Effect of SSIE structure of Cu-exchanged β and Y on the selectivity for synthesis of diethyl carbonate by oxidative carbonylation of ethanol: A comparative investigation

Article abstract of DOI:10.1016/j.cattod.2009.06.013

Cu-exchanged β and Y catalysts were investigated by oxidative carbonylation of ethanol in the gas-phase reaction. Cuβ catalyst has shown better catalytic selectivity for oxidative carbonylation of ethanol to diethyl carbonate (DEC), without the principal by-product 1,1-diethoxyethane (DEE) for CuY catalysts. In order to investigate the effect of zeolite structure on the selectivity for products, computational analysis of molecular dimensions and diffusion parameters of DEC and DEE within Cuβ and CuY catalysts zeolite framework has been performed using molecular mechanics and quantum mechanics methods. The computational analysis results are in good agreement with the experimental results to some extent. DEC having a kinetic diameter of 3.663 A? and the lowest energy barrier was formed preferentially over both zeolites. However, the DEE molecule was not detected among the products over Cuβ because of its greater kinetic diameter 6.059 A? and higher energy barrier. The special architecture of β zeolite did not allow the diffusion of DEE molecules through its pores. The formation of the higher sterically hindered DEE over CuY catalyst could be explained by involvement of the outer surface.

Full text of DOI:10.1016/j.cattod.2009.06.013

Catalysis Today 149 (2010) 202–206
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Effect of SSIE structure of Cu-exchanged
b
of diethyl carbonate by oxidative carbonylation of ethanol: A comparative
investigation
and Y on the selectivity for synthesis
*
Pingbo Zhang, Shouying Huang, Yang Yang, Qingsen Meng, Shengping Wang, Xinbin Ma
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Weijin Road 92,
Nankai District, Tianjin 300072, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Cu-exchanged
reaction. Cu
b
and Y catalysts were investigated by oxidative carbonylation of ethanol in the gas-phase
Available online 21 July 2009
b
catalyst has shown better catalytic selectivity for oxidative carbonylation of ethanol to
diethyl carbonate (DEC), without the principal by-product 1,1-diethoxyethane (DEE) for CuY catalysts. In
order to investigate the effect of zeolite structure on the selectivity for products, computational analysis
Keywords:
Diffusion energies
Zeolites
of molecular dimensions and diffusion parameters of DEC and DEE within Cub and CuY catalysts zeolite
framework has been performed using molecular mechanics and quantum mechanics methods. The
computational analysis results are in good agreement with the experimental results to some extent. DEC
Oxidative carbonylation
Cu
b
˚
having a kinetic diameter of 3.663 A and the lowest energy barrier was formed preferentially over both
CuY
zeolites. However, the DEE molecule was not detected among the products over Cu
b
because of its
zeolite did not
˚
greater kinetic diameter 6.059 A and higher energy barrier. The special architecture of
b
allow the diffusion of DEE molecules through its pores. The formation of the higher sterically hindered
DEE over CuY catalyst could be explained by involvement of the outer surface.
ß 2009 Elsevier B.V. All rights reserved.
1. Introduction
that all of the catalysts prepared by impregnating methods
deactivated quickly because of losing the chlorine anion and the
Diethyl carbonate (DEC) is recognized as an environmentally
benign chemical because of its negligible ecotoxicity and low
bioaccumulation and persistence. Because of its high oxygen
contents (40.6 wt.%), DEC has been proposed as a replacement for
tert-butyl ether (MTBE) as an attractive oxygen-containing fuel
additive [1]. A further significant advantage of DEC over other fuel
oxygenates such as MTBE is that DEC could slowly decompose to
CO2 and ethanol, which have little environmental impact when
released into the environment [2]. In addition, the gasoline/water
distribution coefficients for DEC are more favorable than those for
dimethyl carbonate (DMC) and ethanol [3]. Besides these
applications as a fuel additive, DEC is also drawing attention as
a safe solvent and an additive in lithium cell electrolyte [3–5].
There are several synthetic methods for producing DEC, such as
a phosgene process [6], carbonylation of ethyl nitrite [7], oxidative
carbonylation of ethanol [8–12], and an ester exchange process
[13,14]. Among them, the vapor phase oxidative carbonylation of
ethanol represents one of the proposed favorable processes based
on the ‘‘green chemistry’’ principles. Previous results have shown
remodel of active copper species on the surface [10,15,16].
Recently, Cu-exchanged zeolites have shown to be preferable
active catalysts for the oxidative carbonylation of methanol to
dimethyl carbonate [17–19]. However, the selectivity for the Cu-
exchanged zeolites catalysts was not favorable with the principal
by-products being dimethoxymethane (DMM) and methyl formate
(MF) [20–22]. Up to now, the influences of zeolite pore structure on
the activity and selectivity for DEC synthesis of Cu-exchanged
zeolites have not been investigated.
Computational modelling has been shown an efficient method
of screening possible catalysts for shape-selective reactions [23].
Catalytic conversion and selectivity of molecules in zeolites were
influenced by several factors, molecular dimensions, the pore
structure, diffusion energy, etc. Among those factors, the diffusion
energy of products could play an important role. The information
about interactions at the molecular level is difficult to obtain
experimentally. So computer simulation, using molecular
dynamics techniques, is an attractive alternative to study the
influences of zeolite pore structure on the activity and selectivity
for products.
The main objective of this work was to gain the calculation
and comparison of diffusional energies for possible products
* Corresponding author. Tel.: +86 22 27406498; fax: +86 22 27890905.
E-mail addresses: xbma@tju.edu.cn, xinbinma@gmail.com (X. Ma).
by oxidative carbonylation of ethanol inside the channels of
b
0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2009.06.013

