Suchith Chellappan ,Vaishakh Nair ,Sajith V.,Aparna K.,*

1 Department of Chemical Engineering,National Institute of Technology,Calicut,India

2 Institute of Physical Chemistry,Polish Academy of Sciences,Kasprzaka 44/52,01-224,Warsaw,Poland

3 School of Nano Science and Technology,National Institute of Technology,Calicut,India

Keywords:Biochar Biochar based catalyst Pyrolysis Biomass Sulfonation

A B S T R A C T The development of a benign environmental catalyst for the generation of biodiesel is an area of importance to reduce the overall usage of fossil fuels.In the current work,biochar was produced by slow pyrolysis of Irul wood sawdust.The optimization for biochar generation was carried out for different reaction temperatures and heating durations.The biochar was used to prepare solid acid catalysts by sulfonation process.The characterization of biochar and the sulfonated catalyst were carried out using Elemental analysis,Fourier Transform Infrared spectroscopy(FTIR),Powder X-ray Diffraction(XRD),Thermo Gravimetric Analysis(TGA),Scanning Electron Microscopy(SEM)and Surface area analyzer(BET).The characterization results showed that sulfonation of biochar resulted in biochar based solid acid catalyst containing various functional acidic groups like weak acidic--OHgroups,strong acidic--COOHand SO3Hgroups.The totalacid density and sulfonic acid group density of catalyst were estimated and showed excellent acidic sites concentration which gives a good catalytic activity for biodiesel production through simultaneous esterification and transesterification.The enhanced catalytic activity is due to the high acid density of SO3H groups and the reactant accessibility towards acidic sites as well as the strong affinity between the hydrophilic reactants and the neutral OH groups which are bonded with the polycyclic aromatic carbon rings.The performance of biochar catalyst for the production of biodiesel was evaluated by comparing the yield obtained.The FTIR and Gas Chromatography–Mass Spectroscopy(GC–MS)were also carried out for the analysis of biodiesel produced.

1.Introduction

Sustainable use of natural resources in the present technologically developing era goes hand in hand with the issues like the cleaner environment,cultivation of feed stocks for oil generation,etc.[1].The steep hikes in the petroleum prices due to an imbalance in demand for the petroleum based products have compelled the scientific society to think of alternative renewable fuels[2].Biodiesel is an alternative fuel which can replace diesel,and biodiesel blends have similar properties as that of the conventional diesel.Biodiesel is a mixture of fatty acid methyl esters(FAME)produced from oil feedstocks such as vegetable oil,non-edible oil,waste cooking oil,animal oil/fats,tallow and it can be used in diesel engines with out any furthermodifications[3].There are primarily four methods for the synthesis of biodiesel such as direct use and blending of vegetable oils,microemulsions,thermal cracking and transesterification[4].Among all the above mentioned methods,the most common method used for the synthesis of biodiesel is transesterification.Transesterification is the reaction of oil or fat with alcohol to form esters or glycerol in the presence of a catalyst.Typical refined edible oils such as soybean oil,rapeseed oil,and palm oil,are not considered as raw materials,particularly in developing countries due to lessproduction and high cost[5].Hence nonedible oilfromvarious plants,animalfats,and waste cooking oilare used.The transesterification process can be catalyzed by different ways via the homogenous base,homogenous acid,heterogeneous base,heterogeneous acid,enzyme,ionic liquids,carbon based catalyst,etc.[6–9].

The feedstock oils usually contain high free fatty acid(FFA)content in addition to the Triglyceride(TG).A high percentage of FFA along with transesterification results in unwanted soap formation which requires expensive separation techniques.Therefore,biodiesel production from the oil feedstock would take place in a two-step process.Conversion of the FFA to corresponding esters occurs in the first step(esterification),and the conversion of the remaining TG to alkyl esters,i.e.,biodiesel forms in the second step(transesterification).Different catalysts are required separately foresterification and transesterification.The wide acceptability of homogeneous base catalyst is mainly due to their fast reaction rates.The process has many limitations;a lotofenergy is required for the product purification and catalyst separation,and moreover,these catalysts are not reusable,leading to significant energy wastage and the generation of chemical waste[10].

