Simultaneous transesterification and esterification of unrefined or wasteoils over ZnO-La2O3 catalysts

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					                                                               Applied Catalysis A: General 353 (2009) 203–212

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Simultaneous transesterification and esterification of unrefined or waste
oils over ZnO-La2O3 catalysts
Shuli Yan, Steven O. Salley, K.Y. Simon Ng *
Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI 48202, USA

A R T I C L E I N F O                                A B S T R A C T

Article history:                                     A single-step method was developed for biodiesel production from unrefined or waste oils using a series
Received 10 June 2008                                of heterogeneous zinc and lanthanum mixed oxides. Effects of metal oxide molar ratio, free fatty acids
Received in revised form 17 October 2008             (FFA) and water content in feedstock, molar ratio of methanol and oil, and reaction temperature on the
Accepted 27 October 2008
                                                     yield of biodiesel were investigated. A strong interaction between Zn and La species was observed with
Available online 11 November 2008
                                                     enhanced catalyst activities. Lanthanum promoted zinc oxide distribution, and increased the surface acid
                                                     and base sites. The catalyst with 3:1 ratio of zinc to lanthanum was found to simultaneously catalyze the
                                                     oil transesterification and fatty acid esterification reactions, while minimizing oil and biodiesel
                                                     hydrolysis. A reaction temperature window of 170–220 8C was found for the biodiesel formation. A high
Unrefined and waste oils
Transesterification                                   yield (96%) of fatty acid methyl esters (FAME) was obtained within 3 h even using unrefined or waste oils.
Esterification                                                                                                                        Published by Elsevier B.V.

1. Introduction                                                                           A conventional operation usually uses strong basic or acidic
                                                                                       solutions (i.e., NaOH, KOH and H2SO4) as catalyst and food-grade
   Biodiesel, a renewable fuel with similar combustion properties                      vegetable oils as raw material. These homogeneous catalysts are
to fossil diesel, is normally produced by transesterification of                        quite sensitive to free fatty acids (FFA) and water in the oil
highly refined oils with short-chain alcohols. Biodiesel can                            feedstocks and alcohols. FFA reacts with the basic catalyst
significantly decrease the exhaust emission of CO2, SOx and                             (NaOH, KOH) and forms soaps. This soap formation complicates
unburned hydrocarbons from motor vehicles [1,2]. Biodiesel is                          the glycerol separation, and drastically reduces the methyl ester
environmentally beneficial, and therefore, is a promising alter-                        yield. Water in the feedstock leads to hydrolysis of oils and fatty
native to fossil diesel [3].                                                           acid methyl esters (FAME) in the presence of strong basic or
   Transesterification reaction of triglycerides for the production                     acidic catalysts. Thus, some inexpensive oils, such as crude
of biodiesel is as follows:                                                            vegetable oils, waste cooking oil, and rendered animal fats,

                                                                                       which generally contain a high content of FFA and water, cannot
                                                                                       utilize homogeneous catalysts directly. Furthermore, the water
 * Corresponding author. Tel.: +1 313 577 3805; fax: +1 313 578 5814.                  content in alcohols is also an important issue in traditional
   E-mail address: (K.Y. Simon Ng).                                  processes. Since alcohols are hygroscopic, the recovered alcohols

0926-860X/$ – see front matter . Published by Elsevier B.V.
204                                           S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212

