Transesterification of Waste Cooking Oil Catalysed by Crystalline Copper Doped Zinc Oxide Nanocatalyst

Biodiesel has its unique position in the field of renewable energy as alternate fuel to diesel due to fuel price, energy requirement and petroleum crisis. In this study, biodiesel was produced from Waste Cooking Oil (WCO) using Copper doped Zinc Oxide (CZO) nanocatalysts. The synthesized Copper doped Zinc Oxide nanocatalysts were characterized by X-Ray Diffraction (XRD) and High Resolution Transmission Electron Microscope (HRTEM). Design of experiment was framed using Taguchi method to limit the experiments and to find the optimum reaction conditions. The effect of process parameters such as oil-to-methanol ratio (O/M), catalyst type, catalyst concentration, temperature and time on the transesterification reactions using characterized Copper doped Zinc Oxide nanocatalyst were investigated. The 4% (weight /weight) nanocatalyst concentration, 1:5 Oil to methanol molar ratio at 60°C temperature and 40 minutes of reaction time were found to be optimum, in which the maximum biodiesel yield of 98 % (w/w) was obtained. Hence it was determined that nanocatalysts exhibited good catalytic activities on biodiesel production from Waste Cooking Oil (WCO).


Introduction
Fossil fuels such as petroleum, coal and natural gas are the major sources of increased pollution level. Studieshave shown that biodiesel being less polluting, non-toxic and biodegradable; it can be used directly in most diesel engines without much engine modifications. Biodiesel is commonly produced by transesterification of lipids, animal fats, grease and waste cooking oils [1,2,3,4,5,6,7,8,9,10,11,12,13].Catalyst plays an important role in transesterification reactions. Homogeneous catalysts result in the formation of soap which makes more complication in the reaction and also in the product separation. Heterogeneous catalysts has its own benefits such as ease of separation, easy recovery and can potentially be re-used. Finally, all these benefits leads to a reduced production cost [1,2,3,6]. Due to the nanodimension and morphological structure, the nanocatalyst composites provides higher catalytic activity [14,15]. This research work aims at conversion of WCO into biodiesel using the transesterification process with CZO nanocomposite as heterogeneous catalysts. Since nano particles have a large surface-to-volume ratio compared to bulk materials, they are attractive to use as catalysts for the transesterification reactions.

Materials
Cupric sulphate, Zinc sulphate, Sodium carbonate were the chemicals used for synthesizing nanocatalyst. Methanol was used for transesterification reactions. All chemicals were of analytical grade purchased from Merck, India. Transesterification was carried out using Waste Cooking Oil, Copper doped Zinc Oxide nanocatalyst and methanol.

Catalyst Synthesis
Co-precipitation method was used for synthesizing CZO nanocatalyst. Solution 1was prepared by mixing 28.6% (w/v) of zinc sulfate with 1.52% (w/v) of cupric sulfate in 100 ml of distilled water. Solution 2 was prepared by mixing 5.28% of sodium carbonate in 100 ml of distilled water. Then solution 1 was added in drops into solution 2under continuous stirring. The temperature was maintained as constant at 60°C. The resultant bluish white precipitate was filtered, washed with distilled water and dried in hot air oven at 80°C. The dried precipitate of CZO was calcinated at 500°C in a muffle furnace for 2 hour.

Pre-treatment of Waste Cooking Oil
WCOwas filtered to remove suspended solid materials. Then the oil was heated at 110°C to remove water content.

Gas Chromatography (GC) Analysis of WCO
Gas Chromatography (GC) coupled with Mass Spectroscopy (MS) (Thermo GC-Trace Ultra Ver: 5.0, Thermo MS DSQ II)was used for analysis of fatty acid content in WCO. Capillary non polar column DB 35-MS was used. Temperature program was carried out from 70 to 260°C at a heating rate of 6°C/minute using helium as carrier gas.

Density
Density of WCO was found out using hydrometer. Measured value of density was 902 kg/m 3 .

Kinematic Viscosity
Viscosity of WCO was found out using redwood viscometer. Measured value of kinematic viscosity was 40 mm 2 /s at 40 °C.

Flash point
Flash point is defined as the lowest temperature at which it forms vapour which produces combustible mixture with air.The flash and fire point temperatures depends upon the volatility of oil. Cleveland Open Cup apparatus was used for measuring flash and fire point. Measured value of flash point was 260°C and fire point 280°C.

