A tuneable bifunctional water-compatible heterogeneous catalyst for the selective aqueous hydrogenation of phenols

Article abstract of DOI:10.1002/adsc.201100479

A water-tolerant bifunctional heterogeneous catalyst is able to effectively catalyse the selective hydrogenation of phenol to cyclohexanone in water at atmospheric pressure and room temperature with >99.9% selectivity to cyclohexanone at phenol conversions >99.9%. The catalyst was found to be highly active and reusable, giving identical activities and selectivities after >5 uses. Moreover, this reported simple bifunctional catalyst is also able to hydrogenate a range of substituted phenols in high yields under the investigated aqueous conditions. Copyright

Full text of DOI:10.1002/adsc.201100479

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DOI: 10.1002/adsc.201100479
A Tuneable Bifunctional Water-Compatible Heterogeneous
Catalyst for the Selective Aqueous Hydrogenation of Phenols
a
a,
b,
a
Hongli Liu, Yingwei Li, * Rafael Luque, * and Huanfeng Jiang
a
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Peopleꢀs
Republic of China
E-mail: liyw@scut.edu.cn
b
Departamento de Quꢁmica Orgꢂnica, Universidad de Cꢃrdoba, Edif. Marie Curie, Ctra Nnal IV , Km 396, 14014
a
Cꢃrdoba, Spain
E-mail: q62alsor@uco.es
Received: June 19, 2011; Published online: November 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100479.
Abstract: A water-tolerant bifunctional heterogene-
ous catalyst is able to effectively catalyse the selec-
tive hydrogenation of phenol to cyclohexanone in
water at atmospheric pressure and room tempera-
ture with >99.9% selectivity to cyclohexanone at
phenol conversions >99.9%. The catalyst was
found to be highly active and reusable, giving iden-
tical activities and selectivities after >5 uses. More-
over, this reported simple bifunctional catalyst is
also able to hydrogenate a range of substituted phe-
nols in high yields under the investigated aqueous
conditions.
Traditionally, phenol hydrogenation has been car-
ried out in the gas phase, with supported Pd catalysts
as the most efficient systems for this reaction.
[3–8]
However, high temperatures are normally required
for this process (150–3008C), and the generation of
carbonaceous deposits (coking) in the course of the
reaction leads to deactivation of the catalyst. More re-
cently, liquid-phase phenol hydrogenation protocols
[
9–17]
have been the subject of numerous studies
due to
the relatively low temperatures needed for the reac-
tion to take place as compared to the energy intensive
gas-phase methodologies.
In any case, the main issue of this industrially rele-
vant process is the existing compromise between con-
version and selectivity at high conversions. Selectivity
to cyclohexanone in most cases cannot be switched to
maximise cyclohexanone production due to its further
hydrogenation to cyclohexanol (Cxnol) under the re-
Keywords: green chemistry; heterogeneous cataly-
sis; hydrogenation; metal-organic frameworks; phe-
nols
[
4,16]
action conditions.
drawback of current liquid-phase hydrogenation pro-
Cyclohexanone (Cxnone) is one of the key raw mate- tocols relates to the use of environmentally unfriendly
Another important and inherent
[
15–17]
rials utilised in the synthesis of caprolactam and solvents (e.g., dichloromethane)
and/or expensive
adipic acid, which are important intermediates for the and more sofisticated conditions (supercritical carbon
preparation of nylon 6, nylon 66, and polyamide dioxide as solvent which requires high H and CO
2
2
[
1]
[14,15]
resins. Industrially, cyclohexanone production typi- pressures, >7.0 MPa).