must be dried thoroughly to remove the water from the azeo-                     2. Experiments
tropes. For conventional processes using homogenous catalysts,
the FFA content in the feedstock must be lower than 0.50                        2.1. Materials
(wt%) [4] and water content lower than 0.06 (wt%) [5]. Usually,
highly refined oils are used in conventional methods for bio-                        Food-grade soybean oil was purchased from Costco warehouse
diesel production. According to the calculation of Haas et al. [6],             (Detroit, MI), crude soybean oil from BDI (Denton, TX), crude palm
the cost of oil feedstock accounts for up to a total of 88% of                  oil from Malaysia Palm Oil Board (Selangor, Malaysia) and waste
biodiesel production cost in traditional processes. With recent                 cooking oil was obtained from a local restaurant. The fatty acid
increases in refined oil prices, the cost of oil feedstock                       compositions of these four kinds of oil were determined by GC–MS
accounts for an even higher fraction of the total production                    (Table 1). Oleic acid and methyl alcohol were obtained from the
cost. Thus, it is important to develop new catalytic processes                  Mallinckrodt Chemicals (Phillipsburg, NJ), with water contents of
which can handle unrefined and waste oils directly to lower the                  0.02% and 0.03%, respectively. Zinc nitrate hexahydrate (98%),
cost of biodiesel.                                                              lanthanum nitrate hydrate (98%), and urea (99%) are of analysis
    Recently, an acid- and alkali-catalyzed two-step method for                 grade, and were purchased from Sigma–Aldrich Company (St.
biodiesel production using some unrefined or waste oils as raw                   Louis, MO).
materials has been reported [7,8]. In this method, an acidic catalyst
(H2SO4, HCl) is initially used to convert FFA to the esters, and then           2.2. Catalyst preparation and characterization
in the second stage, transesterification of oil is performed using an
alkaline catalyst (NaOH, KOH). Although this method can utilize                     A homogeneous-coprecipitation method [20] was used to
unrefined or waste oils for biodiesel production, the process                    prepare catalyst samples. First, 2 M Zn(NO3)2 and 1 M La(NO3)3
requires multiple reactions, washing, and separation stages. The                were prepared with distilled water. Then, solutions with varying
strong acidic or basic catalysts used are highly corrosive, and must            ratios of Zn–La (1:0, 1:1, 3:1, 9:1, 0:1) were mixed with a 2 M urea
be removed from the biodiesel product by multiple washing. Thus,                solution. The mixture was boiled for 4 h, and then dried at 150 8C
a significant amount of waste water is generated, together with a                for 8 h, followed by step-rising calcination at 250 8C, 300 8C, 350 8C,
loss of catalyst [9,10].                                                        400 8C, finally at 450 8C for 8 h. The catalysts are noted as Zn10La0,
    Therefore, it is advantageous to develop a new class of                     Zn1La1, Zn3La1, Zn9La1, Zn0La10 according to their catalyst
heterogeneous catalysts, which has a higher tolerance to water                  compositions.
and FFA in oils, and can simultaneously catalyze both the                           Powder X-ray diffraction (XRD) patterns were taken with a
esterification and transesterification reactions. There are reports               Rigaku RU2000 rotating anode powder diffractometer (The
of heterogeneous catalytic esterification of fatty acids [11,12]                 Woodlands, TX) equipped with CuKa radiation (40 kV, 200 mA).
and transesterification of highly refined oils [13–16]. Moreover,                 Scanning electron microscopy (SEM) and energy dispersive
Omota et al. [12] reported a process of fatty acid esterification                spectrometer (EDS) were taken with a Hitachi S-2400 Scanning
using zirconium salts as catalysts. Suppes et al. [13] reported                 Electron Microscope (San Jose, CA). Maximum operating voltage
that some salts and oxides of magnesium, calcium, and zinc are                  used was 25 kV. N2 adsorption and desorption isotherms were
active in transesterification. Serio et al. [14] used Mg–Al calcined             measured at 77 K with a Quantachrome AS-1MP volumetric
hydrotalcites in the tranesterification of refined soybean oil,                   adsorption analyzer. Before adsorption measurements, all the
which contained 0.1 (wt%) FFA, at high temperature. They found                  samples were outgased for 12 h at 300 8C.
that this type of heterogeneous catalyst is more tolerant to                        XPS analysis was performed with a PHI 5500 system
water and the catalytic performance was not affected by the                     (PerkinElmer, Wellesley, MA), using a monochromatic Al Ka X-
presence of 1 (wt%) of water in oils. Recently, Yan et al. [17]                 ray radiation source (1486.6 eV) and AugerScan system control
reported that supported CaO catalysts directly catalyzed the                    (RBD Enterprises, Bend, OR). Elemental concentration on the
reaction of some crude oils into biodiesel when water content in                sample surface was measured by XPS multiplex scan (spot size:
the oils was less than 2 (wt%) and FFA content less than 3.5                    $1 mm diameter).
(wt%). However, further increases of FFA and water content in
oils could inhibit the transesterification. The mechanism for the                2.3. Biodiesel reactions and product analysis
improved tolerance to water and FFA of this type of catalysts
was not fully elucidated. Bournay and Hillion [18] stated in a                     Catalytic transesterification, esterification and hydrolysis reac-
patent that oils with high content of FFA can be directly used                  tions were conducted in a 500 mL stainless steel stirred reactor
with a Lewis acid catalyst (zinc aluminate) for biodiesel                       (Parr 4575 HT/HP Reactor). For transesterification, 126 g of oil,
production. However, that catalyst was quite sensitive to water                 180 g of methanol, and 3 g of catalyst were used. During the
and the limitation of water content in oils is 0.15 (wt%).
Sreeprasanth et al. [19] prepared other types of Lewis acid                     Table 1
catalysts containing zinc and iron and used them in transester-                 Fatty acid methyl esters, FFA and water content of food-grade soybean oil, crude
                                                                                soybean oil, crude palm oil and waste cooking oil.
ifying unrefined and used oils. He found that the catalyst is
active in both esterification and transesterification. However,                   Fatty acid              Food-grade       Crude            Crude         Waste
the catalyst is also active in the hydrolysis of FAME to FFA, thus              components              soybean          soybean          palm          cooking
                                                                                                        oil (%)          oil (%)          oil (%)       oil (%)
the overall yield of biodiesel is decreased. In this paper, we
prepared and characterized a new class of ZnO-La2O3 hetero-                     C 14:0                   0                0.27             0.21          0
geneous catalysts with different Zn:La ratios. The effects of                   C 16:0                  11.07            13.05            41.92         11.58
                                                                                C 16:1                   0.09             0.39             0.23          0.18
metal oxide ratio on the Lewis acid and base sites, and on the
                                                                                C 18:0                   3.62             4.17             3.85          4.26
transesterification of triglyceride, esterification of fatty acid,                C 18:1                  20.26            22.75            42.44         24.84
and hydrolysis of triglyceride and fatty acid methyl esters,                    C 18:2                  57.60            52.78            11.30         53.55
were investigated. These mixed oxide catalyst shows very                        C 18:3                   7.36             6.59             0.04          5.60
                                                                                FFA content              0.02             3.31             0.24          3.78
promising results for processing unrefined and waste oils dire-
                                                                                Water content            0.02             0.27             0.04          0.06
ctly into biodiesel.
                                                        S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212                                                    205

