Generic Biphasic Catalytic Approach for Producing Renewable Diesel

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A generic biphasic catalytic approach for producing renewable diesel from fatty acids and vegetable oils Shaoqu Xie, Chuhua Jia, Arushi Prakash, Mackenzie Irene Palafox, Jim Pfaendtner, and Hongfei Lin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00215 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 16, 2019

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A generic biphasic catalytic approach for producing renewable diesel from fatty acids and vegetable oils Shaoqu Xiea,†, Chuhua Jiaa,†, Arushi Prakashb, Mackenzie Irene Palafoxa, Jim Pfaendtnerb, Hongfei Lina,* aThe

Gene and Linda Voiland School of Chemical Engineering and Bioengineering,

Washington State University, Pullman, WA 99164, USA bThe

Department of Chemical Engineering, University of Washington, Seattle, WA 98195,

USA †Equally

contributed

* Corresponding

author: [email protected]

Abstract: Conversion of fatty acids to diesel-range hydrocarbons suffers from elevated reaction temperature or low selectivity in single liquid-phase processes. Herein the biphasic interfacial catalytic process was developed for the decarboxylation of fatty acids produce alkane hydrocarbons using Pd/C catalyst at the water-organic solvent interface. An exceptionally high carbon yield of 91.7±2.3 % (theoretical maximum 94.4 %) and a high selectivity of ~99 % to n-heptadecane were obtained from the conversion of stearic acid in the cycloalkane/water biphasic solvent system at a relatively low temperature (260 oC). The kinetic study of the conversion of stearic acid and oleic acid in the biphasic catalytic process were investigated and the activation barriers of both reactions were determined and compared to those of the monophasic catalytic processes. Both the experimental studies 1

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and MD simulations were performed to elucidate the synergistic effects of water and various organic solvents, which stabilize the carboxylate group and the hydrophobic hydrocarbon tail in a fatty acid molecule, respectively, and improve the kinetic rates and the selectivity. The application of the biphasic tandem catalytic process (biTCP) approach was further extended to the decarboxylation of a wide selection of saturated and unsaturated fatty acids, triglycerides (e.g. glyceryl trioleate), and oilseed biocrude oil (e.g. canola oil) to produce high-quality diesel fuels. Keywords: Decarboxylation; fatty acid; vegetable oil; biphasic system; interfacial catalysis 1

Introduction According to the recent report issued by DOE/EIA1, it is estimated that the diesel fuel

consumption will increase by nearly 50% until 2040, which indicates the strong demand of diesel for transportation. In order to create a carbon-neutral scheme for the ease of climate change caused by the drastic increase of CO2 emissions, developing liquid fuels derived from renewable resources for the replacement of fossil fuels has become an urgent topic2–4.

The first-generation biofuels for diesel replacement, i.e. biodiesel, are

synthesized through transesterification of vegetable oils or animal fats.

However,

biodiesel is a mixture of oxygenate compounds such as fatty acid esters, resulting in unfavorable cold flow properties, low energy density and poor storage stability5,6.

Such

inferior properties might render biodiesel a less ideal choice in transportation fuels7,8.

For

drop-in

via

replacement,

a

complete

deoxygenation 2


of

oils

and

fats

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hydrogenolysis/hydrotreating seems to be a promising route since it produces renewable fuels in the form of pure hydrocarbons, i.e. “green” diesel or renewable diesel.

The

commercial HEFA (hydroprocessed esters and fatty acids) processes (e.g. Neste Oil’s NExBT, UOP/Eni’s Ecofining) that produce renewable diesel fuels from oils and fats have already been launched and under operation.

However, to date the cost of renewable diesel

fuels is still too high to compete with that of petroleum-based fuels. Due to the huge market demand of transportation fuels, there are tremendous interests in more efficient conversion processes for producing renewable fuels, especially the synergistic efforts in the improvement of the conversion efficiency and the product yield. Upon this venue, the deoxygenation of triglycerides and fatty acids via decarboxylation (CO2) and/or decarbonylation (-CO) has been extensively studied in the past decade9,10. Several groups reported the decarboxylation/decarbonylation of bio-derived fatty acids in organic solvents such as n-dodecane requires relative high temperature (>573 K) and prolonged reaction time (>6 h) to achieve satisfactory outcomes11–18.

On the other hand,

various saturated fatty acids were readily converted to hydrocarbon products over heterogeneous catalysts including Pt/C19, Pd/C20 and activated carbon21 in hot water.

For

instance, Savage and coworkers found that the highest yield of pentadecane (~76%) was obtained from the decarboxylation of palmitic acid without adding H2 in near-supercritical water at 643 K within an hour22.

