Bioresource Technology 99 (2008) 8752–8758

Contents lists available at ScienceDirect

Bioresource Technology

journal homepage: www.elsevier .com/ locate/bior tech

Efficient production of biodiesel from high free fatty acid-containing waste oilsusing various carbohydrate-derived solid acid catalysts

Wen-Yong Lou, Min-Hua Zong *, Zhang-Qun DuanLaboratory of Applied Biocatalysis, South China University of Technology, Guangzhou 510640, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 January 2008Received in revised form 11 April 2008Accepted 11 April 2008Available online 27 May 2008

Keywords:Carbohydrate-derived catalystsStarch-derived solid acid catalystBiodiesel productionWaste oilsHigh FFAs

0960-8524/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.biortech.2008.04.038

* Corresponding author. Tel.: +86 20 87111452; faxE-mail address: btmhzong@scut.edu.cn (M.-H. Zon

In the present study, such carbohydrate-derived catalysts have been prepared from various carbohy-drates such as D-glucose, sucrose, cellulose and starch. The catalytic and textural properties of the pre-pared catalysts have been investigated in detail and it was found that the starch-derived catalyst hadthe best catalytic performance. The carbohydrate-derived catalysts exhibited substantially higher cata-lytic activities for both esterification and transesterification compared to the two typical solid acid cata-lysts (sulphated zirconia and Niobic acid), and gave markedly enhanced yield of methyl esters inconverting waste cooking oils containing 27.8 wt% high free fatty acids (FFAs) to biodiesel. In addition,under the optimized reaction conditions, the starch-derived catalyst retained a remarkably high propor-tion (about 93%) of its original catalytic activity even after 50 cycles of successive re-use and thus dis-played very excellent operational stability. Our results clearly indicate that the carbohydrate-derivedcatalysts, especially the starch-derived catalyst, are highly effective, recyclable, eco-friendly and promis-ing solid acid catalysts that are highly suited to the production of biodiesel from waste oils containinghigh FFAs.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel is defined as the simple alkyl esters of fatty acids pro-duced from vegetable oils and animal fats. There has been anincreasing interest in biodiesel as a green and alternative fuel asa result of recent legislations that require a major reduction ofvehicle emissions, as well as the soaring price of petroleum (Maand Hanna, 1999; Vicente et al., 2004). However, biodiesel has cur-rently not been commercialized all over the world. The major bot-tleneck is the high cost of feedstock used for biodiesel production,which prohibits its widespread application (Kulkarni and Dalai,2006). One way of reducing the cost of biodiesel production is toemploy low quality feedstocks such as waste cooking oils whichare readily available and inexpensive, instead of neat vegetableoil (Watanabe et al., 2005; Zafiropoulos et al., 2007; Canakci,2007). However, such a process is challenging due to the presenceof considerable undesirable components especially free fatty acids(FFAs) and water. Use of alkaline catalysts for transesterification ofsuch feedstock is problematic because the alkali reacts with theFFAs to form large amounts of unwanted soap by-products whichcreate serious problem of product separation and ultimately lowerthe yield substantially (Veljkovic et al., 2006). Homogeneous acidcatalysts do not exhibit measurable susceptibility to FFAs, but aredifficult to recycle and operate at high temperatures, and give rise

ll rights reserved.

: +86 20 22236669.g).

to serious environmental and corrosion problems (Lotero et al.,2005; Canakci and Van Gerpen, 2003). Lipases are generally effec-tive catalysts and are non-polluting (Selmi and Thomas, 1998), butthey are expensive and there exist problems associated with theirusage in the presence of FFAs and short chain alcohols (such asmethanol and ethanol), which denature the enzyme to some ex-tent. Glycerol, which is one of the products of the reaction, mani-fests a serious negative effect on the enzyme (Dossat et al., 1999;Watanabe et al., 2000; Kose et al., 2002).

Solid acid catalysts offer significant advantages of eliminatingseparation, corrosion, toxicity and environmental problems (Clark,2002; Okuhara, 2002), and, therefore, have recently attracted con-siderable attention. A few reports have dwelt on the importanceof solid acids for biodiesel production (Lotero et al., 2005; Kisset al., 2006; Kulkarni et al., 2006). Apart from recyclability and reus-ability, an ideal solid acid catalyst for biodiesel preparation shouldhave high stability, numerous strong acid sites, large pores, hydro-phobic surface and low cost (Lotero et al., 2005). Inorganic-oxidesolid acids such as zeolite and Niobic acid have low densities ofeffective acid sites and readily lose their activities under harsh con-ditions (Van Rhijn et al., 1998; Harmer et al., 1998). In particular,these catalysts have small pore and thus are not suitable for biodie-sel production because of the diffusion limitation of the large fattyacid molecules. Although strongly acidic ion-exchange resins suchas Amberlyst-15 and Nafion-NR50 have abundant sulfonic acidgroups, these resins are expensive and show bad stability (Kisset al., 2006; Kulkarni et al., 2006; Harmer and Sun, 2001; Harmer

W.-Y. Lou et al. / Bioresource Technology 99 (2008) 8752–8758 8753

et al., 2000). Sulphated zirconia, on the other hand, is an efficient so-lid acid catalyst (Kiss et al., 2006; Yadav and Nair, 1999), but isexpensive because zirconium is a rare and costly metal and hightemperatures are required both for the calcination and for reactiva-tion of the catalyst. These limitations of the currently available solidacids have restricted their practical utility in biodiesel production.

