2.2. Catalyst characterization
Elemental analyses were determined by atomic absorption spectroscopy (AAS). Thermogravimetric analysis (TGA) was performed with a STA409 instrument in dry air at a heating rate of 15 C/min. X-ray diffraction (XRD) patterns were carried out with a Bruker D8 Advance powder diffractometer using Cu Ka radiation source (k = 1.5406 Å) at 40 kV and 40 mA from 10 to 90 with a scan rate of 4/min. The microstructures were collected on scanning electronic microscope (SEM) at 2.0 kV (Hitachi S-4800, Japan).
The basic strength of the catalyst was measured with Hammett indicators, including neutral red (pKBH+ = 6.8), phenolphthalein (pKBH+ = 9.8), Nile blue (pKBH+ = 10.1), and 2,4-dinitroaniline (pKBH+ = 15.0)The basicity of the catalyst was determined by method of Hammett indicator–benzene carboxylic acid 0.02 mol/L anhydrous methanol solution) titration Additionally, the total basicity and basic strength of the catalyst was also detected by temperature programmed desorption of carbon dioxide (CO2-TPD) with an AutoChem 2920. About 0.1 g of catalyst was pretreated under a helium stream at 700 C for 2 h. Subsequently the catalyst was saturated with CO2 for 1 h at room temperature. After flushing with He flow for 30 min, the catalyst was heated from room temperature to 900 C with a heating rate of 10 C/min and the desorbed CO2 was detected with a thermal conductivity detector