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**Glycerol: Applications as a Green Chemical**

toc Glycerol (glycerin) is quickly emerging as a key player in the world of green chemistry and has become the focus of a vast range of research and development based on its conversion to green and useful products. Glycerol is a polyol compound with three hydroxyl groups found naturally in fatty acid esters, and is a central part of all lipids. It is colourless, soluble in water, and has low toxicity.

The main source of glycerol production in the past has been via byproducts from soap manufacturing, fatty acid production, fatty ester production, and microbial fermentation, but is currently being overproduced as a byproduct from the transesterification of oils and fats in a process to create [|biodiesel]. Glycerol can replace the use of fossil fuels and provide low energy alternatives in many process applications. These include the use of glycerol in a biofuel cell, synthetic gas and hydrogen production, fuel additive production and as a solvent and are just a few possibilities for the application of utilizing the excess production of glycerol from the biodiesel industry. Not only can these applications serve to improve the feasibility of biodiesel as a reliable alternative to fossil fuels, but can also "green" the current processes in each specific application.

**Properties**
 * Molecular Formula ||> C 3 H 8 O 3 ||
 * Molar Mass ||> 92.09g/mol ||
 * Density ||> 1.261g/cm 3 ||
 * Melting Point ||> 17.8 o C, 291K ||
 * Boiling Point ||> 290 o C, 563K ||
 * Viscosity ||> 1.412 Pa - s ||
 * Flash Point ||> 160-176 o C ||

Greening Biodiesel Production by Utilizing Waste Glycerol
The continuous rising demand for fuel along with the steady decline of petroleum resources has led to extensive promotion of biodiesel production. Biodiesel is most commonly produced by the transesterification of vegetable oils such as rapeseed, soybean and sunflower oils or from animal fat feedstocks. Crude glycerol accounts for 10% of the total product mass through this process, which typically contains about 80% pure glycerol mixed with methanol and water. Glycerol has many commercial and industrial applications; however the market is currently not large enough to accommodate its increasing production, especially because the crude glycerol derived from biodiesel production cannot be used directly due to its impurities. As a result many new processes and research topics have been appearing in an effort to convert this increasing production of glycerol into value added products and to decrease the impact of bio diesel production.

Due to the excess supply of glycerol, biodiesel producers are being forced to pay in order to have the crude glycerol taken away and incinerated. This has caused crude glycerol prices to plummet to negative values and has severely decreased the economics of biodiesel production. On the other hand, although pure glycerol prices initially halved, there has been a steady increase in the market price from 2006 onwards, describing the need for innovative techniques to be developed for the utilization of crude glycerol [__1__]. The advancement of current techniques as well as the creation of new uses for glycerol is necessary to reduce its accumulation in the environment and improve the economics of biodiesel.

In the following sections, we have outlined several applications that will help to improve the efficiency of biodiesel production. By utilizing crude glycerol, we reduce the environmental impact of biodiesel production by eliminating a waste product through the development of new techniques to convert it into value added products. This has two main advantages: it reduces the cost of the transportation and disposal of the waste, and it improves current processes by eliminating the need for fossil fuels as an input. Glycerol biofuel cell technology can be integrated into the biodiesel industry to provide an alternate source of energy to natural gas heating. The process would recycle biodiesel byproduct and increase its overall efficiency by reducing the amount of waste. Another way to recycle glycerol waste is through the production of syngas. Syngas can further be converted into methanol which is another input in the production of biodiesel. The conversion of glycerol to fuel additives replaces the need for petroleum feedstocks as well as improves the properties of biodiesel fuel making it a more attractive alternative for conventional diesel. The excess glycerol from biodiesel production can also be taken advantage of by using it as a green solvent in the replacement of fossil fuel derived or volatile organic solvents.

