Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A DFT study on the reaction mechanism for dimethyl carbonate synthesis from
methyl carbamate and methanol
Yangyan Gao a,b , Weicai Peng a , Ning Zhao a , Wei Wei a , Yuhan Sun a,c,∗
a
b
c
State Key Laboratory of Coal Conversion Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, PR China
Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
Low Carbon Energy Conversion Technology Research Center Shanghai Advanced Research Institute, Chinese Academy of Science, Shanghai, 201203, PR China
a r t i c l e
i n f o
Article history:
Received 20 April 2011
Received in revised form 5 September 2011
Accepted 8 September 2011
Available online 16 September 2011
Keywords:
Density functional theory (DFT)
Dimethyl carbonate
Methyl carbamate
Methanol
a b s t r a c t
Using density functional theory, the reaction mechanism for dimethyl carbonate (DMC) synthesis from
methyl carbamate (MC) and methanol (CH3 OH) with and without catalysts is investigated. And to investigate solvent effects, the reactions are simulated in CH3 OH solvent by using the conductor like solvent
model (COSMO). In our calculation, the uncatalyzed MC methanolysis reaction is kinetically and thermodynamically unfavorable. However, it is obviously catalyzed by acid and base catalysts. By orbital
energy calculation, MC methanolysis reaction is considered as the charge-controlled reaction. Thus, electrostatic potential fitted charges as reaction indicator is used to infer the activity of species after catalyst
Zn(NH3 )2 (NCO)2 initiation. And it is confirmed to be successful in reactivity prediction. The catalytic
cycles involved the selected active species are proposed. By calculating activation energies, the catalytically active species is considered to be Zn(NH3 )2 (NCO)(NHCOOCH3 ), which derives from the reaction
of Zn(NH3 )2 (NCO)2 with CH3 OH. The DMC synthesis from the catalytically active species and CH3 OH is
favored, of which the reaction activation energy is effectively decreased.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
As an environmentally benign building block, dimethyl carbonate (DMC) has gained considerable attention in recent years [1–5].
Recently, a new route of DMC synthesis from urea and methanol
has been developed [6–9]. The synthesis reaction could be divided
into two steps.
O
O
H2N C NH 2
+ CH 3OH
O
H 2N C OCH 3 + CH 3OH
H2N C OCH 3 + NH3
(i)
O
H3CO C OCH 3 + NH3 (ii)
The intermediate methyl carbamate (MC) is produced in the first
step, and further converted to DMC by consecutive reaction with
methanol (CH3 OH). The first step of the reaction is highly selective
even without using any catalyst. While the second step, the reaction
of MC with CH3 OH, is much more difficult to accomplish, and is
considered as the rate-limiting step [10–14].
∗ Corresponding author at: State Key Laboratory of Coal Conversion Institute of
Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, PR China.
Tel.: +86 351 4049612; fax: +86 351 4041153.
E-mail addresses: [email protected], [email protected] (Y. Sun).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.09.006
Zinc oxide (ZnO) is considered as the best catalyst for the DMC
synthesis from urea and CH3 OH. And the highest DMC yield was
up to 30% at the optimal reaction conditions, which are as follows:
reaction time of 6–12 h, and reaction temperature of 170–190 ◦ C
[12]. However, ZnO shows lower activity for the isolated second
step, the reaction of MC with CH3 OH. ZnO is initially considered as
a typical heterogeneous catalyst with the proper base property for
activating CH3 OH to attack MC carbonyl C atom [7,9]. However, our
group reveals that the solid ZnO completely dissolves during the
reaction. And according to the results of FTIR and element analysis,
ZnO is evidently found to be the precursor of homogenous catalyst
Zn(NH3 )2 (NCO)2 [13].
Although there have been several experimental papers investigating the mechanisms of urea methanolysis to produce DMC over
ZnO [11–14], to the best of our knowledge, why the zinc complex
Zn(NH3 )2 (NCO)2 derived from ZnO can lower the energy barrier is
still mysterious. Zhao et al. reported that MC coordinated to Zn2+ in
Zn(NH3 )2 (NCO)2 to activate the carbonyl C atom, yet they did not
give a deep investigation on that [14]. Actually, the available experimental methods have difficulty in elucidating how homogenous
catalyst weakens the reaction barriers. Thus, theoretical calculations are needed to confirm this mechanism. And based on our
calculation, the mechanism proposed by them is kinetically and
thermodynamically unfavorable, so it has still been controversial
by now. In the present work, for further discovering the catalysis
30
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
Table 1
The bond dissociation enthalpies (at 298 K and in kcal/mol) of the O–H and C–O
bonds in CH3 OH.
BDE (CH3 O–H)
BDE (CH3 –OH)
Our calculation
Experiment
Others calculation
98.5
94.3
100.9
90.2
101.0, 99.7
83.7
0.969OCH
H
3
H3CO
H
H
N
1.024
OCH3
H
H
CH3O
C
OCH3
H N H
H
DMC
MC
NH3
O
O
1.217
89 C 1.303
1.3
1.220
1.572 1.3
C 82
H2N
OCH3
1.124
1.862
H2N
OCH3
H
2.086
O
H
OCH3
OH
OCH3
H2N
H
OH
1.496
H3CO
OCH3
1.608
NH2
O
O
1.274
OCH3
1.513
P2
M2
6 1.5
1.47 C 36
1.220
45 1.3
1.3 C 45
OCH3
H3CO
OCH3
OCH3
R3
1.599
1. 361 C 1.372
H
O
H 2N
1.371
OCH3
H3CO
TS2
OCH3
OH
1.379
1.376 C 1.3 68
28
1.0 NH2
1.318 NH2
H
1.220
69 C 1.371
1.3
H2N
P1
OH
OCH3
H3CO
R2
2. Computational methods
TS1
1.345
1.496C 1.3 24
1.222
OCH3
H
H2N 1.023
R1
1.311
1
12 C .305
1.3
1
50 C .348
2.424
0.977
M1
1.216
1 .3
H3CO
1.462
OCH3
72
0.9 OCH3
All the DFT calculations are performed using the Dmol3 program
available in Materials studio 3.2 package [15,16]. The generalized
gradient approximation (GGA) with Perdew–Wang 1991 function
is used [17,18]. Complete linear synchronous transit and quadratic
synchronous transit (LST/QST) calculations are performed to obtain
the structures of TS [19,20]. Intrinsic reaction coordinate (IRC) calculation from TS is carried out to confirm that its correlation with
the desired reactant and product. The following thresholds are used
for the geometry optimization: 1 × 10−5 Hartree for the maximum
energy change, 2 × 10−3 Hartree/Å for the maximum force, and
5 × 10−3 Å for the maximum displacement. The doubled numerical basis set with a set of polarization functions (DNP) is used,
which is comparable to Gaussian 6–31G** [15]. Vibrational frequencies are calculated at the optimized geometries to identify the
nature of the stationary points (no imaginary frequency) and the
TS (only one imaginary frequency). The activation energy (Ea ) is
defined as Ea = H =/ + nRT, where H =/ is the activation enthalpy.
