Synthesizing CâC Skeletal Branched Biobased Chemicals from Sugars

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Branching-first: synthesizing C-C skeletal branched bio-based chemicals from sugars Aron Deneyer, Sam Tlatli, Michiel Dusselier, and Bert F. Sels ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01234 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Branching-first: synthesizing C-C skeletal branched bio-based chemicals from sugars Aron Deneyer, Sam Tlatli, Michiel Dusselier* and Bert F. Sels* Center for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium. *Corresponding Author: Prof. Dr. Michiel Dusselier (E-mail: [email protected]) and Prof. Dr. Bert F. Sels (Email: [email protected]) KEYWORDS: Branching-first, Aldol addition, Hydro(deoxy)genation, Branched bio-based chemicals, Polyols, Bio-gasoline, Heterogeneous catalysis

ABSTRACT: A novel strategy to bio-based chemicals with a branched carbon skeleton is introduced. Hereto, small sugars, such as 1,3-dihydroxyacetone, are coupled catalytically to obtain branched C6 sugars, such as dendroketose, in high yield at mild conditions. By bringing this branching step up front, at the level of the sugar feedstock (branching-first), new opportunities for the synthesis of useful chemicals arise. Here, we show that the branched sugar can be efficiently valorized into (i) new branched polyols and (ii) short branched alkanes. The first route preserves most of the original sugar functionality by hydrogenation with Ru/C, and renders access to branched polyols with three primary alcohol groups. These molecules are potentially interesting as plasticizers, crosslinkers or detergent precursors. The second valorization route demonstrates a facile hydrodeoxygenation of the branched sugars towards their corresponding branched alkanes (e.g. 2-methylpentane). The highest alkanes yields (> 65 mol% C) are obtained with a Rh/C redox metal catalyst in a biphasic catalytic system, following a HDO mechanism. In the short term, commercial integration of these mono-branched alkanes, in contrast to branched polyols, is expected to be straightforward, because of their drop-in character and well-known valuable octane booster role when present in gasoline. Accordingly, the branching-first concept is also demonstrated with other small sugars (e.g. tetroses) enabling the production of branched C6-C8 sugars, and thus also branched C5-C8 alkanes after HDO. INTRODUCTION Hexose based saccharides such as (hemi)cellulose, glucose and sucrose (dimer) are gaining importance as a feedstock for the production of added-value chemicals.1,2 Using sugar-based feedstock – and by extension other biomass resources such as lignin and fatty acids – results in new opportunities for the chemical industry. Either this can produce drop-in products, which can be implemented in current infrastructure and technology, or novel functionalized and added-value chemicals.3–7 Some of the key enabling technologies are gasification8, (hydro)pyrolysis9–11 and fractionation of biomass12–16. The latter preserves the original linear carbon skeleton of classic saccharides (C6). The chemocatalytic synthesis of platform molecules, originating from such linear sugars using homogeneous and heterogeneous catalysts, is an emerging field.17,18 Sorbitol19, isosorbide20, 5-hydroxymethylfurfural21, 2,5-furandicarboxylic acid22, lactic acid23, levulinic acid24, ethylene glycol25–27, γ-

valerolactone28 and n-hexane29,30 are some of the most investigated examples. Opportunities related to these renewable chemicals are widely accepted, ranging from low value fuels to high value chemicals. In addition to these chemocatalytic examples, biocatatytic processes using sugars, e.g. fermentation into bio-ethanol, are known.31 In contrast to sugars with a linear carbon skeleton (e.g. both linear or cyclic hemiacetal glucose), little attention has been paid to form branched sugars, i.e. sugars with tertiary carbon atoms. The main reason for this may be the restricted availability of such sugars in nature. The most important natural branched sugars, D-apiose (3-C-hydroxymethyltetrose) and Dhamamelose (2-C-hydroxymethyl-Ribose), are found in minute amounts in plants (e.g. apiose in parsley).32–34 Also, these sugars are usually not available as such, but rather as part of a more complex biomacromolecule.

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Figure 1. Branched sugar valorization scheme in this work. Focus points are: (i) aldol addition of small C2-4 sugars; (ii) hydrogenation of branched sugars towards branched polyols and (iii) full hydrodeoxygenation of branched sugars towards branched alkanes. Abbreviations: Ox = Oxidation; RA = Retro-Aldol; AA = Aldol-addition; Red = Reduction and HDO = Hydrodeoxygenation.

