Blending Real World Gasoline with Biofuel in a Direct Conversion

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Blending real world gasoline with biofuel in a direct conversion process Edward Nürenberg, Philipp Schulze, Frank Kohler, Marius Zubel, Stefan Pischinger, and Ferdi Schueth ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03044 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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ACS Sustainable Chemistry & Engineering

Blending real world gasoline with biofuel in a direct conversion process Edward Nürenberg†, Philipp Schulze†, Frank Kohler†, Marius Zubel‡, Stefan Pischinger‡ and Ferdi Schüth†* †Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany. * [email protected] Tel:

+49 208 306 2373, Fax: +49 208 306 2995 ‡Institute for Combustion Engines (VKA), RWTH Aachen University, Forckenbeckstraße 4, 52074 Aachen

Keywords: Drop-in biofuels, 2,5-dimethylfuran, 5-hydroxymethylfurfural, PtCo, hydrodeoxygenation, real world applicable fuel blends, fuel characteristics.

ABSTRACT

ACS Paragon Plus Environment

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A method to produce the biofuel 2,5-dimethylfuran (DMF) from cellulose-derived 5-hydroxymethylfurfural (HMF) by hydrodeoxygenation (HDO) using commercial gasoline as solvent to obtain mixtures of gasoline with DMF, appropriate for direct use in present internal combustion engines, is presented. Best results were obtained with gasoline:ethanol mixtures in the ratio 9:1(E10), as ethanol acts as solvent mediator for the dissolution of HMF. Selected potential biofuels are also found to give high DMF yields e.g. several alcohols (81 - 92%) and 2-butanone (94%), while -valerolacton and saturated hydrocarbons show limitations (75 and 37%, respectively). The reaction in gasoline is conducted sequentially up to three times with an initial loading of 10 wt% HMF per step, resulting in a concentration increase of up to 7 wt% DMF for each step, by which a concentration range between 7 and 20 wt% DMF in the final blend is covered. The obtained blends were evaluated by the determination of the derived cetane number (DCN) and a simulated distillation with comparison to premixed blends and proved to be comparable in a wide concentration range of DMF (5-15 wt%). Thus, a potentially directly usable fuel blend is produced in a direct conversion process without the need of costly separation.

INTRODUCTION The tremendous demand on chemical fuels cannot be sustainably covered by biofuels alone, thus the addition of drop-in biomass-derived fuels to the currently dominating fossil fuels while maintaining the existing fuel-infrastructure is a promising approach to reduce greenhouse gas (GHG) emissions from the transportation sector,1 which significantly contributes to the global GHG emissions (26%).1, 2 Currently ethanol and fatty acid methyl esters are used in low concentrations of around 5-10% as drop-in fuel additives for gasoline and diesel, respectively. The implementation of any new biofuel component can only be successful if two general conditions are met: 1. Sustainability should be maintained along the entire process chain of biofuel production, including the cultivation of bio-resources, their harvesting, transport and refinement, the use of land, water, energy, indirect land use change. Finally, the total greenhouse gas emissions have to be taken into account.3-5 According to optimistic calculations, up to 30 % of the worldwide used transportation fuels


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could be sustainably replaced by biofuels, while in the worst case scenario biofuels are found to be not suitable for the sustainable replacement of fossils at all.6 2. Economic competiveness of the biofuel is essential. Along with this point, the implementation in the current automobile vehicle technology and infrastructure without the need of major changes has to be ensured.7 Various concepts are described to produce different biofuels, including furanics, from non-edible and largely available cellulose.8-11 Among those, a promising drop-in biofuel candidate, derived from cellulose via HMF12 is 2,5-dimethylfuran (DMF), see Scheme 1. Its properties regarding energy density, volatility, water miscibility and combustion are closer to gasoline, making it superior compared to the currently established biofuel ethanol.13, 14 Issues might be the formation of gum and a relatively high peroxide number.15,16 Combustion studies in a direct-injection-spark-ignition engine showed that pure DMF can in principle be used as fuel, with minor technical modifications of the engine.17, 18 However, using DMF as an additive is stated to be more attractive,19-21 because compatibility problems can be minimized and strict technical regulations22, 23 can be fulfilled while still meeting a large market due to the vast overall fuel consumption.24 McCormick studied blends of gasoline with 13-16 wt% DMF regarding various physical, combustion and emission properties, including octane rating, volatility, viscosity, water exposure, and found them to be beneficial compared to pure gasoline.15,

