An intracrystalline catalytic esterification reaction between ethylene glycol intercalated layered double hydroxide and cyclohexanecarboxylic acid

Article abstract of DOI:10.1016/j.catcom.2015.01.004

An intracrystalline catalytic esterification between ethylene glycol intercalated NiAl layered double hydroxide and cyclohexanecarboxylic acid is developed. It leads to much higher deacidification efficiency than the bulk catalytic reaction. The intracrys

Full text of DOI:10.1016/j.catcom.2015.01.004

An intracrystalline catalytic esterification reaction between ethylene glycol
intercalated layered double hydroxide and cyclohexanecarboxylic acid
Hao Wang, Weizhuo Duan, Yanqing Lei, Yan Wu, Ke Guo, Xiaodong
Wang
PII:
DOI:
Reference:
S1566-7367(15)00012-6
doi: 10.1016/j.catcom.2015.01.004
CATCOM 4186
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
15 October 2014
9 December 2014
6 January 2015
Please cite this article as: Hao Wang, Weizhuo Duan, Yanqing Lei, Yan Wu, Ke Guo,
Xiaodong Wang, An intracrystalline catalytic esterification reaction between ethylene
glycol intercalated layered double hydroxide and cyclohexanecarboxylic acid, Catalysis
Communications (2015), doi: 10.1016/j.catcom.2015.01.004
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ACCEPTED MANUSCRIPT
An intracrystalline catalytic esterification
reaction between ethylene glycol intercalated
layered
double
hydroxide
and
cyclohexanecarboxylic acid
Hao Wang, Weizhuo Duan, Yanqing Lei, Yan Wu*, Ke Guo and Xiaodong Wang
School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu
610500, P. R. China
*Tel: +86-28-83037338; E-mail: swpuwh@gmail.com
ABSTRACT: An intracrystalline catalytic esterification between ethylene glycol intercalated NiAl layered
double hydroxide and cyclohexanecarboxylic acid is developed. It leads to much higher deacidification
efficiency than the bulk catalytic reaction. The intracrystalline catalytic reaction mechanism has been proposed.
Keywords: Intracrystalline catalytic esterification, layered double hydroxide, deacidification
Due to the acidity or basicity of sheets, LDHs can act
as esterification catalysts[7]. In pervious study[8], LDHs
1. Introduction
LDHs are a class of anionic layered materials that can be
were used as catalysts to remove naphthenic acids from
crude oil via esterification, however, this reaction suffers
from diffuse limitation caused by highly viscous ethylene
glycol (EG), leading to low catalytic efficiency. A
possible pathway to eliminate the diffusion limitation is
that EG anions are pre-immobilized in the interlayer of
LDHs and then naphthenic acids diffuse into the
interlayer and react with EG under the catalysis of metal
ions in sheets. To date, there is no report on this
intracrystalline catalytic esterification.
In this work, firstly, an EG intercalated NiAl LDH
(NiAl-EG) with a Ni/Al ratio of 2 was synthesized by using
nitrate LDH (NiAl-NO3) as precursor and EG as solvent;
KOH was added into EG to facilitate the deprotonation of
EG. Secondly, the intracrystalline catalytic esterification
of NiAl-EG with cyclohexanecarboxylic acid (Cyc) was
performed and compared with the conventional bulk
catalytic reaction. Herein, Cyc in dodecane was used for
the purpose of convenient analysis.
III
represented
by
[MII x(OH)2]x+(An–)x/n·yH2O[1].
M
1–x
Because of the layered structure and interlayer
exchangeable anions, LDHs can be used as “molecular
container” for immobilization of anions. It allows
intracrystalline reactions in which the control of the
reactants diffusion imposed by the structural nature of
LDHs could strongly determine the rate[2], selectivity[3]
and yield[4]. A common intracrystalline reaction is
formed by intercalating one reactant into the interlayer of
LDHs. Prévot et al.[4] found the O-alkylation of benzoate
intercalated LDHs with alkyl halide increased ester yield
and decreased by-products. Another intracrystalline
reaction is formed by intercalating catalytic sites into the
interlayer of LDHs[3]. Shi et al.[5] prepared titanium
tartrate complex intercalated LDH as catalyst to enhance
asymmetric induction in sulfoxidation of pro-chiral
methyl phenyl sulphide through the constrained complex
in the interlayer. However, less attention has been paid on
establishing an interlayer catalytic reaction by virtue of
the catalytic effect of metal sheets.
2. Results and discussion
1

