This document summarizes a study that used reactive molecular dynamics (RMD) simulations to investigate the reaction mechanisms of the methanol to olefin (MTO) process over ZSM-5 zeolite catalysts. Three types of simulation models were used: methanol/acidic zeolite, ethene/methoxylated zeolite, and a complex methanol/ethene/acidic zeolite system. The simulations identified several reaction pathways and intermediates. Comparisons between the initial reactions of the pure systems found carbon chain growth to be the rate determining step, with a lower activation energy than methanol decomposition.

1. Mechanism and kinetic study of Methanol to Olefin (MTO) process by
Reactive Molecular Dynamics (RMD) method
Bai Chen, Liu Lianchi, and Huai Sun*
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai
200240, P.R. China
1. Introduction
Methanol to olefin (MTO), catalyzed by ZSM-5 or SAPO-34, is one of the
promising alternative energy technologies which can effectively transform methanol
into lower alkenes. However, the reaction mechanism of MTO is far from understood.
All of the theoretical studies carried out so far were based on static energy
calculations which were based on static energies calculated based on proposed
reaction pathways. The aim of this work is to understand how chemical reactions are
taking place with entropy contributions.
2. Models and Methods
The ZSM-5 zeolite was represented by a 2x1x1 super cell. The silicon atoms
located at the T12 and T9 sites were replaced by aluminum atomsThe Brønsted acid
sites or the methoxyl groups were added to the Al sites, as shown in Figure 1.
(a) (b)
Figure 1. (a) Plan and (b) vertical view of a typical system, 128T with Si/Al ratio 176/16.
Three types of simulation models were used in this work.
A) Acidic zeolite with fifteen (15) methanol molecules;
B) Methoxylated zeolite with ten (10) ethene molecules;
C) Acidic zeolite with ten (15) methanol molecules and six (6) ethene molecules.
Results and discussions
3. Results and discussion
3.1. Parameterization
We firstly fit parameter of Aluminum atom based on ReaxFFsi/o force field;
training sets include bond, angle, and reaction pathways, we recreate the pathway of
carbon chain growth suggested by Maihom et al. by QM calculations and then add

2. them in the training sets. (Figure 2 and Figure 3)
Figure 2. Comparisons of energy between ReaxFF and QM on the typical bonds and angle in zeolite system. (a)
Dissociation of the Al-O bond in Al(OH)3cluster; (b) energy of H2Al-O-H as a function of the Al-O-H valance
angle; (c) energy of Al(OH)3 as a function of the O-Al-O valance angle; (d) energy of H3SiO(H)AlH3 as a function
of the Si-O-Al valance angle.
(a)

3. (b)
(c)
Figure 3. Comparison of energy differences between FF and QM results of three reaction pathways in 3T model.
(a)First step of step-wise mechanism, decomposition of methanol and formation of surface methoxyl group;
(b)Second step of step-wise mechanism, carbon chain growth and formation of propene molecule; (c)Concerted
mechanism for formation of propene molecule, carbon chain growth while water molecule dissociated.
3.2. Simulations on Simplified Models
MD simulations were performed on Model A and Model B, several new
mechanisms were identified.
3.2.1 Model A (Methanol/Acidic zeolite)
Trajectory and mechanism analysis were performed on Model A:

4. Figure 4. Product distributions as a function of simulation time of model A (Methanol/Acidic zeolite), The high
temperature regions were removed.
CmHn,
Cm+1Hn+2(3)
H2O
CH3-Z
CH3OH
CH3OH - H-Z Z
H-Z H2O + Collision C2H5 C2H4 + H-Z
H-Z
Collision
CH3OH2 + Z CH3
-Z
CH3OH2
CmHn
Cm+1Hn+3
Z
CmHn - H-Z
H-Z
H2 + CmHn-3 CH4 + Z
Scheme 1. Mechanisms for activation of methanol molecules. Species in red color only observed during high
temperature regions.
The CH3 groups act as a reactive intermediate during conversion, which were
generated from collisions upon absorbed methanol molecules.

5. We identified detailed formation of CH3 groups and C2 species
(a) (b) (c)
Figure 5. Formation of free CH3OH2 groups by collision of a methanol molecule. (a) A methanol
absorbed on the acid cite was attacked by a free methanol molecule, (b) collision between the two
molecules, and (c) CH3OH2 group releases and a deprotonated cite formed.
I(a) I(b) I(c)
II(a) II(b) II(c)
Figure 6. Two pathways of formation of first C2 species in model A (only methanol as reactant).
I(a) A methanol absorbed on the acid cite was attacked by a free CH3, I(b) C-O bond of methanol
stretched, water molecule was formed, and I(c) C-C bond was formed, water molecule was
absorbed at the deprotonated site. II(a) A methanol absorbed on the acid cite was attacked by a
free CH3, II(b) C-C bond was formed and one C-H bond from methoxyl group was stretched, and
(c), a acid site was restored and a C2H5 species was produced.
3.2.2 Model B (Ethene/Methoxylated zeolite)

