Kinetic investigation of the effect of nickel and fluorine on the HDN of methylcyclohexylamine over WS2/Al2O3 catalysts

Article abstract of DOI:10.1006/jcat.2001.3241

The effects of incorporation of nickel and fluorine in WS₂/Al₂O₃ catalysts on the kinetics of the hydrodenitrogenation of methylcyclohexylamine were investigated. The catalysts were prepared from ammonium tetrathiotungstat

Full text of DOI:10.1006/jcat.2001.3241

Journal of Catalysis 201, 138–144 (2001)
doi:10.1006/jcat.2001.3241, available online at http://www.idealibrary.com on
Kinetic Investigation of the Effect of Nickel and Fluorine on the HDN
of Methylcyclohexylamine over WS2/Al2O3 Catalysts
Mingyong Sun and Roel Prins1
Laboratory for Technical Chemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
Received January 29, 2001; revised March 29, 2001; accepted March 29, 2001; published online May 31, 2001
The promotive effect of fluorine on HDN over W/Al2O3
or Mo/Al2O3 catalysts was investigated with pyridine (10,
11). Conversion of pyridine involves hydrogenation of the
heterocyclic ring and C–N bond breaking, but these steps
have not been studied separately. In most cases, the hy-
drogenation step and the subsequent C–N bond breaking
have a similar rate (4, 12–14). Consequently, it was not clear
whether the effect of catalyst modification on the HDN
of pyridine arose from both reaction steps or only from
one of them. We investigated the effect of fluorine on the
elementary reactions involved in the HDN of o-toluidine
and methylcyclohexylamine (MCHA) over W/Al2O3 cata-
lysts, and found that fluorine increased the activity of the
hydrogenation of the aromatic ring and the breaking of
the C(sp2)–N bond, while it hardly affected the HDN of
MCHA and the hydrogenation of methylcyclohexene (15).
The HDN of MCHA over NiMo/Al2O3 has been proposed
to take place via two mechanisms (Fig. 1): a Hofmann-type
elimination (path 1) and a nucleophilic substitution (path 2)
(7). Analysis of the data obtained from the HDN of MCHA
over tungsten-based catalysts showed that fluorine slightly
increased the elimination and decreased the substitution
(15).
The incorporation of nickel in molybdenum (or tungsten)
catalysts is traditionally considered to promote the hydro-
genation activity (16, 17). It was also found that addition
of nickel to tungsten catalysts increased the activity for the
HDS of thiophene (18, 19). Reports of the effect of nickel
on C–N bond breaking are scarce. Jian et al. found that
introduction of nickel to Mo/Al2O3 catalysts accelerated
the C–N bond breaking of piperidine (14) and decahydro-
quinoline (8). To better understand the effects of fluorine
and nickel on C(sp3)–N bond breaking, a kinetic investi-
gation of the HDN of MCHA over tungsten catalysts was
conducted in the present study.
The effects of incorporation of nickel and fluorine in WS2/Al2O3
catalysts on the kinetics of the hydrodenitrogenation of methylcy-
clohexylamine were investigated. The catalysts were prepared from
ammonium tetrathiotungstate and fully sulfided. The kinetic data
wereobtained byvaryingtheinitialreactant partialpressureand the
reaction temperature, and a Langmuir-Hinshelwood kinetic model
was used to fit the data. The hydrodenitrogenation of methylcyclo-
hexylamine takes place via two mechanisms, ammonia elimination
to methylcyclohexene, and nucleophilic substitution by SH followed
by hydrogenolysis of the C–S bond of methylcyclohexyl mercaptan
to methylcyclohexane. The first mechanism has a higher activation
energy than the second. Addition of nickel changes the nature of
the site for the elimination; it increases the activation energy of the
reaction and the equilibrium adsorption constant of the reactant on
the active site. Fluorination does not change the intrinsic properties
of the active sites, but it decreases the number of the active sites.
c
2001 Academic Press
Key Words: HDN; fluorination; nickel; WS2/Al2O3; kinetics;
methylcyclohexylamine.