However, it was found that the unsaturated fatty acids

exhibited the much lower selectivities to the paraffin products than the saturated fatty acids in monophasic aqueous solutions19, while the by-products such as ketones, aromatic 3


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compounds, and condensed solid residues were produced. In general, decarboxylation/decarbonylation of fatty acids or triglycerides tends to be faster in water, while it often has a higher selectivity to paraffin products in organic solvents, especially when unsaturated fatty acids are used as the feedstocks.

In order to

compromise the advantages from both approaches, an intuitive idea would be the introduction of a biphasic solvent system containing both water and an organic solvent (Scheme 1).

In this envisaged biphasic catalytic process, two immiscible solvents such

as water and cyclohexane create a water/oil interface. Under room temperature, hydrophobic Pd/AC catalyst and reactants such as fatty acids are distributed in oil phase before the initiation of the decarboxylation. Upon the elevation of the temperature, decarboxylation of fatty acids may occur at the water/oil interface due to the amphiphilic property of fatty acids.

Critically, it is shown later that the “on water” effect will have a

significant promotion effect on the rate of decarboxylation.

As the unsaturated feedstock

is used, a tandem hydrogenation reaction may occur in the bulk oil phase.

After the

decarboxylation reaction is finished, hydrocarbon products can be simultaneously transferred into organic solvent from the water. Besides the “on water” effect, the organic solvent also played a key role in the biphasic system. The use of appropriate organic solvents, with varied viscosities and dielectric constants, may increase the reaction rates and product selectivity in biomass conversion processes23,24. However, the synergistic effects of water and various organic solvents were rarely reported. The proposed biphasic tandem catalytic process (biTCP) may have the following advantages: (a) the hypothetic 4


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“on

water”

effect

could

considerably

improve

the

kinetic

rate

of

decarboxylation/deCOx25,26; (b) the hydrogenation of the unsaturated C=C double bond in bulk organic solvent prevents the occurrence of side reactions27; and (c) the integration of tandem hydrogenation and decarboxylation reactions with in-situ separation of products in a “one-pot” process.

Scheme 1 Schematic illustration on the concept of biphasic tandem catalytic process (biTCP). Hereby the organic solvent is cyclohexane. Yellow and red circles indicate tandem decarboxylation and hydrogenation, respectively. Note that the sizes of reactant, gas molecules and Pd/AC particle are not in scale. Our group has successfully implemented the similar biTCP approaches for effectively converting terpenoid biomass such as Eucalyptus oil and Grindelia biocrude to cycloalkanes, a high-density jet fuel component28,29, as well as for selectively converting sugars to furan derivatives30.

Hereafter, we extend our research on biTCP as a generic

catalytic process for the production of renewable diesel fuels from a wide selection of saturated and unsaturated fatty acids/esters, e.g. vegetable oils. 5


Moreover, the

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mechanistic study of the solvent effect in this biTCP is explored by molecular dynamics (MD) simulations. 2

MATERIALS AND METHODS

2.1 Materials All chemicals were used as received except the self-synthesized catalysts. Stearic acid (Alfa Aesar, 98%), oleic acid (Alfa Aesar, 99%), linoleic acid (Sigma Aldrich, 99%), dodecanoic acid (Sigma Aldrich, 99%), myristic acid (Sigma Aldrich, 99%), palmitic acid (Sigma Aldrich, 99%), erucic acid (Sigma Aldrich, 99%), glyceryl trioleate (Sigma Aldrich, 99%), and canola biocrude oil provided by the local farm (Natural Selection Farms, Sunnyside, WA) were used as the feedstock.

Cyclohexane (ACS grade, Macron Final

Chemicals), n-hexane (Alfa Aesar, 95%), toluene (ACS grade, Macron Final Chemicals), p-xylene (Alfa Aesar, 99%), decalin (Sigma Aldrich, 98%), n-heptane (Alfa Aesar, 99%), ethyl acetate (Sigma Aldrich, 99.5%), and n-dodecane (Alfa Aesar, 99%) were used as the organic solvents. Ultrapure water with a specific resistance of 18.2 MΩ cm-1 was used as the aqueous phase solvent.

Palladium on activated charcoal (Pd/C) (5% Pd basis, Sigma-

Aldrich), platinum on activated charcoal (Pt/C) (5% Pt basis, Sigma-Aldrich), ruthenium on activated charcoal (Ru/C) (5% Ru basis, Sigma-Aldrich), rhodium on activated charcoal(Rh/C) (5% Rh basis, Sigma-Aldrich), nickel on activated charcoal (Ni/C) (16% Ni basis, self-synthesized according to the reference31, the synthesis procedure was provided in the following paragraph), and activated carbon (AC) (self-synthesized through ball-milling bulk activated carbon from Alfa Aesar) were used as the catalysts. 6