ACCEPTED MANUSCRIPT
The XRD patterns of NiAl-NO3 and NiAl-EG are shown
in Fig. 1. Each sample shows the general features of
LDHs[1]. The (003) reflection of NiAl-EG shifts to a
lower angle compared with that of NiAl-NO3. The basal
spacing of (003) reflection (d003) is 1.12 nm, subtracting
the thickness of brucite layer (0.48 nm[1]), the gallery
height is 0.64 nm, larger than the length of EG (0.42 nm).
Therefore, EG anions may be accommodated in the
interlayer as a tilted bilayer with the negative ethyleneoxy
groups attaching to the upper or lower hydroxide
layers[9].
The FT-IR spectra of NiAl-NO3, NiAl-EG and pristine
EG are shown in Fig. 2A. For NiAl-NO3, an apparent
-
band at 1384 cm-1 is assigned to the v3 vibration of NO3
[1]. For NiAl-EG, there are two absorption bands at 2940
and 2880 cm-1 assigned to asymmetric and symmetric
stretching vibrations of -CH2, respectively, and two bands
at 1086 and 1047 cm-1 ascribed to the in-plane bending
vibration of C-C-O[10]. Moreover, rocking vibrations of -
CH2 centered at 880 and 863 cm-1 are observed. The
presence of these bands confirm EG anions within the
interlayer. The band at 1383 cm-1 in NiAl-EG is very
d003=1.61 nm
-
close to that assigned to NO3 , but no nitrogen is detected
by elemental analysis (Table S1†). Therefore, this band is
ascribed to the scissoring, wagging or twisting vibration
of -CH2[11]. The O-H stretching at 3367 cm-1 in pristine
EG shifts to 3415 cm-1 in NiAl-EG, due to the interaction
between ionized guests and layers of LDHs[12].
Additionally, owing to this interaction, the absorption
bands of C-C-O of NiAl-EG shift to higher wavenumber
than those of EG[13].
NiAl-EG-RC
d003=0.88 nm
NiAl-NO3-RC
d003=1.12 nm
NiAl-EG
d003=0.88 nm
NiAl-NO3
The DSC curves of NiAl-EG (Fig. S1b†) shows a sharp
and intensive exothermal peak, resulting from the
decomposition and combustion of organic species in the
interlayer. Moreover, the temperature of this peak (265
10
20
30
40
()
50
60
70
o
oC) is higher than EG’s boiling point (198 C), indicating
Fig. 1. XRD patterns of NiAl-NO3, NiAl-EG, NiAl-NO3-
RC and NiAl-EG-RC
the host-guest interaction.
The above characterization results clearly demonstrate
the successful intercalation of EG into the interlayer.
According to the elemental analysis data (Table S1†), the
stoichiometric formula of NiAl-EG is expressed as
A
EG
-
[Ni0.67Al0.34(OH)2(C2H5O2 )0.33·0.32H2O].
The intracrystalline catalytic esterification reaction was
carried out by adding NiAl-EG (0.9 mmol EG within the
interlayer) to a solution of Cyc (2.5 mmol) and dodecane.
A bulk catalytic reaction over NiAl-NO3 catalyst and a
non-catalytic reaction between EG and Cyc were also
performed under the same conditions, wherein the
amounts of EG, Cyc and dodecane were as same as those
in the intracrystalline reaction (see details in
supplementary data). After reaction, NiAl-EG and NiAl-
NO3 were recovered (see details in supplementary data)
and noted as NiAl-EG-RC and NiAl-NO3-RC,
respectively. The acid concentration of solution was
analysed by GC-MS. The reaction results of the
intracrystalline, bulk and non-catalytic reactions are
listed in Table 1. The deacidification ratio of the
intracrystalline catalytic reaction is extremely higher than
that of the bulk catalytic and non-catalytic reactions.
Table 1. The results of intracrystalline catalytic, bulk
catalytic and non-catalytic reactions
NiAl-EG
NiAl-NO3
4000
3000
2000
Wavenumber (cm-1)
1000
B
CycNa
NiAl-EG-RC
NiAl-NO3-RC
Intracryst
alline
Bulk
catalytic
reaction
Non-
catalytic
reaction
catalytic
reaction
4000
3000
2000
1000
Wavenumber (cm-1)
Deacidification
ratio
Ester yield
69.80%
36.52%
2.33%
2.33%
0
-
Fig. 2. FT-IR spectra of (A) NiAl-NO3, NiAl-EG and EG;
(B) NiAl-NO3-RC and NiAl-EG-RC
2

