Study of Mn-based catalysts for oxidative dehydrogenation of cyclohexane to cyclohexene

The gas-phase oxidative dehydrogenation (ODH) of cyclohexane to cyclohexene in the presence of molecular oxygen has been studied over various Mn-based catalysts. It is found that LiCl/MnOx/PC (Portland cement) catalyst exhibits the highest catalytic performance, and a 42.8% cyclohexane conversion, 58.8% cyclohexene selectivity and 25.2% cyclohexene yield can be achieved under 600 degrees C, 20,000 h(-1) and C6H12/O-2/N-2= 14/7/79. There are good correlations between the selectivities to cyclohexene and the electrical conductivities of Li doped Mn-based catalysts, from which it is deduced that the non-fully reduced oxygen species (O-2(-), O-2(2-), O-) involved in a new phase of LiMn2O4 might be responsible for the high selectivity toward cyclohexene, whereas the Mn2O3 crystal phase results in the COx formation. The selectivity to cyclohexene increases with increasing molar ratio of Li to Mn in LiCl/ MnOx/PC.

Hydroisomerization of long-chain alkane over Pt/SAPO-11 catalysts synthesized from nonaqueous media

SAPO-11 molecular sieves were synthesized from nonaqueous media. The effects of Si and Al sources as well as solvents on the catalytic performance of SAPO-11 were investigated by the hydroisomerization reaction of n-dodecane. The samples were characterized by XRD, XRF, N-2-adsorption, SEM, NH3-TPD, IR-NH3 and Si-29 CP MAS NMR. The SAPO-11 samples synthesized with tetraethoxysilane as the Si source showed higher Si incorporation contents than the SAPO molecular sieves prepared with polymeric Si sources (fumed silica and Si colloidal gel). The reaction results showed that Pt/SAPO-11 catalysts synthesized from ethylene glycol and glycerol media with the monomeric Si and Al sources (tetraethoxysilane, aluminum isopropoxide) exhibited higher catalytic activities than those catalysts with the polymeric Si or Al (pseudo-boehmite) sources, due to the larger external surface area and higher acidity of the former ones. Especially, the catalyst synthesized in an ethylene glycol medium possessed the highest catalytic activity. Over this catalyst, 88% conversion of n-dodecane was achieved at a low temperature of 250 degrees C.

Selective oxidations on recoverable catalysts assembled in emulsions

Catalysts assembled in emulsions are found to be potentially recoverable and efficient for a number of catalytic reactions. The catalysts composed of polyoxometalate anions and quaternary ammonium cations have been designed and synthesized according to the catalytic reactions and by optimizing the structures of cations and anions. The catalysts act essentially as surfactants, which are uniformly distributed in the interface of the emulsion droplets, and accordingly behave like homogeneous catalysts. The catalysts show remarkable selectivity and activity in the oxidation of sulfur-containing molecules to sulfones in diesel and the selective oxidation of alcohols to ketones, using H2O2 as oxidant. For an example, the catalyst demonstrated over 96% efficiency of H2O2 and similar to 100% selectivity to sulfones for the selective oxidation of sulfur-containing molecules in real diesel. Moreover, the catalysts can be separated and recycled by a simple demulsification and re-emulsification.

The synergic effect between Mo species and acid sites in Mo/HMCM-22 catalysts for methane aromatization

The acid properties of Mo/HMCM-22 catalyst, which is the precursor form of the working catalyst for methane aromatization reaction, and the synergic effect between Mo species and acid sites were studied and characterized by various characterization techniques. It is concluded that Bronsted and Lewis acidities of HMCM-22 are modified due to the introduction of molybdenum. We suggest a monomer of Mo species is formed by the exchange of Mo species with the Bronsted acid sites. On the other hand, coordinate unsaturated sites (CUS) are suggested to be responsible for the formation of newly detected Lewis acid sites. Computer modelling is established and coupling with experimental results, it is then speculated that the effective activation of methane is properly accomplished on Mo species accommodated in the 12 MR supercages of MCM-22 zeolite whereas the Bronsted acid sites in the same channel system play a key role for the formation of benzene. A much more pronounced volcano-typed reactivity curve of the Mo/HMCM-22 catalysts, as compared with that of the Mo/HZSM-5, with respect to Mo loading is found and this can be well understood due to the unique channel structure of MCM-22 zeolite and synergic effect between Mo species and acid sites.

