The Transformations of n-Butane over Platinum-Promoted Mn-Aluminophosphate Molecular Sieves

Abstract

The catalytic transformations of n-butane were performed over a platinum-promoted manganese-substituted aluminophosphate molecular sieve (Pt/MnAPO-11), and over a Pt promoted manganese-supported aluminophosphate molecular sieve (Pt/Mn/AlPO4-11). For comparison purposes, the reactions were also carried out over a Pt-supported AlPO4-11 molecular sieve (Pt/AlPO4-11). X-ray diffraction (XRD) was used to determine the crystallographic structure of the unpromoted and Pt-promoted samples. Phosphorous 31-magic angle spinning-nuclear magnetic resonance (31P MAS NMR) spectroscopy was used to study the possible31P–Mn(II) dipolar interaction. Brönsted and Lewis acidity were measured by pyridine chemisorption at different temperatures. Pt dispersions were determined by hydrogen chemisorption. CO chemisorption followed by IR spectroscopy was used to corroborate the dispersion measurements and to evaluate possible changes in the electronic density of the Pt phase. The catalytic results indicate higher yields (and selectivities) for the production of isobutane and isobutene over the Pt/MnAPO-11, compared with those observed over the Pt/Mn/AlPO4-11 and Pt/AlPO4-11 samples. Also, a severe decrease in the yield, in the selectivity and in the turnover frequency (TOF), for the formation of hydrocarbons with less than four carbon atoms, was observed for the Pt/MnAPO-11 system when compared with the Pt/Mn/AlPO4-11 and Pt/AlPO4-11 solids. A larger number of (moderate+strong) Brönsted acid sites was found for the MnAPO-11 solid compared to the Mn/AlPO4-11 and AlPO4-11 samples. For the last two solids, no Brönsted acidity was detected after evacuation of the catalyst at 623 K.31P MAS NMR results showed an increase in the intensity of the side bands, probably due to an anisotropic paramagnetic shift caused by a stronger dipolar interaction between the31P and the paramagnetic 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 mentioned31P–Mn(II) interaction. The addition of platinum decreased the Lewis and Brönsted acidity for all the catalysts studied. The Pt dispersions 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 samples) (ca 2067 cm−1), as well as the drop in the Pt dispersion observed for the manganese containing solids, suggest an ensemble effect as responsible for the differences observed in the yield, selectivity, and TOF of formation of hydrocarbons with <C4. The higher yield and selectivity observed for the formation of iso-C4 and iso-C4=hydrocarbons over the Pt/MnAPO-11 solid, were accounted for in terms of the largest number of (moderate+strong) Brönsted acid sites found on this solid. The catalytic and characterization results suggest the incorporation of manganese into the molecular sieve structure, for the Pt/MnAPO-11 sample.