Mix and wait – a relaxed way for synthesizing ZIF-8

Herein we report the synthesis of a zeolitic imidazolate framework (ZIF-8) by an easy “mix and wait” procedure. In a closed vial, without any interference, the mixture of 2-methylimidazole and basic zinc carbonate assembles into the crystalline product with approx. 90% conversion after 70 h. The reaction exhibits sigmoidal kinetics due to the self-generated water which accelerates the reaction.


Synthesis
ZIF-8 was obtained via simply mixing powders of 2-methylimidazole (HmeIm) and basic zinc carbonate (ZnCarb) in an HmeIm/Zn molar ratio of 2:1. To increase the area of contact between solid surfaces, the large-crystal HmeIm was ground into a powder. For this, a vertical miller (Pulverisette 23, Fritsch, Germany) with two stainless steel grinding balls (10 mm) were used to yield ca. 2 g of the HmeIm fine powder after 5 min at 50 Hz of milling frequency. The fine powder of ZnCarb was used as purchased. Removal of unreacted HmeIm was performed according to the reported procedure 1 by washing, sonicating (1 min), and centrifugation (10 min) of the assynthesized powder in 50 mL of ethanol. After three washes, the sample was dried at 60°C overnight.

Non-shaking experiments
HmeIm (0.36 g, pre-ground) and ZnCarb (0.24 g, as purchased) were placed in a beaker (50 mL) and thoroughly mixed with a spatula for 3 minutes. Several equally prepared mixtures were sealed in 8 mL glass vials and kept undisturbed at ambient conditions for 4, 16, 22, 46 hours, 1 week, and 4 weeks. After the specified elapsed time, the vials were opened and powder X-ray diffraction (PXRD) patterns were quickly recorded. Each experiment was repeated three times to check its reproducibility (Fig. S1). For comparison, a reference mixture was kept in an open vial for one month (Fig. S1).

Shaking experiments
HmeIm (0.36 g, pre-ground) and ZnCarb (0.24 g, as purchased) were placed in a beaker (50 mL) and thoroughly mixed with a spatula for 3 minutes. Several equally prepared mixtures were sealed in 8 mL glass vials and shook at 800 rpm on the shaker Minishaker MS 2 (IKA, Germany) with circular motion of the agitation table at ambient conditions for 4, 8, 12, 16, 22, and 46 hours. After the specified elapsed time, the vials were opened, and PXRD patterns were quickly recorded. Each experiment was repeated three times to check its reproducibility (Fig. S2).

Experiment in the dry atmosphere
The reaction in the dry atmosphere was prepared as follows: HmeIm (0.36 g, pre-ground) and ZnCarb (0.24 g, as purchased) were placed in a glove box under dry argon atmosphere. The reagents were thoroughly mixed in a beaker (50 mL) with a spatula for 3 minutes, sealed in 8 mL glass vial, and kept undisturbed for 48 h. After the specified elapsed time, the vial was opened and the PXRD pattern was quickly recorded (Fig. S3).

Experiments using different precursors
To study the effect of different metal sources, we prepared the following three mixtures: i) HmeIm (0.21 g, pre-ground) and zinc acetate dihydrate (0.29 g, pre-ground), ii) HmeIm (0.18 g, preground) and zinc nitrate hexahydrate (0.32 g, pre-ground), iii) HmeIm (0.31 g, pre-ground) and zinc hydroxide (0.19 g). The first two mixtures react already at mixing, as evidenced by the fluid release. After drying the mixtures the corresponding PXRD patterns were recorded (Fig. S3). The (iii) mixture of HmeIm and zinc hydroxide, in turn, was thoroughly mixed in a beaker (50 mL) with a spatula for 3 minutes, sealed in 8 mL glass vial, and kept undisturbed for 48 h. After the specified elapsed time, the vial was opened and the PXRD pattern was quickly recorded (Fig. S3).

Powder X-ray diffraction (PXRD)
PXRD patterns were collected with Cu Kα radiation (λ = 1.50406 nm) in a Bragg Brentano D8 Advanced diffractometer (Bruker AXS, Germany) equipped with a LYNXEYE XE-T detector. Samples were measured in reflection geometry in a 2Ɵ range from 3° to 50° with a step size of 0.02° with spinning setup.
Powder diffraction measurements of the samples prepared under dry argon atmosphere were performed with a D8 Discover diffractometer (Bruker AXS, Karlsruhe, Germany) operated in transmission geometry (Cu-Kα1 radiation, λ = 0.154056 nm), equipped with a Lynxeye detector. Samples were prepared in borosilicate glass capillaries (diameter 0.5 mm) and were measured in a 2θ range of 5-50° with a step size of 0.009° and 4 s per step.

