Methanol-to-Olefin Conversion Catalyzed by Low-Silica AlPO-34 with Traces of Bronsted 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 Bronsted 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 Bronsted 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.