Heteropolyacids embedded in a lipid bilayer covalently bonded to graphene oxide for the facile one-pot conversion of glycerol to lactic acid

Article abstract of DOI:10.1039/c7ta01334e

A new two-step strategy for the functionalization of graphene oxide (GO) with heteropolyacids (HPAs) and the catalytic activity of the so-obtained materials in the conversion of glycerol to lactic acid (LA) are reported. Covalent bonding of a a-NH-(CH₂)₂-NH₂⁺-CH₂(CH₂)₈CH₃ lipid-like bilayer to the GO surface is followed by embedding of HPAs to give HPMo@lipid(n)/GO hybrid materials, where n represents the length of the diamine carbon chain (n = 2, 4, 6, 8, and 10). The HPAs are tightly surrounded by the lipid bilayer through electrostatic interactions with the protonated amine groups and are highly resistant to environmental changes and leaching from the GO. The hydrophilicity of HPAs and GO and the hydrophobic properties of the lipid bilayer can be easily controlled by changing the length of the alkyl chain, and the redox potential varies as the distance between HPAs and GO varies. The materials show excellent catalytic activity in glycerol cascade conversion to LA, with HPMo@lipid(4)/GO achieving the highest reported efficiency to date, with 90% yield at 97% conversion under mild conditions (1 M glycerol, 60 °C, 3.5 h, and 10 bar O₂). Such high efficiency is attributed to the combination of suitable redox potentials, balanced hydrophilic and hydrophobic properties, and the capillary-like reactor formed by the wall of the lipid bilayer, which enhances the adsorption of O₂ and glycerol around catalytic active sites. HPMo@lipid(4)/GO functions in the absence of an organic solvent and base, with a high concentration of glycerol (9.2 wt%) and in the presence of methanol and other impurities from biodiesel production, with crude glycerol giving almost 87% yield. No structural changes or leaching of HPAs from GO occurs during the reaction or with recycling up to fifteen times.

Full text of DOI:10.1039/c7ta01334e

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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Heteropolyacids Embedded in Lipid Bilayer Covalent to Graphene
Oxide for Facile One-pot Conversion of Glycerol to Lactic acid
M. Tao,a,b N. Sun,aY. Li,a T. Tong,aM.Wieliczko,b S. Wangaand X. Wanga*
Received 00th January 20xx,
Accepted 00th January 20xx
A new two-step strategy for functionalization of graphene oxide (GO) with heteropolyacids (HPAs) and the catalytic activity
of the so-obtained materials in the conversion of glycerol to lactic acid (LA) are reported. Covalent bonding of a-NH-(CH2)2-
DOI: 10.1039/x0xx00000x
www.rsc.org/
+
NH2 -CH2(CH2)8CH3 lipid-like bilayer to the GO surface is followed by embedding of HPAs to give HPMo@lipid(n)/GO hybrid
materials, where n represents the length of the diamine carbon chain (n = 2, 4, 6, 8, 10). The HPAs are tightly surrounded
by the lipid bilayer through electrostatic interactions with the protonated amine groups and are highly resistant to
environmental changes and leaching from GO. The hydrophilicity of HPAs and GO and the hydrophobic properties of the
lipid bilayer can be easily controlled by changing the length of the alkyl chain, and the redox potential varies as the
distance between HPAs and GO. The materials show excellent catalytic activity in glycerol cascade conversion to LA, with
HPMo@lipid(4)/GO achieving the highest reported efficiency to date, with 90 % yield at 97 % conversion under mild
conditions (1 M glycerol, 60 °C, 3.5 h, 10 bar O2). Such high efficiency is attributed to the combination of suitable redox
potential, balanced hydrophilic and hydrophobic properties, and the capillary-like reactor formed by the wall of the lipid
bilayer, which enhances the adsorption of O2 and glycerol around catalytic active sites. HPMo@lipid(4)/GO functions in the
absence of organic solvent and base, with high concentration of glycerol (9.2 wt%) and in the presence of methanol and
other impurities from biodiesel production, with crude glycerol giving almost 87 % yield. No structural changes or leaching
of HPAs from GO occurs during the reaction or with recycling up to fifteen times.
substrates to increase reaction rates in catalysis. Accordingly,
graphene materials, especially chemically-modified graphene
oxide (GO), serve as ideal platforms for anchoring various
active sites, either through strong π-π interaction or chemical
Introduction
The limitations of fossil fuels, including declining reserves and
the impact of their use on climate change, have recently led to
increased focus on the improved utilization of renewable
feedstocks, including biomass-derived molecules.1 One of the
main challenges is the rational design of catalytic materials
with significantly enhanced activity, selectivity and stability in
biomass transformation. Within this realm, graphene materials
have attracted much attention because they can be used not
only as catalyst supports, but also as metal-free catalysts to
increase conversion rate and selectivity.2 Graphene, which
consists of a one-atom-thick planar sheet with an sp2-bonded
carbon structure, exhibits
(theoretically 2630
graphene),3extraordinary electron transport capabilities,4 and
excellent thermal and electrical conductivities.5 This endows
graphene materials with enhanced adsorption ability towards
bonds, including metal nanoparticles, Brønsted-acidic groups,
or ionic liquids. GO or modified GO exhibit activity in hydrolytic
cleavage of cellulose, dehydration of sugar, dehydrogenation
and oxidative depolymerization of lignin as acid or redox
catalysts.6 The synergistic effect between active sites and GO
can promote reactivity, but efficient biomass oxidation
catalyzed by graphene materials has rarely been reported due
to the complexity and variable molecular structures of
components of biomass, aqueous reaction medium, and harsh
reaction conditions.7 There has been only one report on the
conversion of glycerol to lactic acid (LA) by CuPd-Graphene,
which gave 56.2 % conversion of glycerol to produce 49.5 % LA
at 140 ºC for 16 h under basic conditions.8Glycerol is a major
side-product in biodiesel industry with ca. 3×106 tons
produced in 2014. Impurities in crude glycerol render it
problematic in most applications.9 Production of LA from crude
glycerol without purification is of great economic interest.
