Hydrothermal Post-synthesis of HZSM-5 Zeolite to Enhance the Coke-resistance of Mo/HZSM-5 Catalyst for Methane Dehydroaromatization

Hydrothermal post-synthesis was used to modify the micropores and acidity of commercially available HZSM-5 zeolites. The recrystallization and the dynamic incorporation and extraction of the framework Al not only stabilized the framework with high crystallinity, but also inhibited the creation of extra-pores during the post-synthesis in NaOH aqueous solution. The resulted Mo/HZSM-5 catalyst showed rather high catalytic stability and greatly enhanced selectivity towards aromatics for methane dehydroaromatization reaction by effectively inhibiting the coke formation.     

CVD Synthesis in Static Mode of Mo/H-ZSM5 Catalyst for the Methane Dehydroaromatization Reaction to Benzene

Chemical vapor deposition (CVD) in a fused ampoule (static mode) was applied for synthesis of the series of Mo/H-ZSM5 catalysts. Kinetic data, TPO and EXAFS observation were evaluated in terms of effective dispersion of MoO₃ within the zeolite channels in 3% Mo/H-ZSM5 which provided an excellent activity and stability for methane dehydrocondensation reaction to benzene. The same catalyst doped with 0.5% La₂O₃ revealed significant suppression of coke and gave a little lower formation rate of benzene, probably due to formation of La₂O₂CO₃ species staying in equilibrium with gas phase of CO₂. La₂O₂CO₃ (Ia) phase formation was confirmed by X-ray diffraction experiment.     

Highly Stable Performance of Methane Dehydroaromatization on Mo/HZSM-5 Catalyst with a Small Amount of H2 Addition into Methane Feed

With a small amount of H₂ (3 6%) addition into methane feed, coke formation on 6 wt% Mo catalyst during the methane dehydroaromatization reaction was effectively suppressed and the catalyst stability was increased evidently under the reaction conditions of 1023K, 0.3MPa and 2520 mL g-MFI^-1 h^-1 of methane space velocity.     

Creating Mesopores in ZSM-5 Zeolite by Alkali Treatment: A New Way to Enhance the Catalytic Performance of Methane Dehydroaromatization on Mo/HZSM-5 Catalysts

Well-controlled treatment with alkali solution causes the etching of HZSM-5 framework, which results in the formation of the new porosity and channel structure with the coexistence of micropores and mesopores, as evidenced by nitrogen adsorption experiments. The dissolution of the zeolite framework, as revealed by the investigation of solid-state NMR, begins from the crystalline site with Si–O–Si linkages. The inertness of the alkali treatment toward Si–O–Al bond in the framework preserves the specific Brønsted acid site that is defined to be the bridging OH species over Si–O–Al units in zeolite. The Mo-modified catalysts derived from the alkali treatments showed a very high catalytic performance in the conversion of methane to aromatics (MDA) when compared with the conventional Mo/HZSM-5 catalyst. The unique selectivity to aromatics and stability of the catalysts derived from the alkali-treated ZSM-5 are attributed to the coexistence of mesopores and inherent micropores in the zeolites, which optimizes an environment for catalytic reaction and mass transfers. The channel with mainly 3–5?nm in diameters in the zeolites serves as the “aisle” to enhance the diffusion of molecules, especially the aromatics molecules, while the micropores have been identified to be the active cavities for the aromatics formation.     

Effective Coke Removal in Methane to Benzene (MTB) Reaction on Mo/HZSM-5 Catalyst by H2 and H2O Co-addition to Methane

The catalytic dehydro-aromatization reaction over Mo/HZSM-5 catalyst was drastically stabilized by the co-addition of 5.4% H₂ and 1.8% H₂O to methane feed at 750 °C, 0.3 MPa and methane space velocity of 3000 mL g⁻¹ h⁻¹, suppressing the coke formation effectively, compared with single hydrogen or steam addition.     

Methane dehydroaromatization over Mo/HZSM-5: A study of catalytic process

The deactivation process of methane dehydroaromatization (MDA) reaction has been followed by various physical chemistry methods.     

