Synthetic Middle-Distillate-Range Hydrocarbons via Catalytic

Aug 11, 2015 - An integrated process for production of jet-fuel range olefins from ethylene using Ni-AlSBA-15 and Amberlyst-35 catalysts. B. Hari Babu...
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Synthetic Middle-Distillate-Range Hydrocarbons via Catalytic Dimerization of Branched C6−C8 Olefins Derived from Renewable Dimethyl Ether Mayank Behl,† Joshua A. Schaidle,† Earl Christensen,‡ and Jesse E. Hensley*,† †

National Bioenergy Center and ‡Transportation and Hydrogen Systems Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: 2,3,3-Trimethyl-1-butene (triptene) and other branched C6−C8 olefins, having structures characteristic of the products from the low-temperature acid-catalyzed homologation of dimethyl ether (DME), were converted to distillate-range hydrocarbons (C10−C20) with high selectivity via dimerization over a commercial ion-exchange acidic resin (Amberlyst-35) under liquid-phase stirred-batch conditions operating at ambient pressure. Triptene conversion and dimer (2,2,3,5,5,6,6heptamethyl-3-heptene) production were monitored with time at different temperatures (60, 80, and 100 °C). The dimer production rate increased with increasing temperature; however, dimer concentration decreased with increasing temperature due to competing side reactions. Dimerization, as compared to cracking, isomerization, and oligomerization, was the dominant reaction pathway during the first hours of reaction at all temperatures. Dimerization at 100 °C achieved a conversion of 35% and a molar selectivity to the desired dimer of 71% in 2 h. In longer runs (≥16 h), the highest conversion (80%) was achieved at 100 °C whereas the maximum total C10+ production (1.83 g/batch, 52% by weight of the reactant) was achieved at 80 °C. The nucleophilicity and extent of branching of the C6−C8 olefins were found to have a strong effect on dimerization yields. The cloud point, boiling range, carbon-number distribution, and lower heating value of the dimerized product were compared to ASTM specifications for middle-distillate fuels, and the results suggest that approximately 80% of the product has potential as a jet fuel blend stock.

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INTRODUCTION Among the numerous routes being pursued to reduce our dependence on conventional, fossil-fuel-based energy sources, the biomass-to-liquid-fuel approach remains one of the most promising in terms of its immediate impact and compatibility with existing infrastructure.1−4 Gasification of nonedible lignocellulosic biomass and subsequent conversion of the gaseous intermediates to methanol and dimethyl ether (DME) are well-known.1−3 However, the integration of methanol and DME as fuels into our energy infrastructure is limited due to their dissimilarities with long-chain hydrocarbons. Specifically, they differ in mass and energy density, lubricity, volatility, water solubility, and viscosity.5,6 Consequently, petroleum-derived hydrocarbons remain as the main constituent of middledistillate-based fuels. One way to increase the share of renewable biofuels in the distillate range is to first produce ethene or butene via dehydration of ethanol or butanol followed by oligomerization.7−12 This reaction has proven to be successful in producing gasoline-range hydrocarbons; however, achieving selective synthesis of hydrocarbons in the jet/diesel range is challenging due to broad product distributions and low yields of middle distillates.7,8 Another approach to increase the share of renewable biofuels is to use methanol/DME as building blocks for energy-dense hydrocarbons.13−15 Recently, it has been shown that it is possible to selectively produce paraffins and olefins in the C4− C7 range via low-temperature homologation of DME over BEA zeolite and metal-modified BEA zeolites.16−19 Due to the relative stabilities of the intermediate carbenium ion transition © 2015 American Chemical Society

states, the major products comprise four-carbon backbone structures (e.g., 2,3,3-trimethyl-1-butene (triptene) and 2methyl-2-butene). An inherent mechanistic feature associated with this C1 homologation is a termination step that proceeds via hydride transfer, which limits chain growth mainly to C7 or lower molecular weight molecules, with C8 undergoing facile βscission to produce C4 hydrocarbons.17−19 Therefore, the product is limited to gasoline-type fuels. Nevertheless, the product mixture contains a significant fraction of C4−C8 branched olefins that can potentially be coupled to upgrade the product into the distillate boiling range. Herein, we report that it is possible to upgrade triptene (C7) and other branched C6−C8 olefins, such as those derived from low-temperature acid-catalyzed DME homologation, to jet/ diesel-range hydrocarbons (>C9) via catalytic dimerization over a commercially available acidic ion-exchange resin (Scheme 1). We investigated the effect of reaction conditions, such as temperature, solvent type, reaction duration, and reactant structural properties, on the conversion, product distribution, and dimer production under liquid-phase stirred-batch conditions. Finally, we evaluated the fuel potential of the resultant product mixture by comparison to ASTM specifications for boiling range, lower heating value, and cloud point. Received: May 26, 2015 Revised: July 22, 2015 Published: August 11, 2015 6078

