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. 2020 Mar 23;5(12):6956–6966. doi: 10.1021/acsomega.0c00326

Solvent-Free Hydrodeoxygenation of Triglycerides to Diesel-like Hydrocarbons over Pt-Decorated MoO2 Catalysts

Sisira Fangkoch †, Sutida Boonkum †, Sakhon Ratchahat †, Wanida Koo-amornpattana †, Apiluck Eiad-Ua ‡, Worapon Kiatkittipong §, Wantana Klysubun ∥, Atthapon Srifa †,*, Kajornsak Faungnawakij ⊥, Suttichai Assabumrungrat #
PMCID: PMC7114607  PMID: 32258932

Abstract

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In the present work, the solvent-free hydrodeoxygenation of palm oil as a representative triglyceride model compound to diesel-like hydrocarbons was evaluated in a batch reactor using Pt-decorated MoO2 catalysts. The catalysts with various Pt loadings (0.5–3%) were synthesized by an incipient wetness impregnation method. The metallic Pt and MoO2 phases were detected in the XRD patterns of as-prepared catalysts after the reaction and acted as active components for the deoxygenation reactions. The XPS experiments confirmed the existence of metallic Pt and PtOx species. The XANES investigation of Mo L3-edge spectra elucidated a change in the valence state by the transformation of MoO3 into MoO2 species after the deoxygenation reaction. The TEM observation revealed the formation of Pt nanoparticles in the range of 1–3 nm decorated on MoO2 species. The number of acid sites increased with stronger metal–support interactions on increasing the Pt loading. The catalytic performance of the MoO2 catalyst significantly improved with a small amount of Pt decoration. However, the further increase in Pt loading did not relatively increase the deoxygenation activity due to the formation of the agglomerated Pt particles. The high performance of the decorated catalysts could be attributed to the moderate acidity from the Pt dispersed on MoO2 toward decarbonylation and decarboxylation reactions.

1. Introduction

Currently, the utilization of bioresources from agricultural feedstocks for alternative biofuel production has been recognized as an effective process due to the depletion of fossil fuel and global environmental problems. Compared with the various types of bioresources such as lignocellulosic biomass and carbohydrates, vegetable oils, fatty acids, and animal fats have been generally used as starting materials for biofuel production due to their low functionalization degree and simple chemical structure compared with lignocellulosic materials.1,2 Diesel-like hydrocarbons produced via hydrodeoxygenation of triglycerides and fatty acids from agricultural oil feedstock have been recommended as a sustainable replacement for diesel-based petroleum because their fuel properties are comparable to those of the fossil diesel.3−6 Generally, the deoxygenation of oil feedstock containing triglycerides and/or fatty acids in the presence of hydrogen and suitable catalysts could produce diesel-like n-alkanes under moderate conditions (temperature 300–400 °C and H2 pressure 20–50 bar).1,7−12 Hydrodeoxygenation (HDO), decarboxylation (DeCO2), and decarbonylation (DeCO) are the three major reaction pathways occurring in the deoxygenation process.13,14 HDO leads to the elimination of oxygen by generating water and produces n-alkanes with a carbon number similar to that of the fatty acid component in the oil feed. DeCO and DeCO2 pathways result in the oxygen removal by producing the carbon monoxide and water or carbon dioxide, respectively, with the n-alkanes shorter by one carbon atom compared with the original fatty acid.

Various transition metal and metal sulfide catalysts have been utilized for the hydrodeoxygenation of triglycerides, fatty acids, and other oxygenated biomass compounds, especially in liquid-phase reactions under a hydrogen atmosphere such as NiMoS2,15−17 CoMoS2,16,18 Ni,1,7,19−23 Co,1,7,24 Pd,19,25−27 and Pt1,27−29 supported on various materials such as Al2O3, TiO2, SiO2, zeolites, and carbon. In the case of transition metal sulfide catalysts, the sulfiding of the oil feedstock is required to retard the catalyst deactivation.24,30 In particular, sulfur leaching due to the small amount of water generated from HDO and DeCO reactions leads to a shortened catalyst lifetime and sulfur contamination in the liquid product.31 Among the other catalysts, sulfur-free monometallic catalysts such as Ni- and Co-based catalysts exhibited high activity toward DeCO2 and DeCO reactions compared with the NiMo and CoMo sulfides supported on Al2O3 catalysts. However, the formation of a large amount of coke during deoxygenation was the major reason for catalyst deactivation.7 In addition, precious-metal-based catalysts such as Pt and Pd have been extensively investigated for hydrodeoxygenation with and without a solvent due to their high activity under mind conditions.27,32−35 In fact, the Pt- and Pd-based catalysts are favorable for DeCO and DeCO2 routes with a lower hydrogen consumption than for the HDO route.1 Thus, the utilization of these kinds of catalysts under solvent-free conditions and limited hydrogen supply in a batch reactor is crucial for further investigation. Recently, transition metal oxides such as MoO3–x have been reported as active components for oxygen removal from small oxygenated compounds, mainly palmitic acid and oleic acid via HDO under a moderate hydrogen pressure.35−39 The oxygen vacancies on the MoO3–x surface occurred by the reaction between hydrogen and the generated water via HDO, and DeCO has been reported as the active site for breaking the C–O bond.36 In particular, molybdenum dioxide, MoO2, has been confirmed to be an active component for the C–O and C=O breakings for palmitic acid hydrodeoxygenation.37,38 Nevertheless, the deoxygenation of triglycerides using molybdenum oxides under solvent-free conditions was limited because of the complexity of the reaction pathway via the hydrogenation and hydrogenolysis reactions to produce fatty acids and, subsequently, n-alkane production through HDO, DeCO, and DeCO2 reactions. To the best of our knowledge, the investigation of the effect of Pt-decorated molybdenum oxide in the form of MoO2 on the solvent-free hydrodeoxygenation of triglycerides has never been reported in the literature.