To comparing the Ea in the uncatalyzed and catalyzed reactions, our
discussions are based on H =/ and the reaction enthalpy change
(H) instead of activation Gibbs free energy (G =/ ) and Gibbs free
energy change (G).
The bond dissociation energies (BDE) of some species in the
reactions are calculated. To evaluate the reliability of the BDE calculation at chosen level of theory, the BDE for O–H and C–O bonds in
CH3 OH are calculated for comparison with others experimental and
theoretical results [21–24]. The results are listed in Table 1. As the
small discrepancies between our results and others, it is considered
that the calculation accuracy for BDE is appropriate.
In order to simulate the solvent effects, the conductor-like
screening model (COSMO) implemented into Dmol3 is used. The
dielectric constant of CH3 OH is considered as 32.6. Beginning with
the gas-phase geometries, we reoptimize the structures of the
species that involved in the reactions mentioned in this study. And
it is found that the structures of the charged species are sensitive to
the COSMO solvent model. However, the structures of the neutral
species in COSMO solvent model almost keep the same as that in
gas phase. Thus, the TS searching for the reactions involved in the
charged species begin with the reactants reoptimized in COSMO
solvent model. As for the neutral species reactions, the corresponding single-point solvation energies based on gas-phase geometries
are calculated, which are considered as the total energies including
COSMO solvation energies. And for comparison with H, the energies related with the single-point solvation energies are presented
in italics and bolds.
O
1.240
78 1. 3
1.3 C 78
CH3OH
O
mechanism, a density functional theory (DFT) study on the MC
methanolysis catalyzed by the zinc complex is undertook for the
first time, by searching transition state (TS) and stable intermediate
of possible reaction pathways. And to better simulate the experimental system, in which DMC is synthesized from urea and excess
CH3 OH (nurea :nmethanol = 1:20), solvent effects of CH3 OH are considered in this theoretical study. The enthalpies of reactions are
calculated at 450 K, as in the actual experiment ZnO showed the
highest activity at this temperature [12].
O
1.220
69 C 1.371
1.3 (1
.36 OCH
N
5)
3
1.012
H
NH2
M3
P3
Fig. 1. The optimized geometries for various species involved in the Reaction of
(1)–(3) described in Scheme 1 (R-reactant complexes, M-intermediate, TS-transition
states, P-product complexes. Dash lines indicated Coulomb interactions or hydrogen
bonds). Bond length is in angstroms. The “+” and “−” means the formal charges rather
than the calculated ESP fitted charges.
3. Results and discussion
3.1. Mechanism proposal
3.1.1. Reaction mechanism for the uncatalyzed reaction
The MC methanolysis is the reaction of an acyl compound (MC)
with a nucleophile (CH3 OH). In general, this type of reaction proceeds in a stepwise addition–elimination mechanism involving the
tetrahedral intermediate, and the nucleophilic addition step is usually the rate-limiting step. Thus, based on the general mechanism,
a tetrahedral intermediate (M1) is assumed and shown in Fig. 1.
However, after the geometry optimization, M1 turns into a weak
hydrogen-bonded complex between MC and CH3 OH. The instability of M1 results from two reasons. First, the C O double bond in
MC is conjugated with the lone pairs electrons on both the amino
N atom and the methoxy O atom, which largely lowers the electropositivity of the carbonyl C atom and thus lowers its reactivity
towards the nucleophilic attack. Second, CH3 OH is a feeble nucleophile due to the weak polarity of the O–H bond and the steric effect
of the methyl group. Based on our calculations, the MC methanolysis reaction completes in a single step via TS1, which is nearly
tetrahedral. In TS1, the O–H bond of CH3 OH breaks (RO–H : 1.462 Å)
and the split H atom is attached to the MC amino N atom (RN–H :
1.124 Å). The distance between the MC carbonyl C and methoxy O
atoms is 1.862 Å. The central C atom changes from sp2 hybridization to almost sp3 hybridization, as indicated by the change in the
H2 N–C O–OCH3 dihedral angle from 180.0◦ in R1 to 133.2◦ in TS1.
The N–C bond lengthens from 1.389 Å in R1 to 1.572 Å in TS1. While
the changes in the C O double bond and the existing C–OCH3 bond
lengths are slight. Then in P1, the N–C bond completely breaks. The
carbonyl C atom goes back into sp2 hybridization.
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
31
Table 2
The Fukui indices and ESP fitted charges of the atoms in MC and CH3 OH.
Atoms
Cc
Oc
O
C
N
Hm
Ha /Hh
MC
CH3 OH
Fukui
(+)
Fukui
(−)
ESP fitted
charges
0.224
0.216
0.094
0.037
0.127
0.038
0.044
0.036
0.094
0.089
0.098
0.339
0.110
0.036
0.163
0.035
0.046
0.033
0.076
0.064
0.778
−0.537
−0.303
−0.255
−0.867
0.127
0.146
0.146
0.371
0.393
Fukui
(+)
Fukui
(−)
ESP fitted
charges
0.200
0.114
0.441
0.113
−0.574
0.036
0.074
0.089
0.089
0.434
0.073
0.123
0.123
0.128
0.096
0.028
0.028
0.387
Cc is carbonyl C atom, Oc is carbonyl O atom, Hm is methyl H atom, Ha is amino H
atom and Hh is hydroxyl H atom. The maximum of every volume are in bold.
Fig. 2. The total energies including solvation energies along MC methanolysis Reactions (1)–(3). The data in parentheses are H at 450 K for Reaction (1). The total
energies including solvation energies and enthalpies of R1, R2 and R3 are set to
zero.
In Fig. 2, we compare the H at 450 K and the change of
total energies including COSMO solvation energies (short for solvation energies) of this reaction. And there is no obvious difference
between them, indicating the uncatalyzed MC methanolysis is not
sensitive to the solvent effect of CH3 OH. The Gibbs free energy
change of this step is predicted to be 1.8 kcal/mol, which indicates
that MC methanolysis is thermodynamically unfavorable. And Ea
for this reaction is predicted to be 42.6 kcal/mol. The relatively high
reaction barrier probably results from the difficulty in carbonyl C
atom being attacked by CH3 OH, and the transfer of hydroxyl H atom
from hydroxyl O to MC amino N atom as the former is more electronegative. Thus, the influence of these two aspects on reaction
Ea are studied separately by modeling title reactions over an acid
catalyst (H+ ) and a base catalyst. An acid catalyst can activate the
carbonyl C atom and make it easier to be attacked by nucleophile,
whereas a base catalyst can facilitate the O–H bond in CH3 OH breaking.