Recently, a new strategy of preparing novel carbon-based solidacids has been developed by Hara’s research group (Hara et al.,2004; Toda et al., 2005; Takagaki et al., 2006; Okamura et al.,2006): sulfonation of incompletely carbonized D-glucose. Incom-plete carbonization of D-glucose leads to a rigid carbon materialconsisting of small polycyclic aromatic carbon sheets in a three-dimensional sp3-bonded structure (Okamura et al., 2006). Sulfona-tion of such carbon material has been demonstrated to afford ahighly stable solid with a high density of active SO3H sites, whichis physically robust and there is no leaching of SO3H groups duringuse and so displays remarkable catalytic performance for the ester-ification of higher fatty acids. Only recently have we presented thepreliminary characterization of this novel D-glucose-derived solidacid catalyst and its successful use for biodiesel production fromhigher fatty acids and especially waste oils with a high acid value(Zong et al., 2007). Except this preliminary work we previously re-ported, to the best of knowledge, utilization of carbohydrate-de-rived solid acid catalysts for biodiesel production from lowquality waste oils has not been explored in detail so far. In thepresent work, we describe the development of a series of carbohy-drate-derived catalysts prepared from various cheap starting mate-rials, including starch, cellulose, D-glucose and sucrose, andexamine their catalytic activities of both esterification and transe-sterification. Furthermore, such carbohydrate-derived catalysts arehere evaluated for biodiesel preparation from waste cooking oilscontaining 27.8 wt% FFAs by simultaneous esterification andtransesterification. Influences of several crucial variables such asreaction temperature, molar ratio of alcohol to oil, catalyst loadingand reaction time on biodiesel production using the most efficientstarch-derived catalyst, are also studied.

2. Experimental

2.1. Materials

The pre-treated waste cooking oils (FFAs content: 27.8 wt%;acid value: 55.3 mg KOH/g; saponification value: 194 mg KOH/g;water content: 0.03 wt%) was kindly provided by a local companythat collects the waste oils from restaurants by the authority of lo-cal government. Non-oil components of the waste cooking oilswere removed by separation prior to use. Methyl palmitate, methylstearate, methyl oleate, methyl linoleate, methyl linolenate,methyl heptadecanoate, oleic acid and triolein were purchasedfrom Sigma–Aldrich (USA) and were all of over 98% purity. Allother chemicals were also obtained from commercial sources andwere of the highest grade available.

2.2. Catalyst preparation

The carbohydrate-derived solid acid catalysts were prepared viaa modification of the published methods (Toda et al., 2005; Tak-agaki et al., 2006; Okamura et al., 2006). In a typical procedure,10 g of carbohydrate powder (D-glucose, sucrose, cellulose orstarch) was heated for a specified time at an appropriate tempera-ture (P300 �C) under N2 flow to produce a brown-black solid (anincomplete carbonization). The resulting material was then groundto a powder and heated in 100 mL of concentrated H2SO4 (>96%)under N2 flow to introduce SO3H into the aromatic carbon rings.After sulfonation of incomplete carbonization for a certain timeat a given temperature (P100 �C), the mixture was cooled to

30 �C and diluted with 500 mL of distilled water to form a blackprecipitate. Then, the precipitate was collected by filtration andwashed repeatedly with hot distilled water (>80 �C) until impuri-ties such as sulfate ions were no longer detected in the wash water.The resulting black solids (i.e. carbohydrate-derived solid acid cat-alysts) were dried at 60 �C for 48 h in vacuo to remove water ab-sorbed on the surface of the catalyst. Details about raw materials,carbonization temperature and time, and sulfonation temperatureand time are specified for each case. The total yield of the preparedcatalyst is around 45 wt% based on the raw material weight. Thesecarbohydrate-derived catalysts are insoluble in the tested solventsand liquid reactants (water, methanol, n-hexane, t-butanol, oleicacid, triolein and waste cooking oils).

2.3. Catalyst characterization

Scanning electron microscopy (SEM) of the prepared catalystswas performed by using a FEI Quanta 400 FEG electron microscopewith an acceleration voltage of 10 kV. The X-ray diffraction (XRD)analysis was conducted on a Rigaku D/MAX b powder X-ray diffrac-tometer using Cu Ka radiation (k = 0.18415 nm) at 30 kV and 30 mAin the scanning angle (2h) of 2�–60� at a scanning speed of 10�/min.The SO3H groups were determined using X-ray photoelectron spec-troscopy (XPS) with a Kratos Axis Ultra DLD apparatus. A mono-chromatic Al Ka (hm = 1486.6 eV) source operating at 150 W wasused. The photoelectron pass energies for wide and narrow scanswere 160 and 40 eV, respectively. The base pressure of the XPSchamber was 1 � 10�9 Torr. Elemental composition of the preparedcatalysts was determined by elemental analysis (EA) using Elemen-tar vario EL b apparatus. The absolute errors were 60.1% (CHS) and60.2% (O). NH3-TPD spectra were recorded using a XianQuanTP5000 flow unit apparatus to characterize the acid site adsorptiondistribution for each solid acid catalyst. Each sample (50 mg) wasplaced in a quartz reactor and heated to 300 �C in helium (30 mL/min) for 1 h to remove adsorbed impurities. The temperature wasthen cooled to 40 �C and saturated for 2 h with 100 mL/min of10% (v/v) NH3/He as carrier gas. Subsequently, the system wasflushed with 30 mL/min of He for 2 h. The temperature was rampedup to 300 �C at a rate of 10 �C/min. A thermal conductivity detector(TCD) was used to measure the desorption profile of NH3. A blank(no catalyst) study of the TPD ramp was made in order to accountfor baseline drift. As COOH and SO3H groups were present in cata-lyst samples prepared from carbohydrate (Okamura et al., 2006),the acid site densities estimated by NH3-TPD are total amounts ofboth functional groups. According to XPS analysis, it is expectedthat all S atoms in the carbohydrate-derived solid acids are con-tained in SO3H groups (see below). The densities of SO3H groupswere thus estimated by EA. The acid strength of catalyst samplewas examined by ultraviolet–visible diffuse reflectance spectros-copy (UV–vis DRS) on a Shimadzu UV-2501PC spectrophotometer.A mixture of catalyst sample (0.2 g) and BaSO4 powder (a referencematerial for DRS measurements, 1.0 g) was evacuated at 150 �C for1 h to remove adsorbed water. In an Ar-filled glove box, the mixturewas packed into a sealable quartz cell, and benzene (with or with-out color-producing reagent) was then added to the cell. The DRS ofthe mixture in each benzene solution was measured without expo-sure to air. The DRS of the color-producing reagent in the presenceof the carbohydrate-derived catalyst was obtained by subtractingthe spectrum for the mixture in pure benzene from that of the mix-ture in the benzene solution containing the color-producing re-agent. The DRS was also similarly measured using BaSO4 inbenzene with each color-producing reagent and also without addedcolor-producing reagent. The specific surface area and the pore sizeof the catalysts were assayed on Micromeritics Flowsorb III 2310equipment at –196 �C using liquid nitrogen. Prior to the analysisthe catalyst was pre-treated at 120 �C under vacuum for above

8754 W.-Y. Lou et al. / Bioresource Technology 99 (2008) 8752–8758

2 h to remove all adsorbed moistures (mainly water) from the cat-alyst surface and pores.