DOI: 10.1021/acs.energyfuels.5b01175 Energy Fuels 2015, 29, 6078−6087

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Energy & Fuels Scheme 1. Dimerization Reaction of DME-Derived Olefins over a Sulfonic Acid Resin To Produce Distillate-Range Hydrocarbonsa

a

Apart from dimerization, the overall conversion of reactant olefins also involves isomerization, cracking, and oligomerization.

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The percentage yield of the isomer product, Y′(t), was calculated according to

MATERIALS AND METHODS

Batch-dimerization reactions were conducted at local ambient pressure (83 kPa) at three temperatures (60, 80, and 100 °C) and with nonane and pentadecane as solvents for various durations (2−22 h). Experiments were carried out in a 100 mL three-neck round-bottom glass flask that was designed to model a stirred-batch liquid-phase reactor. A hot plate was used to heat the flask, and temperature was maintained using a thermostat. Temperature was measured continuously by a mercury thermometer in contact with the reaction mixture. The batch reaction headspace was continuously purged with nitrogen. A water-cooled reflux condenser operating with cooling water at 9 °C was connected to the flask at the outlet of the nitrogen purge, which limited the loss of reactant and product vapors. One of the necks on the glass flask served as a port for online-sampling. Amberlyst-35 (dry), a macroporous, sulfonated ion-exchange resin (300−800 μm size distribution; proton site (H+) density ≥ 5.0 equiv/kg), supplied as a sample from Rohm & Haas, was used as the catalyst. Chemical reagents were purchased from Sigma-Aldrich and were used as supplied without any purification. 2-Methyl-1-hexene (96%), 2,3,3trimethyl-1-butene (triptene, 98%), 2,3-dimethyl-1-butene (97%), and 2,4,4-trimethyl-1-pentene (96%) were used as reactants. n-Nonane (≥99%) and pentadecane (≥99%) were used as solvents. The amounts of reagents and catalysts used in a typical batch were 3.2−3.4 g of Amberlyst-35, 34−37 mL of solvent, and 5 mL of the reactant olefin. The selected catalyst-to-solvent/catalyst-to-reactant ratios have previously proven to be successful for dimerization over Amberlyst-type catalysts.20 Solvents that have higher molecular weight than the reactants, such as nonane and pentadecane, were selected to minimize the vapor pressure of the mixture. A high solvent-to-reactant ratio (ca. 7:1) was selected to further decrease the vapor pressure. This helped in minimizing any errors in concentration measurements due to possible loss of mixture components via vaporization. The stirring speed was 500 rpm with a PTFE stir bar. This stirring rate was considered sufficient for overcoming any interparticle mass transfer diffusion limitations in macroporous resins.21 For experiments involving the reaction of mixed olefins, the mixture composition was simulated to resemble the product molar ratios that are typically obtained during DME homologation over HBEA at 200 °C (i.e., 2,3,3trimethyl-1-butene:2,3-dimethyl-1-butene:2,4,4-trimethyl-1-pentene = 3:1:2).17−19 Reactant conversion X(t) at time t was calculated according to X(t ) =

CA0 − CAt × 100 CA0

Y ′(t ) =

t CA → 2A 2C 2A × 100 = × 100 CA0 CA0

(3)

CtA′

is the concentration of the isomer product at any given time where t and CA→A′ is the molar amount of reactant used in producing the isomer. The dimer production and total production per batch were defined as per eqs 4 and 5, respectively: t dimer production(t ) = VC2A MW2A

total production(t ) = V

∑ i ≥ C10 +

(4)

