Review on latest developments in biodiesel production using carbon-based catalysts

https://doi.org/10.1016/j.rser.2013.09.003Get rights and content

Abstract

Catalyst plays an important role in biodiesel production. Owing to the advantages of heterogeneous catalysts in terms of separation and reusability over the traditionally used homogeneous catalyst, the research has now been focused on these heterogeneous catalysts in recent years. In order to make the process fully “green”, researchers are trying to prepare catalysts from renewable sources such as biomass. Within this concept the carbon based catalysts have been introduced. Carbon based materials are considered as ideal catalysts due to desirable features such as low material cost, high surface area and thermal stability. They are easily prepared by functionalizing carbon surface with acids or bases; in other cases carbon material was reported to be used as a support. Additionally, the carbon could be produced from most of the waste generated in different industrial processes. Therefore, its utilization as catalyst makes the biodiesel production a “greener” one. Under optimal conditions biodiesel (FAME) yields up to 90–98.3% were reported over various carbon based catalysts.

Introduction

Due to the continuous decline of limited petroleum reserves and the growing environmental concerns, the use of biodiesel in recent years as a fuel in the existing diesel engines has gained much importance [1]. Current generation biodiesel production usually utilizes homogeneous transesterification of vegetable oils with strong alkali (NaOH, KOH) as catalysts [2]. The process has many limitations, a considerable amount of energy is required for the purification of products and catalyst separation, and furthermore these catalysts are not reusable. This results in substantial energy wastage and the production of large amounts of chemical waste. Strong acids such as H2SO4, HCl can also catalyze this reaction but at a much slower rate limiting their industrial applicability [3]. Enzymes such as lipase can also do the same but the process is not economically viable [4]. In order to overcome these issues researchers have utilized different heterogeneous catalysts (both acidic and basic) for transesterification. These catalysts can be readily separated from the products at the end of the reaction and reused for the next reaction cycle [5], [6]. This has been demonstrated by different catalysts reported such as supported alkali metal catalysts [7], alkali and alkaline earth oxides, mixed metal oxides [5], [8], dolomites perovskite-type catalysts and zeolites [9], heteropolyacids [10], Amberlyst-15 [11], H3PW12O40·6H2O [12], WO3/ZrO2 [13], hydrotalcite and so on [14]. Ionic liquids have also been explored as catalysts in transesterification [15]. However, most of the ionic liquid catalysts reported so far utilized complex and expensive synthesis routes, demonstrated poor reusability and were non-biodegradable. Moreover, none of the catalysts were capable of showing activity comparable to alkali metal hydroxides. To address these issues, catalyst research for the production of cost effective biodiesel has been focused towards low cost renewable “green catalyst”. Such a novel catalyst could be prepared either from biomass or from waste generated from it in the households. Recently renewable heterogeneous catalysts such as metal oxides catalysts derived from oyster shells [16], shrimp shell [17], [18], eggshells [19], [20] and carbon-based catalysts [21], [22], [23] have gained much importance owing to their low material costs which could significantly bring down the biodiesel production cost. Many review papers regarding biodiesel feedstocks, properties, characterization and the development of heterogeneous catalysts in biodiesel production have been published in recent years. In some of the review papers the reviewers have divided the heterogeneous catalysts into different categories such as oxides, mixed metal oxides, and zeolites. However, review on the carbon based catalyst has rarely been mentioned in most of the review papers. The focus of this review paper is to present the latest research on carbon based catalysts, specific outcome and its importance in environmentally benign biodiesel production.

Conventional biodiesel production is based on the transesterification of triglycerides and alcohols (Scheme 1). Transesterification is the general term used to describe the important class of organic reactions where one ester is transformed into another ester through interchange of the alkoxy moiety.

