Rhodium(i) diphenylphosphine complexes supported on porous organic polymers as efficient and recyclable catalysts for alkene hydrogenation

Article abstract of DOI:10.1039/C6RA26104C

This paper describes the synthesis and characterization of porous polymeric materials as a support for rhodium(i) cationic coordination compounds and their use as heterogeneous catalysts for alkene hydrogenation. The synthetic strategy was the insertion of a vinyl-moiety in a bis(2-chloroethyl)amine precursor to provide highly porous resins with an enriched modifiable surface. The precursors synthesized were N,N-bis(2-chloroethyl)prop-2-en-1-amine (Alk-POL) and N,N-bis(2-chloroethyl)acrylamide (Acy-POL). The resins were obtained through suspension polymerization of methyl acrylate and divinylbenzene as a co-polymer and cross-linker, respectively. The resin surfaces were functionalized with diphenylphosphine groups followed by Rh(i) metal deposition using [Rh(COD)₂]BF₄ (COD = 1,5-cyclooctadiene) as the catalyst precursor. The Rh-catalysts were characterized by different physicochemical techniques and assessed for their catalytic performances in the heterogeneous hydrogenation of styrene and its derivatives. It was found that the catalytic activities and selectivity of the heterogenized rhodium complex (Rh-Alk-POL and Rh-Acy-POL) in the hydrogenation reactions were comparable to its homogeneous analogue. Analysis of the spent homogeneous resin Rh-Alk-POL catalyst after the first reaction cycle showed the presence of metallic Rh nanoparticles arising from the reduction of the Rh complex. Extensive recycling and Rh leaching studies were carried out for the Rh-Acy-POL catalyst. Both the activity and selectivity could be maintained for at least seven reaction runs and without metal leaching during the reaction cycles. We have also studied the liquid-phase hydrogenation reaction of various styrene m-substituted derivatives. The Rh-Acy-POL catalyst exhibits excellent catalytic activity for hydrogenation of the substrates and only vinyl-group hydrogenation was detected. Finally, the presence of electron-donating/-withdrawing substituents at the meta-position resulted in different rates of vinyl group hydrogenation. This effect was quantified in terms of the Hammett relationship, in which the catalyst displayed a linear correlation between the Hammett substituent constant (σ_meta) and the hydrogenation rate.

Full text of DOI:10.1039/C6RA26104C

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Rhodium(I) diphenylphosphine complexes
supported on porous organic polymers as efficient
Cite this: RSC Adv., 2017, 7, 3398
and recyclable catalysts for alkene hydrogenation†
a
Cristian H. Campos, Julio B. Belmar,b Solange E. Jeria,b Bruno F. Urbano,c
*
Cecilia C. Torresd and Joel B. Aldereteb
This paper describes the synthesis and characterization of porous polymeric materials as a support for
rhodium(I) cationic coordination compounds and their use as heterogeneous catalysts for alkene
hydrogenation. The synthetic strategy was the insertion of a vinyl-moiety in a bis(2-chloroethyl)amine
precursor to provide highly porous resins with an enriched modifiable surface. The precursors synthesized
were N,N-bis(2-chloroethyl)prop-2-en-1-amine (Alk-POL) and N,N-bis(2-chloroethyl)acrylamide (Acy-
POL). The resins were obtained through suspension polymerization of methyl acrylate and divinylbenzene
as
a
co-polymer and cross-linker, respectively. The resin surfaces were functionalized with
1,5-
diphenylphosphine groups followed by Rh(I) metal deposition using [Rh(COD)2]BF4 (COD
¼
cyclooctadiene) as the catalyst precursor. The Rh-catalysts were characterized by different
physicochemical techniques and assessed for their catalytic performances in the heterogeneous
hydrogenation of styrene and its derivatives. It was found that the catalytic activities and selectivity of the
heterogenized rhodium complex (Rh-Alk-POL and Rh-Acy-POL) in the hydrogenation reactions were
comparable to its homogeneous analogue. Analysis of the spent homogeneous resin Rh-Alk-POL catalyst
after the first reaction cycle showed the presence of metallic Rh nanoparticles arising from the reduction
of the Rh complex. Extensive recycling and Rh leaching studies were carried out for the Rh-Acy-POL
catalyst. Both the activity and selectivity could be maintained for at least seven reaction runs and without
metal leaching during the reaction cycles. We have also studied the liquid-phase hydrogenation reaction of
various styrene m-substituted derivatives. The Rh-Acy-POL catalyst exhibits excellent catalytic activity for
hydrogenation of the substrates and only vinyl-group hydrogenation was detected. Finally, the presence of
electron-donating/-withdrawing substituents at the meta-position resulted in different rates of vinyl group
hydrogenation. This effect was quantified in terms of the Hammett relationship, in which the catalyst
displayed a linear correlation between the Hammett substituent constant (smeta) and the hydrogenation rate.
