Different scenarios of glycerin conversion to combustible products and their effects on compression ignition engine as fuel additive: a review

In biodiesel production by trans-esterification, one of the essential compound is glycerin. Global glycerin production is increasing significantly, projecting a global value reduction for glycerol. Consequently the scientific community had been encouraged to investigate converting glycerol into more valuable products. In this research, the primary sources and processes of biodiesel production are surveyed. Where the processes that involve glycerin are reviewed and the diesel engine performance and emissions under variant states are discussed. According to the results of this study, it is reported that the choice of an optimal diesel/biodiesel significantly depends on the materials, additives and the engine condition. Glycerol etherification, carboxylation, and glycerol carbonate, however, had been identified as the widely manufactured and used additives. It is further observed that the use of these such additives has reduced several emissions, which is an important factor. In addition, it is suggested that using glycerin additives improves the properties of biodiesel. Acetone, on the other hand is introduced as one of the most important additives in the combination of diesel and biodiesel fuel due to the reduction of maximum emission. The presence of hydroxyl groups can reduce NOx. Finally, the diethyl ether additive can be mentioned which increases the thermal efficiency and increases the brake-specific fuel consumption (BSFC). ARTICLE HISTORY Received 2 September 2020 Accepted 21 July 2021


Introduction
Today, the need for new energy sources is one of the major problems for all countries in the world. This is a problem not only for oil-importing countries but also for the major oil producers. The unavailability of fossil fuels, termination and environmental problems and price fluctuations of these resources are the most important contemporary human challenge. Greenhouse gas emissions from fossil fuels, including coal, gasoline, and oil, are alarming, while many governments are developing fossil energy use (Karmaker et al., 2019). Given the current environmental challenges and concerns, it makes more sense to find renewable energy sources (Akbarian & Najafi, 2019;Karimi et al., 2020;Karimmaslak et al., 2021;Najafi et al., 2021;Shoar et al., 2021). Renewable energy sources should be able to reduce the negative environmental impacts of fossil fuels. They are economically competitive with fossil fuels and do not reduce the process of food production (Hill et al., 2006; Shoar is similar to that of diesel fuel (Xu et al., 2016). The cetane number and similar thermal value provide the capability to replacing diesel with biodiesel (Noor et al., 2018). Biodiesel can easily be blended with both diesel and alcohol blends (Yilmaz & Atmanli, 2017). Research showed that biodiesel cetane number was higher than that of diesel, but its thermal value was lower than that of diesel (Najafi et al., 2018). The biodiesel cetane number from oilseeds is lower than the biodiesel cetane number from animal oils (Ambata et al., 2018). Besides, the attendance of oxygen in the chemical structure of biodiesel results in cleaner combustion than that of diesel fuel. By biodiesel fuel combustion, emissions of PM, poly aromatic hydrocarbons and SO 2 decrease. Biodiesel has a high flash point. Biodiesel does not have sulfur (Akbarian & Najafi, 2019;Hosseinzadeh-Bandbafha et al., 2018;Noor et al., 2018). Biodiesel can be mixed with diesel fuel in any proportion. It is currently common to use 5% biodiesel (B5) and diesel fuel mixes, and most countries are planning to use B20 fuel (Lewis et al., 2009). Researches showed that biodiesel-diesel fuel mixture up to 20% can be used in the conventional diesel engine. This 20% compounding rate was recommended in order to keep engine performance at a high level and also to reduce environmental pollution (Attia et al., 2018).
Biodiesel is mono-alkyl fatty acid esters (FAAE), produced from triglycerides found in vegetable oils or animal fats. Various methods have been developed for biodiesel production, which was reviewed in Ambata et al. (2018), Mittelbach (2009), and Tabatabaei et al. (2019). Currently, biodiesel production by alkaline transesterification is very common because it costs less and has a higher conversion efficiency. Figure 1 shows the transesterification reaction from triglycerides in the presence of oil (Go et al., 2016;. However, acid catalysts for the trans-esterification process are not widely used in the industry (Boon-Anuwat et al., 2015). According to the theory, 1 mol of triglyceride and 3 moles of alcohol (such as methanol or ethanol) react together with a catalyst (such as NaOH) to produce 3-mole of ester (biodiesel) and 1 mol of glycerol.
In research by Ghazanfari et al. (2017), biodiesel was obtained by the trans-esterification method from oil (palm) and ethanol in the presence of NaOH catalysts. According to this method, the oil was warmed to 70°C and mixed at 600 rpm. In other reactor, ethanol, with 1/3 of the primary weight of the oil (with molar ratio of alcohol to oil of 6:1), was warmed to 70°C in the presence of a 0.1 wt% NaOH catalyst. Then alcohol and catalyst were added to oil and the solution was stirred at 600 rpm for 30 min and 70°C. After 8 h, the solution temperature was reduced to ambient temperature (25°C). Then the acidity of the solution was measured and by adding 46% hydrochloric acid, it was regulated to about 7. The operation of rinsing by using water at 50°C was performed by gravitation for 12 h (Ghazanfari et al., 2017). Figure 2 shows the steps of biodiesel production by trans-esterification.
The conversion efficiency obtained in the transesterification reaction depends on the type and percentage of catalyst purity, alcohol to oil ratio, reaction temperature and time, water mass, and amount of free fatty acids (Pitt et al., 2019). In a study by Ahmad et al. (2019), optimization of effective factors in the biodiesel production process from flaxseed oil was performed by a transesterification method using response surface methodology. They studied factors such as ratio of methanol to oil volume, catalyst weight percentage (KOH), reaction temperature and time. According to the results, the highest amount of biodiesel production was about 99.5% at optimum conditions with reaction temperature of 59°C, catalyst content of 0.51% primary oil, 33 min reaction time and molar ratio of 5.9:1 from methanol to cotton oil (Ahmad et al., 2019). Successful commercialization and marketability of biodiesel require stringent quality assurance standards. Evaluation of the quality of biodiesel is   achieved by determining its chemical composition and physical properties. Properties and limits specified in the biodiesel standard should protect the efficiency and durability of automobile engines or combustion equipment (Carrero & Pérez, 2012). Biodiesel can only be used when it meets the required standards. These standards (Table 1) include a density at 15°C (according to NPEN 14214 standard: 2009), which is adapted from (Knothe, 2006), the amount of iodine (according to EN14111 standard) and some others (Caetano et al., 2012).
Biodiesel is produced from renewable energy sources, including vegetable oils or animal fats. Production of biodiesel from food sources can imperil food security, therefore, global efforts to produce biodiesel from non-food sources have increased. And biomass, which contains raw materials, is considered as an energy source. Plant oils and animal adiposes are green energy sources. Table 2, which is adapted from (Atabani et al., 2013), shows oils available from raw materials as well as their percentages of oil.
Sources of biodiesel production can be divided into three generations (Figure 3). In the first generation, biodiesel was produced from oil sources such as sunflower, soybean, date, coconut, castor, rapeseed, and sesame. By increasing the production of biodiesel from edible oils, there have been concerns about the utilization of arable land for the agricultural generation to produce biodiesel, which could lead to a shortage of food sources.
In the second generation of biodiesel production sources, the tendency to use non-food sources has increased. As such, it was not competitive with food production (Yang et al., 2014). Biodiesel sources in the third generation (Mofijur et al., 2019) do not directly compete with food and also do not lead to land-use change and environmental damage. These sources can make cheap biodiesel fuel available. For example, through the third generation, Isochrysis aff. galbana microalgae can be considered as more viable source for biodiesel production, partly due to the relative ease in producing a high oil yield (Atmanli, 2020). Therefore, third generation resources are a good alternative for the first and second generations (Singh et al., 2011). In the biodiesel production process by trans-esterification,  glycerol is produced as a by-product (Anger et al., 2011). By increasing the biodiesel utilization in the world, the glycerol production amount is increasing. Excess glycerol has created a new challenge. Although there are a wide range of potential applications for crude glycerol, glycerol produced in the trans-esterification process has a low purity, which limits its conversion to other materials (Monteiro et al., 2018). Studies showed that biodiesel production was drastically increasing (Nomanbhay et al., 2018), and consequently, glycerin production was also growing. It was expected that this trend would continue until 2030. Figure 4 is adapted and reproduced from (Nomanbhay et al., 2018). This figure shows the global glycerol production trend by 2030. This amount of glycerin is anticipated to be about 48500 million cubic meters.
Due to the increasing production of glycerin and its environmental problems, it needs to be managed and transformed into more valuable and environmentally friendly products. Therefore, in this research, the conversion of glycerin to other materials is investigated. The production of glycerin occurs in both edible and nonedible oils and its amount is approximately the same as 10% by volume of the transesterification process material produced. Glycerin is produced during the transesterification reaction, which is one of the most efficient and common methods for biodiesel production, and therefore glycerin is produced during different steps of this process. Therefore, various edible and non-edible oils do not play a significant role in the production or non-production of glycerin. However, the use of edible waste oils (second-generation) and non-edible oils (third-generation), which have no competition with human food is recommended. These sources are much more economical and feasible. In this research, the study is conducted with emphasis on non-edible oils, although as mentioned above, glycerin production is due to various stages of the transesterification method and its amount for all various sources is about 10% by volume. Different ways of producing glycerol products are investigated at first. Then the materials from these methods and their applications are analyzed. Finally, the possibility of using glycerin-derived products as an additive for diesel fuel and their impacts on performance and emission of the engine are investigated on a diesel engine.