Methane Aromatization over 2 wt% Mo/HZSM-5 in the Presence of O2 and NO

In the non-oxidative aromatization reaction (temperature = 770 C, flow rate = 34 ml min^-1), 2 wt% Mo/HZSM-5 deactivated after 4 h due to severe coking. We observed that with a suitable amount of O₂ (5.3 vol%) in the methane feed, the catalyst could last for more than 6 h with a ca. 4% yield of aromatics at 770 °C. Depending on the concentration of O₂ or the reaction temperature, there are three reaction zones in the catalyst bed: (i) methane oxidation; (ii) methane reforming; and (iii) methane aromatization. CO and H₂ produced in the first two zones are accountable for stability amelioration of the catalyst. The addition of NO exhibited similar effects on the reaction. Further increase in O₂ (8.4 vol%) or NO (14.2 vol%) concentration would result in CO and CO₂ being the predominant carbon-containing products; C₂H₄ and C₂H₆ were generated in small amounts and no aromatics were detected.     

钇对甲烷芳构化催化剂Mo／HZSM—5性能影响的研究

研究了稀土钇的含量对Ｍｏ／ＨＺＳＭ－５催化剂的活性和选择性的影响，发现稀土钇的加入，不同程度上提高了Ｍｏ／ＨＺＳＭ－５的活性和选择性。特别是，当Ｙ／Ｍｏ＝０．０４时，活性最佳。甲烷在１０２３Ｋ芳构化反应，转化率达１９．６％，苯的选择性达９６．５％，且活性较稳定。     

高分散Mo/HMCM-22催化剂制备及其甲烷芳构化性能

采用两段程序升温焙烧制备了Mo/HMCM-22催化剂,并利用X射线衍射、N2吸附-脱附及X射线光电子能谱对催化剂的结构和Mo物种的分散进行了表征。与传统的一段程序升温焙烧的Mo/HMCM-22催化剂相比,该催化剂具有更大的微孔表面积和微孔容积,Mo物种分散度高,更多的存在于分子筛的孔道中,因而在甲烷芳构化反应中表现出更好的催化性能,具有高的甲烷转化率和芳烃产率。     

Mo/HZSM-5上甲烷无氧芳构化催化剂制备因素的考察

采用Mo/HZSM-5作为甲烷无氧芳构化催化剂,考察了催化剂制备部分影响因素.结果表明,采用较低的nSi:nAl时(25)载体制备的催化剂活性和稳定性较好;浸渍法优于固相反应法;Mo担载的质量分数为4%时,催化剂表现出最高活性;分子筛预先经碱处理,能够明显改善催化剂稳定性.     

A Transient Behaviour in the Initial Production of Aromatic Compounds from Methane Catalyzed by Mo/HZSM-5

The production of aromatic compounds directly from methane using catalysts based on zeolite HZSM-5 impregnated with molybdenum was investigated using a flow reactor at 750 °C. Main products were benzene and naphthalene as well as toluene and others like azulene in smaller amounts. Naphthalene appeared after benzene following an induction period suggesting a consecutive reaction path. A transient behavior in the production of naphthalene was observed along the time on stream, being benzene practically the only product at the steady state. The transient behavior disappeared when increasing contact time, which produced more naphthalene at the expense of benzene.     

Dehydrogenation of Methane over Mo/ZSM-5. Effects of Additives in the Methane Stream

The dehydrogenation of methane was carried out over a Mo/ZSM-5 catalyst. It was revealed that the purity of the methane was very critical for the evaluation of the catalyst activity. In order to study the phenomenon, the effects of the addition of O₂, CO₂, CO or H₂ to the feed were investigated. A small amount of O₂ increased the amounts of aromatic compounds and CO produced. The addition of H₂ scarcely affected the conversion of methane, but it prevented the deactivation of the catalyst, i.e., benzene production remained constant during a 6 h test.     

One-Step Oxidation of Benzene to Phenol over Cu/Ti/HZSM-5 Catalysts

The gas-phase catalytic oxidation of benzene over copper supported on HZSM-5 added with titanium (Cu/Ti/HZSM-5) was carried out by using molecular oxygen as an oxidant. Phenol was effectively produced by the titanium addition to Cu/HZSM-5. The titanium addition to Cu/HZSM-5 induces the easy reduction of Cu²⁺ species to Cu⁺ species in the catalysts, and the produced Cu⁺ species seems to produce the phenol effectively.     

