Taylor C.
Schulz
a,
Mason
Oelschlager
b,
Simon T.
Thompson
a,
Wim F. J.
Vermaas
c,
David R.
Nielsen
b and
H. Henry
Lamb
*a
aDepartment of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA. E-mail: lamb@ncsu.edu; Tel: +1-919-515-6395
bChemical Engineering, School for Engineering of Matter, Transport, and Energy, Arizona State University, Tempe, AZ 85287, USA
cSchool of Life Sciences, Arizona State University, Tempe, AZ 85287, USA
First published on 9th February 2018
A two-step catalytic process for converting cyanobacteria-derived fatty acids (CBFAs) to linear and branched alkanes for biorenewable synthetic paraffinic kerosene was demonstrated. Fatty acids synthesized and secreted into the growth medium by an engineered strain of the cyanobacterium Synechocystis sp. PCC 6803 were recovered from 20 liter photobioreactor cultures by adsorption on hydrophobic resin beads. By design, lauric acid (LA, C12:0, ∼80% w/w) was the main CBFA constituent; however, myristic acid (MA, C14:0, 6–10% w/w), palmitic acid (PA, C16:0, 2–6% w/w), and β-hydroxymyristic acid (BHMA, 2–3% w/w) also were produced. LA and MA model compounds were deoxygenated over a 5 wt% Pd/C catalyst to n-undecane and n-tridecane, respectively, with high yields and CO2 selectivities. Major products of BHMA deoxygenation over Pd/C were n-tridecane and 2-tridecanone. BHMA concentrations typical of the CBFA samples were found to inhibit LA deoxygenation. Because Pd sites responsible for fatty acid decarboxylation are poisoned at sulfur concentrations [S] typical of crude CBFA samples (100–150 ppm), post-recovery purification procedures were developed and evaluated based on their efficacy in reducing S-containing impurities. Deoxygenation of CBFAs was most effective when purification procedures limited [S] to <15 ppm, as evidenced by >80% n-alkane yield and ∼90% CO2 selectivity. The n-alkane products of CBFA deoxygenation were hydroisomerized in the liquid phase (with added n-dodecane) over a 0.70 wt% Pt/CaY catalyst. The resultant mixtures had isoalkane/normal alkane ratios of 0.25–0.50.
As with triacylglycerides (TAGs), catalytic deoxygenation of FAs can be accomplished by hydrodeoxygenation (HDO) over conventional petroleum hydroprocessing catalysts (e.g., NiMo/Al2O3)13 and by decarbonylation (DCN)/decarboxylation (DCX) over supported noble metal catalysts (e.g., Pd/C).14–25 In HDO, oxygen is removed as H2O (requiring 1 mole H2 per mole of O), and the product alkane (CnH2n+2) contains all the (n) carbon atoms in the parent FA. DCN and DCX are the principal pathways for FA deoxygenation over supported Pd. DCN produces CO, H2O, and an alkene (Cn−1H2n−2) with one less carbon atom than the parent FA; subsequent hydrogenation of the alkene requires 1 mole H2 per mole alkane produced. DCX produces CO2 and an (Cn−1H2n) n-alkane. In biofuel applications, DCX may be preferable to either HDO or DCN due to its lower H2 requirement.14 Pd supported on activated carbon (Pd/C) is one of the most effective noble metal catalysts for producing n-alkanes from FAs by DCX. Deoxygenation of stearic acid (SA) over Pd/C under an inert (He) atmosphere proceeds with a CO2 selectivity of ∼90%. Although the initial rate of SA deoxygenation is higher under He, more unsaturated hydrocarbon products are generated, and complete SA conversion requires much longer processing times (4–5 h). By adding ∼10% H2 to the reactor purge stream to prevent catalyst deactivation, complete SA conversion to a >99% n-heptadecane (n-C17) product was achieved in ∼50 min.15
Catalytic hydroisomerization (HI) is required to produce SPK from the normal alkanes (n-alkanes) resulting from deoxygenation CBFAs. SPK can be defined as a mixture of normal, branched, and cyclic paraffins with carbon numbers between 9 and 15.26,27 Conventional jet fuels (e.g., Jet A, JP-5 and JP-8) have freezing points below −40 °C.28,29n-Alkanes in this carbon number range (with the exception of n-nonane) have much higher freezing points. Petroleum-derived jet fuels have lower freezing points because they contain isoalkanes (i-alkanes), cycloalkanes and aromatic compounds. For example, Jet-A aviation fuel consists of 20% n-alkanes, 30% i-alkanes, 30% cycloalkanes, and 20% aromatic compounds (on a weight basis). JP-8 consists of approximately 25% n-alkanes, 30% i-alkanes, 25% cycloalkanes, and 20% aromatics.30 Long-chain n-alkanes can be hydroisomerized to branched alkanes with very high selectivity at moderate conversions (∼40%) over Pt/CaY.31 Increased hydrocracking (loss of isomerization selectivity) over bifunctional metal-zeolite catalysts is typically observed at high n-alkane conversion.31,32 Fortunately, a high i-alkane content is not required to significantly lower the freezing point of SPK mixtures.
