苄胺衍生物自發性氧化反應之動力學研究

Abstract

苄胺氧化反應(benzylamine oxidation reaction; BOR)可作為動力學緩慢且產物價值較低之產氧反應(oxygen evolution reaction; OER)的替代陽極反應，不僅能於溫和條件下合成具有高附加價值之苯甲腈(benzonitrile)，亦可降低電解系統之能量需求，然而現有直接電催化BOR多於鹼性水系電解液中施加陽極電位，以驅動Ni(OH)2氧化形成具BOR催化活性之NiOOH，雖然NiOOH之生成可促進苄胺氧化，但其形成與持續再生所需之鹼性水系環境及陽極電位，可能促使BOR過程之中間體苯甲亞胺(benzylimine)發生水解反應，生成苯甲醛(benzaldehyde)，而苯甲醛可進一步與未反應之苄胺進行縮合反應，形成N-亞苄基苄胺(N-benzylidenebenzylamine; N-BB)，此外，目標產物苯甲腈亦可能經逐步水解，先形成苯甲醯胺(benzamide)，再進一步轉化為苯甲酸(benzoic acid)，導致其選擇性降低，因此NiOOH之生成與持續再生所需之反應條件，與BOR中間體及目標產物之穩定性難以同時兼顧。 為解決上述問題，本研究提出結合亞鐵氰化物/鐵氰化物(Ferro/Ferri; [Fe(CN)6]4−/[Fe(CN)6]3−)液態氧化還原儲庫(redox reservoir; RR)與Ni(OH)2/NiOOH固態RR之混合式RR系統，於電化學階段，以濺鍍銀(Ag)之氣體擴散電極(gas diffusion electrode; GDE)作為陰極工作電極，進行二氧化碳還原反應(CO2 reduction reaction; CO2RR)生成含一氧化碳(CO)與氫氣(H2)之合成氣(syngas)，同時Ferro於陽極氧化為Ferri以暫時儲存氧化當量，隨後Ferri將Ni(OH)2氧化為NiOOH，使氧化當量由液態RR轉移至固態RR，最後，預先生成之NiOOH在無外加電位且未添加電解液之純苄胺中驅動BOR，藉此將NiOOH之生成環境與苄胺氧化環境分離，並於十個循環中實現高達99.06 μmol min−1之苯甲腈平均生成速率，所搭配之CO2RR於−20、−30、−60 mA cm−2之電流密度下，CO法拉第效率皆可維持於70%以上。 此外，本研究利用X光繞射(X-ray diffraction; XRD)分析證實，所使用之Ni(OH)2為β相，且β-Ni(OH)2於氧化過程中會先轉變為β-NiOOH，並於進一步氧化後形成γ-NiOOH，X光吸收近邊結構(X-ray absorption near-edge structure; XANES)比較結果顯示，γ-NiOOH具有較高之平均Ni氧化態，延伸X光吸收精細結構(extended X-ray absorption fine structure; EXAFS)擬合結果顯示，γ-NiOOH之Ni–O與Ni–Ni鍵長分別為1.88與2.81 Å，均短於β-NiOOH之1.90與2.84 Å，動力學分析結果顯示，由β-NiOOH與γ-NiOOH催化之BOR皆遵循一級反應動力學，其中以γ-NiOOH催化BOR時之反應速率常數為0.1203 min−1，高於β-NiOOH之0.1057 min−1，理論計算結果顯示，γ-NiOOH催化BOR時，速率決定步驟之活化能障為1.54 eV，低於β-NiOOH之1.95 eV，與實驗所得之動力學趨勢一致，說明以γ-NiOOH催化BOR時有較快之動力學，最後，不同對位取代苄胺衍生物於γ-NiOOH作用下之氧化反應速率常數，分別為4-甲氧基苄胺之0.1564 min−1、4-甲基苄胺之0.1389 min−1及4-氟苄胺之0.0860 min−1，相較於未取代苄胺(0.1203 min−1)，推電子取代基之存在可提高氧化反應速率，而拉電子取代基之存在則使反應速率降低，顯示取代基電子效應會影響BOR動力學，並展現γ-NiOOH催化不同苄胺衍生物氧化之通用性。

