Thermal stability and water effect on ion-exchange resins in ethyl octyl ether production at high temperature

Article abstract of DOI:10.1016/j.apcata.2013.07.024

Abstract Thermal stability and water inhibition effects were studied at 150 and 190 C on the chlorinated acidic polystyrene-divinylbenzene resins Amberlyst 70, Amberlyst XE804 and Purolite CT482, and the non-chlorinated one Dowex 50Wx2. Catalytic activity in the reaction between ethanol and 1-octanol to form ethyl octyl ether (EOE) was monitored for 70 h in a continuous fixed-bed reactor. Leaching of sulfonic groups at 190 C was found to be negligible for Purolite CT482 and Amberlyst 70, but it was significant for Amberlyst XE804 and Dowex 50Wx2. The activity decay to a steady EOE reaction rate on Purolite CT482 and Amberlyst 70 has been ascribed to the reaction rate inhibition by the formed water. However, water adsorption on the catalyst also modified the resin morphology during the course of the reaction. Adsorbed water swelled the gel-phase and more acid sites became accessible for 1-octanol molecules. As a result, the activity decay of EOE and the longer byproduct di-n-octyl ether syntheses was smaller than that of the shorter one diethyl ether.

Full text of DOI:10.1016/j.apcata.2013.07.024

Applied Catalysis A: General 467 (2013) 301–309
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Thermal stability and water effect on ion-exchange resins in ethyl
octyl ether production at high temperature
∗
Jordi Guilera, Eliana Ramírez, Carles Fité, Montserrat Iborra, Javier Tejero
Chemical Engineering Department, Faculty of Chemistry, University of Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
◦
Article history:
Thermal stability and water inhibition effects were studied at 150 and 190 C on the chlorinated acidic
polystyrene-divinylbenzene resins Amberlyst 70, Amberlyst XE804 and Purolite CT482, and the non-
chlorinated one Dowex 50Wx2. Catalytic activity in the reaction between ethanol and 1-octanol to form
ethyl octyl ether (EOE) was monitored for 70 h in a continuous fixed-bed reactor. Leaching of sulfonic
Received 25 March 2013
Received in revised form 21 June 2013
Accepted 11 July 2013
Available online 22 July 2013
◦
groups at 190 C was found to be negligible for Purolite CT482 and Amberlyst 70, but it was significant
for Amberlyst XE804 and Dowex 50Wx2. The activity decay to a steady EOE reaction rate on Purolite
CT482 and Amberlyst 70 has been ascribed to the reaction rate inhibition by the formed water. However,
water adsorption on the catalyst also modified the resin morphology during the course of the reaction.
Adsorbed water swelled the gel-phase and more acid sites became accessible for 1-octanol molecules. As
a result, the activity decay of EOE and the longer byproduct di-n-octyl ether syntheses was smaller than
that of the shorter one diethyl ether.
Keywords:
Chlorinated resin
Thermal stability
Etherification
Water inhibition
Catalytic activity
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
temperature can be raised to obtain higher reaction rates and,
therefore, to have a more economically feasible reaction units.
Besides, it is well-known that acidic resins suffer different mor-
phological changes, and therefore catalytic performance varies,
depending on the nature of reaction medium. Consequently, their
catalytic activity is highly related to the properties of the reaction
mixture [11]. In the presence of polar substances such as alcohols
and water, non-permanent pores appear and diffusion of reactants
towards the acid centres is enhanced [12]. However, in some reac-
tion systems interactions between water and PS-DVB resin matrix
have opposite effects: on one hand, water competes with reactants
as it adsorbs strongly on the sulfonic groups [13–17]; on the other
hand, as water is a polar compound, it contributes to open the
resin backbone, what enhances the accessibility of reactants to acid
centres. In addition, depending on the water amount, the catalytic
mechanism can change from concerted to ionic which are slower.
In industrial reaction units, the best resin performance takes place
at low water contents (0.1–3 mol water/L) where sulfonic groups
are partially dissociated [18].
Sulfonic polystyrene-divinylbenzene (PS-DVB) resins are used
as catalysts in many industrial applications [1,2]. As a drawback,
they present a low thermal stability as, at high temperature, sul-
fonic groups, the actual catalytic species, undergo deactivation by
leaching [3,4]. Many PS-DVB resins are stable up to 150 C, but the
maximum operating temperature of some resins such as Amberlyst
◦
◦
1
5 is even lower (120 C) [1]. In contrast, fluorinated polystyrene
®
◦
sulfonic resins like Nafion can operate up to 210 C, because flu-
orine atoms increase the thermal stability. In addition, fluorine
atoms confer a higher acid strength which contributes positively to
the catalytic activity [5]. Nevertheless, compared to PS-DVB resins,
Nafion has lower acid capacity and it is more expensive, which
are important drawbacks for industrial use [6,7].
New thermally stable PS-DVB resins Amberlyst 70 and Purolite
CT482 have been recently commercialized to catalyze processes
such as esterification, aromatic alkylation and olefin hydration
®
◦
at temperatures higher than 150 C [3,4]. In these resins, some
hydrogen atoms have been substituted by chlorine. These addi-
tional electron withdrawing atoms increase the acid strength of
ion exchangers and minimize the cleavage of the sulphur bond to
aromatic carbon atoms up to 190 C [8–10]. Thus, in some reac-
tions taking place over acidic ion-exchange resins, the operating
A reaction particularly sensitive to the aforementioned water
effects is the dehydration of linear alcohol to ether, in which water
is a reaction product [19]. This reaction is of industrial interest since
long linear ethers have excellent properties for blending with com-
mercial diesel. They raise the cetane number of blends and reduce
the environmental impact of engine exhaust emissions [20–23].
Additionally, the use of renewable alcohols to produce synthetic
fuels can enhance the bio-fuel percentage in the diesel pool without
a reduction of diesel fuel quality.
◦
∗ Corresponding author. Tel.: +34 93 402 1308; fax: +34 93 402 1291.
