Formation and activity of activated carbon supported Ni2P catalysts for atmospheric deoxygenation of waste cooking oil

Fuel Processing Technology 185 (2019) 117–125

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Research article

Formation and activity of activated carbon supported Ni2P catalysts for atmospheric deoxygenation of waste cooking oil

T

Le Kim Hoang Phama, Thi Tuong Vi Trana, Suwadee Kongparakula, Prasert Reubroycharoenb, ⁎ Surachai Karnjanakomc, Guoqing Guand, Chanatip Samarta, a

Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathumthani 12120, Thailand Center of Excellence on Petrochemical and Materials Technology and Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand c Department of Chemistry, Faculty of Science, Rangsit University, Pathumtani 12000, Thailand d Institute of Regional Innovation, Hirosaki University, Aomori 030-0813, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Atmospheric-pressure deoxygenation Waste cooking oil Nickel phosphide Activated carbon

The atmospheric-pressure hydrodeoxygenation (HDO) of waste cooking oil (WCO) was investigated in a continuous fixed-bed reactor over a series of activated carbon (AC)-supported nickel phosphide catalysts with different initial Ni/P molar ratios (0.5–2.0) and nickel loading levels (1.16–38.90 mmol/g AC). The formation of the Ni2P phase on the AC, which was produced from commercial charcoal, as well as its structural and acidic properties was characterized by hydrogen-temperature programmed reduction (TPR), X-ray diffraction analysis, N2 adsorption–desorption measurements performed at −196 °C, and ammonia-temperature programmed desorption. The effects of the Ni/P molar ratio, nickel loading level, reaction temperature, and gas hourly space velocity (GHSV) on the catalytic activity were elucidated. The complete formation of the Ni2P phase on the AC was observed at a Ni/P ratio of 1.5, while smaller Ni2P crystallite sizes were observed at lower Ni/P ratios. In addition, it was observed that the acidity increased and the specific surface area decreased with an increase in the nickel loading level, presumably because nickel phosphate is not readily reduced to Ni2P. The 5.37-Ni2P/1.5TPR catalyst (Ni loading level of 5.37 mmol/g AC and Ni/P molar ratio of 1.5) exhibited good activity and stability during the HDO of WCO. The high-quality deoxygenated product primarily consisted of n-alkanes at the moderately high temperature of 300 °C and GHSV of 2.33 min−1. Based on the results, we propose that the mechanism underlying the hydrotreatment of WCO involves hydrogenolysis, hydrodeoxygenation, dehydrationdecarbonylation, and hydrogenation. To conclude, the synthesized Ni2P/AC catalyst could readily deoxygenate WCO at atmospheric pressure, producing n-paraffins as the primary component.

1. Introduction Currently, increases in the demand for energy owing to global economic expansion and technological developments as well as because of rapidly increasing population levels are draining the reserves of nonrenewable fossil fuels, causing sharp increases in petroleum prices. These factors are also accelerating climate change and could potentially lead to energy scarcity. Against this background, biofuels, which are fuels derived from biomass and waste, have been proposed as renewable alternatives to fossil fuels [1]. Among the range of biofuels being explored, biodiesel derived from waste cooking oil (WCO), a secondgeneration biofuel, has the greatest potential for replacing fossil diesel owing to its sustainable renewability, mild synthesis conditions, and

⁎

lack of adverse effects on other industries [2]. However, its low stability, high viscosity, and low calorific value, caused by the presence of oxygenates and, in particular, carboxyl compounds, have prevented it from being used directly as a commercial fuel. Therefore, it requires an additional treatment involving catalytic hydrodeoxygenation (HDO) [3]. Nickel phosphide has been used as a catalyst for the HDO reaction, as it is cheap, exhibits high catalytic performance with respect to the hydrotreatment reactions, such hydrodesulfurization and hydrodenitrogenation, and has a long lifetime [4–9]. In addition, Ni2P (one of the various nickel phosphide compounds) has bifunctional active sites [10–12]. Ni2P as catalyst performs just as well as the noble metals, including platinum and palladium, during HDO reactions [13]. When

Corresponding author. E-mail address: [email protected] (C. Samart).

https://doi.org/10.1016/j.fuproc.2018.12.009 Received 31 August 2018; Received in revised form 12 November 2018; Accepted 12 December 2018 0378-3820/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic of tubular fixed-bed reactor system used for HDO of WCO.

