Influence of Alkaline and Alkaline Earth Metal Promoters on the

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Catalysis and Kinetics

Influence of Alkaline and Alkaline Earth Metal Promoters on the Catalytic Performance of Pd-M/SiO (M = Na, Ca or Ba) Catalysts in the Partial Hydrogenation of Soybean Oilderived Biodiesel for Oxidative Stability Improvement 2

Chachchaya Thunyaratchatanon, Apanee Luengnaruemitchai, Jakkrapong Jitjamnong, Nuwong Chollacoop, Shih-Yuan Chen, and Yuji Yoshimura Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01498 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Influence of Alkaline and Alkaline Earth Metal Promoters on the Catalytic Performance of Pd-M/SiO2 (M = Na, Ca or Ba) Catalysts in the Partial Hydrogenation of Soybean Oil-derived Biodiesel for Oxidative Stability Improvement

Chachchaya Thunyaratchatanona,b, Apanee Luengnaruemitchaia,b,c,*, Jakkrapong Jitjamnongd, Nuwong Chollacoope, Shih-Yuan Chenf, Yuji Yoshimurae,f

a

The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12,

Phayathai Rd., Pathumwan, Bangkok 10330, Thailand b

Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn

University Research Building, Soi Chula 12, Phayathai Rd., Bangkok 10330, Thailand c

Center of Excellence on Catalysis for Bioenergy and Renewable Chemicals (CBRC),

Chulalongkorn University, Phayathai Rd., Pathumwan, Bangkok 10330, Thailand d

Petroleum Technology Program, Faculty of Industrial Education and Technology,

Rajamangala University of Technology Srivijaya, 1 Rachadamnoennork Rd., Boryang, Muang, Songkhla 90000, Thailand e

Renewable Energy Laboratory, National Metal and Materials Technology Center (MTEC),

114 Thailand Science Park, Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120, Thailand f

Energy Catalyst Technology Group, Research Institute of Energy Frontier (RIEF), National

Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305–8569, Japan * Corresponding author. Tel.: +662–2184148, Fax: +662–6117220 E-mail address: [email protected] (A. Luengnaruemitchai)

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ABSTRACT Palladium/silica (Pd/SiO2) and Pd-M/SiO2 catalysts, where M stands for sodium (Na), calcium (Ca) or barium (Ba), were studied for their catalytic ability in the partial hydrogenation of soybean oil-derived fatty acid methyl esters (SO-FAMEs) using a semibatch reactor under mild reaction conditions (4 bar hydrogen pressure and 80 °C). The catalytic performance, in terms of turnover frequency (TOF), and the selectivity towards (E)monounsaturated FAMEs (C18:1) were investigated at 11% and 45% diunsaturated FAMEs (C18:2) conversion levels. The catalysts were analyzed by X-ray diffractometry, temperature programmed desorption of carbon dioxide, transmission electron microscopy, pulse chemisorption of carbon monoxide, nitrogen adsorption-desorption and X-ray photoelectron spectroscopy. On the one hand, the highest TOF was observed with the catalyst with the lowest basic sites on the catalyst surface (Pd-Ba/SiO2) in comparison with the other basicmodified catalysts; on the other hand, the highest selectivity towards (E)-C18:1 was evidently noticed. The oxidative stability of the partially hydrogenated SO-FAMEs (biodiesels) obtained using these catalysts could be improved by two- to five-fold compared to that of the feed SO-FAMEs.

Keywords: Biodiesel; Partial hydrogenation; Oxidative stability; Pd catalyst; Basic promoter; XPS

1. INTRODUCTION

The rapidly increasing global population and economic development, which drives the rapid growth of several developing sectors, such as transport, agricultural and industrial, contributes towards the continuous and increasing depletion of the crude oil resources and


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environmental concerns. Moreover, the high consumption level of conventional petro-diesel is the major cause of emission of greenhouse gases to the atmosphere, which then contributes to global warming. To overcome such problems, alternative and sustainable sources of energy are necessary and in demand. Biodiesel, comprised of fatty acid methyl esters (FAMEs), has become one of the most promising renewable fuels owing to its biodegradable nature, sustainable renewability and high cetane number. Additionally, biodiesel has lower exhaust emissions (unburned hydrocarbons and particulate matters) than petroleum-based fuels, which is in accordance with the more earnest environmental emission regulations.1,2 Biodiesel can be produced via transesterification of animal fats,3 vegetable oils,4 waste cooking oils,1,5 or microalgae6 with a short-chain alcohol in the presence of a catalyst, of which methanol is the most commonly used alcohol to yield FAMEs. Apart from the conventional method, it can also be derived without a catalyst or even by autocatalysis process; for instance, the esterification of fatty acids with supercritical ethanol.2 In spite of the many advantages of biodiesel, there are some drawbacks related to its properties, which are its lower oxidative stability and cold flow properties. These fuel properties are directly dependent on the structural features (chain length, degree of saturation and branching of the chain) of the starting oil or fat. Biodiesel with a high content of polyunsaturated FAMEs has a low oxidative stability, whereas biodiesel with a high content of saturated FAMEs exhibits poor cold flow properties. However, monounsaturated FAMEs have the appropriate compositions to reach a compromise between these two properties.7,8 Partial hydrogenation has been applied to transform polyunsaturated FAMEs into monounsaturated FAMEs with a limited level of formation of saturated FAMEs.9,10 In comparison with the (Z)-isomer, the (E)-isomer has a less attractive composition through its higher crystallization point, giving worse cold flow properties.11,12 However, the (Z)-configuration is isomerized to the more thermodynamically stable (E)-configuration during the hydrogenation. Accordingly, the


