Insights into the Mechanism and Reactivity of Zeolite-Catalyzed Alkylphenol Dealkylation

In the stride toward the production of low-carbon-footprint commodity chemicals, the development of a complete wood biorefinery plays a pivotal role. The lignin fraction of wood can be depolymerized and demethoxylated mainly into 4-alkylphenols. These phenolic compounds can further catalytically be C-dealkylated within the H-ZSM-5 zeolite at relatively high temperatures and in the presence of steam, producing phenol and olefins. Experimentally, the dealkylation reaction was found to have two striking features: first, different reactants possess very different reactivity. 4-Ethylphenol (4-EP) is somehow less reactive than 4-n-propylphenol (4-n-PP), which is in turn much less reactive than 4-isopropylphenol (4-iso-PP). Second, cofeeding of steam in the reaction mixture was necessary to prevent rapid and reversible catalyst deactivation. Herein, a combination of static and dynamic density functional theory (DFT) simulations is used to unravel the molecular and mechanistic origin of these observations. Free-energy profiles obtained from static calculations confirm the experimentally observed reactivity sequence, where our computations show that the secondary nature of the alkyl carbon involved in 4-iso-PP dealkylation strongly stabilizes the respective transition states. To investigate the effect of water on the mobility of the reactive species and their interaction with the active site, we investigated the diffusion of phenol along the H-ZSM-5 straight channel in the presence of water loadings from 0 to 3 molecules per zeolite unit cell. We show that water has a strongly beneficial effect in promoting desorption and diffusion of phenol away from the Brønsted acid site through competitive adsorption and by the formation of hydrogen bond chains with the diffusing phenol. This effect could lead to a shorter residence time inside the zeolite, preventing active site poisoning and condensation to bulkier biphenylether moieties.在推进低碳足迹大宗商品化学品（low-carbon-footprint commodity chemicals）生产的进程中，完备的木材生物炼制厂（wood biorefinery）建设发挥着至关重要的作用。木材中的木质素组分可经解聚与脱甲氧基反应，主要生成4-烷基苯酚（4-alkylphenols）；这类酚类化合物可在高温及水蒸气存在的条件下，于H-ZSM-5沸石（H-ZSM-5 zeolite）中经催化碳脱烷基反应，生成苯酚与烯烃。实验研究表明，该脱烷基反应具备两项显著特征：其一，不同反应物的反应活性差异悬殊，4-乙基苯酚（4-Ethylphenol, 4-EP）的反应活性略低于4-正丙基苯酚（4-n-propylphenol, 4-n-PP），而4-正丙基苯酚的反应活性又远低于4-异丙基苯酚（4-isopropylphenol, 4-iso-PP）；其二，向反应体系中共进料水蒸气是防止催化剂快速且可逆失活的必要条件。本研究结合静态与动态密度泛函理论（density functional theory, DFT）模拟，对上述实验现象的分子起源与反应机理进行解析，通过静态计算得到的自由能曲线验证了实验观测到的反应活性顺序，计算结果表明，参与4-异丙基苯酚脱烷基反应的烷基碳为仲碳，这一结构可显著稳定对应的过渡态。为探究水对反应物种迁移能力及其与活性位点相互作用的影响，本研究针对每个沸石晶胞含水0至3个分子的体系，考察了苯酚在H-ZSM-5沸石直孔道中的扩散行为，研究发现，水可通过竞争吸附以及与扩散中的苯酚形成氢键链，显著促进苯酚从布伦斯特酸位点（Brønsted acid site）脱附并向外扩散，从而产生强烈的积极作用，该效应可缩短苯酚在沸石内部的停留时间，避免活性位点被毒化，同时防止苯酚发生缩合反应生成体积更大的二苯醚类基团。