Patent Application
20160237361
Fossil fuels are being replaced by renewable and sustainable energy sources such as biodiesel, biokerosine, and ethanol, which can supply the needs of many types of ground transportation by fueling cars, trucks, and rail. Nevertheless, many biofuels such as biodiesel are not suitable for use in cold climates because they contain long chain hydrocarbons that can solidify at operating temperatures in such environments. Biofuels are also not suitable for use as aviation fuel, both because of their solidification at the low temperatures encountered at high altitude and because they lack the high energy density required of an aviation fuel.
There remains a need to develop methods to convert biofuels into other useful fuel mixtures, such as mixtures of short chain alkanes that can be used for ground transportation in cold climates and for aviation fuel.
The invention provides a series of electrochemical processes carried out by directed electrochemical reactors for use in improving the quality of a biofuel, converting a biodiesel fuel to a fuel containing short chain alkanes, and converting the fatty acid methyl esters of a biodiesel fuel into a mixture of aliphatic hydrocarbons. The reactors can be used individually or combined into systems to modify a biofuel for new uses, such as in aviation or motor vehicles and as a replacement for liquified petroleum gas in heating and cooking.
One aspect of the invention is an electrochemical reactor for cleaning up biodiesel via selective oxidation of alcohols and glycerol, and thereby producing cogenerated power.
Another aspect of the invention is an electrochemical reactor for selective elimination of unsaturation and cleavage of C═C bonds in fuels with a high degree of unsaturation, such as biodiesel derived from rapeseed oil.
Yet another aspect of the invention is an electrochemical reactor for hydrogenation of ester moieties in a biodiesel fuel for the generation of aliphatic hydrocarbons.
Each of the abovementioned reactors utilizes on-line monitoring of fuel components by analytical methods such as high pressure liquid chromatography and mass spectrometry. The reactors also utilize selected electrocatalysts in conjunction with ion conducting membranes.
Still another aspect of the invention is a system for the chemical conversion of a biodiesel to an alkane composition. The system includes a first electrochemical reactor that reduces excess Me0H in a biodiesel source material to yield a first composition containing methyl esters of aliphatic carboxylic acids. The system also includes a second electrochemical reactor that fragments the methyl esters of aliphatic carboxylic acids of the first composition by carbon-carbon double bond cleavage to yield a second composition containing short chain methyl esters of aliphatic carboxylic acids. The system further includes a third electrochemical reactor that hydrogenates the methyl esters of the second composition to yield a third composition comprising alkanes.
Another aspect of the invention is a system for the chemical conversion of a biodiesel to an alkane composition. The system includes a first electrochemical reactor that fragments aliphatic chains of a biodiesel source material by carbon-carbon double bond cleavage to yield a first composition containing short chain methyl esters of aliphatic carboxylic acids. The system also includes a second electrochemical reactor that performs a Kolbe reaction, whereby the aliphatic carboxylic acids of the first composition are decarboxylated to yield a second composition containing alkanes.
Still another aspect of the invention is a system for the chemical conversion of a biodiesel to an alkane composition. The system includes a first electrochemical reactor that fragments aliphatic chains of a biodiesel source material by carbon-carbon double bond cleavage to yield a first composition containing short chain methyl esters of aliphatic carboxylic acids. The system also includes a second electrochemical reactor that hydrogenates the methyl esters of the first composition to yield a second composition comprising alkanes.
Yet another aspect of the invention is a method for the chemical conversion of a biodiesel to an alkane composition. The method includes providing a crude biodiesel composition that contains fatty acid esters and methanol; reducing the amount of methanol by electrochemical oxidation of the methanol to carbon dioxide in a first electrochemical reactor; fragmenting fatty acid chains in a second electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; and hydrogenating the short chain fatty acids and aldehydes in a third electrochemical reactor to yield a composition comprising alkanes.
Another aspect of the invention is a method for the chemical conversion of a biodiesel to an alkane composition. The method includes providing a crude biodiesel composition containing fatty acid esters and methanol; fragmenting the fatty acid chains in a first electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; oxidizing the short chain aldehydes to short chain fatty acids; and performing a Kolbe reaction in a second electrochemical reactor, whereby the short chain fatty acids are decarboxylated to form alkanes.
Still another aspect of the invention is a method for the chemical conversion of a biodiesel to an alkane composition. The method includes providing a crude biodiesel composition containing fatty acid esters and methanol; fragmenting the fatty acid chains in a first electrochemical reactor that cleaves the fatty acid chains at carbon-carbon double bonds to yield short chain fatty acids and aldehydes; and hydrogenating the short chain fatty acids and aldehydes in a second electrochemical reactor to form alkanes.