P. Zhang et al. / Catalysis Today 149 (2010) 202–206
203
˚
and Y zeolites. In the present work, the catalytic performances of
Cu-exchange and Y catalysts by oxidative carbonylation of
ethanol in the gas-phase reaction were investigated. The experi-
mental results were compared with the computer simulation
results to find an explanation at the molecule level for different
behaviours of products in these zeolites in the studied reactions.
was forced to diffuse stepwise, in steps of 0.1 A inside the 12 m
channel between two points, which defined the diffusion path.
These two points were located at the mid-points of the pore
b
apertures of the 12 m channel in zeolites
b and Y. In every position,
the minimized diffusion energies were searched. The diffusion
energies (Edif) were calculated by the following expression:
2. Experimental and methods of calculation
2.1. Experimental
Edif ¼ Ezꢀd ꢀ ðEz þ EdÞ
In the expression, Edif is the molecular diffusion energy in
zeolite, Ezꢀd denotes the system energy of zeolite and diffusing
molecule, Ez is the energy of only zeolite and Ed shows the energy of
only molecule.
The catalysts were prepared by solid-state ion exchange (SSIE)
of the protonated form of each zeolite (Y: Si/Al = 5;
b: Si/Al = 20)
with CuCl in a flow of N2 at a heating rate of 10 8C/min up to the
desired temperature 550 8C and then held for 6 h.
3. Results and discussion
3.1. Catalytic results of Cu-exchange
b and Y catalysts
2.2. Production and analysis of product
The catalytic results of Cu-exchange
oxidative carbonylation of ethanol with CO and O2 were shown in
Table 1. The distribution of products for Cu-exchange and Y
catalysts system was completely different in Table 1. The Cu
catalysts showed better selectivity to DEC based on ethanol
without DEE formation. Although CuY catalysts exhibited similar
conversion of EtOH, the selectivity for DEC was not favorable.
Consequently, the Cub catalysts attracted much attention because
of its relative higher yield of DEC. In addition, substantial amounts
of the by-products, ethyl acetate and acetaldehyde, were produced
b and Y catalysts on
Catalytic activity was measured by a computer-controlled
continuous microreactor system (WFS-3015) with a quartz
tubular reactor of 4 mm inner diameter. The reaction products
collected in the cooling trap were taken out and sampled every
hour, and were analyzed by a gas chromatograph (GC) (4890D,
Agilent) with a FID detector. The uncondensed gas products were
introduced to the gas chromatograph (GC-8A, Shimadzu) through
an on-line six-way valve and analyzed by a TCD detector with a
TDX-01 and Propak-Q packed column. The reaction conditions
were steadily kept at a reaction temperature of 140 8C and a
reaction pressure of 0.64 MPa.
b
b
under CuY catalysts system, while the by-products for Cu
catalysts only contained acetaldehyde.
b
As known, distribution of products over zeolite catalysts was
influenced by several factors, products’ shape, size and the
diffusion energy, etc. Hence, molecular dimensions and diffusion
energy of studied molecules were calculated in succession.
2.3. Methods of calculation
The Material Studio (Version 3.1) software package was used for
computational modelling analysis. The analysis was based on
energies, dimensions and diffusion energies of the molecules in
the zeolite. The molecular dimensions of studied molecules DEC and
DEE were calculated using the Dmol3 module function of Material
Studio 3.1. The diffusion energies of the molecules inside zeolite
channel were computed using the Discover module of Material
3.2. Molecular dimensions
The molecular representations of DEC and DEE were shown in
Fig. 1. The smallest sectional (or critical) diameters of DEC and DEE
were calculated using the Dmol3 module function of Material
Studio 3.1. From Fig. 1, the DEE molecule is approximately like cage
Studio3.1. The FAU (Yzeolite)and BEA (bzeolite)channelsstructure
were from zeolite in Material Studio 3.1 and optimized with SiO
clusters (Si196O384Al96 for a FAU cluster and Si256O512 for a BEA
cluster). It must be pointed out that, for these calculations, an ideal
channelofthezeoliteisused.Additionalfactors,suchasthechemical
compositionofthezeoliteframeworkandtheacidicpropertiesofthe
zeolitetogetherwiththemechanismoftheformationoftheproducts
(primary or secondary) were not taken into account.
Molecular diffusion energies calculations were used by CVFF
force field [24]. During the calculation of diffusion energies,
the product molecules moved from jumping-off point to end point
in the paths, which were chosen in advance. The product molecule
Table 1
Comparison of Cu-exchange
b
and Y catalysts systems on catalytic performance.
Catalyst
Conversion
of EtOH (%)
Selectivity (%)
DEC
DEE
Ethyl acetate
Acetaldehyde
Cu
b
4.3
4.2
78.4
23.9
0
0
21.6
9.1
CuY
56.1
10.9
Reaction condition: T = 140 8C, P = 0.64 MPa, O2 = 4 sccm, CO = 40 sccm,
2 = 26 sccm.
N
Fig. 1. Molecular representations of 1,1-diethoxyethane (DEE) and diethyl carbonate (DEC).