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16 wt. % Ni/C was prepared via wet impregnation using Ni(NO3)2⋅6H2O as the metal precursor and activated carbon (Alfa Aesar -20+40 mesh powder, grinded in ball grinder for 2 hours) as the support. After drying, the impregnated carbon was heated under flowing N2 at 350 ℃ for 3 hours. Then the material was reduced in H2 at 450℃ for 4 hours. 2.2 Catalytic reaction procedures The reactions were carried out in a 45 mL Parr Series 5000 Multiple Reactor System with a 4871 series temperature controller. In general, the feedstock, the solvents (monophasic or biphasic system), and a certain amount of heterogeneous catalyst were placed in the vessels. Reactions conditions for the individual decarboxylation reactions are described in the Figure captions. The vessels were sealed, purged 3 times with 400 psi N2 followed by 3 times with 400 psi H2 and then pressurized with H2 to the set pressure. The mixture was magnetically stirred at 700 rpm while being heated to the set reaction temperature in half an hour and kept at the set temperature for the set reaction time.

After

the reaction, the vessels were quenched with cold water for fast cooling. For the lifetime of the catalyst, we added additional starting material (e.g., 0.15 g stearic acid) into the vessel every 6 hours without unloading all the materials except the offgas after cooling32. After being pressurized with the fresh H2, the catalytic reaction was conducted for the next cycles sequentially. 2.3 Analysis The gas phase products were analyzed by a GC-TCD (GC-2014, Shimadzu) before dissembling the reactor.

High-purity N2, CO2, and H2 were calibrated and the calibration 7


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results were used to quantify the gaseous products.

After dissembling the reactor, the

internal standard (n-dodecane) was added immediately into the liquid-phase products for quantitative analysis.

In the case of using n-dodecane as the solvent or Ru/C as the

catalyst, n-pentadecane or n-heneicosane was employed as the internal standards, respectively.

After the addition of internal standard and 31.25 wt% NaCl for total salting-

out the feedstock, the samples were stirred at 700 rpm for 15 min and then settled for 20 min. The organic phase was decanted and analyzed by a GCMS QP-2020 (Shimadzu) to identify and quantify the unknown components.

All the liquid phases were then filtered

through a 0.45-micron syringe filter before the analysis.

After the determination of the

reagent and products contents, the conversion of the biomass feedstock (α) and the carbon yield of the alkane products (Y) are calculated by the following equations, 𝛼=

𝑛0 ― 𝑛1

𝑌=

𝑛0 17𝑛2 18𝑛0

× 100 %

× 100 %

Where no is the moles of biomass feedstock before reaction, n1 is the moles of biomass feedstock after reaction, and n2 is the moles of alkane hydrocarbons after reaction. The carbon balance values in the experimental reactions are 100 ± 10%. 2.4 Molecular dynamics simulations A simulation box (3.5 nm x 3.5 nm x 7 nm) was packed with molecules of oil (3.5 nm x 3.5 nm x 3.5 nm) and water (3.5 nm x 3.5 nm x 3.5 nm), and one molecule of stearic acid using PACKMOL33.

This generated an interface of 3.5 nm by 3.5 nm. 8


The oil phases

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used in the simulation study were cyclohexane, n-hexane, toluene, n-heptane, ethyl acetate, and n-dodecane.

Water was modelled using the SPC/E water model34.

Models for oils

and stearic acid were obtained from the Automated Topology Builder repository35. All simulations were conducted using the GROMACS 2016.3 simulation package36 and PLUMED 2.4.1 software package37.

Unfavorable contacts in the initial configuration

were removed using a steepest descent algorithm.

Thereafter, all simulations utilized a 2

fs time step by constraining the bonds between hydrogen and other heavy atoms with the LINCS algorithm38.

Electrostatic interactions were calculated with the particle mesh

Ewald (PME) summation method39 and a cutoff value of 0.8 nm. value of 0.8 nm was used.

A van der Waals cutoff

The system was evolved in the NPT ensemble (Temperature

300 K; Pressure 1 atm) for 1 ns using the Donadio-Bussi-Parrinello thermostat40 (Time constant τ = 0.1 ps) and the Berendsen barostat41 (Time constant τ = 1 ps).

Only the

longest box dimension (z-direction) was only allowed to change during the NPT simulation. The production runs were conduction in the NVT ensemble (Temperature 300 K), where the pressure was maintained using the Donadio-Bussi-Parrinello thermostat (Time constant τ = 0.1 ps). For the production runs, three starting configurations were generated.

Using

PLUMED, the molecule of stearic acid was moved to the interface, inside the water phase, and inside the oil phase.

Three replicas (different seeds) of each configuration were

propagated in the NVT ensemble for 50 ns.

Thus, each water-oil combination was

simulated for a total of 450 ns during the production runs. 9


From these simulations, the

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location of the end carbon (COOH group), and the center of all carbon atoms was tracked with reference to the interface of each configuration using PLUMED.

