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Catalysis and Kinetics
An evaluation of published kinetic models for vapor phase methanol conversion to dimethyl ether over the H-ZSM-5 catalyst Ionut BANU, Rodica Ganea, Gabriel Vasilievici, Alin Anghel, Valentina Gogulancea, Gabriela Isopencu, and Grigore Bozga Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01839 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018
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Energy & Fuels
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An evaluation of published kinetic models for vapor phase methanol
2
conversion to dimethyl ether over the H-ZSM-5 catalyst
3
Ionut BANUa, Rodica GANEAb, Gabriel VASILIEVICIb, Alin ANGHELa, Valentina
4
GOGULANCEAa, Gabriela ISOPENCUa, Grigore BOZGAa*
5 6 7 8
a
9
The published kinetic models for methanol etherification over ZSM-5 zeolite were
10
evaluated against own experimental data. The process was investigated over a synthesized H-
11
ZSM-5 zeolite using a laboratory fixed bed reactor. The experiments were carried out at
12
atmospheric pressure, temperatures of 170-270 °C, methanol feed concentrations up to 15 mol %
13
and gas phase velocity (WHSV) values in the range 15-48 h-1. The results showed a decrease of
14
methanol conversion with respect to feed methanol concentration, the single reaction product
15
obtained in significant concentrations being dimethyl-ether (no secondary products were
16
observed in significant concentrations). The methanol conversion measurements were compared
17
with theoretical predictions based on the main kinetic models published for methanol
18
etherification over ZSM-5 zeolites with different acidities. As none of the published models
19
fitted satisfactorily our experimental data, we re-estimated the parameters of the tested models
20
and applied a discrimination procedure in order to identify the most suitable one. The best
21
quality of the fit was obtained by using a LH kinetic model based on the associative surface
22
reaction mechanism. The adequacy of this kinetic model was confirmed by statistical and
23
thermodynamic consistency criteria.
24
Keywords: catalysis, methanol etherification, kinetic model, H-ZSM-5 zeolite
University Politehnica of Bucharest, Faculty of Applied Chemistry and Material Science, Department of
Chemical and Biochemical Engineering, 1-7 Polizu Str., 011061, Bucharest, Romania b
National Research Institute for Chemistry and Petrochemistry, ICECHIM, 202 Spl. Independentei,
060021,Bucharest, Romania
25 26
Introduction
27
Dimethyl ether (DME) emerged in the last decades as a chemical product with potential
28
advantages in the clean energy supply, attracting a large research interest in academia and
29
industry. DME features outstanding fuel properties, exhibiting a high cetane number (55-60) and
30
excellent burning characteristics due to the low level of particulate products, low NOx emissions 1 ACS Paragon Plus Environment
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31
and no release of sulfur compounds. These properties make DME a potential alterative to
32
petroleum diesel fuel, an interesting fuel for combustion cells and a largely applied LPG
33
substitute. LPG–DME blends have been suggested as fuels for use in cooking, heating and LPG-
34
fueled vehicles 1. It can be also used as an aerosol propellant and a liquid refrigerant suitable to
35
replace ozone destroying chlorofluorocarbons. DME is used as an intermediate for synthesis of
36
several oxygenated products such as methyl acetate, dimethyl sulfate etc. It is to be noted as well
37
that DME appears as an intermediate in the methanol conversion to hydrocarbons.
38
As the synthesis gas, the raw material for methanol synthesis, can be directly obtained
39
from biomass, DME has also the advantages of a renewable product 2-6. The economic aspects of
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biomass utilization as raw material for production of syngas and its derivatives were analyzed in
41
an important number of published studies. As known, the bottlenecking step in the DME
42
production from syngas is the methanol synthesis (mainly due to its slower kinetics and higher
43
thermodynamic limitation, as compared with methanol etherification step). Therefore, the
44
economic viability of DME production from biomass is directly dependent on methanol
45
production viability. This issue was analyzed and demonstrated by several authors. Borgwardt 7,
46
estimated a cost of methanol production from biomass amounting to 0.42 USD/gal (~ 0.14
47
USD/kg). For a methanol plant using biomass as carbon source, with a capacity of 1085 million
48
gal/yr (~ 3.2 million t/yr), Li et al.
49
which could be compared to methanol price in the United States. On the European and North-
50
American markets the reported methanol price in July 2018 is around 0.49 USD/kg (~0.419
51
€/kg) ( https://www.methanex.com/our-business/pricing). Besides the economical viability, other
52
reasons for the utilization of the biomass for methanol production are that it permits to valorize
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secondary feedstocks for industrial applications, decreasing, in the same time, the environment
54
impact of the process (Li et al. 9).
8
have reported a methanol production cost of 0.43 ($/gal)
55
DME can be produced at commercial scale by two technologies: (i) a technology using as
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raw material the methanol previously synthesized in a different plant (two-step process or
57
indirect method) and (ii) a technology of DME fabrication directly from synthesis gas,
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integrating the methanol synthesis and methanol dehydration in a single process unit (the one-
59
step process or the direct method). The latter is advantageous, particularly from the
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thermodynamic point of view, as the methanol consumption is promoting the carbon oxides
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hydrogenation to methanol. Furthermore, methanol etherification is less limited by the chemical 2 ACS Paragon Plus Environment
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equilibrium state as compared with the methanol synthesis. The one-step DME synthesis
63
involves the simultaneous use of two catalysts, the methanol synthesis catalyst (MSC) and
64
methanol etherification catalyst (MEC). These are used in the same catalytic reactor, in different
65
spatial distributions (individual grains of the two catalyst in different arrangements, or a bi-
66
functional catalyst prepared by combining the MSC and MEC in the same pellet 4).
67
Published studies have evaluated the catalytic performances of different acidic materials for 10, 11
68
methanol conversion to dimethyl ether (MEC component): alumina
69
mordenite or mordenite modified with metal oxides
70
ZSM-5 (MFI), zeolite, SAPO mesoporous materials or ion exchange resins 2, 14-16.
12, 13
, dealuminated H-
, ferrierite, Y zeolites, Beta-zeolite,
71
Review papers analyzing the methanol etherification catalysts, reaction mechanisms,
72
reaction kinetics and technologies were published by Spivey 17, Bozga et al. 4, Azizi et al. 5 and
73
Saravanan et al. 18.