For example hamameli-tannin, a known astringent and cytotoxic substance against colon cancer, combines a cyclic hamamelose with two gallic acids.35 Branched deoxysugars, mainly available in micro-organisms, are a second important group, but these are also part of larger macromolecules.34,36 Although natural availability is an issue, conceptually, the introduction of a branched C-C skeleton at the start, thus originating from abundant bio-based feedstock could present interesting opportunities for the production of chemicals and allows access to new chemical and physical properties. Branched sugars can be obtained by three routes (Figure S1 in ESI): (i) isolation of natural ones from plants or microorganisms34; (ii) skeletal isomerization of abundant linear sugars such as fructose37 and (iii) C-C coupling of small sugars (C2-4 sugars)38 resembling the known formose chemistry. The latter reaction, where branched and extended sugars are formed, is linked to studies on the origin of life, in the context of the RNA-first model.39,40 Low abundance of branched sugars in biomass on the one hand, and unfavorable equilibrium levels during isomerization on the other, confine the practical applicability of the first two routes. The third route depends on the availability of small sugars (e.g. 1,3-dihydroxyacetone). This is reasonably more feasible as three different ways to such small sugars exist: (i) retro-aldol of linear C6 sugars into small C2-4 sugars (ii) thiazolium catalyzed coupling of formaldehyde41, and, (iii) oxidation of bio-based glycerol into 1,3dihydroxyacetone.42,43 Formaldehyde can be synthesized via methanol from syn gas (CO:H2)44, e.g. possibly obtained after biomass gasification45. The coupling of C3 sugars towards the branched sugar dendroketose has been reported.38,46,47 Further valorization of the latter towards 4-hydroxymethylfurfural; 2,4-dimethylfuran and long branched alkanes (C9-C15) were successfully explored in a multistep approach by Deng et al.38 In the present work, branched sugars are first synthesized by coupling of two 1,3dihydroxacetone C3 sugars, with an extension to C2 and C4 sugars. The different branched sugars were successfully used as feedstock for the one-step synthesis of branched light naphtha on the one hand, and the formation of new branched polyols on the other. Two different applications are thus targeted, one related to drop-in chemicals – C5-C8 branched alkanes as additive for gasoline – and the other related to new functionalized chemicals – branched polyols (Figure 1).

Bio-based strategies leading to short branched alkanes (≤ C9) are limited48–50 as liquid phase cellulose-to-naphtha processes mainly give short linear alkanes (C4-C6)29,30,51, while Fisher-Tropsch of syn gas52 or C-C coupling of biomassderived platform molecules followed by hydrodeoxygenation results in long (linear and branched) alkanes53–56. The latter mainly target diesel and jet-fuel applications. Our work presents a new strategy to selectively form short branched alkanes by complete oxygen removal of prior formed branched sugars. Such branched bio-based additives could help achieve the implementation of green carbon (as additive) in gasoline along government directives.57 In contrast to oxygen removal, preservation of the hydroxyl groups and hydrogenation of the aldehyde leads to novel branched polyols and maximizes atom economy.1,4 The branched C6 polyol could be interesting as a branching agent, monomer for polymers (polyesters) or a precursor to amphiphilic detergents (e.g. uses of sorbitol or pentaerythritol).58–60 EXPERIMENTAL Chemicals and materials. For a list of all used chemicals and materials, the reader is kindly referred to the Electronic Supporting Information (section A in ESI). Aldol addition of small sugars. In a typical coupling experiment, 2.8 mmol C3-sugar, 0.05 g oxide or ion exchanger with 0.13 mmol ion exchange capacity and 9 mL water were loaded in glass vials. Reaction takes places at room temperature under stirring (700 rpm). Upscaling experiments are conducted with 5 g C3-sugar (or equimolar amount of other sugars); 5 g Dowex 550 A (OH-) and 50 ml water under stirring at room temperature for 18 h. After coupling, Dowex 550 A is filtered and the sugar solution is diluted until the desired concentration for hydrogenation or hydrodeoxygenation is obtained. Hydrogenation towards polyols. In a typical hydrogenation experiment, 2.5 g sugar mixture (branched sugar feedstock with a branched/linear sugar ratio of ± 4), 0.25 g Ru/C and 40 mL of water were loaded into a 100 mL batch reactor (Parr Instrument Co.). After flushing with N2, the reactor was pressurized with a H2 pressure of 50 bar at 383 K. Subsequently, a stirring rate of 700 rpm was installed. After a reaction of 3 h, the reactor is cooled, depressurized and opened for sampling and analysis.