The benzylamine oxidation reaction (BOR) can serve as an alternative anodic reaction to the kinetically sluggish oxygen evolution reaction (OER), which produces relatively low-value oxygen (O2). BOR not only enables the synthesis of high-value benzonitrile under mild conditions but also reduces the energy demand of the electrolysis system. However, conventional direct electrocatalytic BOR is typically conducted in alkaline aqueous electrolytes under an applied anodic potential to oxidize Ni(OH)2 to NiOOH, which is catalytically active toward BOR. Although the formation of NiOOH facilitates benzylamine oxidation, the alkaline aqueous environment and anodic potential required for its formation and continuous regeneration may promote the hydrolysis of benzylimine, an intermediate in the BOR process, to benzaldehyde. Benzaldehyde can subsequently undergo condensation with unreacted benzylamine to form N-benzylidenebenzylamine (N-BB). In addition, the target product benzonitrile may undergo stepwise hydrolysis, first to benzamide and subsequently to benzoic acid, thereby lowering the selectivity toward benzonitrile. Therefore, the reaction conditions required for the formation and continuous regeneration of NiOOH are difficult to reconcile with those required to maintain the stability of the BOR intermediate and the target product. To address this issue, a hybrid redox reservoir (RR) system combining a ferrocyanide/ferricyanide (Ferro/Ferri; [Fe(CN)6]4−/[Fe(CN)6]3−) liquid RR with a Ni(OH)2/NiOOH solid RR was developed in this study. During the electrochemical stage, a silver-sputtered gas diffusion electrode (Ag-GDE) was employed as the cathodic working electrode for the CO2 reduction reaction (CO2RR), producing syngas comprising carbon monoxide (CO) and hydrogen (H2). Simultaneously, Ferro was oxidized to Ferri at the anode, thereby temporarily storing oxidation equivalents in the liquid RR. Ferri subsequently oxidized Ni(OH)2 to NiOOH, transferring the stored oxidation equivalents from the liquid RR to the solid RR. Finally, the pre-generated NiOOH drove BOR in neat benzylamine without an externally applied potential and without added electrolyte, thereby separating the reaction environment for NiOOH generation from that for benzylamine oxidation. Over ten cycles, an average benzonitrile formation rate of 99.06 μmol min−1 was achieved. Meanwhile, the coupled CO2RR maintained CO Faradaic efficiencies above 70% at current densities of −20, −30, and −60 mA cm−2. Furthermore, X-ray diffraction (XRD) analysis confirmed that the Ni(OH)2 used in this study was in the β phase. During oxidation, β-Ni(OH)2 was first transformed into β-NiOOH, which subsequently converted into γ-NiOOH upon further oxidation. X-ray absorption near-edge structure (XANES) analysis revealed that γ-NiOOH exhibited a higher average Ni oxidation state. Extended X-ray absorption fine structure (EXAFS) fitting further showed that the Ni–O and Ni–Ni bond lengths of γ-NiOOH were 1.88 and 2.81 Å, respectively, both shorter than the corresponding values for β-NiOOH (1.90 and 2.84 Å). Kinetic analysis showed that BOR catalyzed by both β-NiOOH and γ-NiOOH followed first-order kinetics. For BOR catalyzed by γ-NiOOH, the rate constant was 0.1203 min−1, higher than the corresponding value obtained with β-NiOOH (0.1057 min−1). Theoretical calculations showed that the activation barrier for the rate-determining step of BOR catalyzed by γ-NiOOH was 1.54 eV, lower than the corresponding barrier for β-NiOOH-catalyzed BOR (1.95 eV). This result was consistent with the experimentally observed kinetic trend, indicating that BOR catalyzed by γ-NiOOH exhibited faster kinetics than that catalyzed by β-NiOOH. Finally, the oxidation rate constants of different para-substituted benzylamine derivatives in the presence of γ-NiOOH were 0.1564 min−1 for 4-methoxybenzylamine, 0.1389 min−1 for 4-methylbenzylamine, and 0.0860 min−1 for 4-fluorobenzylamine. Relative to unsubstituted benzylamine (0.1203 min−1), derivatives bearing electron-donating substituents exhibited higher oxidation rate constants, whereas the derivative bearing an electron-withdrawing substituent exhibited a lower rate constant. These results demonstrate that substituent electronic effects influence BOR kinetics and further establish the general applicability of γ-NiOOH to the oxidation of different benzylamine derivatives.