Effects of Fatty Acid Structures on Ketonization Selectivity and Catalyst

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Effects of Fatty Acid Structures on Ketonization Selectivity and Catalyst Deactivation Kyungho Lee, Myoung Yeob Kim, and Minkee Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02576 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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ACS Sustainable Chemistry & Engineering

Effects of Fatty Acid Structures on Ketonization Selectivity and Catalyst Deactivation Kyungho Lee, Myoung Yeob Kim, and Minkee Choi* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291, Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Corresponding author *Minkee Choi. E-mail address: [email protected]

ABSTRACT: Ketonization of fatty acids into fatty ketones is a potential route for producing high-value chemicals including bio-derived lube base oil. In the present work, the catalytic selectivity and deactivation during the ketonization of C18 fatty acids having different unsaturation degrees over a TiO2 catalyst were rigorously investigated. The results demonstrated that the yield of fatty ketone gradually decreased with increasing unsaturation degree, while byproducts such as methyl ketones and olefins were produced owing to McLafferty rearrangement. It was verified that carboxylic acids longer than C5 can be decomposed via this pathway, the rate of which increased with the carbon chain length. In the ketonization of unsaturated fatty acids, the McLafferty rearrangement and cracking produced conjugated polyunsaturated olefins (e.g., dienes), which could be readily decomposed to coke. The results implied that the ketonization of natural fatty acids requires the pre-saturation of C=C bonds for increasing the fatty ketone yield and inhibiting catalyst deactivation. Indeed, the ketonization of a natural fatty acid mixture obtained by palm oil hydrolysis exhibited diminished fatty ketone selectivity and rapid catalyst deactivation, owing to the presence of unsaturated fatty acids. In contrast, the ketonization of a saturated fatty acid mixture obtained by hydrogenative hydrolysis exhibited a high fatty ketone yield (~90%) and negligible deactivation.

KEYWORDS: Ketonization; Fatty Acid; Selectivity; Deactivation; Lube Base Oil

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INTRODUCTION Owing to the global need for finding sustainable energy sources and mitigating anthropogenic CO2 emission, there is an increasing interest in the development of processes that can convert biomass into fuels and chemicals.1-4 Among the various biomass feedstocks, triglycerides (e.g., vegetable oil, microalgae oil, and animal fat), composed of three long-chain fatty acids (typically C14–C22) and a glycerol unit, have been widely used for the production of bio-fuels.4,5 A significant advantage in using triglycerides is their low oxygen contents and highly paraffinic backbone (i.e.,–(CH2)x–) in the fatty acid units, the structures of which are already quite similar to those of petroleum-derived hydrocarbons. This implies that relatively simple catalytic conversions can effectively convert triglycerides into fuels. In this regard, previous researches on triglyceride conversion have mainly focused on the synthesis of biodiesel (e.g., fatty acid methyl esters) via transesterification,5,6 and diesel and jet fuel production via hydrotreating.7-11 However, relatively few efforts have been undertaken for converting triglycerides into more value-added chemical products. Lube base oil is one of the potent chemicals that can be produced from triglycerides.12 It is a liquid containing branched and cyclic paraffins typically having C20–C35 carbon numbers,12,13 which are used to manufacture products including lubricating greases, motor oil, and metal processing fluids. Conventional lube base oils have been produced by solvent refining or hydrocracking of petroleum-derived feedstock.14,15 Their global demands have gradually increased over the past decades; in particular, the requirement for Groups II and III lube base oils with low sulfur contents (< 0.03%) and high paraffin contents (saturates > 90%) have rapidly increased in recent years.16 Owing to the low sulfur contents and highly paraffinic structures, triglycerides are highly relevant sources for the production of these Groups II and III lube base oils.12 Because triglycerides are composed of mainly C14–C22 fatty acid units, which are shorter than the typical hydrocarbons in lube base oils (C20–C35), the carbon backbone of fatty acid units should be elongated for the production of lube base oil. A potential strategy is to dimerize two fatty acids into a fatty ketone containing approximately two times larger carbon numbers, via ketonization or ketonic decarboxylation (dimerization of Cn fatty acid results in C2n-1 fatty ketone, Scheme 1). The produced fatty ketones can be further hydrodeoxygenated and hydroisomerized/cracked for obtaining hydrocarbon mixtures with desirable physical properties. It is noteworthy that the fatty ketones also have specialized applications as in ink vehicles,17 dishwashing detergents,18 and personal care products.19 Ketonization is a reaction that couples two carboxylic acids into a ketone with the elimination of H2O and CO2. The reaction is environment-friendly because non-polluting byproducts are generated, and


