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Monitoring of Biodiesel Quality Parameters by 1HNMR Spectroscopic Techniques

October 01, 2013
A.S. Sarpal, Paulo R Silva, Valnei S. Cunha, and Romeu J. Daroda
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Biodiesel, methyl or ethyl fatty esteris produced by transesterification of fatty oil (triacyl glycerol, TAG) present in the feed stocks of vegetable oil, algae and yeast biomass. The quality of biodiesel B100 is determined as specified in standards ASTM 6751 in USA and EN14214 in Europe. These specifications are intended to produce biodiesel suitable for blending with conventional diesel(B5, B20) to ensure that the product meets the technical requirements of modern diesel engines. The most important specified quality parameters are ester content(96.5%), fatty acid profile, linoleicacid methyl ester(C18:3)(12%w/w), PUFA (polyunsaturated fatty acids) content(more than or =4 double bonds, 1%), iodine value(Iv)(120g/100g), residual mono, di and triglycerides content(total glycerine0.25%) and methanol content(0.2%). The biodiesel quality is dependent on the fatty acid profile of oil constituting the feed stocks used for production of biodiesel. The pour point and cetane number are also important characteristic dependent upon the distribution of saturated and unsaturated fatty esters. Thus quality of both feed stock and product biodiesel needs to be monitord in order to produce biodiesel meeting specification laid down by a country. The limit of quality parameters has been fixed in order to provide better ignition properties and  oxidation stability including the long term stability. The oxidation stability is mostly determined by the Iodine value; which in term depends on the distribution of unsaturated fatty esters of starting material and final product. For instance, the higher iodine value of 120-130g/100g of Soya been and Sun flower biodieel is due to the higher content of C18:N(N=2-3) compared to Jatropha, Gerelium, Corn oil(<120g/100g)etc., although PUFA content is within the limit of specification. The quality of biodiesel from algae is dependent upon the strains used for cultivation as certain strains generate much higher content of C18:3, C20:5, C22:6 esters. The content of residual TAG, which is dependent upon the process, catalyst and reaction parameters of transesterification, is detrimental to the health of the engine. Thus each compositional parameter is significant and need to be controlled in order to produce biodiesel for blending in the conventional diesel. The fatty acid profile, C18:3, PUFA and methanol content are determined by the standard methods based on chromatographic techniques and iodine value by potentiometric methods as mentioned in the specification. Non standard methods but accurate and reliable based on NMR, IR, GC and GC-MS techniques are routinely used in oil industry to monitor the quality of biodiesel.

Nuclear magnetic Resonance techniques (NMR) are extensively used in the oil and fat industry to monitor the transesterification process and quality of biodiesel. Methods based on NMR are quantitative, direct, rapid and convenient; require no standards and pre chemical treatment compared to other methods. Due to multinuclear capability (1H, 13C, 15N, 31P) and ease of 2DNMR and spectral editing analyses, NMR techniques have proved rapid and reliable aid to the bio refinery for quality control of biodiesel. In continuation of our passion for innovation for characterization and development of new methods for determination of parameters related to many quality aspects of biodiesel, it has been demonstrated that most of the specified compositional parameters can be determined with ease and  precisely from a single 1HNMR spectrum of a biodiesel or oil. A reliable and rapid analytical protocol based on 1HNMR spectral analyses on 500 and 600 MHz instruments has been established and validated for the determination of ester content, unsaturated fatty acid profile including omega N-3 and N-6(C18:N=1-3, C20:5, C22:6), saturated(SF) and unsaturated(UF) ester content, PUFE (polyunsaturated fatty esters), free fatty acid(FFA), molecular weight(MWT) and iodine value(Iv) of feed  and product biodiesel. The total TAG or glycerin content can also be estimated. The methods are based on derivation of mathematical equations, which require the percentage integral area of a particular parameter in an assigned chemical shift region. The signals or resonances corresponding to ester protons (OCH3, OCH2, CH2CO), unsaturated protons (CH=CH), saturated and unsaturated fatty ester (C18:N=0-3), terminal CH3 and CH=CH-CH2-CH=CH- protons of omega (N-3) and omega(N-6) PUFE have been  unambiguously assigned in the 1HNMR spectra of biodiesel samples. Since CH2CO signals due to ester and FFA are distinguishable with a little overlapping in the chemical shift regions of 2.32-2.33ppm and 2.35 ppm respectively, their content can be estimated. Since, ester signals due to biodiesel (3.67pp) and oil(4,05-4.38ppm) are distinguishable, the conversion process can be monitored.  The detailed spectral analyses has enabled to estimate compositional parameters of a feed stock and biodiesel.