E-mail address: jtejero@ub.edu (J. Tejero).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.07.024

3
02
J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
comparison. The aim of this paper is to evaluate the thermal
Nomenclature
ai
stability of chlorinated resins, as well as the effect of water on
◦
their catalytic performance, in the temperature range 150–190 C.
activity of reaction i, dimensionless
terminal activity of reaction i, dimensionless
diethyl ether
Besides, their properties are examined and compared to those of
conventional ones.
a
∞
,i
DEE
DNOE
dp
dpore
DVB
Ed
EOE
EtOH
Fj
di-n-octyl ether
particle diameter (␮m)
pore diameter (nm)
divinylbenzene
activation energy of activity decay, kJ/mol
ethyl octyl ether
ethanol
molar flow rate (mol/h)
gel-type resin
2. Material and methods
2.1. Chemicals
OcOH (≥99%, Acros) and EtOH (≥99.8%, Panreac) were used as
the reactants. 1-octene and DNOE (≥97%, Fluka), and DEE (≥99%,
Panreac) were used for analysis purposes. EOE was obtained and
purified in our lab by rectification to 99% purity (GC). Bidistilled
water was also used.
G
+
+
[
H ]
acid capacity (meq H /g)
−1
kd
rate decay constant (h
)
2.2. Catalysts and catalyst characterization
ISEC
M
OcOH
P
inverse steric exclusion chromatography
macroreticular resin
1-octanol
Tested catalysts were supplied by Purolite (CT482), Rohm and
Haas (Amberlyst 70 and XE804) and Aldrich (Dowex 50Wx2).
They were used with the commercial particle sizes distribution
to approach the industrial reactor. Skeletal density (ꢀs) was mea-
sured by helium displacement in an Accupyc 1330 apparatus
pressure
PS-DVB polystyrene-divinylbenzene
q
volumetric liquid flow rate (mL/min)
ri
reaction rate of reaction i at time t (mol/(h kgcat))
reaction rate of reaction i with fresh catalyst
0
(Micromeritics). BET surface area, SBET, was estimated from N2
r
i
2
adsorption-desorption isotherm at 77 K (krypton for SBET < 1 m /g).
(
mol/(h kgcat))
Surface of fresh dried resins was observed by using an H-2300 Scan-
ning Electron Microscope (Hitachi). Acid capacity was measured by
titration against standard base. Water content in the catalysts was
determined by Karl-Fisher titration (Orion AF8) in dry samples prior
to their use in the reactor, and after separation from the reaction
liquid by filtration at atmospheric pressure.
R
1-octanol/ethanol molar ratio (mol/mol)
selectivity to k with respect to j (mol/mol)
OcOH/EtOH
k
Sj
2
SBET
SISEC
SEM
t
BET surface area (m /g)
surface area from ISEC data (m /g)
2
Scanning Electron Microscopy
time, h
The particle size distribution of commercial catalysts was deter-
mined with a LS 13320 Laser Diffraction Particle Size Analyzer
in different environments: air, water, and a mixture of 1-octanol
and ethanol (molar ratio ROcOH/EtOH = 10); the latter was consid-
ered to be representative of the feed composition in catalytic tests.
Prior to particle size analysis dry samples were submerged in the
selected solvent for 2 days to ensure a fully swollen state of the
resin. Swelling degree was calculated by Eq. (1), as the quotient of
the resin volume increase with respect to the resin volume in air.
Vj is the mean particle volume in water or in the alcohol mixture,
and Vo is the volume of dry resin in air. Volumes were computed
by assuming that particles are spherical.
t0
T
Tmax
Xj
VISEC
Vj
time when decay begins (h)
temperature ( C)
maximum operating temperature ( C)
relative conversion of compound j (mol/mol)
◦
◦
3
pore volume from ISEC data (cm /g)
3
resin volume in solvent j (cm )
3
V0
Vsp
resin volume in air (cm )
specific volume of the swollen polymer phase
3
(
cm /g)
Wcat
dry catalyst mass (g)
skeletal density (g/cm )
3
ꢀ
s
V
j
{
Swelling degree} = V − 1
(1)
0
In this work, the reaction of ethanol (EtOH) with 1-octanol
OcOH) to produce ethyl octyl ether (EOE) is studied. EtOH has
The nature and characteristics of the non-permanent pores of
resins were determined by the inverse steric exclusion chromatog-
raphy (ISEC) technique as described elsewhere [24,25].
(
been chosen because it allows introducing bio-fuels to the fuel
market, and OcOH in order to obtain long chain ethers, which
are preferred as diesel fuels. Fig. 1 shows the reaction scheme. In
parallel with EOE formation (1), dehydration reactions of EtOH to
give diethyl ether (DEE) (2) and that of OcOH to di-n-octyl ether
2.3. Catalytic tests
(
DNOE) (3) take place. The chlorinated PS-DVB resins Amberlyst 70,
The experiments were performed in a 20-mL continuous fixed-
bed reactor (PID Eng & Tech) fed by a HPLC pump (Gilson 307).
The reactor bed consisted of resin homogeneously diluted with
Amberlyst XE804, and Purolite CT482 have been used as catalysts.
The PS-DVB resin Dowex 50Wx2 has also been used for the sake of
Fig. 1. Reaction scheme of EOE synthesis from 1-octanol and ethanol.

J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
303
inert quartz. Quartz was used to keep the bed isothermal and to
3. Results and discussion
ensure good contact between reactants and catalyst. Back-mixing
and channelling effects were avoided by using a resin to inert mass
ratio large enough accordingly to previous studies in our lab. During
the experiment liquid samples were taken on-line from the reactor
inlet and outlet. Their composition was determined in an HP6890A
GLC equipped with TCD detector. A 50 m × 0.2 mm × 0.5 ␮m capil-
lary column HP-Pona (Agilent) was used to separate and quantify
the compounds present in the reaction medium. The column was
3.1. Properties and structural parameters of tested resins
The main properties of tested catalysts are shown in Table 1.
Used catalysts are acidic PS-DVB resins of two different classes:
macroreticular and gel-type. In the manufacture of macroreticu-
lar resins (Amberlyst 70, Amberlyst XE804 and Purolite CT482),
styrene and DVB copolymerization is carried out in the presence
of a solvent, called porogen. The formed polymer is not soluble
in the porogen, which is excluded from the resin backbone after
polymerization. After porogen removal large agglomerates of gel
micro-spheres interspersed by a three-dimensional network of
permanent pores appear inside of the resin bead. The reaction
rates of the formation of bulky ethers are not typically affected
by the particle size of the resin used. However they proved to be
mass transfer limited within the densest gel-phase fractions of the
gel micro-spheres [6,12,27,28]. Purolite CT482 photomicrographs
under different magnifications are shown in Fig. 2 as an example of
typical macroreticular resins. On the contrary, in gel-type resins as
Dowex 50Wx2 copolymerization takes place in the absence of any
porogen, and no permanent pores are formed. As seen in Table 1,
chlorinated resins tested in this work are macroreticular. To the
best of our knowledge, gel-type chlorinated resins have not been
commercialized up-to-date.
