具有不同活性中心的固体催化剂的失活模式

 An approach is suggested to distinguish different types of active sites responsible for different reactions on bifunctional catalysts. The model assumes a non-uniform vulnerability of active sites that depends on their location. Problems on the relationship between the dispersion of the active phase and selectivity are discussed. The effect of coke formation on the activity change of different sites is analyzed.     

Improved Performance of W／HZSM-5 Catalysts for Dehydroaromatization of Methane

The dehydroaramatization of methane over W-supported ZSM-5 with varying degrees of Li^ ion-exchanged catalysts was studied with and without oxygen at 1073 K and atmospheric pressure.Catalyst activity and stability were found to be influenced by the catalyst acidity related to BrSnsted acid sites and by the presence of oxygen in the feed. The NH3-TPD and FTIR-pyridine results demonstrated that partially exchanged of H^ ions by Li^ into the W/HZSM-5 catalysts could be used to control the amount of strong acid sites on the catalyst surface. Without oxygen, the 3WHLi-Z (5:1) catalyst that has strong acid sites equal to nearly 74% of the original strong acid sites in the parent HZSM-5 exhibited the highest methane conversion and selectivity towards aromatics. However, the catalyst deactivated in a five hour period. In the presence of oxygen, the catalyst activity and stability could be improved further.The results of this study revealed that a suitable amount of strong Bronsted acid sites as well as oxygen addition in the feed increased the catalyst activity and stability. The 3WHLi-Z(5:1) catalyst exhibited improved performance in the dehydroaromatization of methane.     

Reversible Nature of Coke Formation on Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

Non‐oxidative dehydroaromatization of methane over Mo/ZSM‐5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM‐5 catalysts and a wide range of reaction temperatures and space velocities. High‐pressure operando X‐ray absorption spectroscopy demonstrated that the structure of the active Mo‐phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass‐spectrometry and ¹³C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space.     

钼源对钼基催化剂芳构化性能的影响

 采用乙酸回流MoO3的方法制备了MoAC, 以其为钼源分别制备了Mo/HMCM-22, Mo/HMCM-49和Mo/HZSM-5催化剂,并考察了各催化剂在甲烷无氧芳构化反应中的催化性能. 结果表明,以MWW型分子筛(HMCM-22和HMCM-49)为载体时,催化剂的芳烃收率和催化剂稳定性均比传统浸渍法制备的催化剂明显提高; 而以HZSM-5为载体时,催化剂的甲烷无氧芳构化活性无明显变化.     

程序升温技术研究Mo/HZSM-5催化剂上甲烷无氧芳构化反应积炭的化学特性

对经过程序升温表面甲烷无氧芳构化反应(TPSR)后的Mo/HZSM-5催化剂上的积炭进行了程序升温加氢反应(TPH)和程序升温CO₂反应(TPCO₂),并对相应催化剂上的积炭进行了程序升温氧化反应(TPO)和热重(TG)实验.TPH实验及TPCO₂实验表明,H₂主要对高温烧炭峰发生作用.CO₂则可同时对这两种烧炭峰产生影响,对低温峰积炭的影响尤其明显.对不同Mo含量的催化剂进行上述实验的结果表明,Mo担载量的变化及不同的实验过程都会影响催化剂上两种温峰的积炭在总积炭量中所占的比例.由TG实验数据计算出的动力学结果说明,无论是TPH还是TPCO₂实验都可以降低烧炭过程的一级反应起始温度,并减少其活化能.此外,催化剂中Mo担载量的变化也会对开始温度和烧炭活化能产生明显影响.     

