Carbon Chain Length Increase Reactions of Platform Molecules

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Carbon Chain Length Increase Reactions of Platform Molecules Derived from C5 and C6 Sugars Mats Kal̈ dström,*,† Marina Lindblad,† Kaisa Lamminpaä ,̈ † Susanna Wallenius,† and Sami Toppinen‡ †

Neste Corporation, POB 310, 06101 Porvoo, Finland Neste Jacobs Oy, POB 310, 06101 Porvoo, Finland

‡

ABSTRACT: The production of liquid fuels from lignocellulose-derived platform molecules has attracted much interest in recent years. Platform molecules mostly have a shorter carbon chain length, compared to liquid fuels, which have a typical chain length varying between 4 and 25 carbon atoms, whereas aviation and especially diesel fuel have a carbon chain length exceeding 10 carbon atoms. For this reason, some carbon chain length increase reactions are required. In this article, carbon chain length increase reactions are compared for typical lignocellulose-derived platform molecules. The focus is placed on the ability of the molecules to participate in self-condensation reactions in a controlled manner. Hydrogen plays a key role when producing fuels from platform molecules. Hydrotreatment is applied not only when converting the products from a carbon chain length increase reaction into hydrocarbons but also for modifying the functional groups of the model compounds and, thereby, their reactivity.

1. INTRODUCTION The production of renewable fuels has received much attention in academia and industry over the past few years. Potential platform molecules for fuel production have been listed by the U.S. Department of Energy and the Energy Department of the European Commission.1,2 The incentive for producing liquid fuels, particularly from lignocellulose, has greatly increased during the past decade, because of favorable legislation promoting the utilization of nonfood feedstock. Platform molecules can be formed from lignocellulosic feedstock via different production routes, involving, e.g., hydrolysis, fermentation, dehydration, oxidation, and hydrogenation. A typical chain length of platform molecules originating from lignocellulose-derived sugars varies in the range of 2−6 C atoms. Platform molecules frequently mentioned in the literature, such as levulinic acid (LA), furfural, and 5-hydroxymethylfurfural (HMF), originate from C5 and C6 sugars.1−4 Liquid fuels have a carbon chain length that is typically in the range of C4−C12 for gasoline and C12−C25 for diesel. Aviation fuels have a typical chain length of C9−C16. If the goal is to produce gasoline, diesel, or aviation fuels from lignocellulose sugars via platform molecules, some carbon chain length increase reactions are required. Carbon chain length increase reactions for transforming platform molecules to suitable chain lengths include aldol condensation, ketonization, alkene oligomerization, and the Guerbet reaction. The type of carbon chain increase reactions that occur directly are dependent on the functional groups of the molecules. Aldol condensation typically involves aldehydes and/or ketones with the simultaneous formation of water. Ketonization occurs between carboxylic acids, producing water and carbon dioxide as byproducts. Alkene oligomerization occurs through double © XXXX American Chemical Society

bond chemistry, whereas alcohols such as ethanol can be coupled to 1-butanol through the Guerbet reaction.5,6 In an ideal case, some oxygen is simultaneously split off as water without any external need for hydrogen during the carbon chain increase reactions. Oxygen removal through decarboxylation reduces hydrogen consumption in downstream processes, but simultaneously decreases the carbon yield. Typically, low hydrogen consumption with high carbon yield is preferred. However, depending on the feedstock, it might sometimes be beneficial to remove oxygen as carbon dioxide, and sometimes as water, by adding hydrogen. From a technical and economic point of view, it is also important to try to minimize the number of reaction steps and to avoid the need for additional reactants. For this reason, self-condensation reactions carried out in a single reaction step are often preferred when performing carbon chain length increase reactions. This article discusses carbon chain increase reactions with heterogeneous catalysts for selected lignocellulose-derived platform molecules. The main focus is on self-condensation reactions; reactions requiring additional reactants are only mentioned briefly. Most of the reactions have indeed been performed via heterogeneous catalysis, as solid catalysts are usually preferred over homogeneous catalysts when performing industrial-scale processing. The influence of hydrotreatment on both monomers and products is investigated in terms of carbon chain increase reactivity and carbohydrate energy content. Special Issue: Tapio Salmi Festschrift Received: May 8, 2017 Revised: August 21, 2017 Accepted: August 25, 2017

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DOI: 10.1021/acs.iecr.7b01904 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research This article is divided into three main parts. In the first part, the literature concerning chain length increase reactions of selected platform chemicals is reviewed. In the second part, experimental results for levulinic acid, 2-methylfuran, and αangelica lactone are presented and discussed. In the last section, the hydrogen consumption required for the biofuel applications is calculated and the yields to hydrocarbons are compared.

(volume) of the compounds formed. From the chromatograms, a qualitative division (area %) into monomers, dimers, trimers, and tetramers (including possible contribution from highermolecular-weight products) of the feed molecules was made. The main compounds present in the product mixtures were identified by the gas chromatography−mass spectrometry (GCMS) technique.