reaction, 15 mL of reaction mixture was collected via a two-stage
liquid sampler at different time intervals. When the reaction was
completed, the catalyst was filtered out. The liquid product
obtained was vaporized to remove excessive methanol, and then
settled in a separating funnel. The upper layer in the separating
funnel (mainly containing fatty acid methyl esters) was char-
acterized by a GC–MS spectrometer (Clarus 500 MS System,
PerkinElmer, Shelton, CT) equipped with a capillary column (Rtx-
WAX Cat. no.12426) (Bellefonte, PA). Methyl arachidate (Nu-Chek
Prep Inc, Elysian, MN) was used as an internal standard.
    Esterifications were performed with 126 g of oleic acid, 180 g of
methanol, and 3 g of Zn3La1 catalyst. Yield of oleic acid methyl
ester were determined by GC–MS. And oleic acid concentration
was determined using a Brinkman/Metrohm 809 titrando (West-
bury, NY) according to ASTM D 664.
    Hydrolysis of oil and hydrolysis of biodiesel were conducted
with 285 g of food-grade soybean oil or soybean biodiesel (Wacker
Oil, Manchester, MI), 15 g of water, and 2.3 (wt%) of Zn3La1
catalyst. Yield of FFA was determined by Brinkman/Metrohm 809
titrando (Westbury, NY) according to ASTM D 664. Water content                            Fig. 2. Transesterification activities of Zn10La0, Zn9La1, Zn3La1, Zn1La1 and
was analyzed using a Brinkman/Metrohm 831 KF Coulometer                                   Zn0La10 at 200 8C. Reaction mixtures are 126 g of oil, 180 g of methanol, 3 g of
                                                                                          catalyst. Zn3La1 shows the highest activity compared to other catalysts.
(Westbury, NY) according to ASTM D 6304-00.

3. Results and discussion                                                                    To investigate the effect of catalyst composition on transester-
                                                                                          ification, mixed oxides with different molar ratio of zinc to
3.1. Catalytic activity of zinc lanthanum mixed oxides in oil                             lanthanum were tested at 200 8C (Fig. 2). The times needed to reach
transesterification                                                                        chemical equilibrium were about 60 min, 80 min, 120 min,
                                                                                          150 min and 200 min for Zn3La1, Zn0La10, Zn1La1, Zn9La1 and
    Triglycerides are the major component of vegetable oils. In this                      Zn10La0, respectively. The catalyst with 3:1 molar ratio of zinc to
study, a food-grade soybean oil, which contained 99.5 (wt%) of                            lanthanum showed the highest activity in oil transesterification.
triglycerides, was used as the model for most of the studies.                                The effect of reaction temperature on Zn3La1– catalyzed
    A step-rising heating method (heating rate of 2 8C/min, and held                      transeseterification process was shown in Fig. 3. Since the
at the target temperature for 1 min) was used to compare the                              methanol concentration was kept in excess, a power rate law
catalytic activities of mono-metal oxide (Zn10La0, Zn0La10) and                           model can be written as:
mixed metal oxides (Zn3La1) for the oil transesterification reaction                                a
                                                                                          Àg ¼ k1 Coil
(Fig. 1). All three catalysts show activities in catalyzing the
transesterification reaction when the reaction temperature is                              where r is the reaction rate (for <50.00% FAME yield), k1 is the
higher than 170 8C; while without any catalyst, the transester-                           apparent reaction rate constant, Coil is the oil concentration, and a
ification reaction starts at 220 8C. The mixed oxides catalyst clearly                     is the reaction order. Based on the data in Fig. 3, the apparent
shows an enhanced activity as compared to the pure oxides.                                reaction order to the oil was found to be 1.2. The apparent
                                                                                          activation energy Eapp was 90.9 KJ molÀ1, which was significantly
                                                                                          higher than the reported activation energy (19.9 KJ molÀ1) using