1. INTRODUCTION
Removal of nitrogen from oil fractions is an important
objective for hydrotreating processes. Breaking of different
kinds of C–N bonds is an essential step in accomplishing this
task. According to the type of carbon atom to which the ni-
trogen atom is bonded, there are two kinds of C–N bonds:
C(sp2)–N, like in aniline, and C(sp3)–N, like in cyclohexy-
lamine. The active site for C(sp3)–N bond breaking differs
from the site for C(sp2)–N bond breaking (1–3). The hy-
drodenitrogenation (HDN) of aniline-type molecules (e.g.,
ortho-propylaniline, which is an intermediate in the HDN
of quinoline) goes preferentially via hydrogenation of the
phenyl ring (4–7); thus, eventually the N atom is removed
via C(sp3)–N bond breaking. Nevertheless, only few HDN
studies are concerned with C(sp3)–N bond breaking (2,
7– 9).
Kinetic data can only be interpreted meaningfully if the
catalyst consists of a single phase. Classically prepared
alumina-supported tungsten catalysts are only partially sul-
fided, however (20–24). Thus, introduction of fluorine and
nickel to classically prepared W/Al2O3 catalysts will affect
the proportions of the WS2 active phase and the nonsulfided
1 To whom correspondence should be addressed at Laboratory for
Technical Chemistry, ETH Zentrum, 8092 Zurich, Switzerland. Tel: 41-
1-6325490 Fax: 41-1-6321162. E-mail: prins@tech.chem.ethz.ch.
138
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

HDN OF METHYLCYCLOHEXYLAMINE OVER Ni-W/Al2O3(F)
139
Addition of fluorine and nickel to the catalyst may affect
the strength of adsorption of MCHA as well as its reac-
tivity. As a consequence, the partial pressure of MCHA
in the feed may influence the apparent relative activity of
the catalysts. Figure 2 shows that nickel and fluorine did
not have significant effect on the conversion of MCHA at
high partial pressure of MCHA, while nickel increased and
fluorine decreased the conversion of MCHA at low par-
tial pressure. At high partial pressure of MCHA, the active
sites of both catalysts are fully covered by the reactant, and
the difference in adsorption constants does not have a sig-
nificant influence on the apparent activity. At low partial
pressure, however, the active sites are only partially cov-
ered by the reactant, and a larger adsorption constant gives
a higher coverage and thus a relatively higher apparent ac-
tivity. Comparison of the Ni-ATT/Al2O3-F and ATT/Al2O3
with ATT/Al2O3-F gave the same result about the effect of
nickel and fluorine (not shown). This indicates that nickel
increases and fluorine decreases the adsorption constant of
MCHA, which was confirmed by the kinetic calculations
(see below).
FIG. 1. HDN network of MCHA.
species. Consequently, it is difficult to clarify the promo-
tional effect of fluorine and nickel on the WS2 active phase.
Fully sulfided tungsten catalysts were obtained by using am-
monium tetrathiotungstate (ATT) as the precursor (23, 24),
and such catalysts have been used in the present study.
Influence of Reaction Temperature
2. EXPERIMENTAL
If the activation energy for the HDN of MCHA on
one catalyst differs from that on another, the appar-
ent relative activities of the catalysts should change with
reaction temperature. Figure 3 shows that the activity
of the Ni-ATT/Al2O3-F catalyst is slightly lower than
that of the ATT/Al2O3-F catalyst at 320 C, the same at
345 C, but higher at 370 C. Reaction temperature does not
Details of the preparation of tungsten-only (ATT/Al2O3
and ATT/Al2O3-F) (23) and nickel-promoted (Ni-
ATT/Al2O3 and Ni-ATT/Al2O3-F) (24) catalysts were re-
ported before. All the catalysts contained 10 wt% of tung-
sten; the nickel-promoted catalysts contained 1 wt% of
nickel, and the fluorine content in the fluorinated cata-
lysts was 1 wt% . Prior to HDN reaction, the catalysts were
treated in situ with a mixture of H2S (10 mol% ) and H2
(90 mol% ) at 400 C and 1.5 MPa for 4 h. Then, the temper-
ature was lowered to 370 C, the pressure was increased to
3.0 MPa, and the liquid feed was introduced to the reactor
by means of a high-pressure pump, with n-octane as solvent
and n-heptane as internal standard. Dimethyldisulfide was
added to the feed to generate H2S (6 kPa) in the reaction
stream. The partial pressure of MCHA varied from 2 to
21 kPa, while the partial pressures of n-octane and n-
heptane were kept constant at 67 and 8 kPa, respectively.
The partial pressure of H2 varied slightly from 2890 to
2910 kPa, to keep the total pressure constant at 3.0 MPa.