Glycerin
Glycerol, being a chemical product commercially known as glycerin, is colorless, odorless, and viscous at room temperature (Christoph et al., 2000). It is classified as three-component alcohols with formula C 3 H 5 (OH) 3 according to IUPAC and is known as 1, 2, 3-propantryriol. Glycerol is completely soluble in water, alcohol and in many common solvents such as ether and dioxane, due to its three hydrophilic hydroxyl groups, but it is insoluble in hydrocarbons. This is true for glycerin with 95% purity. Table 3 shows some of the glycerol physical properties at 293 K (Pitt et al., 2019) that make it difficult to use for new products. Usually, glycerol obtained from the trans-esterification process has a purity of 90-92%, adapted from (Christoph et al., 2000).

Glycerol reforming processes (GRP)
Glycerol is a fundamental chemical for biological refineries. Glycerol produced during the biodiesel production process is the primary feedstock for bio-factory (Yazdani & Gonzalez, 2007). Glycerol is a crude substance for the production of intermediates or chemical materials at various industrialization and application sectors  (Monteiro et al., 2018). In general, glycerol can be used for the following usages: (a) Production of chemical products; (b) Manufacture of polymer compounds; (c) Generation of biofuels; (d) Refinement and direct utilization from glycerol.
Therefore, the extension of novel technologies for the utilization of glycerol is essential (Monteiro et al., 2018). Production of chemical materials, foods, polymers, fuel additives, hydrogen, and energy industries such as fuel cells, gasification industries, and anaerobic digestion are important destinations for glycerin conversion from biodiesel production.
Glycerol as an attractive substance (Karam et al., 2008) is used for the synthesis of many products such as surfactants (Pirog et al., 2013) and solvents (Favier et al., 2018). Besides, glycerol as a promising crude substance for microbial surfactants, can create modern antimicrobial materials for use in food, pharmaceutical, health, agricultural and practical applications (Pirog et al., 2013). Due to the high demand for renewable energy sources, glycerol has a tremendous potential to become a valuable material, which can be used as a fuel additive to petroleum derivatives, have a good perspective in the oil industry. Converting glycerol to oxygenated fuels by a variety of methods such as etherification and esterification have attracted attention (Rahmat et al., 2010). For example, glycerol triacetate (GT) compound was used as a fuel additive by glycerol esterification on the CI engine (Akbarian & Najafi, 2019). General methods for converting glycerin to other substances are shown in Figure 5. In the following, methods for converting glycerin to more valuable materials are discussed.

