Oxidative Esterification of Methacrolein to Methyl Methacrylate over Gold Nanoparticles on Hydroxyapatite

Article abstract of DOI:10.1002/cctc.201601560

The catalytic production of methyl methacrylate through the direct oxidative esterification of methacrolein is important in terms of green chemistry and sustainable development. In the present work, Au nanoparticles supported on three needle-like, lamella-like, and rodlike hydroxyapatites were synthesized. We demonstrated that needle-like hydroxyapatite could facilitate the higher dispersion of Au species because of its high specific surface area, and the strong interaction between the Au nanoparticles and the support resulted in the formation of more surface defects because of the existence of partially encapsulated Au particles by the needle-like hydroxyapatite. The surface defects were related closely to the generation of strong basic sites. Compared with the other two materials, the Au supported on the needle-like hydroxyapatite catalyst, which had a large amount of surface acid–base sites, exhibited a much higher catalytic activity and selectivity to methyl methacrylate in the direct oxidative esterification of methacrolein with methanol under mild reaction conditions (i.e., ambient pressure, low reaction temperature of 70 °C, and low methanol/aldehyde ratio of 8:1). The superior catalytic performance of the Au supported on needle-like hydroxyapatite catalyst was attributable to a cooperative effect between abundant acid–base sites for the preferential chemisorption of methacrolein and highly dispersed active Au species for the favorable formation of β-hydride and oxygen activation. The present findings open a new and promising route for the practical production of methyl methacrylate using high-performance hydroxyapatite-supported metal catalyst systems.

Full text of DOI:10.1002/cctc.201601560

Accepted Article
Title: Oxidative esterification of methacrolein to methyl methacrylate
over gold nanoparticles on hydroxyapatite
Authors: Jun Gao, Guoli Fan, Lan Yang, Xinzhong Cao, Peng Zhang,
and Feng Li
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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To be cited as: ChemCatChem 10.1002/cctc.201601560
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Oxidative esterification of methacrolein to methyl
methacrylate over gold nanoparticles on hydroxyapatite
Jun Gao,a Guoli Fan,a Lan Yang,a Xinzhong Cao,c Peng Zhang,c Feng Li a,b *
Presently, the catalytic production of methyl methacrylate
through direct oxidative esterification of methacrolein is
important in terms of green chemistry and sustainable
development. In the present work, three needle-like,
lamella-like, and rod-like hydroxyapatites supported Au
nanoparticles were synthesized. It was demonstrated
that needle-like hydroxyapatite could facilitate the higher
dispersion of Au species, owing to higher specific surface
area, and that the stronger interaction between Au NPs
and the support resulted in the formation of more surface
defects, due to the existence of partially encapsulated Au
particles by the needle-like hydroxyapatite. Meanwhile,
surface defects were found to be closely related to the
generation of strong basic sites. Compared with other
two ones, needle-like hydroxyapatite supported catalyst
with a larger amount of surface acid-base sites exhibited
much higher catalytic activity and selectivity to methyl
methacrylate in direct oxidative esterification of
methacrolein with methanol under mild reaction
conditions (i.e., ambient pressure, low reaction
temperature of 70 oC, and low methanol/aldehyde ratio of
8:1). Superior catalytic performance of needle-like
hydroxyapatite supported catalyst was attributable to a
cooperative effect between abundant acid-base sites for
the preferential chemisorption of methacrolein and highly
dispersed active Au species for the favorable formation
of β-hydride and oxygen activation. The present findings
open
a new and promising route for the practical
production of methyl methacrylate using high-
performance HAP-supported metal catalyst systems.
Introduction
pollution of sulfuric acid used.[2] Therefore, developing a
green approach for the production of MMA is of far-
reaching significance. As an ideal alternative, direct
oxidative esterification of methacrolein (MA) with methanol
to produce MMA using molecular oxygen is attracting
considerable attention,[6] because it is an environmentally
friendly, greatly simplified and cost-effective production
process with good atom economy in comparison to other
production processes.
Methyl methacrylate (MMA) is an important and widely
used polymer monomer or chemical intermediate for the
production of acrylic plastics (e.g. polymethyl methacrylate),
paints, adhesives, and other important fine chemicals.[1]
For this, the production of MMA can be performed by
several methods, such as partial oxidation/coupling of
ethylene, propylene or isobutene and acetone
cyanohydrine (ACh) method. [2-5] Among them, the partial
oxidation/coupling of ethylene has an obvious shortcoming
of low conversion rate, and the isobutene oxidation goes
Recently, many efforts have been made to oxidative
esterification of MA by using heterogeneous Pd-based
catalysts.[6-9] Specially, the addition of second metal Pb can
further improve the catalytic activity,[8,9] regardless of low
selectivity to MMA (< 90%). In addition, for the oxidative
esterification of aldehydes or alcohols, Au-based catalysts
also exhibit high selectivity to esters.[10-13] However, in most
cases, harsh reaction conditions, [11-13] such as the use of a
large excess of methanol, need of basic additives in the
reaction mixture to inhibit the generation of by-products,
high energy consumption, and long reaction time, greatly
limit their practical applications. Therefore, designing new
highly efficient catalyst systems for the oxidative
esterification of MA has been an important and urgent task
in terms of green chemistry and sustainable development.