74
In the DME synthesis by one-step technology, it is recommended to use MEC having
75
sufficiently high activity in the temperature window of the methanol synthesis reaction (usually
76
220- 280oC) in order to keep a limited rate of methanol synthesis catalyst deactivation
77
Although alumina is a cheap and widely available catalyst adequate for methanol etherification
78
in the two-step technology, its catalytic activity is too low to ensure a satisfactory methanol
79
conversion under the temperature conditions of methanol synthesis 20. Additionally, the presence
80
of water, which is a reaction product, deactivates alumina, presumably due to its blocking effect
81
on the acid sites
82
than alumina and more resistant to the presence of water are necessary. Published studies suggest
83
that a good candidate in this regard is the ZSM-5 zeolite, which is one of the most studied
84
zeolites and among the best catalysts reported for methanol etherification
85
catalytic activity and relatively low cost, it presents also the advantage of a tunable acidity and
86
superior resistance to water, as compared with other acidic zeolites
87
correlated the DME productivities of the H-ZSM5 zeolite, H-Y (Faujasite) and γ-alumina with
88
their Weitkamp hydrophobicity index (ratio of toluene adsorption by water adsorption). The best
89
productivities were obtained by using the H-ZSM-5 zeolites having relatively high Si/Al ratios,
90
i.e. increased hydrophobicity, these results evidencing that the acid sites concentration and
91
strength are not the only parameters to be adjusted, another important one being the catalyst
92
hydrophobicity. Published studies highlight that, if calcined at adequate temperatures, the
21
19
.
. Therefore, in the one-step technology etherification catalysts more active
22
23-25
. Besides its high
. Vanoye et al.
24
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HZSM-5 zeolite is hydrothermally stable, a property that makes it usable in processes involving
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reaction-regeneration cycles 26, 27.
95
An experimental study investigating the influence of the main process variables on methanol 28
96
dehydration over pure H-ZSM-5 zeolite was recently published by Ajami and Shariati
97
authors reported maximum methanol conversions (practically equilibrium levels) in the
98
temperature range 240–250°C. For temperatures over 270°C, the authors observed significant
99
amounts of byproducts (no details about their nature were provided).
100 101
. The
A number of published studies evidenced better catalytic performances of H-ZSM-5 in the methanol etherification, as compared with γ-alumina 31
24, 29
; ferrierite zeolite
102
(Vanoye et al., 2013), and different mordenites
103
performances of ferrierite zeolites in the DME synthesis (see Frusteri et al.
104
therein).
30
, HY zeolite
. Recent studies have highlighted also good 32
and references
105
An inconvenience of the H-ZSM-5, as a catalyst for methanol etherification, is the occurrence
106
of the secondary reaction of DME conversion to hydrocarbons over its stronger acid sites; this
107
transformation is significant particularly at temperatures over 270oC
108
this, the strength and surface density of acid sites can be varied to a certain extent in order to
109
improve the zeolite’s catalytic properties
110
aim: partial neutralization of strong acid sites with alkaline metals
111
matrix of alumina (which is diluting the density of strong acid centers) 16, reducing the acidity by
112
increasing the Si/Al ratio
24, 33, 34
17
28
. Nevertheless, to avoid
. Different technical solutions were proposed in this 16, 31
, H-ZSM-5 binding in a
, decreasing the zeolite crystal size by appropriate synthesis
113
techniques , selection of special organic compounds as templates in the zeolite synthesis 36, an
114
alkaline treatment at the synthesis step combined with partial activation 37, working in presence
115
of air 38, and lastly modification of HZSM-5 with a small amount of antimony oxide 39.
116
35
Several studies
24, 34
observed an increase of reaction rate (methanol conversion) with an
117
increase in the Si/Al ratio. Migliori et al. 34 found also that the activation energy of the methanol
118
etherification reaction over H-ZSM-5 zeolite decreased with an increase of the Si/Al ratio,
119
indicating that the temperature sensitivity of the catalytic process is higher when using low Si/Al
120
ratio (high acidity) zeolites. However, it seems that in the hybrid catalyst for direct DME
121
synthesis, (CuZnAl/ZSM-5) the conclusions (regarding the HZSM-5 properties) obtained in
122
methanol etherification experiments do not confirm entirely 40, 41.
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Energy & Fuels
Bonura et al
20, 42
demonstrated experimentally a good performance of H-ZSM-5 zeolite as
124
etherification catalyst in the process of direct DME synthesis from carbon dioxide (3.0 MPa and
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513 K), using Cu–ZnO–ZrO2 as methanol synthesis catalyst, in a fixed bed containing individual
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pellets of the two catalysts. In the latter study, the authors found that the catalytic beds obtained
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by mechanical mixing of individual pellets ensure superior performances as compared with the
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bifunctional catalyst pellets.
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Mechanism and kinetics of methanol etherification on the ZSM-5 zeolite
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The design of the DME synthesis processes requires reliable kinetic models, constructed in
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accord with the real mechanism of catalyst surface reaction. The methanol dehydration reaction
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is described by the global equation: cat → CH 3 − O − CH 3 + H 2 O 2CH 3 OH ←
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(1)
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The mechanism of methanol etherification on zeolite catalysts was investigated in a
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significant number of published studies, by comparing theoretical predictions (i.e. density
136
functional theory) with experimental observations. The general conclusion is that the
137
etherification mechanism is dependent on the acidic sites strength and their density on the
138
surface. It is largely accepted the hypothesis that in the formation of DME are involved the
139
medium strength acid sites, whereas the strong acid sites are responsible for methanol conversion
140
to hydrocarbons 31.
141
The majority of published studies are hypothesizing that, in the etherification process over the
142
zeolite catalysts, there are involved Bronsted acid – Lewis base pairs of zeolite surface. Two
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classes of mechanisms were proposed to explain the methanol etherification on the surface of
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solid acidic catalysts of this type: dissociative (sequential) and associative (non-dissociative or
145
direct). Both of these involve a first step of methanol adsorption (protonation) on the Bronsted
146
acid groups (HX) by a hydrogen bond to the non-participant electrons of the alcoholic oxygen
147
atom to form a methoxonium cation (X-H---HO-CH3), as shown in Fig. 1 43.
148
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Fig. 1. Structure of the intermediary species appearing in methanol dehydration mechanisms 43
151
In the dissociative mechanism, the adsorbed methoxonium cation (protonated methanol)
152
eliminates water transforming into a methoxy intermediate, this being generally supposed as the
153
rate determining step. In a final step the methoxy intermediate reacts with a methanol molecule
154
forming DME. In the associative mechanism, the adsorbed methoxonium cation reacts with a
155
non-adsorbed methanol molecule to form a protonated methanol dimer ((X-CH3OH-H+)2 shown
156
in Fig. 1). Furthermore, this dimer reorganizes internally to form transition states which finally
157
dehydrate to DME. Different variants of these two mechanisms were adopted by different
158
authors to explain the methanol etherification on zeolite catalysts. Kubelková et al.