In this work, CBFAs synthesized by an engineered strain of Synechocystis sp. PCC 6803, comprising primarily lauric acid (LA), were recovered from photobioreactor cultures, purified, and subsequently deoxygenated to n-alkanes using a Pd/C catalyst. Sulfur (S)-containing impurities and β-hydroxymyristic acid (BHMA) were found to negatively impact n-alkane yield and CO2 selectivity. BHMA, a cell membrane constituent in Gram-negative bacteria including Synechocystis,33,34 was present at 2–3% w/w in crude CBFA samples. Sulfur concentrations in crude CBFA samples were typically 100–150 ppm, but levels of up to 450 ppm were encountered. Post-recovery purification methods aimed at removing S-containing impurities and BHMA, including solvent extraction, recrystallization, acid/base hydrolysis, activated carbon (AC) treatment, and preparative-scale C18 liquid chromatography (LC) were investigated, and the most efficacious are described in detail herein. Finally, the CBFA-derived n-alkanes were hydroisomerized over a 0.70 wt% Pt/CaY catalyst to produce mixtures of linear and branched alkanes for biorenewable SPK.
Preparative-scale C18 (reverse-phase) LC was performed by first dissolving crude CBFA samples in an excess (20 mL g−1) of 45% v/v MeOH in water. The resulting solution was then loaded onto C18-functionalized silica adsorbent contained in a vertical glass column with a fritted glass outlet (to retain the adsorbent bed). Approximately 5 g adsorbent was used per 1 g CBFA processed and the solution passed slowly through the column by gravity. Following initial sample loading, the adsorbent bed was the subjected to elution using aqueous MeOH solutions of various concentrations as the mobile phase and two general approaches. Initially, the mobile solution consisted of either 75% or 100% MeOH, added to the column at a ratio of 50 mL g−1 CBFA sample initially loaded onto the column. To provide improved separation, a gradient elution method was later devised wherein a series of mobile phase solutions containing 0 to 100% MeOH in increments of 2.5 or 5% were instead used. In this case, at each step along the gradient, the mobile phase was added to the column at a ratio of 10 mL g−1 CBFA initially loaded onto the column. In this case, most of the CBFA sample was found to be eluted from the column when using a mobile phase containing between 65 and 100% MeOH. In both approaches, all fractions were individually collected as they exited the column and allowed to air dry to recover the treated CBFA sample.
Electrospray ionization-mass spectrometry (ESI-MS) was also used to determine CBFA composition. Dried extract samples weighing 3–5 mg were collected and placed in previously weighed Eppendorf tubes. The weights of the dried samples were determined to within 0.1 mg. The samples were then dissolved in MeOH at a final concentration 5 mg mL−1. Serial 100× and 10× dilutions were made, with the final sample containing 5 μM heptadecanoic acid, 1 mM NH4OH, and 5 μg mL−1 of the FA extract. This sample was then infused at 3 μL min−1via the ESI source of the MicrOTOF-Q MS using a KD Scientific model KDS-100-CE syringe pump. Mass spectra obtained in the negative-ion mode were averaged over 3 min. FA concentrations were determined using heptadecanoate (m/z = 269.25) as an internal standard.
ICP-OES of CBFA samples was performed by the EATS at North Carolina State University, the Goldwater Environmental Lab at Arizona State University and/or by Galbraith Labs (Knoxville, TN) to quantify ppm levels of S, P, Na, Mg, and Ca.
