THE TRANSFORMATIONS OF N-BUTANE OVER PLATINUM-PROMOTED MN-ALUMINOPHOSPHATE MOLECULAR-SIEVES

The catalytic transformations of n-butane were performed over a platin um-promoted manganese-substituted aluminophosphate molecular sieve (Pt /MnAPO-11), and over a Pt promoted manganese-supported aluminophosphat e molecular sieve (Pt/Mn/ALPO(4)-11). For comparison purposes, the rea ctions were also carried out over a Pt-supported AlPO4-11 molecular si eve (Pt/AlPO4-11). X-ray diffraction (XRD) was used to determine the c rystallographic structure of the unpromoted and Pt-promoted samples. P hosphorous 31-magic angle spinning-nuclear magnetic resonance (P-31 MA S NMR) spectroscopy was used to study the possible P-31-Mn(II) dipolar interaction. Bronsted and Lewis acidity were measured by pyridine che misorption at different temperatures. Pt dispersions were determined b y hydrogen chemisorption. CO chemisorption followed by IR spectroscopy was used to corroborate the dispersion measurements and to evaluate p ossible changes in the electronic density of the Pt phase. The catalyt ic results indicate higher yields (and selectivities) for the producti on of isobutane and isobutene over the Pt/MnAPO-11, compared with thos e observed over the Pt/Mn/ALPO(4)-11 and Pt/AlPO4-11 samples. Also, a severe decrease in the yield, in the selectivity and in the turnover f requency (TOF), for the formation of hydrocarbons with less than four carbon atoms, was observed for the Pt/MnAPO-11 system when compared wi th the Pt/Mn/AlPO4-11 and Pt/AlPO4-11 solids. A larger number of (mode rate + strong) Bronsted acid sites was found for the MnAPO-11 solid co mpared to the Mn/AlPO4-11 and AlPO4-11 samples. For the last two solid s, no Bronsted acidity was detected after evacuation of the catalyst a t 623 K. P-31 MAS NMR results showed an increase in the intensity of t he side bands, probably due to an anisotropic paramagnetic shift cause d by a stronger dipolar interaction between the 31P and the paramagnet ic Mn(II) ions on the MnAPO-11 sample, when compared with the Mn/AlPO4 -11 solid. These results suggest a better dispersion of the manganese species on the MnAPO-11 solid, which would facilitate the above mentio ned 31P-Mn(II) interaction. The addition of platinum decreased the Lew is and Bronsted acidity for all the catalysts studied. The Pt dispersi ons were 68%, 59%, 49% for the Pt/AlPO4-11, for the Pt/Mn/AlPO4-11 and for the Pt/MnAPO-11 solids, respectively. The constant value obtained for the IR CO-Pt stretching frequency (Pt/AlPO4-11 and Pt/MnAPO-11 sa mples) (ca 2067 cm(-1)), as well as the drop in the Pt dispersion obse rved for the manganese containing solids, suggest an ensemble effect a s responsible for the differences observed in the yield, selectivity, and TOF of formation of hydrocarbons with <C4. The higher yield and se lectivity observed for the formation of iso-C4 and iso-C4= hydrocarbon s over the Pt/MnAPO-11 solid, were accounted for in terms of the large st number of (moderate + strong) Bronsted acid sites found on this sol id. The catalytic and characterization results suggest the incorporati on of manganese into the molecular sieve structure, for the Pt/MnAPO-1 1 sample. (C) 1998 Academic Press.