Zirconia/Titania Catalysts for Carbon Dioxide Utilisation

Reaction and conversion of CO2 to chemicals is a challenging area of research. The objective of this work is to study and investigate the use of mixed metal oxide Zr/Ti oxide and related catalysts for the conversion and utilisation of CO2. The first reaction studied was propane dehydrogenation using CO2 to produce propene. Then, the study extended to investigate the direct reaction of CO2 as whole molecule with methane, ethane, acetylene, ethylene and propane to synthesis carboxylic acids.
The catalysts were prepared in several ways. Four methods were based on coprecipitation of the mixed catalyst from solutions of zirconium (IV) oxynitrate hydrate and titanium (IV) chloride. Other methods involved impregnation, based on titanium (IV) oxide. Catalysts were characterised by nitrogen adsorption, by powder X-ray diffraction, by ammonia temperature programmed desorption and, ultimately, in terms of catalytic activities.
The powder X-ray diffraction patterns of the impregnated titania-rich Zr/Ti oxide catalysts showed that ZrO2 dissolved in the solid anatase phase of titania. At higher concentrations, the ZrO2 appeared as a separate tetragonal phase. Low zirconia content Zr/Ti oxide catalysts showed significantly increased surface areas and higher acidities than the individual oxides. A range of other metal oxides were added as third metal
oxides in these mixtures, but none had significant impacts on surface areas or on surface acidities.
Propane dehydrogenation is thermodynamically limited. The only possible route is a radical mechanism for H2 removal via a surface process. The catalytic activities at low CO2:propane ratio showed that Zr/Ti oxide exhibited the higher activity than single oxides, but activities were all too low to be of economic significance.
In contrast, using higher CO2:propane ratio improved the propene yield and selectivity to values comparable to those achieved with the industrial chromium based catalyst. The catalyst showed selectivity to C-H bond breaking to form propene over C-C bond breaking to make ethene. The study demonstrated that CO2 was utilised mainly for the reverse water gas shift reaction (RWGS) to remove hydrogen from the catalyst surface.
The study showed that the Zr/Ti oxide catalysts exhibited higher stability compared to the industrial catalysts at slightly higher gas space velocity. Thermogravimetric analysis showed that Zr/Ti oxide catalyst assists coke gasification in the presence of CO2 at 600 oC.
The other mixed oxide catalysts generally showed lower surface acidities and higher selectivities to C-C bond breaking products over the desired propene product.
The second study was the direct reaction of CO2 with CH4 to produce acetic acid. Again, this reaction is thermodynamically unfavourable and the only possible route must involve a radical species by which reactants are concentrated on the catalyst surface. Evidence of methyl surface species formation in the presence of methane was indeed found over the Zr/Ti oxide catalyst. With CO2 methane reacted with CO2 to form acetic acid over Zr/Ti oxide catalysts. The C-C insertion mechanism is proposed by which methyl surface species formed on the catalyst and reacted with CO2. This was followed by hydrogenation to form acetic acid.
Reactions of CO2 with ethane, ethylene, acetylene and propene were also studied, in the hope of observing direct insertion to produce the corresponding carboxylic acid. In fact, lower acids were formed in all cases, suggesting a radical mechanism involving C-C bond breaking over Zr/Ti oxide catalyst.
Interestingly, acetic acid was formed with all these precursor hydrocarbons, and it appears that it occurs via C≡C, C=C, C-C and C-H bond breaking.