Deoxygenation of Triglycerides by Catalytic Cracking with Enhanced

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Deoxygenation of Triglycerides by Catalytic Cracking with Enhanced Hydrogen Transfer Activity Iori Shimada, Shin Kato, Naoki Hirazawa, Yoshitaka Nakamura, Haruhisa Ohta, Kengo Suzuki, and Toru Takatsuka Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03514 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 14, 2016

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Industrial & Engineering Chemistry Research

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Deoxygenation of Triglycerides by Catalytic

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Cracking with Enhanced Hydrogen Transfer

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Activity

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Iori Shimada,*,† Shin Kato,† Naoki Hirazawa,‡ Yoshitaka Nakamura,‡ Haruhisa Ohta,§ Kengo

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Suzuki,§ and Toru Takatsuka⊥

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†

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386-8567, Japan

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‡

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386-8567, Japan

Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano

Graduate School of Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano

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§

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Kanagawa 230-0046, Japan

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⊥Technology

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Kanagawa 220-8765, Japan

Research & Development Department, euglena Co., Ltd., 75-1 Ono, Tsurumi-ku, Yokohama,

Development Unit, Chiyoda Corporation, 4-6-2 Minatomirai, Nishi-ku, Yokohama,

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ABSTRACT: The efficient use of plant oils as alternative fuels was investigated by studying

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triglyceride deoxygenation in catalytic cracking using a fluid catalytic cracking catalyst with

ACS Paragon Plus Environment

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enhanced hydrogen transfer activity. Unsaturated triglyceride deoxygenation was rapid and

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complete but small amounts of oxygenated products such as fatty acids, ketones, and aldehydes

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were produced from saturated triglycerides. Reaction product analysis showed that hydrogen

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transfer reactions between oxygenates and hydrocarbons produced by cracking fatty acid carbon

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chains caused hydrodeoxygenation even in the absence of hydrogen. Catalytic cracking of

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triglycerides with fatty acid carbon chains of various lengths showed that triglyceride

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deoxygenation is not affected by steric hindrance, and probably occurs on zeolite external

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surfaces, whereas secondary cracking of hydrocarbons occurs on the internal surfaces. We

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showed that catalytic cracking can be used for efficient conversion of triglycerides to

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hydrocarbons in the absence of hydrogen.

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1. INTRODUCTION

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The efficient production of alternative fuels from biomass resources is a key technique for

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achieving a sustainable society. Plant oils are attracting increasing attention as alternatives to

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fossil fuels because they can be extracted from various resources such as inedible crops, waste

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cooking oils, and microalgae oils. Plant oils consist mainly of triglycerides and can be upgraded

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to alternative automotive fuels by chemical conversion processes such as transesterification,

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hydrodeoxygenation, and catalytic cracking. The most widely investigated and widespread

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process is transesterification with methanol to yield fatty acid methyl esters (FAMEs), which can

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be blended with petroleum-based diesel fuels.1-5 However, the energy densities of FAMEs are

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lower than those of diesel fuels because of their higher oxygen contents. The high oxygen

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content also results in poor thermal and oxidation stabilities, and this limits the maximum FAME

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content that can be blended with diesel fuels. Deoxygenation of triglycerides and their


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conversion to hydrocarbons are important for improving their energy densities and stabilities.

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Catalytic hydroprocessing is usually used,6-9 in which hydrodeoxygenation of triglycerides

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produces oxygen-free hydrocarbons. The produced hydrocarbons are sulfur free and can be

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blended with petroleum-based fuels at any level. The main disadvantage of this process is the use

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of a pressurized hydrogen atmosphere, which incurs high operating costs. In addition, the main

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reaction products under a pressurized hydrogen atmosphere are saturated hydrocarbons, and they

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require further reforming for conversion to valuable unsaturated hydrocarbons that can be used

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as octane enhancers for gasoline and feedstock for bulk chemicals. A new process for converting

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triglycerides directly to unsaturated hydrocarbons in the absence of hydrogen is therefore needed.

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We investigated the conversion of triglycerides to hydrocarbons without a hydrogen

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atmosphere using a fluid catalytic cracking (FCC) process involving hydrogen transfer reactions.

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There are many reports in the literature of catalytic cracking of triglycerides and they have been

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summarized in review papers.10-14 The catalysts used include oxides, base, zeolites, mesoporous

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materials and their composites, as well as commercial FCC catalysts. Deoxygenation of

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triglycerides to give CO2, CO, and H2O occurs even in the absence of hydrogen. Among the

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deoxygenation reaction paths in the catalytic cracking of triglycerides, decarboxylation, yielding

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CO2, and decarbonylation, yielding CO, result in partial loss of the carbon resources contained in

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the triglyceride feedstock; however, hydrodeoxygenation, yielding H2O, can convert most of the

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carbon resources in the feedstock to hydrocarbons. The selectivity for deoxygenation reaction

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paths therefore significantly affects the conversion efficiency of triglycerides to hydrocarbons,

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but the factors determining deoxygenation selectivities have not been widely investigated.

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Hydrodeoxygenation, which is the most efficient deoxygenation path for triglyceride conversion,

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in catalytic cracking is considered to be limited because catalytic cracking is performed without a


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hydrogen atmosphere. We focused on the hydrogen atoms contained in the triglyceride feedstock

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itself. The amount of hydrogen atoms contained in a triglyceride molecule is enough for

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hydrodeoxygenation of the triglyceride ester bonds if they are used efficiently. We focused on

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hydrogen transfer reactions during catalytic cracking over zeolite catalysts to control the

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distribution of hydrogen atoms among the triglyceride derivatives. Hydrogen transfer reactions

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are bimolecular reactions between a hydrogen donor and a hydrogen acceptor, in which active

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hydrogen species are released from the hydrogen donor and received by the hydrogen acceptor.

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The most common example of a hydrogen transfer reaction in an FCC process is the reaction

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between naphthenes (hydrogen donors) and olefins (hydrogen acceptors), producing aromatics

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and paraffins. Because the conversion of olefins to paraffins results in production of gasoline

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with a lower octane value, FCC catalysts that suppress hydrogen transfer reactions have been

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designed recently. From a different perspective, however, hydrogen transfer reactions have an

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important role in determining the properties of produced hydrocarbons because they determine

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the distribution of hydrogen atoms among the produced hydrocarbons in reactions in hydrogen-

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free atmospheres. Enhancement of the hydrogen transfer activity during catalytic cracking of

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triglycerides is therefore expected to accelerate hydrodeoxygenation of triglycerides using only

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the hydrogens contained in the feedstock. Černý et al. compared the catalytic cracking of six

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model feedstocks consisting of saturated and unsaturated triglycerides, fatty acids, and fatty

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alcohols over USY zeolites.15 The yields of CO2 and CO from the unsaturated triglycerides and

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unsaturated fatty acids were lower than those from the corresponding saturated feedstocks. They

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attributed the different CO2 and CO yields to reduction of carboxyl groups by hydrogen transfer

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reactions because the hydrogen-donating abilities of unsaturated feedstocks are higher than those

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of saturated feedstocks. In contrast, CO2 and CO were not formed in the reactions of fatty


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alcohols because dehydration was the dominant reaction. These results indicate that the

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selectivity for deoxygenation products is affected by the hydrogen transfer activity in the

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reaction fields and the structures of the oxygenated groups. It is therefore important to clarify the

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triglyceride deoxygenation mechanism and the effects of hydrogen transfer reactions on the

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deoxygenation rates and reaction paths over an FCC catalyst to design efficient catalysts for

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triglyceride conversion.

