3.1. Physicochemical Properties of Mixed Metal Oxide (MMO) Catalysts
X-ray diffraction analyses of MMO catalysts were performed to determine the existential state of binary oxide phases in the MMO system. The XRD patterns (Figure 2
) of co-precipitated CaO-MgO, CaO-ZnO, MgO-ZnO, and CaO-La2
showed the characteristic peaks of individual metal oxide crystalline phases. New formation of a homogeneous solid solutions phase was absent in the binary system. For calcium-based MMO catalysts (CaO-MgO, CaO-ZnO, CaO-La2
), two types of diffraction patterns were observed. The first type of characteristic peak was attributed to the CaO phase (JCPDS File No. 37-1497), while the second type corresponded to another joined oxide, such as MgO, ZnO, and La2
(JCPDS File No. 4-0829, 36-1451, and 2-668, respectively). The diffraction patterns observed in CaO-La2
catalyst showed an intense and dominant La2
phase and a low intensity of Ca crystallite peaks. This suggested that La3+
ions in the host lattice were partially substituted by Ca2+
] and resulted in habitation of CaO crystal growth and vigorous dispersion CaO crystals. The XRD patterns of MgO-ZnO MMO catalyst showed the existence of individual MgO and ZnO phases in the binary system. The presence of separated oxide phases for all mixed-metal oxide catalysts were greatly influence by the synthesis technique (co-precipitation) and also the synthesis parameters (concentration of metal salt solution, precipitation agent, temperature, aging, or stirring time) [26
The BET surface area of the MMO catalysts: CaO-MgO, CaO-ZnO, MgO-ZnO, and CaO-La2
were 5.8, 5.7, 9.5, and 7.7 m2
/g, respectively. The low surface areas of the MMO catalysts were closely related to the calcination temperature. A high calcination temperature and longer time of treatment is necessary for MMO catalysts to increase the basicity or active sites of the catalyst. This is because the metal oxides need suffient energy to be decomposed into metal carbonate, However, this scenario has led to sintering of fine crystals, thus promoting cluster agglomeration [27
], and resulted in a reduction of surface area in bimetal oxides [24
]. As observed in Figure 3
, the SEM images of alkaline-based MMO catalysts showed large and aggregated particles on the catalyst surface. Furthermore, elemental analysis of the active metal was determined via EDX analysis. The results showed that the experimental M1
ratio were 1.14, 0.93, 1.56, and 0.98 atomic % for CaO-MgO, CaO-ZnO, MgO-ZnO, and CaO-La2
, repectively, which is in agreement with the intended ratio M1
ratio = 1.
Basicity of catalyst is one of the crucial characteristics to influence the performance of transesterification, whereby the transesterification activity greatly depends upon the number and the strength of basic sites present in a catalyst [28
]. Thus, the basicity characteristic of MMO catalysts were analyzed using temperature-programmed desorption of TPD-CO2
. The basicity profiles of alkaline-based MMOs and pure metal oxide catalysts are displayed in Table 1
. Results showed that calcium-based mixed-metal oxides (CaO-MgO, CaO-ZnO, and CaO-La2
) render strong basic strength (with an intense desorption peak at 500–700 °C), which corresponded to the association of Ca2+
pairs located in a particular position of the binary system. The combination of metal oxide systems for MMO catalysts rendered higher amounts of basic sites compared to single metal oxides (CaO, MgO, La2
, and ZnO). The results showed that CaO-MgO, CaO-ZnO, MgO-ZnO, and CaO-La2
with amounts of basic sites of 1210.5, 1383.0, 333.1 and 737.8 μmol of CO2
/g, while MgO, ZnO, and La2
give lower amounts of basicity, which is less than 200 μmol of CO2
/g. In fact, CaO itself renders the highest basicity at 290.42 μmol of CO2
/g among the pure oxide, whereby it showed comparable basicity content to the binary MgO-ZnO system with the presence of strong basic strength. Thus, combination of Ca2+
with other group metals synergically alter the electronic properties of the catalyst's surface by increasing electron transfer from metal ions (Mn+
) to the oxide phase in metal-metal oxide systems and, thus, increase the electron density of the oxide phase for better basicity behaviour [31
The basic strength of MMO catalysts were indirectly influenced by the electron transfer from the metal groups. It was assumed that positive metal ions (cations) generated from the binary metal oxide system possess Lewis acidity, whereby the high negativity oxygen ions (anions) were prone to nucleophilic attack (proton acceptors) and act as Brønsted bases. The higher the electro-negativity in the oxide phase means the stronger the basic strength that exists in the active site of the catalyst. During the methanolysis of oil, the strong basic active sites provide sufficient adsorptive sites for methanol, in which the (O-H) bonds readily break into methoxide anions and hydrogen cations (Figure 4
). The methoxide anions were then reacted with triglyceride molecules to yield methyl esters [33
3.2. Transesterification Activity of the MMO Catalysed Reaction
The catalytic performance of MMO catalysts (CaO-MgO, CaO-ZnO, CaO-La2
, and MgO-ZnO) have been studied and compared with the individual oxide catalysts (CaO, MgO, ZnO, and La2
) under reaction conditions: catalyst amount of 3 wt %, methanol/oil ratio of 25:1, 120 °C within 3 h. It is interesting to observe CaO-ZnO (94% yield) shows the highest catalytic activity as compared to other alkaline-based MMO catalysts (Figure 5
). As compared to the catalytic activity of single CaO (91%) and ZnO (41%), the synergism effect CaO-ZnO has significantly improved the catalytic performance. Although the CaO-ZnO basicity is weaker than CaO (Table 2
), the amount of basic sites is still higher than CaO. Thus, this will enhance the catalytic performance of CaO-ZnO.
