Support Effect of Metal–Organic Frameworks on Ethanol Production through Acetic Acid Hydrogenation

We present a systematic study on the support effect of metal–organic frameworks (MOFs), regarding substrate adsorption. A remarkable enhancement of both catalytic activity and selectivity for the ethanol (EtOH) production reaction through acetic acid (AcOH) hydrogenation (AH) was observed on Pt nanoparticles supported on MOFs. The systematic study on catalysis using homogeneously loaded Pt catalysts, in direct contact with seven different MOF supports (MIL-125-NH2, UiO-66-NH2, HKUST-1, MIL-101, Zn-MOF-74, Mg-MOF-74, and MIL-121) (abbreviated as Pt/MOFs), found that MOFs having a high affinity for the AcOH substrate (UiO-66-NH2 and MIL-125-NH2) showed high catalytic activity for AH. This is the first demonstration indicating that the adsorption ability of MOFs directly accelerates catalytic performance using the direct contact between the metal and the MOF. In addition, Pt/MIL-125-NH2 showed a remarkably high EtOH yield (31% at 200 °C) in a fixed-bed flow reactor, which was higher by a factor of more than 8 over that observed for Pt/TiO2, which was the best Pt-based catalyst for this reaction. Infrared spectroscopy and a theoretical study suggested that the MIL-125-NH2 support plays an important role in high EtOH selectivity by suppressing the formation of the byproduct, ethyl acetate (AcOEt), due to its relatively weak adsorption behavior for EtOH rather than AcOH.


Preparation of MOF supports
Synthesis of UiO-66-NH 2 ({Zr 6 O 4 (OH) 4 (bdc-NH 2 ) 6 } ∞ ) 1 ZrCl 4 (11.52 g, 49.43 mmol) was dissolved in 2880 ml DMF by sonication for 15 minute. 2aminoterephthalic acid (8.928 g, 49.29 mmol) was added into the solution. After sonication for 15 minute, the mixture was heated at 120 °C for 24 h. The yellow precipitate was collected by centrifugation. It was washed with DMF and MeOH, and dried under vacuum at 160 °C for a night.
After washing with DMF, the blue precipitate was immersed in chloroform for four days. This operation was repeated three times. The precipitate was separated by filtration and washed with MeOH and distilled water. To dry the powder, it was heated at 160 °C in vacuo. To remove unreacted chemicals and solvent, S2 reflux using MeOH was conducted for a day. After that, the sample was moved into DMF. The mixture was kept at 70 °C for 26 h. After separation by centrifugation, the resultant was washed with MeOH and head at 160 °C under vacuum until dryness.

Synthesis of Zn-MOF-74 ({Zn 2 (dobdc)} ∞ (H 4 dobdc = 2,5-dihydroxyterephthalic acid)) 1
Zn(NO 3 ) 2 ·6H 2 O (22.6 g, 76.0 mmol) was dissolved in 1000 ml DMF. 2,5-dihydroxyterephthalic acid (5.00 g, 26.0 mmol) was also dissolved in the mixture and sonicated 10 minute. 50 ml of distilled water was added to this solition and heated at 100 °C for 20 h. The precipitate was separated by decantation and washed with DMF. It was immersed in MeOH for five days. This operation was repeated two times. The precipitate was separated by filtration. After washing with MeOH and distilled water, it was refluxed in MeOH for 30 hours. The sample was dried in air after separation by centrifugation.
The sample was separated by filtration and dried at 250 °C for 6 h under vacuum.
After sonication, the solution was placed in Teflon-lined autoclaves and heated at 180 °C for 24 h. The precipitate was collected and washed with MeOH. The white powder was dried at 120 °C under vacuum for a night.

Synthesis of UiO-66 ({Zr 6 O 4 (OH) 4 (bdc) 6 } ∞ )
ZrCl 4 (1.66 g, 7.1 mmol) and terephthalic acid (1.16 g, 7.0 mmol) were dissolved in 30 ml of DMF and 0.8 ml of 35% HCl. The mixture was heated and stirred at 60 °C for complete dissolution. After that, it was put in Teflon-lined autoclaves and heated at 220 °C for 17h. Resulting white precipitate was separated by filtration and washed with DMF and MeOH. The white powder was dried at 180 °C under vacuum over a night.

S3
Synthesis of UiO-67 ({Zr 6 O 4 (OH) 4 (bpdc) 6 } ∞ (H 2 bpdc = 4,4'-biphenyldicarboxylic acid)) ZrCl 4 (2.3 g, 9.7 mmol) was mixed into 300 ml of DMF and stirred until completely dissolved. 4,4'biphenyldicarboxylic acid (2.4 g, 10.0 mmol) was added into the mixture and stirred for 20 min. Then, 0.25 ml of H 2 O and 50 ml of DMF were also added into the mixture. The mixture was heated at 95 °C for 99 h without stirring. After cooling to room temperature, the precipitate was filtrated and washed with ethanol. The resulting powder was dried at 90 °C under vacuum for 3h.

Synthesis of MIL-125 ({Ti 8 O 8 (OH) 4 (bdc) 6 } ∞ )
Titanium (IV) isopropoxide (0.78 ml, 2.6 mmol), terephthalic acid (1.50g, 9.0 mmol) were added in mixture of 22.5 ml of DMF and 3.0 ml of ethanol. The mixture was stirred until dissolved. Then, it was heated at 130 °C for 15 h in Teflon-lined autoclaves. The precipitate was separated by filtration and washed with MeOH and acetone. At last, the resulting white powder was heated at 70 °C under vacuum.