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partial and selective hydrogenation of polyunsaturated FAMEs towards (Z)-monounsaturated FAMEs is required. In recent decades, supported palladium (Pd) catalysts have been broadly used in liquidphase hydrogenation due to their high catalytic activity compared with other active metal catalysts, such as platinum (Pt), rhodium (Rh), rubidium (Ru) and nickel (Ni).13,14 Likewise, various strategies have been reported for improving the activity and selectivity of the FAME hydrogenation process, including increasing the pore width and morphology of the catalyst support,15,16 alloying with second transition metals,17 adsorption of organic solvents11,18 and the use of an aqueous-phase catalyst.19 However, these approaches are relatively complicated and quite high-priced, and so other strategies, such as tunable acid-base supported catalysts, are more attractive.20–22 Zhao et al.23 investigated Ni catalysts with different acidic and basic supports, alumina (Al2O3), magnesium-aluminium (Mg-Al) mixed oxides and magnesium oxide (MgO), for the hydrogenation of pyridine. The electron density of Ni was found to be increased when using a basic MgO support, leading to an increased heat of adsorption of carbon monoxide (CO) on the Ni while weakening the strength of the H–Ni bond. As a consequence, a decreased hydrogenation activity of Ni/MgO was noticed.23 Conversely, Liu et al.20 developed Pd-silver (Ag) catalysts with reducible Mg-titanium (Ti) mixed oxide supports, using various Mg/Ti ratios. As the concentration of Mg2+ increased, the hydrogenation activity of acetylene and selectivity towards ethane were initially enhanced, but then decreased.20 The catalytic performance of silica (SiO2) supported noble metals with a Mg promoter were previously compared in the partial hydrogenation of soybean oil (SO)-derived FAMEs, where Mg promoted catalysts were found to improve the conversion level of polyunsaturated FAMEs and the selectivity towards (Z)-monounsaturated FAMEs.24


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From the above information, the investigation of the role of other basic modifiers, such as alkaline and alkaline earth metals, is remarkably appealing due to the fact that the hydrogenation process is likely to prefer acidic to basic catalysts.23 In this study, the catalytic performance of SiO2-supported Pd catalysts with three different basic metal promoters for the partial hydrogenation of SO-FAMEs is reported for the first time. The intention of this work was to consider the role of basic modifiers, sodium (Na), calcium (Ca) and barium (Ba), on the partial hydrogenation of SO-FAMEs, which has a high content of polyunsaturated FAMEs (C18:3 and C18:2) that results in a poor oxidative stability. The reactions were conducted in a batch reactor at 80 °C and a 4 bar atmosphere of hydrogen (H2). The hydrogenation activity was expressed in terms of the turnover frequency (TOF), while the selectivity was examined in terms of the composition of (Z)- and (E)monounsaturated FAMEs ((Z)-C18:1 and (E)-C18:1) after a 4-h reaction time. X-ray diffractometry (XRD), temperature programmed desorption of carbon dioxide (CO2-TPD), transmission electron microscopy (TEM), CO pulse chemisorption, nitrogen (N2) adsorptiondesorption and X-ray photoelectron spectroscopy (XPS) analyses were used to characterize the structural properties, basic properties and chemical states of the prepared catalysts. The FAME composition and chemical functional groups of the feed SO-FAMEs and resultant partially hydrogenated biodiesels (PH-biodiesels) were analyzed using gas chromatography with flame ionization detection (GC-FID) and attenuated total reflectance-Fourier transform spectroscopy (ATR-FTIR), respectively. In particular, the fuel properties including the oxidative stability and cold flow properties were also determined according to the standard test methods.25,26

2. EXPERIMENTAL


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2.1. Materials

Palladium ammonium chloride (Pd(NH3)4Cl2.xH2O), which was used as the Pd precursor, was purchased from N.E. Chemcat Corporation, Japan. The SiO2 with a pore width of about 30 nm (Q30-SiO2), which was applied as the solid catalyst support owing to its highest hydrogenation activity in accordance with our previous researches,15,24 was obtained from Fuji Silysia Chemical Co. Ltd., Japan. The SO employed as a starting material for transesterification, was bought from Thai Vegetable Oil Co. Ltd. Methanol (analytical reagent grade) and potassium hydroxide (KOH; 85%) were purchased from RCI Labscan Limited, whilst n-heptane (analytical reagent grade) was obtained from Fisher Scientific. Other chemical reagents, including sodium sulphate anhydrous (Na2SO4; 99%), sodium nitrate (NaNO3; 99%), calcium nitrate (Ca(NO3)2.4H2O; 99%) and barium chloride (BaCl2.2H2O, 99%) were bought from Ajax Finechem Pty Ltd.

