Solid Acid Catalyzed Biodiesel Production From Waste Cooking Oil
Solid Acid Catalyzed Biodiesel Production From Waste Cooking Oil
Solid Acid Catalyzed Biodiesel Production From Waste Cooking Oil
A R T I C L E I N F O A B S T R A C T
Article history: Various solid acid catalysts were evaluated for the production of biodiesel from low quality oil such as
Received 29 January 2008 waste cooking oil (WCO) containing 15 wt.% free fatty acids. The zinc stearate immobilized on silica gel
Received in revised form 28 June 2008 (ZS/Si) was the most effective catalyst in simultaneously catalyzing the transesterification of triglycerides
Accepted 1 July 2008
and esterification of free fatty acid (FFA) present in WCO to methyl esters. The optimization of reaction
Available online 10 July 2008
parameters with the most active ZS/Si catalyst showed that at 200 8C, 1:18 oil to alcohol molar ratio and
3 wt.% catalysts loading, a maximum ester yield of 98 wt.% could be obtained. The catalysts were recycled
Keywords:
and reused many times without any loss in activity.
Biodiesel
Esterification
ß 2008 Elsevier B.V. All rights reserved.
Transesterification
Waste cooking oil
Methyl esters
Solid acid catalyst
Free fatty acids
0926-3373/$ – see front matter ß 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2008.07.005
K. Jacobson et al. / Applied Catalysis B: Environmental 85 (2008) 86–91 87
catalyzed by an acid (generally sulfuric acid) followed by interconnected system of large pores, a moderate to strong acid
transesterification process, in which triglyceride (TG) portion of sites and a hydrophobic surface would be ideal for biodiesel
the oil reacts with methanol and base catalyst (usually sodium or preparation [4]. Therefore, in an attempt to develop a robust solid
potassium hydroxide) to form ester and glycerol. The current acid catalyst that can simultaneously catalyze esterification as well
process increases the production cost of biodiesel as it involves a as transesterification reaction, different types of solid acid
number of steps including washing of the esters to remove acid/ catalysts such as MoO3/SiO2, MoO3/ZrO2, WO3/SiO2, WO3/SiO2–
alkali catalysts in addition to creating contaminated water disposal Al2O3, zinc stearate supported on silica, zinc ethanoate supported
issues. on silica and TPA supported on zirconia are synthesized and
Solid acid catalysts have the strong potential to replace liquid evaluated for biodiesel preparation from waste cooking oil. Also,
acids, eliminating separation, corrosion and environmental pro- influence of various reaction parameters such as molar ratio of
blems. Recently a review was published that elaborates the WCO to alcohol and catalyst loading was studied in the present
importance of solid acids for biodiesel production [4]. We have investigation.
reported the production of biodiesel by simultaneous esterification
and transesterification of high free fatty acid-containing oil on 2. Experimental
supported heteropolyacid catalysts system [8]. A recyclable
diarylammonium catalyst has been recently used for the synthesis 2.1. Materials
of biodiesel from high free fatty acid-containing feedstocks [7]. A
two steps process was employed where diarylammonium catalyst Waste cooking oil (WCO) was obtained from Saskatoon
was used for the esterification of free fatty acid to methyl esters Processing Co., Saskatoon, Saskatchewan, Canada. Ammonium
and the ester–glyceride mixture was further converted to methyl heptamolybdate, Ammonium metatungstate, zirconyl chloride
esters by base-catalyzed transesterification. Sugar catalysts from octahydrate, aluminum nitrate and tetraethoxysilane were
D-glucose have been used for the production of biodiesel from high obtained from Sigma-Aldrich, MO, USA. 12-tungstophosphoric
fatty acid-containing waste oil with a high acid value [9]. A novel acid was purchased from BDH chemicals Ltd., England. Zinc acetate
Fe–Zn, double-metal cyanide (DMC) complexes has been reported and Zinc stearate were purchased from Fisher chemicals and Alfa
as highly active heterogeneous catalysts for production of biofuels Aesar, USA, respectively. Silica was obtained from Engelhard, USA.
