In this contest, our work focus on the possibility to use strong acid zeolites USY, BEA, FAU-X, weak acid materials materials with high density of silanol groups: pure silica mesoporous
Trang 1Contents lists available atScienceDirect
Applied Catalysis A: General
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / a p c a t a
Biodiesel production process by homogeneous/heterogeneous catalytic system using an acid–base catalyst
Anastasia Macarioa,∗, Girolamo Giordanoa, Barbara Onidab, Donato Cocinab,
Antonio Tagarellic, Angelo Maria Giuffrèd
a Department of Chemical Engineering & Materials, Università della Calabria, Rende (CS), Italy
b Department of Materials Science & Chemical Engineering, Politecnico di Torino, Torino, Italy
c Department of Chemistry, Università della Calabria, Rende (CS), Italy
d Department of Biotechnology, M.A.A., Università Mediterranea di Reggio Calabria, Reggio Calabria, Italy
a r t i c l e i n f o
Article history:
Received 1 January 2010
Received in revised form 9 February 2010
Accepted 10 February 2010
Available online 18 February 2010
Keywords:
Biodiesel
Transesterification
Esterification
Zeolites
Acid–base catalysis
a b s t r a c t
The transesterification of triglycerides contained in waste oilseed fruits with methanol has been studied
in heterogeneous/homogeneous systems using acid and base catalysts The acid catalysts (strong acid catalysts: USY, BEA, FAU-X, and weak acid catalysts: MCM-41 and ITQ-6 with Si/Al =∞) were prepared
by hydrothermal synthesis procedures In order to obtain acid–base catalysts, potassium was loaded
on different materials by ionic exchange (obtaining K-MCM-41, K-ITQ-6,) XRD, ICP-MS, IR after CO and
CO2adsorption, thermal analyses and N2adsorption/desorption techniques have been used for catalysts characterization The highest triglycerides conversion and biodiesel yield values were achieved by
K-ITQ-6 catalysts, after 24 h of reaction at 180◦C Deactivation of this catalyst occurs for potassium leaching, but its regeneration and reuse are feasible and easy to perform A possible continuous biodiesel production process has been proposed
© 2010 Elsevier B.V All rights reserved
1 Introduction
The transestrification of vegetable oils, catalyzed by either acids
or bases, consists in three consecutive and reversible reactions
in which triglycerides are converted first to diglycerides, then to
monoglyceride and finally to glycerin In each steps, one ester is
formed In the overall reaction, using methanol as alcohol, 3 mol of
methyl esters are produced for each mole of triglyceride
Industrial processes use 6 mol of methanol for each mole
of triglyceride obtaining methyl esters as biodiesel mixture
(FAME = Fatty Acid Methyl Esters) This large excess of methanol
ensures that the reaction is driven in the direction of methyl esters
The methyl esters mixture (or Biodiesel fuel) has similar
proper-ties of fossil diesel fuel (cetane number, kinematic viscosity) but it
does not contain petroleum products and sulfur compounds
Fur-thermore, it possesses a higher flash point (>130◦C) with respect
the conventional diesel For these reasons biodiesel is recognized
as “green fuel” Finally, it is almost neutral towards CO2emission
because its renewable sources (vegetable oils and biomass)
Industrial current production of biodiesel is carried out by
homogeneous alkali-catalyzed transesterification of vegetable oils
∗ Corresponding author Tel.: +39 0984 49 66 67; fax: +39 0984 49 66 55.
E-mail address: macario@unical.it (A Macario).
with methanol, using sodium hydroxide, potassium hydroxide
or potassium methoxide as catalyst[1] The homogeneous basic transesterification shows a very fast kinetic of reaction but also a collateral saponification reaction that reduces the biodiesel pro-duction efficiency To prevent the biodiesel yield loss due to the saponification reaction, oil and alcohol must be dry and the oil should have a minimum amount of free fatty acids (less than 0.1 wt%) Biodiesel is finally recovered by repeated washing with water to remove glycerol, soap and the excess of methanol By contrary, the acid transesterification allows to obtain a biodiesel production without formation of by-products The drawbacks of
an acid homogeneous transesterification are the corrosive catalyst (H2SO4, H3PO4, and HCl) and the slow reaction rate This may be increased at high temperature and pressure, involving larger costs [2] Methanol and oil are poorly soluble, so the reaction mixture contains two liquid phases Others alcohols can be used, but being more expensive Moreover, an acid pre-treatment is often carried out in the homogeneous alkaline-transesterification of oils having more than the 5 wt% of free fatty acids, in order to improve the biodiesel efficiency production[1–3] Recent works, also, demon-strate as a heterogeneous enzymatic catalyst represents a potential solution to produce biodiesel from very low-quality triglycerides source[4,5], but in these cases the cost of the enzymes has to
be considered All these aspects suggest that an environmental-friendly and cheaper biodiesel production process could be carried 0926-860X/$ – see front matter © 2010 Elsevier B.V All rights reserved.
