Isothermal activation of Mo2O52+-ZSM-5 precursors during methane reactions:: effects of reaction products on structural evolution and catalytic properties
Isothermal activation of Mo2O52+-ZSM-5 precursors during methane reactions:: effects of reaction products on structural evolution and catalytic properties
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DOI：
10.1039/b415166f
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发表时间：
2005-01-01
影响因子：
3.3
通讯作者：
Iglesia, E
中科院分区：
文献类型：
--
作者：
Lacheen, HS;Iglesia, E
在900 ~ 1000 K催化裂解CH4制烯烃和芳烃过程中，Mo-oxo前驱体交换到H-ZSM-5上的渗碳动力学对初始诱导期和稳态速率有很大影响。通过对流出流进行在线时间分辨质谱分析来检验共反应物和激活条件的影响，使用严格的分析来解释平衡对测量速率的影响。在H-ZSM-5中，乙烯共反应物及其在酸位上转化成的较大碳氢化合物导致交换的（Mo2O5）（5+）二聚体的增碳速度比纯CH4反应物快得多，诱导期也比纯CH4反应物短，但稳态热解速率不变，表明CH4和C2H4在交换的Mo-oxo前体增碳过程中形成相似的MoCx簇。在CH4反应前，在973 K下H-2处理导致Mo6+还原为Mo4+，在与CH4初始接触时，Mo6+比（Mo2O5）（5+）前驱体渗碳更快。如果考虑到逆向反应的影响，H-2预处理或CH4 - H-2混合物的使用不会影响稳态热解速率。在纯CH4流中，（Mo2O5）(5+)- ZSM- 5通过自催化过程在沸石通道内转化为活性的MoCx簇，其中在MoOx向MoCx结构的初始转化过程中最初形成的高级碳氢化合物导致下游催化剂部分更快的渗碳。同时，交换态（Mo2O5）（5+）和未交换态MoO3在渗碳初期形成的微量H2O和CO2抑制甚至阻止了渗碳，延长了活化期。C2H4的活化方案也成功地激活了更多难降解的高价金属氧，如WOx和VOx，交换到H-ZSM-5上。在纯CH4反应物中，活性碳化物结构的形成时间小于300 s，而VOx和WOx样品分别为4 ks和16 ks。这些活化方案导致VCx - ZSM5催化剂的活性大约是纯CH4反应物活化的三倍。
The dynamics of carburization of Mo-oxo precursors exchanged onto H-ZSM-5 strongly influence initial induction periods and steady-state rates during catalytic pyrolysis of CH4 to alkenes and arenes at 900 - 1000 K. The effects of co-reactants and of activating conditions were examined by on-line time-resolved mass spectrometric analysis of effluent streams using rigorous analyses to account for equilibrium effects on measured rates. Ethene co-reactants and the larger hydrocarbons to which it converts on acid sites in H-ZSM-5 led to much faster carburization of exchanged (Mo2O5)(5+) dimers and to shorter induction periods than with pure CH4 reactants, but steady-state pyrolysis rates were unchanged, indicating that CH4 and C2H4 form similar MoCx clusters during carburization of exchanged Mo-oxo precursors. H-2 treatment at 973 K before CH4 reactions led to reduction of Mo6+ species to Mo4+, which carburize faster than (Mo2O5)(5+) precursors during initial contact with CH4. This H-2 pretreatment or the use of CH4 - H-2 reactant mixtures did not influence steady-state pyrolysis rates, once contributions from reverse reactions were taken into account. With pure CH4 streams, (Mo2O5)(5+)- ZSM- 5 converts to active MoCx clusters within zeolite channels via autocatalytic processes, in which higher hydrocarbons, initially formed during initial conversion of MoOx to MoCx structures, lead to faster carburization of downstream catalyst sections. Concurrently, H2O and CO2 formed during this incipient carburization of exchanged (Mo2O5)(5+) and unexchanged MoO3 present in trace amounts inhibit and even prevent carburization and lengthen activation periods. Activation protocols with C2H4 were also successful in the activation of more refractory high-valent metal-oxo species, such as WOx and VOx, exchanged onto H-ZSM-5. The formation of active carbide structures occurred in less than 300 s, instead of 4 ks and 16 ks for VOx and WOx samples, respectively, in pure CH4 reactants. These activation protocols led to VCx - ZSM5 catalysts about three times more active than those activated in pure CH4 reactants.