Supported Rh-phosphine complex catalysts for continuous gas-phase decarbonylation of aldehydes

Article abstract of DOI:10.1039/c4dt01629g

Heterogeneous silica supported rhodium-phosphine complex catalysts are employed for the first time in the catalytic decarbonylation of aldehydes in continuous gas-phase. The reaction protocol is exemplified for the decarbonylation of p-tolualdehyde to toluene and further extended to other aromatic and aliphatic aldehydes achieving excellent results in terms of both conversion and selectivity. This journal is

Full text of DOI:10.1039/c4dt01629g

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Supported Rh-phosphine complex catalysts for
continuous gas-phase decarbonylation of
aldehydes†
Cite this: Dalton Trans., 2014, 43,
17230
Received 3rd June 2014,
Phillip Malcho,a Eduardo J. Garcia-Suarez,a Uffe Vie Mentzel,a Christian Engelbrektb
Accepted 30th September 2014
and Anders Riisager*a
DOI: 10.1039/c4dt01629g
www.rsc.org/dalton
Heterogeneous silica supported rhodium-phosphine complex ered that complexes of rhodium and polydentate ligands
catalysts are employed for the first time in the catalytic decarbony- provided improved yields of decarbonylation products from
lation of aldehydes in continuous gas-phase. The reaction protocol aldehydes when reacted in boiling dioxane.6,7 Subsequently,
is exemplified for the decarbonylation of p-tolualdehyde to Madsen et al. performed a thorough investigation on such
toluene and further extended to other aromatic and aliphatic alde- rhodium catalyzed reactions monitoring the effect of mono-
hydes achieving excellent results in terms of both conversion and and multidentate phosphine ligands.7a In this work, they con-
selectivity.
cluded that bidentate phosphine ligands in combination with
Rh gave the most efficient catalyst for the studied reaction;
however, high temperature of approx. 160 °C was a prerequis-
ite.7d Recently, exploratory decarbonylation studies have
further been performed using other transition metals e.g. iron,
platinum or palladium in combination with phosphine
ligands as catalysts with good results.9 Despite the efforts it is
yet not unraveled if these – or the established rhodium cata-
lyzed reactions – can be carried out in continuous flow systems
using heterogenized catalyst formulations. Such systems can
be expected to be highly desirable on an industrial scale com-
pared to homogeneous reaction systems, due to the ease of
separation of products and versatile catalyst use.
In this study, we have examined silica-supported rhodium-
phosphine and iridium-phosphine complex catalysts for con-
tinuous flow, gas-phase decarbonylation of aromatic and ali-
phatic aldehydes. Excellent results in terms of both activity
and selectivity were obtained. The best catalytic system
[Rh(dppp)2]Cl (dppp: 1,2-bis(diphenylphosphino)propane) was
found to be stable for at least 4 h with different substrates
under the examined continuous flow conditions.
1. Introduction
Selective catalytic removal of carbonyl groups from molecules
by decarbonylation is an important transformation in syn-
thetic chemistry and biology.1 In organic synthesis, for
example, an aldehyde can be utilized as an in situ CO source
for
a desired carbonylation reaction, e.g. in a tandem
approach, thus avoiding hazardous CO gas as an external
reagent.2 Selective decarbonylation constitutes also a potential
strategy for the mild upgrading of carbohydrates and furanic
compounds contained in biomass to bio-fuels and higher-
value chemical building blocks. Such a strategy can be envi-
saged to be a viable alternative in bio-refineries to current
deoxygenation methodologies relying on dehydration or
hydrogenolysis.3
The selective catalytic decarbonylation of aromatic and ali-
phatic aldehydes with rhodium complex catalysts was intro-
duced in the mid 1960s by Tsuji and Ohno using
stoichiometric amounts of the Wilkinson catalyst,
Rh(PPh3)3Cl, at room temperature.4 Later, Doughty and Pigno-
let developed a procedure for the decarbonylation of aldehydes
2. Experimental
using catalytic amounts of the Wilkinson catalyst at high temp- 2.1 Materials
eratures, typically 115–180 °C.5 Only in the 1990s was it discov-
All chemicals were purchased from Sigma-Aldrich and used
without purification unless otherwise noted. The metal com-
pounds used for catalyst preparation were prepared under an
inert atmosphere of Argon (grade 5.0, Air Liquide) as described
in the literature.8,10 Silica-90 support (BET = 340 m2 g−1; pore
volume = 0.95 cm3 g−1; pore size = 90 Å; particle size = 70–230
MESH) was calcined for 4 h at 600 °C, dried under vacuum at
110 °C for 24 h and kept under moisture free conditions prior
aCentre for Catalysis and Sustainable Chemistry, Department of Chemistry,
Technical University of Denmark, DK-2800 Kgs, Lyngby, Denmark.
