Production of Liquid Hydrocarbons from Sorbitol by Reduction with

May 21, 2014 - A reaction system has been studied for the production of C12 and C18 hydrocarbons from sorbitol by reduction with a hydroiodic acid (HI...
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Production of Liquid Hydrocarbons from Sorbitol by Reduction with Hydroiodic Acid Dong-Can Lv, Yun-Quan Liu,* Ben-Bin Zhang, and Duo Wang College of Energy, Xiamen University, Xiamen, Fujian 361102, People’s Republic of China ABSTRACT: A reaction system has been studied for the production of C12 and C18 hydrocarbons from sorbitol by reduction with a hydroiodic acid (HI)−phosphorous acid mixture. The effects of reaction conditions on the yield and selectivity of hydrocarbons have been investigated. It was found that both the increase in the water content and decrease in the HI amount lead to the decrease of the yield and increase of selectivity for hydrocarbons and water exhibits greater effect than HI. The yield of hydrocarbons was found to increase with time but did not change too much after the reaction proceeded for 14 h. A maximum yield of 62.07% for hydrocarbons was achieved at a water content of 34%, a sorbitol/hydroiodic acid/phosphorous acid molar ratio of 1:3:8, and a reaction time of 12 h. The possible reaction mechanism, we think, is the umpolung of the C−I bond and eventually the formation of the C−H bond plus the polymerization of unstable halocarbons, which results in the formation C12 and C18 hydrocarbons.

1. INTRODUCTION Liquid fuels have been receiving more and more attention in recent years because of the unceasing depletion of crude oil reserves and the increasing demand for transportation fuels all over the world. Besides being produced from traditional fossil fuels, liquid fuels can also be made from other sources, such as biomass, via either biochemical or thermochemical conversion. The product of biochemical conversion is usually ethanol or butanol,1,2 while thermochemical conversion generates biooil3−5 or syngas, which can be further converted into gasoline or diesel via catalytic upgrading or Fischer−Tropsch (F−T) synthesis. Until now, however, there still exist a lot of challenges for both of these pathways, although the fields have been explored for decades. Another way to obtain hydrocarbons from biomass is via catalytic conversion of cellulose or its derivatives, such as polyols. Huber et al.6 demonstrated that alkanes, especially C6, could be generated by aqueous-phase reforming (APR) of sorbitol over bifunctional catalysts. The mechanism for the reaction was proposed.7,8 In addition, a strategy for dehydration of sugars to furan derivatives, which subsequently undergo aldol condensation with ketones to form C9−C15 alkanes, was also outlined.9,10 It was reported that APR can be tuned for the production of aromatic compounds and branched hydrocarbons in the gasoline range or less highly branched, longer chain hydrocarbons in the diesel or jet fuel range.11 However, there appeared to be some deficiencies in this approach as a result of complex series reactions involved, expensive catalysts used, and relatively severe operating conditions of high temperature and pressure employed. Robinson12 proposed a novel strategy that could produce valuable hydrocarbons from biomass via several catalytic steps. Biomass, such as cellulose or starch, was first hydrolyzed in dilute acid accompanied by simultaneous catalytic hydrogenation to obtain sorbitol. The obtained polyhydric alcohol was then reduced to liquid hydrocarbons and halocarbons by reaction with hydriodic acid with a co-reducing agent, such as phosphorous acid or hypophosphorous acid, which could © 2014 American Chemical Society