2. MATERIALS AND METHODS Levulinic acid (>97%), 2-methylfuran (>98%), α-angelica lactone (99%), 2-pentanone (>98%), and γ-valerolactone (GVL) (99%) were purchased from Sigma−Aldrich. All the chemicals were used without further purification. The catalysts used were commercial or prepared by the authors. Stirred semibatch reactor experiments were conducted with 2-pentanone and angelica lactone, and batch reactor experiments with 2-methylfuran. In the aldol condensation of 2pentanone acid, the ion-exchange resin catalysts Amberlyst CH28 (0.7% Pd) and Amberlyst 36 were used (0.1 g catalyst/1 g feed) under hydrogen and nitrogen flow (6 L/h), respectively. The resin catalysts were dried in an oven at 120 °C overnight under nitrogen flow, prior to testing. The reaction was carried out at 120 °C and 20 bar for 2 h. Double-bond oligomerization of angelica lactone was performed with anhydrous K2CO3 powder (oven-dried overnight at 105 °C) using 0.06 g catalyst/1 g feed. The reaction was done at 70 °C and 2−3 bar under nitrogen flow (10 L/h) for 5 h. Hydroxyalkylation/alkylation of 2-methylfuran was performed with the ion-exchange resin catalysts Amberlyst 16 (wet or dry) and Amberlyst XE586 (wet), using 0.025 g catalyst/1 g feed. The pressure in the reactor was 1 bar, the temperature was 60 °C, and the reaction time was 4 h. Thermal stability tests of levulinic acid, α-angelica lactone, and γ-valerolactone were carried out in a tubular reactor over a bed of quartz sand and under nitrogen flow (3 L/h). Catalytic continuous flow tubular reactor experiments were conducted with levulinic acid and α-angelica lactone. The stability of ion-exchange resin catalysts in the aldol condensation of levulinic acid was tested with Amberlyst CH28 (0.7% Pd) and Amberlyst 36. The reaction with Amberlyst CH28 (80% dried, 4.5 g dry mass) was carried out at 120 °C, 20 bar, and a weight hourly space velocity (WHSV) of 0.5 h−1 under hydrogen flow (3 L/h) and with Amberlyst 36 (dried, 2.5 g) at 120 °C, 20 bar and WHSV 1.0 h−1 under nitrogen flow (3 l/h). To compare the product composition, i.e., the degree of condensation of levulinic acid, Amberlyst CH28 and the acid aluminosilicate based MM-Beta material7 were used as catalysts. The reaction with Amberlyst CH28 (80% dried, 34 g dry mass) was carried out at 120 °C, 20 bar and WHSV of 0.2 h−1 under hydrogen flow (0.5 L/h). MM-Beta was crushed and sieved to a particle size of 0.150−0.355 mm. The catalysts (4 g) were dried under nitrogen flow (3 L/h) at 350 °C. The reaction was carried out at 200 °C, 1 bar, and WHSV = 0.5 h−1 under nitrogen flow (3 L/h). Angelica lactone oligomerization was performed with a supported K-based catalyst crushed and sieved to a particle size of 0.150−0.355 mm. The catalyst was dried under nitrogen flow (6 L/h) at 300 °C for 1 h. The oligomerization was conducted at 177 °C, 2 bar, and WHSV = 0.2 h−1 under nitrogen flow (3 L/h). Liquid product samples were analyzed by gel permeation chromatography (GPC) to obtain an estimate of the degree of oligomerization/condensation. GPC chromatograms show the product distribution according to the conformational size

3. CARBON CHAIN INCREASE REACTIONS Typical platform molecules originating from C5 and C6 sugars were selected for the carbon chain length increase study (Figure 1). The capability of the platform molecules to conduct self-

Figure 1. Platform molecules derived from C5 and C6 sugars.

condensation reactions was compared as hydrogen was added to the compounds. The functionality, and thereby the reactivity, of the model compounds can be modified by hydrotreating the platform molecules to different degrees. Compounds that have not been hydrotreated can be found in the upper part of Figure 1, whereas their hydrotreated counterparts can be found in the lower part of the figure. By hydrotreating furfural it is possible to remove the reactive carbonyl group as water, forming 2methylfuran. On the other hand, by hydrotreating 2methylfuran, the double bonds in the furan ring are removed, forming 2-methyltetrahydrofuran. When hydrotreating HMF, both the hydroxyl and carbonyl groups can be removed forming 2,5-dimethylfuran. In similarity to 2-methylfuran, hydrotreating 2,5-dimethylfuran hydrogenates the furan ring, forming 2,5dimethyltetrahydrofuran. By removing water from levulinic acid, the unsaturated lactone, α-angelica lactone, is formed. αAngelica lactone (α-AL) can be hydrogenated under mild reaction conditions into GVL. In terms of reactivity, furfural, HMF, and levulinic acid are much more reactive and more prone to participate in carbon chain increase reactions, compared to their hydrogenation products: 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and GVL. The formation of humins is an undesired, typically uncontrolled carbon chain increase reaction, in which both furfural and HMF often participate.8−10 Usually, the more hydrogen is added to a platform molecule, the more stable it becomes and, simultaneously, the less likely to participate in controlled or uncontrolled carbon chain increase reactions. B


