Effects of Fatty Acid Compositions on Heavy Oligomer Formation and

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Effects of Fatty Acid Compositions on Heavy Oligomer Formation and Catalyst Deactivation during Deoxygenation of Triglycerides Tae-Hyoung Kim, Kyungho Lee, Myoung Yeob Kim, Yong Keun Chang, and Minkee Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04552 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Sustainable Chemistry & Engineering

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Effects of Fatty Acid Compositions on Heavy Oligomer Formation and Catalyst

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Deactivation during Deoxygenation of Triglycerides

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Tae-Hyoung Kim,† Kyungho Lee,† Myoung Yeob Kim,†,§ Yong Keun Chang,*,†,‡ and Minkee Choi*,†

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†

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34141, Republic of Korea

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‡ Advanced

Department of Chemical & Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon

Biomass R&D Center, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

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*Corresponding authors

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Yong Keun Chang

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e-mail address: [email protected], TEL: +82-42-350-3927, FAX: +82-42-350-3910

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Minkee Choi

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e-mail address: [email protected], TEL: +82-42-350-3938, FAX: +82-42-350-3910

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ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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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ABSTRACT: Deoxygenation of triglycerides (e.g., vegetable and microalgae oils) through hydro-

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upgrading is one of the most important routes to produce oxygen-free hydrocarbon fuels from biomass.

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Even though natural triglycerides are composed of fatty acids of different structures, their effects on

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catalytic selectivity and deactivation during deoxygenation have not been comprehensively investigated. In

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this study, we used three different reactant triglycerides including palm oil, soybean oil, and linseed oil for

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catalytic deoxygenation over Pt/γ-Al2O3. These oils consist of fatty acids with increasing degrees of

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unsaturation in the order of palm oil (number of C=C per fatty acid: 0.55) < soybean oil (1.33) < linseed oil

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(2.18). The catalytic results showed that the increased unsaturation of the triglycerides caused enhanced

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formation of heavy products by oligomerization (mostly dimers) of unsaturated fatty acid derivatives

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through Diels-Alder reaction and/or radical addition. These heavy products were readily transformed to

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coke species, causing catalyst deactivation. In this regard, we demonstrated that the pre-hydrogenation of

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triglycerides at a mild temperature is highly beneficial for steadily obtaining a high yield of diesel-range

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paraffins and retarding catalyst deactivation in deoxygenation.

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KEYWORDS: Triglyceride, Deoxygenation, Diesel, Unsaturation, Deactivation, Coking

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INTRODUCTION

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Extensive efforts have been made to develop processes for converting sustainable biomass resources to

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fuels and high-value chemicals.1-4 Among biomass feedstocks, triglycerides (e.g., vegetable oil and

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microalgae oil) have been widely investigated for producing bio-fuels,3-5 olefins,6,7 and lube base oil.8,9

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Triglycerides are esters built with three fatty acids (typically C14–C22) and one glycerol. An important

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advantage of using triglycerides over other biomass feedstocks is their highly paraffinic structures with low

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oxygen contents, which can be readily upgraded to high-energy-density fuels via various catalytic

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conversions. Such catalytic processes include the production of biodiesel (e.g., fatty acid methyl esters) by

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transesterification,5,10-12 and green diesel12-19 and bio-jet fuel19-24 production through hydro-upgrading.

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Biodiesel production by transesterification of triglycerides with methanol or ethanol is currently the

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major process commercially used. However, oxygen-containing biodiesel is not fully compatible with

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conventional diesel engines and thus should be blended with petro-diesel or the engines have to be

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modified.25 Furthermore, biodiesel has poor oxidative stability and cold-flow properties.26 In contrast,

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deoxygenation of triglycerides by hydro-upgrading can produce oxygen-free hydrocarbons that can

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overcome the intrinsic problems of biodiesel.13,15-17,27 The resultant hydrocarbons are fully compatible with

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conventional internal combustion engines. Catalytic deoxygenation of triglycerides produces n-paraffins

