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covered via vacuum filtration in air, then washed with toluene and
dried overnight under dynamic vacuum. As a final step, the materi-
al was calcined in air immediately before catalysis at 5508C for 6 h
ards (Fluka); phosphorus calibration solutions were prepared and
analyzed separately from those of metals. All final metal and phos-
phorus surface densities are reported relative to the N physisorp-
2
ꢀ1
using a ramp rate of 108Cmin .
tion specific surface area of the bare, partially dehydroxylated SiO2
support (Micromeritics ASAP 2010 CE system).
Syntheses of catalysts Ti-SiO and Zr-SiO were previously reported
2
2
[14]
by us. In brief, stoichiometric amounts of 1,3-dimethoxy-tert-bu-
tylcalix[4]arene and a metal precursor (TiCl (THF) or ZrCl (THF) ,
The 0.25-PPA-Nb-SiO2 and PPA-Nb-SiO2 materials were analyzed
[39]
31
using P cross-polarization magic angle spinning (CP-MAS) NMR
4
2
4
2
Strem) were refluxed in anhydrous, degasified toluene under N2
for 14 h, followed by the addition of silica gel which had previously
undergone partial dehydroxylation as above. The suspensions
were refluxed for 24 h, vacuum-filtered in air, washed with toluene,
and dried under dynamic vacuum overnight. The resulting solids
were air-calcined following the conditions listed above immediate-
ly before use as catalysts.
spectroscopy. Spectra were collected at a spin rate of 5 kHz at am-
bient conditions using a Varian 400 MHz VNMRS instrument
equipped with a 5 mm HPC probe previously calibrated to an am-
monium phosphate standard.
Thioanisole oxidation experiments were performed as follows:
0.59 mmol thioanisole (Sigma) and 0.44 mmol DCB were added to
5.0 mL acetonitrile and 22 mg catalyst in a 20 mL septum-cap vial.
After the vial was sealed, a vent needle was inserted, and the reac-
tors were shaken at 700 r.p.m. on a Glas-Col Heated Vortexer at
258C, 458C or 658C for 10 min. The reaction began after the in-
stantaneous addition of 1.8 mmol H O delivered from a 4.0m solu-
The PPA-Nb-SiO catalyst was prepared using a protocol adapted
2
[15,16]
from previous work by our group.
Nb-SiO2 (480 mg,
0
.125 mmol Nb) were suspended in acetonitrile (12 mL, spectro-
photometric grade, Alfa Aesar), followed by the addition of phenyl-
2
2
phosphonic acid (“PPA”, 0.140 mmol, Sigma) from a 100 mm solu-
tion in acetonitrile, prepared by diluting 10 mL 30% H O (w/w in
2 2
tion in ultrapure H O (18 MW·cm). The suspension was shaken for
H O, Sigma) in 20 mL acetonitrile, drying over 7 g anhydrous
2
2
[14,40]
2
h at 658C at 700 r.p.m. on a Glas-Col Heated Vortexer, then
MgSO (Macron), centrifuging, and decanting.
At the start of
4
vacuum-filtered and washed with acetonitrile (150 mL). The 0.25-
PPA-Nb-SiO2 catalyst was prepared in the same manner, except
with the addition of 0.0340 mmol PPA (P/Nb=0.26 during synthe-
sis conditions). The resulting solids were dried under dynamic
vacuum (<25 mTorr) and stored in a desiccator prior to catalysis.
Diffuse-reflectance UV/Vis (DRUV/Vis) spectroscopy was performed
using a Shimadzu UV-3600 spectrometer equipped with a Harrick
Praying Mantis diffuse reflectance accessory. Spectra were collected
at ambient conditions from 800 to 200 nm using a 1.0 nm sam-
pling step and a slit width of 3.0 nm, calibrated to polytetrafluoro-
ethylene (PTFE) powder as a perfect reflector for baseline measure-
ments. Solid catalyst samples were diluted in a 20:1 ratio with
PTFE using a mortar and pestle. Kubelka–Munk pseudoabsorban-
ces (F(R)) were computed to calculate optical edge energies via the
reaction, initial concentrations were [thioanisole] =0.106m,
0
[H O ] =0.323m, giving molar ratios thioanisole/H O /DCB/M=
2
2 0
2
2
106:323:79:1.0 and a total reaction volume of 5.6 mL. Aliquots
were withdrawn with a syringe at specified time intervals, filtered
using a 0.7 mm glass microfiber filter (Whatman), and contacted
with silver powder (Sigma, <250 mm) to decompose unreacted
[14,15,40]
H O prior to GC analysis.