Currently, supported gold-based catalysts have
received numerous attention due to their unique catalytic
properties in many important types of chemical
reactions.[14-19] However, these catalysts suffer from
problems, such as easy agglomeration of gold
nanoparticles (NPs) and thus rapid decline in the activity
during reactions. It is well known that for supported metal
catalysts, the interactions between metals and supports
can greatly affect electronic structures of active species
through
a complicated multi-step route including the
oxidation of isobutene to methacrylic acid and the following
conversion of methacrylic acid to MMA. Currently, the
production of MMA has been carried out commercially by
the traditional ACh method. This unsustainable method,
however, has several drawbacks including long
technological route, high cost, strong corrosion of
hydrocyanic acid raw material, and serious environmental
[a] J. Gao, Dr. G. Fan, Prof. L. Yang, Prof. F. Li
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
Beijing, 100029, P. R. China.
E-mail: lifeng@mail.buct.edu.cn; Fax: +8610-64425385;
Tel.: +8610-64451226
[b] Prof. F. Li
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, Beijing 100029, China.
[c] Dr. X. Cao, P. Zhang
Key Laboratory of Nuclear Radiation and Nuclear Energy
Technology, Institute of High Energy Physics, Chinese
Academy of Sciences, Beijing 100049, P. R. China
and
thus
significantly
govern
their
catalytic
performances,[20-22] besides the size and geometry of metal
particles and the acid–base property of supports. Specially,
it was reported that strong metal-support interactions
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/cctc.xxxxx.
1
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(SMSI) could form if metals were encapsulated by suitable
supports.[23,24] In this regard, Zhang et. al found that
hydroxyapatite (HAP) could strongly interact with Au
NPs,[24] thereby facilitating the immobilization and
dispersion of active components and inhibiting the growth
of metal NPs.
of HAP for carbonatation. In all cases, no characteristic
diffractions related to metallic Au phase are observed,
mirroring the highly dispersive nature of Au species on
supports, as well as the low actual Au content (Table 1).
Commonly, HAP with the formula Ca10(PO4)6(OH)2,
which belongs to the hexagonal cylinder crystal structure,[25]
is widely used as green, substrate-tolerant, nearly water-
insoluble, and easy to recover catalyst support with
excellent ion exchange/adsorption ability and tunable
surface acid-base property.[26-33] Especially, many base-
catalyzed reactions,[34-36] as well as Guerbet reaction for the
production of n-butanol from ethanol,[37-40] can be efficiently
promoted by HAP-based catalyst systems. In addition, the
lattice oxygen in the structure of HAP may improve the
oxidation activity of supported metal catalysts.[41] Thus, it is
probable that reasonably regulating the microstructure and
surface acid-base property of HAP supports can greatly
enhance catalytic performances of corresponding catalysts.
Taking into account the above scenarios, in this work,
we first synthesize supported Au nanocatalysts based on
three different types of needle-like, lamellar-like and rod-
like HAP supports. The results demonstrate that small and
uniform Au NPs can be tightly anchored onto the surface of
HAP supports and even partially embedded into supports,
thereby resulting in the SMSI and the formation of a large
number of surface defects. The effects of the
microstructures of HAP supports on metal-support
interactions, surface acid-base properties, surface defects,
electronic structures of Au species, and catalytic
performance of catalysts are studied systemically.
Compared with lamella-like and rod-like HPA supported
ones, needle-like HAP supported Au catalyst displays
much higher catalytic performance in the oxidative
esterification of MA in terms of both the activity and the
MMA selectivity under mild reaction conditions (i.e., low
methanol/MA ratio, low reaction temperature and ambient
Figure 1. XRD patterns of different HAP-supported Au samples.
Figure
2 shows the morphologies of three HAP
supports. Nanometric N-HAP crystals present an oriented
array structure of several needles with a width of 100–150
nm (Figure 2a-c). As shown in Figure 2d-f, the morphology
of nano lamellas with a width of 150–200 nm is formed in
the L-HAP. Such structure may be constructed through a
parallel assembly of needle-like crystals elongated in the c
direction, as proved by relatively sharpened diffractions
compared with those for N-HAP (Figure 1). Long and
uniform rod-like fibers with the diameter of 100-150 nm
elongated along the c axis can be found in the case of R-
HAP (Figure 2g-i). Here, the formation of different
morphologies of needle-like, lamella-like and rod-like HAPs
should mainly originate from different basic conditions and
ion concentrations applied in synthesis solutions,[42] thus
leading to different precipitation routes and growth modes
of HAP crystals.
pressure). High catalytic efficiency is attributed to
a
cooperative effect between abundant surface acid-base
sites and highly dispersed active Au species, thereby
facilitating the chemisorption of reactants and the following
activation.
Results and Discussion
Structural characterization
As shown in Figure 1, all of HAP-supported Au samples
present characteristic diffractions of (002), (102), (210),
(211), (112), (300), (202), (310), (222), (213) crystal planes
of hexagonal HAP phase (JCPDS No. 73-0293),
respectively. It is worth noting that an impure CaHPO4
phase (JCPDS No. 70-0360) is detected in the Au/R-HAP.