2.2. Catalyst Preparation

2.2.1. Pd/SiO2 catalyst The Pd/SiO2 was prepared using the incipient wetness impregnation method10 with a Pd loading of 1% by weight (wt.%). Firstly, the SiO2 support was dried in an oven at 110 °C overnight in order to remove any residual adsorbed water. An aqueous Pd(NH3)4Cl2 solution was then impregnated into the dried support under vacuum for 24 h, whereupon the catalyst was dehydrated using a vacuum oven at 110 °C for 6 h, calcined under an oxygen gas flow of 500 mL/min at 300 °C for 3 h, and then reduced under a H2 gas flow of 100 mL/min at the same temperature for 2 h.


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2.2.2. Pd-M/SiO2 catalyst (where M is Na, Ca or Ba) The Pd-M/SiO2 catalysts were prepared using the same method as for the Pd/SiO2 catalyst (section 2.2.1), but additionally with a 4 wt.% loading level of the respective basic metals. The aqueous solution containing the respective basic metal was impregnated into the calcined Pd/SiO2 under vacuum for 24 h, dried in a vacuum oven as in section 2.2.1, and then calcined at 400 °C for 3 h and reduced under a H2 gas flow of 100 mL/min at 400 °C for 2 h.

2.3. Catalyst Characterization

Crystallinity and composition in the crystalline phases of catalyst materials were determined by XRD analysis. The diffraction patterns were recorded on a Rigaku DMAX 2200HV at a scan speed of 1 °/min over a 2θ range of 10–90°. The X-ray source (Cu Kα radiation with a wavelength of 1.54 Å) was generated by a ceramic X-ray tube at 40 kV and 30 mA. The analysis was performed by placing the powder samples onto glass holders, which were then set at the center of the diffractometer. Surface basic site density and basic strength of catalyst samples were measured from CO2-TPD using a Thermo Finnigan TPDRO 1100 series with a thermal conductivity detector (TCD). The catalyst samples were pretreated with N2 at 300 °C for 1 h and then cooled to 30 °C. Thereafter, the samples were purged with CO2 at 30 °C for another 1 h to ensure the maximum adsorption. Subsequently, the CO2-TPD patterns were obtained under a helium (He) gas flow rate of 30 mL/min within the range of 30–950 °C, heating at 10 °C/min. The micro-structure, micro-texture and the composition of the phases of samples were characterized by TEM. The micro-photographs were obtained using a JEOL JEM-2100 model. Furthermore, energy-dispersive X-ray spectroscopy (EDS) was used for elemental identification. Typically, tiny amounts of powder samples were dispersed in ethanol and then


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placed in an ultrasonic sonicator for 30 min. A drop of the sample solutions was added on the holey carbon-covered grids and dried at room temperature (25 °C) for 16 h. Metal dispersion and metal particle size were determined by CO pulse chemisorption using an Ohkura Riken R6015 instrument. Firstly, the fresh catalysts were reduced under a H2 atmosphere at 300 °C for 4 h, and then were purged with He for cooling. The pretreated catalysts were sequentially pulsed by a 10% (v/v) CO in He flow until no more CO was adsorbed. The metal dispersions were calculated using a CO/Pd stoichiometric ratio of 1. Textural properties of the SiO2 support and the respective catalysts were analyzed using a Quantachrome Autosorb–1 MP surface area analyzer. The samples were first degassed at 250 °C for 12 h under a high vacuum condition (≤10 µm Hg/min) in order to eliminate the volatile species adsorbed on the sample surface. The specific surface area (SBET) was calculated by the Brunauer-Emmett-Teller (BET) model. The total pore volume (VTotal) was measured from the adsorbed N2 after complete condensation at the relative pressure of P/P0 = 0.995. The average pore diameter (DAvg) was obtained by the Gurwitch rule (DAvg = 4VTotal/SBET). Surface elements and chemical states of catalysts were identified by XPS, recorded on a Kratos Axis Ultra DLD photoelectron spectrometer equipped with a magnetic immersion lens and charge neutralizer. The analyses were performed under a high vacuum condition (≤5 x 10–7 Torr). A monochromatic Al Kα (hv = 1486.6 eV) source operated at 15 kV was employed as the X-ray radiation source. The recorded spectra were calibrated using the binding energy peak of C 1s at 284.6 eV.

2.4. Transesterification of SO to SO-FAMEs


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The SO (180 g) was added to a 500-mL three-necked round-bottom flask, and heated to 60 °C. Methanol (61 g) and 1.8 g KOH (1 wt.% compared to SO) were prepared by homogeneous mixing and then added into the heated SO (9:1 methanol: SO molar ratio) and maintained at this temperature for 1 h. The solution was then transferred into a separation funnel to allow phase separation of the glycerine and biodiesel overnight. The lower phase (glycerine) was removed and the upper phase (biodiesel) was extracted with distilled water at 60 °C several times to remove the excess methanol, KOH and possible soap. Lastly, the obtained SO-FAMEs, or biodiesel, was kept in a glass bottle containing anhydrous Na2SO4 for absorption the residual water.

2.5. Partial Hydrogenation of SO-FAMEs to PH-Biodiesel

Partial hydrogenation of the SO-FAMEs (from section 2.4) was performed in a 300-mL stainless-steel semi-batch reactor at 80 °C and 4 bar partial H2 pressure. Firstly, 1.304 g of reduced catalyst (1 wt.% compared to biodiesel) was put into the reactor containing 130.4 g of SO-FAMEs. The reactor was then purged with N2 to remove any leftover air, followed by H2 at a flow rate of 150 mL/min, maintaining the H2 pressure at 4 bar. A stirrer (500 rpm) was used to ensure a perfect mix between the feed SO-FAMEs, catalyst and H2. The temperature was gradually increased to the desired point, and subsequently the first hydrogenated SO-FAMEs was instantly collected. Subsequent PH-biodiesel sampling was performed every 30 min during the 4-h reaction time.

