Journal of the American Chemical Society
Article
coworkers have reported that 10 wt % Y/BEA (YO supported
on dealuminated BEA zeolite (DeAlBEA)) is an effective
solution of HfCl with the Hf concentration adjusted to achieve the
4
x
desired metal loadings. Typically, 2 mL of the HfCl solution was used
4
per gram of support. After impregnation, the slurry was dried in air at
room temperature for 5 h, then heated to 323 K under flowing He,
and held at this temperature for 2 h. This step was followed by further
calcination in a muffle oven. The sample was heated from room
temperature to 393 K over a 30 min period, then heated to 823 at 5 K
catalyst for acetic acid conversion to isobutene with 100%
15
conversion and 60% isobutene selectivity. Lewis acid sites
have also been proposed as aldol condensation centers for
producing C6 intermediates over Y O3 and YOx species
2
supported on DeAlBEA. More recently, Shylesh et al. have
demonstrated C−C bond formation via propanoic acid
ketonization and acetone aldol condensation to mesityl oxide
−1
min and held at this temperature for 6 h, and finally cooled to room
temperature. The samples prepared are designated as xHf/SiO or
2
xHf/Silicalite-1, respectively, where x designates the surface
−
2
(
MO) over site-isolated Zr catalyst (Zr/SiO ) containing
concentration of Hf (atoms nm ).
.2. Catalyst Characterization. Powder X-ray diffraction (XRD)
2
1
9
2
Zr−OH sites.
patterns of Silicalite-1 and Hf supported catalysts were recorded using
a Bruker D8 GADDS diffractometer equipped with a Cu Kα target
Previous studies have also found that the local environment
of Lewis acidic metal centers influence their catalytic
performance. For the reaction of acetone to isobutene, Li et
(
40 kV and 40 mA) X-ray source. To observe the morphologies of
Silicalite-1, scanning electron microscopy (SEM) images were
acquired using a Thermo Scientific Helios G4 UC Focused Ion
Dual Beam microscope operated with an accelerating voltage of 2 kV
and a probe current of 25pA using a TLD detector. The specific
surface areas of all samples were determined by acquiring N2
adsorption isotherms at 77 K with a Micrometrics Gemini VII 2390
analyzer. The total surface areas were evaluated by applying the
Brunauer−Emmet−Teller (BET) equation. Prior to analysis, samples
were degassed for 6 h at 453 K. The Hf loadings were analyzed with
inductively coupled plasma optical emission spectroscopy (ICP-OES)
by Galbraith Laboratories, Inc.
al. found that the catalytic efficiency of Y/SiO was much lower
2
than that of Y/DeAlBEA, and Y/Silicalite-1 showed almost no
C−C coupling activity. This much lower activity was ascribed
to the weaker interaction of Y species with SiO and Silicalite-1
2
15
compared with DeAlBEA.
Here, we describe the preparation of isolated Lewis acidic Hf
sites on SiO and Silicalite-1. Hf was chosen for these studies
2
18,20,21
rather than Zr or Y for the following reasons.
The Lewis
acidity of Hf is slightly lower than that of Zr, which should
result in a higher isobutene selectivity without significant
Fourier transform infrared (FTIR) spectra were collected to
characterize surface hydroxyl groups using a Thermo Scientific
20,21
sacrifice in activity,
and compared to Y, Hf grafted on silica
and Silicalite-1 is expected to exhibit higher activity for aldol
Nicolet 6700 IR spectrometer equipped with a liquid N -cooled MCT
2
1
5,22,23
condensation.
The catalysts were characterized by XRD,
detector. Samples (∼36 mg) were pressed into thin self-supporting
wafers and then transferred to a transmission IR cell equipped with
1
IR, H MAS NMR and UV−vis spectroscopy. The structure of
the active Hf sites is proposed to be (Si−O) −Hf−OH for
CaF windows. Prior to the acquisition of spectra, samples were
2
3
−
1
pretreated under dry flowing air (Praxair, ultrazero, 100 mL min )
both Hf/SiO and Hf/Silicalite-1. In addition, for Hf/Silicalite-
2
for 1 h at 773 K to remove impurities and moisture and then cooled
1
, the Hf−OH groups H-bond with adjacent Si−OH groups.
−
1
to 393 K. Spectra were recorded at 393 K with a resolution of 1 cm
and an accumulation of 64 scans.