Therefore, we investigated the solvent-free hydrodeoxygenation of triglycerides to diesel-like hydrocarbons over the Pt-decorated MoO2 catalysts in a batch reactor. Extensive characterizations of catalysts using N2 sorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), NH3 temperature-programmed desorption (NH3-TPD), H2 temperature-programmed reduction (H2-TPR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were carried out to comprehend the correlation between the structural and textural properties and deoxygenation activities of the synthesized catalysts. Their catalytic performances were systematically compared in terms of the triglyceride conversion, gasoline and diesel yields, the relative involvement of HDO (hydrodeoxygenation) and DeCO/DeCO2 (decarbonylation and decarboxylation) activities, and n-alkane product distribution.

2. Results and Discussion

2.1. Textural Properties by BET

The N2 sorption experiments were first conducted to investigate the textural properties of the calcined samples. The N2 adsorption and desorption isotherms of all of the samples are exhibited in Figure 1a. According to the IUPAC classification, it can be found that both the bare MoO3 and Pt-decorated MoO3 samples exhibited a type II isotherm with H3-type hysteresis loops. The pore size distribution calculated from the adsorption branches of the isotherms using the BJH model reveals the presence of uniform mesopores and an average pore diameter for all of the samples except for the 3%Pt-decorated MoO3 catalyst (Figure 1b).

Figure 1.

Figure 1

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(a) N2 adsorption and desorption isotherms and (b) pore size distribution of the MoO3, 0.5%Pt/MoO3, 1%Pt/MoO3, 2%Pt/MoO3, and 3%Pt/MoO3 catalysts.

Table 1 summarizes the specific surface area, average pore diameter, and pore volume of the calcined samples. The BET surface area and pore volume of the bare MoO3 were 3.63 m2 g–1 and 0.0039 cm3 g–1, respectively. A small increase in the surface area from 3.91 to 4.57 m2 g–1 with a larger pore volume in the range of 0.0043–0.0065 cm3 g–1 was observed on increasing the Pt loading from 0.5 to 2%. The pore diameter was in the range of 43–57 nm for the Pt loading in the range of 0.5–2%. Interestingly, an increase in the Pt loading of up to 3% led to a significant increase in the BET surface area of up to 22.46 m2 g–1 accompanied by the highest pore volume of 0.1217 cm3 g–1. This may be due to the formation of small pores with an average pore diameter of 21.7 nm, resulting in the enlargement of the catalyst surface area.

Table 1. Textural Properties and Acidity of Different Catalysts.

catalyst BET surface areaa (m2 g–1) total pore volumeb (cm3 g–1) average pore diameterc (nm) acidityd (μmol NH3 gcat–1)
MoO3 3.63 0.0039 43.2 24
0.5%Pt/MoO3 3.91 0.0043 43.9 50
1%Pt/MoO3 3.88 0.0042 43.3 72
2%Pt/MoO3 4.57 0.0065 57.0 141
3%Pt/MoO3 22.46 0.1217 21.7 152
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a

BET surface area determined from the adsorption branch of the N2 isotherm.

b

Total pore volumes determined from the N2 adsorption at a relative pressure of 0.990.

c

Average pore diameter determined from the desorption branch using the BJH method.

d

Acidity determined by NH3 temperature-programmed desorption in the range of 70–275 °C.

2.2. Phase Identification by XRD

The XRD patterns of all of the fresh catalysts are exhibited in Figure 2. The XRD analyses confirmed the formation of the α-MoO3 phase (JCPDS No. 05-0508) after the calcination of ammonium heptamolybdate in the air at 500 °C for 5 h. The decoration of Pt species into as-synthesized MoO3 samples resulted in the existence of the diffraction peak of metallic Pt at 2θ = 39.8° to the (111) plane of the face-centered cubic structure (PDF#04-0802).40 The disappearance of the diffraction peak at 2θ = 46.2° corresponding to the (200) plane of Pt was due to the overlap with the diffraction peak of MoO3. As demonstrated in Figure 2, the diffraction peak intensity of Pt seemed to be increased with Pt loadings, implying the larger catalyst particle formation.