204
P. Zhang et al. / Catalysis Today 149 (2010) 202–206
Fig. 2. The pore structure of
b and Y zeolites.
˚
type and the smallest sectional diameter of DEE is 6.059 A. And the
calculation of diffusion energies of the molecules inside zeolite
channel is necessary.
DEC molecule approximates fold line type and the smallest
˚
sectional diameter of DEC is 3.663 A.
The pore structures of and Y zeolites are different, as shown in
Fig. 2. zeolite has a three-dimensional interconnected (3D)
channel system with 12-membered ring openings. Interconnected
b
3.3. Diffusion energies
b
The channels dimensions from X direction of
different from those from Y direction. So energy parameters on the
basis of both diffusion channels were calculated over zeolite. The
diffusions of the molecule of DEC in and Y zeolite were taken
graphic example in Figs. 3–5. Energy profile for the diffusion of DEC
in zeolite from X direction was taken example for the calculation
b zeolite are
˚
˚
linear pores with an opening of 7.6 A ꢁ 6.6 A run in X direction, and
communicate through perpendicular connecting openings of
b
˚
˚
5.6 A ꢁ 6.6 A in the Y direction [25,26]. Y zeolite also has a
b
three-dimensional interconnected (3D) channel system with only
˚
˚
one 12-membered ring openings of 7.4 A ꢁ 7.4 A.
b
From molecular dimensions and pore structure of zeolites, DEC
molecule of fold line type easily diffused through both channels of
b
and Y zeolites. Whereas the smallest sectional diameter of DEE
molecule like cage type was larger than pore dimension of zeolite
from Y direction. So DEE molecule hardly diffused through both
channels of zeolite from Y direction. From Table 1 in the reaction
over Cu zeolite, DEC was observed as the main product with
relatively small amounts of acetaldehyde. This may be attributed
b
b
b
˚
˚
˚
˚
to the different architecture of pores (7.7 A ꢁ 6.6 A, 5.6 A ꢁ 5.6 A
interconnected channels, absence of supercages in ) in which DEE
cannot freely move. The results clearly indicated that the pore
b
architecture and pore dimensions of
selective synthesis of DEC.
b
zeolite were suitable for the
Hereinbefore, effect of selectivity of products over
b
and Y
zeolites was investigated from molecular dimensions and pore
structure of zeolites. The results observed on the basis of
molecular dimensions and pore structures of zeolites predicted
that
b zeolite could be a potential catalyst for shape-selective
oxidative carbonylation of ethanol. In fact, molecules maybe
arise from a possible conformational flexibility during diffusing in
the channels of zeolites. And the preferential formation of DEE
against DEC on Y zeolites possibly arises from their differences in
diffusion kinetics although the two molecules could easily diffuse
in enough empty space with small molecular diameters. The
Fig. 3. Molecular representation of DEC in channel of
b zeolite from X direction.