The influence of active site structure and local environment
was investigated for the conversion of acetone to isobutene for
both Hf/SiO2 and Hf/Silicalite-1. The rate of isobutene
formation on Hf/Silicalite-1 was found to be 4.5 times higher
The acid properties of Hf/SiO2 and Hf/Silicalite-1 were
characterized by FTIR spectroscopy of adsorbed pyridine using a
Thermo Scientific Nicolet 6700 IR spectrometer. Prior to the
acquisition of spectra, samples were pretreated at 773 K for 1 h in a
than that on Hf/SiO . The reactions of the intermediates
2
−1
involved in the conversion of acetone to isobutene−diacetone
alcohol (DAA), mesityl oxide (MO), and acetic acid were also
investigated with the aim of identifying the roles of these
species in the overall reaction pathway. The accumulated
information was used to propose a reaction mechanism for the
conversion of acetone to isobutene over isolated Hf sites.
flow of 100 mL min air and then cooled to 393 K to take
background scans. FTIR spectra of adsorbed pyridine were recorded
by injecting 2 μL of pyridine and then purging the IR cell with He at
3
93 K for 20 min to remove any physically adsorbed material.
FTIR spectra were collected at a resolution of 4 cm during the
−1
reaction of acetone as well as mesityl oxide (MO) over Hf/Silicalite-1.
For these experiments, 30 mg of sample was pressed into a thin self-
supporting wafer, transferred into the transmission IR cell, and
−
1
2
. EXPERIMENTAL SECTION
pretreated at 773 K for 1 h in 100 mL min of dry air. Subsequently,
−1
the sample was purged with 30 mL min of He (Praxair, UHP) and
cooled to 306 K to collect the background spectrum. After that, 2 μL
of acetone was fed onto the catalyst under flowing He (Praxair, UHP,
2
.1. Catalyst Preparation. Preparation of Hf Grafted Silicalite-1
and SiO . The silica support (SiO ) was provided by Silicycle.
2
2
Nanosheet-like Silicalite-1 (self-pillared pentasil) was synthesized
−
1
following a modification of a method reported previously by Zhang et
30 mL min ), and the spectra were recorded at constant time
intervals. For the reaction of MO, 0.2 μL MO was injected under
similar reaction conditions.
24
al. Tetra(n-butyl) phosphonium hydroxide (TBPOH) (Sigma-
Aldrich, 40%) and fumed silica (Sigma-Aldrich) were used as the
structure-directing agent and silicon source, respectively. After
crystallization, the as-synthesized material was washed with distilled
water until the pH of the supernatant was lower than 9; the
precipitate was then dried at 343 K in vacuum overnight and calcined
at 823 K for 6 h in a muffle oven. To ensure complete removal of the
residual organic structure-directing agent and decomposed organic
moieties, the calcined material was washed twice with a large amount
of deionized water at room temperature until the pH of the
supernatant was close to 7, then dried at 343 K overnight, and
Prior to carrying out the NMR experiment, all samples were
dehydrated overnight at 353 K under vacuum, and then the material
was packed in a 4 mm zirconia rotor inside the glovebox. The rotor
was then placed inside a Bruker high resolution dual resonance
1
13
1
H/ C magic angle spinning (MAS) probe that has a H frequency of
500 MHz. All NMR measurements were recorded with a spinning rate
1
of 10 kHz. The quantitative H spectra were acquired using the
26
DEPTH pulse sequence with a recycle delay of 2 s, equivalent to
five times the spin−lattice T relaxation, and a radio frequency power
1
25
27
calcined at 823 K for 6 h.
of 80 kHz. The spectra were fitted with Dmfit software.
Hafnium (Hf) was grafted onto Silicalite-1 and mesoporous SiO2
by aqueous impregnation. Prior to introducing Hf, both supports,
Ultraviolet−visible (UV−vis) diffuse reflectance spectra of
Silicalite-1, SiO , and Hf supported samples are collected with a
2
SiO and Silicalite-1, were dehydrated at 353 K for 6 h in a vacuum
drying oven and then stored in vacuum. Afterward, Hf was introduced
Fischer Scientific Evolution 300 spectrometer equipped with a
Harrick Scientific Praying Mantis diffuse reflectance chamber.
Samples were grounded and dehydrated at 473 K for 20 min in a
2
on SiO and Silicalite-1 by impregnating the support with aqueous
2
8
353
J. Am. Chem. Soc. 2021, 143, 8352−8366