Enhancement of the catalytic performance of supported-metal catalysts by pretreatment of the support

We report an interesting finding that the catalytic performance of supported Ag/SiO2 catalysts toward selective catalytic oxidation of CO in hydrogen at low temperatures can be greatly enhanced by pretreatment of the SiO2 support before catalyst preparation. Calcination of SiO2 at appropriate temperatures preferentially removes the H-bonded SiOH, which results in the highly dispersive Ag/SiO2 catalyst and thus improves the catalytic performance. (c) 2005 Elsevier Inc. All rights reserved.

Methane dehydroaromatization over Mo/HZSM-5 catalysts: The reactivity of MoCx species formed from MoOx associated and non-associated with Bronsted acid sites

The catalytic performances of methane dehydroaromatization (MDA) under non-oxidative conditions over 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time at 773 K have been investigated in combination with ex situ H-1 MAS NMR characterization. Prolongation of the calcination time at 773 K is in favor of the diffusion of the Mo species on the external surface and the migration of Mo species into the channels, resulting in a further decrease in the number of Bronsted acid sites, while causing only a slight change in the Mo contents of the bulk and in the framework structure of the HZSM-5 zeolite. The MoQ(x) species associated and non-associated with the Bronsted acid sites can be estimated quantitatively based on the 1H MAS NMR measurements as well as on the assumption of a stoichiometry ratio of 1: 1 between the Mo species and the Bronsted acid sites. Calcining the 6 wt.% Mo/HZSM-5 catalyst at 773 K for 18 h can cause the MoOx species to associate with the Bronsted acid sites, while a 6 Wt-% MO/SiO2 sample can be taken as a catalyst in which all MoOx species are non-associated with the Bronsted acid sites. The TOF data at different times on stream on the 6 wt.% Mo/HZSM-5 catalyst calcined at 773 K for 18 h and on the 6 Wt-% MO/SiO2 catalyst reveal that the MoCx species formed from MoOx associated with the Bronsted acid sites are more active and stable than those formed from MoOx non-associated with the Bronsted acid sites. An analysis of the TPO profiles recorded on the used 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time combined with the TGA measurements also reveals that the more of the MoCx species formed from MoOx species associated with the Br6nsted acid sites, the lower the amount of coke that will be deposited on it. The decrease of the coke amount is mainly due to a decrease in the coke burnt-off at high temperature. (c) 2005 Elsevier B.V. All rights reserved.

The effect of Rh particle size on the catalytic performance of porous silica supported rhodium catalysts for CO hydrogenation

Silica-supported Rh catalysts with different Rh particle dimensions were investigated for CO hydrogenation. The catalysts were characterized by various techniques such as TEM, H-2-TPR and N-2 adsorption to study the catalyst morphology, the size distributions of Rh particles and the silica pores. It was found that the distribution and the size of Rh particles were affected by the silica pores, and the metal grains were enclosed in the pores of the support, and thereby their growth was limited. The catalytic activity and selectivity to C-2-oxygenates for CO hydrogenation were found to be significantly controlled by the Rh particle sizes, and the higher activity and selectivity to C2-oxygenates were obtained over bigger Rh particles, within the range of the reported particle sizes.

Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts

Silver is well known to show peculiar catalytic activities in several oxidation reactions. In the present paper, we investigate the catalytic activity of silver catalysts toward CO-gelective oxidation in H-2. XRD, TEM, TPD, and in situ FTIR techniques were used to characterize the catalysts. The pretreatment of the catalysts was found to have great influence on their performance. The pretreatment in 02 improves the activity of the silver catalyst, whereas He pretreatment at 700 degreesC or direct hydrogen pretreatment shows an inverse effect. Silver catalysts undergo massive structural change during oxygen pretreatment at high temperatures (> 500 degreesC), and there is solid evidence for the formation of subsurface oxygen species. The existence of this silver-subsurface oxygen structure facilitates the formation of active sites on silver catalysts for CO oxidation, which are related to the size, morphology, and exposed crystal planes of the silver particles. Its formation requires a certain temperature, and a higher pretreatment temperature with oxygen is required for the silver catalyst with a smaller particle size. It is observed, for the first time, that adsorbed CO on the surface of silver particles can directly react with subsurface oxygen species at low temperatures (e.g., RT), and the surface oxygen can migrate into and refill these subsurface sites after the consumption of subsurface oxygen by the reaction with CO. This finding provides a new reaction pathway for CO oxidation on silver catalyst. (C) 2004 Published by Elsevier Inc.