Glycerol aqueous phase modification process (GAPR)
The glycerol aqueous phase modification process (GAPR) is a potential pathway for the manufacture of fluid and H 2 fuels. GAPR is usually accomplished in continuous current with a moderate temperature (200-260°C) and high pressure (20-50 bar). The conversion to aqueous phase occurs without any pre-evaporation steps. The GAPR reaction is according to Eq. (1) (Roslan et al., 2019): Fluid alkanes can be directly manufactured from glycerol during a continuous process ( Figure 6). The process implicates the catalytic transformation from glycerol into  H 2 and CO synthesis gas mixtures, which are combined by FT synthesis process (Fischer-Tropsch). Syngas (synthesis gas) can be manufactured from concentrated glycerol at 548 K, pressures of 1-17 bar and 10% weight of Pt-Re catalyst (1:1). The major intermediates when converting glycerol to syngas include acetone and ethanol (Simonetti et al., 2007). As an application of synthesis gas from H 2 and CO produced by the APR method, the production of saturated and unsaturated liquid fuels can be mentioned (Domínguez-Barroso et al., 2019). Using Pt-Ni/Al 2 O 3 catalyst at 200°C, 80.3% of glycerol can be converted to H 2 .
The best way to increase the economic values of biodiesel is to convert raw glycerol into valuable products such as H 2 and syngas. H 2 production from glycerol is accomplished by various processes such as aqueous phase modification, pyrolysis, steam modification, partial oxidation, and dry modification reactions. In largescale industrial applications, Ni-based catalysts are used as the most common catalyst, because this type of catalyst is easily accessible, cheap and has high catalytic activity (Roslan et al., 2019).
The GAPR process is an endothermic process. As such, this method requires high energy consumption; but the advantages of this process are its high speed and selectivity. Due to the harsh conditions and high cost of this method, there has been little research performed on this method, yet this method can convert glycerol into liquid fuels (alkanes).

Glycerol reduction process (GRP)
Glycerol reduction process (GRP) with hydrogen produces ethylene and propylene glycol (Figure 7). Glycerol is hydrogenated by metal catalysts and hydrogen and it is converted to products such as propylene glycol ethylene glycol and 1, 3 -propylene glycol. The major product of the glycerol reduction process is propylene glycol (propanediol).
Hydrogenolysis of glycerol to propane-diol is accomplished by using Ni, Pd, and Cu chromate catalysts. Propanediol conversion is reduced due to excessive hydrogenolysis of propylene glycol at 473 K and 13.79 bar. Besides, with decreasing water content, the efficiency of propanediol increases. Cu chromite catalyst  (CuO.Cr 2 O 3 ) at 473 K is used to generate propanediol from glycerol. The reaction is executed by forming an intermediate, which is acetol (hydroxy-acetone). Figure 8 is adapted and reproduced from (Dasari et al., 2005). Studies of different catalysts at 473 K and 13.79 bar pressure indicated that the highest conversion rate of glycerol to propylene glycol was obtained by chromite copper catalyst.
Besides, the highest glycerol conversion is achievable by using CuNi/Al 2 O 3 and CuNi/ZSM-5 catalysts at 80 and 85%, respectively. With these catalysts, about 25% of the production is propylene glycol. By hydrogen addition, the glycerol conversion achieves above 90% in all catalysts. Single mono-metal catalysts, compared with bimetallic samples from copper, produces higher propylene glycol. Cu/Al 2 O 3 showed the highest performance to convert to propylene glycol (with 70% efficiency) (Freitas et al., 2018). In this method, selectively the intermediate hydroxyl category converts glycerol to the tosyloxy group, afterward separates the changed group by using catalyst hydrogenolysis. By this novel approach, the transformation of glycerol to 1, 3-propanediol was performed in 3 stages: (1) acetylation, (2) tosyloxylation and (3) de-tosyloxylation. Steps for the establishment of 1, 3-Propylene glycol are shown in Figure 9, which is adapted and reproduced from (Wang et al., 2003).
Nanoparticles synthesis (AuNPs), specifically with greenways, due to its bio-adaptability and various usages in different threads, has received a lot of attention. The establishment of permanent AuNPs with a controlled scale mostly needs the utilization of surfactants. These stabilizing factors may damage catalytic actuality and also have noxious effect on the bio-adaptability of AuNPs and surfactants. Therefore, in the study (Parveen et al., 2019), the effects of glycerol and water ratio, pH, temperature, ionic strength on stability and particle size distribution were investigated. Such AuNPs obtained in this way by the reduction method were environmentally friendly and have useful applications in catalytic and medical fields (Parveen et al., 2019). Glycerol reduction is another effective way to produce different products from glycerol under mild conditions. In this method, glycerol is hydrogenated using metal catalysts. This method does not require high-temperature conditions in the glycerol aqueous phase modification process, so it is more economical. Synthesis parameters indicate that the ability to reduce and stabilize glycerol is associated with pH and percentage of glycerol in the reaction medium, respectively. The main products produced by this method include 1, 2-propanediol (propylene glycol) and 1, 3-propane (diethyl and ethylene glycol). A major product of the glycerol reduction process is propylene glycol, which is not harmful to the environment.

