Recent trends on sewage sludge transesterification: review

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.

Turpentine oil and its derivatives are major monoterpene compounds widely used as organic solvent, fragrance, flavoring agent, disinfectant, pesticides, and pharmaceutical applications. Isomerization of turpentine (mostly composed of α-pinene) can results important products such as camphene and limonene. Unfortunately, the synthesis occurred in a very complex reaction pathway, which can produce several side-products such as terpinene, terpinolene, polymers, and hydrates. The aim of this study is to conduct isomerization reaction of turpentine by using heterogeneous and homogeneous catalysts in the optimum operation conditions. The reaction was carried out under acidic solution as it needs protons (H+) to attack the double bond of α-pinene and produce carbocation named pinanyl cation. Then it would be rearranged into more stable isomers that we desired. The procedures of the synthesis were divided into four steps i.e. preparation of catalyst, reaction of turpentine and the catalyst in an acidic solution, separation of water-oil phase by decantation, and analysis of the oil phase product by using Gas Chromatography-Mass Spectrophotometry (GC-MS) method. Titanium dioxide (TiO2), silicon dioxide (SiO2), and zeolite (treated and mixed by either HCl or p-toluenesulfonic acid) are the solid or heterogeneous catalysts used in this experiment while sulfuric acid and formic acid are the homogeneous ones. Other variations were conducted for obtaining the best operation condition i.e. reaction temperature, amount of the catalyst, and value of pH. It was found in heterogeneous acid catalysts; the highest limonene yield is 24.3% from zeolite catalyst with composition of 0.8 gram catalyst per 10 mL turpentine mixed with HCl 1 M at 85°C and agitation time of 6 hours. For homogeneous acid catalysts, the highest limonene yield is 48.2% by using combined sulfuric acid and formic acid at pH of 0.37 under the same temperature and reaction time as heterogeneous condition.

Biodiesel, an environmentally friendly biomass-based fuel, is gaining popularity globally as a cost-effective way to meet rising fuel demand. However, the high cost of raw materials and catalysts continues to drive up biodiesel production. An alternative feedstock with a heterogeneously catalyzed reaction could be the most cost-effective way to stabilize industrial biodiesel growth. Understanding these issues led to the idea of using waste palm oil as a feedstock for biodiesel production. While using waste materials as feedstock for biodiesel is an elegant solution, converting high free fatty acids (FFA) directly into methyl esters has some drawbacks. High FFA processes (acid esterification, then base transesterification) are costly. The commercial processes currently use a homogeneous system with sulfuric acid to catalyze both esterification and transesterification. However, heterogeneous solid acid catalysts are preferred over hazardous mineral acids for high FFA esterification because they are less corrosive, produce less waste, and are easier to separate from reactants and products by filtration, recovery, and reusability. Heterogeneous acid catalysts can also simultaneously catalyze transesterification and esterification reactions. Thus, new waste-based support for heterogeneous catalysts (solid acid catalysts) is required to convert waste oils into biodiesel.

Biodiesel fuel has shown great promise as an alternative to petro-diesel fuel. Biodiesel production is widely conducted through esterification and transesterification reaction, catalyzed by homogeneous or heterogeneous catalysts. In the present endeavour, sulphated zirconia has been prepared using microwave method and characterized by using XRD, FT-IR, BET surface area, SEM and acidity measurement. The utility of synthesized catalyst is demonstrated in catalytic biodiesel production from acid oil feed stock. The influence of different parameters such as the catalyst concentration, oil: methanol ratio, operation time on the yield and properties of the produced biodiesel were studied. The produced biodiesel was characterized by using gas chromatography-mass spectroscopy (GC-MS) and NMR. Keywords: Heterogeneous acid catalyst, sulfated zirconia, acid oil, biodiesel

Metal oxide catalysts for biodiesel production

This work deals with the sustainable biodiesel production from low-cost renewable feedstock (waste and non-edible oils) using a heterogeneous catalyst constituted by potassium loaded on an amorphous aluminum silicate naturally occurring as volcanic material (pumice). The main challenge to biodiesel production from low-quality oils (used oils and greases) is the high percentage of free fatty acids (FFAs) and water in the feedstock that causes undesirable side reactions. The catalytic materials studied were tested in the transesterification reaction when using low-quality oils containing a high proportion of free fatty acids (FFAs) and water. Results indicated that the amount of acid and basic sites on the catalytic surface increases upon increasing potassium loading in the catalyst, displaying better performance for biodiesel production. Indeed, the modification of the aluminum silicate substrate upon potassium incorporation results in a catalytic material containing both acidic and basic sites, which are responsible for both triglycerides transesterification and FFA esterification reactions. The studied catalyst not only showed good performance in the biodiesel production reaction but also good tolerance to FFA and water contained in the feedstock for biodiesel production. The catalytic material was microstructured by 3D printing in order to design a catalytic stirring system with high mechanical strength, efficient and reusable. The use of 3D printing in biofuel production is a novelty that brings good solutions for catalyst production.