Fig. 1. Transesterification activities of Zn10La0, Zn3La1 and Zn0La10 as a function of
temperature. Reaction mixtures were heated at 2 8C/min and maintained at target           Fig. 3. Transesterification with different molar ratio of methanol to oil with Zn3La1
temperatures for 1 min. Zn3La1 shows the highest activity compared to Zn10La0             at 200 8C. Reactants are 3 g of catalyst and 300 g of oil and methanol mixture. 36–42
and Zn0La10.                                                                              of molar ratio of methanol to oil shows high yield of FAME.
206                                                   S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212

NaOH catalyst [21]. The higher activation energy is expected of a
heterogeneous catalytic system.
   The effect of molar ratio of methanol to oil was shown in Fig. 4.
In traditional homogeneous processes, a 6:1 molar ratio of
methanol to oil is usually used for biodiesel production. However,
in Fig. 4 it gave poor yields of FAME in Zn3La1 catalyzed process,
and higher molar ratios of methanol to oil (36:1 to 42: 1) led to
yields higher than 95% within 60 min. The optimal molar ratio is
suggested to be 36:1. The excess methanol promoted the
transesterification reaction forward, and extracted products
(FAME and glycerin) from reactants to renew the catalyst surface

3.2. Catalytic activity of zinc lanthanum mixed oxides in fatty acid

   In unrefined or waste oils, FFA concentration can be very high
(0.5–30%) [22]. Therefore, developing a catalyst which can
simultaneously esterify FFA and transesterify triglyceride into
biodiesel is important. As fatty acids have similar chemical                            Fig. 4. Transesterification at different reaction temperatures with Zn3La1. Reactant
properties, oleic acid was used as a model FFA for the esterification                    mixtures are 126 g of oil, 180 g of methanol, 3 g of catalyst.

study. The esterification reaction with methanol is as follows:

   Fig. 5a presents the results of using Zn3La1 in esterifying pure                     3.3. Catalytic activity of zinc lanthanum mixed oxides in biodiesel and
oleic acid with methanol using a step-rising heating method                             oil hydrolysis
(heating rate of 2 8C/min, and held at the target temperature for
1 min). When the temperature was higher than 140 8C, esterifica-                            Unrefined or waste oils generally contain a high content of
tion activity was observed. At 200 8C, a 96.7% yield of oleic acid                      water. Thus, fatty acid methyl esters and triglycerides hydrolysis
methyl ester can be obtained within 10 min (Fig. 5b).                                   are undesirable side reactions that may decrease the yield of FAME
   Since FFA can exist in considerable amount in unrefined or                            and increase the acidity of reaction mixtures. Fig. 7 illustrates that,
waste oil feedstock, investigation of esterifying FFA with methanol                     without zinc and lanthanum mixed oxides catalyst hydrolysis
in the presence of triglycerides is important. A mixture oil                            reactions were not observed even for reaction temperature up to
containing 5.4 (wt%) of oleic acid and 94.6 (wt%) of food-grade                         250 8C. In the presence of Zn3La1, hydrolysis occurs at tempera-
soybean oil was prepared and reacted with excess methanol at                            tures above 220 8C. As hydrolysis is not desirable, a reaction
200 8C. The results are shown in Fig. 6. Oleic acid content in mixed                    temperature lower than 220 8C is necessary. A final yield of 95.0%
oil quickly decreased in the first 10 min. The yield of FAME                             FAME can be obtained with a mixture of 5.3 (wt%) water and 94.7
approached 95.3% after 60 min. Fig. 6 suggests that, on Zn3La1, the                     (wt%) food-grade soybean oil at 200 8C (Fig. 8). The water contents
esterification reaction is rapid and occurs simultaneously with the                      in the reactant mixture were found to maintain at about 5.2%
transesterification of triglycerides in a single-step process.                           during the process. The FFA contents were very low (<0.01%),

Fig. 5. Esterification activity of Zn3La1 in pure oleic acid (a) yield of oleic methyl esters as a function of temperature. Reaction mixtures were heated at 2 8C/min and
maintained at target temperatures for 1 min. Zn3La1 shows the esterification activity over 140 8C; (b) yield of oleic methyl esters at 200 8C.
                                                          S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212                                    207