In-situ Raman spectroscopy
Raman measurements were performed on a Raman RXN1 TM analyser (Kaiser Optical Systems, France), with a non-contact probe head (working distance of 6 cm, spot size = 1 mm), using an excitation wavelength of l = 785 nm for the collection of Raman spectra every 15 min. For every measurement, five spectra with an acquisition time of 5 s were accumulated. The jar contribution subtraction, baseline correction, and normalization were performed using a script for basic analysis and visualization of in situ Raman monitoring data 2 .

Dry ZIF-8 self-formation
To perform the non-shaking synthesis for in-situ Raman monitoring, HmeIm (0.36 g, pre-ground) and ZnCarb (0.24 g, as purchased) were placed in a beaker (50 mL) and thoroughly mixed with a spatula for 3 minutes. The mixture was sealed in the 3.5 mL PMMA jar, which is transparent to Raman laser radiation, and kept undisturbed until the end of the Raman experiment (70 h). To perform the shaking synthesis, the mixture sealed in the 3.5 mL PMMA jar was shaken for 8 h at 50 Hz using Pulverisette 23 vertical miller (Fritsch, Germany).

Water-assisted ZIF-8 self-formation
Two types of water-assisted experiments were performed: In the first kind of experiment, HmeIm (0.36 g, pre-ground) and ZnCarb (0.24 g, as purchased) were placed in a beaker (50 mL) and thoroughly mixed with a spatula for 3 minutes. Then 50 μL of Milli-Q water was added there and mixed 1 min more. After that, the mixture was sealed in the 3.5 mL PMMA jar and kept undisturbed until the end of the Raman experiment (45 h). In the second kind of experiment, HmeIm (0.36 g, pre-ground) and ZnCarb (0.24 g, as purchased) were placed in a beaker (50 mL) and thoroughly mixed with a spatula for 3 minutes. 50 μL of Milli-Q water was added to the bottom of the 3.5 mL PMMA jar, and the reaction mixture was added to the jar with no further mixing with the water below. The jar was sealed and kept undisturbed until the end of the Raman experiment (48 h).

Calibration of the Raman signal to calculate the conversion of reagents
To use the Raman spectroscopy for quantitative analysis, a 3-component mixture of solid samples was analysed. Several mixtures of basic zinc carbonate, 2-methylimidazole, and reference ZIF-8 were prepared as follows: HmeIm (1 g, pre-ground) and ZnCarb (0.67 g, as purchased) were placed in a beaker (50 mL) and thoroughly mixed with a spatula for 5 minutes. After that, the material was placed in several glass vials (8 mL) and mixed (3 min, spatula) with the equivalent amount of ZIF-8 to a conversion of 6, 32, 55, 76, and 96% of ZnCarb to ZIF-8, preparing three mixtures for each concentration as standards. Several measurements were performed for each mixture, and the results were processed analogously to those obtained in the in-situ mode using a script for analysis of in situ Raman data 2 . The calibration curve is plotted below, where the conversion of HmeIm was calculated from the ratio between the 1481 cm -1 band of HmeIm (I1) and the 1461 cm -1 band of ZIF-8 (I2).

Thermogravimetry (TGA)
TGA measurements were conducted on a TGA/DSC 3 + Thermogravimetric Analyzer (Mettler-Toledo, USA) in an atmosphere of air/nitrogen gas mixture (4/5 of air and 1/5 of nitrogen) in the temperature range of 25-600 °C, at a heating rate of 10 °C/min.

Scanning Electron Microscopy (SEM)
The Scanning Electron Microscopy (SEM) characterization was conducted on an XL 30 ESEM equipped with a tungsten cathode (FEI, Eindhoven, in 2020 electronic upgrade by point electronic GmbH). An Energy-Dispersive X-ray system (EDX, Quantax 200, Bruker Nano GmbH, X Flash 6/60 SDD) was used for the quantitative analyses of the elemental composition. Both, ESEM and EDS investigations, were performed in "high-vacuum mode" of the microscope. The signal was processed with a BackScatter Electron detector (BSE) and a Secondary Electron detector (SE). Prior to the analysis, all samples were coated with carbon. The corresponding EDS spectra were collected at an accelerating voltage of 10 keV and the quantification was performed standardless.
3.6 N2 adsorption measurements N2 adsorption/desorption isotherms were measured at 77 K on a volumetric ASAP 2020 device from Micromeritics in the range of relative pressures p/p0 from 10 −4 to 0.996. For analysis, 0.15 g of material were loaded in the analysis probe, followed by thermal activation at 120 °C for 18 hours under a dynamic vacuum to reach a pressure below 10 -7 mbar. The BET area (ABET) was calculated by following Rouquerol and Llewellyns' rule to select the optimal BET range. 3 The total pore volume (Vp) was calculated from the N2 uptake at p/p0 ≈ 0.9.       Table S1 The EDX evaluation of the as-synthesized ZIF-8 in the dark and bright regions compared with the theoretical values for pure ZIF-8 and ZnCarb.