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M.A. Tejero et al. / Applied Catalysis A: General 517 (2016) 56–66
Amberlyst 35 [A35], Amberlyst 36 [A36], Amberlyst 39 [A39],
Amberlyst 46 [A46] and Amberlyst 70 [A70]), Purolite (Purolite®
CT224 [CT224]) and Aldrich (Dowex 50Wx2 [DOW2], Dowex
50Wx4 [DOW4] and Dowex 50Wx8 [DOW8], supplied as beads of
50–100 mesh).
The use of such small catalyst mass (0.5 g of dry catalyst; cata-
lyst loading <1%) allows us to work free of spurious effects on the
reaction rate as can be seen in the dehydration reactions to ether
of 1-butanol [53] or 1-hexanol [54] on Amberlyst 70, or 1-pentanol
over CT224 and Dowex 50Wx4 [51] carried out in similar setups.
Evaluation of the possible effects of the stirring speed on exter-
nal mass transfer was not within the bounds of this study. However,
performed at the same temperature range, e.g. the esterification of
lactic acid with methanol [55] or that of acrylic acid with 2-ethyl
hexanol [56] over Amberlyst 15. In addition, the dehydration reac-
tions to ether of 1-butanol [53] or 1-hexanol [54] on Amberlyst 70,
or 1-pentanol over CT224 and Dowex 50Wx4 [51] at temperatures
as high as 150 ◦C also shows that external mass transfer influence
Macroreticular resins were used with only a fraction of the
commercial distribution of particle sizes (0.4 < dbead < 0.6 mm), and
gel-type resins with the commercial distribution of particle sizes
(0.15 < dbead < 0.3 mm for Dowex catalysts, and dbead ≈ 0.32 mm for
compounds of similar molecular size to LA, BuOH and BL. Among
tion of lactic acid with methanol over Amberlyst 15 [55], that of
propionic acid with 1-butanol over Amberlyst 35 [60], that of acetic
acid with 1-pentanol over Dowex 50Wx8 [61], or that of succinic
acid with ethanol over Amberlyst 70 [62]. As these works show, in
swollen state the influence of the diffusion on reaction rate is neg-
ligible when macromolecular and gel-type resin beads of similar
size to those of the present study are used.
A wide set of acidic PS-DVB resins with different morphological
properties was used in this study. These include both macroretic-
ular (macroporous) and gel-type (microporous) resins. They also
cover a wide range of acid capacities, with monosulfonated or
conventionally sulfonated resins (which have a concentration of
about one sulfonic group per styrene ring, statistically in para-
substitution [47,48]), oversulfonated (in which the concentration
of sulfonic groups has been increased beyond the usual limit of one
group per styrene ring [49], where the additional sulfonic groups
are predominantly distributed close to the particle surface [50])
it has chlorine atoms in its structure, which confer this catalyst
a higher thermal stability (yet its acid capacity is only less than
3 mmol H+/g). Resin properties and their acronyms can be seen in
Table 1.
2.3. Apparatus and analysis
Experiments were carried out in a 100 mL stainless steel auto-
clave operated in batch mode with an electrical furnace controlling
temperature. One of the outlets of the reactor was connected
directly to a liquid sampling valve, which injected 0.2 L of pres-
surized liquid into a gas–liquid chromatograph (GLC). The liquid
composition was analyzed hourly by using a split mode opera-
tion in a HP6890A GLC apparatus equipped with a TCD detector. A
50 m × 0.2 mm × 0.5 m methyl silicone capillary column was used
to separate and determine reactants and products. The column was
temperature programmed to start at 500C with a 10 ◦C/min ramp
until 250 ◦C and held for 6 min. Helium (≥99.998%, Linde) was used
as the carrier gas with a total flow rate of 30 mL/min. All the species
were identified by a second GLC (Agilent 6890) equipped with a MS
detector (Agilent GC/MS 5973) and chemical database software. A
typical chromatogram of the reaction mixture is shown in Fig. S1
(Supplementary material).
In each experiment, LA conversion (XLA) and selectivity to BL
(SBBLuOH) were estimated by Eqs. (1) and (2), respectively.
ꢀ
ꢁ
mole of LA reacted
mole of LA initially
XLA
=
× 100 %, mol/mol
(1)
(2)
ꢀ
ꢁ
mole of LA reacted to form BL
mole of LA reacted
BL
S
=
× 100 %, mol/mol
BuOH
2.4. Methodology
Reaction rates of BL formation at any time were estimated from the
functions of variation of nBL (number of produced BL mole) versus
time, where w is the mass of dry catalyst
Acid resins were dried at 110◦ C for 2 h at atmospheric pressure
and subsequently at 110◦ C under vacuum overnight (10 mbar).
Residual water content was <3%, as determined by the Fischer
method with a Karl Fischer titrator (Orion AF8).
The reactor was loaded with 70 mL of LA/BuOH mixture (1/3
molar ratio), heated up to the desired temperature and stirred at
500 rpm. Pressure was set at 2.5 MPa with N2 in order to main-
tain the liquid phase in the reactor and also have the pressure
needed to shift liquid samples to the GLC apparatus. When the mix-
ture reached the operating temperature, 0.5 g of dry catalyst were
injected into the reactor from an external cylinder by shifting with
N2. Catalyst injection was taken as time zero.
ꢂ
ꢃ
1 dnBL
mol BL
rBL
=
(3)
w
dt
kg · h
t
Finally, the turnover frequency of BL formation was estimated by
dividing the reaction rate by the number of acid sites per gram of
dry resin (acid capacity), [H+].
ꢂ
ꢃ
rBL
H+
mol BL
eq H+ · h
TOFBL
=
(4)
[
]
A single experiment (Dowex 50Wx2) was replicated twice with
perfect experimental overlap. Therefore, it was concluded that the
experiments are fully replicable and experimental error was less
than 3–5%.
Experiments of 8 h duration were carried out at 80 ◦C. Initial
excess of alcohol was selected to shift the equilibrium to the pro-
duction of the ester. However, the closer to the stoichiometric ratio
is the initial mixture, the higher the amount of ester obtained at
equilibrium. Experimentallywe found that a mixtureAL/BuOHwith
initial molar ratio lower than 1/3 split into two phases due to the
formation of water over the reaction. Therefore, we use an initial
molar ratio AL/BuOH of 1/3 in all the experiments. To work with
such alcohol excess has the advantages of minimizing the possible
formation of humins by polymerization of LA and, since the alcohol
3. Results and discussion
3.1. Morphology of the swollen resins
Ion exchange resins are nearly spherical beads of sulfonated PS-
DVB copolymers. Copolymerization of styrene and DVB gives place