(Cit MW) i (5)

where V is the volume of reactant contents in the reactor, MW2A is the molecular weight of the dimer product, Cti is the concentration of the ith product at time t, and MWi is the molecular weight of the ith product. The total production was defined as the sum of hydrocarbon products having a carbon number greater than or equal to 10. The effect of temperature was studied by keeping all other reaction variables (i.e., substrate concentration, catalyst amount, reaction duration, stirring speed, and reaction pressure) constant. The selection of operating temperatures was based on the previously reported stable operating temperature of the catalyst (≤120−140 °C) and the refluxer’s ability to condense vapor-phase reactants and products.22,23 Gas chromatography−mass spectrometry (GC-MS) was used to quantify reaction products via an Agilent 7890A equipped with a capillary column (Agilent J&W HP-5, nonpolar). The GC was connected to a mass selective detector (MSD, Agilent 5975C) for product identification, which used electron ionization with a 10 μA emission current. A flame ionization detector (FID) was also used for product quantification. The GC was operated with an inlet temperature of 260 °C and a split ratio of 100:1. The GC oven temperature started at 100 °C and then increased to 114 °C at a rate of 10 °C·min−1 and then further increased to 160 °C at a rate of 20 °C·min−1 with a hold of 1.5 min. The temperature was then further increased to 260 °C at a rate of 20 °C·min−1 with a hold of 4.3 min. Samples and standards were diluted by 100 volumes of nonane prior to injection. The concentration of reactants and products in a given sample were calculated based on the area under the FID peak and an FID response factor determined from the calibration curves. Representative GC chromatograms of the product mixture are provided in the Supporting Information (Figure S4). 1-Heptene, 2methyl-1-hexene, 2,3,3-trimethyl-1-butene, 2,3-dimethyl-1-butene, 2,4,4-trimethyl-1-pentene, 1-decene, 1-dodecene, n-tridecane, 1tetradecene, n-pentadecane, and 1-hexadecene were used as reference standards for calibration. The structures of individual major products were determined by comparing their mass spectra with the standard references provided in the NIST database (See Figure S5 in the Supporting Information). Structural assignments to the major products were based on a probability match of at least 70%. For experiments conducted at 100 °C, the carbon balances were greater than 95% at t ≤ 2 h and less than 60% at t ≥ 17 h. In comparison, the

(1)

where C0A and CtA are the reactant’s initial concentration and the concentration at time t, respectively. The percentage yield of the dimer product, Y(t), was calculated according to Y (t ) =

CA → A ′ CAt ′ × 100 × 100 = CA0 CA0

(2)

Ct2A

is the concentration of the dimer product at any given time where t and CA→2A is the concentration of reactant used in producing the dimer. 6079

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Figure 1. (a) Triptene conversion and (b) corresponding dimer and total production at three different temperatures (60, 80, and 100 °C) with nonane as solvent. Dashed/solid curves only serve as a guide to the eye. carbon balances for experiments conducted at 60 and 80 °C were greater than 90% at all times. Products for ASTM analyses were isolated with vacuum-assisted distillation. The still was operated at 20 kPa (∼150 mmHg) and 95 °C for 80−90 min. The boiling range of the final product was determined using ASTM method D2887 for simulated distillation by GC. The carbon-number distribution of the product was estimated from the simulated distillation chromatogram and calibrated FID response factors. The lower heating values of the products were determined by ASTM D240. Weight percent carbon and hydrogen were determined by combustion analysis. The freezing point and cloud point of the product was measured using a Phase Technologies 70X Analyzer following ASTM methods D5972 and D5773, respectively. The cloud point method was adjusted to a slower cooling rate of 1.5 °C·min−1, compared to the typical rate of 15 °C·min−1. Phase transitions were also estimated with differential scanning calorimetry (DSC) on a TA Instruments Q200 differential scanning calorimeter at a rate of 0.5 °C· min−1from 25 to −75 °C. The product fractions of primary, secondary, tertiary, and quaternary carbon species were determined using 13C