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Lubrication properties of

gasoline/DMF blends of 2-20 vol%26 and gasoline/DMF/ethanol blends of various concentrations27 were found to improve wear resistance and reduce friction. Rothamer measured the knocking propensity of gasoline with 5, 10 and 15 vol% DMF and 10 vol% DMF/10 vol% ethanol and reveal all of them to reduce the knocking intensity while the latter blend performed best.28 The synthesis of DMF starting from cellulose is presented in Scheme 1. It involves the depolymerisation of cellulose to glucose, isomerization to fructose, dehydration to 5-hydroxymethylfurfural (HMF) and final hydrodeoxygenation (HDO) of HMF to DMF. Intermediate products are 2,5-di(hydroxymethyl)furan (DHMF) and 2-methyl-5-hydroxymethylfuran (MHMF), see section I. Side products are shown in section II, they involve products from the hydrogenation of the furan ring, ring cleavage with an optional addition of water and hydrogenation,29 the dimerization product of DMF30 (detected by GC-MS, see SI Figure S4) and humins from unspecific polymerization.31


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Scheme 1: Reaction pathway for conversion of cellulose over sugar intermediates to HMF and detailed reaction course of the reaction HMF to DMF in section I. Possible byproducts are shown in section II. The pathway to DMF starting from cellulose over HMF, fructose or glucose has extensively been studied and reviewed.32-36A high dilution of the starting material is necessary, as HMF tends to form polymeric byproducts, humins.31, 37 Consequently the resulting biofuel has to be separated from the reaction solvent by distillation, which is known to be very energy intensive.38 Addressing this problem, we propose a reaction system using real world gasoline, a complex mixture of hydrocarbons and ethanol, as reaction medium to produce a potentially readily usable biofuel blend, which has been investigated previously and proven to be a suitable motor fuel for current internal combustion engines.

EXPERIMENTAL


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The PtCo-alloy catalyst supported on graphitized carbon (PtCo/GC) was synthesized according to a wet impregnation procedure, which is a slightly modification of a published procedure.39 Catalytic experiments were carried out in magnetically stirred homemade stainless-steel batch-autoclaves with volumes of 25 mL and 45 mL and external heating. HMF (98 %) from AVA-Chemicals, Switzerland, PtCo/GCcatalyst and solvent were weighted into the autoclave. The autoclave was then purged for three times with 10 bar of H2 to remove air, afterwards purged to the desired pressure and inserted into a heating block which was pre-heated to reaction temperature. 1-Butanol (99 %) from Alfa Aesar, 1-hexanol (98%) from Fluka, 2-butanone (99 %), ethanol (99.5 %), 2-butanol (99 %), 1-octanol (99%), aliphatin (mixture of linear saturated C10 – C14 hydrocarbons), -valerolacton (99%) and toluene (99.8%) were purchased from Sigma Aldrich and used without further purification. Commercial gasoline with a research octane number of 95 (RON 95) and an ethanol content of approximately 5% (E5) was purchased from a local gas station (ARAL, Mülheim an der Ruhr, Germany) in summer and stored in a cold (-18°C) and dark place, for detailed specifications by GC and NMR see SI Figure S5 and Figure S11, respectively. To remove ethanol from gasoline, 100 mL gasoline were extracted three times with 50 mL of water and finally filtrated over activated molecular sieve (3 Å). Products were quantified by GC, the gasoline samples were additionally analyzed by NMR. Measurements of the derived cetane number (DCN) were carried out by the Advanced Fuel Ignition Delay Analyzer (AFIDA). For detailed description see the SI, section 10. Simulated distillation was performed with an Agilent 7890 gas chromatograph with a programmed temperature vaporizing injector (KAS4, Gerstel GmbH, Germany) equipped with an unpolar in-house coated OV-1 column (50 m, 250 µm I.D.), for further details see SI, section 11. Sequential enrichment studies were performed in a magnetically stirred 200 mL autoclave with external heating. The autoclave was loaded as described above. Although gasoline-like smell was observed during the purging process, no mass loss above 0.05 g was detected. After the first conversion the autoclave was cooled on ice and depressurized. First both water/ethanol and gasoline phases were separated from the solid catalyst by centrifugation. Liquid phases were separated in a separation funnel and filtrated through a syringe filter if needed. Optionally the gasoline phase was washed two times with 25 mL deionized water to remove any containing ethanol. GC and NMR samples were taken and the gasoline phase was loaded with 10 wt% HMF and optionally ethanol. After each step the mass of the


