空间分辨在线质谱揭示Mn-Na2WO4/SiO2催化甲烷氧化偶联自由基偶联机理

doi:10.1016/S1872-2067(25)64694-8

催化学报 ›› 2025, Vol. 74: 167-176.DOI: 10.1016/S1872-2067(25)64694-8

空间分辨在线质谱揭示Mn-Na₂WO₄/SiO₂催化甲烷氧化偶联自由基偶联机理

丁宁徐进^a, 王丹宇^a, 邹世辉^b, 范杰^b, Lyubov Alexandrovna Isupova^c, 郎峻宇^a^,^*(), 杨永^a^,^d^,^*()

^a上海科技大学物质科学与技术学院, 上海 201210, 中国
^b浙江省应用化学重点实验室, 浙江大学化学系, 浙江杭州 310036, 中国
^c俄罗斯科学院西伯利亚分校, 波列斯科夫催化研究所, 拉夫连季耶夫大街5号, 新西伯利亚, 俄罗斯
^d上海科技大学上海市高分辨电子显微镜重点实验室, 上海 201210, 中国

收稿日期:2025-01-07 接受日期:2025-03-18 出版日期:2025-07-18 发布日期:2025-07-20
通讯作者: *电子信箱: yangyong@shanghaitech.edu.cn (杨永),langjy@shanghaitech.edu.cn (郎峻宇).
基金资助:
国家重点研发项目(2023YFB4005200);国家自然科学基金(22272107);国家自然科学基金(22072092);上海科技大学物质学院分析测试中心(SPST-AIC10112914);上海科技大学高性能计算共享服务平台;上海市高分辨电子显微学重点实验室(21DZ2260400)

A spatial-resolved online MS study on OCM reaction catalyzed by Mn-Na₂WO₄/SiO₂ system for radicals coupling mechanistic insight

Ningxujin Ding^a, Danyu Wang^a, Shihui Zou^b, Jie Fan^b, Lyubov Alexandrovna Isupova^c, Junyu Lang^a^,^*(), Yong Yang^a^,^d^,^*()

^aSchool of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
^bKey Laboratory of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou 310036, Zhejiang, China
^cBoreskov Institute of Catalysis SB RAS, Lavrentiev Ave. 5, Novosibirsk 630090, Russia
^dShanghai Key Laboratory of High-resolution Electron Microscopy, ShanghaiTech University, Shanghai 201210, China

Received:2025-01-07 Accepted:2025-03-18 Online:2025-07-18 Published:2025-07-20
Contact: *E-mail: yangyong@shanghaitech.edu.cn (Y. Yang), langjy@shanghaitech.edu.cn (J. Lang).
Supported by:
Ministry of Science and Technology of National Key R&D Program of China(2023YFB4005200);National Natural Science Foundation of China(22272107);National Natural Science Foundation of China(22072092);Analytical Instrumentation Center(SPST-AIC10112914);HPC Platform of ShanghaiTech University;Shanghai Key Laboratory of High-resolution Electron Microscopy (Shanghai Science and Technology Plan, No. 21DZ2260400), SPST, ShanghaiTech University

摘要/Abstract

摘要：

甲烷氧化偶联反应(OCM)是将甲烷直接转化为C₂烃类(如乙烷和乙烯)的催化部分氧化过程, 学界普遍认为OCM遵循"多相-均相"协同催化机制: 即CH₄与O₂在催化剂表面活化生成CH₃^•自由基, 随后气相中的CH₃^•自由基通过偶联生成C₂H₆, 再经脱氢生成C₂H₄. 对于理解自由基行为, 通过产物空间分辨实验方法开展研究并关联催化活性至关重要. 传统催化剂评价流程仅通过对反应器出口尾气采样实现组分分析, 无法获取在反应过程中床层内部气体组分的动态分布信息. 而OCM反应温度最高达到约800 ^oC, 进一步增加了对空间分辨的表征的难度.

本文开发了一种空间分辨在线质谱(MS)系统, 并将其应用于Mn-Na₂WO₄/SiO₂催化的OCM体系. 除反应残气分析外, 该采样系统还能够通过采样毛细管在床层内的连续位移获得反应物与产物的空间分布信息. 以不同设定温度和7个空间采样点为参数, 采集各反应组分在线质谱信号并绘制出关联温度、时间与空间参数的反应活性图谱. 在不同固定位置的采样点上, 随着温度从658升至802 ^oC, CH₄与O₂的消耗量以及C₂和CO_x的生成速率均能呈现单调递增趋势; 而在恒定温度下转化和生成速率也随采样位置延伸而逐步增加, 结果均能与预期一致. 658 ^oC时全域反应活性接近于0; 706 ^oC时, 床层首端与末端信号已呈现显著差异; 显示MnNaWSi催化剂在Na₂WO₄熔点(约698 ^oC)附近有效活化OCM反应. 当温度升至754 ^oC及以上时, 转化和生成速率在毫米级位置呈现明显的阶跃式变化, 分别对各个采样点绘制阿伦尼乌斯图, 结果显示C₂表观活化能在750 ^oC出现明显降低, 而CO_x活化能保持不变, 表明C₂生成速率和选择性在该温区均明显上升并占据主导地位. 对比空间分辨的在线质谱以及空管对照实验结果, OCM反应在高温区以催化和无催化两种形式同时进行. 由空间各采样点的反应物和产物的碳平衡分析, 发现在各设定温度下催化床层内获得的碳平衡值均一致高于没有催化剂的区域. 表明催化剂在作为C-H活化活性中心的同时, 也为自由基提供了过渡吸附位点; 通过这种方式, 催化剂床层有效减缓或截留了来自上游的自由基迁移, 从而提高了催化床层的碳平衡率. 选择性分析与碳平衡分析有力支持了OCM的自由基偶联模型, 同时根据催化床层后碳平衡的变化趋势还进一步表明自由基存在显著自由程, 即在长度 > 2 mm的区域内仍保持有自由基富集. 基于此模型, 本文开发了一种通过紧靠催化剂床层下游位置迅速降低温度促进CH₃^•偶联的方法. 在保持其他条件不变的前提下, 当床层下游区域温度梯度累计降低120 ^oC时, C₂产率最高可提升至三倍.

