Xiexian Guo

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.

Partial oxidation of methane to syngas over nickel-based catalysts modified by alkali metal oxide and rare earth metal oxide

Abstract The NiO/Al2O3 catalyst was modified by alkali metal oxide (Li, Na, K) and rare-earth metal oxide (La, Ce, Y, Sm) in order to improve the thermal stability and the carbon-deposition resistance during the partial oxidation of methane to syngas (POM) reaction at high temperature. The reaction performance, thermal stability, structure, dispersity of nickel and carbon-deposition of the modified NiO/Al2O3 catalyst and unmodified NiO/Al2O3 catalyst were investigated by a series of characterization techniques including flow-reaction, BET, XRD, CO chemisorption and TG analysis. The results indicated that the modification with alkali metal oxide and rare-earth metal oxide improves the dispersion of active component nickel and the activity for the POM reaction over the nickel-based catalysts, and enhances their thermal stability during high temperature reaction and the ability to suppress the carbon-deposition over the nickel-based catalysts during the POM reaction. The nickel-based catalysts modified by alkali metal oxide and rare-earth metal oxide have excellent POM reaction performance (CH4 conversion of 94.8%, CO selectivity of 98.1%, 2.7×104l/kg·h), excellent stability and carbon-deposition resistance.

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 MoO42− species, which is detrimental to methane activation.

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.

Preparation and characterization of three pure magnesium vanadate phases as catalysts for selective oxidation of propane to propene

Three magnesium vanadate phases, i.e., MgV2O6 (metavanadate), α-Mg2V2O7 (pyrovana-date) and Mgs V2O8 (orthovanadate), have been successfully prepared with high purity by the citrate method at a relatively low temperature (550°C). FT-IR, LRS, XRD and SEM techniques have been used to characterize these vanadate phases. The effect of calcination temperature has also been investigated. It was found that the particle size and morphology of the MgV2O6 phase, which is a function of calcination temperature, appear to have a strong effect on the infrared spectra. Furthermore, the catalytic properties of the three phases were examined in the oxidative dehydrogenation of propane. The propene selectivity follows the order: α-Mg2V2O7 > Mg3 Vg2O8 > MgV2O6, which is consistent with their redox properties. This fact suggests that there is some correlation between the catalytic and redox properties of these magnesium vanadate phases.

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.

Characterization of modified ZSM-5 catalysts for propane aromatization prepared by a solid state reaction

Abstract This paper describes a simple preparation method using a solid state reaction for Zn-, Mo-, and Cr-ZSM-5 catalysts for propane aromatization. NH 3 -temperature-programmed desorption, infrared, temperature-programmed reduction and electron spin resonance techniques were used to characterize the interaction of H-ZSM-5 with ZnO, MoCl 5 and CrO 3 , respectively, which leads to introduction of cations into the channels of the zeolite. A solid-exchange mechanism is suggested here. Zn-ZSM-5 is the most active catalyst for propane conversion and gives a better benzene, toluene and xylenes selectivity. Over Mo-ZSM-5, propane mainly undergoes cracking to methane and ethane, and the loading ZSM-5 with Cr 5+ enhances the propane dehydrogenation to propene.

Study on catalysis by carbonyl cluster-derived SiO2-supported rhodium for ethylene hydroformylation

Atmospheric hydroformylation of ethylene was studied under differential conditions over Rh4(CO)12-derived Rh/SiO2 catalysts. The specific activities as functions of Rh dispersions show that ethylene hydroformylation is structure sensitive and ethylene hydrogenation structure insensitive. These structural dependences and in situ IR observations show that Rh0 is the unique active site for catalytic ethylene hydroformylation on Rh/SiO2. The reactions of Rh0-coordinated CO and Rh0-adsorbed CO with C2H4 + H2 at 293 K were monitored by IR spectroscopy. The linear CO adsorbed on Rh0/SiO2 is consumed with formation of propanal, whereas the coordinated CO in Rh6(CO)16/SiO2 and its derivative do not participate in CO insertion. IR study of the thermal decomposition of Rh6(CO)16/SiO2 indicates that the cluster can be stabilized on the surface up to 548 K by gaseous CO under hydroformylation conditions. Moreover, the Rh6(CO)16/SiO2 system exhibits increased catalytic hydroformylation activity with reducing coordinated CO. These results show that coordinative unsaturation on the Rh0 surface is necessary for heterogeneously rhodium-catalyzed hydroformylation and that totally decarbonylated Rh0/SiO2 is most effective. In addition, the oxidation of Rh0 by surface OH− is discussed.