Ligand-modified rhodium catalysts on porous silica in the continuous gas-phase hydroformylation of short-chain alkenes-catalytic reaction in liquid-supported aldol products

Article abstract of DOI:10.1002/cctc.201300305

Ligand-modified Rh complexes were physically adsorbed on the surface of porous silica. The resulting materials were subjected to the continuous gas-phase hydroformylation of C₂ and C₄ alkenes. The ligands used for catalyst modification were bidentate phosphorus ligands known from the literature, namely, sulfoxantphos (1) and a benzopinacol-based bulky diphosphite 2. The tested catalyst materials were active and, in particular, selective as in comparable homogeneous liquid-phase experiments. Long-term stability experiments over 1000h on stream showed minor deactivation. A significant increase in the catalyst mass after the reaction was detected by weighing and thermogravimetric analysis. By using headspace-GC-MS, the mass increase could be attributed to high-boiling compounds, which are formed insitu during the catalytic reaction itself and accumulate inside the pores of the support. Evidence is given that the initially physisorbed catalyst complexes dissolve in the high-boiling aldol side-products, which are suitable solvents for the active catalyst species and provide a liquid-phase environment held by capillary forces on the support. It's all in the pores! Ligand-modified Rh complexes are physically adsorbed onto the surface of porous silica and the resulting solid materials are subjected to continuous gas-phase hydroformylation of C₂ and C₄ alkenes. The catalyst materials were surprisingly active and, in particular, exhibited similar selectivity to liquid-phase reactions.

Full text of DOI:10.1002/cctc.201300305

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DOI: 10.1002/cctc.201300305
Ligand-Modified Rhodium Catalysts on Porous Silica in the
Continuous Gas-Phase Hydroformylation of Short-Chain
Alkenes–Catalytic Reaction in Liquid-Supported Aldol
Products
[
a]
[a]
[a]
[a]
[b]
Andreas Schçnweiz, Jonas Debuschewitz, Simon Walter, Renꢀ Wçlfel, Hanna Hahn,
[b]
[b, c]
[a]
[a]
Katrin M. Dyballa, Robert Franke,
Marco Haumann,* and Peter Wasserscheid
Ligand-modified Rh complexes were physically adsorbed on
the surface of porous silica. The resulting materials were sub-
lyst mass after the reaction was detected by weighing and
thermogravimetric analysis. By using headspace-GC–MS, the
mass increase could be attributed to high-boiling compounds,
which are formed in situ during the catalytic reaction itself and
accumulate inside the pores of the support. Evidence is given
that the initially physisorbed catalyst complexes dissolve in the
high-boiling aldol side-products, which are suitable solvents
for the active catalyst species and provide a liquid-phase envi-
ronment held by capillary forces on the support.
jected to the continuous gas-phase hydroformylation of C and
2
C4 alkenes. The ligands used for catalyst modification were bi-
dentate phosphorus ligands known from the literature, namely,
sulfoxantphos (1) and a benzopinacol-based bulky diphosphite
2
. The tested catalyst materials were active and, in particular,
selective as in comparable homogeneous liquid-phase experi-
ments. Long-term stability experiments over 1000 h on stream
showed minor deactivation. A significant increase in the cata-
Introduction
Hydroformylation is one of the most common homogeneously
catalyzed processes worldwide. With an annual production of
oxo products beyond 10 million tons, this reaction is of high
[
1]
technical relevance in the chemical industry. In hydroformyla-
tion, alkenes react with synthesis gas (syngas), a mixture of H2
and CO, to form aldehydes, which are valuable intermediates
for the synthesis of bulk chemicals such as alcohols, esters,
Scheme 1. Catalytic hydroformylation of terminal alkenes to yield aldehydes
(n and iso) and possible consecutive reaction pathways to bulk chemicals.
[
2]
and amines (Scheme 1). Long-chain alcohols with a doubled
carbon number are accessible through consecutive aldolization
reactions. The majority of today’s hydroformylation processes
utilize homogeneously dissolved, Rh-based organometallic cat-
alytic complexes, which operate under relatively mild reaction
conditions in contrast to cobalt systems, that is, temperatures
in the range of 85–1308C and pressures in the range of 15–
[3]
60 bar. In the well-known Ruhrchemie-Rhꢁne/Poulenc pro-
cess, very efficient catalyst recycling and product separation
has been established by the use of an aqueous/organic bipha-
[4]
sic reaction system. Since the successful start of this process
almost 30 years ago, numerous immobilization techniques for
homogeneous catalysts have been developed and reported,
and many of these concepts have been exemplified for hydro-
[a] A. Schçnweiz, J. Debuschewitz, S. Walter, Dr. R. Wçlfel, Dr. M. Haumann,
[5]
formylation catalysis. Amongst these, the heterogenization of
catalyst complexes by immobilization on porous support mate-
rials has been extensively studied. Such heterogenization can
be achieved by, for example, covalently anchoring the Rh com-
Prof. Dr. P. Wasserscheid
Lehrstuhl fꢀr Chemische Reaktionstechnik
FAU Erlangen-Nꢀrnberg
Egerlandstr. 3, 91058 Erlangen (Germany)
Fax: (+49)9131-85-27421
[6]
plex onto the support by spacer ligands. Furthermore, the
metal complex can be dissolved in an appropriate liquid, and
the resulting solution dispersed onto the surface of a porous
solid support. Depending on the liquid, so-called supported
E-mail: marco.haumann@fau.de
[
b] H. Hahn, K. M. Dyballa, Prof. Dr. R. Franke
Evonik Industries AG
Paul-Baumann-Str. 1, 45772 Marl (Germany)
[7]
[8]
[
c] Prof. Dr. R. Franke
Lehrstuhl fꢀr Theoretische Chemie
Ruhr-Universitꢁt Bochum
liquid-phase (SLP), supported aqueous-phase (SAP) or sup-
[9]
ported ionic-liquid-phase (SILP) catalysts have been obtained.