NMR spectroscopy. A series of distortionless enhancement by polarization transfer (DEPT−NMR) experiments with pulse angles of 45°, 90°, and 135° were performed. NMR samples were prepared by mixing approximately 0.5 mL of product with 0.5 mL of deuterated benzene in an NMR tube. All spectra were acquired with a Bruker Avance III spectrometer with a 14.1 T magnet (13C-resonance frequency = 150.9 MHz; 1H-resonance frequency = 600 MHz) using a Bruker BBO 5 mm probe. Quantitative 13C NMR spectra were acquired with inverse-gated decoupling and a recycle delay of 60 s. Representative 13C NMR spectra obtained from product analysis are provided in the Supporting Information (Figure S6). A ramé-hart model 200 standard contact angle goniometer was used in the Sessile drop mode to measure contact angles between liquid and solid phases. Solvent drops were placed on a flat sheet of Nafion-117 membrane to mimic a sulfonated polymer, as the spherical beads of Amberlyst-35 prevented measurements on the actual catalyst. The Nafion sheet was pretreated by washing in deionized boiling water for 2 h followed by soaking in 0.5 M H2SO4 (80−90 °C) for 1 h and rinsing in deionized boiling water for 1 h. 6080

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RESULTS AND DISCUSSION Effect of Temperature. The role of operating temperature on dimerization of triptene was studied by performing the reaction at three different temperatures. As shown in Figure 1a, the consumption of triptene was greater at higher temperatures for any given reaction time. For example, triptene conversion after 2 h was approximately 4%, 13%, and 35% at 60, 80, and 100 °C, respectively. For a given temperature, the diminishing slope of the conversion−time profile with increasing conversion suggests that the rate of triptene consumption started to decrease as more triptene was consumed indicating a positive reaction order with respect to the reactant. The final conversions, as measured at 60, 80, and 100 °C (at different total reaction times), were approximately 18%, 51%, and 80%, respectively. Figure 1b compares the dimer production (C14) and total hydrocarbon production (C10+) at various times corresponding to the three different temperatures. The initial rate of production was highest at 100 °C. The dimer and total production continued to increase rapidly during the initial hours. At t ≈ 2 h, the total production was 1.1 g and the dimer product comprised approximately 75% by weight of the total production (0.82 g). In comparison, the total production amounts achieved during the same time at 80 and 60 °C were 0.15 and 0.07 g, respectively. For all temperatures, the main product was 2,2,3,5,5,6,6-heptamethyl-3-heptene (ditriptene), a dimer obtained from triptene. This product is characteristic of chain growth reactions on Brønsted acids, such as Amberlyst35, which are known to proceed according to carbocation mechanisms as illustrated in Scheme 2 for the dimerization of Scheme 2. Dimerization of Triptene over a Sulfonic Acid Resin To Produce Ditriptene (2,2,3,5,5,6,6-Heptamethyl-3heptene)

triptene over a sulfonic acid resin.24 This type of reaction is typically initiated by catalyst-assisted proton addition to the olefin resulting in formation of a carbocation on the catalyst surface. A liquid-phase olefin molecule then adds to this surface intermediate to form a dimer that subsequently desorbs back into the liquid phase. The productivity of such a reaction is known to vary considerably with reaction temperature.21 Dimer production at 100 °C started to decline after the first 2 h. After approximately 17 h of reaction, dimer and total production decreased to 0.28 and 0.85 g, respectively (Figure 1b). On the other hand, production at 80 and 60 °C did not encounter any temporal deterioration. Dimer and total production after long reaction times (≥16 h) were 1.21 and 1.83 g at 80 °C, and 0.57 and 0.62 g at 60 °C. The distinct drop in dimer production with increasing conversion at 100 °C suggests that side reactions, mostly cracking of the dimerized product, become significant at higher temperatures. The variation in relative product composition with increasing triptene conversion for the three temperatures is shown in more detail in Figure 2. For the reaction run at 100 °C, Figure

Figure 2. Evolution of products with increasing triptene conversion at (a) 100, (b) 80, and (c) 60 °C. Dashed lines are only to guide the eye.