In this study, Amberlyst-15 as heterogeneous catalyst was used for the reduction of free fatty acid (FFA) from the palm oil mill effluent (POME) for biodiesel production with acid-catalyzed esterification process. The objective of this study was to decrease a high FFA in POME to less than 2 wt.% FFA, for used as a raw material to produce biodiesel in the second-step transesterification process. Amberlyst-15 as an eco-friendly catalyst with non-toxic wastes after reactions, when compared to homogeneous catalysts such as sulfuric acid. Therefore, an esterification reaction with a heterogeneous acid catalyst was carried out to examine the FFA conversions. The conditions of two parameters of Amberlyst-15 catalyst (10–40 wt.%), and 1–8 h reaction time were varied, whereas the methanol to oil molar ratio and the speed of the stirrer were fixed at 5:1 and 300 rpm, respectively. As a result, the FFA sharply decreased from 89.16 wt.% to 1.75 wt.% under the conditions of 40 wt.% of Amberlyst-15, 5 h reaction time, 5:1 molar ratio methanol to oil, speed of the 300 rpm stirrer. The Amberlyst-15 had the potential to reduce high FFA in POME using the esterification reaction.

This work reports the characterization of a vegetable oil extracted from pequi seeds, an agroindustrial residue, and its biodiesel production using ethanol and heterogeneous catalysis. The pequi seeds showed 40.73 wt.% of extractive content, which represents a large amount of the biomass composition. The crude oil extracted from the pequi seeds with ethanol as solvent presented a high content of free fatty acids (FFAs), mainly oleic (54.14%) and palmitic (36.71%) acids, resulting in an acidity value of 13.8 ± 0.1 mg KOH g-1. The esterification/transesterification process was performed using two ion exchange resins as heterogeneous catalysts, a commercial protonic form (assigned as PR) and a zirconium-exchanged (assigned as PRZr). Conversions of 87.1 and 91.4% were achieved for PR and PRZr as catalysts, respectively, at optimal conditions (1:6 oil-to-alcohol molar ratio, 25 wt.% of catalyst, 100 ºC and 1 h). These results indicated that heterogeneous acid catalysts can be successfully applied in biodiesel production from fatty acidrich oils, such as the one extracted from pequi seeds. Also, a simultaneous process involving both oil extraction and biodiesel production was tested using PRZr as catalyst (25 wt.% of catalyst and 100 ºC), but due to the greater amount of ethanol necessary for the oil extraction (1:16 oil to alcohol mass ratio) the conversion reached only 51.5% after 5 h. For that reason, this work proposes a two stage system for biodiesel production that integrates oil extraction (stage one) and the esterification/transesterification reaction (stage two) to achieve a greener process, waste-to-bioenergy.

ABSTRACTBiodiesel has developed attraction of most researchers recently because of its renewable resources and environmental benefits. Transesterification process in the presence of catalysts is the most common way, which is used for biodiesel production. Heterogeneous acid catalysts are considered more reliable than any other catalysts to carry out most vital reactions related to green chemistry (biodiesel production), because the production of biodiesel from solid acid catalysts is considered economically favorable. Nowadays, biodiesel is preparing from low quality feedstock by using solid acids catalysts in many research laboratory throughout the world. This article discusses how much catalyst shapes affect the efficiency of catalyst during catalysis. Different types of supports (zinc oxide, alumina, zirconia, and silica) are used to increase the efficiency of catalysts. Supported Lewis acid, Brønsted acid, and heteropoly acid catalysts show good efficiency for the catalytic transesterification of oil with alcohol. Heteropoly acid catalysts are tremendous and environment friendly acid catalyst and have ability to tolerate contaminations of oil resources such as water contents and free fatty acids (FFAs) contents. Keggin-type heteropoly acids are easily available and having stable structure while Wells–Dawson-type heteropoly acids are included in super acid class, due to these reasons heteropoly acids are considered as best acidic catalysts for biodiesel production by catalytic transesterification process. Therefore, this review also focused on the deactivation, regeneration and advantages of supported solid acid catalysts used for the catalytic production of biodiesel through transesterification.