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gasoline phase was found to be reduced by approximately 10 wt% due to practical handling losses - the amount of other reaction components was adjusted to the reduced mass of gasoline, while the change in gas/liquid ratio in the autoclave was neglected. Reaction products dissolved in commercial gasoline were analyzed using an Agilent 6890 gas chromatograph equipped with a split/splitless injector and a flame ionization detector. The samples were measured on a DB-Waxetr column (Agilent, 30m, 250 µm I.D., 0.25 µm film thickness). Peak assignment of HMF, DHMF, MHMF and DMF was conducted using standard addition; all other peaks were assigned via GC-MS. Analysis of the water phase was performed with an Agilent 6890 gas chromatograph equipped with a split/splitless injector and a thermal conductivity detector. He carrier gas and a poly ethylene glycol coated CW-20M GC column (CS-Chromatographie, 28m, 25 µm I.D., 0.25 µm film thickness) was used. NMR spectra were recorded with a Bruker Nanobay 300A Spectrometer at 300 MHz. 1,4-Dioxane was used as an internal standard for the quantification of DMF in gasoline, for details see SI, Figure S11.

RESULTS AND DISCUSSION The hydrodeoxygenation (HDO) of HMF to DMF was catalyzed with a platinum/cobalt bimetallic alloy supported on graphitic carbon (PtCo/GC) developed in our group,39 as it is known to be highly active and selective, and thus does not lead to overhydrogenation nor to ring-opening.29, 40 The study includes the screening of different solvents to identify potential fuels in which DMF could efficiently be produced. From several solvents tested, real world gasoline was chosen for further studies in order to maximize the initial concentration of the reactant. To reach the envisaged DMF concentration of 20 wt% the reaction was performed sequentially in up to three steps. Selectivity of the catalyst is crucial at this point and justifies the use of expensive Pt, as the concept to reach high concentrations of DMF involves the conversion of HMF in gasoline already containing DMF. Finally, the prepared blends were evaluated regarding their combustion properties by determination of the derived cetane number (DCN) and the boiling range distribution.


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Solvent system. The hydrodeoxygenation of HMF to DMF was studied in different solvents in order to determine a suitable type of solvent media and explore limits of the PtCo/GC catalyzed reaction system. Solvents were chosen based on their applicability as fuels (gasoline, diesel) or biofuels.14 The initial concentration of HMF was set at 5 wt% and the hydrogen pressure was kept at 10 bar.39 The results of the solvent screening are shown in Figure 1.

Figure 1: Screening of solvent media. Reaction conditions: 2 mmol HMF 5 wt%, 50 mg catalyst (1 mol% Pt regarding HMF), p= 10 bar H2 (initial), V= 45 mL, T= 180 °C, t= 2 h, aliphatin= linear saturated hydrocarbons C10-C13. All solvents except of 2-butanone remain inert at the reaction conditions. The catalytic system is found to be highly flexible with respect to solvent choice. Solvents with a relatively low polarity, such as aliphatin (C10 – C13 linear saturated hydrocarbons) and diesel fuel (hydrocarbon mixture C8-C25) are least suitable (37 and 57% DMF yield, respectively), as the starting compound HMF poorly dissolves in these and forms humins before being converted to DMF. The tested alcohols (ethanol, 1-butanol, 2-butanol, 1-hexanol and 1-octanol) showed to be highly suited for the reaction giving high DMF yields between 88 – 95 %, with the exception of 1 octanol (83%); here also solubility problems may occur. 2-Butanone is itself a very promising biofuel candidate,41 and as the only ketone tested gave excellent yields (95 %), but was partly hydrogenated to 2-butanol (8%). This may