and lubricants from vegetable oil by esterification/transesterifica-
tion reactions [10]. These catalysts were hydrophobic (at reaction 2.2. Catalysts preparation
temperatures of about 443 K) and insoluble in most of the solvents
including aqua regia. Solid acid catalyst based on tungsten oxide The hydrous zirconia and zirconia-alumina (1:1 molar ratio)
supported on zirconia was found to be very active for the support was prepared by the hydrolysis of zirconyl chloride and
esterification of palmitic acid with methanol [11]. However, most zirconyl chloride-aluminum nitrate aqueous solution by adding
of the studies on biodiesel synthesis from low quality real aqueous ammonia to a pH of up to 9. The gel was washed with
feedstocks such as WCO have been focused on dual step process deionized water and dried at 120 8C for 12 h. MoO3 supported ZrO2,
using homogenous catalysts. To our knowledge, there are no SiO2 and WO3 supported ZrO2, ZrO2–Al2O3 of 5 and 10 wt.% were
reports on the utilization of solid acid catalysts for the production prepared by impregnation method. The material thus obtained was
of biodiesel from WCO in a single step. dried at 110 8C and calcined at 500 8C. 12-tungstophosphoric acid
ZrO2 support is an interesting material because it possesses (TPA) supported hydrous zirconia was synthesized, as described
acidic, oxidizing and reducing properties on the surface. Generally previously [8]. Zinc Stearate and Zinc ethanoate supported on silica
it has been observed that the acidic properties of zirconia can be gel was prepared using sol – gel method. The preparation method
modified by the impregnating it with species such as tungsten was based on the preparation of zinc ethanoate supported on silica
oxide (WO3) and molybdenum oxide (MoO3). Literature studies gel reported by Nava et al. [29]. In the typical synthesis, tetraethyl
reveal that due to the strong acidity of WO3, MoO3 supported on orthosilicate (TEOS) (98%, check, Aldrich) was employed as the
ZrO2 catalysts, they have been widely used for a wide variety of silica gel source. TEOS was combined with distilled water in the
reactions including esterification, tranesterification, alkylation and molar ratio of 1:4. The hydrolysis of TEOS was carried out in a 500
isomerization [12–15]. The combination of Al2O3 and ZrO2 and cc Parr reactor (Parr Instrument Co.) equipped with a temperature
modification of ZrO2–Al2O3 with WO3 not only provides greater controller at 85 8C under vigorous stirring. After 1 h of stirring,
mechanical strength but also enhances the acidity of the catalyst succinic acid (99%, Aldrich, check) was added to the reaction
[16–19]. The homogeneous acetate and stearate of zinc were found mixture, in a molar ratio of TEOS: SA of 1:0.1. After stirring for 1 h,
to be very effective catalysts for the synthesis of biodiesel [20] due the reaction temperature was decreased to 50 8C and zinc
to the lewis acidity of metal and molecular structure of anion. acetate (ZnE) (99%, Merck) or zinc stearate (ZS) was added to
Further the heterogenize of zinc acetate complex by supporting it the reaction mixture in a molar ratio of TEOS: ZnE/ZS of 1:0.08 and
on functionalized silica leads to higher surface area, thermal the reaction was carried out for another 1 h at 50 8C under stirring.
stability, mild acidity and average pore size in the region of The reaction mixture was cooled to room temperature and the
mesopores [21]. Molybdenum oxide as such or supported on silica resultant gel was dried overnight at room temperature. Finally, the
(MoO3/SiO2) is a well-known solid acid catalyst possessing both material was dried at 80 8C for 24 h and subsequently at 110 8C for
strong Lewis and Bronsted acidity [22]. MoO3/SiO2 catalyst has 3 days. The catalysts were designated as Mo/Zr, WO/Zr, WO/Zr–Al,
been effectively used for various reactions such as nitration, Mo/Si, TPA/Zr, ZS/Si and ZnE/Si referring to MoO3/ZrO2, WO3/ZrO2,
esterification, acylation and transesterification due to the combi- WO3/ZrO2–Al2O3, MoO3/SiO2, TPA/ZrO2, Zinc stearate/SiO2 and
nation of strong acidity and higher dispersion of active MoO3 Zinc ethanoate/SiO2, respectively.