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that can be directly used as a diesel (often called “green diesel” to differentiate from oxygen-containing

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“biodiesel”). These hydrocarbons even outperform petroleum-derived fuels in terms of cetane numbers

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(85–99, compared with 45–55 for petro-diesel).18,26 Alternatively, the resultant n-paraffins can be

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additionally hydrocracked into smaller branched hydrocarbons (typically C8–C16) that can be used as an

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aviation fuel (“bio-jet fuel”).19-24

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Deoxygenation of triglycerides can generally be carried out over supported metal catalysts such as

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Pd,13,14,21,27-29 Pt,13-15,21,22,29 and Ni13,14,29-33 or MoS2-based catalysts (e.g., NiMoS2 and CoMoS2)14-16,21,27,32 3



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under a hydrogen atmosphere. At the initial stage of the reaction, triglycerides containing unsaturated fatty

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acid units are generally hydrogenated and decomposed into three saturated fatty acids by hydrogenolysis

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(Scheme 1). The produced fatty acids are then deoxygenated by hydrodeoxygenation (HDO) and/or

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decarboxylation/decarbonylation (DCO).34,35 The HDO pathway produces n-paraffins containing the same

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carbon number with those of the original fatty acids because the oxygens are removed as H2O. On the other

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hand, DCO produces n-paraffins containing one less carbon atom because the oxygens in the fatty acid

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units are removed as COx. Minor amounts of oxygenates such as fatty acid, fatty alcohol, and fatty acid

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ester are known to be formed by incomplete deoxygenation.27,32,36,37 Along with the production of bulky

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hydrocarbons, light hydrocarbons (C1–C3) are also co-produced due to the hydrogenolysis of the glycerol

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unit and methanation reaction.

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In previous studies, most research efforts have been paid for understanding the effects of the catalyst

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compositions and reaction conditions to achieve increased deoxygenation activities. In those studies, model

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reactants (e.g., unnatural triglycerides,27,37,38 fatty acids,27,35-40 and fatty acid methyl esters38,41-44) containing

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only saturated fatty acid units (e.g., stearic,35,37-40,44 palmitic,36,37,39 and lauric acids36,37) have often been

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investigated. One possible reason is to simplify the analysis of products and/or to reduce the experimental

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difficulty of injecting viscous reactants into a reactor system. However, depending on their origin, natural

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triglycerides include various fatty acid units with different numbers of C=C bonds (or different degree of

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unsaturation). The presence of C=C bonds in the fatty acid units can lead to various side reactions and cause

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rapid catalyst deactivation. In a batch reactor, Murzin and colleagues showed that increased degree of

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unsaturation of feedstock resulted in the substantial formation of oligomers/aromatics and fast catalyst

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deactivation.45,46 Thus far, however, rather limited researches have been carried out to investigate the effects

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of triglyceride structures on product selectivity and catalytic deactivation during deoxygenation in a

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continuous fixed-bed reactor, which is industrially more relevant.

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In the present work, we studied the effects of different fatty acid compositions of triglycerides on

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product selectivity and catalyst deactivation during deoxygenation in a continuous fixed-bed reactor.

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Vegetable oils with various unsaturation degrees (palm oil, soybean oil, and linseed oil) were deoxygenated

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over a Pt/γ-Al2O3 catalyst. This study shows that increased unsaturation in triglycerides can substantially

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reduce the yield of the target n-paraffins by producing heavy byproducts such as oligomers of fatty acid

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derivatives and also cause fast catalyst deactivation due to coke formation. In this regard, we propose that

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pre-saturation of triglycerides at a mild reaction temperature is highly beneficial for the efficient and steady

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deoxygenation of triglycerides into diesel-range hydrocarbons.

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EXPERIMENTAL SECTION

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Catalyst Preparation. As a catalyst for hydro-upgrading, 1 wt% Pt/γ-Al2O3 was prepared by wet

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impregnation of an aqueous solution of Pt(NH3)4(NO3)2 onto γ-Al2O3.22 After the impregnation, the sample

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was dried at 373 K for 12 h, followed by calcination at 673 K for 3 h in dry air. The sample was subsequently

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reduced under a H2 flow at 673 K for 3 h.

