Cr/MgF_2氟化催化剂活性物种的研究

The Active Species of Cr/MgF_2 Catalyst

用混合法制备了Cr/MgF2 氟化催化剂 ,并以CH2 Cl2 氟化合成CH2 F2 为模型反应 ,评价了催化剂经活化、强制失活、空气焙烧再生和HF氟化四个过程的活性 .实验结果发现 ,积炭会抑制催化剂的活性 ,而催化剂活性的降低可能主要是由于Cr的氟化物种减少所致 .XRD结果表明 ,Cr/MgF2 催化剂可能存在具有较高催化活性的CrOxFy.由XPS结果可推断 ,新鲜催化剂的主要活性物种可能是CrOxFy 和Cr(OH) xFy,后者不稳定 ,易转化为晶体的Cr2 O3 和无定形的CrF3 ;再生催化剂可能只含有CrOxFy.表面原子浓度比表明 ,MgF2 以非化学计量的MgFx(x <2 )缺陷结构存在 .这种缺陷结构可能提高了活性物种的活性 .

The activity of Cr/MgF2 catalyst samples, treated in the processes of activation, forced deactivation, regeneration in air and fluorination in HF respectively, were measured in the model reaction of CH2Cl2 with HF to CH2F2. The results showed that the deactivated sample with coking ratio of 2.67% exhibited lower activity, and there was no coking in other samples. When the coke was removed by calcination in air, the original activity of the catalyst did not recovered at once, but refreshed step by step in HE The coking on the catalyst surface restrained its activity, and the main reason might be the decrease of fluoride Cr active species. The XRD results showed that, in Cr/MgF2, there exists CrOxFy (2theta = 15degrees) an active species with higher activity. The XRD peaks of CrF3.3H(2)O (2theta = 19degrees and 22degrees) in the regeneration sample were different from those in the fresh one, indicating that the active phase was changed after regeneration. It can be deduced that the main active species of fresh sample may be CrOxFy and Cr(OH)(x)F-y, and the latter is instable and liable to convert to crystalline Cr2O3 and amorphous CrF3, and the main active species of regeneration sample may be CrOxFy. The surface atom ratio demonstrated that MgF2 on the catalyst surface existed in MgFx (x<2) defection structure I being liable to combine with HF and to promote F/Cl exchange reaction.