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Although acceleration of hydrogen transfer reactions is the opposite strategy to that used

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recently in FCC catalyst design, we have shown the effectiveness of the acceleration of hydrogen

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transfer reactions in the catalytic cracking of polycyclic aromatic hydrocarbons using a residual

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FCC (RFCC) catalyst.16 The RFCC catalyst contained a rare-earth-exchanged USY zeolite; it has

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high hydrothermal stability in the regeneration cycle of an FCC process and therefore maintains

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high hydrogen transfer activity even in an equilibrium state. In this study, we used the RFCC

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catalyst for triglyceride cracking and investigated the effect of hydrogen transfer reactions on

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triglyceride deoxygenation.

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In addition to a rare-earth-exchanged USY zeolite, the RFCC catalyst contains matrix

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components such as kaolin clay particles and alumina binders,17 and the effects of these on

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triglyceride deoxygenation also need to be investigated. It is difficult for triglyceride molecules

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to diffuse directly into zeolite micropores because of size restrictions. Bhatia and coworkers have

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widely investigated catalytic cracking of triglycerides on microporous−mesoporous composite

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catalysts, and achieved higher conversions and improved compositions of produced

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hydrocarbons compared with those obtained from reactions on simple zeolites.18-22 The

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enhancement of triglyceride cracking on microporous−mesoporous hierarchical catalysts has also

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been reported by other researchers.23, 24 The matrix components of an RFCC catalyst, which


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contains macroscale and mesoscale pores, probably also contribute to the initial cracking of

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triglycerides. An understanding of the role of each component in an RFCC catalyst in

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triglyceride cracking will enable the design of catalysts that give efficient conversions.

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Considering the above, the aim of this research was to clarify the deoxygenation reaction

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mechanism in catalytic cracking of triglycerides on an RFCC catalyst with enhanced hydrogen

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transfer activity. We focused on the role of the zeolite and matrix parts of the RFCC catalyst. We

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analyzed the deoxygenation products in detail and compared the deoxygenation rates and

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selectivities in the cracking of triglycerides containing saturated and unsaturated fatty acids,

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which have different hydrogen-donating abilities. We also investigated the effects of steric

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hindrance on the deoxygenation of triglycerides by comparing the deoxygenation rates of

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saturated triglycerides with different carbon chain lengths. The effects of steric hindrance

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indicate the size of the deoxygenation reaction field and clarify the role of the zeolite and matrix

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parts of the RFCC catalyst. These results will provide strategies for the design of catalysts for

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efficient triglyceride conversion.

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2. EXPERIMENTAL SECTION

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2.1. Catalysts. Equilibrium RFCC catalysts (E-cat) with high (E-cat A) and low (E-cat B) rare-

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earth loadings were obtained from commercial RFCC units. In addition, a fresh RFCC catalyst

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was hydrothermally deactivated in a fluidized bed reactor under a 100% steam atmosphere at 800

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°C for 12 h (Steamed-cat C). The rare-earth loading of Steamed-cat C was the same as that of E-

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cat A. Most of the catalytic cracking experiments performed in this study were conducted using

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E-cat A. In the experiments aiming at clarifying the effect of catalytic activity on the reaction

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products, the other catalysts (E-cat B and Steamed-cat C) were used and compared with E-cat A.


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In the petroleum industry, the activity of an FCC catalyst is evaluated based on the unit cell

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size (UCS). The UCS provides a measure of the total number of tetrahedral aluminum sites per

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unit cell, and the tetrahedral aluminum sites in zeolites are the main active sites in FCC catalysts.

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The UCS therefore correlates with the activity (product distribution, coke formation,

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isomerization, and hydrogen transfer) of the FCC catalyst.25-29 The three catalysts used in this

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study were characterized by X-ray diffraction (XRD; RINT 2550, Rigaku Co., Tokyo, Japan)

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using Cu Kα radiation. The UCS value of each catalyst was calculated from the diffraction peaks

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of the (533) and (642) planes; the results are shown in Table 1. Table 1. Unit cell sizes of FCC catalysts used in this study Catalyst Unit cell size (Å)

E-cat A

E-cat B

Steamed-cat C

24.27

24.27

24.56

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2.2. Feedstocks. The plant oil feedstocks were coconut oil (Kaneda Shoji Co., Ltd., Tokyo,

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Japan) and sunflower oil (Wako Pure Chemical Industries, Ltd., Osaka, Japan). Coconut oil

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consists mainly of saturated fatty acids, and sunflower oil contains unsaturated fatty acids. We

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also used model triglyceride compounds containing saturated fatty acids such as tricaprylin

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(Sigma-Aldrich Co., St. Louis, MO, USA), and trilaurin and tripalmitin (Tokyo Chemical

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Industry Co., Ltd., Tokyo, Japan). In addition, we investigated the reactions of a fatty acid, a

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ketone and an aldehyde, which were identified as reaction intermediates, to clarify the reaction

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pathways in triglyceride cracking. We used lauric acid (Wako Pure Chemical Industries, Ltd.),

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12-tricosanone, and dodecanal (both Tokyo Chemical Industry Co., Ltd.) as a model of fatty acid,

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ketone, and aldehyde, respectively.


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The fatty acid compositions of the coconut oil and sunflower oil were determined using the

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method reported by Ichihara et al.30 Reagent-grade hexane, acetone, KOH, sulfuric acid (Wako

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Pure Chemical Industries, Ltd.), and methanol and acetic acid (Kanto Chemical Co., Inc., Tokyo,

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Japan) were used as received. A few drops of each plant oil were mixed with hexane (2 mL) and

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2 M KOH/methanol solution (0.2 mL). The mixture was vortexed at room temperature for 2 min.

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The hexane layer was analyzed using thin-layer chromatography on silica gel plates (Merck

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KGaA, Darmstadt, Germany) to confirm that all the triglyceride molecules were converted to

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FAMEs; the solvent system was hexane/acetone/acetic acid (95:5:0.5 by volume). The developed

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lipids were visualized by spraying with 50 wt% sulfuric acid and heating at 135 °C. The FAMEs

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in the hexane layer were identified using a gas chromatography (GC) system (GC-390B, GL

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Sciences Inc., Tokyo, Japan) equipped with a flame ionization detector (FID). The column used

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for FAME fractionation was 3 m length and packed with Uniport C 80/100 coated with Unisole

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3000 (GL Sciences Inc.). The obtained fatty acid compositions of the plant oils are shown in

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Table 2.