In particular, the SILP concept is ideally suited for continuous
gas-phase processes because of the extremely low vapor pres-
sure of the ionic liquid solvent. Long-term stability of more
4
4780 Bochum (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300305.
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than 800 h on stream has been reported for C hydroformyla-
4
[
10]
tion on application of these SILP systems. Bulky bidentate di-
phosphine and diphosphite ligands have been used to ensure
[
11,12]
high selectivity to the desired linear aldehyde.
In one of the first reports on the SILP-catalyzed hydroformy-
lation of propylene, some of us reported a small decrease of
catalyst activity over time because of the formation of high-
boiling side-products, however, the initial activity was easily re-
[
13]
stored by a vacuum procedure. To investigate the influence
of possible aldol condensation products on the hydroformyla-
tion reaction in the gas phase, we prepared catalyst materials
according to a similar procedure as for the established synthe-
[9]
sis of SILP catalysts (incipient-wetness impregnation) but
without adding ionic liquid. The as-prepared, purely physisor-
Figure 2. Conversion and selectivity over time of the continuous gas-phase
bed Rh complexes on SiO (modified with sulfoxantphos (1)
2
hydroformylation of 1-butene using Rh-1 (^, ^) and Rh-2 (
&
&
, ) catalyst
and a benzopinacol-based bulky diphosphite 2 ligand, respec-
tively; Figure 1) were tested in the continuous gas-phase hy-
complexes on silica 100; parameters: mcat =2.3 g, mRh =0.2 wt%, L/Rh=5,
T=1008C, ptotal =10 bar, p1-butene =1.8 bar, pH2 =pCO =4.1 bar, residence
time=20 s.
picted in Figure 2. Furthermore, the catalyst selectivity towards
the linear aldehyde product, n-pentanal, was 97%. This value is
in good agreement with literature data for Rh-1 catalysts in ho-
[9,14]
mogeneous liquid-phase and SILP systems.
drogenation side-products in the product mixture was below
%. On using the Rh-2 catalyst complex in the same physisor-
The share of hy-
5
bed state on silica 100, a comparable high stability was ob-
served even though 2 is a diphosphite ligand and thus prone
to decomposition by hydrolysis. A maximum degree of conver-
sion of 47.3% related to 1-butene was detected for the Rh-2/
silica100 material compared to 5.1% in the case of Rh-1/
silica100. The higher catalytic activity of the Rh-2-based system
results from the chemical nature of 2. The diphosphite ligand
2 is known to provide a better p-acceptor ability that strength-
ens the Rh-to-ligand bonding, which facilitates CO dissociation
Figure 1. Bidentate ligands used in (SILP) hydroformylation catalysis to ach-
ieve high regioselectivities: sulfoxantphos (1) and benzopinacol-based
ligand (2).
droformylation of short-chain alkenes. In this contribution, we
demonstrate that, under the reaction conditions applied,
a liquid phase that consists predominantly of aldol condensa-
tion products forms in situ inside the pores of the supporting
silica and acts as suitable solvent for the active hydroformyla-
tion catalyst. With the catalytically active species dissolved in
these high-boiling compounds, the catalyst materials showed
the same n/iso selectivities as those typically found for classical
homogeneous systems. Analysis of the spent catalysts after re-
action indicates that the filling of the pores with high-boiling
compounds is a function of the reaction conditions, the
degree of conversion and the porosity of the silica support.
The long-term stability of the Rh-2/SiO2 system was tested
under the optimized conditions for a technically relevant C4
feedstock.
[12]
and alkene association within the catalytic cycle. The hydro-
genation activity of the Rh-2/silica100 system was around 7%,
and the regioselectivity reached a value of 99.5%. Again, this
very high selectivity for the formation of the n-pentanal prod-
uct is in good agreement with previous experiments that used
[15]
Rh-2 in a bulk liquid environment.
A closer look at the
degree of conversion over time on stream (TOS) reveals that
slight deactivation occurred for both catalyst systems. In the
case of Rh-1/silica100, the conversion dropped from 5.1 to
4.3% over 70 h on stream, which corresponds to a decrease in
ꢀ1
turnover frequency (TOF) from 90.5 to 75.2 h . For the Rh-2/
silica100 system, the degree of conversion decreased from 47.3
to 41.5% (which corresponds to a drop in TOF from 750.7 to
ꢀ
1
Results and Discussion
658.6 h ) over the same operation time. Nonetheless, the ob-
served catalyst stabilities are surprisingly high if we take into
account that the applied Rh complexes were initially support-
ed on dry silica (traces of solvent from the incipient-wetness
impregnation are not excluded). No liquid phase has been of-
fered to explicitly provide a solvent environment for the Rh
complexes that are usually applied and known to work best as
homogeneous catalysts.