                                                                                            3.4. Effects of FFA and water on transesterification

                                                                                                FFA and water are usually considered as poisons to both
                                                                                            homogeneous acidic and basic catalysts in traditional biodiesel
                                                                                            production processes [4,5]. In order to examine the effects of FFA
                                                                                            and water, the activity of Zn3La1 catalyst on food-grade soybean
                                                                                            oils with 5.20%, 10.13%, 15.21% and 30.56% of oleic acid, with 1.03%,
                                                                                            3.12% and 5.07% of water, and with 5.16% of oleic acid and 3.10% of
                                                                                            water were examined. Fig. 9a also shows that the addition of oleic
                                                                                            acid accelerated the reaction rate and shortened the time to a high
                                                                                            yield of FAME. For example, without FFA addition, 60 min is needed
                                                                                            to reach completion; while for oil with 5.20% FFA addition, the time
                                                                                            to reach completion is only about 20 min. With further increase in
                                                                                            FFA addition, the time to reach completion decreased. In Fig. 9b,
                                                                                            the effect of FFA addition on the equilibrium yields of FAME using
                                                                                            H2SO4, NaOH and Zn3La1 as catalyst is compared. A sharp decrease
                                                                                            in FAME yield was observed in the processes that use H2SO4 or
Fig. 6. The FAME yield and oleic acid content as a function of time at 200 8C. Reaction     NaOH as catalyst [23–25]. Fig. 9b shows that even with 5.20% FFA,
mixtures are 126 g of the oil containing 5.4 (wt%) oleic acid and 94.6 (wt%)                FAME yield decreased to 78% and 88% in NaOH catalyzed and
triglycerides, 180 g of methanol, 3 g of Zn3La1 catalyst.
                                                                                            H2SO4 catalyzed processes, respectively. However, FAME yield was
                                                                                            maintained at 96.6% in Zn3La1 catalyzed process. More difference
                                                                                            can be observed for 30.56% FFA addition, FAME yield decreased to
                                                                                            10% and 60% in NaOH catalyzed and H2SO4 catalyzed processes,
                                                                                            respectively; while FAME yield was still maintained at 96.0% in
                                                                                            Zn3La1 catalyzed process. The result indicates that in comparison
                                                                                            with NaOH and H2SO4, Zn3La1 has a remarkable tolerance to FFA in
                                                                                            the transesterification reaction. Thus, this class of mixed metal
                                                                                            oxides is very promising for direct conversion of acidic oils to FAME
                                                                                            in a single-step process.
                                                                                                Effects of water on FAME yield using Zn3La1 as catalyst are
                                                                                            shown in Fig. 9c and d. It was found that the addition of water
                                                                                            prolonged the time to reach completion. For example, with 3.12%
                                                                                            water added to oil, the time to completion was prolonged from
                                                                                            60 min to 90 min. Further increase of water led to the increase of
                                                                                            reaction time to equilibrium. However, the equilibrium yield of
                                                                                            FAME was around 97% regardless of the water content. On the
                                                                                            other hand, for H2SO4 and NaOH catalyzed processes, water
                                                                                            showed considerable effect on transesterification activity (Fig. 9d)
                                                                                            [23–25]. When water addition in oils was 5.20%, FAME yields
                                                                                            decreased to 78% and 11% for NaOH and H2SO4 catalyst,
Fig. 7. Hydrolysis of biodiesel and oil in presence or absence of Zn3La1 catalyst.
Reaction mixtures are 285 g of oil or biodiesel and 15 g of distilled water.                3.5. Single-step conversion of unrefined and waste oils

which are beyond the detection limit of the titrator. Fig. 8 suggests                          Zn3La1 was demonstrated to be active in the transesterification
that at 200 8C and in the presence of Zn3La1, there was little or no                        reaction above 170 8C, in FFA esterification above 140 8C, and in
hydrolysis of biodiesel and triglyceride, while a high yield of FAME                        biodiesel and oil hydrolysis over 220 8C. Thus, in order to obtain a
was obtained. The triglyceride hydrolysis (1) and FAME hydrolysis                           high FAME yield, the reaction temperature in this system should be
(2) reactions are:                                                                          limited to the range of 170–220 8C to enhance simultaneous