Glycerol dehydration process (GDP)
From the Glycerol Dehydration reaction, two important acrolein chemicals and 3-hydroxy-propionaldehyde (3-HPA) can be prepared directly. In the primary stage, the glycerol dehydration process results in the establishment of enols that are in balance with hydroxy-acetone and 3-hydroxy-propionaldehyde. Then in the second step, it is formed by the dehydration reaction of acrolein or by the reaction of retro-aldol in the presence of O 2 and formaldehyde. Figure 10 is adapted and reproduced from (Katryniok et al., 2009). Acetaldehyde may be quickly converted to acetic acid.
Different studies on the method of dehydrating glycerol to3-hydroxy propionaldehyde were performed (Table 4). In the study of Lago et al. (2018), modified H-ZSM-5 zeolite catalysts for the dehydration of glycerol to acrolein in the gaseous phase were investigated. Alkaline pretreatment with sodium carbonate aqueous solutions at various concentrations was also carried out with ion exchange from ammonium nitrate solution. Their results showed that at selected concentrations, temperature and time of pretreatment to achieve selective removal of Si from the zeolite structure and keeping Al content, middle porosity was more appropriate. Alkaline pretreatment with Na 2 CO 3 resulted in higher performance compared to that of NaOH pretreatment. Therefore, this led to lower acrolein selectivity and higher selectivity for acetone formation (Lago et al., 2018). The glycerol dehydration process to acrolein and acrylic acid is shown in Figure 10.
In the study of Vieira et al. (2015), the effect of crystal size, acidity, and synthesized products on the efficiency of AL and Ga -MFI zeolite catalysts in glycerol dehydrogenation were studied. The outcome illustrated that the efficiency of these catalysts was very good in glycerol dehydrogenation reaction. Besides, small zeolite particles, especially gallium zeolite catalysts, increased the duration of catalyst effectiveness. The main product of this synthesis was acrolein. Therefore, the acidity and size of the zeolite crystals affected the inactivation and activation of the catalyst in glycerol dehydration.
Different studies on dehydrating glycerol to acrolein were performed (Table 5) (Dasari et al., 2005;Domínguez-Barroso et al., 2019;Freitas et al., 2018;Katryniok et al., 2009;Parveen et al., 2019;Simonetti et al., 2007;Wang et al., 2003). Acrolein is a chemical material and is used as an industrial and pesticide chemical. So dealing with it requires safety tips (Auerbach et al., 2008). The conversion process of glycerol dehydration to acrolein is accompanied by adverse reactions and leading to the establishment of byproducts. This causes the formation of coke on the catalyst and changes its color from white to black. Besides, the catalyst increases weight and becomes inactive, and its efficiency and selectivity for acrolein production are reduced. 90% of glycerol conversion, with 80% selectivity of acrolein, is achievable in supercritical conditions at 34.5 MPa, 673 K and in the vicinity of H 2 SO 4 catalyst. In order to improve the efficiency of this process, the authors propose to increase the concentration of glycerol and H 2 SO 4 and work at higher pressures (Watanabe et al., 2007). After the acrolein product, the product of 3-hydroxy propion aldehyde (3-HPA) is another glycerine-derived substance during the dehydration procedure. 3-HPA is an important industrial intermediate that can be converted into a number of large-scale conventional chemicals. Figure 11 is adapted and reproduced from (Zheng et al., 2008). These include acrolein, acrylic acid, 3-hydroxy propionic acid (3-HPA), malonic acid, acrylamide and 1, 3-propanediol.
3HPA is important for several chemicals and polymer materials and is also momentous for various chemical materials and polymer. Poly 3HP is a polyester that is tolerable to degradability. Figure 12 is adapted and reproduced from (Zaushitsyna et al., 2017). Different studies on dehydrating glycerol to 3-hydroxy propionaldehyde were performed (Auerbach et al., 2008;Sardari et al., 2013;Sardari et al., 2014;Zaushitsyna et al., 2017). Using the E. coli SH254 compound, 3-HP maximum yields were 6.5 mmol L-1 (10.58 gL-1). It is expected that the highest selective conversion and amount of 3-HP production from glycerol will be 6.6 mmol g −1 cdw h −1 and 0.48 mmol −1 , respectively. It is expected that the production of 3-HP can be improved by further control of SH254.
Improve 3 -HP production Raj et al. (2008) 2 Biotransformation of glycerol to 3-hydroxy propion aldehyde: Improved production by in situ complexation with bisulfite in a fed-batch mode and separation on an anion exchanger Combining 3HPAwith bisulfite The potential of combining 3HPAwith bisulfite and then attaching it to the anionic resin to recover the production of 3-HPA solution with remarkable performance was investigated.
The use of resin-bonded functional groups allows for much easier separation of 3-HPA from Bioproducts by glycerol processing.
Production of 3-HPA Sardari et al.
3 Semicarbazidefunctionalized resin as a new scavenger for in situ recovery of 3-hydroxy propion aldehyde during biotransformation of glycerol by Lactobacillus reuteri

Application of Semikarbazide-Functionalised Resin
The application of Semikarbazide resin for 3-HPA adsorption was investigated. And were evaluated as an alternative to the reported methods for topical removal of 3-HPA by its production from glycerol biotransformation with Lactobacillus reuteri.
Semicarbazide applied resin is a non-toxic material for 3-HPA. And has a high connection capacity for -3HPA. Improves the production of 3HPA by using resin. By decreasing the capacity and adsorption time, 3-HPA from the resin is obtained better.

Lactobacillus reuteri
Lactobacillus reuteri cells, as potential biocatalysts for the conversion of glycerol, into potential bio-based chemicals such as 3-hydroxy propionaldehyde, 3hydroxy propionic acid and 1, 3-propanediol, was investigated Under optimum conditions, 19.7 g/L 3HPA was produced from the carbohydrazide compound at a rate of 9.1 g /Lh with a 77% molar yield.
3-hydroxy propionaldehyde, 3-hydroxy propionic acid, and 1,3-propanediol Zaushitsyna et al. (2017) Numerous attempts have been made to develop glycerol conversion processes into precious chemical materials, which suggests that this is a major source for some future glycerol applications. These include the generation of 1,2-propylene glycol, 1,3-propylene glycol, and acrolein. According to these results, alkaline pretreatment is a suitable solution to increase acrolein production. The use of catalysts also improves the production of acrolein and in the meantime, the SiW20-Al / Zr10 catalyst performs well. Another important product from the glycerol dehydration method is 3-hydroxy propion aldehyde, which is an important industrial intermediate, and it turns into some of the most common chemicals on a large scale. However, given the usual use of glycerol, the actual market seems to be unable to consume all produced glycerol. Therefore, in the future, a purification step is needed to achieve the degree of purity on an industrial scale.