159
investigated the mechanism of DME synthesis from methanol over H-Y and H-ZSM-5 catalysts
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using FT-IR spectroscopy, temperature-programmed desorption of methanol and mass
161
spectrometry. The authors adopted a dissociative mechanism, evidencing the apparition of
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methoxonium cations ([CH3OH2]+) on the zeolite surface. These were assumed to dehydrate,
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forming methoxy species, which react with gaseous methanol molecules giving DME (a Rideal -
164
Eley scheme).
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Bandiera and Naccache
45
44
investigated the reaction over a highly dealuminated H-mordenite
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zeolite and postulated that the surface reaction occurs by a dissociative, dual site mechanism.
167
According to this mechanism, a molecule of methanol is protonated on a Brønsted acid site,
168
generating adsorbed [CH3OH2]+ (methoxonium) cations. A second methanol molecule interacts
169
with an adjacent Lewis basic site (oxygen atom), losing a proton and generating an anion
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[CH3O]−. Finally, the two ions combine forming DME and water.
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Lu et al. 14 investigated the kinetics of the direct DME synthesis from syngas over a Cu–ZnO–
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Al2O3/HZSM-5 catalyst in a fluidized bed reactor. The authors adopted a dissociative mechanism
173
for methanol dehydration over HZSM-5 zeolite, where the second step is the reaction between
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the methoxy cation and an adsorbed methanol molecule, leading to DME and adsorbed water
175
molecules.
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Moses and Nørskov
46
analyzed both the dissociative and associative mechanisms for
177
methanol dehydration on a ZSM-22 catalyst, based on density functional theory (DFT)
178
calculations. The authors concluded that the dissociative route is faster compared with the
179
associative one. They also found that the dissociative and associative mechanisms have similar
180
dependencies on catalyst acidity and consequently the dissociative mechanism will also
181
dominate for weaker acids.
182
Blaszkowski and Santen
47
investigated theoretically, using DFT, the methanol
183
etherification over acidic zeolites. Three alternative surface mechanisms were analyzed,
184
involving the methanol interactions with the Brønsted acid and Lewis base sites of acid zeolite
185
catalysts. The most probable mechanism was found to be an associative one, involving the
186
simultaneous adsorption and reaction of two methanol molecules.
187
Ha et al.
48
adopted a version of an associative mechanism, including splitting steps, in a
188
kinetic study of methanol etherification over alkali metals (Na and K) modified ZSM-5 zeolite.
189
According to this study, the protonated methanol dimer, appearing in the associative mechanism,
190
rearranges itself to split either into the methyl carboxonium ion and carbenium ion at the same
191
time, or split into two methyl carboxonium ions. The resulting ions combine in the final steps to
192
form DME. From the results of experimental data fitting, the authors concluded that the rate
193
controlling step should be the decomposition of protonated methanol dimer. The mechanism
194
proposed by Ha was used by Tavan et al. [16] for methanol etherification over H-ZSM-5
195
catalyst.
196
The associative mechanism of methanol etherification over acid zeolites was also confirmed 43, 49
197
by the studies of Iglesia’s group
198
polyoxometalate clusters and zeolite H-BEA zeolite, they proposed, based on experimental
199
observations correlated with DFT calculations, a direct (associative) mechanism 43
. In a first study of methanol dehydration on Keggin 49
. In a
200
following study
201
functional theory(DFT), that on MFI (ZSM-5) catalysts the associative mechanism prevails in
202
typical conditions for methanol dehydration.
it was shown, by IR spectra and theoretical analysis based on density
203
In a ‘van der Waals corrected DFT’ study of the methanol-to-DME reaction on H-ZSM-5
204
zeolite, Ghorbanpour et al. 50 found that the associative route is preferred at lower temperatures, 7 ACS Paragon Plus Environment
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but a transition in the mechanism from associative to dissociative is predicted at higher
206
temperatures, resulting in a temperature interval over which the etherification occurs by both
207
mechanisms. In particular, for H-ZSM-5 zeolite, the authors estimated a temperature limit of 700
208
K, below which the mechanism is associative.
209
Kinetic models for methanol etherification over ZSM-5 zeolites
210
The main kinetic models formulated for the methanol etherification, in accord with the
211
mechanisms described above, are presented in Tables 1 and 2.
212
Based on the associative adsorption mechanism presented above, there were deduced, by
213
different authors, the rate expressions (2) and (3) (Table 1). Also, by adopting the dissociative
214
type mechanisms, Lu et al. [11] proposed the rate expression (4) whereas Carr et al. 49, proposed
215
the rate expression (5).
216
The goal of our study was to investigate experimentally the vapor phase etherification of
217
methanol over a home synthesized alumina-binded H-ZSM-5 catalyst and to test the predictive
218
capabilities of the published kinetic models to fit the experimental data over the whole practical
219
domain of methanol conversions. As the quality of the fit was unsatisfactory for all the tested
220
models, a discrimination study was performed, and a kinetic model was selected appearing as the
221
most appropriate to describe the experimental data.
222 223
Catalyst synthesis and characterization
224
Synthesis of the H-ZSM-5 zeolite
225
A ZSM-5 zeolite has been prepared using hexamethylenediamine (HAD, Aldrich) as a structure
226
directing agent (template). The synthesis process included two main steps as described elsewhere
227
51
228
heating rate of 2°C/min to remove the organic compound occluded in the zeolite framework
229
during the crystallization step. The calcined Na-ZSM-5 sample was converted to the ammonium
230
form by ion exchange with a NH4NO3 solution (1M), at 80oC for 2 h, using a liquid/solid ratio of
231
20 mL/g. The resulting NH4-ZSM-5 powder was used to prepare the formulated catalyst by
232
extrusion.