A histogram of

these values was generated. 3

Results and discussion

3.1 Catalyst Screening Herein stearic acid was used as a model compound for the deoxygenation of fatty acids42.

The deoxygenation of stearic acid was investigated over a variety of catalysts in

the biphasic systems composing of cyclohexane and water under similar reaction conditions. Figure 1 shows that Pd was superior to the other metals and gave the highest yield of heptadecane which was consistent with its relative highest deoxygenation activity10. Activated carbon (AC) and Ni/C show practically no conversion of stearic acid to heptadecane at 260 oC. °C

Other groups reported that high conversions were achieved at 300

and 370 ºC for Ni-based catalyst43,44 and AC21, respectively.

Albeit the cost of Ni is

1000 ∼2500 times lower than those of noble metals, the conversion of stearic acid on a Ni catalyst is still not promising under relatively mild conditions.

Admittedly, development

of new Ni-based catalysts is attractive for upgrading bio-oil and biodiesel to bio-fuels,45,46 especially in the absence of H2.47

It was also observed that the n-heptadecane yield using

Ru/C was quite low due to its high catalytic activity on breaking the carbon chain, and thus a variety of alkanes with different carbon number were obtained, as shown in Figure S1. By contrast with AC and Ni/C, Rh/C and Pt/C exhibited better performance in the production of n-heptadecane, and particularly Rh/C yielded 75.9% of n-heptadecane. 10


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Though Rh/C and Pt/C exhibited relatively high selectivity toward deoxygenation products similar to what Pd/C did,12 the incomplete conversions of stearic acid were observed when Rh/C or Pt/C was used under the identical reaction conditions.

These outcomes indicate

that noble metal catalysts are favored in fatty acid deoxygenation, especially the Pd/C, which exhibited the highest deoxygenation activity among the tested catalysts.

The

deoxygenation of stearic acid over Pd/C or Pd/Al2O3 was usually completed at 300 °C or 350 °C, respectively12,18,48, while the biphasic catalytic process for the deoxygenation of stearic acid using Pd/C in our study produced an impressive n-heptadecane yield (91.7 ±2.3%) with a conversion of stearic acid up to 100 % at 260 ºC. The decrease in the reaction temperature, which resulting in even more significant decrease of the operation pressure, is a considerable progress at present for production of renewable diesel from fatty acids.

100 90 80 70

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 50 40 30 20 10 0

AC Ru/C Ni/C Rh/C Pt/C Pd/C

Figure 1 Performance of the screened catalysts in the decarboxylation of stearic acid. C17 represents heptadecane throughout this paper. Reaction condition: 0.15 g stearic acid (0.53 mmol), 0.05 g catalyst, 22 mL water, volume ratio of cyclohexane to water (C/W) was 2:22, p(H2) = 28 bar at room temperature, 260 ℃, 6 h. 11


ACS Catalysis

The examination of the stability of the Pd/C catalyst found that the carbon yield of nheptadecane only slightly decreased: 91.7 % over the fresh Pd/C (1st use), 85.4% over the third use of the same spent Pd/C, and 85.1 % over the sixth use of the same spent Pd/C, indicating that the activity of the spent Pd/C catalyst was close to the fresh one for the conversion of stearic acid to n-heptadecane, as shown in Figure 2. The catalyst activity was maintained without regeneration, suggesting that the spent Pd/C may be readily reused in a continuous biTCP.

100 90 80

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

70 60 50 40 30 20 10 0

fresh second third fourth fifth sixth

Lifetime of Pd/C

Figure 2 Lifetime of the Pd/C catalyst for the conversion of stearic acid in the biphasic catalytic process. Reaction condition: 0.15 g stearic acid (0.53 mmol), 0.05 g Pd/C, 22 mL water, volume ratio of organic solvent to water was 2:22, p(H2) = 28 bar at room temperature, 260 ℃, 6 h. 3.2 Biphasic Catalytic Process With the same solvents volume, the conversions of stearic acid were much different in the single phase of either water or cyclohexane, as well as in the biphasic system, using 12


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Pd/C at 260 ℃.

However, Figure S2 shows that, the major deoxygenation route over Pd/C

was achieved via decarboxylation no matter in monophasic or biphasic solvents. Compared with the gas chromatographs of the calibration gases (Figure S3), the peak located at ~ 4 min retention time in Figure S2 is confirmed to be N2. We also found that CO2 was the only gaseous product while no CO was detected after the deoxygenation of fatty acids over the Pd/C catalyst. Therefore, we believe that decarboxylation is the dominant deoxygenation pathway during the conversion of fatty acids to alkanes with the Pd/C catalyst, while decarbonylation is negligible.

Likewise, n-heptadecane was the sole

liquid-phase product derived from the decarboxylation of stearic acid in which a cleaved carbon was released as CO2.