With the presence of a high acid value (12 wt % FFA) in crude jatropha oil, it has been presumed that unfavorable side reactions (saponification) between free fatty acids and the active metal (Ca2+
) happen during the transesterification. As expected, the single alkaline-earth metal oxide (MgO and CaO) showed the existence of soap in the final product. This result is in accordance with a previous study [35
]; poor stability of the CaO catalyst is prone to deactivation by acid and forms soap in the reaction. Consequently by coupling two different oxides in one catalyst system, the physicochemical properties of MMO catalysts are superior than pure alkaline oxide with less influence by FFA in the oil feedstock. The mixture of both alkaline metal oxide with transition metal and rare earth metal by co-precipitation methods rendered positive feedback with less soap formation and even gives a high biodiesel yield.
The transition metal oxide (ZnO) and rare-earth metal oxide (La2
) catalysts are of interest due to their activity in simultaneous transesterification and esterification of high-FFA containing vegetable oils. They are found to be water-tolerant compared to alkali and alkaline metal oxide catalysts [24
]. Thus, it is observed that CaO-ZnO catalysts are dual-functional due to the presence of combining Lewis base and Lewis acid metal oxides. This has led to the prevalence of Lewis base and acid catalytic sites in a single catalyst for simultaneous esterification and transesterification reactions. Thus, combination of an active alkaline metal oxide with ZnO is a wise choice for the transesterification reaction, as it can be conducted under mild reaction conditions.
Based on the present study, the CaO-ZnO catalyst showed the highest biodiesel yield (94%) via transesterification of high-acid crude jatropha oil (12 wt % FFA) under reaction conditions of: catalyst amount, 3 wt %; methanol/oil ratio = 25:1; temperature of 120 °C; and time of 3 h. Comparative study for the Ca-Zn binary metal oxide system was depicted in Table 1
. Most of the studies were focused in transesterification of refined vegetable oil with low FFA content. Thus, the slightly lower content of biodiesel for the present study was due to the occurrence of the unfavarouble saponification reaction of FFA with the base catalyst and, thus, formed soap byproducts instead of biodiesel yield [36
]. Although the present CaO-ZnO catalyst showed lower activity than the literature, this catalyst is capable of sustaining and resisting poisioning of FFAs in low-quality oil with a considerably good yield of product.
3.3. Catalyst Reusability
From an economic point of view, the most important features for a catalyst to be industrially competent are stability and reusability. Although single CaO showed comparable catalytic activity as a MMO catalyst, some studies reported that the Ca2+
active metal of the CaO catalyst easily leached into the methanolic solution, which reduces the catalytic activity [35
]. Furthermore, strong surface basicity of CaO favours the neutralization reaction with FFA to form large-molecule calcium soap. This inhibits the continuous cycle of the reaction.
This scenario was affirmed in the present reusability study (Figure 6
), in which jatropha-based biodiesel yield obtained from CaO-catalyzed transesterification was reduced badly from 91% to 58% after four cycles. In contrast, MMO catalytic activities only drop slightly as compared to that of the CaO catalyst. The biodiesel yield capable to maintain around 80% for four runs using calcium-based MMO catalysts (CaO-MgO, CaO-ZnO, CaO-La2
), with CaO-ZnO being the most stable MMO catalyst. In the case of magnesium-based MMOs catalyst (MgO-ZnO), the yield also dropped sharply after the fourth cycle because of the higher solubility of magnesium in the methanol FFA medium [40
], which led to the poor reusability performance. As shown in Figure 7
, the reusability profile was well correlated with the leaching test in biodiesel products via AAS analysis. The Ca content in the first-run biodiesel product (catalysed by CaO) was very high (156 ppm), followed by 87 ppm and 67 ppm for second and third runs, respectively. This finding was compatible with other studies, which decreased CaO activity for continuous runs due to the leaching of Ca2+
active sites. For calcium-based MMO catalysts, low concentrations of Ca (<30 ppm) were found in biodiesel yield for four cycles. It is also shown that the leaching of CaO-ZnO is the lowest amongst the alkaline-based MMO catalysts. This indicated the stronger stability of the binary metal oxide systems. The biodiesel derived by the MgO-ZnO catalysed reaction showed high Mg content (68 ppm) in the first cycle, while second and third cycle products rendered lower Mg content (<15 ppm). This might due to the presence of unstable amorphous MgO on the catalyst surface and easily be detached and solubilised during reaction.
Furthermore, the loss of transesterification activity of catalysts at continuous usage may be due to the blockage of active sites by adsorbed intermediates or product species, such as diglyceride, monoglyceride, and glycerine, and the contamination by O2
O, and CO2
in the air. These adsorbed intermediates sometimes led to the formation of magnesium diglyceroxides or calcium diglyceroxides that poisoned the active phase of the catalysts and, thus, reduced the transesterification rate [42
Although MMO-catalysed transesterification renders lower leaching of active Ca or Mg active metal (<15 ppm), the concentration is still above the biodiesel standard. Thus, simple water washing was done prior to a fuel properties study. For the biodiesel to be used in diesel engines, it has to meet the biodiesel standards ASTM D6751 and EN14214. Results showed that the biodiesel prepared by the MMO-catalysed process complied with the biodiesel standard. The density and kinematic viscosity of biodiesel at 40 °C is in the range of 891.8–890.7 kg/m3 and 3.8–3.9 mm2/s, respectively. The pour point and flash point of biodiesel were −6.0 to −7.0 °C and 140–160 °C, respectively, which indicated high adaptability for various weather conditions. Furthermore, the moisture content (<1.0 wt %,), acid value (0.5 mg KOH/g), and FFA (<1.0 wt %) in this biodiesel was low after washing. In conclusion, this biodiesel is suitable for use in diesel engines.