simultaneously reduce I2 back to hydroiodic acid (HI). Furthermore, the byproducts, halocarbons, could be converted to alkenes by an elimination reaction with NaOH or KOH in boiling alcohol solution. A distinct feature of Robinson’s approach is that the reaction happened under a relatively mild condition of boiling aqueous solution at atmospheric pressure, which does not need expensive catalysts or uses no catalysts at all. The overall process was thought to be innovative and economical, and the liquid hydrocarbons produced could be economically attractive in the near future.13 Recently, a new coupled electrochemical system,14 in which metal ions capable of converting I2 back to HI instantaneously during the reduction of polyols to hydrocarbons, was developed. Although the entire roadmap from biomass to hydrocarbons for this innovative strategy has been proposed by Robinson et al.,12−14 there is still a shortage of data regarding the effects of reaction conditions on selectivity and yield of hydrocarbons during the reduction of sorbitol with HI. In addition, the reaction mechanism has not yet been studied. Therefore, more work on this new strategy is needed. The purpose of this work is to identify the product distribution profile and further investigate the effects of reaction conditions on the yield and selectivity. In addition, a possible reaction mechanism for sorbitol conversion to hydrocarbons by HI reduction will be proposed to understand the process better.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. The chemical reagents used in this study were all in analytical reagent (AR) grade. The purities of Dsorbitol, phosphorous acid, and methylene chloride were 98, 99, and 99%, respectively. The concentration of hydriodic acid employed was 47%. The experiments were carried out in a 100 mL three-neck flask under atmospheric pressure. In each trial, a specific amount of sorbitol, Received: January 23, 2014 Revised: May 20, 2014 Published: May 21, 2014 3802

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phosphorous acid, hydriodic acid, and deionized water was added to the flask, and the mixture was then heated and kept boiling with reflux. The amount of phosphorous acid added was in excess to ensure iodine completely reduced back to hydriodic acid. To study the effects of reaction conditions on the yield and composition of hydrocarbons and halocarbons, the water content in the system was varied from 32 to 48%, reaction time was varied from 2 to 16 h, and the molar ratio of sorbitol/hydriodic acid was varied from 1:2 to 1:6. During the reaction, the mixture was stirred with a magnetic stirrer at 1000 rpm and the reaction temperature was maintained around 120 °C. After the reaction finished, the mixture was cooled and transferred to a separatory funnel to collect the upper layer (oil phase). The upper layer was then diluted with methylene chloride for gas chromatography−mass spectrometry (GC−MS) analysis. A trace amount of hydrocarbons and halocarbons might exist in the lower layer (aqueous phase) but was neglected because the amount was non-detectable by GC−MS. It is worth mentioning that the halogenated hydrocarbons in the oil phase can be further transformed into alkenes by the elimination reaction.15 The details are that the oil phase was first mixed with a mixture containing 30 wt % KOH in hexyl alcohol solution with a ratio of oil/KOH−hexyl alcohol solution of 1:3, in which halocarbons were converted into alkenes. After the reaction was finished, dichloromethane was added to the liquid mixture to extract hydrocarbons. The hydrocarbon-rich dichloromethane layer was then transferred to a vacuum rotary evaporator to separate the heavier hydrocarbons and C6 alkenes from dichloromethane.16,17 2.2. Analysis Methods. The quantitative and qualitative analyses of products were performed with GC−MS (QP2010 SE) made by Shimadzu, which is equipped with a Rtx-5MS capillary column (length, 30 m; internal diameter, 0.25 mm; and film thickness, 0.25 μm). The column was operated in constant flow mode using helium as the carrier gas. The column temperature was initially maintained at 40 °C for 3 min and then increased to 270 °C at a heating rate of 10 °C/min. The column continued to run at the maximum temperature for 3 min. A total of 1 μL of sample was injected into the injection port in a 100:1 split ratio. The GC−MS sampling occurred after 2 min of solvent delay. Identification of the peaks in the chromatogram was based on the comparison to the standard spectra of compounds in the National Institute of Standards and Technology (NIST08 and NIST08s) library. Because it is difficult to obtain the quantitative analytical results from the products using the existing GC equipment, the area percentage of GC−MS was considered as an approximation for indicating the amounts of various iodoalkanes and hydrocarbons in the oil phase.18 Each sample was analyzed 3 times, and the average was taken as the final result.

Figure 1. Composition of the (a) oil phase product and (b) hydrocarbon product.