Figure 2.

Figure 2

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XRD patterns of the fresh (a) MoO3, (b) 0.5%Pt/MoO3, (c) 1%Pt/MoO3, (d) 2%Pt/MoO3, and (e) 3%Pt/MoO3 catalysts.

The catalysts after the reaction tests were subsequently characterized by XRD to identify the active components for the hydrodeoxygenation reaction (Figure 3). Interestingly, the diffraction peaks at 2θ = 26.3, 37.2, 54.4, 60.2, and 67.5° assigned to the planes (011), (020), (022), (031), and (−231) of the MoO2 phase, respectively (JCPDS: 65-1273),37 were observed for all of the spent catalysts. It should be deduced that MoO3 was completely in situ reduced to MoO2 during the reaction. This was similar to the previous study where the MoO3 was transformed into the MoO2 phase at 400 °C under the high pressure of H2.41 Furthermore, the diffraction peak of metallic Pt at 2θ = 39.8° (111) still presented after the reaction and metallic Pt acted as an active site in hydrodeoxygenation reactions. In the case of the 0.5%Pt/MoO3 sample, the diffraction peak of metallic Pt could not be detected due to the low amount of Pt loading. Similar to the fresh catalysts, the diffraction peak of Pt(111) observed in decorated samples became shaper, indicating the growth of metal particles with increasing Pt loading.

Figure 3.

Figure 3

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XRD patterns of the spent (a) MoO3, (b) 0.5%Pt/MoO3, (c) 1%Pt/MoO3, (d) 2%Pt/MoO3, and (e) 3%Pt/MoO3 catalysts.

2.3. Pt Surface Investigation by XPS

The XPS analysis was conducted to confirm the form of Pt species before and after the reaction. The XPS survey spectra of the fresh and spent 1%Pt/MoO3 samples show characteristic peaks of O, C, Pt, and Mo elements (see Figure S1a,b). The presence of carbon could be ascribed to the sample preparation from the carbon tape or coke deposition of the spent sample. As displayed in Figure 4, the XPS profiles of Pt 4f spectra of the fresh and spent samples exhibited the characteristic peaks of Pt 4f as two distinguished peaks of Pt 4f7/2 and Pt 4f5/2 doublets for both samples, indicating that the Pt species predominately existed on the MoO2 species. The peaks were deconvoluted into two oxidation states of metallic Pt0 and oxidized Pt2+. In the case of fresh 1%Pt/MoO3, two peaks occurred at 71.3 and 73.4 eV for Pt0 and those of Pt2+ appeared at 72.7 and 76.2 eV. The 4f peaks of Pt0 for the spent 1%Pt/MoO3 appeared at 70.2 and 73.5 eV and those of Pt2+ occurred at 71.4 and 74.7 eV. The shift observed in 4f peak positions of Pt in the presence of the spent 1%Pt/MoO3 to lower binding energies was due to a change in the electronic structure of metal oxide to metallic species during the deoxygenation. A shift in electron transitions to lower binding energies was reported for the supported Pt–WOx42 and Pt–MoOx35 catalysts. As summarized in Table 2, the presence of a high amount of the Pt0 state was observed for the spent 1%Pt/MoO3, confirming that the surface PtOx species were in situ reduced to metallic Pt in the presence of H2 at 400 °C.

Figure 4.

Figure 4

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XPS profiles of the Pt 4f spectra of (a) fresh 1%Pt/MoO3 and (b) spent 1%Pt/MoO3.

Table 2. Peak Assignment, Binding Energy, and Relative Area of Deconvoluted Peaks from Pt 4f Spectra of the Fresh and Spent 1%Pt/MoO3 Catalysts.

catalyst peak binding energy (eV) relative area (%)
fresh 1%Pt/MoO3 Pt0 71.3/73.4 64.4
Pt2+ 72.7/76.2 35.6
spent 1%Pt/MoO3 Pt0 70.2/73.5 85.9
Pt2+ 71.4/74.7 14.1
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2.4. Mo Oxidation State by XANES