◦
◦
temperature programmed to start at 50 C with a 25 C/min ramp
◦
up to 250 C and held for 6 min. Helium (≥99.998%, Linde) was
the carrier gas. Chemical species were identified by a second GLC
apparatus equipped with mass spectrometer GC/MS 5973 (Agilent)
assisted by a chemical database software.
◦
Catalysts were dried overnight at 110 C under vacuum
(
0.01 bar). Dry samples (0.1–0.7 g) were diluted in quartz (12–15 g).
Reactor feed consisted of an OcOH–EtOH mixture (ROcOH/EtOH = 10).
The large excess of 1-octanol was selected to enhance the formation
of EOE and DNOE in front of DEE, and also to promote the forma-
tion of 1-octenes, and in this way, to study the possible catalyst
deactivation by carbon deposition. Water (1 wt%) was added to the
reactant mixture in some runs to stress its effect on the reaction
rate without the liquid splitting off in two phases. The feed was
◦
preheated in a hot box at 80 C and then fed to reactor at a flow
rate of 0.25 mL/min. The reactor operated isothermally at 25 bars
◦
in the temperature range 150–190 C to assure that the reaction
In dry state, Purolite CT482 showed the highest BET surface
area, typical of medium cross-linked macroreticular resins with
12% DVB, such as Amberlyst 16 or 36 [27], whereas Dowex 50Wx2,
Amberlyst 70 and XE804 showed extremely low BET surface areas,
typical of low crosslinked resins (macroreticular with 8% DVB and
gel-type ones [27]).
took place in the liquid phase.
An additional series of experiments was performed to test the
catalyst reusability. After 48 h on-stream, the reactor was cooled
at room temperature. EtOH was fed at a flow rate of 2 mL/min
for 1 h to remove water and OcOH present in the resins. Subse-
quently, the catalysts were dried for 2 h in a 50 mL/min N2 stream
to remove EtOH. Catalysts dried in this way in the reactor were re-
used two times. It is to be noted that water content of fresh catalysts
In order to characterize the resin morphology in an environ-
ment similar to the reaction medium, particle size distribution was
determined in a mixture of 1-octanol and ethanol (ROcOH/EtOH = 10)
and in water, and related to dry state in air. Table 2 shows that all
resins swelled noticeably at the ambient temperature as compared
with dry state. The highest swelling with regard to air was observed
for Dowex 50Wx2 (swelling degree from 282% in the OcOH–EtOH
mixture to 473% in water). This catalyst is a gel-type resin with
2% DVB, and therefore its polymeric gel-phase is highly flexible.
In polar media, gel-phase micropores are open and accessibility
to active centres is highly improved. It is also observed that the
three macroreticular resins greatly swell in water, with swelling
degrees from 150 to 206%. In this case, besides micropores in the
gel-phase, intermediate pores in the mesopore range appear among
gel-type aggregates. Purolite CT482 showed the lowest swelling
degree among chlorinated resins, probably as a consequence of hav-
ing the highest crosslinking degree. Amberlyst 70 and XE804 also
are highly expanded in water and in the alcohol mixture.
To describe the resin morphology in swollen state, the distri-
bution of non-permanent pores was determined in water by ISEC
technique. This description is assumed to be representative of the
morphology of swollen PS-DVB resins in aqueous alcohol solution.
Table 2 shows the main morphological parameters obtained from
analysis of ISEC data. As seen, chlorinated macroreticular resins
develop wide spaces in the mesopore range with pore diame-
ter ranging from 8.5 nm (Amberlyst XE804) to 19.6 nm (Purolite
CT486), which allow reactants to get easily to internal gel-phase
surface. No mesopores were detected in gel-type resin Dowex
(
2–4 wt%) was some higher than the residual water content after
the reactivating process (<1 wt % [26]).
Due to the small catalyst mass in the reactor bed, conversions
were low (XOcOH < 10%, XEtOH < 25%). Reaction rates to form EOE, DEE
and DNOE were calculated by means of the following equations
where it is assumed that the reactor operated in the differential
regime:
FOcOHXOcOH
FEtOHXEtOH
EOE
EOE
EtOH
r1 = rEOE
=
S
=
S
(2)
(3)
(4)
(5)
OcOH
Wcat
Wcat
FEtOHXEtOH
DEE
r2 = rDEE =
S
EtOH
2
Wcat
FOcOHXOcOH
DNOE
r3 = rDNOE
=
S
OcOH
2
Wcat
{
mole of j reacted to form k}
k
Sj
=
{
mole of j reacted}
In these equations, Wcat is the dry catalyst mass, F the molar
j
flow rate of species j fed into the reactor, X the conversion of species
j
k
j
j, and S the selectivity of reactant j towards product k at the reactor
outlet. The relative error by assuming differential behaviour of the
fixed-bed reactor in Eq. (2) was estimated to be lower than 5%,
within the limits of the experimental analysis error.
5
0Wx2. From ISEC data it is possible to obtain a rough space dis-
tribution within the swollen gel-phase (not detectable by standard
techniques of pore analysis, i.e. N2 adsorption-desorption at 77 K)
and, correspondingly, a distribution of zones of different chain den-
sity [24,25,29,30].
Catalyst activity, a , for reaction i was defined as the ratio of the
i
0
i
reaction rate at time t to the reaction rate for fresh catalyst, r , by
means of Eq. (6).
As Table 2 shows, Dowex 50Wx2 exhibits the highest specific
volume of the swollen gel-phase, Vsp. Then, in decreasing order:
ri
ai =
.
(6)
0
r
i

3
04
J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
Table 1
Properties of tested catalysts.
+
Acid capacity (meq H /g)
Structurea
DVB%
Chlorinated
ꢀs b (g/cm3)
SBETc (m /g)
2
This workd
literature
Tmax ( C)
◦
Catalyst
Dowex 50Wx2
Amberlyst 70
XE804
G
2
8
No
1.426
1.514
1.475
1.538
1.32
0.02
0.27
8.70
5.06
2.65
3.17
4.25
4.83 [19]
2.55 [8]–3.01 [6]
150 [3]
190 [3]
M
M
M
Yes
Yes
Yes
CT482
190 [4]
a
G = gel-type; M = macroreticular.
b
c
ꢀ
s = skeletal density.