Homogenous acids such as H2SO4,HCl have shown high catalytic performance,but their industrial applicability is limited as the purification of the biodiesel poses a serious concern.These homogeneous acid catalysts react with the reactor and corrode,resulting in the degradation of the setup[6].Different heterogeneous base and acid catalysts have been utilized for transesterification to overcome these issues.Heterogeneous catalysts can be separated readily from the products at the end of the reaction and also can be reused for the next reaction cycle.Different types of heterogeneous base catalysts reported are alkali and alkaline earth oxides[11,12],mixed metal oxides[13],supported alkali metalcatalysts[14].Some of the heterogeneous acid catalysts reported so far are Amberlyst-15[15],solid acid inorganic–oxide[16],heteropoly acid(HPA)[17],ion exchange resin[18],WO3/ZrO2[19],zeolites and dolomites perovskite-type catalysts[20,21],hydrotalcite[22],supported noble metal oxide[16],H3PW12O40·6H2O[23]and so on.These heterogeneous catalysts are commonly hydrophilic,and it also reacts with water produced from FFA esterification causing a decrease in their catalytic activity.Moreover,their effective acid site density is low with microporous structure,low stability,and high cost which in fact limited their practical use in biodiesel production.

Enzyme based catalysts such as lipase have also been used,but the viability of the process is not economical[24].Ionic liquids used as catalysts in transesterification have complex and high cost methods of synthesis.Above all the catalyst reusability is poor and also nonbiodegradable[25].

These days many researchers are working on “green catalyst”which are cheap and reusable.The green catalysts are prepared from biomass and solid wastes.Recently,a new class of catalysts derived from incomplete carbonization of natural products(biochar)has been reported to have the better catalytic performance for esterification of FFA,and higher stability than other solid acid catalysts.The biochar based solid acid catalystis the latest and most promising catalystdue to its environmentally safe nature.In the presence of a solid acid heterogeneous catalyst,the two-step biodiesel production can be carried out in single step while reducing further washing,neutralization,and loss of catalysts[26].Biochar is the charring organic matter,produced through the process of carbonization.It is the residual of the fast pyrolysis from the woody mass and also the main product of slow pyrolysis.Biochar material has abundant oxygen content(27 wt%–34 wt%)mostly in the form of phenolic and carboxylic acidic groups.The phenolic,carboxylic and sulfonic group helps in improving the catalytic activity and also improves the adsorption of the molecule on the surface of catalyst[27].Zeng et al.synthesized a strong Bronsted solid acid from partially carbonized agricultural bio-waste peanut shell by sulfonation.The highest conversion of 90.2%was obtained for a methanol/oil molar ratio of 9:1 with a catalyst concentration of 2 mass percent.The optimum oil conversion was obtained at a temperature of 85°C and reaction time of 2 h[28].Ming Li et al.prepared solid acid catalyst by sulfonating pyrolyzed rice husk with concentrated sulphuric acid.The free fatty acid(FFA)conversion reached 98.17%after 3 h,and the fatty acid methyl esters(FAME)yield reached 87.57%after 15 h[29].

In the present study,optimization of Irul wood sawdust(Xylia xylocarpa,a common tree found in south west India)biochar generation based on the pyrolysis temperature and duration of heating were carried out.The generated biochar was further used to develop solid acid catalysts by sulfonation for its application for biodiesel production.The biochar was sulfonated by treating with concentrated sulphuric acid,and this catalyst was used for simultaneous esterification and transesterification of pongamia pinnatta oil.

2.Materials and Methods

2.1.Preparation and analysis of raw materials

Irul wood sawdust(WSD)was collected from a wooden sawmill factory.Biochar was generated via slow pyrolysis of sawdust.Before slow pyrolysis,biomass(WSD)size reduction and sieving were done to get a particle size of range less than 75 μm.The fine particles of size less than 75 μm were then dried in air dry oven at 80 °C for 24 h to remove moisture content.After drying the biomass was stored in a desiccator.Pongamia pinnatta oil(PPO)was used as base oil for transesterification process,in the present work.The average molecular weight of oil was calculated from acid value and saponification value[30].The major composition of fatty acid was obtained from chromatograph of pongamia pinnatta oil as Palmitic acid(C16:0):13.64 wt%,Oleic acid(C18:1):55.74 wt%,Arachidic acid(C20:0):4.4 wt%,Gadoleic acid(C21:1):3.52 wt%,Behenic acid(C22:0):9.81 wt%,Lignoceric acid(C24:0):4.03 wt%.All the reagents like Methanol(HPLC grade),Sulphuric acid(95%–98%)used were bought from Merck India Ltd.Mumbai and used as received.