. The as-synthesized Na-ZSM-5 zeolite powder was calcined in air at 580oC for 8 h, at a
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Table 1. The proposed mechanisms for methanol etherification Associative mechanism, Ha et al. 52
Associative mechanism, Tavan et al. 29
Dissociative mechanism, Lu et al. 14
Dissociative mechanism, Carr et al. 49
→( X −CH3 − OH − H + ) ( a ) + L ( H1) 2H3C − OH ( a ) + 2HX ← 2
→ HXCH 3OH CH 3OH ( a ) + HX ←
( L1) → CH X + H 2 O ( a ) HXCH 3OH ← ( L2) → HXCH 3OHCH 3+ ( L3) CH 3+ X − + HXCH 3OH ← → HXCH 3OH + CH 3 HXCH 3OHCH 3+ ← ( L4) + + → HXCH 3OH CH 3 ← HXCH 3OCH 3 + H ( L5 ) → CH 3OCH 3 + HX HXCH 3OCH 3 ← ( L6 ) + − → HX H + X ← ( L7 )
→ H3C − OH − H + ( a ) + ( X −CH3 − OH − H + ) ( a ) + L ← 2
+CH3+ X − + H2O ( a ) + X −
+ 3
( H 2)
→ H3C − OH − CH3+ ( a ) + H3C − OH − H + ( a ) + CH3+ X − ← + HX
H3C − OH − CH
+ 3
→ H3C − O − CH3 + H ( a ) ←
+
+L
→ HX H + X ← +
−
( H 3) ( H 4) ( H 5)
−
→ HXCH3OH CH3OH ( a ) + HX ← →CH X + H2O ( a ) HXCH3OH ← + 3
−
→ DME − HX CH3OH ( g ) + CH3+ X − ← → DME ( g ) + HX DME − HX ←
( G1) ( G2 ) ( G3) ( G4 )
L-active site
1 pD pW 2 1 k KM pM − 2 k K M pM − p D pW K p pM K p rM = rM = 1+ K M p M + KW pW 2 (1+ K M p M + K W pW ) (3) (2) The rate controlling step (RCS) is surface reaction (H3) RCS is surface reaction (H2)
p2M 1 rM = k − pD p W Kp (4)
p2 1 k KM M − pD pW K p rM = p 1+ K M M pW (5)
RCS is surface reaction (L3)
RCS is the surface reaction (G3)
236 237
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Table 2. Published kinetic models for methanol dehydration to DME
N0 Reference 1
Ha et al. 52
Catalyst
Rate expression
Kinetic parameters
(K or Na) ZSM-5
p p kK p 2M − D W K p r= 2 (1+ K M p M + KW pW ) 2 M
k = 1051⋅ e
−6278.6 T
K M = 7796 ⋅ e KW = 1825 ⋅ e
2
Tavan et al. 29 H-ZSM-5 Pure methanol
p p kK M p M − D W pM K p r= (1+ K M p M + KW pW )
3
Lu et al. 14
4
Tavan and HZSM-5 Hasanvandian Pure methanol 53
p2M pD r =k − p K p W
HZSM-5
p p r =k p2M − D W Kp
mol kgcat s
11954.5 T
12064 T
k = 3812.11⋅ e
K M = 67.42 ⋅ e KW = 2.17 ⋅ e
,
, bar −1
−8345.4 T
13003.2 T
10804.5 T
k = 8.2894 ⋅10 4 ⋅ e
k = 525.32 ⋅ e
, bar −1
−10896 T
,
mol kgcat s
, bar −1
, bar −1 −6367.6 T
,
,
mol g cat s
mol kg cat s
240 241 242
H-ZSM-5-alumina catalyst formulation
243
The catalyst preparation was carried out by the extrusion of NH4-ZSM-5 powder using hydrated
244
alumina of pseudoboehmite type (65 % Al2O3) as binder and aqueous nitric acid (12 wt. % HNO3,
245
Aldrich) as peptizing agent. Further, the dry catalyst extrudates (diameter 2 mm) were calcined in air
246
at 550oC for 6 hours. The final calcination step involved the transformation of ZSM-5 zeolite from
247
ammonium to protonic form and the formation of γ-Al2O3 active phase. The final calcined catalyst
248
designated as H-ZSM-5/γ-Al2O3 (Ex) has the following composition: 60 wt.% of H-ZSM-5 zeolite
249
and 40 wt. % of γ-Al2O3 (matrix).
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Energy & Fuels
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Catalyst characterization
251
X-ray diffraction (XRD) of the as-synthesized zeolite sample was recorded on a computer-controlled
252
DRON DART UM2 diffractometer in the range of 2θ = 4-50o.
253
The nitrogen adsorption/desorption isotherms were recorded at 77 K in the relative pressure range
254
p/po=0.005-1.0 using a Quantachrome NOVA 2200e Gas Sorption Analyzer. Prior to adsorption
255
measurements, the samples were degassed at 300 °C under vacuum. Data processing was performed
256
using NovaWin version 11.03 software.
257
The specific surface area was determined by the standard Brunauer-Emmett-Teller (BET) equation
258
applied to the linear part of the adsorption isotherm. The total pore volume was estimated by the
259
volume adsorbed at a relative pressure p/po close to unity. The mesopores volume and mesopores
260
size distribution were estimated from desorption branch of the isotherm by applying the Barrett-
261
Joyner-Halenda (BJH) model. The Horwath-Kawazoe (HK) method was used for the calculation of
262
the micropores size. The t-plot method was used to estimate micropores volume, micropores surface
263
area and external surface area.
264
The acid strength distribution of zeolite samples was determined by the thermal desorption of
265
diethylamine (DEA). The weight loss in the temperature range of 160-600 °C was recorded using
266
DuPont Instrument ″Thermal Analyst 2000/2100″. The weight loss was used to estimate the
267
concentration of acid sites assuming that each mole of amine corresponds to one mole of proton of
268
zeolite framework.
269
Catalyst testing
270
Liquid methanol of 99.5 % (wt) purity from Sigma Aldrich was used in the etherification 2
271
experiments. According to the study published by Pop et al.
272
with alumina, the influence of the internal diffusion on the global kinetics is avoided for catalyst
273
particle size below 1.5 mm. Ha et al. 52 studied the methanol dehydration over Na modified HZSM-5
274
zeolite (pure state) and found that the influence of internal diffusion on the process kinetics is
for a SAPO-34 catalyst formulated
275
insignificant for zeolite pellets with the diameter below 840 µm (20-40 mesh). Similarly, Kim et al.
276
16
277
840 to 1190 µm. (16-20 mesh). Considering these results, all the experiments reported in this study
278
were performed with catalyst pellets of H-ZSM-5 formulated with alumina (40 wt%) having a size
used Na modified HZSM-5 zeolite (formulated with alumina) as pellets having the size between
11 ACS Paragon Plus Environment
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Page 12 of 30
279
range 100-150 µm. The experimental set-up is presented in Fig. 2. The reaction was carried out
280
isothermally at atmospheric pressure in a quartz tube reactor (inner diameter 4 mm). A quantity of
281
catalyst (0.150 g) was loaded in the reactor between two layers of quartz beads to improve the gas
282
flow distribution across the catalyst bed. The reaction temperature was monitored at the center of the
283
catalyst bed with a Pt/Rh thermocouple having the accuracy of 0.1 oC, placed in a 1 mm external
284
diameter thermowell. A flow of nitrogen saturated with methanol at 30 °C was fed to the fixed-bed
285
reactor operated at atmospheric pressure. In order to get the desired methanol concentration in the
286
feed, the methanol-nitrogen stream was diluted in the right proportion with a stream of pure nitrogen.