Trang 2out using acid or basic heterogeneous catalysts or, better,
hetero-geneous catalyst with both acid and basic properties They could
combine the advantages of the alkaline and acid transesterifications
with those of heterogeneous catalytic process The suitable catalyst
should possess high activity and selectivity, high water-tolerance,
high stability, it should be inexpensive and its production process
should be environmental friendly The activity and selectivity
prop-erties of catalyst generally depend on the amount and the strength
of acid or basic sites Towards organic reaction (like
transesterifica-tion), catalysts with high hydrophobic surface area are preferable
because otherwise water can interact with active sites preventing
the adsorption of organic reactants As it is well known, zeolites and
related materials are suitable materials for these purposes because
they can be easy synthesized and modified in order to affect the
acidity, basicity and hydrophobicity of their surface
In this contest, our work focus on the possibility to use strong
acid zeolites (USY, BEA, FAU-X), weak acid materials (materials with
high density of silanol groups: pure silica mesoporous materials
and delaminated zeolites) and materials with acid–base
proper-ties (K-MCM-41, K-ITQ-6) as catalysts for the transesterification of
waste fruit oilseeds with high amount of free fatty acids (FFAs),
higher than 5 wt% (as low-quality and cheap triglycerides sources),
with methanol to biodiesel There are many different materials
developed as catalysts for the transesterification of triglycerides
to biodiesel, such as solid acids[6–11](Amberlyst-15, H-Zeolites,
Cs-heteropoly acids) or solid basic[9,11–19](KOH-NaX, KI-Al2O3,
Na/NaOH/␥-Al2O3, ETS-10, CaO, NaCs-X, KOH-Al2O3) All these
materials show interesting results but only in respect with acid or
basic catalytic aspect Moreover, in order to achieve good catalytic
performance, some of them can used only in strong conditions,
such as high temperature, high methanol content or in presence
of extracting co-solvent, or starting from high quality triglycerides
source While, the conversion of biological feedstocks to biodiesel
using mesoporous calcium silicate mixed oxides, as heterogeneous
catalysts with acids and basics, is recently published and patented
by Lin et al.[20,21]
Besides the usual zeolites characterization techniques (XRD,
TG-DTA, ICP-MS, N2 adsorption), the study of the adsorption of
CO and CO2 on the catalysts has been carried out by IR
spec-troscopy, in order to identify the nature and the strength of the
catalytic active sites The reaction conditions were not optimized
for the highest reaction yield: all catalytic tests provided to
com-pare the activities of different catalysts The catalyst showing the
best catalytic performances (triglycerides and FFAs conversion and
biodiesel yield) was regenerated and used for more than one
reac-tion cycle The efficiency of catalyst regenerareac-tion procedure has
been evaluated in order to perform a possible continuous biodiesel
production process Chemical composition (free fatty acids,
mono-glycerides, diglycerides and triglycerides content) and total acidity
of the biodiesel final product have been measured
2 Materials and methods
2.1 Materials
The silica sources used for catalysts preparation were:
pre-cipitated silica (BDH) and silica fumed (Aerosil 200 (Degussa) or
silica fumed (Aldrich)) The structure directing agents used were:
cetyltrimethylammonium bromide (CTABr, Aldrich) for
meso-porous materials The mineralizing agents were: sodium hydroxide
(Carlo Erba), tetramethylammonium hydroxide (TMAOH, 25%,
Fluka) and tetraethylammonium hydroxide (TEAOH, 40%, Fluka)
Aluminum hydroxide (98%, Aldrich) was used as metal sources for
Al–BEA catalyst preparation, while sodium aluminate (NaAlO2, 99%,
Carlo Erba) was used as aluminum source for FAU-X preparation
Waste fruit oilseeds with oleic acid (C18:1, 39 wt%) and linoleic acid (C18:2, 30 wt%) as main FFAs content (total free acidity 5.58%)
is used as triglycerides source
2.2 Catalysts preparation Pure silica MCM-41 type material was prepared starting from a gel with the following molar composition:
1SiO2–0.26TMAOH–0.12CTABr–40H2O The crystallization time and temperature were, respectively,
24 h at 140◦C in autoclave
Delaminated zeolite ITQ-6 (Si/Al =∞) sample was prepared by swelling the laminar pure silica PREFER according to the procedure described by Corma et al.[22]
USY zeolite has been supplied by UOP Molecular Sieves BEA catalyst was prepared by hydrothermal synthesis starting from the following molar gel:
50SiO2–10TEAOH–2Na2O–0.5Al2O3–350H2O time and temperature of crystallization were, respectively, 5 days
at 150◦C in autoclave
The molar composition of the gel for hydrothermal synthesis of FAU-X zeolites was the following:
1SiO2–0.6Na2O–0.1Al2O3–40H2O After the synthesis, the solid phases of all syntheses were recov-ered by filtration and washed with distilled water The samples were calcined in air flow at 550◦C for 8 h (heating rate: 5◦C/min) Finally, samples containing potassium (K-MCM-41, K-ITQ-6) were obtained by ionic exchange carried out on correspondent cal-cined material at 60◦C, for two times, with KCl 1 M solution and
a ratio solid/solution equal to 0.01 g/ml After ionic exchange, the dried samples were activated at 300◦C for 8 h Commercial anhy-drous potassium silicate in powder was supplied by Alfa Aesar Co