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3.1. HMF and Furfural. HMF and furfural cannot undergo self-condensation through aldol addition/condensation, bceause they do not possess any α-H atoms. HMF chain length increase reactions are typically linked to unwanted side reactions that occur during HMF formation from glucose. Under the prevailing aqueous acidic conditions, HMF reacts to form levulinic and formic acids, but it also forms solid byproducts called humins. It has been shown that humin formation proceeds through aldol addition/condensation reactions between HMF and 2,5-dioxo-6-hydroxy-hexanal, where the latter is a product from the hydrolytic ring opening of HMF.8 It is notable that, in the experiments, no soluble intermediates between HMF and humins were detected, indicating a high reactivity of 2,5-dioxo-6-hydroxy-hexanal. Similarly to HMF, furfural forms solid products under aqueous acidic conditions. Also, in this case, it has been suggested that the reactions proceed through hydrolytic ring opening and further aldol addition/condensation.9 Another option is the Diels−Alder reaction between two furfural molecules.12 However, the conversions are low, even at 200 °C, and no specific products in liquid phase have been detected. Zang and Chen have recently reported that controlled selfcoupling reactions for HMF and furfural can occur when applying organocatalysis, where the reaction between two carbonyl groups is enabled by converting one of the electrophilic carbonyls to a nucleophilic center.13 HMF was shown to undergo organocatalytic self-condensation into the dimer, C12 furoin (Figure 2), with an ionic liquid derived Nheterocyclic carbene catalyst. In this solvent-free self-coupling conducted at 60 °C, dimers of HMF were produced in nearly quantitative yield. An N,S-heterocyclic carbene catalyst, formed in situ from a thiazolium ionic liquid, is introduced as a lessexpensive alternative. Combined with a base, for example, Et3N, this catalyst was shown to be highly efficient for both HMF and furfural self-coupling reactions into furoins. With this catalyst system, the yield of furfural dimers (C10) achieved was 99% at 60 °C and the yield of HMF dimers (C12) was 97% at 120 °C. In order to enable carbon chain increase reactions by aldol condensation with HMF or furfural, an additional compound possessing α-hydrogen must be introduced. Cross-aldol condensation of HMF or furfural with acetone has been studied more thoroughly. The Dumesic group14,15 has reported the production of C9−C15 unsaturated compounds from HMF and acetone by varying the acetone-to-HMF molar ratio (Figure 2). In corresponding condensation reactions with furfural and acetone, C8−C13 unsaturated compounds are formed. Catalysts used in these cross-aldol condensations include aqueous NaOH in a biphasic reactor system at room temperature or solid base catalysts, such as MgO−ZrO2, in the aqueous phase at 50−80 °C. 3.2. Levulinic Acid. The carbon chain increase reaction routes proposed in the literature for levulinic acid (LA) mostly proceed via GVL as an intermediate and involve two or more reaction steps to achieve carbon chain lengths of C9−C18.16−19 Levulinic acid (4-oxopentanoic acid) has two reactive functional groups−a ketone (CO) and a carboxylic acid (COOH) group−through which direct self-condensation reactions could proceed by aldol condensation and ketonization, respectively. The primary levulinic acid dimers formed in these reactions are depicted in Figure 3. For the production of compounds that are in the fuel range, but larger than LA dimers, higher degrees of condensation should occur in one reaction step.

The boiling points of the selected platform molecules are shown in Table 1. The boiling point of the compounds typically Table 1. Boiling Points of the Platform Molecules Listed in Figure 1a

a

compound

boiling point (Bp) range [°C]

furfural 2-methylfuran 2-methyltetrahydrofuran 5-hydroxymethylfurfural 2,5-dimethylfuran 2,5-dimethyltetrahydrofuran levulinic acid α-angelica lactone γ-valerolactone, GVL

162 63−66 78−80 351−354 92−94 90−92 245−246 184−188 207−208

Data taken from ref 11.

decreases as the degree of hydrotreatment increases. However, there is a slight increase in boiling point when the double bonds are removed from 2-methylfuran, forming 2-methyltetrahydrofuran, and from α-angelica lactone, forming GVL. The boiling points of the feedstock determine whether the reactions are carried out in gas or liquid phase at normal pressure. Generally, liquid-phase reactions give a higher throughput of feed and products per time unit through a fixed catalyst bed, compared to gas-phase reactions, and are therefore usually preferred. Self-condensation products from the platform molecules are shown in Table 2. The products formed from LA are produced by aldol condensation, whereas the products from angelica lactone and 2-methylfuran proceed through a double-bond reaction and alkylation, respectively. Furfural and HMF are coupled via direct condensation. Table 2. Products from Carbon Chain Length Increase Reactions of the Selected Platform Compounds

C


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Figure 2. Dimers of HMF formed by cross-aldol condensation between acetone and two HMF molecules and by organocatalytic self-condensation.

catalyst.16 However, it is known from the literature that AL can form dimers and trimers by base-catalyzed carbon−carbon double-bond oligomerization, using anhydrous K2CO3 as a catalyst under solvent-free conditions.22,23 A free radical reaction mechanism for the C−C bond formation over K2CO3 catalyst was proposed by Xin et al.23 AL was very reactive and shown to produce 94 wt % dimers at 70 °C,22 as well as 65 wt % dimers, 33 wt % trimers, and 2 wt % tetramers at 80 °C.23 The dimer and trimer structures formed are presented in Figure 4. Figure 3. Primary levulinic acid dimers formed by aldol condensation and ketonization.

