Pan Zeyou, Puente-Urbina Allen, Bodi Andras, van Bokhoven Jeroen A, Hemberger Patrick
Laboratory for Synchrotron Radiation and Femtochemistry, Paul Scherrer Institute 5232 Villigen Switzerland
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich 8093 Zurich Switzerland.
Chem Sci. 2021 Feb 18;12(9):3161-3169. doi: 10.1039/d1sc00654a.
The catalytic pyrolysis mechanism of the initial lignin depolymerization products will help us develop biomass valorization strategies. How does isomerism influence reactivity, product formation, selectivities, and side reactions? By using imaging photoelectron photoion coincidence (iPEPICO) spectroscopy with synchrotron radiation, we reveal initial, short-lived reactive intermediates driving benzenediol catalytic pyrolysis over H-ZSM-5 catalyst. The detailed reaction mechanism unveils new pathways leading to the most important products and intermediates. Thanks to the two vicinal hydroxyl groups, catechol (-benzenediol) is readily dehydrated to form fulvenone, a reactive ketene intermediate, and exhibits the highest reactivity. Fulvenone is hydrogenated on the catalyst surface to phenol or is decarbonylated to produce cyclopentadiene. Hydroquinone (-benzenediol) mostly dehydrogenates to produce -benzoquinone. Resorcinol, -benzenediol, is the most stable isomer, because dehydration and dehydrogenation both involve biradicals owing to the position of the hydroxyl groups and are unfavorable. The three isomers may also interconvert in a minor reaction channel, which yields small amounts of cyclopentadiene and phenol dehydroxylation and decarbonylation. We propose a generalized reaction mechanism for benzenediols in lignin catalytic pyrolysis and provide detailed mechanistic insights on how isomerism influences conversion and product formation. The mechanism accounts for processes ranging from decomposition reactions to molecular growth by initial polycyclic aromatic hydrocarbon (PAH) formation steps to yield, , naphthalene. The latter involves a Diels-Alder dimerization of cyclopentadiene, isomerization, and dehydrogenation.
初始木质素解聚产物的催化热解机理将有助于我们制定生物质增值策略。异构现象如何影响反应活性、产物形成、选择性和副反应？通过使用同步辐射成像光电子光离子符合（iPEPICO）光谱，我们揭示了驱动苯二酚在H-ZSM-5催化剂上催化热解的初始短寿命反应中间体。详细的反应机理揭示了通向最重要产物和中间体的新途径。由于邻位的两个羟基，儿茶酚（-苯二酚）很容易脱水形成富烯酮，一种活性乙烯酮中间体，并表现出最高的反应活性。富烯酮在催化剂表面氢化生成苯酚或脱羰生成环戊二烯。对苯二酚（-苯二酚）大多脱氢生成-苯醌。间苯二酚，-苯二酚，是最稳定的异构体，因为由于羟基的位置，脱水和脱氢都涉及双自由基，是不利的。这三种异构体也可能在一个次要反应通道中相互转化，该通道通过脱羟基化和脱羰反应产生少量的环戊二烯和苯酚。我们提出了木质素催化热解中苯二酚的通用反应机理，并提供了关于异构现象如何影响转化和产物形成的详细机理见解。该机理解释了从分解反应到通过初始多环芳烃（PAH）形成步骤进行分子生长以生成萘的过程。后者涉及环戊二烯的狄尔斯-阿尔德二聚化、异构化和脱氢反应。