2.4. Catalytic activity of carbohydrate-derived solid acid catalyst

Esterification activity was evaluated by selecting the esterifica-tion of oleic acid with methanol as the model reaction. The reactionwas conducted by adding 0.14 g of the carbohydrate-derived cata-lyst into a methanol-oleic acid mixture (methanol: 100 mmol;oleic acid: 10 mmol) at 80 �C and a stirring rate of 500 rpm. Ali-quots (200 lL) were withdrawn at specified time intervals fromthe reaction mixture and centrifuged (10,000 rpm, 10 min) to re-move the catalyst, and were then kept for 10 min at 70 �C undervacuum to remove methanol from the samples, followed by addi-tion of anhydrous n-hexane (200 lL) containing 1.0 mM methylheptadecanoate (as an internal standard) for GC analysis.

Transesterification activity was evaluated by selecting thetransesterification of triolein with methanol as the model reaction.The reaction was carried out by adding 0.14 g of the prepared cat-alyst into a methanol–triolein mixture (methanol: 100 mmol; trio-lein: 10 mmol) at 80 �C and a stirring rate of 500 rpm. Prior to GCanalysis, samples (200 lL) were taken and worked up as describedabove.

The impact of stirring rate on both esterification reaction rateand transesterification reaction rate with each catalyst was exam-ined (data not shown). When the stirring rate was 500 rpm orabove, no external mass transfer limitations were observed.

2.5. Biodiesel production from waste cooking oils

Simultaneous esterification and transesterification for biodieselproduction were conducted by adding a fixed amount of the pre-pared catalyst to a mixture of waste cooking oils (27.8 wt% FFAs,5 g) and methanol at a stirring rate of 500 rpm and an appropriatetemperature. Samples (200 lL) were taken, worked up and ana-lyzed as described in Section 2.3. Details about molar ratio ofmethanol to oils, catalyst loading, reaction temperature and reac-tion time are specified for each case.

2.6. Operational stability of starch-derived solid acid catalyst

In order to assess the operational stability of the starch-derivedsolid acid catalyst, the re-use of the prepared catalyst was investi-gated in the production of biodiesel from waste cooking oils(27.8 wt% FFAs). Initially, 0.5 g (10 wt% catalyst loading) of the cat-alyst was added into the mixture of waste cooking oils (5 g) andmethanol (30:1 methanol to oil molar ratio) and the reaction wasconducted at 500 rpm and 80 �C. Then, the reaction was repeatedover P50 cycles (8 h per cycle) under the same reaction conditionsdescribed above. After each cycle of use in the production of bio-diesel, the catalyst was recovered by filtration before re-use. Therelative activity of the catalyst employed for the first batch was de-fined as 100%.

2.7. GC analysis

Reaction mixture were assayed with a GC 2010 gas chromato-graph (Shimadzu Corp., Kyoto, Japan) with a HP-5 capillary column(0.53 mm � 15 m, Hewlett–Packard, USA) equipped with a flameionization detector. The column temperature was held at 180 �Cfor 1 min, then raised to 186 �C at 0.8 �C/min and kept at constanttemperature for 1 min. Nitrogen was used as the carrier gas at aflow rate of 2 mL/min. The split ratio was 1:100 (v/v). The injectorand the detector temperatures were set at 250 �C and 280 �C,respectively. The retention times for methyl myristate, methylpalmitate, methyl heptadecanoate, methyl oleate, methyl linoleate,

and methyl stearate were 2.11, 4.12, 5.88, 7.64, 7.68 and 8.45 min,respectively. The average error for this determination was less than0.5%. All reported data are averages of experiments performed atleast in duplicate.

3. Results and discussion

3.1. Catalytic activity of carbohydrate-derived solid acid catalyst

Recent studies have shown that a novel carbon-based solid acidcatalyst, consisting of small polycyclic aromatic carbon sheets withhigh densities of SO3H groups, can be readily synthesized bysulfonation of incompletely carbonized D-glucose and exhibitsremarkable catalytic performance for esterification reaction as wellas hydration reaction (Toda et al., 2005; Takagaki et al., 2006;Okamura et al., 2006). However, such highly active and stable car-bon-based solid acid could not be prepared by sulfonation of anincompletely carbonized resin, amorphous glassy carbon, activatedcarbon, or natural graphite. Catalyst samples prepared from suchstarting materials showed no significant activity for esterification,hydration or hydrolysis (Okamura et al., 2006). These clearly sug-gest that the catalytic performances of the novel catalysts are sig-nifcantly dependent on the starting materials used for theirpreparation. Therefore, we initially focused on influences of differ-ent starting materials on catalytic and textural properties of novelsolid acid catalysts prepared from them. Several cheap carbohy-drate compounds such as starch, cellulose, sucrose and D-glucosewere selected as starting materials, and were pyrolyzed, accompa-nied by dehydration and dissociation of –C–O–C– at 400 �C for15 h, leading to the formation of polycyclic aromatic carbon rings(incomplete carbonization). SO3H groups were then introducedinto the aromatic carbon rings by sulfonation with concentratedH2SO4 at 150 �C for 15 h, resulting in the four corresponding carbo-hydrate-derived solid acids. Subsequently, the four catalyst sam-ples were examined through methyl oleate (high-grade biodiesel)formation from oleic acid (Fig. 1), one of main ingredients in vari-ous vegetable oils, as an example of the esterification of higherfatty acids. After 3 h reaction, the four corresponding catalysts pre-pared from starch, cellulose, sucrose and D-glucose gave a yield of95%, 88%, 80%, and 76%, respectively. It is obvious that differentraw materials have shown great impact on catalytic activity ofthe resulting catalyst samples. Time-dependent curves of methyloleate yield as depicted in Fig. 1 clearly indicated that the starch-derived catalyst was much more active than the other ones, achiev-ing the maximum yield of 95% only within 3 h, compared to 4–5 hfor the other catalysts. The marked difference in catalytic perfor-mance of the four catalysts prepared from different raw materialsis expected to be closely related to the physical and chemical prop-erties of the catalyst samples. No significant difference in morphol-ogy was observed among the prepared four catalysts by the SEManalysis (data not shown). The particles of all these carbohy-drate-derived catalysts reach micrometre dimensions and do notsignificantly aggregate. All the XRD diffraction patterns exhibitbroad and weak diffraction peaks (2h = 10�–30�; 2h = 35�–50�),which are attributable to amorphous carbon composed of polycylicaromatic carbon sheets oriented in a considerable random fashion(Tsubouchi et al., 2003). Also, no clear difference in the XRD pat-terns was found among the four catalysts. However, the notablydifferent textural properties of the four catalysts summarized inTable 1 could be responsible for such difference in catalytic activityof methyl oleate formation. Elementary analysis (EA) showed thatthe catalyst samples had clearly different compositions. Thestarch-derived catalyst contained a higher content of S elementthan the three catalysts. In the XPS for the catalysts prepared fromstarch, cellulose, sucrose and D-glucose (Fig. 2), the single S 2p peakwas observed at 168 eV, assigned to SO3H groups. It meant that

