The impact of heterogeneous catalyst on biodiesel production; a review

The present review primarily focuses on the perspectives and state-of-the-art of heterogeneous catalysts, nanocatalysts, biocatalysts, bifunctional catalysts, metal-organic frameworks (MOF), and covalent organic frameworks (COF) for biodiesel production. The environmental concern associated with nonrenewable fossil fuels has led to finding alternative energy sources that can be used to meet global energy demands. Biofuels such as biodiesel are one of the energy sources that could replace fossil fuels. The homogeneous acid and base catalysts are generally used for commercial biodiesel production. However, homogeneous catalysts have downsides such as toxicity, corrosion, soap formation, high wastewater output, and non-reusability. Consequently, heterogeneous acid and base catalysts have been introduced that are less sensitive to moisture and free fatty acids (FFAs), easily separated and recovered, and reusable. Recently, novel catalysts such as waste biomass-derived mesoporous heterogeneous catalysts, chemically synthesized heterogeneous catalysts, metal ion-doped heterogeneous catalysts, bifunctional acid-base catalysts, and carbonaceous char-supported hetero catalysts, nanocatalysts, MOF and COF catalysts have potential to replace homogeneous base catalysts, aid in sustainable and cost-effective biodiesel production. This review provides insights into the recent advancement of various catalysts, catalyst preparation and operations, type of catalysts and suitability, catalyst efficiency, life cycle assessment, catalyst-associated challenges, and prospects for 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.

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 continuous increase in the growth of the world's population is also drastically increasing energy demands in both industrial and domestic sectors. Moreover, public awareness about pollution due to overuse of fossil fuels has led to a rise in interest in research into alternative renewable energy sources. Among the various options for renewable energies for road transportation, biodiesel is considered the most suitable replacement for petro-diesel. In terms of industrial applications for biodiesel production, homogeneous catalysts such as NaOH, KOH, H2SO4, and HCl are usually preferred, but their removal is rather complex and somewhat polluting, bringing extra cost to the final product. The application of heterogeneous catalysts, both solid acid and solid base, for the transesterification reaction can resolve the biodiesel purification problems associated with homogeneous catalysts. However, heterogeneous catalysts are currently somewhat slow and inefficient, and present some mass-transfer limitations. In this context, nanocatalysts represent a new generation of heterogeneous catalysts with promising advantages over homogeneous and conventional heterogeneous ones. Other alternatives also exist. Enzymatic catalysis has been used in biodiesel production, employing lipases as biocatalysts. However, for economic reasons and due to issues of reusability and recycling, the lipases must be immobilized on suitable supports, bringing into play the concept of heterogeneous biocatalysis. Like other heterogeneous catalytic materials, this presents similar issues with inefficiency and mass-transfer limitations. A solution may be the use of nanostructured supports for enzyme immobilization as new heterogeneous biocatalysts. This chapter mainly focuses on the application of heterogeneous basic, acidic, and enzymatic catalysts, as well as nanocatalysts and nano(bio)catalysts, in transesterification reactions, and their multiple methods of synthesis.

Biodiesel is getting the attention of researchers due to its low cost and renewability. Researchers have done much work in the field of biodiesel production and its performance on engine. The effect of catalyst on transesterification reaction plays an important in terms of biodiesel yield and quality. Therefore, it is important to investigate the effect of catalyst quality and characteristics on transesterification reaction. In view of the same, the present paper deals with a review on effect of different heterogeneous and homogeneous catalyst on the biodiesel yield. The work of various past researchers has been compared. It is found that homogeneous catalyst gives better results in low temperatures however the heterogeneous catalyst needs high temperature for preprocessing however heterogeneous catalyst can be used again and again.