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Hydrodeoxygenation towards alkanes. In a typical hydrodeoxygenation experiment, 1.25 g of branched sugar feedstock (branched/linear sugar ratio of ± 4), 2.4 g TSA hydrate, 0.5 g Rh/C, 20 mL of water and 20 mL of n-decane were loaded into a 100 mL batch reactor (Parr Instrument Co.). After flushing with N2, the reactor was pressurized with a H2 pressure of 50 bar at room temperature and a stirring rate of 700 rpm was installed. A temperature program, going from room temperature to 383 K (± 12 K min-1) and further to 493 K (2 K min-1), is followed. After a reaction of 3 h, the reactor is cooled, depressurized and opened for sampling and analysis. Small modification to this experiment, with regard to reactor loading, temperature program and/or reaction time, are always mentioned in the caption of the figures and tables. Product yields are expressed as mol% C. Yield (mol% C) =

x 100

(1)

HPLC. HPLC analysis of aqueous samples, which are obtained after coupling or hydrogenation, was performed on an Agilent 1200 series HPLC equipped with a Varian Metacarb 67C column (300 * 6.5 mm, mobile phase: water) and a RID detector. Before analysis, the solution was filtered over a 0.45 µm PES filter. NMR. In addition to HPLC analysis, identification of branched sugars and polyols is carried out by H-decoupled 13 C-NMR on a Bruker Avance 400 MHz spectrometer. Moreover, distortionless enhancement by polarization transfer (DEPT; here DEPT-90 and DEPT-135), is used to make a clear distinction between primary (CH3), secondary (CH2), tertiary (CH) and quaternary (C) signals. GC. GC analysis of n-decane samples, which are obtained after hydrodeoxygenaton, was performed on a Hewlett Packard 5890 GC equipped with an HP 7673 autosampler, a HP-1 column (length: 60 m; diameter: 0.32 mm and film thickness: 1 µm) and a FID detector. RESULTS & DISCUSSION Synthesis of branched C6 sugars: aldol addition of C3 sugars. In order to obtain a branched carbohydrate feedstock, the carbon skeleton is synthesized through aldol addition of small sugars. The C3 ketose 1,3-dihydroxyacetone (DHA) is chosen as model compound to obtain insights into this reaction. Deng et al., described the alkaline ion exchanger Amberlite IRA-900 as catalyst for the aldol addition of DHA towards the branched C6 ketohexose (dendroketose) at room temperature.38 Here, in exploration of a viable and cheap branching solution, different ion-exchangers and oxides, alkaline as well acidic ones, are investigated as catalyst. To understand the mechanics of branched sugar production, a kinetic study with 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLY) is conducted as well. The screening showed the need for alkaline active groups in an aqueous environment. While acidic ion exchangers with SO3--functional groups (e.g. Dowex 50x8-100 and Amberlite IR118) and COO--functional groups (e.g. Dowex MAC-3) do not give any C3 conversion at room temperature, strong alkaline ion exchangers (e.g. Dowex 550 A or Amberlite IRA-400) do (Table S1 in ESI). Specifically, the use of a styrenedivinylbenzene matrix with quaternary amine functional groups and OH- counter ions (Dowex 550 A (OH-)) is characterized by a turnover frequency of 10.10-3 s-1 (DHA per ion-

exchange site) with a selectivity of ± 87.5% at 80% conversion (Figure 2B). Alkaline oxides (MgO, Table S2 in ESI) also perform aldol, while oxides without distinct alkaline groups or those with acidic groups (e.g. TiO2, ZrO2, Al2O3) fail to produce the branched ketohexose (Table S2, entries 5-8). The branched sugar is identified by means of HPLC and 13 C-NMR (Figure S2 in ESI). Its branched nature is evident from the disappearance of two quaternary signals, one from the carbonyl group and one from the branching point, in DEPT-90 and DEPT-135 13C-NMR. Also three CH2-groups and only one CH-group are found, while a linear hexose has two CH2- and three CH-groups. In addition, NMR shows that the dendroketose occurs mainly in its furanose ring form, as depicted in Figure 1. For simplicity, branched sugar skeletons in the next sections are depicted in their free aldehyde or ketone form. Experiments with DHA, GLY and a mixture of both were conducted with DOWEX 550 A (OH-) as catalyst to gain insight into the branching kinetics (Figure 2). The condensation of pure GLY (Figure 2A) results mainly in linear sugars and a small amount of branched sugars (mainly dendroaldose47). The formation of fructose and sorbose as main product is only possible through isomerization of GLY into DHA, followed by aldol addition of one GLY (Figure 2D). Since only small concentrations of DHA are analyzed during reaction, the rate of isomerization, i.e. GLY into DHA, is likely slower than the rate of condensation, i.e. C-C coupling of DHA with GLY. In line with these results, Assary et al. published several thermodynamic properties related to the [DHA]/[GLY] equilibrium, confirming that the tautomerization of GLY into DHA is favored at alkaline conditions (solid catalyst-free).61 The reaction of an equimolar mixture DHAGLY mainly produces linear sugars (Figure 2C), endorsing that the aldol reaction of GLY with DHA is faster than the isomerization of GLY into DHA and the GLY-GLY condensation. Otherwise, a much higher amount of branched ketohexose should be formed. Finally, a kinetic profile of pure DHA conversion results in branched ketohexose as main product (Figure 2B). This means that the rate of condensation in this case, i.e. C-C coupling DHA-DHA, is faster than the rate of isomerization, i.e. DHA into GLY. Taken together, the following order of rates, for alkaline-catalyzed C3 sugar aldol addition at room temperature can be listed: r (GLY – DHA) > r (iso GLY) >> r (GLY – GLY)