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no other solvents or reagents are necessary.20 It recently regained significant scientific attention because of its potential applications in biomass conversions.21-25 Various metal oxides including low-lattice-energy alkaline earth oxides (MgO, CaO, BaO, CaO, etc.) as well as high-lattice-energy metal oxides (TiO2, CeO2, ZrO2, MnO2, etc.) can be used as a catalyst.21-23 The former class of catalysts first forms bulk carboxylate salts, which decompose upon thermal treatment, generating ketones. In contrast, in the latter class of catalysts, the reaction proceeds selectively at the catalyst surface. It is generally accepted that the abstraction of the α-hydrogen of carboxylic acid by a basic site of the catalyst surface is the most important reaction step (Scheme 2).26-31 The generated nucleophile (e.g., enolate species) can react with another carboxylic acid adsorbed on the Lewis acid sites (or coordinatively unsaturated metal cation sites) of catalysts to form β-ketoacids, which can decompose into ketone and CO2.26-31 This proposed acid-base bifunctional mechanism explains why amphoteric metal oxides generally exhibit superior ketonization activities.29-41 Earlier studies on ketonization have mainly focused on the reactions using short and saturated model carboxylic acids (< C7).29,30,37-39,42-44 A major reason is that ketonization has generally been investigated as a tool for elongating the carbon chains of small oxygenate molecules produced from the fast pyrolysis (bio-oil) or hydrolysis of lignocellulosic biomass to produce middle distillate fuels.44-46 Although the ketonization of long-chain fatty acids can be important in bio-derived lube base oil production as well as other fine chemical synthesis, relatively very few studies have been carried out. Corma et al. investigated the ketonization of saturated lauric acid (C12) as a model fatty acid for the production of long-chain hydrocarbons as diesel additives or lube base oil.24 They reported that the yield of desired fatty ketone (laurone, C23) over MgO catalyst is high (> 90%). However, it should be noted that natural triglycerides always contain various fatty acids having different degrees of unsaturation (i.e., different numbers of C=C bonds in fatty acid chains). Considering the harsh reaction conditions of ketonization (573–723 K), the presence of C=C bonds in the fatty acid backbones can cause the formation of various byproducts and catalyst deactivation. Until the present, the effects of fatty acid structures on ketonization selectivity and catalytic deactivation have not been systematically investigated in literature. In the present work, we investigated the effects of different fatty acid structures on ketonization. The C18 fatty acids with different degrees of unsaturation (i.e., stearic, oleic, and linoleic acid) were ketonized over a TiO2 catalyst. TiO2 has been widely investigated as one of representative high-lattice-energy metal oxide catalysts,22,29-30 which have been often additionally modified by H2 pre-treatment29 and alkalinetreatment40,41 to enhance catalytic activity and lifetime. The possible reaction pathways and catalyst deactivation mechanism were rigorously investigated. We also carried out ketonization using a mixture of natural fatty acids pre-synthesized by the hydrolysis of palm oil. It will be demonstrated that an increased


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degree of unsaturation in fatty acids can significantly lower the yield of the desired fatty ketones by producing byproducts such as methyl ketones and olefins, and also accelerate catalyst deactivation via coke formation. Based on these results, we propose that pre-saturation of unsaturated fatty acids is essential for efficient production of fatty ketones.

EXPERIMENTAL SECTION Catalyst preparation. Commercial TiO2 (P25, Sigma-Aldrich, 99.5%) was used as a ketonization catalyst. 5 wt% Pt/C (activated carbon support, Sigma-Aldrich) was used as a catalyst for the presaturation of fatty acids during the hydrolysis of palm oil (i.e., hydrogenative hydrolysis). As a hydrodeoxygenation catalyst for fatty ketones, 1 wt% Pt/γ-Al2O3 was synthesized by the incipient wetness impregnation of an aqueous solution of Pt(NH3)4(NO3)2 (Aldrich) into γ-Al2O3 (Strem, 97%). The impregnated sample was dried at 373 K, calcined under dry air at 673 K (ramp: 2 K min-1) for 2 h, and reduced under H2 at 673 K (ramp: 2 K min-1) for 2 h. Characterization. X-ray diffraction (XRD) pattern was collected using a Bruker D2-phaser diffractometer with CuKα radiation (operated at 30 kV and 10 mA) and a LYNXEYE detector. The data were collected at a resolution of 0.01° and a count time of 2.5 s at each point. The N2 adsorption– desorption isotherm was measured using a BELSORP-max volumetric analyzer (BEL Japan) at liquid N2 temperature (77 K). Before the measurements, TiO2 was degassed under vacuum for 4 h at 673 K. The specific surface area was determined in the P/P0 range 0.10–0.30 using the Brunauer–Emmett–Teller (BET) equation. Transmission electron microscopy (TEM) images were taken with a FE-TEM (JEOL Ltd.) operating at 200 kV after mounting the samples on a carbon-coated copper grid (300 mesh) using ethanol dispersion. The NH3 and CO2 temperature-programmed desorption (TPD) profiles were recorded using BELCAT (BEL Japan) equipped with a thermal conductivity detector. Typically, 0.1 g of sample was degassed in He (50 cm3 min-1) at 673 K for 1 h. Then, the sample was equilibrated under 5% NH3/He or 99.5% CO2 (30 cm3 min-1) at 323 K for 1 h. The weakly adsorbed NH3 or CO2 was removed by flowing He (30 cm3 min-1) for 1 h at the same temperature. For the measurement of the TPD profiles, the temperature was increased up to 923 K (ramp: 10 K min-1) under a He flow. The acidity of TiO2 was also analyzed using FT-IR after the adsorption of pyridine as a base probe molecule. The FT-IR spectrum was collected on a Thermo Nicolet NEXUS instrument using a laboratory-made in situ cell with CaF2 window. A self-supporting wafer consisting of 15 mg sample was placed in the in situ IR cell and connected to a vacuum system. The wafer was degassed at 723 K for 2 h under vacuum, and the background spectrum of the sample was collected. Pyridine vapor was introduced to the cell at 323 K for 1 h, and the cell was