Transesterification is the easiest and the most cost effective way to produce biodiesel. The reaction can be catalyzed by an acid or a base, in principle transesterification proceeds most efficiently on base catalysts. The overall economy of biodiesel production depends mainly on two crucial factors: (i) feedstock and (ii) catalyst (determines the no of steps and synthesis route). Nearly all the biodiesel plants are currently using refined vegetable oils such as soybean, rapeseed, and cottonseed as main feedstock and contribute nearly 80% of the overall biodiesel production cost [24]. In order to overcome these limitations and ensure economic viability in biodiesel production, biodiesel manufacturers are focusing their attention towards heterogeneous catalysis and low-cost alternatives feedstock such as waste cooking oil and non-edible oil (such as Jatropha and Pongamia) [25], [26]. As mentioned before, transesterification proceeds most efficiently on homogeneous base catalysts such as NaOH and KOH. However, due to some major drawbacks such as separation difficulty from product the focus is now on the development of heterogeneous catalysts which are easy to recover and could be reused for consecutive cycle. The schematic representations of biodiesel production with homogeneous and heterogeneous catalysts have been illustrated below Fig. 1, Fig. 2). Table 1 below illustrates an overview on the different aspects of biodiesel production using different types of catalyst. The use of carbon based catalyst makes the biodiesel production even more environmentally benign. Fig. 3 represents the schematic for the preparation of carbon based catalyst from biomass and its utilization as a catalyst in biodiesel production. This demonstrates that the carbon based catalysts could not only reduce the cost of biodiesel making but also could be used as “green catalyst”.

Activated carbon (AC) also called activated charcoal, activated coal or carbo activatus is the most well known form of carbon as catalyst or catalyst supports. AC is a type of amorphous carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions [28].

Prior to its development as a catalyst/catalyst support, the earliest use of AC could be traced far back into the history of the Ancient Egyptians who utilized its adsorbent properties for purifying oils and medicinal purposes. Charcoal was used for drinking water filtration in ancient India and is still used. By the early 19th century both wood and bone charcoal were used in large-scale for the decolorization and purification of cane sugar. However, it was not until the beginning of the First World War that the potential of activated carbon was realized. During the 1939–1945 war, significant development took place – leading to the development of more sophisticated chemically impregnated carbon for the entrapment of both war and nerve gases. In the 19th century in Europe, powdered activated carbon was first produced commercially by wood as a raw material. In the United States, the first production of activated carbon used black ash as the source after it was accidentally discovered that the ash was very effective in decolorizing liquids [28], [30], [31], [32], [33], [34], [35], [36]. Nowadays AC is mainly employed in filtering air and gases, wastewater treatment, removal of liquid-phase contaminants, including organic pollutants, heavy metal ions, organic dyes [29], [30], [32], [37] and as catalyst support [40]. The application of these carbons has been considered as a major unit operation in chemical and petrochemical industries [29]. In addition to serving as an adsorbent, high porosity carbons have recently been applied in the manufacture of high-performance layer capacitors. Because of the introduction of rigorous environmental regulations and the development of new applications, the demand for porous carbons has increased [29].

The raw materials used for producing AC are materials with high carbon content (such as coal, wood, peat, coconut shells and petroleum residues [30], [31]). These carbon-based materials are converted to AC by thermal decomposition in a furnace using a controlled atmosphere and heat by physical activation or chemical activation [32], [33] involving the following steps: (1) removal of all water (dehydration), (2) conversion of the organic matter to elemental carbon, driving off the non-carbon portion (carbonisation) and (3) burning off tars and pore enlargement (activation) [34]. Activated carbons are divided into three categories based on their physical characteristics: (a) powdered activated carbon (PAC), (b) granular activated carbon (GAC) and (c) extruded activated carbon (EAC) [35]. The basic structural unit of activated carbon is closely approximated by the structure of pure graphite. “Activated carbon is a crude form of graphite with a random or amorphous structure which is highly porous over wide range of pore sizes from visible cracks and crevices to cracks and crevices of molecular dimensions”. Structurally activated carbon is a disorganized form of graphite due to impurities and the method of preparation (activation process) where the layers are held by carbon–carbon bonds. Its precise atomic structure, however, is unknown. A much more recent suggestion is that activated carbon has a structure related to that of the fullerenes, in other words that it consists of curved fragments containing pentagons and other non-hexagonal rings in addition to hexagons, as illustrated in Fig. 4. Such a structure would explain the microporosity of the carbon and many of its other properties [38].