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not be an issue as 2-butanol is also considered as a biofuel. -Valerolactone was not highly suitable as solvent medium under the applied reaction conditions, as the DMF yield was slightly lower (75%). Toluene and commercial RON 95 gasoline lead to very good yields of DMF, 92 and 95 % respectively. Except of 2-butanone none of the single-component solvents underwent chemical transformation according to GC analysis of the liquid phase. Additionally, the consumption of hydrogen was measured by the pressure drop after the reaction and approximated by the ideal gas law. It was found to be in good accordance with the yields determined by GC. In case of gasoline RON 95 the consumption of hydrogen slightly exceeded the required amount for HDO. It was estimated that approximately 1% of the solvent might be hydrogenated, see SI Table S1. However, due to the complexity of the solvent this effect could not be analyzed in detail by GC. In comparison experiments in absence of HMF and harsher reaction conditions (15 bar H2, 2 h) hydrogenation of naphthalene and methylnaphtalenes was observed (see SI Figure S8 and S9), while benzene derivatives remain inert. This is not unexpected, as naphthalenes are known to be more reactive towards hydrogenation than aromatic six-rings. However, under reaction conditions no significant hydrogenation of this or any other compounds was detected. A partial hydrogenation of gasoline would lead to a fuel with a higher heating value and thus is not considered to be highly problematic. Moreover, hydrogenation of aromatic components might be even desired, because reduction of the amount of aromatics and especially naphthenic rings in fuel mixtures decreases the formation of soot.42, 43

If this step is carried out along the gasoline production chain, it might be discussed as a reasonable integration point to add biofuel

components by direct conversion at this stage. Although the used PtCo/GC catalyst does not lead to substantial hydrogenation of aromatics, which was not desired in our study, the reaction conditions might be similar. Based on this data set, it can be concluded that the PtCo/GC-catalyzed HDO of HMF to DMF system is highly flexible towards solvent media, as long as the reactant HMF can be properly dissolved. This allows the addition through conversion of DMF to a broad variety of (bio)fuel blends, whose properties might be estimated in advance by experiment or simulation.44 However, further studies are focused on the synthesis of DMF in RON 95 gasoline in order to gain a fuel blend with up to 20 wt% DMF.


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Reactant concentration. In case of full conversion 1 wt% of 5-HMF will lead to 0.76 wt% of DMF due to detachment of water. Ideally, one should use a high starting concentration of HMF in order to achieve reasonable blend concentrations. However, raising the concentration of HMF is challenging, as this results in the formation of humins and tar. Figure 2 shows the yield of DMF at different initial concentrations of starting material HMF. The pressure was raised to 25 bar, to make sure that enough hydrogen is present in the batch autoclave to perform the HDO also at high initial concentrations of HMF. Other reaction parameters were kept constant. The concentration of platinum, which is the most valuable part of the bimetallic supported catalyst, is kept constant in relation to HMF at 0.4 mol% Pt per mol of HMF.

Figure 2: Yields of DMF in the gasoline phase at different initial HMF concentrations. HMF conversion= 100 %. Conditions: 5 g reaction mixture composed of gasoline:ethanol in a ratio of 9:1 (E10) and the respective amount of HMF, 0.4 mol% Pt per mol HMF , p= 25 bar H2 (initial), T= 185 °C, V= 25 mL, t= 2 h, 600 rpm, yields by GC. = resulting black solid formed at initial HMF concentrations above 15 wt%. In all cases full conversion of the reactant HMF was monitored. High yields of DMF were achieved at HMF concentrations of 5 and 10 wt%, 98 and 78 %, respectively. Many of the possible byproducts and intermediates cannot be quantified at low concentrations, due to the high complexity of the gasoline mixture. At higher HMF starting concentrations of 15 and 20 wt%, the yield of DMF sharply decreased to give 19 and 7 %. Additionally some intermediate products were identified (12 and 4% of 2-methyl-5-hydroxymethylfuran, MHMF),