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Characterization. Elemental analysis of the catalyst was performed using inductively coupled plasma

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atomic emission spectroscopy (ICP-AES) with OPTIMA 7300 DV (Perkin-Elmer). N2 adsorption-

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desorption isotherm was collected using BELSorp-max (BEL Japan) at 77 K after vacuum-degassing the

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catalyst at 473 K for 12 h. The Brunauer-Emmett-Teller (BET) area was calculated in the P/P0 range of

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0.10–0.30. H2 chemisorption was measured at 323 K using ASAP 2020 (Micromeritics). Before measuring

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the isotherm, the catalyst was vacuum-degassed at 573 K for 6 h, reduced by H2 at 673 K for 2 h, and then

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evacuated at 673 K for 2 h. The chemisorption amount was determined by extrapolating the linear regime

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(10–30 kPa) of the isotherm to zero pressure. The diameter of Pt cluster was estimated from the metal

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dispersion (H/Pt) using the equation:47 5



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dchem (nm) = 1.13/(H/Pt)

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(1)

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High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were

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collected using a FEI Talos F200X STEM at 200 kV, after mounting the catalyst on a Cu grid. More than

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200 crystallites were counted to analyze the particle size distribution. The surface-area-weighted mean

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cluster diameter was calculated using the equation:47

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dSTEM (nm) = Σnidi3/ Σnidi2

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The amounts of Brønsted and Lewis acids were quantified with Fourier transform infrared spectroscopy

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(FT-IR) after adsorption of pyridine as a base probe molecule. The FT-IR spectrum was measured with a

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Thermo Nicolet NEXUS spectrometer. Typically, 15 mg of the sample was pressed into a self-supporting

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wafer and then placed within an in situ IR cell having CaF2 windows. After vacuum-degassing at 723 K for

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4 h, background spectrum was collected. Pyridine adsorption was performed at 423 K for 1 h using a room-

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temperature pyridine vapor source, followed by evacuation at the same temperature for 2 h to remove

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weakly physisorbed pyridine. The IR spectrum was collected with a resolution of 4.0 cm-1. The amounts of

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Brønsted and Lewis acids were analyzed using IR bands at 1545 cm-1 (molar extinction coefficient: 1.67

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cm μmol-1) and 1445 cm-1 (2.22 cm μmol-1), respectively.48 The amount of coke deposited in the used

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catalyst was quantified by thermogravimetric analysis (TGA) under an air flow using Setaram TGA 92

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(ramp: 5 K min-1).

(2)

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Analysis of the Fatty Acid Compositions in the Triglycerides. In this study, palm oil (Sigma-

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Aldrich), soybean oil (Sigma), and linseed oil (Aldrich) were used as reactant triglycerides for catalytic

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deoxygenation. To analyze the fatty acid compositions of these triglycerides, transesterification with

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methanol was carried out to obtain fatty acid methyl esters, which were analyzed using gas chromatography

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(GC).11 Typically, 1 g of the triglycerides was reacted with 4.5 g of methanol (methanol/triglyceride ratio

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~6) for 1 h at 338 K using 1 wt% NaOH as a catalyst. The upper phase of the product and methyl 6


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heptadecanoate (Sigma-Aldrich, 99%) as an internal standard were dissolved in chloroform, and then

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analyzed by GC (Agilent 7890) equipped with a flame ionization detector and HP-5 column (30 m  0.32

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mm  0.5 μm).