2.2.Biochar production

The sawdust biochar was produced using a slow pyrolysis setup.WSD was placed in a stainless steel batch reactor(height of 11 cm and inner diameter 6.5 cm).A hole of diameter 0.05 cm was made in the middle of the reactor to release the excess pressure.The reactor was tightly closed using a metal lid,which is connected to a nitrogen cylinder for continuous purging of nitrogen gas.The reactor was kept inside a muffle furnace to heat externally,and the corresponding temperature was noted down using a digital temperature controller.

2.3.Effects of reaction parameters for biochar production

The functional properties of biochar depend highly on the production condition and feedstock.A detailed study of the physicochemical properties of biochar produced is necessary,to obtain a higher yield of biochar and also to understand the effect of its pH for catalyst preparation.Hence the effect of reaction temperature and the heating time interval of pyrolysis was studied by carrying out the pyrolysis at a heating rate of 10 °C·s-1and 1 h of vapor residence time for different reaction temperatures and heating durations.The reactor was allowed to cool for approximately 2 h,on reaching the residence time of slow pyrolysis experiment.The biochar thus obtained was stored in a desiccator to avoid moisture absorption.

The mass percentage of biochar yield was calculated using the following equation:

For high biochar yield,the biomass was pyrolyzed at lignin degradation temperature which was obtained from the TGAplots.The pyrolysis of biomass was done at two reaction temperatures(400 °C and 600 °C)with various heating durations of 1,2 and 4 h.The pH of the biochar generated at different reaction temperatures and heating durations was determined by mixing it with deionized water in the ratio 1:10,in a beaker[31].The mixture was continuously stirred using a magnetic stirrer till equilibrium is obtained.The pH was measured using a pH meter at intervals of 2 h till concurrent values were attained.The physicochemical properties of WSD biochars obtained at different pyrolysis temperatures with various heating durations were measured and also characterized by FT-IR and TGA.From these studies,the suitable biochar prepared at optimal parametric condition was labelled as BC-WSD and selected for catalyst preparation.

2.4.Catalyst preparation and characterization

Catalyst functionalization was done to obtain a solid acid catalyst by introducing SO3H groups into polycyclic aromatic sheets of biochar via sulfonation.Biochars as such synthesized were manually grounded using a mortar and pestle.10 g of the sample was sulfonated using 100 ml of concentrated sulphuric acid(95%–98%)for1 h in a 250 mlconical flask,heated at 90°C in a water bath.The mixture was cooled to ambient temperature,and 100 ml distilled water was added to it.The mixture was further stirred in a shaker for 50 °C at 120 r·min-1for 24 h,followed by filtration.The precipitate was then thoroughly washed with hot deionized water until the sample was free from sulfate ions.The sample was dried at 80°C for 24 h in an air-drying oven after filtration.

The properties ofBC-WSDand sulfonated biocharcatalyst(BC-WSDSO3H)were studied by Elemental analysis,FT-IR,and XRD.The elemental analysis was done using an elemental analyzer(Elementar Vario EL III)which gives the percentage compositions ofcarbon,hydrogen,nitrogen,sulfur and correspondingly the oxygen percentage composition was calculated.The surface morphology of materials was studied by Scanning Electron Microscope(Make:HITACHI SU6600)with an acceleration voltage of 10 kV,and magnification at 20.0 μm to get a vivid view of pore diameter and density.Thermogravimetric analysis(Make:HITACHI STA 7200)was done to determine degradation temperatures and thermal stability of materials.The surface area of the biochar catalyst before and after sulfonation was determined by N2adsorption/desorption experiments performed using porosimeter(Make:Micromeritics Asap 2020).The samples were first degassed at 150°C for 12 h.X-ray diffraction(XRD)analysis(Make:Rigaku Mini flex 600)was done to identify the crystallographic structure of carbon powder samples.FTIR(Make:Agilent Technologies FTIR-4700)was done to determine the possible functional groups present in the sample.The total acid density of catalyst(BC-WSD-SO3H)was estimated by back titration method.1 g of the sample was taken in 50 ml solution of 0.05 mol·L-1NaOH.The sample mixture was stirred for one day,and then 5 ml of filtrate was pipetted and titrated against 0.1 mol·L-1HCl.The total acidic density of catalyst was calculated from the amount of NaOH solution used.