3. RESULTS
Influence of Partial Pressure of MCHA
Under the same reaction conditions, the conversion
of MCHA decreases with increasing initial partial pres-
sure of MCHA because of self-inhibition. As an example,
Fig. 2 gives the conversion of MCHA at different initial
partial pressures on the ATT/Al2O3, Ni-ATT/Al2O3 and
FIG. 2. Comparison of the apparent activities of Ni-ATT/Al2O3 (1),
Ni-ATT/Al2O3-F (ᮀ), and ATT/Al2O3 ( ) in the HDN of MCHA at 320 C
Ni-ATT/Al2O3-F catalysts as a function of weight time. and 3.0 MPa for different partial pressures of MCHA in the feed.

140
SUN AND PRINS
FIG. 3. Comparison of the apparent activities of Ni-ATT/Al2O3 (1),
Ni-ATT/Al2O3-F (ᮀ), and ATT/Al2O3-F ( ) in the HDN of MCHA at
different temperatures (21 kPa MCHA, 3.0 MPa).
FIG. 4. Influence of the initial partial pressure of MCHA on the se-
lectivity of MCHE (Ni-ATT/Al2O3, 370 C, 3.0 MPa).
change the relative activities of the fluorine-containing and
fluorine-free Ni-ATT/Al2O3 catalysts significantly. This in-
dicates that the reaction over the Ni-ATT/Al2O3-F catalyst
has a higher activation energy than over the ATT/Al2O3-
F catalyst, while fluorination does not affect the activation
energy to a significant extent.
of the studied catalysts for the HDN of MCHA were
obtained by simulating the experimental data with
Langmuir-Hinshelwood rate expressions. The HDN of
MCHA proceeds along two pathways (Fig. 1). Therefore,
the overall HDN conversion is the sum of the two reac-
tions. Reaction conditions and catalyst composition can
change the relative concentrations of MCH and MCHE
in the HDN product of MCHA, indicating that the active
sites for these two mechanisms are different (7, 15). In the
Langmuir-Hinshelwood equation, the two pathways then
require their own rate as well as adsorption constants. How-
ever, a change in the initial partial pressure of MCHA did
The HDN of MCHA proceeds according to a Hofmann-
type elimination, which gives methylcyclohexene (MCHE)
as the product, and according to a nucleophilic substitu-
tion, which gives methylcyclohexane (MCH) as the product
(Fig. 1) (7, 15). In the presence of MCHA, the hydrogena-
tion of MCHE to MCH does not proceed to a significant
extent due to the strong inhibition effect of MCHA on
the hydrogenation (15). As a consequence, the selectiv-
ity of MCHE is almost independent of the conversion of
MCHA up to high MCHA conversion (Fig. 4). The selectiv-
ities of MCHE and MCH thus reflect the relative reaction
rates of these two mechanisms, elimination and substitu-
tion. Figure 5 shows the influence of the reaction tempera-
ture on the selectivityofMCHE in the HDN ofMCHA over
the Ni-ATT/Al2O3 and ATT/Al2O3 catalysts. The selectivity
of MCHE is higher at 370 C than at 320 C, indicating that
the elimination mechanism has a higher activation energy
than the substitution mechanism. The increase in selectivity
of MCHE with temperature is higher for the Ni-ATT/Al2O3
catalyst than for the ATT/Al2O3 catalyst, meaning that the
difference in activation energy between these two mecha-
nisms is larger for the former catalyst than for the latter.
Kinetic Parameters
The Langmuir-Hinshelwood model has often been used
to express the kinetics of the HDN reactions occurring
FIG. 5. Effect of the reaction temperature on the selectivity for
MCHE of Ni-ATT/Al2O3 (heavy line) and of ATT/Al2O3 (thin line) in
on sulfide catalysts (6, 8, 25–27). The kinetic parameters the HDN of MCHA (21 kPa MCHA, 3.0 MPa).