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evacuated at the same temperature for 1 h to remove the weakly adsorbed pyridine. The IR spectrum was recorded at a resolution of 4.0 cm-1. The amounts of coke in the used-catalysts after ketonization were analyzed by thermogravimetric analysis (TGA) in air using a TGA N-1000 instrument (Thermo). Catalytic reaction. Ketonization was carried out in a down-flow plug-flow reactor (quartz, 10.9 mm inner diameter) at 653 K under atmospheric pressure. As a model fatty acid reactant, stearic acid (C18:0, Sigma-Aldrich, 95%), oleic acid (C18:1, Sigma-Aldrich, 90%), and linoleic acid (C18:2, Sigma-Aldrich, 58−74%) were used. Typically, 0.14–5.0 g of sieved catalysts (150–200 µm) were loaded into the reactor and pretreated at 653 K for 2 h under a He flow. The fatty acids preheated at 353 K were introduced using a HPLC pump (0.05 g min-1) and He was used as a carrier gas (40 cm3 min-1). All the tubings located after the fatty acid container (even including the head of HPLC) were carefully wrapped with heating tape and gently heated at 353 K to inhibit the solidification of fatty acids. A weight hourly space velocity (WHSV) was controlled in the range of 0.5–20 g gcat.-1 h-1. The liquid products were collected using a trap heated at 373 K. The products were dissolved in chloroform and analyzed using an offline gas chromatography (GC) equipped with a flame ionization detector (FID) and an HP-5 capillary column (Agilent, 30 m × 0.25 mm). GC-MS analysis of the liquid products was also carried out using a HP5890 GC (HewlettPackard) equipped with a HP-5 column (Agilent, 30 m × 0.32 mm) and a mass spectrometer (HP 5989B, Hewlett-Packard). The gas products were directly analyzed using an online GC equipped with a FID and a GS-GasPro capillary column (Agilent, 30 m × 0.25 mm) as well as a TCD and a Carboxen-1000 column (Supelco, 1.5 m × 1/8 inch). For calculating the product selectivity and yield, only the hydrocarbons and oxygenate compounds were considered as the products. Inorganic byproducts such as CO2 and H2O were not included in the calculation. The ketonization of carboxylic acids having different chain lengths, namely, butanoic acid (Aldrich, 99%), pentanoic acid (Aldrich, 99%), hexanoic acid (Aldrich, 99%), octanoic acid (Aldrich, 98%), and dodecanoic acid (Aldrich, 98%) was carried out in a similar manner using a fixed WHSV of 0.5 g gcat.-1 h-1. Palm oil hydrolysis was carried out in a Teflon-lined stainless steel autoclave with a nominal volume of 150 cm3. Typically, 20 g of palm oil and 80 g of H2O were loaded into the reactor, and hydrolysis was carried out under magnetic stirring (900 rpm) at 473 K for specified time intervals. After the reaction, the mixture was cooled to room temperature, and the wax-like products (i.e., fatty acids) were collected using a separatory funnel after gentle heating at 353 K. Hydrogenative hydrolysis of palm oil was carried out in the presence of 5 wt% Pt/C as a hydrogenation catalyst under H2 atmosphere. For this experiment, 20 g of palm oil, 80 g of H2O, and 55 mg of 5 wt% Pt/C were loaded into the autoclave. Then, 15 bar H2 was loaded into the headspace of the autoclave. The reaction was carried out at 473 K under stirring (900 rpm) for specified time intervals and the fatty acid products were separated similarly. The fatty acids dissolved