In addition, due to the fact that the positions for the (104)
plane of two different crystalline CaCO3 phases (JCPDS
No. 72-1652 and JCPDS No. 86-2343) partially coincide
with that for the (210) diffraction of hydroxyapatite, the
relatively high intensity of the (210) diffraction line
demonstrates the presence of CaCO3 impurity in the Au/R-
HAP. Two Ca-containing impurities possibly originate from
the weak basicity in the synthesis solution and the affinity
Figure 2. SEM and TEM images of hydroxyapatite supports with
different morphologies: N-HAP (a,b,c), L-HAP(d,e,f) and R-HAP
(g,h,i).
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Figure 3 shows TEM and HRTEM images of different
supported Au samples. In each case, a large number of
small black NPs with narrow particle distributions are well
dispersed on the HAP support. However, smaller Au
clusters or atomically dispersed mono–layer Au species
cannot be found regardless of supports, mainly due to the
limitation of preparation method, as well as relatively high
Au loading of about 1.8 wt.% (Table 1). Typically, the lattice
fringes of single particle clearly indicate an exposed (200)
plane of Au0 NPs with an interplanar spacing of about
0.203 nm in the case of Au/R-HAP, while the lattice fringes
related to (202) plane of hexagonal HAP phase with an
interplanar spacing of about 0.262 nm are clearly observed.
According to the size distributions of Au NPs, the average
diameter of Au NPs on N-HAP (2.60 nm) and L-HAP (2.40
nm) are smaller than that on the R-HAP (4.55 nm), in spite
of their identical Au loadings.
Figure 3. TEM images, HRTEM images and particle size distributions of Au/N-HAP (a,b,c), Au/L-HAP (d,e,f) and Au/R-HAP (g,h,i) samples.
The inset in (b) shows the HRTEM image of a single particle on the Au/N-HAP.
Table 1 Composition, structural and textural data of different samples.
[c]
[d]
+
+ [e]
Au [a]
(wt %) molar ratio
Ca/P [a]
Dis [b]
(%)
DTEM
SBET
Au /(Au0+Au
)
Specific acidity [f]
(mmol /g)
0.351 (0.60) [g]
0.342 (0.48)
0.280 (0.39)
Specific basicity [h]
(mmol/g)
0.753 (0.328) [i]
0.456 (0.187)
0.356 (0.104)
Samples
(nm)
2.60
2.40
4.55
(m2/g)
molar ratio
0.36
Au/N-HAP 1.82
Au/L-HAP 1.73
Au/R-HAP 1.75
1.43
1.55
1.58
32.3
27.6
26.0
84
53
0.42
14
0.49
[a] Analyzed by ICP-AES. [b] Au dispersion degree by CO pulse chemisorption.[c] Average particle size of Au NPs based on by
TEM analysis. [d] BET specific surface areas. [e] Calculated based on XPS of Au 4f core levels. [f] The amount of total acid sites
determined by NH3-TPD. [g] The proportion of strong and super strong acid sites in the total amount of acid sites. [h] The amount
of total base sites determined by CO2-TPD. [i] The amount of strong and super strong base sites determined by CO2-TPD.
The Cs-STEM is an excellent technique to precisely
observe the microscopic structure and morphology. Thus, Cs-
STEM images of Au NPs on different HAP supports were
examined (Figure 4). It is interesting to note that for Au/N-HAP
sample, some Au NPs are partially encapsulated by the N-HAP
support to a large extent (Figure 4a,b), forming an embedded
structure of Au NPs with a large interface between Au NPs and
the support. The above phenomenon implies the formation of
SMSI. Similar result also is observed for Au/L-HAP with
partially encapsulated Au NPs by the L-HAP and small
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exposed surface of Au NPs (Figure 4c,d); however, embedded
structure of Au NPs in the Au/R-HAP is not as pronounced as
those in Au/N-HAP and Au/L-HAP (Figure 4e, f), as evidenced
by a smaller interface between Au NPs and the support.
Moreover, in the case of Au/N-HAP, Au NPs clearly show the
distortion of lattice fringes, i.e., dislocation and rugged steps,
indicative of the existence of more surface defects. Such
surface defects on Au NPs can be formed by the structural
reconstruction of the HAP support. It is well accepted that
surface defects on metal particles are thought to have a crucial
impact on the adsorption and activation of reactant molecules,
probably responsible for the excellent catalytic performance in
some heterogeneous catalytic reactions in a way. [43]
Positron annihilation spectroscopy (PAS) is a powerful tool
for detecting different types and relative densities of defects in
solids.[44] Table
2 shows four kinds of positron lifetime
components (τ1, τ2, τ3 and τ4) and their relative intensities for
supported Au samples. Among them, the component of the
longest life (τ4) is assigned to inter-crystallite spaces, while the
third component (τ3) is related to defects inside the crystallites
or to positronium-surface bound state.[45,46] The component of
the shortest life (τ1) with the similar τ1 values for three samples
is associated with small size bulk defects of particles (e.g.
monovacancies),[47,48] while the second component (τ2) is
generally caused by large size defects located on the surface
boundary and interface of particles.[49,50] Further, the higher
relative intensity ratio of T2 component to T1 component for
Au/N-HAP than those for other two supported samples
apparently reflects that Au/N-HAP possesses
a higher
concentration of surface defects, well consistent with the Cs-
STEM results. Accordingly, it is assumed that the formation of
surface defective structures is related to the presence of SMSI.
Figure 5. XPS of Au 4f regions for supported Au samples.