The sulfonic acid group density was calculated by back titration method.0.1 g of catalyst was mixed with 30 ml of 0.6 mol·L-1NaCl solution for 2 h for facilitating the exchange of Na+and H+in the form of--SO3H.The filtrate from this mixture was titrated against 0.1 mol·L-1NaOH with methyl orange as an indicator to get a color change from pale red to bright yellow.A blank titration was also performed to determine the unconsumed initial volume of NaOH solution and further--SO3H density was calculated.

2.5.Transesterification reaction and biodiesel analysis

The transesterification reaction was done in a three-necked glass reactor(batch reactor).A condenser was fitted to one of the necks of the reactor to condense the methanol vapors by circulating chilled water continuously.All the reactor joints were sealed using Teflon tapes to prevent the leakage of methanol vapors to the atmosphere.The reactor consists of an electrical motor driven stirrer in the center to agitate the reaction mixture,and a thermometer was placed in one of the necks.A constant temperature bath was used to maintain the reaction temperature,and bath temperature was measured using a thermometer.

The biodiesel was synthesized from Pongamia pinnatta oil by transesterification using BC-WSD-SO3H as a catalyst,by taking various parametric conditions.A mixture of oil,methanol,and catalyst was taken in the reactor.The optimum parameters adopted for the synthesis of biodiesel in the present work are(1)oil to methanol ratio:9:1,(2)catalyst amount:2 wt%,(3)reaction temperature:85°C and(4)reaction time:2 h,as reported elsewhere[28].Once the reaction was completed,the reaction mixture was transferred to a separating funnel and allowed to settle for 24 h.The biodiesel was separated from the top layer of residue based on the density difference and was further filtered.The biodiesel was water washed with deionized to water to remove unreacted methanol.

The FFA conversion percentage was calculated from the acid value of initial raw oil and an acid value of final biodiesel product.

The acid value was determined(in triplicates)by titration method using standard KOH solution and was calculated using the equation:

where N represents Normality of KOH;V represents the volume of KOH(titre value);W represents the mass of oil taken(g),the equivalent mass of KOH is 56.1.

The percentage of methyl esters was determined by gas chromatography GC–MS system(JEOL GC MATE II),equipped with a capillary column.The carrier gas used was helium(99.99%purity)with a constant flow rate of 1 ml·min-1.The initial column oven temperature was maintained at 50°C for one minute,which was then heated up to 250 °C at a rate of 10 °C·min-1,and the final temperature was maintained at 250 °C for 15 min.1 μl of the sample was injected into GC column,and the scanning range was varied from 50 to 600 g·mol-1.When the FAME of the biodiesel sample were notified,the areas of the peaks were determined to find out the FAME content of the samples.

The FAME was calculated using the following equations:

As per the mentioned equation,∑A denotes the total peaks area,ASIdenotes peak area of methyl heptadeconoate(internal standard),CSIis the concentration of methyl heptadeconoate(mg·ml-1),VSIis the volume of internal standard solution used(ml)and w is the mass of biodiesel sample(mg).

where WBiodieselis the mass of raw biodiesel is produced and WRawoilis the mass of raw oil used.The comparison of FTIR spectra of raw oil and biodiesel was done to check the conversion of oil to methyl esters by transesterification.

3.Results and Discussion

3.1.Biomass analysis

The physicochemical properties of Irul wood sawdust(WSD)were estimated by proximate analysis and elemental analysis and also determining the heating value which isshown in Table 1.From the proximate analysis,it was observed that WSD feedstock contains volatile matter of 71.1 wt%,ash content of 1.4 wt%,and fixed carbon content of 20.8 wt%.Elemental analysis of WSD showed less than 1%of nitrogen and sulfur.The above analysis shows that WSD is an environmentally friendly feedstock suitable for slow pyrolysis.