As an amphipathic molecule, stearic acid was hypothesized

to show faster decarboxylation rate in biTCP than that in single phase.

As seen in Figure

3, the rate of the decarboxylation through the biphasic approach is much faster than that in either water or cyclohexane solvent. After 1 h reaction, the yield of n-heptadecane in the biTCP is almost tripled compared to that in the single phase counterparts.

A high carbon

yield of 91.7±2.3 % was achieved in the biTCP within 6 h whereas the yields of the singlephase reactions are not satisfied towards the desired n-alkane product. Thus the biTCP was superior to the monophasic catalytic processes for the decarboxylation of fatty acids.

13


ACS Catalysis

100 90

Biphasic Cyclohexane Water

80 70

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60 50 40 30 20 10 1

2

3

4

5

6

Reaction time (h)

Figure 3 The yield of n-heptadecane as a function of time from the decarboxylation of stearic acid in monophasic and biphasic solvents. Reaction condition: 0.15 g stearic acid (0.53 mmol), 0.05 g Pd/C, p(H2) = 28 bar at room temperature, 260 ℃; for monophasic system, 24 mL water (W) or cyclohexane (C); for biphasic system, 22 mL water, volume ratio of C/W was 2:22. The solubilities of stearic acid in water within the temperatures ranging from 0 °C to 60 °C were very poor, so that most of stearic acid often existed in the form of solid.49 The dissolved stearic acid form aggregates as micelles in water, as demonstrated by the tyndall effect in Figure S4(a). The poor solubility of fatty acids can be observed in some hydrocarbons at low temperatures.50,51 However, stearic acid used in our study was completely dissolved in cyclohexane, as shown in Figure S4(b). Upon the temperature elevation, the stearic acid used in our study (0.53 mmol) is predicted to be completely dissolved in water at 260 °C.52 The mutual solubility of cyclohexane and water increased with increasing the temperature53 or by the aid of nonionic surfactant.54 However, the reaction system composing of 2 mL cyclohexane and 22 mL water remained as the biphasic 14


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state.

In a biTCP, besides the common water-in-oil (W/O) and oil-in-water (O/W) types,

other different dispersion types may also be observed, such as W/D, D/W, D/O, O/D, and (W+O)/D types, where D represents the surfactant phase.54–56 When the volume ratio of cyclohexane to water (C/W) was 2:22, the water phase was continuous and the system is predicted to be a cyclohexane-swollen micellar solution of water.54 Since water occupied a large volume fraction, phase inversion did not occur from an O/W to a W/O type at higher temperatures (higher than the phase inversion temperature). The dissolved stearic acid monolayer at the oil-water interface had a concave tendency towards cyclohexane at 260 oC

and formed the O/W dispersion type. The decarboxylation of stearic acid was carried

out at the interface between water and cyclohexane, similar to the liquid/liquid interface of emulsions where the biphasic hydrodeoxygenation or condensation reactions were catalyzed.57,58

The carboxyl group is relatively hydrophilic while the alkyl group is

hydrophobic, making the stearic acid molecule to lie at the interface between aqueous phase and organic phase, where the catalyst was wrapped in the micelles. Under such circumstances, the fatty acid molecules may interact with the catalyst active sites efficiently, thus accelerating the decarboxylation reaction.

The high selectivity in the biTCP may

also be caused by the direct contact of catalyst with the carboxyl group in stearic acid. Therefore, the hypothetic “on water” will considerably improve the kinetic rate of decarboxylation.

In contrast, the decarboxylation rate in cyclohexane is slower than that

in water because no emulsions formed in cyclohexane whereas stearic acid formed a small amount of emulsions in water. After the decarboxylation, the product, n-heptadecane, was 15


ACS Catalysis

surmised to form new emulsions with water and the remaining ingredients thus further accelerated the reaction rate. This hypothesis will be discussed later. 3.3 Kinetic Studies The effect of Pd/C dosage on the n-heptadecane yield was studied by varying the amount of catalyst at 260 ℃. In the absence of catalyst, a very low, negligible yield was observed, as shown in Figure 4. The n-heptadecane yield increased with increasing the Pd/C dosage at first. When the [Pd]/[SA] ratio was 0.88%, the n-heptadecane yield reached the maximum (91.7 ±2.3%) and as the [Pd]/[SA] ratio continued to increase, the equivalent yields were obtained. Therefore, 0.05 g Pd/C will be used in the following experiments. These results indicate that the decarboxylation of stearic acid is a highly selective heterogeneous catalytic reaction in the biTCP.