On the correlation between microstructural changes of Ag-H-ZSM-5 catalysts and their catalytic performances in the selective catalytic reduction of NOx by methane

The silver catalyzed, selective catalytic reduction (SCR) of nitrogen oxides (NOx) by CH4, is shown to be a structure-sensitive reaction. Pretreatment has a great affect on the catalytic performances. Upon thermal treatment in inert gas stream, thermal induced changes in silver morphology lead to the formation of reduced silver species of clusters and particles. Catalysis over this catalyst indicates an initially higher activity but lower selectivity for the CH4-SCR of NOx Reaction induced restructuring of silver results in the formation of ill-defined silver oxides. This, in turn, impacts the adsorption properties and diffusivity of oxygen over silver catalyst, results in the decrease in activity but increase in selectivity of Ag-H-ZSM-5 catalyst for the CH4-SCR of NO.. (c) 2004 Elsevier B.V. All rights reserved.

Liquid phase oxidation of toluene to benzaldehyde with molecular oxygen over copper-based heterogeneous catalysts

We conducted the liquid phase oxidation of toluene with molecular oxygen over heterogeneous catalysts of copper-based binary metal oxides. Among the copper-based binary metal oxides, iron-copper binary oxide (Fe/Cu = 0.3 atomic ratio) was found to be the best catalyst. In the presence of pyridine, overoxidation of benzaldehyde to benzoic acid was partially prevented. As a result, highly selective formation of benzaldehyde (86% selectivity) was observed after 2 h of reaction (7% conversion of toluene) at 463 K and 1.0 MPa of oxygen atmosphere in the presence of pyridine. These catalytic performances were similar or better than those in the gas phase oxidation of toluene at reaction temperatures higher than 473 K and under 0.5-2.5 MPa. It was suggested from competitive adsorption measurements that pyridine could reduce the adsorption of benzaldehyde. At a long reaction time of 4 It, the conversion increased to 25% and benzoic acid became the predominant reaction product (72% selectivity) in the absence of pyridine. The yield of benzoic acid was higher than that in the Snia-Viscosa process, which requires corrosive halogen ions and acidic solvents in the homogeneous reaction media. The catalyst was easily recycled by simple filtration and reusable after washing and drying.

CO hydrogenation to light alkenes over Mn/Fe catalysts prepared by coprecipitation and sol-gel methods

CO hydrogenation to light alkenes was carried out on manganese promoted iron catalysts prepared by coprecipitation and sol-gel techniques. Addition of manganese in the range of 1-4 mol.% by means of coprecipitation could improve notably the percentage of C-2 (=) similar to C-4 (=) in the products, but it was not so efficient when the sol-gel method was employed. XRD and H-2-TPR measurements showed that the catalyst samples giving high C-2 (=) similar to C-4 (=) yields possessed ultra. ne particles in the form of pure alpha-(Fe1-xMnx)(2)O-3, and high quality in lowering the reduction temperature of the iron oxide. Furthermore, these samples displayed deep extent of carburization and different surface procedures to the others in the tests of Temperature Programmed Surface Carburization (TPSC). The different surface procedures of these samples were considered to have close relationship with the evolving of surface oxygen. It was also suggested that for the catalysts with high C-2 (=) similar to C-4 (=) yields, the turnover rate of the active site could be kept at a relatively high level due to the improved reducing and carburizing capabilities. Consequently, there would be a large number of sites for CO adsorption/dissociation and an enhanced carburization environment on the catalyst surface, so that the process of hydrogenation could be suppressed relatively to a low level. As a result, the percentage of the light alkenes in the products could be raised.