2.3 Catalysts characterization Powder X-ray diffraction (XRD) data were recorded using a Phillips PW 1710 diffractometer with CuK␣radiation The samples were scanned in the range of 2 from 1◦ to 8◦ (for mesoporous materials) or from 5◦ to 45◦ (for microporous materials) in steps
of 0.005◦with a count time of 1 s at each point XRD analyses of all synthesized samples show the characteristic diffraction peaks (pat-terns not reported), confirming that the expected phases have been obtained for all materials BET surface area and physical properties
of samples were evaluated by N2adsorption/desorption isotherms carried out at 77 K on a Micromeritics ASAP 2020 sorption ana-lyzer Thermal decompositions of as-synthesized samples were investigated by SHIMADZU DTG-60 instrument, between 20◦C and
850◦C, at a ramp of 5◦C/min in air with a flow rate of 5 ml/min Chemical composition of samples was evaluated by an Elan DRC-e ICP-MS instrument (PerkinElmer SCIEX) Samples were introduced
by means of a quartz nebulizer The ICP torch was a standard torch (Fassel type torch) with a platinum injector For the quantitative analysis, calibration curves were built on six different concentra-tions in a calibration range of 1–5000g/l and having composition similar to that of solution samples Standard solutions were pre-pared by diluting a solution of Na and K (1000 mg/l)
For IR measurements, self-supporting wafers were prepared and activated under dynamic vacuum (10−4Torr) for 1 h at 573 K, in
an IR cell allowing in situ thermal treatments, gas dosage and IR measurements to be carried out both at room temperature and at
a nominal temperature of 77 K, presumably in fact around 100 K
Trang 3Spectra were collected on a Bruker IFS 55 Equinox instrument
equipped with a MCT cryodetector working with 2 cm−1resolution
Difference spectra are obtained after subtraction of the spectrum
of the naked sample
2.4 Transesterification reaction
The alcoholysis of triglycerides with anhydrous methanol
(99.9%, Sigma–Aldrich) performed in batch Teflon-steel autoclaves
immersed in a temperature-controlled bath The reaction
temper-ature was varied from 100◦C to 180◦C In a typical experiment
5.0 g of oil were mixed with 2.74 ml of methanol (molar ration
oil:methanol equal 1:20) and 0.15 g of catalyst (5 wt% respect to
oil) Then the mixture was transferred in the autoclave and stirred
(650 rpm) The autoclave was close and heated in the bath until the
desired temperature The reaction was been carried out under
auto-geneous pressure in the batch-autoclave system for the required
time To analyze the reaction progress, products and reactant were
separated from catalyst and glycerin by centrifugation in hexane
(95%, Sigma–Aldrich) To analyze the chemical composition of the
biodiesel produced we used the ASTM D-6584-00 method Through
this method, it is possible to quantify triglycerides (TG),
diglyc-erides (DG), monoglycdiglyc-erides (MG), free fatty acids and methyl ester
by unique chromatogram The Rtx®-Biodiesel fused silica
capil-lary column (10 m× 0.32 mm × 0.10 m) was used in an Agilent
6890 GC instrument equipped with FID detector Before analysis,
trimethylsilyl-trifluoroacetamide (98%+ Acros Organics) was added
to the mixture as silyating agent Tricaprin (Sigma–Aldrich, puriss
p.a standard for GC) was added as external standard 1l of sample
was injected and analyzed by the following oven temperature
pro-gram: 1 min at 140◦C, 10◦C/min to 360, 8 min at 360◦C To correct
the area obtained from the GC, in order to calculate the exact
triglyc-erides conversion and biodiesel yield, the response factors for each
compounds have been calculated using the correspondent standard
compound (puriss p.a standard for GC): oleic acid, methyl oleate,
monoglycerides, diglycerides and triolein The biodiesel yield (in
percentage) was calculated according to the following equation:
By=
ABio/RBio [(ABio/RBio)+ (AFFA/RFFA)+ 2(ADG/RDG)+ 3(ATG/RTG)]
×100
where Aiis the area of the peak correspondents to each compounds
and Ri is the related response factor Total acidity value of final
biodiesel mixture was calculated by the Standard European Method
EN14104 and compared with the European Quality Standard
Spec-ification of Biodiesel EN14214-UNI10946
2.5 Recovery and regeneration of catalyst
At the end of each reaction cycle, two centrifugation processes
were carried out in order to separate the catalyst from the reaction
mixture and the glycerol to biodiesel mixture The recovered
cata-lyst has been washed with ethanol, in order to remove all organic
compounds (triglycerides and esters tracks), and then with water,
in order to remove ethanol After drying at 120◦C for 1 night, the
catalyst was reused for another reaction cycle In order to check
the presence of organic compounds, a thermal analysis has been
carried out on the regenerated catalysts
3 Results and discussion
3.1 Catalytic test results: screening of all tested catalysts
For a first comparison among all catalysts tested, the same
reaction conditions were employed for the starting experiments:
Fig 1 Catalytic performance (triglycerides conversion and biodiesel yield) obtained
by all catalysts tested at 100 ◦ C, at 5 wt% respect to the oil, for 24 h and with a molar ratio between oil:methanol equal to 1:20.
