Prospects of novel heterogeneous base catalysts and nanocatalysts in achieving sustainable biodiesel production

Homogeneous base catalyst has wide acceptability in biodiesel production because of their fast reaction rates. However, postproduction costs incurred from aqueous quenching, wastewater and loss of catalysts led to the search for alternatives. Heterogeneous base catalyst is developed to cater these problems. The advantages of heterogeneous catalyst are their high basicity and non-toxicity. This work compared the production of biodiesel using two different kind of catalysts that is homogeneous catalyst (sodium hydroxide, NaOH and potassium hydroxide, KOH) and heterogeneous catalysts (calcium, oxide, CaO catalyst derived from chicken and ostrich eggshells). Transesterification of waste cooking oil (WCO) and methanol in the presence of heterogeneous base catalyst was conducted at an optimal reaction condition (calcination temperature for catalyst: 1000 °C; catalyst loading amount: 1.5 wt%; methanol/oil molar ratio: 10:1; reaction temperature: 65 °C; reaction time: 2 hours) with 97% biodiesel yield was obtained. While, the homogeneous base catalyst gave higher biodiesel yield of 98% at optimum operating condition (catalyst concentration: 0.75 wt%; methanol/oil molar ratio: 6:1; reaction temperature: 65 °C; reaction time: 1 hours). The slight difference in the biodiesel yield was due to the stronger basic strength in the homogeneous catalyst and were not statistically not different (p=0.05). However, despite these advances, the ultimate aim of producing biodiesel at affordable low cost and minimal-environmental-impact is yet to be realized.

Biodiesel is one of the biofuels that has been examined the most in the research and has a promising future as a replacement for fossil fuels. A catalyst is often needed to increase the manufacture of biodiesel, which is produced by transesterification or esterification. A number of difficulties with product separation, a significant amount of wastewater effuents released from the downstream filtration, and the unwanted soap created as a result of the reaction between feedstock with a high FFA composition and homogeneous base catalysts are typical problems faced by commercially available homogeneous catalysts. The use of heterogeneous magnetic catalysts in FAME production has become a focus of interest for many researchers due to its efficient catalyst separation, high catalyst recovery rate, and shorter processing time. The development of magnetic acid and base heterogeneous catalysts, as well as their efficiency in producing biodiesel, were the primary subjects of this study. Additionally, several techniques for producing magnetic particles were evaluated. Method of heterogeneous catalysts in promoting biodiesel synthesis through transesterification procedure was also investigated. To generate more self-sustaining biodiesel industries, further investigation is needed to employ waste materials as a basis for the manufacturing of heterogeneous magnetic acid and base catalysts.

Lactic acid (2-hydroxypropionic acid, CH3CHOHCOOH) is one of the platform chemicals derived from biomass. It is used in the food industry and in the manufacture of biodegradable plastics and useful chemicals. Recently, various examinations were carried out not only by fermentation but also by the chemical methods using heterogeneous and homogenous catalysts. This chapter focuses on the chemical processes with heterogeneous catalysts in lactic acid and lactate ester productions from sugars. Bronsted basic catalysts and Lewis acid catalysts gave lactic acid in high yields. In the lactic acid productions from triose, lactic acid ester is obtained with high yields of nearly 100 % in alcohols around 100 °C using Sn-β zeolite, Sn–carbon–silica, and H-USY catalysts. In the lactic acid production from hexose, lactic acid ester or a lactate salts was obtained from glucose, fructose, and sucrose with the comparatively high selectivity of about 50 % by several catalytic processes, that were in water around 50 °C using heterogeneous basic catalysts, such as activated hydrotalcite catalyst, in hydrothermal water around 300 °C using homogeneous basic catalysts, such as NaOH and ZnSO4, and in alcohols around 160 °C using heterogeneous Lewis acid catalysts, such as Sn-β zeolite.

With the rapid development of industry and the increasing demand for transportation, traditional sources of energy have been excessively consumed. Biodiesel as an alternative energy source has become a research focus. The most common method for biodiesel production is transesterification, in which lipid and low carbon alcohol are commonly used as raw materials, in the presence of a catalyst. In the process of transesterification, the performance of the catalyst is the key factor of the biodiesel yield. This paper reviews the recent research progress on homogeneous and heterogeneous catalysts in biodiesel production. The advantages and disadvantages of current homogeneous acid catalysts and homogeneous base catalysts are discussed, and heteropolyacid heterogeneous catalysts and biomass-derived base catalysts are described. The applications of the homogeneous and heterogeneous catalyst derivatives ionic liquids/deep eutectic solvents and nanocatalysts/magnetic catalysts in biodiesel production are reviewed. The mechanism and economic cost of current homogeneous acid catalysts and homogeneous base catalysts are also analyzed. The unique advantages of each type of catalyst are compared to better understand the microscopic details behind biodiesel. Finally, some challenges of current biodiesel catalysts are summarized, and future research directions are presented. This review will provide general and in-depth knowledge on the achievements, directions, and research priorities in developing novel homogeneous/heterogeneous catalysts for the green and cost-effective production of biodiesel.

