Deactivation of medium pore zeolite catalysts by butadiene during n-butene isomerization

The deactivation of the 10-membered ring zeolites MFI, FER and TON was studied during the skeletal isomerization of n-butene in the presence of butadiene. The rate of deactivation per butadiene molecule was found to increase with the concentration of butadiene and the contact time. It decreased in the presence of Pt and hydrogen. FER and TON deactivated much faster than MFI. This was attributed to two factors: (i) Deactivation of TON occurred by pore-blocking, whereas direct poisoning of the acid sites by coke deposits took place on MFI. In the case of FER, coke was only deposited in the 10-membered ring channels, whereas the access to the 8-membered ring channels was blocked. (ii) The build-up of carbonaceous deposits on MFI was slower than that on the other zeolites, because MFI had a high activity for cracking oligomeric coke-precursors into smaller fragments, which desorbed. Although this cracking activity is responsible for the moderate selectivity of MFI for butene isomerization, it is beneficial with respect to the stability against deactivation by butadiene.