100

80

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60

40

20

0

-20

0.0

0.4

0.8

1.2

1.6

2.0

[Pd]/[SA] (%)

Figure 4 The effect of Pd/C loading on the yield of heptadecane from stearic acid in the biphasic catalytic process. Reaction condition: 0.15 g (0.53mmol) stearic acid, 22 mL water, volume ratio of C/W was 2:22, p(H2) = 28 bar at room temperature, 260 oC, 6 h . The dispersion of Pd/C is ca. 20%. The effective catalyst loading NPd/NSA mol% is calculated 16


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as follows: NPd/NSA mol% = ([mass of the Pd/C catalyst]*[5 wt%]*[Pd dispersion])/(MPd* NSA) Catalytic decarboxylation of stearic acid in the pure water or cyclohexane solvent were not as efficient as that in the C/W (cyclohexane/water) biphasic solvent. As we discussed earlier, in the biTCP, stearic acid tends to accumulate at the C/W interface since water attracted the hydrophilic head of stearic acid and its hydrophobic tail were surrounded by cyclohexane.

Therefore, an adequate ratio water and cyclohexane that provides an ample

liquid/liquid interfacial surface area seem to be the prerequisite for the efficient decarboxylation of stearic acid. Predictably, as the C/W volume ratio increased, the trend of the yields of n-heptadecane followed a “volcano” like shape with a highest plateau at the C/W ratios of 12:12 to 2:22 (Figure 5). Although there is lack of a visualization detection technique for subcritical/supercritical liquids, we suspect that the decrease in the yield, when the C/W ratio was either less than 2:22 or larger than 12:12, was caused by the insufficiency of water or cyclohexane in the biphasic system to create amphiphilic emulsions at elevated temperatures. In a liquid/liquid biphasic system, emulsified solvent usually provides a larger interfacial area, which is of importance for interfacial catalytic reactions, than non-emulsified one. The amphiphilic emulsion system including both O/W and W/O dispersion types which may stabilize the liquid-liquid interfacial catalytic reactions at elevated temperatures.54 Hence the optimization of solvent ratio in the biTCP is a crucial factor for the efficient decarboxylation of fatty acids. There was a wide window to tune the volumetric ratio between cyclohexane and water, suggesting that the biTCP was 17


ACS Catalysis

controllable for commercial implementation.

100

80

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60

40

20

0

24:0 20:4 16:812:128:16 4:20 2:22 1:23 0:24

Cyclohexane/water volume ratio

Figure 5 The effect of volumetric ratio of cyclohexane to water on the n-heptadecane yield after the conversion of stearic acid. Reaction condition: 0.15 g stearic acid (0.53 mmol), 0.05 g Pd/C, p(H2) = 28 bar at room temperature, 260 ℃, 6 h. The solvents volume was controlled at 24 mL. Surprisingly, the hydrogen partial pressure has a significant effect on the decarboxylation reaction, as shown in Figure 6. In the absence of H2, the n-heptadecane yield was only 38.6%. The n-heptadecane yield increased with the increase in hydrogen partial pressure and reached the highest theoretical value when the hydrogen pressure was greater than 28 bar. The Savage group reported that the 5 % Pt/C catalyst was active in hot water for the decarboxylation of stearic, palmitic, and lauric acids without the addition of H2,19 while in this study H2 has been an indispensable part of the decarboxylation reaction at 260 oC.

The decarboxylation of stearic acid does not consume H2 though the activation

of dihydrogen by Pd metal is fairly easy.59,60 Indeed, H2 can form molecular dihydrogen complexes with Pd(0),61 which may accelerate the cleavage of C-C bond to release CO2. 18


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Page 19 of 43

We observed that higher hydrogen pressure (40 bar) would not decrease the product yield in the biphasic reactions.

100 90 80

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

70 60 50 40 30

0

13

18

23

28

40

Hydrogen pressure (bar)

Figure 6 The influence of hydrogen pressure on the decarboxylation of stearic acid. Reaction condition: 1) From 0 to 28 bar, 0.15 g stearic acid (0.53 mmol), 0.05 g Pd/C, 22 mL water, volume ratio of C/W was 2:22, 260 ℃, 6 h, ptotal=28 bar at room temperature, nitrogen was used as the makeup gas to ensure a constant total pressure of 28 bar at various hydrogen partial pressures, 2) At 40 bar, 0.15 g stearic acid (0.53 mmol), 0.05 g Pd/C, 22 mL water, volume ratio of C/W was 2:22, 260 ℃, 6 h, ptotal=40 bar at room temperature, no nitrogen. A highlight of this study is the lower reaction temperature compared to the other prevailing fatty acid conversion processes.

