Enhanced performance of methane dehydro-aromatization on Mo-based HZSM-5 zeolite pretreated by NH4F

Enhanced performance of methane dehydro-aromatization reaction (MDA) were achieved on a Mo-based HZSM-5 zeolite catalyst in which HZSM-5 were pretreated by a proper amount of NH₄F (Mo/HZ(F)). The results of NH₃-TPD and ²⁷Al MAS NMR demonstrated that the number of Brönsted acid sites decreased on the HZSM-5 zeolite and Mo/HZSM-5 catalyst after NH₄F treatment. TGA and TPO measurements showed that the Mo/HZ(F) catalysts were highly resistant to coke deposition, which resulted mainly from the elimination of the Brönsted acid sites after the pretreatment of the HZSM-5 zeolite with NH₄F.     

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.     

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.     

MAS NMR, ESR and TPD studies of Mo/HZSM‐5 catalysts: evidence for the migration of molybdenum species into the zeolitic channels

NH₃‐TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo‐modified HZSM‐5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of ¹H MAS NMR resonance originating from Brønsted acid sites in the zeolites and a distinct splitting of Mo⁵⁺ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Brønsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization.     

Study on the induction period of methane aromatization over Mo/HZSM-5: partial reduction of Mo species and formation of carbonaceous deposit

The induction period of dehydrogenation and aromatization of methane over Mo/HZSM-5 was studied by combining a pulse reaction method with TPSR, UV laser Raman, and ¹³C CPMAS NMR techniques. BET and XRD results showed that Mo species were well dispersed on/in the zeolite. TPSR in CH₄ stream revealed that Mo species were reduced in at least two different stages before the formation of benzene. TPR results were in agreement with TPSR results. The two stages might be attributed to the reduction of two kinds of Mo⁶⁺ species to low valence Mo species. One was polymolybdate MoO₃, and the other was crystalline MoO₃. UV Raman spectra showed the existence of octahedrally coordinated polymolybdate species. XRD, however, did not detect any crystalline MoO₃, possibly because they were too small to be detected with this technique. Pulse reaction results indicated that pre-reduction of the catalyst and formation of carbonaceous deposit could shorten the induction period. It is concluded that the formation of active sites during the induction period via partial reduction of Mo⁶⁺ species and formation of carbonaceous deposit on partially reduced Mo species is of significance for methane aromatization over Mo/HZSM-5. This revised version was published online in August 2006 with corrections to the Cover Date.     

Methane and ethane activation without adding oxygen: promotional effect of W in Mo-W/HZSM-5

The conversions of methane and ethane over Mo/HZSM-5 and W/HZSM-5 catalysts are compared. A reaction model for hydrocarbon formation over Mo/HZSM-5 catalysts is proposed, which involves heterolytic splitting of methane and a molybdenum-carbene intermediate. Ethene is shown to be the initial product of methane conversion, and it undergoes further reaction to form aromatics in a solid acid environment. The promotional effect of addition of tungsten in the Mo-W/HZSM-5 catalyst in methane conversion reaction suggests the formation of Mo-W mixed oxide. The product selectivity patterns of Mo/HZSM-5 and W/HZSM-5 catalysts in ethane conversion reaction are consistent with a dual-path model involving dehydrogenation and cracking (or hydrogenolysis) of ethane. The rates of both these reactions over Mo/HZSM-5 are higher than over W/HZSM-5.     

Methane aromatization using Mo-based catalysts prepared by microwave heating

Mo/HZSM-5 and Cu–Mo/HZSM-5 catalysts for the non-oxidative aromatization of methane have been prepared by microwave heating method. The effects of Mo loading, the molar ratio of Cu/Mo and preparation method on the catalytic performance of catalysts were studied. The results were compared with those for the methane aromatization over catalysts prepared by conventional heating. Both two kinds of catalysts have the maximum methane conversion when the Mo loading is 6%. The catalysts prepared by microwave heating exhibited higher selectivity to benzene than that prepared by conventional heating. The addition of metal Cu to Mo/HZSM-5 catalyst prepared by microwave heating enhanced the lifetime of catalyst, and gave rise to a little increase in methane conversion. The molar ratio of Cu/Mo influenced the methane conversion, and the maximum value was attained when Cu/Mo = 0.05, whereas no significant influence on the benzene selectivity was observed with the increase molar ratio of Cu/Mo. N₂ adsorption results showed that the catalysts prepared by microwave heating have the larger surface area and the similar pore volume compared with the catalysts prepared by conventional heating. This fact revealed that the more Mo species located on the outer surface of catalysts prepared by microwave heating is the main reason why they have better catalytic performance. XRD analysis indicated that the Mo species are highly dispersed on HZSM-5 zeolite. The addition of Cu influenced the dispersion. The actual active phase Mo₂C can be identified on the catalyst surface after reaction. TEM analysis revealed the carbonaceous deposition to have the form of carbon nanotube after reaction, with a uniform size range of 10–20 nm. TG analysis indicated that carbonaceous deposition on the catalysts prepared by microwave heating is lower than that by conventional heating, and the metal Cu further prompts the stability of catalyst. Most of the carbonaceous deposition on catalysts prepared by microwave heating is formed at low temperature and it is easy to burn-off. Coke accumulation at high temperature is the main reason of catalyst deactivation. The carbonaceous deposition formed on the catalysts for non-oxidative aromatization of methane is different from those formed on the catalysts for partial oxidation of methane.     