To investigate the electronic structures of Au species in
samples, XPS of Au 4f region were examined. In the fine 4f
spectra of Au species (Figure 5), there are two spin−orbit Au 4f
7/2 and Au 5/2 core levels at about 83.0 and 86.5 eV,
respectively, indicative of the existence of metallic Au0 species.
In comparison with that for bulk metallic Au0 (84.0 eV for Au 4f
7/2) reported in the literature, however, the binding energy (BE)
value shifts to 82.6 eV for Au/N-HAP, 82.8 eV for Au/L-HAP
and 83.2 eV for Au/R-HAP, respectively, indicative of the
presence of the SMSI.[51] Meanwhile, a second component at
higher BE values of 84.1 eV for Au/N-HAP, 84.3 eV for Au/L-
HAP and 84.8 eV for Au/R-HAP in the Au 4f 7/2 core level is
associated with cationic Au+ species.[52] Obviously, the relative
proportion of Au+ in total Au species decreases gradually in the
following order: Au/R-HAP> Au/L-HAP > Au/N-HAP (see Table
1), which suggests the enhanced metal-support interactions.
As for Au/N-HAP, both a more negative shift of the BE value for
Au 4f 7/2 of Au0 specie and a higher proportion of Au0 species
in total Au species indicate a more intense electron transfer
from the support to Au species, which results from a stronger
interaction between Au species and the support, well
consistent with the above Cs-STEM results.
Figure 4. Cs-STEM images of Au/N-HAP (a,b), Au/L-HAP (c,d) and
Au/R-HAP (e,f) samples, showing clear interface between Au NPs and
supports. Images are colorized in order to highlight the uncovered
surface of Au NPs (blue broken lines) and some surface defects of Au
NPs (red polylines).
Table
2 Positron lifetimes and relative intensities of
different components for supported Au samples
[b]
Samples τ1 [a] (ns)
τ2 [a] (ns) τ3 [a] (ns) τ4 [a] (ns) I2/I1
Au/N-HAP 0.175(40.4) 0.360(53.5) 1.51(4.2)
Au/L-HAP 0.160(47.9) 0.325(47.2) 1.46(3.5)
5.75(1.9)
5.69(1.4)
1.32
0.99
0.97
Au/R-HAP 0.171(47.9) 0.367(46.3) 1.72(4.4)
5.35(1.4)
[a] Values in parentheses are proportions of different components. [b]
Relative intensity ratio of T2 component to T1 component.
As for supported catalysts, it is generally accepted that the
high specific surface area of supports can greatly facilitate the
dispersion of metal particles. [53, 54] In our case, the BET specific
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surface areas of Au/N-HAP and Au/L-HAP are approximately 6
and 4 times as that of Au/R-HAP (Table 1). Accordingly, the
higher surface area of N-HAP support can mainly provide a
beneficial dispersing effect for Au NPs, thus inhibiting the
growth and agglomeration of Au species on the surface and
resulting in the largest dispersion degree of Au species (Table
1), although Au NPs can be largely embedded into the N-HAP
support stemming from SMSI. Thus, the average diameter of
Au NPs on Au/N-HAP and Au/L-HAP is much smaller than that
on the Au/R-HAP. As for Au/L-HAP sample, to a large extent,
partially coated nanostructure of Au NPs hinders the exposure
of Au species, although Au/L-HAP has higher surface area and
smaller average size of Au NPs than Au/R-HAP. Therefore, the
dispersion degree of Au species for Au/L-HAP is slightly larger
than that for Au/R-HAP.
NH3-TPD also was examined to determine the strength and
amount of surface acidic sites on samples. As shown in Figure
7, there are mainly three desorption ranges in the TPD profiles:
o
the desorption below 250 C is attributed to desorption of NH3
from weak acidic sites; the second one at 250-400 oC is
assigned to NH3 desorption from medium-strength acidic site;[65]
o
the third one above 400-650 C is ascribed to NH3 desorption
from strong acidic sites. Specially, Au/N-HAP also presents a
small desorption in the range 650–800 oC, owing to desorption
of NH3 from super strong acidic sites. Such super strong acidic
sites are probably related to the synergistic interaction between
POH groups and metal Ca2+ ions in the structure of N-HAP
support, as well as the high coordination number of metal Ca2+
ions. According to integral areas of desorption peaks, the
amount of total acidic sites increases gradually from Au/R-HAP
to Au/L-HAP and Au/N-HAP (Table 1), probably due to the
lowering of the Ca/P molar ratio originating from the formation
Surface acid-base properties of supported Au samples
2-
of more HPO4 groups in the structure of HAP, as well as the
Surface acid-base property of catalysts can play an
important role in affecting the catalytic performance in many
heterogeneous catalytic systems.[55,56] As for amphoteric HAP
support, surface acidic and basic sites are usually considered
to be related to POH groups or metal ions and OH- groups or
O2- ions of PO43- groups, respectively.[57,58]
increased specific surface area. Notably, the proportion of
strong and super strong acidic sites in the total amount of acid
sites increases gradually from 39 % for Au/R-HAP to 48 % for
Au/L-HAP and 60 % for Au/N-HAP.