The invention provides a series of directed electrochemical reactors useful for modifying biofuels in several ways. One type of reactor can be used to clean up biodiesel via selective oxidation of alcohols and glycerol, while simultaneously generating power. Another type of reactor can selectively eliminate unsaturation found in many biofuel sources, such as rapeseed oil, by cleavage of C═C bonds, yielding short chain hydrocarbons. A third type of reactor hydrogenates ester moieties and generates aliphatic hydrocarbons from fatty acid esters. Each of these reactors is designed to perform specific tasks and can be outfitted with on-line monitoring of fuel components using known analytical tools such as high pressure liquid chromatography and mass spectrometry. The reactors utilize selected electrocatalysts in conjunction with ion conducting membranes, such as the perfluorinated sulfonic acid prototype represented by DuPont's Nafion® series. The electrochemical reactions of the invention can be integrated into existing biodiesel production technology or can be added to it as a post-production refining process, e.g., to convert biodiesel to aviation fuel.
As used herein, a biofuel is a hydrocarbon-based mixture obtained from plant, animal, or microbial sources which can be used as a fuel in internal combustion engines, jet engines, for heating or cooking, or for generating other forms of energy such as electricity. One type of biofuel is biodiesel, which is a mixture of mono-alkyl esters of long chain fatty acids. Biodiesel is typically produced by transesterification of fats using methanol as the alcohol, but other short chain alcohols such as ethanol, (iso)propanol, or butanol also can be used. Most vegetable oil sources of biodiesel contain fatty acids having a chain length from about 16 to about 22 carbons in length and from 0 to 3 C═C double bonds, most commonly 18 carbons in length and 1 or 2 C═C double bonds. The methods according to the present invention can convert such mono-alkyl esters into short chain alkanes having, for example, from about 1 to about 16 carbon atoms in length, or from about 2 to about 12 carbon atoms in length, or from about 3 to about 10 carbon atoms in length. The short chain alkane product can include straight chain unbranched alkanes, but also can contain branched chain, cyclic, unsaturated, or aromatic hydrocarbons. In certain embodiments, the short chain alkane product of the present invention is essentially free of aromatic hydrocarbons, or is essentially free of unsaturated hydrocarbons, or is essentially free of mono-alkyl esters of long chain fatty acids, or is essentially free of alkanes having a chain length of 12 or more, 14 or more, 15 or more, 16 or more, 17 or more, 18 or more, 20 or more, or 22 or more carbon atoms. In some embodiments, the short chain alkane product consists essentially of linear alkanes having a chain length of about 8-11 carbon atoms.
In Reactor I, a polyvalent transition metal ion complex mediates the methanol oxidation process at the immobilized Pt catalyst active sites for anodic oxidation of methanol. To illustrate the mechanism, ethanol electro-oxidation on a Pt/C catalyst was studied in the presence of millimolar quantity of a polyvalent transition metal ion complex (lead acetate) using chronoamperometry over a one hour period. The results are presented in
In Reactor II, methyl esters of fatty acids derived from inedible sources, such as rapeseed oil, whose primary constituent is erucic acid, are cleaved at C═C double bonds by electrocatalytic oxidation with oxygen in the presence of Cu catalysts (homogenous mediated catalysis). The Cu2+/Cu+ redox couple has been applied to a C—C double bond cleavage reaction of styrene [1]. In E-reactor II of
In Reactor III methyl esters are reduced to alkanes. See, e.g.,
(ii) The alcohol can then be transformed to an alkane product by an electroreductive method on a lead cathode [4], according to Scheme II.
Optionally, the number or capacity of Reactors III can be reduced if all or a portion of the methyl esters are converted to alkanes directly by catalysis.
Traditionally, the excess alcohol in raw biodiesel is removed with a flash evaporation process or by distillation. Such processes have several disadvantages, including energy consumption and product quality. Further, the flavor components resulting from decomposition of long chain organic compounds are contamination to the environment. Worse, a “burn on” effect may occur, resulting in highly viscous concentrates that tend to adhere to evaporator heat transfer surfaces [5].