Glycerol Oxygen Biofuel Cells
Glycerol makes for a viable fuel for fuel cells because of the degree of its availability from the production of biodiesel. Glycerol is an alternative to traditional methanol and ethanol fuels because it is a non-toxic, non-flammable, safer, higher energy density fuel. With the growing need for green energy, scientists have explored the option of implementing more complex fuels for conventional precious metal based polymer exchange membrane (PEM) fuel cells. However, complex fuel PEM fuel cells produce minimal power outputs due to low oxidation efficiencies which is a recurring and a strong deterrent from further advancing the technology. Glycerol is no different when used for a PEM fuel cell; it is futile in yielding power at low temperatures because of the precious metal’s inability to oxidize the liquid fuel. Therefore, enzymatic biofuel cells have come to the forefront of green alternatives. A combination of alcohol dehydrogenase and aldehyde dehydrogenase based bioanodes have replaced the existing metal catalysts and have resulted in improved oxidization of glycerol fuel to create higher power densities which can operate at higher fuel concentrations when compared to standard PEM fuel cells. Fuel cells have been commonly used in power generation and electric vehicles. Because of the current infrastructure in place, it is impractical to expect glycerol to replace fossil fuel as the world’s leading energy resource. Furthermore, a glycerol oxygen biofuel cell unwanted byproducts such as dihydroxyacetone and glyceraldhyde. A prospective application for the glycerol oxygen biofuel cell is to incorporation it into the production of biodiesel. In doing so, the integration would green the process by reducing waste product and increasing efficiency. The recycling process would enable effective and economic biodiesel production.

**How a Polymer Exchange Membrane Fuel Cell Works**
The discovery of the fuel cell began with the familiarity that water could be separated into its elements, hydrogen and oxygen, when put under an electric current through a process called [|electrolysis]. The reverse process was hypothesized; reacting hydrogen and oxygen to produce electricity and water. The fuel cells are similar to gas turbines or gasoline engines in how they convert stored energy into working energy. A fuel cell provides a direct current (DC) voltage that can then be harnessed and used for a wide variety of electrical applications. The polymer exchange membrane fuel cell uses a simple electrochemical reaction to produce useful energy. There are four fundamental elements of a generic PEM fuel cell:


 * 1) **Anode**: the negative side of the fuel cell, attracts loose electrons from the oxidized fuel and leads them outside of the system where it can then be used to power an external circuit. Also, the anode equally distributes the fuel onto the surface of the catalyst where it will react with the oxygen.
 * 2) **Cathode**: the positive side of the fuel cell, enables the reaction by equally distributing the oxygen and conducting the electrons back from the external circuit onto the catalyst where the formation of water can be completed.
 * 3) **Electrolyte**: the proton exchange membrane which blocks the negatively charged electrons and exclusively conducts the positive charges. The membrane is typically hydrated to increase its stability.
 * 4) **Catalyst**: facilitates the reactions between the fuel and oxygen. The design of the fuel cell can maximize the surface area to increase its contact with both the fuel and oxygen.

The process begins when the fuel is fed to the anode. It then comes in contact with the catalyst which strips the fuel of its electrons, oxidizing the fuel. The electrons are then free to travel through an external circuit. At the same time, oxygen gas (O 2 ) is split at the cathode into two individual oxygen atoms. Both oxygen atoms have a negative charge which attracts the positive fuel ions through the membrane. Voltage across a fuel cell can be increased by combining individual fuel cells into what is called fuel cell stack.

**Glycerol as an Alternative Fuel**
Glycerol is inexpensive and broadly available making it an attractive fuel alternative to the depleting fossil fuels. Additionally, glycerol is a byproduct in the production of biomass. Energy density is also an essential characteristic for a potential fuel. When compared to ethanol (5.442kWh/L liquid), methanol (4.047kWh/L liquid) or glucose (4.125kWh/L saturated solution), glycerol has the highest energy density (6.260kWh/L) 2]. Glycerol is non-toxic, non-flammable and non-volatile. The concentration of glycerol also affects its properties and consequently, its power output. At 98.9 wt. % glycerol fuel cell operates at about 40% of its capable performance at 0.8 wt. % glycerol 2]. The higher concentration of glycerol increases its viscosity which results in the decrease in mass transport through the membrane. The Stokes-Einstein expression proves the diffusion coefficient of a molecule is inversely proportional to viscosity, meaning the more viscous the fluid, the less capable it is of maximizing its ability to be oxidized and in turn the energy density that can be produced. Overall, glycerol is an improved substitute for ethanol or methanol because of its greater potential to generate more energy.