Only a few studies have been reported in the literature on these more straightforward alternatives for carbon chain increase reactions based on direct condensation of levulinic acid.20,21 A patent application submitted by Blessing and Petrus20 described the aldol condensation of levulinic acid in hydrogenating conditions on a metal-containing, strong acid, ion-exchange resin catalyst (Amberlyst CH28, 0.7% Pd). In the reaction done at 130 °C, 20 bar, and WHSV = 0.5 h−1, the conversion of levulinic acid was 30 wt %, producing mainly a mixture of dimers (26 wt %). The predominant dimers formed were the primary linear and branched aldol condensation products shown in Figure 3, but with the carbon double bond being hydrogenated. The main intention of Blessing and Petrus20 was to produce biofuel components from the levulinic acid dimers by further converting them to esters. Ketonization of levulinic acid using red mud bauxite mining waste as the catalyst was proposed by Karimi et al.21 The active catalyst consists of reduced iron oxide, silicate, aluminate, and carbide phases. Red mud can act as a multifunctional acid/base and hydrogenation catalyst under a hydrogen atmosphere. The multifunctionality of the catalyst allowed several parallel and subsequent reaction pathways to occur at 200 °C and 55 bar H2, producing a complex product mixture and, eventually, compounds with a high degree of oligomerization, causing coke formation on the catalyst. In addition, the selectivity for 2,5,8nonatrione, i.e., the ketonization product of levulinic acid (Figure 3), was low. The addition of water to the reaction mixture (50:50 wt %)tested at 365 °C and 55 bar H2 suppressed the extent of oligomerization, but did not improve the selectivity for ketonization. Nevertheless, in their study, both hydrogenating conditions and the addition of water were needed to control carbon chain increase reactions for levulinic acid. 3.3. α-Angelica Lactone. Very few reports on controlled oligomerization of angelica lactone (AL) can be found in the literature, but it has been proposed that AL is very prone to polymerization and coke formation in the presence of an acid

Figure 4. Formation of dimers and trimers of angelica lactone by carbon−carbon double-bond oligomerization.

Hydrotreating the dimers and trimers obtained by the C−C coupling of α-AL and β-AL resulted in branched alkanes suitable for use as transportation fuels.22−25 3.4. 2-Methylfuran. Carbon chain length increase reactions with 2-methylfuran can proceed via various reaction routes (Figure 5). In the hydroxyalkylation/alkylation route, 2methylfuran reacts first with a co-reactant, such as aldehyde or ketone and then with another 2-MF molecule, producing a structure of 12 carbon atoms or more. For this route, different molecules have been examined including acetone, butanal, 2pentanone, furfural, and ethyl levulinate as co-reactants. The catalysts used include para-toluene sulfonic acid, MCM-41, zeolites, and ion exchange resins (Amberlyst 15, Dowex 50WX2-100, Nafion-212).26,27 A special case of the hydroxyalkylation/alkylation route is 2MF trimerization. As early as the 1960s, it was shown that 2methylfuran forms 5,5-bis(5-methyl-2-furyl)pentan-2-one (trimer) through a condensation reaction in the presence of sulfuric acid.30 The reaction proceeds via the hydrolytic ring opening of one 2-methylfuran molecule to an aldehyde (4oxopentanal), followed by hydroxyalkylation/alkylation of the formed aldehyde with two molecules of 2-methylfuran.26,28,29 This trimerization can be done in aqueous sulfuric acid (50− 12−38 (wt %) 2-MF−H2SO4−H2O) at 75% yield and a highpurity (96%) trimer can be achieved after distillation.26 D


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Figure 5. Examples of hydroxyalkylation/alkylation routes for 2-methylfuran (2-MF): upper row shows examples with a co-reactant and lower row shows examples formed via self-condensation.