In order to determine surface acidity, in situ FT-IR of
pyridine adsorbed on supported Au samples were measured
(Figure 6). In each case, a strong band at about 1454 cm-1 is
assigned to the vibration of pyridine adsorbed on Lewis acidic
sites associated with metal Ca2+ ions,[59-62] while a weak band
at 1545 cm-1 is assigned to the vibration mode of pyridine
adsorbed on Brønsted acidic sites related to the POH group in
the HAP structure.[63] And, an observable small shoulder band
at 1609 cm-1 is probably associated with pyridine adsorbed on
strong Lewis acidic sites.[64] Meanwhile, four absorption bands
for physisorbed (1439, 1565, 1580 cm-1) and H-bonding
adsorbed (1591cm-1) pyridine are observed in all samples.[64]
Specially, the enhancement in the relative intensity of
absorption band at 1454 cm-1 from Au/R-HAP to Au/L-HAP and
Au/N-HAP clearly reveals the gradually increased amount of
surface Lewis acidic sites.
Figure 7. NH3-TPD profiles of different supported Au samples.
Figure 6. In situ FT-IR spectra of pyridine adsorbed on different
samples: (a) Au/N-HAP, (b) Au/L-HAP and (c) Au/R-HAP.
Figure 8. CO2-TPD profiles of supported Au samples: (a) Au/N-HAP, (b)
Au/L-HAP and (c) Au/R-HAP.
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On the other side, to probe surface basicity of supported Au
samples, CO2-TPD was examined (Figure 8). Clearly, all of
supported Au samples show four desorption ranges: the first
desorption (I) below 250 °C is assigned to the CO2 desorption
from weak basic sites; the second desorption (II) at 250-470°C
to that from medium-strength basic sites; the third desorption
(III) at 470-600 °C to that from strong basic sites; the forth one
(IV) above 600 °C to that from super strong basic sites.[66,67]
Among them, weak basic sites are related to
hydrogenocarbonate species originating from surface hydroxyl
groups, while medium-strength and strong basic sites are
(1792 h-1) is about 2.2 times and 3.4 times as those for Au/L-
HAP and Au/R-HAP, respectively, indicating
a
greatly
enhanced catalytic activity. If the methanol/MA molar ratio is
further increased from 8:1 to 40:1, a complete MMA conversion
can be achieved over the Au/N-HAP after reaction for 2 h. In
contrast, other single metal oxides (CaO, SiO2, Al2O3, and MgO)
supported Au reference catalysts with the similar metal
loadings show greatly lowered catalytic activities with much
lower conversions (Table 3), although relatively higher MMA
selectivities are obtained over basic CaO and MgO supported
catalysts in comparison with Au/L-HAP and Au/R-HAP.
3-
mainly related to surface carbonate coordinating with PO4 (or
HPO42-) groups and O2- ions in the structure of HAP supports.[63]
Although the Ca/P molar ratio in samples is slightly lowered in
the following order: Au/R-HAP > Au/L-HAP > Au/N-HAP, the
amount of total basic sites shows a great increase. Specially,
Au/N-HAP provides a much higher amount of strong and super
strong basic sites (Table 1). The above results suggest that the
great encapsulation of Au NPs by the N-HAP support, i.e., the
formation of SMSI, probably leads to the formation of more and
stronger basic sites on the surface, which is closely associated
with the presence of more surface defects on the Au/N-HAP.
Catalytic performance of supported Au catalysts
Figure. 9 MA conversion (A) and MMA selectivity (B) with the reaction
time over supported Au catalysts in the oxidative esterification of MA.
Reaction conditions: methanol/MA molar ratio, 8:1; oxygen pressure, 0.1
MPa; reaction temperature, 70 oC.
Table 3 Catalytic oxidative esterification of MA over catalysts.[a]
Scheme 1. Main reaction pathways for the oxidative esterification of MA
with methanol.
TOF [b]
( h-1)
Conv.
(%)
Selectivity (%)
Catalyst
MMA
2
Acetal
97
Others [c]
1
Scheme 1 shows possible main reaction pathways that
happen in the course of oxidative esterification of MA with
methanol. Generally, the first step is the condensation between
MA and methanol to produce hemiacetal.[9,13] Further,
hemiacetal is oxidized to form MMA in the presence of
molecular oxygen. Meanwhile, further condensation between
hemiacetal and methanol can occur, and thus acetal by-
product is generated.
N-HAP [d]
--
38
Au/N-HAP
Au/L-HAP
Au/R-HAP
Au/N-HAP [e]
Au/N-HAP [f]
Au/N-HAP [g]
Au/CaO
1792
1101
397
2501
2918
--
68
48
36
87
100
34
13
5
98
76
43
99
99
4
0
18
53
0
2
6
4
1
0
1
95
3
1
29
90
47
6
7
The catalytic oxidative esterification of MA with methanol
was conducted over different supported Au catalysts. Figure 9
shows MA conversion and MMA selectivity with the reaction
Au/SiO2
6
35
85
5
18
9
Au/Al2O3
3
25
13
Au/MgO
20
83
12
o
time at 70 C, and the results are summarized in Table 3 (GC-
[a] Reaction conditions: methanol/MA molar ratio, 8:1; reaction
temperature, 70 oC; oxygen pressure, 0.1 MPa; reaction time, 2
h. [b] TOF of MMA produced, which was given as the overall
formation rate of MMA normalized by the number of active
metallic Au sites within the initial 30 min. [c] By-products:
isobutyric acid, methacrylic acid, and isobutyl aldehyde. [d] N2
atmosphere. [e] Methanol/MA molar ratio, 20:1. [f] Methanol/MA
molar ratio, 40:1. [g] N2 atmosphere.