The scientific community is being forced to consider alternative renewable fuels such as biodiesel as a result of the sharp increases in the price of petroleum and the increased demand for petroleum-derived products. Transesterification is a technique used to create biodiesel where a variety of edible oils, non-edible oils, and animal fats are used. For this, either a homogeneous or heterogeneous catalyst is utilized. An appropriate catalyst is chosen based on the quantity of free fatty acid content in the oil. The main distinction between homogeneous and heterogeneous catalysts is that compared to the heterogeneous catalyst, the homogeneous catalyst is not affected by the quantity of free fatty acids in the oil. Early methods of producing biodiesel relied on homogeneous catalysts, which have drawbacks such as high flammability, toxicity, corrosion, byproducts such as soap and glycerol, and high wastewater output. The majority of these issues are solved by heterogeneous catalysts. Recent innovations use novel heterogeneous catalysts that are obtained from biomass and biowaste resources. Numerous researchers have documented the use of biomass-derived heterogeneous catalysts in the production of high-quality, pure biodiesel as a potentially greener manufacturing method. The catalysts were significantly altered through conventional physical processes that were both cost- and energy-effective. The present review is intended to analyze catalysts from biowaste for making biodiesel at a minimal cost. The most recent methods for creating diverse kinds of catalysts—including acidic, basic, bifunctional, and nanocatalysts—from various chemicals and biomass are highlighted in this review. Additionally, the effects of various catalyst preparation methods on biodiesel yield are thoroughly explored.

Chapter 3 - Applications of heteropoly acids as heterogeneous catalysts

The need for fossil fuels and the emissions generated from these fuels are increasing daily. Researchers are concerned with global warming as well as climate change; and energy sustainability and material usages are important issues today. Waste cooking oil (WCO) can be processed into biodiesel as an alternative fuel to replace diesel. Production of biodiesel using WCO as the feedstock has been of growing interest for the last two decades. A number of research papers related to the improvements in production, raw materials and catalyst selection have been published. This paper reviews the various types of heterogeneous solid catalyst in the production of biodiesel via the transesterification of WCO. The catalysts used can be classified according to their state presence in the transesterification reaction as homogeneous or heterogeneous catalysts. Homogeneous catalysts act in the same liquid phase as the reaction mixture, whereas heterogeneous catalysts act in a solid phase with the reaction mixture. Heterogeneous catalysts are non-corrosive, a green process and environmentally friendly. They can be recycled and used several times, thus offering a more economic pathway for biodiesel production. The advantages and drawbacks of these heterogeneous catalysts are presented. Future work focuses on the application of economically and environmentally friendly solid catalysts in the production of biodiesel using WCO as the raw material.

Fossil fuel depletion, increased world energy demand, and the environmental crisis linked to petroleum-based energy instigated the quest for its substitute. The sustainability of biodiesel affords it a high prospect over fossil fuels. It has been receiving attention as a result of its biodegradability, renewability, low toxicity, and good transport and storage properties. The main shortcomings of biodiesel are the production cost and choice of catalyst. Three types of catalysts mainly used for biodiesel production are basic, acidic, or enzyme. Industrial production of biodiesel typically employed homogeneous catalysts due to their ability to facilitate the reaction quickly. However, catalyst separation and biodiesel purification are tormenting, requiring a large amount of water. Thus, heterogeneous catalysts, with several advantages over homogenous catalysts, have been searched. Heterogeneous catalysts can be separated from the products effortlessly, thus allowing for recycling. Furthermore, the process is simpler, cheaper, and more environmentally benign. This review aims to evaluate the performance of different types of catalysts in the transesterification reaction, with special emphasis on heterogeneous base catalysts. The review also gives insight into the key catalytic properties that need to be tailored economically and eco-friendly to reduce cost, to give better biodiesel yield/conversion. Additionally, the various conditions necessary for the optimum yield of biodiesel have also been explored. Lastly, the future challenges and prospects of heterogeneous base catalysts are proposed. Keywords: Biodiesel, Catalyst performance, Heterogeneous catalyst, Transesterification

An ever-increasing energy demand and environmental problems associated with exhaustible fossil fuels have led to the search for an alternative renewable source of energy. In this context, biodiesel has attracted attention worldwide as an alternative to fossil fuel for being renewable, non-toxic, biodegradable, carbon-neutral; hence eco-friendly. Despite homogeneous catalyst has its own merits, currently, much attention has been paid to chemically synthesize heterogeneous catalysts for biodiesel production as it can be tuned as per specific requirement, easily recovered, thus enhance reusability. Recently, biomass-derived heterogeneous catalysts have risen to the forefront of biodiesel productions because of their sustainable, economical and eco-friendly nature. Further, nano and bifunctional catalysts have emerged as a powerful catalyst largely due to their high surface area and potential to convert free fatty acids and triglycerides to biodiesel, respectively. This review highlighted the latest synthesis routes of various types of catalysts including acidic, basic, bifunctional and nanocatalysts derived from different chemicals as well as biomass. In addition, the impacts of different methods of preparation of catalysts on the yield of biodiesel are also discussed in details.