E-mail: ar@kemi.dtu.dk; Fax: +45 45883136; Tel: +45 45252233
bDepartment of Chemistry, Technical University of Denmark, DK-2800 Kgs, Lyngby,
Denmark
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt01629g
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to its use. The concentrations of CO and CO2 in the effluent rate of 10 ml min−1, and the reaction pressure was 1 bar in all
were determined by a BINOS instrument placed after conden- experiments. The product mixture was condensed directly
sation of liquids in a cold trap kept at 0 °C.
below the reactor in a condenser kept at 0 °C and analyzed
offline by GC-FID using a HP 5890 Series II chromatograph
equipped with a SGE BP1 non-polar 100% dimethyl polysilox-
2.2 Catalyst preparation
The supported silica catalysts [Rh(dppp)2]Cl/SiO2, RhCl3·xH2O/ ane capillary column (50 m × 0.32 mm × 0.25 ml). Standard
SiO2, [Rh(COD)Cl]2/SiO2 and [Ir(dppp)2]Cl/SiO2 were prepared curves were used to quantify the conversion and product yield.
by impregnation using the following procedure: a 10 ml
2.5 Metal depletion analysis
Schlenk flask was charged with 1.5 ml of dichloromethane
and 1.5 ml of ethanol followed by the appropriate amount of a
metal compound to obtain the desired metal content. The
solution was then stirred for 15 min to ensure complete sol-
vation, where after 1.0 g of silica support was added and the
suspension stirred for 1 h. Finally, the volatile solvents were
removed under vacuum at 70 °C over a period of 30 min to
obtain the supported catalyst. The catalyst was kept under an
inert atmosphere of Argon until its use.
The condensed reaction mixtures were subjected to Inductively
Coupled Plasma (ICP) analysis using a Perkin-Elmer Elan 6000
ICP-MS equipped with a cross-flow nebulizer with a LOD of
1 ng kg−1 to evaluate for rhodium metal leaching from the
catalysts during reaction. Quantification of rhodium was made
using a series of rhodium standards.
3. Results and discussion
2.3 Catalyst characterization
Thermal stabilities of the catalysts were determined by thermo- The prepared supported catalysts with rhodium/iridium com-
gravimetric analysis (TGA) on a combined TGA/DSC, Mettler plexes immobilized on silica were tested for the very first time
Toledo, STARE System with a GC100 gas controller. The in continuous flow gas-phase decarbonylation of aromatic and
heating rate was 5 °C min−1 from room temperature to 260 °C, aliphatic aldehydes. When the catalysts were tested at 250 °C
where the temperature was kept for 6 h.
in runs lasting up to 4 hours the yield was found to depend on
Attenuated total reflectance infrared (ATR-IR) spectra of the the metal, the ligand employed and/or the metal loading. The
catalysts were recorded on a Thermo Scientific, Nicolet iS5 results are compiled in Table 1.
spectrophotometer with a Specac Golden Gate ATR diamond
cell. The spectra were recorded in the range of 4000–400 cm−1 aldehydes under homogeneous conditions,7 it was selected as
with a resolution of 2 cm−1
the optimal candidate for testing it in continuous flow after
Since [Rh(dppp)2]Cl is known to efficiently decarbonylate
.