MS spectra of typical experiments are shown in Figures S1 and
S2). Clearly, the structure of HAP supports exerts an important
impact on the oxidative esterification of MA. Notably, Au/N-
HAP generates a high MMA selectivity of ~98 % at 68 %
conversion after reaction for 2.0 h. In this case, the MMA
selectivity almost keeps unchanged within the whole reaction
process. For Au/L-HAP and Au/R-HAP, however, the
conversions only reach 48 % and 36 %, respectively, together
with low MMA selectivities (76 % for Au/L-HAP and 43 % for
Au/R-HAP). In two cases, more acetal by-product is generated.
Noticeably, the TOF (turnover frequency) value for Au/N-HAP
6
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(Figure S4), demonstrating that the oxidative esterification of
MA obeys first-order kinetics in the presence of excess
methanol. According to the linear relationship between lnk (k:
reaction rate constant) and reaction temperature (Figure 11),
the apparent activation energy of the oxidative esterification of
MA determined from Arrhenius equations is about 45 kJ/mol for
Au/N-HAP, 50 kJ/mol for Au/L-HAP and 65 kJ/mol for Au/R-
HAP, respectively. The aforementioned results further confirm
the excellent catalytic performance of Au/N-HAP catalyst.
In order to investigate the reusability of as-synthesized
supported Au/N-HAP catalyst, the spend catalyst was collected
and washed with ethanol and deionized water for several times,
dried overnight, and reused in next cycling tests. As shown in
Figure S5, after recycling for four times, the MMA conversion
only reduced slightly from 68 % to 64 %. Meanwhile, there is
no observable change in the MMA selectivity. Further,
elemental ICP-AES analysis shows that the weight loss of Au is
only about 0.86 wt % in the total Au content in the Au/N-HAP
after four cycles. The above results demonstrate the high
stability of the present Au/N-HAP catalyst, due to the presence
of SMSI.
Figure. 10 Effect of the reaction temperature on the MA conversion (A)
and the MMA selectivity (B) over the Au/N-HAP with the reaction time.
Conditions: methanol/MA molar ratio, 8:1; oxygen pressure, 0.1 MPa.
Reaction mechanism for oxidative esterifiction of MA
Previously, it was reported that surface acidic sites on
catalysts facilitated the oxidative transformation of aldehydes
and the further formation of acetals in the course of oxidative
esterification of benzaldehyde with methanol.[11] In our case,
compared with other two supported catalysts, Au/N-HAP with
relatively more surface strong acidic sites shows better
catalytic performance with much higher MMA selectivity and
MA conversion. In addition, under inert N2 atmosphere, pure N-
HAP support and Au/N-HAP catalyst also are found to exhibit
moderate conversions of MA with high selectivities to acetal by-
product (>95 %) (Table 3). It demonstrates that surface acidity
should play an important role in controlling the condensations
between MA (or hemiacetal) and methanol.
Based on the above characterizations and catalytic tests,
we tentatively propose Lewis-acid (I) and Brønsted acid (II)
jointly promoted mechanism of the condensation between MA
and methanol on the Au/N-HAP (Scheme 2). On the hand,
surface Lewis acidic sites as electrophilic centers or electron
acceptors can facilitate the activation of the C=O bond in MA
via its lone electron pair in oxygen,[68-71] thereby leading to the
condensation by direct nucleophilic attack of methanol to the
carbonyl group in MA and the following formation of hemiacetal
intermediate. On the other side, the proton from Brønsted acid
sites on the support also can interact with MA through
coordinating with the oxygen atom of carbonyl group to weaken
the C=O bond, thus forming protonated MA. Subsequently, the
electrophilic carbon of carbonyl group in protonated MA is
susceptible to be attacked by nucleophilic methanol species,
which can greatly promote the occurrence of the condensation
between MA and methanol. At last, hemiacetal intermediate is
formed.
Figure. 11 Ln(reaction rate constant) vs. reciprocal of reaction
temperature over different supported Au catalysts in the oxidative
esterification of MA.
As shown in Figure 10, the reaction temperature has a
significant effect on the MA conversion over the Au/N-HAP.
When the reaction temperature is elevated from 50 to 70 oC,
the MA conversion is significantly increased due to the
enhanced reaction rate, as well as the nature of endothermic
reaction. No obvious change in the MMA selectivity with the
reaction temperature is observed.
Based on the MMA
conversions over different HAP supported Au catalysts at
different temperatures (Figure S3), one can note that -ln(1-x) (x:
MA conversion) linearly increases with the reaction time
7
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Scheme 2. Proposed Lewis acid (I) and Brønsted acid (II) jointly promoted mechanism of the condensation between MA and methanol over Au/N-HAP.
Scheme 3. Proposed Lewis base (I) and Brønsted base (II) jointly promoted mechanism of hemiacetal oxidation over Au/N-HAP.
Further, hemiacetal intermediate can be oxidized to
produce MMA product in the presence of active Au NPs and
molecular oxygen. Herein, surface basic sites can play an
undergoes the β-hydride elimination to form metal–hydride
species at the catalyst interface and MMA product. At last,
activated oxygen in the form of Au-O-O-Au species on the
surface rapidly reacts with the metal-hydride and hydrogen
atom in adsorbed water molecule, while original Brønsted basic
sites are recovered to finish the catalytic oxidation process.