Catalytic investigations
After deposition of the Rh-1 catalyst complex on porous
silica 100 (calcined at 6008C for 12 h prior to catalyst prepara-
tion) by incipient-wetness impregnation, the resulting material
was tested in the continuous gas-phase hydroformylation of 1-
butene. Surprisingly, this essentially solvent-free catalyst com-
plex showed considerable stability over 70 h on stream in this
purely physisorbed state with only minor deactivation as de-
To elucidate these remarkable catalyst stabilities, we had
a closer look at the full product spectrum during continuous
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catalytic operation. In addition to the C aldehyde products
If we compare the signal intensities in the headspace-GC–
MS spectra, it is evident that under the conditions applied
5
measured by online GC, traces of aldol compounds were de-
tected for both the Rh-1 and Rh-2 catalyst systems. We weigh-
ed the catalyst materials after 70 h on stream, which revealed
an increase in mass of 5.2 and 29.1% for the two catalysts, re-
spectively, in comparison to the material initially put into the
reactor. Combined headspace-GC–MS analysis of the spent cat-
alysts showed the presence of considerable amounts of alde-
hyde and aldol products alongside with other high-boiling
compounds, which clearly remained inside the porous materi-
als after recovery from the reactor. The main signals can be at-
tributed to linear pentanal and the C10 aldol condensation
product, 2-propyl-2-heptenal (Figure 3). The latter is formed by
during continuous hydroformylation (ptotal =10 bar, p1-butene
=
1.8 bar, T=1008C) the produced pentanals undergo consecu-
tive reactions to a considerable extent to form predominantly
the C10 aldol condensation product. The relatively low vapor
pressure of these aldols (and the other high-boiling com-
pounds) clearly leads to an in situ formation of a liquid aldol
phase inside the smallest pores of the porous silica 100 sup-
port.
Notably, the catalyst materials remained free-flowing pow-
ders even after prolonged time on stream, which indicates that
a vapor–liquid equilibrium of high-boiling compounds is
reached under steady-state reaction conditions. This assump-
tion will be discussed in detail below. As the tested catalytic
materials showed high regioselectivities similar to the behavior
known from classical homogeneous systems, it is proposed
that the high-boiling products dissolve the initially physisorbed
Rh-1 and Rh-2 catalyst complexes, respectively, to provide
a liquid-phase environment for the catalytically active species
inside the pores of the support. According to the literature,
purely heterogeneous catalysts suffer from low hydroformyla-
tion activity and rather high hydrogenation activity if applied
[16]
in gas-phase contact. Without the presence of a liquid reac-
tion phase, in which the organometallic catalyst complex is dis-
solved, poor regioselectivity is often found as well. Early publi-
cations report active and selective hydroformylation catalysts if
Rh-TPP (triphenylphosphine) complexes are used on silica sup-
[7]
ports. In these cases, TPP is a liquid under the reaction condi-
tions applied and serves as both the ligand and the solvent of
the Rh catalyst complexes. As the melting points of 1 and 2
are higher than the reaction temperature, this contribution can
be excluded in the present study. According to the literature,
Figure 3. Headspace-GC–MS analysis of used Rh-1 (c) and Rh-2 (d) cat-
alyst materials. Both catalysts showed stable catalytic performance over 70 h
on stream in the continuous hydroformylation of 1-butene. The main prod-
ucts are labeled with chemical structures; side signals are labeled with
names as different isomers may be present.
selective and stable hydroformylation catalysts can be ob-
0
tained from Rh /SiO in the presence of syngas by in situ for-
2
I
the consecutive reaction of the linear aldehyde product proba-
bly catalyzed by the surface acidity of the silica support
mation of Rh (CO) species from Rh nanoparticles followed by
2
[17]
ligation of a phosphine ligand, such as TPP. Even xantphos-
0
(Scheme 2). The intermediate aldol addition product, 3-hy-
modified Rh /SiO systems have been published recently that
2
droxy-2-propylheptanal, was not detected. Further signals
were found to behave similarly to comparable liquid-phase or
SILP catalysts with respect to kinetic and spectroscopic charac-
were attributed to iso-pentanal, C alkenes, ketones, and di-
8
[18]
pentyl ether, which originate from other consecutive reaction
pathways of the produced linear pentanal. The other signals
visible in the spectrum in Figure 3 stem from support or ligand
decomposition compounds (indicated by *) as determined by
reference measurements of the freshly prepared catalyst mate-
rials. Traces of organic solvents used in the catalyst preparation
were also detected.
terization. These catalytic materials, however, were investi-
gated under reaction conditions that should strongly minimize
aldol formation and condensation, that is, ptotal =2 bar and
alkene conversion <1%. The fact that ligand-modified Rh
complexes are active and highly regioselective in liquid aldols
is not surprising as this principle is applied on a million-ton-
scale in the industrial low pressure oxo (LPO) process, devel-
[19]
oped by the Union Carbide Corporation (UCC). However, to
the best of our knowledge, the concept of the immobilization
of ligand-modified Rh complexes by the in situ formation of
high-boiling compounds in the confined space of porous sup-
port materials for gas-phase application has not been reported
elsewhere.
[20]
By using the Kelvin equation [Eq. (1)], which describes the
change in vapor pressure of a pure compound at curved
liquid–vapor interfaces, such as, for example, in the capillaries
of a porous support, it is possible to estimate whether the con-
Scheme 2. Consecutive reaction of linear pentanal to form a C10 aldol con-
densation product, 2-propylhept-2-enal.
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densation of pentanals and/or C10 aldols occurs under the con-
ditions of a specific catalytic experiment inside the pores of
silica 100. Using highly porous materials as the catalyst sup-
port, the vapor pressure of the hydroformylation products is
reduced significantly, which promotes the condensation of
low-volatility compounds that would be gaseous without the
confinement. Therefore, the following estimations are relevant
for the present study.
pore fraction of the support. The condensation of produced
pentanals might occur in the micropores for Rh-2 because of
the pore size distribution of silica 100 as here the observed n-
pentanal partial pressure is above the value of 0.24 bar. In the
case of Rh-1/silica100, pentanal condensation should not be
relevant because of the significantly lower degree of conver-
sion. This situation changes on considering the aldol products.