2
2
BET surface area in dry state (N2 for SBET ≥ 1 m /g; Kr for SBET < 1 m /g).
d
Titration against standard base.
Fig. 2. Microphotography of CT482. Magnifications: 25 (A), 600 (B) and 4000 (C).
Table 2
Morphological parameters from analysis of ISEC data and particle diameter of resins in air and fully swollen in water and an alcohols mixture.
Catalyst
ISEC analysis
dp (␮m)
Swelling degree (%)
2
3
3
Aira
OcOH/EtOHb
OcOH/EtOHb
SISEC (m /g)
VISEC (cm /g)
dpore (nm)
Vsp (cm /g)
Water
Water
Dowex 50Wx2
Amberlyst 70
XE804
–
66
243
214
–
–
13.2
8.5
2.68
1.15
0.83
1.08
252
590
505
527
451
857
710
715
394
843
697
675
473
206
178
150
282
192
163
110
0.22
0.52
1.05
CT482
19.6
a
◦
Dried at vacuum overnight (T = 110 C).
ROcOH/EtOH = 10.
b
Amberlyst 70, Purolite CT482 and Amberlyst XE804. Therefore, the
lower the DVB% the higher Vsp. Fig. 3 shows the space distribu-
tion in the swollen gel-phase. Among chlorinated resins, Amberlyst
equivalent to pores of 2.6 nm diameter [28]), similarly to low
cross-linked gel-type resin Dowex 50Wx2. In resins with such low
polymer density, gel-phase is highly expanded in polar media and
the accessibility of large molecules, such as OcOH, to a higher num-
ber of acid sites is favoured [27,30,31]. Amberlyst XE804 showed
−
2
7
0 has wider spaces between chains (polymer density 0.4 nm
,
−
2
a slightly denser polymer (0.8 nm , equivalent to pores of 1.5 nm
diameter), similarly to the macroreticular Amberlyst 39 (8% DVB)
and gel-type with 4% DVB Purolite CT124 [30]. Finally, in line with
swelling data, Purolite CT482 showed the densest swollen gel-
−
2
phase (1.5 nm , equivalent to pores of 1.0 nm diameter), typical of
medium cross-linked macroreticular resins [27]. BET surface area,
swelling degree and ISEC data suggest that Purolite CT482 morphol-
ogy is similar to conventional medium cross-linked macroreticular
resins.
The acid capacity of tested chlorinated resins ranges from 2.55
+
to 4.25 meq H /g (Table 1). These values are lower than those of
+
non-chlorinated monosulfonated (typically 4.8–5 meq H /g) and
+
oversulfonated (5.3–5.6 meq H /g) resins [27]. The substitution of
hydrogen by chlorine atoms lessens the acid capacity, because there
are less available phenyl rings for the further process of sulfonation
in the resin synthesis on a weight basis [8]. Concerning tested chlo-
rinated resins, it is observed that the higher acid capacity (Purolite
CT482 > Amberlyst XE804 > Amberlyst 70), the higher resin stiff-
ness. This behaviour is usual in non-chlorinated PS-DVB resins,
in which oversulfonated resins show higher acid capacity and
Fig. 3. ISEC pattern in water for used acidic ion-exchange resins.

J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
305
Table 3
Reaction rates, activity and selectivity at t = 0 (fresh catalyst) and after 70 h on-stream (ROcOH/EtOH = 10, q = 0.25 mL/min, P = 25 bar).
EOE
EtOH
DEE
EtOH
EOE
OcOH
DNOE
OcOH
Octene
Catalyst
t (h)
r1 = rEOE
r2 = rDEE
r3 = rDNOE
a1
a2
a3
S
S
S
S
S
OcOH
(
mol/h kg)
(mol/h kg)
(mol/h kg)
◦
T = 150
C
CT 482
0
2.89
2.55
2.11
1.78
1.88
1.54
2.60
2.30
0.442
0.328
0.380
0.285
0.228
0.155
0.251
0.200
3.08
2.88
2.50
2.12
2.89
2.50
6.40
5.90
0.767
0.794
0.735
0.758
0.804
0.832
0.839
0.852
0.234
0.205
0.265
0.242
0.196
0.168
0.162
0.148
0.304
0.294
0.286
0.288
0.240
0.232
0.168
0.159
0.647
0.664
0.678
0.683
0.739
0.752
0.815
0.819
0.049
0.042
0.036
0.029
0.021
0.016
0.023
0.022
70
0
70
0
70
0
70
0.882
0.844
0.819
0.885
0.743
0.670
0.678
0.743
0.937
0.846
0.864
0.922
XE804
Amberlyst 70
Dowex 50Wx2
◦
T = 190
C
CT 482
0
0
0
0
0
0
0
0
28.2
23.0
24.8
17.5
18.5
15.2
24.0
15.9
4.58
3.31
3.60
2.53
2.20
1.77
1.83
0.865
35.8
33.0
26.1
20.2
32.0
28.4
49.1
33.4
0.755
0.777
0.775
0.776
0.808
0.812
0.867
0.858
0.245
0.223
0.225
0.224
0.192
0.188
0.133
0.142
0.250
0.236
0.257
0.236
0.213
0.203
0.189
0.186
0.635
0.676
0.541
0.546
0.737
0.758
0.774
0.784
0.115
0.088
0.202
0.218
0.050
0.039
0.037
0.030
7
7
7
7
0.816
0.706
0.822
0.662
0.728
0.703
0.802
0.717
0.891
0.774
0.887
0.680
XE804
Amberlyst 70
Dowex 50Wx2
rigidity than their monosulfonated analogues [27]. However,
their acid capacity decreases as DVB% increases unlike the non-
chlorinated ones. Finally, acid strength of chlorinated resins is
higher than that of non-chlorinated monosulfonated analogues [8].
As a result of their morphology, Amberlyst 70 is a suitable cat-
alyst in hydrophilic liquid systems [6,8,10,16,19,27], whereas the
permanent porosity of Purolite CT482 allows its use in lipophilic
reaction media too.
formation. Among macroreticular resins, reaction rate of DNOE syn-
thesis decreased in the order CT482, Amberlyst 70 and Amberlyst
XE804. It is to be noted that a short period of constant activity was
observed before the activity decay for the three reactions starts,
being more noticeable on Amberlyst XE804 and Dowex 50Wx2 (Fig.
