catalysts Review Biochars and Their Use as Transesterification Catalysts for Biodiesel Production: A Short Review John Vakros Department of Chemistry, University of Patras, GR 26504 Patras, Greece; [email protected]; Tel.: +30-26-1099-7143 Received: 16 October 2018; Accepted: 14 November 2018; Published: 20 November 2018 Abstract: Biodiesel can be a significant alternative for diesel. Usually, it is produced through transesterification with a base catalyst. Using heterogeneous catalysts for transesterification, the process can be more efficient. Among the possible catalysts that can be used, biochars combine high performance for transesterification and valorization of waste biomass. Biochars are cheap materials, and are easy to activate through chemical treatment with acid or base solutions. In this short review, the application of biochar as solid heterogeneous catalysts for transesterification of lipids to produce biodiesel is discussed. Keywords: biodiesel; biochar; acid modification; base modification; transesterification; pseudocatalytic transesterification 1. Introduction Two great challenges that people are called to face in our time is to meet the increasing demand for energy and to reduce the wastes released in the environment. The energy challenge is approached with CO2 neutral fuels like biofuels. Among them, biodiesel has significant advantages and it is preferable for using in existing engines and transport infrastructures. Biodiesel is usually produced through transesterification/esterification of lipids [1]. This process requires the presence of a catalyst. On the other hand, biomass is a class of solid wastes with interesting properties. Biomass can be used directly as fuel, or through gasification or pyrolysis, and produce valuable products. What is more attractive is that the by-product of the pyrolysis process is a carbon rich solid called biochar. Biochars have unique properties and can have significant applications in environmental protection [2]. In this short review, the potential use of biochars as heterogeneous catalysts for transesterification/esterification reaction for producing biodiesel will be discussed. 2. Biodiesel Diesel is a liquid fuel containing C8–C25 hydrocarbons and is produced from petroleum by fractional distillation at temperatures 200–350 ◦C. It is used in diesel engines for transportation and heating. Gasoline and diesel cannot be used in the same engine, so renewable fuel sources to diesel are required for partial substitution of diesel. The two main alternatives are biodiesel and green diesel [3,4]. Biodiesel and green diesel are both produced from triglycerides but biodiesel has a significant amount of oxygen while green diesel consists of hydrocarbons. The green diesel is produced through selective deoxygenation under high pressure of hydrogen (10–80 bar) and moderate temperatures (260–360 ◦C), and requires catalysts with hydrogenation ability like Ni or precious metals [5–7]. Biodiesel is a renewable liquid fuel and a potential alternative to diesel. It is a yellowish liquid derived from lipids through transesterification reaction with methanol or ethanol in the presence of basic catalysts. The chemical form of biodiesel is methyl esters of fatty acids and has 90% of the Catalysts 2018, 8, 562; doi:10.3390/catal8110562 www.mdpi.com/journal/catalysts Catalysts 2018, 8, x FOR PEER REVIEW 2 of 13 Catalysts 2018, 8, 562 2 of 14 basic catalysts. The chemical form of biodiesel is methyl esters of fatty acids and has 90% of the energy energydensity densityof the diesel. of the Biodiesel diesel. Biodiesel is non-toxic, is non-toxic, with low withor no low sulfur or noand sulfur aromatic and compounds. aromatic compounds. The main Thebiodiesel main advantages biodiesel advantages are that it is are carbon that itneutral is carbon and neutralit has the and same it has properties the same with properties the diesel, with which the diesel,make it which easier make to replace it easier without to replace engine without modifica enginetion. modification.Biodiesel can be Biodiesel produced can from be produced many sources from manylike straight sources vegetable like straight oil, animal vegetable oil/fats oil, or animal waste cooking oil/fats oil. or wasteThe most cooking popular oil. method The most for biodiesel popular methodproduction for biodieselis the transesterification production is the process transesterification [8]. process [8]. AccordingAccording toto thisthis method,method, aa vegetablevegetable oiloil oror a fat feedstock reacts with an alcohol, like methanol oror ethanol,ethanol, inin thethe presencepresence ofof aa catalystcatalyst toto produceproduce esters.esters. The scheme of the reactionreaction cancan bebe seenseen inin FigureFigure1 1.. TheThe triglyceridetriglyceride