直接催化甲烷(CH4)氧化转化制备甲醇(DMTM)是具有较高绿色化学原子经济性的反应过程,且可在常温下进行,是潜在的实现CH4转化升级的重要过程.作为"圣杯反应",DMTM性能通常显著受氧化剂影响,使用氧气(O2)作为氧化剂一步实现DMTM仍然极具挑战性.至今,双氧水(H2O2)仍是被报道最多的具有较高CH4转化速率和甲醇(CH3OH)选择性的绿色氧化剂.为了深入理解氧化剂如何影响DMTM反应性能,本文基于密度泛函理论计算和微观动力学分析研究了在Cu-ZSM-5,Cu-MOR和Cu-SSZ-13三种具有不同微孔尺寸的单核铜分子筛上DMTM反应机理,以确定H2O2作为氧化剂在DMTM反应中的优势和局限性. 通过理论计算对比在反应条件下O2和H2O2的O-O键活化以及CH4的C-H键活化过程,发现在单核Cu分子筛中,H2O2的O-O键通过水介导机制极易被活化,并形成活性Z[Cu(OH)2]+中心,该活性中心所含的表面羟基自由基能够在低温下活化CH4的C-H键,进而高效的形成CH3OH.随着分子筛微孔孔径增大,其更有利于表面羟基自由基的形成并促进C-H键的活化.相反,在低温下O2的O-O键难以被具有不同微孔孔径的单核Cu分子筛催化断键,难以生成表面活性氧物种,进而抑制了CH4的C-H键活化.与O2相比,H2O2能显著提升甲烷的活化转化. 虽然H2O2能使得CH4被快速转化为CH3OH,但计算结果表明由于活性中心Z[Cu(OH)2]+上C-H键优先通过自由基机理活化,CH3OH较弱的C-H键更易断键,从而难以协调CH3OH选择性和CH4转化之间的竞争.同时,较弱的O-H键使得Z[Cu(OH)2]+将优先催化H2O2自分解.尽管长期而言,使用H2O2作为氧化剂存在上述瓶颈,但动力学分析表明,相比于现有报道工作,以H2O2作为氧化剂时,通过改进催化剂和优化反应条件,仍能显著提升甲烷转化率和甲醇选择性.综上,本文研究了甲烷直接制甲醇过程中氧化剂对于反应性能的影响机制,为进一步设计改进甲烷转化催化剂、优化反应条件和探索新的转化途径提供借鉴.
The efficiency of direct catalytic oxidation of methane to methanol(DMTM)is significantly influ-enced by oxidants.However,realizing a one-pot DMTM using dioxygen remains challenging.Hy-drogen peroxide is still the most frequently reported green oxidant for DMTM,with high selectivity for methanol.To gain insight into the influence of oxidants on DMTM performance,we computa-tionally investigated the reaction mechanisms involved in DMTM using H2O2 at mono-copper sites in three types of Cu-exchanged zeolites with different micropore sizes.We identified the advantages and limitations of H2O2 as an oxidant.In contrast to the O-O bond in O2,the O-O bond in H2O2 can be easily broken to produce reactive surface oxygen species,which enable the facile C-H bond activa-tion of methane at a low temperature.However,because of the radical-like process of C-H bond activation at mono-copper sites,actualizing the preferential C-H bond activation of methane is kinetically challenging compared to that of methanol.Moreover,the lower O-H bonding energy of H2O2 would result in self-decomposition of H2O2.Despite these bottlenecks,kinetic analysis shows that improving catalysts to boost the DMTM performance using H2O2 is a promising approach.