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Table 2. Fatty acid compositions of coconut oil and sunflower oil used in this study Coconut oil

Sunflower oil

Number of carbon atoms

Number of double bonds

(wt%)a

(mol%)

(wt%)a

(mol%)

Caproic acid

6

0

0.8

1.3

−

−

Caprylic acid

8

0

9.4

13.0

−

−

Capric acid

10

0

6.4

7.6

−

−

Lauric acid

12

0

45.9

46.9

−

−

Myristic acid

14

0

17.3

15.7

−

−

Palmitic acid

16

0

9.0

7.3

7.3

7.9

Stearic acid

18

0

2.5

1.9

3.7

3.6

Oleic acid

18

1

6.8

5.0

32.6

32.3

Linoleic acid

18

2

1.9

1.4

56.4

56.2

Linolenic acid

18

3

−

−

Trace

Trace

Fatty acid

a

Weight distribution of FAMEs after transesterification.

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2.3. Catalytic activity test. Catalytic activity tests were conducted in a fixed-bed microactivity

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test reactor; it has been described in a previous publication.31 In each trial, the catalyst (2–6 g)

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was placed in the reactor and maintained at reaction temperatures between 450 and 500 °C. The

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plant oils or model compounds were fed into the reactor by a microfeeder while being heated

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electrically in a preheating line. The feed injection time was 75 s and the weight ratio of the

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catalyst to the oil (cat/oil) was varied between 1.5 and 6.0. N2 gas was added during feed

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injection at 19 mL min−1. After each test, the catalyst was stripped by purging with N2 gas at 5

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mL min−1 for 15 min. During the reaction and stripping step, the liquid products were collected

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in a cold trap with two receiving vessels connected in series and maintained at 0 °C and −15 °C,


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respectively. Simultaneously, the gaseous products were collected in a gas burette by water

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displacement. In all the runs, the overall mass balances were between 92% and 103%.

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In this study, the residence time was expressed as the inverse of the weight hourly space

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velocity (WHSV) or that of the gas hourly space velocity (GHSV). The WHSV is a typical index

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of space velocity in the field of catalytic cracking and was calculated as the feed injection rate (g

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h−1) divided by the catalyst weight (g). The GHSV was calculated using the following equation,

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and is based on the number of molecules passing through the reactor under the assumption of an

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ideal gas.

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𝑤𝑤feed 𝑅𝑅𝑅𝑅 𝑀𝑀feed × 𝑃𝑃 GHSV = 𝑉𝑉cat

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Here, wfeed is the feed injection rate (g s−1), Mfeed is the molar weight of the feedstock (g mol−1), R

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is the gas constant (J mol−1 K−1), T is the reaction temperature (K), P is the atmospheric pressure

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(Pa), and Vcat is the catalyst volume (m3). For the coconut oil and sunflower oil, Mfeed was

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calculated based on the average of the fatty acid compositions shown in Table 1. The inverse of

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the GHSV was used as an index of the residence time to enable comparison of reaction rates for

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different feedstocks on the basis of the number of molecules rather than the weight.

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The amounts of H2, N2, CO, and CO2 in the gaseous products were determined using a GC

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system (GC-8A, Shimadzu Corp., Kyoto, Japan) equipped with a packed column

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(SHINCARBON-ST 50/80, Shinwa Chemical Industries, Ltd., Kyoto, Japan) and a thermal

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conductivity detector, with Ar as the carrier gas. The gaseous hydrocarbons, liquid hydrocarbons,

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and oxygenates in the liquid products were investigated using a GC system (GC-2014, Shimadzu

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Corp.) with a capillary column of length 60 m (BP1, SGE Analytical Science Pty. Ltd., Victoria,

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Australia) and a FID, with He as the carrier gas. The liquid hydrocarbons and oxygenates were


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also analyzed using a GC-mass spectrometry system (GCMS-QP2010 Plus, Shimadzu Corp.)

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equipped with a capillary column of length 60 m (Rxi-1ms, Restek Corp., Bellefonte, PA, USA),

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with He as the carrier gas. Biphenyl (99.5%, Sigma-Aldrich Co.) was used as an internal

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standard in the GC-FID assay. Quantification of the hydrocarbons and oxygenates was based on

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the effective carbon number theory.32 The amount of coke deposited on the catalyst was

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determined from the difference between the weight of the reactor before and after the catalytic

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test. Hydrocarbon products were classified based on their carbon numbers or boiling points into

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the following groups: gaseous hydrocarbons (C1–C4), gasoline (C5 to 216 °C), heavy liquid

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hydrocarbons (> 216 °C), and coke.

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3. RESULTS AND DISCUSSION

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3.1. Catalytic cracking of coconut oil and sunflower oil. The catalytic cracking of coconut

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oil and sunflower oil using E-cat A was investigated at 450–500 °C. Figure 1 shows the reaction

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product yields from the catalytic cracking of each plant oil. Hydrocarbons were the main reaction

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products in all the experiments. As the oxygen-containing products, CO2 and CO were detected.

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Some oxygenates, e.g., fatty acids, ketones, and aldehydes, were detected only in the reaction

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products from coconut oil. Water droplets were visible in the liquid products from both plant oils

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but this was not quantified. Assuming that all the oxygen atoms contained in the feedstock were

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converted only into the detected oxygenates, CO2, CO and water, the water yields in C and D in

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Figure 1 can be calculated from the oxygen balance to be 13% and 8%, respectively. These

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values are almost consistent with the “unknown” yields in Figure 1, indicating that the “unknown”

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yields in C and D in Figure 1 can be explained by water formation. However, in the cases of A

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and B in Figure 1, the calculated water yields under the same assumption are 12% and 11%,


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respectively, which cannot explain all of the “unknown” yields. This may be the result of the

218

formation of large-molecule products that were not detected by GC-FID because of their high

219

boiling points. Alternatively, the relative response factors of the oxygenates in the GC-FID assay

220

may have been overestimated. Nevertheless, most of the reaction products were quantified in all

221

the experiments and we can identify the trends in triglyceride deoxygenation based on these

222

results.

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Figure 1. Reaction product yields from catalytic cracking of coconut oil and sunflower oil at

225

various temperatures (E-cat A, WHSV = 16 h−1); A: coconut oil at 450 °C; B: coconut oil at 470

226

°C; C: coconut oil at 500 °C; and D: sunflower oil at 470 °C.