W.-Y. Lou et al. / Bioresource Technology 99 (2008) 8752–8758 8755

essentially all S atoms in the prepared catalyst samples were con-tained in SO3H groups. Hence, the densities of SO3H sites in the car-bohydrate-derived catalysts could be calculated based on the Scontent in catalyst compositions. Table 1 clearly indicated thatthe starch-derived catalyst afforded higher densities of SO3Hgroups than the other three catalysts, which is consistent withthe observation that the starch-derived catalyst gave a faster reac-tion rate as shown in Fig. 1. It was interestingly noted that the acidsite densities of all the four catalysts estimated by NH3-TPD wereslightly higher (0.14 mmol/g or so) than the SO3H site densities,possibly resulting from small amounts of COOH groups presentin the carbohydrate-derived catalysts. It has been confirmed byHara’s group using 13C MAS NMR that the COOH groups are presentin the catalyst prepared from D-glucose (Takagaki et al., 2006;Okamura et al., 2006). Additionally, the starch-derived catalystmanifested clearly larger BET surface area, pore volume and poresize than the catalysts prepared from the other three startingmaterials, which could also well explain for higher activity ofstarch-derived catalyst in esterifying oleic acid to methyl oleate.Surprisingly, for all the four carbohydrate-derived catalysts thedensities of SO3H groups as main functional sites were as high as1.47–1.83 mmol/g despite the small surface areas (4.1–7.2 m2/g).These densities were much too high to be attributed to SO3Hgroups attached to the catalyst surface. This suggests that SO3Hgroups in the amorphous carbon bulk can also participate in theesterification. Therefore, the incorporation of large reactant mole-cules into the bulk of amorphous carbon can greatly affect thereaction. Thanks to the relatively large pore volume and pore size,the starch-derived catalyst made reactants more accessible to theSO3H sites in the amorphous carbon bulk than each of the threecatalysts prepared from cellulose, sucrose and D-glucose, thus dis-

00

20

40

60

80

100

Yie

ld o

f m

ethy

l ole

ate

(%)

Reaction time (h)

Starch-derived catalystCellulose-derived catalyst

Sucrose-derived catalystD-glucose-derived catalyst

1 2 3 4 5 6

Fig. 1. Time-dependent curves of methyl oleate formation catalyzed by variouscarbohydrate-derived catalysts prepared from different starting materials (D-glu-cose, sucrose, cellulose and starch). Reaction conditions: 10 mmol oleic acid,100 mmol methanol, 80 �C, 500 rpm, 0.14 g catalyst.

Table 1Textural properties of various solid acid catalysts

Catalyst (composition) S content (wt%) Acid site density (mmol/g

Total SO3H

D-Glucose-derived catalyst(CH1.14O0.39S0.030)

4.7 1.60 1.47

Sucrose-derived catalyst(CH1.06O0.30S0.029)

5.1 1.71 1.59

Cellulose-derived catalyst(CH1.01O0.28S0.031)

5.4 1.82 1.68

Starch-derived catalyst(CH0.85O0.23S0.032)

5.9 1.97 1.83

Sulphated zirconia 2.5 0.4 –Niobic acid (Nb2O5 � nH2O) – 0.3 –

playing high efficiency in catalyzing the esterification of higherfatty acids as illustrated in Fig. 1. Therefore, among the carbohy-drate compounds tested, starch was thought to the best startingmaterial for preparing highly active carbohydrate-derived solidacid catalysts. Certainly, further detailed investigations are re-quired to get a deep insight into the observation that the fourraw materials resulted in such significant difference in catalyticand textural properties of the catalysts prepared from them.