the desired reaction temperature, at which point they were considered to be ready for the deoxygenation of WCO. In addition, the catalyst samples were passivated for further characterization in a N2 flow for 4 h. The various samples are designated as “x-Ni2P/AC-y-TPR”, where “x” is the Ni loading level in mmol/g AC, “y” is the Ni/P molar ratio, and “TPR” is the Ni2P preparation method.

the HDO of guaiacol was investigated using various monometallic phosphides supported on silica, the catalytic performances in terms of the turnover frequencies could be placed in the following order: MoP < WP < Fe2P < Co2P < Ni2P [4]. Moreover, Ni2P/SiO2 exhibits high selectivity for n-C11 and n-C12 during the HDO of methyl laurate [14]. In addition, during the HDO of palmitic acid, bulk Ni2P and Ni2P supported on γ-Al2O3 were found to be suitable catalysts for the formation of pentadecane (C16) via decarbonylation and decarboxylation [15]. However, the HDO of complex feedstock, such as WCO, has not been studied in detail. In addition, most HDO reactions have been performed under extreme conditions, such as those involving high pressures. Therefore, HDO at atmospheric pressure needs to be investigated, as it would reduce the cost of the equipment required. Moreover, the process would also be safer. Pd supported on nitrogendoped porous carbon as catalyst has been used for the decarbonylation of vanillin at atmospheric pressure [16]. In this study, the catalytic activity of Ni2P with respect to the HDO of WCO at atmospheric pressure was investigated. Activated carbon (AC) produced from biochar was used as the catalyst support, and the Ni2P itself was prepared by the phosphidation of nickel phosphate. The effective utilization of both WCO and biochar is in keeping with the concept of green carbon science [17].

2.1.2. Characterization of catalyst samples Powder X-ray diffraction (XRD) analyses were performed to determine the crystal structures of the catalyst samples. The measurements (X'Pert, Philips) were performed using a Cu-Kα X-ray source with a wavelength of 1.54 Å. The radiation was generated at 40 kV and 20 mA, and the scans were performed for the range of 20–80° using a step size of 0.02° min−1. The crystallite sizes were calculated using Scherrer's equation. The reducibility of each catalyst sample was investigated by H2– TPR, which was performed in a quartz reactor by loading 0.1 g of the test catalyst in the presence of 5% (v/v) H2 in N2 at a flow rate of 30 mL/min and heating the sample from 100 to 900 °C at 10 °C min−1. The amount of H2 consumed was determined using a thermal conductivity detector. The acidic properties were characterized using ammonia (NH3)-temperature programmed desorption (TPD), which was performed using a TPD/R/O100a (BELCAT, BEL) system. The number of acid sites corresponded to the amount of desorbed NH3 over temperatures of 100–500 °C. The specific surface areas of the Ni2P/ AC catalyst samples were calculated using the Brunauer-Emmett-Teller (BET) method. The samples were pretreated to remove the moisture present and then allowed to adsorb N2 at −196 °C.

2. Experimental 2.1. Production of catalyst 2.1.1. Synthesis of AC and Ni2P/AC catalysts The AC was prepared by the chemical activation of biochar with potassium hydroxide (KOH). The biochar (obtained from the local community) was mixed with solid KOH in a ratio of 1:4 (w/w) in an agate mortar and activated at 700 °C (heating rate of 3 °C/min) for 2 h. AC was obtained after washing the sample with 10 wt% hydrochloric acid (HCl) until the pH became neutral and subsequently drying it at 120 °C for 3 h. The Ni2P/AC catalyst samples were prepared by temperature program reduction (TPR). First, 1 g of AC was wet-impregnated with a mixed solution of (NH4)2HPO4 and Ni(NO3)2·6H2O in different Ni/P molar ratios (0.5–2.0) and Ni loading levels (1.16–38.90 mmol Ni/g AC). The catalyst samples were then aged for 12 h at ambient temperature, dried at 110 °C for 12 h, and calcined and reduced in a single step at 600 °C (heating rate of 3 °C min−1) for 4 h in a hydrogen (H2) flow at 40 mL/min. Finally, the reduced catalyst samples were cooled to

2.2. Catalytic activity with respect to HDO of WCO The Ni2P/AC catalyst being tested (1 ± 0.01 g) was packed in a tubular reactor with a bed volume of 8.58 cm3. The catalyst was then reduced at 600 °C in a H2 flow of 20 mL/min for 1 h. Once the reduction process was complete and the catalyst had been cooled to the desired reaction temperature, it was used for the HDO of WCO at atmospheric pressure in a tubular fixed-bed reactor (schematic shown in Fig. 1). The composition of the WCO sample used in this study was analyzed by gas chromatography–mass spectrometry (GC–MS). The results are shown in Table 1. During each trial, the feed consisted of 1.2 wt% WCO in n-heptane and was delivered by an high-pressure liquid chromatography (HPLC) pump at a flow rate of 0.8 mL/min. The H2 gas was fed in the downward and concurrent modes using a mass flow controller. The liquid products 118