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Catalytic Deoxygenation of Triglycerides. Single-step catalytic deoxygenation (Scheme 2a) was

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carried out in a stainless steel down-flow fixed-bed reactor (10.9 mm inner diameter). Typically, 2.4 g of 1

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wt% Pt/γ-Al2O3 catalyst (150–300 μm) were pretreated at 653 K under a H2 flow for 3 h. Deoxygenation

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was carried out at 653 K by introducing the triglycerides at the rate of 2.4 g h-1 with a HPLC pump (WHSV

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= 1.0 h-1). All of the tubings and the triglyceride container were carefully wrapped with heating tape (353

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K) to inhibit the solidification of the triglycerides. The H2 flow rate was 46 mL min-1 and the total pressure

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was fixed at 20 bar using a back pressure regulator. The liquid products were collected using a liquid

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condenser preheated at 353 K and separated from condensed water by decanting. Then, the products and n-

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dodecane (Sigma-Aldrich, >99%) as an internal standard were dissolved in chloroform, and analyzed using

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GC. For the quantification of unconverted triglycerides, a high performance liquid chromatography (HPLC)

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system (Agilent 1260 Infinity) with an evaporative light scattering detector and a Chromolith®

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Performance-Si column (100 mm × 4.6 mm) was used.49 The elemental compositions of the liquid products

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were analyzed using an elemental analyzer (Thermo scientific FLASH 2000). The effective hydrogen-to-

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carbon ratio (H/Ceff) was estimated using the equation,

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H/Ceff = {n(H) - 2n(O) - 3n(N) - 2n(S)}/n(C)

(3)

where n(H), n(O), n(N), n(S), and n(C) indicate the molar amounts of H, O, N, S, and C, respectively.4,50

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The yield of heavy products (undetectable either by GC or HPLC) was calculated by subtracting the

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yields of the GC-detectable products and glycerides from the yield of the total liquid products. To analyze

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the molecular size distributions of the heavy products, gel permeation chromatography (GPC) was carried 7



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out. The liquid products were dissolved in tetrahydrofuran (Daejung, HPLC grade) and analyzed using

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Agilent 1260 Infinity equipped with two PLgel 5 μm mixed-C (300 mm  7.5 mm) columns and a refractive

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index (RI) detector. Heptadecane (Sigma-Aldrich, 99%), deoxygenated dimer acid, and palm oil were used

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as reference materials to assign the GPC peak positions. Deoxygenated dimer acid was prepared by catalytic

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deoxygenation of the commercially available dimer acid (Aldrich, average Mn ~570). Dimer acid dissolved

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in octane (30 wt%) was deoxygenated over a 1 wt% Pt/γ-Al2O3 catalyst in similar conditions mentioned

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above (liquid injection rate of 2.4 g h-1; 653 K; H2 flow rate of 46 mL min-1; 20 bar). From the collected

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liquid products, the octane solvent was removed using a rotary evaporator at 353 K. Elemental analysis

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confirmed that the oxygen content in the deoxygenated dimer acid was less than 0.7 wt% (dimer acid: 11.0

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wt%), which indicated almost complete deoxygenation.

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Two-step catalytic deoxygenation was carried out in two fixed-bed reactors connected sequentially

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(Scheme 2b). In both reactors, 2.4 g of 1 wt% Pt/γ-Al2O3 catalyst (150–300 μm) was loaded and pretreated

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at 653 K under a H2 flow for 3 h. In the first reactor, pre-hydrogenation of the triglycerides was carried out

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at a mild reaction temperature (473 K). In the second reactor, deoxygenation was carried out at 653 K as in

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the case of the aforementioned single-step deoxygenation. For the reaction, triglyceride was injected at a

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rate of 2.4 g h-1 using a HPLC pump (WHSV = 1.0 h-1). The H2 flow rate was 46 mL min-1 and the total

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pressure was fixed at 20 bar. Liquid products were collected and analyzed in the same way to the single-

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step deoxygenation.