Influence of binder on catalytic performance of Ni/HZSM-5 for hydrodeoxygenation of cyclohexanone

The influences of binders (alumina, silica sol, kaolin) on the performance of Ni/H-ZSM-5 for hydrodeoxygenation of cyclohexanone were investigated in a fixed-bed reactor. N₂ sorption, X-ray diffractions, H₂-temperature-programmed reduction, transmission electron microscopy, ²⁷Al MAS NMR, and temperature-programmed desorption of ammonia were used to characterize the catalysts. The obtained results exhibited that porosity and acidity of the catalysts were strongly influenced by the binders. The most outstanding catalytic performance was observed on catalyst with alumina binder, which bears a well-developed pore structure and more acid sites than the others. Thus, alumina was chosen as the optimum binder to Ni/HZSM-5.     

Dual effects of supported W catalysts for dehydroaromatization of methane in the absence of oxygen

The screening of a series of W-based catalysts on different supports i.e. HZSM-5, Hβ, USY and Al₂O₃ for the dehydroaromatization of methane (DHAM) revealed that HZSM-5 emerged as the best support. Next, the performance of W/HZSM-5 and W-H₂SO₄/HZSM-5 catalysts for the DHAM reaction was compared to study the effect of acidic treatment in the impregnation method. The results showed that the optimum activity of W-H₂SO₄/HZSM-5 catalyst exceeded that of W/HZSM-5 catalyst. Finally, the influence of Si/Al ratio in the W-H₂SO₄/HZSM-5 catalyst was studied and the catalyst with Si/Al ratio = 30 was found to be the most promising for the DHAM reaction. The remarkable activity of the catalyst is attributed to the presence of dual effects: suitable content of octahedral polymeric and tetrahedral monomeric tungstate species accompanied by proper amount and strength of acid sites in the catalyst.     

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. I. Performance of Catalysts

With incorporation of Zn (or Mn, La, Zr ) into the W/HZSM-5 catalyst, highly active and heat-resisting W/HZSM-5-based catalysts were developed and studied. Under reaction conditions of 0.1 MPa, 1073 K, GHSV of feed-gas CH₄+10% Ar at 960 h^–1, the conversion of methane reached 18–23% in the first 2 h of reaction, and the corresponding selectivity to benzene, naphthalene, ethylene and coke was 56–48, 18, 5 and 22%, respectively. Addition of a small amount of CO₂ (2%) to the feed-gas was found to significantly enhance the conversion of methane and the selectivity of benzene, and to improve the performance of coke-resistance of the W/HZSM-5-based catalysts. Heavy deposition of carbon on the surface of the functioning catalyst was the main reason leading to deactivation of the catalyst. Reoxidation by air may regenerate the deactivated catalyst effectively. In comparison with the Mo/HZSM-5 catalyst, the promoted W/HZSM-5-based catalyst can operate under reaction temperature of 1073 K, and gain a methane conversion approximately two times as high as that of the Mo/HZSM-5 catalyst operating at 973 K. It can also operate at 973 K and have about the same methane conversion as that of the Mo/HZSM-5 catalyst at the same reaction temperature. Its main advantage is its heat-resistant performance; the high reaction temperature did not lead to loss of W component by sublimation.     