As a result of the lack of physical data for 2-propyl-2-heptenal,
n-decanol was chosen as a comparable C10 compound for
a first approximation. Under the reaction conditions, the satu-
ꢀ
ꢁ
2
ꢁ s ꢁ M
1
p ¼ p ꢁ exp ꢀ
ꢁ r
ð1Þ
s
RT ꢁ d1
pore
ration partial pressure of n-decanol is already very low (p =
s
ꢀ
3
1
3ꢃ10 bar). If we take the confined space in the pores of
The parameters used in Equation (1) are saturation vapor
silica 100 into account, the condensation vapor pressure is fur-
ꢀ
3
pressure p , surface tension s, molar mass M, universal gas con-
ther reduced to 0.6ꢃ10 bar. If we assume that the value for
10 aldol is of the same order of magnitude, it is not surprising
s
stant R, temperature T, liquid density d, and pore radius rpore.
C
l
Even though the Kelvin equation is only valid for pure com-
pounds and a complex gas mixture is present in the case of
the hydroformylation catalysis under investigation, the equa-
tion was applied here for a first estimation of possible pore
condensation phenomena. The surface tension s was calculat-
ed from an empirical formula according to Brock and Bird
based on critical parameters (p , T , and the normal boiling
that a considerable amount of aldol and other high-boiling
products are found in the pores of the spent catalyst materials
as shown in Figure 3. The detected amount of pentanals might
be a result of capillary condensation or dissolved matter in the
higher boiling C10 aldols.
To evaluate the conditions under which the formation and
accumulation of low-volatility organic compounds inside the
porous network of the catalytic materials occur, both the sup-
porting silica and alkene substrate were varied. The micro-/
c
c
[
21]
point temperature TNBP) [Eqs. (2) and (3)].
ꢀ
ꢁ
11=9
T
Tc
mesoporous silica 100 was replaced by a macroporous SiO2
2
=3
1=3
s ¼ ðPcÞ ꢁðTcÞ ꢁQ 1 ꢀ
ð2Þ
material, namely, Trisopor 423, which was expected to suppress
the extent of pore condensation by providing pores with an
average diameter of 423 nm instead of 11 nm as in the case of
silica 100. After immobilization of the Rh-1 and Rh-2 catalyst
with
"
ꢂ
ꢃ#
TNBP
Tc
Pc
ꢁ
ln
complexes on this macroporous SiO support, the resulting
2
1:01325
TNBP
Q ¼ 0:1196 1 þ
ꢀ 0:279
ð3Þ
materials were applied in the continuous hydroformylation of
ethylene. Ethylene was used as the alkene feed as possible C6
aldol products exhibit considerably higher volatility than the
1
ꢀ
Tc
All other necessary physicochemical data are compiled in
Table 1 (values stem from the FLUIDAT database provided by
Bronkhorst High-Tech B.V., the Netherlands). From Equation (1),
the vapor pressure of n-pentanal was calculated as 0.74 bar
C10 aldol compounds that originate from the hydroformylation
of 1-butene.
The results obtained are presented in Figure 4. It is clear
that, after an initial activation period, the physisorbed catalysts
showed stable performance over the testing period of 70 h on
stream without major deactivation. The reaction conditions
Table 1. Characteristics of n-pentanal and n-decanol at T=1008C and
p=10 bar. Values labeled with * were taken from the FLUIDAT database,
Bronkhorst High-Tech B.V., the Netherlands.
(
p=20 bar, T=808C) and a relatively low ethylene partial pres-
sure in the feed gas were set with respect to a preferably low
degree of conversion, which minimizes the probability of aldol
product formation. However, also in this case, postreaction
headspace-GC–MS analysis of the spent catalytic materials re-
Compound
p
s
p
c
T
c
T
[K]
NBP
s
d
l
M
ꢀ
1
ꢀ3
ꢀ1
[
bar] [bar] [K]
[m Nm
]
[kgm
]
[g mol ]
n-Pentanal 0.93* 35.5* 554.0* 375.8* 16.65
n-Decanol 0.013* 22.3* 700.0* 513.1* 23.72
685.7* 86.1
790.7* 158.3
vealed the presence of C aldol compounds to a significant
6
extent. The main signal in the chromatogram depicted in
Figure 5 can be attributed to 2-methyl-2-pentenal, and further
signals stem from propanal and the hydrogenated C aldol de-
6
with respect to the reaction conditions applied, that is, T=
008C, p=10 bar, and a mean pore radius of silica 100 of 6 nm
rivative 2-methylpentanal. Again, no aldol addition compounds
were detected, which suggests that the formation of aldol con-
densation products seems to be highly favored under the reac-
tion conditions applied.
1
as determined from N adsorption measurements. Regarding
2
the micropores only (mean pore radius of 1 nm), the value fur-
ther reduces to 0.24 bar. For the observed degree of conver-
sion, an n-pentanal partial pressure of 0.10 and 0.52 bar in the
feed gas was determined for the Rh-1 and Rh-2 catalyst sys-
tems. These partial pressures are below the estimated conden-
sation pressure of 0.74 bar, which takes into account the total
It might be quite surprising that the relatively low-boiling C3
aldehyde can be detected postreaction inside the macropo-
rous catalyst material as its vapor pressure should be high
enough to exclude condensation. However, as 2-methyl-2-pen-
tenal condensed under the reaction conditions, the amount of
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Table 2. Summary of the catalytic results and catalyst mass increases de-
termined by weighing and TG analysis. Degrees of conversion are related
to the linear alkene substrate, and TOF values are in molaldehyde per molRh
per hour.
