IDENTIFICATION OF
ADULTERANT AND ALCOHOL ROUTE IN BIODIESEL USING MID-INFRARED ABSORPTION
SPECTROSCOPY
Maryleide
Ventura da Silva
Universidade Estadual de Mato
Grosso do Sul, Brazil
E-mail:
maryleide.ventura@gmail.com
Sandro
Marcio Lima
Universidade Estadual de Mato
Grosso do Sul, Brazil
E-mail:
smlima@uems.br
Luis
Humberto da Cunha Andrade
Universidade Estadual de Mato
Grosso do Sul, Brazil
E-mail:
luishca@me.com
Euclésio Simionatto
Universidade Estadual de Mato
Grosso do Sul, Brazil
E-mail:
euclesio@uems.br
Márcio
da Silva Figueiredo
Universidade Estadual de Mato
Grosso do Sul, Brazil
E-mail:
marciofigueiredo@ufgd.edu.br
Submission:
09/03/2013
Revision:
26/09/2013
Accept:
10/10/2013
ABSTRACT
Mid-infrared absorption
spectroscopy was used to analyze soybean oil, ethylic and methylic
soybean biodiesel, and blends prepared with soybean oil mixed with biodiesel,
in order to evaluate this method as an alternative to assess oil as impurities
or adulterant in biodiesel. We also aimed to determine whether the biodiesel
was prepared by the ethyl or methyl routes, by inspecting the infrared spectra.
The C-O functional groups between 1100 and 1200 cm-1 are different
for oil and biodiesel, which allows them to be used to distinguish impurities
(residual oil) in biofuel. The peak C-O-C at 1017 cm-1 is characteristic for methylic biodiesel, and the peak O-C-C at 1035 cm-1
for ethylic biodiesel. These vibrational modes can therefore
be used to indicate the route used to prepare the
biofuel. Results
indicated that infrared spectroscopy is appropriate for
monitoring the quality of biofuel for commercial sale.
Keywords: biodiesel, oil-biodiesel
blends, infrared spectroscopy.
1.
INTRODUCTION
Biodiesel is a
fuel derived from renewable energy sources, and has been
proposed as an alternative to fossil fuels, mainly
due to its environmental benefits (Meher et al. 2006, Nabi et al. 2006, Varun et al. 2013,
Silva et al. 2013). It has been marketed in several countries, and
in Brazil it is currently sold mixed with up to 8% diesel. Some
studies have indicated that the use of biodiesel will increase within the next
few years, making it essential to determine the properties of biodiesel
(quality, purity, origin, etc.).
The
most common chemical method
used to obtain
biodiesel is the transesterification process, in which triglyceride molecules are reacted with
alcohol molecules in the presence of a catalyst (Silva et al. 2012, Kucek et al.
2007). Because this is a reversible reaction, the
conversion from triglyceride to ester is not totally efficient, so that
triglycerides are often present as residues in the ester. The ester may also contain
residual alcohol molecules and traces of catalyst or glycerol that were not
eliminated during the purification process. Thus, the
processes of production and purification may result in a biofuel that is likely
to cause an engine to deteriorate (Meher et al. 2006, Knothe 2010). For
these reasons, the National Agency of Petroleum, Natural Gas and Biofuels
(ANP), regulated by the Brazilian Association of Technical Standards (ABNT),
established acceptable limits of impurities in biodiesel: the standard indicates
that 96.5% must be ester, i.e., 3.5% is the limit of
all impurities combined (Resolution from the Agência Nacional do
Petróleo, Gás Natural e Biocombustíveis (ANP), 2013). This
regulation is based on the international standards of the American Society for
Testing and Materials (ASTM), the International Organization for Standardization (ISO) and the Comité Européen de Normalisation (CEN). These specifications require efficient technical
methods for quality analysis.
The methods most
commonly used for monitoring the
quality of biofuel in order to detect
the presence of adulterants are chromatography and mid-infrared optical absorption spectroscopy (Lira et al. 2010, Richard et al.