2a shows the changes in relative concentrations of the dimer and the main side products. Most of the side products observed were lighter than C14 (C10−C12 hydrocarbons), and were highly branched, including products such as 2,2,3,5,6-pentamethyl-3heptene and 2,3-dimethyl-3-octene. Products lighter than C10 were negligible. Isomers of triptene were not detected. The production of C2−C4 molecules, which are the stoichiometric co-products of the C10−C12 hydrocarbons, was assumed but was not quantified. During the initial stages of the reaction (conversion < 35%; time < 2 h), the relative dimer content in 6081

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Figure 3. Product composition (C10+) and dimer production as a function of temperature at (a) two equivalent reaction times (t) and (b) at two equivalent conversions (X).

the product mixture remained in excess of 70% while no trimers or heavier products were observed. The decrease in dimer concentration at higher conversion is met with an increase in concentration of lighter products indicating dimer cracking as a competing side reaction. The corresponding concentration− time profiles are provided as a reference in Figure S1a. In comparison, the extent of formation of cracked products was less severe at lower temperatures. Though cracking was also observed at 80 and 60 °C, the generation of cracked products was comparatively much lower at increased conversion as shown in Figure 2b,c (and Figure S1b,c). This result suggests that, at higher reaction temperatures, the dimer product forms quickly but is also more susceptible to cracking. The extent of

cracking is also expected to increase with the average C−C bond density per molecule in the reacting mixture because additional energetically favorable modes become available for βscission.25 Thus, the comparatively faster dimer formation at 100 °C further contributes to cracking. The relative amounts of dimer and side products as a function of temperature compared at equivalent reaction time and conversion are shown in Figure 3a,b, respectively. The dimer production amounts, in relation to temperature, time, and conversion, are also shown. During the initial stages of the reaction, the dimer production increased with time and temperature but the relative dimer content in the product mixture (i.e., dimer selectivity) decreased. For example, the 6082

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Energy & Fuels dimer production after 2 h increased from 0.07 to 0.82 g while the dimer selectivity decreased from 100% to 71% at 60 and 100 °C, respectively. Comparing the dimer production and product selectivity at equivalent conversion further illustrates the impact of temperature on triptene dimerization (Figure 3b); dimer production and selectivity are higher at 60 °C than at 100 °C at an equivalent conversion of 18%. Specifically, dimer selectivity decreased from 92% to 75%, and dimer production decreased from 0.57 to 0.43 g going from 60 to 100 °C, respectively. It is relevant, however, that it took approximately 16 h to achieve a conversion of 18% at 60 °C, whereas it took 1 h to achieve the same conversion at 100 °C (Figure 1a). Effect of Olefin Nucleophilcity and Skeletal Structure. The performance of triptene dimerization was compared with that of 2,3-dimethyl-1-butene and 2,4,4-trimethyl-1-pentene to determine differences in reactivity due to carbon length and structure. These olefins are also among the major products of low-temperature acid-catalyzed DME homologation.17−19 Parts a−c of Figure 4 compare time-varying fractional concentrations of these three different reactants and the corresponding yields of different product types generated during their dimerization. It can be seen that 2,4,4-trimethyl-1-pentene (Figure 4a) and 2,3-dimethyl-1-butene (Figure 4b) were more reactive than triptene (Figure 4c), but the overall reaction was less selective to dimerization. As shown in Figure 4a, the resulting dimer yield for 2,4,4-trimethyl-1-pentene was quickly eroded due to faster dimer cracking. For 2,3-dimethyl-1-butene dimerization, the competing isomerization reaction resulted in formation of a less reactive nonterminal olefin (i.e., 2,3-dimethyl-2-butene from isomerization of 2,3-dimethyl-1-butene as shown in Figure 4b and Figure S2). Unlike the other olefins, triptene is comparatively less reactive, but it appears to exhibit more resistance to side reactions (Figure 4c). For triptene, isomerization and cracking were restrained and dimerization was the dominant reaction. This is likely due to the relatively higher energy barriers associated with isomerization and βscission of triptene.18 In order to understand why triptene was more amenable toward dimerization, we compared the structural and electronic properties of the three olefins and the resultant transitions states. It has been proposed that the overall reaction rate can be determined by the mutual nucleophilic−electrophilic donor− acceptor interactions between the olefin and the corresponding carbocation.24 The reactivity of a free olefin toward an adsorbed carbocation depends upon the π-electron donor ability of the unsaturated CC bond in the olefin and the ease of formation and electron affinity of the carbocation. It is important to note here that the initial transition state resulting from adsorption of each of the three olefins tested is a tertiary carbocation, which forms relatively easily and has higher stability than primary and secondary carbocations.24 However, because dimerization is a carbon−carbon bond forming reaction, dimer production not only depends on the stability and ease of formation of the carbocation, but also on the nucleophilicity or π-basicity (or the electron-donation ability) of the incoming olefin. Pertaining to these highly branched olefins, steric hindrance plays an important role in determining the overall basicity of the C C bond. Triptene can be obtained from isobutene if one of the α-methyl groups in isobutene is substituted by a much bulkier tertiary-butyl group. This results in considerable shielding of the CC bond, which leads to the lowest nucleophilicity among the three olefins (explained further in later text). In the