Methanol-to-Olefin Conversion over Zeolite Catalysts: Active Intermediates and Deactivation

Methanol-to-olefin (MTO) conversion over various zeolites with different topologies, Si/Al molar ratios, and crystallite sizes were investigated to verify the effects of pore shape and size, acidity, and external surface area on the catalytic activity, product selectivity, and deactivation. The IR and electron spin resonance (ESR) study of zeolite catalysts used in MTO also proceeded to deduce the active intermediates formed in their cages or pores. The zeolites with 8 membered-ring (MR) pore entrances such as CHA, ERI, LTA, and UFI commonly exhibited high selectivity to lower olefins due to their small entrances, but the CHA catalyst with the smallest cage maintained its activity longer than other 8MR zeolites. The slow condensation of polymethylbenzene (PolyMB) to polyaromatic hydrocarbons (PAH) on MOR zeolite with a high Si/Al molar ratio due to its low concentration of strong acid sites resulted in a slow deactivation. The extremely small crystallites of H-SAPO-34 and H-ZSM-5 less than 100 nm showed an adverse effect in MTO; while the large crystallites above 1,000 nm also exhibited poor catalytic performance because of their small external surface. The study of IR regarding the adsorbed and occluded materials on zeolites demonstrated the effect of pore shape and size on the active intermediates: the zeolites with larger pores and cages allowed the formation of alkylbenzenes with long alkyl groups which preferred to be condensated to PAH. The well-resolved hyperfine splitting of ESR spectra observed on H-SAPO-34 used in MTO clearly illustrated the presence of hexamethylbenzenium radical cations. The small intersections of phosphorous-modified H-ZSM-5 allowed the formation of tetramethylbenzenium radical cations in MTO. The formation of PolyMB radical cations, their role as active intermediates and the effect of topology, acidity, and crystallite size of zeolites on their deactivation were discussed.     

Ethylene Dehydroaromatization over Ga-ZSM-5 Catalysts: Nature and Role of Gallium Speciation

Bifunctional catalysis in zeolites possessing both Brønsted and Lewis acid sites offers unique opportunities to tailor shape selectivity and enhance catalyst performance. Here, we examine the impact of framework and extra-framework gallium species on enriched aromatics production in zeolite ZSM-5. We compare three distinct methods of preparing Ga-ZSM-5 and reveal direct (single step) synthesis leads to optimal catalysts compared to post-synthesis methods. Using a combination of state-of-the-art characterization, catalyst testing, and density functional theory calculations, we show that Ga Lewis acid sites strongly favor aromatization. Our findings also suggest Ga(framework)–Ga(extra-framework) pairings, which can only be achieved in materials prepared by direct synthesis, are the most energetically favorable sites for reaction pathways leading to aromatics. Calculated acid site exchange energies between extra-framework Ga at framework sites comprised of either Al or Ga reveal a site-specific preference for stabilizing Lewis acids, which is qualitatively consistent with experimental measurements. These findings indicate the possibility of tailoring Lewis acid siting by the placement of Ga heteroatoms at distinct tetrahedral sites in the zeolite framework, which can have a marked impact on catalyst performance relative to conventional H-ZSM-5.     

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM‐5

Non‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.     

Mo/HZSM-5催化剂上甲烷无氧芳构化反应——HZSM-5分子筛脱铝的影响

采用两种不同的脱铝方法对HZSM-5分子筛进行了预处理,并利用MAS NMR和吸附吡啶的FT-IR对分子筛的结构和酸性质进行了表征,考察了分子筛的脱铝程度对Mo基催化剂上甲烷芳构化反应性能的影响.结果表明,HZSM-5分子筛的酸性过强或B酸量不足,均会导致催化剂严重积炭,但积炭成因不同.母体HZSM-5分子筛上的强B酸中心的存在可促使催化剂上反应中间物种深度脱氢,造成催化剂在反应过程中严重积炭.经水热处理的HZSM-5分子筛,骨架铝脱出严重,造成B酸活性中心不足以及部分微孔阻塞,不利于C2中间物种芳构化,导致芳烃选择性显著降低.经高温N2处理的HZSM-5分子筛,骨架铝脱出相对缓和,在消除母体分子筛上强B酸中心的同时,保留了较多的弱B酸中心,既可满足C2中间物种芳构化反应的需要,又可有效抑制催化剂积炭,导致甲烷芳构化反应性能显著改善.     