[
2]
cally involves either the oxidation of cyclohexane or
Recently, organic reactions in aqueous media have
the hydrogenation of phenol. The former route re- attracted a great deal of attention because water is
quires high temperatures and high pressures and gen- benign, relatively cheap, and easily available. The sep-
erates a large number of by-products. In the latter aration of products, however, should be the key con-
route, cyclohexanone is generally obtained via one- or sideration when employing water as the solvent for
two-step processes. The one-step selective hydrogena- organic reactions. Nevertheless, for relatively insolu-
tion of phenol to cyclohexanone is comparatively ble cyclohexanone and its derivatives, products could
more advantageous than the two-step procedure (via be naturally separated from water. In this regard, a
cyclohexanol) in terms of energy efficiency and capi- selective phenol hydrogenation in aqueous media will
tal costs as it combines an improved hydrogen utilisa- be highly desirable, combining the use of an ideal in-
tion while avoiding the highly endothermic dehydro- expensive solvent with a facile and straightforward
genation step from cyclohexanol to cyclohexanone.
Adv. Synth. Catal. 2011, 353, 3107 – 3113
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3107

Hongli Liu et al.
COMMUNICATIONS
separation of reagents and products from the reaction observed selectivity could be obtained at remarkably
media. mild reaction conditions including atmospheric pres-
To date, only few studies have reported aqueous sure and temperatures as low as 258C (room tempera-
phenol hydrogenation protocols, which generally pro- ture). Furthermore, various substituted phenols, in-
vided cyclohexanol/cyclohexanone mixtures (or even cluding methylphenol derivatives, p-dihydroxyben-
only cyclohexanol) instead of being completely selec- zene, and naphthol, could also be effectively hydro-
[
9–13]
tive to cyclohexanone.
The combination of an genated to their partial hydrogenation products under
aqueous highly selective protocol (>95% Cxnone) at mild conditions.
high phenol conversions (>80%) under “green” and
MIL-101 was selected as support owing to its many
mild conditions that can be catalysed by a water-toler- outstanding physical and chemical properties. It has a
ant, active and reusable heterogeneous system in one- large specific surface area and pore size as well as
step could have an important impact on the produc- good chemical resistance to water and organic sol-
tion of cyclohexanone from phenol hydrogenation.
vents, a highly desirable and most promising combina-
[
20–22]
Many heterogeneous catalysts have been screened tion for catalytic applications.
In addition, MIL-
for the selective hydrogenation of phenol. These in- 101 possesses numerous potentially unsaturated chro-
[
13,15]
[7,8]
[10]
[23]
clude Pd/C,
MgO,
Pd/Al O ,
Pd/hydrophilic C, Pd/ mium sites,
which could provide Lewis acid sites
2
3
[
6,8]
[16]
and mesoporous Ce-doped Pd. The nature upon removal of the terminal water molecules. The
of the catalyst support was reported to have a strong deposition of metal nanoparticles on MIL-101 could
impact on product selectivity. However, the surface potentially render a bifunctional catalyst which com-
requirements for a high cyclohexanone selectivity bines a high acidity with highly dispersed metallic
have not been well established in the literature to sites able to effect hydrogenation/dehydrogenation.
date, and many of the existing reports are even con-
tradictory. While some authors have suggested that a the reported procedures.
MIL-101 was synthesized and purified according to
[
19]
Supported Pd catalysts
support and/or modifier with acidic properties is cru- were prepared via simple impregnation of activated
[
15]
cial for cyclohexanone formation,
others suggest MIL-101 using Pd ACHTUNTGNERNUG( NO ) as precursor (Pd solution in
3 2
that the basicity of the support is essential to achieve acetone; see Supporting Information), in a similar
[
8]
[21]
high cyclohexanone yields.
way to that previously reported. Different Pd load-
More recently, a combination of a supported Pd ings were utilised for the preparation of a series of
catalyst (Pd/C, Pd/NaY) and a solid Lewis acid Pd/MIL-101 materials, namely 1, 3, 4, 5 and 6 wt%
(
AlCl , SnCl ) has been reported for the liquid-phase Pd. A novel impregnation methodology for the MIL-
3 2
hydrogenation of phenol with very high cyclohexa- 101 Pd nanoparticle decoration was also developed
[
15]
none selectivity at complete conversions. Neverthe- (see Supporting Information for more details) so that
less, the employed Lewis acids (e.g., AlCl ) are highly originally prepared Pd-MIL-101 catalysts using the
3
[
21]
moisture sensitive and hence demand moisture-free protocol described in ref. have also been prepared
and environmentally unfriendly organic solvents (e.g., for comparative purposes.
dichloromethane), reactants and anhydrous catalysts
as well as also a dry atmosphere for their handling.