Direct conversion of crude glycerol can be achieved via
fermentative processes,10 but only a limited number of
microorganisms are tolerant to complex impurities in the
crude mixture.11 Chemical conversion of crude glycerol is used
to produce hydrogen and syngas, while direct transformation
a
large specific surface area
m2/g
for single-layer
a.Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal
University, Changchun 130024, P. R. China
*E-mail: wangxh665@nenu.edu.cn Address here.
bDepartment of Chemistry, Emory University, Atlanta, Georgia 30322, USA
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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into valued-added products is lacking.11 There has been only the support material beyond immobilization need to be
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DOI: 10.1039/C7TA01334E
one report on chemical transformation of crude glycerol to LA studied further.
using homogeneous Ir complex as catalyst under alkaline
In the course of our research, we aimed to combine the
condition with 98 % conversion and 96 % LA selectivity at 130 benefits of GO and HPAs to fabricate a new HPA/GO hybrid
ºC for 24 h.12 Significant challenges remain in the design of with some advantages in production of LA. First, our synthetic
more selective and highly active catalysts based on graphene strategy differs from prior reported routes for HPAs/GO which
and GO to improve reactivity for applications in bio-based use redox methods,20 and instead heteropolyanions are
industry, especially under mild reaction conditions in green immobilized in a lipid-like bilayer which is covalently bound to
solvents such as water.
graphene oxide. Second, HPMo maintains its negative charge
Heteropolyacids (HPAs), also called polyoxometalates without being reduced by oxidative graphene, and retains its
(POMs), with turnable Brønsted- or Lewis-acidity and redox higher redox capacity. Meanwhile the functional groups of the
properties, have been demonstrated to display outstanding lipid-like bilayer on the GO surface can enhance adsorption of
catalytic performance in a wide range of acid-, base- or redox- oxygen or glycerol molecules and concentrate them around
catalyzed reactions.13 Thus far,HPAs have only been used as HPMo. Furthermore, such a lipid-like layer imparts stability
single acidic or redox catalysts separatelyin organic towards pH and temperature changes and improves the
transformations. Cascade or tandem reactions, where two or resistance of HPA against leaching.21 Third, the bilayer may
more individual reactions are often carried out in one pot, elicit a so-called “microenvironment effect,” with hydrophilic
have attracted much attention, as they can maximize spatial and lipophilic surroundings serving as
a
biomimetic
and temporal productivity with consumption of minimal environment for improvement of the activity by positioning
resources.14 The most important cascade transformation in substrates towards target products.22 The benefits to be
biomass utilization is the oxidation of glycerol directly to LA.15 gained from these heterogeneous catalysts include lower
LA can be produced through biological process-fermentation catalyst loading, decreased leaching of active sites from the
of sugars, albeit slowly, requiring two to four days to achieve support, promotion of HPA electron-transfer by graphene
profitable yields.16 Alternatively, high-efficiency glycerol oxide, and enhanced adsorption of substrate and oxygen.
conversion has been achieved only under hydrothermal
Herein, we report a new strategy for immobilizing HPAs into
conditions (high temperature and pressure) requiring a large a lipid-like bilayer that is covalently anchored to GO. HPMo
excess of inorganic hydroxides without any recycling. The few molecules surrounded by this lipid-like bilayer were
reports on glycerol conversion to LA without added base energetically stable on the GO surface. In addition, such a
employ noble metals (mostly Pt and Au) as catalysts combined HPMo-functionalized nanocatalyst facilitates the aerobic
with Lewis acid supports.17 However, even using noble metals oxidation of glycerol with superior activity, as measured by
as catalysts at high temperature or using long reaction times TOF of 257 h-1 (based on glycerol conversion over 3.5 h)
did not result in satisfactory efficiency due to the difficulty in compared to our previous results.19 Furthermore, the
conversion of glycerol to LA without base.18 To the best of our microenvironment effect of HPMo@lipid(n)/GO hybrids
knowledge, the highest yield of LA from glycerol oxidation improved the selectivity to lactic acid from glycerol through
achieved thus far is 72 %, catalyzed by combined Pt/Sn-MFI at tandem reactions. This new strategy not only demonstrates
100°C for 24 h.17b Therefore, new base-free and noble metal- the production of lactic acid from glycerol catalyzed by HPAs,
free catalytic processes for LA production, especially in one- but also illustrates the potential for wider application of this
pot transformations are extremely desirable for economic synthetic methodology in heterogeneous catalytic materials
profitability, durability, and environmental sustainability. HPA- based on HPAs.