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indicating that the reaction was not complete, see scheme 1. Formation of large amounts of black solid was observed at initial HMF concentrations above 15 wt%. This could be explained by the formation of a substantial water phase during HDO in which HMF is present in high concentrations and thus forms substantial amounts of polymeric humins as byproducts. In general a lower concentration of the starting material HMF leads to better selectivity towards DMF, because side reactions causing the formation of humins proceed slower. Humins are not monitored by the analytical methods used in this study (GC, NMR) and thus do not appear in the overall mass balance. For practical application, a lower reactant loading could be beneficial to reach high selectivity and reduce losses by humin formation. A high overall DMF concentration could still be reached by repeating the reaction multiple times in the same solvent batch – a concept described in the following section. The studies described in the following were carried out at an initial HMF concentration of 10 wt%, as this was found to be a compromise between acceptable yields of DMF and a sufficient initial loading of the reactant HMF. Sequential enrichment. Since the starting concentration of HMF cannot be raised above 10 wt% it is impossible to produce a fuel blend with more than 7.6 wt% DMF in a single reaction step. The envisaged DMF concentration of 20 wt% can, however, be reached by conducting the reaction in a sequential manner (Figure 3): first HMF catalytically reacts in gasoline to DMF, which results in a biphasic mixture of gasoline and DMF in the organic phase and an aqueous phase formed by the water produced during the HDO reaction. Then the phases are separated: the organic phase is isolated from water and solid catalyst (1st). Recycling of the catalyst was not part of the study at this stage. Water is removed to avoid a high concentration of HMF in the water phase which would lead to increased formation of humins. The reaction is repeated after the gasoline/DMF blend is loaded with fresh HMF and catalyst (2nd step). Finally, water and catalyst are removed to gain the desired biofuel blend. The possible third step is omitted for clarity. In principle this procedure can be repeated as often as required. Problems which might arise are the correct quantification of DMF, if the reaction mixture is already highly concentrated with this product.


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Figure 3: Concept of sequential enrichment of gasoline with DMF. Results of the sequential enrichment of DMF in gasoline. For fuel quality tests described in the final section 5, appreciable amounts of the fuel are required. For the sequential enrichment, the reaction batch was thus scaled up. The catalyst concentration was lowered to 0.1 mol% platinum per mol of HMF, while the reaction time was increased to three hours to achieve reasonable yields of DMF, conditions which had been found during optimization studies, see SI, Figure S13. The hydrogen pressure was adjusted to 35 bar to provide a sufficient amount of hydrogen in the batch autoclave, see SI Figure S14. Figure 4 shows the DMF yields for each enrichment step of the commercial RON 95 gasoline as well as the concentration of DMF in the resulting fuel blend. Lowering the catalyst concentration led to DMF yields of 77 % corresponding to 6 wt% concentration of DMF in the resulting fuel blend after the first reaction step. The second conversion step yielded 74 % DMF leading to a concentration increase to 12 wt% DMF. Unexpectedly, the DMF yield decreased substantially in the third enrichment step (41 %). This can be explained by the change in ethanol concentration: In this reaction system a low amount of ethanol, which is present in RON 95 gasoline in a concentration of approximately 5 vol%, is crucial for the HDO of HMF to


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DMF. Ethanol acts as a solvent mediator for the dissolution of HMF in non-polar gasoline. Since water separation took place after each step, ethanol was extracted and thus removed from the reaction system. GC analysis shows the water phase to contain 68 ± 2% ethanol in the first step as well as a decrease of the ethanol concentration in the gasoline phase, see SI Figure S10. This can explain the low yields of DMF in the third step. To prove this conclusion, the reaction was carried out with thoroughly water-extracted gasoline with no detectable ethanol and was found to give low yields of DMF (20%).

Figure 4: Results of subsequent blending reactions. HMF conversion= 100 %. Conditions: 45 g gasoline RON 95, HMF 10 wt%, PtCo/GC catalyst: 0.1 mol% Pt regarding HMF, p= 35 bar H2 (initial), V= 200 mL, T= 185 °C, t= 3 h, 600 rpm, yields by NMR. Therefore, to ensure constant performance of the reaction system, the ethanol concentration in the reaction mixture needs to be kept constant. For further studies the amount of ethanol present in the reaction mixture was controlled precisely at 10 % by weight, resulting in a fuel blend commonly known as E10. In order to achieve this, the RON 95 gasoline was extracted with water and ethanol was added. After each reaction step the mixture was again extracted with water, and ethanol was added in addition to HMF and fresh catalyst. Traces of gasoline components and DMF could be found in the aqueous extract, see SI Figure S7. Of course in the present setup we are removing one biofuel (ethanol) to replace it with another, DMF – in technical application


