www.acsprf.org

This project explores the development of solid, copper based catalysts for C-N and C-O coupling reactions.  Most existing catalysts are homogeneous or soluble in nature, and therefore difficult to separate from the reaction media for product purification or catalyst recycle. 

The project has been broken into three parts.  In this first part, solid copper oxide nanoparticles of different crystal shapes are being prepared and evaluated as catalysts for catalytic C-N coupling reactions using amines and aromatic halides.  The overall goal is to assess whether the solid nanoparticle surfaces are catalytically active, as in traditional heterogeneous catalysts, or whether the nanoparticles must dissolve to produce soluble, molecular copper ions that act as homogeneous catalysts.  A hypothesis is that by synthesizing copper oxide nanoparticles of different shapes, which present different crystal surfaces, one can assess whether the nature of the solid copper surface affects the reaction, consistent with a heterogeneous catalytic mechanism.  It was found that differently shaped nanoparticles, which have different surface areas (15, 29, and 45 m²/g) have different catalytic reactivity in typical C-N coupling reactions (e.g. imidazole and iodobenzene) and C-O coupling reactions (phenol and iodobenzene). Additional studies are directed at whether these results correlate with ease of dissolution of the nanoparticles or differing reactivity of the presented crystal faces.

In the second part, ”heterogeneous” copper catalysts, prepared by ion-exchanging aluminosilicate zeolites (NaY) with Cu^II ions, are used to assess the heterogeneity/homogeneity of C-N coupling reaction. Kinetic studies of the C-N coupling reactions using different sized reagents are being used to test assess whether the reaction occurs shape-selectively within the zeolite pores or whether copper ions leach from the zeolite particles, into solution. Using reactants larger than the zeolite pore size, the reaction is much slower than when using an electronically similar but is faster when using the non-size-discriminating copper oxide nanoparticles, these data suggest that the catalytic reactions occur within the zeolite pores and a truly heterogeneous catalyst has been developed. Elemental analysis of the zeolite before and after the reaction showed no change of the copper loading after the reaction, consistent with a heterogeneous process. A downside to this approach is that the inorganic bases used in the catalytic reaction degrade the crystallinity of the zeolite catalyst.

In the third part, CuO supported on mesoporous CeO2 has been developed as a supported copper catalyst that should be stable in the presence of strong bases.  Using potassium carbonate as base, this material is effective in the C-N coupling of imidazole and iodobenzene.  Yields are moderate, 50-85% over several hours of reaction.  The catalyst can be recovered and effectively reused, although some copper species are lost from the solid under reaction conditions.  Control experiments suggest these soluble species are not catalytically active.  When using potassium hydroxide as base, reaction rates are significantly increased, although the increase is largely associated with side reactions, such as conversion of iodobenzene to phenol.

Ongoing studies will focus on elucidating the nature of the reactivity in the nanoparticle case, as well as development of other truly heterogeneous solid-supported copper coupling catalysts.