Received 31st October 2016
Accepted 26th November 2016
DOI: 10.1039/c6ra26104c
www.rsc.org/advances
polymer resins,7,8 clay–polymer composites,9 among others.
Especially in the use of synthetic hybrid-polymeric materials,
1. Introduction
Merrield resins arise as an attractive type of support for the
covalent immobilization of coordination metal complexes
catalysts for selective hydrogenation10,11 due to their versatile
surface functionalization. The main disadvantage of Merrield
materials is its low surface area which restrict the accessibility
at the reactive moieties introduced into them as a result of the
crosslinking density of the polymer matrix.5
Hybrid materials have become an excellent alternative for use as
a support and/or catalyst in hydrogenation processes for ne
chemicals because their organic/inorganic nature confers the
ability to be used in very specic processes.1–5 Some examples of
hybrid materials are metal organic frameworks (MOFs),6 hybrid-
a
´
´
´
Departamento de Fısico-Quımica, Facultad de Ciencias Quımicas, Universidad de
In recent years, suspension polymerization is used to
produce crosslinked-polymers with a controllable pore struc-
ture and a high surface area compared to Merrield resins.12–14
These materials contain well-developed micropores and meso-
pores, therefore should decrease the diffusion limitations and
signicantly improve the accessibility to the reactive pending
groups and produce highly dispersed catalysts.15–17 In this way,
´
´
Concepcion, Concepcion, Chile. E-mail: ccampos@udec.cl
b
´
´
´
Departamento de Quımica Organica, Facultad de Ciencias Quımicas, Universidad de
´
´
Concepcion, Concepcion, Chile
cDepartamento de Polımeros, Facultad de Ciencias Quımicas, Universidad de
´
´
´
´
Concepcion, Concepcion, Chile
d
´
Departamento de Ciencias Quımicas, Facultad de Ciencias Exactas, Universidad
Andres Bello, Sede Concepcion, Autopista Concepcion-Talcahuano 7100,
´
´
Talcahuano, Chile
Canali et al.18 have reported
a
porous polystyrene/
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra26104c
polymethacrylate-based resin with supported Jacobsen's chiral
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Mn–saliciden–ethylenediamine complexes for alkene enantio- Aldrich®) were distilled at a reduced pressure prior to use: TEA
selective oxidation. Drake et al. have explored the use of vinyl- with CaH2, both MA and DVB in presence of p-tert-butylcatechol
benzyl chloride-based resins which have been aminated using as polymerization inhibitor. 2,20-Azobis(isobutyronitrile) (AIBN,
N,N,N0-trimethylethylenediamine to produce coordinated Pt(II) Aldrich®) was recrystallized from methanol. The compounds
catalysts for the hydrosilylation of 1-octene using methyldi- allyl bromide (98% Aldrich®), acryloyl chloride (99% Merck®),
chlorosilane at room temperature.19
potassium diphenylphosphide (KPPh2 solution 0.5 M in THF,
Heterogeneous metal catalysts are widely used in the Aldrich®), bis(1,5-cyclooctadiene)rhodium(I) tetrauoroborate
hydrogenation of alkenes.20,21 Liquid-phase hydrogenation of (Aldrich®), diisopropylethylamine (DIEA, 99% Merck®), bis(2-
unsaturated organic compounds with C]C bond typically chloroethyl)amine hydrochloride (98% Aldrich®), anhydrous
involve transition metals such as Pd, Pt, Rh, Ni, Ru, among sodium sulfate (Na2SO4, Merck®), hydrochloric acid (HCl, 37%
others. Palladium supported on carbonaceous materials is Merck®), sodium chloride (NaCl, Merck®), sodium hydroxide
widely used as a catalyst in this reaction.22 However, problems (NaOH, Merck®), hydroxyethylcellulose (average Mw 250 000,
in chemoselectivity are occasionally encountered when the Aldrich®), styrene (98%, Aldrich®), m-nitrostyrene (98%,
substrate contains functional groups sensitive under the reac- Aldrich®), m-chlorostyrene (99%, Aldrich®), m-methylstyrene
tion conditions of hydrogenation.23–25 To address this issue, (99%, Aldrich®), m-aminostyrene (99%, Aldrich®) and the
rhodium is the most employed noble metal as homogeneous solvent n-hexanes (PA, Merck®) were used as received. The
catalyst for alkene hydrogenation reactions because possess the Scheme 1 shows the synthetic rout employed to prepare the
ability to activate the hydrogen molecule to provide a specic immobilized catalysts.
transfer of a hydride atom to the alkene moiety during the
catalytic cycle.26,27 Phosphines molecules are used as ligands in
the hydrogenation of several types of olens by Rh catalysts4,28,29
2.2 Co-monomers synthesis
N,N-Bis(2-chloroethyl)-2-propen-1-amine
(Alk-N(CH2CH2-
and the literature contains several reports about the hydroge-
nation ability of cationic [Rh(diene)(PR3)2]+ complexes under
relatively mild reaction conditions.27,30 From this background,
we are interested in investigating the performance of diphe-
nylphosphine–rhodium complexes based immobilized on the
porous polymeric-resins in the hydrogenation of the styrene
and its derivates. The aim of the present work is the design and
synthesis of new polymeric-resins having functional moieties
able to covalently bond Rh(I) diphenylphosphine catalysts.
