Tuning the zeolite acidity enables selectivity control by suppressing ketene formation in lignin catalytic pyrolysis

Abstract

Unveiling catalytic mechanisms at a molecular level aids rational catalyst design and selectivity control for process optimization. In this study, we find that the Brønsted acid site density of the zeolite catalyst efficiently controls the guaiacol catalytic pyrolysis mechanism. Guaiacol demethylation to catechol initiates the reaction, as evidenced by the detected methyl radicals. The mechanism branches to form either fulvenone (c-C5H4 = C = O), a reactive ketene intermediate, by catechol dehydration, or phenol by acid-catalyzed dehydroxylation. At high Brønsted acid site density, fulvenone formation is inhibited due to surface coordination configuration of its precursor, catechol. By quantifying reactive intermediates and products utilizing operando photoelectron photoion coincidence spectroscopy, we find evidence that ketene suppression is responsible for the fivefold phenol selectivity increase. Complementary fulvenone reaction pathway calculations, along with 29Si NMR-MAS spectroscopy results corroborate the mechanism. The proposed, flexible operando approach is applicable to a broad variety of heterogeneous catalytic reactions.

The elusive phenylethynyl radical and its cation: synthesis, electronic structure, and reactivity

“…33 This suite of techniques 34 allows to unravel reaction mechanisms relevant in combustion, 35,36 astrochemistry [37][38][39] and catalysis. 40,41 In addition, experimental vibrational information of the ground and excited states, and in some cases the singlet-triplet energy splitting (DE ST ), has been obtained for the ethyl, 42 vinyl, 43 phenyl, 44 and benzyl cations. 45 In particular, the ethynyl cation in its triplet ground state was unveiled via direct photoionization of the ethynyl radical.…”

Molecular Tuning of Reactivity of Zeolite Protons in HZSM-5

“…The latter efforts are heavily focused on controlling the zeolitic structure . These include isomorphous replacement of the Al atom with other heteroatoms, selective siting of Al in the zeolitic framework, controlling the proximity of Al sites , as well as their interaction with nonframework protons . A complementary and yet little explored, molecular-level approach is to adsorb an inert modifier molecule on the BAS site in amounts 1 equiv or less (i.e., 1 or fewer molecules per BAS) and relate the reactivity trend to the properties of the modifier.…”

CO Cofeeding Affects Product Distribution in CH3Cl Coupling over ZSM‐5 Zeolite: Pressure Twists the Plot

“…This capability enables isomer-specific identification of stable products and reactive intermediates, such as radicals or ketenes, desorbed from the catalyst surface. [3] The mechanism of methanol to hydrocarbons (MTH), a prominent alternative to oil-based olefin production, has long been subject to debate. [4,5] While it is generally accepted that an autocatalytic mechanism is established after an oxygenate involved induction period, [4a] questions remain about the initial CÀ C bond formation steps.…”