Liquid products were analyzed using a Shimadzu GC-2010 Plus GC-FID with a Restek RTX-5 capillary column (30 m length × 0.32 mm inner diameter × 1 μm film thickness), a constant linear velocity of 30.3 cm s−1, and a split ratio of 50:1. CBFA deoxygenation products were analyzed by ramping the oven at 10 °C min−1 from 120 to 300 °C and holding at 300 °C for 7 min. Products from BHMA (and related compounds) deoxygenation were analyzed with the following temperature program: ramp at 10 °C min−1 from 120 to 195 °C, cool at 20 °C min−1 to 157 °C, hold at 157 °C for 3 min, ramp to 300 °C at 20 °C min−1, and hold for 7 min at 300 °C. The cooling stage in this temperature ramp was utilized to enhance separation of 2-C13-OH and 2-C13-ONE. Product concentrations were determined using a three-point calibration method with a C10 internal standard. Response factors for n-alkenes other than 1-C17-ene were assumed to be equivalent to their corresponding n-alkanes, which shown to be true for FID response factors for shorter n-alkanes and n-alkenes.37n-Alkane yields were determined by dividing the total moles of n-alkanes produced from the reaction by the total reactant moles added to the reactor.
HI products were analyzed using a Shimadzu GC-2010 GC-FID with an EconoCap EC-1 capillary column (30 m length × 0.32 mm inner diameter × 1 μm film thickness). The oven temperature program comprised 3 segments: 5 min hold at 30 °C, ramp from 30 °C to 280 °C at 10 °C min−1, and a 5 min hold at 280 °C. The weight percentage of each alkane species was estimated from its area percentage. Alkanes in the C14–C32 range have equivalent GC-FID response factors (within error);38 therefore, we assumed that the area percentage of each species equaled its weight percentage in the mixture.
Sample | Post-recovery purification | [S] (ppm) | [P] (ppm) |
---|---|---|---|
a AC: activated carbon treatment. b C18 LC: preparative-scale C18 liquid chromatography. c X% MeOH: methanol concentration in C18 column eluent. For single values, samples were eluted with only one eluent concentration. For multiple values, stepwise gradient elution was employed, and the reported values represent the elution fractions comprising the sample. | |||
FA01 | None | 157 | 8 |
FA02 | None | 159 | <21 |
FA02A | Recrystallization | 109 | <20 |
FA03 | 2× HCl/n-hexane extraction | — | — |
FA03A | 2× HCl/n-hexane extraction; ACa | — | — |
FA04 | C18 LCb (75% MeOH)c | 73 | 5 |
FA05 | C18 LC (100% MeOH) | 198 | 16 |
FA06 | None | 454 | — |
FA06A | C18 LC (75, 80, 85% MeOH) | 75 | 8 |
FA06B | C18 LC (75, 77.5, 80, 82.5% MeOH) | 12 | — |
FA07 | None | 104 | — |
FA07A | Alkaline hydrolysis | 39 | — |
FA07B | 2× alkaline hydrolysis | <10 | — |
FA08A | AC; C18 LC (75% MeOH) | 60 | — |
FA08B | AC; C18 LC (100% MeOH) | 89 | — |
FA09 | Alkaline hydrolysis; 4× AC | 102 | — |
FA10 | 4× alkaline hydrolysis; 4× AC | 29 | — |
Sample | Percent of total fatty acids (w/w) | |||||
---|---|---|---|---|---|---|
Fatty acida | FA01 | FA02A | FA03A | FA06A | FA06B | FA07B |