2.6. Feed SO-FAMEs and PH-Biodiesel Analysis


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The FAME composition of the feed SO-FAMEs and the derived PH-biodiesels were determined by GC-FID using a Hewlett Packard 5890 Series II and an HP–88 (100 m x 0.25 mm x 0.2 µm) fused-silica capillary column with He as the carrier gas at a flow rate of 70 mL/min. About 0.2 µL of sample was injected into the oven (155 °C) with an injector and detector temperature of 200 °C and 230 °C, respectively, with a split ratio of 75:1. After a 20min isothermal period, the temperature was increased to 220 °C at 2 °C/min and held at 220 °C for 10 min. The total analysis time was 62.5 min. The type and the amount of each FAME composition was identified by reference to the retention time and calculated from the fraction of area under the peak of interest, respectively. The chemical functional groups of the feed SO-FAMEs and PH-biodiesel samples were analyzed by ATR-FTIR using a Thermo Scientific iD7 ATR accessory for Thermo Scientific Nicolet iS5 Fourier transform infrared spectrometer. The liquid samples were placed onto the monolithic diamond crystal plate. The IR spectra were recorded in the range of 3500–650 cm–1 at a 4 cm–1 resolution with 32 scans. Oxidative stability of the feed SO-FAMEs and biodiesel products, corresponding to the induction period (IP; h), was measured by the Rancimat method according to EN 1421425 using a Metrohm 743 instrument. Feed SO-FAMEs and the PH-biodiesels (after a 4-h reaction time) were tested in heating blocks at 110 °C under an air flow rate of 10 L/h. During this time, the volatile compounds, which were the byproducts of fatty acid ester degradation, were released and recorded by the conductivity cells. The inflection point of the conductivity vs. time curve denotes the IP (h). The cloud point (CP) and pour point (PP) are the important cold flow characteristics used for determining the operating capability of fuels. The CP, which is the temperature at which wax crystals start to cloud the fuel, was evaluated in accordance with ASTM D2500.26 The PP, which is the lowest temperature at which a fuel becomes immovable making the fuel


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unusable, was investigated according to ASTM D97.26 The tests were performed using a CPP 5Gs automated CP and PP analyzer. Samples were put into the test jar and the temperature was continuously decreased. These characteristics were reported on screen to one decimal place.

3. RESULT AND DISCUSSION

3.1. Characterization and Properties of Each Catalyst

Representative XRD patterns of silica support and the reduced catalysts with different basic modifiers are presented in Figure 1. The characteristic peaks were determined according to ICDD reference standards, PDF-2/Release 2012 RDB. All diffraction profiles exhibited a broad peak around a 2θ of 22°, indicating the typical peak of amorphous SiO2 (DB 00-001-0649). Likewise, the four prominent peaks centered about a 2θ value of 40.1°, 46.5°, 68.1° and 82.2°, corresponding to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections of Pd0, respectively, were observed in all the Pd-based catalysts (DB 00-001-1201).10,21 Except for the presence of the distinctive Pd0 peaks, the other peaks from the different forms of the respective basic modifiers (Na, Ca and Ba) were also detected. In the case of Pd-Na/SiO2, there were four other weak reflections at a 2θ of 31.7°, 37.9°, 45.4° and 56.3°. The two former peaks were attributed to NaNO3 (DB 00-001-0840), while the two later peaks were assigned to NaNO2 (DB 00-001-0883). For Pd-Ca/SiO2, the three diffraction peaks located at a 2θ of 28.7°, 46.9° and 56.3°, attributed to Ca(NO3)2, were notably found (DB 00-0011215). In the case of Pd-Ba/SiO2, analogously, the four characteristic peaks of PdCl2 were detected at a 2θ value of 16.1°, 29.2°, 37.1° and 43.2° (DB 01-079-8422), while the other two


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observed reflections centered at a 2θ value of 21.9° and 34.8° corresponded to BaCl2 (DB 9009003). In accordance with these results, it was conspicuous that the compositions of the basic modifiers were related to the corresponding basic precursors (section 2.1), not metal forms, even though all the catalysts had been reduced prior to characterization and activity testing. This likely reflects that these basic compound precursors are difficult to eliminate from the catalyst surface at the temperature used for the calcination and reduction stages (section 2.2.2).27 Representative TEM micrographs of the Pd-based catalysts are illustrated in Figure 2. The bright region and the dark spot represent the SiO2 support and metal species, respectively. It was noticeable that the distribution of Pd on the SiO2 support was not strongly uniform, leading to broad size distribution (Figure 2A). In the case of the basic-promoted Pd catalysts, the types of metal were difficult to identify from the many dark spots of various dimensions (Figure 2B–D). To confirm the existence of basic metals, TEM-EDS analysis was applied. The EDS spectra of all the catalysts revealed the characteristic peaks of Si, O, Pd and Cu, referring to the SiO2 support, metal active compounds and copper grid, consecutively. Also, the basic modifiers (Na, Ca and Ba) could be detected in the catalysts (Figure 2B–D). Because the dark spots had unclear edges that varied considerably in size and shape, the metal particle sizes could not be precisely measured from these TEM images. Thus, CO pulse chemisorption was employed to determine the sizes of these metals. The metal dispersion (%) and particle sizes for all the catalysts are summarized in Table 1. The Pd dispersion and particle size for Pd/SiO2 were around 2.75% and 17.6 nm, respectively. The abundant surface silanol groups are generally considered to be beneficial to strengthen the interaction between SiO2 support and Pd particles, resulting in the reducing of Pd migration and further growth. Nevertheless, the existence of alkaline promoters caused the