or POM-catalyzed cascade conversions have not been
thoroughly explored. In the course of our work developing
multifunctional active sites for cascade conversion of glycerol
Results and discussion
directly to LA, we found that the H3PMo12O40 (HPMo) is active
in the one-pot conversion of glycerol to LA due to its redox
ability and Brønsted-acidity.19 The activity of HPAs was
The HPMo@lipid(2)/GO material was prepared according to
the procedure shown in Scheme 1: first, the surface of
graphene was oxidized by successive treatments with H2SO4 +
improved with loading onto carbon materials, with 94 % HNO3 and H2SO4 + H2O2 mixtures to give graphene oxide
functionalized with COOH groups; subsequently, coupling of
1,2-diaminoethane to the surface carboxylic groups results in
amide bond formation to generate the covalently-linked GO-
NH2 monolayer; finally, functionalization with 1-bromodecane
and loading of HPMo via self-assembly to form a lipid-like
bilayer on the surface of GO gives the HPMo@lipid(2)/GO
hybrid material. The same strategy was employed with
diamines of various carbon chain length yields
selectivity at 98 % conversion under optimized conditions (60
°C, 5 h, 5 bar O2). In the above work, carbonized willow catkins
were selected as carbon precursors, which acted only as a
support and did not present any synergistic effects in the
oxidation of glycerol. In addition, the influence of
hydrophilic/hydrophobic micro-environment on activity and
selectivity has not been investigated deeply. Specific effects of
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(NH4)3PMo12O40 (Fig.1d) shows similarly shifted bands at 1077,
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DOI: 10.1039/C7TA01334E
-1
986, 889 and 799 cm , further confirming that HPMo
molecules were embedded in the lipid like bilayer by
interaction with ammonium ion. Furthermore, the peaks at
1250-1500 and 3000-2800 cm-1 illustrate the presence of an
alkyl group belonging to 1-bromodecane. The IR spectra of
other HPMo grafted on GO with different layer thickness was
also characterized in Fig. S1. The Raman spectra of the samples
are shown in Fig. 2. The Raman spectrum of graphene, GO and
GO-NH2 typically exhibit one peak at 1585 cm-1 for G-band and
1363 cm-1 for D-band,26 respectively, while G/D intensity ratios
change after oxidation and modification with -COOH and -NH2.
The Raman spectrum of HPMo shows bands at 980 and 870
cm-1 for the stretching modes of Mo=O and Mo-O-Mo,
respectively.27 The Raman spectrum of HPMo@lipid(2)/GO is
similar to that of HPMo but with some shifts to 983 and 877
cm-1, respectively. This indicates that the Keggin structure of
HPMo remains intact in the hybrid material and corroborates
the electrostatic interactions between HPAs and -NH2,28
indicated by the IR spectra. In addition, the peaks
corresponding to GO at 1585 cm-1 and 1363 cm-1 show that the
graphene layers are not damaged during chemical synthesis of
HPMo@lipid(2)/GO. The XRD patterns of as-prepared catalysts
(Fig. S2) also support the results of FTIR. All the characteristic
diffraction peaks of HPMo can be nearly matched with those
of HPMo@lipid(2)/GO, and HPMo@lipid(2)/GO hybrids
showed highly crystallinity little change due to the interaction
between HPMo and GO-NH2.
Scheme
1
.
Synthetic strategy for assembly of
HPMo@lipid(n)/GO hybrid catalyst materials.
HPMo@lipid(n)/GO (with n = 2, 4, 6, 8 and 10) to determine
the influence of lipid bilayer thickness.
As shown in Fig. 1a, the sharp peaks at 1640 and 3438 cm-1,
attributed to vibrations of the carboxylate groups (COO-) of
the deprotonated COOH23 and of the primary hydroxyl
groups,24 respectively, indicate the presence of COOH in GO.
The IR spectrum of the diamino-modified grapheme oxide (GO-
NH2) shows additional at 2933, 3008, and 1653 cm-1 for C-H
stretch vibrations in the alkyl chain of 1,2-diaminoethane,25 N-
The 31P MAS NMR spectrum of HPMo@lipid(2)/GO (Fig. S3a)
presented two peaks at -5.22 and -6.05 ppm originating from
the central PO4 unit of PMo12O403- anions (-5.43 ppm). The split
of one resonance is attributed to the electrostatic interaction
between PMo12 anions and H2N+ groups on the GO surface.29
Furthermore, the 13C MAS NMR demonstrated the presence of
the organic layer in HPMo@lipid(2)/GO hybrids. 13C MAS NMR
spectrum of HPMo@lipid(2)/GO (Fig. S3b) gave peaks at 20,
40, 60, and 80 ppm, which are attributed to alkyl C-CH3, C-CH2-
, N-CH2-, and N-C, respectively. The peak around 130 ppm is
attributed to the graphitic sp2 carbon, while the peak of 180
H
vibrations, and C=O vibrations of an amide bond,
respectively (Fig. 1b), clearly confirming the formation of
covalent amide bonds between ethyl groups and -COOH. The
IR spectrum of HPMo@lipid(2)/GO (Fig.1c) shows four peaks at
1076, 986, 889, and 780 cm-1 which correlate to the
characteristic stretching vibrations of the Keggin framework,
which are observed at 1060, 965, 882, and 748 cm-1 in the free
3-
PMo12O40 anion (Fig. 1e). These shifts are attributed to the
interaction between HPMo and lipid layers. The IR spectrum of
Fig. 1 FTIR spectra of the material components: (a)
unmodified GO, (b) after modification to GO-NH2 (c) the
final HPMo@lipid(2)/GO (d) the ammonium salt of HPMo,
(NH4)3PMo12O40 and (e) the free HPA H3PMo12O40.