For this purpose we used two co-monomers derived from N,N-
bis(chloroethyl)amine: N,N-bis(2-chloroethyl)-2-propen-1-amine
(Alk-N(CH2CH2Cl)2) and N,N-bis(2-chloroethyl)acrylamide (Acy-
N(CH2CH2Cl)2) as shown in Scheme 1. In fact, the polymer
synthesis procedure including chloroethyl-monomers allows
obtaining porous polymer-supports with an improved accessi-
bility to its functionalities. Subsequently diphenylphosphine
ligands were prepared into the polymer-resins and formed
cationic rhodium(I) complexes, which were employed in
heterogeneous liquid phase styrene hydrogenation. Herein, the
best catalytic system was studied in seven recycles studies.
Finally, the catalytic hydrogenation of a series of m-substituted
styrene substrates was investigated and the catalytic data were
subjected to Hammett treatment.
Cl)2). A solution of bis(2-chloroethyl)amine hydrochloride in dry
in CH3CN solution (50 mL, 0.22 mol Lꢀ1) was slowly added
(10 min) to a dry CH3CN (10 mL) solution of the allyl bromide
1.4 mL (17 mmol) and DIEA (40 mmol). The mixture was stirred
for 12 h at room temperature. Removal of solvent led to a solid
which was dissolved in CH2Cl2 and washed with an HCl 0.1 mol
Lꢀ1 (50 mL ꢁ 2) and water (50 mL ꢁ 2). The organic layer was
dried overnight in anhydrous Na2SO4. Finally, removal of
solvent from the ltrate led to a yellowish viscous compound
with 58% yield. 1H NMR (CDCl3): d 2.83 (t, 4H, –N–(CH2–)2), 3.16
(d, 2H, –CH2–N–), 3.46 (t, 4H, –(CH2–Cl)2), 5.12 (dd, 2H, CH2]
CH–), 5.76 (m, 1H, ]CH–CH2–); 13C NMR (CDCl3): d 42.1 (s, C5
ꢂ C7), 55.9 (s, C4 ꢂ C6), 57.8 (s, C3), 118.0 (s, C1), 134.8 (s, C2).
N,N-Bis(2-chloroethyl)acrylamide (Acy-N(CH2CH2Cl)2).
A
solution of the acryloyl chloride in dry CH2Cl2 (50 mL, 0.11 mol
Lꢀ1) was added dropwise (0.5 h) to a stirred, ice-cooled solution
of bis(2-chloroethyl)amine hydrochloride (1.0 g, 5.6 mmol) with
TEA (2.0 mL, 11.2 mmol) in dry CH2Cl2 (50 mL). The mixture
was further stirred for 2 h. The reaction mixture was washed
with HCl 0.1 mol Lꢀ1 (50 mL ꢁ 2) and water (50 mL ꢁ 2). The
organic layer was dried overnight in anhydrous Na2SO4. Finally,
removal of solvent from the ltrate led to colorless viscous with
1
90% yield. H NMR (CDCl3): d 3.24 (t, 4H, –N–(CH2–)2), 3.64 (t,
4H, –(CH2–Cl)2), 5.58 (dd, 1H, CH2]CH–), 6.34 (dd, 1H, CH2]
CH–); 6.62 (m, 1H, ]CH–CO–); 13C NMR (CDCl3): d 42.7 (s, C5 ꢂ
C7), 52.9 (s, C4 ꢂ C6), 126.2 (s, C1), 131.0 (s, C2), 162.8 (s, C3).
2. Experimental section
2.1 Materials
All of the air-sensitive reactions were performed under an inert
atmosphere (Ar or N2) in a Schlenk ask. Tetrahydrofuran (THF,
2.3 Synthesis of porous polymer-resins
Merck®) and toluene (Merck®) were dried in metallic sodium/ Porous polymer-resins were synthesized via suspension poly-
benzophenone, dichloromethane (CH2Cl2, 99.8% anhydrous, merization using a modied procedure reported in detail by
Aldrich®) and acetonitrinile (CH3CN 99% Merck®) were dried Wei et al.14 The co-monomers (MA and x-N(CH2CH2Cl)2 where x:
as required by contact with calcium hydride (Sigma®) for Alk or Acy) were rst mixed well with the porogen (toluene) and
several days and then distilled immediately before use. Trie- DVB as crosslinker, in mass ratio 2 : 39 : 60, to form an organic
thylamine (TEA 95% Aldrich®), methyl acrylate (MA, 99% phase in which the initiator, AIBN, was added (1 wt%
Aldrich®) and divinylbenzene (DVB, technical grade, 80% monomers-based). The organic phase mixture was then added,
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Scheme 1 Immobilization rout for the Rh-based catalysts.
