# On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol
## 2 Materials and Methods
### Characterizations  
The XRD patterns of the catalysts were obtained using a Bruker D8 Advance diffractometer (Woltzbach, Bruker Company, Karlsruhe, Germany) with Cu K\(\alpha\) radiation, scanned over a 20 range of 5-85\({}^{\circ}\) and at a scanning rate of 10\({}^{\circ}\)/min. The SEM of the catalysts was examined using a field emission scanning electron microscope (SEM, Nova NanoSEM 450, FEI, Hillsboro, OR, USA) with a magnification of 50,000 times. The BET specific surface area was determined by N\({}_{2}\) adsorption-desorption isotherm measurements performed on a Micromerities ASAP 2460 instrument (Micromerities Company, Norcross, GA, USA). The sample was first vacuum-dried at 60 \({}^{\circ}\)C for over 12 h, then approximately 100 mg was weighed and placed in a U-type tube. It was degassed under vacuum at 120 \({}^{\circ}\)C for 6 h as pretreatment. Subsequently, N\({}_{2}\) adsorption and desorption were carried out at 77 K. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) equation, while the pore size distribution was determined from the desorption data via the Barrett-Joyner-Halenda (BJH) method. The CO\({}_{2}\)-TPD of the catalyst was performed using the VDSorb-91i chemisorption analyzer (Ward Instrument Co., LTD, Quzhou, Zhejiang, China). Approximately 60 mg of the sample was pretreated at 500 \({}^{\circ}\)C for 1 h, cooled to 40 \({}^{\circ}\)C, and then exposed to CO\({}_{2}\) (30 mL/min). Helium purging was performed until the baseline stabilized, followed by heating to 750 \({}^{\circ}\)C for testing (10 \({}^{\circ}\)C/min). The Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS) were conducted using the Thermo Scientific Nicolei iS20 instrument (Thermo Fisher Scientific, Waltham, MA, USA). In these experiments, an in situ diffuse reflectance cell was used and detected by an MCT/A detector (Thermo Fisher Scientific, Waltham, MA, USA). Spectral analysis was carried out within the wavenumber range of 4000 cm\({}^{-1}\) to 650 cm\({}^{-1}\), with high precision of 4 cm\({}^{-1}\) and 32 scans. Approximately 20 to 30 milligrams of catalyst were loaded into the in-situ battery. The sample was first introduced with pure argon gas, heated up to 400 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min for 1 h, and then slowly cooled to 20 \({}^{\circ}\)C. Meanwhile, the background spectrum at the corresponding temperature was recorded. After continuing to purge with argon gas for 10 min, CO\({}_{2}\) gas was introduced. After adsorbing CO\({}_{2}\) for 20 min, Ar is blown into the battery. After blowing for 5 min, it is heated to 400 \({}^{\circ}\)C at a rate of 10 \({}^{\circ}\)C/min. The real-time spectrum of the temperature rise process after adsorbing CO\({}_{2}\) was recorded.  
The alkali strengths of the solid catalysts were determined using the Hammett indicator method, and the surface alkalinity was investigated using non-aqueous titration (specific operational steps can be found in the Supporting Information). Following the representation method of the Hammett function, the surface alkali strength of solid catalysts is denoted by H\({}_{0}\). Indicators employed for measuring surface alkalinity included bromothymol blue (Brb, pKa = 7.2), phenolphthalein (Phe, pKa = 9.3), alizarin yellow R (AyR, pKa = 11.0), and 2,4-dinitroaniline (Din, pKa = 15.0). When titrating the alkalinity, cyclohexane was used as the solvent and phenolphthalein as the indicator. The solid surface was titrated with a benzoic acid/ethanol solution until it turned colorless, indicating the endpoint. The alkali content was then calculated based on this titration [22; 34].