When 2-MF trimerization was performed in the presence of solid acid catalysts, the formation of a tetramer product was also detected. Under the same conditions (85 °C, 3 h), paratoluene sulfonic acid and dry Amberlyst-15 produced mainly tetramers, whereas aqueous sulfuric acid selectively produced trimers. By adding water to the reaction media, the selectivity over Amberlyst-15 changed from tetramers to trimers, but, at the same time, the conversion decreased. In addition, other catalysts were studied, and the results were similar: adding water to the reaction mixture increases the yield of trimers and decreases the conversion.29 The trimer selectivity can also be enhanced by conducting the reaction in an ethanol−water solvent.28 3.5. 2,5-Dimethylfuran. 2,5-dimethylfuran (DMF) is not very active in carbon chain increase reactions, because of the methyl groups at both α-positions.30 However, it is used to make p-xylene via Diels−Alder cycloaddition with ethylene and subsequent dehydration.31 Moreover, DMF has the potential to be used as an oxygenate blended with gasoline or diesel fuel. The advantages of DMF over ethanol include higher energy density, higher boiling point, and immiscibility in water.32,33 On the other hand, the compatibility of DMF with fuels systems, its toxicity, and influence on engine emissions (e.g., NOx, HC, and CO) must be studied further.33 3.6. 2,5-Dimethyltetrahydrofuran and 2-Methyltetrahydrofuran. Similarly to DMF, 2,5-dimethyltetrahydrofuran (DMTHF) and 2-methyltetrahydrofuran (MTHF) are rather stable compounds and do not easily participate in carbon chain length increase reactions. However, they have potential in solvent applications or as biobased fuel additives, because of their stability and low volatility.34−37 DMTHF and MTHF are not easily polymerized compared to THF, which readily participates in polymerization reactions.38 The methyl groups of MTHF and DMTHF function as sterical hinders, preventing polymerization reactions from occurring. 3.7. γ-Valerolactone. As such, γ-valerolactone (GVL) is rather stable and does not easily undergo self-condensation to form longer carbon chains. However, GVL can produce longer carbon chains via two different reaction pathways (Figure 6). GVL can undergo pyrolytic elimination of ester to pentenoic acid, which can either (a) experience decarboxylation to form

Figure 6. Reaction routes for producing longer carbon chains from γvalerolactone (GVL).

butenes or (b) go through hydrogenation to form pentanoic acid. Butene can further oligomerize to form alkenes, while pentanoic acid can form 5-nonanone through ketonization with simultaneous formation of carbon dioxide and water. GVL decarboxylation has been extensively studied over acid catalysts.39 Catalyst deactivation is a major problem during butene formation through GVL decarboxylation, but this can partly be overcome by adding water to GVL.

4. EXPERIMENTAL RESULTS AND DISCUSSION Carbon chain increase reactions for platform molecules based on our own experimental results are discussed in more detail in the following paragraphs. 4.1. Thermal Reactions of Levulinic Acid, α-Angelica Lactone, and γ-Valerolactone. When performing catalytic chain increase reactions, it is important to know at which temperature thermal reactions start to occur. Thermal reactions typically cause undesirable side reactions such as oligomer formation, yielding products with chain lengths exceeding 25 C atoms. The thermal stability of LA, AL, and GVL was investigated in a plug-flow reactor at a temperature range of 70−350 °C (Figure 7). Oligomers were observed from AL at 70 °C (4%) and from LA at 150 °C (0.2%), whereas no oligomers were found from GVL, even at 350 °C. The density of the samples also increased with increasing oligomer content. An increase in density indicates the formation of highermolecular-weight products. At 300 °C, there was a pressure buildup in the reactor with levulinic acid, most probably due to E


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related to the possible formation of high-molecular-weight products and their accumulation in the resin pores, preventing the access of LA to the active sites. Identification of LA dimers formed on the metal-containing resin catalyst at increased hydrogen pressures confirmed the gradual removal of the most-reactive CC and CO sites. The main products identified by GC-MS are shown in Figure 8. The illustration is highly simplified, as many isomer structures also exist.

Figure 7. Oligomer formation determined by gel permeation chromatography (GPC), as a function of temperature in a tubular reactor over a bed of quartz sand. [Legend: AL = angelica lactone, LA = levulinic acid, GVL = γ-valerolactone.]

extensive formation of oligomers plugging the quartz bed. The pressure buildup inhibited gas from flowing through the reactor, which meant that the experiment had to be stopped. 4.2. Levulinic Acid Chain Length Increase Reactions. In our study on the self-condensation of levulinic acid, the objective was to find catalyst solutions capable of producing LA dimers and oligomers within the fuel range−without adding any solvents, such as water, and minimizing the contribution from levulinic acid thermal reactions. The potential of ion-exchange resin catalysts to increase the degree of aldol condensation was evaluated in more detail. Since the use of ion-exchange resin catalysts is restricted by their rather low maximum operational temperature, commonly being between 120 °C and 140 °C, we expanded the study of alternative catalysts to include more thermally stable inorganic-oxide-based materials. The effect on the degree of condensation of using a hydrogenating or inert atmosphere during aldol condensation was studied by using 2-pentanone as a model compound. The experiments were done in a semibatch reactor with a metalcontaining ion-exchange resin catalyst (Amberlyst CH28) in hydrogen flow and a nonmetal resin catalyst (Amberlyst 36) in nitrogen flow, respectively. The results showed that the hydrogenation activity in resin catalyst restricted aldol condensation products almost completely to dimer formation, while the nonmetal resin catalyst was able to form oligomer products−i.e., mainly trimers and tetramers. This proved the ability of C5 ketones to achieve a higher degree of aldol condensation with the microporous ion-exchange resin catalyst under non-hydrogenating conditions. Since a long catalyst lifetime is valuable in industrial processing, the stability of the two ion-exchange resin catalysts presented above was studied for aldol condensation of LA in a continuous flow reactor at 120 °C for 750 h. The metalcontaining resin catalyst showed stable performance and also, as expected, high selectivity for LA dimer formation. On the other hand, with the nonmetal resin catalyst, very fast deactivation was observed as the conversion decreased to zero within 200 h. It is proposed that the hydrogenation activity of palladium reduces part of the reactive sites in LA dimers and thereby controls their further growth. Stable performance indicates that the resin pores allow for the effective diffusion of LA dimers. Deactivation under nonhydrogenating conditions can now be

Figure 8. Main LA dimers identified by GC-MS from aldol condensation on Amberlyst CH28 (0.7% Pd) with the dimer composition moving downward with increasing hydrogen pressure.