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reduction of silanol group content, especially for Pd-Na/SiO2, leading to the weakening of the interaction between support and Pd particles and consequently increased Pd particle size. For Pd-Ba/SiO2, the metal dispersion level was slightly worsen compared with that of the unmodified Pd/SiO2 catalyst, attributing to an insignificant influence of basic covered catalyst surface on the SiO2-Pd interaction.28,29,30 This was supported by the presence of quite largedark regions in the TEM micrographs (Figure 2). In contrast, the metal dispersion level in PdCa/SiO2 was obviously increased (1.63-fold), whereas the metal particle size was correspondingly smaller, which was due to an appropriate basic content on catalyst surface.31,32,33 The CO pulse chemisorption result was in accordance with the well-dispersed dark spots on the SiO2 support in the TEM analysis (Figure 2C). To identify and quantify the surface basic sites of the catalysts, CO2-TPD was employed. The quantitative basic site densities, measured from the CO2-TPD profiles (Figure 3), of all the materials are summarized in Table 1. The absence of a CO2 desorption peak in the TPD profile implied the absence of basic sites in the pure SiO2 support (Figure 3a).34 After metal impregnation, all the catalysts presented different desorption peaks, indicating the presence of basic sites of different strengths. The CO2 desorption peaks at a temperature of less than 300 °C were assigned to the interaction of CO2 with sites of a weak basic strength which have been proposed to correlate with the OH− group on the surface.35−36 Weak basic sites were observed in the basic-promoted Pd/SiO2 catalysts, where desorption peaks within the 300–600 °C range were attributed to basic sites of medium strength, corresponding to Mn+-O2− pairs. The strong basic sites, which are related to isolated O2− anions, were located above 600 °C in the CO2-TPD profiles,35−37 and could be found in all the catalysts, which might be due to the catalyst reduction. The total basic densities of these materials were ranked in the order: Pd-Na/SiO2 > Pd-Ca/SiO2 >> Pd-Ba/SiO2 > Pd/SiO2 (Table 1). Thus, basic sites with different strengths on the catalyst surface could be


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customized using different kinds of modifiers. The unique basic properties of these catalysts would likely affect the way that FAMEs bound with the active species on the surface. The metal coverage of the SiO2 surface and M/Pd atomic ratio are tabulated in Table 1. The level of Pd located on each catalyst surface was slightly dissimilar within the range of 0.54–0.63 atom/nm2, albeit the same Pd loading level was applied for all the catalysts. This was due to the variety of catalyst surface areas with promoter addition (SBET; Table 1). For the basic-modified catalysts, the basic metal contents were correlated with the amount of the corresponding basic metal impregnation (4 wt.%), which resulted in an increment in the total content of basic metals in the order of Na (10.80 atom/nm2) > Ca (6.68 atom/nm2) > Ba (1.67 atom/nm2). Likewise, the atomic ratios of M/Pd were the same order as the basic metal contents covered on the SiO2 surface (M; Table 1). The N2 adsorption-desorption isotherms for the SiO2 support and Pd/SiO2-based catalysts are illustrated in Figure S1 in the Supplementary Information (SI). As depicted in Figure S1a, the SiO2 support distinctly exhibited a typical type IV isotherm with a H1 hysteresis loop, in accordance with the categorization of IUPAC. This hysteresis loop at a relative pressure (P/P0) range of 0.9–1.0 signified the large mesopores of the support. Nevertheless, the isotherms of these materials were not altered after catalyst preparation (Figure S1b–e; SI). The textural properties of the SiO2 support and Pd-M/SiO2 catalysts are summarized in Table 1, where all the catalysts presented a slightly lower SBET, VTotal and DAvg than those of the pure SiO2 support. This results from the metal deposition and the process of catalyst preparation. In brief, the textural properties, which are generally decisive for the properties of hydrogenation catalysts, might not be the predominant determinants for governing the hydrogenation activity of FAMEs in this study owing to the insignificant difference between all the catalysts.