To further investigate the oxidation state of Mo in MoO3 and 1%Pt/MoO3 samples before and after the deoxygenation experiments, the X-ray absorption near-edge structure (XANES) analysis was conducted. Figure 5 exhibits the normalized and first-derivative Mo L3-edge XANES spectra of all of the Mo standards and the MoO3 and 1%Pt/MoO3 catalysts before and after the deoxygenation reaction. The edge position, white line, and spectral features were used to identify the form of samples compared with the Mo foil, bulk MoO2, and bulk MoO3 standards. As demonstrated in Figure 5, the edge and shape of the spectra of the as-prepared MoO3 and 1%Pt/MoO3 samples by the decomposition of ammonium heptamolybdate at 500 °C have almost the same characteristic features as the bulk MoO3 standard, indicating the complete transformation of ammonium heptamolybdate into the bulk MoO3 sample. This observation is in agreement with the XRD analysis of all of the fresh samples. In contrast, it was found that the adsorption edges of the MoO3 and 1%Pt/MoO3 catalysts after the deoxygenation reaction shifted to lower energy. As demonstrated in the first-derivative spectra in Figure 5, the transformation of MoO3 to MoO2 forms was clearly detected by the spectral features and edge position. These results implied that a change in the valence state occurred during the deoxygenation process and the MoO2 form acted as an active component for the reaction.

Figure 5.

Figure 5

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Normalized and first-derivative Mo L3-edge XANES spectra of (a) Mo foil, (b) standard MoO2, (c) standard MoO3, (d) fresh MoO3, (e) fresh 1%Pt/MoO3, (f) spent MoO3, and (g) spent 1%Pt/MoO3 catalysts.

2.5. Support and Metal Interaction by H2-TPR

The H2 temperature-programmed reduction (H2-TPR) experiments were conducted to investigate interactions of the Pt and Mo species using calcined samples (Figure 6). The H2-TPR profile of MoO3 exhibited two major reduction peaks in the wide temperature range of ca. 630–900 °C, indicating the reduction behavior of pure MoO3 to metallic Mo. The sharp peak at ca. 780 °C is due to the reduction of MoO3 to MoO2 and the broad peak at a higher temperature of ca. 855 °C is assigned to the reduction of MoO2 to metallic Mo, which was consistent with the previous study.43 The H2-TPR profiles of all of the Pt-decorated MoO3 catalysts showed a reduction peak at around 260 °C, confirming the characteristic reduction of surface PtOx, which was detected by the XPS analysis. A H2 consumption at ca. 500 °C for all of the decorated samples might correspond to the H2 spillover from the Pt species to MoO3 resulting in the partial reduction to MoO3–x. This suggested that the Pt loading had strong metal–support interactions.44,45 In addition, the reduction peak of MoO3 to metallic Mo at high temperatures in the wide range of ca. 700–900 °C was quite different from that of bare MoO3, suggesting that the Pt species changed the reduction behavior of MoO3 species. Although the reduction peaks of MoO3 to MoO2 occurred at temperatures higher than 700 °C, the MoO2 phase detected after the reaction (see in Figure 3) was attributable to the severe operating conditions (T, 400 °C; initial H2P, 40 bar; and time, 3 h). As summarized in Table S1, it was found that the calculated H2 consumption from the characteristic reduction of PtOx was in the range of 0.82–0.96 mmol gcat–1 and the highest value was obtained for the 1%Pt/MoO3 sample. The lower H2 consumption for the 2 and 3%Pt/MoO3 samples was expected to be due to the lower metal dispersion and agglomeration of the higher PtOx contents. This observation was in agreement with a former investigation.46

Figure 6.

Figure 6

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H2-TPR profiles of (a) MoO3, (b) 0.5%Pt/MoO3, (c) 1%Pt/MoO3, (d) 2%Pt/MoO3, and (e) 3%Pt/MoO3 catalysts.

2.6. Acidity by NH3-TPD

The acidity of the bare MoO3 and Pt-decorated MoO3 catalysts was examined using NH3 temperature-programmed desorption (NH3-TPD). The catalysts were first in situ reduced in the presence of H2 at the same reaction temperature to change the surface PtOx to metallic Pt before NH3-TPD implementation. As exhibited in Figure 7, it was found that the NH3 desorbed in the wide temperature range of 70–250 °C for the bare MoO3 and 0.5–2%Pt-decorated MoO3 catalysts and at 100–275 °C for the 3%Pt-decorated MoO3 catalyst. These characteristic desorption peaks were ascribable to NH3 desorption on acidic sites of the pre-reduced catalysts. The peak intensity of ammonia desorption for 0.5–3%Pt-decorated MoO3 catalysts was much higher than that for bare MoO3. This NH3 desorption behavior implied that the number of acidic sites on the catalyst surface relatively increased by the decoration of Pt species into the MoO3 species. In addition, the desorption peak of ammonia for the 3%Pt-decorated MoO3 catalyst shifted to higher temperatures, indicating that the ammonia strongly adsorbed on the catalyst surface, implying stronger acidic sites generated for the higher Pt loading. Based on the literature, it was reported that the high hydrodeoxygenation activity was due to the synergetic effect between acidic and metallic sites.32 It was evident that the calculated number of acidic sites significantly increased with Pt loading and the calculated values using the ammonia standard are summarized in Table 1.