a Cn:m refers to the carbon number of the FA (n) and the degree of unsaturation (m) (number of C–C double bonds in the alkyl chain). b Authentic standard not available to confirm identification. c Largest contribution peak eluting immediately after C12:0. | ||||||
C10:0 | 0.60 | 0.00 | 0.42 | 0.05 | 0.00 | 0.27 |
C10:1b | 0.00 | 0.00 | 0.00 | 0.11 | 0.00 | 0.30 |
C12:0 | 79.09 | 80.33 | 81.58 | 77.02 | 78.82 | 82.03 |
C14:0 | 6.60 | 10.10 | 7.61 | 10.94 | 0.19 | 8.47 |
C14:1 trans | 0.00 | 0.00 | 0.00 | 0.02 | 0.00 | 0.00 |
C14:1 cis | 0.13 | 0.00 | 0.00 | 0.10 | 0.06 | 0.00 |
C16:0 | 2.78 | 2.60 | 3.18 | 3.94 | 0.00 | 3.43 |
C16:1 trans | 0.00 | 0.00 | 0.00 | 0.05 | 0.00 | 0.00 |
C16:1 cis | 1.30 | 0.51 | 0.78 | 1.95 | 0.00 | 0.77 |
C16:3 cisb | 0.00 | 1.35 | 0.00 | 0.42 | 0.00 | 0.70 |
C16:4 cis | 0.00 | 0.59 | 0.00 | 0.95 | 0.00 | 0.00 |
C18:0 | 0.35 | 0.00 | 0.34 | 0.02 | 0.00 | 0.00 |
C18:1 cis | 1.46 | 0.00 | 0.35 | 0.08 | 0.00 | 0.00 |
C18:2 cis | 1.97 | 1.42 | 0.74 | 0.73 | 0.00 | 0.54 |
C18:3 cis 6 | 1.66 | 1.66 | 0.65 | 0.17 | 0.00 | 0.24 |
C18:3 cis 9 | 0.12 | 0.00 | 0.08 | 0.03 | 0.00 | 0.00 |
C20:1 cis | 0.00 | 0.00 | 0.15 | 0.00 | 0.00 | 0.08 |
Unknowns | 2.08 | 1.04 | 2.64 | 2.50 | 20.93c | 1.48 |
Total FA content (mg g−1) | 755 | 749 | 767 | 800 | 888 | 895 |
Sample | Percent of total fatty acids (w/w) | ||||||
---|---|---|---|---|---|---|---|
Fatty acida | FA03A | FA06 | FA06A | FA06B | FA07B | FA09 | FA10 |
a Cn:m refers to the carbon number of the FA (n) and the degree of unsaturation (m) (number of C–C double bonds in the alkyl chain). | |||||||
C10:0 | 0.43 | 0.40 | 0.00 | 0.00 | 0.41 | 0.40 | 0.42 |
C12:0 | 79.09 | 76.54 | 74.05 | 97.04 | 75.71 | 82.84 | 78.33 |
BHLA | — | 2.76 | — | 0.00 | 4.44 | 0.65 | 0.91 |
C14:0 | 8.21 | 6.92 | 9.80 | 0.25 | 8.02 | 7.00 | 8.16 |
BHMA | 1.80 | 2.74 | 0.95 | 0.14 | 2.48 | 0.11 | 0.26 |
C16:0 | 4.61 | 2.94 | 6.08 | 0.75 | 3.30 | 1.78 | 3.38 |
C16:1 | 1.02 | 1.02 | 4.19 | 0.61 | 0.88 | 0.11 | 0.99 |
C18:0 | 1.56 | 0.14 | 0.72 | 0.43 | 0.95 | 0.07 | 0.36 |
C18:1 | 1.13 | 1.01 | 2.51 | 0.23 | 0.78 | 0.41 | 1.10 |
C18:2 | 1.38 | 2.64 | 1.30 | 0.07 | 0.76 | 2.38 | 2.60 |
C18:3 | 0.76 | 1.65 | 0.39 | 0.06 | 0.42 | 2.02 | 1.95 |
Total FA content (mg g−1) | 798 | 807 | 849 | 914 | 973 | 779 | 897 |
ICP-OES analysis of FA01 found 157 ppm S, 7.94 ppm P, 33.9 ppm Na, 8.52 ppm Mg, and 30.2 ppm Ca. Among these impurities, S and P are potential catalyst poisons, and alkali and alkaline earth metals may act as catalyst modifiers and/or promoters. Sulfur concentrations in crude CBFA samples were typically in the 100–150 ppm range, and phosphorus concentrations were 8–20 ppm (Table 1). FA06 was more heavily contaminated with S. As demonstrated below, S impurities severely degrade the deoxygenation performance of the Pd/C catalyst. Consequently, several post-recovery purification procedures, including preparative-scale C18 LC and alkaline hydrolysis, were developed and evaluated for their efficacies in reducing S concentrations.
