N. Redondo, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
method for processing naphthenic acid since the treated crude are
thermally stable. It can be emphasized that esterification processes
employing basic catalysts occur at lower temperatures than those em-
ploying acid catalysts [11,12].
2.2. Catalysts characterization
The HTs were characterized by means of a N2 adsorption isotherm
at 77 K in an ASAP 2010 apparatus from Micrometics and X-ray dif-
fraction (XRD). The surfaces areas were obtained using BET metho-
dology. Previously the sample were pre-treating under vacuum at
400 °C overnight for HTcalc and 120 °C for HT and HTcalcRH. X-ray
diffraction measurements were recorded with Siemens D 5000 equip-
ment with Cu Kα radiation.
The layered double hydroxide (LDH) play a critical role in the
process of catalytic reaction by proton elimination. Hydrotalcites (HTs)
are well-known layered double hydroxides (LDH) that have the general
2+
n−
formula [M
M3x+(OH)2][(A ).mH2O], where M2+ and M3+ re-
(1-x)
x/n
presents metal components with divalent and trivalent cations, re-
spectively. For the divalent metal component, magnesium is the most
common, although many other metal species with a cation size like that
of Mg are also available, such as Co, Cu, Ni, Ca, Zn, and Mn. The tri-
valent metal component is generally occupied by Al, although Fe and Cr
2.3. Reaction measurements
The esterification reaction was performed in a mechanically stirred
stainless-steel batch reactor of 500 mL. The reagents used were 3-cy-
clopentyl propionic acid (98% Sigma Aldrich) and methanol (99.8%
Cicarelli) diluted in n-heptane (95% Cicarelli).
can be also employed. Some available anions, such as CO32−, NO3−
,
Cl−, SO42−, and OH−, are located between the interlayer along with
water molecules to compensate the excessive positive charges. As a
result, HTs have a unique layered structure with positively charged
brucite like layers and compensating anions [13,14].
In all the experiments, the pressure was maintained at 3.5 kg/cm2 to
prevent methanol evaporation from the reactive mixture. The reactor
was heated externally by an electric resistance to keep the reaction
temperature constant. To ensure efficient mixing a stainless propeller
type stirrer driven by a 0.25 HP motor was used.
The objective of this work is to develop a solid catalyst based on
Mg/Al LDH, which is active at low temperatures in the esterification of
3-cyclopentyl propionic acid, representative of naphthenic acids.
Different activation treatments, as calcination or calcination followed
by rehydration, were applied to the samples. Additionally, it is an aim
to carry out the kinetic study of the esterification reaction is carried out
in a batch stirred reactor with a solid catalyst in suspension by agita-
tion.
The compounds 3-cyclopentyl propionic acid, methanol and ester
were analysed by gas chromatography using an Agilent Technology GC-
7890A chromatograph with flame ionization detector and a non-polar
Agilent J & W DB-5 column (30 m length, internal diameter of 0.32 mm
and film thickness of 0.5 μm). The concentration of the compounds in
each sample was determined using n-octane as internal standard.
The conversion is defined as:
2. Material and methods
c (t = 0) − c (t = t)
x (t) =
100
c (t = 0)
(1)
2.1. Catalysts preparation
Where, c (t) is the acid molar concentration as a function of time.
Mg-Al Hydrotalcite catalysts with an Mg/Al ratio equal to 4 were
synthesized to ensure that the obtained solid has basic properties [14].
Hydrotalcite (HT) was synthesized by homogeneous precipitation using
the urea hydrolysis method [15]. The method consisted of preparing
stock solutions of Al (III) and Mg (II) nitrates (0.5 mol/L, each one) by
dissolving Al(NO3)3.9H2O (Merck, purity > 98.5%) and Mg
(NO3)2.6H2O (Merck, purity > 99.9%) in distilled water, respectively.
A solution of urea (CO(NH2)2, 2 mol/L) (Sigma-Aldrich, purity > 99%)
was also prepared beforehand. Then, these solutions were mixed in
appropriate proportions, according to the stoichiometry required in the
solid, taking into account that in the generated solution, once the stock
solutions were mixed, the urea concentration should be 0.5 mol/L,
while the total concentration of cations [Mg(II) + Al(III)] should be
5.0 × 10−2 mol/L and the ratio Al/(Al + Mg) should be 0.2. The so-
lutions containing urea-Mg(II)-Al(III) were aged in a thermostatted bath
at 90 °C for 24 h in polypropylene (PP) bottles.
The procedure used in each experiment consisted in the addition to
the reactor of 3-cyclopentyl propionic acid (0.005 M) and methanol in
n-heptane (0.04 M) solutions and a determined mass of catalyst with an
average particle diameter of 100 μm. Then, N2 was injected with an on-
off valve until a constant pressure of 3.5 kg/cm2 was reached, in-
creasing the temperature of the reactor up to the corresponding reac-
tion temperature. Once the reaction temperature was reached, this time
was recorded as the start of the reaction. The following operating
conditions were varied to study the effect of mass of catalyst (0.3 g,
0.5 g and 1.5 g), stirred speed (700, 800 and 900 rpm), temperature (70,
80, 90 °C) and particle diameter (44 μm, 66 μm, 100 μm).
3. Calculations
3.1. Kinetics modelling approach
Under these conditions, a homogenous alkalinization rate de-
termined by the rate of urea hydrolysis was reached. The gel was pre-
cipitated, centrifuged three times, with alternating washes with cold
distilled water, and dried in an oven at 70 °C overnight. Different ac-
tivation treatments were implemented: 1) only drying, without calci-
nation; 2) calcination at 500 °C and 3) calcination at 500 °C and sub-
sequent rehydration. The solid obtained without further treatment is
identified as Htnot calc. On the other hand, HT calcined at 500 °C during
5 hs it is identified as HTcalc. Finally, HTcalRH is designated to calcined
and rehydrated HT. The rehydration was carried out by dilution of 1 g
of HTcalc in 100 mL of distilled water in a bath thermostatted at 80 °C,
stirred for 4 h and finally centrifuged for 10 min at 160 rpm. Finally, the
largest amount of supernatant water was discarded and the solid ob-
tained was dried in an oven at 90 °C until there were no traces of liquid
in the sample. Once the catalysts were synthesized, pellets of approxi-
mately 1 cm of diameter were formed by means of a hydraulic press, to
subsequently sieve in order to obtain a determined particle diameter.
The kinetic model is based on the following assumptions: a) the
contribution of non-catalysed reaction rate was negligible compared to
the catalysed reaction rate b) the direct reaction follows the pseudo-n
order reaction rate law, while the reverse reaction is neglectable.
dcA
dt
rA = −
= k. cAn
(2)
It is considered that far from the equilibrium, at the beginning of the
reaction, the inverse reaction rate is negligible.
Where cA: acid molar concentration, k: reaction rate constant, n:
reaction order.
E
a
k = k0. e− R.T
(3)
Where Ea is the activation energy, k0: pre-exponential coefficient, R:
gas constant and T: temperature (K).
Applying natural logarithm to both members of Eq. 2, n can be
calculated from the slope and the reaction rate constant (k) from the
2