3.2.Effects of reaction parameters for biochar production and its characterization

Biochar was produced by slow pyrolysis of biomass feedstock(WSD).The effect of reaction parameters such as temperature and heating duration of pyrolysis on the yield of biochar was studied(Table 2).The acidic nature of biochar generated was evaluated bymeasuring the pH.From the Table 2 and Fig.1 it was observed that the biochar yield at 400 °C is 39.6%and that at 600 °C is 27.45%for 1 h heating duration.The decrease in the biochar yield with an increase in pyrolysis temperature is due to the thermal cracking of volatile components into lower molecular weight liquids and gases rather than to biochar.The pH value at 400 °C is 4.45 and that at 600 °C is 9.11 for the 1 h heating duration(Fig.2).At high pyrolysis temperature,the increase in pH is mainly due to the decrease of organic functional groups such as--COOH and--OH.At 400°C,it was observed that with an increase in heating duration the biochar yield decreased while the pH is increased.At 600°C,the temperature effect dominated the heating duration and showed only marginal changes in the biochar yield and pH.

Table 1Physicochemical properties of WSD

Table 2Physicochemical properties of biochars with different pyrolysis temperature and heating duration

Fig.1.Effect of biochar yield with different pyrolysis temperature and heating duration.

Fig.2.Effect of pH of biochar with different pyrolysis temperature and heating duration.

Similarly,from Table 2,it was observed that the fixed carbon content of biochar increased with increase in temperature.The low percentage of hydrogen and oxygen can be attributed to the cleavage of weak bonds within the feedstock structure.Nitrogen is resistant to heating and is not volatilized easily,while sulfur content decomposes at higher temperature[31].Fig.3 shows the FT-IR spectra of biochars pyrolyzed at different temperatures and heating time.The FT-IR spectrum of WSD shows the presence of a peak at 1604 cm-1which are attributed to aromatic compounds present in the lignin structure.The absence of peaks at 1030 cm-1shows that the presence of cellulose is very low.FT-IR spectra of biochar synthesized at 400°C at different heating durations showed a significant decrease in the intensity of the peak at 1604 cm-1.The FT-IR spectra of biochar generated at 600°C and 400°C for 1 h heating duration showed a significant decrease in the peak of lignin as compared to the FT-IR spectrum generated for other heating durations.There was no significant change in the FT-IR spectra of biochargenerated at600°C at higher heating duration[32].TGA plots for different biochars are shown in Fig.4.The TGA plot of biochar synthesized at 400°C for 1 h and 2 h heating duration showed significant mass loss when compared to samples,which may be due to the presence of high amount of volatile compounds.It was observed that the mass loss is less at higher temperatures and higher heating time due to the decomposition of the volatile matters by pyrolysis.For the preparation of the highly acidic sulfonated catalyst,the pH of the biochar is expected to be more acidic so that more--SO3H ions will get attached on sulfonation.From the Figs.1 and 2,it was observed that biochar with high yield and more acidic nature is generated at a pyrolysis temperature of 400°C with a heating duration of 1 h.Hence for the preparation of the acid catalyst,suitable temperature and time for pyrolysis is 400°C and 1 h respectively and has been termed as BC-WSD,in further discussions.

Fig.3.FT-IR of biochars with different temperatures and heating duration.

Fig.4.TGA curves of biochars with different temperatures and heating duration.

3.3.Characterization of catalyst

3.3.1.Thermo gravimetric analysis

Fig.5.(a)TGA and(b)DTG curves of WSD,BC-WSD and BC-WSD-SO3H.

Thermo Gravimetric Analysis(TGA)was used to determine the temperature range of pyrolysis.The comparison of TGA and DTG curves of biomass(WSD),biochar(BC-WSD)and biochar based catalyst(BC-WSD-SO3H)shows the evident difference.The analysis of thermal stability of WSD as a function of temperature shows a reduction in mass due to the release of volatile matters.Fig.5 depicts the TGA curve of the wooden sawdust.Below 200°C,the mass reduction of WSD is primarily due to the evaporation of the moisture content and volatile matter from the solid structure.In the temperature range,200 °C to 400 °C rapid decrease in mass loss was observed,which is mainly due to the decomposition of cellulose and hemicellulose.Above 400°C,the reduction of wooden sawdust is attributed to the decomposition of lignin[33–35].The TGA curve of BC-WSD shows that it is thermally stable.The TGA curve of the catalyst(BC-WSD-SO3H)shows a significant mass loss until 300°C,which can be attributed to the formation of volatile matter during the sulphonation of BC-WSD.The mass loss around a temperature of 300°C can be attributed to the loss of--SO3H groups.The catalyst was observed to be slightly stable till 400°C[5,36,37].The presence of a new peak in the DTG curve of the catalyst shows the effect of sulfonation during the activation of biochar.

