46439-AC10
Probing the Mechanism of Sorptive Reconstruction Upon Ethylene and CO binding to CuAlCl4
In the first year of this study of the molecular mechanism(s) for binding small molecules to network solids we have:
- Investigated the melt structure of (et)CuAlCl4.
- Constructed a microbalance for kinetic gas sorption measurements.
- Begun an investigation of the mechanism of the hydration of ZnCl2.
1. Melt Structure of (et)CuAlCl4.
In preliminary work investigating the mechanism of ethylene sorption into CuAlCl4 we discovered one- and two-equivalent ethylene adducts. The one-equivalent adduct forms at low pressures of ethylene (<50 Torr) and the two-equivalent adduct forms at higher pressures. Comparison of the crystal structures of these two phases provides an apparent correlation of structure, forming the basis of our hypothesized sorptive reconstruction reaction mechanism. Preliminary studies of the kinetics of this sorption implied a first order dependence on the reactive gas, and data are consistent with an associative reaction mechanism. Nevertheless interpretation of the kinetics based on accepted solid-state rate laws was severely complicated because room temperature liquid phase(s) formed as ethylene was sorbed. In other work, we established that melt structures of materials are similar to that of their crystalline parent. Therefore as part of this project we are conducting synchrotron and neutron scattering experiments to determine the melt structure of (et)CuAlCl4.
Neutron scattering data of molten (et)CuAlCl4 were collected on the GLAD instrument at the intense pulsed neutron source. Ethylene adducts with the fully deuterated and a H/D null scattering mixture were measured. The structure factor of the fully deuterated melt shows an intense low-Q diffraction peak at 0.94 Å-1 that is followed by a broad shoulder out to 1.5Å-1. By contrast the null scattered sample, which highlights the scattering from non-hydrogen components, exhibits a strong low-Q peak at 1.5Å-1. The intense peak observed at 2.0Å-1 is essentially constant for both the deutrated and null scattering samples consistent with assignment largely to the metal halide scattering. Other broad diffraction features are observed at higher reciprocal space. These low reciprocal space peaks are of most interest being the primary signatures of intermediate range order. There is a high correlation between the diffraction patterns of the melt and the known crystalline structure. Clearly preserved are (hk0) diffraction features corresponding to chain-to-chain stacking. An initial model suggests that upon melting a significant extent of the chain structure remains intact, with some loss of chain-to-chain registry. However, greater chain-to-chain registry may be maintained in this melt than has been observed for other melt systems. Our initial hypothesis is that hydrogen bonding between the olefins and the chlorides accounts for weak higher dimensional organization in the melt.
2. Temperature Controlled Microbalance
A significant effort in the first year of this project involved constructing an environmentally controlled microbalance. A commercial quartz microbalance head was purchased and the corresponding sample chamber, and temperature control system was constructed in-house. Lab-view software was utilized for instrument control. To test the efficacy of this system for the desired kinetic measurements on the lab bench before moving it inside a glove box where the studies of CuAlCl4 will be conducted, we began investigating the hydration of zinc chloride.
3. Hydration of ZnCl2
Like many metal halides, ZnCl2 is hygroscopic. While there are many literature reports investigating the dehydration of metal hydrates, few studies have examined the hydration reaction. Having observed structural analogies between ZnCl2 phases and the one crystallographically characterized hydrate, ZnCl2×1.3H2O, this system seemed to be viable to test the above microbalance. In addition, this hydration study is of scientific value, given a thorough knowledge of hydrate formation is anticipated to be of fundamental importance to developing potential water splitting catalysts. Surprisingly, when ZnCl2 placed on the microbalance is exposed to a stream of water saturated nitrogen at room temperature, more than twenty equivalents of water per zinc are sorbed. On desorption by exposure to dry nitrogen, excess water is lost yielding the known 1.3 hydrate. Interestingly, however, we have not yet established any conditions under which the hydration reaction proceeds via a solid-solid reaction. In all cases examined, a melt/liquid phase forms on sorption, with the solid hydrate phase forming only after desorption. Controlling the reactant temperature to precisely determine the water vapor pressure, and developing temperature control of the reaction chamber, we are establishing the equilibrium binding constants as a function of temperature. With water vapor pressure controlled to 5 Torr, there appears to be an inflection in the sorption around three equivalents of water at room temperature. By contrast at 50°C only 1.8 equivalents of water are sorbed, and at 10°C eight equivalents of water are sorbed. The microbalance effectively allows for measurement of the kinetics of the sorption process as well, which are consistent with declaratory solid-state reaction kinetics. More precise gas flow control is being constructed to reliably complete the kinetic study.