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The surface elements and chemical states of the Pd-based catalysts were investigated by XPS. To examine the surface elements, the survey scan of XPS was commonly applied. As depicted in Figure S2 (SI), the interesting peaks of O 1s, Si 2p and Pd 3d could be visualized for all catalysts, suggesting the achievement of Pd impregnation. Likewise, the characteristic peaks of Na 1s, Ca 2s and Ba 3d could be detected in the corresponding basic-modified Pd/SiO2 catalysts (Figure S2B–D, SI). As for Pd-Ca/SiO2, it was noticeable that the identical peak of Ca 2p, which is generally located at a binding energy (BE) of around 346 eV, disappeared from the survey spectra (Figure S2C, SI), which was attributed to the low-intense peak along with the Pd 3d apposition (BE ~335 eV).22,38–40 However, the Ca 2p peak could be found and further inspected in the high-resolution XPS spectra (Figure 6B). In addition to the XPS survey-scan analysis, the narrow-scan technique was performed to determine the chemical states of catalysts. The XPS spectra with decomposition for Pd, O and the basic elements (Na, Ca and Ba) are illustrated in Figures. 4–6 in turn. The XPS spectrum of Pd/SiO2 (Figure 4A) clearly showed two pairs of doublets. The main doublet at 335.7 eV and 340.8 eV were assigned to Pd0 3d5/2 and Pd0 3d3/2, respectively, while the other doublet at 337.6 eV and 343.8 eV was attributed to PdO 3d5/2 and PdO 3d3/2, respectively, indicating that the Pd particles were exposed to atmospheric oxygen.22,40,41 As observed in Figure 4B–D, the presence of basic modifiers resulted in the electron transfer from the basic sites to the adjacent Pd atoms, accounting for the increased electron density of Pd and so the slight downshift of the binding energy of Pd 3d (Pd0 around 335.3–335.5 eV) in comparison with that of the metallic state (Pd0 at 335.7 eV). As for Pd-Na/SiO2 and Pd-Ca/SiO2 catalysts, the high-resolution XPS spectra were the same as Pd/SiO2 in which the two doublets of Pd0 and PdO were detected. In the case of Pd-Ba/SiO2, the presence of basic modifier (BaCl2) resulted in the formation of another chemical state, excluding Pd0 and PdO, which was PdCl2 owing to the high electronegativity of chlorine (Cl).


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The chemical states of O 1s were examined (Figure 5A–D), where the O 1s spectra were curve-fitted into three identical peaks. For all the catalysts, the two main peaks centered at a BE of 533.3 eV and 536.7 eV were attributed to the Si–O (amorphous SiO2 support) and O– Pd bonds, respectively,42–44 the latter being compatible with the Pd 3d spectra. In addition, two other weak peaks at a BE of around 535.3 eV and 531.4 eV were present in the catalysts and were attributed to oxygen within the surface hydroxyls (OH−) and the sub-oxide lattice (O−), respectively.42,45,46 The appearance of the OH− state might result from H2 adsorbed onto the surface oxygen ions during the catalyst reduction under a H2 atmosphere, whereas the O− state was commonly generated in the metal-rich environment due to the impregnation of various metal kinds.47 To investigate the environment of the basic modifiers in the catalysts, the decomposition of high-resolution XPS spectra was performed (Figure 6A–C). For all three basic-modified catalysts, the characteristic peaks were shifted to higher BEs than those reported in the literature,46–48 suggesting the relocation of electrons from the basic sites to the Pd atoms.22,41,51 For Pd-Na/SiO2, the XPS spectrum of Na 1s was curve-fitted into three peaks, attributed to Na2O, NaOH and NaNO3/NaNO2 (Figure 6A), whereas a doublet assigned to Ca(NO3)2 and BaCl2 was clearly evident for Pd-Ca/SiO2 and Pd-Ba/SiO2, respectively (Figure 6B and C). Here, the largest content was basic compound corresponding to the basic precursor instead of basic metal, despite the catalyst being reduced under a H2 gas flow.

3.2. Analysis of the Feed SO-FAMEs

The SO-FAMEs (from section 2.4) was applied as a starting material to study the consequence of partial hydrogenation using various types of catalysts. The synthesized SOFAMEs consisted of proximate 99.7% fatty acids (by proximate analysis), with the five main


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constituents being methyl linoleate (C18:2, 52.58%), (Z)-methyl oleate ((Z)-C18:1, 23.84%), methyl palmitate (C16:0, 11.66%), methyl linolenate (C18:3, 5.54%) and methyl stearate (C18:0, 3.73%). As mentioned previously (section 1), the high degree of unsaturation, from the C18:3 and C18:2 compositions, will cause a substandard oxidative stability, whereas a high level of saturation of biodiesel will result in poor cold flow properties. Therefore, the diminution of the unsaturated FAME contents without an increased saturated FAME composition is required using partial hydrogenation. The ATR-FTIR spectrum of the SO-FAMEs in the range of 3500–650 cm−1 is shown in Figure S3a (SI). No vibration band was detected in the 3500–3000 cm−1 range, attributed to the axial deformation characteristic of OH groups, which resulted suggested that the synthesized SO-FAMEs had no detectable level of water molecules and/or unreacted methanol.52 The band located at 3008 cm−1 was assigned to the =C–H stretching vibration of the (Z)-carbon-carbon double bond (alkene groups), indicating the existence of unsaturated FAMEs with a (Z)-configuration. The bands at 2922 cm−1 and 2853 cm−1 suggested the presence of methyl (CH3) or methylene groups in the ester chains of the SO-FAMEs. The sharpest peak, at 1741 cm−1, was attributed to C=O groups with a stretching mode of vibration. This result obviously supported the transesterification of the SO with methanol to FAMEs.52,53 The peaks at 1460 cm−1 and 1361 cm−1 were scribed to the bending vibrations of the CH3 and/or CH2 aliphatic groups, while the band located at 1436 cm−1 referred to the asymmetric stretching of O–CH3 in the methyl ester. The ATR-FTIR spectrum also exhibited peaks at 1244 cm−1, 1195 cm−1, 1169 cm−1 and 1016 cm−1, which were attributed to the stretching vibrations of C–O and C–O–C. More specifically, the vibration bands appeared at 1169 cm−1 and 1016 cm−1 corresponded to the C–O symmetric stretching vibration and C–O anti-symmetric stretching vibration, consecutively. The peak centered at 722 cm−1 was assigned to (Z) isomers of –CH, which were from the oleic and linoleic acid components of