3.3.2.Elemental analysis

The comparison of the elemental composition of WSD,BC-WSD,and BC-WSD-SO3H are shown in Table 3.It is evidently seen that sulfur content and oxygen content percentage increases in case of BC-WSD-SO3H as compared to WSD and BC-WSD.This also shows that the catalyst has a good amount of active sites which is further confirmed in terms of total acid density and sulfonic group density as shown in Table 5.

Table 3Elemental composition of WSD,BC-WSD and BC-WSD-SO3H

3.3.3.SEM and BET analyses

Fig.6.SEM Images of BC-WSD and BC-WSD-SO3H.

The surface morphology of BC-WSD and BC-WSD-SO3H revealed by SEM images is shown in Fig.6.The heterogeneous surfaces of both materials show the presence of cracks,crevices with irregular shapes and rough edges.The SEM images of BC-WSD shows the presence of large particles whereas that of BC-WSD-SO3H shows smaller size particles.The significant decrease in the particle size is due to the breakdown of different organic compounds during the sulfonation[5,36,38].The BET analysis(Table 4)showed that biochar catalyst has a higher surface areaas compared to biochar,which is confirmed by the SEM images.The catalyst shows unique properties with surface area 3.30 m2·g-1and average pore diameter of 101.02 nm.

Table 4Surface area and porosity of biochar catalyst

3.3.4.XRD analysis

The XRD analysis of BC-WSD and its biochar based catalyst(BC-WSD-SO3H)was done as shown in Fig.7.The XRD pattern of biochar exhibited one broad(2θ =20°–30°)diffraction peak corresponding to the diffraction of C(002).The peak indicates the presence of amorphous carbon structure having randomly oriented carbon sheets.The narrowing of the C(002)peak points indicates the formation of a more ordered graphite like carbon lattice structure.The presence of open graphite like structure in the catalyst indicates it as the stable state of the carbonaceous material.Upon sulfonation,the peak intensitiesfurther decreased and were found to be the lowest for the catalyst.The reduced peak intensities are due to attachment of abundant--SO3H groups to the sp2carbon network leading to an increased disorder among graphitic carbon sheets[5,28,36,38–40].

Table 5Chemical formula,total acid density and sulfonic acid group density of BC-WSD-SO3H

Table 6GC–MS analysis results of biodiesel produced using BC-WSD-SO3H catalyst

Table 7Fuel properties of oil and biodiesel

Fig.7.XRD patterns of BC-WSD and BC-WSD-SO3H.

3.3.5.FT-IR analysis

FT-IR spectra were employed to characterize the functional groups on the BC-WSD and BC-WSD-SO3H.For a solid acid catalyst to be efficient,the presence ofOH,COOH,and SO3Hacidic sites are essentially required.As shown in Fig.8,two peaks at1036 cm-1and 1170 cm-1in the spectra of BC-WSD-SO3H are attributed to the SO2asymmetric and symmetric stretching modes,respectively.The FT-IR peak at 3352 cm-1corresponds to stretching vibration of hydroxyl(--OH)groups.The band at 1632 cm-1is attributed to the--C--O stretching mode of the COOH groups.From the FT-IR spectra,the presence of functional groups like OH,COOH,and SO3H on the surface of the solid acid catalyst is well established[5,28,36–39].

Fig.8.FTIR Spectrum of BC-WSD and BC-WSD-SO3H.

3.4.Biodiesel analysis

The biodiesel was produced from pongamia pinnatta oil by transesterification using BC-WSD-SO3H as a catalyst by taking various parametric conditions.The optimum parameters considered for the synthesis of biodiesel in the present work was adopted from literature[28]as mentioned earlier.The FT-IR spectra of PPO and biodiesel synthesized from it,shown in Fig.9 was analyzed to verify the ester formation by transesterification.The observed absorption bands are as follows:The peaks observed at 3006.1 cm-1refers to stretching asymmetric and symmetric CH3(--CO--O--CH3)vibrations.The peaks observed at 2920.4 cm-1,2853.3 cm-1,1459.3 cm-1,721.5 cm-1are stretching CH3,stretching CH2,bending(--C--H--)bonding,and rocking CH respectively,whereas that observed at 1239.3–1163 cm-1attributes to the stretching(--C--O--vibration)ester groups.The presence of stretching(C=C)group was observed at 2337.0 cm-1.The peak of(C=O)carbonyl ester group at 1742.5 cm-1was observed.The FTIR of biodiesel was similar that of PPO however with a little move towards higher or lower frequency.Another confirmation on the transformation of the oil to its corresponding ester is that range under the absorption bands of the stretching C=O and C--H band,and also the C--H bonding band of the methyl esters was much lower in the oil as compared to biodiesel.Thus the experimental results show the success of transesterification for the synthesis of PPO based biodiesel[38,41].