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reduced at the high temperature. In the case where the amount of phosphorus precursor was excessively high, the reduction process occurred simultaneously with the conversion of the phosphate into a complex mix of phosphorus oxides (P2O5, PO2, and P4O7), which were then continuously reduced to phosphine and phosphorus over the temperatures of 300–900 °C [19,20]. The peak at 800–900 °C could be assigned to the reduction of phosphate (ePO4) [13]. The extra nickel precursor was reduced by being converted into various types of nickel oxides, which were then transformed into an intermediate, namely, Ni0, that reacted with the phosphide groups to form the Ni-rich phosphide phases, such as Ni12P5, at the higher temperatures [12,24,25]. The amount of nickel loaded (mmol/g AC) on the AC support also affected the formation of Ni2P. Fig. 2 shows the reducibilities of the Ni2P/AC-1.5-TPR catalyst samples with the different nickel loading levels and a constant Ni/P molar ratio of 1.5. At low nickel loading levels (1.16 and 5.37 mmol/g AC), most of the TPR peaks were observed at temperatures of 200–650 °C and corresponded to complete phosphidation. On the other hand, at higher nickel loading levels (16.80 and 38.90 mmol/g AC), the peak intensities increased and the peaks were shifted to higher temperatures (200–800 °C), owing to the reduction of Ni12P5. The phosphate peak was not detected in the TPR profiles corresponding to low Ni loading levels (1.16 and 16.8 mmol/g AC) as the nickel was well distributed on the support and because phosphidation occurred to a high degree in these cases. In contrast, at higher Ni loading levels, the Ni particles underwent agglomeration, which hindered the phosphidation reaction. However, at higher Ni loading levels, the amount of phosphorous available was also high. This probably accelerated the reaction at the high temperatures. The products from the side reaction affected the TPR profile, Therefore, the integrated area corresponding to the high temperatures was not only indicative of the amount of H2 consumed but also of these undesirable products. Fig. 3 shows the NH3-TPD profiles of the Ni2P/AC catalyst samples with various Ni/P ratios and Ni loading levels. The profiles were used to characterize the acidic properties of the samples. In general, three distinct desorption peaks were observed, at 130–140 °C, at 230–250 °C, and at temperatures higher than 300 °C. These could be assigned to the Brønsted acid sites of the PeOH groups, the Lewis acid sites of the Niδ+ (0 < δ < 1) in the Ni2P phase, and the Lewis acid sites of the Ni-rich phosphide [14,26–28]. The contributions of the acid sites responsible for the desorption peaks are summarized in Table 2. The NH3 desorption peak of (ePO4) was small but was observed for all the catalyst samples, owing to the reduction of the phosphate precursor during the catalyst synthesis step. The acidity of the Ni2P phase [Brønsted acid sites from the PeOH groups and Lewis acid sites from Niδ+ (0 < δ < 1)] was also clearly observed in the case of all the catalysts. However, at a Ni/P molar ratio of 2.0, an additional desorption peak belonging to the Lewis acid sites of Ni12P5 was also seen. The Ni2P/AC catalyst samples with Ni/P molar ratios of 0.5, 1.0, and 1.5 primarily contained Ni2P acid sites, and the total acidity of the samples increased with an increase in the Ni/P ratio (see Table 2). However, further increasing the Ni/P molar ratio from 1.5 to 2 decreased the total acidity of the Ni2P/AC catalyst samples. This can be attributed to the presence of the nickel-rich phosphide (Ni12P5). The NH3-TPD profiles of the Ni2P/AC catalyst samples with the different Ni loading levels are also shown in Fig. 3. The Ni2P/AC catalyst samples with low Ni loading levels (1.16 and 5.37 mmol/g AC) exhibited only two distinct peaks, which were ascribable to the acid sites of Ni2P and nickel-rich phosphide (Ni12P5). The total acidity of the samples increased with an increase in the Ni loading level (see Table 2). Meanwhile, the Ni2P/AC catalyst with a Ni loading level of 38.9 mmol/ g AC showed a distinct peak belonging to the Ni12P5 phase, as mentioned above. In addition, the total acidity of the Ni2P/AC catalyst sample with a Ni/P molar ratio of 16.8 mmol/g AC was lower than