Fig. 2 Raman spectra of the samples including graphene,
GO, GO-NH2 and HPMo@lipid(2)/GO.
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ppm belongs to C=O.30
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DOI: 10.1039/C7TA01334E
(a)
(b)
(d)
Structural and component changes of samples during the
synthesis were followed by X-ray photoelectron spectroscopy
(XPS). Compared to graphene, there were three new carbons
for GO with different chemical states at C-O (286.5 eV), C=O
(287.6 eV), and O-C=O (289 eV).31 After functionalization with
diamine, the C-O (286.5 eV) peak disappeared completely and
a new peak for C-N appeared at 286.1 eV. There were no
obvious changes in the C1s spectrum of HPMo@lipid(2)/GO
compared to that of GO-NH2, indicating HPAs were not
interacting with carbon atoms. As shown in Fig. 3c, the
spectrum of GO-NH2 shows two peaks at 399.8 and 401.1 eV,
corresponding to C-NH2 and C-NH-C, while the absence of the
399.8 eV peak and appearance of a new peak at 398.1 eV is
attributed to H2N+32 for HPMo@lipid(2)/GO. This indicates that
HPMo molecules interact electrostatically with protonated
H2N+ groups. XPS combined with IR and Raman spectra
demonstrate the strong interactions between HPAs and the
protonated amino moieties of the lipid bilayer.
(c)
two layers
Figure. 4 shows the typical TEM and EDX of the resulting
samples during the preparation of HPMo@lipid(2)/GO. The
unmodified graphene sheet showed a clean wall (Fig. 4a),
which became thicker after modification with 1,2-
diaminoethane (Fig. 4b). After functionalization with HPMo, a
Fig. 4 TEM micrographs of Graphene (a), GO-NH2 (b);
HPMo@lipid(2)/GO with 30 wt% loading (c) and EDX of
HPMo@lipid(2)/GO (d).
layer of particles with sizes ranging from 50 to 150 nm is
observed to cover the GO-NH2, which could be attributed to
the assembly of HPMo (Fig. 4c). Elemental analysis of carbon,
phosphorous and molybdenum in the material by EDX
supports the uniform distribution of HPMo nanoparticles on
the surface of GO with ~ 30 wt% loading (Fig. 4d).
12 3 4
5 6
(b)
(a)
Graphene
GO
The surface areas of HPMo@lipid (n)/GO with different lipid
bilayer chain lengths are summarized in Fig. S4 and showed an
increase from HPMo@lipid (2)/GO to HPMo@lipid (4)/GO
which correlates to the longer carbon chain length. The
surface area did not increase with increasing thickness of the
lipid layer using longer chain lengths (n = 6, 8, 10), likely due to
steric crowding of these long carbon chains.
1
3
In contrast, the lipophilicity could be enhanced and altered
depending on the length of alkyl chains (Fig. S5). GO
nanosheets, which are hydrophilic when unmodified due to
the oxygen-containing surface, are converted to hydrophobic
materials upon alkyl-functionalization, with longer chains
imparting greater hydrophobicity. This is evident from the
increasing water contact angle for different samples with
increasing alkyl chain lengths from two to ten, giving 90.2 ±
0.3, 94.3 ± 0.5, 97.6 ± 0.2, 101.3 ± 0.2 and 107.7 ± 0.8 °,
respectively.
2
(c)
GO-NH2
GO-NH2
Fig. S6 shows the cyclic voltammetry (CV) of HPMo and
HPMo@lipid(n)/GO in carbon paste. The original PMo12O40
HPMo@lipid(2)/GO
HPMo@lipid(2)/GO
3-
exhibits three reversible peaks, which correspond to the redox
processes of the Mo6+ species in the HPA framework. The first
Fig. 3 (a) X-ray photoelectron spectra of graphene, GO,
GO-NH2, and HPMo@lipid(2)/GO.(b) C1s XPS for graphene,
GO, GO-NH2, and HPMo@lipid(2)/GO. The binding energy
were assigned to 1 as C-C (284.6 eV); 2 as C=C (285.5 eV); 3
as C-N (286.1 eV); 4 as C-O (286.5 eV); 5 as C=O (287.6);
and 6 as O-C=O (289 eV), respectively. (c) N 1s XPS spectra
for GO-NH2, and HPMo@lipid(2)/GO with the binding
6+
wave corresponds to the [PMo12 O40]3-/[PMo116+Mo5+O40]4-
couple with E1/2 = 0.34 V, where E1/2 = (Epa+Epc)/2. For
HPMo@lipid(n)/GO (n = 2, 4, 6, 8, and 10), E1/2 values shifted
to 0.61, 0.40, 0.33, 0.19 and 0.15 V, respectively. The increase
in oxidative ability of HPMo@lipid(n)/GO for n = 2, 4 compared
to HPMo can be attributed to the electrostatic and charge-
transfer interactions between HPMo and the graphene
+
energies were attributed to 1 as -NH3 (398.1 eV), 2 as C-
NH2 (399.8 eV) and 3 as C-NH-C (401.1 eV), respectively.