Accordingly, in the absence of strong basic sites, further
condensation between hemiacetal and methanol easily take
place and thus more acetal by-product is formed in the cases
of Au/L-HAP and Au/R-HAP, as evidenced by the experimental
results.
important role in controlling the hemiacetal oxidation.
A
possible mechanism of hemiacetal oxidation over the Au/N-
HAP promoted by surface cooperative effect between Lewis/
Brønsted basic sites and active Au species also is proposed
(Scheme 3). First, hydroxyl group in hemiacetal can interact
with Lewis basic sites adjacent to active metallic Au sites. Then,
the hydrogen in hydroxyl group is abstracted to the support and
the resulting negatively charged alkoxide intermediate
coordinates with active Au species at the interface.
Subsequently, unsteady β-hydride formed through the metal-H
bond is eliminated to generate a metal-hydride species,[72]
while MMA product is produced. Finally, molecular oxygen is
activated by metallic Au species through a double linear
model,[73,74] and thus both adsorbed proton and metal-hydride
on the surface are oxidized at the metal-support interface to
generate water. Meanwhile, Brønsted basic sites can uniquely
activate the hydroxyl group of hemiacetal to promote the
formation of metal alkoxide. That is, hydroxyl hydrogen in
hemiacetal interacts with Brønsted base (OH-), resulting in the
formation of alkoxide intermediate at the interface and water
molecule located at the other Brønsted base site. Similarly, the
β-H atom in alkoxide coordinating with Au species also
Among three supported catalysts, Au/N-HAP possesses
the largest amounts of surface acidic and basic sites, which
greatly promote the chemisorption-activation of MA and
reaction intermediate molecules. Meanwhile, Au/N-HAP
catalyst provides the most amounts of catalytically active Au0
sites on the surface due to the highest dispersive nature of Au
species, which plays
a crucial factor for facilitating the
formation of β-hydride and the oxygen activation in the course
of the oxidation of hemiacetal. Accordingly, Au/N-HAP
constructs the strongest cooperative effect between Au species
and surface acid-base sites, thus resulting in the superior
catalytic performance. As a result, compared with Au/L-HAP
and Au/R-HAP, Au/N-HAP catalyst exhibits much higher
catalytic performance.
8
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10.1002/cctc.201601560
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Conclusions
graphite-filtered Cu-Ka source (λ = 0.15418 nm) as the X-ray
source and the scanning rate of 10o/min.
Elemental analysis of samples was performed by using a
Shimadzu ICPS-7500 inductively coupled plasma atomic emission
spectroscopy (ICP-AES). The sample was dissolved by aqua regia
before measurements.
In summary, three types of needle-like, lamella-like and rod-
like HAPs supported Au catalysts are successfully synthesized.
A series of characterizations including XRD, SEM, TEM, Cs-
STEM, XPS, in situ FT-IR adsorption, TPD results demonstrate
the formation of well-dispersed Au0 NPs and surface defects on
catalysts. Surface defects on catalysts caused by the
encapsulation of Au NPs by HAP supports are found to be
helpful to the generation of strong basic sites. Compared with
other HAP supported ones, needle-like HAP supported catalyst
shows much better catalytic performance in terms of both the
activity and the MMA selectivity under mild reaction conditions.
It is believed that the cooperation between surface acid-base
sites and highly dispersive active Au species constitutes an
efficient catalytic reaction environment, thus being beneficial to
the chemisorption of MA, and the formation of β-hydride and
the following oxygen activation in the oxidative esterification of
MA. The present work develops new high-performance
supported Au catalysts with abundant surface acid-base sites
for practical industrial production of MMA by one-step direct
oxidative esterification of MA under mild reaction conditions.
Low temperature nitrogen adsorption-desorption experiments
were performed on Micromeritics ASAP 2020 sorptometer
apparatus at -196 °C. The Brunauer–Emmett–Teller (BET) method
was used to calculated specific surface areas.
Transmission electron microscopy (TEM) and high resolution
transmission electron microscopy (HRTEM) tests were conducted
using a JEOL 2100 electron microscope with the accelerating
voltage of 80~200 kV. Spherical aberration corrected scanning
transmission electron microscope (Cs-STEM) images was recorded
using a Cs-corrected JEOL JEM-ARM200F instrument.
Positron annihilation spectroscopy (PAS) was characterized by
a fast/slow coincidence ORTEC system using 22Na as radioactive
positron source (13×10-6 Ci) with a time resolution of 210 ps full
width at half maximum in the transmission mode at the Beijing
Synchrotron Radiation Facility (BSRF).
CO pulse chemsorption of samples was carried out on a
Micromeritics AutoChem II 2920 instrument with
a thermal
conductivity detector (TCD). The sample (0.1 g) was heated to
200 °C under He flow (40 ml/min) and then held for 1h. After the
temperature was reduced to 50 oC, pure CO gas was fed according
to a pulse titration procedure until the signal peak area was
constant. The dispersion of Au was determined based on the
CO/Au adsorption ratio of 1:1.
Experimental Section
Synthesis of HAP supports and supported catalysts
Synthesis of needle-like HAP. Calcium oxide (0.28 g) was
added into 100 mL of distilled water under vigorous stirring at
nitrogen atmosphere for 30 min to obtain Ca(OH)2 suspension.