(pure GLY)

r (GLY – DHA) > r (DHA – DHA) > r (GLY – GLY)

(GLY-DHA)

r (DHA – DHA) > r (iso DHA)

(pure DHA)

The mechanism of aldol addition with alkaline catalysts starts with α-H proton abstraction from the C3 sugar, resulting in the formation of a negatively charged enolate (Figure 2D).62 The kinetic data suggests that proton abstraction is be favored for DHA in contrast to GLY. This is likely caused by a combination of steric hindrance, i.e. C-H of a primary carbon for DHA vs. a secondary carbon for GLY; and charge stabilizing groups for the negatively charged enolate. Secondly, the (nucleophilic) enolate attacks a carbonyl group, either from GLY or DHA.62 If both are present, it seems that the enolate has a distinct preference for GLY instead of DHA, which could be caused by steric hindrance. This scheme explains why the formation of linear sugars is favored, except when pure DHA is used.



Branched ketohexoses are thus most easily accessed from alkaline aldol addition of pure DHA feedstock. Further valorization experiments towards target polyols or alkanes were always conducted with an identical starting feed, i.e. a sugar mixture made with Dowex 550 resin and with a final branched/linear sugar ratio of approximately 4 (78% of dendroketose). Preserving most functional groups: branched polyols. Hydrogenation of dendroketose to its branched polyol was investigated with heterogeneous Ru/C catalysts under H2 atmosphere at 383 K (Figure 3A). The HPLC profile (Figure 3B) demonstrates the selective conversion of the mixture of

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linear and branched sugars to their corresponding linear and branched polyols with high selectivity (yield polyols > 98%). The branched nature of the sugar does not impede the hydrogenation, as these high yields are in line with comparable experiments on glucose and xylose with Ru catalysts.63,64 Hydrogenation of the linear sugar part of the feed (ketoses) results in the formation of mannitol and sorbitol due to the creation of an additional chiral carbon and the racemic nature of such hydrogenation products. The same holds true for the branched sugar as its products are divided over two polyol diastereoisomers. These are not enantiomers and thus different retention times in classic (non-chiral) HPLC can be obtained.

Figure 2. Product profiles and mechanism of the alkaline-catalyzed aldol reactions using 2.8 mmol triose; 0.05 g Dowex 550 A (OH-) and 9 ml water at room temperature. Kinetics of (A.) pure glyceraldehyde (GLY); (B.) pure 1,3-dihydroxyacetone (DHA); and (C.) an equimolar mixture of GLY and DHA. (D.) Reaction scheme of the aldol addition starting with H abstraction by the alkaline catalyst, which is recovered in the end.62 Identification of the branched aldohexose or dendroaldose was not possible through small product formation, but was high likely formed based on the work of Harsch et al.47 The attack of GLY-enolate on DHA is not drawn because no related sugar formation was visualized on HPLC or mentioned in literature.


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Figure 3. Formation of C6 polyols through hydrogenation of a typical C6 branched sugar feedstock (branched/linear sugar ratio ≈ 4) using 2.5 g feedstock; 40 ml H2O and 0.25 g Ru/C; 3 h at 383 K under 50 bar H2 atmosphere. (A.) Reaction scheme. (B.) HPLC chromatogram before reaction, viz. linear and branched sugars, and after, viz. linear and branched polyols with diastereomer formation. Numbers refer to Figure 3A. (C.) 13C-NMR (H-decoupling), DEPT-135 13C-NMR and DEPT-90 13C-NMR of the polyol mixture confirms the presence of a branched carbon skeleton with functional alcohol groups. The other signals present, are likely derived from the linear polyols in minor amounts versus the branched products.