species on high surface area silica support [23–28]. Our earlier
work on the supported 12-tungstophosphoric acid (TPA) catalysts 2.3. Catalyst characterization
system, revealed that TPA supported on zirconia was the most
promising catalyst for the production of biodiesel due to the lewis Specific surface area and pore size measurements of the
acid sites generated by the strong interaction of TPA and surface catalysts were performed using Micrometrics adsorption equip-
hydroxyl groups of zirconia [8]. Solid acids catalysts having ment (Model ASAP 2000) at 78 K using liquid nitrogen. Prior to the
88 K. Jacobson et al. / Applied Catalysis B: Environmental 85 (2008) 86–91
2.5. Catalysts reusability and leaching tests 3.1.1. Textural analysis of catalysts
Now we proceeded to examine the textural properties of the
The catalysts separated from the reaction mixture by filtration catalysts. As shown in Table 3, the surface area and the average
were initially washed with hexane to remove non-polar com- pore diameter of catalysts varied in a wide range from 35 to
pounds such as methyl esters on the surface. Further, the catalysts 457 m2g 1 and 20 to 83 A8, respectively. ZS/Si catalyst had the
were washed with methanol to remove polar compounds such as largest average pore diameter among all the prepared solid acid
glycerol and finally dried at 80 8C overnight. The leaching of the catalysts. The primary requirement of an ideal solid acid catalyst
catalyst into the reaction mixture was investigated by inductively for biodiesel synthesis is large interconnected pores that would
coupled plasma-mass spectrometry (ICP-MS). minimize diffusional limitations of molecules having long alkyl
chains [4]. A typical triglyceride molecule has a diameter of
3. Results and discussion approximately 58 A8. Comparing the textural properties of most
active catalysts in this work (ZS/Si, ZnE/Si and 5% Mo/Si), it can
The prepared solid acid catalysts were evaluated for the be seen that ZS/Si has the largest pore diameter and it can
synthesis of biodiesel from WCO under identical reaction accommodate a bulky triglyceride molecule easily. Thus, based
conditions such as reaction temperature of 200 8C, stirring speed on the highest activity (esterification as well as transesterifica-
of 600 rpm, 1:6 molar ratio of oil to alcohol, and 3% w/w catalyst. tion reaction) and average pore diameter, ZS/Si catalysts was
The best catalyst was chosen based on the yield of maximum selected to further study in detail the physico-chemical
methyl ester with minimum free fatty acid content in the product. properties and the effect of various reaction parameters on
The methyl ester yield (with an experimental error of 2 wt.%) ester yield.
Table 1
Ester yield (wt.%) of different solid acid catalysts. Reaction conditions: reaction temperature 200 8C, molar ratio of oil to alcohol 1:6, stirring speed 600 rpm and catalyst
loading 3% w/w
5% Mo/Zr 10% Mo/Zr 5% WO/Zr–Al 10% WO/Zr–Al 5% WO/Zr 10% WO/Zr ZS/Si ZnE/Si 5% Mo/Si 10% Mo/Si 10% TPA/Zr
1 42 47 23 38 13 40 50 48 26 56 18
2 47 54 26 51 27 48 71 68 72 57 28
3 50 59 26 52 31 52 75 75 74 58 32
4 55 62 26 54 36 56 79 79 75 59 36
5 58 63 27 57 40 58 80 80 76 59 40
10 65 71 27 65 42 67 81 80 79 60 43
K. Jacobson et al. / Applied Catalysis B: Environmental 85 (2008) 86–91 89
Table 3
Textural properties of various solid acid catalysts
5% Mo/Zr 113 – 35
10% Mo/Zr 127 – 30
5% WO/Zr–Al 209 – 41
10% WO/Zr–Al 194 – 40
5% WO/Zr 100 – 50
10% WO/Zr 63 – 47
ZS/Si 35 – 83
ZnE/Si 457 138 21 Fig. 2. Thermal analysis curves of pure zinc stearate and zinc stearate supported
5% Mo/Si 265 118 33 over silica gel.
