2
22
R. Silbernagel et al. / Journal of Molecular Catalysis A: Chemical 394 (2014) 217–223
recyclable than 1i–Rh, it still compared favorable with immobilized
Wilkinson-type Rh complexes immobilized via chelate phosphine
linkers, some of which required nearly 100 h for full conversion in
cycle 13 [5a].
accordance with the reported ones. Surface coverages were deter-
mined by ICP-MS using a Perkin-Elmer Optima 3000 dual-view
ICP-MS instrument. For this purpose, 10 mg of 1i was immersed
in 10 L of HF and diluted with 15 mL of 2% HNO . The atoms Zr,
3
Si, and P were measured to determine the surface coverage of 1 on
ZrP. Quantitative compositional analyses were carried out on a four
spectrometer Cameca SX50 electron microprobe at an accelerating
voltage of 15 kV at a beam current of 20 nA. All quantitative work
employed wavelength-dispersive spectrometers (WDS). Analyses
were carried out after standardization using very well character-
ized compounds or pure elements. Pressed powder micro pellets
were prepared by pressing a few milligrams of powder between
the highly polished surfaces (0.25 m) of hardened steel dies, and
transferring the pellets onto double-sided conductive carbon tape.
The pellets were carbon coated before analysis to make them elec-
trically conductive. The analyses of the pressed powder pellets were
carried out with a 20 m diameter beam while the stage was being
moved 20 m every 2 s, repeated over a ten spot traverse. This
ensured representative sampling and minimized possible thermal
damage to the samples.
3
. Conclusion
In summary we have demonstrated that a Wilkinson-type
hydrogenation catalyst can be tethered irreversibly to zirconium
phosphate nanoplatelets. It has been shown that the linker is evenly
“
distributed” on the outside of the nanoplatelets and therefore the
catalyst is dispersed homogeneously on the surface. The ligand or
catalyst do not intercalate into the layers of the zirconium phos-
phate support. The catalyst immobilized by the phosphine linker
can be easily recycled by allowing it to settle after the catalytic reac-
tion. The most favorably sized nanoplatelets of the support do not
clump during the catalytic reaction but retain their high specific
surface area. Therefore, most importantly, the obtained catalyst
exhibits record speed in hydrogenating the standard substrate 1-
dodecene and it can be recycled 15 times in a batchwise manner
without major loss of catalytic activity. Following this successful
first study, which yielded the most active and recyclable immo-
bilized hydrogenation catalyst so far, the application of chelate
phosphine linkers [5a–c,6,7,20] and rigid scaffold systems [5d,8]
is planned. Another point of interest is that we are not restricted to
silanes as linkers as isocyanates, epoxides, phosphates and phos-
phonic acids also bond to the surface of ␣-ZrP [24]. Furthermore,
the effect of varying the nanoplatelet sizes of the support material
on the immobilized catalysts will be investigated.
The hydrogenation reactions were monitored using the appara-
tus described in Ref. [5c] by reading the consumption of hydrogen
during the course of the reaction. The standard conditions listed in
Scheme 3 and Refs. [5] were applied.
4.2. Immobilization of 1 on ZrP to give 1i
4
00 mg (1.33 mmol) of thoroughly dried ␣-ZrP is placed into a
Schlenk flask and 200 mL of dry, degassed toluene is added. The
white opaque liquid is stirred vigorously under an argon atmo-
sphere and 105 mg (0.267 mmol) of 1 is added via syringe. Then
◦
4
. Experimental
the reaction mixture is heated to 95 C for 12 h while stirring vig-
orously. The flask is cooled and stirred for 7 more hours. Then the
contents are transferred to four 50 mL centrifuge tubes and cen-
trifuged at 5000 rpm for 15 min. The supernatant is decanted and
a white solid remains. 100 mL of toluene is added to the solid for
washing, and the mixture is shaken thoroughly and centrifuged.
The washing process is repeated three more times. Finally, all
toluene is removed and the white solid is dried under vacuum at
4
.1. General remarks
The 1H, 13C, and 31P NMR spectra of liquids were recorded at
99.70, 125.66, and 202.28 MHz on a 500 MHz Varian spectrom-
4
29
eter. Si NMR spectra of liquids were recorded at 79.37 MHz on
a 400 MHz Inova spectrometer. The C, Si, and P spectra were
recorded with H decoupling if not stated otherwise. The solid-state
13
29
31
1
◦
70 C for 12 h. The solid material 1i (0.3326 g, 83% with respect to
NMR spectra were measured with a Bruker Avance 400 widebore
NMR spectrometer equipped with a 4 mm MAS probehead. For the
CP/MAS and MAS measurements H high-power decoupling was
ZrP) is then triturated with a mortar and pestle for 2 min and placed
in a Schlenk flask for longterm storage.
1
applied. The recycle delays were 5 s for CP/MAS, and 10 s for MAS
spectra. For more measurement details, see Ref. [18b]. XRPD exper-
iments were performed from 2 to 40 (2ꢁ angle) as described in Ref.
4.3. Generation of 1i–Rh
◦
◦
10 mg (0.01 mmol) of Wilkinson’s catalyst ClRh(PPh3)3 are
placed in a Schlenk flask under nitrogen and dissolved in 10 mL
of toluene. Then the dark red liquid is transferred via cannula to
a second Schlenk flask containing 30 mg of 1i (corresponding to
0.077 mmol of 1) in 10 mL of toluene. During this process the reac-
tion mixture turns orange-brown and is stirred for 24 h at room
temperature. Then the stirring is stopped and the solid 1i–Rh set-
tles to some degree. The supernatant appears orange/pink while
the settled solid is orange/brown. The supernatant is removed via
syringe and 5 mL of toluene is added. The sample is stirred again
for 20 min and allowed to settle. The supernatant is removed and
two more wash cycles are performed. After the third wash no more
catalyst is removed from the support, as evidenced by the col-
orless supernatant and by 31P NMR. 1i–Rh is then suspended in
5 mL of toluene and employed for catalyzing the hydrogenation of
1-dodecene.
[
15] using a Siemens D8 X-ray diffractometer system with a copper
anode source (K␣1, ꢂ = 1.5406 A˚ ) and a filtered flat LiF secondary
beam monochromator. The divergence, receiver, and detector slit
widths were 2 mm, the scatter slit width was 0.6 mm. The inter-
layer distances were determined using Bragg’s Law for the (0 0 2)
diffraction plane of the diffraction pattern of ␣-ZrP. Transmission
electron micrographs (TEM) of the samples were acquired using
a JEOL 2010 transmission electron microscope at an acceleration
voltage of 200 kV. Samples were prepared by drop casting a ca.
0
.01% (w/w) suspension of the nanoparticles on a formvar/carbon
coated copper grid from Ted Pella. All reactions were carried out
using standard Schlenk techniques and a purified N2 atmosphere,
if not stated otherwise. Reagents purchased from Sigma Aldrich or
VWR were used without further purification. Solvents were dried
by boiling them over Na, distilled, and stored under N . In addition,
toluene is stored under nitrogen over 3 A˚ molecular sieves. The ZrP
2
nanoplatelets with sizes between 100 and 400 nm were synthe-
sized as described in Refs. [15–17] and dried thoroughly in vacuo at
RT to remove water adsorbed on the surface. The linker was synthe-
sized following the procedure given in [19b] and the data were in
4.4. Generation of ZrP–Rh
The material ZrP–Rh is prepared following the procedure
described for 1i. Wilkinson’s catalyst (10 mg, 0.01 mmol) is reacted