Oxidative dehydration of glycerol to acrylic acid (ODG)
Acrylic acid is one of common products made of glycerin. The dehydration reaction is coupled with aerobic oxidation to transform acrolein to acrylic acid directly. In this process, the oxidative exothermic reaction is accompanied by the dehydrating endothermic reaction. Figure 13 is adapted and reproduced from (Li & Zhang, 2016). An investigation by dos Santos et al. (2019) performed the A catalyst with wt % 8 Nb2O5 was applied to semi-porous silica zirconium with different amounts of phosphoric acid (Nb / P = 0.1-1). To alter their tissue properties and to modify their acidic properties.
Therefore, the catalyst with Nb / P = 0.2 had the most favorable acidic distribution and was able to achieve a less damaged structure, which resulted in the conversion of glycerol and acrolein production to 74%, after 2 h, at 350°C.
Glycerol conversion and acrolein production 74%   oxidative dehydration of glycerol for the generation of stable acrylic acid using H, Fe-MCM-22 catalysts, in which H, Fe-MCM-22 catalysts were reported to be highly active in the dehydration oxidation of glycerol and its conversion to acrylic acid. Fe is a marvelous metal, mostly due to its Fe 3+ or Fe 2+ regeneration cycle, which has made it affordable and low costly response, also it is low toxic than V, W or Mo (dos Santos et al., 2019). Balancing tissue properties and regeneration of H, Fe-MCM-22 catalysts leads to a better dehydration function of glycerol. This is especially true for low Fe amount catalysts. The performance of H-Fe-MCM-22 catalysts is related to other catalysts caused by regenerated molecules or blended oxides. Even in more difficult moods, it also acts as an enhancer of the molar ratio of oxygen to glycerol. Low iron catalysts increase the selectivity of acrylic acid. Vanadium-based catalysts also have good potential and are used for glycerol conversion and acrylic acid production without significant reduction. Vanadium is a non-toxic and widely available metal and it has the  potential to achieve sustainable manufacture from acrylic acid. But results showed that He-MCM-22 was a promising catalyst for sustainable manufacture from acrylic acid instead of V 23 catalysts (dos Santos et al., 2019).
In general, the ODG to acrylic acid is a thermally equilibrated process because it has both exothermic and endothermic phases. Acrylic acid is a widely used substance derived from glycerin and the toxicity of this substance is low. In the production of this material, H, Fe-MCM-22 has been reported as the best catalyst because of its high activity. Compared to other glycerol conversion methods, less research has been performed on this method.

Glycerol etherification (GE)
Glycerol ethers are manufactured from esterification of glycerol via iso-butene in the presence of homogeneous acidic catalysts (Vlad et al., 2011). Figure 14 is adapted and reproduced from (Zheng et al., 2008). By reacting glycerol with isobutene in etherification, the production of mono-ether, di-ether and tri-ether can be attained. Different syntheses have been undertaken to convert di-ethers and tri-ethers from glycerin to higher ethers. 'Superior ethers' may be consumed as an octane booster for car fuel (Behr & Obendorf, 2002).
Oxygenated compounds can be considered as fuel additives with the function of octane resonator. For example, methyl tri-butyl ether has explosive properties and is added to the fuel as octane enhancers. Glycerol tri-butyl ethers (GTBEs), as additives for diesel fuel and biodiesel, increase the fuel octane number and they are soluble in nonpolar fuels. Alcohols and ethers offer different and specific benefits in providing clean fuel. Methyl tri-butyl ether (MTBE) has been the preferable product, but a positive future outlook is predictable for less volatile ethyl tri-butyl ether (ETBE) and methyl tri-ethyl ether (TAME) (Ancillotti & Fattore, 1998;Noureddini, 2001).
As a result, it is highly desirable to add low-cost glycerol to valuable chemicals or substances. Glycerol etherification with acetic acid is used to produce glycerol acetate as a bio-additive from biodiesel, which provides a promising approach to glycerol use. This pathway leads to the establishment of glyceryl-monoacetate (MAG), glyceryl-di-acetate (DAG) and glyceryltri-acetate (TAG), which are widely used in degradable polyesters as well as cosmetics (Rahmat et al., 2010).
Many of the recent methods that use glycerol as a raw material are in the expanding phase yet, and some have been commercialized. Accordingly, the straight transformation of glycerol to important chemical materials have engrossed many considerations. Glycerol ethers are produced by using different catalysts during the etherification process; Based on the comparison of results in the above table, when BEA di-silicate zeolite catalysts react with glycerol, they produce high amounts of di and tributyl-glycerol. Therefore, these ethers may be utilized as oxygen fuels to improve diesel fuel.

Polymerization to poly-glycerol
Poly-glycerol is a branched structure, it is liquid and has high viscosity. It is completely solvable in liquid and polar solvents, for example, ethanol. Besides, it is nonvolatile at normal temperatures. Glycidol is a derivative of glycerol and is used in the controlled synthesis of poly-glycerol. For example, one type of rapidly emerging polymers is PGs. PEG is a promising alternative for medical applications (Ampatzidis et al., 2014;Gheybi et al., 2018). Poly-glycerols (sometimes called 'poly-glycidols') represent classes of extremely bio-matchable and multihydroxyl polymers that can be propounded as polyethylene glycol (PEG). Figure 15 is adapted and reproduced from (Thomas et al., 2014). Also, Figure 16 is adapted and reproduced from (Jamróz-Piegza et al., 2006). Different architectures of monomers lead to the formation of polyglycerols. Glycidol polymerization results in over-excited state poly-glycerols.
A novel type of exothermic polymers based on modified polymers poly-glycidol (1, 2, 3-expoxypropanol) has been proposed. During heating, the water reactivity and solubility of the polymer (glycidol-ethyl glycidyl carbamate) are obtained by modifying the hydrophilicity of the hydroxyl groups from the poly-glycidol chain with ethyl isocyanate.
Linear poly-glycidols with high molar masses are provided using activated monomeric anionic  polymerization, ethoxy ethyl-glycidyl ether, and tertbutyl-glycidyl ether, with a composed system of tetraoctyl ammonium bromide as initiator and tri isobutyl-ammonium as an activator of monomer. The polymerization reaction is shown in Figure 17, which is adapted and reproduced from (Gervais et al., 2010).
Pharmaceutical polymeric compounds derived from branched poly-glycerol sulfate and mono-methyl auristatin E were evaluated as anticancer drugs. Intermixed polymers (PDCs) offer promising ways to treat cancer, according to (Rades et al., 2019). Sangkhum et al. (2019) investigated the oxidation of Ca-Mg-Al compound for selective glycerol
The etherification of glycerol with benzyl alcohol was investigated using ZSM-5 zeolites.  (Sangkhum et al., 2019). In another study by Zhang et al. (2010), high-energy PEI polymers led poly-glycerol to the delivery of genes. To improve the efficiency of gene transfection and prevent cancer cells, branched polymers, namely PG6-PEI25k and PG6-PEI800, are designed as novel gene pathways.
Currently, a number of poly-glycerols are commercially available in a variety of applications, including cosmetics and pharmaceuticals. Biocompatibility is a prominent feature of these materials in determining their use.