6. Figure 7. Product distributions as a function of simulation time of Ethene//Methoxylate zeolite system, the high
temperature regions were removed.
- H-Z
CH3CH2CH2 + Z CH3CHCH3 + Z
CH3-Z
C2H4 C2H4, CH3 C3H6
C2H4 - H-Z, CH3-Z, - Z
Z C2H4 Z
- H-Z, CH3-Z
C3H5 + H-Z
- H-Z, CmHn Z
Cm+2Hn+3 Cm+2H + H2 + H-Z
Scheme 2. Mechanisms for carbon chain growth start from ethene. Species with red color only exist in high
temperature regions.
In this model, we identified unprotonated Al center could play a role of catalysis
center, it can activate hydrocarbons in the system. Intramolecular hydrogen transfer
were observed, this may affect product distribution.
3.2.3 Comparison of initial reactions in models A and B
By comparing pre-exponential factors, activation energies, and effective collision
possibility, we found different from QM’s prediction, the actual rate-determining step
is the 2nd step which has lower activation energy – carbon chain growth,

7. Figure 8. Arrhenius extrapolation for pure ethene and pure methanol system, temperature range of 1500-1800K for
the Ethene/Methoxylate zeolite system and 1600-2000K for Methanol/Acidic zeolite system.
(a) (b)
Figure 7. Representation of collisions between reactants of both bad direction and good direction, (a) the first and
second pictures represent collisions between methanol molecule and H-Site in good or bad direction; (b) the third
and fourth pictures represent a collision between ethene molecule and CH3-Site in good or bad direction.
 ESA   ESA 
ECP     
 TASA  A  TASA  B
Where ESA is the effective molecule surface area, TASA is the total accessible
surface area.
The ECP values are 9.1% for methanol and Brønsted hydrogen, but only 1.8% for
ethene and methoxyl group.

8. 3.3 Model C (Methanol/Ethene/Acidic zeolite)
Figure 9. Product distributions as a function of simulation time of Complex system, numbers of species were
averaged in 15 simulations. The high temperature regions were removed.
15 parallel simulations were performed to gain statistic insights, the commonly
referred chain-growth mechanism, methoxyl + ethene mechanism, is observed as a
small contribution to the total production. Most ethene molecules are activated by
zeolite (unprotonated acidic sites) directly and the activated ethene molecules react
with other species in the system vigorously.

9. [5/5]
- H-Z,
CH3-Z, - Z C3H6
CH3CHCH3+Z [6/62]
C3H5
- H-Z, CH3-Z
[3/62]
[5/5]
C2H4 Z CmHn,
- H-Z
CH3CH2CH2+Z
[16/62] [36/62]
- H-Z,
[62/67] H2O
Z
[5/67] C2H3-OH2
Cm+2Hn+3
CH3-Z
Z C2H4O
[10/16]
C2H4 H-Z
2 4 H-Z CH3OH
C2H4 + H-Z
CH3OH2-Z
Z, [6/6]
(Chain growth…) H2O
- H-Z
[27/210] Collision
[183/210]
[165/186]
C2H5 + H-Z CH3-Z CH3OH2 + Z
Z
[27/27]
[3/27] -Z
CH3-Z
[6/27]
CH3
H2O
CmHn H-Z,
-Z
[9/27]
[9/27]
Cm+1Hn+3 CH4
Scheme 3. Mechanisms collection from Complex model, numbers in parenthesis indicate times reactions occurred
in 15 parallel Complex system simulations. Species with red color only exist in high temperature region. Blue
arrows indicate reactions only occured in simplified models.
The catalysis of unprotonated center was validated by QM calculations:

10. 1.24
1.35
10
Energy (kcal/mol)
5
TS C2H3 + H-Z
C2H4 + Z
0
-5 C2H4…Z
C2H3…H-Z
-10
Figure 10. QM validation of hydrogen transfer process from ethene to deprotonated site, 3T
cluster at B3LYP/6-31g(d,p) level, open shell.
4. Conclusions
The main novel contributions are:
1) Augmented ReaxFF for simulation of catalytic reactions in acidic zeolite.
2) Using three simulation models and programmed data analysis techniques,
complete reaction networks of MTO process are obtained, new reaction
mechanisms or conclusions are proposed.
3) By considering the entropy contributions, the rate-determining step for MTO is
not the activation of methanol, as suggested by static QM calculations, but the
C-C chain formation and growth.
4) New observed mechanisms were identified and made comparison with
experiment.