The system of Reactors 1-111 described above has several advantages compared to traditional chemical reactions used for similar purpose. Conventional hydrogenation of esters is carried out under high hydrogen pressure and elevated temperature in slurry-phase or fixed-bed reactors. Such methods involve high energy consumption, low selectivity, and low product yield [8,9,10]. The simple mediator method used in the Reactor I needs only a low concentration of a transition metal complex (such as 0.5 mM lead acetate). In addition, the transition metal complex is not consumed and remains as a redox couple once the fuel cell runs. The mediator catalyst used in Reactor II is a common and inexpensive salt, CuC12. No reducing agents are required in Reactor II for the activation of molecular oxygen. Reactor III uses moderate temperature and atmospheric pressure conditions for hydrogenation of methyl esters. Proton donors can be generated by a highly efficient water electrolysis process. Specificity for the electroreduction of esters and alcohols to final alkane products is provided through the use of two specific and exclusive catalysts.
Fabrication and installation of the electrochemical reactors required by the invention are simple and convenient. The electrochemical catalytic systems are operated in an environmentally friendly manner, and there is no noise contamination produced. Conditions of mild temperature and relatively low pressure during the running of the reactors offer them long-term durability. The overall process offers high product yield and efficient energy conversion. The traditional refining process of biodiesel always has been highly energy intensive and has faced technical hurdles. The present system of three electrochemical reactors has the potential to overcome these problems and achieve the use of aviation biofuels as a commercially viable energy source, which will reduce emissions of NOx and unburned carbon dramatically and lower the cost of production of aviation fuel significantly.
The invention also contemplates other combinations of electrochemical reactors for the processing of biofuels. In one such embodiment, the cloud point of a liquid fuel mixture is reduced through a series of electrochemical reactions.
The lipid content harvested from a biological source such as a plant source or a microbial source such as algae or bacteria flows into an electrochemical processing cell where fatty acids and triglycerides are cleaved into lower molecular weight components. This cleavage occurs at carbon-carbon double bonds which, as the inventors have shown, can be preferentially broken through electrocatalytic oxidation. Once cleaved, the fragments can be used as liquid fuel. However, the properties of these fragments can lead to a high cloud point. To reduce the fuel cloud point, oxygen can be removed through a secondary electroreduction process. The inventors have demonstrated that methyl esters can be reduced to alcohols using an electrochemical cell with magnesium electrodes.
A series of directed electrochemical reactors enable higher efficiency and greater selectivity for conversion of bio-sourced oils such as those containing triglycerides, aliphatic free acids and esters into kerosene grade fuels. Specifically tailored electrochemical reactors eliminate unsaturation (cleavage of C═C bonds) in free acids, esters and triglycerides; followed by either hydrogenation of ester moieties for generation of aliphatic hydrocarbons or oxidation of free acids to alkanes via the Kolbe process.
The inventors are unaware of any viable process for conversion of bio-derived oils to kerosene-type alkanes in terms of energy balance, flexibility towards feedstock and modularity. Some of the steps in the system of the present invention, especially the selective cleavage of the unsaturation at C8-C10 part of the aliphatic backbone, have no analog in conventional catalytic routes. Conventional routes for bio-oil upgradation (conversion to alkane derivatives) typically involves hydro-deoxygenation [11]. However the conventional sulfide based hydrotreating catalysts contaminate products by incorporation of sulfur, deactivate rapidly by coke formation, and are potentially poisoned by trace amounts of water [12,13]. A new acid aqueous process based on select transition metal catalysts has been proposed for bio-derived compounds such as polyols [14], and limonene [15]. However these are largely aqueous processes where phase transfer catalysis has yet to be incorporated, and later expensive separation steps are invariably needed.
The entirely electrochemical process described herein allows for complete flexibility and smooth transition between reactors without expensive cleanup steps. It also provides for complete elimination of all oxygenated moieties from the derived bio-oil fragments to yield alkanes suitable for use as fuel.
As shown in the overall schematic of the process (
An entirely electrochemical system of processing a biofuel according to this embodiment has a number of highly desirable characteristics and advantages over non-electrochemical approaches:
In order to develop each of the reactor components, model compounds can be used to mimic the real bio-oil feedstock input. For the process described in
E-Reactor I (100) carries out C═C bond cleavage. The C═C double bond cleavage reaction of the aliphatic backbone in bio-derived ester or acid (such as in the case of erucic acid or ester) can be promoted via electrocatalytic oxidation with oxygen in the presence of Cu-catalysts (115) (homogenous mediated catalysis) in the anode compartment (118). The Cu2+/Cu+ redox couple concept was applied in a C═C double bond cleavage reaction of styrene [16]. As shown for E-reactor I in
E-reactor 1 can be followed with catalytic oxidation of aldehyde moieties to either acid or ester terminal groups. In the case of conversion to acid terminal ends, this can be conducted using conventional Tolien's reagent (Ag catalyst in conjunction with NH3). For conversion of the fractions containing ester terminal groups (such as those obtained from erucic ester) the aldehyde end groups obtained in step 1 can first be converted to ester groups using conventional base catalyzed esterification in conjunction with methanol. Both conversion steps have greater than 95% yield and typically use an 85-95° C. single pot process followed with phase separation between aqueous and oil phases. These steps are conventional.