The novel branched polyols, to the best of our knowledge detailed here for the first time, were identified by 13C-NMR (Figure 3C). Roughly two times six signals between 60 and 80 ppm represent the carbons of similar C-OH groups. The existence of 12 signals confirms the presence of two diastereomers, in line with HPLC. In addition, DEPT-135 and DEPT-90 13CNMR confirmed the presence of a branched carbon skeleton. More specifically, the disappearance of the 76 ppm signals in DEPT-135 (one for each diastereomer), highlights the presence of the branching point. Also two CH-groups and three CH2-groups are visualized in Figure 3C for each diastereomer via negative CH-signals in DEPT-135 13C-NMR and absence of CH2-signals in DEPT-90 13C-NMR, respectively. The potential usefulness of these novel functionalized polyols is, in contrast to drop-in chemicals, uncharted terrain, although structural similarities with commercial chemicals indicate opportunities. For example the most common C6 polyol (sorbitol) is used in a range of applications such as food sweetener, cosmetics and personal care.65 In addition, the new branched polyol, has three primary alcohol groups, while sorbitol and pentaerythritol respectively possess two and four. Such primary alcohols are points of functionalization (ether, ester) e.g. of use in surfactants as well as plasticizers, or as crosslinking or branching point for many polymer chemistries. Also, pentaerythritol is used in the painting industry (alkyd paints and varnishes).58 In spite of this potential upside, possible shortfalls can for instance be low (improved) application performance (vs. linear polyols or pentaerythritol) or too high

production costs (e.g. branched sugar synthesis from more expensive/less available small sugars). Therefore, further studies, investigating the applicability of these polyols in daily life applications, are needed. Complete defunctionalization: proof of concept HDO of branched sugar feedstock. Branched C6 alkanes, if available, could immediately be mixed into specific streams in the chemical industry (drop-in). Not only in light of governmental directives to stimulate renewable carbon, a straightforward bio-based route to such branched alkanes is desired. For example for gasoline (desired octane number (ON) > 87), nhexane (n-Hex) is characterized by an octane number of 3066, while branched 2-methylpentane (i-Hex) totals 7566. Transforming sugars into alkanes however entails hydrodeoxygenation (HDO) and a low atom economy. HDO of the branched mixture was studied using a biphasic dual catalytic system, designed by Op de Beeck et al., for direct cellulose hydrodeoxygenation into alkanes.29 Although several HDO technologies for bio-based saccharides are known (Tomishige et al., Wang et al., Ma et al., etc.)30,67–70, this catalytic technology is unique as a biphasic system is used, where each phase has its own essential catalytic function, viz. Brønsted acidity in the aqueous phase and metal redox sites in the organic phase. The sugars in such process first dehydrate to furan-intermediates before HDO through a series of oxygenates yields n-hexane. Table 1 entry 1 shows a typical HDO of cellulose using a custom poly-acid-modified Ru/C catalysts in n-decane and tungstosilic acid in the aqueous phase. When the branched



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1 ), crucial in the case of cellulose HDO,29 is limited and a slight improvement in total yield was even recorded for an intermediate rate of 2 K min-1 starting from 383 K towards 493 K (Figure 4A). The final temperature proves to be a more essential parameter, viz. Figure 4B. Analysis of the organic phase for instance shows that at low temperature (393 K), only 20 mol% C is obtained, consisting mainly of oxygenates such as 2,4-DMTHF, while HDO is more effective and complete at higher temperatures. Optimally, at 493 K (reached at 2 K min-1 from 383 K), 34% i-Hex (ON = 75); 8% n-Hex (ON = 30); 15% n-Pen (ON = 61); 8% other C4-6 alkanes (ON > 75) and 6% oxygenates are obtained (mol% C), totaling 72% of ndecane-soluble products. Based on this product distribution, gasoline fuel properties (for one), such as the octane number, can be estimated. Final properties will depend on the degree of enrichment in current fossil gasoline. In a final optimization, catalyst loadings were varied (at 493 K, Figure 4C). Acidity, i.e. tungstosilic acid (TSA) in the aqueous phase, and redox capacity, i.e. Rh/C in the organic phase, can separately be reduced by respectively 87.5% and 50%, while maintaining a reasonably efficient HDO. The acid/redox balance of active sites, calculated from TSA concentration (acid) and CO-accessible Rh-metal in chemisorption (redox), seems to be optimal around 60 (Figure S3 and S4 in ESI), corresponding to the reference in Figure 4C.

sugar is used as a feedstock, the formation of 2-methylpentane (i-Hex) appears as the main product in HDO (13%; Table 1, entry 2-3). In addition, n-hexane from the linear sugar fraction, n-pentane through decarbonylation of a primary alcohol and several intermediate oxygenates from partial HDO (e.g. tetrahydrofuran, tetrahydropyran and oxepane related molecules such as 2,4-DMTHF) are observed by GC(-MS) analysis. Remarkably, the branched/linear alkane ratio (i-Hex vs. nHex) is lower than 2 even though the branching ratio of the feedstock sugars is 4. This indicates that the yield of i-Hex from the dendroketose is lower than to be expected based on n-Hex yield from glucose HDO (in cellulose, Table 1 entry 1). Since the poly-acid-modified Ru/C catalyst was tailored to cellulose conversion in previous work, a wider screening of different metal catalysts for the branched sugar was performed (Table 1, entry 4-8). Notably higher alkane yields (51%) are obtained with Rh/C, with a striking 31% of i-Hex. In total, 65 mol% C of n-decane-soluble products are analyzed. A blank reaction with activated carbon is unable to catalyze alkane formation, highlighting the crucial role of the metal (Table 1, entry 8-9). Complete defunctionalization: optimization of branched sugar HDO. A systematic study of the influence of heating rate, temperature and catalyst amount, on the Rh/C HDO system is presented. The influence of slow heating (0.5 K min-