S2, Supporting information). As shown in Table 3, activity decay of
EOE synthesis ranges from 18 to 34%, from 10 to 32% in DNOE for-
mation and from 20 to 30% in DEE synthesis. In general, selectivity
hardly changes with time.
Apparent activation energies can be estimated by Arrhenius
relationship from reaction rates at the two temperatures both on
fresh catalysts and after 70 h on-stream (Table S1). Apparent acti-
vation energies for the three reactions of ether formation are in the
range 90–100 kJ/mol, what shows a high sensitivity to temperature.
Apparent activation energies obtained for CT482 and Amberlyst 70
do not change with time, that of Amberlyst XE482 decreased mod-
erately, and for Dowex 50Wx2 decreased by 10–15% with respect
to the values found over fresh catalysts.
Observed activity decays could be ascribed to several causes:
(1) thermal instability of resins, (2) changes of activity caused by
the interaction of the polymeric matrix with the water formed as
byproduct, and/or (3) deposition of alkene oligomers on the resin
surface.
3
.2. Catalytic tests
The catalytic performance has been studied in the liquid-phase
reaction between 1-octanol and ethanol. Experiments were car-
ried out at 150 C and 190 C in the fixed bed reactor at a flow rate
of 0.25 mL/min (WHSV = 17–120 h ), representative of the indus-
trial case. They lasted 70 h to evaluate possible catalytic activity
variation. Besides EOE, DEE and DNOE were formed. C8 alkenes
from 1-octanol dehydration were also detected, but in very small
amounts (<1%, w/w, at 190 C; <0.25%, w/w, at 150 C). Results are
gathered in Table 3. The evolution of reaction rates vs. time at 150 C
and 190 C is shown respectively in Figs. S1 and S2 (Supporting
Information).
◦
◦
−1
◦
◦
◦
◦
As Table 3 shows, reaction rates of EOE and DNOE syntheses
were of the same order of magnitude on the macroreticular resins at
◦
1
50 C, whereas that of DEE was much lower, since 1-octanol was in
3.3. Hydrothermal stability
large excess in the reactor feed (ROcOH/EtOH = 10). On gel-type resin
Dowex 50Wx2, results are qualitatively similar, but the reaction
rate of DNOE synthesis was about 3-fold higher than that of EOE,
which is about 10-fold higher than reaction rate of DEE synthesis. As
for EOE synthesis, Purolite CT482 showed the highest reaction rate,
followed by Dowex 50Wx2, Amberlyst XE804 and 70. Reaction rate
of EOE synthesis decreased continuously with time and the same
effect was observed for DNOE and DEE formation reactions (See
Fig S1, supporting information). Activity decay for EOE synthesis
was of the same order on all resins, by 12–18% with regard to the
fresh catalyst. Decay for DNOE synthesis was of 6–15%, and for DEE
synthesis it was of 25–35%. Selectivity to ethers changed slowly
with time, showing that EOE and DNOE are more favoured with
time on-stream with regard to DEE. The fact that the more bulky
ethers are preferably formed from each alcohol can be explained
by the large excess of OcOH in the reaction medium and the very
important effect of the resin morphology in EOE synthesis [32].
Hydrothermal stability tests were carried out by adding bidis-
tilled water to the reactor feed for 24 h at the selected temperature.
Afterwards, acid capacity was measured by titration and compared
to that of fresh catalyst. The difference correspond to the lost of
acid sites (Table S2, Supporting Information). The conventional PS-
◦
DVB resin Dowex 50Wx2 retained acid capacity fully at 150 C,
◦
but it greatly decreased at 190 C, which is far above its maximum
operating temperature. Amberlyst 70 and Purolite CT482 showed
◦
◦
negligible desulfonation at 150 C and 190 C, in agreement with
the manufacturer’s tests [3,4]. These data confirm that introduc-
tion of chlorine atoms into the resin backbone improves its thermal
stability. Finally, Amberlyst XE804 lost sulfonic groups at both tem-
peratures, what indicates that it is not as suitable as the other two
chlorinated resins for catalyzing high temperature processes.
Since hydrothermal tests showed that resins retained the acid
◦
capacity at 150 C, except for the slight desulfonation of Amberlyst
◦
At 190 C, Purolite CT482 was the most active resin for both EOE
and DEE formation, followed by Amberlyst XE804, Dowex 50Wx2
and Amberlyst 70. Dowex 50Wx2 was the most active to DNOE
XE804, the activity decay observed at this temperature cannot be
ascribed to the loss of sulfonic groups by thermal instability. As a
result, interaction with the reaction medium has to be considered.

3
06
J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
◦
Activity drop at 190 C after 70 h on-stream agrees well with the
higher leaching of acid groups found in hydrothermal experiments
3.4. Reusability tests
(
Table 3). Some differences in the activity decay of catalysts have
Accordingly to literature, activity decay of thermally stable
resins in alcohol dehydration reactions can be ascribed to the pre-
ferred adsorption of the water formed in the reaction on active sites
[9,13,14,16,18]. In order to confirm the inhibitory effect of water,
Purolite CT482 and Amberlyst 70 were washed and dried in the
reactor at room temperature after 48 h on-stream and reused twice.
Reaction rate of formation of the three ethers, and selectivity and
activity of catalysts at 48 h for fresh and reused catalysts follows
a similar evolution with time (Table S3, Supporting Information).
As example, Fig. 4A and B display the activity evolution of Purolite
CT482 and Amberlyst 70, respectively, in EOE synthesis during the
been observed at this temperature which could be partly ascribed
to their thermal stability. After 70 h on-stream, the non-chlorinated
resin Dowex 50Wx2 showed higher activity decay at 190 C (34%)
than at 150 C (12%) for EOE synthesis. Similar decays have been
found in DNOE synthesis (32% at 190 C, and 8% at 150 C). The decay
found at 190 C is is of the same order as sulfonic group leaching
and could be explained because its low maximum operating tem-
perature is 150 C. Similarly, activity decay of Amberlyst XE804 at
1
◦
◦
◦
◦
◦
◦
◦
◦
90 C (29%) was almost twice that observed at 150 C (16%) for EOE
◦
synthesis, whereas for DNOE synthesis decays are 23% at 190 C and
1
◦
◦
5% at 150 C. For both catalysts, activity decay in DEE synthesis is
three cycles at 190 C. Fresh (1st cycle) and reused catalyst (2nd and
of the same order at both temperatures.