The BET surface areas of the support and the catalysts were being supported on silica by a simple impregnation method.
derived from the nitrogen adsorption–desorption isotherms Firstly, the influence of metal loading from 0.3 to 3 wt% was
performed at −196 °C (liquid nitrogen) in a Micromeritics studied. As expected, the toluene yield increased with metal
ASAP 2020 pore analyzer. Prior to measurement, samples were loading from 18 to 74% after 1 h of reaction confirming the
degassed overnight by heating at 200 °C in vacuo (<10−4 Pa) to rhodium metal to be catalytically active under continuous flow
ensure a clean surface before adsorption.
reaction conditions (Table 1, entries 1 and 3). Due to the high
The structure and morphology of the catalysts were exam- reaction temperature and in order to avoid possible issues
ined by transmission electron microscopy (TEM) on a Tecnai related with the decomposition of the employed catalysts, the
G2 T20 from FEI Company (Oregon, USA) before and after de- catalytic reaction was performed at a lower temperature
carbonylation. Prior to analysis the samples were crushed into (215 °C) (Table 1, entry 2) showing low to non-activity, hence it
a fine powder and subsequently supported on plain carbon was decided to perform the decarbonylation reaction at
film coated copper grids purchased from Agar Scientific 250 °C. Then, other Rh precursors were employed containing
(Stansted, UK). Elemental analysis was achieved by energy dis- Cl− and COD (1,5-cyclooctadiene) as ligands in order to study
persive X-ray spectroscopy (EDX) using an X-MaxN detector the influence of the presence of a phosphine ligand in the
from Oxford Instruments (Abingdon, UK).
catalytic system (Table 1, entries 4 and 5). RhCl3·xH2O/SiO2
gave toluene yield of 17% (TOF = 234 h−1) after 1 h reaction
which was comparable to [Rh(dppp)2]Cl/SiO2 (Table 1, entries
2.4 Catalytic reactions under continuous gas-phase flow
In a typical experiment, 300 mg of catalyst was placed between 1 and 4), whereas [Rh(COD)Cl]2/SiO2 gave by far the lowest
two quartz wool plugs in a stainless steel, fixed-bed reactor yield of only 3% (TOF = 42 h−1). It is noteworthy that the
having an inner diameter of 7 mm. The selected substrate was RhCl3·xH2O/SiO2 catalyst was deactivated after 1 h of reaction,
introduced into the reactor by a HPLC pump diluted to 50 vol% while the [Rh(dppp)2]Cl/SiO2 catalyst with the dppp ligand
in 2-propanol or n-pentane at a flow rate of 0.05 ml min−1 via maintained its activity for at least 4 h (Table 1, entries 1 and
heated lines and evaporated before reaching the reactor. The 4). This point signifies the important role of the ligand in cata-
reactor was heated by an electric furnace to the desired reac- lyst stabilization during the course of the reaction.
tion temperature measured through a thermocouple posi-
After confirming that the best catalytic activity was
tioned inside the oven close to the catalyst bed. Helium (grade obtained with the [Rh(dppp)2]Cl/SiO2 (TOF = 246 h−1) catalyst,
5.0, Air Liquide Denmark) was used as a carrier gas with a flow the corresponding iridium catalyst was also tested in the
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Table 1 Decarbonylation of aldehydes with supported silica catalystsa
Entry
Catalyst
Metal (wt%)
Substrate
Yieldb,c (%)
Yieldd (%)
TOFb (h−1
)
1
[Rh(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
RhCl3·xH2O/SiO2
[Rh(COD)Cl]2/SiO2
[Ir(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
[Rh(dppp)2]Cl/SiO2
SiO2
0.3
0.3
3
0.3
0.3
3
3
3
3
3
p-Tolualdehyde
p-Tolualdehyde
p-Tolualdehyde
p-Tolualdehyde
p-Tolualdehyde
p-Tolualdehyde
o-Tolualdehyde
m-Tolualdehyde
p-Anisaldehyde
1-Nonanal
18
3
74
17
3
23
95
31
56
94
80
0
25
5
53
5
1
2
83
9
44
85
57
0
246
42
102
234
42
30
132
42
2e
3
4
5
6
7
8
9
10
11f
12
80
126
114
0
3
0
p-Tolualdehyde
p-Tolualdehyde
a Reaction conditions: 300 mg catalyst, 0.05 ml min−1 liquid flow (50 vol% of substrate in 2-propanol), 10 ml min−1 He as carrier gas, p = 1 bar,
T = 250 °C. b After 1 h. c Selectivity towards the decarbonylation product >99% for all tested substrates. d After 4 h. e 215 °C. f n-Pentane was
employed as the solvent.