HDN OF METHYLCYCLOHEXYLAMINE OVER Ni-W/Al2O3(F)
141
not affect the selectivity of MCHE to a significant extent
(Fig. 4), indicating that the adsorption constants of MCHA
on the two sites are equal. Therefore, it is possible to sim-
plify the Langmuir-Hinshelwood equation by using one and
the same adsorption constant for both sites. The adsorption
ofthe N-free productson the active site ismuch weaker than
MCHA and ammonia. Thus,
d PMCHA
d
(k1 + k2)KMCHA PMCHA
=
[1]
[2]
1 + KMCHA PMCHA + KNH PNH
3
3
PNH = PMCH + PMCHE
,
3
where is the weight time, k1 and k2 are the rate constants
for path 1 and path 2, respectively, KMCHA and KNH are the
3
adsorption constants of MCHA and NH3, respectively, and
PMCHA, PNH , PMCH, and PMCHE are the partial pressures of
3
MCHA, NH3, MCH, and MCHE, respectively. Results of
the nonlinear regression of the experimental data according
to Eqs. [1] and [2] showed that KNH was about an order
3
FIG. 6. Conversion versus weight time in the HDN of MCHA over
Ni-ATT/Al2O3 (21 kPa MCHA, 3.0 MPa).
of magnitude smaller than KMCHA, and that its standard
deviation was large. Therefore, the adsorption of ammonia
was ignored and Eq. [1] simplified to
d PMCHA
d
(k1 + k2)KMCHA PMCHA
1 + KMCHA PMCHA
of the plots indicates that the above simplification is jus-
tified. Thus, the sums of k1 and k2 were obtained for all
catalysts, and the results are given in Table 1. Once k1 + k2
was determined, the adsorption constant KMCHA could be
determined uniquely by varying the initial partial pressure
of MCHA from 2 to 21 kPa and by nonlinear regression
analysis of the experimental data according to Eq. [3]. The
results are also presented in Table 1. Figure 7 presents the
experimental data for three different initial partial pres-
sures of MCHA and the simulation results (drawn lines)
for the Ni-ATT/Al2O3 catalyst. The statistical evaluation
of the goodness-of-fit gave the standard deviations of the
parameters, which are also presented in Table 1 (in paren-
theses).
=
.
[3]
Direct simulation of the partial pressure of MCHA ver-
sus weight time with the above equation did not give unique
parameter values. The experimental data fitted the model
equally well with a variety of combinations of (k1 + k2) and
KMCHA. To get unique parameters one of the parameters
must be determined independently, as Jian and Prins al-
ready pointed out before (6, 8). At large partial pressure of
MCHA, KMCHAPMCHA
1, and Eq. [3] simplifies to
k1 + k2
XMCHA
=
,
[4]
PMo CHA
where XMCHA is the conversion of MCHA and PMo CHA
is the initial partial pressure of MCHA. As an example,
Fig. 6 gives the conversions of MCHA over the Ni-
ATT/Al2O3 catalyst versus weight time when the initial par-
tial pressure of MCHA is 21 kPa. The very good linearity
Figure 4 shows that the selectivity of MCHE is constant
when the conversion of MCHA is lower than 50% . At low
conversion of MCHA, path 1 and path 2 constitute parallel
reactions (7, 15), and the selectivities of MCHE and MCH
at low conversion of MCHA are equal to k1/(k1 + k2) and
TABLE 1
Kinetic Parameters Obtained by Fitting the Conversion of MCHA at 3.0 MPa with
a Langmuir-Hinshelwood Expression, k1 + k2 in kPa mol min
1
1
g
1, KMCHA in kPa
ATT/Al2O3
ATT/Al2O3-F
Ni-ATT/Al2O3
Ni-ATT/Al2O3-F
370 C
345 C
320 C
k1 + k2
KMCHA
k1 + k2
KMCHA
k1 + k2
KMCHA
1.44 (0.03)
0.49 (0.05)
0.47 (0.01)
0.72 (0.10)
0.113 (0.004)
1.06 (0.25)
1.47 (0.03)
0.27 (0.02)
0.47 (0.01)
0.47 (0.08)
0.113 (0.004)
0.60 (0.15)
1.79 (0.03)
0.61 (0.07)
0.53 (0.01)
1.17 (0.34)
0.111 (0.001)
3.0 (0.4)
1.64 (0.03)
0.60 (0.06)
0.46 (0.01)
0.99 (0.26)
0.100 (0.001)
2.1 (0.2)

142
SUN AND PRINS
FIG. 8. Temperature dependence of the rate constants of path 1 (k1)
and path 2 (k2) and the equilibrium adsorption constant of MCHA for the
FIG. 7. Comparison of the experimental data (points) and model pre-
HDN of MCHA over the Ni-ATT/Al2O3 catalyst at 3.0 MPa.
diction results (lines) for the HDN of MCHA over the Ni-ATT/Al2O3 at
320 C and 3.0 MPa with three initial partial pressures.
(Figs. 4 and 5), and it is affected to a greater extent by ad-
dition of nickel and fluorine than the substitution path (Ta-
ble 2). The change in the total conversion of MCHA caused
by catalyst modification is mainly due to the change in ac-
tivity for the elimination path. The activation energy of the
elimination reaction (path 1) is higher than that of the sub-
stitution reaction (path 2) for all catalysts studied (Table 3),
indicating that the elimination mechanism is relatively fa-
vored at higher reaction temperatures. This explains why
the selectivity of MCHE is higher at 370 C than at 320 C
(Fig. 5).