**Enzymatic Catalysts in a Biofuel Cell**
A biofuel cell has the same four fundamental elements as a conventional PEM fuel cell. The cathode is separated from the anode by a polymer electrolyte membrane which allows for proton transportation while inhibiting electron transfer. The difference between a biofuel fuel cell and a PEM fuel cell is the catalyst. PEM fuel cells commonly include metal such as platinum, palladium, or gold; all of which limit its own capability for oxidization by absorbing the glycerol fuel. Another issue with a glycerol PEM fuel cell is the partial oxidation which produces 28.6% glycerate and reduces the possible energy density production 2]. PEM fuel cells also yield other toxic partially oxidized byproducts. Biofuel cells use biological molecules to advance the electrochemical reaction instead of conventional precious metals. Biological molecules like enzymes are used in the glycerol oxygen biofuel cell. Enzymes work like any other catalyst by lowering the activation energy and providing an alternative reaction pathway. They follow a lock and key model where only specific substrates or reactants can enter the enzyme. Enzyme isolation techniques increase volumetric activity and catalytic capacity leading to higher power densities by overcoming cellular membrane disturbance. Pyrroloquinoline quinine-dependent alcohol dehydrogenase (PQQ-ADH) has the ability to oxidize various kinds of alcohols. The enzyme contains the prosthetic group (PQQ) and carbon subunits to enable electron transport between the enzyme and the electrode. Alcohol dehydrogenase (PPQ-ADH) and aldehyde dehydrogenase (PQQ-AldDH) have been proven to be able to oxidize glycerol at lower temperatures and utilize 86% of the energy density of the glycerol 2].

**Oxidation of Glycerol and its Effectiveness**
In low temperature fuel cells, the fuel cell's ability to oxidize the fuel at the anode is used to theoretically compare energy density outputs. Typical PEM fuel cells made for glycerol and glycerol derivatives have not been developed due to the low amount of oxidation. Glycerol has a total of 14 electrons; four of which are oxidized by the metal catalyst resulting in a low energy density 2]. Enzymes in biofuel cells however, can increase energy density by providing enzymatic pathways which lead to a greater extent of oxidization. The glycerol oxygen biofuel cell has the ability to produce power densities of 1.21mW/cm 22] and the capability to work with 98.9% fuel concentrations 2]. When compared to an ethanol biofuel cell with the same enzymes, glycerol is able to generate more power because it is a triol 2], thus it has more aldehyde oxidation sites than ethanol or methanol. Prior experiments with highly concentrated fuels showed that biofuel cells performed poorly mainly due to the properties of solid fuels like sugars or the solvent properties of fuels 2]. The glycerol/O 2 biofuel cells yields higher power densities when compared to traditional ethanol fueled biofuel cells because of its improved efficiency and ability to oxidize the fuel.

**Biofuel Cell Integration Difficulties**
Fuel cells have the potential to be the green alternative to fossil fuel engines, however, like many new technologies, they present various difficulties. Cost is a major reason why the development of commercial fuel cells continues to be relatively sluggish when compared to the advancements in fossil fuel technologies. The components and materials used for a fuel cell are costly. Infrastructure and transportation for not only glycerol but other fuel cell fuels such as ethanol or hydrogen are not as established as those currently in place for oil and gas. However with further advancements in fuel cell research and technologies, there is the potential for an inexpensive alternative whether it is in the chemical process or in the materials used in the fuel cell. The properties of glycerol also depend on the temperature. If the biofuel cell is able to operate at a greater range of temperature, its durability will make it a more attractive alternative. The glycerol/O 2 biofuel cell has the limitation of only operating with low temperatures and membranes which frequently degrade as the operating temperature rises. Another limitation of glycerol/O 2 biofuel cells is the byproducts produced. Since glycerol is a carbon based molecule, byproducts can include carbon based pollutants such as CO 2 and CH 4. Because of this, it would be unreasonable to use glycerol fuel cells on a large scale without improving technology to seperate or diminish the pollutants produced. The technology of fuel cells and glycerol as a potential fuel has the prospect of enabling the shift from the current dependency on fossil fuels to new renewable energy.

The Conversion of Glycerol into Syngas and Hydrogen
Syngas (synthetic gas or synthesis gas) is a gas mixture containing varying amounts of hydrogen and carbon monoxide, and is used as an intermediate in the production of synthetic natural gas, ammonia, methanol, as well as synthetic petroleum. With a H 2 /CO ratio of 2:1, syngas can be used as feedstock in [|Fischer Tropsch Synthesis], in which syngas is converted into [|synfuels] (gasoline, diesel, kerosene, methane, etc) or chemicals such as ethylene or propylene [__3__]. Hydrogen alone can be used in refinery hydrotreating operations, ammonia production and fuel cells as a greener alternative to conventional fuels. Lastly, the energy acquired from the thermal cracking in these processes can be used to produce electricity [__3__].

