Organometallic Complexes in Supported Ionic-Liquid Phase Catalysts
FULL PAPER
in the catalytic SILP system could only be attributed to a
structural modification in the active SILP catalyst in the ini-
tial reaction phase which prolonged the residence time of
the product in the catalytic system.
To investigate the change in the PHIP activity of the
SILP catalyst during the hydrogenation reaction in further
detail, subsequent PHIP-hydrogenation experiments were
performed with three different SILP catalysts (SILP-1,
SILP-2, and SILP-3) which contain the same rhodium com-
plex. Strongly enhanced PASADENA signals were observed
with the SILP-1 catalyst in the early reaction phase (Fig-
ure 4a). The spectra of the hydrogenation catalyzed by the
Figure 5. Single-scan 1H NMR spectra of the propene hydrogenation at
758C detected in situ in the PASADENA experiments at flows of
30 mLminÀ1 para-H2 and 20 mLminÀ1 propene over activated SILP cata-
lysts with a) SILP-1, b) SILP-2, c) SILP-3. d) Spectrum acquired at flows
of 1 mLminÀ1 para-H2 and 1 mLminÀ1 propene using SILP-3 catalyst.
are shown in Figure 5. The SILP-1 system with the thinnest
IL coating produces now the largest PHIP signals (Fig-
ure 5a), while the SILP-2 and the SILP-3 systems with
thicker IL films exhibit smaller PHIP signals (Figure 5b and
5c). These first quantitative measurements clearly demon-
strate that the thickness of the IL layer plays a determining
role in the PHIP enhancement after the catalyst activation
period.
To elucidate a possible correlation of the catalyst activity
and the intensity of the PHIP signals, reference experiments
at low flow rates (Figure 5d) were performed where only
thermally polarized propene and propane signals were ob-
served owing to the long residence time in the reactor. By
comparing the signals corresponding to the product propane
and the reactant propene, a conversion of about 66% was
evaluated for both SILP-1 and SILP-2 catalysts. A higher
propene conversion of 91% was achieved with the SILP-3
catalyst owing to the higher loading with the rhodium com-
plex. These studies of catalyst activity further support the
notion that the PHIP signal intensity is determined primari-
ly by the thickness of IL film and less by the amount of rho-
dium complex present in the activated SILP catalyst.
It has already been demonstrated that the signal intensity
in PHIP enhanced NMR spectra depends on the catalyst ac-
tivity, the spin-lattice relaxation time T1 of the PHIP-polar-
ized product, and the residence time of the product in the
catalytic system.[14,17] When the reaction conditions (flow
rates and temperature) are time-invariant, the T1 relaxation
time of the polarized product would be constant. In the ex-
amined systems, the three SILP catalysts contain the same
rhodium complex in altered amounts and with different IL
loadings. As in these three catalysts the same active species
would be present, a comparison of the PHIP performance of
these catalysts would provide detailed information on the
structural changes in the supported IL layer. The observa-
tion that in the early stage of the reaction the thickness of
the IL layer has no measurable influence on the PHIP
signal intensity and higher catalyst loading contributes to
larger PHIP signals implies a surface enrichment of the rho-
Figure 4. Single-scan 1H NMR spectra of the propene hydrogenation at
758C detected in situ with flow rates of 30 mLminÀ1 para-H2 and
20 mLminÀ1 propene in the early hydrogenation phase with a) SILP-1,
b) SILP-2, and c) SILP-3 catalyst. d) Spectrum acquired at identical reac-
tion conditions but at flow rates of 30 mLminÀ1 regular H2 and
20 mLminÀ1 propene using the SILP-3 catalyst and has been magnified
10 times. The signals from the para-H2 derived protons in propane and
the CH2 signals in propene are marked.
SILP-2 catalyst with the same amount of rhodium complex
but higher IL loading showed the same order of enhance-
ment for the PHIP signals (Figure 4b). In contrast to this,
SILP-3 catalyst with an increased amount of the rhodium
complex produced PHIP signals twice as large (Figure 4c).
These results indicate that in the initial phase of the reac-
tion, the intensity of the PHIP signals from the SILP-cata-
lyzed hydrogenation is mainly determined by the amount of
the rhodium complex present in the SILP catalyst and not
by the thickness of the IL layer on the support. The spec-
trum from a reference experiment at the same reaction con-
ditions as in the PASADENA experiments, but with regular
H2 gas is shown in Figure 4d. No considerable propane
signal could be observed in this experiment, indicating a
really low product conversion. A PHIP-enhancement factor
of about 410 was evaluated by comparing the data in Fig-
ure 4c and 4d from the SILP-3 catalyst. This result further
corroborates the effectiveness of the PHIP experiments with
SILP catalysts.
After the initial reaction phase (10 to 30 min), the PHIP
experiments were continued and the PHIP enhancement
factors of the three different catalysts were further evaluat-
1
ed. H PHIP NMR spectra from the activated SILP catalysts
Chem. Eur. J. 2011, 17, 13795 – 13799
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13797