Bioresource Technology 101 (2010) 9570–9576

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Bioresource Technology

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Biodiesel production from waste cooking oil catalyzed by TiO2–MgO mixed oxides

Zhenzhong Wen a, Xinhai Yu a,b,*, Shan-Tung Tu a, Jinyue Yan c,d, Erik Dahlquist c

a Key Laboratory of Safety Science of Pressurized System (MOE), School of Mechanical Engineering, East China University of Science and Technology, Shanghai 200237, Chinab State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai 200237, Chinac School of Sustainable Development of Society and Technology, Mälardalen University, SE-721 23 Västerås, Swedend School of Chemical Science and Engineering, Royal Institute of Technology Stockholm, SE-100 44 Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 26 February 2010Received in revised form 11 July 2010Accepted 15 July 2010Available online 8 August 2010

Keywords:BiodieselWaste cooking oilSolid catalystStabilityHeterogeneous

0960-8524/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.biortech.2010.07.066

* Corresponding author at: Key Laboratory of SaSystem (MOE), School of Mechanical Engineering, Easand Technology, Shanghai 200237, China. Tel./fax: +8

E-mail address: yxhh@ecust.edu.cn (X. Yu).

a b s t r a c t

Mixed oxides of TiO2–MgO obtained by the sol–gel method were used to convert waste cooking oil intobiodiesel. Titanium improved the stability of the catalyst because of the defects induced by the substitu-tion of Ti ions for Mg ions in the magnesia lattice. The best catalyst was determined to be MT-1-923,which is comprised of an Mg/Ti molar ratio of 1 and calcined at 923 K, based on an assessment of theactivity and stability of the catalyst. The main reaction parameters, including methanol/oil molar ratio,catalyst amount, and temperature, were investigated. The catalytic activity of MT-1-923 decreased slowlyin the reuse process. After regeneration, the activity of MT-1-923 slightly increased compared with thatof the fresh catalyst due to an increase in the specific surface area and average pore diameter. The mixedoxides catalyst, TiO2–MgO, showed good potential in large-scale biodiesel production from waste cookingoil.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel is biodegradable, renewable, and non-toxic. It has arelatively high flash point and can be blended with fossil diesel fuelor used in pure form (Marchetti et al., 2007; Leung et al., 2010). Themerits of biodiesel make it a good alternative to petroleum-basedfuel and have led to its use in many countries, especially in envi-ronmentally sensitive areas (Meher et al., 2006; Dubé et al.,2007). Refined vegetable oils, such as soybean oil or rapeseed oil,are the main feedstocks for biodiesel production. However, biodie-sel from such oils are not cost-competitive because the feedstockcost is high, accounting for roughly 60–80% of total biodiesel cost(Pandey, 2008; Glišic et al., 2009). Using cheaper feedstocks, suchas waste cooking oil (WCO), animal fats, and crude vegetable oils,have been suggested to lower the cost. Among these, WCO hasbeen the focus of studies since a growing quantity has been pro-duced as a consequence of economic development (Zhang et al.,2003; Jacobson et al., 2008; Demirbas, 2009). Thus, biodiesel de-rived from WCO can efficiently and effectively contribute to envi-ronmental enhancement.

Generally, the production of biodiesel can be catalyzed by bothhomogeneous acid and base catalysts (Lopéz et al., 2005). Due to itshigh catalytic efficiency and low corrosive properties,

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fety Science of Pressurizedt China University of Science6 21 6425 3513.