287
The two flow rates of nitrogen were kept constant by using electronic mass flow controllers. The
288
pressure drop in the reactor was monitored by using a differential manometer. The concentrations in
289
the reactor effluent were measured on-line by gas chromatography, using a Varian CP-3800 GC
290
equipped with methanizer, FID and TCD detectors. A capillary column (Poraplot Q HT) was used to
291
separate methanol and DME products, using hydrogen as carrier gas. 7 3 MF C 1
4 9
MF C 2
8
10
5
11
2
6
Purge
1 Transfer line to cromatograph
292 293 294
Fig. 2. Experimental set-up (1- gas cylinder; 2, 3 – mass flow controllers; 4- methanol bubbler; 5-thermostated bath; 6-buffer vessel; 7pressure indicator; 8-furnace; 9-reactor; 10-catalyst bed; 11-innert bed)
295 296 297
12 ACS Paragon Plus Environment
Page 13 of 30
298
Results and discussions
299
Structural characterization
300
X-ray diffraction pattern of the synthesized zeolite (Na-ZSM-5 powder sample) in Fig. 4
301
shows the formation of a well-crystallized zeolite exhibiting the MFI type structure. As known, MFI
302
framework has a particular structure characterized by a void space of tridimensional interconnecting
303
channels of 10-rings, of 0.51x0.55 nm and 0.53x0.56 nm respectively.
304
The XRD analyses have evidenced characteristic diffraction peaks at 2θ values of 7.9o, 8.8o, 23.1o
305
and 23.9o, respectively, representing (011), (020), (051) and (033) planes of crystalline structure, in
306
accordance with reported data. The characteristic peaks in the ranges of 2θ=7-9o and 23-25o
307
respectively, were used to calculate the crystallinity of the zeolite sample 54, 55. The Si/Al ratio of the
308
ZSM-5 zeolite framework, evaluated by X-ray diffraction data, is around 40.
309
Hydrated alumina of pseudo-boehmite type structure was used as a conventional binder for the
310
extrusion of ZSM-5 zeolite. XRD - pattern in Fig. 3 illustrates the formation of γ -Al2O3 phase by
311
calcination of hydrated alumina precursor.
312
p -B - (2 31 ),(00 2)
γ-(400) γ-(311)
p-B (2 51 )
p-B - (05 1),(2 00 )
p-B - (0 31 )
p-B - (12 0)
p-B (02 0)
313
I (u.a.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
γ-(440) o
500 C
0
20
40
60
80
100
2θ CuKα
314 315
Fig. 3. XRD-pattern of hydrated alumina precursor and its calcined form
13 ACS Paragon Plus Environment
Energy & Fuels
γ−Al2O3(400)
111
151 133
051
200, 020
303
101, 011
501
H-ZSM-5 (P) H-ZSM-5/γ-Al2O3(Ex)
I (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 14 of 30
0
10
20
30
40
50
2θ CuKα
316 317
Fig. 4. X-ray diffraction pattern of synthesized ZSM-5 zeolite sample
318
As shown in Fig. 4 in the XRD – pattern of the catalyst extrudates, can be identified the
319
diffraction peaks characteristic for the MFI-type framework, as well as a diffraction peak at 2θ = 45o
320
(400) associated with the γ-Al2O3 phase. As the decrease of the diffraction peaks intensities is more
321
significant than the diminishing due to the dilution effect of zeolite phase, this can indicate a partial
322
amorphisation of the zeolite structure, during the catalyst preparation process. It is also worth to be
323
noted that the diffraction peaks at lower angles are affected, mainly due to their sensitivity to the
324
presence of some material in zeolite channels.
325
Textural characterization
326
The N2 adsorption/desorption isotherms and pore size distribution curves of ZSM-5 (P) and H-ZSM-
327
5/ γ -Al2O3 (Ex) samples are presented in Figs. 5 and 6.
328
The N2 sorption isotherm of ZSM-5 (P) sample is considered of type I, characteristic of
329
microporous materials displaying a significant hysteresis loop, which is an indicative of the
330
mesoporosity, as it was determined by the BJH-plot.
14 ACS Paragon Plus Environment
Page 15 of 30
0.0010
220
H-ZSM-5 (P)
H-ZSM-5/γ-Al2O3 (Ex)
0.0008
180 160
Dv(d) (cc/nm/g)
Volume adsorbed (cc/g)
200
140 120
adsorption desorption
100 80
0.0006
BJH-adsorption BJH-desorption 0.0004
0.0002
60 40
0.0000
20 0.0
0.2
331
0.4
0.6
0.8
0
1.0
2
4
6
332
8
10
12
14
16
18
20
Pore diameter (nm)
Relative pressure (p/po)
Fig. 5. N2 adsorption/desorption isotherm and BJH ads-des-pore size distributions of H-ZSM-5 (P) sample
333 334
The two pore size distribution (PSD) curves, derived from adsorption and desorption branches of the
335
isotherm, are quite similar in the lower pore size region. The presence of the inter-crystalline
336
mesopores of 3.8 nm diameter, observed on the BJH-desorption curve, can be associated with the
337
formation of zeolite particle aggregates. The step down characteristic in the desorption isotherm
338
reflect a “cavitation or tensile strength” effect associated with the spontaneous evaporation of
339
metastable pore liquid. This effect is evidenced by the presence of mesopores of 6.5 nm diameter. In
340
contrast, the pore size distribution derived from the adsorption isotherm does not reveal this
341
behavior.
0.0010 240
H-ZSM-5/γ-Al2O3 (Ex)
200 180
adsorption desorption
160
H-ZSM-5/γ-Al2O3 (Ex)
0.0008
BJHdes-Dv(d) (cc/nm/g)
220
Volume adsorbed (cc/g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
140 120 100
0.0006
0.0004
0.0002
80 0.0000 60 0.0
342 343 344
0.2
0.4
0.6
Relative pressure (p/po)
0.8
1.0
0
2
4
6
8
10
12
14
16
18
20
Pore diameter (nm)
Fig. 6. N2 adsorption/desorption isotherm and BJHdes-pore size distribution of H-ZSM-5/ γ -Al2O3 (Ex) sample
15 ACS Paragon Plus Environment
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Page 16 of 30
345
Textural properties of H-ZSM-5 (P) and H-ZSM-5/γ-Al2O3 catalyst extrudates (Ex) respectively are
346
summarized in Table 3.