Figure 7.

Figure 7

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NH3-TPD profiles of (a) MoO3, (b) 0.5%Pt/MoO3, (c) 1%Pt/MoO3, (d) 2%Pt/MoO3, and (e) 3%Pt/MoO3 catalysts.

2.7. Morphology by SEM

SEM observations were performed to further study on the morphology of the as-prepared catalysts. The SEM images of the fresh and spent MoO3 and 1 and 3%Pt/MoO3 catalysts are represented in Figure 8. It can be seen that MoO3 consists of the angular-shaped particles with different particle sizes, similar to the previous study.47 The microstructures of the fresh catalysts did not change by the addition of Pt species into the MoO3 species. On the other hand, the irregularities of angular-shaped particles were detected, in particular, the observation of micropores for the Pt-decorated MoO2 catalysts (see Figure S2b,c) after the hydrodeoxygenation experiments. It has been reported that H2-reduced MoO3 catalysts were accompanied by an increased specific surface area, which was the highest when MoO3 was reduced to MoO2 by reduction degrees of 60–70% at 400 °C.48−50 This may be due to the formation of a porous structure inside the large angular-shaped particles observed in this present study. Moreover, as shown in Figure S3a,b, the results of EDS measurements of the prepared Pt-decorated catalysts before and after the reaction revealed the existence of Pt species on the MoO3 species.

Figure 8.

Figure 8

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Typical SEM images of the fresh (a) MoO3, (c) 1%Pt/MoO3, and (e) 3%Pt/MoO3 catalysts and the spent (b) MoO3, (d) 1%Pt/MoO3, and (f) 3%Pt/MoO3 catalysts.

2.8. Particle Size Distribution by TEM

The spent 1%Pt/MoO3 catalyst was selected to examine the particle size distribution of Pt by TEM measurements due to the highest activity among the other catalysts (Figure 9). TEM images exhibited the spherical shape and uniform dispersion of metallic Pt decorated on MoO2 species in the narrow range of 1–3 nm. Furthermore, the observed lattice fringes of ca. 0.0224, 0.225, and 0.234 nm correspond to the Pt(111) plane, indicating that the Pt species located on MoO2 species have a face-centered cubic (fcc) (111) lattice spacing.51 This was consistent with the XRD analysis. Moreover, some agglomerated Pt particles with an average metal particle of 3.3 ± 0.6 nm were detected in the fresh 3%Pt/MoO3 sample (see Figure S4).

Figure 9.

Figure 9

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Typical TEM images (a–c) and particle size distribution (d) of the spent 1%Pt/MoO3 catalyst.

2.9. Catalytic Performance of the Pt-Decorated MoO2 Catalysts

The solvent-free hydrodeoxygenation of refined palm oil as the representative triglyceride model compound was conducted under the following conditions; a temperature of 400 °C, an initial H2 pressure of 40 bar, and a reaction time of 3 h over the bare MoO3 and Pt-decorated MoO3 catalysts. The bare MoO3 catalyst was initially investigated by varying the reaction temperatures to 370, 400, and 430 °C to find a suitable reaction temperature since the temperature significantly affected the conversion and product yields. As demonstrated in Table S2, the triglyceride conversion was 100% for all of the temperatures, indicating that the triglyceride completely transformed into oxygenated intermediates and desired products. The liquid product obtained from the deoxygenation reaction at a reaction temperature of 370 °C became solidified under ambient conditions and mainly consisted of fatty acids and a small amount of fatty acid alcohols. This result was confirmed by GC–MS analysis. This result suggested that the hydrogenation of carbon double bonds in unsaturated triglycerides occurred, followed by the carbon–oxygen bond cleavage toward the hydrogenolysis of saturated triglycerides to generate the primary oxygenated intermediates (fatty acids and fatty acid alcohols) and propane.1,52 Subsequently, the produced fatty acids underwent hydrodeoxygenation (HDO), decarbonylation (DeCO), and decarboxylation (DeCO2) to generate n-alkanes. The gasoline and diesel yields obtained were only 1 and 3%, respectively, at 370 °C. The increase in the reaction temperature significantly improved the deoxygenation activities. As revealed in Table S2, the total product yields increased from 4 to 22.3% with the temperature increasing from 370 to 430 °C. However, the decrease in the diesel yield from 10.1 to 8.8% on increasing the temperature from 400 to 430 °C would be associated with the promotion of cracking due to the formation of light hydrocarbons at a high reaction temperature (430 °C). Therefore, the reaction temperature of 400 °C was chosen to investigate the effect of Pt decoration on MoO2 by varying the Pt loading.