at a 1 : 3 ratio (v/v), to the aqueous phase containing 0.2% ask (100 mL) containing dry and degasied THF (50 mL).
hydroxyethyl cellulose and 20% NaCl, and tꢃhe polymerization Under dry N2, the required amount of KPPh2 was added and the
ꢃ
was allowed to proceed at 70 C and 85–90 C for 14 and 4 h, mixture stirred for 2 h. The mixture was then le stirring for
respectively. The resulting MA/x-N(CH2CH2Cl) co-polymer 48 h, at room temperature, under N2 gas. The quantity of KPPh2
materials were washed with hot water and extracted thor- used were such that a 1 : 2 molar ratio of porous polymer-resin-
oughly with acetone in a Soxhlet apparatus, and the acetone- N(CH2CH2Cl) functionality : KPPh2. The reaction was quenched
ꢃ
swollen solid were nally dried in vacuum at 80 C.
in degasied H2O (100 mL), ltered and the solids washed with
both degasied H2O and THF, respectively. Each phosphinated
resin (2.0 g) was placed in a Schlenk ask with dry and
degasied THF (20 mL) and mixed with a suitable amount of
[Rh(COD)2]BF4 dissolved in dry and degasied THF (5 mL). The
quantity of the rhodium used was stoichiometrically equivalent
2.4 Phosphination of porous polymer-resins and catalyst
synthesis
Initially the a suitable amount of MA/x-N(CH2CH2Cl) co-
polymer materials was charged into a cooled (0 ꢃC) Schlenk
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to the diphenylphosphine content of the phosphinated resin
used, and was calculated from the values detailed in Table 1.
The mixture was stirred for 48 h, at room temperature, under
N2 atmosphere. Finally the obtained materials were ltered
and repeatedly washed with THF, acetone and water, respec-
tively. The light-orange materials prepared were stored in
a desiccator under an N2 atmosphere prior to the activity
measurements. The catalysts obtained were labeled as Rh-Alk-
POL and Rh-Acy-POL for both alkylated and acylated co-
monomers precursors, respectively.
2.6 Catalytic activity
The catalytic assays of the alkenes hydrogenation were per-
formed in a stainless steel (100 mL) Parr-type semi-batch reactor
at a substrate concentration of 0.01 mol Lꢀ1 using methanol
(25 mL) as the solvent and stirring at 700 rpm under 5.0 bar of
H2 pressure at 25 ꢃC. All of the catalytic runs were conducted in
the absence of external mass transfer limitations and were
repeated three times for each experiment. The catalysts were in
the form of ne powders (>30 mm), assuming a negligible effect
of pore diffusion limitations. In the catalyst mass studies, the
alkene/Rh molar ratio was varied, the concentration of PPD was
maintained constant, and the mass of the catalyst was modi-
ed. No signicant differences in the initial reaction rates were
noted upon varying the catalyst mass over the range of 0.01 to
0.07 g because the reaction rate was proportional to the catalyst
mass, indicating that the gas–liquid and liquid–solid external
mass transfer limitations were absent.32,33 The pseudo-kinetic
constants (k) were calculated using a pseudo-rst-order kinetic
model for a batch reactor under similar conditions, as reported
in previous studies.34 The reactants and products were analyzed
by gas chromatography and mass spectrometry using a GC-FID
instrument (HP-4890) with a semi-capillary column HP-5 and N2
as the carrier gas. The recycling assays were performed by
ltering the catalyst from the reaction medium. The ltered
catalyst was washed three times consecutively with methanol
2.5 Characterization
The chemical composition of the catalysts was determined by
inductively coupled plasma optical emission spectroscopy
(ICP-OES, PerkinElmer 7300DV). The samples (10 mg) were
chemically digested by dissolution in a mixture of 15 mL HCl
and 5 mL HNO3 in an autoclave for 2 h at 150 ꢃC. The 13C- and
31P-decoupled NMR spectra were obtained on a Bruker AMX-300
spectrometer (300 MHz for 1H, 75 MHz for 13C and 121 MHz for
31P) using trimethylsilane and H3PO4 as internal standards. The
solid-state 13C and 31P CP-NMR spectra were recorded at
100 MHz and 162 MHz, respectively, uses a Bruker AV 400 WB
spectrometer. The XRD patterns were recorded in a RigakuD/
max-2500 diffractometer with the Cu Ka radiation at 40 kV
and 100 mA. The N2 adsorption–desorption isotherms at
ꢃ
ꢃ
(50 mL ꢁ 3) to clean the surface and then dried at 100 C for
ꢀ196 C were performed in a Micromeritics ASAP 2010 appa-
24 h. The activity of the catalyst was monitored by the following
conversion levels and selectivity:
ratus. The specic surface areas were determined by the BET
(Brunauer–Emmett–Teller) equation, using the adsorption data
in the relative pressure range of 0.05 to 0.3, and the pore-size
distributions were estimated using the BJH method. The cata-
lysts morphology was examined by the transmission electron
microscopy (TEM). The X-ray photoelectron spectra (XPS) were
recorded using a VG Escalab 200 R spectrometer equipped
with a hemispherical analyzer and using non-monochromatic
Mg Ka X-ray radiation (hn ¼ 1253.6 eV). The binding energies
(BE) were calculated with respect to the C-component of the
C 1s peak xed at 284.8 eV. The data analysis was performed
with the “XPS Peak” soware. The peaks were decomposed
into a sum of Gaussian/Lorentzian (G/L ¼ 90/10) aer the
subtraction of a Shirley-type baseline. The surface P/C, Cl/C, Rh/
C and Rh/P atomic ratios were estimated from the integrated
intensities of the P 2p, Rh 3p, C 1s, and Cl 2p lines aer the
background subtraction and were corrected by the atomic
sensitivity factors.31
½alkeneꢄ ꢀ ½alkeneꢄ ðmol Lꢀ1Þ
0
t
Xð%Þ ¼
ꢁ 100
½alkeneꢄ ðmol Lꢀ1Þ
0
½target alkeneꢄ ðmol Lꢀ1Þ
t
Sð%Þ ¼
ꢁ 100
½target alkeneꢄ þ ½other productsꢄ ðmol Lꢀ1Þ
t
t
where [alkene]0 corresponds to the initial concentration of the
alkene, and [alkene]t is the concentration at different times.
