Selmi Erim Bozbag

Synthesis of nanostructured materials using supercritical CO2: Part I. Physical transformations

Nanostructured materials have been attracting increased attention for a wide variety of applications due to their superior properties compared to their bulk counterparts. Current methods to synthesize nanostructured materials have various drawbacks such as difficulties in control of the nanostructure and morphology, excessive use of solvents, abundant energy consumption, and costly purification steps. Supercritical fluids especially supercritical carbon dioxide (scCO2) is an attractive medium for the synthesis of nanostructured materials due to its favorable properties such as being abundant, inexpensive, non-flammable, non-toxic, and environmentally benign. Furthermore, the thermophysical properties of scCO2 can be adjusted by changing the processing temperature and pressure. The synthesis of nanostructured materials in scCO2 can be classified as physical and chemical transformations. In this article, Part I of our review series, synthesis of nanostructured materials using physical transformations is described where scCO2 functions as a solvent, an anti-solvent or as a solute. The nanostructured materials, which can be synthesized by these techniques include nanoparticles, nanowires, nanofibers, foams, aerogels, and polymer nanocomposites. scCO2 based processes can also be utilized in the intensification of the conventional processes by elimination of some of the costly purification or separation steps. The fundamental aspects of the processes, which would be beneficial for further development of the technologies, are also reviewed.

Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature

Direct partial oxidation of methane into methanol is a cornerstone of catalysis. The stepped conversion of methane into methanol currently involves activation at high temperature and reaction with methane at decreased temperature, which limits applicability of the technique. The first implementation of copper-containing zeolites in the production of methanol directly from methane is reported, using molecular oxygen under isothermal conditions at 200 °C. Copper-exchanged zeolite is activated with oxygen, reacts with methane, and is subsequently extracted with steam in a repeated cyclic process. Methanol yield increases with methane pressure, enabling reactivity with less reactive oxidized copper species. It is possible to produce methanol over catalysts that were inactive in prior state of the art systems. Characterization of the activated catalyst at low temperature revealed that the active sites are small clusters of copper, and not necessarily di- or tricopper sites, indicating that catalysts can be designed with greater flexibility than formerly proposed.

Methane to methanol over copper mordenite: yield improvement through multiple cycles and different synthesis techniques

Copper mordenite was used for the conversion of methane to methanol in a cyclic operation. Repeated cycling was possible using catalysts having different loadings prepared via aqueous ion exchange using copper(II) acetate (Cu-MORA) and solid state ion exchange using copper(I) chloride (Cu-MORS). For Cu-MORA, the yield increased by at least 30% on the second cycle and remained constant afterwards. Linear combination fitting of the XANES identified a similar increase in the fraction of CuI formed upon reacting with methane on the second cycle. For Cu-MORS residual chlorine initially hindered the production of methanol. Successive cycles removed chlorine and yielded significantly more methanol per copper than Cu-MORA. Over successive cycles of Cu-MORS, the fraction of the CuII species that was reduced to CuI upon reacting with methane correlated well with the amount of methanol produced. Although commonly done, analyzing only one reaction cycle is not representative of the long-term performance of methane to methanol over copper mordenite. Copper species equilibrate with cycling.