Product identities were initially de-
2
2
termined using GC-MS (Shimadzu QP2010, Zebron ZB-624 capillary
column) and quantified using GC-FID (Shimadzu 2010, TR-1 capilla-
ry column) via calibrated standards. For two experiments, reactions
over Nb-SiO were performed as above at 458C, except with the
2
addition of 0.0067 or 0.025 mmol (1.2 or 4.3 equiv. w.r.t. Nb) of tri-
phenylphosphine oxide (“TPPO,” Sigma) during preparation; reac-
tion mixtures with TPPO were equilibrated at 458C for 1 h prior to
addition of H O . For a separate subset of experiments, the oxida-
1/2
x-intercepts of corresponding indirect Tauc plots ([F(R)·hn] vs. hn
2
2
[21b,23]
(
eV)) following literature protocol.
tion of methyl phenyl sulfoxide was examined at 458C following
the same protocol as above except with batch reaction solutions
prepared as follows: 0.60 mmol methyl phenyl sulfoxide (Sigma)
was delivered from a 0.150m solution in acetonitrile to a 20 mL
vial containing catalyst (22 mg), followed by the addition of aceto-
nitrile (1.0 mL) and DCB (0.44 mmol).
X-ray absorption near-edge structure (XANES) spectroscopy was
performed at the Sector 5, Bending Magnet Station D (5BM-D) of
the DuPont-Northwestern-Dow Collaborative Access Team (DND-
CAT) of the Advanced Photon Source at Argonne National Labora-
tory. Spectra were collected at the niobium K-edge (18,985.6 eV)
following tuning of the beam energy via a Si(111) monochromator
The oxidation of benzothiophene derivatives was performed as fol-
ꢀ4
ꢀ1
(
10 eV resolution) and calibrating using a pure niobium foil in
lows: separate 3000 ppm (e.g. mgL ) solutions of benzo[b]thio-
transmission mode. Nb-SiO2 and PPA-Nb-SiO2 powder samples
were ground using a mortar and pestle and packed into 10 mmꢁ
phene (“BT,” Alfa Aesar), dibenzothiophene (“DBT,” Alfa Aesar), and
4,6-dimethyldibenzothiophene (“DMDBT,” Alfa Aesar) were pre-
pared in acetonitrile. For a typical reaction, Nb-SiO or Ti-SiO (6.0–
2
1
mmꢁ1 mm voids bored-through self-supporting 13 mm dia.,
mm-thick aluminum discs. The sample discs were then loaded
2
2
12.0 mg) were weighed into 20 mL septum-cap vials, to which
fresh acetonitrile (6.6 mL), acetonitrile solution (3.5 mL) of BT, DBT,
or DMDBT, and DCB (0.44 mmol) were added. The reactors were
sealed and shaken at 700 r.p.m. for 5 min at 258C, 458C, or 658C,
after which H O (0.16–0.24 mmol) was added from a 4.0m solution
into a controlled atmosphere cell, evacuated at room temperature
(
ꢀ30 inHg gauge), and purged with purified argon for three cycles,
then evacuated continuously for 2 h at 1208C. The sample cell was
cooled to 508C under dynamic vacuum, then backfilled with argon
and sealed immediately before spectroscopy. The cell was oriented
at 458ꢁ58 relative to the incident beam, and spectra were collect-
ed at ambient temperature using a four-channel Canberra SII
Vortex ME4 fluorescence detector.
2
2
in acetonitrile as prepared above. At the start of reaction, reaction
volumes were approximately 10.2 mL, and concentrations of reac-
tive species were as follows: [BT] =7.7 mm, [H O ] =23 mm, with
0
2
2 0
initial molar ratios BT/H O /M=32:99:3.1; [DBT] =5.6 mm,
2
2
0
Metal and phosphorus content was determined via inductively
coupled plasma optical emission spectroscopy (ICP-OES) using a
Thermo ICAP 7600 spectrometer. Catalysts were digested using
[H O ] =18 mm, with initial molar ratios DBT/H O /M=32:102:3.1;
2 2 0 2 2
and [DMDBT] =4.9 mm, [H O ] =15 mm, with initial molar ratios
0
2
2 0
DMDBT/H O /M=31:100:3.1. GC sample collection and analysis
2
2
1
.0 mL concentrated hydrofluoric acid (HF, 48% w/w in H O,
was performed as before.
2
Macron) diluted to 11.0 mL in 0.9% w/w HNO in H O. [CAUTION:
Epoxidation of cyclohexene with in situ PPA poisoning was execut-
ed following analogous protocols for previously reported by our
3
2
HF is a severe contact poison. Handle and store concentrated HF
solutions with extreme care.] Metal and phosphorus calibrants
were prepared via serial dilution of commercially available stand-
[14–16]
group.
Cyclohexene (Sigma) was purified over a column of
activated Al O (Selecto Scientific, 63–150 mm) immediately before
2
3
ChemCatChem 2017, 9, 1 – 12
9
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&
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