Target alkene corresponds to the hydrogenation of vinyl moiety
of styrene or meta-substituted styrene substrates. The turnover
frequency (TON) of the catalysts was calculated as follows:
substrate hydrogenated ðmolÞ
TON ¼
Rh ðmolÞ
3. Results and discussion
3.1 Supports and catalysts characterization
Table 1 Textural and physical properties of polymer-based supports
and Rh catalysts
Polymer supports were synthesized via the copolymerization of
DVB with the as-prepared N,N-bis(2-chloroethyl)-2-propen-1-
amine and N,N-bis(2-chloroethyl)acrylamide co-monomers,
respectively, as illustrated in Scheme 1. These compound were
inserted into the polymer matrices and then allow the synthesis
of the catalyst systems as seen in Table 1. Both systems showed
N2 adsorption–desorption isotherms with H1 hysteresis loop,
which is a typical for mesoporous structure and cylindrical pores
as shown in Fig. 1. In the same way, the catalysts isotherms
ICP
Sample
SBET (m2 gꢀ1
)
Rha (wt%)
Pa (wt%)
Rh/Pb
Alk-POL
Acy-POL
Rh-Alk-POL
Rh-Acy-POL
420
380
330
270
—
—
—
—
—
—
0.63 (1.0)
0.95 (1.0)
0.50 (0.61)
0.59 (0.61)
1.26 (1.66)
1.63 (1.66)
Nominal values in brackets. b wt% ratio.
a
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Fig. 1 N2 adsorption/desorption isotherms of supports and Rh catalysts. (A) Alk-POL based materials and (B) Acy-POL based materials.
remain a mesoporous structure of supports aer the post- carbons from the ligands and –CH3 moiety attributed to MA
modication synthesis. However, the BET specic surface area methoxyl-group at chemical shis between 20 and 60 ppm are
of polymers and catalysts gradually decrease according to the detected. The signals at 120–160 ppm can be assigned to
parameters provided in Table 1. This phenomenon is mainly DVB crosslinker aromatic carbons. Both catalysts showed an
due to the introduction of bulky –P(Ph)2 ligands and Rh complex increased contribution of the aromatics signals at 140 ppm
that partially block the pores and reduce the adsorption of N2.
attributed to the presence of diphenylphosphine ligands.
The amount of rhodium determined by ICP-OES in the Rh- Finally, the signal at 184 ppm ascribed to carbonyl contribution
Alk-POL fresh catalysts is 0.67 wt%, corresponding to 65.1 of MA is detected.
mmol of complex per gram of sample, while the catalyst Rh-Acy-
In 31P NMR characterization (Fig. 2B) for both catalysts,
POL the amount is 0.95 wt% corresponding to 93 mmol of a broad signal at chemical shis between 20 and 45 ppm is
complex per gram of sample. According to these values the observed. In the case of system Rh-Alk-POL showed two
Rh-Alk-POL system showed both lower Rh loading and Rh/P contributions, the rst one at 20.2 ppm attributed to the pres-
mass ratio. This behavior could be attributed to the uncom- ence of species of P–Rh35 and the second one at 38.4 ppm
plete chlorine substitutions during the support's modication attributed to P]O species.36,37 This result is in agreement with
exhibiting a decrease in the (Ph2)2P loading conrmed by XPS the lower metal loading detected by ICP-AES. It must be noted
(vida infra). In the same way, the Rh/P mass ratio decreases that the NMR analysis of sample Rh-Alk-POL was conducted
compared with the nominal value, most likely due to uncoor- in air atmosphere, and hence the P]O signal appears due to
dinated P moieties remaining from the catalyst synthesis. Rh- the oxidation of unreacted Ph2P groups with air to produce
Acy-POL catalyst showed the expected Rh and P contributions phosphine oxide. Rh-Acy-POL catalyst showed a signal at
indicating that the support modications were made according 27.0 ppm attributed to the coordinate P–Rh as was reported by
38
´ ´
Roman-Martınez et al. whereas the signal of P]O species do
to the nominal catalyst loading.