100◦C, 24 h of reaction, 650 rpm, 5 wt% of catalyst respect to the triolein, molar ratio equal to 1:20 between oil and methanol
As observed from Fig 1, the strong acid catalysts (USY, BEA and FAU-X) do not reach an appreciable triglycerides conversion
in the reaction conditions Commercial potassium silicate, appears
as the best catalysts showing a triglycerides conversion of the 82% and biodiesel yield of the 79% Among the synthesized catalysts reported inFig 1, the most promising are K-MCM-41 and K-ITQ-6 Their triglycerides conversion is similar, 58% and 64% respectively, whereas the biodiesel yield differs significantly, being 56% for K-ITQ-6 and only 3% for K-MCM-41
The pure silica mesoporous material, Si-MCM-41, has not been used as catalyst in the mentioned reaction due to very weak acidity
of its surface While, concerning the pure silica delaminated zeolite ITQ-6, due to its high weak acid silanols groups density, its catalytic activity at 100◦C until 24 h of reaction can be detected but, in any case, has been low: the triglycerides conversion was closed to 20% Starting from these results, in order to improve the catalysts per-formance, time and temperature of reaction have been increased Operating at 180◦C and following the reaction for 72 h, for acid catalysts (FAU-X, BEA, USY) an increasing of triglycerides conver-sion has not been observed Finally, large increase of triglycerides conversion and biodiesel yield have been observed for K-MCM-41 and K-ITQ-6 catalysts, maintaining the temperature at 100◦C and increasing the reaction time until 72 h (seeFig 2(a)) In particu-lar, K-ITQ-6 catalyst gave a triglycerides conversion of 98% and a biodiesel yield of 74% (Fig 2(b)) The triglycerides conversion of K-MCM-41 reached a value of 90%, but the main products were the free fatty acids (32%) and monoglycerides (42%) whereas the biodiesel yield grew only up to the 15% Instead for K2SiO3catalyst a faster reaction kinetic has been observed: after 17 h of reaction, the triglycerides conversion was 73% (Fig 2(a)) and the biodiesel yield was the 68% (Fig 2(b)) After almost 30 h of reaction, the potassium silicate shows its best catalytic performance (83% of triglycerides conversion and 80% of biodiesel yield) and they do not change for longer reaction time A further increase of the reaction temperature (up to 180◦C) for K-ITQ-6 caused a faster kinetic of transesteri-fication (Fig 3) At 180◦C, the K-ITQ-6 catalyst gave the 97% of triglycerides conversion and the 80% of biodiesel yield after 24 h
of reaction Following the reaction until 96 h, the triglycerides con-version was observed to grow up to 99%, the biodiesel yield and the FFAs conversion to increases up to 90% The composition of the biodiesel mixture at different reaction time is reported inTable 1
At 180◦C, the amount of free fatty acids, tri-, di- and monoglyc-erides decreases and simultaneously the amount of methyl esters increases when reaction time increases up to 48 h In these
Trang 4con-Fig 2 Triglycerides conversion and biodiesel yield, as function of time, obtained at 100◦ C, with the 5 wt% of catalyst respect to the oil and with a molar ratio between oil:methanol equal to 1:20.