Subsequently, a solution of Na3PO4·12H2O (1.14 g) dissolved in 20
mL of distilled water was slowly added into the above suspension,
and continuously stirred for 2 h under ultrasonic condition. The
resultant suspension was filtered, and the obtained solid was dried
at 70 °C overnight and then calcined at 400°C for 4h at a ramping
rate of 2°C/min at air. The obtained HAP was labeled as N-HAP.
Synthesis of lamella-like HAP. Calcium oxide (0.28 g) and
NH4H2PO4 (0.35 g) were dissolved in a mixture solvent of N,N-
dimethylformamide (DMF, 25mL) and deionized water (10 mL).The
solution obtained was aged at 120°C for 24 h. Subsequently, the
resultant suspension was filtered. At last, the obtained solid was
dried and calcined according to the identical procedure to N-HAP.
The obtained HPA was labeled as L-HAP.
X-ray photoelectron spectroscopy (XPS) experiments were
performed on VG ESCALAB 250 electron spectrometer at the
vacuum degree of 2×10-9 Pa. The Al Kα (hv =1486.6 eV) was used
as the target source. Binding energies were calibrated using C1s
peak at 284.6 eV.
In situ Fourier transform infrared (FT-IR) of pyridine adsorption
test was performed on a Nicolet 380 type spectrophotometer. First,
the sample (40 mg) were pressed into a thin slice and put into the
in situ IR cell. Then in N2 atmosphere, the sample was heated to
200 oC and held for 1h. When the temperature of the cell was
reduced to 25 oC, pyridine was introduced and balanced at this
temperature for 2 h. FT-IR spectra were recorded under vacuum
conditions.
Temperature-programmed desorption was tested using
Chemisorb 2720 instrument with a TCD. The sample (0.1g) was
first heated to 200 oC in a He flow (40 ml/min) and held for 1 h.
After the temperature was cooled down to 50 oC, the sample tube
was put into pure NH3 (or CO2) for 1h, and purged with a He flow
for 1 h. Finally, the temperature was increased to 700 °C at a rising
rate of 5 oC/min.
Synthesis of rod-like HAP. The synthesis procedure was
similar with that of L-HAP, except that the solvent was replaced by
a pure DMF (35 mL). The obtained HAP was labeled as R-HAP.
Synthesis of supported Au catalysts. Supported Au
catalysts were synthesized by a urea homogeneous deposition-
precipitation method. Typically, polyvinylpyrrolidone (PVP, 0.3 g)
was dispersed in 100 mL of deionized water with stirring. Then, 2.1
mL of HAuCl4 solution (24.28 M) was added into the above solution
under stirring for 30 min. Afterwards, HAP support (0.5g) was
added into HAuCl4 solution and stirred for 30 min. Then, a urea
solution (0.1 M) was added into the above suspension until pH =
9.0. Subsequently, the suspension was aged at 60°C for 1 h.
Finally, the white precipitate was filtered, washed with deionized
water for several times until pH was 7, dried at 70°C overnight and
reduced at 300 °C (2°C/min) for 2 h in a muffle furnace at air.
Catalytic test
The catalytic oxidative esterification of MA was carried out in a
50 mL glass reactor with a magnetic stirrer, where the catalyst (0.1
g), MA (1.04 ml) and methanol (4 ml) were charged. Before
reaction, the reactor was purged with ultrahigh purity O2 flow (30
ml/min) for three times, and each time was held for 2 min. Then,
the temperature of the reactor was increased to 70 oC.
Subsequently, the O2 was fed into the reactor and the relative
pressure was kept at 0.1 MPa. At last, the reaction was performed
with a stirring speed of 1000 rpm. A schematic diagram of a
reaction apparatus is presented at supporting information (Scheme
S1). After 2h, the reactor was cooled in an ice bath, and the
Characterization
X-ray diffraction (XRD) experiments were carried out on
Shimadzu XRD-6000 powder diffraction spectrometer with
9
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10.1002/cctc.201601560
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827–830.
reactants were centrifuged and quantitatively analyzed by gas
chromatography (GC) using an Agilent GC7890B equipped with
DB-WAX capillary polarity chromatographic column (30.0 m×320
mm×0.25 mm) and hydrogen flame ionization detector. Further, the
liquid products were determined by GC-MS using a Shimadzu QP
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Acknowledgment
We gratefully thank the financial support from National
Natural Science Foundation of China (21325624;
21521005) and the Fundamental Research Funds for the
Central Universities (buctrc201528).
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Keywords: Oxidative esterification • Methyl methacrylate •
Acid-base sites • Hydroxyapatite • Gold.
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Entry for the Table of Contents
FULL PAPER
Three needle-like, lamella-like, and
rod-like hydroxyapatite supported
gold nanocatalysts were employed in
direct oxidative esterification of
methacrolein to produce methyl
Jun Gao, Guoli Fan, Lan Yang,
Xinzhong Cao, Peng Zhang,Feng
Li*
Page No. – Page No.
methacrylate.
Supported
Au
nanocatalyst over the needle-like
hydroxyapatite presented a greatly
enhanced catalytic performance, due
to surface cooperative effect between
Oxidative esterification of
methacrolein
methacrylate
nanoparticles
hydroxyapatite
to
over
methyl
gold
on
abundant
acid-base
sites
for
of
preferential
chemisorption
methacrolein and highly dispersed
active Au species for the favourable
formation of β-hydride and oxygen
activation.
12
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