A review of magnetic solid catalyst development for sustainable biodiesel production

A heterogeneous base catalyst transesterification process with a calcium oxide (CaO) catalyst was performed to produce high-purity methyl ester (ME) from pretreated sludge palm oil (PSPO) derived from sludge palm oil (SPO). Additionally, a comparative analysis was conducted with potassium hydroxide (KOH) as a homogeneous base catalyst to assess the distinctions between heterogeneous and homogeneous base catalysts. The response surface methodology (RSM) was utilized to determine the optimal and recommended conditions for both transesterification processes. For heterogeneous transesterification, a varying CaO catalyst loading (10–60 wt.%), methanol (25–65 wt.%), and reaction time (60–180 min) were essential parameters. Meanwhile, homogeneous transesterification involved investigating the KOH catalyst loading (1–3 wt.%), methanol (1.8–5.5 wt.%), and reaction time (20–60 min). For the heterogeneous-base-catalyzed reaction, the recommended conditions were as follows: a molar ratio of methanol to oil of 5.83:1 (41.61 wt.%), 31.3 wt.% CaO, and a reaction time of 119.0 min, which resulted in a ME purity of 96.51 wt.%. The optimal conditions for homogeneous transesterification were a molar ratio of methanol to oil of 0.49:1 (3.45 wt.%), a 40 min reaction time, and a 1.39 wt.% KOH concentration, which achieved 96.59 wt.% ME and met the standard.

The relevance of heterogeneous catalysis in biodiesel production cannot be overemphasized, as heterogeneous catalysts have eliminated the demerits associated with a homogeneous catalysts. Some heterogeneous catalysts experience drawbacks such as partial recoverability and reusability, energy and waste conservation issues during biodiesel processing and leaching of active catalyst sites. This paper highlights and summarizes several heterogeneous catalysts used in biodiesel production. The catalyst preparation, reaction conditions, feedstock, and biodiesel yield for the heterogeneous base and acid catalysts were emphasized. The inability of heterogeneous base catalysts to trans-esterify low-grade oil with high free fatty acid (FFA) is a primary concern; the cost of processing low-grade oil with high FFA using heterogeneous acid catalysts is also a big issue. Nano-doped heterogeneous catalysts with unique properties were recommended because they can process oil with high FFA transesterification, improve reaction efficiency, simplify production, reduce the leaching of active sites, enable better biodiesel yield by minimizing energy and waste, and increase catalyst recoverability, activity, selectivity and durability.

The effective transesterification process to produce fatty acid methyl esters (FAME) requires the use of low-cost, less corrosive, environmentally friendly and effective catalysts. Currently, worldwide biodiesel production revolves around the use of alkaline and acidic catalysts employed in heterogeneous and homogeneous phases. Homogeneous catalysts (soluble catalysts) for FAME production have been widespread for a while, but solid catalysts (heterogeneous catalysts) are a newer development for FAME production. The rate of reaction is much increased when homogeneous basic catalysts are used, but the main drawback is the cost of the process which arises due to the separation of catalysts from the reaction media after product formation. A promising field for catalytic biodiesel production is the use of heteropoly acids (HPAs) and polyoxometalate compounds. The flexibility of their structures and super acidic properties can be enhanced by incorporation of polyoxometalate anions into the complex proton acids. This pseudo liquid phase makes it possible for nearly all mobile protons to take part in the catalysis process. Carbonaceous materials which are obtained after sulfonation show promising catalytic activity towards the transesterification process. Another promising heterogeneous acid catalyst used for FAME production is vanadium phosphate. Furthermore, biocatalysts are receiving attention for large-scale FAME production in which lipase is the most common one used successfully This review critically describes the most important homogeneous and heterogeneous catalysts used in the current FAME production, with future directions for their use.

