Linsheng Wang

Dehydrogenation and aromatization of methane under non-oxidizing conditions

The dehydrogenation and aromatization of methane on modified ZSM-5 zeolite catalysts has been studied under non-oxidizing conditions with a fixed bed continuous-flow reactor and with a temperature programmed reactor. The results show that benzene is the only hydrocarbon product of the catalytic conversion of methane at high temperature (973 K). The catalytic activity of ZSM-5 is greatly improved by incorporating a metal cation (Mo or Zn). H2 and ethene have been directly detected in the products with a mass spectrometer during TPAR. A carbenium ion mechanism for the activation of methane is suggested.

Methane dehydrogenation and aromatization in the absence of oxygen on MoHZSM-5: A study on the interaction between Mo species and HZSM-5 by using 27Al and 29Si MAS NMR

By using a high-resolution solid state nuclear magnetic resonance spectrometer with 27Al and 29Si probes, the interaction between Mo species and HZSM-5 of frsol|Mo/HZSM-5 catalysts has been studied. The results show that there is a strong interaction between Mo species and HZSM-5 zeolite. The framework aluminum in the zeolite can be easily extracted by the introduction of Mo species. The extractability of framework aluminum by Mo species increases with increasing Mo loading and the calcination temperature. The extraction process leads to the formation of non-framework Al at first and then a new crystalline phase of Al2(MoO4)3. The dealumination of the catalyst having a Mo loading of 15% and had been calcined at 973 K is so severe that all the aluminum in the framework are extracted and no framework Al could be detected by 27Al MAS NMR. The catalyst, therefore, lost its catalytic activity for methane dehydrogenation and aromatization in the absence of oxygen. The SiAl ratio measured from 29Si MAS NMR further confirms the dealumination process observed by 27Al MAS NMR. The MAS NMR results give us an evidence that Al2(MoO4)3 crystallites are much less active for the reaction.

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.

Dehydrogenation and aromatization of methane in the absence of oxygen on Mo/HZSM-5 catalysts before and after NH4OH extraction

Mo/HZSM-5 catalysts show good catalytic reactivity in the absence of oxygen for the dehydrogenation and aromatization of methane at 973 K. The active Mo species were investigated by combining catalytic studies on Mo/HZSM-5 catalysts before and after NH4OH extraction with XRD, BET, NH3-TPD and TPR analysis. The XRD patterns show that Mo species are well dispersed on the zeolite surface. The specific surface areas decrease with increasing Mo loading but they can be restored to a large extent by NH4OH extraction. NH3-TPD results suggest that the Mo species prefer to deposit on the strong acid sites of HZSM-5 zeolite. TPR profiles show that there is a kind of Mo species which is easily reduced. No TPR peaks could be obviously observed if the Mo/ HZSM-5 catalysts were extracted by NH4OH solution. The results of NH4OH extraction experiment and other relevant characterization studies suggest that there are several kinds of Mo species deposited on the surface. By referring to the Mo species on Al2O3 supported MoO3 samples, we propose that the dissolvable Mo species in NH4OH solution are MoO3 crystallites and their aggregates in octahedral coordination, while the unsoluble Mo species mainly are Al2(MoO4)3 and MoO42− in tetrahedrally coordinated form. The catalytic performance of Mo/HZSM-5 catalysts before and after NH4OH extraction illustrates that Mo species in small MoO3 crystallites with octahedral coordination form are active for methane activation in the absence of oxygen on Mo/HZSM-5 catalysts, while Mo species in tetrahedrally coordinated form is less active for the reaction.

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.

Activity and stability enhancement of MoHZSM-5-based catalysts for methane non-oxidative transformation to aromatics and C2 hydrocarbons: Effect of additives and pretreatment conditions

Abstract The effect of additive and pretreatment condition on the activity and stability of Mo HZSM-5-based catalysts for methane non-oxidative transformation to aromatics and C 2 hydrocarbons is investigated. Activity and selectivity of Mo HZSM-5-based catalysts can be modified by the addition of a second metal component, and the catalytic performance of the catalyst is very sensitive to the atmosphere of pretreatment at high temperature. High methane conversion of over 10% and quite stable activity have been obtained at a reaction temperature of 973 K over Mo HZSM-5 , MoW HZSM-5 and MoZr HZSM-5 catalysts pretreated by air at a temperature of 873 K. Catalyst pretreatment under air atmosphere at high temperature may lead to an increase in the pore size of the catalysts and a decrease in methane vonversion and activity maintenance. However, the catalysts can be stabilized if they were pretreated under an atmosphere of methane. A new Mo phase may be formed on the catalysts under methane atmosphere, which is a prerequisite for the formation of aromatics and C 2 hydrocarbons.