It was observed that carbon−carbon double bonds were easily saturated under hydrogenating conditions. Increasing the hydrogen pressure removed the ketone carbonyl groups CO from diacid dimers by direct hydrodeoxygenation (HDO) and in secondary lactone formation. Since the porosity of ion-exchange resin catalysts is primarily related to their micropores in the gel phase, this property is proposed as the main reason for the fast fouling of resin pores under nonhydrogenating conditions. Strong acidity, on the other hand, is seen as beneficial for high activity in the aldol condensation of levulinic acid. Based on these assumptions, alternative inorganic-oxide-based materials with strong acidity, but with a pore structure that allowed for an easier diffusion of product molecules, were seen as potential candidates to achieve a higher degree of aldol condensation with LA. Zeolite-based microporous materials possess strong acidity. Improved accessibility to these strong acid sites can be achieved by embedding small zeolite crystallites in a mesoporous matrix. A representative of this type of hierarchical material (MMBeta)7based on beta zeolite embedded in the regular, tubular pores of MCM-41was selected for evaluation as an alternative acid aluminosilicate-based catalyst. With this catalyst, it was possible to improve the selectivity for compounds of sizes corresponding to those of LA trimers and tetramers (Figure 9). In addition, even without any hydrogenation activity, the catalyst performance was relatively stable during the testing period, which lasted for 400 h. The reaction temperature of 200 °C used for MM-Beta was at the threshold for the thermal stability of levulinic acid (Figure 7), and no higher temperatures were studied. Even if the conversion was slightly higher on Amberlyst CH28 at 120 °C, the hydrogenation activity effectively restricted the aldol condensation reaction to dimer formation. At the elevated reaction temperature used with MM-Beta, a rather complex reaction mixture was obtained containing byproducts such as F


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A heterogeneous, metal-oxide-supported, potassium-based catalyst exhibited good activity in oligomer formation. Under the most severe reaction conditions used (177 °C, WHSV 0.2 h−1), the liquid product mixture contained high amounts of AL dimers, trimers, tetramers, and higher oligomers (Figure 10). The supported K-based catalyst produced an oligomer mixture with a higher degree of oligomerization, compared to K2CO3. In addition, it was observed that α-AL was partly transformed to the β-AL isomer. This transformation is important, since it is proposed that both isomers participate in dimer formation. The reaction temperature of 177 °C was at the threshold for the thermal stability of angelica lactone (Figure 7) and no higher temperatures were studied. The lower reactivity of the supported potassium-based catalyst and the higher reaction temperature required were expected, since it is well-known that the interaction of active sites with an oxide support can decrease their strength, compared to corresponding bulk compounds−in this case, supported K species and bulk K2CO3, respectively. Also, the number and accessibility of active sites can be higher on bulk compounds. Nevertheless, the high selectivity for oligomer formation makes the supported K-based catalyst a good candidate for the oligomerization of AL. 4.4. 2-Methylfuran Chain Length Increase Reactions. As mentioned in the previous chapter, the trimer selectivity of 2-methylfuran oligomerization can be enhanced by conducting the reaction in an ethanol−water solvent. However, water and 2-methylfuran are not miscible with each other, to a large extent.40 In practice, to be able to conduct the experiments easily in continuous flow reactors, the reactants should be miscible. Ethanol is miscible with water and 2-methylfuran and is thus a suitable solvent for the process. In experiments carried out by Chavez-Sifontes et al.,28 a 2-MF/solvent ratio of 50/50 (wt %) was used. In our experiments, we tried decreasing the amount of ethanol. Therefore, the solubility of water and ethanol with 2-MF was tested before the experiments. These results are shown in Table 3. According to this data, only 2

Figure 9. Qualitative product distribution (GPC area %) of liquid phase sample in levulinic acid carbon chain increase experiment over aluminosilicate-based MM-Beta catalyst, compared to Amberlyst CH28 (0.7% Pd).

angelica lactone and some aromatic compounds. The latter indicates that secondary reactions of LA derivatives may occur, complicating the overall reaction scheme. 4.3. Angelica Lactone Chain Length Increase Reactions. The product distribution obtained in our oligomerization experiment with α-AL as feed over anhydrous K2CO3 under reaction conditions similar to those reported in the literature (70 °C) is shown in Figure 10. In our experimental

Table 3. 2-Methylfuran−Ethanol−Water Miscibility Tests

Figure 10. Qualitative product distribution (GPC area %) of liquidphase sample in α-angelica lactone carbon chain increase experiment over supported potassium-based catalyst, compared to anhydrous K2CO3 powder.