Syngas is most commonly produced from steam reforming of natural gas, coal or liquid hydrocarbons. The disadvantages of this are that high amounts of energy are needed for the reaction to take place and the inputs are non-renewable petroleum-based resources. The production of syngas from glycerol provides a greener alternative that will reduce fossil fuel needs. Crude glycerol, derived as a bi-product of biodiesel production, can be used in many of these processes thereby providing a cheaper alternative to pure glycerol and coincidentally making biodiesel more economically feasible. Methanol production from syngas can also be used as an input for biodiesel production which could further improve its economics. Pyrolysis and steam reforming/gasification are the typical methods that are being used to convert glycerol to syngas; however some lower energy alternatives have also been studied. A particularly interesting example is the catalytic partial oxidation of glycerol. The use of low energy or autothermal methods is necessary to stabilize the economics of glycerol-related chemistry and hinder the use of fossil fuels that would otherwise be needed to provide heat to the process.

**Glycerol Pyrolysis**
Glycerol pyrolysis involves the thermochemical decomposition of glycerol at high temperatures with the absence of oxygen. It is an irreversible process where the chemical composition and the physical phase are changed simultaneously. The pyrolysis process yields liquid fuels at low temperatures (400-600 o C) and gaseous products at high temperatures (>750 o C), where temperatures at around 800 o C have been shown to produce the highest concentration of CO and H 2. Other gaseous products include CO 2, CH 4 , C 2 H 4 and C 3 H 8 , while the liquid products obtained are methanol, ethanol, acetone, and acetaldehyde. A small amount of char is also produced. The ideal decomposition of glycerol can be seen below:



The process is carried out in a fixed bed reactor at atmospheric pressure and high temperature. Packing material and a carrier gas are used to ensure uniform distribution of reactant throughout the reactor bed. Carrier gas is allowed to flow before the reactor reaches the desired temperature, at which point glycerol is fed to the reactor. Product leaving the reactor is then condensed and separated into liquid and gaseous portions. Once the reactor is cooled, char developed in the reaction can be weighed and discarded. Studies have been in order to determine the optimum conditions for the maximum production of syngas.

Valliyappan [__3__] looked at pyrolysis carried out in a fixed bed down-flow reactor at atmospheric pressure. The experiments were conducted in three major phases to study the effects of carrier gas flow rates, temperature, and particle diameter in the packing bed. It was shown that an increase in carrier gas flow rate had little effect on the production of syngas, though decreasing the gas product yield. Increased reaction temperatures more than doubled gas yield, and tripled hydrogen yield leading to an increase in syngas production from 70-93%, where the dramatic increase is due to the thermal cracking of hydrocarbons to hydrogen. Decreasing the particle diameter also showed a significant increase in both gas and syngas yields. Thus, a maximum syngas yield of 93% was achieved at a temperature of 800 o C, nitrogen flow rate of 50mL/min, and a packing bed of 0.21-0.35mm quartz particles. The energy acquired per mol of glycerol fed in this case was 111.18kJ.

**Steam Gasification (Steam Reforming)**
Steam gasification is a process that will convert glycerol to syngas (CO and H 2 ), CO 2, and CH 4 through a high temperature reaction with a controlled amount of steam. The process is very similar to that of pyrolysis; however in this case, due to the excess steam used, an additional water-shift reaction will occur to convert carbon monoxide to carbon dioxide and hydrogen [__3__]. Therefore, a much higher H 2 /CO ratio will be achieved. Reaction pathways for H 2 and CO production by steam reforming are depicted in the figure to the right.