Catalyst material
Substrate
Dmweighing
DmTG
X
[%]
alkene
TOF
[h ]
ꢀ
1
[
%]
[%]
[
[
a]
a]
Rh-1/silica100
Rh-2/silica100
1-butene
1-butene
ethylene
ethylene
1-butene
5.2
28.6
2.6
7.0
3.0
3.0
28.1
1.3
6.7
2.6
5.1
47.3
1.1
5.7
4.2
90.5
750.7
6.9
37.5
66.3
[
[
[
b]
b]
a]
Rh-1/Trisopor423
Rh-2/Trisopor423
Rh-2/Trisopor423
[
p
a] Reaction conditions: T=1008C, ptotal =10 bar, p1-butene =1.8 bar, p 2
H
=
=
CO =4.1 bar, residence time=20 s; [b] T=808C, p=20 bar,
p
ethylene
1
.0 bar, p 2 =pCO =9.5 bar, residence time=30 s.
H
Figure 4. Conversion over time in ethylene hydroformylation using Rh-1 (^)
and Rh-2 (
cat =2.3 g, mRh =0.2 wt%, L/Rh=5, T=808C, p=20 bar, pethylene =1.0 bar,
=pCO =9.5 bar, residence time=30 s.
&
) complexes on macroporous Trisopor 423; parameters:
materials. If we compare the Rh-1 and Rh-2-based catalysts,
the latter showed a more pronounced mass increase, which
corresponds to the higher catalytic activity. The higher catalytic
activity clearly led to the enhanced formation of high-boiling
compounds, which then remained to a certain extent in the
pores of the catalyst support. This result was found for both
ethylene and 1-butene and was also reflected by the peak in-
tensities of the headspace-GC–MS analysis shown in Figures 3
and 5, respectively. Another explanation for the larger mass in-
crease observed with Rh-2-based catalyst materials could be
the higher Lewis acidity of the Rh complex as a result of the
better p-accepting ability of ligand 2, which thereby promotes
the aldol formation. Additionally, the influence of the support
porosity was substantial as the replacement of silica 100 by Tri-
sopor 423 while using the same Rh-2 catalyst complex in the
continuous hydroformylation of 1-butene under identical reac-
tion conditions leads to a significantly lower mass increase ac-
companied by a drastic decrease in catalyst activity (compare
entries 2 and 5; additional details can be found in the Support-
ing Information). Following these investigations, the extent of
pore filling with aldol and other high-boiling compounds
during the course of the catalytic reaction seems to depend
on a) the activity/nature of the immobilized catalyst species,
m
H
p 2
Figure 5. Headspace-GC–MS analysis of Rh-1 (c) and Rh-2 (d) catalyst
materials on macroporous SiO support after continuous hydroformylation
of ethylene for 70 h. The main signals are attributed to propanal, 2-methyl-
-pentenal, and 2-methylpentanal, respectively.
2
2
b) the porosity/texture of the SiO support, and c) the vapor
2
propanal detected by headspace-GC–MS possibly derived from
pressure of possible aldol compounds.
the dissolved compound in condensed C aldols.
6
In addition to simply weighing the catalyst materials before
and after the reaction, the amount of high-boiling compounds
accumulated in the pores was quantified by using thermogra-
vimetric (TG) analysis. The difference in relative mass loss be-
tween the catalyst materials before and after use in the contin-
uous gas-phase experiment was attributed to the mass in-
crease caused by high-boiling products and compounds dis-
solved therein. The results obtained from TG measurements
are summarized in Table 2 together with the mass increase de-
termined by weighing and results from the catalytic experi-
ments for comparison.
Analysis of the start-up behavior
A comparison of the start-up behavior of the tested catalyst
systems, that is, the development of the degree of conversion,
X, within the first hours of time on stream, revealed that stable
operation was reached after different times, which depended
on the catalyst complex and support material. The maximum
degree of conversion, Xmax, was obtained almost spontaneously
for Rh-2/silica100, whereas an activation behavior was ob-
served for all other systems (Figure 6). In particular, the macro-
porous SiO -based catalysts showed pronounced activation pe-
2
The increases of catalyst mass determined by TG analysis
and weighing were of the same order of magnitude, even
though the Dm values obtained from TG measurements were
in general somewhat smaller, in particular for the Rh-1 based
riods, taking up to 15 and 30 h for Rh-1 and Rh-2 on Triso-
por 423 to reach the maximum conversion.
For the case of steady-state operation at maximum conver-
sion, the presence of a thermodynamic equilibrium between
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Figure 6. Relative degree of conversion (X/Xmax) over time on stream for Rh-
1
(^) and Rh-2 (
&
) on silica 100 and the same catalyst complexes on Triso-
Figure 7. Conversion over time in ethylene hydroformylation using Rh-1 (^)
por 423 (open symbols). Reaction parameters are given in the caption of Fig-
ures 1 and 4, respectively.
and Rh-2 ( ) immobilized in 2-methyl-2-pentenal on Trisopor 423. 2-Methyl-
&
2-pentenal was added in the catalyst preparation prior to catalytic reaction;
parameters: mcat =2.36 g (Rh-1) and 2.46 g (Rh-2), mRh =0.2 wt%, L/Rh=5,
m
2-methyl-2-pentenal =2.6 wt% (Rh-1) and 7.0 wt% (Rh-2), T=808C, p=20 bar,
p
ethylene =1.0 bar, p 2 =pCO =9.5 bar, residence time=30 s. The results from
H
the condensation and evaporation of aldehyde and aldol prod-
ucts inside the porous catalysts is assumed. It takes longer to
reach this equilibrium state if a macroporous instead of
micro-/mesoporous support is used. This is because of the
lower capillary pressure and generally lower degree of conver-
sion characteristic of these catalytic materials. As a conse-
quence, lower amounts of aldehyde and aldol products are
formed during a certain time interval, which thus extends the
time on stream needed to reach the maximum conversion.