Fig 3 Triglycerides conversion, biodiesel yield and FFAs conversion, as function of
time, obtained using K-ITQ-6 catalyst at 180 ◦ C, at 5 wt% respect to the oil and 1:20
molar ratio between oil:methanol.
ditions, the triglycerides conversion is almost complete, whereas
the biodiesel yield only slightly increases, because the reaction
proceeds with the conversion of mono- and diglycerides to free
fatty acids (seeTable 1) The temperature increasing does not affect
appreciably the catalytic performance of pure silica ITQ-6 material:
the triglycerides conversion grew to 38% but the biodiesel yield was
closed to 3% This means, most probably, that the weak acid silanol
groups of the ITQ-6 surface are able to esterificate the free fatty
acids but they are not able to transesterificate the triglycerides
Fig 4 Comparison between K-ITQ-6 and commercial KOH catalysts Conditions for
KOH: 24 h at 70 ◦ C, methanol:oil molar ratio 10:1—catalyst amount: 5 wt%; condition for K-ITQ-6: 48 h at 180 ◦ C, methanol:oil molar ratio 20:1—catalyst amount: 5 wt%.
Further investigations have not been applied to this catalyst due to its low catalytic performance
Due to the promising results obtained by K-ITQ-6 catalyst, a comparison of its catalytic performance with those of commercial KOH homogeneous basic catalyst could be interesting For this pur-pose, in theFig 4the biodiesel yield, the FFAs and the triglycerides conversion obtained for K-ITQ-6 and KOH catalysts are reported The main result noticeable by this comparison is, even if the time and temperature of reaction is different for two catalysts, when the triglycerides conversion is complete, K-ITQ-6 permits to obtain higher biodiesel yield and higher FFAs conversion than KOH cat-alyst Moreover, no soap products are detectable in the reaction mixture using K-ITQ-6 catalysts by GC analysis (results not showed)
Table 1
Triolein conversion, biodiesel yield and chemical composition of biodiesel mixture obtained using 5 wt% of K-ITQ-6 catalyst at 180 ◦ C and with triolein:methanol molar ratio 1:20.
Time [h] Triglycerides conversion [%] Biodiesel yield a [%] Methyl esters b [%] Free fatty acids [%] MG c [%] DG c [%] TG c [%]
a B y ={(ABio /R Bio )/[(A Bio /R Bio ) + (A FFA /R FFA ) + 2(A DG /R DG ) + 3(A TG /R TG )]} × 100.
b Weight percentage of methyl esters, obtained directly by GC, after correction with response factors.
c
Trang 5Fig 5 N2 adsorption isotherms of mesoporous (a) and delaminated ITQ-6 (b) catalysts, before and after ionic exchange.
and three clear phases are visible after centrifugation: catalyst,
glycerol and biodiesel, and their separation is easy
Finally, the total acidity of biodiesel mixture, measured by
the European Standard method (EN14104), is equal to 0.23 mg
KOH/g after 48 h of reaction, when the amount of free fatty
acids is 2.42 wt% This is a good value of biodiesel final acidity,
considering that the maximum value accepted by the European
Quality Standard Specification of Biodiesel (EN14214-UNI10946)
is 0.5 mgKOH/g (0.8 mgKOH/g for the American ones ASTM6751)
Moreover, also the maximum amount of diglycerides and
triglyc-erides are respected, while only the amount of monoglyctriglyc-erides is
higher (see legend inTable 1)
3.2 Catalysts characterization
Main physico-chemical properties of tested catalysts are
sum-marized inTable 2 For the microporous materials (USY, FAU-X
and BEA) the low triglycerides conversion may be ascribed to the
fact that the higher density of active acid sites is inside to the
catalyst structure Then, the big molecule of triglycerides
can-not enter inside the microporous channels of the catalysts and
the very low concentration of acid sites on the external surface
of these catalysts is not enough to obtain an appreciable
triglyc-erides conversion In fact, the pore dimension of acid microporous materials used (FAU-X, USY and BEA) does not exceed 6 Å (see Table 2) while the spherical molecular diameter of triolein (triglyc-eride of oleic acid C18), calculated by empirical correlation[23], is
14 Å This aspect is also confirmed by a previous study[9]where the activity of a strong acid zeolites (BEA) has been increased (of the 10%) by lantanium ionic exchange, due to the fact that the
La species are able to increase the presence of “external” basic sites In any case, the more acid La-BEA catalyst are not able to reach a triglycerides conversion upper to 50%, meaning that the strong acid zeolites are not suitable as heterogeneous catalyst for biodiesel production Moreover, the high hydrophilicity of the sur-face of these acid catalysts can be another reason of their inactivity towards the transesterification reaction The Si/Al molar ratio of these catalysts, in fact, ranging between Si/Al = 5–50 in the start-ing synthesis gel Then, the surface of final catalyst can be very hydrophilic, due to presence of aluminum and the water can covers the surface of the acid solid, preventing the adsorption of organic substrate Further investigations have been carried out only on the samples that have showed the best or an appreciable catalytic performance, that is the mesoporous materials and delaminated zeolites
Therefore, textural properties of these catalysts have been ana-lyzed by ICP-MS, N2adsorption/desorption and thermogravimetric thermal analyses After ionic-exchange with K+ the amount of potassium loaded is 0.22 wt% and 1.59 wt% for MCM-41 and ITQ-6, respectively (Table 2)
The N2adsorption/desorption isotherm, BET surface area and pore diameter of the mesoporous sample after potassium loading
by ionic exchange, has been reported inFig 5(a) Both BET value and pore size of mesoporous structures decrease after K+loading, indicating the presence of the cation For ITQ-6 delaminated zeolite, the ionic exchange, carried out to load potassium cation on the highly hydroxilated surface of this material, strongly reduces the final BET value (Fig 5(b)), indicating a great amount of potassium loaded (as confirmed by ICP elemental analysis)
The hydrophobicity of mesoporous samples and delaminated zeolites were estimated from the weight loss of physisorbed water
at 200◦C The K-MCM-41 loses the 6.80% and the K-ITQ-6 loses only the 3.21% (Fig 6) These results indicate that the delaminated zeolite containing potassium is more hydrophobic than K-MCM-41 (thermogravimetric analysis results) This can be one explanation why K-MCM-41 shows a lower catalytic activity towards the trans-esterification reaction, respect to the K-ITQ-6 catalyst
Trang 6Fig 7 IR spectra related to the adsorption of CO at 77 K on K-ITQ-6 outgassed at 573 K Section (a) OH stretching region; section (b) CO stretching region.