Benzene Alkylation with Propane over Mo Modified HZSM-5

Benzene alkylation with propane has been studied over HZSM-5 loading 3.1–15.4 wt% Mo in continuous-flow microreactor under 350 °C and atmospheric pressure with the highest activity obtained at 6.7 wt% Mo loading. C_7–9 aromatics were obtained as main products while the total amount of benzene rings kept unchanged. i-Propylbenzene and n-propylbenzene are formed primarily, while toluene, ethylbenzene, and ethyl-toluene are formed secondly from the propylbenzenes. Catalytic performance of 6.7 wt% Mo/HZSM-5(38) partially poisoned by NH₃ shows that the strong acid sites play a crucial role in the alkylation. Low SiO₂/Al₂O₃ ratio of HZSM-5 in the Mo modified catalysts gives high propane conversion. Two hydrothermal treatment methods were applied to the 6.7 wt% Mo/HZSM-5(38) catalyst, caused decrease of propane conversion but result in different product distribution. A possible reaction mechanism concerning bifunctional active centers resulted from combination of loaded Mo species and strong acid centers on HZSM-5 is proposed.     

Methane Aromatization over Cobalt and Gallium -Impregnated HZSM-5 Catalysts

The influence of the catalyst acidity, the ratio of cobalt in the catalyst on the conversion of methane and the stability were evaluated using a fixed-bed microreactor at atmospheric pressure and at a flow rate of 1500 mL/g h (GHSV 600 h^?1). The reaction was conducted at 973 K and 1023 K over gallium and cobalt -impregnated HZSM-5 catalysts. The 2%Ga–2% Co/HZSM-5 catalyst exhibited remarkable stability with no significant deactivation for 100 h on stream, and yielded a maximum conversion of methane to benzene equal to 9.9%. These catalysts were thoroughly characterized using XRD, N₂ adsorption measurements, TPD of NH₃ and FT-IR. The acidity changes severely affected aromatization, and resulted in drastic modifications in product distribution. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane aromatization reaction, while the superfluous strong Bronsted acid sites, which can be decreased by adding Ga and Co, are shown to be related with the aromatic carbonaceous deposits on the catalysts. After adding Ga and Co the strength of Lewis acid sites of the catalyst increased. But the total amount of the acidity on the catalyst decreased.     

Stable and Selective Dehydrocondensation of Methane Towards Benzene on Modified Mo/HMCM-22 Catalyst by the Dealumination Treatment

A modified Mo/HMCM-22 catalyst by the dealumination treatment (Mo/HMCM-22-D) exhibited remarkable performance for the catalytic dehydrocondensation of methane with a higher selectivity of benzene and a lower selectivity of coke, in comparison with the same Mo catalyst supported on parent HMCM-22 (Mo/HMCM-22). Excellent catalytic stability as well as a high benzene formation rate of 1500 nmol/(g-cat·s) was obtained on a 6%Mo/HMCM-22-D catalyst at 1023 K, 3 atm and 2700 ml/(g·h) owing to the efficient suppression of coke formation. Dealumination of the HMCM-22 zeolite was characterized by XRD, ²⁷Al and ¹H MAS NMR and NH₃-TPD techniques. It was found that the dealumination treatment of HMCM-22 zeolite resulted in an effective suppression of acid sites, particularly the Brønsted acid sites (proton form in Al--O--Si) owing to the removal of tetrahedral framework aluminum, while the microporous structure and the zeolite framework remained unchanged. It was suggested that the stable and selective dehydrocondensation of methane towards benzene is based on the suppression of coke formation owing to the effective decrease of strong Brønsted acid sites by the dealumination treatment of the HMCM-22 zeolite.     

硅烷化和有机弱碱改性Mo/HZSM-5对甲烷无氧芳构化催化性能的影响

催化剂的形态及晶粒的组装对其催化性能有重要影响,采用硅烷化处理对Mo基催化剂表面酸性进行毒化制备了核壳型（Mo基催化剂@Silicalite-1）复合材料;采用四丙基氢氧化铵或正丁胺有机弱碱对Mo/HZSM-5进行刻蚀,然后经过脱硅再结晶分别制备了表面富硅型中空结构Mo/HZSM-5微球和表面富硅、核内含有多级孔道的Mo/HZSM-5微球。采用XRD、TEM、N₂等温吸脱附和NH₃-TPD对催化剂结构进行表征,并考察了三种不同后处理方法对Mo基催化剂在甲烷无氧芳构化反应中催化性能的影响。硅烷化和有机碱处理均能够调变Mo/HZSM-5催化剂的表面酸性,而经有机碱处理以后,催化剂结晶度、介孔比表面积和孔容均具有不同程度的增加,三种不同后处理方法均能改善Mo/HZSM-5催化剂的反应稳定性,对产物的分布也产生了显著影响。     

Phosphorus induced hydrothermal stability and enhanced catalytic activity of ZSM-5 in methanol to DME conversion