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in chloroform were analyzed using an offline gas chromatography (GC) equipped with a HP-5 capillary column. The residual triglycerides were also quantified by HPLC (Agilent Infinity 1260-ELSD equipped with evaporative light scattering detector and Chromolith Performance-Si 100-4.6 column). The fatty acid mixtures produced from palm oil were used as a reactant for the ketonization. The ketonization was carried out in a manner similar to that mentioned above. The fatty ketones produced by the hydrogenative hydrolysis of palm oil and subsequent ketonization were converted to oxygen-free hydrocarbons via hydrodeoxygenation. The reaction was carried out in a down-flow plug-flow reactor (stainless steel, 10.9 mm inner diameter). Prior to the reaction, a mixture of 1.2 g of sieved 1 wt% Pt/γ-Al2O3 catalyst (150–200 µm) and 3 g of quartz diluents were loaded into the reactor and pretreated at 673 K for 2 h under H2. The reaction was carried out at 573 K under 20 bar H2. The liquid injection rate of fatty ketones was 2.4 g h-1 (WHSV: 2 g gcat.-1 h-1), and the H2 flow rate was 68 cm3 min-1. The wax products were collected using a liquid trap heated at 373 K and analyzed using an offline GC with HP-5 column.

RESULTS AND DISCUSSION Ketonization of model fatty acids. In the present study, TiO2 (P25) was used as a catalyst for ketonization. TiO2 has been widely investigated as a representative high-lattice-energy metal oxide catalyst.22,29-30 XRD analysis revealed that it contains both anatase and rutile phases (Figure 1a). An earlier quantitative analysis indicated that P25 TiO2 is composed of more than 70% anatase with a minor amount of rutile.47 The TiO2 catalyst was composed of 20–30 nm crystallites (Figure 1b) and exhibited a BET surface area of 58 m2 g-1. The NH3 and CO2 TPD profiles (Figure 1c) revealed that the TiO2 surface exhibits both acidity and basicity, indicating its amphoteric nature. Quantification of the acid and base sites based on the NH3 and CO2 TPD profiles, respectively, revealed that the TiO2 has 484 µmol g-1 acid sites (8.3 µmol m-2) and 123 µmol g-1 base sites (2.1 µmol m-2). The pyridine IR spectrum of TiO2 (Figure 1d) showed four major bands at 1607, 1577, 1494, and 1445 cm-1, which could be assigned to the pyridine coordinated to the Lewis acid centers on the TiO2 surface.48 No band for protonated pyridine (1640–1630 and 1540–1500 cm-1) was observed, indicating the absence of strong Brønsted acid sites on the TiO2. C18 fatty acids with different degrees of unsaturation, namely, stearic acid (C18:0), oleic acid (C18:1), and linoleic acid (C18:2), were used as a reactant for ketonization (in the notation “C18:n,” 18 indicates the carbon chain length and n indicates the number of C=C bonds) at 653 K. In Figure 2, the fatty acid conversion and product selectivities measured after 12 h reaction were plotted as a function of space time (1/WHSV). All the fatty acids showed more or less similar conversion behaviors as a function of space


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time (Figures 2a-c), indicating that the different unsaturated degrees in fatty acids do not significantly affect the ketonization activity. For all the fatty acids, acid anhydrides (C36) were the dominant products at very low conversion levels (< 20%) (Figures 2d-f). They are likely to have been produced by the dehydrative condensation of two fatty acids. As the fatty acid conversion increased, fatty ketones were detected as a major product, and the selectivity toward anhydride rapidly decreased. The result indicates that the anhydride is not a dead-end product and can also participate in ketonization reaction. As the conversion of fatty acids increased above ~60%, the selectivity toward fatty ketones slightly decreased. This is because other byproducts such as C16 olefins, C19 methyl ketones, and various cracking products were formed. The detailed product distributions at complete fatty acid conversions (WHSV: 0.5 g gcat.-1 h1

) are summarized in Table 1. Because more C16 olefins and C19 methyl ketones were detected as the fatty acid conversion

increased, they are likely to be the secondary decomposition products of fatty ketones. Several research groups also detected similar byproducts,49,50 the production of which was attributed to the McLafferty rearrangement (Scheme 3a). In McLafferty rearrangement, the molecules containing keto group undergoes β-cleavage with a gain of γ-hydrogen. The reaction produces enol and 1-alkene, wherein the enol can be further tautomerized to methyl ketone. We detected substantial formation of 1-hexadecene (C16 olefin) and 2-nonadecanone (C19 methyl ketone) as a byproduct during the ketonization of stearic acid (C18:0) (Table 1), which are exactly the products expected from this reaction scheme. We could rule out the possibility of olefin formation via direct fatty acid decarbonylation, because it should produce C17 olefins from C18 fatty acids. C17 olefins have not been detected in our reaction products (Table 1). Separate ketonization experiments with saturated carboxylic acids of different lengths (Figure 3) demonstrated that such McLafferty rearrangement became more pronounced as the carbon length of acids increased. Pentanoic acid (C5) was the shortest carboxylic acid that exhibited the McLafferty rearrangement, producing 2-hexanone and propylene. Ketonization with butanoic acid (C4) did not produce McLafferty rearrangement products (i.e., 2-pentanone and ethylene) during ketonization, indicating that the presence of electron-donating alkyl group in the γ-position is important for facilitating this rearrangement. In addition, we carried out McLafferty rearrangement with and without a TiO2 catalyst, using a pre-synthesized C11 symmetric ketone (6-undecanone, 97%, Aldrich) as a model reactant (Figure S1). The results showed that the conversion of 6-undecanone via McLafferty rearrangement was inappreciable in the absence of a catalyst, but became significant in the presence of a TiO2 catalyst. The present result clearly showed that McLafferty rearrangement is also catalyzed by a TiO2 catalyst (not a simple thermal decomposition).