208                                                       S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212

                                                                                             re-examine the temperature windows for the three reactions in
                                                                                             fixed bed reactors to optimize the FAME yield. Fig. 10 [26,27]
                                                                                             illustrates the possible reaction pathway in converting unrefined
                                                                                             and waste oils into biodiesel in the presence of Zn3La1 at 200 8C.
                                                                                             FAME is formed through triglyceride transesterification and FFA
                                                                                             esterification reactions; while being consumed through FAME
                                                                                             hydrolysis reaction. Triglyceride hydrolysis can also occur which
                                                                                             lead to lower FAME yield and higher total acid number of the
                                                                                             resulting products. At 200 8C, the reaction rates of FAME hydrolysis
                                                                                             and triglyceride hydrolysis were not significant.
                                                                                                 Several unrefined and waste oils without any pretreatment
                                                                                             were converted directly using the Zn3La1 catalyst at 200 8C. Fig. 11
                                                                                             illustrates the FAME yields from waste cooking oil, unrefined
                                                                                             soybean oil, unrefined palm oil, food-grade soybean oil with 5.2%
                                                                                             oleic acid and 3.1% water, and food-grade soybean oil. The FFA and
                                                                                             water contents of these oils are shown in Table 1. It is remarkable
                                                                                             that the equilibrium yield of the different oils were all very high
                                                                                             around 96%. For this Zn3La1 catalyst, the presence of FFA and water
Fig. 8. The FAME yield and water content as a function of time at 200 8C. Reaction
mixtures are 126 g of the oil containing 5.3 (wt%) oleic acid and 94.7 (wt%)
                                                                                             did not significantly affect the equilibrium yield.
triglycerides, 180 g of methanol, 3 g of Zn3La1 catalyst.
                                                                                             3.6. Effect of metal oxide composition on catalyst structure

transesterification and esterification, while limiting hydrolysis                              3.6.1. XRD, SEM, EDS and BET
reactions. It should be noted that in a fixed bed reactor con-                                   The XRD patterns and EDS of zinc and lanthanum metal oxides
figuration, the optimal operating temperature will be a function                              are given in Fig. 12 and Table 2. The XRD pattern of Zn10La0
of contact time with the catalyst. Therefore, it is necessary to                             corresponds with hexagonal wurtzite structure of zinc oxide. The

Fig. 9. Effect of FFA and water additions on transesterification (a) yield of FAME in the presence of different FFA addition; (b) effect of FFA content on equilibrium yield of
FAME; (c) yield of FAME in the presence of different water addition; (d) effect of water addition on equilibrium yield of FAME. Reaction conditions: (1) acidic catalysis process,
sulfuric acid amount is 3%, molar ratio of methanol to oil is 6:1, reaction temperature is 60 8C, and reaction time is 96 h [19]; (2) alkaline catalysis process, NaOH amount is 1%,
molar ratio of methanol to oil is 6:1, reaction temperature is 25 8C, and reaction time is 8 h [20]; (3) heterogeneously catalytic process, catalyst amount of Zn3La1 is 2.3 (wt%),
molar ratio of methanol to oil is 36:1, reaction temperature is 200 8C, and reaction time is 1.5 h.
                                                         S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212                                                      209

                                Fig. 10. Reactions pathways of transesterification, esterification, and hydrolysis of unrefined and waste oils.

Fig. 11. FAME yield of crude palm oil, crude soybean oil, waste cooking oil, food-
grade soybean oil and food-grade soybean oil with 3.1% water and 5.2% oleic acid            Fig. 12. XRD patterns of pure and mixed zinc and lanthanum mixed metal oxides.
addition. Reaction mixtures are 126 g of oil, 180 g of methanol, 3 g of catalyst. Note      Note the transition of bulk ZnO structures to mixed ZnO-La2O3 structures as La
that 96% yield can be obtained within 3 h.                                                  content increases.

pattern of Zn0La10 shows a mixture of La2CO5 and LaOOH. The                                 Zn1La1) have higher activities than pure zinc oxide (Zn10La0),
diffraction patterns observed for Zn9La1, Zn3La1 and Zn1La1 show                            which correlate well with the effects of lanthanum on enhancing
lower intensity than Zn10La0, and mixed ZnO, La2CO5 and LaOOH                               the dispersion of ZnO.
are found in Zn9La1, Zn3La1 and Zn1La1.                                                        The XRD pattern of Zn3La1 shows a mixture of ZnO, La2CO5 and
    The mean grain size of ZnO in Zn9La1, Zn3La1 and Zn1La1                                 LaOOH phases. Various polar crystal planes of ZnO, such as ZnO
was calculated by the Deby-Scherrer equation based on the                                   (1 0 2), (1 0 3) and (1 1 2), could be observed. Methanol molecules
reflection peak of ZnO (1 0 1) in Fig. 12. The bulk molar ratios of                          prefer to stick on ZnO (1 0 2) and (1 0 3) which contain higher
zinc to lanthanum were determined by EDS (Table 2). The mean                                concentration of oxygen atoms than zinc. While on ZnO (1 1 2),
grain size of ZnO was found to decrease with the addition of La                             where concentration of zinc atoms is higher, adsorption of
(Table 2), suggesting that a strong interaction between the                                 carbonyl groups are favored [28,29]. Therefore, these polar
La and Zn species enhances the ZnO dispersion. Figs. 1 and 2                                surfaces can be attributed as the active centers for transesterifica-
show that the mixed ZnO-La2O3 catalysts (Zn9La1, Zn3La1 and                                 tion and esterification reactions.