3.2. Influences of the preparation variables on the catalytic activity ofcarbohydrate-derived catalyst

In the course of preparing such carbohydrate-derived catalysts,it was found that carbonization temperature and time, and sulfona-tion temperature and time also significantly affected the catalyticperformance of the resulting catalysts. Hence, subsequent workwas made on the effects of the above-mentioned preparation vari-ables on the esterification of oliec acid catalyzed by the resultingstarch-derived catalyst. As can be seen in Table 2, with increasingcarbonization temperature up to 400 �C, the resulting starch-de-rived catalyst became more active and led to the improvement inthe yield of methyl oleate within 3 h. Further increase in carboniza-tion temperature above 400 �C resulted in a clear decline in cata-lytic activity of the prepared catalyst. This may be because thesample carbonzied at lower temperature (below 400 �C) has higherdensities of OH groups and more water from the esterification canbe adsorbed on the surface of the resulting catalyst, and thereforethe formed water layer will prevent the access of relatively hydro-phobic oleic acid to the catalyst. Also, the sample carbonized at low-er temperature (below 400 �C) is soft aggregate of small polycylicaromatic carbon with SO3H rather than rigid carbon material, and

) Surface area(m2/g)

Average pore volume(cm3/g)

Average pore size(nm)

4.1 0.44 4.0

5.0 0.52 5.1

5.7 0.65 6.4

7.2 0.81 8.2

218 0.18 2.7128 – –

176 174 172 170 168 166 164 162

150

200

250

300

350

400

450

500

D-glucose-derived catalyst

Sucrose-derived catalyst

Cellulose-derived catalyst

Starch-derived catalyst

CP

S

Binding energy (eV)

SO3H

Fig. 2. S 2p XPS spectra for the catalysts prepared from starch, cellulose, sucroseand D-glucose.

0

20

40

60

80

100

120

140

160

180

0

100

200

300

400

500

600

700

800 Esterification activity

Con

centra

ted H 2

SO 4

Niobic

acid

Sulphat

ed zi

rcon

ia

Starc

h-der

ived ca

talys

t

Cell

ulose-d

erive

d cata

lyst

Sucros

e-der

ived ca

talys

t

D-glu

cose

-der

ived ca

talys

t

Tra

nses

teri

fica

tion

act

ivit

y (µ

mol

/min

· g)

Est

erif

icat

ion

acti

vity

(µm

ol/m

in· g

)

Transesterification activity

Fig. 3. Esterification and transesterification activities of various catalysts tested.Reaction conditions: 10 mmol oleic acid or triolein, 100 mmol methanol, 80 �C,500 rpm, 0.14 g catalyst.

8756 W.-Y. Lou et al. / Bioresource Technology 99 (2008) 8752–8758

therefore the sulfopolycyclic aromatic compounds can be readilyleached from the solid at high reaction temperature or with higherfatty acids as reactants. On the other hand, the sample carbonized athigher temperature (above 400 �C) becomes more rigid due to thegrowth of large polycylic aromatic carbon sheets and the stackingof carbon sheets, and thus the resulting catalyst contains lowerdensities of SO3H sites as well as less flxible structure. 400 �C repre-sents a balance between these conflicts and so the resulting catalystsample carbonized at 400 �C displays higher activity in methyl ole-ate formation. At the optimal carbonization temperature of 400 �C,the suitable carbonization time was thought to be 15 h on accountof higher catalytic activity of the resulting catalyst (Table 2). Simi-larly, under the preparation conditions tested the most effectivecatalyst was produced when sulfonation was conducted at 150 �Cfor 15 h (Table 2). There is no doubt that the textural propertiesof the starch-derived catalysts such as the compositions, the acidsite density, and the inner porous structure will be affected whenthe variables of carbonization and sulfonation are changed. The ef-fect profiles of such variables on the textural properties of the pre-pared catalysts are expected to be capable of further explaining theresults described above, and are the subjects of ongoing investiga-tions in our laboratory.

3.3. Comparison of the catalytic activities of various catalysts

It is of particular interest to compare the catalytic activities ofthe carbohydrate-derived catalysts with those of the concentratedH2SO4 (>96%) and the two typical strong solid acid catalysts, sulph-ated zirconia and Niobic acid (Nb2O5 � nH2O) that are widely usedin industrial acid-catalyzed processes (Okuhara, 2002; Ecormieret al., 2003), in the esterification and transesterification reactionsinvolved in biodiesel production from waste oils. The esterificationand transesterification activities were evaluated through the ester-ification of oleic acid (10 mmol) and transesterification of triolein(10 mmol) with methanol (100 mmol) at 80 �C, respectively. Theresults for 0.14 g of all the tested catalysts were shown in Fig. 3.Obviously, the four carbohydrate-derived catalysts exhibited muchhigher esterification and transesterification activities (on a weightbasis unless otherwise specified) than those of sulphated zirconiaand Niobic acid. Although the two typical solid acid catalystspossess much larger specific surface areas than the four carbohy-drate-derived catalysts, the acid site densities of the four carbohy-drate-derived catalysts are 4–6 times lager than the two solid acidcatalysts (Table 1). Therefore, the superior catalytic activity of thecarbohydrate-derived catalysts can be explained on the basis oftheir markedly greater densities of active acid sites. Also, the largerpore volume and size of the carbohydrate-derived catalysts, whichwill result in accessibility of large molecular reactants (oleic acid ortriolein) to the active acid sites in the bulk of the catalysts, can wellaccount for their considerably excellent catalytic performance forboth reactions compared to Sulphated zirconia. It is well known

Table 2Influence of the preparation variables on the catalytic activity of the starch-derived cataly

Carbonization temperature Carbonization time

Temperature (�C) Yieldb (%) Time (h) Yieldb (%)

300 70.1 10.0 71.2350 81.3 12.5 81.3400 94.9 15.0 94.9450 85.4 17.5 73.3500 80.2 20.0 61.3

a Catalyst activity was measured in the model reaction, esterification of oleic acid withthe table were set as follows: carbonization temperature, 400 �C; carbonization time, 1methanol, 100 mmol; starch-derived carbohydrate catalyst, 0.14 g; stirring rate, reaction

b Yield of methyl oleate.