改性Mo/HZSM-5催化剂甲烷无氧芳构化反应条件研究

采用Mo担载的质量分数为6%、经过先碱后酸预处理的Mo/HZSM-5作为催化剂,在不同反应温度和反应空速下,比较了甲烷的转化率、苯的生成速率和积炭的收率.结果表明,当反应温度为700℃,反应空速为1 400 mL/(g·h)时,甲烷无氧芳构化性能最佳.     

Marked Enhancement of the Methane Dehydrocondensation Toward Benzene Using Effective Pd Catalytic Membrane Reactor with Mo/ZSM-5

Steady state product formation rates of benzene, hydrogen, naphthalene, toluene in methane dehydrocondensation reaction on 3wt% Mo loaded ZSM-5 catalyst was enhanced 2–10 times by the removal of hydrogen using Pd membrane for 100h at 883K. The amount of permeated hydrogen through the Pd membrane was measured before and during the methane dehydrocondensation reaction. About 50–60% of hydrogen from the total hydrogen produced during the methane dehydrocondensation was selectively removed by the Pd membrane, owing to which the equilibrium of the methane dehydrocondensation was shifted toward the product side.     

A new way to enhance the coke-resistance of Mo/HZSM-5 catalyst for methane dehydroaromatization

A thermal dealumination method was applied to modify HZSM-5 zeolites, and the Mo/HZSM-5 catalyst pre-dealuminated in N₂ stream exhibited rather high catalytic activity and stability in the methane dehydroaromatization reaction (MDA). ²⁹Si NMR, FT-IR and TPO measurements show that the thermal treatment of the HZSM-5 in inert atmosphere induced partial removal of tetrahedral coordinated Al from the zeolite lattices leading to elimination of the original excess strong Brönsted acid sites (known as responsible for the coke formation), and thus significantly promoted the coke-resistance of the Mo/HZSM-5 catalyst.     

焦化苯中噻吩在酸性沸石催化剂上的催化裂解性能

研究了焦化苯中噻吩在酸性沸石催化剂上的催化裂解性能. 结果表明：噻吩在HZSM-5沸石催化剂的作用下被分解生成硫化氢气体逸出,进而达到脱硫的目的. 通过对不同温度和压力下的催化脱硫性能进行考察,认为HZSM-5沸石催化剂对脱除苯中噻吩具有较高的活性及较好的活性稳定性,且温度、压力是影响催化剂活性和稳定性的重要因素. 以含270 mg/L噻吩的焦化苯为原料,在反应温度为320~380℃、反应压力为3.5~6.0 MPa、质量空速为4~12 h-1的条件下,能彻底脱除其中的噻吩.     

Interaction between ammonium heptamolybdate and NH4ZSM-5 zeolite: the location of Mo species and the acidity of Mo/HZSM-5

Mo/HZSM-5 catalysts show high reactivity and selectivity in the activation of methane without using oxidants. Mo/HZSM-5 catalysts with Mo loading ranging from 0 to 10% were prepared by impregnation with an aqueous solution of ammonium heptamolybdate (AHM). The samples were dried at 393 K, and then calcined at different temperatures for 4 h. The interaction between Mo species and NH₄ZSM-5 zeolite was characterized by FT-IR spectroscopy, differential thermal analysis (DTA) and temperature programmed decomposition (TPDE) and NH₃-TPD at different stages of catalyst preparation. The results showed that if Mo/HZSM-5 catalysts were calcined at a proper temperature, the Mo species will interact with acid sites (mainly with BrØnsted acid sites) and part of the Mo species will move into the channel. The Mo species in the form of small MoO₃ crystallites residing on the external surface and/or in the channel, and interacting with BrØnsted acid sites may be responsible for the methane activation. Strong interaction between Mo species and the skeleton of HZSM-5 will occur if the catalyst is calcined at 973 K. This may lead to the formation of MoO ₄ ^2– species, which is detrimental to methane activation.     

Promotion effects of Pt and Rh on catalytic performances of Mo/HZSM-5 and Mo/HMCM-22 in selective methane-to-benzene reaction

The promotion effects of Pt and Rh on catalytic performances of Mo/HZSM-5 and Mo/HMCM-22 in selective methane-to-benzene reaction were studied in the presence of additive H₂. The selectivity to naphthalene was effectively suppressed and highly selective and stable benzene formation was obtained by the addition of noble metal to the Mo/HZSM-5 and Mo/HMCM-22 catalysts, due to the suppression of carbon deposition on the Brønsted acid sites of zeolite.