Table 1 Chemical composition of WCO sample used in this study. Component

Chemical structure

Oleic acid (18:1) Palmitic acid (16:0) Octadecanoic acid (18:0) Tetradecanoic acid (14:0) Hexadecanoic acid 2-hydroxy-1,3propanediyl ester (35:0) Di-(9-octadecenoyl)-glycerol (39:2) Oleic acid, 3-hydroxypropyl ester (21:1) Octadecanoic acid 2-hydroxy-1,3propanediyl ester (39:0) Hexadecanamide (16:0) Palmitoyl chloride (16:0) Others

Content (mol %)

C18H34O2 C16H32O2 C18H36O2 C14H28O2 C35H68O5

17.45 18.26 6.64 0.28 15.01

C39H72O5 C21H40O3 C39H76O5

20.74 4.06 4.69

C16H33NO C16H31ClO

2.83 3.21 6.83

Remark: Fatty acid (x/y) means fatty acid had x carbon number and y number of double bonds.

were collected every 20 min and separated from the heptane using a rotary evaporator. Subsequently, their chemical composition was determined by GC–MS (Shimadzu-2010) analysis performed using a HP5MS column. The oven temperature was programmed to remain at 50 °C for 3 min, ramp at 15 °C min−1 to 280 °C, and stay at 280 °C for 5 min. The injection temperature was 300 °C, and the injector split ratio was set to 10:1. The flow rate of the carrier gas, He, was 1 mL/min. The oxygen removal rate was calculated based on the GC–MS results as follows:

Tfeed − Tproduct ⎤ × 100 Oxygen removal = ⎡ ⎢ ⎥ Tfeed ⎣ ⎦ where Tfeed and Tproduct are the contents of all the oxygenated compounds as determined by the GC–MS analyses of the WCO sample and its liquid products, respectively [18]. The gaseous products were also identified using a GC system (Shimadzu, GC-8A) equipped with a thermal conductivity detector and packed ShinCarbon 80/100 column. The injection temperature, initial temperature, and final temperature were all kept constant at 50 °C. 3. Results and discussion 3.1. Characterization of Ni2P/AC catalyst samples The reducibility of each Ni2P/AC catalyst sample was investigated by H2-TPR analysis. The results for representative examples are shown in Fig. 2(a–d). The Ni2P catalyst precursors were prepared by coimpregnation, which resulted in a greater degree of intimate contact between Ni2+ and the phosphate groups on the AC surface. It has been suggested that phosphidation occurs through the simultaneous step-bystep reduction of nickel phosphate [19]. The complexity of the TPR curve for the reduction of nickel phosphide as catalyst is related to the concurrent formation of H2O and PH3 [19–22]. The peak at temperatures of 100–300 °C was assigned to the reduction of the hygroscopic precursor, which led to the decomposition of the nitrogen-containing salts [12,23]. Moreover, the broad reduction peak could be deconvoluted into three distinct peaks at 300–400 °C, 400–700 °C, and 700–900 °C; these corresponded to the reduction of nickel oxide, nickel oxy-phosphates [Ni2P4O12, Ni2P2O7, Ni(PO3)2], and nickel phosphate, respectively [20]. In the case of a high phosphate content, the TPR peak was observed at a higher reduction temperature, owing to the reduction of phosphorous. In addition, Ni2+ was reduced to Ni metal during the reduction of phosphorous. The presence of Ni metal aided the reduction of phosphorous, resulting in the broad reduction peak being observed at 300–900 °C. Further, a high Ni content also resulted in a broad peak because the volume fraction of the Ni-rich phosphide phase was 119

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Fig. 2. Representative H2-TPR profiles of various Ni2P/AC catalyst samples with different Ni/P ratios and Ni loading levels (mmol/g AC): (a) Activated carbon, (b) 5.37-Ni2P/AC-0.5, (c) 5.37-Ni2P/AC-1.0, (d) 5.37-Ni2P/AC-1.5, (e) 5.37-Ni2P/AC-2.0, (f)1.16-Ni2P/AC-1.5, (g) 16.80-Ni2P/AC-1.5, and (h) 38.90-Ni2P/AC-1.5.