Methane activation without using oxidants over supported Mo catalysts

Abstract The catalytic properties of Mo/HSAPO-34, Mo/H-ZSM-5 and Mo/HY catalysts for methane conversion without using oxidants were compared. Mo/H-ZSM-5 catalyst has the best performance in terms of catalytic activity and stability. This catalyst is very selective for benzene production and the benzene yield is the highest of the three catalysts studied. The catalytic activity and benzene selectivity over the Mo/HSAPO-34 catalyst, however, decreased with increasing time on stream. As the catalyst deactivated, the selectivity to ethene increased. The Mo/HY catalyst is active for methane conversion but rapid deactivation of the catalyst resulted in negligible product yield. These results are discussed in terms of the shape-selectivity effect of the support taking into consideration the high reaction temperature. Preliminary studies on alkali metal doped Mo/HSAPO-34 catalysts showed no observed effect on catalyst deactivation.

Bifunctional catalysis of Mo/HZSM-5 in the dehydroaromatization of methane with CO/CO2 to benzene and naphthalene

The catalytic dehydrocondensation of methane to aromatics such as benzene and naphthalene was studied on the Mo carbide catalysts supported on micro- and mesoporous materials such as HZSM-5 (0.6 nm) and FSM-16 (2.7 nm). The Mo catalysts supported on H-ZSM-5 having appropriate micropores (0.6 nm size) and Si/Al ratios (20-70) exhibit higher yields (90-150 nmol/g-cat/s) and selectivities (higher than 74% on the carbon basis) in methane conversion to aromatic products such as benzene and naphthalene at 973 K and 1 atm, although they are drastically deactivated because of substantial coke formation. It was demonstrated that the CO/CO2 addition to methane effectively improves the catalyst performance by keeping a higher methane conversion and selectivities of benzene formation in the prolonged time-on-stream. The oxygen derived from CO and CO2 dissociation suppresses polycondensation of aromatic products and coke formation in the course of methane conversion. XAFS and TG/DTA/mass-spectrometric studies reveal that the zeolite-supported Mo oxide is endothermally converted under the action of methane around 955 K to nanosized particles of molybdenum carbide (Mo2C) (Mo-C, coordination number = 1,R- 2.09 å; Mo-Mo, coordination number = 2.3–3.5;R = 2.98 å). The SEM pictures showed that the nanostructured Mo carbide particles are highly dispersed on and inside the HZSM-5 crystals. On the other hand, it was demonstrated by IR measurements of pyridine adsorption that the Mo/HZSM-5 catalysts having the optimum SiO2/Al2O3 ratios around 40 show the maximum Brönsted acidity among the catalysts with the SiO2/Al2O3 ratios of 20–1900. There is a close correlation between the activity of benzene formation in the methane aromatization and the Brönsted acidity of HZSM-5 due to the bifunctional catalysis.

Catalytic dehydroaromatization of methane with CO/CO2 towards benzene and naphthalene on bimetallic Mo/zeolite catalysts : Bifunctional catalysis and dynamic mechanism

In recent years a great challenge and intriguing problem in hetergogeneous catalysis is the catalytic dehydrocondensation of natural gas into useful petrochemical feed stocks such as ethylene by the oxidative coupling of methane over Li/MgO and Sm 2 O 3 catalysts and the lower hydrocarbons by the two-step conversion of methane on Co and Ru catalysts, while they have not been feasible yet due to the lower yields and selectivities.

Methane direct conversion to aromatic hydrocarbons at low reaction temperature

Conversion of pure methane and natural gas with different methane purity to aromatic hydrocarbons at. 773 and 873 K have been investigated. Conversion of methane to aromatics under non-oxidizing conditions can be initiated by higher hydrocarbon mixtures in the feed and, some special coke deposited on Mo/HZSM-5 catalyst at lower reaction temperature. Methane conversion of about 10–20% is obtained at 773 K. The possible reaction mechanism and product phase transformation process for conversion of pure methane and natural gas at lower temperature are proposed. The thermodynamic limitation for methane conversion under non-oxidizing conditions may be circumvented.