Investigating the Effect of Trace Levels of Manganese Ions During Solvothermal Synthesis of Massey University Framework-16 on CO2 Uptake Capacity

The effects of impurities on reaction precursors for metal–organic framework (MOF) synthesis have not been studied in extensive detail. The impact of these impurities can be an important factor while considering scale-up of these materials. In this work, we study the apparently positive impact of the presence of manganese ions for the synthesis of a Co-based MOF, Massey University Framework-16 (MUF-16). The presence of a trace amount of manganese in the reaction mixture led to consistently high CO2 uptake across multiple batches. Characterization including X-ray diffraction, scanning electron microscopy, Fourier transform infrared-attenuated total reflectance, ultraviolet–visible spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure spectroscopy led us to hypothesize that the differences in CO2 adsorption among materials with differing synthesis routes arise from variations in the local environment around the cobalt metal center. Aided by density functional theory calculations, we speculate that manganese ions get inserted into the structure during crystallization and act as catalysts for ligand substitution, improving the possibility for octahedral coordination of cobalt with the ligand, thus leading to Co-based pristine structures with higher CO2 uptakes.


Section 2. SCXRD
Single crystal XRD analysis was performed for one high performing batch (S2(II), CO2 uptake of 2.09 mmol/g at 100 kPa @30⁰ C) and one low performing batch (R3, CO2 uptake of 1.03 mmol/g at 100 kPa @30⁰ C).In both cases, the resolved structures have similar unit cell parameters.
Table S1 shows SCXRD structure parameters for batches R3 and S2(II) and the structure reported by Qazvini et al. [1] (CCDC 1948901).Both the batches synthesized in this work show agreement with each other and with the structure reported in literature.
These structures are reported as CIF files in the "Structure Files" folder in the supplementary information.

Batch R3
Batch S2(II)  Table S2: Space group and unit cell parameters obtained from SCXRD analysis for batches R3, S2(II) and as reported by Qazvini et al.

Section 3. Thermogravimetric Analysis
TGA curves for multiple batches are given in Figure S3.High performing batches (S1, S2(II), S3 and Mn containing batches) seem to have a relatively higher decomposition temperature (≥350 ⁰C) while T2, R2 and R3 exhibit a significant loss in weight below 350 ⁰C (R1 shows the decomposition step at a higher temperature).This indicates that high performing batches in general have slightly better thermal stability.At 350 ⁰C, about 80 weight percent of the material leaves as gaseous byproducts of combustion, leaving metal oxide as residue.The ~80% weight loss is in line with our expectations since stoichiometrically the MOF contains about 14 wt% Cobalt.

Section 4. X-ray Photoelectron Spectroscopy
Cobalt XPS spectra for some additional batches are given in Figure S5.Batches S2(II).T1 and R2 do not show a prominent Co 2p3/2 satellite.M2_0.5, the only low performing batch with Mn, also shows the feature.XPS does not detect the presence of Manganese in the Mx_y series.However, MUF-16(Co/Mn) shows the Mn can be inserted into the MOF backbone (Figure S6).The atomic percentage for MUF-16(Co/Mn) is given in Table S2.Co:Mn ratio is

Section 7. Energy of mixing calculations
These structures are reported as CIF files in the "Structure Files" folder in the supplementary information.

Section 8. Proposed mechanism for impact on Mn on MOF crystallization
A potential mechanism for the apparent catalysis by Mn 2+ ions of Co-MOF formation is shown below.The MUF-16(Co) structure is represented here by a two-dimensional trimeric fragment.Mn 2+ undergoes ligand substitution significantly faster than Co 2+ , although both metal ions are classified as substitution-labile.[2] On this basis we suggest that a binuclear intermediate (structure 1) can be formed rapidly by two successive steps at the Mn 2+ center.The last step to form the "complete" MOF, is essentially a carboxylate ligand exchange reaction, assisted for Co 2+ (which normally undergoes dissociative ligand substitution) by the compensatory delivery of the MOF-associated ligand by Mn 2+ (which favors associative ligand substitution).[2] We further suggest that Mn 3+ , which also serves as a catalyst for formation of the complete MUF-16(Co) structure, is reduced to the active Mn 2+ by Co 2+ ions, as that process is very strongly driven (by more than 3.0 V).

Figure 1 (
Figure 1(b) shows PXRD patterns for batches containing high amounts of manganese.The patterns show agreement with all other PXRD patterns indicating that Mn can get inserted into the structure with similar lattice.SEM images for batches with high uptakes (T1, M2_4.9, M3_0.7) are given in FigureS2.SEM images show that the high performing batches show sheet like structures whereas low performing batches show a mix of needle like and sheet like structures.

Figure S1 :
Figure S1: (a) PXRD patterns for batches containing trace amounts of manganese (b) PXRD patterns for MUF-16(Mn) synthesized with no cobalt salts and MUF-16(Co/Mn) synthesized with 1:1 molar ratio of Co and Mn in the reaction mixture.
However, a clearer indication of the possible presence of loosely bound species in the low performing batches comes from looking at the weight profile between 200 -350 ⁰C.Batches S1, S2(II), S3, Mx_y (except M2_0.5) show high CO2 uptakes and also show a sharp decline in weight at the decomposition temperature.However, the low performance batches show a more gradual mass decline starting at about 200 ⁰C.

Figure S5 :
Figure S5: TGA analysis for (a) batches from R, S and T series (b) batches from Mx_y series.The weight percentage is calculated based on dry weight after activating in-situ at 150 • C for 2 hours.

Figure S6 :
Figure S6: X-ray photoelectron spectroscopy highlighting (orange band) the Co 2p3/2 satellite for (a) Series R, S and T. Batches S2(II) and T1 show CO2 uptakes > 1.5 mmol/g at 100 kPa at 30 ⁰C.Batch R2 shows an uptake of 1.3 mmol/g at those conditions.(b) MOF batches with addition of Mn ions where Mx_y indicates addition of Mn x+ ions at a concentration of y mmol Mn x+ /mol Co 2+ .All batches except M2_0.5 show high uptake performance.

Figure S11 :
Figure S11: (A) Overall scheme proposed for Mn 2+ -accelerated elimination of defects in the MUF-16(Co) structure.(B) An example of binuclear Mn-mediated exchange of carboxylate ligands required for this operation.