Characterizing Bond Activation in Alkanes and Alkyl Halides by Metal and Metal Cluster Cations using Spectroscopy and Photofragment Imaging

University Of Massachusetts Amherst, Amherst MA

Investigators

Abstract

In this project, funded by the Chemical Structure, Dynamics, and Mechanisms-A program of the Chemistry Division, Professor Ricardo Metz and his students at the University of Massachusetts Amherst are studying how alkanes and alkyl halides can be converted to more industrially useful chemicals. Although natural gas is mostly methane (CH4), it also has up to 6% ethane and 4% propane. Ethane and propane are extremely useful, as they can be converted into ethylene and propylene, which are then used to produce plastics like polyethylene and polypropylene, or other higher value chemicals. Industrially, the production of ethylene and propylene require a great deal of energy and they produce over a pound of carbon dioxide for every pound of ethylene produced. The Metz group uses specialized instruments in which streams of molecules under high vacuum interact with laser light to produce products like ethylene or propylene. Both graduate and undergraduate students are involved in this research project. In addition to gaining and disseminating knowledge about the chemistry of molecular systems, the students are receiving training in advanced laser techniques and vacuum technologies, as well as computational chemistry, which is used to help interpret the experimental results. Professor Metz is also helping to introduce eighth-grade girls to research in STEM fields through the Eureka! program. Complexes of metal ions and metal cluster ions with ethane and propane are produced in a laser ablation/flow reactor source, cooled via supersonic expansion, mass selected in a mass spectrometer and their structure and bonding is characterized using vibrational and electronic spectroscopy. Interaction with the metal weakens the carbon-hydrogen (C-H bonds) in the ethane or propane ligand; this weakening is key to the subsequent catalytic activation of the bond. The experiments measure the extent of C-H bond weakening and polarization by the metal, as they lead to lower frequencies for the corresponding C-H stretch vibrations. These experiments also reveal the identity of reaction intermediates and products in cases where several isomers could be involved, as in competitive C-H and C-C bond insertion. These studies are being carried out for several metals and for clusters with differing numbers of metal atoms to determine the electronic and structural factors that lead to size-dependent reactivity. Complementary velocity map imaging studies of metal halide, sulfide and carbene cation photodissociation measure kinetic energy release and fragment anisotropy, determining accurate bond strengths and probing the couplings between excited electronic states. Photodissociation of a metal halide are also being used to initiate a bimolecular reaction between a metal cation and an alkane, allowing the team to study reaction dynamics with controlled impact parameter and orientation, as a function of translational energy. The broader impacts of this work include potential benefits from improved catalysts as well as training undergraduate and graduate students in mass spectrometry and laser spectroscopy and in broader skills such as problem solving, experimental design, data acquisition and analysis and communication of their results. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.