A study of manganese-silicoaluminophosphate molecular sieves

Surface and chemical characterization were performed on a manganese-substit uted silicoaluminophosphate molecular sieve (MnAPSO-11), and on a manganese -supported silicoaluminophosphate molecular sieve (Mn/SAPO-11). For compari son purposes, the characterization process was also carried out over the pa rent SAPO-11 molecular sieve. Different characterization techniques were us ed: XPS, redox cycles, P-31 MAS NMR, and acidity measurements. The transfor mations of n-butane were carried out over the corresponding platinum promot ed solids (Pt/MnAPSO-11, Pt/Mn/SAPO-11 and Pt/SAPO-11). Platinum dispersion was measured by H-2 chemisorption. XPS results indicated that manganese wa s better dispersed on the MnAPSO-11 solid than on the supported Mn/SAPO-11 catalyst. Redox cycles showed a strong difference between the H-2 (or O-2) consumed by each solid. The Mn/SAPO-11 consumed nearly three times as much H-2 (or O-2) per Mn atom as the MnAPSO-11 solid. P-31 MAS NMR results showe d an increase in the intensity of the side bands, probably due to an anisot ropic paramagnetic shift caused by a stronger dipolar interaction between t he P-31 and the paramagnetic Mn(II) ions on the MnAPSO-11 sample, when comp ared with the Mn/SAPO-11 solid. These results suggest a better dispersion o f the manganese species on the MnAPSO-11 solid, which would facilitate the above mentioned P-31-Mn(II) interaction, in agreement with the XPS results. Acidity was measured by pyridine chemisorption at different temperatures. A larger number of (moderate + strong) Bronsted acid sites was found for th e MnAPSO-11 solid compared with the SAPO-11 and Mn/SAPO-11 samples. The add ition of platinum decreased the acidity. The Pt dispersions were 83%, 68% a nd 54% for the Pt/SAPO-11, Pt/Mn/SAPO-11 and Pt/MnAPSO-11 solids, respectiv ely. The catalytic results indicate higher yields for the production of iso butane and isobutene over the Pt/MnAPSO-11. A severe decrease in the yield of formation of hydrocarbons with less than four carbon atoms (undesirable side reaction) was also observed for the Pt/MnAPSO-11 system compared with the Pt/SAPO-11 and Pt/Mn/SAPO-11 systems. An ensemble effect is suggested a s responsible for the differences observed in the yield of formation of hyd rocarbons with less than four carbon atoms. The higher yield and selectivit y observed for the formation of isobutane (iso-C4) and isobutene (iso-C4=) hydrocarbons over the Pt/MnAPSO-11 solid, was accounted for in terms of the largest number of (moderate + strong) Bronsted acid sites found on this so lid. The catalytic and characterization results suggest the incorporation o f manganese into the molecular sieve structure for the substituted MnAPSO-1 1 solid. (C) 1999 Elsevier Science B.V. All rights reserved.