sample

2-methylfuran (wt %)

ethanol (wt %)

water (wt %)

miscible

1 2 3 4 5 6 7 8

80 80 80 80 73 69 66 61

20 18 14 10 22 22 29 31

0 2 6 10 6 9 5 8

yes yes no no no no yes yes

wt % of water is soluble in the 80/20 2-MF/ethanol solution. When the ethanol amount is increased to 30 wt %, up to 8 wt % of water can be dissolved. The effect of water and ethanol on 2-methylfuran oligomerization was tested in batch reactor experiments. Three experiments were done where the reaction medium was either 2-MF, 2-MF−ethanol, or 2-MF−ethanol−water (Figure 11). In the experiments, dried catalyst was used to ensure that no water was coming from the catalyst. The catalyst used in all three experiments was ion-exchange resin Amberlyst 16. Figure 11 shows that the conversion of 2-methylfuran is similar when only 2-MF or 2-MF−ethanol (80/20 wt %) were

procedure, a higher degree of oligomerization of AL was achieved, compared to results reported in the literature. The challenge with the K2CO3 catalyst was related to stability, as K2CO3 was partly transformed to KHCO3 during the reaction in our experiment. The goal for AL oligomerization was thus to find a more robust heterogeneous catalyst with high activity for the formation of AL dimers, trimers, and tetramers, in order to optimize the yield of compounds suitable as traffic fuel components after HDO. G


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Figure 12. Qualitative product distribution (GPC area %) of liquid phase sample in 2-methylfuran carbon chain increase experiment over Amberlyst XE586 (wet) and Amberlyst 16 (wet). Figure 11. Qualitative product distribution (GPC area %) of liquid phase sample in 2-methylfuran carbon chain increase experiment conducted in various reaction media over Amberlyst 16. The amount of water and ethanol are not taken into account in the graph.

4.5. Summary. Several carbon chain increase routes with platform molecules have been reported in the literature. However, in order to minimize the process complexity, the preferred reaction route would be self-coupling, i.e., a one-step condensation/oligomerization reaction without the need for any additional reactants. For industrial production of biofuel components, a process solution based on heterogeneous catalysts is to be preferred. Of the primary platform molecules (HMF, furfural, LA), only LA fulfilled the criteria given above, regarding the reaction route and catalyst. With a metal-containing ion-exchange resin catalyst, selective production of LA dimers can be obtained. The hydrogenating activity was shown to effectively restrict further carbon chain growth and simultaneously provide the stable performance of the catalyst. The target to extend the condensation of LA to products in the trimer and tetramer carbon chain range was shown to be successful with an acid hierarchical catalyst containing small zeolite crystallites. With a suitable combination of acidity and porosity in this catalyst, stable performance was achieved without adding any hydrogenating activity. Self-condensation of furfural and HMF is more complex and cannot easily be carried out over solid catalysts, thus making it less attractive, from an industrial process point of view. Of the secondary platform molecules (AL, 2-MF, GVL), AL and 2-MF were active in self-coupling reactions and both have heterogeneous catalyst alternatives available. The oligomerization of AL into mainly dimers, trimers, and tetramers was shown to be successful with a more robust and chemically stable oxide supported K-based catalyst, compared to the salt K2CO3 reported thus far. Trimers and tetramers of 2-MF are easily obtained with an ion-exchange resin catalyst. One specific property of 2-MF is the ability to tune the trimer-to-tetramer ratio in the product. This can be done not only by adding water and/or ethanol to the reaction mixture, but also by the choice of catalyst properties. The importance of large pores for the formation of tetramers was demonstrated here by using an ionexchange resin catalyst, where the location of the active sulfonic acid groups was restricted to the walls of the permanent mesopores. The heterogeneous oxide catalyst solutions presented for LA and AL required elevated temperatures, compared to those

used as the reaction medium. In contrast, when both ethanol and water were added, the conversion decreased dramatically. The addition of ethanol to the reaction medium did not affect the conversion, but the selectivity for trimers increased, compared to pure 2-MF as a reactant. The dehydration of ethanol to ethylene and water at 100 °C proposed by ChavezSifontes et al.28 was not verified in our experiment and might be of minor importance, because of the lower reaction temperature (60 °C) used. Ethanol as a polar compound modifies the acidity of the active sulfonic acid groups in the resin catalyst and this may also be a reason for the altered selectivity. The conversion obtained when water was added with ethanol to the reaction medium was unexpectedly low, but the presence of water supports the high selectivity for trimers that was observed. The change in selectivity with water in ethanol, in our case, cannot be directly compared to the selectivity obtained with only ethanol added, because of the relatively high difference in conversion levels. As a result of the promising results obtained with conventional ion-exchange resin catalysts, the influence of the location of the active sites was evaluated by using a corresponding ion-exchange resin catalyst, Amberlyst XE586, where sulfonation was restricted to the walls of the permanent mesopores, leaving the micropores in the gel phase unsulfonated.41 The product distribution obtained is shown in Figure 12, with the result from Amberlyst 16 as a reference. The conversion was in the same range for the two resin catalysts, despite the much lower number of active sites on Amberlyst XE586 (∼1 mequiv H+/gcat), relative to Amberlyst 16 (∼5 mequiv H+/gcat). This indicates better availability of active sites in permanent mesopores, compared to active sites in gel-phase micropores, where the availability is dependent on the polarity of the reaction medium. Compared to Amberlyst 16, selectivity for tetramers increased significantly with Amberlyst XE586, where the reaction occurs only in the mesopores. H