Ester Content

The approach is based on two methods mainly related to (a)determination of relative group molecular weights and (b)calculation of proportionality constant as described in the following equation:

Where IA is the percentage integral area of chemical shift region of OCH3 group (3.67ppm) of biodiesel or OCH2 group(sn1,sn3)(4.05-4.38ppm) of oil  and K is a constant independent of the nature, source and composition of biodiesel or oil.

The value of K for biodiesel has been estimated from the individual spectra of variety of biodiesel; blends of biodiesel with varying proportion of biodiesel of Soya, rape seed, sunflower, jatropha, cotton, gereleum, corn, kartuma, castor, canola and palm; blends of biodiesel and oils; NIST 2772 &  INMETRO biodiesel standards and  blends of standards of unsaturated fatty including PUFE. Similar approach was adopted for oils of different types and sources. The constant Ks are independent of the nature and source of biodiesel and equally applicable to blends of different biodiesel and blends of biodiesel and oil. The excellent correlation between the results obtained by equation 1 and specified standard methods based on GC and GC-MS techniques has been obtained for ester content of oil and biodiesel.The method is applicable to biodiesel with iodine value 10 to 140. However, a small variation in the results has been obtained for biodiesel of palm, castor, coconut oil etc, which have iodine value in the range of 10 to 25 and molecular weight in the range of 250-260. The second method based on estimation of group molecular weight is recommended for biodiesel having low molecular weight and iodine value as given in the equation 2.

The ester content of oil i.e. fat and biodiesel can also be estimated by the following equation which is based on the estimation of relative group molecular weight (GMWT):

Where IA is the integral area of one proton of the OCH3 or OCH2group at 3.67ppm(biodiesel) or 4.0-4.38ppm(oil), MWT is the average molecular weight 293 for biodiesel or 874 for oil estimated from the 1HNMR spectra of different biodiesel or oils and their blends, and GMWT is the relative group molecular weight  calculated from the integral intensity of different chemical shift regions corresponding to various functional groups in the NMR spectra. The average molecular weight by NMR has been validated from the GC-MS fatty acid profile. Results of ester content and molecular weight by the equations 1&2 have been found to be accurate and obtained  excellent correlation with the specified standard methods.

Iodine Value

As discussed, Iodine value is an important parameter for quality control of biodiesel and its feed stock as it is related to the oxidation stability. It has been recommended to be estimated by the potentiometric method EN14111. The method is tedious, time consuming, require health hazardous chemicals and depends upon number of parameters such as black room residence time, purity and standardization of chemicals in order to get accurate values. We have developed a direct, fast and accurate emethod  for the determination of iodine value of oil and biodiesel. The unsaturated proton content of biodiesel or fat has been correlated with the iodine value determined from the unsaturated fatty ester profile by GC-MS.  Iodine value(Iv) of biodiesel and fat can be estimated  from the following equation 3:

Where IUSis the integral area percentage of unsaturated protons between 5-6 ppm in the IHNMR spectra of biodiesel or fat.

The value of constant ´K` has been found to be 15.6. The constant K is independent of the nature and source of both oil and biodiesel. The method has been applied to all types of edible and non edible oil and biodiesel including non edible oil national resources of Brazil (radish, castor, palm kern etc) with iodine value in the range of 10 to 140 and excellent correlation with other methods have been obtained. The method has also been found to be quite accurate for fish oil and its products with iodine value of 220 or more. Since blends of biodiesel such as soya or sun flower with other biodiesel with low iodine value are used in order to meet the limit of iodine value of 120, the NMR method has given accurate results with better precision for  blends. The method can also be applied to solvent extracts of algae and yeast biomass. In a similar way the equations for unsaturated FAME profile have been developed and validated by GC and GC-MS methods.

A.S. Sarpal is with the Instituto Nacional de Metrologia, Qualidade e Tecnologia– INMETRO, located in Brazil, and can be reached at Sarpal.as2@gmail.com.

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