It should also be pointed out that the oil phase cannot be used directly in engines as a fuel because of the existence of iodine in it. Instead, the halocarbons have to be removed, which were further converted to alkenes as fuel or dialkyl ethers as fuel additives.18 In our study, iodoalkanes were converted to hexenes by the elimination reaction. The composition of hydrocarbon products obtained from the oil phase is shown in Figure 1b. As seen, none of the iodoalkanes existed in the final products, while 1-hexene and its isomers were detected. The hydrocarbon products include isomers of C6H12, C12H16, C12H18, C12H20, C12H22, C18H24, C18H26, C18H28, and C18H30, all of which can be used directly as fuels. 3.2. Effect of the Water Content. Figure 2 presents the yield and selectivity of hydrocarbons as a function of the water content for the case of a sorbitol/hydroiodic acid/phosphorous acid molar ratio of 1:3:8 and a reaction time of 12 h. As seen, water has a great effect on the reaction. When the water content was below 34%, the yield of hydrocarbons increased with the increase of water because water provides an environment for the ionization of reactants; however, when the water content is varied between 36 and 46%, the increase of water leads to the decrease of the hydrocarbon yield from 46.16 to 9.48%. The optimum water content that could maximize the hydrocarbon yield was 34%, which corresponded to a maximum hydro-

3. RESULTS AND DISCUSSION 3.1. Composition of the Hydrocarbon Products. The composition of the oil-phase products is shown in Figure 1a, which was obtained at the condition of a sorbitol/hydroiodic acid/phosphorous acid molar ratio of 1:3:8 and a water content of 34% for a reaction time of 12 h. As seen, the products from reduction of sorbitol were mainly 2-iodine hexane, 1,6diiodohexane, and isomers of C12H16, C12H18, C12H20, C12H22, C18H24, C18H26, C18H28, and C18H30, which are basically consistent with the results reported by Robinson et al.13 Although the components were almost the same, the composition could be a little different with different operating conditions (e.g., varied with the changing of the molar ratio of reactants). In fact, all of the heavier hydrocarbons produced have an aromatic ring or unsaturated bonds, and the C12 hydrocarbons have the characteristics of low volatility, high degree of branching, cyclic structure, and partial unsaturation, which can be used as gasoline fuel, while the C18 hydrocarbons are likely to have the properties of fuels in the diesel or jet fuel range. 3803

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Figure 2. Effect of the water content on the yield and selectivity of hydrocarbons.

Figure 3. Effect of the molar ratio of sorbitol/hydroiodic acid on the yield and selectivity of hydrocarbons.

carbon yield of 62.07%. No reaction occurred when the water content was higher than 48%. This finding is different from what was observed by Robinson,12 which we believe could be due to the difference in the feed ratio. We also found that the selectivity of hydrocarbons had a rising trend with the increasing of the water content and the products were all hydrocarbons when the water content was 48%; that is, the selectivity was 100% for hydrocarbons. However, the corresponding yield of hydrocarbons at this water content was extremely low (almost zero), which means that too high of a water content in the reaction mixture is not favored by the formation of hydrocarbons. This phenomenon can be explained by the fact that a little or more water added to the system could promote the formation of halocarbons and hydrocarbons but

an excess amount of water may inhibit the conversion of unstable halocarbons to hydrocarbons via umpolung of the C−I bond and polymerization. Thus, there must exist an optimum water amount for the reaction. 3.3. Effect of the Molar Ratio of Sorbitol/Hydroiodic Acid. Several reaction systems with different sorbitol/hydroiodic acid molar ratios were studied under the optimal water content of 34%. The ratios studied include 1:2, 1:3, 1:4, 1:5, and 1:6. The number of moles of phosphorous acid in each system was 2.6 times of that of HI to ensure complete regeneration of iodine back to HI. A similar trend was also observed in section 3.1 when these reaction systems were at different water contents. However, the optimum water content that could maximize hydrocarbons varies slightly with the feed 3804

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Figure 4. Effect of the reaction time on the yield and selectivity of hydrocarbons.