continuous flow set-up, as such catalysts also previously have para-position were attempted and tested under the reaction
proven active for decarbonylation (Table 1, entry 6).9b Com- conditions. Unfortunately, the aromatic aldehydes with elec-
pared to the 3 wt% Rh catalyst (Table 1, entry 3) [Ir(dppp)2]Cl/ tron withdrawing groups (5)–(9) were poorly soluble in the
SiO2 showed much lower catalytic activity yielding only 23% applied solvents (i.e. 2-propanol and n-pentane) to provide
toluene (TOF = 30 h−1) after 1 h reaction. This result can be comparable data. The obtained results are summarized in
rationalized by inspection of FTIR spectra of the two catalysts Table 1 (entries 7 to 10).
measured before and after reaction, respectively, where a
When the more electronically activated substrates with
characteristic peak at ν = 700 cm−1 belonging to the ligand ortho and para substituents were applied, larger conversion
(dppp) disappeared after the reaction (see ESI, S1†). In was obtained compared to the meta-substituted tolualdehydes.
addition, the spectra showed that the phosphine was removed Previously it has been shown for homogeneous decarbonyla-
to a larger extent from the iridium center than the rhodium tion reactions with Rh-complexes that upon addition of elec-
center as the signal from the Ir sample collided with the signal tron withdrawing substituents to the aromatic ring reaction
for the pure silica.
rates are increased.7d This is further elaborated when the
The results obtained with the [Rh(dppp)2]Cl/SiO2 catalyst methyl substituent was exchanged with a methoxy group.
with p-tolualdehyde (Fig. 1, (3)) encouraged us to test the versa- However, in the current study with an immobilized catalyst the
tility of the prepared catalyst with other aromatic and aliphatic activity was apparently 25% lower for p-anisaldehyde than for
aldehydes. Thus, the analogous o- and m-tolualdehyde (1) and p-tolualdehyde resulting in TOFs of 80 and 102 h−1, respect-
(2) were also used to study steric effects on the catalytic ively. On the other hand, the higher activity towards o-tolualde-
activity. In addition, related aromatic aldehydes with both elec- hyde decarbonylation (Table 1, entry 7; TOF = 132 h−1) was
tron donor (4) and electron withdrawing groups (5)–(9) in the possibly related to steric hindrance induced by the methyl
Fig. 1 Aldehydes applied for the decarbonylation reaction.
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group in the ortho-position, thus making the expected rate- first 2 h of reaction, before the activity decreased to around
determining reductive elimination step more facile yielding 85% after an additional hour of reaction in both the cases. A
the desired product.
similar trend in activity decrease was also noticed for the other
In order to extend the scope of the continuous flow decar- tested substrates.
bonylation process and the versatility of our catalysts, an ali-
In order to determine if the loss in activity over the 4 h reac-
phatic aldehyde, namely n-nonanal (Fig. 1 (10)) was also tested tion period was related to metal depletion from the catalysts,
under the selected reaction conditions. Excellent results were representative reaction samples from n-nonanal decarbonyla-
obtained with a TOF as high as 126 h−1 (Table 1, entry 10). A tion were collected after each reaction hour and analyzed by
blank experiment was also carried out in the absence of the ICP-MS. The results revealed Rh amounts of 7, 11, 5 and
Rh complex and as expected no conversion was observed after 3 ppb, respectively, corresponding to a negligible total metal
the reaction time (Table 1, entry 12).