homogeneous base catalysts, such as sodium, potassium hydrox-ides and alkoxides, are preferred to homogeneous acid catalysts(Verziu et al., 2008; Kawashima et al., 2009). However, the use ofhomogeneous base catalysts results in an undesirable reaction ofthe catalyst with the free fatty acids contained in waste and usedfrying oils, leading to the formation of soap (Ma and Hanna,1999; Kawashima et al., 2008) and lowering biodiesel conversion.Compared with homogenous catalysts, heterogeneous catalystscan provide clean and recyclable catalyst systems. Several solidheterogeneous catalysts have been used in biodiesel productionfrom WCO containing free fatty acids (Jacobson et al., 2008; Britoet al., 2009; Yan et al., 2009; Pugnet et al., 2010; Park et al.,2008; Sun et al., 2010). Among these, Mg–Al hydrotalcite showsthe best activity under mild reaction conditions. Unfortunately,its catalytic activity is unstable because some alkali cations leachout in the fatty acid methyl ester (FAME) phase. As heterogeneousbase catalysts for biodiesel production, Al–Zn spinel (Pugnet et al.,2010) and Zr–La (Sun et al., 2010) show good activity and stability.However, a reaction temperature exceeding 473 K is necessary toguarantee high FAME yield, resulting in an increase in productioncost and energy consumption. Hence, the search for a solid catalystwith stable activity, which could yield products at a short reactiontime under mild reaction conditions (lower temperature and pres-sure at the level of high biodiesel yield), is significant for industrialapplications.

Metal-doped MgO has been extensively studied as an importantsolid base catalyst in various organic syntheses, such as aldol con-densation (Díez et al., 2006), propane oxidation (Trionfetti et al.,

Z. Wen et al. / Bioresource Technology 101 (2010) 9570–9576 9571

2008), oxidative methylation of acetonitrile (Bothe-Almquist et al.,2000), and isobutanol decomposition (Lopez et al., 2008), all ofwhich exhibited excellent activity. To the best of our knowledge,several reports have been conducted on biodiesel synthesis usingmetal-doped MgO catalysts. Recently, Macleod et al. (2008) re-ported that Li-doped MgO can potentially replace conventionalhomogeneous catalysts for large-scale biodiesel production. Inour previous study (Wen et al., 2010), Li-doped MgO producedthe high biodiesel yield (93.9%) within 2 h at 333 K, but metalleaching from the catalyst was significant. As the valence of Tiion (4) is higher than that of the Mg ion (2), Ti addition may theo-retically bring about defects on the MgO surface and, hence, im-prove the activity and stability of the catalyst. However, thereare few reports on biodiesel production using MgO with Ti addi-tion. Moreover, all metal-doped MgO catalysts reported to datehave been used for biodiesel synthesis starting from refined vege-table oil and not WCO.

In this study, the preparation of TiO2–MgO mixed oxides forbiodiesel synthesis from WCO has been investigated by comparingdifferent Mg/Ti molar ratios and calcination temperatures. The ef-fects of Ti addition were interpreted in terms of the structural andtextural properties of the resulting oxides. The effects of methanol/oil ratio, catalyst amount, and temperature were also studied forthe best catalyst. Furthermore, deactivation and regeneration ofthe optimized catalyst were investigated in detail.

2. Methods

2.1. Materials

Anhydrous methanol (99.9%) and n-hexane (97%) were pur-chased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, Chi-na). WCO was collected at a public restaurant in East ChinaUniversity of Science and Technology, Shanghai, China. The physi-cal and chemical properties of the WCO are listed in Table 1. Priorto the reaction, WCO was dried at 393 K for maximum reduction ofthe water content. Standard analytical reagents, such as methylundecanoate, methyl palmitate, and methyl oleate, were obtainedfrom Fluka. Methyl stearate, methyl linoleate, and methyl linole-nate were purchased from Tokyo Chemical Industry Co., Ltd.

2.2. Catalyst preparation

Pure MgO was prepared by hydration of the low surface areacommercial MgO (Sinopharm Chemical Reagent Co., Ltd., Shanghai,China) with distilled water. Then, the resulting Mg(OH)2 wasdecomposed and stabilized at 773 K under He flow for 18 h to ob-tain MgO with a high surface area. The TiO2–MgO mixed oxides

Table 1Physical and chemical properties of waste cooking oil.