Water gas shift reaction:

Overall reaction:

The objective of steam gasification is to increase the selectivity of hydrogen, thus increasing the H 2 /CO ratio and decreasing the amount of char produced. The same set up is used as in pyrolysis however no carrier gas is needed for the process, and water is added directly to the glycerol flow stream. The effects of varying steam to glycerol ratios have been studied for both pure and crude glycerol forms. Research conducted by Valliyappan et al [__3__] concluded a maximum gas production of 89.5 mol% containing 93 mol% syngas with a H 2 /CO ratio of 1.94 using pure glycerol, and 83.3 mol% syngas and a H 2 /CO ratio of 1.67 with crude glycerol. The experiment was carried out at a temperature of 800 o C and a steam to glycerol ratio of 25:75. 117kJ/mol of glycerol was produced. The use of a catalyst (Ni/Al 2 O 3 ) dramatically increased hydrogen and total gas production in both cases. Compared to pyrolysis, hydrogen production is increased by 15% through steam gasification, due to the additional water gas shift reaction.

**Enhancing Pyrolysis and Gasification**
As shown above, steam gasification and pyrolysis can easily be carried out at high temperatures to produce a high yield of syngas and hydrogen. However, there are ways to enhance syngas production using less energy as follows:

> >
 * 1) By using catalysts it is possible to obtain the same products at much lower temperatures of around 400 o C, reducing the need for input energy and thus "greening" the process . Catalysts provide an alternative path for the reaction process thus lowering the activation energy. Specific catalysts will also increase the selectivity towards syngas, and more specifically hydrogen production.
 * 1) Microwave heating can be used as a heat source which ensures even distribution of energy throughout the reactor and takes much less time to reach the target temperature . Conventional heating involves the use of a furnace or bath which heats the walls of the reactor by conduction or convection, whereas microwaves are able to target the compounds without having to first heat the entire furnace or bath. Not only does this save time, but energy as well. Without the use of a furnace, no fuel is needed to produce heat. This in turn reduces the consumption of fossil fuels and leads to a greener process.
 * 1) The addition of water as in steam gasification increases the selectivity of hydrogen. An H 2 /CO ratio of 2:1 is ideal so that can the product can be directly used in the Fischer Tropsch process to then convert syngas into useful fuels. Though theoretically, high amounts of water would only increase H 2 production and decrease CO production as the water-shift reaction takes place, there is a maximum amount of water that can be added. This is due to the fact that the water will eventually quench the reaction. Maximum H 2 production therefore takes place with a 2:3 steam/carbon ratio [__6__].

**Autothermal Catalytic Partial Oxidation (CPOx)**
Autothermal catalytic partial oxidation allows glycerol to be converted into syngas without the addition of process heat. The process was initially applied to convert methane to syngas. Partial oxidation involves a fuel-air mixture being partially combusted in a reformer which provides the heat for the desired reaction to take place. In catalytic partial oxidation, the use of a catalyst reduces the required temperature. The selectivity of the catalyst chosen for CPOx is important since non-equilibrium chemicals such as acrolein, acetaldehyde and hydroxyacetone can be produced from the decomposition or dehydration of glycerol. In order to produce a high yield of syngas, Schmidt et al conclude that Rh-Ce catalysts should be used. From the overall reaction of glycerol with steam, it can be seen that a significant amount of heat is required for the reaction to take place.

Overall reaction:

In CPOx, heat is generated through the partial combustion of glycerol. The combustion of glycerol is highly exothermic and for 1 mol of glycerol combusted, heat can be generated to drive 4.5 moles of glycerol to syngas.

Glycerol combustion:

Schmidt et al [__6__] performed CPOx by mixing droplets of the glycerol-water mixture with air at room temperature for reactive flash volatilization. Reactive flash volatilization rapidly converts nonvolatile liquids to volatile compounds through thermal decomposition, where volatility is defined by the ability of a substance to vapourize. This mixture is then introduced over a Re-Ce catalyst which oxidizes the glycerol at temperatures over 600 o C to produce syngas and non-equilibrium products. It was shown that a H 2 /CO ratio of 2:1 could be achieved with yields of 40% using a steam/carbon ratio of 2:3. The set up for the experiment is shown to the right.

