Unsaturated Renewable Oil Transformation into Novel Biofuel Compositions via an Olefin Metathesis-Transesterification-Hydrogenation Sequence

O uso sequencial de reações de etenólise, transesterificação e hidrogenação de um óleo renovável modelo para a preparação de potenciais biocombustíveis é reportado. Conversões de até 70% foram obtidas na etenólise de óleo de oliva usando uma pressão relativamente baixa de etileno (2 bar) e tolueno como solvente. O óleo de oliva etenolisado foi transesterificado quantitativamente com metanol e, posteriormente, hidrogenado com hidrogênio molecular sobre Pd/C, resultando em novas composições de biocombustíveis, incluindo diferentes frações de biogasolina, bioquerosene e biodiesel.


Introduction
In recent years there has been an increasing interest in the search for new technologies or in the improvement of current ones aiming the development of more sustainable processes.In this context, one of the key areas of interest is the search for new alternatives to fossil fuels, which are nonrenewable raw materials and its reserves are dwindling and bound to end in the future.As a consequence, this situation represents a steady threat to our supply chain, including chemicals, energy and materials.3][4][5][6][7] Although the use of edible oils is considered polemical due to the competing demand for the food source, non-edible oils, known as the second generation feedstock, are promising substitutes for traditional edible food crops. 8efin metathesis with oleochemicals is a wellestablished research field. 6,7,9It is well known that several metathesis catalysts (Figure 1) are not deactivated by substrates with different functional groups, including those with oxygen, enabling the application of vegetable oils and derivatives as substrates. 102][13] Due to the fact that ethylene can be easily obtained from bioethanol, 14 it makes FAME ethenolysis especially interesting for the production of renewable 1-alkenes (α-olefins) with and without the ω-ester group. 15In comparison, the direct ethenolysis of vegetable oils has been less explored. 16,17oth approaches have the potential of providing an interesting platform for the transformation of biomass into renewable chemicals, energy and materials. 13However, despite the good tolerance to functional groups, olefin metathesis catalysts are poisoned by impurities present in small quantities in vegetable oils, which are intuitively supposed to be increased in its derivatives, more specifically in the monoalkyl esters, due to the alcoholysis step.In this context, the utilization of α-olefin mixtures for the preparation of potential biofuels, obtained from direct vegetable oil ethenolysis, has not yet been explored.
The ethenolysis of unsaturated oils produces α-olefin feedstocks with and without the ω-ester group.α-Olefins without the ω-ester group could be used as high yield catalytic reforming source for the preparation of gasoline/ kerosene-type biofuels and direct hydrogenation provides access to the corresponding alkanes (paraffins). 14,18urthermore, α-olefins are important feedstocks for detergents, poly(α-olefins), epoxies, alkyl aromatics, esthetic products, flavors and fragrances. 5,19Biofuels in the range of biogasoline and biokerosene could be obtained by a consecutive transesterification/hydrogenation sequence of the α-olefin mixture. 14,20,21The chemical similarity with fossil gasoline and kerosene would avoid the necessity to adapt existing motors, which, together with the possibility of mixing these biofuels into fossil fuels, allow its introduction into the market in a nondisturbing way. 4 Besides, these biofuels will not contain aromatic and sulfur impurities.Therefore, addressed in this study is the use of olefin metathesis as key catalytic reaction for the transformation of olive oil (chosen as a model unsaturated oil substrate due to its high oleic acid content) into renewable fuels (Scheme 1).This study does not advocate the use of olive oil as a raw material but aims to investigate the possibility of using this catalytic strategy with highly unsaturated oils.