Property Waste cooking oil

Palmitic acid (wt%) 8.5Stearic acid (wt%) 3.1Oleic acid (wt%) 21.2Linoleic acid (wt%) 55.2Linolenic acid (wt%) 5.9Others (wt%) 4.2Water content (wt%) 1.9Density (cm3 g�1) 0.91Kinematic viscosity (313 K) (mm2 s�1) 4.2Saponification value (mg KOH g�1) 207Acid value (mg KOH g�1) 3.6Iodine number (g I2 100 g�1) 83Sodium content (mg kg�1) 6.9Peroxide value (meq kg�1) 23.1

catalysts were synthesized by the sol–gel method. Briefly, SolutionA, a tetrabutyl titanate solution, was prepared in absolute ethanolwith a certain acetate quantity. Solution B contained Mg(NO3)2 inan ethanol/water mixture (1/1, v/v). Nitric acid was used for pHadjustment. Solution B was slowly (3 ml min�1) added into Solu-tion A with continuous stirring for about 3 h. The mixture was sub-sequently aged at 318 K for 12 h to form the gel. The gel was thendried at 393 K for 24 h and calcined with air at a set temperature(723–973 K). The prepared catalysts are hereafter denoted as MT-x-y, where x is the Mg/Ti molar ratio and y is the calcination tem-perature (K). Pure TiO2 was prepared according to the sol–gelmethod without Mg(NO3)2 in Solution B. The drying condition isthe same as above and the calcination temperature was 873 K.

2.3. Catalyst characterization

Powder X-ray diffraction (XRD) patterns of the samples were re-corded on a Rigaku D/Max-rC powder diffractometer using Cu Karadiation (40 kV and 100 mA). The diffractograms were recordedin the 2h range of 10–80� with a 2h step size of 0.02� and a steptime of 10 s.

The specific surface area of each sample was determined by N2

adsorption at 77 K using a Micrometrics Tristar 3000 instrument.The samples were outgassed in a vacuum at 473 K for 24 h priorto analysis.

Fourier transform infrared (FTIR) spectroscopy was done on aNicolet 6700 spectrophotometer equipped with a DTGS detector.The FTIR spectroscopy measurements were performed by mixingsamples into KBr pellets (32 scans with 4 cm�1 resolution).

Samples were analyzed by UV–Vis diffuse reflectance spectros-copy (DRS) using a Scan UV–Vis–NIR spectrophotometer (VarianCary 500) equipped with an integrated sphere assembly. Polytetra-fluoroethylene was used as a reflectance sample.

Thermogravimetry differential thermal analysis (TG-DTA) wasrecorded using an SDT Q600 instrument. The dried samples (priorto calcination) were heated up to 1073 K from room temperature(heating rate of 10 K min�1) in an air stream.

To investigate the leaching of metal content (Ti and Mg) in theFAME phase, the catalyst was separated and centrifuged from theliquid reaction mixture after reaction completion, and then the li-quid mixture was analyzed with inductively coupled plasma atom-ic emission spectroscopy (ICP-AES, TJA IRIS 1000).

2.4. Catalytic tests

The transesterification reaction of WCO and methanol was car-ried out in liquid phase under autogenous pressure in a 100 mlstainless steel batch reactor. This reactor was magnetically stirredand equipped with a heater and a corresponding thermal control-ler. The reaction procedure is as follows: the catalyst was dispersedin methanol contained in the reactor and agitated for 10 min atroom temperature. Then, WCO was added into the mixture of the5 wt% catalyst (based on the weight of WCO) and methanol at amethanol/oil molar ratio of 30. The reactor was sealed and subse-quently heated to 423 K for 6 h under vigorous stirring at a fixedspeed of 1500 rpm. After reaction completion, the reaction mix-tures were collected and centrifuged. The upper layer of the centri-fuged mixture was collected for further purification. Aftermethanol was evaporated, the sample was washed with hot dis-tilled water, dissolved in petroleum ether, and then washed againwith hot distilled water in the refining process. Moisture from thewashed biodiesel was subsequently removed using anhydrous so-dium sulfate.

The biodiesel yield (FAME yield) was measured using a gaschromatograph (GC, FULI9790, China) equipped with a flame ioni-zation detector and a capillary column (30QC3/AC20-0.5, 30 m � id

10 20 30 40 50 60 70 80

RutileMgTiO3

AnatasePericlase

Inte

nsity

, a.u

.