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surface, which influence the electronic structure and redox rare due to mass transferring difficulty in the former. As a
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DOI: 10.1039/C7TA01334E
homogeneous catalyst, HPMo forms a uniform system with
glycerol molecules and the reaction rate could be enhanced
with faster stirring to increase the frequency of molecular
collisions between HPMo and glycerol. Under our aerobic
oxidation reaction conditions, however, dioxygen suffers from
gas-liquid and gas-solid transferring difficulties. With
increasing stirring speed, dioxygen more easily escapes from
solution, resulting in lower concentration of dissolved oxygen
and consequently lower activity. Loading HPMo on reduced
GO (r-GO) giving (HPMo/r-GO) through redox procedure33a
only led to negligible increase in activity compared to HPMo
with 20 % LA yield at 66 % conversion. The activity and LA yield
over HPMo@lipid(2)/GO were almost 1.5 times and 4 times
greater than HPMo/r-GO, showing that surrounding of HPMo
Fig. 5 Oxidation of glycerol in the presence of various
catalysts.
on GO support and the valence of HPMo strongly affect
dehydrogenation and dehydration activity as well as product
selectivity.
couples of HPMo.33
These results show that strong
The difference between HPMo/r-GO and HPMo@lipid(2)/GO
reflect the influence of surface properties of GO on glycerol
conversion and LA selectivity. These differences were
emphasized at an early stage (12 vs 34 % for glycerol
conversion and 7 vs 37 % for LA selectivity after 30 min, 30 vs
61 % for glycerol conversion and 15 vs 68 % for LA selectivity
after 1.5 h), showing an important delay probably due to
different phenomena (lower concentration of O2 around active
sites, slower diffusion rates of substrates or different
accessibility to the active sites in heterogeneous conditions).
For HPMo/r-GO, HPMo molecules were attached on r-GO
surface through chemisorption generated by the charge
transfer from HPA anion and GO34 (Scheme S1). The electron
density decrease and hydrophilic surface for HPMo/r-GO
favored adsorption of oxygen, but were detrimental to
adsorption of glycerol. The adsorption test showed that 0.009
mol/g of glycerol and 9×10-7mol/g of oxygen could be
adsorbed by HPMo/r-GO. Adsorption by HPMo@lipid(2)/GO is
4.5 times higher for glycerol at 0.041 mol/g and 20 times
greater for oxygen at 1.8×10-5mol/g than in HPMo/r-GO. This
result demonstrates that the lipid-like bilayer of
HPMo@lipid(n)/GO affects the adsorption of substrates,
wherein the hydrophobic surroundings and hydrophilic HPAs
collaboratively generate an adsorbent microregion, much like
an enzymatic microenvironment, with high affinity for glycerol.
The thickness of the lipid layer likely influences the adsorption
of
interactions between GO and HPMo indeed occur and can
significantly alter the redox properties of HPMo toward the
aerobic oxidation of glycerol, but longer alkyl chains could
hinder electron transfer from GO to HPMo, as evidenced by
the lower redox potentials for HPMo@lipid(n)/GO with n = 6,
8, 10.
In order to evaluate the catalytic dehydrogenation and
dehydration activity of HPA-graphene hybrids in cascade
reactions, we tested them in the conversion of glycerol under
mild reaction conditions (1 M glycerol, 1.95 wt% of catalyst, 60
°C, 3.5 h, 10 bar O2). As shown in Fig. 5, three catalysts
including graphene, GO and GO-NH2 exhibited low activity for
glycerol conversion and low selectivity to LA, giving about 10,
15, and 23 % glycerol conversion and 3, 7, and 10% LA
selectivity, respectively. The conversion of glycerol was
significantly higher at 52 % with free HPMo as catalyst with 23
% LA selectivity. The TOFGly (TOFGly = converted glycerol
(mol)/mol of HPMo × Time) of 148 h-1 was a promising
indicator that immobilization of this homogeneous catalyst
could render it more practical for industrial use as one
component of a separable, recyclable heterogeneous catalyst
material. Interestingly, the HPMo@lipid(2)/GO exhibited
enhanced activity with 95 % LA selectivity at 90 % conversion
and TOFGly of 257 h-1. Furthermore, the TOFGly and LA yield
over heterogeneous HPMo@lipid(2)/GO were 2-fold and 7-
fold greater than those with homogeneous HPMo. Higher
activity with a heterogeneous versus homogeneous catalyst is
(b)
(a)
Scheme 2. The schematic drawing of O2 and glycerol adsorption (a) on the lipid-like bilayer of HPMo@lipid(n)/GO and (b) on
HPMo/r-GO.
