Using water adsorption measurements to access the chemistry of defects in the metal-organic framework UiO-66

Tailoring defects in metal–organic frameworks is important for enhancing sorption and reaction properties. Defects in UiO-66 have been characterized for the first time by using water adsorption measurements. We found that the defect-induced hydrophilicity, quantitatively expressed by the Henry constant and the saturation water uptake, correlates well with the catalytic performance in the cyanosilylation of benzaldehyde.


Experimental section
Materials: All chemicals were purchased from commercial suppliers (Sigma-Aldrich, Alfa Aesar, Acros Organics and others) and used without further purification.
Characterization Methods: Powder X-ray diffraction (PXRD) measurements for AA samples were recorded in flat mode with an X'Pert Empyrean series 2 PANalytical equipment in Bragg-Brentano geometry, with a PIXcel position sensitive detector and a CuKα radiation source (λ = 1.54178Å) at room temperature.Instead, TFA samples were recorded in capillary mode.FTIR spectra were measured with a Bruker Alpha FTIR spectrometer equipped with a single-reflection platinum ATR module under inert gas conditions.Nitrogen sorption measurements were performed with a Quantachrome NovaTouch instrument with nitrogen gas at 77 K. Prior to the measurements, all samples were evacuated in dynamic vacuum (p = 10 -3 mbar) at 150 °C for 12 h.Thermogravimetric analysis was performed with a Mettler Toledo TGA/STA 409 PC apparatus with a continuous heating ramp of 10 °C/min applied under inert conditions in Ar flow.3-5 mg of sample were used in a temperature range from 30 to 1000°C.Water isotherms were measured on a BEL Japan BELSORP.Prior to the measurements, samples were activated to 170°C under secondary vacuum for 16 hours.Prior to performing water adsorption, water (analyte) was flash frozen with liquid nitrogen and then evacuated under dynamic vacuum 3 times in order to remove dissolved gases from the water reservoir.The measurement temperature was controlled with a water bath at 25 °C and with the dosing manifold controlled at 40°C to avoid unwanted condensation effects.Helium was used to estimate the dead volume prior to the water adsorption-desorption measurements.Adsorption equilibrium was assumed when the variation of the cell pressure was 0.5% for a minimum period of 300 seconds.

Reference UiO-66
The reference sample (UiO-66 ref) was synthesized with a slightly modified procedure of Shearer et al.. Typically 0,945 g of ZrCl 4 1.34 g of terephthalic acid and 0,715 ml of HCl were mixed in 24,35 ml of DMF.The solution was kept in a oven for 24 h at 180°C in a 50 ml teflon liner autoclave.The resulting material were then collected via centrifugation washed 3 times with fresh DMF (3 x 30 ml) and solvent exchange with MeOH (3 x 30 ml).The product was heated under vacuum at 250°C overnight, at 300°C for two hours and then stored in a glovebox.

Defective UiO-66 (Acetic acid modulation)
Briefly, for the AA modulation, 169 mg of ZrCl 4 (0,73 mmol) and either 121 mg of terephthalic acid (0,73mmol) were dissolved in 38ml of DMF.For each sample 16AA, 32AA, 64AA, 100AA, 200AA we added a different amount of acetic acid: 0.7, 1.4, 2.8, 4,2 and 8,4 ml, respectively.All the samples were kept in a glass jar in the oven at 120°C for 24h.The resulted materials were collected by centrifugation, washed three times with DMF (3 x 30 ml) and solvent exchange with MeOH (3 x 30 ml) in three days.Finally, the solids were activated at 250°C overnight under dynamic vacuum ( ≈ 10 -3 mbar) and stored in a glovebox.

Defective UiO-66 (trifluoro acetic acid modulation)
In a typical synthesis, 3,5 g of ZrCl 4 as well as 2,5 g of terephthalic acid with 1,5 ml of HCl were mixed in 155 ml of DMF.Instead of AA, 1 eq ( 1,15 ml ) and 10 eq ( 11,5 ml ) of trifuoro acetic acid (TFA) were used as modulator respect to the amount of zirconium.The resulted materials were collected by centrifugation, washed three times with DMF (3 x 30 ml) and solvent exchange with MeOH (3 x 30 ml) in three days.Then the samples were activated at 250°C for 12 h and at 320°C for 3 h under dynamic vacuum.All the samples were stored inside a glovebox.

Catalytic experiments
In a typical catalytic experiment 62µL (0.5 mmol) trimethylsilylcyanide (TMSCN), 25 µL (0.25 mmol) benzaldehyde and ~3mg solid catalyst were stirred (~500 rpm) in a finger schlenk under Ar atmosphere at 40 °C in 1 mL DCM.The reaction was followed by taken aliquots at given time intervals that were analyzed by GC and GC-MS.For the analysis by GC the catalyst were filtered of by the use of syringe filters.The sum formula of the obtained materials are calculated from TGA data based on the method proposed by Valenzano et al.. 1 Due to low incorporated amount of modulator and small differences in the TGA curves in the first three AA samples (16AA, 33AA, 67AA) the sum formula is considered the same.

Fig
Fig. S8.Determination of Henry's Constant

Table S1 :
Reusa bility studies for UiO-66 ref, 1TFA and 10TFA which were used as catalysts in the cyanosilylation of benzaldehyde at T = 40 °C in CH 2 Cl 2 as solvent.Yield was obtained after 20 h reaction time.Hot filtration test conducted for UiO-66 ref, 1TFA and 10TFA catalysts.The inserted line indicates the time of the hot filtration.

Table S2 :
Proposed sum formula for the defective UiO-66 samples.