化活性和产物选择性.为了提高异构产物选择性，特别是多支链异构产物的选择性，本文研究了Zn改性的Pt/SAPO-11催化剂在正辛烷临氢异构化中的反应性能，与未改性的Pt/SAPO-11催化剂相比，Zn改性的Pt/SAPO-11催化剂表现出更高的二甲基烷烃选择性.
通过动态合成方法合成SAPO-11分子筛，进而制备Zn改性的Pt/SAPO-11催化剂.XRD表征表明，Pt和Zn的引入基本没有引起SAPO-11特征峰改变，并且未发现Pt和Zn氧化物的特征峰，说明金属组分得到很好的分散.氮气吸附-脱附实验表明，随着Zn添加量的增加，载体的比表面积和孔体积略有减少.NH₃-TPD和吡啶红外光谱用于表征Zn的加入对催化剂酸性的影响，结果表明，少量Zn的添加可以增加酸量，这是由于Zn的引入可以产生L酸性位；而随着Zn的增加，总酸量减少，这是由于过多的Zn覆盖了载体的酸性位.H₂-TPR和H₂-TPD表征发现，Zn的引入使得Pt在高温处的还原峰减小，说明Zn改变了Pt在载体上的位置，使其主要负载在分子筛外表面.由于Zn²⁺是阳离子并具有较小的离子半径，它可以作为竞争吸附剂，首先进入分子筛孔道从而抑制了Pt进入分子筛孔道.H₂-TPD表明，Zn的引入使得H₂的脱附温度逐渐降低，这与Zn增加了Pt的分散度有关；但是当Zn的添加量达到0.7%时，H₂的脱附温度升高，同时溢流氢增加.
以正辛烷为模型化合物，在固定床高压微型反应装置中对一系列催化剂进行了评价，考察了Zn改性的Pt/SAPO-11催化剂和未改性的Pt/SAPO-11催化剂的催化性能.与未改性催化剂相比，Zn改性的催化剂在相同转化率条件下，异辛烷的选择性基本相同，但是二甲基己烷的选择性明显提高.当Zn的添加量为0.5%时，二甲基己烷选择性达到最大值，继续提高Zn的添加量，其选择性下降.值得注意的是，在相同反应条件下，未改性的催化剂具有更高的正辛烷转化率.我们分析认为，由于Zn的添加促进了二甲基己烷的产生，而其具有较大的分子尺寸，在反应中存在扩散问题，影响了改性催化剂的异构化活性.本文工作表明，适宜的Zn添加量有利于提高二甲基烷烃的选择性，对于提高汽油辛烷值具有重要意义.

关键词: Pt/SAPO-11催化剂, 锌, 正辛烷, 临氢异构化, 二甲基烷烃

Abstract:

Zinc-modified Pt/SAPO-11 catalysts were prepared by incipient wetness impregnation and assessed in the hydroisomerization of n-octane. Their physicochemical properties were investigated using powder X-ray diffraction, scanning electron microscopy, nitrogen adsorption-desorption, pyridine-adsorbed infrared spectroscopy, temperature-programmed desorption of NH₃, temperature-programmed reduction of hydrogen, temperature-programmed desorption of hydrogen, transmission electron microscopy, and X-ray photoelectron spectroscopy. The addition of zinc resulted in high dispersion of platinum. Zinc acted as a competitive adsorbent, changed the location of platinum. The catalyst with a zinc loading of 0.5% gave the highest selectivity to dimethylhexanes, but the conversion was lower than those achieved with the other catalysts. Dimethylhexanes have large molecular diameters, and therefore their diffusion may be difficult. This weakens the catalytic activity of the zinc-modified catalysts and lowers the n-octane conversion.

Key words: Pt/SAPO-11 catalyst, Zinc, n-Octane , Hydroisomerization, Dibranched alkanes

杨志超, 柳云骐, 赵晋翀, 苟建霞, 孙凯安, 刘晨光. Zn改性的Pt/SAPO-11催化剂及其提高二甲基烷烃选择性[J]. 催化学报, 2017, 38(3): 509-517.

Zhichao Yang, Yunqi Liu, Jinchong Zhao, Jianxia Gou, Kaian Sun, Chenguang Liu. Zinc-modified Pt/SAPO-11 for improving the isomerization selectivity to dibranched alkanes[J]. Chinese Journal of Catalysis, 2017, 38(3): 509-517.

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https://www.cjcatal.com/CN/Y2017/V38/I3/509

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