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Table 1
Journal Name
.
P
hysical and chemical properties of different catalysts with different amino chains.
Oxygen
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DOI: 10.1039/C7TA01334E
Glycerol
uptake
TOF Conversion Selectivity
surface
areas
Redix
potentials
E1/2(V)
water
contact
angle (°)
catalyst
Adsorption(
mol/g)
Pa (×10-
5mol/g)
h-1
(%)
to LA (%)
(m2/g)
-
H3PMo12O40
0.34
0.61
0.40
0.33
0.19
0.15
-
26.3 ± 0.5
90.2 ± 0.3
94.3 ± 0.5
97.6 ± 0.2
101.3 ± 0.2
107.7 ± 0.8
-
2 (0.02)
180 (1.8)
300 (3.0)
280 (2.8)
240 (2.4)
200 (2.0)
9 (0.09)
0.006
0.041
0.050
0.044
0.032
0.022
0.009
35
52
90
97
91
85
73
66
24
95
93
86
81
70
30
HPMo@lipid(2)/GO
HPMo@lipid(4)/GO
HPMo@lipid(6)/GO
HPMo@lipid(8)/GO
HPMo@lipid(10)/G
HPMo/r-GO
202.9
280.2
207.7
196.5
181.2
-
244
258
224
197
152
63
Reaction conditions: 1 M of glycerol, 0.01 mmol of catalyst, 60 °C, 3.5 h, 10 bar of O2, 800 rpm. Turnover frequency
calculated as TOF= (concentration of formed LA, mol/ L)/[(concentration of used HPA, mol L-1)×(reaction time, h)].
oxygen and glycerol. As for glycerol: HPMo@lipid(4)/GO (0.050 >HPMo@lipid(8)/GO (85 %) >HPMo@lipid(10)/GO (73 %)
mol/g)
>HPMo@lipid(2)/GO
(0.041
mol/g)
~
>HPMo/r-GO (66 %) (Table 1)
The difference between
HPMo@lipid(6)/GO (0.044 mol/g) >HPMo@lipid(8)/GO (0.032
HPMo@lipid(2)/GO
and
mol/g) >HPMo@lipid(10)/GO (0.022 mol/g); for oxygen: HPMo/carbon from our previous report (80 % conversion and
HPMo@lipid(4)/GO (3.0 × 10-5mol/g) >HPMo@lipid(6)/GO (2.8 55 % of LA selectivity) was attributed to the superior the
×
10-5mol/g) >HPMo@lipid(8)/GO (2.4
×
10-5mol/g) electron-transferring capability of the GO support to HPMo
>HPMo@lipid(10)/GO (2.0 × 10-5mol/g) >HPMo@lipid(2)/GO compared to the carbon microtubes. . This difference results in
(1.8
10-5mol/g), respectively. With increasing n, the the shift to higher E1/2 value of 0.61 V for HPMo@lipid(2)/GO
hydrophobicity increased but adsorption of glycerol compared to the unchanged value of 0.34 V for HPMo/carbon.
decreased. Maximum LA selectivitywas achieved with the n = 2 hybrid
×
Therefore, the hydrophobic surroundings of HPMo hybrids with decreasing selectivity as HPMo@lipid(2)/GO (95 %)
have an opposing effect on concentration of glycerol and >HPMo@lipid(4)/GO (93 %) ~ HPMo@lipid(6)/GO (86 %)
oxygen around active sites of HPMo. HPMo@lipid(2)/GO was >HPMo@lipid(8)/GO (81 %) >HPMo@lipid(10)/GO (70 %)
the exception among the HPMo@lipid(n)/GO series, which was >HPMo/r-GO (30 %) ~ HPMo (24 %)(Table 1). The selectivity to
attributed the shorter length of the alkyl chain. This suggests LA was opposite to the hydrophobicity of HPMo@lipid(n)/GO,
that HPMo@lipid(n)/GO could act as a capillary-like reactor showing that the production of LA from glycerol requires
(Scheme 2a), in which the lipid carbon chains form a capillary- hydrophobic surroundings. As in our previous reports,19
like wall surrounding HPMo, providing a space for O2 and glycerol was catalytically oxidized by O2 to dihydroxyacetone
glycerol. Therefore, the combination of hydrophobic or (DHA) via HPMo, then dehydrated to pyruvaldehyde (PRV) by
hydrophilic properties and capillary-like reactor of the Brønsted-acid site of HPMo, with the so-formed PRV
HPMo@lipid(n)/GO result in the differences in adsorption of undergoing another intramolecular 1,2 hybrid shift after
oxygen and glycerol, with HPMo@lipid(4)/GO showing the hydration of PRV to finally generate LA. The highly
highest adsorption capability. The adsorption of glycerol by hydrophobic surface of HPMo@lipid(n)/GO could exclude
HPMo@lipid(n)/GO and HPMo/r-GO was also tested by IR (Fig. water molecules from the capillary reactor, making realization
S7). After adsorption, one new peak at 3287 cm-1 was of this last step of the PRV to LA more difficult. Therefore, the
observed due to -OH groups of glycerol and the characteristics stronger hydrophobic surroundings of longer alkyl chains do
peaks for HPMo shifted to 1080, 999, 916 and 841 cm-1 for not favor LA generation. Enhanced LA selectivity was achieved
HPMo@lipid(2)/GO and 1078, 987, 905 and 820 cm-1 for using HPMo@lipid(n)/GO instead of HPMo and HPMo/r-GO.