2011, Jiang et al. 2010). Chromatography is an efficient
technique for the analysis of biodiesel; however, it is very expensive and
destructive. In contrast, mid-infrared spectroscopy in a detection range of 4000-400 cm-1 is a
nondestructive technique and it affords good
reproducibility, with very rapid acquisition time and high spectral resolution,
requiring a minimum amount of sample for analysis (Meher et al. 2006, Knothe et al. 2005).
Mid-infrared
spectroscopy has been widely used to analyze biodiesel and diesel blends,
mainly because they are sold at gas stations (Aliske et al. 2007, Monteiro et
al. 2008, Pimentel et al. 2006, Lira et al. 2010).
Some studies have evaluated the efficiency
of the transesterification
process via mid-infrared spectroscopy (Dubé et al. 2004, Richard et al. 2011). More
recently, I. P. Soares and coauthors used the mid-infrared
spectroscopy combined with chemometric tools for identification of oil as adulterant in biodiesel
(Soares et al. 2008). Thus, it is of great importance to
the development of biodiesel fuel to use the
mid-infrared absorption spectroscopy to detect contaminants, such
as soybean oil, that can
be originated both from
the transesterification
process as well as by
the contamination purpose.
The present study
investigated
the potential application of mid-infrared
spectroscopy to assess the
presence of residual soybean oil in methylic
and ethylic biodiesel,
and to provide
information about the alcohol route used to prepare
the biodiesel. This oil is regarded as an impurity and/or adulterant present
in biodiesel, since its presence in biodiesel may result either
from an incomplete transesterification reaction or from
purposeful addition.
2.1.
Preparation of biodiesel and oil-biodiesel
blends
Biodiesel was prepared by
using Sadia® refined soybean oil, in a transesterification process with potassium hydroxide (KOH) as catalyst, following two different
routes: methylic and ethylic. The molar ratio of alcohol:oil and the quantity of
catalyst used in the reaction were, respectively,
16:1 with 0.7% KOH and 24:1 with
0.6% catalyst for the ethylic and methylic
routes. This high concentration of alcohol is to
guarantee a high conversion to ester. The amount of catalyst required
for the transesterification
reaction was determined from the acidity of soybean oil
through potentiometric titration. For the transesterification process, the alkaline reagent
(KOH) was previously dissolved in the alcohol, and
the soybean oil pre-heated at 60ºC was added to the mixture. The
solution was mixed with a magnetic stirrer at 60ºC for 80 min. After this period, the
sample was distilled in an evaporator under reduced pressure to remove excess
alcohol. The product was then placed in a funnel for 24
h to allow the biodiesel and glycerol phases to separate. With the
biofuel separated from the glycerol, the biodiesel was washed with distilled
water to remove residues from the reaction,
including impurities from free fatty
acids, diglycerides, triglycerides, the catalyst and residual glycerol. This process was repeated five times until the
residual water remained totally transparent.
To perform mid-infrared spectroscopy on the biodiesel and oil:biodiesel blends, we
prepared samples with different volumetric ratios of soybean oil (the raw material used in obtaining the
ester) blended in biodiesel. Table 1 shows the proportions (in volume and percentage) of soybean oil
(SO) added to methylic biodiesel (MB), ethylic
biodiesel (EB), and commercial
biodiesel (B). We prepared three types of blends: soybean
oil in ethylic biodiesel
(SOEB), soybean oil in methylic biodiesel (SOMB), and soybean oil in commercial
biodiesel (SOB). The commercial biodiesel was acquired from a
gas station in Dourados, Mato Grosso do Sul, Brazil.
Table 1. Biodiesel and soybean oil concentrations
used to prepare the blends. In the definition of nomenclature, “y” is “M” for the methylic route and “E” for the ethylic route. For
the commercial biodiesel “y” was omitted.
Biodiesel (B) mL (%) |
Soybean oil
(SO) mL (%) |
Nomenclature |
5 (100) |
0 (0) |
B |
4.95 (99) |
0.05 (1) |
SOyB1 |
4.90 (98) |
0.10 (2) |
SOyB2 |
4.85 (97) |
0.15 (3) |
SOyB3 |
4.80 (96) |
0.20 (4) |
SOyB4 |
4.75 (95) |
0.25 (5) |
SOyB5 |
0 (0) |
5 (100) |
SO |
2.2.