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In the ketonization of oleic acid (C18:1), we mainly observed the formation of a conjugated 1,3diene (i.e., 1,3-hexadecadiene) rather than that of 1-hexadecene (Table 1). Because the C=C double bond of oleic acid was initially located at the middle of the fatty acid backbone (ninth carbon), the dominant formation of 1,3-diene implied that the C=C bond migrated in the course of McLafferty rearrangement to form a conjugated 1,3-diene (Scheme 3b). Because we used amphoteric TiO2 catalyst, it is likely that the acid sites on the TiO2 surface catalyzed the C=C bond migration. In the ketonization of linoleic acid (C18:2), 1,3,5-triene was expected as a major product of McLafferty rearrangement but it was not detected (Table 1). Rather, 1,3-diene was still observed as a major byproduct. This may be attributed to the instability of triene at such harsh reaction conditions, which can be readily converted to coke and diene via hydrogen transfer. As shown in Figure 2, a higher degree of unsaturation of the original fatty acids (C18:0 < C18:1 < C18:2) facilitated the formation of terminal olefins and methyl ketones; thus, the selectivity toward the desired fatty ketones decreased. The result indicated that the presence of unsaturated C=C bonds in fatty ketones accelerated their decomposition via the McLafferty rearrangement. In addition to the olefins and methyl ketones produced by McLafferty rearrangement, minor amounts of cracking products were also detected (Figure 2 and Table 1). It is likely that acid sites on TiO2 surfaces catalyzed the cracking reactions. These cracking products include short-chain hydrocarbons (≤ C15) and ketones (C20–C30, other than methyl ketones). These products were clearly distinguishable from the products from McLafferty rearrangement; they have a wide-range of carbon length distributions (do not have specific carbon lengths). The formation of cracking products was also more pronounced as the degree of unsaturation in fatty acids increased (C18:0 < C18:1 < C18:2). This seems natural because a general acid-catalyzed cracking mechanism involves the β-scission of carbocations formed by the acidification of unsaturated C=C bonds.51 Similar to the case of the McLafferty rearrangement, the cracking products formed during the ketonization of saturated stearic acid (C18:0) mainly consisted of mono-alkenes. On the other hand, the cracking products obtained during the ketonization of unsaturated oleic (C18:1) and linoleic acid (C18:2) mainly contained polyunsaturated olefins (e.g., dienes) (Table 1). In Figure 4, the time-on-stream reaction results are shown for the ketonization of stearic acid (C18:0), oleic acid (C18:1), and linoleic acid (C18:2) at a fixed space velocity (WHSV: 0.5 g gcat.-1 h-1). Under the present reaction conditions, the initial conversions of all the fatty acids were nearly complete (> 99%). The initial selectivity toward fatty ketones (C35) decreased in the order C18:0 (89%) > C18:1 (85%) > C18:2 (80%). As explained above, this trend can be explained by the fact that an increased unsaturation in fatty acids can facilitate the side reactions such as the McLafferty rearrangement and cracking. With timeon-stream, the completely saturated stearic acid (C18:0) exhibited negligible deactivation, and the product


