Radical Involvement and Active Sites in the Isomerization of 1-Butene

Understanding the identity and location of active sites during catalyzed reactions is critical to enhance catalyst performance and mitigate deactivation. In the isomerization of 1-butene to iso-butene over ferrierite (FER), general consensus in literature exists that the reaction proceeds over strong Brønsted acid sites (BAS) during the first 24 hours on-stream. The formation of carbonaceous deposits inside the zeolite pores during this start-up phase renders the catalyst pores inaccessible beyond 24 h on-stream. Yet, FER maintains high catalytic performance over another 200 h. The nature of active sites and mechanism by which the isomerization continues remains an ongoing debate.

Carbonaceous deposits themselves have been suggested to be catalytically active during 1-butene isomerization. In a proposed pseudo-monomolecular mechanism, a benzylic carbocation located on a deposit reacts with a butene molecule to yield iso-butene. However, the presence of benzylic carbocations has not been experimentally confirmed (van Donk, S.; et al., J. Catal 2002, 212, 86). Others have demonstrated radical species derived from carbonaceous deposits to be active during ethanol to hydrocarbon conversion over HZSM-5 (Pinard, L.; et al., Catal. Today 2013, 218, 57).

Nitrogen physisorption of spent FER after butene isomerization confirmed that the internal pores of the zeolite are inaccessible after carbonaceous deposits have formed. FER possesses a unique 2-dimensional channel structure of perpendicularly intersecting 8-membered and 10-membered ring channels that offers an optimal pore size to stabilize the active coke species. Therefore, the active sites are likely located at the FER pore mouths. Pyridine adsorption followed by infrared spectroscopy was used to locate and quantify the available BAS. The loss of Brønsted acidity during the initial hours of the reaction indicates that BAS inside the zeolite pores cannot be a catalytic site beyond 6 h on-stream. Electron Paramagnetic Resonance analysis of our spent FER showed highly stable carbonaceous radicals over several months. Radicals and Brønsted-acidic benzylic cations both might be active species in butene isomerization. To elucidate the effect of the radicals on the reaction, 2,2,6,6-tetramethylpiperidine was added to the feed as a radical scavenger. Since this compound is also a base, its radical scavenging effects were distinguished from its basicity effects by using triethyl amine as a secondary probe molecule.

Attaining insight into the character of active sites will facilitate further investigation on how active sites can be stabilized, consequently increasing catalyst lifetime. This understanding can enable the design of optimized catalysts for similar hydrocarbon chemistry.