The sites for these two reactions are different in na-
ture. The two nickel-containing catalysts have significantly
higher activation energies for the elimination reaction than
the two nickel-free catalysts, while the fluorine-containing
and fluorine-free catalysts show no difference in activa-
tion energy (Table 3). The same is true for the heat of
adsorption of MCHA on the active sites (Table 4). These
k2/(k1 + k2), respectively. Thus, the ratio of k1 to k1 + k2 and
the individual values of k1 and k2 can be obtained, and they
are given in Table 2.
The activation energies of the HDN reaction paths 1 and
2 for the studied catalysts were obtained according to the
Arrhenius equation. The results are given in Table 3. The
standard deviations in parentheses were obtained through
a statistical calculation using the MicroMath Scientist pro-
gram. Similarly, the heats of adsorption of MCHA and
their standard deviations were calculated and are given in
Table 4. As an example, Fig. 8 gives the plots of the tem-
perature dependence of the rate constants of paths 1 and
2 and of the equilibrium adsorption constant of MCHA on
the Ni-ATT/Al2O3 catalyst.
4. DISCUSSION
The elimination of ammonia from MCHA is the major results indicate that the addition of nickel to the tung-
path in the HDN of MCHA over the WS2/Al2O3 catalysts sten catalyst changes the nature of the active sites whereas
TABLE 2
Rate Constants of the Paths 1 and 2 for the HDN of MCHA at 320 to 370 C, 3.0 MPa
1
1
over Different Catalysts (kPa mol min
g )
ATT/Al2O3
ATT/Al2O3-F
Ni-ATT/Al2O3
Ni-ATT/Al2O3-F
370 C
345 C
320 C
k1/(k1 + k2)
0.82
1.18
0.26
0.80
0.38
0.09
0.79
0.089
0.024
0.88
1.29
0.18
0.86
0.40
0.07
0.84
0.095
0.018
0.86
1.54
0.25
0.81
0.43
0.10
0.74
0.082
0.029
0.89
1.46
0.18
0.85
0.39
0.07
0.81
0.081
0.019
k1
k2
k1/(k1 + k2)
k1
k2
k1/(k1 + k2)
k1
k2

HDN OF METHYLCYCLOHEXYLAMINE OVER Ni-W/Al2O3(F)
143
TABLE 3
Arrhenius Constants in the HDN of MCHA at 3.0 MPa, k = A exp( Ea/RT)
ATT/Al2O3
ATT/Al2O3-F
Ni-ATT/Al2O3
Ni-ATT/Al2O3-F
Path 1
Ea (kJ/mol)
A (kPa mol min
Path 2
Ea (kJ/mol)
A (kPa mol min
164 (7)
3E + 13
166 (7)
4E + 13
186 (10)
1E + 15
184 (5)
1E + 15
1
1
1
1
g
g
)
151 (10)
6E + 11
146 (8)
1E + 11
137 (9)
3E + 10
143 (10)
8E + 10
)
fluorine does not affect the intrinsic properties of the sites. and fluorine did not change the nature of the site for the
Elimination of ammonia from MCHA requires a pair of substitution. However, fluorine decreased the rate constant
basic and acidic sites. The basic site can be a surface sul- of path 2 (k2), while nickel had no influence on k2. The num-
fur atom and the acidic site a neighboring sulfur vacancy ber of the active sites and their intrinsic activity determine
or an SH group attached to a sulfided tungsten atom. It the rate constant. Therefore, it is concluded that fluorine
is believed that nickel and tungsten in the sulfided cata- does not change the intrinsic properties of the site for sub-
lyst form the Ni–W–S structure (18, 19, 28, 29). The na- stitution reaction, but decreases the number of the sites, and
ture of the surface sulfur atom, sulfur vacancy, and SH that nickel has no influence on the substitution mechanism.
