In situ MAS NMR–UV/Vis investigation of H-SAPO-34 catalysts partially coked in the methanol-to-olefin conversion under continuous-flow conditions and of their regeneration

In situ13C MAS NMR and UV/Vis spectroscopy was applied to study the nature of organic deposits formed under continuous-flow conditions of the methanol-to-olefin (MTO) conversion on the silicoaluminophosphate H-SAPO-34. Simultaneously, the volatile reaction products were analyzed by on-line gas chromatography. At reaction temperatures of 573 and 623 K, the occurrence of 13C MAS NMR signals of polyalkylated aromatics (18, 125–135 ppm) was accompanied by increasing UV/Vis bands of monoenylic (300 nm), dienylic (345 nm), and trienylic carbenium ions (430 nm). Under these conditions, the yield of propene was higher than that of ethene. At 673 K, a strong UV/Vis band of polycyclic aromatics, such as polymethylanthracenes (400 nm), and a change of the product selectivity from propene to ethene occurred, which indicates a starting catalyst deactivation. Quantitative evaluation of the in situ13C MAS NMR spectra of H-SAPO-34 used as catalyst at 623 and 673 K gave a mean number of ca. 0.4 aromatic rings per chabazite cage with 4.1 and 1.1 alkyl groups per aromatic ring, respectively. According to former studies, this decrease of the mean number of methyl groups per aromatic ring may be the reason for the above-mentioned change of the product selectivity from propene to ethene. Regeneration of the used H-SAPO-34 catalyst with synthetic air (20 vol.% oxygen) at 673 and 773 K led to a removal of up to 90% of all organic deposits. The remaining organic compounds on the regenerated H-SAPO-34 catalysts were found to consist of phenolic species and dienylic and trienylic carbenium ions.