乙烷添加对 Mo/HZSM-5 催化剂上甲烷芳构化反应的影响

 研究了乙烷添加对 Mo/HZSM-5 催化剂上甲烷芳构化反应性能的影响. 在所考察的反应条件下, 未观察到乙烷添加对甲烷转化的促进作用, 乙烷本身反而生成甲烷, 同时导致更高的积炭生成速率, 使 Mo/HZSM-5 催化剂更快地失活. 但添加乙烷加速了钼活性中心的形成, 缩短了芳构化反应的诱导期, 使苯的生成提前.     

多级结构ZSM-5沸石分子筛的合成及其Mo基催化剂在甲烷无氧脱氢芳构化中的应用

以经盐酸预处理的碳纳米管为第二模板,在不添加其它有机溶剂的情况下,仅通过控制晶化条件,即采用变温水热晶化法合成具有多级结构的ZSM-5分子筛.通过x射线衍射、红外光谱测试、透射电镜和N2吸附对合成的分子筛进行了表征,结果表明,该合成分子筛呈近球形,是由纳米棒自组装形成的具有多级结构的亚微米球.该分子筛改性后用于甲烷无氧脱氢芳构化反应,显示出良好的催化性能,甲烷转化率最初达到19%,反应至24 h时甲烷转化率仍保持在10%左右,并且保持了较高的芳香物选择性(达到50%以上).     

无第二模板剂法合成多级结构ZSM-5分子筛微球及其在甲烷无氧芳构化反应中的应用

在无第二模板剂的条件下,采用简单的水热晶化法,通过控制条件合成出具有多级结构的ZSM-5(ZSM-5-HW)分子筛,并运用X射线衍射、红外光谱、扫描电镜和N2吸附-脱附等技术对合成的分子筛进行了表征.结果表明,所得样品是由棒状晶体组装而成的具有介孔结构的分子筛微球.用等体积浸渍法制备了Mo/HZSM-5-HW催化剂并用于CH4无氧芳构化反应,表现出较高的催化活性和稳定性.在实验条件下,CH4初始转化率为18.5%,而苯收率最高可达9.5%;反应24h后,两者仍然分别保持在10.2%和5.5%左右.     

不同载体成型的 Mo/ZSM-5 催化剂上甲烷无氧芳构化反应

 以不同氧化物为载体, 通过挤条成型法制备了 Mo/ZSM-5 催化剂, 考察了其催化甲烷无氧芳构化反应性能. 采用吸附吡啶红外光谱、氨程序升温脱附和氢程序升温还原等方法对催化剂进行了表征. 结果表明, ZnO 的添加使催化剂的强酸量减少, 强 B 酸比例降低, Mo 物种还原能力提高, 因而催化剂表现出较高的甲烷芳构化活性和较低的积炭选择性.     

单原子催化中的活性位构建方法研究进展

近年来,单原子催化剂因具有多相催化剂的结构稳定、易分离和均相催化剂的“孤立位点”等潜在优点而受到了人们的广泛关注.因此我们着重对比说明了单原子催化剂多种制备方法的形成机制以及制备过程的优缺点,并结合相应反应对其催化活性、催化机理进行介绍.通过对单原子催化剂的制备研究现状的概述,以期对加快其工业化进程起到积极的促进作用.     

蒸气相法ZSM-5分子筛的合成及其负载的Mo催化剂在甲烷芳构化中的应用

采用蒸气相法使无定形凝胶在水蒸气中结晶形成ZSM-5分子筛,通过X射线衍射、红外光谱、热重、扫描电镜以及能量色散型X射线荧光分析对合成的分子筛进行了表征.结果表明,蒸气相法比水热法更易合成出粒径均匀、形貌规则的小颗粒分子筛(粒径为150～250nm),将其负载Mo后用于甲烷无氧芳构化反应中,表现出更好的催化性能,初期甲烷转化率达17.5%,同时表现出更高的稳定性和容炭能力.     