综上, 本研究采用微反应器结合可移动毛细管在线质谱采样系统, 获得了反应床层内气体组分的空间分布信息. 结果表明, 在本研究反应条件下, 通过空间分辨在线质谱技术能够间接观测自由基并推导其行为特征; 而直接将反应活性与温度分布、采样空间共同关联, 还能够揭示反应中不同路径产物的竞争机制. 因此空间分辨在线质谱能够对反应动力学研究提供特殊的洞察信息, 是反应开发与工艺优化的有力工具.

关键词: 甲烷氧化偶联, Mn-Na₂WO₄/SiO₂催化剂, 在线质谱, 空间分辨动力学分析, 自由基偶联模型

Abstract:

Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that directly converts methane into C₂ products. For this high temperature reaction, understanding the radical behavior through experimental investigation is important in correlating the catalytic activity and the products. In this work, a spatial resolution online mass spectrometry (MS) system was developed and applied to a Mn-Na₂WO₄/SiO₂ catalyzed OCM system. In addition to the residue gas analysis, the system obtained the distribution information of the reactants and products in the reactor. At various setting temperatures, all species online MS signals were collected at different positions, mapping the reaction activity covering parameters including temperature, time and space. The distribution behavior of the catalytic activity, selectivity, and apparent activation energy were kinetically analyzed. Selectivity and additional carbon balance analysis strongly supported the radical coupling model of OCM and indicated that after the catalytic bed layer, there is a significant length in the reactor (> 2 mm) filled with radicals. Based on the result, a designed new method by tuning the temperature field in the reactor was found effectively to improve the catalytic activity, especially the C₂ yield from 702 to 773 °C.

Key words: Oxidative coupling of methane, Mn-Na₂WO₄/SiO₂ catalyst, Online mass spectrometry, Spatial-resolved kinetic analysis, Radical coupling model

丁宁徐进, 王丹宇, 邹世辉, 范杰, Lyubov Alexandrovna Isupova, 郎峻宇, 杨永. 空间分辨在线质谱揭示Mn-Na₂WO₄/SiO₂催化甲烷氧化偶联自由基偶联机理[J]. 催化学报, 2025, 74: 167-176.

Ningxujin Ding, Danyu Wang, Shihui Zou, Jie Fan, Lyubov Alexandrovna Isupova, Junyu Lang, Yong Yang. A spatial-resolved online MS study on OCM reaction catalyzed by Mn-Na₂WO₄/SiO₂ system for radicals coupling mechanistic insight[J]. Chinese Journal of Catalysis, 2025, 74: 167-176.

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图/表 8

Fig. 1. Schematic diagram of the reaction device. Upper inset: the heated region of the catalytic bed with a Fe-Cr-Alloy spiral wire tightly winding around the tube together with the ceramic armored/isolated K-type thermocouple.

Fig. 2. The normalized average flow rate of CH4 (a), O2 (b), C2 (c), and COx (d) for every point (left scale) and the corresponding conversion and yield (right scale) at different temperature. 20 mg Mn-Na2WO4/SiO2, CH4:O2:Ar = 1:1:4, GHSV = 12 L·h-1·gcat-1.

Fig. 3. (a) Average ratio of ethylene to ethane for every point at 706, 754, 773, and 802 °C. (b) The normalized average flow rate of CH4, O2, C2H4, C2H6, CO, CO2 (left scale) and the corresponding C balance (right scale) for every point at 706 °C. 20 mg Mn-Na2WO4/SiO2, CH4:O2:Ar = 1:1:4, GHSV = 12 L·h-1·gcat-1.

Fig. 4. Spatial-resolved C balance deviation plot, with results obtained at 754, 773, and 802 °C overlapping together. Reaction conditions: 20 mg Mn-Na2WO4/SiO2, CH4:O2:Ar = 1:1:4, GHSV = 12 L·h-1·gcat-1.

Fig. 5. The average C2 selectivity for every point at 706, 754, 773 and 802 °C. (a) Ratio of C2 to methane conversion. (b) Ratio of C2 to total C products. Reaction conditions: 20 mg Mn-Na2WO4/SiO2, CH4:O2:Ar = 1:1:4, GHSV = 12 L·h-1·gcat-1.

Fig. 6. Spatial-resolved Arrhenius plots of CO2 (a) and C2H6 (b) from 0 to 6 mm, with CH4:O2:Ar = 1:1:4, GHSV = 12 L·h-1·gcat-1. The fitting curve for CO2 covers the temperature range from 706 to 802 °C, while that for C2H6 covers from 754 to 802 °C (Extension lines are indicated by dotted lines). (c)The corresponding apparent activation energy (Ea) of CO2 and C2H6 at different positions.

Scheme 1. Energy diagram of radical coupling model in gas phase.

Fig. 7. Spatial-resolved C2 yield, COx yield and C2 selectivity at different temperatures for diving setup (a) and constant setup (b). Note: C2 yield and COx yield are obtained by deducting the 0 mm product rate.

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空间分辨在线质谱揭示Mn-Na₂WO₄/SiO₂催化甲烷氧化偶联自由基偶联机理

A spatial-resolved online MS study on OCM reaction catalyzed by Mn-Na₂WO₄/SiO₂ system for radicals coupling mechanistic insight

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