3.1.2. MC activation in the presence of an acid catalyst (H+ )
For the charged species, all the calculations are carried out in
COSMO solvent model. H+ bonded with the carbonyl O atom pulls
the electron density in the C O bond towards the electronegative O
end to increase the electropositivity of carbonyl C atom. The corresponding reaction pathway is presented as Reaction (2) in Scheme 1
and the optimized geometries of species involved in the reaction
is illustrated in Fig. 1. The potential energy surface is shown in
Fig. 2 (dash lines). In this reaction pathway, a tetrahedral intermediate (M2) is formed via TS2. In TS2, CH3 OH binds to the MC
carbonyl C atom via O atom with the newly formed C–OCH3 bond
of 1.496 Å. The hydroxyl H atom interacts with the amino N atom
with a distance of 1.222 Å. In M2, the CH3 OH hydroxyl O–H bond
breaks and the split H binds to the MC amino N atom with the bond
length of 1.028 Å. The central C atom is in sp3 hybridization and
the C O double bond elongates to 1.379 Å, which is close to a typical C–O bond. The N–C bond lengthens to 1.559 Å, and the newly
formed C–OCH3 bond further shortens to 1.376 Å. The formation
of M2 suggests that in the presence of H+ the carbonyl C atom is
more active to be attacked. The energy barrier for this step is predicted to be 29.1 kcal/mol, 12.4 kcal/mol lower than that for the
uncatalyzed reaction. Then NH3 dissociates from the tetrahedral
M2, leading to the product complex P2. At the end of the reaction,
NH3 overcomes binding energy of 12.3 kcal/mol to get the rid of the
interaction with the protonated DMC. In a word, H+ increases the
electropositivity of carbonyl C atom to facilitate the formation of
the tetrahedral intermediate (M2), which succeeds to decrease the
reaction energy barrier.
3.1.3. CH3 OH activation over a base catalyst
A base catalyst is supposed to promote CH3 OH to form the anion
CH3 O− , which is a much stronger nucleophile than CH3 OH itself
and can easily attack the MC carbonyl C atom. In this reaction,
all the calculations are also carried out in COSMO solvent model.
At the beginning, CH3 O− and MC form the energy minima of the
reactant R3 endothermically by 1.1 kcal/mol. In R3, CH3 O− keeps
away from MC with the sp3 hybridization carbonyl C atom, as the
H2 N–C O–OCH3 torsion angle is predicted to be 179.4◦ . Then R3
converts into the tetrahedral intermediate M3 without any transition structure. And this reaction step is predicted to be slightly
endothermic. Afterwards, in the elimination step, NH2 − departs
from the tetrahedral M3. Due to the strong basicity of NH2 − , which
interacts with the carbonyl C atom too strongly to dissociate, the
elimination step is endothermic by 14.5 kcal/mol. In brief, although
the endothermic elimination step is hard to proceed, the supposed
base catalyst eliminates the reaction barrier for the addition step.
Fortunately, over some solid base catalysts, such as CaO and La2 O3
[11,25], the NH2 − elimination can be relatively easy due to its
intense interaction with the metal ion. And in experimental investigation, base catalyst is also considered to be potential catalyst for
MC methanolysis [26,27].
Comparing the above two catalytic mechanism, we conclude
that acid and base catalyst can potentially catalyze the MC
methanolysis, furthermore a base catalyst is more effective by facilitating the formation of the strong nucleophile CH3 O− , provided
that NH2 − can be efficiently eliminated.
3.1.4. Reactivity indicator
To better understand the activation and reactivity of the
species in reaction, the reactivity indicator will be introduced.
According to Klopman, a chemical reaction can be orbital or chargecontrolled [28]. And several subsequent studies have shown that
the Fukui function dominates the reactivity indicator for an orbitalcontrolled reaction, where F(+) and F(−) govern nucleophilic and
electrophilic attacks, respectively [29]. As for a charge-controlled
reaction, the electrostatic potential (ESP) is appropriate for the
description of the activity and reactivity of the species [30]. Thus
the Fukui indices and ESP fitted charges for all the atoms in MC and
CH3 OH are calculated and listed in Table 2.
If the MC methanolysis reaction were orbital-controlled, it
would occur at the sites with the maximum Fukui indices. The
maximum F(−) indices in CH3 OH and MC are the hydroxyl O atom
and the carbonyl O atom, respectively. And the maximum F(+)
indices are the hydroxyl H atom and the carbonyl C atom. This
32
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
H
CH3O
O
+
NH2
O
TS1
C
NH3
OCH3
+
Reaction(1)
C
CH3O
OCH3
OH
OH
+
C
H3CO
+
H
O
OH
TS2
H
C
H3CO
OCH3
NH2
NH3
C
CH3O
+ NH3
Reaction(2)
- H+
OCH3
O
M2
C
NH2
OCH3
C
OCH3
CH3O
OCH3
+
+
O
CH3OH
Base
Catalyst
CH3O
NH2
C
H3CO
C
+
+ H+
O
O
OCH3
NH2
OCH3
+ NH
2
C
CH3 O
NH3
Reaction(3)
OCH3
M3
Scheme 1. The proposed mechanisms of the MC methanolysis Reaction (1) without the presence of catalyst, (2) acid-catalyzed (H+ ), (3) base-catalyzed. The “+” and “−”
means the formal charges rather than the calculated ESP fitted charges.
H O
O
H
+
H2 N
H3CO
C
TS4-1
OCH3
H2N
C
O
OCH3
OCH3
TS4-2
NH3 +
H3 CO
C
Reaction (4)
OCH3
M4
Scheme 2. The proposed mechanism of the MC methanolysis reaction under orbital-controlled.
Relative Energy (kcal/mol)
suggests that the most possible reaction is between MC carbonyl
C O double bond and CH3 OH hydroxyl O–H bond. This mechanism
is presented in Scheme 2. Two transition states are located for this
route. The activation energies for the addition and elimination steps
are calculated to be 50.8 and 27.8 kcal/mol (54.5 and 27.5 kcal/mol),
respectively. The detailed reaction energy surface is shown
in Fig. 3.
If the reaction were charge-controlled, it would occur at the
atoms with the maximum ESP. By comparing ESP fitted charges
TS4-1
TS4-2
40.5
49.1
41.4
44.7
of these atoms, we conclude that the reaction would occur at the
CH3 OH hydroxyl O–H bond and the N–C bond in MC. The reaction pathway based on such a hypothesis is discussed above as
Reaction (1) of Scheme 1, of which reaction barrier is predicted to
be 42.6 kcal/mol (41.5 kcal/mol). The Ea difference of 8.2 kcal/mol
(13.0 kcal/mol) between Reaction (1) and addition step in Reaction (4) indicates that the reaction pathway under charge control
is more favorable.
Moreover, as Klopman proposed, the energy difference between
the lowest unoccupied molecular orbital (LUMO) of the electrophilic and the highest occupied molecular orbital (HOMO) of
the nucleophilic (ELUMO − EHOMO ) is large and the energy difference
between the various occupied molecular orbitals of each molecules
(EHOMO − EHOMO-1 ) is small, the reaction between them is chargecontrolled. Thus, the frontier orbital energies of MC and CH3 OH are
calculated. The energy difference between LUMO of MC and HOMO
of CH3 OH is 6.537 eV, much larger than that between various occupied molecular orbitals of CH3 OH (EHOMO − EHOMO-1 = 1.771 eV).
This also suggests that the MC methanolysis is charge-controlled.