or partially displaced by the hydrogen atoms when reacting with HI. The reaction pathway is depicted in Figure 5, and it

composition. Figure 3 shows the yield and selectivity of hydrocarbons at different molar ratios of sorbitol/hydroiodic acid. These results indicated that the yield of hydrocarbons increased insignificantly, while the selectivity decreased dramatically when the amount of HI increased. This phenomenon may be explained by the fact that more HI could promote the formation of total halocarbons; however, only additional unstable halocarbons would be converted to hydrocarbons via umpolung of the C−I bond and polymerization, which causes a big increase in halocarbons but a relatively small increase in hydrocarbons. When the scales of the y axis in Figures 2 and 3 are compared, it can also be seen that the effect of composition of the feed mixture on the yield and selectivity of hydrocarbons is smaller than that of the water content. 3.4. Effect of the Reaction Time. Figure 4 presents the effects of the reaction time on the yield and selectivity of hydrocarbons at a molar ratio of sorbitol/hydroiodic acid/ phosphorous acid of 1:3:8 and a water content of 38%. As shown, the yield increased gradually with time from 40.47% at 2 h to 62.07% at 14 h but had almost no change after 14 h because equilibrium might be reached. However, the trend for selectivity was opposite the yield; that is, the selectivity decreased steadily from 89.82 to 79.97% when the reaction time was increased from 2 to 18 h. As also seen, the rate of increase in the yield was more than twice the rate of decrease in selectivity. This may be explained by the fact that the reaction rate from sorbitol to halocarbons is much higher than that from halocarbons to hydrocarbons because the substitution reaction of sorbitol with HI is much easier than the umpolung of the C− I bond. In other words, the equilibrium constant for converting sorbitol to halocarbons might be much greater than that from halocarbons to hydrocarbons, which needs to be confirmed by our kinetic study in the future. 3.5. Reaction Pathways. Usually the products would be halogenated alkanes when alcohols react with halogen acid. In this work, however, the existence of 2-iodine hexane suggested that the iodine ion attached to the carbon chain was displaced

Figure 5. Schematic for the production of the C−H bond from the C−OH bond in the carbon chain.

may involve two steps: hydroxyl displaced by iodine ion, followed by iodine ion displaced by proton. Let us take the first hydroxyl group in sorbitol as an example. In the first step, the hydroxyl group is reversibly protonated by the strong acidic HI, which yields oxonium ion. The oxonium ion could spontaneously lose a water molecule, which gives a carbocation intermediate, and the latter would react with iodine ion to form an alkyl iodide product.19 In the second step, the larger radius and lower electronegativity of iodine in strong acidic conditions lead to the umpolung19−21 of the C−I bond; therefore, the iodine atom in the carbon chain becomes electrophilic, which is then attacked by the nucleophilic iodide ions, causing the exchange of the attached iodine atom with hydrogen, thus resulting in the formation of I2 and the C−H bond. The derived reaction process agrees well with a previous study.18 As mentioned before, sorbitol can be generated by the hydrolysis of cellulose accompanied with simultaneous hydrogenation22 and sorbitol has a straight-chain structure with six hydroxyls attached to its six carbon atoms separately. On the basis of the process depicted in the previous paragraph, sorbitol would have been transformed into hexane. However, 2-iodine hexane and 1,6-diiodohexane existed in the oil phase. This 3805

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generated beside hydrocarbons and halocarbons in the reaction process. Yield (%) = (area %hydrocarbons × massoil phase × 182)/ (massinitial sorbitol × 84). Selectivity (%) = [2(area %C12H16/160 + area %C12H18/162 + area %C12H20/164 + area %C12H22/166) + 3(area %C18H24/240 + area %C18H26/242 + area %C18H28/244 + area %C18H30/246)]/[2(area %C12H16/160 + area %C12H18/162 + area %C12H20/164 + area %C12H22/166) + 3(area %C18H24/240 + area %C18H26/242 + area %C18H28/244 + area %C18H30/246) + area %C6H13I/212 + area %C6H13I2/338].

could be due to the reason that these two molecules are relatively stable in the solution environment. Also, C12 and C18 hydrocarbons existed in the product, which accounted for most of the oil phase. The molecular structures of C12 and C18 hydrocarbons obtained from GC−MS are shown in Figure 6.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-592-5952780. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (NSFC) (Contract 21276214). REFERENCES

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Figure 6. Molecular structures of C12 and C18 hydrocarbons.