HPMo/r-GO. For Unlike in HPMo/r-GO, where oxygen, glycerol This was attributed to the relative hydrophobic surface, which
and HPMo are dispersed separately on the surface of r-GO could promote the leaching of formed LA from HPMo active
(Scheme 2b), the capillary-like reactor of HPMo@lipid(n)/GO site without further conversion. Therefore, balancing the
minimizes the mass-transferring difficulty between gas phase hydrophobic and hydrophilic properties is essential for the
and liquid.38 As mentioned above, the oxidative ability of design of multifunctional active solid catalysts for this glycerol
HPMo@lipid(n)/GO with different carbon chain lengths as n = conversion reaction, where HPMo@lipid(4)/GO was the best
2, 4, 6, 8, 10 with the highest E1/2 value measured when n = 2 candidate in direct aerobic production of LA from glycerol in
as 0.61 V while the highest conversion of 97 % was achieved the absence of base.
with n = 4, and decreasing conversion with increasing and
To optimize LA production catalyzed by HPMo@lipid(4)/GO,
decreasing chain length as HPMo@lipid(4)/GO (97 %) the temperature, reagent concentrations, amount of catalyst,
>HPMo@lipid(2)/GO (90 %) ~ HPMo@lipid(6)/GO (91 %) reaction time, and amount of oxygen were varied (Fig. S8). As
6 | J. Name., 2012, 00, 1-3
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ARTICLE
expected (Fig. S8), increasing the temperature led to a environment of HPA supported solid catalysts wVaisewsAtruticdleieOdnliinne
decrease in selectivity, indicating that LA was easily over- detail in glycerol conversion, which coDuOldI: 1p0r.o10v3i9d/eC7vTaAl0u1a33b4leE
oxidized at higher temperature. The best glycerol conversion information for other biomass conversions through cascade
and LA yield were achieved as 97 % and 90 %, respectively, at reactions. Unlike previously-reported oxidation-reduction
60 °C. In the investigation of the influence of glycerol methods, this novel strategy involves first covalently anchoring
concentrations (Fig. S8b), conversion and LA selectivity initially a lipid-like bilayer on the GO surface, then embedding HPMo
increased and then decreased as the amount of glycerol into the bilayer to form the HPMo@lipid(n)/GO hybrid
increased. This result could be attributed to the increased material. Apart from the benefits of resistance of HPMo
dihydroxyacetone yield as the amount of glycerol increased, against leaching and good stability towards pH and
and the dihydroxyacetone required more time to be temperature changes, such HPMo@lipid(n)/GO provides
transformed to LA. The optimal concentration of glycerol was capillary-like assembly with hydrophobicity during catalytic
1.0 mol/L (9.2 wt%), higher than the reported usage of 3 wt% reactions. Furthermore, the oxidative ability of HPMo (E1/2
=
of glycerol. This result indicates that our HPMo@lipid(4)/GO is 0.34 V) can be enhanced by GO support and tuned by changing
tolerant to high concentration of glycerol. The conversion and the length of alkyl chain in the lipid bilayer, with n = 2 giving
selectivity increased with longer reaction time (Fig. S8c), the highest E1/2 = 0.61 V. In the cascade conversion of glycerol
amount of catalyst (Fig. S8d), and amount of oxygen (Fig. S8e). to LA, the capillary reactor of HPMo@lipid(n)/GO enhances
The highest selectivity to LA (93 %) was achieved with 1.95 adsorption of glycerol and oxygen, leading to improved
wt% of HPMo@lipid(4)/GO, 1 mol/L of glycerol at 60 °C for 3.5 glycerol conversion. Balancing the hydrophobicity of the lipid
h, and the amount of oxygen was 10 bar.
bilayer and hydrophilicity of HPMo, as well as optimizing redox
HPMo@lipid(4)/GO catalyst was successfully reused fifteen potentials by changing the alkyl chain length gave the highest
times with only minor loss of catalytic activity and selectivity selectivity to LA, where HPMo@lipid(4)/GO presented 93 %
for LA formation (Fig. S9). After each catalytic run, the catalyst selectivity to LA at 97 % conversion ( 1 mol/L glycerol, 60 °C,
was recovered by centrifugation, washed with ethanol and 3.5 h, 10 bar O2). Not only does the HPMo@lipid(4)/GO system
dried before reuse. The total leaching amount of every runs exhibit the highest activity for this transformation reported in
plus the former ones was given in Fig. S9, which showed that the literature to date, but also fulfills the main principles of
the total leaching amount of catalyst after 15 runs was only 8 “green chemistry” by employing mild, solvent-free conditions
wt% of initial usage. To investigate the potential leaching of renewable feedstocks and benign oxidant, low catalyst loading
HPMo, the reaction mixtures after the catalytic experiment and recyclability. Furthermore, the system is extremely
were studied by UV-Vis spectroscopy. The spectra of did not tolerant towards impurities in crude glycerol, eliminating the
exhibit any characteristic peaks of HPMo in the range of 200- need for feedstock purification. Compared to sugar
400 nm, indicating that no leaching of HPA took place under fermentation, this system presents
a reduced-cost and
our reaction conditions (Fig. S10). Therefore, the minor loss of environmentally-friendly route to the efficient production of
catalytic activity of HPMo@lipid(4)/GO can be explained by LA from glycerol.
operation during recycling. The IR-spectrum of the used
HPMo@lipid(4)/GO did not significantly differ from that of
fresh HPMo@lipid(4)/GO indicating that the structure of the
Experomental
heteropoly anion remains intact (Fig. S11). TEM after the
reaction also confirmed that the catalyst did not coacervate
and remained dispersed (Fig. S12).
