Ordered mesoporous materials in catalysis

The quest for an efficient process to convert methane efficiently to fuels and high value-added chemicals such as olefins and aromatics is motivated by their increasing demands and recently discovered large reserves and resources of methane. Direct conversion to these chemicals can be realized either oxidatively via oxidative coupling of methane (OCM) or nonoxidatively via methane dehydroaromatization (MDA), which have been under intensive investigation for decades. While industrial applications are still limited by their low yield (selectivity) and stability issues, innovations in new catalysts and concepts are needed. The newly emerging strategy using iron single sites to catalyze methane conversion to olefins, aromatics, and hydrogen (MTOAH) attracted much attention when it was reported. Because the challenge lies in controlled dehydrogenation of the highly stable CH4 and selective C–C coupling, we focus mainly on the fundamentals of C–H activation and analyze the reaction pathways toward selective rou...

Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects

This article is a review of the current knowledge of the chemical physics of carbon dioxide (CO2) conversion to fuels using light energy and water (CO2 photoreduction) on titania (TiO2)-based catalysts and Ti-species in porous materials. Fairly comprehensive literature reviews of CO2 photoreduction are available already. However, this article is focused on CO2 photoreduction on Ti-based catalysts, and incorporates fundamental aspects of CO2 photoreduction, knowledge from surface science studies of TiO2 and the surface chemistry of CO2. Firstly, the current state of development of this field is briefly reviewed, followed by a description of and insights from surface state and surface site approaches. Using examples such as metal-doping of TiO2, dye-sensitization, oxygen vacancies in TiO2 and isolated-Ti centers in microporous/mesoporous materials, the utility of these approaches to understand photoinduced reactions involved in CO2activation is examined. Finally, challenges and prospects for further development of this field are presented. Enhanced understanding of the CO2 : TiO2 system, with a combination of computational and experimental studies is required to develop catalysts exhibiting higher activity towards CO2 photoreduction.

Photoinduced activation of CO2 on Ti-based heterogeneous catalysts: Current state, chemical physics-based insights and outlook

The NH3-SCR activity of the small-pore zeolites, Cu-SSZ-13, Cu-SSZ-16, and Cu-SAPO-34, is investigated using a high-throughput reactor system. These copper exchanged small-pore zeolites have high SCR activity between 150 and 500 °C and are shown to be much more hydrothermally stable than the medium-pore zeolite, Cu-ZSM-5. The degree of copper exchange, the dimensionality of the framework, and heteroatom framework substitution all impact the SCR activity and hydrothermal stability of the materials. Of the small-pore zeolites tested, Cu-SSZ-13 and Cu-SAPO-34 display superior SCR performance, both before and after high-temperature hydrothermal treatment.

The ammonia selective catalytic reduction activity of copper-exchanged small-pore zeolites

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

Dehydro-oligomerization of Methane to Ethylene and Aromatics over Molybdenum/HZSM-5 Catalyst

Thermal and hydrothermal stability of framework-substituted MCM-41 mesoporous materials

Framework Ti-substituted and Ti-grafted MCM-41 mesoporous material has been prepared by direct hydrothermal synthesis and a post-synthesis grafting method. The materials have been tested as catalysts for cyclohexene oxidation with aqueous H2O2 and tert-butyl hydroperoxide (TBHP). With aqueous H2O2 in methanol, the major products were cyclohexene diol and its methyl ethers. No cyclohexene oxide was produced. Titanium leaching was a serious problem, and the catalyst lost its activity irreversibly after only one cycle of reaction. With TBHP, the selectivity for cyclohexene oxide was near 100%, titanium leaching was negligible, and the catalyst could be repeatedly used after regeneration without suffering significant activity loss. However, the reaction rate was lower than when H2O2 was used. Framework substituted material and catalysts prepared by Ti-grafting onto a MCM-41 support behaved similar, but the Ti-grafted MCM-41 is somewhat more active. The turnover frequency (TOF) per mole of Ti decreases with an increase of Ti content in the catalyst. This is caused by a reduced Ti dispersion within the silica matrix.

Ti-containing MCM-41 catalysts for liquid phase oxidation of cyclohexene with aqueous H2O2 and tert-butyl hydroperoxide

A comparison of post-synthesis alumination and sol-gel synthesis of MCM-41 with high framework aluminum content

The influence of modification of Mo-HZSM-5 catalyst by Pt on methane non-oxidative transformation to ethylene and aromatics is investigated. Carbon accumulation on the catalyst is studied by means of DTA and TG methods. It is demonstrated that the addition of Pt to Mo-HZSM-5 considerably enhances the catalyst stability and reduces the carbon deposition on the catalyst. In the meanwhile, Mo-HZSM-5 is shown not to be a good catalyst for methane non-oxidative conversion because the total efficiency for methane conversion to useful products is even lower than it is in the oxidative coupling process.

Laiyuan Chen

Papers

Dehydro-oligomerization of Methane to Ethylene and Aromatics over Molybdenum/HZSM-5 Catalyst

Thermal and hydrothermal stability of framework-substituted MCM-41 mesoporous materials

Ti-containing MCM-41 catalysts for liquid phase oxidation of cyclohexene with aqueous H2O2 and tert-butyl hydroperoxide

A comparison of post-synthesis alumination and sol-gel synthesis of MCM-41 with high framework aluminum content

Promotional effect of Pt on non-oxidative methane transformation over Mo-HZSM-5 catalyst