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the SO.53,54 Nevertheless, no peak was observed at 968 cm−1, the C–H out of plane in (E)configuration, in SO-FAMEs.10 This result was in accord with the GC-FID analysis. In the case of total biodiesel yield, it was approximately 97.5% obtained from complete transesterification of soybean oil, attributing to a loss of the SO-FAMEs after washing as well as filtration from residual-water adsorbent.

3.3. Partial Hydrogenation of SO-FAMEs

The FAME composition during partial hydrogenation on the pure SiO2 support is shown in Figure S4 (SI), where the five lines were assigned to C18:0, C18:1, C18:2, C18:3 and other compositions. Noticeably, there was hardly any alteration in the FAME composition due to the lack of active species. Hence, the SiO2 support had no influence on this reaction. The hydrogenation activity of all the catalysts was examined in terms of the TOF using Eq. (1), which was compared at a diunsaturated FAME (C18:2) conversion level of 11% and 45%. TOF h = -1

C18:2 conversion×Amount of biodiesel g×(MW of C18:2)-1

metal loading×metal dispersion×Wcatalyst g×(MW of metal)-1

÷ time (1)

At an 11% C18:2 conversion level, where corresponded to the final reaction time (4 h) of the least active catalyst (Pd-Na/SiO2), the FAME compositions in the PH-biodiesel were evidently altered compared to those in the feed SO-FAMEs, with a diminution of the C18:3 and C18:2 contents, an increased C18:1 level and only a limited level of C18:0 formation (Table 2). Nevertheless, the alteration of the FAME compositions was slightly observed due to the low level of catalytic conversion. The TOF of the catalysts were ranked in the order: Pd-Ba/SiO2 >> Pd/SiO2 > Pd-Ca/SiO2 > Pd-Na/SiO2. This result indicated that Pd-Ba/SiO2 had the highest ability to decrease the C18:2 content within the least reaction time (~0.2 h). With respect to Pd/SiO2, the high hydrogenation activity obtained was due to its high


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hydrogen adsorption capacity.55,56 The two other basic metals (Ca and Na) revealed a worse hydrogenation activity than the unmodified Pd/SiO2 catalyst. At a 45% C18:2 conversion level, only two catalysts, Pd/SiO2 and Pd-Ba/SiO2, reached approximately 45% (Table 2), and Pd-Ba/SiO2 clearly had a the higher TOF (1.43-fold) than Pd/SiO2, which was due to the less-required reaction time. Nevertheless, the existence of a high C18:2 content was still noticed, resulting from the mild reaction temperature used. In the case of selectivity towards (E)-C18:1/(Z)-C18:1 ratio, it was noteworthy that the (E)-C18:1 selectivity was directly correlated with the catalytic activity, as described by TOF, at both C18:2 conversion levels (Figure 7A and B). The higher the TOF of catalyst, the larger was the (E)-C18:1 formation. At a C18:2 conversion around 11% (Figure 7A), the (E)-C18:1 selectivity, in terms of the ratio of (E)-C18:1 to (Z)-C18:1, of all the catalysts was obtained in the range of 0.04–0.05, indicating C18:1 formation with little (E)-isomer (≤1.5%). An increased level of (E)-C18:1 selectivity (~0.2) was clearly found at the higher C18:2 conversion level of 45%, which was due to the isomerization of the (Z)- to (E)-configuration during hydrogenation (Figure 7B). This result conformed well to that obtained from the ATRFTIR analysis (Figure S3, SI). It was notable that the C–H out of plane in (E)-configuration peak at 968 cm−1 was distinctly observed in most of the PH-biodiesels, especially for the PdBa/SiO2 and Pd/SiO2 catalysts (Figure S3c and f, SI).10 Next, the function of the basic modifiers (Na, Ca and Ba) on the partial hydrogenation of the SO-FAMEs was further investigated and elucidated sequentially (section 3.4).

3.4. Evaluation of the Role of the Basic Metal Modifiers (Ba, Ca and Na)

The SO-FAMEs contained many double bonds located in various positions, where the hydrogenation reaction can take place. In this sense, the kind of active species and the