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distributions did not change appreciably (Figure 4a). In contrast, oleic acid (C18:1) and linoleic acid (C18:2) exhibited substantial catalyst deactivation, and the fatty ketone yields decreased with time-onstream. After a 40 h reaction, the coke contents in the used catalysts were analyzed using thermogravimetric analysis in air. The result revealed that the coke amounts increased in the order C18:0 (0.33 wt%) < C18:1 (0.56 wt%) < C18:2 (1.25 wt%), which is consistent with the observed catalyst deactivation behavior. This can be attributed to the fact that polyunsaturated olefins (e.g., dienes and trienes) produced by the McLafferty rearrangement/cracking of C18:1- and C18:2-derived fatty ketones can be readily converted to coke species. During the ketonization of C18:1 and C18:2, the yields of methyl ketones and terminal olefins gradually increased with time-on-stream, although the yield of fatty ketones decreased upon catalyst deactivation (Figures 4b and c). Because they are the secondary decomposition products of fatty ketones, their productions were expected to be suppressed upon the deactivation of a TiO2 catalyst. The unexpected trend may imply that the deposited coke can provide additional active sites for McLafferty rearrangement, which might accelerate the decomposition of fatty ketones. We analyzed the remaining acidity/basicity of the TiO2 catalyst after the ketonization of three C18 fatty acids for 40 h. The NH3 and CO2 TPD profiles of the used TiO2 catalysts are shown in Figure 5. The results revealed that the NH3 TPD profile of the TiO2 significantly changed after the ketonization reactions (Figure 5a), while the CO2 TPD profile did not change appreciably (Figure 5b). After the ketonization of more unsaturated fatty acids, the TiO2 exhibited more significant decreases in the NH3 desorption peak area. Particularly, the NH3 desorption peak at a high temperature range (~723 K) decreased significantly, whereas the peak intensity at low temperature (~523 K) slightly increased. The result indicated that the coke species were mainly deposited on the strong acid sites of the TiO2. These results demonstrate that the deactivation of an amphoteric TiO2 catalyst is mainly owing to the loss of acid sites rather than base sites. Ketonization of fatty acids obtained by palm oil hydrolysis. The ketonization of the three model fatty acids (C18:0, C18:1, and C18:2) revealed that the ketonization of unsaturated fatty acids can cause formation of a substantial amount of byproducts and rapid catalyst deactivation owing to coke formation. Considering that natural triglycerides generally contain significant amounts of unsaturated fatty acids,5 it is likely that the fatty acid mixtures produced by the direct hydrolysis of natural triglycerides would exhibit a significant loss of fatty ketone yield and rapid catalyst deactivation during ketonization. To verify this, we investigated the ketonization of fatty acid mixtures obtained by the hydrolysis of palm oil. The result of palm oil hydrolysis without any catalyst at 473 K were shown in Figure 6a. HPLC analysis indicated that almost complete hydrolysis of palm oil (remaining triglyceride content < 1 wt%)


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could be achieved within 16 h at 473 K. The separated fatty acid mixture was mainly composed of palmitic acid (C16:0, 44%), oleic acid (C18:1, 42%), and linoleic acid (C18:2, 8.4%), which indicated the significant presence of unsaturated fatty acids. The fatty acid mixture was ketonized over a TiO2 catalyst and the time-on-stream data are shown in Figure 6b. The reaction result clearly showed the formation of a substantial amount of byproducts (> 20%) owing to McLafferty rearrangement/cracking and rapid catalyst deactivation owing to coke formation. Ketonization of fatty acids obtained by palm oil “hydrogenative” hydrolysis. The aforementioned results verified that unsaturated fatty acids are not suitable for selective and stable ketonization. In this respect, the pre-saturation (i.e., C=C bond hydrogenation) of fatty acids was expected to be highly efficient for increasing the fatty ketone yield and lifetime of a ketonization catalyst. To examine this assumption, we carried out “hydrogenative” hydrolysis of palm oil under H2 in the presence of 5 wt% Pt/C as a catalyst. As shown in Figure 7a, the pre-saturation of the fatty acids is rapid; even after 2 h, all the produced fatty acids were completely saturated. The hydrolysis rate was also enhanced compared with conventional hydrolysis without a catalyst (Figure 6a). After hydrolysis for 16 h, the resultant fatty acid mixture was composed of completely saturated acids, mainly palmitic acid (C16:0, 45%) and stearic acid (C18:0, 52%). HPLC analysis also confirmed that 16 h-reaction could fully hydrolyze palm oil (remaining triglyceride content < 1 wt%). As shown in Figure 7b, the ketonization of this fatty acid mixture exhibited a significantly improved fatty ketone yield (91%) and inappreciable catalyst deactivation. The present results clearly demonstrated that pre-saturation of fatty acids is crucial for selective and steady ketonization. Hydrodeoxygenation of fatty ketones. The fatty ketones produced from the hydrogenative hydrolysis of palm oil and subsequent ketonization (the products obtained in Figure 7b) could be converted to oxygen-free hydrocarbons via hydrodeoxygenation over 1 wt% Pt/γ-Al2O3 as a catalyst. After the hydrodeoxygenation (Figure 8), the collected liquid products were mainly composed of nparaffins having C31, C33, and C35 chain lengths (> 83%). These hydrocarbons are likely to be produced by the hydrodeoxygenation of fatty ketones formed by the coupling of C16/C16, C16/C18, and C18/C18 acid pairs, respectively. The formation of small amounts of C14–C19 hydrocarbons could be attributed to the hydrodeoxygenation of methyl ketones and olefins produced by the McLafferty rearrangement during ketonization. The very minor amounts of C20–C30 hydrocarbons with a broad size distribution could be assigned to the cracking products formed during the ketonization. The present results supported that the ketonization products (i.e., mainly fatty ketones) can be readily converted to oxygen-free hydrocarbons via hydrodeoxygenation without an appreciable change in the carbon-length distributions. It is reasonably


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expected that the obtained long-chain hydrocarbons can be further hydroisomerized/cracked to produce branched hydrocarbons that are suitable as lube base oil.