Figure 4 (without 2-methyl-2-pentenal) are plotted for comparison (gray).
period is detected if the applied catalyst complexes are already
dissolved in the C aldol product during catalyst preparation.
6
In other words, by the addition of a particular amount of aldol
product prior to the hydroformylation reaction, the catalytic
materials start in the same state that a purely physisorbed
system would reach only after several hours time on stream.
These findings confirm the assumption that the homogeneous
hydroformylation reaction indeed proceeds in liquid aldol
products inside the porous support. As the filling of the pores
with high-boiling compounds is supposed to be a dynamic
process, a model can be developed based on the experimental
results that describes the start-up behavior of purely physisor-
bed catalysts.
Catalyst behavior with aldol doping
To prove that the physisorbed catalyst species are dissolved by
a condensed product phase in the pores of the support during
the course of the continuous hydroformylation reaction, cata-
lyst materials were prepared with a defined amount of 2-
methyl-2-pentenal deliberately added during catalyst prepara-
tion. According to the mass increase determined by weighing
the spent materials after 70 h on stream, 2.6 and 7.0 wt% of 2-
methyl-2-pentenal were used as the supported liquid on Triso-
por 423 to dissolve the Rh-1 and Rh-2 complexes, respectively,
right from the beginning of the catalytic experiment. These
amounts correspond to a theoretical degree of pore filling of 3
and 10 vol.% in relation to the total pore volume of the sup-
port. We subjected these materials to continuous hydroformy-
lation of ethylene, and a very similar maximum conversion was
obtained as in the case of the purely physisorbed catalyst com-
plex for the Rh-2-based system (Figure 7).
Pore filling model
Based on the experimental results described above, the as-
sumed dynamic process of pore filling with condensed aldol
(and other high-boiling) products at the beginning of a contin-
uous gas-phase experiment with a purely physisorbed catalyst
complex is shown in Figure 8. According to the catalyst prepa-
ration, the initial material consists of the Rh catalyst complex,
excess ligand, and traces of solvent from the material impreg-
nation attached to the support surface (Figure 8a). On applica-
tion of the reaction conditions, the catalytic production of al-
dehydes starts and with that—probably catalyzed by acidic si-
lanol groups at the support surface—the consecutive forma-
tion of aldol products. As a result of their low volatility and the
capillary pressure that exists in the porous network, these
higher boiling products condense to some extent inside the
pores of the support. According to classical pore condensation
behavior, the micropores are filled first followed by the larger
pores. The condensed aldol phase then dissolves the initially
physisorbed ligand-modified Rh complex to provide a liquid
In the case of Rh-1/Trisopor423 doped with 2-methyl-2-pen-
tenal, the degree of conversion was higher than that of the
system without C aldol. This result might be attributed to an
6
insufficient dissolution of the Rh catalyst complexes in the
aldol phase generated at a low degree of conversion, whereas
it is assumed that all catalyst complexes are dissolved if 2-
methyl-2-pentenal is added during the catalyst preparation.
The major difference between the systems with and without
added 2-methyl-2-pentenal is, however, that no activation
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boiling reaction products, the
Rh-2/silica100 material was sub-
jected to
a continuous gas-
phase hydroformylation of an in-
dustrial C4 feedstock. This feed
consists of 28% 1-butene, 44%
2-butenes, ꢂ0.1% iso-butene,
and 28% inert butanes. The cat-
alytic experiment was performed
at Evonik Industries AG, Marl,
Germany. The conversion over
time is depicted in Figure 9,
which shows highly active and
selective catalyst operation for
Figure 8. Illustration of the process of pore filling with aldehyde and aldol products during the catalytic hydrofor-
mylation reaction using ligand-modified Rh complexes in porous silica supports.
1000 h on stream under the ap-
plied optimized conditions. At
a high syngas/butene ratio of 8
reaction phase for supported homogeneous catalysis (Fig-
ure 8b). Herein, the Rh species are supposed to behave similar-
ly to that if the reaction takes place in a classical organic sol-
vent, which shows the same n/iso selectivities. As the share of
the dissolved catalyst molecules is still low at the beginning of
a long-term experiment, the initial degree of conversion is low
if macroporous supports are used. As the catalytic reaction
proceeds, more aldehyde and aldol are formed, which in turn
condense within the pores until a thermodynamic equilibrium
between condensation and evaporation is reached under the
conditions applied. From this point on, the degree of pore fill-
ing remains constant at a level that is characteristic for a given
set of reaction parameters.
From the continuous hydroformylation experiments of ethyl-
ene using macroporous Trisopor 423 as the support, it can be
concluded that the start-up/pore-filling period described
Figure 9. Conversion (related to all butenes) over time in continuous gas-
phase hydroformylation of industrial C feedstock using Rh-2/silica100 cata-
lyst material. Parameters: mcat =12.0 g, mRh =0.2 wt%, L/Rh=10, T=100–
1208C, p=10 bar, pbutenes =1.6 bar (1.3 bar), pbutanes =0.6 bar (0.5 bar),
above takes several hours. In the final state, that is, after the
pore filling has reached equilibrium and a defined share of the
pore volume is filled with condensed high-boiling compounds,
steady-state operation is achieved under the reaction condi-
tions applied (Figure 8c). Notably, it might be possible that
some organometallic complexes remain undissolved on the
support inner surface if the final degree of pore filling is at
a relatively low level. This might explain the lower catalytic ac-
tivity found in the case of Rh-1/Trisopor423 compared to the
same system in which a predefined amount of 2-methyl-2-pen-
tenal was added during catalyst preparation (Figure 7).
