The Blessings and Curses of Carbonaceous Deposits in Ferrierite-Catalyzed Skeletal 1-Butene Isomerization

Conspectus Carbonaceous deposits have long been cast as the villain in zeolite catalysis: clogging pores, poisoning active sites, and gradually strangling catalyst performance during hydrocarbon conversion. Yet in a fascinating paradox, these same “poisons” can enhance catalyst performance in several industrially important reactions through synergistic interactions with zeolite acidity. Two prominent examples stand out: methanol-to-hydrocarbon/olefin reactions (MTH/MTO) and skeletal isomerization of linear butenes to iso-butene. Understanding this counterintuitive behavior has been a challenge in zeolite catalysis for decades. Skeletal butene isomerization requires a Brønsted acidic catalyst of intermediate acid strength. Zeolites with 10-ring (10-R) windows deliver high selectivity to iso-butene while suppressing oligomerization reactions. Ferrierite (framework type FER, with perpendicularly intersecting 8-R and 10-R channels) emerged as the superior catalyst, exhibiting the highest iso-butene selectivity and remarkable resistance to deactivation. The catalytic behavior of FER presents a remarkable contradiction. Within 24 h, around 6 wt % carbonaceous deposits accumulate and reduce the accessible micropore volume by up to 98%. One would expect immediate deactivation. Instead, catalyst performance continues to improve for up to 100 h, reaching peak activity only after the pores are effectively inaccessible. Deactivation finally occurs after 100–200 h on stream. This led Guisnet and colleagues in 1995 (Guisnet, M.; et al.J. Chem. Soc., Chem. Commun.1995, 31, 1685−1686, 10.1039/c39950001685.) to propose that carbocationic deposits themselves are the active species. However, the formation mechanism, precise location, and nature of these deposits and their interaction with zeolite acidity have remained points of debate. In this Account, we describe our systematic investigation of the reaction location, reactive species, and mechanisms governing catalyst activation and deactivation. Through probe molecule adsorption, operando spectroscopy, and operando titration experiments, we demonstrate that catalysis occurs at the pore mouths, where monoaromatic carbonaceous deposits interact with internal Brønsted acid sites. Rather than functioning as classic carbocations, these deposits are presumed to act as weakly acidic “proton-on-demand” reservoirs that mediate proton transfer to reactants outside the pores. This mechanism explains why conventional Brønsted acid sites become undetectable by ex situ methods after just 2 h, yet catalytic activity not only persists but improves far beyond this point. We further reveal that catalyst deactivation stems from external carbonaceous deposits that compress the zeolite structure and block the access to pore mouths, while internal deposits cause unit cell expansion. Oxidative regeneration successfully restores activity across multiple cycles without zeolite framework degradation, and strategic incomplete regeneration retaining around 1 wt % residual coke accelerates catalyst startup. These findings establish that effective skeletal butene isomerization requires a synergy between three elements: Brønsted acidity from the zeolite framework, structural confinement at the pore mouths, and carbonaceous deposits that mediate acid-catalyzed reactions from the inside of the zeolite outward. This understanding provides a framework for rational catalyst design and regeneration strategies, with implications extending to other zeolite-catalyzed hydrocarbon transformations where “beneficial coke” plays an activating role.