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one could remove the HDO-water by adsorption on e.g. zeolite A molecular sieves, which could be regenerated afterwards.45 In this case ethanol would remain in the organic phase and could act as solubility mediator in the next enrichment step or remain in the fuel as blending component. Optionally, a phase separation with subsequent recovery of ethanol, which is present in the aqueous phase in a high amount of 68 %, by distillation can be considered. The results of the sequential enrichment of E10 gasoline with DMF are shown in Figure 5. In addition, the amount of catalyst was increased (0.23 mol% Pt per mol HMF) to achieve higher yields, for optimization data see SI Figure S13. As expected, due to sufficiently high concentration of ethanol in the reaction media, no substantial change in activity towards the main product DMF was detected over the three subsequent reaction steps, showing only a slight decrease in DMF yield from 93 over 84 to 80 %, and resulting overall in DMF concentrations of 8, 14 and 20 wt%, respectively. The decrease of the HMF yield by a few percent might be explained by slow over-hydrogenation or dimerization of DMF as well as by losses due to the extraction process. However, quantitative statements are not possible, since these products could not be quantified in the complex gasoline mixture. Moreover, the presence of byproducts in small quantities will not necessarily have much impact on the quality of the fuel blend, as natural gasoline is known to be a mixture with a large number of different substances, whereas individual relatively low concentrated components do not significantly affect the overall fuel performance, as long as the overall fuel standards are met.7 Evaluations of the performance of the so produced blends are presented further below. For practical applications small scale impurities of course have to be investigated more thoroughly.


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Figure 5: Results of subsequent blending reactions with a controlled ethanol concentration. HMF conversion= 100 %. Conditions: 40 g gasoline E0 RON 95, 5 g ethanol, 10 wt% HMF, PtCo/GC catalyst: 0.23 mol% Pt regarding HMF, p= 35 bar H2 (initial), V= 200 mL, T= 185°C, t= 2h, 600 rpm, yields by NMR. Recycling of catalyst. For studies on reusability of the PtCo-alloy catalyst supported on graphitic carbon, conditions giving an initial yield of 56 % DMF were chosen. The results are shown in Figure 6. After each run the catalyst was separated from the biphasic reaction mixture by centrifugation, washed with ethanol, methyl-tert-butylether and dried at 50°C in vacuum overnight before being used in the next run. Recycling of the catalyst reveals a drop in activity already after the first run, giving half of the initial yield in the second run (27 % DMF). This trend proceeds in the runs three and four, which show yields of 13 and 5 %, respectively. Interestingly, PtCo supported on hollow carbon spheres (HCS) has shown better recycling ability.39 In this study PtCo-alloy is used on a different support. Better recyclability might be achieved by the variation of the support material. The mass of used catalyst was 12 % higher compared to the initially loaded, even though handling losses were present. This suggests the formation of carbonaceous species, which adsorb on the carbon-based catalyst. Further investigations have to be carried out to reveal the mechanism of the catalyst deactivation.


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Figure 6: Results of recycling runs. Conditions: 4.5 g solvent mixture composed of gasoline:ethanol in a ratio of 9:1 (E10), 10 wt% HMF, 0.2 mol% Pt per mol HMF , p= 25 bar H2 (initial), T= 185 °C, V= 25 mL, t= 2 h, 600 rpm, yields by NMR. Fuel blend quality - Derived Cetane Number (DCN). In order to evaluate the quality of the obtained fuel blends in comparison with premixed blends, the auto ignition resistance properties were measured. The auto ignition quality can be described by the cetane number (CN) and is a key property of diesel as well as gasoline-like fuels.46 Relevant for the gasoline-like fuels considered here, is the resistance against engine knock, described by the research octane number (RON). CN and RON are empirically inversely correlated,47, 48 thus qualitative statements of the RON of a test fuel can be made by the determination of the CN. The determination of the CN49 and RON50 in standard test motors requires a relative large amount of test fuel. Calculation of the ignition resistance using only 40 mL of test was possible by the advanced fuel ignition delay analyzer (AFIDA). The AFIDA is a measuring device to determine the ignitability of diesellike fuels by the constant volume combustion chamber method (CVCC).51 This yields the derived cetane number (DCN), but is still well comparable to the CN obtained in a classical way.52 The resulting derived cetane numbers of the biofuel blends obtained by reaction (enriched gasoline) and premixed blends (reference blends) are shown in Figure 7. As expected the fuel’s resistance against auto ignition increases with increasing DMF content since the DCN of DMF is 10.952 (RON= 11953). Here the DCN of the fuel blend can be roughly derived from the linear blending rule.47 The DCNs of