The 13C and 31P solid-state NMR characterization are shown not appear. These results conrm the COD ligand displacement
in Fig. 2. For all of the systems 13C NMR spectra (Fig. 2A) display during the catalyst synthesis providing the complex
the characteristic bands of the polymer backbone, –CH2 (PP)–Rh(COD)BF4 complexes on the resins surface.
Fig. 2 Solid state NMR spectra of polymer-based supports and Rh catalysts. (A) 13C CP/MAS and (B) 31P CP/MAS and rotational sidebands are
denoted by asterisks (*).
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The XPS method has been used for qualitative character-
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izations of the obtained catalysts. The binding energies for Rh
3d5/2, Cl 2p, P 2p and C 1s as well as atomic ratios of Rh/P
and Cl/C on the surface of the samples are given in Table 2.
The Rh 3d5/2 binding energies (BE) conrm the presence of
Rh(I) oxidation state.38,39 During the catalysts synthesis,
Rh(COD)2BF4 precursor was not oxidized providing the ligand
substitution pathway. In the same way, Rh-Alk-POL system
showed BE for Cl 2p at 198.2 eV attributed to the organic
chlorine contributions. In the synthesis design of Alk-POL and
Acy-POL supports,
a stoichiometric quantity of the co-
monomers (Alk-NClCl or Acy-NClCl) to produce the desire
immobilized catalysts were used. However, the Rh-Alk-POL
catalyst showed a remained a fraction of CH2–Cl on the
surface as a consequence to an uncomplete support's modi-
cation. The Rh-Acy-POL catalyst showed BE for Cl 2p at traces
level as shown in Table 2. Both results are in agreement with
ICP-AES characterization and conrm the Rh catalyst immobi-
lization on the polymer's surface.
Fig.
3
Hydrogenation of STY with Rh-based catalysts. Reaction
conditions: 25 ꢃC, 5.0 bar H2, at STY/Rh ¼ 200 mole ratio in methanol
(25 mL) at 30 min of reaction. The homogeneous catalysts were
prepared in situ adding Alk- or Acy-L into the reactor at diphosphine/
Rh ¼ 0.5 mole ratio. For the supported catalysts 100 mg of hybrid
catalyst were used. The selectivity corresponds to EBZ production.
Table 2 includes also display the surface Rh wt% and F factor
determined by XPS employing the same methodology reported
40,41
´
´
by Roman-Martınez et al.
The values of F indicate a high
dispersion degree in the location of the complex in both
supports (where F ¼ 1 correspond to the homogeneous distri-
bution of the complex in the porous materials41). These results
are in agreement with the decrease on the SBET as a conse-
quence of Rh complex anchoraged in the resins.
For both Rh supported catalysts, P 2p BE at 132.2–132.4 eV
were detected. Typically, these values are assigned to (Ph2)2P
coordinated with Rh(I) species.38 The main advantage in the
alkyl-phosphine immobilization is the P atom oxidation by
O2.36,42,43 Unfortunately, it was not possible to distinguish
between coordinated-phosphine (P–Rh) and phosphine oxide
(P]O) due to closer BE of the most intense peak P 2p at 132.0–
132.6 eV (ref. 38 and 41) for P–Rh and 132.8–133.2 eV for P]O.43
However, Rh-Alk-POL catalysts showed an Rh/P atomic ratio
lower than in the nominal content meaning that unreacted/
oxidized diphenylphopshine species are present, in agreement
with 31P solid-state NMR characterization.
without the diphenylphosphine ligand was used. As can be
observed in Fig. 3 the conversion obtained aer 30 minutes
of reaction (35%) shows the low activity for [Rh(COD)2]BF4
catalyst. Secondly, homogeneous catalyst Rh-Alk-L and Rh-Acy-L
references were prepared in situ from the diphenylphosphines-
based ligands and the [Rh(COD)2]BF4 precursor. These catalysts
showed high activity under the same reaction conditions.