Fig. 3. Representative NH3-TPD profiles of Ni2P/AC catalysts with different Ni/P ratios and Ni loading levels (mmol/g AC): (a) 5.37-Ni2P/AC-0.5, (b) 5.37-Ni2P/AC1.0, (c) 5.37-Ni2P/AC-1.5, (d) 5.37-Ni2P/AC-2.0, (e) 1.16-Ni2P/AC-1.5, (f) 16.80-Ni2P/AC-1.5, and (g) 38.90-Ni2P/AC-1.5. 120

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Table 2 Physicochemical properties of Ni2P/AC catalyst samples. Catalysta

Acidic properties (mmol/g)

Total acidity (mmol/g)

Crystallite size of Ni2P (nm)b

SBET (m2/g)

20.9 21.5 22.1 n.d. 26.3 30.9 29.2 31.5 31.6 31.9 41.5 47.5 41.9 51.0

1324 < 0.1 156 17 538 < 0.1 612 0.1 456 0.1 163 < 0.1 38 < 0.1

Peak temperature (°C)

Fresh 1.16-Ni2P/AC-1.5 Spent 1.16-Ni2P/AC-1.5 Fresh 5.37-Ni2P/AC-0.5 Spent 5.37-Ni2P/AC-0.5 Fresh 5.37-Ni2P/AC-1.0 Spent 5.37-Ni2P/AC-1.0 Fresh 5.37-Ni2P/AC-1.5 Spent 5.37-Ni2P/AC-1.5 Fresh 5.37-Ni2P/AC-2.0 Spent 5.37-Ni2P/AC-2.0 Fresh 16.80-Ni2P/AC-1.5 Spent 16.80-Ni2P/AC-1.5 Fresh 38.90-Ni2P/AC-1.5 Spent 38.90-Ni2P/AC-1.5

140–160

220–250

> 300

0.10

0.87

n.d.

0.97

0.06

0.81

n.d.

0.87

0.04

1.14

n.d.

1.18

0.03

1.27

n.d.

1.30

0.11

0.80

0.08

0.99

0.15

0.31

0.13

0.59

0.18

0.26

0.67

1.11

n.d. = not detected. a Spent catalyst samples were analyzed after HDO of WCO (T = 300 °C, GHSV = 2.33 min−1, P = 1 atm). b Crystallite size was determined by XRD analysis.

spent catalysts exhibited characteristic peaks similar to those of the fresh catalysts. Further, the differences in the crystallite sizes of the fresh and spent Ni2P/AC catalysts were not significant; this was true for all the Ni loading levels. Thus, it can be concluded that the degree of catalyst deactivation was low, owing to sintering and the loss of only a few active sites. The BET specific surface areas (SBET) of the catalyst samples are shown in Table 2. Increasing the Ni/P ratio from 0.5 to 1.5 increased the BET surface area 3.93-fold from 155.9 to 612.4 m2/g. However, increasing the Ni/P ratio further from 1.5 to 2.0 decreased the BET surface area 1.34-fold from 612.4 to 456.3 m2/g. This was probably because the higher Ni content (Ni/P ratio of 2.0) allowed for the sintering of Ni to occur, which led to the formation of large particles; these covered the pores and thus decreased the accessible surface area, especially the internal surface area. Meanwhile, increasing the Ni loading level from 1.16 to 38.9 mmol/g clearly decreased the surface area 34.8-fold from 1324.0 to 38 m2/g. Again, this was because the Ni particles underwent agglomeration and completely filled the pores in the AC. After the HDO of WCO, the spent catalyst samples exhibited lower surface areas, which was probably because of coking owing to the secondary reactions, including cracking and polymerization, occurring during the HDO reaction using the Ni2P catalyst. The characterization of the spent catalyst samples indicated that the samples had not undergone a significant degree of deactivation, owing

those of the samples with Ni/P molar ratio of 1.16 and 5.37 mmol/g AC. As stated above, XRD analyses were performed to confirm the formation of Ni2P phases in the Ni2P/AC catalyst samples (see Fig. 4). The diffraction peaks at 2θ of 40.7°, 44.6°, 47.4°, 54.1°, and 56.2° could be assigned to Ni2P [10,15,29], while those at 2θ of 38.3°, 41.7°, and 48.9° were associated with Ni12P5 [30]. In addition, the broad peak at 2θ of 30–40° was assigned to amorphous nickel pyrophosphate (Ni(PO3)2) [31]. The XRD patterns of the 5.37-Ni2P/AC catalyst samples with various Ni/P molar ratios before (fresh) and after (spent) use in the HDO reaction of WCO are shown in Fig. 4a. All the fresh catalyst samples clearly exhibited a Ni2P phase, with Ni12P5 forming only at a Ni/P ratio of 2.0, owing to the high nickel content in this case. The crystallite sizes of the different Ni2P/AC catalyst samples are shown in Table 2 and increased 1.43-fold from 22.05 to 31.64 nm as the Ni/P ratio was increased from 0.5 to 2.0. Furthermore, the Ni2P crystallite sizes of the fresh and spent samples were not significantly different. The XRD spectra contained distinct peaks related to the Ni2P phase at all the Ni loading levels (Fig. 4b), while peaks related to the Ni12P5 phase were observed at the highest Ni loading level (i.e., for the 38.90Ni2P/AC-1.5 sample). This was in keeping with the H2-TPR and NH3TPD profiles, which also indicated the presence of Ni12P5. Notably, increasing the Ni loading level from 1.16 to 38.9 mmol/g AC increased the Ni2P crystallite size two-fold from 20.9 nm to 41.9 nm, owing to the agglomeration of the Ni particles. However, the XRD spectra of the