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To estimate the deactivation rate of each reaction, the time-on-stream results were fitted with powerlaw equation:51 -rdeac = -da/dt = kdeacad

(4)

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where a is the relative rate (defined as the paraffin yield at t divided by the initial paraffin yield at t = 0),

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kdeac is the deactivation rate constant, and d is the order of deactivation (d = 1, 2 and 3).51 8


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RESULTS AND DISCUSSION

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Characterization of the Catalyst. As a catalyst for triglyceride deoxygenation, 1 wt% Pt was

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supported on a γ-Al2O3 support, which had a BET surface area of 200 m2 g-1 and a pore volume of 0.46 mL

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g-1. ICP-AES analysis revealed that the Pt/γ-Al2O3 catalyst contained 0.95 wt% Pt, which is very close to

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the nominal loading. The catalyst showed a H/Pt ratio of 1.0 in the H2 chemisorption. This corresponded to

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the spherical Pt clusters having 1.13 nm diameter on average (dchem).47 HAADF-STEM image (Figure 1a)

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showed the presence of highly dispersed Pt clusters on the γ-Al2O3 support. The surface-area-weighted

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mean cluster diameter (dSTEM) was determined as 1.23 nm. The diameters of the Pt clusters determined both

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by H2 chemisorption (dchem) and HAADF-STEM (dSTEM) were highly consistent. The pyridine IR spectrum

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of the Pt/γ-Al2O3 catalyst is shown in Figure 1b. The IR band at 1445 cm-1 could be assigned to the

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coordinately bonded pyridine on the Lewis acid sites (170 μmol g-1). The absence of the IR band at 1545

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cm-1 indicated the absence of Brønsted acid sites.

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Fatty Acid Compositions of the Triglycerides. Table 1 shows the fatty acid compositions of the palm

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oil, soybean oil, and linseed oil. The fatty acids are denoted by “Cα:β”, where α indicates the number of

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carbon atoms and β indicates the number of C=C bonds in the fatty acids. The analysis showed that all the

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triglycerides were mainly composed of C16 and C18 fatty acids. The average number of C=C bonds per fatty

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acid (or degree of unsaturation) increased in the order of palm oil (0.55) < soybean oil (1.33) < linseed oil

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(2.18).

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Single-Step Catalytic Deoxygenation. For the single-step deoxygenation, three different triglycerides

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(palm oil, soybean oil, and linseed oil) were hydrotreated over the Pt/γ-Al2O3 catalyst in a fixed bed reactor

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(Scheme 2a). The product yields as a function of time-on-stream are plotted in Figure 2. The liquid products

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from all the triglycerides were mostly composed of saturated normal C15 and C17 paraffins during the early 9



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reaction period (4 h, Table 2). Considering that C16 and C18 fatty acids were the dominant components of

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all triglycerides, the major formations of C15 and C17 n-paraffins with one less carbon implied that the

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oxygens in fatty acids were mostly removed by DCO (removal of oxygens by CO/CO2) rather than by HDO

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(removal of oxygens by H2O) (Scheme 1). This result is consistent with earlier studies showing that noble

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metal catalysts (Pt22,44 and Pd14,27,28,52) prefer DCO over HDO. The conversion of all the triglycerides gave

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approximately an 80 wt% yield of liquid products compared to the original mass of the triglycerides (Figure

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2). The yield was very similar to the theoretical paraffin yields (80.5 wt%, 81.6 wt%, and 82.2 wt% for the

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palm oil, soybean oil, and linseed oil, respectively), which were calculated assuming that the glycerol unit

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in the triglycerides is decomposed to light gaseous products (C1–C3) and the fatty acids units are mainly

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deoxygenated by DCO.

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With time-on-stream, deoxygenation of the three triglycerides showed markedly different deactivation

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behaviors (Figure 2). The yields of the paraffins gradually decreased with the reaction time, the degree of

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which increased in the order of palm oil < soybean oil < linseed oil. In the case of the palm oil deoxygenation,

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no significant change in the product distribution was observed up to 180 h (Figure 2a). The slightly

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increased oxygenate formation (Table 2), however, indicated a slight deactivation of the catalyst. The liquid

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products collected during the palm oil deoxygenation contained a very small oxygen content regardless of

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the reaction time (

Effects of Fatty Acid Compositions on Heavy Oligomer Formation and

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