Even though there is hardly any relevant literature found, we can still infer from the molecular structures of C12 and C18 hydrocarbons, that the polymers may be formed by an unstable C6 intermediate product. However, the mechanism of formation of C12 and C18 polymers is not fully certain, which needs more sophisticated instruments to determine. Thus far, what we know is that the selectivity of hydrocarbons to halocarbons largely depends upon which and how many hydroxyls in polyhydric alcohol were replaced by iodine ions, and this can be tuned by changing the water content, the feed mixture composition, and the reaction time.

4. CONCLUSION A new method for the preparation of heavier liquid hydrocarbons (such as C12H16, C12H18, C12H20, C12H22, C18H24, C18H26, C18H28, and C18H30) in one step from sorbitol has been explored, and a maximum hydrocarbon yield of 62.07% was achieved. It was found that the water content in the feed has the most critical effect on the formation of hydrocarbons and the possible reaction mechanism of forming hydrocarbons is the umpolung of the C−I bond plus the polymerization of unstable halocarbons. Our future work will be focusing on the study of reaction kinetics and electrochemical regeneration of I2 back to HI, so that a more economical and environmentally friendly method can be developed.

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APPENDIX Experiments in Figure 2 were conducted for the case of sorbitol/hydroiodic acid/phosphorous acid molar ratio of 1:3:8 and a reaction time of 12 h. Experiments in Figure 3 were conducted under a water content of 34%. Experiments in Figure 4 were conducted at conditions of sorbitol/hydroiodic acid/phosphorous acid molar ratio of 1:3:8 and water content of 38%. It is assumed that there are no other products 3806

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(15) McMurry, J. Organic Chemistry; Cengage Learning: Stamford, CT, 1999; pp 232−233. (16) Robinson, J. M.; Banuelos, E.; Barber, W. C.; Burgess, C. E.; Chau, C.; Chesser, A. A.; Garrett, M. H.; Goodwin, C. H.; Holland, P. L.; Horne, B. O.; Marrufo, L. D.; Mechalke, E. J.; Rashidi, J. R.; Reynolds, B. D.; Rogers, T. E.; Sanchez, E. H.; Villarreal, J. S. Chemical conversion of biomass polysaccharides to liquid hydrocarbon fuels and chemicals. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1999, 44, 224−227. (17) Gordon, R. D. Preparation of ethers. U.S. Patent 3,914,320 A, Oct 21, 1975. (18) Mitchell, H. K.; Williams, R. J. A study of reduction with hydriodic acid: Use in micro determinations of hydroxyl groups. J. Am. Chem. Soc. 1938, 60, 2723−2726. (19) Seebach, D. Methods of reactivity umpolung. Angew. Chem., Int. Ed. Engl. 1979, 18, 239−258. (20) Igarashi, T.; Tayama, E.; Iwamoto, H.; Hasegawa, E. Carbon− carbon bond formation via benzoyl umpolung attained by photoinduced electron-transfer with benzimidazolines. Tetrahedron Lett. 2013, 54, 6874−6877. (21) Chan, A.; Scheidt, K. A. Conversion of α,β-unsaturated aldehydes into saturated esters: An umpolung reaction catalyzed by nucleophilic carbenes. Org. Lett. 2005, 7, 905−908. (22) Fukuoka, A.; Dhepe, P. L. Catalytic conversion of cellulose into sugar alcohols. Angew. Chem., Int. Ed. 2006, 45, 5161−5163.

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