Mid-Infrared Absorption Spectroscopy
For infrared spectroscopy we used a
Fourier Transform Spectrophotometer model Nexus 670, from Thermo Nicolet, with
a frequency range between 1500 and 750 cm-1. The samples
were deposited in a cell made of
ZnSe for attenuated total reflectance (ATR), which was coupled to the
spectrophotometer. To perform the measurements we initially used 10 μL of
sample deposited in the ATR cell; and we then used 20 mL, and finally 30 mL. Thus, three spectra were determined for each sample and the
simple arithmetic mean of the data was calculated. We used a resolution of 0.5
cm-1 with 16 scans for each measurement.
Figure 1 shows the
mid-infrared absorption spectra obtained for the SO, EB, MB and B samples. In this frequency interval it is
possible to note significant differences in peak
positions and absorption intensities for the functional groups present in the samples. The most important
absorption peaks and bands were identified, and the
respective functional groups and vibrational modes
are listed in Table 2. This classification is based on appropriate references (Smith 1999, Soares et
al. 2011, Soares
et al. 2008).
Figure 1. Absorption spectra in the
fingerprint region of the samples
of soybean oil (SO), ethylic biodiesel (EB), methylic biodiesel
(MB) and commercial biodiesel (B).
Table 2. Wavenumber, functional groups and vibrational modes for the main observed
peaks in triglyceride
and biodiesel. The samples columns are indication if the
absorption intensity is weak (w) or strong (s).
Peaks/Bands |
Wavenumber (cm-1) |
Functional Group |
Vibrational Mode |
Sample |
||
SO |
EB |
MB |
||||
1 |
845-964 |
C-O-C |
Stretch |
w |
w |
w |
2 |
1017 |
C-O-C |
Stretch |
|
|
s |
3 |
1035 |
O-C-C |
Stretch |
|
s |
|
4 |
1080-1130 |
C-O-C |
Asymmetric Stretch |
s |
s |
w |
5 |
1143 |
C-O |
Asymmetric Stretch |
s |
|
|
6 |
1161-1245 |
C-C-O |
Asymmetric Stretch |
|
s |
s |
7 |
1302-1320 |
C-O |
Stretch |
|
s |
|
8 |
1363-1378 |
CH3 |
Bend |
w |
s |
w |
9 |
1436 |
H3C-O |
Bend |
w |
w |
s |
The chemical structures of both the triglyceride
and biodiesel consist of long asymmetrical chains. The main difference is that in
triglyceride there are three fatty acids chemically
bonded to the glyceride, while in
biodiesel the fatty acid molecule is bonded to the
alcohol radical used in the
reaction (Kucek et
al. 2007, Talebian-Kiakalaieh et al. 2013).
Thus, several molecular vibrational modes belonging to the oils are also found in biodiesel, together with those due to the alcohol radical used. This
can be observed in Figure 1, where both
the absorption bands 1 and 4 (Table
2) correspond to the C-O-C functional
group that is present in the triglyceride and
biodiesel. This functional
group in oil comes from the
part of the molecule that has the carbonyl bonded to CH,
while in biodiesel it corresponds
to the fragment of the molecule that has the carbonyl attached
to the alcohol radical used in the reaction.
Peak
Absorption
peak 5 (Fig. 1)
represents the C-O bond, and was present
only in the SO sample. Band 6 represents the C-C-O bonds,
which shift in frequency from 1161 to 1245 cm-1 between
the SO and EB spectra; this band arises from fatty acids that
are present in both triglyceride and biodiesel. So, in the EB spectrum this absorption band can be an
indication of non-transesterified oil, which is an
impurity in the biofuel. The shift in frequency occurs due to the asymmetry in the
chemical chains and also due to the difference in the size of the oil and
biodiesel chains. The C-O bond, responsible for
peak
Figure
2. Molecular structure of triglyceride
(a) ethylic ester (b) and methylic ester (c).