M4
14.5
17.2
CH3OH
+MC
3.2. Catalyst initiation
DMC
P4
2.2
1.9
0.0
+NH3
2.3
3.4
R4
-9.4
-5.4
Reaction Process
Fig. 3. H energy surface at 450 K for MC methanolysis with the single-point solvation energies (bold and italic) under orbital control. The total energies including
COSMO solvation energies and enthalpies of CH3 OH and MC are set to zero.
Previous experimental studies of our group reveal that the actual
catalyst for MC methanolysis is Zn(NH3 )2 (NCO)2 [13]. Here we
model this zinc complex and investigate its possible catalytic mechanisms. The zinc complex Zn(NH3 )2 (NCO)2 is denoted as Z. Prior to
being used in the catalytic reaction, Z as catalyst undergoes the initiation step via association or dissociation processes. Generally, Zn2+
forms 4, 5 or 6 coordinated complexes with various ligands [31].
However, in the process of MC or CH3 OH association to Z, none 5
or 6-coordinated structure is located. The species formed in this
way are the hydrogen bonded complexes, in which MC or CH3 OH
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
33
Scheme 3. Initiations of Zn(NH3 )2 (NCO)2 and the definition of equilibrium constants.
interacts with the isocyanate (NCO− ) or NH3 ligands of Z. The complexes are labeled as A and C respectively and shown in Scheme 3,
as well as the equilibrium constants defined as KA and KC .
It is noteworthy that in complex A, the associated CH3 OH probably further react with near NCO− to form a new zinc complex,
Zn(NH3 )2 (NCO)(NHCOOCH3 ), in which the MC lost one of amino H
atom directly binds to Zn2+ via the amino N atom. It is denoted as
B. The complex B as catalyst, rather than catalyst Z, will be involved
in the catalytic cycle. Thus, the complex B formation process is also
considered as an initiation reaction. The formation reaction is like
the formation of MC from HNCO and CH3 OH, which is investigated
in Appendix A and denoted as Reaction (5) in Scheme 4. As for
the formation process of complex B, it is presented as Reaction (6).
The equilibrium constant for the complex B formation is defined as
KA × KB . The ESP fitted charges of the atoms of NCO− in Table 3 suggest that the reaction with CH3 OH will occur at N C double bond.
The maximum charge difference is between the N atom in NCO−
and the CH3 OH hydroxyl H atom. This is different from the Reaction
(5), in which the maximum charge difference is between the C atom
of NCO− and the hydroxyl O atom. This probably results in the different transition states. As shown in Fig. 4, the O–H bond of CH3 OH
in TS5 elongates but not breaks, while in TS6 it breaks and the split
H atom binds to the N atom of NCO− . The formation of complex B
completes in one step, and its enthalpy profile is presented in Fig. 5.
Ea of this step is calculated to be 42.1 kcal/mol (42.2 kcal/mol),
much higher than that for Reaction (5). This is mostly due to the
O
O
C
OCH3
+
TS5
N
Reaction (5)
C
NH2
H
OCH3
H
P5 (MC)
O
OCN
N
NH3
Z
C
H
O
+
Zn
H3 N
C
OCH3
TS6
OCN
HN
OCH3
Reaction (6)
Zn
H3N
NH3
P6 (B)
Scheme 4. The proposed mechanisms for methanolysis reactions of HNCO and NCO− in Z.
34
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
O
O
O
1.179
C
H
1.222
N
0.
2.069
9
96
H
H
1.192
6 1.7
30 C 24
.
1
H
O
OCN
N
C
O
1.216 1.192
OCN
Zn
H 3N
R5
H
1.837
17
1.2
OCH3
CH3
H
OCH3
N
0.995
TS5
1.018
1.468 OCH
1.251
41 1 . 3
1.3 C 80
OCH3
N
H 3N
N
O
C
1.251 1.180
Zn
H3N
NH3
P6 (B)
R6
2.436
1.168
OCN
Zn
NH3
3
H
NH3
TS6
Fig. 4. The optimized geometries for various species involved in the Reactions (5) and (6) described in Scheme 4. Bond length is in angstroms.
Table 3
The Fukui indices and ESP fitted charges of the atoms in HNCO and NCO− of complex
Z.
Atoms
H
N
C
O
HNCO
Table 4
The enthalpies, Gibbs free energies (in kcal/mol) and the equilibrium constants for
the complexes formation reactions at 450 K.
NCO− in Z
Complex
H450
G450
K
A
B
C
Z
D
E
F
G
−14.0
−5.30
−16.7
19.4
−14.2
−2.8
5.5
5.8
−2.06
−2.72
−2.18
−1.44
4.18
−0.98
8.12
6.29
9.98
20.8
11.4
4.99
9.44 × 10−4
2.99
1.16 × 10−4
8.90 × 10−4
Fukui (+)
Fukui (-)
ESP fitted charges
ESP fitted charges
0.200
0.238
0.294
0.247
0.113
0.363
0.202
0.321
0.361
−0.577
0.526
−0.328
−0.841
0.599
−0.437
The maximum of every volume are in bold.
interaction of CH3 OH with NH3 , which keeps the O atom of
CH3 OH far away from the C atom of NCO− . However, the formation of R6 and complex B are exothermic by complexation energy
of 14.0 kcal/mol (8.7 kcal/mol) and 21.2 kcal/mol (19.5 kcal/mol),
which probably promote the reaction of CH3 OH with Z.
The third initiation mechanism is a dissociation process. At our
chosen level of calculation, the BDE of Zn–NH3 in Z is predicted to be
only 20.8 kcal/mol. Thus one of the NH3 ligands can easily dissociate
from Zn2+ leading to the unsaturated zinc complex (Z ). Then, MC
or CH3 OH coordinates with Zn2+ in Z . Previous experiments show
that Zn2+ usually binds to the ligands through the O and N atoms
[31]. Consequently, there is only one kind of complex denoted as
D is formed in CH3 OH coordination to Z . As for MC, three possible
isomers are formed in this process. They are denoted as E, F and G in
Scheme 3, depending on the different coordination atoms. And the
different coordination results in the different thermodynamic stability. Compared the Gibbs free energies, the complex E, in which
the MC coordinates to Zn2+ via carbonyl O atom, is considered to
be the most thermodynamically stable. The coordination equilibrium constants, KE , KF and KG , as well as KD for the coordination of
CH3 OH with Z , are defined in Scheme 3 and their values are listed
in Table 4. Based on the above data, complexes A, B, C and Z are
considered to be thermodynamically stable, whereas all of the complexes initiated by the dissociation process are thermodynamically
unstable.
The chosen reactivity indicator (ESP fitted charges) is used in
judging the activation of CH3 OH and MC in these complexes. Taking
the reaction sites of MC methanolysis into account, the ESP fitted
charges of hydroxyl O and H atoms, as well as the MC amino N and
carbonyl C atoms are listed in Table 5. In complex D, the O atom in
CH3 OH is less negative (−0.558 e) than that in free CH3 OH (−0.626).