Figure 4. Reactant concentrations and product yields resulting from conversion of (a) 2,4,4-trimethyl-1-pentene, (b) 2,3-dimethyl-1butene, and (c) 2,3,3-trimethyl-1-butene (triptene) in pentadecane at 100 °C. “Other products” refers to both identified C10−C12 cracked products (including 2,2-dimethyl-3-octene, 2,2,4,6,6-pentamethyl-3heptene, 2,2,3,5,6-pentamethyl-3-heptene, and 2,3,5,5,6-pentamethyl3-heptene) and unidentified C2−C4 cracked products, and carbonnumber-determined and quantified (structure unidentified) side products (C10−C12). Dashed lines are only to guide the eye.

case of the 2,4,4-trimethyl-1-pentene, this tert-butyl is separated from the CC bond by several angstroms due to the presence of a methylene bridge. Hence, the shielding is significantly reduced, resulting in higher nucleophilicity of the CC bond. On the other hand, 2,3-dimethyl-1-butene is obtained if the α6083

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between the nucleophilic parent olefin and the electrophilic carbocation intermediate. Effect of Solvent/Reaction Medium. Figure 5 compares the production of ditriptene in both nonane and pentadecane at

methyl group in isobutene is substituted by a much larger isopropyl group, which is adjacent to the CC bond. This results in a decrease in the nucleophilicity of 2,3-dimethyl-1butene, with a value that is less than that of 2,4,4-trimethyl-1pentene but greater than triptene. The preceding inference is further supported by an empirical nucleophilicity scale developed by Mayr et al.26−30 This scale assigns a numerical value, called N, to the different π-bases. The value of parameter N is used to predict the rate constants of a reaction that involves a nucleophile and an electrophile as per the free energy relationship derived by Mayr (log(k) = s(N + E)).27−29 Accordingly, a higher value of N indicates stronger nucleophilicity and higher C−C bond formation reactivity. The relevant values of the parameter N for triptene, 2,3-dimethyl-1-butene, and 2,4,4-trimethyl-1-pentene are provided in Table 1. Table 1. Comparison of Nucleophilicity Values for Different Olefins

Figure 5. Effect of solvent on dimer production: ditriptene concentration as a function of time for nonane (red) and pentadecane (green) at 60 and 100 °C. Dashed lines are only to guide the eye.

60 and 100 °C. The initial dimerization rate and the dimer content at any given time were found to be higher in nonane, despite full miscibility of solvents, reactants, and products. Table 2 compares the contact angle of nonane and pentadecane with a flat, water-saturated hydrophilic surface (Nafion-117). The contact angle, as measured, is much lower for nonane (25.5°) than pentadecane (40°). This observation suggests that one of the possible causes for differences in dimerization activity could be the extent of catalyst wetting achieved by the different solvents. Pentadecane is considerably more hydrophobic than nonane whereas Amberlyst-35 is hydrophilic.32 Therefore, it is possible that nonane interacts with the surface to a greater extent than pentadecane and, as a result, nonane reduces the transport barrier for triptene between bulk liquid and catalyst. Differences in relative solvent/catalyst interaction (“wettability”) have been observed in other studies as well.33,34 For example, Chen et al. found that liquid-phase reactions over cobalt−silica composites were directly influenced by the extent of catalyst wettability in reactions such as ethylbenzene oxidation and fructose dehydration.33 The dominant factor that influences the morphological properties of Amberlyst-type resins is the polarity of the solvent.23 However, in the present case, both solvents are nonpolar and did not cause significant swelling of the ionomer resin (see Figure S3 for photographs). Therefore, differences in the free volume of the catalyst beads, leading to changes in reactant/product bulk mass transfer, are not likely for the two solvents. Hence, we infer that the difference in the wettability of Amberlyst in these two solvents, instead of the changes to the catalyst morphology, is the likely cause of the observed activity differences. Fuel Property Analysis. The dimer product mixtures were evaluated for potential compatibility with distillate-range transportation fuels by examining the boiling range, carbonnumber distribution, heat of combustion, and cloud point.