Table 1. Screening different metals (on carbon) as catalyst in the one pot hydrodeoxygenation of branched sugars into alkanes. A representative sugar mixture with a branched/linear sugar ratio of ± 4 is used.a Yield (mol% C) Entry 1 c, d 2

Substrate

Redox catalyst

6

13

73

8

4

18

49

5

4

23

49

3

8

8

45

n-Hex

12

/

42

6

13

8

10

Ru/C

12

13

htTSA(2)Ru/C

3 4

Sum of n-dec. soluble organics

i-Hex

Cellulose

c

Other

C4-6OHx

n-Pen

C4-6 alk.b

5e

branched sugar

Raney Ni

/

/

1

1

5

7

6

feedstock

Pd/C

/

3

1

1

28

33

7

Pt/C

1

8

3

2

31

44

8

Rh/C

8

31

7

5

14

65

9

C

/

/

/

1

/

1

a

conditions: 1.25 g sugar; 2.4 g TSA; 0.5 g redox catalyst (5 wt% metal); biphasic system: water/n-decane (20:20); 50 bar H2 at RT; temperature program: RT to 423 K (12 K min-1) and from 423 K to 493 K (0.5 K min-1); total reaction time of 3 h. b Other C4-6 alkanes: n-butane; 2-methylbutane; cyclopentane; 3-methylpentane; methylcyclopentane and cyclohexane. c 4.8 g TSA instead of 2.4 g TSA. d Data is obtained from Op de Beeck et al. e 0.3 g redox catalyst instead of 0.5 g.


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ACS Sustainable Chemistry & Engineering through decarbonylation of primary alcohols, leading to methane (indeed analyzed) and C4-5 alkanes (2% n-but; 3% i-pen; 15% n-pen).

Figure 5. Proposed reaction scheme for hydrodeoxygenation of branched ketohexose into 2-methylpentane (i-Hex).

Figure 4. Influence of reaction conditions, (A.) heating rate (with max rate = 10 K min-1); (B.) temperature and (C.) catalyst loading, on HDO efficiency. Standard conditions (reference): 1.25 g sugar; 2.4 g TSA; 0.5 g Rh/C; biphasic system: water/n-decane (20:20); 50 bar H2 at RT; temperature program: RT to 383 K (12 K min-1) and from 383 K to 493 K (2 K min-1); total reaction time of 3 h.

Complete defunctionalization: reaction network. The catalytic biphasic system for cellulose with modified Ru/C catalysts followed a pathway with HMF as key intermediate before HDO ensued, instead of sorbitol in other known systems.30 An analogous reaction scheme is proposed for branched feedstocks in the biphasic approach using Rh/C (Figure 5), corroborated in part by the product range of Table 1 and GC-MS analysis of intermediates. Firstly, dendroketose is dehydrated to 4-HMF. The furan ring of 4-HMF is then reduced and additional oxygen removal occurs at the 1 and 6 positions, resulting in 2,4-DMTHF. Removal of the final oxygen is presumably performed by ring opening the cyclic ether into a diol, and its subsequent HDO. Carbon loss can occur

Figure 6. Influence of starting substrate (sugars or polyols) on HDO efficiency. Conditions: 1.25 g substrate; 2.4 g TSA; 0.5 g Rh/C; biphasic system: water/n-decane (20:20); 50 bar H2 at RT; temperature program: RT to 383 K (12 K min-1) and from 383 K to 493 K (2 K min-1).