However, a different selectivity on the products distribution was
detected. The STY hydrogenation could be following the reac-
tion pathways which are shown in Scheme 2. The [Rh(COD)2]
BF4 and Rh-diphosphine catalysts are highly chemoselective
towards alkene hydrogenation.44–47 The over-hydrogenated
product could be attributed to the presence of metallic Rh
nanoparticles indicating that the electronic state of rhodium in
the complex was modied during the hydrogenation
process.42,48,49 TEM analysis was employed to conrm that
3.2 Catalytic activity
Fig. 3 shows the catalytic data obtained for diphenylphosphine-
based Rh(I) catalyst at 30 minutes of reaction (see the synthesis
details in ESI†) for hydrogenation of styrene (STY). Firstly, for
comparison purposes
a reference [Rh(COD)2]BF4 catalyst
Table 2 XPS data of the immobilized Rh catalysts
Rh content
(wt%)
Rh 3d5/2 Cl 2p P 2p C 1s
(eV)
Sample
(eV) (eV) (eV) Rh/P Cl/C XPSa ICP Fb
Rh-Alk-POL 308.9
Rh-Acy-POL 309.1
198.4 132.2 284.8 0.33 0.009 1.60 0.67 2.4
Trace 132.4 284.8 0.45
—
2.95 0.95 3.1
a
b
Rh content determined by XPS.40,41 F ¼ Rh wt% (XPS)/Rh wt% (ICP).
Scheme 2 Reaction pathways in the catalytic hydrogenation of STY.
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a consequence of catalyst immobilization as was widely re-
ported.7,18,30,36,38,40,41,44,45,47 Despite this, both systems showed
a similar product distribution as their homogenous equiva-
lents. Fig. 4B shows the TEM characterization where the Rh-Alk-
POL showed the presence of metallic Rh NPs as a consequence
of the Rh-complex reduction at the reactions conditions.
3.3 Recycling test
Fig. 4 TEM characterization post-reaction. (A) Rh-Alk-L and (B) Rh-
The catalyst Rh-Acy-POL showed the best results to be employed
as recyclable catalyst. The stability of this catalyst was evaluated
under reaction conditions (180 min) performing a run without
the presence of the substrate. In this case, no leaching was
observed as shown in Table 3. In Fig. 5, the STY conversion
levels and recycling test for the Rh-Acy-POL catalyst is shown.
The catalyst is active for the STY hydrogenation reaction
reaching a conversion of 98% in 120 minutes. The conversion
data tted with pseudo-rst order's kinetics with respect to the
substrate.
The recycling studies, aer the rst run, the Rh-Acy-POL was
recovered from the reaction mixture by ltration, washed with
MeOH to remove traces of the previous mixture and engaged in
a new catalytic run. A complete conversion of the substrate was
obtained with the same selectivity to EBZ even at 7 consecutives
cycles. The post-reaction characterization showed Rh 3d5/2 and
P 2p BE similar than the fresh catalyst in agreement with the
preservation of Rh metal loading even at seven recycles. These
results probed that Rh-Acy-POL catalyst is stable and recyclable
for its use as heterogeneous system for the hydrogenation of
alkenes.
Alk-POL.
metallic Rh NPs are formed, for both homogeneous and
heterogeneous diphenylphosphine-based Rh(I) catalyst, as
shown in Fig. 4.
Only Rh-Alk-L catalysts showed the presence of metallic Rh
NPs mainly ascribed to Alk-L ligand which decrease the stability
of the Rh-complex catalysts under reaction conditions at 30
minutes compared with Acy-L. This behavior could be explained
by the nature of the N atom on the ligand, where the amide
nature of Acy-L ligand provide greater stability to the Rh-center
decreasing its reducibility under H2 atmosphere. Finally, Rh-
Alk-L catalyst showed ECX and EBZ as hydrogenation products
while Rh-Acy-L achieved the maximum selectivity to EBZ (100%).
A similar trend for the immobilized Rh-complex catalyst
was detected. Both systems showed a decrease in the activity
compared with the homogeneous catalysts which is
Table 3 XPS and ICP characterization post-reaction for Rh-Acy-POL
catalyst
3.4 meta-Substituted styrene hydrogenation
Rh 3d5/2
(eV)
P 2p
(eV)
Rh content
(wt%)
Sample
We also investigated the hydrogenation of meta-substituted
styrene molecules (m-X-STY where X: NH2, CH3, Cl or NO2) over
Rh-Acy-POL catalyst as shown in Table 4. All the kinetics showed
pseudo-rst order's kinetics with respect to the m-X-STY
substrates consumption and only produced the hydrogenation
Fresh
Used without STY
Used aer last run
309.1
309.0
308.9
132.4
132.3
132.2
0.95
0.96
0.92
Fig. 5 Reaction profiles for hydrogenation of STY and recycles studies using Rh-Acy-POL as catalyst. (A) Conversion level vs. time and (B)
recycles runs at 120 minutes. Reaction conditions: 25 ꢃC, 5.0 bar H2, at STY/Rh ¼ 200 mole ratio in methanol (25 mL).
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Table 4 Catalytic data for meta-substituted substrates hydrogenation
over Rh-Acy-POL catalyst. Reaction conditions: 25 ꢃC, 5.0 bar H2, at
m-X-STY/Rh ¼ 200 mole ratio in methanol (25 mL)
Conversiona
[ꢂ2%]
Selectivityb
[ꢂ2%]
km-x-STY
c
ꢀ1
d
Substrate
[hꢀ1 mmolRh
]
smeta
STY
68
83
76
65
60
100
100
100
100
100
29
46
39
26
22
—
0.71
0.37
m-NO2-STY
m-Cl-STY
m-CH3-STY
m-NH2-STY
ꢀ0.07
ꢀ0.16
a
b
30 min of reaction. Calculated for vinyl-group hydrogenation.
c
Pseudo-rst-order constant. d Extract from Hansch et al.50
Fig.