CONCLUSIONS We investigated the effects of different fatty acid structures on the selectivity and catalyst deactivation during ketonization. C18 fatty acids with different degrees of unsaturation (i.e., stearic, oleic, and linoleic acid) were used as a model reactant for ketonization over an amphoteric TiO2 catalyst. The results demonstrated that the yield of the desired fatty ketones gradually decreased with increasing unsaturation (i.e., the presence of C=C bond) of fatty acids owing to the formation of methyl ketones/olefins via McLafferty rearrangement and various cracking products. In the presence of the unsaturated C=C bonds in the fatty acid chains, the McLafferty rearrangement and cracking reactions produced conjugated polyunsaturated olefins (e.g., dienes), which can be readily converted to coke species causing rapid catalyst deactivation. Based on these results, we proposed that the ketonization of natural fatty acid mixtures requires pre-saturation of C=C bonds for increasing the fatty ketone yields and inhibiting the catalyst deactivation. We demonstrated that the ketonization of the fatty acids directly obtained by the hydrolysis of palm oil exhibited a substantial loss in the fatty ketone yield and a rapid deactivation, owing to the significant presence of unsaturated fatty acids (oleic and linoleic acids). In contrast, the ketonization of the saturated fatty acids obtained by the “hydrogenative” hydrolysis of palm oil exhibited a substantially higher fatty ketone yield (~90%) and negligible catalyst deactivation.

ACKNOWLEDGMENTS This research was supported by the Advanced Biomass R&D Center (ABC) of Global Frontier Project funded by the Ministry of Science, ICT, and Future Planning (ABC-2015M3A6A2066121), and Basic Science

Research

Program

through

the

National

Research

Foundation

of

Korea

(NRF-

2017R1A2B22002346).

Supporting Information Results of McLafferty rearrangement using 6-undecanone as a reactant


11


Page 12 of 37

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Page 16 of 37

Figures & Tables

Scheme 1. Catalytic Processes for Transformation of Fatty Acid into Lube Base Oil

Scheme 2. Proposed Reaction Mechanism for Ketonization of Carboxylic Acids28


16

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Scheme 3. Decomposition of (a) Saturated and (b) Unsaturated Fatty Ketones into Alkenes and Methyl Ketones via McLafferty Rearrangement

Figure 1. (a) XRD pattern, (b) TEM image, (c) NH3/CO2-TPD profiles, and (d) pyridine IR spectrum of TiO2 ketonization catalyst.


17


Page 18 of 37

Figure 2. Fatty acid conversion as a function of space time (1/WHSV) in (a) C18:0, (b) C18:1, and (c) C18:2 ketonization over TiO2 at 653 K. Product selectivities as a function of conversion in the ketonization of (d) C18:0, (e) C18:1, and (f) C18:2.

Figure 3. Product selectivities in the ketonization of saturated carboxylic acids having varied chain lengths (reaction conditions: 653 K, WHSV: 0.5 g gcat.-1 h-1).


18

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Figure 4. Product yields as a function of time-on-stream (stacked area graph) in the ketonization of (a) stearic acid (C18:0), (b) oleic acid (C18:1), and (c) linoleic acid (C18:2) (reaction conditions: 653 K, WHSV: 0.5 g gcat.-1 h-1).


19


Page 20 of 37

Figure 5. (a) NH3-TPD and (b) CO2-TPD profiles of the fresh and used TiO2 catalysts (i.e., after ketonization of C18:0, C18:1, and C18:2 fatty acids for 40 h). The values in parentheses indicate the quantified amounts (µmol g-1) of acid (a) and base sites (b).


20

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Figure 6. (a) Conversion (line) and fatty acid distributions (bar) obtained during hydrolysis of palm oil at 473 K without catalyst. (b) Product yields as a function of time-on-stream (stacked area graph) in the ketonization of fatty acid mixture obtained by palm oil hydrolysis (the products obtained in Figure 6a).


21


Page 22 of 37

Figure 7. (a) Conversion (line) and fatty acid distributions (bar) obtained during “hydrogenative” hydrolysis of palm oil at 473 K under 15 bar H2 in the presence of 5 wt% Pt/C catalyst. (b) Products yields as a function of time-on-stream (stacked area graph) in the ketonization of a fatty acid mixture obtained by hydrogenative hydrolysis of palm oil (products obtained in Figure 7a).


22

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Figure 8. Hydrocarbon distributions obtained after complete hydrodeoxygenation of fatty ketones over 1 wt% Pt/γ-Al2O3 (reaction conditions: 573 K, 20 bar H2, WHSV: 2 g gcat.-1 h-1). The reactant fatty ketones were obtained by the hydrogenative hydrolysis of palm oil and subsequent ketonization (product obtained in Figure 7b).