Glycerol esterification
Glycerol esterification has been one of the practical methods of converting glycerol into valuable substances. The esterification reaction is carried out by carboxylic acids, carboxylation and adding nitrate to glycerol. By glycerol esterification via acetic acid and catalysts, glycerol is converted to glycerol-monoacetate, glyceryl -diacetate, and glyceryl-tri-acetate. Glycerol conversion can be caused even under the absence of the catalyst in this reaction. Therefore, glycerol esterification is a selfcatalyzed reaction, in which the acidic protons present in acetic acid can catalyze this reaction itself (Gao et al., 2015). With the zeolite (BEA), the pore volume can be more than doubled, so that by etherification, the conversion of glycerol was 98%. And while the selectivity was 99% for di-and tert-butyl glycerol (DTBG + TTBG).

Carboxylation reaction for glycerol carbonate preparation
In general, glycerol carbonate is synthesized by the reaction between glycerol and fusogen. However, due to the great noxiously of fusogen, trans-esterification-based alternatives to di-alkyl or alkylene carbonates have been investigated. Industrial synthesis of glycerol carbonate requires several stages. Ethylene oxide reacts with carbon dioxide, which results in the production of annular ethylene carbonate. Glycerol is also reacted to produce glycerol carbonate and ethylene glycol. This function includes the utilization of homogeneous basic catalysts, for example, sodium bicarbonate or sodium hydroxide ( Figure 18). It has problems such as neutralization and also causes problems in the recovery of the product with low-pressure distillation method (Alvarez et al., 2012). Trans-esterification of glycerol by annular carbonates or alkyl carbonates is thermodynamically favorable for the production of glycerol carbonate from glycerol. Increasing the temperature may enhance the chemical    balance of the reaction of glycerol with dimethyl carbonate (Li & Wang, 2011). Moreover, glycerol carbonate can be produced by the following methods (Figures 19-21) (Li & Wang, 2011).
The production of glycerol carbonate by using aluminum waste as a low-cost catalyst resulted in the highest conversion of glycerol at 500°C (Das & Mohanty, 2019). Glycerol carbonate is polar, colorless, non-toxic and has high boiling temperature. Another method for the production of carbonate derivatives of glycerol results from the reaction between urea and glycerol. In this process, glycerol carbonate is separated from glycerol by 90% yield (with100% purity) and the reaction provides more conversion and more energy savings, 29.1%, and 37.1% respectively (Lertlukkanasuk et al., 2013). Alvarez et al. (2012) investigated the synthesis of glycerol carbonate by trans-esterification of glycerol in a continuous system using hydrotalcites as a catalyst. Glycerol esterification method, as an efficient method to obtain the hydrotalcite used in α-Al 2 O 3 or γ -Al 2 O 3 , was employed. They are active in converting glycerol to glycerol carbonate and glycerol di-carbonate.
In addition, it was demonstrated that activated catalysts led to greater production of glycerol dicarbonate manufactured in glycerol carbonate. An enhancement in Mg amount in the catalyst resulted in better glycerol conversion and better glycerol dicarbonate yield, although the HTr4-Alpha catalyst was less stable than the HTr2-Alpha catalyst. This was maybe due to the presence of excess MgO (H) in the catalyst. Therefore, it was deduced that hydrotalcite combinations were promising for the continuous trans-esterification reaction (Alvarez et al., 2012). Zheng et al. (2015) investigated the trans-esterification of glycerol with calcined Ca-Al hydrocalumite dimethyl carbonate. Ca-Al hydrodalumite (Ca/Al = 2-6) was synthesized by high crystallization using a conventional Coprecipitation technique in N 2 space. According to results, calcined Ca-Al hydrocalumite was an efficient catalyst for glycerol trans-esterification with dimethyl glycerol carbonate and had high selectivity of glycerol carbonate compared with usual conditions. The highest conversion rates of glycerol and GC selectivity were 93% and 97%, respectively.
Different studies on the carboxylation reaction for glycerol carbonate were performed (Alvarez et al., 2012;Alvarez et al., 2012;Das & Mohanty, 2019;dos Santos et al., 2019;Elhaj et al., 2019;Gao et al., 2015;Goncalves et al., 2015;Gonzalez-Arellano et al., 2015;Lertlukkanasuk et al., 2013;Li & Wang, 2011;Sangkhum et al., 2019;Wang et al., 2017;Zhang et al., 2010;Zheng et al., 2015) (Table 7). The carboxylation reaction results in the production of glycerol carbonate, which is used as a solvent for plastics and resins, for example, cellulose acetate, nitro-cellulose, and poly-acrylonitrile. Besides, this compound has the adhering property to metal surfaces and resistant to oxidation, hydrolysis, and pressure, and has a lubricating effect. According to the results of the above table, various studies have been carried out to produce glycerol carbonate. Among them, Wang et al. (2017) reported the highest amount of glycerol carbonate production. The Na 2 SiO 3 solid base catalyst at 200°C was a noteworthy example for industrial applications in GC synthesis that illustrated glycerol conversion of 97.7%. This comparison is expressed based on the highest amount of glycerol carbonate production efficiency.

Reaction of adding nitrate to glycerol
Glycerol can be treated by using nitrating material to form di-nitro-glycerol solution. This solution is converted to glycidyl nitrate by an annular agent from dinitro-glycerol, which is also polymerized to poly (glycidyl   nitrate). Poly (glycidyl nitrate) is known as a high-energy polymer appropriate for utilization in propellants, explosive materials, gas generators and pyrotechnics (Zheng et al., 2008). Besides, the glycerol nitrate reaction can produce glycerol carbonate. In this method, glycerol reacts with urea to produce glycerol carbonate (Figure 22).
In Ashok et al. (2013), under anaerobic conditions, 3-HP production of glycerol by KpC (Klebsiella pneumonia glpK dhaT) resulted in an increase of NADH cell levels and decrease of glycerol consumption. Adding nitrate enables KpC to successfully regenerate NAD + , but the active presence of the enzymes was effective in reducing the carbon flow pathway to produce 3-HP and 1, 3-PDO. A heavy compound of KpC glpK dhaT, which contained both GlpK and DhaT, ran a large amount of glycerol toward the generation of 3-HP in the presence of nitrate under anaerobic conditions. Glycerol can be polymerized into poly-glycidyl nitrate using nitrating factors. This polymer has the potential to be used in fire-cracking materials and propulsions.  Researches showed that successful production of 3-HP from glycerol required maintaining an appropriate inhibitory balance and optimal enzyme activity related to glycerol metabolism. However, not many researches have been reported on glycerol nitration and therefore more studies need to be done.