Fig. 4. Representative XRD diffractograms of (a) 5.37-Ni2P/AC catalyst samples at different Ni/P ratios and (b) Ni2P/AC-1.5 catalyst samples with different Ni loading levels before and after HDO of WCO (T = 300 °C, GHSV = 2.33 min−1, P = 1 atm). 121

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Fig. 5. Deoxygenated liquid products obtained after HDO of WCO using various Ni2P/AC catalyst samples. (a) 5.37-Ni2P/AC with different Ni/P ratios, (b) Ni2P/AC1.5 with different Ni loading levels (mmol/g AC) (conditions for both: T = 300 °C, GHSV = 2.33 min−1, P = 1 atm); (c) 5.37-Ni2P/AC-1.5 at different reaction temperatures (GHSV = 2.33 min−1, P = 1 atm); and (d) 5.37-Ni2P/AC-1.5 at various GHSV values (T = 300 °C, P = 1 atm).

produced at low Ni loading levels (1.16 and 5.37 mmol/g AC), with the oxygen removal level also being high. At a Ni loading level of 1.16 mmol/g AC, the main hydrocarbons were cyclic compounds, such as C21H36. However, at higher loading levels (5.37–38.9 mmol/g AC), the reaction products were mainly linear hydrocarbons, with the highest alkane yield being obtained at a Ni loading level of 5.37 mmol/ g AC. The hydrocarbons primarily consisted of alkanes because the unsaturated hydrocarbons, such as alkenes, acted as substrates for the undesired reactions, including polymerization. At a Ni loading level of 38.9 mmol/g AC, the gas product contained CO and CO2, whereas the side reactions were caused by the presence of hydrocarbon gases, such as ethylene and ethane. The effect of the reaction temperature on the HDO of WCO while using the 5.37-Ni2P/AC-1.5 catalyst samples was evaluated at a GHSV of 2.33 min−1 and operating pressure of 1 atm (Fig. 5c). Increasing the reaction temperature enhanced the hydrocarbon fraction, since the HDO reaction requires energy for breaking the chemical bonds present, especially the CeO bond, to yield the hydrocarbon compounds. The oxygen removal rate increased from 55% to 89% when the reaction temperature was increased from 260 to 320 °C. The main hydrocarbons were n-alkanes. However, at a reaction temperature of 320 °C, a significant amount of aromatic hydrocarbons (yield of 1%) were obtained because this high reaction temperature promoted additional reactions, such as cyclization and aromatization. Fig. 5d shows the effect of changing the GHSV on the catalytic activity for the HDO reaction. The measurements were performed using the 5.37-NiP2/AC-1.5 catalyst at 300 °C. The hydrocarbon yield decreased with an increase in the GHSV, because the cleavage of the CeO bonds requires a long retention time [33]. The highest hydrocarbon yield was observed at a GHSV of 2.33 min−1. As is also the case for the GHSV, the retention time on the catalyst bed is determined by the flow rate of the carrier gas. The highest n-alkane yield was observed at the lowest GHSV (2.33 min−1) and gradually decreased with an increase in the GHSV. Meanwhile, the lowest GHSV (2.33 min−1) resulted in low deoxygenation activity as compared to the case at a GHSV of