In accordance with Mayr’s scale, the most reactive olefin should be the one with the highest nucleophilicity value, i.e., 2,4,4-trimethyl-1-pentene. Our results are in agreement with this prediction as 2,4,4-trimethyl-1-pentene exhibited the highest initial dimerization rates, although this high reactivity comes with a trade-off of lower stability of the dimer resulting in loss of selectivity (Figure 4a). Dimers produced from the smaller olefins, triptene and 2,3-dimethy-1-pentene, exhibited higher stability and cracked to a much lesser extent. Nonetheless, the dimer yield from 2,3-dimethyl-1-butene was lower than triptene. One of the possible reasons for the observed low dimer yield in the case of 2,3-dimethyl-1-butene is the relative ease of rearrangement of the adsorbed carbocation, which can undergo facile hydride or methyl shifts, resulting in double-bond isomerization and formation of less reactive nonterminal alkenes such as 2,3-dimethyl-2-butene. This observation concurs with the previously reported comparisons that found that terminal alkenes and isobutene were more reactive than nonterminal olefins.31 Similar to isobutene, triptene is resistant to the rearrangements leading to isomerization under the mild conditions employed for the current dimerization reaction since it involves a higher energy barrier alkyl shift, which differs from the facile hydride shifts for double-bond isomerization as commonly observed in other olefins (further detail in Figure S2). This resistance to isomerization for triptene further contributes to its high dimer selectivity. A comparison of the dimer yield and side products revealed that neither cracking nor isomerization impeded the triptene dimerization (Figure 4c). Thus, we infer that the high selectivity of triptene toward dimerization was due in part to (1) the high stability of the tertiary carbocation intermediate and (2) the controlled acid−base interaction 6084

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Energy & Fuels Table 2. Photographs and Contact Angles for Nonane and Pentadecane on Pretreated Nafion 117

Figure 6. Carbon-number distribution of the product mixture obtained from triptene feed and mixed olefin feed after 16 h of reaction.

Scheme 3. Structures of Some Identified Dimers and Cracked Products of Mixed Olefins

Figure 6 shows the carbon-number distribution of the product mixtures after 16 h of reaction (≈80% conversion) at 100 °C from a single olefin (triptene) and from a mixed olefin feed comprised of 2,3 dimethyl-1-butene, 2,3,3-trimethyl-1-butene, and 2,4,4-trimethyl-1-pentene (present in an initial molar ratio of 1:3:2). The majority of products consisted of C10−C20 hydrocarbons, which constituted approximately 85% of the final product from triptene and 90% of the final product from mixed olefins. Scheme 3 shows some identified structures of product dimers and cracked products. Hydrocarbons heavier than C20 potentially result from oligomerization of the cracked products, obtained from the dimers. The boiling ranges of the final products are shown in Figure 7 and listed in Table 3. The initial boiling points (IBPs) of the product mixtures were 203 °C for triptene feed and 192 °C for mixed olefin feed, respectively. Since the limit on final boiling point in the ASTM D7566 for jet fuel containing synthetic hydrocarbons is 300 °C, approximately 80% of the dimerized product from both feeds is potentially useful for jet blending.

Similarly, approximately 95% of the total product can actually be blended with diesel. Higher boiling fractions are potentially suitable for heating oil and lubricant applications. The freeze point of the products as measured by ASTM D5972 was below the instrument limit (−80 °C). The freeze points of the products, as measured by ASTM D5773 (slower cooling rate), were −75 °C for the triptene product and approximately −55 °C for the mixed olefin product. (An exact value could not be determined for the mixed olefin product due to variability in the values detected on replicate analyses). For mixed olefin product, DSC analysis (with an even slower cooling rate) indicated a glass transition temperature at −48 °C, but freezing did not occur down to −80 °C. A low freeze point is critical for jet applications given the low ambient temperatures at high altitude. The cloud point measurements indicate that both of the product mixtures met or exceeded the requirements for Jet-A grade fuel. DEPT-13C NMR measured the relative amounts of tertiary carbon species at 55% and 50% in the products from triptene and mixed olefin feed, 6085

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NIST database; however, the presence of such cycloparaffins as low-concentration components may explain the relatively modest LHV of these products.