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polarity of the catalyst surface are considerably influential.21 Here, Pd0 was applied as an active species for hydrogenation due to the fact that the dissociative adsorption of H2 molecules onto Pd surfaces occurs with little or no activation energy barrier.57 As a result, hydrogenation using Pd catalysts takes place rapidly compared to that with other metals. In addition to the active metal species, the polarity of the catalyst surface is an important feature that affects the way molecules approach the hydrogenation sites. Therefore, the acid-base nature of a catalyst affects its hydrogenation activity and favors the selective production of hydrogenated compounds.21 Three types of basic metals (Na, Ca and Ba) were evaluated for their modulation of the catalytic function of Pd/SiO2 on the partial hydrogenation of SO-FAMEs. As expected, the hydrogenation activities obtained were visibly different for the different base metal doped Pd/SiO2 catalysts (Figure 7A and B), being ranked in the order: Pd-Ba/SiO2 >> Pd-Ca/SiO2 > Pd-Na/SiO2, which was principally due to the dissimilarity of their basic sites together with the basic compounds on the catalyst surface. The hydrogenation procedure can be described as follows. The polyunsaturated FAMEs adsorb over the surface of the SiO2 support and Pd0, where acidic sites (protonic sites) on the SiO2 support are preferable for their adsorption due to the fact that unsaturated C=C bonds are more basic (electron-rich) in nature. These C=C bonds in the FAMEs were substituted by hydrogen atoms and then the adsorbed-reacted molecules were desorbed from the SiO2 surface and Pd0. At this point, the SiO2 support covered with acidic sites might enhance the C=C adsorption as well as hydrogenation activity.58 The high performance of pure SiO2 supported Pd0 was clear at both C18:2 conversion levels (Figure 7A and B). The addition of alkaline and alkaline earth metals strongly influenced the SiO2 surface and Pd particles. In terms of the SiO2 support, the presence of the basic metal component caused an increase in the basic properties of the SiO2 surface through a decrease in the


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number of acidic sites and so would weaken the interaction of the C=C bonds with the SiO2 surface. For Pd particles, electrons from the basic catalyst surface were transferred to the adjacent Pd particles resulting in an increased electron density of Pd, confirmed by the XPS results (section 3.1). Simultaneously, the electron-rich Pd particle weakened the adsorption of reacted FAME molecules, making them easier to be desorbed from the catalyst surface and so, as a result, increased opportunity for other C=C bonds of FAME molecules to be hydrogenated. For Pd-Na/SiO2, with the lowest catalytic performance, this was due to it having the highest basic sites on the catalyst surface (10.8 atom/nm2). The high content of basic Na might considerably decrease the acidic sites over SiO2 and so restrict the adsorption of C=C bonds. Likewise, Pd-Ca/SiO2 exhibited a lower TOF compared to the unmodified Pd/SiO2 as a result of the high level of basic sites covering the catalyst surface (6.68 atom/nm2). In addition to the electronic effect, another possible influence on the catalyst’s performance was the vicinity of Pd particle to the alkali metal particle, as demonstrated by the M/Pd atomic ratio. As noticed in Table 1, Pd-Na/SiO2 had the highest Na/Pd atomic ratio compared to the other two basic-modified catalysts, indicating that an approach of the large FAME molecule to the hydrogenation site might be impeded by the surrounding Na components.20,21 Therefore, it was not surprising that Pd-Na/SiO2 had the lowest hydrogenation activity in this study. For Pd-Ba/SiO2, the Ba components only slightly altered the acidic properties of the SiO2 support due to it having the lowest density of basic sites (1.67 atom/nm2). Thus, the adsorption of C=C bonds was nearly the same as on the Pd/SiO2. However, some of the Ba atoms were in close contact with Pd particles and so could transfer electrons to those Pd sites. As described earlier, these electron-rich Pd particles would then weaken the adsorption of reacted FAME molecules leading to their being desorbed more easily. In the same way as the


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other two basic-modified catalysts, the high hydrogenation activity of Pd-Ba/SiO2 could be observed as a result of the number of basic components surrounding Pd particles (Ba/Pd~3.1). The small amount of Ba components might not hinder the access of FAME molecules to the Pd active site and so the hydrogenation activity was quite high. For the catalysts selectivity, the lowest (E)-C18:1 formation was found using the PdNa/SiO2 at a 11% C18:2 conversion level (Figure 7A), due to its highest basic coverage on the catalyst surface. The weakened adsorption of C=C bonds on the catalyst would result in a lower level of (E)-isomer formation.59 The selectivity at a 45% C18:2 conversion level resulted in a higher level of (E)-C18:1 with the Pd-Ba/SiO2 catalyst than with Pd/SiO2 (Figure 7B). The effect of the basic modifiers on the partial hydrogenation of SO-FAMEs is summarized in Figure S5 (SI) as the correlation between the polyunsaturated FAME conversion level and the selectivity towards (E)-C18:1 (in terms of the ratio of the (E)-C18:1 to (Z)-C18:1 content) after a 4-h reaction time. This correlation indicated that the improvement in the hydrogenation level using basic modifiers needs to consider the kind of basic metal as well as its nature. The impregnation of basic metals at too high a level of basic sites, i.e. with Na and Ca, did not enhance the hydrogenation activity compared to the unmodified Pd/SiO2 catalyst. In contrast, a better catalytic activity could be obtained when an appropriate basic modifier was incorporated, as demonstrated by Pd-Ba/SiO2 for Ba. Nevertheless, the selectivity towards (E)-C18:1, which is the less favorable isomer, was produced at the highest level with the Ba-modified catalyst in this study. In addition, leaching contents of Na, Ca and Ba in the PH-biodiesels obtained from inductively coupled plasma optical emission spectrometry (ICP-OES) technique showed that the metal leaching was negligible (

Influence of Alkaline and Alkaline Earth Metal Promoters on the

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