4
H
p 2
=pCO =3.9 bar (4.1 bar) before (and after) variation of the syngas/butene
ratio from 6 to 8 after 170 h on stream, residence time=48 s (43 s).
(parameter change after 170 h on stream) the catalyst showed
a typically high regioselectivity of 99.4% to linear pentanal,
comparable to the results obtained with pure 1-butene feed-
stock (Figure 2).
In such cases in which the aldol product is used explicitly for
the immobilization of the Rh complex prior to the catalytic re-
action, a particular degree of pore filling is already present in
the as-prepared material. This starting point significantly short-
ens the dynamic process of (additional) pore filling under the
reaction conditions. The equilibrium between the condensa-
tion and evaporation of high-boiling compounds is reached
within a shorter time on stream.
The initial degree of conversion of 38% was well maintained
within the first 170 h on stream. After increasing the syngas/
butene ratio, the catalyst activity increased almost instantane-
ously, probably as a result of the higher syngas availability in
the liquid catalyst phase. We attribute this rather unexpected
observation to the assumption that mass transfer limitations
might play a role in this particular case because of the relative-
ly high degree of pore filling with aldol products (45 vol.%).
Regarding the comparative catalyst systems in which pore fill-
ing is below 10 vol.% and thin films of aldol provide a high ex-
change surface, similar syngas/alkene variation resulted in
a more common catalyst behaviour, which resulted in a lower
catalyst activity for a higher syngas/alkene ratio (see Support-
Long-term stability experiment
To study the long-term stability of catalyst materials obtained
as described by dissolving Rh complexes in supported, high-
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ing Information). For the continuous C4 hydroformylation,
slight deactivation was observed over a prolonged reaction
time. The extent of this deactivation behavior is comparable to
that observed for Rh-2/silica100 with a pure 1-butene feed. A
temperature variation to 110 and 1208C resulted in slightly
lower n/iso selectivities and more pronounced catalyst deacti-
vation. Even though the extraordinary long-term stability of
Experimental Section
Chemicals
Rh(CO) (acac) (acac=acetylacetonate), methanol, and dichlorome-
2
thane (HPLC grade) were purchased from Sigma–Aldrich and used
without further purification. The sulfoxantphos ligand 1 was syn-
thesized by sulfonation of 9,9-dimethyl-4,5-bis(di-tert-butylphosphi-
[14]
[
10]
no)xanthene (Sigma–Aldrich) according to a literature procedure.
a comparable SILP system was not achieved, the observed
behavior is quite remarkable for an initially purely physisorbed
catalyst complex with a labile diphosphite ligand such as 2.
One possible explanation for deactivation might be the forma-
tion of water as a side-product of the aldol reaction. During
the long-term test, considerable amounts of water are formed
in the pores, which possibly lead to the gradual hydrolysis of 2
over time.
The benzopinacol ligand 2,2’-[(3,3’-di-tert-butyl-5,5’-dimethoxybi-
phenyl-2,2’-diyl)bis(oxy)]bis-(4,4,5,5-tetraphenyl-1,3,2-dioxaphos-
pholane) (2) was supplied by Evonik Industries AG, Marl, Germany.
Silica gel 100 (63–200 mm) was purchased from Merck, and Triso-
por 423 (100–200 mm) from VitroBio GmbH. For the long-term sta-
bility experiment silica gel 100 (200–500 mm) was used. The SiO
2
materials were pretreated at 6008C for 18 h before use in catalyst
preparation. 1-Butene (99.5%), ethylene (99.95%), CO (99.97%),
and H (99.999%) were purchased from Linde AG. The industrial C
2
4
In addition, inductively coupled plasma (ICP) measurements
of spent catalyst materials often revealed a decreased Rh con-
tent, which was of the same order of magnitude as the loss of
catalytic activity observed during the catalytic runs. This loss of
Rh is probably caused by liquid entrainment of the condensed
aldol phase from the support over the duration of a continuous
experiment.
feedstock was provided by Evonik Industries AG and contained
2
8% 1-butene, 44% 2-butenes, ꢂ 0.1% iso-butene, and 28% inert
butanes. 2-Methyl-2-pentenal (97%) was purchased from Sigma–
Aldrich.
Catalyst preparation
All syntheses were carried out using standard Schlenk techniques
under Ar (99.999%). Rh(CO) (acac) was dissolved in methanol or di-
2
chloromethane, for the reaction with 1 or 2, respectively, and
stirred for 5 min. Ligand 1 or 2 was added in a fivefold molar
excess (ligand/Rh=5) dissolved in methanol or dichloromethane,
respectively, and the resulting solution was stirred for 5 min. In the
long-term stability experiment, a tenfold molar excess of 2 (ligand/
Rh=10) was used. After addition of the appropriate amount of cal-
cined silica 100 or Trisopor 423 (see Supporting Information), the
suspension was stirred for another 10 min. The organic solvent was
removed by using a rotatory evaporator, and the dry powder was
Conclusions
Catalytic materials comprising Rh complexes with sulfoxant-
phos (1) and benzopinacol-based ligands (2), Rh-1 and Rh-2,
respectively, dispersed on the internal surface of porous SiO2
materials have been prepared and successfully tested in the
continuous gas-phase hydroformylation of short-chain alkenes.