3.3 Infrared spectroscopy analyses results
Due to the good catalytic performance showed by K-ITQ-6, we
have characterized the catalyst by FT-IR analysis, in order to have
additional information on the nature of the acid and basic catalytic
sites
3.4 Adsorption of CO at 77 K
Fig 7reports IR spectra related to increasing coverage of CO
adsorbed at 77 K onto K-ITQ-6, previously outgassed at 573 K In the
OH stretching region (section (a)), the decrease of the isolated SiOH
stretching mode at 3750 cm−1 is observed, accompanied by the
increase of the broad band at 3660 cm−1, due to the same species
interacting via H-bonding with CO The shift, which is an indirect
measure of the Brønsted acidity, is about−90 cm−1, i.e the value
measured for isolated silanols in all silica ITQ-2 and amorphous
sil-ica Aerosil[24] The stretching mode of CO molecules interacting
with silanols is observed at 2156 cm−1 (section (b)), as expected
[24] Another band increases at 2138 cm−1, due to “liquid-like” CO,
probably mainly physisorbed in ITQ-6 microporosity.Fig 8reports
the IR spectra related to increasing coverage of CO adsorbed at 77 K
onto K-MCM-41 The inset in section (a), which refers to the OH
stretching region, reports the spectrum of the sample outgassed at
573 K before adsorption of CO Besides the band at 3748 cm−1, due
to isolated silanols, a broader band is observed at 3698 cm−1, which
is ascribed to silanols perturbed by hydrocarbon moieties, due to
templates residues
Upon adsorption of CO, the band at 3698 cm−1is not perturbed (section (a), body of the figure) The band due to isolated silanols, instead, decreases and that due to H-bonded silanols increases
at 3660 cm−1 The measured shift is some −90 cm−1, revealing the same Brønsted acidity of silanols as in the case of K-ITQ-6 The stretching mode of CO molecules interacting with silanols is observed at 2156 cm−1(section (b)), as in the case of K-ITQ-6 At low coverage, a shoulder is hardly discernible at 2162 cm−1, due
to CO interacting with K+ions[25] The band due to physisorbed
“liquid-like” CO is observed at 2138 cm−1and its relative intensity with respect to the band at 2156 cm−1is weaker than in the case
of K-ITQ-6 This is ascribed to the absence of microporosity in the MCM-41 sample, at variance with K-ITQ-6
3.5 Adsorption of CO2at room temperature Fig 9reports the IR spectra related to the adsorption of CO2at room temperature on K-ITQ-6 (section (a)) and K-MCM-41 (sec-tion (b)), previously outgassed at 573 K In the case of K-ITQ-6, a composite band between 1600 cm−1and 1700 cm−1 is observed, accompanied by a second one at 1349 cm−1 These bands are not depleted by outgassing at RT and they are ascribed to carbonate species, formed by interaction of CO2with basic oxygen sites They are not formed upon adsorption of CO2on K-MCM-41 (section (b))
In conclusion, only on the surface of ITQ-6 exchanged with potassium, both acids and basics sites are present, which are responsible of its higher catalytic activity, with respect to the K-MCM-41
Table 2
Main physico-chemical properties of catalysts tested.
Catalyst type Si/Al gel molar ratio BET surface area [m 2 /g] Pore diameter [Å] K + loaded [wt%]
a
Trang 7Fig 8 IR spectra related to the adsorption of CO at 77 K on K-MCM-41 outgassed at 573 K Section (a) OH stretching region; section (b) CO stretching region.