E-reactor 2 (200) carries out Route 1 (Kolbe Process), the direct conversion of acids to alkanes. A decarboxylative dimerisation known as the “Kolbe Reaction” proceeds via a radical reaction mechanism, yielding aliphatic products directly [17,18]. Generally, the reaction can be represented as:
R1COO−+R2C00+→R1—R2+2CO2 (1)
This reaction can be carried out using metered amounts of acetic acid. R1 can be a methyl group, and R2 can be a fragment, with two terminal acid groups obtained from previous step. However, if RI and R2 are different, then alkanes R1—R1 and R2—R2 are also likely products. Although use of the Kolbe process has the potential of saving a lot of effort in conversion of the acid to alkane products, there remains the potential problem of making a mixture of alkanes with various lengths of C chains, causing difficulty for separation and purification. Process design can utilize a proton exchange membrane (240) [18]. The important consideration is using metered amounts of H2O (270) in the cathode (220) compartment for H2 evolution. This careful metering of H2O is important, as the electro-osmotic drag in this reaction is towards the cathode and hence has the added advantage of removing excess water from the oils at the anode (210). In addition the use of Pt/C catalyst is mandated due to the use of proton exchange membranes. However, the catalyst loading can be kept low using careful membrane electrode assembly strategies. The final design consideration is to prevent reaction of long aliphatic acid components obtained from cleavage of unsaturation in the starting long chain acid or ester (230) (erucic acid). Therefore, acetic acid (260) can be used as input for preferential reaction with individual aliphatic fractions obtained from E-reactor 1. Two important strategies can be used.
In strategy 1, catalysis on a nanocluster surface can be used with different oxidative sites (Pt and alloying element), while in the second strategy individual elements can be used as catalytic sites in a well-coordinated crystalline environment.
E-Reactor 3 (300) carries out Route 2, the conversion of methyl esters to aviation fuels of alkanes. Route 2, shown in
Three separate reactor designs can be used. In the first, the electrodes are separate from the ion conducting membrane. Though simple to implement, this rendition requires the greater use of free electrolyte, which presents problems later when purification of the fragments is conducted. Second is the use of half membrane electrode assemblies, wherein either the anode or the cathode would be bonded to the membrane. This method allows for the electrode where the primary process is occurring to be in a high throughput flow through mode. Third is the formulation of an anode and cathode bonded membrane electrode assembly (MEA). Here, electrolyte conductivity and that of the charged species is not a problem; however, proper mass transport of the reactants to the appropriate interfacial reaction zone has to be ensured.
Design of E-Reactor 1. The membrane electrode assembly can be designed using porous carbon electrodes (uncatalyzed) using various thicknesses of Nafion® (Dupont) membranes. Carbon electrodes with various levels of porosity can be used for correlating with mass transport measurement (electrochemical polarization measurements). Flow through plates can be designed using graphite plates for effective turnover numbers and charge transfer efficiencies. Both parameters can be measured under glavanostatic conditions with aid of an online GS-MS set up.
Design of E-Reactor 2. Membrane electrode assemblies can be designed with Pt/C electrodes on anode and cathode sides (e.g., 60% on C, 0.4 mg/cm2 loading). The metering arrangement for cathodic hydrogen evolution reaction can be designed in keeping with water activity at the anode stream containing the aliphatic acid moieties.
Strategies can be implemented for promoting radical initiated reactions between aliphatic acid components obtained from E-reactor 1 and acetic acid derived radicals. This is intended to prevent formation of larger chain aliphatic chains due to reaction between the aliphatic components from E-reactor 1. Two approaches can be used.
Approach 1. Pt alloys with Cr, Ni and Co can be employed with the same electrode loadings as described above.
Approach 2. Use of crystalline chalcogenide catalyst, specifically RhxSy, prepared using a thiol based process, with the same loadings as described above.