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CONCLUSION Results from this work demonstrate that it is technically feasible to upgrade triptene and other branched olefins generated from the homologation reactions of DME to distillate-range fuels in the liquid phase over Amberlyst-35. Reactions include isomerization, dimerization, and cracking. Among the competing reactions, dimerization was favored throughout the reaction duration (>16 h) at 60 and 80 °C. At 100 °C, reactions were fastest, but cracking became appreciable after 2 h. A maximum conversion (80%) was obtained at 100 °C whereas a maximum dimer and total C10+ yield were observed at 80 °C. Among three different olefins examined (i.e., 2,3-dimethyl-1-butene, triptene, and 2,4,4-trimethyl-1-pentene), triptene was uniquely amenable to the selective cationic dimerization pathway with low rates of skeletal isomerization and β-scission, while 2,3dimethyl-1-butene favored isomerization and the dimer obtained from 2,4,4-trimethyl-1-pentene was susceptible to cracking. The extent of catalyst wetting by solvent affected the dimerization rate with lower molecular weight solvents being more productive at all times and temperatures. Approximately 85−90% of the dimerized products obtained from triptene and an olefin mixture were in the range of C10−C20 with a boiling range, cloud point, and lower heating value that are potentially suitable for application as a distillate-range fuel. It is possible to further tune the product composition toward a specific jet fuel blend by optimizing the reaction duration and the composition of the initial olefin mixture. For example, including a lighter olefin such as isobutene (a major product from DME homologation) to the reactant mixture can lower the boiling range of the final product resulting in boiling curves that are more similar to Jet A. Future work will investigate the effects of catalyst acid strength and the effect of lighter olefins on the overall conversion, initial rates, product selectivity, and fuel properties.

Figure 7. Boiling curves for the product mixtures obtained in this work (per ASTM D2887) compared to those of commercial fuels.35,36 Dashed lines are only to guide the eye.

respectively. The observation of low cloud point is therefore further supported in accordance with a published correlation between high tertiary carbon content and low cloud point.37 Table 3 summarizes the properties of the dimerization products and compares them to jet and diesel specifications. The lower heating values, as measured per ASTM D240, were 42.9 MJ·kg−1 for the product from triptene and 41.7 MJ·kg−1 for the product obtained from mixed olefin feed. The results imply that the dimerized product from triptene may be suitable for application as a blend stock for distillate-range fuel. Given that the synthetic fuel from triptene does not contain aromatics, unlike Jet-A, the lower heating value of the synthetic fuel from triptene was lower than anticipated. Close inspection of the GC-MS results indicate that cycloparaffins, which have lower gravimetric heating values than branched olefins, may be present in the synthetic fuel (Figure S5). Identification of these compounds is ambiguous due to low match quality with the

Table 3. Comparison of Different Fuel Properties of the Synthetic Hydrocarbons from This Work with the Known Values of Typical Commercial Fuels

a

For pure ditriptene C14H28. 6086

DOI: 10.1021/acs.energyfuels.5b01175 Energy Fuels 2015, 29, 6078−6087

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Energy & Fuels

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b01175. Additional figures illustrating changes in the product composition (as percentage) with time at three different temperatures (Figure S1), changes in concentration of 2,3-dimethyl-1-butene and its isomerization (Figure S2), supplementary catalyst characterization results (Figure S3), representative gas chromatograms (Figure S4), representative mass spectra of the identified products (Figure S5), and 13C NMR spectra (Figure S6) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the Department of Energy’s Bioenergy Technology Office under Contract No. DE-AC3608-GO28308. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Erica Gjersing for performing NMR-related measurements and Gina Chupka for carrying out the DSCbased cloud point analysis. We also thank the Dow Chemical Co. for providing catalyst samples.

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DOI: 10.1021/acs.energyfuels.5b01175 Energy Fuels 2015, 29, 6078−6087