that Niobic acid (providing acidic OH groups) is a non-porous solidacid and displays only marginal acidity in the presence of water. Sothe negative effect of the formed by-product water in the esterifi-cation reaction on Niobic acid can be partially responsible for itslow esterification activity towards oleic acid. Additionally, Niobicacid showed only slight transesterification activity towards trio-lein, which might be attributable to the inaccessibility of morehydrophobic triolein to the effective acid sites attached on the rel-atively hydrophilic surface of the catalyst. As can be clearly seenfrom the data depicted in Fig. 3, the esterification and transesteri-fication activities of the three carbohydrate-derived catalysts fromD-glucose, sucrose and cellulose were much higher than half thoseof concentrated H2SO4, while the activities of Sulphated zirconiaand Niobic were clearly below 19% of those of concentratedH2SO4. Amazingly, the starch-derived catalyst manifested almostcomparable esterification and transesterification activities withthose of concentrated H2SO4. Also, ultraviolet–visible diffusereflectance spectroscopy (UV–vis DRS) analysis indicates that thefour carbohydrate-derived catalysts have a strong acid strengthwith pKa being between –11 and �8, which is comparable to thatof concentrated H2SO4 (pKa = �11.9). On the other hand, the con-centrated H2SO4 as a homogeneous liquid acid catalyst cannot bereadily recycled and presents a threat to the environment andthe operator’s health, especially when it is employed on a largescale, whilst the carbohydrate-derived solid acid catalysts them-selves are relatively non-toxic, eco-friendly, and can be easily recy-

sta

Sulfonation temperature Sulfonation time

Temperature (�C) Yieldb (%) Time (h) Yieldb (%)

100 83.8 10.0 76.3125 89.8 12.5 84.4150 94.9 15.0 94.9175 86.6 17.5 86.6200 78.8 20.0 80.5

methanol. The reaction parameters that were not specifically changed as indicated in5 h; sulfonation temperature, 150 �C; sulfonation time, 15 h; oleic acid, 10 mmol;temperature and time for the esterification, 500 rpm, 80 �C and 3 h, respectively.

W.-Y. Lou et al. / Bioresource Technology 99 (2008) 8752–8758 8757

cled and re-used many times though concentrated H2SO4 is em-ployed during their preparation. Therefore, the carbohydrate-de-rived catalysts in this study, especially the starch-derivedcatalyst, have clear potential for use as a replacement for H2SO4

in both esterification and transesterification involved in biodieselproduction from waste oils.

As can be clearly seen in Fig. 3, for all the tested catalysts, theesterification activity towards oleic acid was much higher thanthe transesterification activity towards triolein under identicalreaction conditions, which might be due to the following threepossible reasons. First, fatty acids, particularly unsaturated onessuch as oleic acid, are more soluble in methanol than triglyceridessuch as triolein, and, therefore, the esterification reaction pro-ceeded faster. Second, acid-catalyzed esterification reaction re-quires relatively low activation energy compared totransesterification reaction, thus resulting in a faster reaction rateunder identical reaction conditions. The third reason is probablyrelated to the reaction mechanism of esterification and transesteri-fication. Methanolysis of fatty acid proceeds through simple ester-ification, while methanolysis of triglyceride proceeds throughcomplicated transesterification which consists of three consecutiveand reversible reaction steps. It is well known that transesterifica-tion of triglyceride with methanol is a stepwise reaction, and in thereaction sequence triglyceride is converted stepwise to di- andmonoglyceride and finally glycerol. When tri-, di- and monoglycer-ide come in contact with the acid sites, respectively, transesterifi-cation takes place and generates one mole of fatty acid methylester in each step. The transesterification in each step is reversible.Owing to these reasons, the rate of triolein transesterification wasmarkedly slower than that of oleic acid esterification, which is inaccordance with much lower activity of transesterification thanesterification as illustrated in Fig. 3. Similar observations were alsodescribed by other research groups (Kulkarni et al., 2006; Warabiet al., 2004). Thus, the feedstocks with a higher content of FFAs ap-pears preferable to that with a lower FFAs content for biodieselproduction by acid-catalyzed simultaneous esterification andtransesterification on account of substantially faster reaction rate.

3.4. Biodiesel production from waste cooking oils with carbohydrate-derived solid acid catalyst

In view of the excellent catalytic performance, the carbohy-drate-derived catalysts were further tested for the conversion ofwaste cooking oils containing high free fatty acids (27.8 wt% FFAs),in the presence of methanol, to fatty acid methyl esters that

0 2 4 6 8 10 12 140

20

40

60

80

100

Yie

ld o

f fa

tty

acid

met

hyl e

ster

s (%

)

Reaction time (h)

Starch-derived catalystCellulose-derived catalystSucrose-derived catalyst

D-glucose-derived catalystSulfated zirconiaNiobic acid

Fig. 4. The biodiesel production from waste cooking oils containing 27.8 wt% FFAsusing various solid acid catalysts. Reaction conditions: methanol to oil molar ratio,20:1; catalyst loading, 10 wt% based on the weight of waste oils; stirring rate,500 rpm; reaction temperature, 80 �C.

constitute biodiesel by simultaneous esterification and transesteri-fication. As transesterification of oil (triglyceride) using heteroge-neous acid catalysts is well known for its reversibility and slowreaction rate, use of excess methanol can improve the reaction rateand favour forward reaction, maximizing the yield of fatty acidmethyl esters (Furuta et al., 2004). Thus, the production of biodie-sel using various carbohydrate-derived catalysts was initially con-ducted at a relatively high molar ratio of methanol to oil (20:1,mol/mol), combined with reaction temperature of 80 �C, stirringrate of 500 rpm and catalyst loading of 10 wt% based on the weightof the used waste oils. For comparison, the results for Sulphatedzirconia and Niobic acid were also shown in Fig. 4. The four carbo-hydrate-derived catalysts gave much faster reaction rates than Sul-phated zirconia and Niobic acid, achieving the maximum yield(above 80%) of fatty acid methyl esters within 8–12 h, comparedto the yield of 44% for Sulphate zirconia and the yield of only16% for Niobic acid within 14 h or a longer time. Of all the testedcatalysts, the starch-derived catalyst proved to be the most effec-tive in catalyzing the conversion of the waste cooking oils to bio-diesel by simultaneous esterification and transesterification, andafforded the yield of around 83% only within 8 h, consistent withthe higher catalytic activity of the starch-derived catalyst in themodel esterification and transesterification reactions. Since it wasthe best-performing catalyst for biodiesel production, the starch-derived catalyst was selected further to investigate in detail the ef-fect of various variables on the methyl ester yield for optimizationof biodiesel production process.