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the synthesized fuel and reference fuel are in good accordance at low concentrations of 5 to 15 wt% DMF in gasoline, but deviate at higher concentrations. This behavior could be explained by the enrichment of byproducts in low concentrations during the enrichment, or by additional reactions occurring in the complex gasoline matrix. However, the DCN of the mixture of gasoline with DMF blended by direct conversion is found to be even lower than that of the premixed blend. Since a low DCN results in a high RON, this deviation is not regarded as problematic, as specifications for gasoline-like fuels do not include an upper level for the research octane number; the higher, the better is the knock resistance.54 This emphasizes the high potential of this method to produce premixed fuels with a high knock resistance.

Figure 7: Derived cetane numbers of fuel blends obtained through direct conversion a premixed reference blends Boiling range distribution. Volatility of automotive fuel is crucial for proper combustion and therefore highly affects engine performance.55 The volatility of a complex hydrocarbon mixture for use as gasoline is described by the boiling range distribution and is subject of regulations by standardization norms in the European Union (EN 228) and the United States (ASTM D4814). The boiling range distribution of the produced fuel blend was assessed by gas chromatography. A series of n-alkanes between C5 to C20 was injected via a cool injection system (KAS 4 from Gerstel) with a programmed temperature vaporizing method on a non-polar, low interacting column (OV-1). Fuel blend samples were measured similarly; peaks in the retention time around a certain alkane were manually combined to peak-groups.


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For detailed method specifications see SI, part 11, Figure S15 and S16. It has to be pointed out, that this procedure is not a standard method neither for distillation, nor for simulated distillation. It is applied in a comparative way to reveal differences between synthesized and premixed blends, but statements concerning fuel standards should be evaluated carefully. Results of the simulated distillation of a premixed reference blend containing 20 wt% DMF, synthesized blends with DMF concentrations of 20 wt% (DMF20), 14 wt% (DMF14), 7 wt% (DMF7) and pure gasoline are shown in Figure 8. As an accurate determination of the concentration of ethanol is not possible in gasoline mixtures, it was extracted with water for all measurements. Synthesized blends with DMF concentrations of 7 and 14 wt% will not be discussed in detail, as their observed characteristics go in line with the enriched blend with the highest DMF concentration of 20 wt%. For tabulated data see SI, section 11 Table S2. DMF20 synthesized by direct conversion contains less light-boiling components (12.4 %) in comparison to the reference and pure gasoline (18.6 % and 22.1 %, respectively). This can be explained by evaporation of lightboiling compounds during the three-step enrichment procedure. This deviation does not conflict with fuel-specification norm EN 228, which requires 20-50% boil-off at 70 °C, but most probably might be avoided when working under technical conditions in a closed system. DMF can clearly be seen in the boiling range group of n-heptane (bp = 98 °C). Shares of DMF of up to 20 wt% do not conflict with EN 228, as at least 75% should have been evaporated at 150 °C which holds for the synthesized blend DMF20 (81.3 %) and the reference blends (85.7 % for the premixed blend and 83.3 % for pure gasoline). EN 228 requires complete boil-off after a temperature of 210 °C, this is not achieved by all tested fuels, as none of them is completely evaporated in the range of n-dodecane (bp = 216 °C). The synthesized blend DMF20 deviates from EN 228, with only 97.1 % boil-off (premixed blend = 99.7 %, pure gasoline = 99.7 %, DMF7 = 98.6, DMF14 = 97.8) but still might be suitable for use according to norm ASTM D4814, which demands a complete boil-off after 225 °C. In course of the enrichment of gasoline with DMF, low amount of the high-boiling dimer of DMF is formed, see SI Figure S4 and S16. The boiling point resolution of the used method is limited by the injected comparative substances. Thus, it cannot be concluded with full certainty at which exact temperature complete boil-off is reached and whether the directly synthesized fuel blend DMF20 could meet the boiling point


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specifications of ASTM D4814. However, both methods allow a distillation residue of 2 vol%, meaning the synthesized blends could still remain in this range, although further investigations on this subject have to be carried out to prove this tentative presumption.