Cloud Point and Pour point
It is the temperature at which the oil becomes cloudy in appearance when it is cooled. At low temperatures when oil is cooled down, the impurities tend to separate out of the oil in the form of minute crystals that result in cloudiness. After the cloud point, the oil is cooled further, thus more and more of impurities will crystallize where the oil becomes gel. Measured value of cloud and pour point was -2°C and -8°C.

Acid Value
Acid value is defined as the number of milligrams of potassium hydroxide required to neutralize the free acid present in one gram of oil sample. Measured acid value was 1.56mg KOH/g.

Saponification value
Saponification number of oil is the number of milligrams of potassium hydroxide required to saponify one gram of oil. Measured saponification value by titration was 6.17mg KOH/g.

Iodine value
Iodine value is the number of grams of iodine consumed by 100g of oil. A known weight is allowed to react with an excess of standardized iodine solution and the iodine consumed by oil was estimated by blank titration with a standardized sodium thiosulphate solution. Measured value of iodine by titration was 5.92 I2g/100g.

Transesterification of WCO
The transesterification is the basic process and it is accepted for large scale production of biodiesel [16]. The experiments were conducted in a round bottom flask equipped with heating, condensing and stirring facility as shown in Fig. 1. The process was carried out by mixing desired amount of catalyst, methanol and WCO. To prevent the methanol loss during a reaction, a water-cooled condenser was used. Transesterfication reaction was carried out for varied catalyst concentration (2-8% w/w), Oil to Methanol (O/M) ratio (1:2-1:5mol), temperature (40-70°C) and reaction time (30-60 minutes). After the desired duration the reaction mixture was allowed to settle and then transferred to a separating funnel. The biodiesel was allowed to separate from the mixture. Then, the unreacted I S S N 2 3 2 1 -8 0 7 X V o l u m e 1 2 N u m b e r 2 7 J o u r n a l o f A d v a n c e s i n c h e m i s t r y 5800 | P a g e D e c e m b e r 2 0 1 6 w w w . c i r w o r l d . c o m methanol in the methyl esters were removed by heating the product to 110 o C and maintained for 1 hr. This biodiesel was then collected and stored in an air-tight bottle at room temperature as shown in Fig. 2. Catalyst can be recovered, washed, calcined and reused. It is observed that minimum amount of catalyst is consumed in the reaction [17,18].

Design of Experiments (DOE)
Design of experiments (DOE) is an approach to study the relationship between factors affecting a process and the output of that process. This relationship is required to optimize the process inputs in order to obtain the maximum output. Here the effect of process parameters on the yield is investigated.
Taguchi method is used to reduce the number of experiments in case of large number of experimental situation. On the basis of Taguchi design, Orthogonal array was formed with 4 parameters at 4 levels as shown in table 1. Signal to noise ratio was determined for the experiments in order to find the quality of production. There are three forms of signal to noise (S/N) ratio that are common interest for optimization of static problems.
Smaller In order to maximize the percentage of yield, the optimum level for a factor is the level that gives the highest value of η in the experimental design was chosen.

Characterization of CZO nanocatalyst by XRD
XRD investigates crystallite size and structural phase. The most intense peak for the catalyst is at Bragg's angle of 36.6467° and this was used to calculate the average CZO crystallite size (D) using Scherrer equation [15].

= cos
Where k = shape factor (0.9), FWHM (β) = line broadening at half the maximum intensity in degrees, λ = wavelength of x-rays, 2θ = Braggs diffraction angle. However, the results were used to calculate average crystallite size. Calculated average crystallite size was found to be 18.75 nm.  Three strongest peaks at 2θ value of 36.6467°, 32.1564° and 34.8484° confirms the presence of copper as shown in the Fig. 3. Indexed peaks with hkl planes were shown in Fig. 4.

Fig. 4. Peaks indexed with hkl planes
XRD analysis reported that it is highly-symmetric, singly-crystalline. Its hexagonal lattice matched catalysts lattice structure.

3.3.Characterization of CZO nanocatalyst by HRTEM Analysis
Morphology and atomic structure of the sample were studied by HRTEM. From the HRTEM images, the copper particles were seen on the surface of zinc oxide which is clear from each level. The electrons in the energy levels were focused on the catalyst as shown in Fig. 5. HRTEM image of prepared CZO at 20nm was shown in the Fig. 6.  Crossed lattice fringes are formed by the intersection of two or more sets of lattice fringes. Crossed lattice fringes will be visible when the nanocrystal is oriented close to a zone axis.

