The widespread usage of fossil fuels upset the earth’s carbon cycle. Climate change is the most serious environmental issue of the twenty-first century, and renewable resources must be exploited to investigate sustainable energy and fossil fuel substitutes [1]. To effectively control the global temperature and achieve the goal of carbon neutrality, it is necessary to promote the use of sustainable energy and encourage the utilization of low-carbon emission technologies [2]. Because some carbon in biomass fuel is obtained from the carbon cycle in the biosphere. Compared with fossil fuels, the use of biodiesel can greatly reduce carbon dioxide emissions [3]. Biodiesel is a kind of biodegradable renewable biomass energy. Biodiesel contains no aromatic hydrocarbons, and its combustion products contain almost no sulfide [4]. The raw material of the first-generation biodiesel comes from edible oil-producing crops, such as soybean & peanut [5,6], rapeseed [7], and so on [8]. Using edible oil as raw material for biodiesel production is too costly, while non-edible oil does not compete with people for food, with lower prices [9]. The raw materials of the second-generation biodiesel are the nonedible oil, such as Ricinus communis seeds [10], Jatropha curcas [11,12], tobacco seeds [13], and acer truncatum bunge seed [14]. Edible oil plays an essential role in the manufacture of biodiesel, but it also has some drawbacks, such as disagreements over the creation of fuel from food and the high price of edible oil [15]. Non-edible oil has a cheap production cost and a large oil yield, and it may grow in difficult terrain. Castor oil is one of the preferred non-edible oils for biodiesel production [16,17]. Castor contain 40–55 wt% oil on average, and its plant can grow on barren land. Because castor oil contains hydroxyl groups, even at low reaction temperatures, castor oil and ethanol have high miscibility, which can make the reactants fully contact [18].
There are numerous biodiesel preparation methods, including direct mixing, pyrolysis, microemulsion, and chemical methods, among others. The chemical process of transesterification is the most extensively employed, and transesterification catalysts are widely available [19]. The homogeneous base catalysis method has the benefits of short reaction time, whereas it requires high-quality of raw materials. If raw materials contain high FFAs (free fatty acids) and moisture, the saponification reaction with alkali will consume the catalyst and reduce the yield. At present, there are many limitations in the technology of commercial production of biodiesel by homogeneous transesterification [20]. Homogeneous acid catalysts have modest requirements for the quality of raw materials. However, the catalytic reaction rate is slow, which is damaging to equipment and generates a substantial amount of acidic effluent during product treatment. Homogeneous catalysis has some flaws, such as difficult catalyst separation and reuse, as well as substantial environmental contamination, which hastens the transition to heterogeneous catalysis [21]. Renewable, easy to separate from the product, less corrosive equipment, and reusable are the advantages of heterogeneous catalysts. Nevertheless, there are still some problems in heterogeneous catalysts, such as low acidity or alkalinity of reaction sites, deactivation, and low porosity [22]. Corro et al. proposed earlier to catalyze the esterification of high-content-FFAs in jatropha curcas crude oil with methanol by photocatalyst. With ZnO/SiO2 as a heterogeneous photocatalyst, the conversion rate was 96 % under ultraviolet irradiation [23]. F. Nadeem et al. used a solid alkali catalyst derived from eggshell to photocatalysis transesterification of waste edible oil with methanol under sunlight to prepare biodiesel, and the yield was 86.8 %. There are about 190 kinds of semiconductor photocatalysts. However, most of them are not ideal. Due to the photo-corrosion phenomenon (CdS), the photocatalysts have poor stability, large band gap (TiO2) and do not absorb visible light, or the cost is high (Ag/AgCl), thereby they cannot be applied to industrial production [24]. Zinc oxide has lots of advantages, for example non-toxicity, low production cost, easy crystallization, high activity, and high photosensitivity, which makes it one of the most widely used metal oxides. However, pure ZnO has a large energy gap (Eg = 3.2 eV), lacks activity in the visible region, and has a high electron-hole recombination rate [25]. Massaro et al. loaded ZnO on halloysite and found that ZnO improved the absorption capacity of the ultraviolet–visible spectrum because of halloysite [26]. Zhou et al. prepared α-Fe2O3/g-C3N4@halloysite by a two-step calcination method. Catalyst for photocatalytic degradation of dibenzothiophene. Halloysite fundus augments the specific surface area (SBET) of the sample, increases active sites, and prevents the agglomeration of g-C3N4 [27]. Constructing heterojunction is an effective method to prevent electron-hole pair recombination [28,29]. Guo et al. prepared CuO/ZnO photocatalyst by improved precipitation method to catalyze transesterification of pre-esterified waste edible oil to prepare biodiesel. CuO and ZnO form heterojunction, which is beneficial to inhibit electron-hole recombination, and improve oxygen vacancy and photocatalytic performance [30]. Tin dioxide, a metal oxide semiconductor, has been commonly applied in the fields of photocatalysis, batteries, sensors, detectors, etc. However, tin dioxide has a wide band gap (Eg = 3.6 eV) and photogenerated charges that are easy to recombine with holes, with its low photocatalytic activity [31]. The heterojunction of SnO2 with other suitable materials (such as ZnO) is beneficial to the effective separation of photogenerated charges and can enhance its optical and electrical properties [32].
Halloysite is a kind of natural clay with abundant reserves and low price, which can be used for large-scale production. Halloysite is mainly used as the carrier of catalyst, which can reduce the agglomeration phenomenon of the supported catalyst, improve the absorption and utilization of light source and thus improve the catalytic activity, and is an ideal carrier for constructing efficient composite photocatalyst [33]. Zhu et al. [34] supported calcium and zinc on halloysite by impregnation method to catalyze palm oil to produce biodiesel, and the yield was 95.22 %. Liu et al. [35] supported MoS2/Fe on halloysite to catalyze photo-Fenton reaction, and the decomposition rate of aniline aerofloat was about 99 %. Lou et al. [36] supported Ag3PO4 on halloysite for photocatalytic decomposition of methylene blue and tetracycline, with degradation rates of 98 % and 92 %.
In this study, zinc oxide and tin dioxide were supported on halloysite by the wet firing method, and a reusable halloysite supported bimetallic oxide heterojunction photocatalyst ZnO/SnO2@halloysite with high activity and low cost was prepared. The ingredient, SBET, light absorption characteristics, element distribution, and morphology of the catalyst were analyzed by TG-DTG (Thermogravimetric-Derivative Thermogravimetric), N2-adsorption-desorption, XRD (X-ray Diffraction), UV–vis (Ultraviolet–visible spectroscopy) and SEM-EDS (Scanning Electron Microscope-Energy Dispersive Spectrometer). The experimental factors affecting biodiesel yield were optimized by the Taguchi method, like catalyst dosage, molar ratio of ethanol to oil, reaction time, and reaction temperature. The contribution of reaction factors to biodiesel yield was determined according to variance analysis. Finally, the kinetic equation and reaction mechanism of photocatalytic transesterification with ZnO/SnO2@halloysite photocatalyst are presented. It provides a new insight into preparing biodiesel by photocatalytic transesterification.