P. Zhang et al. / Catalysis Today 149 (2010) 202–206
205
Fig. 6. Energy profile for the diffusion of DEC in
b zeolite from X direction.
the special architecture of
diameter of DEE.
b zeolites and smallest sectional
As known, the active center of Cu-exchanged zeolites catalysts
is formed by the reaction that Cu+ atom of CuCl replaced the H+ of
the OH group in the zeolite with the releasing of HCl. Most
Brønsted acidic sites were presented in the inner surface of pore
Fig. 4. Molecular representation of DEC in channel of
b zeolite from Y direction.
of diffusion energies in Fig. 6. The diffusion energy profiles of other
products in zeolites were not particularized one by one.
The energy parameters calculated from the diffusion energy
channel of
b zeolite [27]. The active centers of Cu-exchanged
zeolite catalysts were formed by the interaction of gaseous CuCl
and H+ of Brønsted acidic sites in the zeolite. Therefore, most active
profiles for DEC and DEE in
Table 2. Although there was more empty space available in channel
of Y than in , the energy barrier values calculated for DEC and DEE
in channels of Y zeolite were higher than those obtained for
zeolite in Table 2. DEC had a lower energy barrier in comparison
with DEE in zeolite channel from X and Y directions, which
b and Y zeolites channel were listed in
center of Cu
of zeolite, and there existed almost no active center in the outer
surface. From this point, the reason why DEE is not produced on the
outer surface of Cu was easily understood.
b catalyst located in the inner surface of pore channel
b
b
b
b
b
And in case of Y zeolites, the higher sterically hindered DEE over
CuY catalyst had the higher yield among the products, which was
somewhat different from the experimental results.
supported the products distribution in experimental results from
Table 1. It was found that DEC had the lowest energy barrier
(ꢀ38.91 kcal/mol) in
products. This computational analysis of diffusion energy for DEC
in zeolites channel was in good agreement with the yield of DEC
over Cu catalyst. Although DEE had low diffusion energy barrier
in zeolites channel from X and Y direction, DEE was absent
among the products over Cu catalyst. It could be interpreted by
b
zeolites channel from Y direction among
Spoto et al. have studied in detail the state of copper in copper–
zeolite materials made by interactions of H-form zeolite with
gaseous CuCl [28]. They discovered that gaseous CuCl reacts with
H+ of the most acidic Brønsted sites of the zeolite at 300 8C as the
following schemes, which were connected with the Al atom. It was
difficult to react with only H+ from the HO–Si–. Therefore, the more
Al content in the zeolites facilitates the more catalyst active
centers forming, and the more active centers maybe advanced the
performance of the catalysts.
b
b
b
b
(1)
(2)
In CuY catalyst system, the content of Al was much more than
that of Cub because Si/Al (5) ratio of CuY was lower than that of Cub
Table 2
Diffusion energies of products in channel of
b
and Y zeolites.
Edif -Y)
Product
Edif
(
b-X)
(
b
Edif(Y)
molecule
(kcal/mol)
(kcal/mol)
(kcal/mol)
DEC
DEE
ꢀ38.61
ꢀ34.81
ꢀ38.91
ꢀ33.91
ꢀ32.81
ꢀ26.71
Fig. 5. Molecular representation of DEC in channel of Y zeolite.

206
P. Zhang et al. / Catalysis Today 149 (2010) 202–206
(20). Sotherewerea massofactivecentersofYzeolite, whichexisted
not only in the inner surface of pore channel but also on the outer
surface of the frame [29]. Therefore, the formation of DEE over CuY
catalyst could be explained rightly by involvement of the more
active centers on the outer surface, which was not influenced by the
pore structure of zeolite and molecule diffusion in the channel.
Hence, the results of computational analysis for sterically hindered
DEE over CuY catalyst are reasonable considered from this point.
(Grant No. 05YFJMJC09800) and the Program of Introducing Talents
of Discipline to Universities (No. B06006) are gratefully acknowl-
edged.
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4. Conclusion
Cu-exchanged
DEC by oxidative carbonylation of ethanol in the gas-phase
reaction. Cu catalyst has been shown better catalytic selectivity
for oxidative carbonylation of ethanol to DEC, without the principal
by-product 1,1-diethoxyethane (DEE) for CuY catalyst. Cu
b and Y catalysts showed different selectivity to
b
b
catalyst was predicted to be a shape-selective catalyst for synthesis
of DEC by oxidative carbonylation of ethanol.
The calculated values have been compared with the experi-
mental results obtained in the oxidative carbonylation reaction of
ethanol over Cu-exchanged zeolites catalysts. In the case of the
reaction, the computational analysis results were in good
agreement with the experimental results achieved in the oxidative
carbonylation reaction of ethanol over Cu
catalysts on the whole. The kinetic diameter of DEC is 3.663 A and
the lowest energy barrier was obtained for DEC in channel from Y
direction. So DEC was formed preferentially over Cu catalyst.
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The financial supports from the National Natural Science
Foundation of China (NSFC) (Grant Nos. 20576093 and
20876112), the Committeeof Tianjin Science and Technology (CTST)