Table 2
Specific surface area, mean grain size and lattice constants of zinc lanthanum oxides as a function of Zn:La ratio. Note the Zn:La ratio. Note the decrease of ZnO grain size as La
content increase.

Catalyst        XRD structure                Mean grain               Lattice constants for ZnO phase                              Zn:La                     Specific
                                             size of ZnO (nm)                                                                      (bulk molar ratio)        surface area (m2/g)
                                                                      a (A)           ˚
                                                                                   c (A)            ˚
                                                                                               Vol (A3)        Density (C)

Zn10La0         ZnO                          >100                     3.25         5.21        47.63           5.68                 1.0: 0                   16.3
Zn9La1          ZnO                            27.6                   3.25         5.36        48.62           5.56                 8.9: 1.0                 16.8
Zn3La1          ZnO, La2CO5, LaOOH             17.1                   3.25         5.23        47.81           5.65                 3.5: 1.0                 15.7
Zn1La1          ZnO, La2CO5, LaOOH              9.8                   3.33         5.10        49.12           5.50                 1.2: 1.0                 14.9
Zn0La10         La2CO5, LaOOH                   –                     –            –            –              –                   0: 1.0                    12.2
210                                                        S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212

                                                                                                   The XRD patterns of Zn10La0, Zn9La1, Zn3La1 and Zn1La1 show
                                                                                                a shift of the diffraction peaks of ZnO (1 0 0), (0 0 2), (1 0 1) based
                                                                                                on MDI’s JADE and DATASCAN. Lattice constants of ZnO crystal
                                                                                                were calculated (Table 2). The increase of lattice constants in a and
                                                                                                c directions suggests partial incorporation of La 3+ ion with ZnO
                                                                                                crystal resulting in lattice distortion [30]. Thus, crystal growth of
                                                                                                wurtzite ZnO was inhibted and ZnO was highly dispersed [31].
                                                                                                   Specific surface area of catalysts ranges from 12.2 m2/g to
                                                                                                16.8 m2/g (Table 2), and do not show a direct correlation with
                                                                                                lanthanum loading. The particle size and morphology of Zn3La1
                                                                                                are shown in Fig. 13. Small particles and some big aggregations are

                                                                                                3.6.2. XPS
                                                                                                   The XPS data of Zn, La and O elements on the surface of Zn10La0,
                                                                                                Zn9La1, Zn3La1, Zn1La1 and Zn0La10 are shown in Table 3. The
                                                                                                binding energy of 1021.9 eV, 835.0 eV, 530.6 eV and 528.9 eV can
Fig. 13. A representative SEM image of Zn3La1 showing aggregations of catalyst                  be attributed to Zn 2+, La 3+, Oad (adsorbed oxygen) and Olat (lattice
particles.                                                                                      oxygen), respectively. For metal oxides, lattice oxygens on the

Table 3
The binding energy and surface percentage of Olat, Zn2+, La3+ as a function of Zn:La ratio. Note the highest total surface percentage of Olat, Zn2+ and La3+ observed for the Zn3La1

Samples        Binding energy (eV)                                 Surface percentage (at.%)                                                           Surface atom ratio
                                                                                   2+      3+             2+        3+              2+            3+
               La3d        Olat         Oad         Zn2p           Olat       Zn         La          Zn        and La    Olat, Zn        and La        Zn2+:La3+     (Zn2+ + La3+):Olat

Zn10La0          –         528.9        530.6       1021.9         19.9       10.2       –           10.2                30.1                           –            0.5
Zn9La1         836.8       530.2        531.8       1021.6         23.2       24.4       2.1         26.5                49.7                          11.6          1.1
Zn3La1         835.7       530.2        531.5       1021.4         30.5       26.8       2.8         29.6                60.1                           9.6          1.0
Zn1La1         835.3       530.1        531.4       1021.1         32.9        4.2       2.9          7.1                40.0                           1.4          0.2
Zn0La10        835         530.3        531.7          –           37.5        –         7.9          7.9                45.4                           –            0.2

                                   Fig. 14. Schematic representation of possible mechanism for esterification of fatty acid with methanol.
                                                 S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212                                  211

                         Fig. 15. Schematic representation of possible mechanism for transesterification of triglyceride with methanol.