Preliminary C18 LC experiments indicated that CBFA fractions eluted in the range of 75 to 85% MeOH–water were LA-rich and contained lower levels of S and P than those eluted with 100% MeOH (see FA04 and FA05, Table 1). Subsequently, CBFA samples were purified by C18 LC using stepwise gradient elution to achieve almost complete removal of β-hydroxy FAs and S-containing impurities. A heavily S-contaminated CBFA sample (FA06) was loaded on the C18 column and eluted using a series of MeOH–water solutions, increasing from 0 to 100% MeOH in steps of 5%; fractions eluting between 75 to 85% MeOH were collected and pooled (FA06A). GC-FID and ESI-MS analyses indicated that FA06A contained 80–85% total FAs with ∼76% LA, ∼10% MA, and ∼5% PA. S and P concentrations were 75 ppm and 8 ppm, respectively.
Another portion of FA06 was loaded and eluted from the C18 column using a similar MeOH–water gradient, however, in steps of 2.5%. Fig. 1 illustrates the eluted mass, LA composition, and S content of individual fractions collected in the 72.5 to 85% MeOH range. More polar hydroxy FAs typically eluted before LA. The mass of eluted CFBAs reached a maximum at 75% MeOH and then decreased nearly linearly as the MeOH concentration was increased to 85%. LA concentration reached a maximum of ∼95% w/w (total mass) at 77.5% MeOH. S concentration was ∼300 ppm in the 72.5% MeOH fraction, decreased abruptly to <25 ppm, and then increased to ∼75 ppm in the 85% MeOH fraction, suggesting that there were multiple different S-containing impurities in the sample. Fractions eluted using 75% to 82.5% MeOH that contained >80% w/w LA and <25 ppm S and were pooled to produce FA06B. ESI-MS indicated that FA06B contained 91% total FAs with ∼97% LA; however, LA recovery (77%) was relatively low. GC-FID evidenced a similarly high FA content. (If the anomalous peak eluting immediately after C12:0 were taken to be LA (due to possible overloading of the capillary column), the GC-FID LA content would be equivalent to that measured by ESI-MS.) Sample FA06B also had a remarkably low S content: 12 ppm.
Fig. 1 Eluted mass (open squares), LA concentration (closed squares), and S concentration (closed circles) during C18 chromatography of FA06B. |
Although preparative-scale C18 LC proved an effective strategy for removing β-hydroxy FAs and S-containing impurities from CBFA samples, it was time and resource intensive and gave relatively low yields. Consequently, simpler batch chemical processes were investigated, and alkaline hydrolysis was found to be effective at removing S-containing impurities. A single alkaline hydrolysis step performed on FA07 (a crude CBFA sample with a typical S content) resulted in a 64% reduction in S concentration with 98% LA recovery (FA07A). Consecutive alkaline hydrolysis steps resulted in a sample (FA07B) containing less than 10 ppm S (below the ICP-OES detection limit at ASU). GC-FID analysis indicated that FA07B contained ∼90% total FAs with a typical chain-length distribution; however, relatively high concentrations of BHLA and BHMA were detected by ESI-MS.
Fig. 2 Temporal evolution of CO2 during catalytic deoxygenation of (a) LA and LA/BHMA mixtures and (b) MA, BHMA and a MA/BHMA mixture over 5 wt% Pd/C in flowing 5% H2 at 300 °C and 15 bar. |
Reactantb | Mass (g) | CO + CO2 yield (%) | n-Alkane yield (%) | CO2 selectivity (%) | H2 generationf |
---|---|---|---|---|---|
