Full Papers
Homogeneous catalyst systems based on Pd–phosphine
complexes and strong acid-promoters are most frequently ap-
plied in alkoxycarbonylation of olefins to provide high activity
under relatively mild conditions. However, the choice of phos-
phine ligand can strongly influence the reaction selectivity to-
methoxycarbonylation reaction. The textural properties of the
support materials and the prepared SILP catalysts are compiled
in Table 1.
[
13]
wards esters or polyketones. The catalytic system developed
Table 1. Textural properties of the supports and the prepared SILP cata-
[
a]
for the Alpha process comprises a strong mineral acid with
lysts.
II
pK ꢀ4, a Pd precursor, and the so called “Alpha ligand” 1,2-
a
Support
Pd
IL loading (a)
S
BET
2
V
t
bis(di-tert-butylphosphinomethane)benzene (DTBPMB) devel-
À1
3
À1
[
wt%]
[m g
]
[cm g ]
[
15]
oped by Shaw et al.
The DTBPMB (P–P) ligand has been
SiO
2
-100
–
0
–
347
204
187
210
196
198
242
252
165
360
208
0.98
0.62
0.57
0.63
0.56
0.57
0.72
0.68
0.48
0.98
0.60
shown to be unique for the ethylene methoxycarbonylation re-
action, yielding MP with high selectivity (>99.9%) at very high
conversion rates, and provides unprecedented stability to the
.08
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.3
–
0.16
.23
0.31
0
[
13]
catalytic species involved in the reaction mechanism.
0
0
0.34
0
–
0
.38
.26
The Alpha process is carried out in homogeneous solution
in a continuously stirred tank reactor under moderate reaction
conditions. Although the catalyst system is well-established
and highly efficient, the product isolation—and mainly the cat-
alyst recovery—still remains challenging as in many other ho-
mogeneous catalytic processes. Different approaches (besides
SILP technology) have been developed in the past to over-
come these inherent drawbacks in homogeneous catalysis in-
.22
2
SiO -500
.23
0.2
[
2 2 2
a] SILP formulation: SiO -100 or SiO -500 support, Pd(OAc) precursor,
DTBPMB ligand (P–P/Pd=5:1), IL loading (a) defined as the ratio between
the volume of IL and the pore volume of the support.
[
16–18]
cluding, for example, biphasic systems
and the use of su-
[
19]
percritical CO2. Indeed, we recently reported an efficient bi-
phasic system for the ethylene methoxycarbonylation by using
Brønsted acid ionic liquids (BAILs) as both reaction media and
acid promoter, in which excellent recyclability, stability, and re-
The BET surface area (SBET) as well as the total mesoporous
volume (V ) decreased regularly by increasing the IL loading al-
t
though a less significant change was observed with increasing
Pd loading, as also expected. TGA of the prepared SILP re-
vealed mass loss in agreement with IL loadings (see the Sup-
porting Information). The pore size distribution of the prepared
[
20]
covery of the catalytic system was achieved.
In this work, we report for the first time the application of
SILP technology to Pd-catalyzed continuous, gas-phase ethyl-
ene methoxycarbonylation. Excellent results in terms of catalyt-
ic activity and selectivity towards MP were achieved by using
the DTBPMB ligand. Different SILP catalyst formulations with
respect to metal loading, ligand loading, IL loading, and nature
of the support were investigated to evaluate the catalytic per-
formance and the stability of the SILP system.
SILPs (with SiO -100 support) revealed a gradually smaller
2
mean average pore diameter with increasing IL, suggesting
that the IL homogenously distributed on the support (see the
Supporting Information).
Catalytic performance of SILP catalysts
The prepared SILP catalysts were examined for continuous,
gas-phase ethylene methoxycarbonylation at 858C and 5 bar
pressure by using fixed-bed technology. In a first approach,
catalyst parameters such as Pd loading, P–P/Pd ratio, and IL
nature and loading were systematically investigated and opti-
mized to achieve the best performance concerning catalytic
activity and stability. Notably, with all examined catalyst sys-
tems the only detected product was MP, confirming that the
reaction selectivity was exceptionally high as also expected for
the DTBPMB ligand system (see above).
Results and Discussion
Preparation and characterization of the SILP catalysts
The SILP catalysts were prepared with the desired amount of
BAIL1 and the commercially available IL2 (Figure 1) with sup-
port pore filling degrees or IL loading (a) corresponding to
10–30 porevol% (i.e., a=0.1–0.3). IL2, comprising an analo-
gous alkyl imidazolium cation as BAIL1 except for the sulfonic
acid group, was selected for comparative purposes to pinpoint
the crucial role of the acid functionalization of BAIL1 in the
Composition of the IL
The crucial role of the Brønsted acid functionalization of the IL
on catalytic performance was demonstrated by comparing the
prepared SILP catalysts with BAIL1 and the non-acidic IL2
under the selected reaction conditions. As shown in Figure 2,
the SILP with BAIL1 yielded a high catalytic activity corre-
À1
sponding to a maximum turnover frequency (TOF) of 210 h
À1 À1
Figure 1. ILs employed in the SILP preparation.
and space time yield (STY) of 0.24 kgL h , but the
&
ChemCatChem 2017, 9, 1 – 7
2
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