surface are Lewis base sites and metal ions are Lewis acid sites [32].             tetrahedral intermediate (Fig. 13). During esterification the
Base sites are considered catalytic sites for transesterification                   tetrahedral intermediate eliminates water molecule to form one
reactions [33,34], and acid sites are considered as the active sites               mole of methyl ester. The transesterification takes place between
for esterification reactions [34,35]. Therefore, it is interesting to see           the adsorbed methanol and triglyceride. The transesterification
if there is a correlation between surface concentrations of Olat,                  mechanism can be extended to di- and mono-glyceride. Methanol
Zn2+, La3+ and the activities of esterification and transesterification              is adsorbed on the Lewis base site (BÀ) of the catalyst and forms
reactions.                                                                         oxygen anion. The nucleophilic attack of alcohol to the esters
    The effect of La3+ concentration on the fraction of basic and acid             produces a tetrahedral intermediate (Fig. 14). Then the hydroxyl
sites is shown in Table 3. Percentages of surface lattice oxygen                   group breaks and forms two kinds of esters. In both cases,
(base site) increased with the lanthanum content. On the other                     esterification and transesterification use of excess methanol favors
hand, surface content of Zn2+ and La3+ in Zn3La1 are higher than                   forward reaction and thus maximizes the FAME yield.
Zn10La0, Zn9La1, Zn1La1 and Zn0La10, suggesting that there is an
optimal La loading to maximize surface acid sites. Combining both                  4. Conclusion
the acid and base sites, Zn3La1 again show the highest total surface
percentage of Olat, Zn2+ and La3+, as compared to other catalysts.                     The synthesis of FAME from unrefined and waste oils was
This correlates well with the transesterification activities (Figs. 1               investigated using a series of zinc and lanthanum mixed oxides
and 2) and XRD findings (Table 2).                                                  catalysts. There was a strong interaction between zinc and
    Shifts of binding energies of Olat, Zn2p and La3d are presented in             lanthanium species, and the catalyst with 3:1 of zinc to lanthanum
Table 2. Binding energies of Olat in Zn9La1, Zn3La1 and Zn1La1 are                 which has shown a higher activity than pure metal oxides. We
higher than Zn10La0, while Zn2p in Zn9La1, Zn3La1 and Zn1La1                       have found that at 200 8C, Zn3La1 was highly tolerant to FFA and
are lower than Zn10La0. Moreover, binding energies of La3d                         water, active in both transesterification and esterification
decrease with the La content. The shift of binding energy can be                   reactions, and with no hydrolysis activity. Within 3 h, 96% yield
attributed to the electron transfer from lattice oxygen atoms to                   of FAME was obtained even with crude palm oil, crude soybean oil,
metal atoms. This suggests that La3+ acts as an electron donor,                    waste cooking oil, food-grade soybean oil with 3% water and 5%
enhancing the interaction of reactant molecules with catalyst                      oleic acid addition. A temperature window was suggested
surfaces [36].                                                                     between 170 8C and 220 8C to change unrefined and waste oils
                                                                                   into FAME based on Zn3La1 catalyst. Lanthanum promoted ZnO
3.7. Reaction mechanism of simultaneous transesterification and                     dispersion, increased the surface amounts of acid and base sites,
esterification                                                                      thus enhanced the catalyst ability in both transeseterification and
                                                                                   esterification reaction. The zinc and lanthanum mixed oxides
   The reaction mechanism of simultaneous esterification and                        catalyst allows the direct use of unrefined and waste oils for
transesterification is suggested in Figs. 14 and 15. The esterifica-                 biodiesel production. Using this class of catalysts, which is
tion takes place between the adsorbed fatty acids and free                         relatively inexpensive because of low raw materials and
methanol. The interaction of the carbonyl oxygen of fatty acid with                manufacturing cost, significantly simplifies the oil pretreatment
the Lewis acidic site (L+) of the catalyst forms carbocation. The                  process and product purification process, and greatly decreases
nucleophilic attack of alcohol to the carbocation produces a                       the production cost of biodiesel.
212                                                       S. Yan et al. / Applied Catalysis A: General 353 (2009) 203–212

Acknowledgements                                                                            [15] C.V. McNeff, L.C. McNeff, B. Yan, D.T. Nowlan, A.E. Gyberg, B.J. Krohn, R. Fedie, T.R.
                                                                                                 Hoye, M. Rasmussen, Appl. Catal. A: Gen. 343 (2008) 39–48.
                                                                                            [16] T. Lacome, G. Hillion, B. Delfort, R. Revel, S. Leporg, G. Acakpo, US, 2005.
   Financial support from the Department of Energy (grant                                   [17] S.L. Yan, H.F. Lu, B. Liang, Energy Fuels 22 (2007) 646–651.
DEFG36-05GO85005) and Michigan’s 21st Century Job Fund is                                   [18] L. Bournay, G. Hillion, European (2003).
                                                                                            [19] P.S. Sreeprasanth, R. Srivastava, D. Srinivas, Appl. catal. A 314 (2006) 148–159.
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Description: Simultaneous transesterification and esterification of unrefined or waste oils over ZnO-La2O3 catalysts