Unraveling the Nature of Sites Active toward Hydrogen Peroxide Reduction in Fe-N-C Catalysts

Fe-N-C catalysts with high O₂ reduction performance are crucial for displacing Pt in low-temperature fuel cells. However, insufficient understanding of which reaction steps are catalyzed by what sites limits their progress. The nature of sites were investigated that are active toward H₂O₂ reduction, a key intermediate during indirect O₂ reduction and a source of deactivation in fuel cells. Catalysts comprising different relative contents of FeN_xC_y moieties and Fe particles encapsulated in N-doped carbon layers (0–100 %) show that both types of sites are active, although moderately, toward H₂O₂ reduction. In contrast, N-doped carbons free of Fe and Fe particles exposed to the electrolyte are inactive. When catalyzing the ORR, FeN_xC_y moieties are more selective than Fe particles encapsulated in N-doped carbon. These novel insights offer rational approaches for more selective and therefore more durable Fe-N-C catalysts.     

Revealing Active Sites and Reaction Pathways in Methane Non-Oxidative Coupling over Iron-Containing Zeolites

Non-oxidative coupling of methane is a promising route to obtain ethylene directly from natural gas. We synthesized siliceous [Fe]zeolites with MFI and CHA topologies and found that they display high selectivity (>90 % for MFI and >99 % for CHA) to ethylene and ethane among gas-phase products. Deactivated [Fe]zeolites can be regenerated by burning coke in air. In situ X-ray absorption spectroscopy demonstrates that the isolated Fe³⁺ centers in zeolite framework of fresh catalysts are reduced during the reaction to the active sites, including Fe²⁺ species and Fe (oxy)carbides dispersed in zeolite pores. Photoelectron photoion coincidence spectroscopy results show that methyl radicals are the reaction intermediates formed upon methane activation. Ethane is formed by methyl radical coupling, followed by its dehydrogenation to ethylene. Based on the observation of intermediates including allene, vinylacetylene, 1,3-butadiene, 2-butyne, and cyclopentadiene over [Fe]MFI, a reaction network is proposed leading to polyaromatic species. Such reaction intermediates are not observed over the small-pore [Fe]CHA, where ethylene and ethane are the only gas-phase products.     

一种获得高活性和高稳定性甲烷无氧芳构化Mo/HZSM-5催化剂的前处理方法

 利用氢气在较低温度(623 K)下对6%Mo/HZSM-5催化剂进行预处理,考察了经低温还原预处理的催化剂在甲烷无氧芳构化反应中的催化性能. 结果表明,经过预处理的催化剂比未经预处理的催化剂表现出更好的甲烷转化活性和稳定性. 1H MAS NMR和EPR表征结果表明,在623 K氢气气氛中对Mo/HZSM-5催化剂进行预处理不会造成Mo物种在分子筛外表面的进一步分散,也不会导致外表面的Mo物种向孔道内迁移. 但是,低温还原预处理可以有效地促进Mo物种由易还原的六方密堆积结构向难还原的面心立方结构转化,后者在反应中具有更好的活性和稳定性.     

Mo/HZSM-5催化剂上甲烷芳构化反应行为的改善-催化剂制备因素及反应物添加剂的考察

考察了6Mo/HZSM-5催化剂的不同制备因素对其甲烷芳构化反应性能的影响,包括甲烷气氛下以不同的程序升温速率(从600℃到700℃)预还原催化剂,及采用热溶液(70～80℃)浸渍制备催化剂等方法.在t=700℃,P=0.1MPa,SV=1500mL/(g-cat·h)的反应条件下,发现适当的程序升温速率预还原和热溶液浸渍制备等方法可以改善6Mo/HZSM-5催化剂的甲烷芳构化反应性能,提高催化剂的反应活性及稳定性.同时还考察了1.5%二甲醚作为反应物添加剂在不同反应温度、不同Mo担载量的HZSM-5催化剂上的甲烷芳构化反应性能,并与CO2,H2O的反应结果进行了对比.实验结果表明,在P=0.1MPa,SV=1500mL/(g-cat·h)的反应条件下,较高的反应温度及适量增加催化剂Mo担载量有利于进一步改善1.5%二甲醚/甲烷共进料情况下的芳构化反应行为;反应6h后催化剂的TEM照片显示,二甲醚与甲烷共进料改变了催化剂上积碳的形貌.