The Conversion of Glycerol into Fuel Additives
A useful application of glycerol is its use as an alternate feedstock to petroleum in producing fuel additives. There are several advantages to enriching gasoline and diesel fuels with these additives such as keeping engine parts clean and free of build up, preventing their corrosion, and reducing incomplete combustion which improves fuel economy and reduces green house emissions. These results are largely due to their effect on the viscosity of the fuel as well as boosting octane numbers (increased satiability, reduced premature ignition) and providing more local oxygen during combustion. This encourages complete combustion and antiknock properties, in some cases also providing lubricating effects. The general process of converting glycerol to fuel additives involves the deprotonating of a hydroxyl group of the glycerol molecule in the presence of heat and/or catalyst before accepting electrons from a specific substrate which depends on the particular process and desired product. In theory an acid based catalyst causes the protonation of the glycerol hydroxyl group, creating a leaving group. This is followed by the nucleophilic attack by the hydroxyl group of another glycerol molecule or substrate. It is also found that high ratios of substrate to glycerol help improve the conversion rate of glycerol as well as moderate temperatures around 70-120 o C. Increasing the temperature beyond this point often leads to a significant decrease in selectivity by promoting the undesirable dealkylation of higher ethers.

**Green Potential for Glycerol Derived Additives**
These processes are just a few applications for converting glycerol into valuable fuel additives. The scope of this topic and its potential is far greater, and can be seen by walking down the fuel additive isle of any automotive store. There are countless such products currently produced from petroleum that could be replaced by the glycerol byproduct of a renewable fuel source. In short, as research continues in this area more and more of these fuel additives after market and used for pretreating fuels before the pump will only continue to spread. These products have already exhibited a large improvement in the properties of biodiesel such as cold properties, density and viscosity, through the application of these glycerol additives which have also proven to be very compatible with these bio fuels. Not only does the conversion of glycerol to fuel additives reduce the impact of a biodiesel byproduct, but it also helps to make the biodiesel a more attractive green product itself and provides alternate renewable feedstocks for fuel additives currently produced using petroleum.

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Etherification is the process of converting an alcohol or molecule containing an OH functional group into an ether molecule containing two alkyl groups bonded to one oxygen. The etherification of glycerol involves the reaction of glycerol with ether substrates such as Isobutylene(IB) producing Mono-tert-butylglycerols(MTBG), Di-tert-butylglycerols(DTBG), and Tri-tert-butylglycerol(TTBG) as set forward by. MTBG and DTBG can include several isomers and are considered to represent all the possible mono- and di-tert-butyl-ethers where MTBG is considered undesirable and TTBG and DTBG are the desirable products. This process was carried out between 60-90 o C, Liquid phase pressurized isobutylene was injected into the reactor, previously charged with glycerol and a catalyst (propyl-SO 3 H- functionalized mesostructured silica at 5%wt) where temperature was raised until the vapour phase was achieved with a desirable pressure for selective production for a period of 1-4 hours. The ratios of reactant to glycerol, temperature, and pressure all vary depending on process but in general a high IB/G ratio and mild temperature are required to yield selective production of TTBG AND DTBG. Etherification produces Di-tert-butyl glycerol ether and Tri-tert-butyl glycerol ether which when added to fuel provide advantages such as:
 * Etherification**


 * 1) Diesel and biodiesel reformulation
 * 2) Improving cold properties. Diesel vehicle operation in cold weather can be a problem as conventional diesel fuel can gel which is undesirable for fuel lines and fuel injectors and is subject to a property called Cold Filter Plugging Point which is the temperature at which the fuel gels to a large enough extent it plugs the fuel filter and inhibits fuel flow to the pistons.
 * 3) Reducing contaminant emissions of particulate matter, hydrocarbons, carbon monoxide and unregulated aldehydes. This is due in large part to increased local oxygen content encouraging complete combustion.
 * 4) Reducing fuel viscosity. A high viscosity fuel may cause extreme pressures in the injection systems and will cause reduced atomization and vaporization of the fuel spray which are important properties for ensuring complete combustion of fuel.
 * 5) Providing an alternative to commercial tertiary alkylethers (MTBE and ETBE). This is desirable as the tertiary alkylethers currently used for oxganate additives to raise octane numbers have been proven as a contaminate issue in ground water and are produced using natural gas.
 * 6) Decreasing cloud point of diesel fuel when blended with biodiesel. Cloud point is the temperature at which wax crystals in the fuel (paraffin base) begin to settle out with the result that the fuel filter becomes clogged.
 * 7) Improving properties such as blending components and good solubility in diesel

Etherification can also occur with glycerol being the only reactant, and is carried out at 533K in a batch reactor at atmospheric pressure under N 2 in the presence of a 2 wt.% catalyst (cesium impregnated on pure mesoporous silica materials). This process yields a conversion rate of glycerol of 80%, with a selectivity of 90% for triglycerol under these conditions. The process produces polyglycerols which provide excellent lubricity reducing engine friction and helps make up for the lost lubrication due to the lowering of fuel viscosities when improving cold properties and cloud points.