to metal sintering, as the difference in the crystalize sizes of the fresh and spent samples was low. However, they exhibited temporary deactivation owing to coking, and it should be possible to reactivate them using different techniques [32]. Therefore, it should be possible to recover the activity of the spent catalyst samples. 3.2. Catalytic HDO of WCO The activities of the various samples with respect to the HDO of WCO are shown in Fig. 5. The Ni/P ratio affected the structure of the nickel phosphide formed, which, in turn, affected the catalytic performance (Fig. 5a). Hydrocarbons were obtained during the first 20 min of the reaction on stream for all the Ni2P/AC catalyst samples (i.e., for all Ni/P ratios). The yield of the hydrocarbons gradually decreased owing to the active sites being covered with the impurities present in the WCO sample. The highest hydrocarbon yield (77.4%) as well as the highest oxygen removal rate (88%) was obtained at a Ni/P molar ratio of 1.5 for all the times on stream (20, 40, and 60 min). The hydrocarbon yield was strongly affected by the presence of the Ni2P phase in the catalyst. That is to say, the sample with a Ni/P ratio of 1.5 exhibited a distinct Ni2P-phase-related peak in its XRD spectrum, which was indicative of its high hydrocarbon selectivity. The hydrocarbons present were primarily n-alkanes, with cyclic hydrocarbons, such as aromatic and cycloalkane compounds, also present in trace amounts. This suggested that the deoxygenation reaction involved only a limited number of undesired pathways, such cyclization and aromatization. At a Ni/P molar ratio of 2.0, the Ni2P/C catalyst (i.e., 5.37-Ni2P/AC-2.0) showed a lower catalytic performance than that of the catalyst with a Ni/P molar ratio of 1.5 (i.e., 5.37-Ni2P/AC-1.5). This was because of the presence of Ni12P5 in the former. Therefore, a Ni/P molar ratio of 1.5 was determined to be the optimal one because it resulted in the highest oxygen removal rate and good selectivity for hydrocarbon formation. For the Ni2P/AC-1.5 samples, the Ni loading level within the range 1.16–38.9 mmol/g AC clearly affected the catalytic performance with respect to the HDO of WCO (Fig. 5b). Primarily hydrocarbons were 122

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3.51 min−1, owing to catalyst deactivation. The long retention time probably promotes secondary reactions such as oligomerization and coking. Therefore, the retention time changes with the GHSV, affecting both the deoxygenation of the hydrocarbons and coking. Overall, the optimized conditions for the HDO of WCO using the Ni2P/AC catalysts was determined to be a Ni/P molar ratio of 1.5, Ni loading level of 5.37 mmol/g AC, reaction temperature of 300 °C, and GHSV of 2.33 min−1. These conditions led to > 85% of the oxygen being removed from the WCO through decarbonylation and decarboxylation reactions. With respect to the deoxygenation mechanism, it is likely that it occurs through two different pathways, namely, decarboxylation and decarbonylation. The cracking reaction also occurs as a side reaction. The CO / (CO + CO2) ratio is indicative of the reaction pathway, as shown in Table 3. A high reaction temperature (> 300 °C) would lead to the decarboxylation pathway while the decarbonylation pathway would be the preferred one at high Ni loading levels and low temperatures (≤300 °C). Therefore, the reaction could be performed at atmospheric pressure. Decarbonylation at atmospheric pressure has been reported previously [16,34]. The liquid product primarily consisted of n-alkanes in the diesel fraction (C15–19). The proposed reaction pathway is shown in Fig. 6. When subjected to hydrogenolysis, the triglycerides formed intermediate products, including aldehydes, ketones, and alcohols. These intermediate compounds were further dehydrated to alkenes. However, as hydrogenation was performed in a H2 atmosphere, n-alkanes were formed. Different mechanisms are probably responsible for the formation of acid anhydrides as the key component. The oxygen is removed by the decarbonylation of acid anhydrides [35]. However, no acid anhydrides were detected in the liquid product whereas aldehydes and ketones were. Therefore, the decarbonylation of aldehydes is the most likely mechanism for the HDO of WCO at atmospheric pressure using Ni2P as catalyst. In addition, the Ni2P/AC catalyst was benchmarked against different Ni-based support catalysts, as shown in Table 4. The performance of Ni2P/AC was higher than those of the other Ni catalysts. However, it should be kept in mind that, in the present study, the HDO of WCO was performed under atmospheric conditions.