From waste to fuel: Challenging aspects in sustainable biodiesel production from lignocellulosic biomass feedstocks and role of metal organic framework as innovative heterogeneous catalysts

Metal oxide catalysts for biodiesel production

One of the most important needs in the biodiesel industry is the availability of catalysts. The catalyst most commonly used in making biodiesel is a homogeneous catalyst. However, the use of this catalyst has a weakness, namely that it is difficult to separate from the reaction mixture so that it will be wasted as waste which can cause environmental pollution. To overcome this, it is necessary to substitute a homogeneous base catalyst with a heterogeneous base catalyst. In this study, biodiesel production was carried out through the transesterification reaction of palm oil and methanol using three types of heterogeneous catalysts, namely zeolite, ZnO, and Al2O3. The results showed that the three types of catalysts can be used to become catalysts in the biodiesel production process. Of the three catalysts, the most effective use for biodiesel production from palm oil is the Al2O3. The characteristics of the biodiesel produced also meet the quality standards set out in the Indonesian National Standard.

Waste cooking oil from restaurants and hotels is expected to be thrown away. As a result, waste cooking oils (WCO) which have no pre-treatment will unfavourably pollute the earth. One of the approaches to treat and use the waste cooking oil is by its conversion into biodiesel. This paper illustrates the study on the active reaction of waste cooking oil transesterification into biodiesel and identifies the best set of input parameters for the most economical yield of biodiesel. The sample of WCO for the present study was collected from the restaurants. Methanol was used as a reactant along with two homogeneous base catalysts(KOH, NaOH) and one heterogeneous base catalyst(CaO). The input parameters selected for the study were methanol-to-oil molar ratio, catalyst type, reaction temperature and reaction time. The orthogonal experimental array L9 was selected for the present study with four parameters and three levels. The optimum set of input parameters for the transesterification process is; 8:1 methanol-to-oil molar ratio, KOH as a catalyst, 55 °C reaction temperature and 120 min of reaction time. The optimum yield of FAME was 92.5% with 1.0% catalyst concentration by weight.

In recent years, biodiesel has gained international attention as a source of alternative fuel due to characteristics like high degradability, no toxicity, low emission of carbon monoxide, particulate matter and unburned hydrocarbons (Al Zuhair, 2007; Vicente et al., 1998). Biodiesel is a mixture of alkyl esters and it can be used in conventional compression ignitions engines, which need almost no modification. As well, biodiesel can be used as heating oil and as fuel (Mushrush et al., 2001; Wardle, 2003). So far, this alternative fuel has been successfully produced by transesterification of vegetable oils and animal fats using homogeneous basic catalysts (mainly sodium or potassium hydroxide dissolved in methanol). Traditional homogeneous catalysts (basic or acid) possess advantages including high activity (complete conversion within 1 h) and mild reaction conditions (from 40 to 65 °C and atmospheric pressure). However, the use of homogeneous catalysts leads to soap production. Besides, in the homogeneous process the catalyst is consumed thus reducing the catalytic efficiency. This causes an increase in viscosity and the formation of gels. In addition, the method for the removal of the catalyst after reaction is technically difficult and a large amount of wastewater is produced in order to separate and clean the products, which increases the overall cost of the process. Thus, the total cost of the biodiesel production based on homogeneous catalysis, is not yet sufficiently competitive as compared to the cost of diesel production from petroleum. An alternative is the development of heterogeneous catalysts that could eliminate the additional running costs associated with the aforementioned stages of separation and purification. In addition, the use of heterogeneous catalysts does not produce soap through free fatty acid neutralization and triglyceride saponification. Therefore, development of efficient heterogeneous catalysts is important since opens up the possibility of another pathway for biodiesel production. The efficiency of the heterogeneous process depends, however, on several variables such as type of oil, molar ratio alcohol to oil, temperature and catalyst type. Thus, the objective of this chapter is to present a review of the effect of the aforesaid variables on important characteristics of biodiesel such as methyl esters content. Some characterization techniques for both, biodiesel and heterogeneous catalysts will also be addressed.

A novel two-step transesterification process catalyzed by homogeneous base catalyst in the first step and heterogeneous acid catalyst in the second step

Acid-base solid catalysts synthesized with structurally controlled uniform active sites can lead to unique catalysis. In this study, a CePO4 catalyst was synthesized using a hydrothermal method and found to exhibit high catalytic performance for the chemoselective acetalization of 5-hydroxymethylfurfural with alcohols, in sharp contrast to other homogeneous and heterogeneous acid and/or base catalysts. In the presence of CePO4, various combinations of carbonyl compounds and alcohols are efficiently converted into the corresponding acetal derivatives in good to excellent yields. Mechanistic studies show that CePO4 most likely acts as a bifunctional catalyst through the interaction of uniform Lewis acid and weak base sites with 5-hydroxymethylfurfural and alcohol molecules, respectively, which results in high catalytic performance.