Y. Lin et al.
Molecular Catalysis 502 (2021) 111405
to the strength of Lewis acid centers, but also to the large amount of base
sites. Han’s group [23] reported the mechanism of using a porous
catalyst formed by hafnium-phytic acid (Hf-phy) to catalyze the
hydrogen transfer reduction reaction. Hf4+-O2- and Hf4+ in Hf-phy
activate the hydroxyl group on isopropanol and the aldehyde group
on the substrate respectively, and then the activated aldehyde group
interacts with isopropanol to form a six-membered ring intermediate.
Gonell et al. [38] reported the co-adsorption of cyclohexanone and
mL of formic acid. After ultrasonication and dissolution,The mixture was
added to an autoclave vessel and heated at 120 ◦C for 48 h.After reac-
tion, the mixture was cooled down to room temperature.The resulting
solid was filtered and washed with an excess of ethanol. Finally the
obtained solid was activated at 80 ◦C in vacuum for 12 h.
2.2.3. Hf-UiO-(OH)2
HfCl4 (160 mg, 0.5 mmol) and 2,5-dihydroxyterephthalic acid (99
mg, 0.5 mmol) were added into a 30 mL of DMF solution, followed by
adding 1.5 mL of formic acid. After ultrasonication and dissolution,The
mixture was added to an autoclave vessel and heated at 120 ◦C for 48 h.
After reaction, the mixture was cooled down to room temperature.The
resulting solid was filtered and washed with an excess of ethanol. Finally
the obtained solid was activated at 80 ◦C in vacuum for 12 h.
propoxide on two neighboring surface Zr atoms leading to
a
seven-membered ring TS, which is different from the previous reaction
model, in which the two reactant fragments adsorb on the same Zr
center [23,37]. Although some previous works [21–25] discussed the
hydrogen transfer ability of Hf-MOFs, no related research addressed the
selectivity of its catalytic transfer hydrogenation. In addition, the
mechanism of Hf-MOFs catalyzing the transfer hydrogenation of ketones
(aldehydes) still needs further study. Rojas-Buzo et al. [25] reported that
Hf-MOF-808 is a direct hydrogen transfer, instead of a sequential metal
hydride formation and subsequent carbonyl reduction. At the same time,
it is proposed to form a six-membered intermediate between alcohol,
carbonyl compound and Lewis acid center.
2.2.4. Hf-UiO-66
HfCl4 (160 mg, 0.5 mmol) and 1,4-benzenedicarboxylic acid (83 mg,
0.5 mmol) were added into a 30 mL of DMF solution, followed by adding
1.5 mL of formic acid. After ultrasonication and dissolution,The mixture
was added to an autoclave vessel and heated at 120 ◦C for 48 h. After
reaction, the mixture was cooled down to room temperature. The
resulting solid was filtered and washed with an excess of ethanol. Finally
the obtained solid was activated at 80 ◦C in vacuum for 12 h.
This paper found that even in the presence of other easily reduced
groups (such as halogens, nitriles, nitro groups and unsaturated carbon
moieties), Hf-MOF can achieve hydrogenation of carbonyl groups with
high efficiency and specific selectivity (>99%). The catalyst has good
stability, and the catalytic performance remains basically unchanged
after being recycled five times. Compared with Hf-UiO-66, Hf-MOF-808
has more defects, so it has more active Lewis acid-base sites to promote
this hydrogen transfer reaction. Furthermore, we perform DFT compu-
tation to study catalytic mechanism of Hf-MOF-catalyzed MPV reaction.
A cluster model containing six Hf (IV) ions are employed to describe the
reaction site, from which the TS and product are derived.
2.3. The general procedure for the selective hydrogenation of ketones or
aldehydes
A mixture of 0.2 mmol of carbonyl compound and 20 mg of Hf-MOF
was added to isopropanol (1 mL), and the mixture was stirred and
reacted at 120 ◦C for 12 h under N2 atmosphere. After the reaction was
completed, Hf-MOF was isolated from the solution by centrifugation and
wash by EtOAc (1 mL × 3). Then the resulted solid was reused for
another reaction cycle directly. The organic layer was collected and
removed in vacuo to afford the crude product, which was analyzed by GC
and GC-MS.
2. Experimental
2.1. General
All chemical reagents are obtained from commercial suppliers and
used without further purification. GC-MS was performed on an ISQ
Trace 1300 in the electron ionization (EI) mode. GC analyses are per-
formed on an Agilent 7890A instrument (Column: Agilent 19091J-413:
2.4. Computational Details
The Hf-MOF cluster model containing 6 Hf atoms is employed in our
computation, which is based on the UiO-66 structure 1406507(Zr) with
Zr replaced by Hf. The terephthalic acid ligands were replaced by formic
acid to obtain the initial structural model. All DFT calculations were
performed at the B3LYP level [39,40] using Guassian 09 W software
package [41]. The LANL2DZ and 6-31 g(d) basis sets are used for Hf and
rest atoms, respectively.
30 m × 320 μm × 0.25 μm, carrier gas: H2, FID detection. The crystal
structure of the synthesized catalysts was recorded by X-ray diffraction
(XRD) using a D8ADVANCED X-ray diffractometer, employing a scan-
ning rate of 0.1 s-1. Scanning electron microscopy (SEM) spectras were
taken using a Hitachi S-4800 apparatus on a sample powder previously
dried and sputter-coated with a thin layer of gold. BET surface areas
were recorded with N2 adsorption/desorption isotherms at 77 K on a
Micromeritics ASAP 2920 instrument. Before measurements, the sam-
ples (> 100 mg) were degassed at 150 ◦C for 12 h. The Raman spectra
were obtained using confocal Raman spectroscopy (inVia-Reflex)
employing 785 nm radiation (3 mW).
Two formate ligands coordinated to the same Hf (IV) were removed
so that cyclohexanone and isopropanol can be introduced as substrates.
After optimizing the reactant, we used the multi-coordinate driven
(MCD) method [42] to drive the reactant to the product using four active
coordinates, namely the distances of C1-H1, C2-H1, O2-H2, and O1-H2.
The resulting product structure was further optimized.
With the optimized reactant and product structures available, we
used MCD method [42] to find a low barrier reaction path connecting
the reactant and product [43]. As long as the minimum energy path is
smooth and continuous, the structure corresponding to the maximum
energy along this reaction path provides an approximate TS structure.
The energy difference between the TS and the reactant state yields an
estimate of the reaction barrier.
2.2. Synthesis of Hf-MOFs
2.2.1. Hf-MOF-808
A mixture of HfCl4 (160 mg, 0.5 mmol), 1,3,5-benzenetricarboxylic
acid (110 mg, 0.5 mmol), DMF/formic acid (20 mL/20 mL) was soni-
cated for 30 min and then added to an autoclave vessel and heated at
120 ◦C for 48 h. After reaction, the mixture was cooled down to room
temperature. The resulting solid was filtered and washed with an excess
of ethanol. Finally the obtained solid was activated at 80 ◦C in vacuum
for 12 h.
3. Results and discussion
3.1. Catalytic performance studies
2.2.2. Hf-UiO-66-NH2
The hydrogen transfer reduction of p-nitroacetophenone was
selected as the model reaction. After screening reaction time, tempera-
ture and catalyst loading, the reaction can be provided >99% yield of 2a
HfCl4 (160 mg, 0.5 mmol) and 2-aminoterephthalic acid (90 mg, 0.5
mmol) were added into a 30 mL of DMF solution, followed by adding 1.5
2