group on the Ni–W–S structure differs from that on the The substitution reaction is catalyzed by surface SH groups
WS2 structure (30, 31). The activation energy of the elim- created by dissociative adsorption of H2S on the catalyst
ination reaction is larger (Table 3), and the adsorption of surface (7). Fluorination of the alumina support leads to a
MCHA is stronger (Table 4) on the Ni–W–S sites than on poorer dispersion of tungsten (10, 11, 15, 35); thus there are
the WS2 sites. Although fluorination of the alumina sup- fewer surface tungsten atoms to dissociate H2S. That nickel
port affects the dispersion and sulfidation of molybdenum has no influence on the substitution reaction indicates that
(32–34) and tungsten (10, 11, 23, 24, 35), there is no ev- the active site for the substitution mechanism is only related
idence that fluorine changes the structure of the active to the surface SH groups, and that Ni has no influence on
phase. The fact that fluorine did not change the activa- their number and properties.
tion energy and the heat of the adsorption of MCHA on
For heterogeneouscatalyticreactions, the apparent activ-
the active site excludes a change in nature of the sites in- ity of a catalyst is determined by the rate constant (product
duced by fluorination. Our catalysts were prepared from of the number of active sites and their intrinsic activity)
ATT and shown to be fully sulfided (23, 24), therefore, flu- and the coverage of the reactant (determined by equilib-
orine mainly changed the size and the stacking of the WS2 rium adsorption constant and partial pressure of the reac-
slabs (10, 23, 24, 36). The change in the morphology of the tant). Two catalysts with different adsorption constants may
catalyst surface induced by fluorination may affect the en- exhibit different relative activities when using feeds with
tropy change for the adsorption of MCHA. At one tem- different partial pressures of reactant. Figure 2 shows that
perature, the heat of adsorption and the change in entropy the reaction rate of the HDN of MCHA is larger over the
together determine the adsorption constant. Fluorination Ni-ATT/Al2O3 catalyst than over the ATT/Al2O3 catalyst
did not change the heat of adsorption, but it decreased when using a low concentration feed, but it is smaller when
the adsorption constant moderately. The moderate change using a high concentration feed. Therefore, one cannot sim-
in the adsorption constant may be ascribed to an entropy ply say that the effect of nickel on the HDN of MCHA is
effect.
positive or negative. It is necessary to perform a kinetic in-
All the catalysts have similar activation energies for re- vestigation to reveal the effect of the catalyst modification
action 2 (Table 3), meaning that the incorporation of nickel on the nature of the active sites.
TABLE 4
Temperature Dependence of KMCHA = K0 exp( 1 H/RT) at 3.0 MPa
ATT/Al2O3
ATT/Al2O3-F
Ni-ATT/Al2O3
Ni-ATT/Al2O3-F
1H(kJ/mol)
49 (1)
50 (12)
101 (9)
80 (7)
1
K0 (kPa
)
5E
5
2E
5
4E
9
2E
7

144
SUN AND PRINS
15. Sun, M. Bussell, M. E., and Prins, R., Appl. Catal., submitted for pub-
5. CONCLUSION
lication.
16. Perot, G., Brunet, S., and Hamze, N., Proc. 9th Int. Cong. Catal. 1, 19
The apparent activity cannot fully reflect the effect of
catalyst modification on the catalytic performance. The ap-
parent activity depends on the reaction conditions and feed
composition. A kinetic investigation can reveal a change in
(1988).
17. Ozkan, U. S., Zhang, L., Ni, S., and Moctezuma, E., J. Catal. 148, 181
(1994).
18. Louwers, S. P. A., and Prins, R., J. Catal. 139, 525 (1993).
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Niemantsverdriet, J. W., J. Catal. 196, 180 (2000).
20. Ng, K. T., and Hercules, D. M., J. Phys. Chem. 80, 2094 (1976).
21. Breysse, M., Cattenot, M., Decamp, T., Frety, R., Gachet, C., Lacroix,
tungsten catalyst affects the intrinsic properties of the ac-
tive site for the elimination of ammonia from MCHA, while
it has no effect on the nature of the site for the substitution
M., Leclerq, C., Mourques, L. de, Portefaix, J. L., Vrinat, M., Houari,
of NH2 by SH in MCHA. Fluorine does not change the
intrinsic properties of the active sites, but it changes the
dispersion of the active sites on the surface. The change in
the morphology of the surface affects the entropy of the
adsorption of MCHA, and thus moderately influences the
adsorption constant of MCHA.
M., Grimblot, J., Kasztelan, S., Bonnelle, J. P., Housni, S., Bachelier,
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