ACCEPTED MANUSCRIPT
EG conversion
91.09%
7.22%
-
7.22% EG (Table 1) in the bulk catalytic reaction was
converted, despite of the same amount of EG in both
reactions. The remarkably enhanced esterification
reactivity for NiAl-EG could be attributed to the
following reasons. First, EG anions are confined in the
nano-sized interlayer space of LDH, greatly limiting the
freedom of EG and hence enhancing the collision
probability between EG and Cyc. Wang et al.[2] also
attributed the faster reaction rate of intracrystalline
oxidation between thiosulfate intercalated ZnAl LDH and
hexacyanoferrate (III) than that of bulk reaction to the
same reason. Second, EG anions in the interlayer have
higher nucleophilicity than EG molecules in liquid,
favourable to attacking the less electrophilic carbon in
carbonyl of Cyc. An et al.[3] found the proline anions
intercalated in LDH were more nucleophilic than
nonionized proline and thus attacked the acetone more
readily, resulting in the easier formation of carbinol amine
intermediate and much faster asymmetric aldol reaction
rate. Third, the pre-immobilization of EG in LDH can
eliminate the diffusion limitation from highly viscous EG
and enlarge the interlayer space to facilitate the Cyc
molecules diffusing into the interlayer.
The XRD patterns of LDHs after reaction are displayed
in Fig. 1. In each case, the layer structure is remained.
NiAl-EG-RC shows a d003 of 1.61 nm and gallery height
of 1.13 nm that is almost twice of the molecular size of
Cyc (0.62 nm). Therefore, Cyc anions in NiAl-EG-RC
may be a tilted bilayer arrangement. The XRD patterns of
NiAl-NO3 and NiAl-NO3-RC show no obvious
differences.
The FT-IR spectra of LDHs after reaction are shown in
Fig. 2B. Sodium cyclohexanecarboxylate (CycNa) was
used as reference. The absorption bands of -CH2 in NiAl-
EG-RC at 2929 and 2854 cm-1 are very close to those in
CycNa (2927 and 2852 cm-1). Additionally, two bands at
1569 and 1352 cm-1 in NiAl-EG-RC are corresponding to
the asymmetric and symmetric vibrations of C=O in -
COO-[3], respectively, suggesting Cyc anions in the
interlayer. The DSC curve of NiAl-EG-RC (Fig. S2b†)
shows a strong exothermal peak with maximum value at
343 oC, due to the decomposition and combustion of Cyc
anions in the interlayer. These characterization results
confirm the presence of Cyc anions in the interlayer of
NiAl-EG-RC. According to the elemental analysis result
(Table S1†), the formula of NiAl-EG-RC is expressed as
-
[Ni0.68Al0.34(OH)2(C7H11O2 )0.33·0.23H2O]. NiAl-NO3-RC
and NiAl-NO3 display the similar absorption bands,
indicating no changes after bulk reaction, which agrees
with TG-DSC result (Fig. S1a† and Fig. S2a†).
Both the unchanged NiAl-NO3 after bulk catalytic
reaction and the higher deacidification ratio than the non-
catalytic reaction demonstrate the catalytic effect of LDH.
By GC-MS, ester was detected in the liquid product of
intracrystalline and bulk catalytic reactions. To determine
the detailed structure of this unusual ester, it was
extracted from liquid product (see details in
supplementary data) and further characterized by FT-IR
(Fig. S3†), 1H NMR (Fig. S4a†) and 13C NMR (Fig.
S4b†). All the results demonstrate that the esterification
of EG with Cyc mainly produces 2-hydroxyethyl
cyclohexanecarboxylate (CAS No. 16179-44-5).
Scheme 1 A potential mechanism of intracrystalline
For NiAl-EG, the Cyc in feed are removed through two
paths. The first one, Cyc molecules enter the gallery of
LDH and react with pre-intercalated EG under the
catalysis of metal in sheets. Hence they are converted into
uncharged esters which diffuse to the solution. It is
proved by that esters were only detected in solution and
no band at 1733 cm-1 assigned to vC=O of ester (Fig. S3†)
was observed in FT-IR spectrum of NiAl-EG-RC. The
second one, after the consumption of EG, Cyc anions
occupy the interlayer and Cyc intercalated LDH is
formed. The Cyc removal ratios of the two paths can be
calculated by the formula of NiAl-EG-RC and
deacidification ratio, which are 36.52% (Table 1) for the
first path and 33.28% for the second. It means that the
deacidification ratio through esterification path or ester
yield of the intracrystalline reaction is 14 times as large as
that of bulk reaction. In other words, 91.09% EG (Table
1) in the interlayer was converted into ester while only
catalytic esterification in NiAl-EG
A potential mechanism of this intracrystalline catalytic
esterification is proposed, as shown in Scheme 1.
First, due to the relatively high electronegativity of Ni,
hydroxyl groups in the sheets show acidity[14] and thus
the proton of hydroxyl could combine with carbonyl
oxygen in Cyc to increase the electrophilicity of carbon in
carbonyl. Lei et al.[15] also suggested that the weak acid
sites on LDH sheets could interact with the carbonyl
group of acetone to polarize the C=O bond and hence
increase the density of positive charge on the carbon of
carbonyl group. Moreover, due to the electron donation
effect of guest organic anions, the O-H bonds of
hydroxyls in the sheets are weakened and protons are
readily produced, which enhances the acidity of layer[16].
To verify the catalytic effect of acidity, EG intercalated
3