This suggests that CH3 OH in complex D is less nucleophilic. Thus
the CH3 OH initiated in dissociation process is considered to be
inactivated. As for MC, after the initiation reactions, the positive
charge of the carbonyl C atom follows the order of B > C > E > G > the
free MC > F, and the negative charge of the N atom follows the
same order. Generally speaking, the more electropositive carbonyl
C atom, as well as the more electronegative amino N atoms, is in
favor of the MC methanolysis. Thus, in the complexes B, C, and E as
both the MC carbonyl C and N atoms are more electropositive and
electronegative respectively, the MC in them are considered to be
activated. Conversely, MC in the complex G and F are not activated.
This judgment is different from the reaction mechanism proposed
by Zhao et al. [14]. They proposed that the activated MC binds to
Zn2+ via the amino N atom, which is actually the MC in complex F.
For further confirming our guess, the catalytic cycles involving the
complexes B, C, E and F are studied in the following section.
Table 5
The ESP fitted charges of the MC carbonyl C and N atoms, and the CH3 OH hydroxyl
O and H atoms in various complexes.
Complex
MC
C
Fig. 5. H energy surface at 450 K for Reactions (5) and (6) with the single-point
solvation energies (bold and italic). The total energies including COSMO solvation
energies and enthalpies of reactants are set to zero in each reaction.
A
B
C
D
E
F
G
CH3 OH
N
1.038
0.890
−1.210
−1.113
0.861
0.710
0.746
−1.070
−0.864
−0.923
O
H
−0.626
0.510
−0.558
0.446
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
OCN
Zn
OCN
NCO
NCO
H3N
O
CH3OH
NH3
NH3
Zn
2+
Zn
NH3
H 3N
NH3
B-P4
O
CH3OH
2+
Zn
C
N
35
OCH3
O
H
H
Z
C
NH
OCH3
B
CH3
B-R
B-TS
O
OCN
NCO
Zn
OCN
H 3N
O
H
H2 N
H2 N
Zn
NH3
NH3
H 3N
HNCO
B-P3
C
NH2
H3 CO
Zn2+
Zn2+
DMC
B-P2
C
OCH3
B-P1
NH2
OCH3
OCH3
B-M
Scheme 5. The catalytic cycle started from the complex B. For the sake of brevity, in some structures Zn2+ replace Zn(NH3 )2 (NCO)2 indicating the species interaction with
Zn2+ .
3.3. Catalytic cycles
The catalytic cycles involving the complexes B, C, E and F are
named as Path B, C, E and F, respectively. The relevant reactant
complexes (R), intermediates (M), transition states (TS) and product
complexes (P) in the reactions are denoted as X − Yn, where X = B,
C, E and F, Y = R, P, M, and TS, and n stands for the order of presence.
3.3.1. Path B
At the beginning of Path B shown in Scheme 5, Z reacts with
CH3 OH to form the complex B, which is discussed in details in Section 3.2. Then the complex B further reacts with CH3 OH to produce
DMC. And at the end of the cycle, HNCO is introduced to regenerate
Z. Thus the whole catalytic cycle accomplishes.
The H energy surface for Path B is illustrated in Fig. 6. The
sum of the enthalpies of the reactants (one HNCO and two CH3 OH
molecules) and catalyst Z is set to zero. The optimized structures
for stationary points and TS in Path B are illustrated in Fig. 7.
The methanolysis of the deprotonated MC in complex B is a twostep reaction via B–TS and an intermediate (B–M). In complex B,
the deprotonated amido directly binds to Zn2+ via N atom, thus
the electron density within the N–C bond concentrates on the N
end to increase the electropositivity of carbonyl C atom (1.038 e).
TS6+CH3OH
+HNCO
28.1
Relative Energy (kcal/mol)
33.5
B-P2+DMC
+HNCO
B-TS
B-P1
+HNCO
+HNCO 6.0
1.2
8.3
2.1
-6.9
-1.7
-2.2
-5.2
Z+2CH3OH
+HNCO
0.0
B-M
+HNCO
-8.7
-14.0
R6+CH3OH
+HNCO
-16.1
-18.5
-19.5
-21.2
B+CH3OH
+HNCO
-26.0
-28.2
B-R
+HNCO
-17.4 Z+DMC
-32.3 -27.5 +NH3
-30.4
B-P4
B-P3 +DMC
+DMC
Reaction Process
Fig. 6. H energy surface at 450 K for the catalytic cycle of Path B described in
Scheme 5 with the single-point solvation energies (bold and italic). The total energies
including COSMO solvation energies and enthalpies of reactants are set to zero in
each reaction.
Firstly, the amino N atom interacts with the CH3 OH hydroxyl H
atom to form the reactant complex B–R. Due to the hydrogen bond
interaction between the NH3 ligand H atom and the MC carbonyl
O atom, the electron density within the C O bond is abstracted
away from the C end, which further increase the electropositivity of carbonyl C atom. Next in B–TS, the CH3 OH hydroxyl O–H
bond breaks. The split H atom binds to the deprotonated MC amino
N atom with a bond length of 1.129 Å. At the same time, The O
atom of CH3 OH is close to the deprotonated MC carbonyl C atom
with the distance of 2.457 Å. The torsion angle of H2 N–C O–OCH3
changes from 180.0◦ to 159.0◦ . B–M is then formed, in which the
central C atom of the deprotonated MC is in sp3 hybridization.
The newly formed ester C–OCH3 bond is 1.458 Å in length and
the N–C bond lengthens from 1.353 Å in B–R to 1.567 Å. The addition step is endothermic by 23.0 kcal/mol (19.1 kcal/mol) with Ea
of 27.4 kcal/mol (34.3 kcal/mol). It is 15.2 kcal/mol (7.2 kcal/mol)
lower than the Ea for uncatalyzed MC methanolysis. Subsequently,
B–M converts into B–P1 and DMC is formed. This elimination step
is endothermic by 3.0 kcal/mol (9.0 kcal/mol). And finally DMC gets
rid of the interaction with B–P2 by overcoming the binding energy
of 3.4 kcal/mol (3.9 kcal/mol).
To regenerate the catalyst Z, one molecule of HNCO is introduced into the catalytic cycle. After geometry optimization, the
N–H bond of HNCO splits and the split H binds to NH2 − of B–P2
to form a new NH3 ligand. Consequently, the complex B–P3 with
three NH3 ligands is located. This is followed by the formation of
the 5-coordinated complex (B–P4), in which the split NCO− directly
coordinates to Zn2+ . No transition state is located in the process
from B–P2 to B–P4. And the regeneration reaction is exothermic by
31.6 kcal/mol (38.3 kcal/mol), which promotes the whole catalytic
cycle.
By consuming one NCO− ligand of Z, the DMC is synthesized via
a series of methanolysis reaction in Path B. On the other hand, Path
B can be considered as the reaction of one HNCO with two CH3 OH
molecules catalyzed by Z. In brief, provided that the catalytically
active complex B could be formed, the DMC formation in Path B is
more feasible than that in the uncatalyzed one.
3.3.2. Path C
As shown in Scheme 6, the catalytic cycle described as Path C
begins with the formation of complex C. Optimized structures of
the stationary points and TS on the reaction pathway are illustrated
in Fig. 8.
