CO2 adsorption in Y zeolite: a structural and dynamic view by a novel principal-component-analysis-assisted in situ single-crystal X-ray diffraction experiment

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.


S1. Preparation of zeolite Y single crystals
Two batches of SOL were prepared using different amounts of TEA. a) 4.76Na2O : 1.0Al2O3 :3.5 SiO2 :454 H2O: 8 TEA b) 4.76Na2O : 1.0Al2O3 :3.5 SiO2 :454 H2O: 5 TEA The first solution was prepared adding 6.468g di NaAlO2 to 170ml of hot NaOH (2M) solution. Then 15.19g (a) or 24.30g (b) of TEA were added to 100g of the solution. In a second becker the solution was prepared adding 0.58g of fumed SiO2 in 10ml of distilled water. The solutions were aged one night then the first solution was filtered and added to the second. Each batch was divided into two sealed bottles. For each batch one bottle was put in oven @75°C and another @100°C.
Batch (b) was left in the oven for 8 days while batch (a) was left for 13 days. In all the bottles, clear octahedral crystals were found at the bottom of the flask. The larger crystals, of about 50 µm, were found in batch (a)@100°C while in batch (b)@100°C the crystals were of about 30 µm in size. In both batches left @75°C the crystals were less in number and less regular. No evident signs of twinning were present and the crystals formed from (a)@100°C and (b)@100°C are approximately all of the same size. For the measurements one crystal from batch (a)@100°C was chosen.
S2. X-ray diffraction data analysis of zeolite Y single crystals Figure S1 Cell parameter vs. temperature of Zeolite Y measured from 300 to 200 K.    Supporting information, sup-3

Figure S4
Structure of Zeolite Y with CO2 and Na atoms labeled. The name of Na atom sites traditionally used in the literature are identified by numbers for sake of clarity (NaI becomes Na1, NaI' becomes Na11 and NaII' becomes Na21) and the different positions assumed during the temperature ramp are indicated by letters from a to c for position I' (Na11a is the nearest to Na1) and from a to e for position II'. Detail of the CO2 in the inset. CO2 is adsorbed in two positions in the large channel of the zeolite, in particular one CO2 molecule is coordinated to Na21 (NaII'), the other CO2 position is more disordered.
S3. PCA analysis of in situ X-ray diffraction data analysis of zeolite Y S4. In PCA case 1 and 2, hkl intensities are given as input in the PCA ( Figure S5 to S8). In these cases, by plotting the loadings of the reflections on PC1 vs. PC2 (top left figures) we obtain a scatter plot showing which reflections (indicated by sequential numbers) have the major weight on the two principal components.
By plotting the loadings on two separate plots ( The variable θ, i.e. the amount of adsorbed CO2, was obtained by two methods. At first the sum of the CO2 occupancies from structural refinement vs. 1/T was used since it is a direct measurement of the absorbed CO2. Of course, this method requires the knowledge of the structure to carry out the refinement. Then the PC1 scores vs. 1/T trend was used, in the assumption that it describes uniquely the variance due to CO2 adsorption. As discussed earlier, the method has the great advantage of being fast without any a priori knowledge of the system. As a drawback, PC scores are not determined in absolute scale and the have a sign ambiguity, which limits their use in this case. The thermodynamic parameters were obtained by the van't Hoff equation (Eq.2): The slope and the intercept of the plot of left side of Eq. 2 vs 1/T allow the calculation of the enthalpy and entropy of the process. In particular, we used = 2 2 where occCO2 are the values of the refined occupancy of CO2 from the XRD experiment and = 1 1 where scorePC1 are the scores of the first principal component from the PCA. When PC scores are used, due to the arbitrariness of the sign, the right sign has to be assigned to the scores by a priori knowledge of the type of reaction.  Table S1 Data from manual refinement with SHELX.