General considerations
All the manipulations involving air and/or moisturesensitive chemicals were performed using standard Schlenk techniques under either argon or nitrogen atmosphere.Toluene (Vetec) was purified by distillation with sodium/benzophenone, and stored over molecular sieves under inert atmosphere.Dichloromethane (Vetec), n-butylvinyl ether (Aldrich), methanol (Vetec), KOH (Vetec), anhydrous MgSO 4 (Vetec), aqueous H 2 SO 4 (Vetec), palladium supported on activated carbon (Pd/C, 5 wt.%,Aldrich), deuterated chloroform (99.8%,Cambridge Isotope Laboratories), ethylene (99.999%,Air Liquide) and hydrogen (99.999%, White Martins) were used as received.The Grubbs first (G1) and second (G2) generation and Hoveyda-Grubbs second-generation (HG2) metathesis catalysts were purchased from Aldrich and used as received.Olive oil (Monde) was purchased from local stores and purified either by passing through an aluminum oxide column or as follows: activated charcoal (5.0 g) was added to a solution of olive oil (30.0 g) in dichloromethane (90 mL) and the resulting mixture was stirred overnight, Scheme 1. Strategy applied to the transformation of unsaturated oils into renewable fuels.followed by filtration through silica and, subsequently, a basic aluminum oxide column.Solvent removal under reduced pressure afforded a slightly yellowish oil.The ethenolysis and hydrogenation reactions were performed in a 300 mL Parr reactor.

Ethenolysis
Typical procedure: prior to use, the purified olive oil (2.00 g) was degassed by five freeze-pump-thaw cycles and then dissolved in 15 mL of toluene.The oil solution was transferred to the reactor vessel under an argon atmosphere.An ethylene pressure of approximately 2.0 bar was applied and the temperature increased to 50 °C.After temperature stabilization, the ethylene pressure was released and the metathesis catalyst added (in 5 mL of toluene).The reactor was pressurized with the desired ethylene pressure and the pressure valve closed.At the end of the reaction, the ethylene pressure was released and 80 μL of n-butylvinyl ether (dissolved in 1.0 mL of toluene) was added.The reaction mixture was stirred for 10 min and then passed through a small silica column.The products were analyzed by 1 H nuclear magnetic resonance (NMR) spectroscopy.

Transesterification
Transesterification was performed according to a two consecutive steps acid-base literature procedure. 22The products were analyzed by 1 H NMR.

Hydrogenation
Hydrogenation reactions were performed using a modified literature procedure. 21Ethenolyzed/transesterified product (10.0g, dissolved in 10 mL of toluene) and 0.5 g of Pd/C were added to the Parr reactor.The temperature was raised to 100 ºC and a 5 bar hydrogen pressure was applied.After stirring the reaction mixture at 200 rpm for 100 min, the solids were filtered off and toluene was removed by fractional distillation.The products were analyzed by 1 H NMR and gas chromatography-mass spectrometry (GC-MS).

H NMR analysis
The 1 H NMR spectra were recorded on a Varian Inova 300 equipment at 300 MHz at ambient temperature, using deuterated chloroform as solvent.The chemical shifts are given in parts per million (ppm) and referenced to the tetramethylsilane (TMS) signal (0.0 ppm).The spectra were recorded on a pulse angle of 45 o , with 64 scans and 1.0 s relaxation time.Experiments varying the relaxation time from 0.3 to 2.0 s resulted in conversion variations of less than 3% (for experimental details about the calculation of conversions see Supplementary Information).