347
Table 3. Textural properties of ZSM-5 samples
Sample
348 349 350
SBET
Sext
Sm
2
2
2
(cm /g)
(m /g)
(m /g) (m /g)
Vt 3
VM 3
Vm 3
(cm /g) (cm /g)
Dm
DM
(nm)
(nm)
H-ZSM-5 (P)
450
155
295
0.32
0.20
0.11
0.45
-
H-ZSM-5/γ-Al2O3 (Ex)
353
203
150
0.39
0.30
0.09
-
3.65
SBET = BET specific surface area; Sext = t-plot external surface area; Sm = t-plot micropores surface area; Vt = total pore volume; VM = BJH mesopores volume; Vm:= t-plot micropores volume; Dm= HK micropores diameter; DM =BJH mesopores diameter
351
The H-ZSM-5 (P) sample resulted from the zeolite synthesized as described above has a high
352
BET specific surface area (450 m2/g), which is in agreement with literature data obtained by using
353
nitrogen as standard adsorptive
354
2
micropores surface area (295 m /g) and a large pore volume (0.32 cm3/g). The relative large external
355
surface (155 m2/g) can be connected to the adsorption in the inter-crystalline space (textural porosity)
356
arising from the zeolite crystalline particle aggregation 56.
55
. The porous texture of this sample is characterized by a high
357
The formulation of the zeolite by extrusion is accompanied by a decrease in BET surface area
358
(from 450 to 353 m2/g), more significantly in micropores surface area (from 295 to 150 m2/g) and an
359
increase in total pore volume (from 0.32 to 0.39 cm3/g). The external surface area (203 m2/g) and
360
mesopores volume (0.3 cm3/g) of the catalyst extrudates are higher as compared with the
361
corresponding parent zeolite (Sext=155 m2/g; VM = 0.2 cm3/g), which is attributed to the contribution
362
of the alumina matrix. The mesoporosity generated during the formulation process, of the zeolite-
363
alumina composite, is characterized by a pore size distribution with a maximum value positioned at
364
3.65 nm.
365
Table 4 presents the acidic properties of the H-ZSM-5/γ-Al2O3 (Ex) including the acid
366
strength distributions (defined as the amount of DEA desorbed) corresponding to weak (160-300oC),
367
medium (300-440oC) and strong (440-580oC) acid sites, respectively.
16 ACS Paragon Plus Environment
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368
Energy & Fuels
Table 4. Acidic properties of H-ZSM-5/γ-Al2O3 catalyst
Acidic strength distribution (mmol DEA/g)
H-ZSM-5/γ-Al2O3 (Ex)
Total acidity
Weak
Medium
Strong
(mmol DEA/g)
0.36
0.21
0.14
0.71
369
The diethylamine (DEA) desorption at low temperature (below 300oC) can be associated with silanol
370
groups on the external surface or that can arise from zeolite lattice defects. The higher temperature
371
region (> 300oC), corresponding to medium and strong acid sites, can be assigned to acid sites
372
generated by substitution of Si4+ with Al3+ in zeolite lattice. The resulting negative charge is
373
compensated by a proton corresponding to the hydrogen form of the zeolite (H-ZSM-5).
374
The theoretical acidity of the H-ZSM-5 zeolite associated with the framework aluminum
375
corresponding to the Si/Al=40 is 0.82 mmol/g. Therefore, the concentration of acid sites of the H-
376
ZSM-5/γ-Al2O3 catalyst extrudates, mainly the concentration of acid sites associated with the
377
diethylamine desorbed at high temperature (> 300oC) should be around 0.5 mmol/g. It can be
378
observed that the catalyst extrudates have a significant concentration of weakly acid sites and a
379
considerably lower concentration of strong acid sites. This result, obtained by using the amine
380
adsorption/desorption measurements, could suggest that the used method underestimates the acidity
381
of the H-ZSM-5 based catalyst.
382
Etherification experiments
383
Before its use in etherification experiments, the catalyst was conditioned for 2 h under a
384
nitrogen stream at 300 oC, and finally under a methanol-nitrogen stream for approximately 24 h at
385
290 oC, with the aim of stabilizing the activity. The reproducibility of the measurements was checked
386
by repeating the methanol conversion measurements in identical operating conditions. The results
387
presented in Fig. 7 show good reproducibility of experiments.
388
Fig. 8 presents methanol conversion- temperature curves that emphasize the influence of feed
389
methanol concentration, at two different gas flow rates. As observed, the increase of methanol
390
concentration leads to a decrease of methanol conversion (methanol etherification rate).
17 ACS Paragon Plus Environment
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Page 18 of 30
0.9 0.8 0.7 0.6 0.5 0.4 0.3
Exp 1 Exp 2 Exp 3
0.2 0.1 180
391 392
200
220
240
260
280
300
o
Temperature, C
Fig. 7. Reproducibility test for methanol etherification (WHSV 24 hr-1, concentration 8 % mol) 0.7 0.6
0.6 Feed concentration 8 mol % Feed concentration 13.5 mol %
Feed concentration 14.7 mol % Feed concentration 8.8 mol %
0.5
0.5
0.4
0.4 0.3 0.3 0.2
0.2
0.1
0.1 0 180
393 394
A 200
220
240
B 260
0 180
o
200
220
240
260
o
Temperature, C
Temperature, C
Fig. 8. Feed methanol concentration influence. A – WHSV 24 hr-1; B- WHSV 34 hr-1.
395
The results presented in Fig. 9, measured at constant feed methanol concentration and variable
396
flowrate, show a decrease of methanol conversion with the increase of feed flow rate, due to the
397
decrease of the residence time inside the catalyst bed. In the same time, this result is suggesting that
398
external diffusion does not have a significant influence on the global process kinetics at the level of
399
catalyst pellet.
18 ACS Paragon Plus Environment
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Energy & Fuels
Methanol conversion
Page 19 of 30
400 401 402 403 404
Fig. 9. Feed flowrate influence (feed methanol concentration 14 mol %) The influence of internal diffusion on the overall kinetics at the level of the catalyst pellet was evaluated using Weisz-Prater criterion 57. WP =
rM ,exp ⋅ρ p R p2 Deff ⋅ C M
,
(6)
405
where: rM ,exp - reaction rate; ρp - catalyst pellet density; Rp – radius of the catalyst pellet; Deff –
406
effective diffusion coefficient; CM- methanol molar concentration in gas phase.
407 408
The experimental data allowed the evaluation of the average reaction rate inside the catalyst bed:
rM ,exp =
FM ,0 ⋅ X M
(7)
mcat
409
FM,0 – molar methanol feed rate; XM – methanol conversion; mcat – weight of the catalyst. Considering
410
an average value of the effective diffusion coefficient Deff = 5⋅10-7 m2 s-1
411
there were obtained values for WP criterion between 0.05 – 0.2, indicating a negligible influence of
412
the internal diffusion on the overall process kinetics.