An ever-increasing energy demand and environmental problems associated with exhaustible fossil fuels have led to the search for an alternative renewable source of energy. In this context, biodiesel has attracted attention worldwide as an eco-friendly alternative to fossil fuel for being renewable, non-toxic, biodegradable, and carbon-neutral. Although the homogeneous catalyst has its own merits, much attention is currently paid toward the chemical synthesis of heterogeneous catalysts for biodiesel production as it can be tuned as per specific requirement and easily recovered, thus enhancing reusability. Recently, biomass-derived heterogeneous catalysts have risen to the forefront of biodiesel productions because of their sustainable, economical and eco-friendly nature. Furthermore, nano and bifunctional catalysts have emerged as a powerful catalyst largely due to their high surface area, and potential to convert free fatty acids and triglycerides to biodiesel, respectively. This review highlights the latest synthesis routes of various types of catalysts (including acidic, basic, bifunctional and nanocatalysts) derived from different chemicals, as well as biomass. In addition, the impacts of different methods of preparation of catalysts on the yield of biodiesel are also discussed in details.

A review on the waste biomass derived catalysts for biodiesel production

Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis

As a promising renewable fuel, biodiesel has gained worldwide attention to replace fossil-derived mineral diesel due to the threats concerning the depletion of fossil reserves and ecological constraints. Biodiesel production via transesterification involves using homogeneous, heterogeneous and enzymatic catalysts to speed up the reaction. The usage of heterogeneous catalysts over homogeneous catalysts are considered more advantageous and cost-effective. Therefore, several heterogeneous catalysts have been developed from variable sources to make the overall production process economical. After achieving optimum performance of these catalysts and chemical processes, the research has been directed in other perspectives, such as the application of non-conventional methods such as microwave, ultrasonic, plasma heating etc, aiming to enhance the efficiency of the overall process. This mini review is targeted to focus on the research carried out up to this date on microwave-supported heterogeneously catalysed biodiesel production. It discusses the phenomenon of microwave heating, synthesis techniques for heterogeneous catalysts, microwave mediated transesterification reaction using solid catalysts, special thermal effects of microwaves and parametric optimisation under microwave heating. The review shows that using microwave technology on the heterogeneously catalysed transesterification process greatly decreases reaction times (5–60 min) while maintaining or improving catalytic activity (>90%) when compared to traditional heating.

Biodiesel is a promising alternative fuel that can be produced from vegetable oil through the esterification process. Conventional homogeneous acid or base catalysts result in a lower biodiesel yield and a longer reaction time and require separation from the product, which lead to inefficient production. This research was aimed at utilising the heterogeneous catalyst potassium iodide (KI)/γ-aluminium oxide (γ-Al2O3) for esterification of waste cooking oil into biodiesel. The catalyst was prepared using the impregnation method in which calcination was conducted at 500°C for 3 h. The esterification operation condition was set to a temperature of 65°C for 3 h and a molar ratio of oil to methanol of 1:14. Moreover, the heterogeneous catalyst was compared with a homogeneous catalyst with respect to the yield of biodiesel. The results showed that at a 2% (g catalyst/g oil) catalyst concentration, the esterification using the catalyst KI/γ-Al2O3 produced a higher yield of biodiesel (88·03%) compared with using γ-aluminium oxide (58·86%) and a conventional catalyst (82·23%). The reusability study of the catalyst KI/γ-Al2O3 showed that after the first reuse, the catalyst could still produce biodiesel with a yield of 61·30%. After the second reuse, the yield of biodiesel was 52·61%, and the third reuse produced a yield of 21·50%.