2 Theta, degree

b

c

d

a

Fig. 1. XRD patterns of TiO2–MgO mixed oxides with different Mg/Ti molar ratios:(a) MT-0.5-873, (b) MT-1-873, (c) MT-2-873, and (d) MT-3-873.

400 600 800 1000

f

e

d

c

b

a

Tra

nsm

ittan

ce, a

.u.

Wavenumber, cm-1

-O-Mg-O-Ti-O-

Fig. 2. FTIR spectra of TiO2–MgO mixed oxides with different Mg/Ti molar ratios:(a) TiO2, (b) MT-3-873, (c) MT-2-873, (d) MT-1-873, (e) MT-0.5-873, and (f) MgO.

9572 Z. Wen et al. / Bioresource Technology 101 (2010) 9570–9576

0.32 lm, SGE, Australia). Helium was used as the carrier gas, andmethyl undecanoate was used as an internal standard. The biodie-sel yield was measured according to our previous study (Wen et al.,2010).

To investigate reusability and regeneration, catalysts were cen-trifuged from the reaction mixture, washed with methanol fourtimes, and dried at 393 K. After the forth reuse, the catalysts werecalcined at a set temperature for regeneration. The FAME yield wasthen measured according to the methods mentioned above.

3. Results and discussions

3.1. Influence of Mg/Ti molar ratio

Table 2 shows the physical properties and catalytic perfor-mance of the TiO2–MgO mixed oxides with different Mg/Ti molarratios. A higher level of biodiesel conversion corresponded togreater metal leaching. The addition of Ti resulted in a decreasein FAME yield and a concomitantly lower extent of metal leaching.The MgO catalyst exhibited the highest FAME yield of 89.6% andthe highest leached Mg content of 198 ppm, 1.13 and 8.61 timesthose of MT-1-873, respectively. With an increase in Mg/Ti molarratio from 0.5 to 1, the FAME yield increased substantially from52.2% to 79.1% with a slight increase in leached Mg content from19 to 23 ppm. Beyond a Mg/Ti molar ratio of 1, the increase inFAME yield slowed down, while the leached Mg content increasedremarkably from 23 to 103 ppm. Thus, the Mg/Ti molar ratio of 1 isreasonable. Compared with Mg, the leaching of Ti was negligible atthe ppm level. No obvious catalytic activity could be observed forTiO2 with the FAME yield below 1%. For all samples, the poresare mesoporous with the pore diameters exceeding 10 nm. Thesurface area and pore volume deceased with an increase in Mg/Timolar ratio, eventually giving the lowest values for MT-3-873,which are approximately 20.7% and 26.5% those of MgO, respec-tively. A similar trend of decrease in MgO surface area during Lidoping was also reported by Díez et al. (2006) and Wen et al.(2010). A decrease in specific surface area is unfavorable for mostcatalysis reactions. However, in this case, MT-3-873 showedroughly similar catalytic activity as MgO, which might be relatedto the changes in the active sites on the catalyst surface due to Tiaddition.

Fig. 1 shows the XRD patterns of TiO2–MgO mixed oxides withvarious Ti content. For all samples, the diffraction peaks of the peri-clase and anatase phases were clear, indicating that Mg and Tiwere partially present in the form of periclase and anatase crystal-lites, respectively. Peaks for MgTiO3 (geikielite) were also clearlyobserved in all samples. To further investigate the species presenton the catalyst surface, samples were analyzed by FTIR spectros-copy. As shown in Fig. 2, for the samples with high Ti content,MT-1-873 and MT-0.5-873, the FTIR spectra show bands at 560,477, and 452 cm�1, characteristic of the bending modes of –O–Ti–O–Mg–O– (Lopez et al., 2008). For pure magnesia, the bands

Table 2Physical properties and catalytic activities of TiO2–MgO mixed oxides.