6
Experimental log(km-X-STY/kSTY
)
vs. smeta constants of
of alkene moiety avoiding the hydrogenation of –NO2 or
hydrodechlorination for m-NO2-STY and m-Cl-STY, respectively.
The kinetic data displayed dependence between reaction rate
and the electronic nature of the substituent in meta position.
Assuming the pseudo-rst model, catalytic rate constant
(km-X-STY and kSTY) were calculated as shown in Table 4. The
substrates. Reaction conditions: 25 ꢃC, 5.0 bar H2, at m-X-STY/Rh ¼
200 mole ratio in methanol (25 mL).
4. Conclusions
m-X-STY substrates showed the tendency km-NH -STY < km-CH -STY
<
2
3
New heterogeneous rhodium-diphenylphosphine supported
catalysts on porous resins have been prepared and character-
ized. The rhodium catalysts based in the precursor N,N-bis(2-
chloroethyl)-2-propen-1-amine was a less stable than N,N-
bis(2-chloroethyl)acrylamide and this difference was reasonably
attributed to the instability of phosphinated ligand under the
reaction conditions. As shown by TEM analysis, on the immo-
bilized N,N-bis(2-(diphenylphosphino)ethyl)-2-propen-1-amine
based catalyst, the rhodium(I) species are converted to
rhodium(0) nanoparticles preventing the selective hydrogena-
tion reaction. The amide based catalyst showed an excellent
catalytic performance during the hydrogenation of styrene
producing ethylbenzene as the only reaction product. The
catalyst was highly stable at the reactions conditions and
no-leaching was detected, even aer 7 consecutive reaction
cycles. Finally, the catalytic activity of this system for the
hydrogenation of m-substituted styrene showed a chemo-
selective hydrogenation of alkene function due to the electronic
nature of substituent group. Thus, substituted styrene
with electron-withdrawing groups showed very high activities,
while styrene derivatives with electron-donating groups pre-
sented lower activities. This catalyst proved to be active and
highly chemoselective for the hydrogenation of alkenes and
its immobilization via covalent immobilization on porous
resin support may indeed be used as a strategy leading to stable
catalysts.
kSTY < km-Cl-STY < km-NO -STY. For the electron-donating (EDG)
2
groups the catalytic rate constant showed lower values in
comparison with m-styrenes derivates with the electron with-
drawing group (EWG). With these results, a correlation analysis
between electronic nature of substrates and the experimental
catalytic constants was performed. The kinetic data was corre-
lated with Hammett smeta constant, as shown in Fig. 6, since
previous report has showed a linear correlation between
sHammett of m-substituted nitroarenes and its the catalytic
constants for hydrogenation reaction catalyzed by metallic Rh
NPs.3 The empirical values of smeta were dened by Hammett
from the ionization constants of benzoic acids as: smeta: log(Kx)
ꢀ log(KH) where KH is the ionization constant for benzoic acid
in water at 25 ꢃC and Kx is the corresponding constant for
a meta-substituted benzoic acid. The values used in our
adjustment were compiled by Hansch et al.50 The rational of this
analysis was based on the hydrogenation reaction mechanism
proposed for Halpern.51 The [Rh(diene)Ln]+ catalysts operates in
4 stages: (1) oxidative addition of H2 promoting Rh(I) / Rh(III),
(2) the substrate insertion into the coordination cloud, (3)
migratory insertion of a hydride to the alkene substrate and (4)
the reductive elimination of the hydrogenated substrate to
retrieve Rh(I) in the catalyst. It is well known that the rate
determining step (RDS) is the insertion of a hydride on the C]C
alkene moiety.26
The hydride insertion could induce a carbanion character on
C]C bond at the transition state, hence the presence of EWG
increases the affinity for the hydride, enhancing the catalytic
activity and instead EDG decrease the hydrogenation rate of the
vinyl group. A good linear correlation (R2 ¼ 0.9698) between
the experimental log log(km-X-STY/kSTY) and the smeta was found.
The following logarithmic equation describes the relationship
Acknowledgements
The authors thank CONICYT for the nancial support (FON-
DECYT postdoctoral 3140130 and FONDECYT Initiation
´
11160468). We also gratefully acknowledge the help of Dr Jose
between
km-X-STY/kSTY and the electronic nature of the
´
´
Luis G. Fierro of the Instituto de Catalisis y Petroleoquımica
(ICP-CSIC) – Sustainable Energy and Chemistry Group (Marie
Curie 2 Cantoblanco Madrid – Spain) for the XPS analysis.
substrates, showing to be a statistically accurate model with
99% condence (p ¼ 0.0072).
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