The change in properties of ZSM-5 samples was achieved by treatment with phosphorus compounds (trimethyl phosphite or phosphoric acid) and the resultant materials were characterized by N₂ adsorption, NH₃-TPD, ²⁷Al, and ³¹P MAS NMR techniques. The phosphorus-treated HZSM-5 (P/ZSM-5) samples exhibited lower acidity, higher hydrothermal stability and improved dimethyl ether (DME) selectivity in methanol conversion when compared to the phosphorus-free HZSM-5. ²⁷Al, and ³¹P MAS NMR results revealed that the added P indeed interacted with the ZSM-5 framework and is responsible for the changes observed in the catalytic properties. The interaction caused the decrease in strong acid sites on one hand and creation of new acid sites (NH₃-TPD) on the other, in P/ZSM-5 samples. The studies indicated the need of optimizing the P loading, where the positive role of P on the catalytic activity was observed to be maximum at P/Al molar ratio of 1.05.     

Study of W/HZSM-5-Based Catalysts for Dehydro-aromatization of CH4 in Absence of O2. II. Action of Promoters Zn and Li

By correlating the results of the NH₃-TPD characteristic study and the catalyst activity assay of the W/HZSM-5-based catalysts, we confirmed that the intensity and concentration of the surface B-acid sites have pronounced effects on the catalyst performance for dehydro-aromatization of methane (DHAM). It was found experimentally that, by addition of a proper amount of Mg²⁺, the strong B-acid sites at the catalyst surface could be effectively eliminated, whereas the addition of a proper amount of Zn²⁺ or Li⁺ resulted not only in eliminating most of the strong surface B-acid sites but also in generating a kind of new medium-strong acid sites, mostly B-acid sites, simultaneously. The latter could serve as the catalytically active sites for dehydro-aromatization of methane; on such medium-strong surface B-acid sites, the formation of coke would be also alleviated to a greater extent. By simultaneous addition of Mg²⁺ and Zn²⁺, optimized adjustment in surface acidity of the catalyst could be realized. On the other hand, the doping of the Zn²⁺ or Li⁺ component to the tungsten oxide matrix would facilitate inhibiting aggregation of the W-containing active species and improving dispersion of the W component at the surface of the catalyst, thus leading to a pronounced decrease in the reduction temperature for the hard-to-be-reduced W⁶⁺ species and an increase in quantity of the reducible W⁶⁺ species at the reaction temperature for DHAM, as has been evidenced by the results of a H₂-TPR study on the reducibility of the Zn²⁺ (or La³⁺, Li⁺, Mn²⁺)-promoted W/HZSM-5 system. The above two roles that Zn²⁺ and Li⁺ as promoters played both contributed to the persistence of high methane conversion and benzene selectivity, and the alleviation of coke deposition, as well as the prolongation of the catalyst lifetime.     

Bifunctional Catalysis of Mo/HZSM-5 in the Dehydroaromatization of Methane to Benzene and Naphthalene XAFS/TG/DTA/MASS/FTIR Characterization and Supporting Effects

The direct conversion of methane to aromatics such as benzene and naphthalene has been studied on a series of Mo-supported catalysts using HZSM-5, FSM-16, mordenite, USY, SiO₂, and Al₂O₃as the supporting materials. Among all the supports used, the HZSM-5-supported Mo catalysts exhibit the highest yield of aromatic products, achieving over 70% total selectivity of the hydrocarbons on a carbon basis at 5–12% methane conversion at 973 K and 1 atm. By contrast, less than 20% of the converted methane is transformed to hydrocarbon products on the other Mo-supported catalysts, which are drastically deactivated, owing to serious coke formation. The XANES/EXAFS and TG/DTA/mass studies reveal that the zeolite-supported Mo oxide is endothermally converted with methane around 955 K to molybdenum carbide (Mo₂C) cluster (Mo-C, C.N.=1,R=2.09 Å; Mo-Mo, C.N.=2.3–3.5;R=2.98 Å), which initiates the methane aromatization yielding benzene and naphthalene at 873–1023 K. Although both Mo₂C and HZSM-5 support alone have a very low activity for the reaction, physically mixed hybrid catalysts consisting of 3 wt% Mo/SiO₂+HZSM-5 and Mo₂C+HZSM-5 exhibited a remarkable promotion to enhance the yields of benzene and naphthalene over 100–300 times more than either component alone. On the other hand, it was demonstrated by the IR measurement in pyridine adsorption that the Mo/HZSM-5 catalysts having the optimum SiO₂/Al₂O₃ratios, around 40, show maximum Brönsted acidity among the catalysts with SiO₂/Al₂O₃ratios of 20–1900. There is a close correlation between the activity of benzene formation in methane aromatization and the Brönsted acidity of Mo/HZSM-5, but not Lewis aciditiy. It was found that maximum benzene formation was obtained on the Moz/HZSM-5 having SiO₂/Al₂O₃ratios of 20–49, but substantially poor activities on those with SiO₂/Al₂O₃ratios smaller and higher than 40. The results suggest that methane is dissociated on the molybdenum carbide cluster supported on HZSM-5 having optimum Brönsted acidity to form CH_x(x>1) and C₂-species as the primary intermediates which are oligomerized subsequently to aromatics such as benzene and naphthalene at the interface of Mo₂C and HZSM-5 zeolite having the optimum Brönsted acidity. The bifunctional catalysis of Mo/HZSM for methane conversion towards aromatics is discussed by analogy with the promotion mechanism on the Pt/Al₂O₃catalyst for the dehydro-aromatization of alkanes.     

