4Department of Biotechnology, Dr. B. R. Ambedkar National Institute of Technology, Jalandhar, India
Abstract
This chapter will describe the recent developments in the efficient utilization of various carbon-based materials as catalysts for the valorization of glycerol waste from the biodiesel industry. Carbon-based materials are generally prepared from biomass, a renewable feedstock and produced in a large amount from numerous sources such as agricultural wastes, forest residues, and food wastes. Biomass materials consist of cellulose, hemicelluloses, and lignin biopolymer, which act as a carbon source for carbon materials. Since a glycerol glut exists in the global market due to rapid growth in biodiesel production, the utilization of value-added products from glycerol is very important for the competitive market of biodiesel with conventional diesel. This chapter thus discusses the basic principles, mechanisms, and advancement in prominent techniques for glycerol valorization along with synthesis, characterization, and function of different carbon-based catalysts. The future prospects of these carbon materials as catalysts for industrial waste utilization are very promising.
Keywords: Waste to energy, biomass waste, carbon materials, glycerol valorization, biodiesel industry up gradation
4.1 Introduction
With the continuously rising population, rapid industrialization, and improving the standard of living, worldwide energy consumption is increasing at a high rate and projected to become more than double (~1.1 × 1,021 J per hour) by 2050 [1]. Despite enormous efforts to develop alternative energy resources, fossil fuels are estimated to continue as the major energy source for the near future. The widespread utilization of fossil fuels to meet the growing demand for energy has adverse effects due to significant greenhouse gases emission and the exhaustion of these resources. Therefore, the growth and utilization of sustainable resources of energy have become a subject of international importance [2].
Recently, biomass has received tremendous interest from researchers as an alternative feedstock for sustainable and clean energy production [3]. Biodiesel production using biomass is a sustainable approach due to its use as renewable, non-toxic, and biodegradable fuel with a small emission of air pollutants as compared to petroleum-based fuels [4]. Biodiesel is environmentally friendly due to small carbon monoxide and particulate matter emissions and does not release hydrocarbons [5]. The increased production of oxygen in the case of biodiesel leads to complete combustion [5]. The vital feedstock for biodiesel production is animal fat, vegetable oil, or waste cooking oil. The productions of biodiesel have increased drastically for the past few years due to its direct use in diesel engines without modifications [6]. However, the formation of biodiesel by transesterification of animal fats or vegetable oils as a raw feedstock produces approximately 10 wt% glycerol (1,2,3-propanetriol) as a by-product which increases its manufacturing cost [7]. This is bottleneck of the technology. A glycerol glut exists in the global market due to the fast growth in biodiesel production. Therefore, the biodiesel industry needs to produce valuable products from glycerol and make it more competitive with conventional diesel fuel. It is also important from an environmental and economic viewpoint [8].
The continuous increase in biodiesel demand will also generate a huge amount of glycerol and therefore, it needs to be utilized [9]. According to 2011 data, globally the total amount of glycerol produced by the biodiesel industry is 66.2%. In the world, the largest biodiesel-producing countries are the United States and Brazil [10]. In the USA, biodiesel consumption increases from 878 million gallons in 2011 to 1,725 million gallons in 2019. To address this issue, the US Department of Energy declared glycerol as a major building block platform chemical for the future. Therefore, the transformation of glycerol to valuable products is a rapidly growing research area that decreases the expenditure of biodiesel production [7, 11].
The transformation of glycerol into valuable products is essential for the industries because a huge amount of glycerol that is generated through biodiesel production and fermentation of sugars can be utilized judiciously. Moreover, glycerol is a non-hazardous, biodegradable, and bio sustainable compound. Economic and technical analyses indicate that the low cost of glycerol and its multifunctional structure could open a new market for valuable commodity chemicals. Furthermore, glycerol conversion into profitable chemicals and fuels (syngas/hydrogen) will facilitate the substitution of petroleum-based products as well as promote the utilization of clean energy resources [8]. However, glycerol must be purified before any attempt to transform it into value-added products. Many different reaction pathways (biological, thermocatalytic, etc.) have been investigated to derive the various chemicals starting from glycerol. Of the various methodologies, the use of catalysis represents an efficient approach for this purpose.
Recently, several novel catalytic transformation processes have been reported in the literature. Glycerol is mainly transformed into useful chemicals using steam reforming, esterification, hydrogenolysis, dehydration, oxidation, carboxylation, etherification, and acetalization [9, 12, 13]. A large variety of valuable derivatives, such as fine chemicals, polymers, fuels, and fuel additives have been achieved. A wide spectrum of catalysts such as metal, metal salt, metal oxides, resins, zeolite, heteropolyacids, sulfonic acid, metal sulfides, and carbon catalysts have been utilized for the efficient conversion of glycerol into value-added products [12, 13]. Carbon-based catalysts have added benefits compared to metal-based catalysts due to their high durability, better textural properties, easy availability, and low cost. Carbon-based catalysts have been used as a catalyst as well as support for different transformation processes. This chapter explains the fundamentals, mechanism, and latest development in the transformation of glycerol into valuable products over carbon-based catalysts. The earlier reported works demonstrated the possibility of the establishment of several new catalytic approaches and value-added products from glycerol. Interestingly, there are still a lot of opportunities available for researchers to design novel carbon-based catalysts and develop new methodologies for glycerol conversion.
4.2 Production of Biodiesel and Crude Glycerol
Worldwide biodiesel production has been rising extensively in the last few years. In 2018, the top five biodiesel producing nations were the United States (US), Argentina, Brazil, Germany, and China [10]. Both the US and Brazil have accounted for a combined share of nearly 87% of the total production in 2018. It is predicted that in the near future, the annual production of biodiesel will rise by approximately 4.5% [10]. Biodiesel is mainly produced from a broad diversity of materials that contains triglycerides as a key component. This can be produced by vegetable oils from jatropha, sunflower, babassu, soybean, macauba, palm, oilseed radish, canola, crambe, castor bean, peanut, and lupine [4]. Alternatively, it can also be produced from photosynthetic algae, animal fat, and waste cooking oil. The transesterification or esterification of triglycerides with alcohol (methanol) produces methyl esters of fatty acids (biodiesel) and glycerol as depicted in Figure 4.1. The catalysts used for transesterification are mainly alkalis, acids, or enzymes.
The transesterification reaction takes place in three steps as shown in Figure 4.2 in which triglycerides react with methanol. First, the reaction involves the conversion of triglycerides into diglycerides which then get converted into monoglycerides. Finally, the monoglycerides transformed into glycerol. The overall reaction is reversible, so, plenty of alcohol is required to favor the reaction in the forward direction. The theoretical stoichiometric ratio for the conversion is 1:3 (triglycerides: alcohol), but for practical applications surplus amount of alcohol is required to shift the equilibrium in the direction of the product. Overall, for the formation of 9 kg of biodiesel, approximately 1 kg of glycerol is generated [5].
The flow chart for the formation of biodiesel and byproduct glycerol is shown in Figure 4.3.
The commonly used catalyst during biodiesel production is NaOH which leads to the generation of faulty smell, dark-colored glycerol with high pH. The crude