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Industrial & Engineering Chemistry Research used in the reactions with ion-exchange resins and alkali metal salt, respectively. At these reaction temperatures, i.e., at the threshold of thermal stability of platform molecules, it cannot be excluded that thermal reactions could occur to some extent and contribute to the products formed in the carbon chain increase reactions. Each hydrogenation step prior to the formation of a platform molecule will decrease its reactivity, because of the removal of reactive oxygen-containing functional groups or carbon double bonds. For GVL, MTHF, DMF, and DMTHF, the ring opening of the internal ester or ether is required to form functional groups active in condensation or oligomerization reactions. The ring opening of GVL is less demanding, compared to that of the three other molecules. The presence of methyl groups at both α-positions in DMF and DMTHF further restricts their reactivity. The chemical stability of these four compounds makes them suitable as solvents. They have also been proposed as biofuel components as they possess some physical properties, similar to gasoline.

CnHmOk + (n + 1 + k −

5. HYDROGEN CONSUMPTION FOR PRODUCING HYDROCARBONS Current specifications of transportation fuels would require the conversion of biogenic platform molecules or their oligomers to hydrocarbons. Therefore, a major challenge is their high oxygen content, and, at the same, a deficiency of hydrogen (see Figure 13). For most of the compounds listed in Table 2, the

m )H 2 → CnH 2n + 2 + k H 2O 2

Figure 14. Hydrogen requirement and mass yield in conversion of platform compounds and their oligomers to hydrocarbons. Hydrogen requirement is based on the theoretical hydrogen amount needed for producing alkane and water from the dimer oxygenates.

Although the energy yield to hydrocarbons is higher than the mass yield, a substantial amount of the energy content of the hydrogenated product can be considered to originate from the hydrogen introduced. The lower heating value (LHV) of hydrogen for the complete HDO of furfural dimer is 42% of the LHV of the corresponding hydrocarbon product (decane). This means that almost half of the heating value of decane originates from the added hydrogen. The calculation is based on the theoretical amount of hydrogen needed for removing double bonds and oxygen (producing alkane and water) and on the LHVs of hydrogen (119.96 MJ/kg) and decane (44.24 MJ/ kg).42 Therefore, in order to fulfill the requirements for a renewable fuel, production routes via HDO of such oxygen-rich compounds may also require that hydrogen originate from renewable sources.

6. CONCLUSIONS Carbon chain length increase reactions are necessary for producing gasoline, diesel, and jet fuel from lignocellulosederived platform molecules. The type of carbon chain increase reactions that occur is dependent directly on the functional groups of the platform molecules. Some functionality is needed for carbon chain increase reactions to occur, but it can be difficult to steer compounds exhibiting several different functional groups simultaneously in the desired direction or in a specific chemical reaction pathway. Sterical hinders such as methyl groups may have a great influence on the capability of platform molecules to take part in carbon chain length increase reactions. The functionality, and thereby the reactivity, of model compounds can be modified by hydrotreating the platform molecules to varying degrees. Hydrogen plays a key role in producing fuels from platform molecules, both in modifying the reactivity of platform molecules and in removing oxygen from the final products. After removing all oxygen from

Figure 13. Van Krevelen diagram of the platform compounds and their oligomers. The H:C and O:C ratios in the figure originate directly from the composition of the molecules.

oligomerization reactions do not change the O:C and H:C atomic ratios, since no cleavage of water or carbon dioxide occurs. Therefore, the symbols of monomers and oligomers in Figure 13 overlap. Even at a theoretical yield, the complete removal of oxygen as water and saturation of all double bonds requires a large amount of hydrogen, 8.7%−12.6% of the mass of the sugar-derived platform compounds. For comparison, the HDO of tripalmitoylglycerol requires only ∼3 wt % hydrogen. In addition, because of the large amount of oxygen removed, the mass yield becomes low (Figure 14). The figure is based on basic stoichiometry: I


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Industrial & Engineering Chemistry Research the final products, a substantial amount of the energy content in the hydrocarbon originates from the added hydrogen, as shown by some basic calculation examples.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +358 50 458 2760. E-mail: [email protected]. ORCID

Mats Käldström: 0000-0003-4175-1137 Kaisa Lamminpaä :̈ 0000-0002-5227-1123 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to show our gratitude to Michel Schellevis for his invaluable work as a trainee at Neste. We are also grateful to Maaria Seläntaus and others at the Department of Chemistry at the Neste R&D. The technicians at Neste R&D are also greatly acknowledged for conducting the experimental work presented in this paper.

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ABBREVIATIONS AL = angelica lactone DMF = 2,5-dimethylfuran DMTHF = 2,5-dimethyltetrahydrofuran HDO = hydrodeoxygenation HMF = 5-hydroxymethylfurfural LA = levulinic acid LHV = lower heating value 2-MF = 2-methylfuran MTHF = 2-methyltetrahydrofuran WHSV = weight hourly space velocity

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Carbon Chain Length Increase Reactions of Platform Molecules

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