3rd cycle) showed the same pattern. After two cycles using reused
catalyst, both resins show a very similar behaviour to the fresh cat-
alyst. So, it can be concluded that Purolite CT482 and Amberlyst 70
could be recovered after 48 h on-stream and reused without any
noticeable activity loss. This fact confirmed that catalyst decay on
these two resins was caused mainly by the inhibitory effect of water
formed in the EOE formation.
Some inferences about the water influence on the leaching of
sulfonic groups can be drawn by comparing acid capacity loss
in hydrothermal experiments and activity decay. Sulfonic group
leaching rate in PS-DVB resins is not uniform; the most active sites
are lost faster [33]. As a result, the early loss of a small number of
acid sites should cause a high drop on the catalytic activity. Data
show that the active sites loss after 24 h in water stream was equiv-
alent to the activity drop after 70 h in the alcohol stream for Dowex
3.5. Catalytic tests with alcohol–water feed
5
0Wx2 and Amberlyst XE804. As a result, sulfonic group hydrolysis
seems to be faster in water than in alcohols, in agreement with the
open literature [34]. However, since activity decay is much higher
than acid sites leaching on the two catalysts at the low tempera-
ture of explored range, other causes than desulfonation have to be
taken into account to explain activity drop.
A set of experiments was performed on Amberlyst 70 and
Purolite CT482 by adding water to the alcohol mixture feed.
As an example, Fig. 5 shows the activity of Amberlyst 70 with
ethanol–octanol and ethanol–octanol–water feeds (that of Puro-
lite CT482 can be found in Fig. S3, Supporting Information). The
reaction rate of fresh catalyst in ethanol–octanol mixtures was
taken as the reference for catalysts activity. As seen, activity to
EOE was lower in the presence of water. Unlike experiments with
ethanol–octanol feed, the activity to EOE formation was almost
constant along time on Purolite CT482 but it decreased slightly on
Amberlyst 70. The water content of resins was determined after
each experiment by titration. As Table 4 shows, water content
within the resin increased with time-on-stream, which could be
related to the continuous decrease of the catalytic activity to EOE
with ethanol–octanol feed. It is also seen that higher water contents
were found in resins when feeds contained water. Purolite CT482
retained higher water amounts than Amberlyst 70. However, the
number of water molecules per sulfonic group is similar on the
Over Purolite CT482 and Amberlyst 70, catalytic activity decay
◦
◦
at 150 C and 190 C is similar for the three reactions, especially
on Amberlyst 70. As desulfonation was not observed in the hydro-
thermal stability experiments with these resins, activity decay
cannot be accounted for sulfonic groups leaching. Instead, it could
be attributed to carbon deposition by chemical species present in
the reaction medium, particularly C -alkene oligomers. C alkenes
8
◦
8
◦
formation rose by 6-fold from 150 C to 190 C, but the catalytic
activity decay was of the same order at both temperatures, what
excludes such relationship. On the other hand, it can be assumed
that the formed ethers (DEE, EOE and DNOE) do not deactivate
the catalyst as their effect on the reaction rate was found to be
negligible in ether syntheses as in that of di-n-pentyl ether from
1
-pentanol previously studied [19].
two resins (2.78–2.98 mol H O/mol sulfonic group). These water
2
Fig. 4. Evolution of activity to EOE formation along the time on CT482 (A) and on Amberlyst 70 (B) at T = 190 ◦C, ROcOH/EtOH = 10, q = 0.25 mL/min, P = 25 bar (ꢀ, 1st cycle; ꢁ,
2
nd cycle; ꢂ, 3rd cycle).

J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
307
◦
Fig. 5. Activity to DEE (ꢁ), EOE (ꢀ) and DNOE (ꢂ) formation vs. time on Amberlyst 70 at T = 190 C, q = 0.25 mL/min, P = 25 bar, ROcOH/EtOH = 10. (A) Pure alcohols fed and (B) 1%
(
w/w) water fed.
contents are far from those of a resin saturated by water (4.2 mol
EOE is nearly independent on time on-stream. As Table 4 shows,
the amount of adsorbed water is similar in both resins. The differ-
ent TOF could be explained by the fact that water probably acts as a
solvent inside pores even at small quantities, and a transition takes
place from concerted to ionic catalytic mechanism (slower) where
the hydrated proton is the true catalytic agent takes place [14]. The
effect of the mechanism change on the rate would be more notice-
able on the three dimensional networks of sulfonic groups in the
denser gel-phase of Purolite CT482 as a result of their proximity
[35].
H O/mol sulfonic group) [26]. This fact can be due to the high affin-
2
ity of alcohols, which compete with water, for the acid sites.
As Table 4 shows, turnover frequency (TOF) is similar at the
beginning of ethanol-octanol fed experiments on the two catalysts;
however TOF of Purolite 482 is a bit lower after 70 h, and with
ethanol–octanol–water feed it is lower by 20% than activity at the
same time on stream with ethanol-octanol feed. Thus, the effect of
water on the activity drop of Purolite CT482 was stronger than on
Amberlyst 70. This pattern can be attributed to the higher acid site
density in the gel-phase of Purolite CT482. With respect to activ-
ity of Amberlyst 70, a short initial flat period is observed (4 h) and
afterwards a continuous decay, whereas for CT482 the flat period
is longer (16 h) and the decay rate is higher, so that after 70 h the
activity level of Amberlyst 70 is slightly higher than that of CT482.
In experiments where water was fed, the activity level of CT482 is
always clearly lower.
Without water in the feed, both OcOH and EtOH swell the resins
and compete for adsorbing on acid centres and the highest reac-
tion rate is achieved with fresh catalyst. Then, a part of formed
water adsorbs gradually on the resin and the reaction rate starts
to decrease. The phase-equilibrium between water in the liquid
phase and adsorbed on acid sites is not likely to be reached dur-
ing the experiments without feeding water. On the contrary, when
water was fed, the water amount in the liquid phase was enough to
reach water-resin quasi-equilibrium, and the reaction rate to form
3.6. Catalytic activity for DEE, EOE and DNOE syntheses
Morphological changes that take place in the resins with time-
on-stream by the action of water can modify their catalytic
behaviour. As the amount of water adsorbed on resins increases
with time-on-stream, accessibility of OcOH and EtOH to acid sites
is also modified. Fig. 5 displays the activity to DEE, EOE and DNOE
along time for Amberlyst 70 (that for CT 482 can be found in Fig.
