Synthesis of Zeolitic Imidazolate Framework-8 Using Glycerol Carbonate

In this study, we show that glycerol carbonate (GlyC), a bio-based derivative of glycerol, can be used as a suitable green solvent for the synthesis of metal–organic frameworks (MOFs). In particular, a zinc-based zeolitic imidazolate framework-8 (ZIF-8) was synthesized by exploring several different experimental conditions (in terms of temperature, reaction time, and reactants’ concentrations) to find that the yield of the reaction and the quality of the products, measured in terms of crystallinity, surface area, and porosity, were in line with those obtained in the most commonly (non-green) used solvents. GlyC was also found to be reusable for several cycles, maintaining the same original quality as a solvent for the synthesis. Finally, some indicators for the assessment of the greenness of a process (E-factor and PMI) revealed a milder environmental impact of GlyC with respect to other solvents.


Table of Contents
• Glycerol carbonate characterization by NMR spectroscopy • Sodium 2-methylimidazolate preparation and characterization • Characterization of the reaction by-products

Glycerol carbonate characterization by NMR spectroscopy
The NMR spectra were collected on a Bruker Avance-400 (100MHz 13 C) spectrometer using deuterated dimethyl sulfoxide (DMSO) as solvent.

Glycerol carbonate
UV-vis spectra were also acquired to characterize the solution, after ZIF-8 precipitation, in the optimized reaction conditions ([Hmim] =20 mM, [Zn(OAc)2] = 10 mM, [NaOH] = 0.01M, 24 h, GlyC as a solvent).ZIF-8 was removed, and a proper amount of the solution was diluted 1:100 with distilled water for UV-vis measurements.As shown in Figure S2, also in this case a strong absorption appeared between 300 nm and 500 nm.A control experiment was carried out using an aqueous solution of 2methylimidazole.In addition, experiments were also performed using different amounts of base at a fixed concentration of 2-methylimidazole (1.0 M) in water.

S6
The formation of sodium 2-methylimidazolate is also confirmed by NMR spectroscopy.The NMR spectrum was collected on a Bruker Avance-400 (300MHz 1 H) spectrometer using CD3Cl as a solvent and are reported in Figure S4 (red line). 1 H NMR spectra confirm the formation of sodium 2-methyl imidazolate from 2-methylimidazole and NaOH in CDCl3.CDCl3 allows the study of this reaction compared to D2O where hydrogen-deuterium exchange is predominant.The solution turns from colorless to yellowish color as a consequence of the formation of 2-methylimidazolate.The 1 H NMR spectrum of 2-methylimidazole in CD3Cl is reported for comparison (blue line).Abs Wavelenght (nm)

Characterization of the reaction by-products
This section shows and discusses the 13 C NMR spectra to gain more insights into the reaction mechanism.The NMR spectra were collected on a Bruker Avance-300 spectrometer.Several experiments were carried out in order to investigate the main side reactions that can take place in the reaction system.First, the reaction of glycerol carbonate with NaOH was investigated.In detail, in a closed glass vial, 5 mL of GlyC and NaOH (0.01 M in GlyC) were mixed at 100 °C for 24 h under magnetic stirring.This reaction yielded glycerol (colorless solution) and carbon dioxide as products (Figure S7), following the reaction mechanism proposed in Figure S8.For the investigation of the reaction between GlyC and 2-methylimidazole a stoichiometric mixture of GlyC (5 mL) and 2-methylimidazole (1.0 M) was mixed under continuous stirring at 100°C for 24 h in a closed vial.The 13 C-NMR spectrum of the reaction product is reported in Figure S10 and compared to that of the solution of 2-methylimidazole (1.0 M) in freshly prepared GlyC (Figure S9).
The reaction mixture turned yellowish at 100 °C. Figure S10 reports all signals for both GlyC and 2methylimidazole.Further signals were identified and will be investigated more in detail in a future work.
The spectrum of the solution after ZIF-8 preparation is not reported due to the excess GlyC (in this case only the characteristic signals of GlyC can be appreciated by NMR spectroscopy).The latter is also related to the low amount of side products formed during the reaction.
Figure S3 illustrates a possible reaction mechanism for the formation of sodium 2-methylimidazolate.