No apparent loss of crystallinity in powder X-ray
diffraction (XRD) patterns (Supporting Information,
[
18]
A better understanding of these contradictions Figure S1) was found upon Pd incorporation on MIL-
prompted us to perform a series of investigations on 101 regardless of the metal content in the materials,
structure-reactivity relationships with the aim to pro- indicating that the framework of MIL-101 was well
vide a definite insight into the reaction mechanism of preserved under the investigated conditions. N ad-
2
phenol hydrogenation. These studies were subse- sorption results further confirmed the stability of the
quently shown to offer the possibility for the design porous structure after metal doping (Supporting In-
of water-tolerant efficient heterogeneous catalytic sys- formation, Table S1 and Figure S2).
tems based on a synergism between acid and metallic
TEM micrographs showed the materials had mostly
sites on a porous surface which can provide unprece- a monomodal distribution of Pd particles, with nano-
dent yields of cyclohexanone in the aqueous hydroge- particle sizes typically between 2–3 nm (Supporting
nation of phenol (similar to that typical of bioprocess- Information, Figure S3 and Figure S4). In any case,
ing conditions).
the observed nanoparticles were homogeneously dis-
Herein, we show that excellent Cxnone yields (> persed in the samples and no significant formation of
9.9% to cyclohexanone at >99.9% phenol conver- aggregates was observed regardless of the Pd content.
9
sion) can be achieved using a unique water-tolerant This is believed to be related to the high porosity and
bifunctional heterogeneous catalytic system based on surface area of the support.
palladium nano-sized particles supported on a highly
The actual Pd contents of materials were close to
acidic metal-organic framework (MOF) such as chro- those in theory prepared (Supporting Information,
mium terephthalate MIL-101 (MIL, Matꢅrial Institut Table S1). XPS measurements of the Pd/MIL-101 ma-
[
19]
Lavoisier) {Cr
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(F,OH)O
A
H
U
G
R
N
U
G
A
H
U
G
R
N
N
(CO )] }.
The terials have also been included in the Supporting In-
3
2
6
4
2
3
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A Tuneable Bifunctional Water-Compatible Heterogeneous Catalyst for the Selective Aqueous Hydrogenation
Table 1. Surface acidity of Pd/MIL-101 materials measured at 2008C using a chromatographic methodology.
À1
À1
[a]
À1
Catalyst
PY [mmolg ]
DMPY [mmolg ]
Lewis acidity [mmolg ]
Parent MIL-101
2020
2058
1992
1220
1530
1082
970
33
47
31
43
76
51
35
1987
2011
1961
1177
1454
1031
935
1
3
5
6
5
5
% Pd/MIL-101
% Pd/MIL-101
% Pd/MIL-101
% Pd/MIL-101
% Pd/MIL-101 reused 5 times
% Pd/MIL-101-AMINE GRAFTED
[b]
Al-MCM-41
Beta zeolite (SiO /Al O =25)
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(Si/Al=20)
450
956
746
200
397
380
250
559
366
2
2
3
ZSM-5(SiO /Al O =30)
2
2
3
[
[
a]
b]
Lewis acidity values can be obtained as difference between PY (total acidity) and DMPY (Brçnsted acidity) data.
The thermal stability of the amine species was reported to be stable at least up to 2208C in ref.
[23]
formation, Figure S5 to Figure S8. Several interesting provided the highest cyclohexanone yield (with a high
observations could be drawn from the experimental selectivity) (Supporting Information, Table S3).
results. Firstly, and most importantly, the presence of
However, Pd/MIL-101 prepared by using a previ-
[
21]
Pd(0) metal in the materials as indicated by the bands ously reported procedure was not as efficient in the
[
24]
at ca. 335–336 eV, typical of Pd metal
was con- hydrogenation reaction (Supporting Information,
firmed. Secondly, Cr(2p) spectra exhibited asymmet- Table S3) as it was in Suzuki and Ullmann cou-
[
21]
ric bands in the 574–580 eV range [Cr(2p) ] and 585– plings.
