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Methanol‐to‐Olefin Conversion Catalyzed by Low‐Silica AlPO‐34 with Traces of Brønsted Acid Sites: Combined Catalytic and Spectroscopic Investigations

The reasons for the high activity and long catalyst lifetime of AlPO‐34 with a very low molar silicon content [nSi/(nAl+nP+nSi)=0.01] in the methanol‐to‐olefin (MTO) conversion were studied by using in situ FTIR spectroscopy, in situ UV/Vis spectroscopy, and solid‐state NMR spectroscopy. The MTO activity of the low‐silica AlPO‐34 is explained by traces of accessible Brønsted acid sites existing in an isolated manner and characterized by a turnover frequency of 7.0 s−1 after a time‐on‐stream (TOS) of 1 h at 673 K. A contribution of Lewis acid sites to the MTO reaction over low‐silica AlPO‐34 could be excluded. The high selectivity of low‐silica AlPO‐34 to light olefins owes to a hydrocarbon pool consisting of benzene‐type carbenium ions, as well as dienes. The very few Brønsted acid sites on low‐silica AlPO‐34 produce a much lower total coke content compared with SAPO‐34, leading to an extended catalyst lifetime of the former material.