WITHDRAWN: Impact of heterogenous acid catalyst on diesel engine performance and emissions using Chlorella-SP biodiesel

Esterification of free fatty acids using ammonium ferric sulphate-calcium silicate as a heterogeneous catalyst

Biodiesel (methyl esters) has been produced using numerous catalysts to enhance its quality and related productivity. Generally, the raw materials for biodiesel production and its catalysts significantly impact the produced biodiesel’s quality. In addition, the heterogeneous catalysts are promising as catalysts in the transesterification for biodiesel production and can be used continuously during this production. In particular, these catalysts are essential for green biodiesel production because of their high activity, thermal stability, and reusability. Hence, several homogeneous and heterogeneous (acidic and alkaline) catalysts for biodiesel production, particularly the naturally derived heterogeneous catalysts, are reviewed in this article. Further, the different heterogeneous catalysts for biodiesel production have been studied extensively as replacements for the respective homogeneous catalyst. Specifically, this replacement is aimed at the simultaneous esterification and transesterification of the nonedible and low-cost biomasses under moderate conditions producing biodiesel. Moreover, this study analyzes biodiesel’s impact and long-term performance in various applications. Finally, it also reports the advancements in biodiesel production in terms of the catalysts used in it and its processes to aid further developments in biodiesel production.

Biodiesel has received tremendous attention as a sustainable energy source. This review presents an overview of various catalysts utilized in biodiesel production and compares their potential for producing biodiesel. Presented here are the excellent features of the various catalysts while highlighting their drawbacks. For instance, production of biodiesel with homogeneous base catalysts is easy but it can only be used with refined oils having low levels of free fatty acid (FFAs). When homogeneous acid is used in esterification, it causes reactor corrosion. Water and FFAs do not affect heterogeneous acid catalysts. Thus, transesterification of triglycerides into biodiesel and converting FFAs into biodiesel through esterification can be catalyzed more efficiently using a heterogeneous acid catalyst. Biocatalysts are also being used to produce biodiesel from oils with high FFAs. However, heterogeneous acid catalysts and biocatalysts are not suitable for industrial application due to serious mass transfer limitations. Biodiesel yield and conversion were compared over various catalysts in this paper. Also presented are the effects of different reaction parameters on biodiesel yield over different catalysts. The correct interplay of factors like reaction temperature, time, alcohol-to-oil molar ratio, and catalyst loading produces optimal process conditions that give the highest biodiesel yield.

Biodiesel is a renewable, clean-burning, and biodegradable fuel which can be synthesized from readily available domestic and natural sources, such as edible, non-edible and waste cooking oils, which may serve as a substitute to petro-diesel. It is produced by catalytic transesterification of fats and oils. A number of researches has been devoted to discovering a benign catalyst, especially heterogeneous acid catalyst that could convert non-edible and waste cooking oils with high free fatty acid into biodiesel, in an attempt to reduce the cost of production. The cost of production of biodiesel is still far higher than that of conventional petro-diesel, owing to the cost of edible oil currently being used, processes involved, and cost of conventional heterogeneous catalysts employed. This study assessed the role of various catalysts; homogeneous, heterogenous and enzyme-catalyzed transesterification reactions, in terms of their advantages and disadvantages in biodiesel production in order to establish very promising catalysts. Some methods of heterogeneous acid catalysts were also highlighted. Amongst the common heterogeneous catalyst, carbon-based solid acid catalysts were recommended as very promising solid acid catalyst that can utilize the non-edible oils in biodiesel production. The advantages of carbon-based solid acid catalysts include cheap readily available raw materials for their synthesis, easier production processes, relative stability, high reusability and potential for utilizing waste and non-edible oils for biodiesel production. 
 Nnaji, J. C. | Department of Chemistry, Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria

This study aims to optimize the reduction of free fatty acids (FFAs) in palm fatty acid distillate (PFAD) using hydrodynamic cavitation reactors (HCRs) in series and a solid acid catalyst for biodiesel production. Hydrodynamic cavitation is used to accelerate the esterification of FFAs using a heterogeneous acid catalyst. There are three HCRs units, and each HCR composed of a 3D-printed rotor and stator, is separated by flanges and equipped with a basket for holding Amberlyst-15 catalyst. Through response surface methodology (RSM), the esterification process is optimized by adjusting its optimal parameters, namely, methanol (2–12 wt%), circulation time (30–170 min), and rotor speed (1000–3000 rpm). The optimal conditions for achieving a maximum methyl ester purity of 89.76 wt% in converting FFA in first-step esterified oil are 9 wt% methanol (molar ratio of methanol to oil of 4:1), 133 min of circulation time, and 2000 rpm of rotor speed. An 82.48 wt% biodiesel yield is achieved from the HCRs in series under the optimal conditions. Scanning electron microscope images reveal that after the esterification process, there are minor cracks and defects on the catalyst’s resin surface, indicating the presence of residual reactants. Further examination of the catalyst after the esterification process, reveals an average absorption pore diameter of 341.41 Å and BET surface area of approximately 41.68 m2/g. Although there were slight physical changes in the catalyst, HCRs technology offers a viable FFA reduction process that could enhance biodiesel production efficiency. Moreover, the optimized conditions achieved in this study contribute to the advancement of biodiesel production processes and provide insights into the performance of the catalyst used.