Fig 9 Difference IR spectra related to the adsorption of CO2 at RT on K-ITQ-6
(sec-tion (a)) and K-MCM-41 (sec(sec-tion (b)) outgassed at 573 K Curve 1: in contact with
CO (p = 47 mbar); curve 2: outgassed at RT after contact with CO
3.6 Proposed reaction mechanism
In theFig 10the reaction mechanism proposed for the reaction catalyzed by acid–basic K-ITQ-6 catalyst is reported
As it is possible to notice, the presence of potassium loaded
by ionic exchange to the silanol groups of the pure silica delam-inated zeolite allows the formation of homogeneous methoxide (Fig 10(a)) that promotes the carbonyl carbon atom attack of triglycerides, and the homogeneous Brønsted basic catalysis takes place[26]
Simultaneously, the free weak Brønsted acid silanol groups
of the catalyst can protonate the carbonyl group of TG and FFAs, increasing their electrophilicity and rendering they more susceptible to the alcohol nucleophilic attack, allowing the transesterification and esterification reaction by a well known homogeneous Brønsted acid catalysis mechanism[27] The acid esterification of free fatty acid by weak acid Brønsted silanol groups
of catalyst prevents the saponification reaction and the biodiesel separation and purification can be carried out more easily For this reason the FFAs conversion is higher than the value obtained by commercial KOH (as showed in theFig 4) Moreover, the simul-taneous presence of basic and acidic sites on the K-ITQ-6 catalyst surface could also explain why for an interesting heterogeneous basic catalyst (the ETS-10), reported by a Suppes et al.[16], the high free fatty acids concentration (about the 30 wt%) inhibit the basic solid catalyst
3.7 Reusability of catalyst The reusability was studied only for K-ITQ-6 and K2SiO3, due
to the fact that these catalysts having best performance For K-ITQ-6 catalysts the amount of recovered material was about 80% of the initial quantity (calculated weighting the dried sample before and after reaction) While, concerning commercial K2SiO3, even if during the first reaction cycle the catalytic performance are better that that of K-ITQ-6, the solid is partially dissolved in the reaction media and only less than the 40 wt% can be recovered Moreover, the biodiesel yield after the second cycle strongly decreases to 63% Triglycerides conversion achieved during the second reaction cycle by K-ITQ-6 was only the 56%, whereas the biodiesel yield was
Trang 8Fig 10 (a) Proposed potassium methoxide mechanism formation (b) Proposed transesterification Brønsted acid mechanism.
35%, after 24 h of reaction at 180◦C (5 wt% of catalyst and molar ratio
oil:methanol equal to 1:20) Comparing these results with those
obtained after the first reaction cycle, a clear potassium leaching
has been taken Moreover, the results of ICP analysis carried out
on the used catalyst show that, after the first use, the amount of
potassium on K-ITQ-6 zeolite decreases from 1.59 wt% to 0.51 wt%,
with a leaching of potassium of the 78%
This means that the maximum amount of potassium in the final
biodiesel does not reach the 0.1 wt% (result obtained by mass
bal-ance on potassium presents on K-ITQ-6 catalyst before and after
reaction) However this metal content can be easily reduced by a
bland washing of biodiesel with distilled water
3.8 Regeneration of catalyst
The thermal analysis carried out on the regenerated catalyst,
after one reaction cycle, shows that the regeneration procedure
allows to obtain a free catalyst surface: no mass loss is detected
(Fig 11) a part of physisorbed water By29Si NMR analysis
car-ried out on regenerated catalyst the amount of terminal silanol
groups measured are quite the same of the amount detected on
the fresh ITQ-6: 27± 3% and 28 ± 5%, respectively This result
sug-gested to use the regenerated catalyst for a new ionic exchange
cycle in order to prepare new catalyst (K-ITQ-6) for biodiesel
pro-duction After this ionic exchange, the same amount of potassium
was loaded (1.62± 4 wt%) and the same catalytic performance of
new K-ITQ-6 catalyst has been obtained: 92% of biodiesel yield and
complete triglycerides conversion after 48 h at 180◦C
A possible flow-sheet of continuous process for biodiesel
production using heterogeneous acid–basic K-ITQ-6 catalyst is
reported in theFig 12 Two reactors are need: the first one for
transesterification–esterification process and the second one for catalyst regeneration Each fixed-bed reactor is supplied by catalyst regeneration lines, consisting of hexane feed for organic com-pound removing, water feed for hexane removing and, finally, KCl 1 M solution feed for potassium exchange and fresh catalyst preparation When the catalyst is exhaust, the reactor stops to work and the catalyst regeneration process starts, simultaneously, the second reactor, terminated the catalyst regeneration, starts to work for a new biodiesel production process with the fresh cata-lyst
Fig 11 TG and DTA curves of regenerated ITQ-6 catalysts.