58
and ρp = 1300 kg m-3,
413
The test of published kinetic models
414
In order to evaluate the suitability of the published kinetic models to our experimental data, a
415
set of methanol dehydration process simulations were carried out, based on these kinetic models. The 19 ACS Paragon Plus Environment
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Page 20 of 30
416
simulations were performed using a plug-flow pseudo-homogeneous reactor model, for a WHSV of
417
23 hr-1, temperatures in the range of 170-270 °C and a methanol feed concentration of 14 mol %. The
418
published values of the parameters involved in the considered kinetic models are given in Table 2.
419
The simulation results are presented in Fig. 10. The interphase concentration and temperature
420
gradients, estimated in preliminary calculations, from the mass and heat balance around the catalyst
421
pellet 53, proved to be negligible for the working conditions of this study (as suggested also from the
422
experimental results presented in Fig. 9).
423
As seen from Fig. 10, none of the published kinetic models predicts with sufficient accuracy our
424
experimental data. This result is explained by the differences among the zeolites used in the
425
construction of these models, combined with the strong dependency of the catalytic performances on
426
the synthesis conditions and acidity of respective zeolites. The kinetic models published by Tavan et
427
al.
428
conversion values whereas the model published by Ha et al.
429
lower methanol conversion values as compared with our experimental ones.
430
The differences between our results and those predicted by the kinetic model of Tavan and
431
Hasanvandian
432
matrix than the HZSM-5 activity in the pure (as synthesized) state
433
fact that this kinetic model was based on experimental data measured at constant feed composition
434
(pure methanol). Another kinetic model for the same catalyst was also published by Tavan et al. 29.
435
However, it seems that there are some inconsistencies in the published values of kinetic parameters
436
for this model. Finally, the kinetic model published by Lu et al.
437
data for DME synthesis in a fluidized bed reactor. As known, the flow and mixing phenomena in
438
fluidized bed reactors are very complex and different of those in the fixed bed, so the accuracy of this
439
kinetic model could be affected by the hydrodynamics description used in its deduction. In
440
conclusion, this comparison study is evidencing relatively large differences between the predictions
441
of the published kinetic models, even for catalysts having close properties.
29
(pure HZSM-5) and Ha et al.
53
52
(Na modified ZSM-5) are predicting higher methanol 52
(K modified ZSM-5) is predicting
could be explained by the lower activity of the HZSM-5 formulated in the alumina
14
16
. A second reason could be the
was deduced using experimental
20 ACS Paragon Plus Environment
Page 21 of 30
1 0.9 0.8
Methanol conversion
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
0.7 0.6 0.5
This work Ha et al [48] (NaZSM) Ha et al [48] (KZSM) Tavan et al [26]-HZSM Tavan and Hasanvandian [49]-HZSM Lu et al [11]
0.4 0.3 0.2 0.1 0 180
190
200
210
230
240
250
260
270
280
o
Temperature, C
442 443 444
220
Fig. 10. Methanol dehydration simulation results
Estimation of the kinetic parameters
445
Due to the poor predicting capabilities of the published kinetic models, we performed a
446
model discrimination study in order to identify the most suitable model to our experimental
447
measurements. In this aim, the parameters of all kinetic models were re-estimated.
448 449
The temperature dependencies of the adsorption equilibrium constants are defined by van’t Hoff expression:
450
∆H a,J K J = K J0 exp ; J= M, D, W RT
451
The preexponential factors, Kj0, appearing in the adsorption equilibrium constants are directly
452 453
(8)
dependent on the entropy variations in the adsorption process 59, 60:
∆Sa,J K J0 = exp R
(9)
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454
Page 22 of 30
The entropy variation ∆Sa,J is negative and smaller than the gas phase entropy of the adsorbate J 60, 61:
455 T
− ∆Sa,J < Sg,J ; S g , J (T ) = S g , J ,0 +
456
∫
C p , J (T )
298
T
dT
(10)
457
In order to compare the prediction quality of models with different number of parameters, we used a
458
model selection criterion (MSC) given by equation (11). A model with a higher value of MSC will be
459
more appropriate 61, 62: N
∑ (X 460
MSC=
M , j ,exp
-X M ,exp ) 2
j=1 N
∑ (X
M , j ,exp
-X M , j )
2
2p N
(11)
j=1
461
X M , j ,exp , X M , j - experimental and calculated values of methanol conversion; N-number of
462
experimental determinations; p-number of model parameters.
463
The estimation calculations were performed by the ‘lsqcurvefit’ function of the Matlab
464
programming environment, using our experimental data. The model parameters thus estimated are
465
given in Table 6.
466
The numerical values for correlation coefficient (R2) and of the model selection criterion (MSC),
467
presented in Table 6, indicate that, among the tested kinetic models, the associative adsorption one
468
given by rate expression (2) (proposed by Ha et al.
469
comparison between the calculated values of methanol conversion (with the kinetic model so
470
selected) and the measured ones is presented in Fig. 11.
52
) provides a better quality of the fit. A
471
Average relative sensitivities of methanol conversion in respect with model parameters were
472
also calculated. These were obtained by averaging the methanol conversion sensitivities in the
473
experimental points, using a relation similar with the one published by Weijers and Vanrolleghem 63:
474
475
Si , X = Si
qi ,n XM
;
Si =
1 N
N
å
j= 1
¶XM,j ¶ qi
;
XM =
1 N
N
å
X M , j ,nom
(12)
j= 1
The derivative was calculated numerically by centered differences formula: 22 ACS Paragon Plus Environment
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Energy & Fuels
¶XM,j
476
¶ qi
=
X M+ , j - X M- , j qi+ - qi-
; qi+ = (1 + ε)qi ,nom ; qi- = (1- e )qi ,nom
(13)
477
In the previous relations, ε is the relative perturbation. and θi are kinetic parameters. The subscript
478
‘nom’ indicates the values corresponding to the nominal values
479
whereas X M+ , j and X M- , j are the calculated values of XM corresponding to the perturbed values of
480
parameter θi ( qi+ and qi- ), the other parameters being kept at nominal values.
481
The sensitivity values presented in Table 5 are evidencing that all the parameters of the kinetic model
482
(2) have a significant influence on the methanol conversion, except the adsorption enthalpy of
483
methanol which is relatively less important. However, we kept this parameter, working with the
484
kinetic model structure, as proposed by his authors.