Energy Dispersive X-Ray (EDX) Analysis
Energy Dispersive X-Ray Analysis was done. From the spectrum it is clear that it confirms the presence of copper and zinc. Composition of elements in the synthesised catalyst is given in the table 3. Composition table shows that zinc occupies a major part. Each peak in the spectrum represents the presence of each element. Each peak is indexed by their chemical formula shown in Fig. 7 [19].

3.4.Gas Chromatography (GC) Analysis of WCO
Composition of fatty acids in WCO were identified using GC-MS. Fatty acids in WCO were quantified by comparing their retention times and peak areas to those of standard fatty acids shown in table 3 [20,21].

Comparison of Properties
Properties comparison is one of the methods to determine the standard of biodiesel. Biodiesel property values can be checked by verifying it with the ASTM D6751 standard shown in table 4 [22,23,24,25,26].  Biodiesel produced seems to be matched with that of the ASTM D6751 biodiesel standard.

DOE Experimental results
Biodiesel conversion was investigated based on the Design Of Experiments i.e., range of the parameters such as 2-8% catalyst concentration, 1:2 to 1:5 oil to methanol ratio, 40-70°C reaction temperature and 30 -60 minutes reaction time. Biodiesel yield depends on the process operating conditions [27]. Orthogonal array is used to design experiments with four parameters at four levels, L-16 shown in table 5.

CT-Confirmation test
Confirmation test was done to validate the taguchi method. In this study, Minitab 17 software was used for automatic design to analyze the results and also to optimize the experimental conditions.

Effect of catalyst concentration
Nanocatalyst posses large surface area, which make them more suitable for the catalytic activity. The effects of the catalyst concentration on methyl ester yield were investigated by changing the weight ratio of the catalyst in the range of 2-8%. When the catalyst concentration was increased from 2wt % to 8 wt% the conversion reached 97.7%. Thus transesterification reaction was strongly affected by catalyst concentration. The results show that they are in agreement with the literatures [28,29,30].

Effect of oil to methanol ratio
Oil to methanol ratio is one of the important parameter which affects the transesterification.

Effect of reaction temperature
Based on the Design Of Experiments, the transesterification of WCO using CZO nanocatalyst was carried out at different temperature ranging from 40°C to 70°C. The result indicates that the biodiesel conversion was high at lower temperature with only 2% catalyst. Increase in the temperature from 40°C to 50 and 60°C increases the yield with 4, 6 and 8%catalyst loading. The maximum biodiesel yield was obtained at 60°C. Then the conversion dropped on further increase in temperature of 70°C. The optimal temperature for transesterification of WCO using CZO nanocatalyst was found to be 60 °C. Thus transesterification occurs at various temperatures. The results show that they are in agreement with the literatures [29,30,31].

Effect of reaction time
Reaction time is another important criterion that will affect the yield of biodiesel. The effect of reaction time on conversion was studied from the Design Of Experiments framed and its corresponding yield obtained. Reaction was carried with variation in time from 30-60 minutes. Results show that there was an increase in conversion when the reaction time was in the range 30 to 40 min.
Thus level of conversion increases with reaction time. The results show that they are in agreement with the literatures [30,31].

Response of parameters with respect to Signal to noise ratio
Basically during the process, noise factors will arise which is an uncontrollable one. In order to control the noise factors, the optimal control factor settings should be identified from the variation of response of parameters with respect to Signal to Noise ratio shown in Fig. 8.  Based on table 6,  parameters are ranked and given in table 7. To have a minimum signal-to-noise ratio, the control factors have to shift from mean to the target. This is achieved by taguchi design.   One of the most important benefit of using heterogeneous catalyst is its reusability. Experiments were carried out with the regenerated catalyst and the effect of reusability was studied. Catalyst once used for the reaction was collected, dried and utilized for the successive cycles. Stable catalytic activity was maintained for 5 successive cycles. Thus the catalyst life is 5 times. From the catalytic activity of CZO in the reaction, it is confirmed that it is an effective catalyst for production of biodiesel from WCO [14,32].

Conclusion
CZO nanocatalyst prepared shows good catalytic activity for the transesterification of Waste Cooking Oil. The 4% (w/w) nanocatalyst concentration, 1:5 Oil to Methanol ratio, 60°C temperature and 40 minute of reaction time were found to be optimum, in which the maximum biodiesel yield obtained was 97.7% (w/w). Thus CZO shows great potential in the field of WCO processing.