A 5%Pd-MIL-101 catalyst prepared using
3/2
5
90 eV [Cr(2p) ] which could be deconvoluted in 2 this methodology provided a poor 25.8% phenol con-
1/2
main components attributed to Cr
hedral coordination (small contribution <15% total (Table 2, entry 10). These results highlight the effi-
Cr, ca. 577 eV) and pentacoordinated Cr(III) (main ciency of the newly developed recipe (which features
ACHTUNGERTNNUNG( III) species in octa- version with a 99.6% selectivity to cyclohexanone
AHCTUNGTRENNUNG
component >70%, ca. 578 eV), respectively. All find- a dropwise careful addition of the palladium precur-
ings were in good agreement with the original report sor solution and an intermediate ultrasonication step,
from Ferey et al. but in this case the coordination of see Supporting Information) in the preparation of
the Cr ACHTUNGTRENNUNG( III) species and availability of the supported highly active catalysts for phenol hydrogenation.
Pd nanoparticles will be critical for the catalytic activ-
ity of the materials.
Table 2 summarises the results of the optimum
5%Pd/MIL-101 material on the aqueous hydrogena-
The acidity of the materials was also determined tion of phenol under different conditions. Blank runs
using a previously reported pulse chromatographic and those using the parent MIL-101 gave no conver-
[
25]
methodology as well as PY-DRIFTs. Data included sion in the systems (Table 2, entries 1 and 2), confirm-
in Table 1 demonstrate that the Pd/MIL-101 materials ing as expected the need of a metal to perform the
possess an unprecendently high Lewis acidity, which hydrogenation of phenol. Comparatively, a complete
was found to decrease in general with an increase in conversion of phenol could be achieved within 11 h
the Pd content in the materials. In any case, this acidi- (>99.9% Cxnone selectivity) at ambient conditions
ty was remarkably superior to that of other acid cata- (1 bar H and 258C) in neat water as indicated in
2
lysts including Starbonꢆ acids, mesoporous aluminosi- Table 2 (entry 3). Such a high activity (with >99.9%
licate materials and/or zeolites (Table 1 and Support- selectivity at the same time) in the liquid-phase hy-
ing Information), which have been previously report- drogenation of phenol at atmospheric pressure and
ed as highly active catalysts in various acid-catalysed room temperature in water constitutes a unique find-
[26,27]
processes.
ing for aqueous heterogeneously catalysed protocols.
In view of the interesting bifunctionality of the ma- We became aware of similar results from other labo-
[28]
terials, Pd/MIL-101 catalysts with different Pd load- ratories while this manuscript was under review. It
ings were subsequently screened in the hydrogenation is worth mentioning that although Wang et al. report-
of phenol in water (Supporting Information). Pd/ ed a selectivity of 96% at >99% phenol conversion
MIL-101 materials were recently demonstrated to be under similar conditions, the completion time (72 h)
highly active and selective in Suzuki and Ulmann cou- was much longer than that achieved in our study
[
21]
plings of aryl chlorides in aqueous media. Results (Table 2, entry 3).
of the phenol hydrogenation pointed to an optimised
performance of 5%Pd/MIL-101 in the reaction, which alyst in this reaction.
Commercial Pd/C has been widely employed as cat-
[
10,13,15]
In this study, a commercial
5% Pd/C (Alfa Aesar) was also tested under identical
Adv. Synth. Catal. 2011, 353, 3107 – 3113
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Hongli Liu et al.
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[a]
Table 2. Results of the hydrogenation of phenol in water media.
