Uptake of Ethyl Xanthate to Metal Organic Frameworks

As the mining industry spreads to new areas in the arctic regions, the need for re-useable efficient methods for mine chemicals’ recycling increases. Especially in the case of xanthates, which are used as collectors for many metals from ore. Xanthates are very toxic to aquatic life either directly or indirectly and cause potentially severe health problems to humans after long-term exposure. In the present work, potassium ethyl xanthate (KEX) was observed to coordinate into metal organic frameworks (MOFs). HKUST-1 and its post-synthetically modified forms were observed to behave most effectively of the studied MOFs at low concentrations of KEX. Differences in the uptake of KEX were detected regarding the synthesis method in the case of MIL-100(Fe) synthetized by solvothermal and mechanochemical methods. Other studied MOFs, UiO-66 and MIL-100(Al)/MIL-96(Al), were not observed to be effective in KEX uptake.


Synthesis
MOFs studied were synthetized following methods found from literature. If MOFs were needed in larger amounts, the syntheses were upscaled by multiplying the amounts of reagents. Reagents were purchased from Aldrich, FluoroChem, VWR, Strem Chemicals, and Acros Organics. All reagents and solvents were used as received.
Activation of the MOFs was conducted in vacuum at 90 ᵒC overnight.

HKUST-1 and its modified forms
HKUST-1 was synthetized as described in ref. [1] but slightly modified as the synthesis was conducted in higher temperature. 1.8 mmol (435 mg) of Cu(NO3)2·3H2O was dissolved in 6 mL of deionized water. 1.0 mmol (221 mg) of H3btc was dissolved in 6 mL of ethanol (99 %, Altia). Solutions were combined and transferred into 25 mL Teflon lined autoclave and kept in 120 ᵒC for 24h. After cooling to room temperature within 6 h, the turquoise product was washed with deionized water three times. The nature of the MOF was verified with PXRD.
Post-synthetic modification was conducted with vacuum dried HKUST-1 following the procedure in refs. [2] and [3].

MIL-100 (Fe)
MIL-100(Fe) was synthetized by mixing 1:1 Fe(NO3)2·9H2O:H3btc and adding water as a solvent. When a 25 mL Teflon lined autoclave was used, 0.567 mmol of each reagent (119.1 mg of H3btc and 229.1 mg of Fe(NO3)2·9H2O) and 17 mL of deionized water were used. The autoclave was kept at 200 ᵒC for 8 h and cooled to room temperature within 20 h. The product was washed with EtOH, DMF and deionized water and dried in air. The synthesis method was adopted from ref. [4]. The nature of the product was confirmed with PXRD.

Mechanochemical MIL-100(Fe)
Same reagents were used in the mechanochemical synthesis as in the solvothermal synthesis of MIL-100(Fe) in 1.5:1 (H3btc:iron salt) molar ratio. The synthesis method was adopted from ref. [5]. Both solid reagents (630 mg of H3btc and 808 mg of Fe(NO3)2·9H2O) were put into an agate mortar and ground for 10 min. During the grinding the reaction mixture changed from white and fluffy to pale yellow and sticky. Reaction mixture was transferred to 25 ml Teflon lined autoclave and kept at 160 ᵒC for 4 h, followed by cooling to room temperature in 14 h. Red orange product was washed once with deionized water and twice with EtOH.

MIL-100(Al)
MIL-100(Al) was synthesized according to the instruction given in ref. [6]. H3btc (2.40 mmol, 505 mg) and Al(NO3)2·9H2O (4.23 mmol, 900 mg) were weighed to beakers, followed by addition of 5 mL deionized water. After 10 min stirring, solutions were combined and stirring continued at room S3 temperature for 1 h. 0.2 mL of glacial acetic acid was added dropwise to the reaction mixture and stirring continued for 5 min. Reaction mixture was transferred to a 25 mL Teflon lined autoclave. Autoclave was placed into an oven and temperature was slowly increased to 210 ᵒC (40 ᵒC/h). The temperature was held at 210 ᵒC for 30 min. Autoclave was cooled rapidly by placing it to an ice bath. As soon as it was possible, the autoclave was opened, and the Teflon liner was allowed to cool down to room temperature in a fume hood. Pale yellow product was washed three times with MeOH and dried in air. The product was confirmed to be MIL-100(Al) with PXRD.
Due to the rapid cooling in the ice bath, some of the product escaped from the Teflon lined inner container. Because of this, the following syntheses were cooled in fume hood after 30 min at 210 ᵒC in the oven. The resulting products were a mixture of MIL-100(Al) and MIL-96(Al) according the PXRD patterns.