Originally, catalytic activity of AC is due to its surface oxides and unique surface properties but its main use is in the form of a catalyst support. AC is a material that has all the required characteristics to be used as a catalyst support and additionally it has unique properties like its heat resistance, stability in both acidic and basic media, the possibility of easy recovery of precious metals supported on it and the possibility of tailoring both its textural and surface chemical properties. Table 2 presents a comparison of the properties of AC with the other commercially available support materials. It is well established in the literature that the surface oxygen groups, which form anchoring sites for metallic precursors as well as for metals, dominantly determine the properties of activated carbon as a catalyst support material [1], [3], [4], [5], [6]. The acidic groups on the surface decrease the hydrophobicity of the carbon, leading to the accessibility of the surface to aqueous metal precursors [39]. The use of AC as a catalyst support has become popular during the last decade owing to its low material cost and desirable properties like very high surface area: 800–1500 m2/g and thermal stability. Fig. 5 presents the total U.S. sales of catalyst support materials. Auer et al. published an excellent review on this topic [40]. Ni/Pd catalysts dispersed on AC supports are the most well known examples of such type of supported catalysts. The physical and surface chemical properties of AC can be modified via different procedures. It is evident from the literature that by changing the surface chemistry of AC support, (i) the dispersion of active metallic phase(s), (ii) the calcination/reduction properties of active metal(s) and (iii) the interaction between metals and extent of alloy formation for bimetallic catalysts can be modified [41], [42]. Moreover, due to its close structural resemblance with graphite it is also possible to functionalise AC with groups like SO3H, Ph-SO3H, etc. similar to graphite, fullerenes, nanotubes or graphene [43], [44], [45], [46], [47], [48], [49], [50], [51]. This property was utilized by researchers to produce sulfonated-active carbons with –SO3H groups. These sulfonated-ACs are amongst the most extensively studied AC-catalysts that are believed to substitute conc. H2SO4 in industries in the near future; they are reported to catalyze reactions like esterification, cellulose hydrolysis, nitration, transesterification, etc. by acting as a solid acid [22], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64]. In addition there are numerous other reactions employing ACs as catalyst or catalyst support [40]. Biodiesel production is one such area where it has been suitably modified to serve as a catalyst.

Section snippets

Carbon-based catalysts in biodiesel production

In order to effectively cover the different types of carbon based (AC) catalysts reported in the literature for biodiesel synthesis, we have divided them into two categories: (i) functionalized catalysts (cover all types of AC catalysts where the active part is covalently attached to the support AC material) and (ii) supported catalyst (this category covers all the catalysts where porous carbon material or AC was used as a support for active catalysts such as CaO and KOH).

Discussion

Directly sulfonated-ACs are most studied of all the carbon based catalysts discussed here. The basic idea is to obtain a –SO3H containing solid material that can substitute homogeneous conc. H2SO4 usually used as a catalyst in esterification and transesterification reactions during biodiesel synthesis. According to the studies by different researchers, it has been well established that any carbon rich material such as sugars, glycerol, cellulose, Kraft lignin, vegetable or petroleum oil

Conclusion

The two key reactions in biodiesel production are the Esterification and Transesterification. These reactions are influenced mainly by the type of feedstock oil, reaction conditions, catalyst used and alcohol to oil molar ratio. Use of carbon-based (AC) catalysts in these reactions opens doors for cost minimization and environmentally benign biodiesel production by eliminating problems associated with the conventionally used reaction schemes (employing homogeneous catalysts commonly H2SO4, KOH

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