Date Published: March 01, 2019
Publisher: International Union of Crystallography
Author(s): Eleonora Conterosito, Luca Palin, Rocco Caliandro, Wouter van Beek, Dmitry Chernyshov, Marco Milanesio.
In situ single-crystal X-ray diffraction data were used to unravel the structural dynamics and enthalpy and entropy of adsorption of CO2 into Y zeolite. A principal-component-analysis- (PCA) based approach is applied in an innovative way to single-crystal X-ray diffraction data analysis, allowing one to selectively detect the information from the subset of active atoms. The potential of and limitations of PCA in single-crystal diffraction are discussed.
PCA is applied here for the first time to the analysis of single-crystal data; it has the advantage of processing all data sets at the same time and does not require one to solve and refine the crystal structures from each data set. It is exploited here three times with two different approaches. PCA was first applied directly on experimental (case 1) and on simulated (case 2) intensities to extract the dynamic trends within the data; in these cases the variables are the Miller indices identifying the diffraction spots and the samples are the hkl intensities in the data sets collected at different temperatures. PCA was then applied in a different way to the occupancies obtained by a structural refinement (case 3) to facilitate the interpretation of the results from a traditional crystallographic approach; the variables in this case are the atomic species, and the PCA cases are again the data sets collected at the various temperatures.
The calculation of the enthalpy and entropy of CO2 adsorption was then performed, extending to in situ XRD the approach proposed by Garrone et al. for in situ FT–IR data (Garrone et al., 2017 ▸; Garrone & Areán, 2005 ▸), using the refined occupancies as an indicator of CO2 adsorption. The calculation of these parameters is possible because the timing of the experiment, described in Section 3, allowed collection of each data set at equilibrium, without evident evolution of the system during individual measurements. This is confirmed and demonstrated by the good values of Rint and R(sigma) [see Fig. S2 (top)] of each data set (they would be higher and more irregular in the temperature range where adsorption occurs if the system were evolving within the same data set). Moreover the 2D raw data frames collected at the beginning and at the end of the data set would have given many inconsistent intensities for equivalent reflections [Fig. S2 (bottom)] if the system were evolving.