Selective oxidation of glycerol
The catalytic conversion of glycerol into high-value products has attracted scientists' attention. Among various methods, the selective oxidation of glycerol via oxygen molecule to di-hydroxy-acetone, glyceric acid, glyceryl aldehydes, and tartaric acid are challenging in research GC Okoye and Hameed (2016) 10 Microbial removal of carboxylic acids from 1, 3-propanediol in glycerol anaerobic digestion effluent by PHAs-producing consortium.
The further kinetic study revealed that more than 80% of the carboxylic substances of 1, 3-PDO were retained after the reduction of carboxylic acids. Therefore, this study provides a successful solution for the removal of carboxylic acids from 1, 3-PDO contained in ADE glycerol and also the production of PHAs as a secondary product.
Crude glycerol containing DBU and ionic compounds DBU / glycerol / CO 2 (DGC), reacts directly with dimethyl carbonate (DMC). and industrial applications. Extensive catalysts for the selective oxidation of hydroxyl groups in glycerol have been reported, for example, single metals Au, Pt, and Pd NPs and bimetallic Au-Pt, Au-Pd, Pt-Bi, Pt-Sb and Cu-Pt . Glycerol oxidation consists of four basic steps: Di-hydrogenation of OH-group, Di-hydrogenation of CH group, aldehyde oxidation and C-C bond cleavage. Studies showed that the increased catalytic activity of glycerol oxidation to carboxylic acids (mainly glycyrrhizic acid and Tartronic acid) was possible by polarizing the Pt surface and forming Pt-OH. From a thermodynamic point of view, high-energy  molecule oxygen reacted exothermically with organic compounds. Most organic compounds were stable in the vicinity of oxygen due to their high activation energy. Hydroxylated molecules and oxygen were activated on the surface of the solid catalyst and the hydrogen removal reaction was carried out and converted to the carbonyl group. Figure 23 is adapted and reproduced from . Hydrotalcite catalyst supported by platinum catalyst (Pt/HT) was determined as an extremely active heterogeneous catalyst for the oxidation of glycerol in net water under atmospheric oxygen pressure and at a high glycerol-metal molar ratio. Moreover, the high selectivity of glyceric acid (78%) was attained at 25°C and 1 atm. The Pt/HT catalyst also selectively oxidized the hydroxyl groups 1,2-propylene glycol to obtain carboxylic acid from glycerol (Tsuji et al., 2011). Selective oxidation of glycerol requires a temperature of 60-80°C. Generally, platinum and palladium are highly active catalysts for oxidation of glycerol. Di-hydroxy-acetone is obtained by aerobic oxidation of glycerol in the vicinity of Pt-Bi-C and in high acidic ambiance. More research is needed to attain comprehensive results.

Important compounds derived from glycerol
A list of important compounds that can be synthesized from glycerol is shown in Table 8.

Effects of modified glycerin on engine performance and emissions
After having described methods of producing valuable substances from glycerol, the effects of the utilization of these materials as diesel fuel additives in diesel engines are investigated. The purpose of this research is to investigate the consequence of these additives on engine performance and emissions, which are identified by examining the results of reported studies. Clearly, an additive that can improve engine performance while decreasing emissions is paramount. In Table 9, the role of these additives on engine performance and emissions is investigated.