The presence (after short reaction times) and subsequent disappearance of cyclic ethers such as 2,4-DMTHF was analyzed (Figure 6). Besides n-Hex and i-Hex, from respectively linear and branched C6 sugars, shorter C4-5 alkanes are ana-



lyzed by GC as well. Although the 4-HMF pathway seems likely, HDO of the branched polyol (previous section) gives almost identical results as the branched sugar feedstock (Figure 6, 29% i-Hex). This conflicts with cellulose conversion using this system, where sorbitol is converted to isosorbide a stable, HDO-resistant molecule, instead of alkanes. As it is chemically impossible to create a double dehydrated cyclic structure from the branched polyol, both polyol HDO and 4HMF HDO routes to alkanes seem to be occurring. This is further corroborated by analysis of the aqueous phase at short reaction time (30 min, HPLC), showing a mixture of sugars (22%) as well as polyols (17%) of approximately 39 mol% C. The efficient HDO of these polyols is a prerequisite to obtaining the final elevated alkane yields. Expanding the branching-first strategy towards C5-8 alkanes. Since gasoline or refinery streams are typically characterized by a mixture of C5-C12 alkanes, preferentially with a branched carbon skeleton, we sought to expand the branchingfirst methodology to obtain longer products. A range of C2 to C6 sugars containing a keto-group (considering the triose coupling mechanism (Figure 2D)) was subjected to aldol C-C coupling, followed by the biphasic HDO process. In this way, GC-analysis of the final organic phase yields information both about the alkane production as well as the coupling mechanism. An overview of investigated combinations is visualized in Figure 7 with attention to the distribution of alkane lengths, the branched/linear ratio and the most important products (detailed product compositions, see ESI Table S3). In general, a broad range of alkanes (C4-C8), linear and branched, are synthesized in the naphtha region. Three trends from widening the substrate scope arise: (i) coupling of pure C3 (DHA), pure C4 (erythrulose - ERY) or mixture of C3 – C4 ketoses results in a high degree of branching (ii) coupling with glycolaldehyde (C2) only produces extended linear sugars and alkanes; (iii) coupling with fructose (C6) only occurs to a minor extent, possibly caused by its dominant cyclic furanose occurrence form at room temperature. The most interesting alkane distribution is obtained from a C3-C4 sugar equimolar feedstock (Figure 7A): branched 2-methylpentane (from C3-C3; 13.6 mol% C); 3-methylhexane (C3-C4; 7.6 mol% C); 2methylhexane (C3-C4; 7.8 mol% C); and 3-methylheptane (C4C4; 5.4 mol% C) are formed in addition to n-hexane (9.0 mol% C), n-heptane (6.7 mol% C) and n-octane (2.5 mol% C). Linear alkanes here are formed through some ketose to aldose isomerization, before aldol addition (Figure 7C). Besides sugar isomerization, decarbonylation of primary alcohols influences the final alkane products (methane, smaller alkanes). The C3-C4 combination results in a branched/linear alkane ratio of 1.40 and a total alkane yield of 63.6 mol% C. In line with cellulose HDO, other products such as cyclic alkanes and oxygenates are only formed in minor amounts (Figure 7B). Although the contribution of these molecules is limited in the final product mixture, they can probably have interesting performances in gasoline additives.71 The alkane distribution reveals information on the aldol mechanism of different sugars. All possible aldol combinations are visualized in the ESI (section G). Glycolaldehyde seems to only undergo nucleophilic attacks, and thus results in extended linear sugars. The triose DHA can perform as well as undergo a nucleophilic attack and therefore will always lead to

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branched sugars. Tetrose ERY behaves similarly to DHA. Theoretically, proton abstraction can take place on two positions of ERY, but one of the two seems not to occur. Sustainability assessment. Recently, our group developed a straightforward sustainability assessment and applied it to sugar-derived molecules. Taking into account four sustainability criteria - (i) functionality; (ii) reaction efficiency; (iii) toxicity and (iv) end-of-life route - polyols obtained an excellent evaluation.1 In line with these findings, the novel branched polyols have high functionality and can be efficiently synthesized from branched sugars. Though their favorable sustainability profile, the integration of these molecules in the current chemical industry is still uncharted terrain and rather suggestive. In contrast, short branched alkanes, used in gasoline applications, are characterized by a higher toxicity and a total loss of the original sugar functionality.5 Nevertheless, their drop-in character facilitates the integration of bio-based carbon in the short term, which is imposed by local regulation (e.g. EU energy directive, integration of 10% biofuels in the current transportation market by 2020). In addition, the here presented branching-first route to bio-based alkanes is compared with a traditional sugar HDO to linear alkanes and their subsequent isomerization route, listing the most striking (dis)advantages in Table 2. Table 2. Comparison between traditional HDOisomerization and branching-first route for bio-gasoline production. Traditional route

Branching-first

(HDO – isomerization)

(aldol addition – HDO)

Substrate availability glucose, sucrose, starch and (hemi)cellulose (+)

small C3-C4 sugars and glycerol (-)

Thermodynamics isomerization eq. - limit amount of isohexanes (-)

no limitation - no eq. for aldol addition and HDO (+)

Alkane range C5-C6 (-)

C5-C8 (+)

Branching and gasoline fuel property mono- and di-branched (+) e.g. 2,3-dimethylbutane (ON = 104)


mono-branched (-) e.g. 2-methylpentane (ON = 75)