At the begging of catalytic cycle, MC associates to Z to form complex C. This step is exothermic by 16.7 kcal/mol (12.5 kcal/mol). In
complex C, MC carbonyl O atom interacts with NH3 ligands H atoms.
36
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
O
Zn
O
1.251
41
1.3 C
2+
1.3
NH
H
H
0.9
B
1.129
91
1.053
1.216
2.097
OCN
OCH3
H3CO
OCN
Zn
NCO
Zn
H3 N
NH3
H3N 2.174 NH3
NH3
Z
B-P4
B-P3
B-P1
OCN 1.902 NCO
2.026
NCO
Zn
OCH3
OCH3
B-M
2.200
1.868
H3N
Zn2+
Zn2+
NH3
H
HN
1.3
48
NH2
B-TS
O
H2N
OCH
H 1.454 3
CH3
B-R
54 C
1.3
2.457
8
O
1.968
1.446
67 C
1.5
OCH3
NH
OCH3
45
OCH3
O 1.293
1.233
1.356
1
3
1.4 C
Zn2+
1.
N
Zn
90
O
1.246
53 1.
1.3 C 37 8
2+
Fig. 7. The optimized geometries for several species involved in the catalytic cycle of Path B.
Z
Z
OCN
NCO
NCO
OCN
Zn
O
O
1.230
73 1.3
1. 3 C 59
20 1.378
1.5 C
H 2N
OCH3
OCH3
H 2N
H
OCH3
H 1.474
H 0.986
1.219
5 C 1. 34
4
3
5
1.
1.032
O
NH3
H3N
O
H2N
OCH3
1.958
1.108
NH3
H 3N
1.222
Zn
OCH3
O
NH3
H3C
1.4
47
1.227
39C
1.3 1
.3
O
39 OCH3
1.459
CH3
C-R
C-TS
C-P2
C-P1
Fig. 8. The optimized geometries for several species involved in the catalytic cycle of Path C described in Scheme 6.
Hence, the electron density in the C O double bond is pulled away
from the carbonyl C atom to increase its positive charge (0.890 e in
Table 5). And the amino N atom is more electronegative (−1.113 e).
Nevertheless, compared with the atoms in complex B, the MC carbonyl C and amino N atoms in complex C are less electrostatically
positive and negative, respectively. For this reason, methanolysis
of the MC in complex C is probably more difficult than that of the
deprotonated MC in complex B. In order to verify this conjecture,
the H energy surface is searched and shown in Fig. 9.
The complex C interacts with CH3 OH to form C–R. This step is
further exothermic by 8.0 kcal/mol (7.7 kcal/mol). Followed by it,
O
O
Zn
H3N
CH3OH
H 2N
C-TS
C
C
NH3
H2N
OCH3
OCH3
Z
H
C
C-R
DMC+NH3
C-TS
Zn
Zn
H 3N
NH3
NH3
NCO
OCN
NCO
OCN
NH3
H3N
O
O
H 3N
H3CO
OCH3
O
OCH3
CH3
C-P2
Z+DMC
+NH3
Z+MC
+CH3OH
2.3
3.4
0.0
C-P1
C+CH3OH
-13.6
-8.0
-16.7
-12.5
C
C
18.6
25.4
OCH3
Relative Energy (kcal/mol)
NCO
OCN
Z
Z
MC
a tetrahedral carbon intermediate is assumed. However, it is too
unstable to exist, which probably results from the steric effects.
CH3 OH and MC are in the interaction with the different NH3 ligands,
which prevent them getting close enough to form the intermediate.
In addition, it is noted that the hypothesized intermediate converts
into the hydrogen-bonded complex between NH3 and the cis–trans
DMC after geometry optimization. In previous sections, all of the
DMC mentioned is the cis–cis conformer. The differences between
the two conformations are described in detail by Katon and Cohen
[32,33]. The reason for the formation of cis–trans DMC is also the
steric effect. The cis–trans conformer is more advantageous for the
C-P1
Scheme 6. The catalytic cycle of MC methanolysis catalyzed by Zn(NH3 )2 (NCO)2 in
Path C. For the sake of brevity, in some structures Z is used, indicating the species
interaction with NH3 ligands of Zn(NH3 )2 (NCO)2 .
C-P2
-17.3
-13.1
C-R
-24.7
-20.2
Reaction Process
Fig. 9. H energy surface at 450 K for the catalytic cycle of Path C with the singlepoint solvation energies (bold and italic). The total energies including solvation
energies and enthalpies of reactants are set to zero in each reaction.
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
37
DMC formation in the limited area between NH3 ligands.
cis-trans
O
H3 C
O
CH3
O
38.1
45.2
O
C
O
CH3
Relative Energy (kcal/mol)
H3C
O
C
E-TS
+NH3
cis-cis
The complex consisted of the cis–trans DMC is denoted as C–P1
and the other as C–P2. The processes from C–R to C–P1 and C–P2 are
investigated respectively, yet the TS searching is successful in the
process from C–R to C–P1. In the only transition structure C–TS, the
CH3 OH hydroxyl O–H bond breaks with the distance between the
O and H atoms of 1.474 Å. The split H atom binds to the MC amino N
atom with the bond length of 1.108 Å. And the O atom of CH3 OH is
close to the MC carbonyl C atom with the distance of 1.958 Å. At the
same time, the N–C bond of MC lengthens from 1.373 Å to 1.520 Å.
C–TS is nearly tetrahedral with the H2 N–C O–OCH3 dihedral angle
of 144.7◦ . Then in C–P1, the N–C bond of MC completely breaks
and the new C–OCH3 ester bond is formed with the bond length
of 1.345 Å. Ea for this step is 44.2 kcal/mol (45.6 kcal/mol), which
is slightly higher than that for the uncatalyzed MC methanolysis
(42.6 kcal/mol 41.5 kcal/mol). It is right in our conjecture that the
MC methanolysis in Path C has a higher Ea than that in Path B.
Besides the reason we make the conjecture, the other reason for the
high Ea is the steric effect arising from the interaction of MC and
CH3 OH with NH3 ligands, which results in an association energy
cost in reactants approaching to each other.
Subsequently, since the cis–cis DMC is more thermodynamically
favorable than the cis–trans one (G: 2.8 kcal/mol), C–P1 further
converts to C–P2, the latter is considered as the final product. The
process of C–P1 converting to C–P2 is exothermic by 3.7 kcal/mol.
Finally, DMC and NH3 overcome the binding energy of 19.6 kcal/mol
(16.5 kcal/mol) to get rid of the interaction with catalyst Z. Thus
catalyst Z is regenerated. To sum up, Ea of MC methanolysis in
Path C is 2.5 kcal/mol (4.1 kcal/mol) higher than that in the uncatalyzed reaction. It indicates the impossibility of DMC synthesis in
this pathway and the MC associated to Zn(NH3 )2 (NCO)2 via the
carbonyl O atom is not activated enough to synthesis DMC.