227 228

Catalytic cracking of coconut oil at a higher temperature resulted in higher yields of

229

hydrocarbons; this reflects involvement of the deoxygenation reaction. Higher reaction

230

temperatures also resulted in lower coke yields, unlike the case for common FCC reactions. This

231

is probably because of the formation of heavy products with high boiling points, which cannot

232

volatilize at the reaction temperatures. At higher reaction temperatures, the heavier products can

233

volatilize and are stripped from the catalyst; for example, the reaction products of boiling point

234

480 °C are deposited on the catalyst and regarded as coke products in the reaction at 470 °C,


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whereas they are stripped from the catalyst and are included in the heavy liquid hydrocarbon or

236

oxygenate yields in the reaction at 500 °C, resulting in a lower coke yield. The “coke” yield in

237

Figure 1 therefore contains not only highly aromatized coke products but also heavy compounds

238

with boiling points around the reaction temperatures.

239

For sunflower oil, oxygenate formation was lower and the hydrocarbon yield was higher than

240

those for coconut oil at the same temperature (Figure 1B and D). The sunflower oil was

241

converted to smaller hydrocarbons than the coconut oil was, although the sunflower oil contained

242

larger fatty acids than the coconut oil did, as shown in Table 1. These results suggest that

243

triglycerides containing unsaturated fatty acids are deoxygenated and decomposed more rapidly

244

than those containing only saturated fatty acids. The rapid cracking of unsaturated fatty acids can

245

be explained by a mechanism involving a carbenium ion intermediate; this is the standard theory

246

for catalytic cracking of hydrocarbons.33, 34 In this mechanism, unsaturated hydrocarbons easily

247

interact with protons on the Brønsted acid sites of the catalyst surface and form carbenium ions,

248

resulting in consecutive β-scission and hydride transfer reactions. The protonation of saturated

249

hydrocarbons requires an acid with a high proton-donor strength because it proceeds via unstable

250

carbonium ions. The cracking of saturated hydrocarbons is therefore much slower than that of

251

unsaturated hydrocarbons. Furthermore, our results suggest that unsaturated fatty acids also

252

accelerate the deoxygenation of triglycerides in catalytic cracking reactions. The deoxygenation

253

mechanism and the effects of unsaturated fatty acids on the mechanism are discussed in the

254

following sections.

255

Figure 2 shows the yields of hydrocarbons with different carbon numbers from the catalytic

256

cracking of coconut oil and sunflower oil. The formation of unsaturated hydrocarbons such as

257

light olefins (propylene and butene) and monocyclic aromatic hydrocarbons was observed in


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both cases. This shows that catalytic cracking of triglycerides competes with hydrocracking,

259

which produces mainly saturated hydrocarbons and requires further reforming to produce

260

unsaturated hydrocarbons. The reaction products from the coconut oil contained high yields of

261

normal paraffins (n-paraffins) with carbon numbers 11, 13, and 15. The data in Table 1 show that

262

the coconut oil consisted mainly of saturated fatty acids with carbon numbers 12, 14, and 16. The

263

composition of the reaction products from coconut oil cracking therefore reflected the fatty acid

264

composition of the reactant, i.e., n-paraffins were produced that were one carbon atom shorter

265

than the fatty acids in the reactant. Correlations between the fatty acid composition of the

266

triglyceride feedstock and the n-paraffins yields were also observed by Boocock et al. in the

267

catalytic cracking of coconut oil on activated alumina catalysts.35 Here, the formation of higher

268

paraffins reflecting the fatty acid composition of the reactant is a result of decomposition of the

269

ester bonds between the fatty acid and the glycerol backbone. This suggests that in the catalytic

270

cracking of saturated triglycerides, ester bond decomposition is faster than cracking of the carbon

271

chains of the fatty acids. In contrast, the formation of higher paraffins or olefins reflecting the

272

fatty acid composition of the reactant was hardly observed in sunflower oil cracking. This

273

suggests that cracking of the unsaturated bonds in the fatty acid is as fast as, or faster than,

274

decomposition of the ester bonds; this is consistent with a previous report.36


14

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275 276

Figure 2. Carbon number distributions of hydrocarbons produced from catalytic cracking of

277

coconut oil (a) and sunflower oil (b) (470 °C, E-cat A, WHSV = 16 h−1).

278 279

3.2. Catalytic cracking and deoxygenation of saturated triglycerides. The catalytic

280

cracking of trilaurin, which is a model saturated triglyceride consisting of C12 fatty acids, was

281

investigated to clarify the deoxygenation reaction mechanism in triglyceride cracking. The

282

composition of the reaction product from trilaurin cracking was similar to that from coconut oil

283

cracking; the main reaction products were hydrocarbons; CO2, CO, H2O, coke, and some

284

oxygenates were also produced. The yields of hydrocarbons, CO2, CO, and coke at different

285

residence times are shown in Figure 3a. The produced oxygenates were mainly lauric acid (C12

286

fatty acid), dodecanal (C12 aldehyde), 12-tricosanone (C23 ketone), and 2-tridecanone (C13

287

ketone), and their yields at different residence times are shown in Figure 3b. In addition to these

288

oxygenates, acrolein, methyl laurate, vinyl laurate, and 3-tetradecanone (C13 ketone) were

289

formed, but their yields were so small that they are not shown in Figure 3. The yields of

290

hydrocarbons, CO2, CO, and coke increased with increasing residence time; this reflects

291

involvement of the deoxygenation reaction. All the oxygenates produced were shown to be

292

reaction intermediates because their yields decreased with increasing residence time and they


15


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Page 16 of 52

293

disappeared after a sufficient residence time. This suggests that catalytic cracking achieved

294

complete deoxygenation of triglycerides. Among the oxygenates, the yield of lauric acid was

295

remarkably high at a short residence time and decreased greatly with increasing residence time,

296

indicating that fatty acids are initially produced by catalytic cracking of saturated triglycerides

297

and are then deoxygenated to hydrocarbons via ketones and aldehydes.

298 299

Figure 3. Reaction product yields from the catalytic cracking of trilaurin at 470 °C on E-cat A:

300

(a) hydrocarbons, CO2, CO, and coke, and (b) oxygenates.