Sample Mg/Ti molar ratioa Surface area (m2 g�1) Pore diameter (nm)

MgO – 144.3 13.1MT-0.5-873 0.52 73.5 10.6MT-1-873 0.93 60.6 12.8MT-2-873 2.01 42.9 17.5MT-3-873 3.22 29.8 21.4TiO2 – 76.6 11.9

a Measured by ICP-AES.b Reaction conditions: temperature of 423 K, methanol/oil molar ratio of 30, catalyst

were undetectable. This result is in good accordance with XRD datasince the formation of MgTiO3 was observed. A similar observationwas also reported by Lopez et al. (2008). They prepared TiO2–MgOoxides by the sol–gel method and found that MgTiO3 could beformed by the parent oxides due to the substitution of Ti for Mgions in the magnesia lattice. Therefore, this confirms the same sub-stitution behavior in our study.

As the Mg/Ti molar ratio decreased from 3 to 1, Fig. 1 shows thatthe XRD peak intensity representing the anatase and rutile phasesincreased, while those for periclase decreased. For periclase, the ra-tio of the peak intensity at 2h = 43� [corresponding to (2 0 0) reflec-tion] to those of other characteristic peaks decreased with an

Pore volume (cm3 g�1) Biodiesel yield (%)b Metal leaching (ppm)a

Mg Ti

0.49 89.6 198 –0.32 52.2 19 70.18 79.1 23 90.17 80.2 76 70.13 85.3 103 90.44 <1% – 3

amount of 5 wt%, reaction time of 6 h, and stirring speed 1500 rpm.

Z. Wen et al. / Bioresource Technology 101 (2010) 9570–9576 9573

increase in Ti content. The unit cell of periclase is a face-centeredcubic lattice. The number of Mg ions achieves the biggest valueof 2 on the (2 0 0) plane, resulting in a maximum probability ofsubstitution by Ti ions. Thus, the peak intensity for the (2 0 0)reflection decreased more considerably than those of others.

When the Mg ions are substituted by Ti ions in the magnesialattice, some vacancies are created to maintain charge neutralityin the ionic crystal. The Ti ion has a valence of 4, higher than thatof 2 for the Mg ion. These vacancies induce defects that are impor-tant for heterogeneous catalysis (Tanabe et al., 1989). To clarify theformation of defects by Ti addition, the UV–Vis spectra of the sam-ples were obtained (see Fig. 3). As for the TiO2–MgO mixed oxidesof MT-1-873, the absorption bands were detected at 274 and380 nm, assigned to be due to the surface O2� ions of coordinationnumbers 3 and 4 (Hattori, 1995; Verziu et al., 2008). These twoabsorption bands (274 and 380 nm) were not observed for TiO2,MgO and other TiO2–MgO samples except MT-1-873. We can con-clude that the addition of Ti to MgO can result in an obvious forma-tion of defect species. Among the ion pairs of different coordinationnumbers, the ion pairs of 3-fold Mg2+-3-fold O2� (Mg2þ

3c —O2�3c ) are

widely accepted as the most reactive (Hattori, 1995). Therefore,the TiO2–MgO mixed oxides are expected to exhibit higher cata-lytic activities than MgO. However, contrary to this assumption,MgO was the most active catalyst among all the samples (Table2). It also showed the highest leached Mg content into the polarphase (methanol). The leaching behavior generated the largestamount of methoxide magnesium as the homogeneous catalyst,thus promoting biodiesel yield due to the superior mass transferfor this homogeneous catalyst. To further investigate the effect ofmetal leaching on the catalytic activities, the MgO and MT-1-873were separately mixed with methanol for 6 h under the same tem-perature, weight ratio of catalyst to methanol and stirring speed aslisted in Table 2. The methanol was then separated from the cata-lyst and used for reaction. The reaction conditions such as temper-ature, methanol/oil molar ratio, stirring speed, and reaction timeare the same as Table 2. The FAME yield of 53.2% and 2.1% wereachieved for the separated methanol from MgO and MT-1-873,respectively. This result definitely indicates that the highest cata-lytic activity of MgO is most likely attributed to the largest amountof homogeneous methoxide magnesium by the metal leaching. Asshown in Table 2, although MT-1-873 is slightly inferior to MgO inthe FAME yield, the leaching Mg content of 23 ppm for MT-1-873 isremarkably lower than that of 198 ppm for MgO. Therefore, it canbe concluded that the defects induced by Ti addition can improvethe stability while maintaining an acceptable catalytic activity.