[b]
Entry
Catalyst
PH2 [MPa]
T [8C]
t [h]
Conversion [%]
Selectivity [%]
C=O
OH
1
2
3
4
5
6
7
8
–
0.1
0.1
0.1
0.1
0.1
0.5
1.0
0.1
0.1
0.1
25
25
25
25
30
30
30
40
25
25
11
11
11
11
10
6
3
7
11
11
–
–
–
–
–
–
–
5.4
–
–
–
0.9
–
0.4
MIL-101
Pd/MIL-101
Pd/C
Pd/MIL-101
Pd/MIL-101
Pd/MIL-101
Pd/MIL-101
1%Pd/MIL-101
Pd/MIL-101
>99.9
48.5
>99.9
>99.9
>99.9
>99.9
3.1
>99.9
94.6
>99.9
>99.9
>99.9
99.1
[
c]
[
9
1
>99.9
99.6
c]
0
25.8
[
[
[
a]
Phenol (1.0 mmol), 5 wt% Pd/MIL-101 (5 mol% relative to phenol), H O (4 mL).
2
b]
C=O denotes cyclohexanone; OH, cyclohexanol.
c]
[21]
Pd@MIL-101 prepared following the previously reported procedure in ref.
reaction conditions for comparative purposes. The ob- included in Figure 1 (see also Supporting Information,
served conversion of phenol for Pd/C was remarkably Table S4) indicate that no efficiency loss was observed
inferior (below 50%), with a considerable production in the phenol hydrogenation for up to five runs. These
of cyclohexanol (>5%), under the investigated condi- findings were in good agreement with AAS experi-
tions as compared to the water-tolerant Pd/MIL-101 ments on the reaction mixtures for which no Pd
catalyst (Table 2, entry 4).
traces (<0.5 ppm) were detected in solution as well
An increase in hydrogen pressure or reaction tem- as with the TEM experiments of the catalyst after the
perature could significantly increase the rates of reac- fifth run which showed a minimum aggregation of
tion and reduce the time to reaction completion to a particles in the materials after subsequent uses (Sup-
few hours at >99.9% cyclohexanone selectivity (3 porting Information, Figure S9).
and 6 h at 10 and 5 bar H , respectively; Table 2, en-
The scope of the presented methodology was then
2
tries 5–7). The systems also reached completion in subsequently extended to the aqueous hydrogenation
shorter times of reaction (typically 4-7 h) at higher of a series of substituted phenols under similar reac-
temperatures (40–508C) but the selectivity to cyclo- tion conditions using the optimum 5%Pd/MIL-101
hexanone was slightly reduced to 99% (Table 2, catalyst. The results summarised in Table 3 prove that
entry 8).
the reported protocol was also amenable to the ring
After the hydrogenation reaction (1 bar H , 258C), hydrogenation of various substrates with high activi-
2
5
%Pd/MIL-101 was separated from the reaction mix- ties and excellent selectivities towards partial hydro-
ture by centrifugation, thoroughly washed with etha- genation products. The hydrogenation of methylphe-
nol and water and reutilised as catalyst in subsequent nol derivatives provided methylcyclohexanones in
runs under identical reaction conditions. The results high yields while the most interesting dihydroxyben-
zene derivative yielded
a 93.8% selectivity to
hydroxycyclohexanone at complete conversion of the
AHCTUNGTRENNUNG
starting material (Table 3, entries 1–5).
The hydrogenation of naphthol provided a relative-
ly good selectivity (ca. 91%) for the hydrogenation of
the non-hydroxylated ring (Table 3, entry 6), possibly
due to the differences in the resonance stabilization
energy between the transition states, in agreement
[
10,14]
with previous literature reports.
In view of the practical applicability of the proto-
col, the next obvious step was to investigate the
mechanism of the reaction in water. Liu et al. recently
proposed a two-step mechanism for the dual activa-
tion in phenol hydrogenation and further inhibition of
the Cxnone hydrogenation to Cxnol by the coordina-
tion of the Lewis acid to the carbonyl group of
Figure 1. Reuses of the 5% Pd-MIL-101 catalyst in the hy-
drogenation of phenol. Conditions: phenol (1.0 mmol),
[
15]
5
0
wt% Pd/MIL-101 (5 mol% relative to phenol), 258C, PH2
=
Cxnone. The addition of a Lewis acid was claimed
to be essential for the observed remarkably superior
.1 MPa, t=11 h, H O (4 mL).