301 302

Oxygenate formation was hardly observed at a residence time of around 5 s, indicating that

303

deoxygenation was almost complete (Figure 3a and b). However, the yields of CO2 and CO at

304

this residence time were only 5.2 wt% and 3.4 wt%, respectively. These weight-based yields of

305

CO2 and CO correspond to production of only 1.4 mol of CO2 (0.69 moles) plus CO (0.71 moles)

306

from 1 mol of trilaurin (Table 3), indicating that the combined number of CO2 and CO molecules

307

produced was less than that of ester bonds contained in the feedstock (three ester bonds per

308

trilaurin). This suggests the formation of H2O by hydrodeoxygenation in addition to

309

decarboxylation and decarbonylation, even in a hydrogen-free atmosphere. Hydrodeoxygenation


16

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310

instead of decarboxylation and decarbonylation results in suppression of carbon loss and efficient

311

conversion of triglycerides to hydrocarbons. When hydrodeoxygenation was dominant, carbon

312

loss as CO2 and CO from trilaurin was suppressed to only 3.6% (Table 3). Table 3. Yields of oxygen-containing products from catalytic cracking of trilaurin (470 °C, GHSV = 0.21 s−1, WHSV = 8.0 h−1) Oxygenates

CO2

CO

Weight-based yield (wt%)

< 1.3

5.2

3.4

Oxygen-based yield (O%)

< 0.8

23.2

11.9

Carbon-based yield (C%)

< 1.2

1.8

1.8

Mole-based yield (mol/molfeed)

< 0.04

0.69

0.71

313 314

We investigated the yields of intermediate hydrocarbons produced from the deoxygenation

315

reaction to confirm the involvement of hydrodeoxygenation. The trilaurin feedstock contained

316

lauric acid, with a carbon number of 12. As mentioned above, oxygenates with a carbon number

317

smaller than 12 were not observed, except acrolein, which was produced from the glycerol

318

backbone. Decomposition of the ester bonds was faster than cracking of the saturated fatty acid

319

carbon chains. The deoxygenation of C12 oxygenates is expected to produce hydrocarbons with

320

carbon numbers between 10 and 12; their yields are shown in Figure 4. Among the n-paraffins, a

321

remarkably high yield of undecane (C11) was observed. The formation of n-paraffins one carbon

322

atom shorter than the fatty acid in the feedstock is consistent with the results for coconut oil

323

cracking shown in Figure 2. The yield of n-undecane increased sharply at short residence times

324

and then decreased slightly. The yields of C10, C11 and C12 olefins also initially increased and

325

then decreased. These results indicate that these products are intermediates and are converted to


17


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Page 18 of 52

326

smaller hydrocarbons by secondary cracking. The reason for the low yields of intermediate

327

olefins is that secondary cracking of olefins is much faster than that of paraffins.33, 34

328 329

Figure 4. Yields of C10−12 hydrocarbons from catalytic cracking of trilaurin at 470 °C on E-cat

330

A: (a) n-paraffins and (b) olefins.

331 332

3.3. Reactions of intermediates. As shown in Figure 3b, the main reaction intermediates in

333

saturated triglyceride (trilaurin) deoxygenation are a fatty acid (lauric acid), ketone (12-

334

tricosanone), and aldehyde (dodecanal). We investigated the reactions of these intermediates on

335

E-cat A to clarify the details of the mechanism of triglyceride deoxygenation. The product

336

distributions in the catalytic cracking of trilaurin, lauric acid, 12-tricosanone, and dodecanal are

337

shown in Figure 5. The overall product composition and oxygenated product composition from

338

the catalytic cracking of lauric acid were similar to those from the catalytic cracking of trilaurin.

339

Ketones and aldehydes were produced from the catalytic cracking of lauric acid, but fatty acids

340

were not observed among the products of the catalytic cracking of 12-tricosanone and dodecanal.

341

These results suggest that fatty acids are the primary products of triglyceride cracking, and

342

ketones and aldehydes are secondary products produced from the fatty acids. The formation of


18

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343

ketones from fatty acids involves ketonic decarboxylation, in which two fatty acids are converted

344

to symmetric ketones, CO2, and H2O.37-40 The formation of aldehydes from fatty acids is

345

probably the result of reduction, producing H2O. The deoxygenations of ketones and aldehydes

346

were faster than those of triglycerides and fatty acids; this suggests that the rate-determining step

347

in triglyceride deoxygenation may be fatty acid conversion.

348


19


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Page 20 of 52

349

Figure 5. Reaction product composition from catalytic cracking of various feedstocks (470 °C,

350

E-cat A, WHSV = 16 h−1); A: trilaurin; B: lauric acid; C: 12-tricosanone; and D: dodecanal; (a)

351

overall products; (b) oxygenates; and (c) C10-12 paraffins and olefins.

352 353

The hydrocarbons produced from the catalytic cracking of trilaurin, lauric acid, 12-tricosanone,

354

and dodecanal are compared in Figure 5c. The cracking of trilaurin and lauric acid produced

355

mainly C11 paraffins and C11 olefins, as a result of direct decarboxylation or decarbonylation-

356

dehydration of the fatty acid.41-44 In contrast, the cracking of 12-tricosanone produced large

357

amounts of C10 paraffins and C10 olefins, as a result of γ-hydrogen transfer of symmetric

358

ketones.38, 45 The cracking of dodecanal produced a large amount of C12 paraffins. This is the

359

result of hydrodeoxygenation of the aldehyde because decarbonylation of dodecanal produces

360

C11 hydrocarbons whereas hydrodeoxygenation produces C12 hydrocarbons. The high yield of

361

C12 paraffins is evidence that hydrodeoxygenation is the primary reaction in dodecanal cracking.

362

The formation of C12 hydrocarbons from lauric acid can also be explained by

363

hydrodeoxygenation via dodecanal. The C10 paraffins produced from 12-tricosanone and the C12

364

paraffins produced from dodecanal included many branched paraffins, whereas the C11 paraffins

365

from trilaurin and lauric acid consisted mainly of linear paraffins. These results suggest that the

366

C10 paraffins from 12-tricosanone and the C12 paraffins from dodecanal were produced via

367

olefins, because olefins easily form carbenium ions and are isomerized by methyl shifts of the

368

carbenium ions.33, 34

369

The oxygen-based yields in the catalytic cracking of trilaurin, lauric acid, 12-tricosanone, and

370

dodecanal are shown in Table 4. Trilaurin cracking gave the highest CO2 and CO yields among

371

the four feedstocks. The yields of CO2 and oxygenates from dodecanal were lower than that from


20

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372

lauric acid, whereas the CO yields from the two feedstocks were similar. This suggests that the

373

yield of H2O from dodecanal should be higher than that from lauric acid because of fast

374

hydrodeoxygenation of the aldehyde. The low yields of CO2 and CO from 12-tricosanone are

375

probably the result of strong hydrogen transfer activity, because the O/H atomic ratio of 12-

376

tricosanone is much lower than those of the other feedstocks. Table 4. Oxygen-based yields [O%] of oxygen-containing products from catalytic cracking of various feedstocks (470 °C, E-cat A, WHSV = 16 h−1) Feedstock

Trilaurin

Lauric acid

12-Tricosanone

Dodecanal

Oxygenates

11.4

9.6

n.d.a

0.6

CO2

19.7

12.8

2.5

5.6

CO

9.1

6.4

2.0

6.6

a

Not detected

377 378

3.4. Effect of hydrogen transfer activity. As shown in the previous section, the primary

379

products of triglyceride cracking are fatty acids, and they are converted to hydrocarbons via

380

ketones or aldehydes. The formation of ketones can be explained by ketonic decarboxylation

381

whereas the formation of aldehydes must involve reduction of fatty acids, which requires

382

hydrogens. In addition, the conversion of dodecanal to C12 paraffins, shown in Figure 5c, also

383

requires hydrogens. These results indicate that hydrogenation proceeds even in a hydrogen-free

384

atmosphere. We assumed that the hydrogenation occurs via hydrogen transfer reactions on the

385

FCC catalysts. To verify this hypothesis, we investigated the catalytic cracking of coconut oil on

386

RFCC catalysts with different hydrogen transfer activities. Figure 6a shows the relationship

387

between the butane/butene ratio and yield of oxygenates. The butane/butene ratio is used as an

388

index representing the hydrogen transfer activities of the different catalysts. A higher


21


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Page 22 of 52

389

butane/butene ratio indicates higher hydrogen transfer activity because the hydrogen transfer

390

reaction converts olefins to paraffins, and because the proportions of unsaturated and saturated

391

hydrocarbons in gasoline obtained from cracking reactions corresponds to that in the C4

392

fraction.46-48 The results shown in Figure 6a therefore suggests that the yield of oxygenates

393

decreases with increasing hydrogen transfer activity.