200 250 300 350 400 450 500

Abs

, a.u

.

Wavelength, nm

a

b

c

d

e

f

Fig. 3. Diffuse reflectance UV–Vis spectra of TiO2–MgO mixed oxides with differentMg/Ti molar ratios: (a) MgO, (b) MT-0.5-873, (c) MT-1-873, (d) MT-2-873, (e) MT-3-873, and (f) TiO2.

3.2. Influence of calcination temperatures

Since the calcination temperature largely affects the structuraland catalytic properties of the resultant catalysts, the catalyst pre-cursor was calcined at various temperatures in order to optimizethis parameter. The Mg/Ti molar ratio was fixed at 1, since thiswas the best value based on an assessment of the activity and sta-bility (see Table 3). The calcination temperature (723–973 K) forthe TiO2–MgO mixed oxides had an important influence on cata-lytic activity (Table 3). The MT-1-923 catalyst, which was calcinedat 923 K, exhibited the highest FAME yield (79.9%). At the sametime, the leached Mg content decreased with increasing tempera-ture. The leached Mg content was 47 ppm at 723 K, roughly 2.5times that at 973 K. Samples were analyzed by TG and XRD to fur-ther investigate the effects of the calcination temperature. The TGresults of the dried catalyst precursor with a Mg/Ti molar ratio of 1are illustrated in Fig. 4. Two typical weight loss regions were pres-ent at different temperature ranges: 373–603 and 623–833 K. Theformer was mostly linked to the loss of physically adsorbed watermolecules and decomposition of Ti(OH)4 (Inbana et al., 2006). Thelatter was mainly attributed to the decomposition of Mg(NO3)2 andthe re-structuring caused by TiO2 (Umdu et al., 2009). Therefore, ahigh temperature was favorable for the reaction of two parent oxi-des (Bokhimi et al., 1999). The exothermic peak at about 625 K wasassigned to the combustion of the organic derivatives in the sam-ples (Enhessari et al., 2010). Beyond 833 K, the weight kept con-stant. An exothermic peak appeared at 920–940 K, which wasmost likely related to the atomic rearrangement (Lopez et al.,2008). To confirm this tendency observed in this study, XRD pat-terns of TiO2–MgO mixed oxides were measured as a function ofcalcination temperature. As shown in Fig. 5A, at low temperatures(723–823 K), the sample showed periclase and anatase phases,while rutile and magnesium–titanate phases could be detected athigh calcination temperatures beyond 873 K. To further under-stand the surface states of TiO2–MgO samples calcined at differenttemperatures, the UV–Vis spectra were investigated. As depicted inFig. 5B, the bands at 274 and 380 nm were observed only with thecalcination temperature exceeding 873 K. Both the XRD and UV–Vis results indicate that the substitution of Ti ions for Mg ions inthe magnesia lattice is generated when the calcination tempera-ture exceeds 873 K in this case. This substitution explains whythe leached Mg content decreased with increasing temperature.As shown in Table 3, the specific surface area and pore volume de-creased with an increase in calcination temperature, most likelyleading to an inferior catalytic activity for the sample calcined at973 K.

3.3. Influence of reaction parameters on the FAME yield

The effect of methanol/oil molar ratio on biodiesel synthesiswas studied for MT-1-923. The results are summarized in Table4. The FAME yield increased remarkably from 52.0% to 85.6% whenthe methanol/oil molar ratio increased from 20 to 50. This increaseslowed down beyond the ratio of 50. Since excessive methanolbrings about difficulties in the heating and elimination process,the molar ratio of methanol to oil was reasonably maintained ataround 50.

The effect of the amount of catalyst on the performance of MT-1-923 was investigated. The catalyst weight was varied in therange of 5–15 wt%. As shown in Table 4, the biodiesel yield in-creased with an increase in catalyst content from 5 to 10 wt%.Upon further increase in the catalyst amount, the FAME yield be-came constant. Hence, a catalyst amount of 10 wt% was deemedreasonable for this application.