S3, Supporting Information). Reaction rates, activity and selectivity
can be found in Table S4. It is observed that activity drop shows dif-
ferent pattern depending on the ether. As a rule, activity decay was
higher as the ether is less bulky: DEE > EOE > DNOE. When water
is fed together with the alcohol mixture, it is seen that EOE and
DNOE syntheses tend to similar activity levels, but DEE formation
continues decreasing.
In ethanol–octanol feed, fresh ion-exchange resins were not
fully swollen and the accessibility to acid centres was in some
extent hindered. It is expected that steric restrictions play a major
role for larger ether formation (DNOE > EOE > DEE). Resins swelled
progressively with reaction time and the void spaces appearing
between polymer chains favoured the diffusion of OcOH and bulky
ethers EOE and DNOE. Thus, activity drop was less pronounced as
the size of the ether increased. The effect of water was especially
stressed in ethanol–octanol–water experiments. The adsorption of
water caused remarkable activity decay for DEE synthesis, but in
the case of EOE and DNOE it was partially balanced by the higher
accessibility to acid centres of 1-octanol. Accordingly, the activity
after 70 h time-on-stream was reduced by 30–35% in EOE and DNOE
syntheses, and 57% in DEE formation, in relation to activity of fresh
catalysts in ethanol–octanol feed.
Table 4
◦
Water content inside resin and TOFs as a function of time on stream. T = 190 C,
ROcOH/EtOH = 10, q = 0.25 mL/min, P = 25 bar.
Catalyst
Time
on-stream
Water
content
(w/w, %)
Mol H2O
per sulfonic
group
TOF
(h−1)
(h)
Amberlyst 70
0
2.1
7.2
9.5
0.45
1.62
2.19
2.98
6.98
24
70
70
5.74
5.40
a
12.5
CT482
0
4
0
0
3.6
10.6
13.2
17.5
0.49
1.55
1.98
2.78
6.94
2
7
7
5.41
4.71
a
a
1
% (w/w) water in the feed.

3
08
J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
Table 5
Parameters of first-order activity decay function for Amberlyst 70 and CT482. ROcOH/EtOH = 10, q = 0.25 mL/min, P = 25 bar.
◦
−1
kd,i (h
Reaction
T ( C)
)
a∞
t0 (h)
Ed,i (kJ/mol)
Amberlyst 70
EOE synthesis
−
−
−
−
−
−
2
2
2
2
2
2
150
5.17 × 10
5.56 × 10
3.38 × 10
5.31 × 10
2.53 × 10
2.81 × 10
0.710
0.721
0.496
0.651
0.674
0.688
4
6
0
0
6
3.0
20
1
90
150
90
150
90
DEE synthesis
1
DNOE synthesis
4.6
1
12
CT482
−1
−1
−2
−1
−1
−1
EOE synthesis
150
90
150
90
150
90
1.89 × 10
2.21 × 10
5.22 × 10
1.04 × 10
4.90 × 10
5.95 × 10
0.865
0.810
0.626
0.696
0.927
0.914
2
14
0
0
0
6.4
28
1
DEE synthesis
1
DNOE synthesis
7.8
1
12
Summarizing, water effects on the reaction between OcOH
and EtOH are complex as this study shows. It is seen that
the period of time necessary to get a steady activity in this
4. Conclusions
The thermal stability study of acidic PS-DVB resins shows that
Dowex 50Wx2 and XE804 lose a relevant quantity of acid centres
case is extremely long. On Purolite CT482 and Amberlyst
◦
◦
7
0, which show very good hydrothermal stability at 190 C,
at 190 C. Leaching of active sites appears to be enhanced by the
released water acts as a solvent and increased the accessi-
bility of bulky molecules to the active centres. Consequently,
catalytic activity to produce long chain ethers is less hindered
in the presence of water than to short ones. In the particular
case of Dowex 50Wx2 and Amberlyst XE804 the presence of
action of the water formed in the reaction between OcOH and EtOH
to form EOE.
On the contrary, desulfonation is not significant for Amberlyst
◦
70 and Purolite CT482 at 190 C. As a consequence of the adsorption
of water which competes with ethanol and 1-octanol for sulfonic
groups, reaction rate on thermally stable resins Amberlyst 70 and
Purolite CT482 decreases with time-on stream up to a constant
activity level lower than that of fresh resins. Both resins recover
completely their activity as soon as water is removed from the
reaction medium and therefore they could be reused. Reused resins
showed a similar kinetic behaviour in the reaction system of EOE
formation.
Ion-exchange resins are not completely swollen in the absence
of water. As a consequence, diffusion of OcOH and bulky ethers are
hindered. However, with time on-stream, released water acts as
solvent and swells partially the resin. Thus, the catalytic activity
drop is less pronounced for the ethers with more steric restrictions
(EOE and DNOE) than for the smallest one (DEE).
◦
significant desulfonation at 190 C makes the situation more com-
plicated.
Finally, activity decay patterns were modelled for Amberlyst
7
0 and Purolite CT482 in runs with ethanol–octanol feed. Activ-
ity drop in Dowex 50Wx2 and Amberlyst 804 was not modelled
since it was partly due to leaching of sulfonic groups. Lit-
erature supplies relationships between resin activity and the
amount of water in the liquid-phase. Some are essentially
empirical, but equations based on exponential, or Langmuir
and Freundlich equilibrium approaches have been also used
[
36,37]. Still, in our experiments the water amount in the liq-
uid phase was always small and often around the threshold
of chromatographic detection. So that, activity as a function
of time was fitted to first and second order activation decays
functions. Best results were found by assuming a first order
decay with terminal activity. In this way, for the reaction
i:
Acknowledgements
Financial support was provided by the State Education,
Universities, Research & Development Office of Spain (project
CTQ2010-16047). The authors kindly thank Dr. Karel Jerabek (Insti-
tute of Chemical Process Fundamentals, Prague) for ISEC analysis,
and Rohm and Haas France, and Purolite to provide ion exchange
resins.