Trang 9Fig 12 Flow-sheet of proposed continuous process for biodiesel production.
4 Conclusion
Microporous acid catalysts are not suitable for
transesterifica-tion reactransesterifica-tion of triglycerides because of the diffusion limitatransesterifica-tion of
reactants inside micropores
Modification of ITQ-6 surface with K+ ions, carried out by
ionic-exchange, produced a catalyst with weak Brønsted acid
sites (silanols) and basic sites, responsible of catalytic
activ-ity in transesterification of triglycerides and esterification of
FFAs The catalyst is active towards the conversion of waste
oil with high free acidity (5.58%) without formation of
unde-sired by-product (soap-compounds) and promotes a simultaneous
homogeneous/heterogeneous and acid/base catalysis This may
open the way to use of low-quality oil to perform a cheaper
biodiesel production by acid–basic solid catalysts The
heteroge-neous catalyst can be easy and efficiently regenerated and reused
for new reaction cycle This opens the possibility to perform a
con-tinuous catalytic process for biodiesel production
References
[1] F Ma, M.A Hanna, Bioresour Technol 70 (1999) 1–15.
[2] H Fukuda, A Kondo, H Noda, J Biosci Bioeng 92 (2001) 405–416.
[3] L.C Meher, D Vidya Sagar, S.N Naik, Renew Sust Energy Rev 10 (2006)
248–268.
[4] A Macario, M Moliner, A Corma, G Giordano, Micropor Mesopor Mater 118
(2009) 334–340.
[5] A Macario, G Giordano, L Setti, A Parise, J.M Campelo, J.M Marinas, D Luna,
Biocatal Biotrans 25 (2007) 328–335.
[6] E Lotero, Y Liu, D.E Lopez, K Suwannakarn, D.A Bruce, J.G Goodwin, Ind Eng.
Chem Res 44 (2005) 5353–5363.
[7] A.A Kiss, A.C Dimian, G Rothenberg, Adv Synth Catal 348 (2006) 75–81 [8] G.D Yadav, J.J Nair, Micropor Mesopor Mater 33 (1999) 1–48.
[9] Q Shu, B Yang, H Yuan, S Qing, G Zhu, Catal Commun 8 (2007) 2159– 2165.
[10] Y.M Park, D.-W Lee, D.-K Kim, J.-S Lee, K.-Y Lee, Catal Today 131 (2008) 238–243.
[11] A Brito, M.E Borges, R Arvelo, F Garcia, M.C Diaz, N Otero, Int J Chem React Eng 5 (2007) A104.
[12] W Xie, H Li, J Mol Catal A: Chem 255 (2006) 1–9.
[13] W Xie, X Huang, H Li, Bioresour Technol 98 (2007) 936–939.
[14] H.-J Kim, B.-S Kang, M.-J Kim, Y.M Park, D.-K Kim, J.-S Lee, K.-Y Lee, Catal Today 93–95 (2004) 315–320.
[15] M Di Serio, R Tesser, L Pengmei, E Santacesaria, Energy Fuels 22 (2008) 207–217.
[16] G.J Suppes, M.A Dasari, E.J Doskocil, P.J Mankidy, M.J Goff, Appl Catal A: Gen.
257 (2004) 213–223.
[17] X Liu, H He, Y Wang, S Zhu, X Piao, Fuel 87 (2008) 216–221.
[18] E Leclercq, A Finiels, C Moreau, JACOS 78 (2001) 1161–1165.
[19] K Noiroj, P Intarapong, A Luengnaruemitchai, S Jai-In, Renew Energy 34 (2009) 1145–1150.
[20] V.S.-Y Lin, J.A Nieweg, C Kern, B.G Trewyn, J.W Wiench, M Pruski, Prep Symp.
Am Chem Soc Div Fuel Chem 51 (2006) 426–427.
[21] V.S.-Y Lin, J.A Nieweg, J.G Verkade, C.R Venkat Reddy, C Kern, Patent WO/2008/013551.
[22] A Corma, U Diaz, M.E Domine, V Fornés, J Am Chem Soc 122 (2000) 2804–2809.
[23] Perry’s Chemical Engineers’ Handbook, McGraw-Hill International Editions, sez.17-20, Table 17-10.
[24] B Onida, L Borello, B Bonelli, F Geobaldo, E Garrone, J Catal 214 (2003) 191–199.
[25] S Bordiga, G Turnes Palomino, C Paze’, A Zecchina, Micropor Mesopor Mater.
34 (2000) 67–80.
[26] E Lotero, J.G Goodwin Jr., D.A Bruce, K Suwannakarn, Y Liu, D.E Lopez, Catal-ysis, vol 19, Royal Society of Chemistry, 2006, pp 41–83.
[27] Y Liu, E Lotero, J.G Goodwin Jr., J Catal 243 (2006) 221–228.