394 395

Figure 6. (a) Yield of oxygenates and (b) aldehyde/ketone ratio from catalytic cracking of

396

coconut oil on various catalysts (450 °C, WHSV = 16 h−1).

397 398

We also investigated the effect of hydrogen transfer reactions on the selectivity of

399

deoxygenation routes in the coconut oil cracking. Figure 6b shows the relationship between the

400

aldehyde/ketone ratio and butane/butene ratio in the catalytic cracking of coconut oil on three

401

different catalysts. The aldehyde/ketone ratio increased with increasing butane/butene ratio. As

402

mentioned above, aldehyde formation from a fatty acid requires hydrogens, whereas ketone

403

formation proceeds via ketonic decarboxylation and does not require hydrogens. The results

404

shown in Figure 6b therefore suggest that the hydrodeoxygenation route in fatty acid

405

deoxygenation is accelerated by hydrogen transfer reactions. The hydrogens released during the


22

Page 23 of 52

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406

cracking of carbon chains16 can probably be received by oxygenate intermediates, with

407

acceleration of hydrodeoxygenation.

408

3.5. Deoxygenation reaction mechanism. The mechanism of triglyceride deoxygenation

409

during thermal cracking49-54 and catalytic cracking with metal oxide catalysts37,

55

410

widely investigated. In catalytic cracking with acidic zeolite catalysts, which are the most

411

common active species in modern FCC catalysts, the formation of CO2, CO, and H2O by

412

triglyceride deoxygenation has been widely reported, but there are few reports of the formation

413

of oxygenated products.36, 56-59 This may be because acidic zeolite catalysts are so active that

414

deoxygenation is too fast to be observed under the standard reaction conditions for an FCC

415

process. Most of the reported oxygenated products are fatty acids; this can be explained based on

416

the thermal cracking mechanism of triglycerides. A reaction mechanism that is specific to zeolite

417

catalysts has not been reported. We investigated the catalytic cracking of saturated triglycerides,

418

which are deoxygenated more slowly than most plant oils that contain unsaturated fatty acids,

419

using an equilibrium RFCC catalyst that had been sufficiently deactivated compared with the

420

fresh catalyst. We observed the formation of several intermediate oxygenates, including ketones

421

and aldehydes, which enabled us to clarify the detailed reaction paths of triglyceride

422

deoxygenation on RFCC catalysts. To investigate the specific features of catalytic cracking

423

deoxygenation of triglycerides on zeolite catalysts with enhanced hydrogen transfer activity, we

424

compared the reaction products and the reaction paths of triglyceride deoxygenation for catalytic

425

cracking on the RFCC catalyst investigated in this study with those for thermal cracking or

426

catalytic cracking on metal oxide catalysts reported in the literature.

has been

427

Figure 7 summarizes the deoxygenation reaction paths in triglyceride thermal cracking and

428

catalytic cracking on metal oxide catalysts proposed in the literature37, 53-55 and modified on the


23


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Page 24 of 52

429

basis of the results of this study. Initially, triglycerides undergo thermal transformation via a β-

430

elimination mechanism and are converted to two fatty acids, an aldoketone, and acrolein (R1).37,

431

54

432

anhydrides,49-51, 57 but these intermediates are so unstable that they were not detected in our

433

experiments. The aldoketone is also unstable and is rapidly converted to olefins by

434

decarbonylation (R2).51 Acrolein decomposes to olefins such as propylene (R3), aromatizes via a

435

Diels–Alder reaction (R4), and polymerizes to form coke (R5).60

This reaction is reported to proceed via unsaturated glycol difatty acid esters and fatty acid

436 437

Figure 7. Schematic diagram of deoxygenation reaction mechanism in catalytic cracking of

438

triglycerides; red dashed arrows represent hydrogen transfer hydrodeoxygenation reaction paths

439

that are specific to reactions on RFCC catalysts with enhanced hydrogen transfer activity.

440


24

Page 25 of 52

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441

The fatty acids produced by R1 are deoxygenated via decarboxylation (R6) or dehydration and

442

decarbonylation (R7).41-44 Ketonic decarboxylation (R8) has also been reported in the

443

literature,37-40 and observed in this study. The symmetric ketones decompose to olefins and 2-

444

ketones and subsequently to olefins and acetone via γ-hydrogen transfer (R9 and R10).38, 45 The

445

produced paraffins and olefins were further cracked to smaller fractions (R11–R13).

446

In addition to fatty acids and ketones, we observed the formation of an aldehyde, i.e.,

447

dodecanal. However, aldehyde formation in thermal and catalytic cracking reactions has rarely

448

been reported. In thermal cracking, aldehyde formation has only been observed above 600 °C,42,

449

61, 62

450

that the aldehyde formation observed in this study is specific to the reactions on zeolite catalysts

451

with enhanced hydrogen transfer activity and is different from that in thermal cracking. In the

452

previous section, we confirmed that the hydrogen transfer reaction contributes to the reaction

453

route producing aldehydes. The specific features of triglyceride deoxygenation on zeolite

454

catalysts with enhanced hydrogen transfer activity are therefore formation of aldehydes as

455

intermediates and involvement of hydrodeoxygenation, even in a hydrogen-free atmosphere.

456

These features suggest that reduction, hydrogenation, and dehydration (R14–R17 in Figure 7)

457

occur, although alcohol dehydration (R16) is so fast that alcohols were not detected in this study.

458

Reduction (R14) and hydrogenation (R15) can be explained by a hydrogen transfer mechanism

459

in which active hydrogen species are released during cracking of intermediate hydrocarbons and

460

transferred to fatty acids and aldehydes. Recently, Grecco et al. observed the formation of

461

alcohols and aldehydes in the catalytic cracking of crude soybean oil on nanocrystalline β-zeolite

462

catalysts, but the amounts produced were much lower than those of fatty acids.59 The reason for

463

the production of low amounts of alcohols and aldehydes is that the hydrogen transfer activity of

which is much higher than the reaction temperature in this study. These conflicts indicate


25


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Page 26 of 52

464

β-zeolites is much weaker than that of the USY zeolites contained in the RFCC catalyst used in

465

this study.63 The reaction paths R14–R17 are coincident with that proposed for the hydrocracking

466

reactions of triglycerides on metal oxide catalysts,64 indicating strong hydrogenation activity of

467

hydrogen transfer reactions during catalytic cracking and the possibility of efficient conversion

468

of triglycerides to hydrocarbons with suppressed carbon loss without using a hydrogen

469

atmosphere.