dai
−
= kd,i(a − a∞)
(7)
dt
whose integrated form is
ai = a∞,i + (1 − a∞,i) exp(−kd,i(t − t ))
(8)
0
where a
is the terminal activity, kd,i is the rate constant of
∞,i
Appendix A. Supplementary data
decay, and t the time when decay starts. The parameters of
0
Eq. (8) for EOE, DNOE and DEE syntheses on both catalysts are
shown in Table 5. As seen, terminal activities roughly agree
with the values found at large time-on-stream in alcohol–water
experiments. Therefore, it can be assumed that activity stabi-
lizes after a long period of time at lower reaction rates than
fresh catalyst. Rate decay constants are of the same order of
magnitude on both resins and increase in the order DEE, EOE,
DNOE syntheses on Purolite CT482, and DNOE, DEE, EOE syn-
theses on Amberlyst 70. As apparent activation energies show,
temperature dependence of rate decay constant is low except
for DEE synthesis. Finally, it is to be noted that t0 appear for
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2
013.07.024.
References
[
1] M.A. Harmer, Q. Sun, Appl. Catal. A: Gen. 221 (2001) 45–62.
[2] K. Tanabe, W.F. Hölderich, Appl. Catal. A: Gen. 181 (1999) 399–434.
[
[
[
3] Dow Chemical website, www.dow.com (10/2012).
4] Purolite website, www.purolite.com (10/2012).
5] S.R. Samms, S. Wasmus, R.F. Savinell, J. Electrochem. Soc. 143 (1996)
1498–1504.
◦
[6] R. Bringué, M. Iborra, J. Tejero, J.F. Izquierdo, F. Cunill, C. Fité, V.J. Cruz, J. Catal.
44 (2006) 33–42.
EOE and DNOE syntheses, being higher at 190 C. On the con-
trary for DEE synthesis decay starts as soon as the reaction
begins.
2
[
7] J. Zhang, L. Zhang, H. Liu, A. Sun, R. Liu, Wiley-VCH, Singapore, Weinheim
(Germany), 2012, pp. 293.

J. Guilera et al. / Applied Catalysis A: General 467 (2013) 301–309
309
[
[
8] P.F. Siril, H.E. Cross, D.R. Brown, J. Mol. Catal. A: Chem. 279 (2008) 63–68.
9] D.M. Alonso, J.Q. Bond, J.C. Serrano-Ruiz, J.A. Dumesic, Green Chem. 12 (2010)
92–999.
[23] T. Dogu, D. Varisli, Turk. J. Chem. 31 (2007) 551–567.
[24] K. Jerabek, Anal. Chem. 57 (1985) 1595–1597.
[25] K. Jerabek, Anal. Chem. 57 (1985) 1598–1602.
9
[
[
[
10] D.M. Alonso, J.Q. Bond, D. Wang, J.A. Dumesic, Top. Catal. 54 (2011) 447–457.
11] G.V. Samsonov, V.A. Pasechnik, Chem. Rev. 38 (1969) 547.
12] J. Guilera, R. Bringue, E. Ramírez, M. Iborra, J. Tejero, Ind. Eng. Chem. Res. 51
[26] M. Iborra, J. Tejero, F. Cunill, J.F. Izquierdo, C. Fité, Ind. Eng. Chem. Res. 39 (2000)
1416–1422.
[27] J. Guilera, R. Bringué, E. Ramírez, M. Iborra, J. Tejero, Appl. Catal. A: Gen. 413-14
(2012) 21–29.
(
2012) 16525–16530.
[
[
[
[
13] B.C. Gates, J.S. Wisnouskas, H.W. Heath, J. Catal. 24 (1972) 320–327.
14] B.C. Gates, W. Rodriguez, J. Catal. 31 (1973) 27–31.
15] R. Thornton, B.C. Gates, J. Catal. 34 (1974) 275–287.
[28] C. Casas, R. Bringué, E. Ramírez, M. Iborra, J. Tejero, Appl. Catal. A: Gen. 396
(2011) 129–139.
[29] B. Corain, K. Jerabek, P. Centomo, P. Canton, Angew. Chem. 116 (2004) 977–980.
[30] J. Tejero, F. Cunill, M. Iborra, J.F. Izquierdo, C. Fité, J. Mol. Catal. A: Chem. 182–183
(2002) 541–554.
16] R. Bringué, J. Tejero, M. Iborra, J.F. Izquierdo, C. Fité, F. Cunill, Top. Catal. 45
(
2007) 181–186.
[
[
[
17] A.A. Kiss, F. Omota, A.C. Dimian, G. Rothenberg, Top. Catal. 40 (2006) 141–150.
18] J.M. Aragon, J.M.R. Begas, L.G. Jodra, Ind. Eng. Chem. Res. 32 (1993) 2555–2562.
19] R. Bringué, E. Ramírez, C. Fité, M. Iborra, J. Tejero, Ind. Eng. Chem. Res. 50 (2011)
[31] K. Jerabek, L. Holub, 13th International IUPAC Conference on Polymers and
Organic Chemistry, Montreal, 2009.
[32] R. Bringué, E. Ramírez, M. Iborra, J. Tejero, F. Cunill, J. Catal. 304 (2013) 7–21.
[33] L. Petrus, E.J. Stamhuls, G.E.H. Joosten, Ind. Eng. Chem. Prod. Res. Dev. 20 (1981)
366–371.
[34] N. Bothe, F. Döscher, J. Klein, H. Widdecke, Polymer 20 (1979) 850–854.
[35] B.L. Yang, S.B. Yang, R.Q. Yao, React. Funct. Polym. 44 (2000) 167–175.
[36] B.L. Yang, M. Maeda, S. Goto, Int. J.Chem. Kinet. 30 (1998) 137–143.
[37] U. Limbeck, C. Altwicker, U. Kunz, U. Hoffmann, Chem. Eng. Sci. 56 (2001)
2171–2178.
7
911–7919.
[
[
[
20] G.C. Pecci, M.G. Clerici, F. Giavazzi, F. Ancillotti, M. Marchiona, R. Patrini,
Proceedings IX Int. Symp. Alcohol Fuels 1 (1991) 321–335.
21] G. A. Olah, Cleaner and cetane enhancing diesel fuel supplements. US Patent
US005520710A (1996).
22] J. Song, Z. Huang, X. Qiao, W. Wang, Energy Convers. Manage. 45 (2004)
2
223–2232.