As shown in the Table 1, the yield of n-

heptadecane from the decarboxylation of stearic acid was 5.4 %, 10.6 %, 19.8 %, and 31.3% at 230, 240, 250, and 260℃, respectively, suggesting that the reaction rate declined sharply as the reaction temperature decreased. However, no by-products except CO2 were observed. Table 1 also shows that the hydrogenation/decarboxylation of oleic acid yielded a similar 19


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Page 20 of 43

amount of n-heptadecane compared to the decarboxylation of stearic acid at the same temperature, which proved that double bond has little effect on the decarboxylation. The absence of by-products indicates the in situ hydrogenation of the unsaturated C=C double bond of oleic acid in the organic solvent prevent the occurrence of side reactions.62

In

comparison, only 44.0~66.1% corresponding alkanes were produced from the unsaturated fatty acids in the monophasic tetradecane solvent over the Pt/NMC(nitrogen-doped mesoporous carbon) catalyst.62

Activated carbon also acted as a catalyst for both

decarboxylation and hydrogenation of oleic acid to produce the corresponding alkanes in relatively low yields with the in situ generated hydrogen in supercritical water at 400 oC.63 Thus the ~100% selectivity to n-heptadecane from oleic acid in our biTCP is exceptional and can be attributed to the rapid saturation of the C=C double bonds with external dihydrogen which prevents the side reactions.

Table 1 Product distribution after the conversion of stearic acid or oleic acid in the biphasic catalytic process. Reaction condition: 0.53 mmol stearic acid or oleic acid, 0.05 g Pd/C, 22 mL water, volume ratio of C/W was 2:22, p(H2) = 28 bar at room temperature, 1 h. Entry

Fatty acid

Temperature (K)

1 2 3 4 5 6 7 8

Stearic acid Stearic acid Stearic acid Stearic acid Oleic acid Oleic acid Oleic acid Oleic acid

503 513 523 533 503 513 523 533

n-heptadecane yield C18 yield (%) (%) 5.4 10.6 19.8 31.3 6.5 12.1 18.8 32.0

20


0 0 0 0 0 0 0 0

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-1

1 h Stearic acid 1 h Oleic acid

-2

Ln k

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Estearic acid=123 kJ/mol -3

Eoleic acid=122 kJ/mol

-4

-3

-3

-3

-3

1.9x10 1.9x10 1.9x10 2.0x10 2.0x10

-3

1/T (K-1)

Figure 7 The Arrhenius plots (ln k vs inverse temperature, 1/T) of the decarboxylation of stearic acid and oleic acid in the biTCP. Reaction conditions: 0.53mmol stearic acid or oleic acid, 0.05 g Pd/C, 22 mL water, volume ratio of cyclohexane to water was 2:22, p(H2) = 28 bar at room temperature, 1 h. Figure 7 shows that the activation energies of the conversion of oleic acid and stearic acid are almost identical, 122 and 123 kJ/mol, respectively, in the biTCP, which implies that the hydrogenation of the C-C double bond in oleic acid is much faster than the decarboxylation, and the latter reaction step contributes the major energy barrier of the overall reaction of converting oleic acid to heptadecane. It is noted that the activation energy of oleic acid conversion in our biTCP is much lower than that reported in the literature64, 148 kJ/mol.

The lower activation energy of decarboxylation enables the

lower operation temperature in the biTCP. 3.4 Effects of Organic Solvents 21


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Page 22 of 43

Organic solvent has significant effects on the efficiency of the biphasic catalytic process for the decarboxylation of stearic acid.

Figure 8 shows that the straight-chain

hydrocarbons, such as n-hexane, were favored for the decarboxylation of stearic acid in the highest n-heptadecane yield. Stearic acid and n-hexane have the similar structure of the hydrophobic, saturated, straight hydrocarbon tails that stabilizing the liquid-liquid interface. However, Chronic exposure to n-hexane is one of the well-known causes of peripheral neuropathy65,66 while occupational exposure to cyclohexane had no adverse effects on the peripheral nervous system.67 Therefore, cyclohexane is the preferred solvent.

Due to the

high catalytic activity of palladium, toluene and p-xylene were unstable organic solvents in the biTCP and were partly saturated with H2, which diminished the hydrogen-use efficiency and thus lowered the reaction rate of fatty acid decarboxylation.68,69

Decalin

was not as efficient as cyclohexane for the decarboxylation, perhaps because the bi-cyclic structure of decalin is bulky and therefore the mass transfer resistance of stearic acid in decalin is higher than that in cyclohexane. sensitive

to

hydrolysis

at

high

The polar solvent, ethyl acetate, was very temperature

decarboxylation/decarbonylation in the presence of Pd/C.70

and

was

degraded

via

On the other hand, because

of its hydrophilic feature, ethyl acetate cannot stabilize the liquid-liquid interface.

As a

result, the lowest yield of n-heptadecane was obtained as ethyl acetate was used as the organic solvent in the biTCP. efficiency of the biTCP.