The cell flow through system can be designed for maximizing the TOF and yield per pass. In this case a careful optimization can be made using on line GC/MS equipment and cell operating conditions.
Design of E-Reactor 3 Assembly. This includes two separate electrochemical cells. The first reactor (E-reactor 3.1) can be designed for conversion of the ester moieties to alcohol followed with a second tandem reactor (E-reactor 3.2) for its conversion to alkanes. The membrane electrode assembly for reactor 3.1 can comprise a RuO2 anode and a Mg cathode. In this case, RuO2 and Mg inks can be directly deposited into the Nafion membrane in conjunction with a solubilized form of the Nafion polymer. Since 02 evolution is occurring at the anode electrode no carbon black is required on that electrode layer. This avoids difficulties in preventing carbon corrosion. At the cathode, Mg catalyst can be used in the same loading as prescribed above. Short periods of voltage reversal are can be used to keep the appropriate TOF's. This is from the perspective of cleaning the Mg electrode at a regular interval (every 30 s). Since voltage swings are mandated, no carbon is added to the Mg side of the electrode as well. Since 02 evolution is involved in the first cell, no carbon plates are used. Instead the bi-polar plates in this reactor can be constructed with Ti flow through plates. The cell flow through system can be designed for maximizing the TOF and yield per pass. In this case a careful optimization can be made using on line GC/MS equipment and cell operating conditions.
Reactor 3.2 can be fitted with a Pb/C cathode for electroreduction of alcohol with an interpenetrating proton-conducting phase, typically a sulfonated polymer such as Nafion, and a RuO2 anode electrode. These components transport electrons, protons and gas or solution phase reactants and products to and from the electrocatalyst site. The oxygen evolution anode can be similar to the one used in reactor 3.1. As mentioned earlier, this anode can be directly deposited on the membrane surface (Nations®, Dupont) and does not contain any carbon black. Ti based bipolar plates can be used for E-reactor 3.2. This is due to oxygen evolution anode electrode being used as the proton source.
Oxygen from E-reactors 3.1 and 3.2 can be recirculated to E-Reactor 1.
The cell flow through system can be designed for maximizing the TOF and yield per pass. In this case a careful optimization can be made using on line GC/MS equipment and cell operating conditions.
Various techniques can be adopted for enhancing conversion efficiency. For example, the concentration of Cu ion catalyst can be optimized based on a balance of high performance and easy separation from the final products. Solvents with boiling point or viscosity different from the reactant (erucic acid) and the obtained fractions can be used to ensure convenient post-reaction separation.
In E-reactor 2 (Kolbe Process), careful control of the dissociation of acid moieties can be obtained via adjustments to solvent composition. Central to the success of the Kolbe process is promotion of the reaction between acetic acid and the aliphatic fractions obtained from E-reactor 1. Considering the mechanism of the Kolbe process, the reaction is initiated by the carboxylate ion undergoing an initial discharge to form an adsorbed radical. RCOO−→RCOO (ads)+e−. Steric considerations can be exploited via the use of Pt alloys and chalcogenide electrocatalysts. However this process can be further promoted by careful adjustment of the solvent conditions to promote dissociation of the acid.
Data have been collected to demonstrate the direct anodic cleavage of the styrene moiety to yield corresponding olefins. The electrolysis reaction was carried out in an H type cell (shown in
Conversion of methyl esters to alkanes proceeds by first transforming the methyl ester into an alcohol followed by a second electroconversion into alkane. The reduction of methyl 1-octanoate to octanol was demonstrated using an electrolysis process with a LiClO4 electrolyte and t-BuOH as a proton source, with magnesium and silver electrodes. The products of this electroreduction were shown using a GC-MS technique. In addition to being quantitative, GC-MS also provides detailed structural information for an analyte as a result of diagnostic fragmentation patterns resulting from ionization of the molecule.
While the present invention has been described in conjunction with certain preferred embodiments, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. It is therefore intended that the protection granted by Letters Patent hereon be limited only by the definitions contained in the appended claims and equivalents thereof.
This application is a divisional of Ser. No. 13/811,843, filed Mar. 6, 2013, which is a US National Phase Application of PCT/US2011/045215, filed Jul. 25, 2011, which claims the priority of U.S. Provisional Application No. 61/367,092 filed Jul. 23, 2010. Each of the aforementioned applications is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61367092 | Jul 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13811843 | Mar 2013 | US |
| Child | 15141182 | US |