10% Mo/Si 141 58 35
10% TPA/Zr 242 – 27
ZS/Si (used) 34 – 88 of zinc stearate. The thermal behavior of ZS/Si catalyst showed
weight loss at two temperature regions i.e. 75 -90 8C and 275–
475 8C, respectively. The loss between 75–90 8C occurred due to
3.1.2. X-ray diffraction and thermal analysis the physisorbed water and second loss from 275–475 8C corre-
Pure zinc stearate is a homogenous lewis acid catalyst and one spond to combined loss due to dehydroxylation of silica and
of its major drawbacks is the difficulty to separate it from reaction decomposition of supported zinc complex. The TG analysis showed
products and to reuse [20]. To overcome this shortcoming, we have that the thermal stability of supported zinc complex is higher than
immobilized zinc stearate (ZS) on the surface of silica by sol-gel those of the pure zinc complex. These results indicate that the
process. The X-ray diffraction pattern of pure zinc stearate and zinc support has a stabilizing effect on the decomposition of catalytic
stearate immobilized on silica gel is presented in Fig. 1. It can be active zinc stearate. Chin et al. [21] reported similar results in the
seen from the diffraction patterns that the major peak correspond- thermal studies of pure and supported zinc acetate complex.
ing to ZS coincide in pure as well as supported ZS. However, the
peaks in ZS/Si are broader than those of pure ZS caused by silica gel 3.2. Effect of reaction parameters
support and the high dispersion of ZS on support. The carbon and
ICP-MS analysis of ZS/Si catalyst showed a Zn: C ratio of 1:3 3.2.1. Catalyst loading
indicating the presence of interaction between stearate and Catalysts loading and molar ratio of oil to alcohol are the
succinate during synthesis due to stronger acidity of succinic acid important reaction parameters that need to be optimized to
than zinc stearate. However, the X-ray diffraction studies showed increase the ester yield. The effect of ZS/Si loading (1, 3 and 5% w/
peaks due to formation of zinc stearate only without showing any w) on ester yield was studied at a molar ratio of oil to alcohol of 1:6
peaks due to formation of new compound formed by interaction as shown in Fig. 3. Increase in catalyst loading from 1 to 5% w/w did
between succinic acid and zinc stearate. show a variation in the initial activity; however, the final methyl
The thermal analysis curves of pure ZS and ZS/Si are shown in ester yield (80 wt.%) was similar irrespective of the loading. Thus,
Fig. 2. The first peak at 80 8C in the DTG curve of pure ZS is related it is clear that the catalyst loading does not have much influence to
to loss of water molecules included in the stearate. The major loss improve the yield of methyl ester. The reaction was further studied
centered approximately at 230 8C is related to the decomposition with 3% w/w of catalyst loading for further optimization of molar
ratio of alcohol to oil.
Fig. 3. Effect of catalyst loading on ester yield using ZS/Si catalyst. Reaction
Fig. 1. XRD patterns of pure zinc stearate, zinc stearate supported over silica gel and conditions: reaction temperature 200 8C, molar ratio of oil to alcohol 1:6, stirring
used zinc stearate supported over silica gel catalysts. (*) zinc stearate. speed 600 rpm.
90 K. Jacobson et al. / Applied Catalysis B: Environmental 85 (2008) 86–91
Fig. 4. Effect of oil to alcohol molar ratio on ester yield using ZS/Si catalyst. Reaction
conditions: reaction temperature 200 8C, stirring speed 600 rpm, catalyst loading
3% w/w.
4. Conclusions
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