Results and Discussion
In general, reports on the cross metathesis of natural oil derivatives are focused on the use of methyl oleate as substrate. 6,7,11Therefore, we have chosen olive oil to perform our studies due to its high oleic acid content. 2,7,11,23owever, it is worth mentioning that other possibilities of either edible or non-edible oils containing basically the same or even higher amounts of unsaturated chains could be employed as well (Table 1).
The tests for olive oil ethenolysis optimization were performed using commercially available ruthenium alkylidene complexes as catalysts (Table 2).In a first attempt, alumina-purified oil, G1 and 45 bar of ethylene pressure were reacted for 120 min.A considerably low conversion of 14% was obtained (Table 2, entry 1).Increasing the toluene to oil molar ratio from 1:4 to 1:15 resulted in a slight improvement of the ethenolysis conversion (Table 2, entry 2).This effect, most likely due to enhanced ethylene solubilization, has previously been demonstrated to improve FAME ethenolysis. 24urther improvement in the conversion was obtained after decreasing the oil to catalyst ratio from 2000:1 to 100:1 (Table 2, entry 3).These results suggest that small amounts of impurities in the oil deactivate the catalytic species and, therefore, are responsible for lowering the conversion when smaller amounts of catalysts were applied.Previous reports have shown that substrate purification is crucial for the improvement of metathesis conversions. 25For instance, researchers from Materia Inc. recently reported an impressive improvement in the propenolysis conversion of soybean oil FAME upon treatment with Magnesol ® , which allowed the use of very low catalyst loadings. 17In this project, we rather used activated carbon as purifying Vol. 25, No. 12, 2014   agent.After treating the olive oil with activated carbon, instead of alumina, significant conversion improvements were obtained, a clear indicative that activated carbon better removes catalyst-poisoning impurities present in the oil than alumina does.For instance, when the aluminapurified oil was used, a 52% conversion was only possible to be reached after 2 h with an oil to catalyst ratio of 100:1.Treatment of the oil with activated carbon allowed obtaining basically the same conversion after only 30 min with a lower catalyst loading (Table 2, entries 3 and 4).
With the positive influence of both oil purification and substrate solubilization, the catalyst screening was the following parameter to be studied.Both Grubbs first (G1) and second (G2) generation metathesis catalysts show similar performance in the ethenolysis of olive oil under the same experimental conditions (Table 2, entries 5 and 6).However, the Hoveyda Grubbs second-generation metathesis catalyst (HG2) was much less efficient, resulting in a conversion of only 19% (Table 2, entry 7).Similar results for the ethenolysis of methyl oleate were reported by Grubbs and co-workers. 26The olive oil conversion remained constant when the reaction time was varied from 30 to 60 min with catalyst G1 (Table 2, entry 5 and 8), indicating that the ethenolysis reached its equilibrium at or before 30 min.Increasing the ethylene pressure from 1 to 2 bar resulted in the highest conversion of 70% (Table 2, entries 8 and 9), which is attributed to olive oil self-metathesis suppression.A further ethylene pressure increase resulted in slightly reduced conversions (Table 2, entry 10 and 11), which was most likely the result of enhanced ethylene self-metathesis and/or catalyst decomposition, decreasing the conversion of the desired cross-metathesis products. 26he importance of the appropriate purification of the substrate can be highlighted comparing the ethenolysis conversion of an olive oil FAME (synthesized in our lab) and the same reaction using commercial methyl oleate, using the same purification procedure (passing through an alumina column) and similar reaction conditions (Table 2, entries 12 and 13).When the synthesized olive oil FAME was used as substrate a much lower conversion was obtained as compared to the conversion obtained when commercial methyl oleate was used.The same is true when comparing the conversions obtained in the ethenolysis of olive oil and olive oil FAME (Table 2, entries 4 and 12).This observation is attributed to residual impurities remaining in the FAME after the methanolysis step.
Figure 2 shows the 1 H NMR spectra of the substrates and products of each catalytic reaction applied in the current biofuel strategy.The olive oil ethenolysis (Table 2, entry 4) resulted in the appearance of the typical terminal olefinic hydrogen signals between 4.8 and 6.0 ppm (Figures 2a  and 2b).
The α-olefins obtained in the ethenolysis were used as feedstock for biofuel preparation, using sequential transesterification and hydrogenation reactions.The respective methyl esters were obtained after transesterification with methanol following a double step process, as confirmed by 1 H NMR (Figure 2c). 22urther hydrogenation with 5 wt.%Pd/C and 5 bar of hydrogen afforded biofuels with different compositions (Table 3). 21Complete olefin hydrogenation was obtained, as confirmed either by 1 H NMR (Figure 2d) of the product mixture or gas chromatography analysis (Supplementary Information).The biofuels were obtained as mixtures of biogasoline, biokerosene and biodiesel components (Table 3).As expected, higher biogasoline/biokerosene contents were obtained when feedstocks with higher ethenolysis conversions were used.If desired, the biogasoline, biokerosene and biodiesel fractions might be easily separated by fractional distillation.Compositions determined by GC-MS; b numbers between parentheses show the ethenolysis conversion (%) determined by 1 H NMR; c not reported in Table 1.

Table 1 .
8nsaturated fatty acid compositions in edible and non-edible oils8

Table 3 .
Composition of olive oil-based potential biofuels produced after sequential ethenolysis, transesterification and hydrogenation Component C 10 -C 18