 * Acetylation/ Esterification **

Acetylation is the process of replacing the hydrogen from an OH group with an acetyl group CH 3 CO. Esterification is the process in which the alcohol group of a molecule reacts with an acid to form an ester product RCO 2 R' where R and R' are alkyl groups. The esterification of glycerol with acetic acid as described by Melero et al, yields the main acetylation products: mono-acetyl-glycerol (MAG), di-acetylglycerol (DAG), and tri-acetyl-glycerol (TAG). The reaction is performed in the liquid phase at temperatures of 125 o C in a stirred Teflon-lined stainless steel autoclave with a 9:1 molar ratio of acetic acid to glycerol over a propylsulfonic acid modified mesostructured catalyst. Using this process at the above determined ideal conditions produces glycerol conversions over 90% and combined selectivities toward di- and triacetylglycerol of over 85% after 4 h. This process produces Diacetyl glycerol and Triacetyl glycerol which are used as fuel additives and exhibit advantages such as:
 * 1) Improving the cold and viscous properties of liquid fuels (including biodiesel)
 * 2) Providing antiknock additives for gasoline which increase the stability of fuel due largely to highly branched hydrocarbon molecules. Increased stability helps to reduce preignition which interrupts engine cycles and can cause damage.

**Acetalation**
The acetalation process set forward by Garcia et al, involves mixing glycerin and acetone with P-toluenesulfonic acid mono-hydrate and is heated to reflux (vapours given off are cooled back to liquid, and fall back into the vessel) for 16 hours. As the wet acetone is distilled in the process, more dry acetone is introduced to the reactor keeping the liquid level constant in the reactor. The reaction is stopped by adding Na 2 CO 3 to neutralize the reaction. The formed salts are eliminated by filtration and solvents removed in a rotary evaporator (device used for the efficient and gentle removal of solvents from samples by evaporation). The acetal methylacetate (2 in below diagram) is obtained by reacting (1) with acetic anhydride in triethylamine. The reaction mixture is then dissolved in ethyl acetate, washed with a saturated solution of NaHCO 3, water and brine then dried over anhydrous MgSO. The solvent was evaporated in a rotary evaporator. Under reduced pressure, traces of water and acetic acid are eliminated from the mixture by vacuum distillation. In vacuum distillation, the liquid mixture to be distilled is reduced to less than its vapour pressure which is typically less than atmospheric pressure, causing evaporation of the most volatile liquids. The reaction yields a mixture of acetal glycerol and triacetyl glycerol (2 and 3) that cannot be separated.

This process produces acetal glycerol and triacetyl glycerol which are used as additives to:
 * 1) Improve viscosity
 * 2) Prevent an increase in the density of the fuel. This is also beneficial for the vaporization of the fuel in pistons via the fuel injectors. Heavy fuels do not vaporize as well and will not combust as easily or completely.

**Using Glycerol as a Sustainable Solvent**
Another application of glycerol is its use as a green solvent. Currently, most solvents are prepared from fossil fuel reserves, but these reserve supplies are depleting so renewable alternatives must be found to replace them. Glycerol is an organic waste produced by the biodiesel industry making it highly available, this brings about the concept of using glycerol as a green solvent presenting a new way to make use of this waste. Glycerol is a polar protic solvent and is completely soluble in water and short chain alcohols, and insoluble in hydrocarbons. It has many properties that make its use as a solvent advantageous as opposed to other green solvents such as its biodegradability, large availability, renewability and easy handling and storage. It exhibits properties of both water including its low toxicity and low cost, and of ionic liquids such as its high boiling point, low vapour pressure and low solubility in scCO₂. These properties are in alignment with many of the [|12 Principles of Green Chemistry]. Recently, researchers have been working on this concept and have been successful in demonstrating the feasibility and necessity of using glycerol as a solvent. It has been found to have beneficial effects on reaction rates and selectivity, as well as being good for product extraction. Using glycerol as a solvent presents a new tool for the replacement of volatile organic solvents by greener alternatives.