a Typical reaction conditions: 0.335 g 5 wt% Pd/C, 22.5 g n-C12 solvent, 5% H2, Prxtr = 15 bar, Trxtr = 300 °C, 4 h reaction time. b 5.6 mmol reactant used, except 1.18 g 2-C13-ONE (5.9 mmol). c Average of two deoxygenation experiments. d Sum of n-C11 and n-C13 produced during deoxygenation. e 10% (w/w) of reactant BHMA, remainder LA or MA. f Net moles H2 generated per mole of reactant(s) charged. | |||||
LAc | 1.13 | 96.5 | 91.7 | 88.5 | −0.13 |
MA | 1.28 | 90.6 | 91.2 | 89.8 | −0.15 |
BHMAc | 1.37 | 93.7 | 51.2d | 86.6 | 0.04 |
BHMA/LAe | 1.14 | 83.9 | 86.4d | 91.0 | −0.09 |
BHMA/MAe | 1.25 | 90.3 | 86.8d | 90.1 | −0.14 |
2-C13-OH | 1.14 | 7.1 | 14.8d | 18.3 | 0.65 |
2-C13-ONE | 1.18 | 6.3 | 11.5d | 16.3 | −0.22 |
BHMA was deoxygenated over 5 wt% Pd/C under equivalent reaction conditions. On-line QMS data evidence rapid evolution of CO2, CO and H2 at 300 °C (Fig. 3). Net generation of H2 is observed, contrary to the H2 consumption typically found during FA deoxygenation. Conversion of BHMA is complete after 3 h. As illustrated in Fig. 2b, initial deoxygenation of BHMA occurs very rapidly when compared to MA. A shoulder on the main CO2 evolution peak at ∼1.3 h reaction time suggests that MA may be an intermediate product. Post-reaction analysis of liquid products by GC-FID (Fig. S2, Table S1†) identified n-C13 and 2-C13-ONE as major deoxygenation products; n-C11 and 2-C13-OH were minor products. A long-retention-time (presumably, high molecular weight) unknown compound was also detected by GC-FID. We infer that the net generation of H2 results from dehydrogenation of 2-C13-OH to 2-C13-ONE. In separate experiments (Table S1†), 2-C13-ONE and 2-C13-OH were reacted under equivalent conditions. Neither 2-C13-ONE nor 2-C13-OH was deoxygenated to n-C13 in appreciable yield; therefore, the formation of these products (and the unknown species) explains the low n-alkane yield from BHMA deoxygenation (Table 4).
Fig. 3 H2, CO, and CO2 evolution from BHMA semi-batch deoxygenation over 5 wt% Pd/C at 300 °C and 15 bar in flowing 5% H2. |
Proposed reaction pathways for BHMA, 2-C13-ONE, and 2-C13-OH over 5 wt% Pd/C are illustrated in Scheme 1. As shown, sequential HDO and DCX reactions yield n-C13; however, direct deoxygenation of BHMA to n-C13 (without MA as an intermediate) cannot be excluded. In parallel, direct DCX of BHMA produces 2-C13-OH that is rapidly dehydrogenated to 2-C13-ONE. Alternatively, BHMA may be dehydrogenated to β-ketomyristic acid before undergoing DCX to 2-C13-ONE.
Scheme 1 Suggested reaction network for BHMA deoxygenation over 5 wt% Pd/C at 300 °C and 15 bar in flowing 5% H2. |
Deoxygenation of BHMA-FA mixtures resulted in CO2 selectivities of ∼90%. As expected, n-alkane yields (∼86%) were lower than for FA deoxygenation (Table 4). Moreover, the presence of BHMA at 2 and 10 wt% (of total FAs) inhibited the kinetics of LA deoxygenation, as illustrated in Fig. 2a. Deoxygenation of BHMA results in a small CO2 evolution peak just prior to the main LA deoxygenation peak. BHMA addition causes a decrease in CO2 peak intensity (greater than the expected ∼10% decrease) and a delay in CO2 peak time. BHMA addition also leads to a corresponding delay and decrease in CO peak intensity. In contrast, BHMA addition (10 wt%) did not affect MA deoxygenation kinetics significantly, as evidenced in Fig. 2b. Slower kinetics for LA deoxygenation may be ascribed to competitive adsorption of BHMA.