Discussion about effects of additives on the performance and emissions of diesel engines
The use of glycerin in the production of a valuable additive is very economical due to the use of 10% of the production of the transesterification process material besides the 90% biodiesel. But some parameters can influence the economic condition. Therefore, depending on the type of production equipment and available primary resources for biodiesel production, the economic parameter is slowly changing, but in general, the use of glycerin as waste material and its conversion into a suitable and valuable additive to reduce diesel engine pollution and improve engine performance is very important. It is observed that most research has been performed on acetone, isopropanol, propanol, diethyl ether and poly-oxymethylene dimethyl ethers. In the following, the effects of these additives are discussed. Adding acetone to diesel and biodiesel fuel and using it in diesel engine reduces the emissions and most NOx reduction occurs. Therefore, adding this substance can solve one of the problems (i.e. the production of high NOx) caused by the use of biodiesel. The main reason for this decrease is due to the low heat value of acetone, which reduces heat and therefore reduces NOx production. On the other hand, the addition of this substance reduces the soot, which can be due to the presence of oxygen in the acetone combination. Therefore, oxidation can reduce soot. Besides, the reduction of CO, PAH (aromatic hydrocarbons) and PM occur. Adding acetone to diesel has a better effect on soot reduction compared to butanol and ethanol. It adds acetone effect on engine performance because it improves braking thermal efficiency. Moreover, adding acetone improves energy efficiency and productivity.
Acetone delays the combustion time and improves the required temperature for combustion, which is due to the endothermic properties of acetone. Therefore, the use of this additive can be effective in providing engine benefits and reducing emissions. Adding isopropanol to diesel and biodiesel fuel and using it in diesel engine reduces CO and THC. However, according to most of the reported research in Table 9, it increases NOx, which can be one of the major disadvantages of isopropanol. Although a study reported that changing the type of injecting and double injecting could reduce NOx production, NO X increase, especially at full load, is generally greater. The high thermal value of isopropanol, which releases high heat, is the main parameter in increasing NOx production. Another study also reported that using the EGR method could reduce NOx and soot simultaneously. By using isopropanol, most reduction is observed in UBHC and CO emission. Adding isopropanol to the diesel engine increases BSEC. This combination delays combustion and reduces the peak pressure. It also reduces BTE but increases the brake thermal efficiency. Injector penetration results in higher fuel injection than diesel fuel. Therefore, it can be concluded that the use of isopropanol in engines containing EGR due to NOx reduction can be recommended. But compared to acetone, it has a low performance in reducing emissions and engine performance.
Adding propanol to diesel fuel and biodiesel mixture reduces some of the diesel engine emissions. Most studies show that the addition of propanol reduces NOx and soot emissions. Adding this substance reduces the temperature of the exhaust gas (EGT). Lower temperature and lower heat value are the main reasons for the reduction of NOx emissions. However, adding propanol increases some of the emissions such as SOF, THC and CO. Therefore, higher monoxide production can be negative factors for propanol use. The addition of propanol also affects the engine performance. Its addition reduces the combustion time. At low injection pressure, the BSFC value also decreases. Important and effective factors in the addition of propanol are the decrease in viscosity and density.  1 A l g a y y i me ta l .
iso-butanol and n-butanol of butanol -acetone The combination of diesel fuel with n-butanol has lower UHC and NOx production. The combination of diesel fuel and ISO-BA produces much less CO.
In n-butanol and iso-BA combined with diesel fuel, the spray penetration is slightly higher than that of pure diesel fuel. Combining iso-BA with diesel improves the peak of cylinder pressure and braking power. Algayyim et al. (2017) 2 A l g a y y i me ta l .  Peak pressure and HRR are reduced when using pilot spraying. Li, Lee, et al. (2019) 15 Li, Liu, et al. (2018) isopropanol The potential for soot emission reductions is significantly increased by the IBE ratio. IBE30 combined with EGR is capable of simultaneously reducing NOx and soot emissions.
The brake thermal efficiency of IBE15 is higher than pure diesel. Li, Liu, et al. (2018) 16 Shaafi and Velraj (2015) isopropanol The presence of oxygen in soybean biodiesel, as well as better incorporation of nanoparticles, significantly reduces the amount of CO and UBHC. However, under full load conditions, there is a slight increase in NOx.
The braking thermal efficiency is higher in the case of the D80SBD15E4S1 alumina fuel mixture.
Shaafi and Velraj (2015) 17 Sen (2019) propanol The addition of propanol to diesel fuel resulted in reduced NOx emission and soot.
The combustion duration of the propanol mixture is shorter than diesel fuel. And with increasing spray pressure, the combustion time is slightly reduced. The lowest BSFC is found at low spray pressure.
The DSC and TGA results confirm that Propanol reduces the starting temperature of the mixtures. Propanol increases specific brake energy consumption (BSEC). Also, it increases brake specific fuel consumption (BSFC).
Tri-propylene-glycol mono-methyl ether TPGME inhibits soot formation or increases soot oxidation during and after the end of fuel injection. And effectively eliminates the smoke from the engine.
Alone TPGME is not sufficient to enable long flame combustion.
Dumitrescu et al.
25 Imtenan et al. (2015) diethyl ether The performance and variability of greenhouse gas emissions from compounds modified from basic fuels (biodiesel blend from Jatropha and diesel) improved slightly compared to diesel.
The physical and chemical properties of the mixtures are improved by additives. 10% of the additive was better than 5% of the additive. Diethyl ether works better in terms of engine performance than n-butanol.
26 Beatrice et al.   Compared to diesel fuel, BSFC has increased with increasing DMC mixing levels. Yang et al. (2016) Besides, cloud point improves with the addition of this material. On the other hand, adding propanol reduces the noise of the diesel engine. Adding diethyl ether to biodiesel and diesel fuel reduces some of the emission parameters. In general, THC and CO engines emissions increase with blending this material in diesel fuel. These pollutants are especially increased in EGR engines, due to the reduced inlet oxygen to the combustion chamber. The rate of heat emission is reduced by diethyl ether. Therefore, NOx production is reduced due to the addition of this substance and this decrease has been reported at higher loads. The addition of diethyl ether to blend of diesel fuel and hydrogen increases NOx because hydrogen fuel has a high heat value, which increases temperature and NOx emission. Besides, the use of diethyl ether in fuel blend up to 10% improves the engine performance. The addition of diethyl ether has better engine performance in comparison to that of butanol. The addition of diethyl ether increases BSFC and thermal efficiency. Compared to diesel, it has a longer combustion delay. Cloud point temperature occurs at below 25°C. With the addition of diethyl ether, it has been reported an increase in BTE, which is due to the high thermal efficiency of this material. However, this action reduces BSEC. Adding poly-oxy-methylene dimethyl ethers reduces the emission of soot, CO, and HC. The effect of adding this substance to diesel fuel is much more severe in reducing HC, which is due to its high oxygen concentration. Yet, adding it does not have much effect on NOx. If EGR is used, NOx emissions are high due to the increase in inlet air temperature to the combustion chamber. Therefore, the disadvantages of using this additive are its nonefficiency in reducing NOX. Adding poly-oxy methylene dimethyl ethers improves engine efficiency. This additive improves the combustion process due to the high oxygen concentration in it, and also increases the brake thermal efficiency. Adding this material also increases the cetane number of fuels. Therefore, it is found that this additive performs very well on the diesel engine combustion process.

Conclusion
The global production of glycerol is increasing significantly and leading to its economic evaluation in the market. Since the generation of new glycerol is quicker than the market demand, the value of glycerol decreases significantly due to overproduction. The production capacity and demand for biodiesel fuel have been increasing in recent years and consequently, global production of glycerol is on the increase. It has little direct utilization and its fuel value is low. Glycerol is a suitable molecule for the synthesis of various products, for example, surfactants, emulsifiers, solvents, lubricants, cosmetics, etc. Besides, glycerol is non-toxic, edible, and biodegradable and has a very different structure that enables the synthesis of a wide array of valuable derivatives using different methods. Among various methods of producing valuable materials from glycerol, the choice of the best method depends on criteria such as the desired product, equipment required and other economic and environmental parameters. Therefore, depending on these parameters, the required method to produce the desired product is chosen. The methods of etherification, carboxylation, and glycerol carbonate preparation have been more widely used, respectively. This is due to being more industrial and producing different products. The use of derived additives from glycerin (with a variety of ways) shows that it plays a role in engine performance and emission parameters. For this reason, different studies on these materials should be carried out under different engine operating conditions. In most study cases, the use of these additives has reduced several emissions parameters, which proved their importance. The presence of hydroxyl groups can reduce NOx. Using glycerin additives to improve the properties of biodiesel can be important. Therefore, they must be added to the base fuel under different engine conditions and in different amounts. Among these additives, acetone is one of the most important additives in the blend of diesel and biodiesel fuel due to the maximum reduction of emissions. Diethyl ether additive is also a useful additive due to its increased thermal efficiency and BSFC. Hence, wide experimental investigations will be needed to obtain an appropriate additive.