In
order to analyze the oil-biodiesel blends by infrared spectroscopy, we plotted the difference
between the biodiesel+oil spectrum and the biodiesel
spectrum. Figure 3 (a), (b) and (c) shows the curves obtained for the methylic, ethylic, and commercial biodiesel samples. In all
samples, the same soybean oil was used to prepare the blend. From the observed difference, it is possible to
distinguish the presence of oil in biodiesel, as noted in the region between
1050 and 1350 cm-1. Comparing the curves in Figure 3 shows that by adding oil to
the biodiesel, the absorption at 1143 cm-1 due to the functional
group C-O in the oil decreases, and the absorption around 1191-1196 cm-1
due to the vibration of C-C-O in biodiesel increases. This results from an
apparent shift in frequency of the absorption peak in this region, which is due
to the asymmetry of the molecule.
(a) |
(b) |
(c)
Figure 3. Absorption subtraction spectra between
the biodiesel (B) and the blends SOMB
(a), SOEB (b) and SOB (c).
In order to more easily visualize the results,
a new plot of the relationship between
the absorption intensities of
greatest differences between the
blends SOEB and SOMB was
constructed, as shown in Figure 4. The
parameter was determined for each sample, in which I1096 corresponds to the absorption intensity at 1096 cm-1,
and Iy represents the intensity of the absorption at y = 1017 cm-1 for MB and y = 1035 cm-1 for EB. Thus, the values of d indicate the
relative change in the absorption
intensity of the blend in relation
to the absorption intensity of soybean oil (impurity) at 1096 cm-1. Analysis of Fig. 4 indicates that for
the SOMB blends, the d values
increased ~ 55% when only 5% soybean oil was added to the methylic
biodiesel. In contrast, for
blends made by the ethylic route, the d values
decreased ~ 31% when 5% oil was added to the EB. These results indicate that
the relationship used for these analyses is useful to indicate the presence of
soybean oil as an impurity in biodiesel, independently of the route used to
prepare the biofuel.
Figure 4., with I1096 corresponding to the absorption intensity of soybean oil and biodiesel, and Iy representing the absorption
intensity in y = 1017 cm-1
for MB and y = 1035 cm-1
for EB.
Our
results indicate that the mid-infrared spectra, specifically the vibrational
modes C-O, C-O-C and C-C-O
can provide useful information about the presence of triglycerides in
biodiesel. With this method of analysis, it was also possible to assess, through the absorption
intensities in the fingerprint region of the spectra, the route used to produce the biofuel. This study reinforces the usefulness of this method for identifying markers for
impurities that may be present in blends
of oil:biodiesel.
ACKNOWLEDGMENTS
The authors thank Fundect-MS,
CAPES, FINEP, and CNPq for partial financial support,
and the Applied Optics
Group (GOA) of the Universidade Federal of Grande Dourados (UFGD) for
providing the commercial biodiesel sample.
REFERENCES
Aliske, M. A.; Zagonel, G. F.; Costa, B. J.;
Veiga, W.; Saul, C. K. (2007) Measurement of
biodiesel concentration in a diesel oil
mixture. Fuel, v. 86, p. 1461-1464.
Dubé,
M. A.; Zheng, S.; Mclean, D. D.; Kates, M. (2004) A comparison of
attenuated total reflectance-FTIR spectroscopy and GPC for monitoring biodiesel
production. J. Am. Oil Chem. Soc.,
v. 81, p. 599–603.
Jiang,
S. T.; Zhang, F. J.; Pan, L. J. (2010) Sodium phosphate as a solid catalyst for biodiesel preparation. Brazilian Journal of Chemical Engineering,
v. 27, p. 137-144.
Knothe, G. (2010) Biodiesel and renewable diesel: A comparison. Progress in Energy and Combustion Science, v. 36, p. 364-373.
Knothe,
G.; Van Gerpen, J.; Krahl, J. (2005) The biodiesel
handbook, AOCS Press: Illinois.