Methane dehydro‐aromatization over a Mo/phosphoric rare earth‐containing penta‐sil type zeolite in the absence of oxygen

The dehydro‐aromatization of methane over a Mo‐modified penta‐sil type high‐silica zeolite containing phosphoric and rare earth oxide (abbreviated as Mo/HZRP‐1) was investigated. As a modification of HZSM‐5, HZRP‐1 is also a good support for the preparation of Mo‐based zeolite catalysts, and is active for methane dehydro‐aromatization. Mo/HZRP‐1 catalysts are more active at high Mo loadings compared with Mo/HZSM‐5 catalysts. ²⁷Al MAS NMR spectra of Mo/HZRP‐1 reveal that there are two kinds of framework Al in HZRP‐1. It is suggested that only the tetrahedral coordinated Al atoms in the form of Al–O–Si species in the zeolite, in the proton forms, are responsible for the formation of aromatics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methane activation without using oxidants over Mo/HZSM-5 zeolite catalysts

The effect of Mo loading, calcination temperature, reaction temperature and space velocity on the catalytic performance of methane dehydrogenation and aromatization without using oxidants over Mo/HZSM-5 has been studied. The XRD and BET measurements show that Mo species are highly dispersed in the channels of the HZSM-5 zeolite, resulting from the interaction between the Mo species and the zeolite, which also leads to a decrease in its crystallinity. The Brønsted acidity, the channel structure and the state and location of Mo species in the zeolite seem to be crucial factors for its catalytic performance. It was found that 2% Mo/HZSM-5 calcined at 773 K showed the best aromatization activity among the tested catalysts, the methane conversion being 9% at 1013 K with the selectivity to aromatics higher than 90%. The experimental results obtained from the variation of space velocity gave evidence that ethylene is an initial product. On the basis of these results a possible mechanism for methane dehydrogenation and aromatization has been proposed in which both the heterolytic splitting of methane in a solid acid environment and a molybdenum carbene-like complex as an intermediate are of significance.     

A novel method for enhancing on-stream stability of fluid catalytic cracking (FCC) gasoline hydro-upgrading catalyst: Post-treatment of HZSM-5 zeolite by combined steaming and citric acid leaching

This article describes a novel modification method consisting of steaming and subsequent citric acid leaching to finely tune acidity and pore structure of HZSM-5 zeolite and thereby to enhance the on-stream stability of the zeolite derived fluid catalytic cracking (FCC) gasoline hydro-upgrading catalyst. A series of dealuminated HZSM-5 zeolites and their derived catalysts were prepared and characterized by X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), ²⁷Al MAS NMR, nitrogen adsorption, temperature programmed desorption of ammonium (NH₃-TPD) and infrared (IR) spectroscopy of chemisorbed pyridine. The results showed that the citric acid leaching could preferentially remove the extra-framework Al (EFAl) species formed by steaming treatment and thus reopen the EFAl-blocked pore channels of the steamed zeolite. The steaming treatment at a suitable temperature and subsequent citric acid leaching not only decreased the strength of acid sites to a desirable degree but also increased the ratio of medium and strong Lewis acidity to medium and strong Brönsted acidity, both of which conferred the resulting catalyst with superior selectivity to aromatics, good hydroisomerization activity and gasoline research octane number (RON) preservability, as well as enhanced on-stream stability. The results fully demonstrated that the treatments by steaming and followed citric acid leaching can serve as an important method for adjusting the physicochemical properties of HZSM-5 zeolite.