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Figure 7. Expansion of branched sugars synthesis through coupling with glycolaldehyde (GLYCO); 1,3-dihydroxyacetone (DHA); erythrulose (ERY) and/or fructose (FRU), followed by a complete hydrodeoxygenation. (A.) Schematic overview with representation of alkanes, of which the most important ones are visualized, in the pie charts; (B.) minor side products after hydrodeoxygenation and (C.) important side reactions during coupling (sugar isomerization) and hydrodeoxygenation (decarbonylation). Conditions coupling: 55.6 mmol (small) sugar (equimolar in case of mixtures); 5.00 g DOWEX 550 A (OH-) ion exchange resin; 50 ml water; magnetic stirrer; room temperature; total reaction time of 18 h. Conditions hydrodeoxygenation: 1.25 g substrate; 2.4 g TSA; 0.5 g Rh/C; biphasic system: water/n-decane (20:20); 50 bar H2 at RT; temperature program: RT to 383 K (12 K min-1) and from 383 K to 493 K (2 K min-1); total reaction time of 3 h.



CONCLUSION This contribution aims to introduce novel routes to useful bio-based chemicals with a branched carbon skeleton, by prior introduction of branching in the sugar substrates (‘branchingfirst’). Hereto, an efficient strategy for coupling small sugars to obtain branched C6-C8 sugars is devised, making use of a mild, alkali resin-catalyzed aldol addition in water. A kinetic study of the coupling process showed that ketose sugars are a necessary condition to obtain larger branched sugars. Using 1,3-dihydroxyacetone as a substrate, for instance leads mainly to the branched sugar dendroketose, in high yields and with an overall 4:1 branched to linear sugar product ratio. Bringing this branching step up front, chemical opportunities arise in form of new branched functionalized molecules or alternative reaction pathways for defunctionalized, but branched, drop-ins. Here we demonstrate the concept by converting the branched sugar feedstock (i) into a novel branched polyol via hydrogenation and (ii) into short branched alkanes by full hydrodeoxygenation. The branched polyols from dendroketose (in fact diastereomers) are efficiently synthesized by a Ru/C catalyst under reductive atmosphere and were identified via (DEPT) 13C-NMR. The three primary alcohols of this branched bio-based chemical, on top of its polar character may be promising for many potential applications (e.g. plasticizer, crosslinker, amphiphilic detergent or building block in alkyd resins). The short branched alkanes, today an important part of gasoline, are successfully made from dendroketose by hydrodeoxygenation in a biphasic catalytic system, characterized by an aqueous phase with acidity and an organic phase with redox capacity. The highest alkane yields, 34 mol% C i-hex and > 65 mol% C alkanes in total, are obtained with Rh/C as redox catalyst. Rhodium is significantly better in comparison to other metals. Notably, evidence points out that the selective alkane production route can rely both on an 4-HMF HDO pathway as well as a polyol HDO pathway. The scope of the branching-first concept is expanded to branched C6-C8 sugars, by coupling with C3 and C4 ketoses, viz. 1,3-dihydroxyacetone and erythrulose. HDO of the thereof derived branched sugar feedstock leads to an interesting mixture of bio-based C6-C8 alkanes, characterized by a branched/linear ratio > 1. The latter can be valuable as biobased drop-in octane booster additive for gasoline.

ysis and writing was done by A.D. supported with the critical input of M.D. and B.F.S.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT A.D. acknowledge the Agency for Innovation by Science and Technology (IWT) for a Ph.D. grant. M.D. acknowledge Research Foundation-Flanders (FWO) and BOFZAP for financial support.

ABBREVIATIONS GLYCO, glycolaldehyde; DHA, 1,3-dihydroxyacetone; GLY, glyceraldehyde; ERY, erythrulose; FRU, fructose; HDO, hydrodeoxygenation.

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ASSOCIATED CONTENT Supporting Information (ESI). More experimental details, additional (reaction) schemes, dendroketose 13C-NMR data, supporting aldol addition data and supporting hydrodeoxygenation data are available in ESI.

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AUTHOR INFORMATION Corresponding Author

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* E-mail: [email protected] (M.D.) * E-mail: [email protected] (B.F.S)

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Author Contributions

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B.F.S, M.D. and A.D. conceived the branching-first principle. All experimental work (catalyst preparation, aldol addition, hydrogenation, hydrodeoxygenation, GC-analysis, HPLC-analysis and NMR) was performed by A.D., assisted in part by S.T. Data anal-

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For Table of Contents Use Only

A new branching-first principle is presented for sugars, synthesizing new molecules - branched C6 polyols - or well-known drop-in molecules - short branched alkanes - in a different way.


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