Structural changes of FCC catalyst from fresh to regeneration stages and associated coke in a FCC refining unit: A multinuclear solid state NMR approach

The changes in zeolitic structure (Y zeolite) of fresh, equilibrium, spent and regenerated FCC catalysts obtained from an Indian FCC refining unit were investigated by 29Si magic angle spinning (MAS) and cross polarization (CP)-MAS, 27Al MAS and multiple quantum MAS (MQMAS) NMR. Deconvoluted 29Si MAS NMR along with 29Si CP-MAS NMR of these four catalysts provide the detailed local structural changes occurring at all Si(nAl) sites. 27Al MAS NMR provides the relative changes of tetrahedral (Td) and octahedral (Oh) aluminum sites of the FCC catalysts at different stages of the cracker. Second order quadrupolar effect (SOQE) and quadrupole coupling constant (CQ) obtained from both computation of 27Al MQMAS and simulation of 27Al 1D MAS spectra of these catalysts give imposition of structural asymmetries in Td and Oh sites that can be correlated with catalytic activities. Local structural changes are explained by NMR and are supplemented by Si/Al ratio, shrinkage in unit cell size and crystallinity obtained from XRD and BET. Si(3Al) sites of Y zeolite are found to be more deactivated and may be the primary sites of coking. 13C single hard pulse excitation (SHPE)-MAS, CP-MAS, cross polarization-dipolar dephasing (CPDD) NMR and CP dynamics analysis of hard coke extracted from the spent catalyst from the FCC unit reveal essential structural properties and nature of coke.