Instruments
XPS spectra were recorded on an Escalab-MK II
photoelectronic spectrometer with Al Kα (1200 eV). IR spectra
(4000-400 cm−1) were acquired in KBr discs on a NicoletMagna
560 IR spectrometer. XRD patterns of the samples were
collected on a Japan RigakuDmax 2000 X-ray diffractometer
with Cu Kα radiation (λ= 0.154178 nm). The measurements
were obtained in the step of 0.04º with account time of 0.5 s
and in the 2θ rang of 5-90 º. Raman spectroscopy was
obtained on a Renishaw-UV-vis Raman System 1000 equipped
with a CCD detector at room temperature. The air-cooled
frequency doubled Nd-Yag laser operating at 532 nm was
employed as the exciting source with a power of 30 MW. The
31P MAS NMR spectra were obtained using a Bruker AM500
spectrometer at 202.5 MHz. SEM micrographs were recorded
on a scanning electron microscope (XL30 ESEM FEG 25kV). EDX
spectra were obtained using 20 kV primary electron voltages
to determine the composition of the samples. The redox
potential was measured by cyclic voltammetry (CV) on a CS
The tolerance towards impurities was tested using crude
glycerol from biodiesel production, comprising 71 wt% of
glycerol, 28 wt % of methanol, and other minor organic
mixtures, without any purification. HPMo@lipid(4)/GO was
found to be highly efficient in the transformation of crude
glycerol, with almost the same conversion (96 %) and
selectivity (91 %) under mild conditions (1 mol/L of crude
glycerol, 1.95 wt% catalyst, 10 bar O2, 800 rpm, 60 °C, 3.5 h).
This result shows that HPMo@lipid(4)/GO is a methanol-
tolerant catalyst capable of converting crude glycerol, and the
esterification of LA with methanol was hindered.
Conclusions
In summary, a new strategy for immobilization of HPAs on GO
has been developed and demonstrated to be highly effective
for producing catalytic materials for the cascade conversion of
glycerol to LA. For the first time, the effect of micro-
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Journal Name
Corrtest electrochemical workstation equipped with carbon H2SO4(0.1 % w/w) in H2O/acetonitrile (1/2 v/v) (1.0 mL min-1)
View Article Online
DOI: 10.1039/C7TA01334E
paste electrode (graphite powder (SP) and mineral oil mixed as the eluent at 50 °C.
with 4:1 in weight) and saturated calomel as reference
electrode in electrolyte of 0.1 M sulfuric acid solution.
Catalyst preparation
Synthesis of GO19: Graphene (0.5 g) was dispersed in
Acknowledgements
This work was supported by the National Natural Science
concentrated sulfuric acid and concentrated nitric acid with a
volume ratio of 3:1 under stirring and sonicating for 1 h.
Thereafter, the solution was stirred for another 24 h at room
temperature. The reaction mixture was diluted, filtered and
washed with deionized water. After that, it was added to 50
mL of a 4:1 (volume ratio) mixture of concentrated sulfuric
acid and hydrogen peroxide and stirred for 30 minutes at 70
°C. The obtained oxidized graphene was washed with
deionized water until the media of the effluent became
neutral. Finally, the product was dried at 80 °C.
Synthesis of functionalized-GO with amino group (GO-NH2):
0.2 g of GO was suspended in 20 mL of 2:1:1 (volume ratio)
mixture of Dimethyl Formamide (DMF), chloroform, and
methanol, and then sonicated for 1 h. 1,2-diaminoethane (0.03
mol) along with 10 mol of 4-Dimethylaminopyridine(DMAP)
and 10 mol of 1-(3-Dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (EDCI) was added into the reaction mixture as
catalyst for amide formation, then sonicated for 1 h and stirred
for 48 h at 50 °C. The reaction mixture was centrifuged for 20
min at 4000 rpm to separate the functionalized GO and
remove unbound 1,2-diaminoethane. The products were
washed with 1:1 (volume ratio) mixture of chloroform and
methanol until all the 1,2-diaminoethane was removed. As-
prepared GO-NH2 was dried overnight under vacuum at 30 °C.
The other amino groups with different chain length including
1,4-diaminobutane, 1,6-diaminohexane, 1, 8-octanediamine
and 1,10-decamethylenediamine were synthesized as the
same method as 1,2-diaminoethane.
Foundation of China (No. 51578119), the major projects of Jilin
Provincial Science and Technology Department (20086035,
20100416, and 20140204085GX).
Notes and references
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