Sample | CBFA conc. (wt%) | CO + CO2 yieldc (%) | Alkane yieldc,d (%) | CO2 selectivity (%) | H2 generatione |
---|---|---|---|---|---|
a Reaction conditions: 0.335 g 5 wt% Pd/C, 22.5 g n-C12 solvent, 5% H2, P = 15 bar, T = 300 °C. b Deoxygenated for 8 h reaction time. All other sample deoxygenated for 4 h reaction time. c Calculated using an average FA distribution: 81% C12:0, 11% C14:0, 6% C16:0, and 2% C18:0. d Includes all C11 to C17n-alkane products, except n-C12 solvent. e Net moles H2 generated per mole of CO + CO2 produced. | |||||
FA01b | 4.90 | 47.4 | 41.7 | 31.7 | −0.88 |
FA02A | 4.75 | 53.6 | 52.1 | 47.2 | −0.46 |
FA03 | 4.96 | 35.5 | 32.1 | 22.0 | −1.07 |
FA03A | 4.53 | 53.6 | 52.8 | 71.2 | −0.32 |
FA04 | 4.75 | 76.0 | 62.2 | 70.2 | −0.44 |
FA05 | 4.74 | 53.6 | 34.4 | 16.1 | −0.64 |
FA06A | 4.77 | 61.6 | 59.5 | 62.0 | −0.69 |
FA06B | 4.63 | 81.7 | 88.3 | 88.0 | −0.23 |
FA07B | 4.74 | 83.2 | 83.1 | 84.8 | −0.09 |
FA08A | 4.79 | 66.2 | 70.0 | 78.7 | −0.19 |
FA08B | 4.78 | 53.7 | 54.1 | 65.3 | −0.33 |
FA09 | 4.75 | 51.8 | 55.4 | 57.7 | −0.47 |
FA10 | 4.82 | 81.0 | 78.8 | 84.1 | −0.18 |
Fig. 4 Temporal evolution of CO2 (a) and CO (b) during deoxygenation of FA04, FA05, FA06B and FA07B over 5 wt% Pd/C in flowing 5% H2 at 300 °C and 15 bar. |
As demonstrated above, catalyst performance in CBFA deoxygenation is negatively impacted by feedstock impurities. Although β-hydroxy FAs inhibit FA deoxygenation, poisoning of the Pd/C catalyst by S-containing impurities is the dominant effect. As illustrated in Fig. 5, CO2 selectivity and n-alkane yield decline precipitously with increasing S concentration. An increase in S concentration from ∼0 ppm (reagent-grade LA) to ∼100 ppm reduces both CO2 selectivity and n-alkane yield by ∼33%. Moreover, for a S concentration of ∼150 ppm (typical of crude CBFA samples), CO2 selectivity drops to <40% and total alkane yield to <50%. Correlating the available ICP-OES data with the semi-batch CBFA deoxygenation results indicates that it is necessary to limit [S] to <15 ppm to obtain deoxygenation performance equivalent to LA over 5 wt% Pd/C.
The strong correlation of n-alkane yield with CO2 selectivity suggests that S has a greater poisoning effect on Pd-catalyzed DCX than DCN. Indeed, as illustrated in Fig. 6, the initial DCX rate is more sensitive to S poisoning than the initial DCN rate. When [S] in the feedstock increases from ∼0 to 50 ppm, the initial CO2 evolution rate declines by almost a factor 5; the initial DCX rate is approximately constant for higher CBFA S concentrations (50 < [S] ≤ 200 ppm). In contrast, the initial CO evolution rate is relatively constant over the entire range of [S]. Previously, Immer and Lamb found that CO selectively (and reversibly) inhibits the DCX pathway in SA deoxygenation using 5 wt% Pd/C under similar conditions.14 The profound effect of S on the rates of Pd-catalyzed reactions can be traced to site blockage and electronic effects. For example, Miller et al. determined that dosing a Pd membrane with H2S created a surface Pd4S layer with each S atom blocking 4 surface Pd atoms.40 Density functional theory calculations showed that H2 dissociative chemisorption on Pd4S surface was always an endothermic process, rather than the exothermic dissociative H2 adsorption exhibited over clean Pd(111).
Semi-batch deoxygenation of ∼11 g of FA08A at 46.6 wt% in n-C12 was conducted using 1 g of 5 wt% Pd/C catalyst under typical reaction conditions. The temporal CO2 and CO evolution curves are shown in Fig. 7. At the higher initial concentration, deoxygenation occurred primarily via DCN despite the ∼3 times greater catalyst mass. Previous research demonstrated that higher initial FA concentrations in semi-batch FA deoxygenation result in lower DCX rates due to self-poisoning by CO.39 This first deoxygenation experiment (1st run) converted approximately half of the CBFAs (based on CO + CO2 yield). Conversion of unreacted CBFAs was achieved by repeating the experiment using 1 g of fresh catalyst (2nd run). This resulted in a fivefold increase in CO2 selectivity and produced a liquid n-alkane product that subsequently was subjected to HI.