The observed regioselectivities to the linear aldehyde were 97
and 99.5%, respectively, which are comparable to those typi-
cally obtained in classical liquid-phase and supported ionic-
liquid-phase (SILP) catalysis. A significant mass increase of up
to 30% was revealed by weighing the catalytic materials after
the reaction in the case of Rh-2/silica100. By using headspace-
GC–MS analysis, this increase in catalyst mass could be attrib-
uted to high-boiling compounds that were generated during
the hydroformylation process itself. Herein, the aldol condensa-
tion products were clearly assigned as the main components.
Dissolved in a condensed phase of high-boiling compounds
inside the pores of the supporting material, the catalytically
active species operate in a homogeneous fashion. A model
that explains the process of pore filling during the catalytic re-
action and possible differences in the start-up behavior of the
tested catalyst materials has been presented. The amount of
condensed high-boiling compounds was found to depend on
ꢀ4
further dried under vacuum (1ꢃ10 bar) overnight. In the case of
aldol addition prior to catalysis, 2-methyl-2-pentenal was mixed
with the Rh-1 and Rh-2 solution, respectively, before addition of
the solid support. No drying under high vacuum was performed
for these catalytic materials (for details see Supporting Informa-
tion).
Catalytic experiments
The reaction setup applied in our laboratories for continuous gas-
phase hydroformylation comprised a fixed-bed reactor, upstream
feed dosing, evaporation and mixing, downstream pressure regula-
tion, and online GC. The entire rig was heated by using electric
heating devices. The catalyst material was filled into a tubular reac-
tor and purged three times with He at room temperature. The rig
was pressurized with He and left under pressure for 15 min while
monitoring the pressure. If no pressure drop was observed, the re-
actor was heated to the reaction temperature under He pressure.
After adjustment of the molar flows, the feed gas (alkene and
syngas) was allowed to enter the reactor. The alkene substrate was
fed into the rig by using an HPLC pump, and syngas was dosed by
using mass-flow controllers. Evaporated alkene was combined with
the preheated reaction gases in a mixing unit, which was filled
with glass beads to ensure proper mixing and isothermal condi-
tions. The gas mixture could then either enter the reactor or exit
the system through a bypass. The reactor consisted of a stainless-
steel tube (12 mm diameter, 500 mm length) equipped with
the catalyst activity and porosity of the applied SiO support.
2
The catalysts were remarkably stable under the optimized con-
ditions, but long-term experiments (up to 1000 h) still revealed
some decay in the system activity. This finding distinguishes
these product-supported systems from classical SILP catalysts,
in which the presence of an ionic liquid film provides a more
robust stabilizing liquid phase for the active Rh complex.
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a porous frit for catalyst placement and a thermocouple to mea-
sure the inner reactor temperature. After the reactor unit, the gas
mixture passed a 7 mm filter to avoid contamination of the tubing
with catalyst or solid particles. A backpressure regulator valve
Research) under the HY-SILP project (no. 01RC1107 A) and its
framework program Research for Sustainable Development
(
FONA) for financial support.
(
Samson) was used to maintain the desired reaction pressure and
outlet gas flow. After the regulator valve, the gas stream was split
and a minor flow was passed through a 1 mL sampling loop
mounted on an online GC (Agilent 7890A). Samples were taken at
regular intervals by injecting the volume of the sampling loop
through a six-port valve into the GC.
Keywords: aldol reaction · hydroformylation · immobilization ·
rhodium · supported liquid
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[
[
1
ꢀ1
stant column flow 4.5 mLmin , detector temperature 2608C. Tem-
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3663.
ꢀ1
ing to 2008C at 408Cmin , hold time 4 min, total time 10 min. In
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7
4 mLmin , detector temperature 2508C. Temperature profile: ini-
[
tial temperature 508C, hold time 15 min, heating to 2008C at
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Headspace-GC–MS analysis was performed by using a Varian 450-
GC combined with a Varian 220-MS. A Combi PAL GC autosampler
[
(
CTC Analytics) equipped with heatable agitator was used for
sample injection. Spent catalyst (0.5 g) was put into a headspace
vial and heated to 1308C for 15 min. Gas samples of 500 mL were
injected by using a preheated syringe (1308C). A FactorFour VF-5
MS capillary column (Varian, length 30 m, inner diameter 0.25 mm)
was used for compound separation. The ionization mode was set
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ꢀ
1
split ratio 10:1, constant column flow 1.0 mLmin . Temperature
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ꢀ
1
1
1
008C at 58Cmin , hold time 10.5 min, heating to 2008C at
1
ꢀ
08Cmin , hold time 0.5 min, total time 36 min.
The mass loss of the catalytic materials was measured by TG analy-
sis by using a Setsys 1750 Cs Evolution (Setaram Instrumentation)
instrument equipped with a 350 mL quartz crucible. The TG signal
was monitored over the following temperature program: initial
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ꢀ1
temperature 308C, He carrier gas flow 200 mLmin , hold time
ꢀ
1
1
0 min, reduce He carrier gas flow to 50 mLmin , hold time 2 min,
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1
ꢀ1
heating to 9008C at 108Cmin and carrier gas flow 50 mLmin
hold time 90 min, cooling to 308C at 308Cmin and carrier gas
flow 200 mLmin , final temperature 308C, final carrier gas flow
,
ꢀ
1
ꢀ1
ꢀ
1
5
mLmin .
[
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Acknowledgements
Received: April 23, 2013
Revised: June 4, 2013
Published online on August 16, 2013
The authors gratefully thank the Bundesministerium fꢀr Bildung
und Forschung (BMBF, German Federal Ministry of Education and
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