Figure 8: Left: Integrals of peak groups expressed by GC area at retention times of corresponding alkanes. *= temperature is derived from the boiling points of respective n-alkenes. Right: Cumulative expression of data on the left to generate a simulated distillation curve of a premixed blend (blue triangle up), synthesized blends obtained in the enrichment procedure shown in fig. 5 (green triangle down: 20 wt% DMF, black triangle left: 14 wt% DMF, black triangle right: 7wt% DMF and yellow square: pure gasoline). Hatched area: limits of boiling range according to EN 228. **= boil-off constructed by cumulative addition of the peak groups. All lines are guide for the eyes. CONCLUSION AND OUTLOOK A concept for efficient production of biofuel blends with conventional fuels was developed, which relies on direct transformation of the biofuel precursor in the conventional fuel as solvent, in order to reduce energy for separation. The concept was successfully demonstrated for the HDO reaction of HMF to the biofuel DMF in conventional E10 gasoline to produce blends with concentrations of DMF of up to 20 wt%. The maximum initial concentration of the starting material HMF is limited to 10 wt% due to massive formation of humins/coke at higher concentrations. This problem was solved by performing the reaction sequentially up to three times, whilst removing the water from the previous step and addition of reactant and catalyst for the following step in each cycle. This led to a stepwise enrichment of gasoline


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with DMF. Crucial for good overall performance was the presence of ethanol in the reaction system to ensure dissolution of HMF and therefore preventing the formation of humins during the course of the reaction. Taking this into account the ethanol concentration in gasoline was kept at 10 wt%, corresponding the commonly known biofuel blend E10. Auto ignition resistance properties of the produced fuel blends were compared to those of premixed blends by the determination of the derived cetane number and were found to be in a good accordance over a broad concentration range from 5 to 15 wt% DMF. However, the derived cetane numbers differ at higher concentrations (20 wt% DMF). Provisional determination of the boiling range distribution has shown, that the produced fuel blend contains small amounts (< 3%) of high boiling components, but still might be compatible with the fuel volatility norms EN 228 and ASTM D4814. While in the current study the general feasibility of the approach has been demonstrated, further analysis of the fuel quality is required, especially regarding the levels of soluble gums56 and the boiling point distributions to estimate the suitability of the produced biofuel blends for the direct use in internal combustion engines.15 In addition, it is highly desirable to expand the reaction system towards fructose and glucose as starting materials, as HMF is currently too valuable to be used as fuel precursor. Fructose should be dehydrated to HMF,57 while glucose has to undergo a Lewis-acid catalyzed isomerization step58 before being converted to fructose or directly to HMF.59 Both reactions can proceed in ethanol - thus this reaction stream can principally be fed into the HDO system discussed here, which also contains ethanol as solvent component. Subsequently, the catalytic system studied in batch mode has to be transformed in an flow system, inevitable for technical applications.60, 61 ACKNOWLEDGEMENT We are very grateful to Silvia Palm for EDX measurements and to Dino Richter for mass spectroscopy measurements. This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities.


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59. Mun, D.; Huynh, N. T. T.; Shin, S.; Kim, Y. J.; Kim, S.; Shul, Y.-G.; Cho, J. K., Facile isomerization of glucose into fructose using anion-exchange resins in organic solvents and application to direct conversion of glucose into furan compounds. Research on Chemical Intermediates 2017, 43 (10), 5495-5506. 60. Braun, M.; Antonietti, M., A continuous flow process for the production of 2,5-dimethylfuran from fructose using (non-noble metal based) heterogeneous catalysis. Green Chemistry 2017, 19 (16), 3813-3819. 61. Luo, J.; Arroyo-Ramírez, L.; Gorte, R. J.; Tzoulaki, D.; Vlachos, D. G., Hydrodeoxygenation of HMF over Pt/C in a continuous flow reactor. AIChE Journal 2015, 61 (2), 590-597. ASSOCIATED CONTENT Supporting Information 16 Figures and 2 Tables are providing additional information about the catalyst, analytical details of gasoline by GC and NMR, balances of hydrogen and ethanol, optimization of reaction parameters, derived cetane number and boiling range distribution measurements of the resulting fuel blends. TOC

SYNOPSIS A novel approach combining chemistry and combustion engineering to gain a readily usable fuel blend for today‘s vehicles.


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Blending Real World Gasoline with Biofuel in a Direct Conversion

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