UiO-66
UiO-66 was synthetized with the same method as in ref. [7]. 0.13 mmol of H2bdc (21.6 mg) and ZrCl4 (30.3 mg) were weighed into a beaker followed by addition of DMF (13.6 mL). The mixture was then stirred and 1.3 mmol of glacial acetic acid was added. After this the reaction mixture was 20 min in an ultrasound bath. Reaction mixture was transferred to a 25 mL Teflon-lined autoclave it was held at 120 ᵒC for 24h. After cooling to room temperature within 10 h, the product was washed three times with DMF and MeOH. Product was dried in air. The identity of the product was confirmed with PXRD.

PXRD
Measurements were conducted for pristine and activated MOFs as well as 1:1 ratio (MOF:KEX) samples. Samples were prepared similarly to the solid-state NMR measurements. 10 mg of MOF in question was weighed and appropriate amount of KEX in ultrapure water was added. After 40 min the sample was either filtered with glass sinther or centrifugated, after which the sample was measured.
The room-temperature powder X-ray diffraction (PXRD) measurements were made by PANalytical X´Pert PRO diffractometer using Cu Kα radiation (λ = 1.5418 Å; 45 kV, 40 mA). For a routine PXRD experiments lightly hand-ground powder samples were prepared on zero-background signal generating Si-plate using petrolatum jelly as an adhesive. Diffraction intensities were recorded from a spinning sample. Diffraction data were acquired by an X´Celerator detector using 2θ range of 3−60° with step size of 0.017° and counting time of 50 s per step. Data processing and Pawley fits were performed with the program X´pert HighScore Plus (v. 4.9). [8] The unit cell parameters of the powder samples were refined by Pawley analysis using the corresponding single crystal structure parameters retrieved from the Cambridge Structrural Database (CSD). [9] Variables for the fits were as follows: zero-offset, polynomial background, sample displacement, unit cell parameters and peak profile parameters including peak width, shape, and asymmetry. Table S1. Crystallographic data of HKUST-1, HKUST-1_activated, HKUST-1_3PA and HKUST-1_4PA determined by whole pattern Pawley fits compared with corresponding single crystal data (SCXRD).

MIL-100(Fe)mechanoactivated
CIGXIA [12] Temperature (     Ref. [13], unit cell of isostructural MIL-100(Cr) was used in comparison.      All measurements were conducted in D2O. 5 mg of MOF in question was suspended into 600 µL of D2O in an NMR-tube with two drops of benzene, and appropriate amount of KEX was dissolved in 1 mL of D2O in a test tube. The ratio of MOF and KEX was 1:1 (with respect of moles). During the titration experiment, KEX was added 0.2 equivalents at a time in 5 min intervals until titration was complete. The titration was continued until the signal of the aromatic protons of the benzene was appr. 1:1 with respect of an aromatic signal observed due to the MOF. In some cases, there were no aromatic proton signal observed from the MOF, thus titration was continued until no significant changes were seen in the spectra.

Solid state NMR
Samples for solid state measurements were prepared from pristine HKUST-1 and UiO-66. In addition, KEX and copper xanthate were measured. Samples of MOF:KEX from 1:1 to 1:4 in the case of HKUST-1 and MOF:KEX 1:1 in the case of UiO-66 were prepared in ultrapure water. To a small beaker 200 mg of MOF was weighed and appropriate amount of KEX in 10 mL of ultrapure water was added. The S36 resulting suspension was stirred at room temperature for 40 min (1:1), 80 min (1:2), 120 min (1:3), or 160 min (1:4) after which the samples were filtered with a glass sinther and transferred to CPMAS rotor immediately. Measurement time was 9 days for HKUST-1 samples and 2 days for UiO-66 samples. 13 C CPMAS NMR spectra of the MOFs, ethyl xanthate, copper ethyl xanthate and ligands were recorded at room temperature with Bruker Avance 400 MHz spectrometer equipped with a SB 4 mm CPMAS probe, using 4 mm ZrO2 rotors. The solid samples were spun at a rate of 10 kHz. The CP contact time was 2 ms and relaxation delay 5 or 6 s depending on the measurement. Adamantane was used in the calibration of the instrument.

FTIR
FTIR measurements were conducted for activated MOFs and 1:1 ratio (MOF:KEX) samples. Samples were prepared similarly to the solid state NMR measurements. 10 mg of MOF in question was weighed and appropriate amount of KEX in ultrapure water was added. After 40 min the sample was either filtered with glass sinther or centrifugated, after which the sample was measured. Bruker Alpha Platinum ATR was used to measure the spectra and data was handled with Opus 7.0 program.