23


Table 1. Product Distributions after Ketonization of C18 Fatty Acids

McLafferty rearrangement

Ketonization

Reaction

Cracking

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

Page 24 of 37

Feed Conversion (%)a Productsa

C18:0 100

C18:1 C18:2 98.8 97.5 Yield (C%)a

C35 ketone

88.9

n.a.

n.a.

C35= ketone

n.a.

83.4

75.0

Hexadecene

4.6 (4.4)b

n.a.

n.a.

Hexadecadiene

n.a.

5.2 [4.7]c

8.0 [7.7]c

C19 methyl ketone

4.0

n.a.

n.a.

C19= methyl ketone

n.a.

4.1

6.7

≤ C15 paraffin

0.1

0.4

0.4

≤ C15 monoene

1.0

0.3

0.4

≤ C15 diene

0.1

3.5

4.5

C20−C30

1.3

1.7

2.5

Fatty ketones

C16 olefins

Methyl ketones

Hydrocarbons

Ketones a

Liquid products collected after12 h ketonization were analyzed using GC-MS.

b

(): % of 1-hexadecene.

c

[]: % of 1,3-hexadecadiene.


24

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Table of Contents Graphic

Synopsis Catalytic selectivity and deactivation during ketonization of fatty acids of different structures were rigorously investigated for producing high-value chemicals.


25


Scheme 1. Catalytic Processes for Transformation of Fatty Acid into Lube Base Oil 80x80mm (300 x 300 DPI)


Page 26 of 37

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Scheme 2. Proposed Reaction Mechanism for Ketonization of Carboxylic Acids28 85x80mm (300 x 300 DPI)



Scheme 3. Decomposition of (a) Saturated and (b) Unsaturated Fatty Ketones into Alkenes and Methyl Ketones via McLafferty Rearrangement 85x78mm (300 x 300 DPI)


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Figure 1. (a) XRD pattern, (b) TEM image, (c) NH3/CO2-TPD profiles, and (d) pyridine IR spectrum of TiO2 ketonization catalyst. 85x81mm (300 x 300 DPI)



Figure 2. Fatty acid conversion as a function of space time (1/WHSV) in (a) C18:0, (b) C18:1, and (c) C18:2 ketonization over TiO2 at 653 K. Product selectivities as a function of conversion in the ketonization of (d) C18:0, (e) C18:1, and (f) C18:2. 170x91mm (300 x 300 DPI)


Page 30 of 37

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Figure 3. Product selectivities in the ketonization of saturated carboxylic acids having varied chain lengths (reaction conditions: 653 K, WHSV: 0.5 g gcat.-1 h-1). 80x66mm (300 x 300 DPI)



Figure 4. Product yields as a function of time-on-stream (stacked area graph) in the ketonization of (a) stearic acid (C18:0), (b) oleic acid (C18:1), and (c) linoleic acid (C18:2) (reaction conditions: 653 K, WHSV: 0.5 g gcat.-1 h-1). 85x164mm (300 x 300 DPI)


Page 32 of 37

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Figure 5. (a) NH3-TPD and (b) CO2-TPD profiles of the fresh and used TiO2 catalysts (i.e., after ketonization of C18:0, C18:1, and C18:2 fatty acids for 40 h). The values in parentheses indicate the quantified amounts (µmol g-1) of acid (a) and base sites (b). 80x130mm (300 x 300 DPI)



Figure 6. (a) Conversion (line) and fatty acid distributions (bar) obtained during hydrolysis of palm oil at 473 K without catalyst. (b) Product yields as a function of time-on-stream (stacked area graph) in the ketonization of fatty acid mixture obtained by palm oil hydrolysis (the products obtained in Figure 6a). 85x130mm (300 x 300 DPI)


Page 34 of 37

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Figure 7. (a) Conversion (line) and fatty acid distributions (bar) obtained during “hydrogenative” hydrolysis of palm oil at 473 K under 15 bar H2 in the presence of 5 wt% Pt/C catalyst. (b) Products yields as a function of time-on-stream (stacked area graph) in the ketonization of a fatty acid mixture obtained by hydrogenative hydrolysis of palm oil (products obtained in Figure 7a). 85x130mm (300 x 300 DPI)



Figure 8. Hydrocarbon distributions obtained after complete hydrodeoxygenation of fatty ketones over 1 wt% Pt/γ-Al2O3 (reaction conditions: 573 K, 20 bar H2, WHSV: 2 g gcat.-1 h-1). The reactant fatty ketones were obtained by the hydrogenative hydrolysis of palm oil and subsequent ketonization (product obtained in Figure 7b). 80x65mm (300 x 300 DPI)


Page 36 of 37

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83x31mm (300 x 300 DPI)


Effects of Fatty Acid Structures on Ketonization Selectivity and Catalyst

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