稀土La改性X分子筛催化异丁烷/丁烯烷基化反应

Isobutane/butene alkylation catalyzed by rare earth La modified X-zeolites

采用液相离子交换法,通过改变交换和焙烧次数制备了5种不同浓度稀土La改性的X分子筛催化剂,使用连续进料的固定床反应器评价其催化异丁烷/丁烯烷基化反应的性能,分析了分子筛物相结构的变化,考察了分子筛的酸性.结果表明,催化剂制备过程对催化剂结构和性能影响显著,La3＋改性后X分子筛结晶度下降,但酸度显著增强,随La^3＋交换次数增加,分子筛的B酸量增多,L酸量减少;5种催化剂中,焙烧前离子交换2次、焙烧后再交换3次、再焙烧所制催化剂催化性能最佳,丁烯的初始转化率为89.94%, C8收率可达66.71%,这归因于酸性增加加快了氢负离子转移,降低了碳正离子上发生重复烷基化的可能性,抑制了大分子生成.反应温度和烯烃空速对反应影响显著,温度从80℃升至100℃,副反应裂解生成的C5-C7从9.64%增加到36.74%;丁烯进料空速从0.1 h^-1降至0.05 h^-1时,低聚生成的C9＋从7.2%增至31%.

Five kinds of rare earth La-modified X-zeolite catalysts were prepared by liquid-phase ion exchange procedures with different time of ion exchange and calcination. Their catalytic performance were tested in isobutane/butene with a continuously operation fixed bed reactor. The crystallographic structure of catalysts were characterized by XRD, and the acidity were examined by NH3-TPD and infrared spectroscopy for pyridine adsorbed（Py-IR）. The results showed that the preparation process had significant effects on the structure and properties of the catalyst. The structure of zeolites did not change fundamentally after the introduction of lanthanum, but their crystallinity decreased significantly. The distribution of acid sites depend strongly on the lanthanum ion exchange procedures, especially the rehydration process of zeolite after the first calcined for ion exchange, which could significantly improve the strong B acid capacity of zeolite. The introduction of La^3＋ could increase the acidic sites of the zeolite, enhance the strong B acid, and reduce L acid of the catalyst. Among the five catalysts, catalytic performance of 2＋3 when its ion exchange twice before calcination and thrice after calcinations occurred and it was calcinated again exhibited best, the initial conversion rate of butene was 89.94% and the yield of C8 products was 66.71%. This is because the enhancement of acidity due to the modification of La facilitate the hydrogen transfer, which is the determine step of alkylation, resulting in inhibiting the side reactions. The reaction temperature and feeding speed have strong effects on alkylation catalyzed over the La-modified X-zeolite. C5-C7 products increased from 9.64% to 36.74% when the reaction temperature was increased from 80 ℃ to 100 ℃. When the feed rate of butene was reduced from 0.1 h^-1 to 0.05 h^-1, the oligomeric generated C9＋ products increased from 7.2% to 31%.