2
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A Tuneable Bifunctional Water-Compatible Heterogeneous Catalyst for the Selective Aqueous Hydrogenation
[a]
Table 3. Hydrogenation of hydroxy-aromatic derivatives with 5 wt% Pd/MIL-101.
Entry
1
Substrate
T [8C]
t [h]
Conversion [%]
Selectivity [%]
9.4
35
7
>99.9
90.6
2
3
4
5
6
35
35
35
25
35
7
96.5
95.3
4.7
0
7
50.8
>99.9
14
7
85.0
97.3
93.8
9.1
2.7
6.1
90.9
>99.9
>99.9
7
[
a]
Substrate (1.0 mmol), 5 wt% Pd/MIL-101 (5 mol% relative to substrate), H O (4 mL), P (0.1 MPa).
2
H2
activity and selectivity to Cxnone in dichloromethane coordinatively unsaturated site and interaction of the
À
and compressed CO2.
other edge (upon protonation) with anionic [PdCl ]
4
[23]
In principle, our reported results seemed to agree followed by reduction to Pd(0) nanoparticles. The
with their proposed mechanism but further investiga- application of the grafting method to our system ren-
tions were needed to complete these studies due to dered materials with no hydrogenation activity under
the use of a polar reaction medium such as water in identical conditions to those previously optimised
this process which could somehow affect the acidity (Supporting Information, Table S5) in spite of the
of the material in the reaction.
only slightly reduced acidity of the grafted materials
Two simple investigations were ultimately per- as compared to untreated Pd/MIL-101 (measured by
formed with this purpose. Firstly, pyridine (PY) was PY titration and PY-DRIFTs, Table 1). This reduction
added in different quantities (0.32 and 1.6M) to the in acidity confirms the successful grafting of ethylene-
catalyst with the aim to theoretically reduce/suppress diamine onto some coordinatively unsaturated Cr ACHTUNGTRENNUNG( III)
the acidity of the materials via coordination to the sites of the MIL in good agreement with FT-IR data
chromium Lewis sites of the catalyst. This PY addi- (Supporting Information, Figure S12).
tion indeed led to inferior activities (reduced conver-
sion of phenol) in the hydrogenation catalysed by and Pd(3d) in these materials showed that both
%Pd/MIL-101 catalyst (Supporting Information, Cr(2p) spectra and Pd(3d) spectra were significantly
Nevertheless, a closer look to the XP spectra of Cr
5
Table S5). FT-IR and XPS measurements of the mate- different and less intense. Deconvolution of Cr(2p)
rials with different PY contents showed that this basic spectra showed the formation of an additional band
molecule was coordinated mostly on the Cr Lewis at remarkably reduced binding energies (~10% total
sites of the MIL-101 as demonstrated by the decrease Cr content, ca. 574.3 eV) which might be due to the
in acidity observed in the materials and the increase small contribution of grafting of both amine groups in
in the proportion of Cr ACHUTNGREUNNG( III) hexacoordinated species EN to neighbouring Cr ACHTUNGTRENNGNU( III) coordinatively unsaturat-
as compared to the unmodified material (XPS, Sup- ed sites (Supporting Information, Figure S13). The
porting Information, Figure S10). Metallic Pd sites re- first thing that could be observed in the Pd(3d) spec-
mained unaltered as shown in the Pd(3d) XP spectra tra was a surprising reduction of the intensity of the
of the PY-5%Pd/MIL-101 (Supporting Information, peaks (Figure S14), accompanied by the presence of
Figure S11). This reduction in activity could in theory a well defined band corresponding to N(1s) in the
account for the observed decrease in Lewis acidity.
survey spectra. A quantitative estimation of the Pd
In addition to this experiment, ethylenediamine content by XPS showed a striking 0.1% Pd in the sur-
EN) was grafted on the Pd/MIL-101 using a previ- face of amine-grafted materials as opposed to a 4.8%
(
ously reported amine grafting method (see Supporting Pd measured in the untreated Pd/MIL-101 (more
[
23]
Information and Figure S12). Hwang et al. original- than 10 times less Pd). This unusual observation can
ly reported this methodology to stabilise the immobi- only be explained by the grafting and/or coverage of
lisation of palladium nanoparticles on MIL-101 via the Pd nanoparticles by the added EN that could po-
grafting of one amine group from EN onto a Cr AHCUTNGRNENUG( III) tentially render the Pd particles not accessible to the
Adv. Synth. Catal. 2011, 353, 3107 – 3113
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Hongli Liu et al.