The influence of reaction temperature on the catalytic activityof MT-1-923 is shown in Table 4. The FAME yield increased with

Table 3Effects of calcination temperatures on the surface area, pore properties and catalytic activities of TiO2–MgO mixed oxides.

Sample Surface area (m2 g�1) Pore diameter (nm) Pore volume (cm3 g�1) Biodiesel yield (%)a Metal leaching (ppm)b

Mg Ti

MT-1-723 136.9 7.7 0.29 68.1 47 5MT-1-773 112.2 9.2 0.22 69.3 33 3MT-1-823 96.5 9.7 0.19 77.5 26 5MT-1-873 60.6 12.8 0.18 79.1 23 9MT-1-923 36.3 16.1 0.16 79.9 19 7MT-1-973 18.5 20.9 0.12 60.8 19 5

a Reaction conditions: temperature of 423 K, methanol/oil molar ratio of 30, catalyst amount of 5 wt%, reaction time of 6 h and stirring speed of1500 rpm.

b Measured by ICP-AES.

300 400 500 600 700 800 900 1000 1100

40

50

60

70

80

90

100

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Wei

ght L

oss,

%

Temperature, K

Hea

t Flo

w, W

/g

Fig. 4. TG-DTA analysis of the MT-1-873 precursor.

10 20 30 40 50 60 70 80

RutileMgTiO3

Inte

nsity

, a.u

.

2 Theta, degree

AAnatasePericlase

a

b

c

d

e

f

200 250 300 350 400 450 500

f

e

d

c

b

a

Abs

, a.u

.

Wavelength, nm

B

Fig. 5. XRD patterns (A) and diffuse reflectance UV–Vis spectra (B) of TiO2–MgOmixed oxides with different calcinations temperatures: (a) MT-1-723, (b) MT-1-773, (c) MT-1-823, (d) MT-1-873, (e) MT-1-923, and (f) MT-1-973.

9574 Z. Wen et al. / Bioresource Technology 101 (2010) 9570–9576

an increase in the reaction temperature. At 433 K, the methyl esteryield was 92.3%. Beyond 443 K, the FAME yield decreased. A hightemperature is favorable for biodiesel conversion because the reac-tion is endothermic. On the other hand, the amount of methanolevaporation rose with increasing temperature, resulting in a de-crease in the methanol amount available for the reaction. Thesetwo combined effects resulted in the best reaction temperatureof 433 K.

3.4. Effect of water content on the FAME yield

As most solid base catalysts are sensitive to water, the effect ofthe water content of WCO on the activity of the best catalyst (MT-1-923) was studied under the best reaction conditions mentionedabove (methanol/oil molar of 50, catalyst amount of 10 wt% andreaction temperature of 433 K).The FAME yield of 85.3% was ob-tained without any pre-treatment on WCO (water content of1.9 wt%). The FAME yield reached 91.7%, 91.1%, and 89.9% withthe water content of 0.5, 1.0, and 1.5 wt%, respectively. Hence,the FAME yield decreased slightly with the increase in water con-tent. This slight decrease might be caused by the hydrolysis ofmethyl ester in the reaction process (Lopéz et al., 2005).

3.5. Catalyst deactivation and regeneration

Table 5 presents the results of the reuse and regeneration ofMT-1-923. For the fresh sample, the biodiesel yield was 92.3%.After each reaction, the catalyst was separated, washed by metha-nol, and then dried for reuse. The FAME yield decreased slowlyfrom 92.3% to 81.2% when this process was repeated 1–4 times,indicating that this catalyst was stable for biodiesel productionstarting from WCO. This stability resulted from the defects induced

by the substitution of Mg ions. Apart from the leaching of activespecies, there are two other reasons behind catalyst deactivation:surface poisoning and pore-filling (Lopéz et al., 2005). Becausethe catalysts were washed four times to reduce the effect of surfacepoisoning, pore-filling may be the likely candidate for the decreasein catalytic activity in the reuse process. Fig. 6 shows the TG-DTAanalysis of the MT-1-923 catalyst after the first use. Weight loss oc-curred at 423–773 K with an exothermic peak at around 603 K.This peak corresponds to the decomposition of organic species (Ha-mad et al., 2008), indicating that the pores of the catalyst are filledby organic species, such as triglycerides and glycerol. Pore fillingled to the low accessibility of the active sites to the reactants,which reduced the FAME yield.