Figure 10 shows the product yields and reaction rates over the bare MoO2 and Pt-decorated MoO2 catalysts. The solvent-free hydrodeoxygenation reaction over all of the catalysts resulted in a 100% conversion of triglycerides with different product yields (Figure 10a). Over all of the Pt-decorated catalysts, the diesel- and gasoline-range n-alkane yields significantly increased compared with that on bare MoO2, implying that the hydrodeoxygenation reaction proceeded over Pt metallic sites accompanied by MoO2 species in the presence of hydrogen. By comparing the catalytic activities with various Pt loadings, the hydrodeoxygenation activities in terms of total product yields decreased in the order of 1%Pt/MoO2 (56.4%) > 2%Pt/MoO2 (55.2%) > 3%Pt/MoO2 (54.5%) > 0.5%Pt/MoO2 (46.2%) > MoO2 (18.4%). It should be noted that the increase in Pt loading from 0.5 to 1% resulted in an increase of gasoline and diesel yields probably due to the sufficient Pt loading and cooperative effects between Pt and MoO2 phases. However, the further increase in the Pt loading from 2 to 3% seemed to decrease the product yields although the number of acidic sites significantly increased. This could be due to the fact that the large amount of Pt is likely to partially cover the active MoO2 sites, causing the agglomeration of larger Pt particles, resulting in a reduction of the n-alkane production rate (see Figure 10b), which was calculated based on the summation of desired products against the amount of catalyst and time.

Figure 10.

Figure 10

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(a) Gasoline and diesel yields and (b) reaction rates over the MoO2, 0.5%Pt/MoO2, 1%Pt/MoO2, 2%Pt/MoO2, and 3%Pt/MoO2 catalysts. All experiments were conducted at a temperature of 400 °C, an initial H2 pressure of 40 bar, and a reaction time of 3 h.

Furthermore, the reusability of the 1%Pt/MoO2 catalyst was investigated at a temperature of 400 °C, an initial H2 pressure of 40 bar, and a reaction time of 3 h. The spent catalyst was rinsed with hexane to remove the oil product and dried overnight at 100 °C before the second and third hydrodeoxygenation tests. It was found that the triglyceride conversion remained constant at 100%; meanwhile, the gasoline and diesel yields dropped after the third consecutive experiment (see Figure S5a,b). The degradation of the catalyst after the first run was likely due to the formation of organic impurities on the active sites during the deoxygenation under solvent-free conditions.53

To evaluate the hydrodeoxygenation behavior using the bare MoO2 and Pt-decorated MoO2 catalysts, the percentage relative involvements of HDO and DeCO/DeCO2 reactions are estimated and depicted in Figure 11a. As reported in the literature, DeCO and DeCO2 were the major reaction pathways compared with HDO over the Pt-based catalysts.1 It was found that the percentage relative involvement of DeCO/DeCO2 was significantly greater than that of HDO over all of the catalysts, suggesting that DeCO and DeCO2 were the major reaction pathways for all of the catalysts. By comparison with the percentage relative involvement of HDO, the bare MoO3 was highly selective to HDO among the other decorated catalysts. The reducible transition metal oxides such as MoO3 and MoO2 species have been suggested as the catalysts for C=O and C–O bond scission in oxygenated biomass compounds toward hydrodeoxygenation.37,38,54,55 In contrast, the addition of Pt into MoO2 resulted in a noteworthy increase in the percentage relative involvement of DeCO and DeCO2, indicating that the active Pt sites promoted the active MoO2 sites at higher Pt loading (Figure 11a). As summarized in Table S3, the C16 and C18 fatty acids are the major fatty acid components in the representative triglycerides used in this study. Therefore, as demonstrated in Figure 11b, the n-alkanes of n-C15 and n-C17 were found as the major constituents of the deoxygenated product due to the high selectivity of DeCO and DeCO2 reactions over the Pt-decorated MoO2 catalysts.

Figure 11.

Figure 11

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(a) Relative involvement of HDO and DeCO/DeCO2 and (b) n-alkane product distribution over the MoO2, 0.5%Pt/MoO2, 1%Pt/MoO2, 2%Pt/MoO2, and 3%Pt/MoO2 catalysts. All experiments were conducted at a temperature of 400 °C, an initial H2 pressure of 40 bar, and a reaction time of 3 h.

2.10. Deoxygenated Product Analysis by FTIR

To confirm the removal of oxygen atoms in refined palm oil, the functional groups of the liquid products were analyzed by Fourier transform infrared (FTIR) spectroscopy (see Figure 12). The peaks at 1704 cm–1 in the FTIR spectrum correspond to the carboxylic functional groups of fatty acids. On the other hand, the carbonyl groups and ester groups of triglycerides were detected in the FTIR spectrum at 1746 and 1165 cm–1, respectively.56 In the deoxygenation process, triglycerides were first hydrogenolyzed to fatty acids and further deoxygenated to n-alkane. Palmitic acid and stearic acid are major oxygenated intermediates detected by GC–MS analysis. The FTIR spectrum of refined palm oil exhibited the adsorption peaks corresponding to triglycerides. Interestingly, for the liquid product after the deoxygenation process, the peaks intensity of triglycerides and fatty acids in the FTIR spectrum significantly decreased with an increase in the Pt loading. This result confirmed that the triglyceride and fatty acids partially converted into n-alkanes under solvent-free conditions via DeCO and DeCO2 reactions.

