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heterogeneous acidic catalysts, they should be supported on porous
materials [16–18]. Different studies have been performed using
immobilized HPA on a wide number of materials such as MCM-
41 [19], zirconia [20], activated carbons, silica and alumina [21] for
esterification reactions.
Previous works, carried out in our labs, demonstrated the suit-
ability of using activated carbon fibers (ACF) to adsorb HPA showing
high adsorption capacities. HPA were adsorbed mainly on the
super-micropores, achieving a high dispersion [18]. The selection
of ACF in liquid reactions is highly advantageous compared to con-
ventional activated carbons since they show smaller hydrodynamic
resistance, due to their fibrous shape, high mass transfer rates and
their easy removal from the reaction medium [22]. Hence, the main
aim of this work is to analyze the suitability of the use of supported
HPA on an industrially and environmentally interesting reaction
such as biodiesel synthesis through the study of free fatty acid
esterification reaction.
with a thermostatically controlled bath at different temperatures
(between 30 and 60 ◦C). Once the palmitic acid was dissolved,
the catalyst was added. The amount of catalyst varied from 3 to
80 mg for unsupported HPA studies and between 50 and 200 mg
for supported HPA on ACF. These experimental conditions were
maintained during 6 h, although additional experiments were per-
formed up to 24 h. In addition, the used catalysts were reused in
successive cycles.
To determine the progress of the reaction, three differ-
ent aliquots were analyzed at 1, 3 and 6 h (also at 24 h for
longer duration experiments). Reaction yield was determined
by titration using
a NaOH 0.1 M (Aldrich) standard pattern.
Moreover, the nature of the reaction products was analyzed
by gas chromatography coupled to mass spectrometry (Shi-
madzu, GCMS QP5050A with a polydimethylsiloxane column CBPI
PONA-M50-042 from Shimadzu, 100 mm × 0.25 mm × 0.5 mm),
so methyl palmitate was confirmed as the only reaction
product.
In order to analyze the regeneration process, additional treat-
ments were performed with the used catalysts. The treatments
tested were (i) thermal treatment at 300–400 ◦C during 6 h; (ii)
washing step with ethanol (25 ml, 40 ◦C and 30 min) and (iii) wash-
ing step with ethanol in acidic medium (25 ml ethanol/20 l H2SO4
98 wt%, 40 ◦C and 30 min). In the last two processes, catalysts were
dried at 110 ◦C for 12 h.
2. Experimental
2.1. Materials
For this study, palmitic acid (Sigma Aldrich) was selected as fatty
acid for the synthesis of the corresponding monoalkyl ester. Among
all vegetables oils, the one obtained from African palm is the high-
est oil production per hectare and year, showing a lower and stable
price compared to other vegetable oils for food use [23]. In addition,
its methyl ester shows one of the highest cetane index (higher than
petroleum diesel) which gives more value to biodiesel obtained
from this compound [24]. However, the main disadvantage of the
use of palm oil is its high FFA content (around 5–8 wt%, of which
44 wt% corresponds to palmitic and 38% to oleic acid-). This aspect
makes its application in a single basic transesterification step pro-
cess ineffective. For this reason, it would be more appropriate to
subject the palm oil to a previous acidic esterification stage to elim-
inate the presence of FFA. Thus, the application of HPA supported
on ACF will be used for this reaction.
2.4. Characterization of the supports and catalysts
Additional characterization of the prepared catalysts was per-
formed in a previous work, using gas adsorption, DRX and TEM
[18].
2.4.1. Chemical analysis
The elemental analysis of HPA, ACF and the fresh and used
catalysts (P, Mo, W) was determined by inductively coupled
plasma spectrometry (ICP 7700x Agilent) and hydrogen, carbon
and nitrogen contents were carried out by a Carlo Erba EA 1110
CHNS-O. For ICP–MS analysis the solid samples were dissolved
in acidic mediun and digested under reflux for 6 h. Aliquots of
the obtained solution were diluted to 50 cm3 using deionized
water.
For the pre-esterification stage, the following materials were
selected: palmitic acid (Sigma Aldrich), methanol (Sigma Aldrich),
three commercially available ACF from Osaka Gas Co. (samples A10,
A15 and A20) and two HPA, phosphomolybdic acid (HPMo) and
phosphotungstic acid (HPW).
2.4.2. Temperature-programmed desorption (TPD)
2.2. Catalyst preparation
The total amount of acid sites present on the catalysts
was estimated by temperature-programmed desorption (TPD) of
chemisorbed NH3 recorded with a quadrupole mass spectrometer
(Balzers QMS 100) connected to a quartz micro-reactor. 0.05 g of
the catalyst sample were placed in the micro-reactor with a heating
rate of 20 ◦C/min up to 200 ◦C under a He flow rate of 60 cm3/min,
and then cooled down to room temperature. Then, a flow of 10 vol%
NH3 in He (60 cm3/min) was passed over the samples until no
further uptake of ammonia was observed. Next, the gas flow was
switch to a pure He flow and the sample was heated up to 120 ◦C.
In this step, the physically adsorbed ammonia was desorbed. After
that, the ammonia TPD profile was recorded upon heating the sam-
ple at a rate of 10 ◦C/min up to 350 ◦C under He atmosphere. The
acid sites were evaluated assuming an interaction 1:1 between NH3
and the acid site.
A series of different catalysts were prepared by an impregnation
method using the above ACF and HPAs, which has been described
in a previous work [18]. Among the prepared supported catalysts,
the most used in this manuscript were a 44 wt% HPMo supported
on A20 ACF (HPMo/A20) and a 52 wt% HPW supported on A20
ACF (HPW/A20) (the amount of supported HPA was determined
by difference of the initial weight of ACF and the final weight
of impregnated and dried sample). In order to analyze the sup-
port’s pore size effect on the reaction, two catalysts were using
the A10 and A15 supports, containing a 27 and 32 wt% of sup-
ported HPMo, respectively. Additionally, to analyze the influence
of the HPA amount on the reaction yield, four different samples
were studied: 12 and 24 wt% and 10 and 21 wt% for HPW/A20 and
HPMo/A20, respectively.
In order to obtain a better characterization of the ACFs sup-
port TPD experiments were carried out in a furnace coupled
2.3. Catalytic activity tests
to
a mass spectrometer (VG Quadrupoles). In these experi-
ments, samples were heated up to 900 ◦C (heating rate 20 ◦C/min)
under a helium flow rate of 60 cm3/min. The quantification of
gases that evolved as CO and CO2 was analyzed in a mass
spectrometer.
Reaction was carried out in a glass flask reactor where 1 g
of palmitic acid (3.8 mmol) and 15 cm3 of methanol were added
(molar ratio 1/97) [25]. The flask reactor was sealed to avoid
evaporation and placed on a shaker at constant stirring speed