Fig. 7 Temporal evolution of (a) CO2 and (b) CO from semi-batch deoxygenation of FA08A (11.0 g) over 5 wt% Pd/C (1 g) at 300 °C and 15 bar under flowing 5% H2. |
Deoxygenation of ∼14 g FA10 was conducted in fed-batch mode14 using 1 g of 5 wt% Pd/C catalyst at higher total pressure (23.4 bar) in flowing 2 vol% H2. The higher operating pressure was necessary to suppress evaporation of the n-C12 solvent and n-alkane products. Fig. 8 shows on-line QMS data recorded during this experiment. After an initial start-up transient, a quasi-steady state characterized by high CO2 selectivity was achieved after 6 h time on stream (TOS). Although there was a gradual decrease in CO2 selectivity with TOS, DCX remained the dominant reaction pathway until feeding was stopped after ∼22 h TOS. Overall CO2 selectivity and hydrocarbon yield were 81.6% and 73.5%, respectively. The hydrocarbon product comprised 99.2% n-alkanes.
Preliminary HI experiments on n-C11 and n-C11/n-C12 mixtures were conducted to determine the optimum reaction conditions and batch time to maximize isomer yield using a 0.70 wt% Pt/CaY catalyst. As shown in Table 6, HI of n-C11 over Pt/CaY at 300 °C for 180 min resulted in moderate conversion with high selectivity to C11 isomers (i-C11). HI of a 50% w/w n-C11/n-C12 mixture under equivalent conditions achieved a similar n-alkane conversion and HI selectivity demonstrating that n-C12 addition did not significantly affect the reaction kinetics. HI of n-alkanes resulting from semi-batch deoxygenation of FA08A in n-C12 resulted in ∼20% conversion with very high selectivity to isomerized products. The HI product (Fig. S3†) was a clear colorless liquid at room temperature. The low yield of C11 isomers resulted from the low initial n-C11 concentration and lower than expected conversion. HI of FA10 deoxygenation products in n-C12 resulted in higher conversion and i-C11 product yield (Table 6). The i/n-alkane ratios of the FA08A and FA10 HI products were 0.23 and 0.51, respectively. For comparison, Jet-A and JP-8 have i/n-alkane ratios of 1.72 and 1.33, respectively. Although the CBFA-derived product mixtures are not directly comparable to petroleum-derived aviation fuels by this metric, these experiments provide proof of concept that CBFA-derived n-alkanes can be hydroisomerized over Pt/CaY to produce biorenewable SPK.
Sample | n-C11 | n-C11/n-C12 | FA08A n-alkanesb | FA10 n-alkanesc |
---|---|---|---|---|
a Batch reaction using 0.350 g 0.7 wt% Pt/CaY at 300 °C and 500 psig H2 (initial pressure at 25 °C). b FA08A semi-batch deoxygenation product (25.8 g). c FA10 fed-batch deoxygenation product (14 g) + n-C12 (11 g). d Mass recovery in liquid products >98%. | ||||
P f (psig) | 490 | 450 | 490 | 450 |
Reaction time (min) | 180 | 176 | 109 | 180 |
Conversion (%) | 58.9 | 60.2 | 19.6 | 35.8 |
HI selectivity (%) | 92.7 | 90.1 | 94.9 | 91.9 |
Liquid product composition (wt%)d | ||||
C4–C10 | 4.3 | 6.0 | 1.1 | 2.9 |
i-C11 | 54.6 | 26.7 | 2.7 | 7.7 |
n-C11 | 41.1 | 20.3 | 12.7 | 18.2 |
i-C12 | — | 27.6 | 15.1 | 24.1 |
n-C12 | — | 19.4 | 65.6 | 44.5 |
i-C13 | — | — | 0.2 | 0.4 |
n-C13 | — | — | 1.1 | 0.7 |
i-C15 | — | — | 0.2 | 0.5 |
n-C15 | — | — | 0.4 | 0.4 |
i-C17 | — | — | 0.4 | 0.3 |
n-C17 | — | — | 0.5 | 0.3 |
Footnote |
† Electronic supplementary information (ESI) available: S1: catalyst characterization. Table S1: product moles of BHMA, 2-C13-OH, and 2-C13-ONE deoxygenation over 5 wt% Pd/C at 300 °C and 15 bar under 5 vol% H2. Fig. S1: photo of crude CBFA sample (FA03). Fig. S2: chromatograms of standards and BHMA deoxygenation product with species identification: (a) 2-C13-ONE, (b) 2-C13-OH and (c) BHMA reactor contents. Fig. S3: photo of vial containing SPK product from FA08A. See DOI: 10.1039/c7se00558j |
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