Zn2+
O
H 2N
OCH3
2.070
OCH3
H
80
0.9
E-R
H
E-TS
-2.8
1.9
-3.4
-5.9
E-R
+NH3
OCH3
7
1.63
1.062
2.3
3.4
-5.6
-6.1
carbonyl C atom and amino N atom are less positive and negative
respectively. It suggests that MC is less active in complex E. The
interaction of deprotonated amino N atom with Zn2+ in complex B,
as well as that of the carbonyl O atom with NH3 ligands in complex
C, is more in favor of MC activation.
Subsequently, CH3 OH reacts with the complex E to synthesize
DMC via E–TS and a tetrahedral carbon intermediate E–M. In E–TS,
the CH3 OH hydroxyl H atom breaks from the O atom and binds to
the MC amino N atom with the N–H bond length of 1.178 Å. At the
same time, the O atom of CH3 OH is close to the MC carbonyl C atom
with the distance of 2.048 Å. The torsion angle of NH2 –C O–OCH3
in MC changes from 180.0◦ to 153.1◦ . Then in E–M, the carbonyl C
atom changes into sp3 hybridization. The C O double bond and N–C
bond elongates to 1.288 Å and 1.637 Å, respectively. And the new
C–OCH3 ester bond is predicted to be 1.422 Å. Ea of this addition step
is 48.7 kcal/mol (50.8 kcal/mol), which is higher than that in Path
B and C. The high activation barrier results from the comparatively
low reactivity of MC and the hydrogen bond interaction between
CH3 OH and NH3 ligands, which keeps the O atom of CH3 OH away
from the MC carbonyl C atom.
After that, E–M converts into E–P1 without any transition structure. In E–P1, the NH3 dissociates from the central C atom with
the distance between the N and C atoms of 2.048 Å, and DMC coordinates to the Zn2+ via the carbonyl O atom. The central C atom
changes back into sp2 hybridization with the C O double bond of
1.232 Å. The elimination reaction is exothermic by 19.7 kcal/mol
(20.4 kcal/mol).
Finally, catalyst Z regenerates in the following two step. Firstly,
E–P1 further converts into E–P2, in which NH3 as the ligand
substitutes DMC to coordinate to Zn2+ . In this step Z is formed
again. At last, DMC overcomes the binding energy of 7.9 kcal/mol
OCN
O
H2N
E-P2
+NH3
Fig. 10. H energy surface at 450 K for the catalytic cycle of Path E described in
Scheme 7 with the single-point solvation energies (bold and italic). The total energies including solvation energies and enthalpies of reactants are set to zero in each
reaction.
OCN
NCO
H 3N
C
1.232
OCH3
1.422 OCH
3
NH3
H3
39 C
1.3 1.3
26
CO
H
E-M
H3 N
O
1.411
E-P1
NCO
Zn
Zn
2.048
1.319
E-P1
+NH3
Reaction Process
1.288
OCH3
H2 N
Z+DMC
+NH3
E+CH3OH
+NH3
-9.7
-5.6
Zn2+
1.244
1.378
6
7
C
1.4
1.178
16.3
14.5
Z+MC
+CH3OH
O
1.241
1
65 C .359
1.3
E-M
+NH3
19.4
15.2
0.0
3.3.3. Path E
As for Path E, it is the catalytic cycle started with the formation
of complex E. The H energy surface for Path E is shown in Fig. 10,
where the sum of the enthalpies of the reactants (MC and CH3 OH)
and catalyst Z is set to zero. Optimized structures of the stationary
points and TS on the reaction pathway are illustrated in Fig. 11.
As shown in Scheme 7, at the begging of the catalytic cycle, Z
loses one NH3 ligand leading to the unsaturated complex Z . This
dissociation step is endothermic by 19.4 kcal/mol (15.2 kcal/mol).
Then MC coordinates to the Zn2+ to form the complex E. This coordination step is exothermic by 22.2 kcal/mol (17.1 kcal/mol). In
complex E, via Zn2+ interaction with carbonyl O atom, the electron
density within the MC carbonyl C O double bond is pulled away
from the carbonyl C atom to increase its electropositivity (0.861 e).
And the MC amino N atom is more electronegative (−1.070 e) than
in free MC. However, compared with the MC in complex C and B,
Zn2+
Z'+NH3+MC
+CH3OH
NH3
O
1.218
OCH3
OCH3
41 C .34 8
1
1. 3
H3CO
Fig. 11. The optimized geometries for several species involved in the catalytic cycle of Path E.
E-P2
38
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
MC
OCN
CH3OH
Zn
NCO
Zn
O
O
OCN
H 3N
Zn2+
NH3
OCN
C
H 2N
C
NH3
NH3
OCH3
OCH3
H2 N
OCH3
H
Z
E
E-R
DMC
E-TS
OCN
NCO
OCN
Zn
H3N
Zn2+
NCO
Zn
NH3
H3 N
O
C
NH3
OCH3
C
O
O
OCH3
H 3N
C
H3CO
OCH3
OCH3
H3 CO
E-P2
E-P1
E-M
Scheme 7. The catalytic cycle of MC methanolysis catalyzed by Zn(NH3 )2 (NCO)2 in Path E. For the sake of brevity, in some structures Zn2+ replace Zn(NH3 )2 (NCO)2 indicating
the species interaction with Zn2+ .
MC
NCO
Zn
Zn
H 3N
O
OCN
NCO
OCN
OCH3
N
H2
H2N
NH3
C
NH3
F
Z
CH3OH
DMC
O
O
Zn
OCN
Zn2+
NH3
OCN
H3N
F-P
C
OCH3
Z'+NH3+MC
+CH3OH
19.4
15.2
Z+MC
+CH3OH
0.0
Scheme 8. The catalytic cycle of MC methanolysis catalyzed by Zn(NH3 )2 (NCO)2
in Path F. For the sake of brevity, in some structures Zn2+ replace Zn(NH3 )2 (NCO)2
indicating the species interaction with Zn2+ .
F-P
+NH
Fig. 12. H energy surface at 450 K for the catalytic cycle of Path F described in
Scheme 8 with the single-point solvation energies (bold and italic). The total energies
including COSMO solvation energies and enthalpies of reactants are set to zero in
each reaction.
Compared with the free MC, in complex F the MC carbonyl
C atom is less electropositive (0.710 e) and the amino N atom
(−0.864 e) keeps almost the same. It indicates that the charge
spread within the conjugated part is less well. In F–TS, the O–H
bond in CH3 OH lengthens to 1.052 Å but not break. The distance
between the O atom of CH3 OH and the MC carbonyl C atom is
1.740 Å. And the C O double bond shortens to 1.119 Å. It is worth
noting that the N–C bond in MC breaks with the distance between
them of 2.133 Å, which is different from all the other transition
1.121
Zn
F-R
2.3
3.4
Reaction Process
O
H
Z+DMC
+NH3
-17.7
-13.4
OCH3
OCH3
F-TS
F+CH3OH
+NH3
5.3
9.8
F-R
+NH3
-7.7
-5.2
C
N
H2
OCH3
46.8
55.3
2+
16
1.4
N
H2
C 1.348
OCH3
0. 9
72OCH3
O 1.119
Zn2+
2.133
NH2
1.300
OCH3
1.740
1.595
H 1.053OCH3
H
F-R
C
F-TS
O
NH3
OCN
Zn
OCN
1.3
64
3.3.4. Path F
As shown in Scheme 8, at the beginning of Path F, one NH3 ligand dissociates and then MC coordinates to Z via amino N atom.