Table 4Effects of reaction parameters on the performance of the MT-1-923.

Methanol/oil molarratio

Catalyst amount(wt%)

Temperature(K)

Biodiesel yielda

(%)

20 5 423 52.030 5 423 79.940 5 423 83.550 5 423 85.660 5 423 85.350 6 423 86.950 8 423 89.950 10 423 91.250 12 423 91.250 15 423 89.350 10 403 22.350 10 413 67.650 10 433 92.350 10 443 91.6

a Reaction conditions: stirring speed of 1500 rpm and reaction time of 6 h.

Table 5Catalytic activities of the MT-1-923 with reuse and regeneration.

Catalytic run Biodiesel yield (%)a

Washing only bymethanol

Two-stepwashing

Fresh 92.3 92.3First reuse 86.6 90.1Second reuse 85.2 89.7Third reuse 81.9 86.2Fourth reuse 81.2 82.5Regeneration after the fourth

reuse93.8 –

a Reaction conditions: methanol/oil molar ratio of 50, catalyst amount of 10 wt%,temperature of 433 K, reaction time of 6 h and stirring speed of 1500 rpm.

300 400 500 600 700 800 900 100095

96

97

98

99

100

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

Wei

ght L

oss,

%

Temperature, K

Hea

t Flo

w, W

/g

Fig. 6. TG-DTA analysis of MT-1-923 after the initial use.

Z. Wen et al. / Bioresource Technology 101 (2010) 9570–9576 9575

As the efficiency of reuse depends on washing methods, a two-step washing method (after the catalyst was separated, the catalystwas washed by methanol four times and subsequently by n-hex-ane once before being dried at 393 K) was used to investigate theeffect of the washing method. As shown in Table 5, the biodieselyields by two-step method were higher than those by only meth-anol washing. However, this two-step washing will bring about acost increase in the reuse process. Detailed investigations on thedesirable washing method in terms of catalytic activity and wash-ing cost will be carried out in our future work.

Catalyst regeneration is an important process as it presentsadvantages to industrial production costs. The regeneration of

MT-1-923 after the fourth reuse was investigated. Interestingly,Table 5 shows that a higher FAME yield (93.8%) can be achievedfor the regenerated catalyst compared with that of the fresh sam-ple (92.3%). The BET surface area, pore volume, and average porediameter of the regenerated MT-1-923 are 42.7 m2 g�1,0.21 cm3 g�1, and 21.4 nm, which are larger than those of the freshsample. This gave rise to the increase in biodiesel yield since theBET surface area and pore diameter are beneficial to most reactionsassociated with organic macromolecules (Lopéz et al., 2005). Themechanism behind this behavior is currently being investigatedin our lab for future publications. The results of deactivation andregeneration definitely indicated that the TiO2–MgO mixed oxidecatalyst has a huge potential in large-scale, improved biodieselproduction starting from WCO.

4. Conclusions

The TiO2–MgO mixed oxide catalysts prepared by the sol–gelmethod were successfully used for biodiesel production. The addi-tion of Ti resulted in the substitution of Ti ions for Mg ions in themagnesia lattice, thus leading to defects on the catalyst surface.These defects can improve the stability while maintaining anacceptable catalytic activity. The best catalyst was determined tobe MT-1-923, which is comprised of an Mg/Ti molar ratio of 1and calcined at 923 K, based on an assessment of the activity andstability of the catalyst. For the MT-1-923, the catalytic activity de-creased slowly within the reuse processes. After regeneration, theactivity of MT-1-923 slightly increased compared with that ofthe fresh catalyst due to an increase in the specific surface areaand average pore diameter. The mixed oxides catalyst, TiO2–MgO, showed tremendous potential in large-scale biodiesel pro-duction from waste cooking oil.

Acknowledgements

We are grateful for the financial support given by the SwedishResearch Links Programme of the Swedish Research Council (VR)and the China Natural Science Foundation (Contract No. 20606011).

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