485
Table 5. Values of the sensitivities for the calculated parameters Sensitivity
θi Si , X ,relation (12)
of model parameters ( qi ,nom ),
k0
E
KM,0
dHa,M
KW,0
dHa,W
0.381
0.192
0.333
0.085
0.207
0.146
486 487
The estimated values of activation energy and adsorption enthalpies for methanol and water are
488
compared with several published values for H-ZSM-5 and similar catalysts, in Table 7. As can be
489
seen, the values of the three parameters are well fitted within the ranges of published data.
490
The adequacy of the estimated values for the kinetic model parameters can be also tested
491
based on thermodynamic reasons. At 298 K, the gas phase entropies are Sg,M,0=239.7 J/(mole K) for
492
methanol and Sg,W,0 = 188.84 J mol-1K-1 for water
493
values for preexponential factors KM,0 and KW,0 for the rate expression (2) (Table 6) one obtains a
494
methanol adsorption entropy of -79.4 J mol-1 K-1 and a water adsorption entropy of -234.8 J mol-1K-1.
495
So, the calculated values for adsorption entropy, based on the rate expression (2), fulfill the
496
thermodynamic criterion (10).
64
. Using the relation (10), and the numerical
23 ACS Paragon Plus Environment
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Page 24 of 30
497
Table 6. The estimated kinetic parameters for methanol etherification (for the rate expressions
498
presented in Table 1)
Parameter
Rate expression (2) Ha et al. 52
Rate expression (3), Tavan et al. 29
Rate expression (4), Lu et al. 14
Rate expression (5), Carr et al. 49
k0, kmol kg-1 s-1 E, J mol-1
1.209·108(1±0.100)
1.684·1014 (1±0.058)
9.753·10-4 (1±0.166)
2.435·105 (1±0.346)
1.038·105 (1±0.038)
1.614·105 (1±0.067)
3.279·104 (1±0.012)
7.147·104 (1±0.327)
KM,0, bar-1
7.113·10-5 (1±0.082)
1.154 ·10-11 (1±0.154)
3.093 ·10-3 (1±0.031)
4.162 ·10-5 (1±0.073)
dHa,M, J mol-1
-5.294·104 (1±0.157)
-1.113·105 (1±0.151)
-3.362·104 (1±0.663)
-1.12·104 (1±0.115)
KW,0, bar-1
5.392 ·10-13 (1±0.083)
2.873 ·10-9 (1±0.089)
2.317 ·10-13 (1±0.152)
dHa,W, J mol-1
-1.451·105 (1±0.116)
-1.162·105 (1±0.096)
-4.251 ·104 (1±0.337)
R2 = 0.962, MSC= 25.5
R2 = 0.956, MSC= 22.7
R2 = 0.91, MSC= 12.9
R2 = 0.91, MSC=13.35
499 500 501 1
0.8
0.6
0.4
0.2
0
502 503
0
0.2
0.4
0.6
0.8
1
Experimental methanol conversion
Fig. 11. Parity diagram for associative kinetic model
504 505 24 ACS Paragon Plus Environment
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506
Energy & Fuels
Table 7. Published values for activation energy and adsorption enthalpies for methanol and water
H-ZSM-5
E, kJ/mole 147.5
∆HaM, kJ/mole -65
∆HaW, kJ/mole -74
Ha et al. 52
(K,Na) H-ZSM-5
52.2 - 55
99 - 105
92-100
Tavan and Hasanvandian 53
H-ZSM-5
69.4
108.11
89.83
Pop et al. 2
H-SAPO34/γ-Al2O3
80.1
-63.89
-49.06
115
90
Authors
Catalyst
Blaszkowski and Santen 47
Lee et al.
65
H-ZSM-5
Bercic and Levec 10
γ-Al2O3
143.7
70.5
42.1
This work
H-ZSM-5/γ-Al2O3
103.8
-52.94
-145.1
507 508
According to Vannice et al.
59
, the adsorption entropy should be in the range
509
0 ( ∆H 0ads in cal mol-1 and ∆S0ads in cal mol-1 K-1). As it can be 10 ≤ - ∆S0ads ≤ 12.2 - 0.0014 ∆H ads
510
easily checked, these thermodynamic restrictions are also fulfilled, both for methanol and water
511
estimated adsorption entropies.
512
Conclusions
513
Due to its high catalytic activity and selectivity, as well as its good hydrothermal resistance, the
514
ZSM-5 zeolite is one of the most appreciated catalysts for methanol etherification. A review of the
515
published studies, approaching the surface mechanism and kinetics of this catalytic process,
516
illustrated that two proposed mechanisms are able to explain the experimental observations and
517
theoretical analyses: the dissociative mechanism which involves a first step of methanol dissociation
518
on the Bronsted acid sites and the associative mechanism, which is hypothesizing that two molecules
519
of methanol associates on the acidic sites form a protonated methanol dimer as an etherification
520
intermediate. The studies published in the last period indicate that the two mechanisms could occur
521
concurrently, but in the typical conditions for methanol etherification the associative mechanism is
522
prevailing, the dissociative mechanism being dominant only at high temperatures. Methanol
523
etherification experiments conducted in vapor phase over a synthesized H-ZSM-5/γ-Al2O3 catalyst,
524
in the ranges typical for the DME direct synthesis process, evidenced a negative influence of
525
methanol concentration on the reaction extent. Methanol conversions close to chemical equilibrium 25 ACS Paragon Plus Environment
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Page 26 of 30
526
have been attained at temperatures around 270 °C, with no significant concentrations of by-products
527
detected in the reactor effluent. The results confirmed the published data regarding the good activity
528
and DME selectivity of the H-ZSM-5 zeolite formulated in an alumina binder. The experimental data
529
so obtained were used to evaluate the prediction capabilities of the main published kinetic models
530
proposed for the vapor phase methanol etherification over ZSM-5 zeolites with different acidities.
531
The results evidenced a poor quality of the predictions and significant differences between the
532
predictions of kinetic models developed for same catalysts. This is an expected result due to the
533
strong dependency of catalytic performances of the zeolite, on the precise composition and synthesis
534
conditions, combined with the diversity of zeolites used in the deduction of these models. A model
535
discrimination study indicated that a kinetic model based on the associative mechanism is the most
536
appropriate to describe the dependencies observed in our experimental data. The parameter values
537
and model quality proved to be consistent with the usual thermodynamic and statistical criteria.
538 539
Acknowledgment
540
This work has been funded by University Politehnica of Bucharest, through the “Excellence
541
Research Grants” Program, UPB – GEX. Identifier: UPB–EXCELENȚĂ–2016, The synthesis of
542
dimethyl ether from methanol over a H-ZSM-5 based catalysts, Contract number 400.
543 544
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