Figure 12.

Figure 12

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FTIR spectra of (a) refined palm olein, (b) heptadecane standard, and the deoxygenated liquid product over the (c) MoO2, (d) 0.5%Pt/MoO2, (e) 1%Pt/MoO2, (f) 2%Pt/MoO2, and (g) 3%Pt/MoO2 catalysts. All experiments were conducted at a temperature of 400 °C, an initial H2 pressure of 40 bar, and a reaction time of 3 h.

3. Conclusions

We have successfully prepared the Pt-decorated MoO2 catalysts with various Pt loadings (0.5–3%) by an incipient wetness impregnation method and their catalytic activities for triglyceride hydrodeoxygenation were investigated without solvent addition in a custom-made shaking-batch-type reactor under the following operating conditions: temperature, 400 °C; initial H2 pressure, 40 bar; and reaction time, 3 h. Using XRD, XPS, and XANES investigations, it was confirmed that the active components for hydrodeoxygenation under solvent-free conditions were in the form of metallic Pt and MoO2 generated during the reaction. The number of acidic sites obtained from NH3-TPD significantly increased by the addition of Pt species into the MoO2 species. In addition, stronger metal–support integration was achieved by Pt decoration. It was found that the catalytic performance of MoO2 for triglyceride hydrodeoxygenation was significantly improved by the decoration of a small amount of Pt with the uniform distribution on the MoO2 phase. Moreover, a large amount of Pt species cause a slight decrease in product yields, suggesting that the excess Pt species are likely to partially cover the active MoO2 sites and cause the agglomeration of larger Pt particles. High decarbonylation (DeCO) and decarboxylation (DeCO2) activities, as the major pathways in the deoxygenation, were observed, attributed to the moderate acidity from the Pt dispersed on MoO2.

4. Experimental Section

4.1. Catalyst Synthesis

Molybdenum trioxide (MoO3) was synthesized by dissolving 10 g of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O, Carlo Erba, purity 99.0%) in 50 mL of deionized water. Subsequently, the resultant solution was heated and stirred at 110 °C until a solid powder formed. The obtained solid was combusted at 350 °C for 2 h. Finally, the obtained sample was calcined at 500 °C for 5 h in stagnant air.

The various Pt loadings (0.5–3%) decorated on as-prepared MoO3 catalysts were prepared by an incipient wetness impregnation method using the corresponding Pt precursor as 3.4 wt % diamminedinitritoplatinum(II) in dilute ammonium hydroxide (Sigma Aldrich). The samples were dried at 110 °C after impregnation overnight and subsequently calcined at 500 °C for 5 h in stagnant air.

4.2. Catalyst Characterizations

The specific surface area, total pore volume, and average pore diameter of the as-synthesized samples before the reaction were determined by a nitrogen adsorption and desorption technique at −196 °C (BEL Japan, Bel Sorp mini II). The samples were degassed, and the moisture content was removed at 150 °C for 12 h before N2 sorption implementation.

The phase identification of as-synthesized samples before and after the deoxygenation tests was confirmed by X-ray diffraction (XRD) performed on an X-ray diffractometer (D8 ADVANCE, Bruker, Ltd., Germany) using Cu Kα radiation over the range of 10° < 2θ < 80°.

The form of Pt decorated on MoO2 was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis ULTRADLD (Kratos) spectrometer equipped with an Al Kα radiation source. The C 1s feature at 284.8 eV was used to calibrate the binding energy of Pt 4f.

The X-ray absorption near-edge structure (XANES) was used to further investigate the Mo oxidation state. XANES spectra at the Mo L3-edge (2500–2550 eV) were acquired in the transmission mode at Beamline 8 of the Synchrotron Light Research Institute (SLRI), Thailand,57 using InSb(111) double-crystal monochromators for scanning the photon energy. Normalization of the XANES spectra was performed using the ATHENA program.58

The H2 temperature-programmed reduction (H2-TPR) was performed to investigate support–metal interactions using BELCAT-B Instruments, Japan, using approximately 100 mg of calcined samples. To remove the moisture content, the samples were first heated to 190 °C for 30 min under an inert gas before H2-TPR implementation. The reduction behavior was analyzed by the temperature-programmed method using a 5 vol % H2/Ar at a heating rate of 10 °C min–1 in the temperature range of 100–1000 °C. The H2 consumption was recorded using a thermal conductivity detector (TCD).