Thus, the selection of the organic phase is critical for the

In general, the hydrophobic alkane products could be easily

separated from the interface and dissolved in the nonpolar organic solvent, which could 22


Page 23 of 43

accelerate the reaction and allowed the synthesis of the targeted products with a high selectivity.71,72

100

80

Yield of C17 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

60

40

20

0 e e e e e e te in exan exanDecal Toluepn-XylelnAcetaHeptanodecan H n Cycloh D y n h t E

Figure 8 Performance of different organic solvents for the decarboxylation of stearic acid. Reaction condition: 0.15 g stearic acid (0.53 mmol), 0.05 g Pd/C, 22 mL water, volume ratio of organic solvent to water was 2:22, p(H2) = 28 bar at room temperature, 260 ℃, 6 h. Using molecular dynamics (MD) simulations, we analyzed the distribution of a stearic acid molecule in the biphasic solvent systems.

As shown in Figure 9 (B and C), it is

evident that the best performing organic solvent (n-dodecane; red) in terms of yield percentage stabilizes stearic acid at the interface more strongly than other organic solvents. In fact, both the head group and the body of the acid molecule are closer to the interface and they are retained at the interface for longer (denoted by a higher peak in the probability distribution curve) than other oil phases. Other organic solvents that give high yields (nheptane, cyclohexane, and n-hexane) can restrain stearic acid at the interface; albeit less strongly than n-dodecane. n-Hexane and n-heptane, which perform slightly worse than 23


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Page 24 of 43

cyclohexane, interact more with the body of stearic acid as evidenced by the probability distribution of the center-of-mass extending further into the oil phase.

Figure 9 (A) Simulation setup showing the oil (cyclohexane) and water phases. Carbon (blue), hydrogen (white), and oxygen (red) atoms are shown explicitly. Probability distribution of the distance of the (B) center of carbons, and (C) the carbon in the head group (COOH) of stearic acid from the interface for all water-oil combinations. Negative values indicate the oil phase, and positive values indicate the water phase. To guide the readers’ eye, the interface is denoted by a dashed line at 0 nm. In sharp contrast, the worst performing organic solvent (ethyl acetate; purple) traps stearic acid in the oil phase and prevents stearic acid from visiting the interface.

Even in

the configurations where stearic acid is placed at the interface, the oil phase quickly dissolves the stearic acid molecule, removing it from the interface.

In the case of toluene,

another organic solvent which gives low yields, the stearic acid head group approaches the surface occasionally.

However, the body of the acid and (occasionally) the head group

spend a lot of time diffusing through the oil phase.

This shows that toluene cannot

strongly restrain the acid at the interface. In light of the results presented in Figures 8 and 9, we can conclude that organic solvents that successfully restrain the entire acid group at the interface result in higher yields of product during the biphasic reaction when compared to organic solvents that allow 24


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ACS Catalysis

the fatty acid molecule to diffuse in the oil phase. Moreover, such an assessment of these oil phases from MD simulations requires information about both the mobility of the head group and the mobility of the whole acid to obtain the complete picture. 3.5 Feedstock Flexibility Finally, to be competitive in terms of process economics, the broad applicability of the biTCP for utilizing a wide selection of fatty acid/ester feedstocks73 should be established to produce diesel-range straight-chain paraffins.

As shown in Table 2, a variety of

different saturated and unsaturated fatty acids with various carbon chain lengths were able to be processed in the biTCP.

The hydrogenation and decarboxylation of other fatty acids

were performed in a similar way as stearic acid, resulting in yields up to ~ 90% of the corresponding hydrocarbons in 6 h.

It was proved that the decarboxylation in the biTCP

is not sensitive to the carbon chain length of fatty acids.

The C-C double bonds in the

unsaturated fatty acid molecules were easily saturated through hydrogenation, which avoided the side reactions such as condensation and aromatization18.

As mentioned

before, the high selectivity of decarboxylation was attributed to the interfacial catalysis in the biTCP, as demonstrated by less than 1 % of the yield of the liquid-phase by-products with respect to the individual fatty acids.

Table 2 Product distribution after the conversion of various feedstocks in the biphasic catalytic processes. Reaction condition: 0.15 g fatty acid, 0.05 g Pd/C, 22 mL water, o

volume ratio of C/W was 2:22, p(H2) = 28 bar at room temperature, 260 C, 6 h. 25


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Entry

Feedstock (X:Y)a

Conversion %

Page 26 of 43

Decarboxylation yield %

1 2 3 4 5 6 7

Stearic acid (18:0) >99 Oleic acid (18:1) >99 Linoleic acid (18:1) >99 Dodecanoic acid (12:0) >99 Myristic acid (14:0) >99 Palmitic acid (16:0) >99 Erucic acid (22:0) >99 b Glyceryl trioleate 8 >99 (3 × 18:1) 9 Canola biocrude oilc >99 a Note: In the parentheses, the left term represents represents the number of double bond. b0.12

Hydrodeoxygenation yield %

91.7 93.7 93.8 88.7 91.8 91.4 91.3

Generic Biphasic Catalytic Approach for Producing Renewable Diesel

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