Probably because of the steric effect of amido, the amino N atom
keeps a certain distance away from the Zn2+ . The coordination bond
between Zn2+ and the MC amino N atom is 2.305 Å in length. The
H energy surface is shown in Fig. 12, in which the sum of the
enthalpies of the reactants (MC and CH3 OH) and catalyst Z is set to
zero. And the optimized structures of stationary points and TS on
the reaction pathway are illustrated in Fig. 13.
After the initiation step, CH3 OH interacts with F to form F–R.
Then it converts to F–P via F–TS without any intermediate. Based on
previous discussion, the formation of the tetrahedral carbon intermediate probably results from two aspects, the steric effects and
the well spread charges within the conjugated part (N–C O–O).
F-TS
+NH3
Relative Energy (kcal/mol)
(9.5 kcal/mol) to get rid of the interaction with the complex Z.
Thus catalyst Z is regenerated. In brief, since the Ea in Path E
is 6.1 kcal/mol (5.2 kcal/mol) higher than that in the uncatalyzed
reaction, the MC methanolysis reaction will not proceed in this
reaction pathway.
H3N
C
1.210
1.348
OCH3
2.225
F-P
Fig. 13. The optimized geometries for the species involved in Path F.
OCH3
Y. Gao et al. / Journal of Molecular Catalysis A: Chemical 351 (2011) 29–40
ESP fitted charges
charges and Ea based on the gas-phase enthalpy changes. The relationship presented in Fig. 14 clearly shows the negative correlation
between the Ea for MC methanolysis reaction and the positive
charge of MC carbonyl C atom, as well as the negative charge of
amino N atom. Thus, the ESP fitted charge as reaction indicator is
confirmed to be successful in reactivity prediction of initiated active
species.
carbonyl C atom
amino N atom
1.0
39
0.5
Acknowledgement
The authors acknowledge the financial support from State Key
Program for Development and Research of China.
-1.0
Appendix A. Appendix
25
30
35
40
45
50
55
60
Reaction activation energy
Fig. 14. ESP fitted charges of the MC carbonyl C and amino N atoms as functions of
MC methanolysis activation energies in Path B, C, E and F.
structures mentioned above. Then in F–P, the CH3 OH hydroxyl O–H
bond breaks (2.225 Å) and the new C–OCH3 ester bond is formed
with the bond length of 1.364 Å. This process is exothermic by
10.0 kcal/mol (8.2 kcal/mol) with the Ea of as high as 55.4 kcal/mol
(60.5 kcal/mol). This is the evidence for the MC not being activated
in complex F. It confirms our guess at the end of Section 3.2, the MC
coordinated to Zn2+ via N atom is not being activated. At last, DMC
overcomes the binding energy of 20.0 kcal/mol (16.8 kcal/mol) to
get rid of the interaction with catalyst Z. Then catalyst Z is regenerated.
4. Conclusions
This study reveals the mechanism for MC methanolysis reaction
without catalyst and in the presence of the Zn (NH3 )2 (NCO)2 catalyst. Solvent effects of CH3 OH have influence on the structures of
the charged species, but not the neutral one. In the COSMO solvent
model, the MC methanolysis is considered to be catalyzed by acid
and base catalyst. And via the formation of CH3 O− , the proper base
catalyst is more effective by facilitating the nucleophilic addition
of CH3 OH and MC to proceed barrierlessly.
In the presence of Zn(NH3 )2 (NCO)2 , all the identified MC
methanolysis pathways are discussed to obtain a deep insight in
favorable mechanisms. The single-point solvation energies of all
the species are calculated based on gas-phase structures. There is
discrepancy between the reaction enthalpy change and the change
of single-point solvation energy along the reaction process, but not
more than 7 kcal/mol. However, based on the two kinds of energies, we deduce the similar conclusions by comparing the energy
barriers in reactions.
In
Path
B,
the
catalytically
active
species
is
Zn(NH3 )2 (NCO)(NHCOOCH3 ) derived from the reaction of NCO−
in Zn(NH3 )2 (NCO)2 with CH3 OH. Provided that the complex B
could be formation, the energy barrier of DMC synthesis from
the catalytically active species and CH3 OH is only 26.5 kcal/mol
(34.3 kcal/mol), which is effectively decreased compared with
the other intermediate methanolysis. It indicates that the DMC
synthesis from urea and CH3 OH over ZnO should be the most
effective. And in experiments, this synthesis method indeed has
the highest DMC yield [12,14].
Based on the energies of frontier molecular orbitals, the title
reaction is considered to be charge-controlled. As the trend of
energy barriers in the reactions is the same in gas-phase and
COSMO solvent model, we discuss the relationship between
The reaction of HNCO and CH3 OH
Based on the Fukui indices and ESP fitted charges for each atom
in HNCO (listed in Table 3 for comparison with NCO− ), we conclude
that the most possible reaction will occur at the N C double bond
regardless of under orbital or charge control. And by orbital energy
calculation, it is also considered to be charge-controlled reaction.
In order to be compared with the formation of complex B, the
reaction pathway is presented in Scheme 4 Reaction (5). The geometric parameters of the species involved in the reaction, except
MC shown in Fig. 1, are shown in Fig. 4 and the potential energy
surface is presented in Fig. 5 in black line.
HNCO initially interacts with CH3 OH to form the reactant complex R5. Then, R5 further converts into product MC (P5) via TS5.
In TS5, via the hydroxyl O atom CH3 OH binds to the C atom of
HNCO with the new formed C–OCH3 ester bond of 1.724 Å. And
the hydroxyl O–H bond lengthens to 1.217 Å but not break. At the
same time, the N C double bond becomes N–C single bond with
the bond length of 1.306 Å. Finally, in P5 (MC), the hydroxyl H atom
transfers from the methoxy O atom to the amino N atom to form
the new N–H bond of 1.012 Å in length. Due to p–␲ conjugation,
the N–C bond lengthens to 1.369 Å, while the new formed C–OCH3
bond shortens to 1.371 Å. In this reaction, the new bonds formation
(the C–OCH3 ester bond and the N–H bond) and the old bond breaking (N C double bond and hydroxyl O–H bond) occur in one step.
Hence, it is considered as a concerted reaction. The formation of MC
is predicted to be exothermic by 20.7 kcal/mol (25.2 kcal/mol). Ea
of this step is 26.2 kcal/mol (29.5 kcal/mol), which is 16.4 kcal/mol
(12.0 kcal/mol) less than that of Reaction (1), so the reaction should
be facile. This is consistent with the experimental conclusion, in
which above the urea pyrolysis temperature urea methanolysis via
HNCO is believed to proceed readily even without catalyst [12].
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