COMMUNICATIONS
reactants on the catalyst surface. These hypotheses for 4 h, followed by treatment in a stream of H
correlate well with the negligible activity observed
at 2008C
2
for 2 h to yield Pd/MIL-101.
for-amine grafted materials, which may have sufficient
acidity to activate the phenol ring via electrophilic ar-
General Procedure for the Hydrogenation of Phenols
omatic substitution but a complete lack of any acces-
Hydrogenation of phenol was performed in a 20-mL Teflon-
lined stainless steel autoclave. Typically, phenol (1.0 mmol),
sible Pd metal sites to activate H and carry out the
2
first and quick hydrogenation step (hence no hydroge- catalyst (0.05 mmol of Pd), and 4 mL of water were loaded
nation activity). into the reactor. The autoclave was sealed, cooled to 08C by
to
From these simple studies, we can conclude that placing it in an ice bath, and purged three times with H
2
remove the air. Then the reactor was pressurized with H2
the mechanism by Liu et al. is reasonably plausible as
well as completely independent of the polarity and
nature of the reaction media in which the hydrogena-
tion is carried out (i.e., organic solvents, compressed
gases and water). A suppression of the Lewis acidity
in the materials reduces the efficiency of the systems
and loaded in an oil bath, which was preheated to the target
temperature. After the reaction, the autoclave was cooled to
0
8C in an ice bath. The gaseous products were collected
with a gas bag, and subsequently analyzed by GC. The cata-
lyst was separated from the solution by centrifugation, and
then washed with water and appropriate organic solvent.
The organic phase was subsequently extracted with the ap-
(
which however remain active), while making unavail-
able the metal in the reaction renders completely in- propriate organic solvent, dried over Na SO , and concen-
2
4
active catalysts. The activation of H and consequent- trated under vacuum. The reaction products were quantified
2
ly the first partial hydrogenation step of the benzene and identified by GC-MS analysis.
[29]
ring to an enol has clearly been confirmed as rate-
determining step in the liquid-phase hydrogenation of
phenol. Furthermore, the water compatibility and sta-
bility of this MIL-101 material could make possible
its utilisation under typical aqueous fermentation con-
ditions. The catalyst also shows a great potential for
other catalytic applications due to its simplicity, effi-
ciency, stability, reusability and ability to work at very
Acknowledgements
This work was supported by NSF of China (20803024,
2
0936001, and 21073065), the Doctoral Fund of Ministry of
Education of China (200805611045), the Guangdong Natural
Science Foundation (S2011020002397 and
mild reaction conditions. Current investigations in our 10351064101000000), the Fundamental Research Funds for
laboratories show that the high conversions and selec- the Central Universities (2009ZZ0023, 2011ZG0009), the
tivities to Cxnone in the hydrogenation of phenol can Guangdong Provincial Engineering Research Center for
Green Fine Chemicals and the program for New Century Ex-
cellent Talents in Universities (NCET-08-0203). R.L. grateful-
ly acknowledges support from the Spanish MICINN via the
concession of a RyC contract (RYC-2009-04199) and funding
under projects P10-FQM-6711 (Consejeria de Ciencia e In-
novacion, Junta de Andalucia) and CTQ2011-28954-C02-02
also be obtained under microwave irradiation at sig-
nificantly reduced times of reaction (typically 1 hour)
under similar conditions. We further envisage our
methodology extended to the aqueous hydrogenation
of many other important chemicals including some of
the top biomass-derived bio-platform molecules and
other chemistries such as the production of other
small molecules (amides, imides, etc.) and polymers
as well as to related petrochemical processes of high
interest at milder conditions (e.g., low temperature
hydroisomerisation of n-alkanes for the production of
branched alkanes to increase the octane number of
gasolines).
(
MICINN).
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