The acidity of the synthesized catalysts was determined by NH3 temperature-programmed desorption (NH3-TPD) technique using the same instrument as for H2-TPR. The samples were initially pre-reduced at 400 °C for 2 h in 5 vol % H2/Ar to transform the surface PtOx into Pt phases. NH3 adsorption was subsequently conducted for 2 h after cooling the samples to 50 °C in an inert gas, followed by the excess NH3 removal under the supply of He for 1 h. The temperature-programmed desorption was carried out in the range of 50–400 °C at a heating rate of 10 °C min–1 under the supply of He. The peak intensity from a thermal conductivity detector (TCD) was used to calculate the number of acidic sites from the amount of NH3 desorption compared with the NH3 standard.

The morphologies of the catalysts before and after the reaction were examined by field-emission scanning electron microscopy (FE-SEM) on a JEOL, JSM-7600F, instrument equipped with an energy-dispersive X-ray spectrometer (EDS; X-MaxN 50, Oxford Instruments).

The particle size distribution and morphology of the Pt-decorated catalyst after the reaction were observed by field-emission transmission electron microscopy (FE-TEM) at 300 kV on a JEOL, JEM-3100F, instrument. The sample preparation was performed by dispersing the catalyst powder in ethanol for 20 min under ultrasonic conditions and subsequently dropping the solution on a carbon-coated Cu grid at least three times.

4.3. Catalytic Deoxygenation Evaluation

A locally available refined palm oil with a low free-fatty-acid content was employed as a model compound for triglycerides, and their catalytic deoxygenations under solvent-free conditions were conducted in a stainless-steel shaking-batch-type reactor with an internal volume of 10 cm3. The fatty acid compositions of the feedstock were experimentally determined via the transesterification of refined palm oil with methanol over a sodium phosphate (NaPO4) catalyst corresponding to the fatty acid methyl ester (FAME) composition. The fatty acid compositions are listed in Table S3. In the deoxygenation activity evaluation, 2 g of refined palm oil with 20 wt % as-synthesized catalysts without the pre-reduction process was loaded into the stainless-steel reactor. Subsequently, the air inside the reactors was removed to prevent explosion and oil combustion by H2 purging three times, and they were pressurized to an initial H2 pressure of 40 bar. The sealed reactors were immediately placed into an electric furnace at an operating temperature of 400 °C for 3 h with a shaking speed of 150 rpm to minimize the effect of mass transfer resistance. After the hydrodeoxygenation test, the reactor was suddenly stopped by subjecting the reaction to initial conditions by quenching in an ice-cool bath. The experiments were conducted in duplicate and the average values are presented.

4.4. Analysis of the Deoxygenated Liquid Product

The deoxygenated liquid products were filtrated to remove the catalyst and were subsequently analyzed by a gas chromatography (GC) system equipped and a flame ionization detector (FID) (Clarus 580, Perkin Elmer) with a capillary column (DB-1HT, 30 m × 0.32 mm × 0.1 μm). The quantities of n-alkanes ranging from n-C8 to n-C18 and unreacted triglycerides (TGs) in the liquid product were calculated using the calibration curves of n-alkane and triglyceride standards. The GC conditions were similar to those of a previous study.1,7,11 The triglyceride conversion, gasoline yield, and diesel yield were defined by the following equations

4.4. 1
4.4. 2
4.4. 3

To directly evaluate the involvement of hydrodeoxygenation (HDO), decarbonylation, and decarboxylation (DeCO/DeCO2) reactions, the percent relative involvements of HDO and DeCO/DeCO2 reactions were calculated using eqs 4 and 5.

4.4. 4
4.4. 5

In addition, the n-alkane product distribution of each component was calculated according to a previous study52 using eq 6.

4.4. 6

Furthermore, to confirm the deoxygenation activities under solvent-free conditions, the functional groups of the refined palm oil, n-alkane standard, and products were identified by Fourier transform infrared (FTIR) spectroscopy (Jasco FT/IR 6800).

Acknowledgments

The authors acknowledge the financial support from the Agricultural Research Development Agency (Public Organization) (CRP6205012040). Also, the financial support from the Mahidol University and the “Research Chair Grant” National Science and Technology Development Agency (NSTDA) are gratefully acknowledged. We thank the Beamline 8 staff for experimental support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00326.

  • XPS survey spectra of the fresh and spent Pt-decorated catalysts, calculated H2 consumption from H2-TPR experiments, SEM images of the spent catalysts with high magnification, EDS measurements, TEM image of fresh 3%Pt/MoO3, effect of reaction temperature over the bare MoO3 catalyst, reusability study of the 1%Pt/MoO2 catalyst in the three consecutive experiments, and fatty acid composition of refined palm olein (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c00326_si_001.pdf (667.3KB, pdf)

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