Theoretically, one mole of triglyceride requires three moles ofmethanol to convert it to the corresponding fatty acid methyl es-ters. In addition, transesterification of triglycerides present inwaste oils is a reversible reaction. Hence, the excess of methanolrelative to oil can shift the equilibrium towards methyl ester for-mation. The yield of methyl esters (reaction conditions: 10 wt%catalyst loading; 80 �C) markedly increased from 50% to 92% after8 h reaction with the increase of molar ratio of methanol to oilfrom 5:1 to 30:1, however, further increase in methanol to oil mo-lar ratio to 40:1 showed only slight enhancement (<2%) in themethyl ester yield. Therefore, the optimal molar ratio of methanolto oil was shown to be 30:1. The excess of methanol used in theprocess can be collected and reused. The catalyst loading is oneof the important parameters that affect the yield of methyl estersand the cost of biodiesel production. As expected, the yield ofmethyl esters after 8 h (reaction conditions: 30:1 methanol/oil mo-lar ratio; 80 �C) greatly increased with increasing catalyst loadingup to 10 wt% (based on the oil weight unless otherwise specified).However, when the catalyst loading was further increased to14 wt%, the yield of methyl esters (about 93%) was similar to thatachieved with 10 wt% catalyst loading. Taking into account theyield and the cost, 10 wt% catalyst loading was thought to be suit-able for the conversion of waste oils to biodiesel. Reaction temper-ature shows the significant effect on the yield of methyl esters after8 h (reaction conditions: 30:1 methanol/oil molar ratio; 10 wt%catalyst loading). The yield remarkably increased with increasingreaction temperature within the range of 65–80 �C. However,when reaction temperature was further increased from 80 �C to100 �C, the initial reaction rate increased to some extent but the fi-nal yield of methyl esters after 8 h displayed no appreciableimprovement. Thus, 80 �C was selected as the optimal reactiontemperature for the process. Under the optimized conditions de-scribed above (30:1 methanol to oil molar ratio, 10 wt% catalystloading, 80 �C reaction temperature), the reaction time-dependentprofile of the methyl ester yield with the most effective starch-de-rived catalyst indicated that the yield clearly increased withincreasing reaction time up to 8 h, and thereafter remained almostconstant at about 92%, which probably represents a near-equilib-rium yield of methyl esters. The FFA content in the reaction system

8758 W.-Y. Lou et al. / Bioresource Technology 99 (2008) 8752–8758

was substantially reduced to around 0.5 wt%. In the production ofbiodiesel from waste cooking oils containing high FFAs, the transe-sterification activity of the starch-derived solid acid catalyst wasmuch lower than its esterification activity (Fig. 3), and conse-quently it required a relatively long time to get a near-equilibriumyield of methyl esters. It is well known that the alkali catalyst forthe transesterification is much more effective than the acid cata-lyst. Therefore, the use of the alkali catalyst for the transesterifica-tion after the use of the starch-derived solid acid catalyst for theesterification can further improve the yield and the efficiency ofbiodiesel production from waste cooking oil containing high FFAs,and this is now under investigation in our laboratory.

3.5. Operational stability of starch-derived solid acid catalyst

The catalyst recycling is a crucial step as it reduces the cost ofbiodiesel production. The efficiency of the catalyst also dependson its reusability. In order to evaluate the reusability, the starch-derived catalyst was recovered for further conversion of waste oilsunder the optimized conditions by simple filtering. It was surpris-ingly found that the starch-derived catalyst still retained about 93%of its original catalytic activity, even after fifty cycles of successivere-use, indicating the excellent operational stability. Generally, theactivities of most solid acid catalysts with hydrophilic surface areseriously hindered by water produced during the esterificationreaction because of the formation of water layer on the hydrophilicsurface preventing the access of relatively hydrophobic substrate(Kiss et al., 2006). In the case of the starch-derived catalyst, theXRD diffraction pattern mentioned above, which exhibits broadand weak diffraction peaks (2h = 10�–30�; 2h = 35�–50�), showedthat similar to the previously described observation (Tsubouchiet al., 2003; Takagaki et al., 2006), the catalyst was composed ofpolycylic aromatic carbon sheets. Therefore, the surface of thestarch-derived catalyst is relatively hydrophobic and it is less likelyto form a water layer on its surface. So the more hydrophobic fattyacids or glycerides could readily access the catalyst. This mightpartially explain for the observation that the starch-derived cata-lyst was especially robust and was less hindered by the by-productwater during biodiesel production from waste cooking oils con-taining high FFAs and the in-depth understanding of the observa-tion is on the way in our laboratory.

4. Conclusions

The present study clearly showed that the carbohydrate-de-rived solid acids, prepared from D-glucose, sucrose, cellulose andstarch through sulfonation of incompletely carbonized carbohy-drate, are non-toxic, inexpensive, recyclable and promisingeco-friendly catalysts. The starting materials, carbonization tem-perature and time, and sulfonation temperature and time for cata-lyst preparation all had significant impact on the catalytic andtextural properties of the prepared catalysts. The carbohydrate-derived catalysts prepared at the optimized conditions displayedmuch higher activities than typical Sulfated zirconia and Niobicacid for both esterification and transesterification. More impor-tantly, various carbohydrate-derived catalysts, especially starch-derived catalyst, were shown to be highly effective in convertinghigh FFA-containing waste oils to biodiesel by simultaneous ester-ification and transesterification. Under the optimized reaction con-ditions, usage of the most effective starch-derived catalyst forbiodiesel production from waste cooking oils containing 27.8 wt%FFAs afforded the methyl ester yield of about 92% after 8 h. Thiscatalyst also manifested very excellent operational stability. Aswell as being potentially useful for biodiesel production, the carbo-hydrate-derived catalysts may find wide applications as a hetero-geneous strong acid catalyst.

References

Canakci, M., 2007. The potential of restaurant waste lipids as biodiesel feedstocks.Bioresource Technology 98, 183–190.

Canakci, M., Van Gerpen, J., 2003. A pilot plant to produce biodiesel from high freefatty acid feedstocks. Transactions of the ASAE 46, 945–954.

Clark, J.H., 2002. Solid acids for green chemistry. Accounts of Chemical Research 35,791–797.