470

3.6. Catalytic cracking of triglycerides with fatty acid carbon chains of different lengths.

471

We investigated catalytic cracking of saturated triglycerides with carbon chains of different

472

lengths to clarify the effects of steric hindrance in catalyst pores on deoxygenation and cracking

473

of triglycerides. The yields of hydrocarbons, CO2, and CO from tricaprylin (C8), trilaurin (C12),

474

and tripalmitin (C16) are shown in Figure 8. The inverse of the GHSV was used as an index of

475

the residence time to enable comparison of reaction rates of different feedstocks on the basis of

476

the number of molecules. Figure 8a shows that the weight-based yields of hydrocarbons from the

477

model triglycerides were similar, regardless of the carbon chain length, and even in the case of

478

coconut oil, which contains carbon chains of various lengths. Figure 8b and c show that there

479

was also consistency among the oxygen-based CO2 and CO yields, respectively, for different

480

feedstocks. The consistency among the deoxygenation reaction rates of saturated triglycerides

481

with fatty acid carbon chains of different lengths indicates that diffusion into active sites and the

482

deoxygenation reaction are not affected by steric hindrance caused by the catalyst pore size. The

483

pore diameter of the USY zeolite in E-cat is 7.4 Å, and the molecular dimensions of triglycerides

484

are estimated to be from 6.2 to 43.7 Å,36 suggesting that triglycerides cannot directly penetrate

485

the zeolite pores. The above findings suggest that the deoxygenation of triglycerides does not

486

occur inside the zeolite pores but in the large region that is not affected by steric hindrance. The


26

Page 27 of 52

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487

active sites for deoxygenation are presumably the acid sites close to the zeolite pore mouths, the

488

outer surfaces of the zeolites, or the surfaces of the matrix components of the RFCC catalysts

489

such as kaolin clays and alumina binders.

490 491

Figure 8. Reaction product yields from catalytic cracking of different feedstocks at 470 °C on E-

492

cat A: (a) yields of hydrocarbons, (b) oxygen-based yields of CO2, and (c) oxygen-based yields

493

of CO.

494 495

Figure 8 shows that the results for sunflower oil deviate from the lines for saturated

496

triglycerides. The catalytic cracking of sunflower oil produced higher hydrocarbon and lower

497

CO2 yields than did saturated triglycerides at the same residence time; this indicates efficient

498

conversion of triglycerides to hydrocarbons. Table 5 shows the yields of non-hydrocarbon gases

499

from different feedstocks at a residence time around 3 s. The H2 yield from sunflower oil

500

cracking was much higher than that from saturated triglycerides. This is because of fast cracking

501

of the double bonds in the unsaturated fatty acids in sunflower oil. One possible reason for the

502

low CO2 yield from sunflower oil cracking is participation of the reverse water gas shift reaction.

503

The effects of the forward and reverse water gas shift reactions in triglyceride cracking have


27


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Page 28 of 52

504

been widely investigated.43, 65 However, as shown in Figure 8c and Table 5, the CO yield from

505

sunflower oil cracking was not higher than that from saturated triglycerides, although the reverse

506

water gas shift reaction converts CO2 to CO. The low CO2 yield from sunflower oil therefore

507

cannot be explained by involvement of the reverse water gas shift reaction. Another possible

508

reason for the low CO2 yield is reduction of the intermediate oxygenated products. The high H2

509

yield from sunflower oil cracking indicates a strong reductive atmosphere in the cracking field,

510

in which the fatty acids are reduced to aldehydes and alcohols and form water (R14–R17 in

511

Figure 7) rather than undergoing decarboxylation to form CO2 (R6). One mole of sunflower oil

512

produced approximately 1 mol of CO2 (0.40 mol) plus CO (0.63 mol), which is less than the

513

amount of CO2 or CO produced from 1 mol of saturated triglycerides (Tables 3 and 5). These

514

results confirm that in catalytic cracking of unsaturated triglycerides, the fast decomposition of

515

unsaturated

516

hydrodeoxygenation of oxygenated intermediates and suppresses carbon loss caused by

517

decarboxylation and decarbonylation reactions. Deoxygenation paths are therefore affected by

518

the hydrogen-donating ability of the feedstock, in agreement with the results reported by Černý

519

et al.15

carbon

chains

produces

active

hydrogen

species,

which

accelerates


28

Page 29 of 52

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Table 5. Yields of non-hydrocarbon gases from catalytic cracking of various feedstocks at 470 °C on E-cat A at residence time around 3 s. Feedstock

Tricaprylin

Trilaurin

Tripalmitin

3.5

3.1

3.0

2.4

3.3

0.011

0.003

0.007

0.006

0.018

CO2

6.9

4.9

3.5

3.3

1.8

CO

4.0

3.0

2.4

2.7

1.8

H2

0.03

0.01

0.03

0.02

0.08

CO2

0.75

0.66

0.66

0.50

0.40

CO

0.69

0.63

0.71

0.66

0.63

GHSV−1 (s)

Coconut oil Sunflower oil

Weight-based yield (wt%) H2

Mole-based yield (mol/molfeed)

520 521

As shown in Figure 4a, the catalytic cracking of trilaurin (C12 triglyceride) gave a remarkably

522

high yield of n-undecane (C11). High yields of Cm−1 n-paraffins from Cm triglycerides (m is the

523

carbon number of the fatty acid in the feedstock) were also observed in the catalytic cracking of

524

tricaprylin (C8 triglyceride) and tripalmitin (C16 triglyceride). The numbers of moles of Cm−1 n-

525

paraffins produced from 1 mol of Cm triglycerides for the three model triglycerides (C8, C12, and

526

C16) are shown in Figure 9. The Cm−1 n-paraffin production rates for the three feedstocks

527

differed; the triglycerides with longer fatty acid carbon chains produced more of the

528

corresponding n-paraffins. This result seems inconsistent with the results shown in Figure 8, i.e.,

529

that deoxygenation of the saturated triglycerides proceeds at the same rate regardless of the

530

carbon chain length. As shown in Figure 4a, the Cm−1 n-paraffins are intermediates and they are

531

further cracked to smaller hydrocarbons. The inconsistencies among the Cm−1 n-paraffin yields in


29


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Page 30 of 52

532

Figure 9 indicate differences among the secondary cracking rates of the Cm−1 n−paraffins. The

533

lower yields of smaller paraffins suggest that secondary cracking of smaller paraffins proceeds

534

more rapidly. The effect of shape selectivity on the secondary cracking rates in catalytic cracking

535

were investigated by Idem et al.55 The differences among the secondary cracking rates can be

536

explained by assuming that the secondary cracking of Cm−1 n-paraffins occurs at the internal

537

surfaces of the zeolite pores, therefore the diffusion of larger paraffins to the active sites is

538

restricted compared with that of smaller paraffins. This assumption is plausible because the

539

cracking of n-paraffins requires strong acid sites.66

540 541

Figure 9. Yields of Cm−1 n-paraffins from catalytic cracking of Cm triglycerides with carbon

542

chains of different lengths (m) at 470 °C on E-cat A.