Fig.10 shows the GC chromatogram of biodiesel synthesized with BC-WSD-SO3H as a catalyst.The GC chromatogram shows the presence of different methyl ester peaks.Table 6 shows the different ranges of individual methyl esters[41].The mass percentage of 14,17-Octadecedienoic acid,methyl ester was found to be maximum while the mass percentage of pentacosanoic acid methyl ester was found to be the least.Fig.11 shows that the presence of compounds with carbon atoms in the range C17–C19is maximum,while the compounds with carbon atoms in the range C12–C15is minimum.About 95.6%of biodiesel yield was obtained from the catalytic reaction.The fuel properties like kinematic viscosity,density,specific gravity, flash point, fire point,cloud point and pour point were measured and compared with the standards as shown in Table 7.The proposed mechanism of biochar based solid acid catalyst is shown schematically in Fig.12.

3.5.Catalyst reusability

The main motive behind our work is to bring down the cost of the production.The catalyst like homogeneous(KOH,NaOH,H2SO4)is unable to use again for the next batch production.To narrow down the problem on the cost of the production,heterogeneous catalyst plays a major role.The reusability of the sawdust biochar catalyst was carried out by collecting the catalyst through centrifuge method after a cycle of production.The catalyst was washed with deionized water and then with acetone.The washed catalyst was kept for drying in a hot air oven for overnight at 80°C.The dried catalyst was used for further more three cycles,and the yields were observed as 90.7%,88.2%and 85.7%as shown in Fig.13.

4.Conclusions

Fig.9.GC chromatogram of PPO and PPO based biodiesel.

Fig.10.FT-IR spectrum of PPO and PPO based biodiesel.

Fig.11.Percentage of methyl esters with different compound range.

In view of environmental consideration,biodiesel produced from simultaneous esterification–transesterification of pongamia pinnatta oil and methanolin the presence ofbiomass derived catalystcan be considered as a source of clean and renewable energy.The effects of reaction temperature and a heating duration of slow pyrolysis for the production of biochar were studied,and it was concluded that the heating duration does not have much effect on pH.The biochar pH increases with increase in temperature whereas the biochar yield decreases.A biochar based solid acid catalyst BC-WSD-SO3H was developed by sulfonation and characterization was done with different techniques.The catalyst shows unique properties with surface area 3.30 m-2·g-1and average pore diameter of 101.02 nm.The catalytic activity shows excellent results with a total acid density of 1.71 mmol·g-1and sulfonic acid group density of0.8 mmol·g-1.The FFA conversion of64.5%and biodiesel yield of 95.6%was obtained when the transesterification was carried out at 85°C with oil to methanol ratio of 9:1 and a catalyst weight of 2 wt%at 2 h of reaction time.Thus a new environmentally benign catalyst has been derived from biomass at a cheaper rate which can be used in the production of biodiesel.

Nomenclature

BC-WSD Wood sawdust biochar

BC-WSD-SO3H Biochar based catalyst

BET Brunauer–Emmett–Teller

FAME Fatty acid methyl esters

FFA Free Fatty Acids

FT-IR Fourier Transform Infrared Spectroscopy

Fig.12.Proposed mechanism of biochar based solid acid catalyst.

Fig.13.Effect of catalyst run cycle on%biodiesel yield.

HPA Heteropoly acid

HPLC High Performance Liquid Chromatography

KOH Potassium Hydroxide

PPO pongamia pinnatta oil

SEM Scanning Electron Microscope

TG Triglycerides

TGA Thermogravimetric Analysis

WSD Wood sawdust

XRD X-ray diffraction

Acknowledgments

For analytical services,we thank STIC CUSAT,SAIF IITMadras,Department of chemistry IIT Madras and NIT Calicut.