0.0 0.0 0.0 0.0 0.0 0.1 1.0 2.0 0.5 0.0 0.0 0.0 96.1 2.8 0.0 0.0 0.0 5.4 2.9 4.7 0.1 0.6 17.9 0.0 0.0 0.0 0.0 0.0 0.0 2.9 0.0 0.0 0.0 0.0 0.0 0.0 0.1 3.6 0.0 0.0 0.0 14.7 28.1 10.2 1.6 7.7 26.7 0.0 0.0 0.0 0.9 0.0 0.0 27.1 0.2 0.0 0.0 0.0 0.0 0.0 0.0 3.3 0.0 21.3 16.3 25.4 12.6 1.9 0.8 2.1 8.7 1.4

4. Conclusions The catalytic deoxygenation of WCO was performed successfully at atmospheric pressure using a series of Ni2P/AC catalyst samples. The presence of the Ni2P phase in the catalyst played an important part in determining the oxygen-removal efficiency as well as the composition of the liquid products, as it promoted the catalytic deoxygenation process via decarbonylation and decarboxylation reactions. The optimized Ni/P molar ratio and Ni loading level (in terms of alkane production and the removal efficiency of oxygen) were 1.5 and 5.37 mmol/ g AC, respectively, as these resulted in the highest volume fraction of the Ni2P phase in the catalyst. The catalytic activity of the Ni2P/AC catalysts with respect to the deoxygenation reaction not only depended on the Ni2P phase but was also affected by the reaction conditions, including the temperature and GHSV. The proportion of oxygen atoms in the WCO sample was reduced by > 85%, resulting primarily in the formation of n-alkanes in the diesel range (C15–19). The optimized reaction temperature and GHSV were 300 °C and 2.33 min−1, respectively. To end, we propose that the deoxygenation process proceeds through triglyceride hydrogenolysis and dehydration reactions.

Remark: Operating pressure (P) of 1 atm and reaction time of 40 min.

2.33 2.33 2.33 2.33 2.33 2.33 2.33 3.51 4.66 2.33 2.33 2.33 300 300 300 260 280 300 320 300 300 300 300 300 1.16-Ni2P/AC-1.5 5.37-Ni2P/AC-0.5 5.37-Ni2P/AC-1.0 5.37-Ni2P/AC-1.5 5.37-Ni2P/AC-1.5 5.37-Ni2P/AC-1.5 5.37-Ni2P/AC-1.5 5.37-Ni2P/AC-1.5 5.37-Ni2P/AC-1.5 5.37-Ni2P/AC-2.0 16.80-Ni2P/AC-1.5 38.90-Ni2P/AC-1.5

0.0 0.0 0.0 0.0 0.0 1.8 0.0 0.1 0.0 0.0 0.0 0.0

n-Alkane (C20-C22) n-Alkane (C11-C14)

n-Alkane (C15-C19)

n-Alkene

Branched alkane

Cyclic compound

Aromatic hydrocarbon

0.04 1.0 1.0 1.2 1.3 1.0 0.03 0.05 1.0 1.0 1.0 1.0

CO / (CO + CO2) (%) Type of hydrocarbon in liquid product (%) Space velocity (min−1) Reaction temperature (°C) Catalyst

Table 3 Hydrocarbon yields in liquid product and CO/(CO + CO2) ratio after HDO of WCO using Ni2P/AC catalyst samples with different Ni/P molar ratios and Ni loading levels at different reaction temperature and GHSV values.

L.K.H. Pham et al.

Acknowledgements The authors gratefully acknowledge financial support from a project jointly funded by National Research Council of Thailand through NRCTNSFC Joint fund (Contract No. NRCT/2558-104). The characterization experiments was performed at the Advance Research Center (ARC), Thammasat University, and the Department of Chemistry, Faculty of 123

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Fig. 6. Proposed pathway for HDO of WCO using Ni2P/AC catalysts. Table 4 Comparison of catalytic performances of different Ni-based support catalysts. Active phase

Support

Feedstock

Conditions

Conversion (%)

Reference

Ni

ZSM-5

Palmitic acid

96.0

[36]

Ni

Triolein

92.5

[37]

20% Ni-5% Cu

Hexagonal mesoporous silica (HMS) γ-Al2O3

Waste lipid

100

[38]

Ni

CNT

Palmitic acid

97.25

[39]

Ni2P

C

Up to100

[30]

Ni2P

Activated carbon

Pure palmitic acid with 5% H2/Ar as carrier gas Waste cooking oil

T = 300 °C, P = 20 bar in high-pressure reactor T = 300 °C, t = 2 h, using vacuum batch reactor T = 400 °C, t=3h P = 580 psi in H2 flow, flow rate of 60 mL/ min T = 240 °C, P = 2 MPa H2 in autoclave reactor T = 350 °C, P = 1 atm T = 320 °C P = 1 atm in fixed bed reactor

89%

This work

Science and Technology, Chulalongkorn University, L.K.H. Pham would like to acknowledge the financial support provided by an International Student Scholarship from Thammasat University, Thailand.

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