**Glycerol as a Medium for Organic Transformations and Catalysis**
It has been found that using glycerol as a solvent can remove the need for a catalyst in certain reactions under standard conditions such as in the aza-Michael reaction between p-anisidine (1) and n-butyl acrylate (2), or in the Michael addition of indole (4) to nitrostryrene (5), while still retaining a high yield. In comparison to using other solvents or water in these reactions, pure or technical grade glycerol was capable of obtaining the desired product at a high yield under catalyst free conditions, as depicted in the figures below. Liquid-liquid phase extractions with ethyl acetate can then be used to isolate the product and for the recycling of the glycerol. [__12__] Eliminating the need for a catalyst promotes the principles of green chemistry supporting the necessity of using glycerol as a green solvent.

**Scheme 1** Aza-Michael reaction of //p//-anisidine in different solvent systems. **Scheme 2** Michael reaction of indole in different solvent systems. Glycerol has also been used as a solvent, in the electrophilic activations of aromatic aldehydes with indoles or 1,3-cyclohexanediones, replacing the need for acid catalysts. Products formed in these reactions were insoluble in glycerol making it easy to isolate the products by filtration after dilution in water. An example is depicted below. Advantages of using glycerol in these reactions are the lack of the use of an acid catalyst, the easy separation of reactants and products, and there is no use of a volatile organic solvent. Some problems that may arise with this reaction procedure are that organic solvents must be used in order to extract products that cannot precipitate in glycerol and glycerol does not work as a solvent for some less reactive substrates and the addition of a Lewis acid would be required to improve the yield. [__12__]

**Scheme 3** Reaction between 4-nitrobenzaldehyde and 2-methylindole in different solvent systems and under catalyst-free conditions.

Some cases have also been reported showing the potential for glycerol in improving the reaction selectivity of a reaction and in being successfully used as a solvent for biocatalysis. Glycerol is also capable of facilitating the isolation of reaction products by using mixtures of 25-75 wt% of glycerol in water to obtain high extraction yields. This is due to the strong interactions between glycerol and water which decrease the solubility of the product in glycerol. Furthermore in instances when water will form an [|azeotropic mixture] with a substance, glycerol can be used as the solvent instead making distillation possible. Cases where the reactants are immiscible in the catalytic glycerol phase, result in a very low reaction rate and the formation of side products. This can be overcome by using glycerol in the presence of the AP (aminopolysaccharide) catalyst (a recyclable homogeneous catalyst); this allows for the selective extraction of a reactions product from the glycerol catalytic phase without the assistance of an organic solvent as is generally the case when water is used. When the reaction products are soluble in the glycerol phase, supercritical carbon dioxide, scCO₂, was found to be a successful extraction solvent, eliminating the need for the assistance of a petrochemically derived volatile organic solvent. [__12]__

**Glycerol as a Solvent for Separation**
An alternative to the current method of [|purification of bioethanol]is to use glycerol, for the extractive distillation production of [|anhydrous] ethanol, in replace of fossil fuel derived chemicals. Conventional extractive distillation uses two distillation columns in order to separate the azeotrope of ethanol-water that is created during the process, but only one distillation column is necessary in the alternative process using glycerol. The anhydrous bioethanol is collected directly at the top of the column and the glycerol and water are separated at the bottom. This process recovers the ethanol, water, and glycerol with more than 99% purity, as opposed to the 95.5% purity obtained by the conventional distillation method. [__12__]

**Obstacles of Using Glycerol as a Green Solvent**
The following are some of the obstacles that must be overcome in order to successfully use glycerol as a green solvent along with some of the possible solutions.
 * 1) Glycerol’s high viscosity can lead to mass transfer problems but this is overcome if reactions occur at temperatures greater than 60°C
 * 2) The chemical reactivity of its hydroxyl groups can lead to the formation of side products but this is overcome by using glycerol in a chemically inert environment (not extremely acidic or basic)
 * 3) Its coordinating properties may induce problems when transition metal complex catalysts are used [__12__]

Glycerol has many other potential uses as a green solvent, such as, but not limited to material synthesis due to its high boiling point and low vapour pressure which allows material synthesis to be performed at a high temperature. Reasons behind the promoting effect of glycerol are still unknown,but some possibilities could be due to its strong hydrogen bond network or due to the presence of impurities in glycerol. Using glycerol as a solvent is an excellent way to utilize waste which is generated by the biodiesel industry. It can be seen through the above examples that glycerol is an important green solvent for the chemical industry.