Dossat, V., Combes, D., Marty, A., 1999. Continuous enzymatic transesterification ofhigh oleic sunflower oil in a packed bed reactor: influence of the glycerolproduction. Enzyme and Microbial Technology 25, 194–200.

Ecormier, M.A., Wilson, K., Lee, A.F., 2003. Structure-reactivity correlations insulphated-zirconia catalysts for the isomerisation of alpha-pinene. Journal ofCatalysis 215, 57–65.

Furuta, S., Matsuhashi, H., Arata, K., 2004. Biodiesel fuel production with solidsuperacid catalysis in fixed bed reactor under atmospheric pressure. CatalysisCommunications, 721–723.

Hara, M., Yoshida, T., Takagaki, A., Takata, T., Kondo, J.N., Hayashi, S., Domen, K.,2004. A carbon material as a strong protonic acid. Angewandte ChemieInternational Edition 43, 2955–2958.

Harmer, M.A., Farneth, W.E., Sun, Q., 1998. Towards the sulfuric acid of solids.Advanced Materials 10, 1255.

Harmer, M.A., Sun, Q., 2001. Solid acid catalysis using ion-exchange resins. AppliedCatalysis A: General 221, 45–62.

Harmer, M.A., Sun, Q., Vega, A.J., Farneth, W.E., Heidekum, A., Hoelderich, W.F.,2000. Nafion resin-silica nanocomposite solid acid catalysts. Microstructureprocessing property correlations. Green Chemistry 2, 7–14.

Kiss, A.A., Dimian, A.C., Rothenberg, G., 2006. Solid acid catalysts for biodieselproduction – towards sustainable energy. Advanced Synthesis and Catalysis348, 75–81.

Kose, O., Tuter, M., Aksoy, H.A., 2002. Immobilized Candida antarctica lipase-catalyzed alcoholysis of cotton seed oil in a solvent-free medium. BioresourceTechnology 83, 125–129.

Kulkarni, M.G., Dalai, A.K., 2006. Waste cooking oil – an economical source forbiodiesel: A review. Industrial and Engineering Chemistry Research 45, 2901–2913.

Kulkarni, M.G., Gopinath, R., Meher, L.C., Dalai, A.K., 2006. Solid acid catalyzedbiodiesel production by simultaneous esterification and transesterification.Green Chemistry 8, 1056–1062.

Lotero, E., Liu, Y.J., Lopez, D.E., Suwannakaran, K., Bruce, D.A., Goodwin, J.G., 2005.Synthesis of biodiesel via acid catalysis. Industrial & Engineering ChemistryResearch 44, 5353–5363.

Ma, F.R., Hanna, M.A., 1999. Biodiesel production: a review. Bioresource Technology70, 1–15.

Okuhara, T., 2002. Water-tolerant solid acid catalysts. Chemical Reviews 102, 3641–3665.

Okamura, M., Takagaki, A., Toda, M., Kondo, J.N., Domen, K., Tatsumi, T., Hara, M.,Hayashi, S., 2006. Acid-catalyzed reactions on flexible polycyclic aromaticcarbon in amorphous carbon. Chemistry of Materials 18, 3039–3045.

Selmi, B., Thomas, D., 1998. Immobilized lipase-catalyzed ethanolysis of sunfloweroil in a solvent-free medium. Journal of the American Oil Chemists Society 75,691–695.

Takagaki, A., Toda, M., Okamura, M., Kondo, J.N., Hayashi, S., Domen, K., Hara, M.,2006. Esterification of higher fatty acids by a novel strong solid acid. CatalysisToday 116, 157–167.

Toda, M., Takagaki, A., Okamura, M., Kondo, J.N., Hayashi, S., Domen, K., Hara, M.,2005. Green chemistry – biodiesel made with sugar catalyst. Nature 438, 178.

Tsubouchi, N., Xu, C.B., Ohtsuka, Y., 2003. Carbon crystallization during high-temperature pyrolysis of coals and the enhancement by calcium. Energy andFuels 17, 1119–1125.

Van Rhijn, W.M., De Vos, D.E., Sels, B.F., Bossaert, W.D., Jacobs, P.A., 1998. Sulfonicacid functionalised ordered mesoporous materials as catalysts for condensationand esterification reactions. Chemical Communications, 317–318.

Veljkovic, V.B., Lakicevic, S.H., Stamenkovic, O.S., Todorovic, Z.B., Lazic, M.L., 2006.Biodiesel production from tobacco (Nicotiana tabacum L.) seed oil with a highcontent of free fatty acids. Fuel 85, 2671–2675.

Vicente, G., Martinez, M., Aracil, J., 2004. Integrated biodiesel production: acomparison of different homogeneous catalysts systems. BioresourceTechnology 92, 297–305.

Warabi, Y., Kusdiana, D., Saka, S., 2004. Reactivity of triglycerides and fatty acids ofrapeseed oil in supercritical alcohols. Bioresource Technology 91, 283–287.

Watanabe, Y., Pinsirodom, P., Nagao, T., Kobayashi, T., Nishida, Y., Takagi, Y., Shimada,Y., 2005. Production of FAME from acid oil model using immobilized Candidaantarctica lipase. Journal of the American Oil Chemists Society 82, 825–831.

Watanabe, Y., Shimada, Y., Sugihara, A., Noda, H., Fukuda, H., Tominaga, Y., 2000.Production of biodiesel fuel from vegetable oil using immobilized Candidaantarctica lipase. Journal of the American Oil Chemists Society 77, 355–360.

Yadav, G.D., Nair, J.J., 1999. Sulfated zirconia and its modified versions as promisingcatalysts for industrial processes. Microporous and Mesoporous Materials 33, 1–48.

Zafiropoulos, N.A., Ngo, H.L., Foglia, T.A., Samulski, E.T., Lin, W., 2007. Catalyticsynthesis of biodiesel from high free fatty acid-containing feedstocks. ChemicalCommunications, 3670–3672.

Zong, M.H., Duan, Z.Q., Lou, W.Y., Smith, T.J., Wu, H., 2007. Preparation of a sugarcatalyst and its use for highly efficient production of biodiesel. Green Chemistry9, 434–437.