543 544

Differences among the amounts of Cm−1 n-paraffins produced from triglycerides with fatty acid

545

carbon chains of different lengths were also observed in the catalytic cracking of coconut oil.

546

Table 6 shows the molar ratios of the saturated fatty acids in the coconut oil feedstock and the

547

corresponding n-paraffins in the reaction products. For carbon numbers between 12 and 18, the

548

ratios of n-paraffins produced from the corresponding fatty acids increased with increasing


30

Page 31 of 52

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


549

carbon number, in agreement with the results shown in Figure 9; however, this trend was not

550

observed for carbon chains shorter than 12 because smaller n-paraffins (m = 8 and 10) were

551

produced not only by deoxygenation of Cm fatty acids but also by cracking of larger fatty acids.

Table 6. Relationship between fatty acid compositions of feedstock and yields of Cm−1 n-paraffins in catalytic cracking of coconut oil (470 °C, E-cat A, WHSV = 16 h−1, GHSV = 0.42 s−1)a Carbon number of fatty acid, m

8

10

12

14

16

18

Fatty acid composition of feedstock, fm (mol/molfeed)

0.39

0.23

1.41

0.47

0.22

0.06

Cm−1 n-paraffin yields, ym (mol/molfeed)

0.036

0.030

0.111

0.058

0.033

0.012

0.13

0.08

0.12

0.15

0.22

Ratio of Cm−1 n-paraffins in reaction product to 0.09 Cm fatty acids in feedstock, ym/fm a

The fatty acid composition and n-paraffin yields correspond to the data shown in Table 1 and Figure 2a, respectively. 552 553

The results confirm that the deoxygenation of triglycerides occurs in the large region that is not

554

affected by steric hindrance, whereas secondary cracking of hydrocarbons occurs on the internal

555

surfaces of the zeolites. It was also confirmed that the hydrodeoxygenation of triglycerides

556

proceeds via hydrogen transfer reactions between oxygenated intermediates and hydrocarbons

557

produced by cracking of the fatty acid carbon chains. The active hydrogen species produced

558

during hydrocarbon cracking on the internal surfaces of the zeolites can be transferred to the

559

zeolite surfaces and consumed in hydrodeoxygenation of the oxygenated intermediates.

560

Identification of the active hydrogen species and clarification of the mechanism of active

561

hydrogen species transfer will be important in designing efficient catalysts for triglyceride

562

conversion, and will be the subjects of our future work.

563

4. CONCLUSIONS


31


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 32 of 52

564

The catalytic cracking of triglycerides with saturated and unsaturated fatty acids using RFCC

565

catalysts with enhanced hydrogen transfer activity was investigated. In the cracking of

566

unsaturated triglycerides, deoxygenation proceeded rapidly and light hydrocarbons, including

567

light olefins and monocyclic aromatic hydrocarbons, were produced. Deoxygenation of saturated

568

triglycerides was slower than that of unsaturated triglycerides and oxygenated compounds such

569

as fatty acids, ketones, and aldehydes were detected in the reaction products. High yields of n-

570

paraffins with one carbon atom less than the fatty acids in the feedstock were detected in the

571

cracking of saturated triglycerides. This suggests that decomposition of ester bonds is faster than

572

cracking of fatty acid carbon chains in catalytic cracking of saturated triglycerides. In contrast,

573

cracking of unsaturated bonds in fatty acids is as fast as, or faster than, decomposition of ester

574

bonds.

575

Among the oxygenates produced by saturated triglyceride cracking, aldehydes are a specific

576

feature of catalytic cracking using zeolite catalysts. The formation of dodecanal as well as C12

577

olefins in the catalytic cracking of trilaurin suggests that hydrodeoxygenation proceeds via

578

reduction of fatty acids to aldehydes and hydrogenation–dehydration of aldehydes to olefins. The

579

dominance of the hydrodeoxygenation reaction was confirmed based on the yields of CO2 and

580

CO. The hydrodeoxygenation reaction proceeds by hydrogen transfer reactions between the

581

oxygenates and hydrocarbons produced from the cracking of fatty acid carbon chains.

582

Hydrodeoxygenation can convert all the carbon resources contained in the feedstock to

583

hydrocarbons, whereas decarboxylation and decarbonylation result in partial loss of carbon

584

resources during conversion. The results obtained in this study suggest that efficient conversion

585

of triglycerides to hydrocarbons can be achieved by catalytic cracking without using a hydrogen

586

atmosphere.


32

Page 33 of 52

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


587

The active sites of FCC catalysts for triglyceride conversion were investigated by focusing on

588

the effects of steric hindrance. In the cracking of saturated triglycerides with fatty acid carbon

589

chains of different lengths, the deoxygenation reaction rates were consistent, but the secondary

590

cracking rates of intermediate paraffins were different. These results suggest that the

591

deoxygenation of triglycerides occurs in large regions that are not affected by steric hindrance

592

(probably at the acid sites close to the zeolite pore mouths, on the external surfaces of zeolites, or

593

on the surfaces of the matrix components), whereas the secondary cracking of hydrocarbons

594

occurs on the internal surfaces of the zeolites. Hydrodeoxygenation of triglycerides consumes

595

active hydrogen species produced from the cracking of intermediate hydrocarbons, therefore, to

596

achieve efficient conversion of triglycerides to hydrocarbons, it is necessary to identify the active

597

hydrogen species and clarify the mechanism of active hydrogen species transfer; this will be the

598

subject of our future work.

599

AUTHOR INFORMATION

600

Corresponding Author

601

*Tel: +81-268-21-5466. Fax: +81-268-21-5391. E-mail: [email protected].

602

Notes

603

The authors declare no competing financial interest.

604

ACKNOWLEDGMENT

605

Part of this work was financially supported by the Program to Disseminate Tenure Tracking

606

System from the Japanese Ministry of Education, Culture, Sports, Science and Technology

607

(MEXT), and the Grant-in-Aid for Young Scientists (B), Grant Number JP15K21038, from the

608

Japan Society for the Promotion of Science (JSPS).


33


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

609 610 611 612 613

Page 34 of 52

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Page 42 of 52

Table of Contents/Abstract Graphics

772


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Carbon Loss

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