ACCEPTED MANUSCRIPT
MgAl LDH was synthesized and used as esterification
3. Conclusion
catalyst under the same reaction conditions. Its
deacidification ratio is 52.32% and lower than that of
NiAl-EG (69.80%), indicating that more acidic NiAl
sheets show better catalytic effect.
Second, the carbonyl carbon with increased
electrophilicity is then attacked by EG anions with
enhanced nucleophilicity in the interlayer and an
intermediate [A] is produced. Li et al.[7] reported that the
deprotonated EG helps to the esterification between EG
and naphthenic acid in vacuum gas oil.
Third, [A] is converted into another intermediate [B]
through the loss of water from the hydroxyl of [A] and
proton of Cyc. Dodecane is an apolar aprotic solvent, in
which Cyc could not ionize. Therefore, Cyc anions should
be generated from the intracrystalline esterification.
Fourth, [B] may lose proton to compensate the sheets
and form ester product accompanying with the recycle of
catalyst. The similar recycle of H+ from LDH sheets was
also reported by Lei et al.[7].
The intracrystalline catalytic esterification between NiAl-
EG and Cyc has been developed. NiAl-EG is synthesized
via ion exchange under mild conditions by the aid of
KOH. The immobilized EG in LDH can react with the
Cyc diffusing into the interlayer and produce ester
entering liquid. Cyc anions occupy the interlayer after EG
anions are consumed. Deacidification ratio, ester yield
and EG conversion of intracrystalline reaction reach up to
69.80%, 36.52% and 91.09%, respectively, extremely
higher than those of the bulk catalytic reaction over NiAl-
NO3 (2.33%, 2.33% and 7.22%). The confined interlayer
enhances the collision probability between EG anions and
Cyc. EG anions pre-intercalated in LDH display increased
nucleophilicity and eliminate the diffusion limitation,
moreover, the enlarged interlayer space facilitates Cyc
diffusing into the interlayer.
■ Acknowledgments
Finally, Cyc anions enter the interlayer of LDH as
counter ions, and ester products diffuse to the solution.
To confirm the intracrystalline reaction, a reaction with
equimolar amount of EG (in the interlayer of NiAl-EG)
and Cyc was performed. Ester product was detected in the
solution, whereas EG was not detected. It suggests the
esterification occurs in the interlayer instead of in the
solution through the exchange between Cyc and EG.
The FT-IR and XRD characterization results for LDH
collected with different reaction time are given in Fig.
S5†, S6† and Table S2†. It is found that Cyc and EG
could co-exist in the interlayer of LDH when the reaction
time is short, due to the rapid reaction and consumption of
EG. Moreover, the concentration of Cyc in liquid at
different reaction time is shown in Fig. S7†. It declines
sharply before 20 min due to the quick diffusion and
reaction of Cyc in the interlayer. Chen et al.[17] and
Wang et al.[2] also found the similar result during the
intracrystalline oxidation reactions in LDH. It is worth
noting that no EG was detected by GC-MS in the solution
at different reaction time, which proves that EG may react
with Cyc in the interlayer rather than in solution.
According to the above mechanism and analysis
results, the steps of this intracrystalline catalytic
esterification could be proposed (Scheme S1†): (1) Cyc
molecules transfer from the bulk solution to the edge of
NiAl-EG; (2) Cyc molecules diffuse into the interlayer;
(3) the esterification occurs between Cyc and pre-
intercalated EG anions in the interlayer under the
catalysis of metal in sheets; (4) the uncharged esters
diffuse to the edge of LDH; (5) the esters enter solution,
leaving Cyc anions in the interlayer.
The authors acknowledge the financial support from the
National Natural Science Foundation of China
(20906075) and the Foundation of State Key Laboratory
of Oil and Gas Reservoir Geology and Exploitation
(PLN1127).
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5

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Graphical abstract
An intracrystalline catalytic esterification
between EG anions pre-immobilized in
layered
double
hydroxide
and
cyclohexanecarboxylic acid was constructed.
Apparently higher deacidification ratio than
that of the bulk catalytic reaction was
obtained.
6

ACCEPTED MANUSCRIPT
Highlights:
 Ethylene glycol (EG) intercalated
LDH was synthesized.
 Intracrystalline esterification occurs
between
EG
in
LDH
and
cyclohexanecarboxylic acid.
 LDH sheets act as esterification
catalyst.
 This reaction shows much higher
deacidification efficiency than bulk
reaction.
7