convertsconverts toto di-di- and and mono-glyceride mono-glyceride and and finally finally to to fatty fatty esters. esters. The The reaction reaction needsneeds thethe presencepresence ofof aa catalystcatalyst andand anan excessexcess ofof alcohol.alcohol. The reaction is aa three-stepthree-step reversiblereversible reactionreaction withwith diglyceridediglyceride andand monoglyceridemonoglyceride asas intermediateintermediate products.products. Most of thethe kinetickinetic studiesstudies forfor transesterificationtransesterification reactionreaction considerconsider thethe reactionreaction asas pseudo-homogeneouspseudo-homogeneous firstfirst order and itit waswas assumedassumed thatthat thethe methanol and catalyst are sufficientsufficient forfor thethe reaction toto shift the reaction equilibrium towardstowards thethe formationformation ofof productsproducts andand thatthat thethe reversereverse reactionreaction isis negligible.negligible. Also,Also, withwith aa high excess ofof methanol,methanol, thethe changechange inin concentrationconcentration ofof methanolmethanol inin thethe reactionreaction wouldwould bebe negligiblenegligible [[9].9]. Figure 1. Transesterification reaction scheme. Figure 1. Transesterification reaction scheme. In most cases, the catalyst is a strong base like NaOH or KOH [10]. The strong base is dissolved in methanolIn most and cases, the catalyst the catalyst is working is a strong homogenous. base like NaOH The reaction or KOH takes [10]. place The instrong the interface base is dissolved between thein methanol alcoholic solutionand the andcatalyst the oilis working and needs homoge intensenous. stirring. The reaction This is the takes most place economical in the interface process becausebetween itthe requires alcoholic a lower solution ratio and between the oil oil and and needs methanol intense (usually stirring. 1:6) This and is low the temperatures most economical and atmosphericprocess because pressures, it requires producing a lower ratio 98% conversionbetween oil yieldand methanol in relative (usually low reaction 1:6) and times low (betweentemperatures 1 to 3and h). atmospheric The amount pressures, of the catalyst producing used 98% is 1% conversionw/w. When yield the in reaction relative completes,low reaction the times glycerol (between and the1 to excess3 h). The of methanolamount of have the catalyst to be removed. used is 1% Gravity w/w. When separation the reaction is used completes, for the removal the glycerol of glycerol and andthe excess this requires of methanol about have 4–8 h. to Thebe removed. upper layer Gravit containsy separation biodiesel is used while for the the second removal layer of containsglycerol glycerol,and this requires about 90% about of the 4–8 base h. The catalyst upper and layer 70% contains of the excessbiodiesel methanol. while the The second biodiesel layer layer contains also containsglycerol, someabout contaminants, 90% of the base like catalyst methanol, and free 70% glycerol, of the andexcess residual methanol. catalyst. The Thesebiodiesel contaminants layer also arecontains water some soluble contaminants, and can be removedlike methanol, through free hot glycerol, water washing. and residual This catalyst. step is the These most contaminants problematic step,are water because soluble water and hascan tobe beremoved removed through completely hot water from washing. biodiesel This and step also is the neutralized most problematic before step, because water has to be removed completely from biodiesel and also neutralized before disposal. The process is complicated and the following parameters can influence the yield of the disposal. The process is complicated and the following parameters can influence the yield of the transesterification/esterification reaction [11]. transesterification/esterification reaction [11]. Catalyst amount: An excess of catalyst amount is required for shorter reaction times and also to Catalyst amount: An excess of catalyst amount is required for shorter reaction times and also to prevent the reversible reaction. For homogeneous catalysis, an amount of % wt/wt is the optimum. prevent the reversible reaction. For homogeneous catalysis, an amount of % wt/wt is the optimum. A higher amount of catalysts increase the cost and makes separation between the catalyst, methanol A higher amount of catalysts increase the cost and makes separation between the catalyst, methanol and biodiesel more difficult. and biodiesel more difficult. Reaction temperature: The activity of the catalyst depends on the reaction temperature. Reaction temperature: The activity of the catalyst depends on the reaction temperature. Higher Higher reaction temperatures increase the reaction rate. In homogeneous catalysis, the temperature reaction