depletion of 0.04% of the initial Rh content. Thus, metal
To get additional information on catalyst stability the con- depletion did not cause the observed activity decay. In order to
tinuous decarbonylation were performed with the aldehyde examine the deactivation behavior more closely, the best per-
substrates for 4 h. Fig. 2 shows the product yields obtained as forming catalyst, [Rh(dppp)2]Cl/SiO2, was further subjected to
a function of time-on-stream. The best results (and catalyst TEM (Fig. 3) and EDX analysis (ESI, S2 and S3†) before and
stability) were attained in the decarbonylation of o-tolualde- after catalytic reaction. As can be seen by TEM, the porous
hyde and n-nonanal resulting in approx. 95% yield during the structure of the silica support was evident in the catalyst
before decarbonylation (Fig. 3a). EDX of the catalyst (based on
six separate measurements) revealed a homogeneous element
distribution of Rh and P on the support and a Rh content of
2.0 0.3 mol% before the reaction. This Rh loading was pre-
served (2.2 0.3 mol%) after reaction, while the P/Rh molar
ratio was found to be lowered from 3.4 0.3 before the reaction
to 2.7 0.4 after the reaction (i.e. about 20% mass loss). The
decrease in P/Rh molar ratio correlated with the activity loss
observed during catalysis as a function of time, and can thus
most likely be connected to ligand degradation under the
applied reaction conditions (250 °C). The thermal instability of
the ligand was confirmed by supplementary TGA measure-
ments performed with the [Rh(dppp)2]Cl/SiO2 catalyst (ESI,
S4†). Here a mass loss corresponding to approximately 10 wt%
was found during heating up to 260 °C, which correlated very
well with about 20% of the phosphine ligand being lost.
As is evident from the TEM pictures, the Rh-ligand complex
decomposition during catalysis resulted in the generation of
Fig. 2 Yield of products as a function of time after decarbonylation
Rh nanoparticles with an average size of 3.2 0.7 nm (Fig. 3b).
reactions with [Rh(dppp)2]Cl/SiO2 (3 wt% Rh). Reaction conditions:
300 mg catalyst, 0.05 ml min−1 liquid flow (50 vol% of substrate in The generated nanoparticles had high crystallinity with a face-
2-propanol), 10 ml min−1 He as carrier gas, p = 1 bar, T = 250 °C.
centered cubic (FCC) structure (ESI, S5†). The corresponding
Fig. 3 TEM micrographs of the [Rh(dppp)2]Cl/SiO2 (3 wt% Rh) catalyst (a) before and (b) after decarbonylation. A histogram of determined Rh nano-
particle sizes is inserted in (b).
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Fig. 4 TEM micrographs of the [Rh(dppp)2]Cl/SiO2 (3 wt% Rh) catalyst (a) before and (b) after decarbonylation reaction in n-pentane.
Fast Fourier Transformations (FFT) revealed inter planar dis- stability of the catalytic system as well as to implement the
tances of 1.96 and 2.80 Å, respectively, which are in good process for other high demanded substrates.
agreement with the values of the (200) and (110) atomic planes
for metallic Rh.10 To examine if Rh nanoparticle formation
was facilitated by 2-propanol – which is a well-known hydrogen
donor – prior to the decarbonylation reaction, and to rule out
Acknowledgements
The Danish Council for Independent Research-Technology and
Production Sciences (project no. 10-081991) is thanked for
financial support of the work. The authors also thank Assoc.
Prof. Jens E.T. Andersen (DTU Chemistry) for the ICP-MS
analysis.
a possible contribution to the observed catalytic activity, an
analogous decarbonylation experiment was performed using
n-pentane as a solvent where formation of Rh nanoparticles is
unlikely before the decarbonylation reaction. The results
employing n-pentane were similar to the ones obtained using
2-propanol as the solvent (Table 1, entries 3 and 11),
suggesting that the formation of Rh nanoparticles occurred
during the course of the decarbonylation reaction, leading to
decay in the catalytic activity with time and not prior to the
decarbonylation reaction. In Fig. 4, TEM images of the catalyst
before and after reaction are shown. It is clear that metal nano-
particles are formed during the course of the reaction as
shown in Fig. 4b. Hence, the observed catalyst instability can
be linked to catalyst decomposition leading to the generation
of metal nanoparticles and dppp ligand loss during the course
of the reaction, which in turn affects the catalytic performance
negatively.
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This work reports the first selective decarbonylation gas-phase
reaction conducted in a continuous flow setup using silica-
supported rhodium- and iridium-phosphine catalysts. Several
aromatic and aliphatic aldehydes were applied successfully in
the decarbonylation process. The catalytic performance was
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the substrates. The preservation of the intact metal-ligand
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