Synthesis of KF/CaO as a catalyst for the production of bio-fuel from cracking of Cornus wisoniana oil

2.1 Materials

Raw oil obtained from C. wisoniana fruit by cold pressing process. The fatty acid compositions of C. wisoniana oil were listed in Table 1. KF and CaO were purchased from Sigma–Aldrich Co., Ltd.

2.2 Catalyst preparation

The catalyst KF/CaO was prepared by impregnation method. Prior to immerse, the carrier of CaO 10.0 g was baked in muffle for 2 h at 500°C. Then CaO was immersed in 30–55 mL KF solution with certain amount (3, 3.5, 4, 4.5, 5, and 5.5 g) mechanical stirring for 5 h followed by 1–6 h calcination in muffle furnace at different temperature (300, 400, 500, 600, 700, and 800°C). The catalysts were preserved in dryer for use once again.

2.3 Characterization

The structure, composition, and morphologies of KF/CaO were examined by XRD (D/max2500PC diffract meter, RIGAKU, Japan) and scanning electron microscopy (SEM, XL-30ESEM instrument, PHILIPS, Holand). The specific surface area of KF/CaO was measured by the Brunauer–Emmett–Teller (BET) method with the instrument of NOVA 2000e (Quantachrome, USA). The basic properties of KF/CaO were measured by CO₂-TPD (TP-5080, XIANQUAN) and Hammett indicator.

2.4 Catalytic cracking of C. wisoniana oil

Catalytic cracking of C. wisoniana oil was carried out in a homemade cracking reactor. C. wisoniana oil (200 g) after pretreated was taken into reactor and mixed with KF/CaO (1.2 wt%) catalysts that were prepared under different process conditions. A bimetallic thermometer at the bottom of the reactor determines the temperature change of the reactants. Catalytic cracking was carried out with C. wisoniana oil and KF/CaO catalyst at 500°C for 45 min. The yield of bio-fuel oil was calculated, which based on the carbon balance. The equation of bio-fuel yield is as follow.

Yield of bio-fuel = mass of bio-fuel oil/mass of raw C. wisoniana oil

2.5 Analysis of bio-fuel oil

Reaction products were periodically sampled and analyzed using GC–MS (BRUKER DALTONICS INCORPORATED) with HP-5MS type capillary column (30 m × 0.05 µm × 0.32 nm) and hydrogen flame ion detector. The initial oven temperature was 50°C with an equilibrium time of 2 min. Then the oven was heated at a speed of 5°C/min to 280°C, the temperature was remain for 20 min. Nitrogen as carrier gas, its velocity is 30 mL/min. A total of 0.1 µL sample was injected into the GC–MS. The samples were dissolved into hexane at a concentration of 0.1 g per 100 mL. The peak of light olefin was obtained compared with peaks of catalyzed by several KF modified catalyst loading different carriers.

3.1 Characterization of KF/CaO catalyst

The SEM photograph (Fig. 1a) showed that the CaO carrier was tiny block structure. However, the CaO carrier (specific surface = 11.6 ± 0.5 m²/g) modified by KF (Fig. 1b) presented porous microstructure 15, which could significantly increase catalytic efficiency due to larger specific surface (107 ± 0.5 m²/g) and more basic sites (H = 15.0–18.4). As seen in Fig. 1c, the CO₂-TPD was in agreement well with that of SEM pattern. The KF/CaO catalyst revealed strong basicity. According to the diffraction peak in XRD pattern (Fig. 1d), KF was successfully loaded on CaO carrier during the impregnation stage. The new crystal structure of KCaF₃ formed at lower calcination temperature (approximately 400°C). With the calcination temperature increasing, the KCaF₃ content increased until the calcination temperature reached 600°C. Subsequently, the KCaF₃ was decomposed at higher calcination temperature and the intensity of CaO diffraction peak is accordingly increased.

3.2 Catalytic cracking of Cornus wilsoniana oil

The effect of the preparation technical parameters of KF/CaO catalyst on the yield of biofuel was discussed. As shown in Fig. 2a, the biofuel yield increased with the increase of KF loading amount. The biofuel yield reached its maximum at 40% loading amount of KF, and then gradually decreased. The results implied that strong base sites are harder to create with low loading amounts. Meanwhile, the active sites are more dispersed on the CaO surface at a low loading of KF. However, if CaO is loaded with too much KF, the KF cannot disperse properly and the excess KF could cover the active sites on the composite surface, which caused the decrease of catalytic activity 16.

The relation between biofuel yield and calcinations temperature (Fig. 2b) is consistent with XRD pattern. At 600°C, the best crystal structure of KF/CaO was formed, and it obtained the best catalytic activity. The effect of calcination time on the yield of biofuel was shown in Fig. 2c. As calcinations time went on, more basic sites gradually formed, which improved the catalytic activity of KF/CaO. But if KF/CaO was calcinated for more than 4 h, the surface of CaO carrier formed compact structure due to modulizing. The compact surface texture blocked the formation of basic sites, which resulted in catalysts deactivation 17.

Based on the above results, the bio-fuel yield reached 82.7% when KF/CaO obtained under optimal process conditions acted as basic catalyst in cracking reaction. Subsequently, the reusability of KF/CaO and CaO are presented in Fig. 3.

The results showed the loss of catalyst's mass in each recovery was minimal and after the ninth repetition the conversion ratio around 65% was maintained. On the contrary, not only catalytic activity but also reusability of CaO catalyst is lower than KF/CaO catalyst. This was mainly due to two reasons. On the one hand, the specific surface of KF/CaO is larger than CaO, which increased the contact between oil and catalyst, leading to improve catalytic effectiveness. On the other hand, based on the previous TPD characterization, KF and CaO combined to form KCaF₃ crystal that provided more basic sites (activated center). In addition, free K⁺ and Ca²⁺ concentration in the biofuel was determined by atomic absorption spectrometry after removal of the catalyst CaO or KF/CaO. The result showed that the concentrations of dissolved K⁺ and Ca²⁺ from KF/CaO and CaO catalysts were 16.7(K⁺), 25.4 and 136 mg L⁻¹, respectively. It indicates that KF/CaO catalyst is more stable than CaO in the reaction system as a result of the existence of KCaF₃ crystal phase 18. Since less loss of active component, the lifetime of KF/CaO was longer than CaO, which consisted with the previous reusability test.

3.3 Compositions and fuel properties of bio-fuel oil from C. wisoniana oil

Through GC–MS analysis of cracking products (Fig. 4), the main chemical compositions were listed in Table 2, including alkane, olefin, ester, and acid. Most of them were short-chain hydrocarbons (C5-C22). It further proved the macromolecular of C. wisoniana oil was converted to small molecule hydrocarbons after cracking reaction, polymerization reaction, and isomerization.

The main properties of the bio-fuel oil obtained from C. wisoniana oil are shown in Table 3. For comparison purposes, Table 3 also gives the specified values for the petroleum-based diesel and biodiesel from transesterification reaction. The result showed that the bio-fuels derived from C. wisoniana oil possess acceptable values of the given fuel properties when compared with those for the petroleum-based fuel and biodiesel. By using basic catalyst, the cracking products showed a relatively low acid value. In addition, the bio-fuel oil exhibited better low temperature fluidity and oxidation stability because it contained abundant short-chain hydrocarbons.