Kucek,
K. T.; Oliveira, M. A. F. C.; Wilhelm, H. M.; Ramos, L. P. (2007) Ethanolysis of refined soybean oil assisted by sodium
and potassium hydroxides. Journal of American Oil Chemists’ Society,
v. 84, p. 385-392.
Lira, L. F. B.; Albuquerque, M. S.; Pacheco,
J. G. A.; Fonseca, T. M.; Cavalcanti, E. H. S.; Stragevitch, L.;
Pimentel, M. F. (2010) Infrared
spectroscopy and multivariate calibration to monitor stability quality
parameters of biodiesel. Microchemical Journal, v. 96, p. 126-131.
Meher L. C.; Sagar D. V.; Naik S. N. (2006) Technical
aspects of biodiesel production by
transesterification - a review. Renewable
and Sustainable Energy Reviewer, v. 10, p. 248-268.
Monteiro,
M. R.; Ambrozin, A. R. P.; Lião, L. M.; Ferreira, A. G. (2008) Critical review
on analytical methods for biodiesel characterization. Talanta, v. 77, p. 593.
Pimentel, M. F.; Ribeiro, G. M. G. S.; Cruz, R. S.;
Stragevitch, L.; Pacheco-Filho, J. G. A.; Texeira, L. S. G. (2006) Determination
of biodiesel content when blended with
mineral diesel fuel using infrared
spectroscopy and multivariate calibration. Microchemical Journal,
v. 82, p. 201-206.
Resolution
from the Agência Nacional
do Petróleo, Gás Natural e Biocombustíveis (ANP), Nº 4 from
02.02.2010. See: <http://nxt.anp.gov.br/nxt/gateway.dll/leg/resolucoes_anp/2010/fevereiro/ranp%204%20-%202010.xml?f=templates$fn=document-frame.htm$3.0$q=$x>.
Richard, R.; Li, Y.;
Dubreuil, B.; Thiebaud-Roux, S.; Prat, L. (2011) On-line monitoring of the transesterification
reaction between triglycerides and ethanol using near spectroscopy combined
with gas chromatography. Bioresource
Technology, v. 102, p. 6702- 6709.
Silva, G. A. M.; Da Rós, P. C. M.; Souza, L.
T. A.; Costa, A. P. O.; Castro, H. F. (2012)
Physico-chemical, spectroscopical
and thermal characterization of
biodiesel obtained by enzymatic route as
a tool to select the most efficient emmobilized lípase. Brazilian Journal of Chemical Engineering,
v. 29, p. 39-47.
SILVA, T. A.
R.; Santos, D. Q.; Lima, A. P.; Borges
Neto, W. (2013) Model for
the correction of the specific gravity of biodiesel from residual oil. Independent
Journal of Management & Production, v. 4, p. 136-147.
Smith, B. Infrared Spectral Interpretation: A Systematic Approach. New York:
CRC Press, 1999. 200p.
Soares,
I. P.; Rezende, T. F.; Silva, R. C.; Castro, E. V. R.; Fortes, I. C. P. (2008) Multivariate calibration by variable
selection for blends of raw soybean oil/biodiesel from different sources using fourier transform infrared spectroscopy (FTIR) spectra
data. Energy & Fuels, v. 22(3),
p. 2079-2083.
Soares, I. P.; Rezende, T.
F.; Pereira, R. C C.; Santos, C. G.; Fortes, I. C. P. (2011) Determination of biodiesel adulteration with raw vegetable oil from
ATR-FTIR data using chemometric tools. J.
Braz. Chem. Soc., v. 22,
p. 1229-1235.
Talebian-Kiakalaieh, A.; Amin, N. A. S.; Mazaheri, H. (2013) A review on novel processes of biodiesel production from waste
cooking oil. Applied Energy, v.104,
p.683–710.
Varun, N. K.; Chauhan, S. R. (2013) Performance and
emission characteristics of biodiesel from different origins: A review. Renewable and Sustainable Energy Reviews,
v. 21, p. 633–658.