Catalytic study of methanol-to-olefins conversion in four small-pore silicoaluminophosphate molecular sieves: Influence of the structural type, nickel incorporation, nickel location, and nickel concentration

The selectivity of four small-pore silicoaluminophosphate molecular sieves, including SAPO-34 (CHA), SAPO-35 (LEV), SAPO-17 (ERI), and SAPO-18 (AEI), toward light olefins in general and ethylene in particular has been investi gated for the methanol-to-olefins reaction using gas chromatography. This s tudy was prompted by earlier electron spin resonance and electron spin-echo modulation results on nickel-modified SAPO materials in which NI(I) was in corporated into both framework and ion-exchanged sites. These seemingly sim ilar materials behaved significantly differently with respect to reducing a gents and adsorbates. Attention was focused on whether the catalyst perform ance is influenced by the structural type, the presence of a transition met al ion (Ni) either in the framework or at ion-exchanged positions, or the a mount of incorporated transition metal ion. Our results show that these fac tors indeed play an important role in the catalytic behavior. Among the pro tonated H-SAPO-n materials, the highest combined distribution of ethylene, propylene, and butenes (C-2-C-4 olefins) was obtained with H-SAPO-34, and t he lowest with H-SAPO-35, which also had the shortest lifetime for catalyti c activity. H-SAPO-18 turned out to be the best catalyst in terms of lifeti me for catalytic activity. Incorporation of N(II) into the framework increa sed the lifetime, the overall distribution of C-2-C-4 olefins tin the case of NiAPSO-34), and the selectivity of the catalysts toward ethylene (in the cases of NiAPSO-34 and NiAPSO-18), whereas incorporation of Ni(II) by mean s of solid-state ion exchange (NiH-SAPO-n) increased only the selectivity t oward ethylene. Also, the increase in ethylene selectivity was more promine nt in synthesized than in ion-exchanged samples. Among the Ni-loaded sample s, NiAPSO-34 was found to be the best catalyst in terms of both ethylene se lectivity and lifetime, whereas NiH-SAPO-18 exhibited the worst ethylene yi eld and NiH-SAPO-35 the shortest lifetime. Finally, the effect of the amoun t of Ni was investigated in NiAPSO-34. It appears that the selectivity towa rd ethylene does not increase linearly with NiAPSO samples prepared with an increasing amount of Ni in the reaction gel. In fact, there seems to be an optimum Ni concentration for which ethylene selectivity reaches a maximum and above which ethylene selectivity decreases. This optimum concentration is the same as was found in earlier studies to yield the strongest Ni(I) si gnal, as observed by ESR.