catalysts

catalysts Article

Support Effect on the Performance of Ni2P Catalysts in the Hydrodeoxygenation of Methyl Palmitate Irina V. Deliy * , Ivan V. Shamanaev , Pavel V. Aleksandrov, Evgeny Yu. Gerasimov , Vera P. Pakharukova, Evgeny G. Kodenev, Ilya V. Yakovlev , Olga B. Lapina and Galina A. Bukhtiyarova Boreskov Institute of Catalysis, Pr. Lavrentieva 5, 630090 Novosibirsk, Russia; [email protected] (I.V.S.); [email protected] (P.V.A.); [email protected] (E.Y.G.); [email protected] (V.P.P.); [email protected] (E.G.K.); [email protected] (I.V.Y.); [email protected] (O.B.L.); [email protected] (G.A.B.) * Correspondence: [email protected]; Tel.: +7-383-326-9410 Received: 11 October 2018; Accepted: 31 October 2018; Published: 3 November 2018

Abstract: The effect of support nature, SiO2 and γ-Al2 O3 , on physicochemical and catalytic properties of nickel phosphide catalysts in methyl palmitate hydrodeoxygenation (HDO) has been considered. Firstly, alumina-supported nickel phosphide catalysts prepared by temperature-programmed reduction method starting from different precursors (phosphate–Ni(NO3 )2 and (NH4 )2 HPO4 or phosphite–Ni(OH)2 and H3 PO3 ) were compared using elemental analysis, N2 physisorption, H2 -TPR, XRD, TEM, NH3 -TPD, 27 Al and 31 P MAS NMR techniques and catalytic experiments. The mixture of nickel phosphide phases was produced from phosphate precursor on alumina while using of phosphite precursor provides Ni2 P formation with the higher activity in methyl palmitate HDO. Besides, the comparative study of the performances of Ni2 P/SiO2 and Ni2 P/Al2 O3 catalysts demonstrates the apparent superiority of alumina-supported Ni2 P in the methyl palmitate hydrodeoxygenation. Considering the tentative scheme of methyl palmitate transformation, we proposed that cooperation of Ni2 P and acid sites on the surface of alumina provides the enhanced activity of alumina-supported Ni2 P through the acceleration of acid-catalysed hydrolysis. Keywords: nickel phosphide; acidity; Ni2 P/γ-Al2 O3 ; Ni2 P/SiO2 ; methyl palmitate; hydrodeoxygenation; biofuel; green diesel

1. Introduction Depletion of fossil oils resources, as well as the environmental issues of increased carbon emission, stimulates the development of new catalytic technologies for the production of transportation fuels from renewable [1–3]. Triglyceride-based feedstocks, such as non-edible vegetable oils, animal fats and waste cooking oils, are the attractive resources that give a mixture of C14 -C18 alkanes in the hydrodeoxygenation (HDO) process [2,4,5]. This product called green diesel has a high cetane number and stability, low density and can be mixed with the fossil-derived fuels to improve their quality [6–8]. Recently, noble metal-based and sulphide hydrotreating catalysts have been studied intensively in the HDO of triglycerides and model compounds, such as aliphatic esters [3–5]. However, the practical application of noble metal catalysts, despite their high HDO activity, is restricted by the high cost and shortage of noble metals. As to hydrotreating sulphide catalysts, the S-containing agent has to be added to the feed to avoid deactivation that, in turn, leads to the appearance of undesirable S-containing products [9–11]. Besides, the catalytic properties of supported noble metals and sulphide phase can be deteriorated by the reaction products–carbon oxides and water [12–14]. Thus, new systems containing base metals or their carbides, nitrides and phosphides have emerged as the catalysts for HDO of Catalysts 2018, 8, 515; doi:10.3390/catal8110515

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Catalysts 2018, 8, 515

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aliphatic esters and vegetable oils in recent years [2–4,15,16]. Among them, silica-supported nickel phosphide catalysts attract the particular attention of researchers due to their high stability and activity in HDO reactions [3,16–28]. Effects of the precursor, initial Ni/P ratio and preparation conditions on the catalytic properties of Nix Py /SiO2 systems in the HDO of aliphatic esters were studied using mainly methyl oleate [18,20], methyl laurate [15,17,19,28,29] and methyl palmitate [21,24–27]. In most studies, the activity of Nix Py /SiO2 catalysts was shown to be higher than the activity of Ni/SiO2 samples and the decrease of Ni/P ratio in the impregnation solution improved the catalytic activity and selectivity towards direct HDO (formation of Cn hydrocarbons through H2 O elimination) [21,24,27]. It was stated that the Ni2 P phase is the most active in hydrodeoxygenation of fatty acid methyl esters among the variety of supported nickel phosphides such as Ni3 P, Ni2 P, Ni5 P4 and Ni12 P5 [17,20,23,27]. The optimal calcination and reduction conditions of Nix Py /SiO2 precursors for obtaining the most active catalysts in HDO were found [25,26,30]. In our previous study we have found out the synergetic effect of Ni2 P/SiO2 and γ-Al2 O3 physical mixture in hydrodeoxygenation of methyl palmitate, which was explained by the cooperation of the metal sites of Ni2 P/SiO2 and the acid sites of γ-Al2 O3 in consecutive metal-catalysed and acid-catalysed reactions of methyl palmitate conversion [31]. Therefore, we proposed, that the employment of γ-Al2 O3 as the support can improve the activity of the Ni2 P catalyst due to the ability of the former to accelerate the aliphatic esters hydrolysis [32,33]. It is well known, that support nature can influence the activity of the catalysts by affecting the active component properties, namely active phase and dispersity, at the stage of catalyst preparation [22,34–37] or as a result of the participation of the support in catalytic conversions [33,35,37–40]. The support effect on the catalytic properties of nickel phosphide catalysts has been studied in the hydrodechlorination of chlorobenzene [41,42], hydrodesulfurization of thiophene [43] and hydrodeoxygenation of guaiacol [44]. Recently, deoxygenation of methyl laurate over nickel phosphide catalysts supported on SiO2 , CeO2 , TiO2 , γ-Al2 O3 and SAPO-11 was also investigated [22]. However, in this study different nickel phosphide phases were formed depending on the support: Ni2 P was produced on SiO2 , CeO2 , TiO2 and SAPO-11, while Ni3 P and Ni12 P5 were received on γ-Al2 O3 . The authors concluded that the catalyst activity of nickel phosphide catalysts in methyl laurate HDO depends on several factors, namely surface density of Ni sites, the electron property of metal sites, the Ni2 P crystallite size and the synergism between the Ni sites and the acid sites. The aim of our current study is to elucidate the real support effect, without the interference of other crucial factors, like phase composition, Ni content or Ni2 P particle size through the comparative study of the performance of Ni2 P/SiO2 and Ni2 P/Al2 O3 catalysts in the methyl palmitate hydrodeoxygenation. Firstly, the alumina-supported nickel phosphide catalysts differing in the precursor and reduction temperature have been investigated by elemental analysis, N2 physisorption, H2 -TPR, XRD, TEM, NH3 -TPD, 27 Al and 31 P MAS NMR. It turned out that the reduction of the phosphite-type precursor at 550–650 ◦ C makes it possible to support Ni2 P nanoparticles on the alumina surface with the particle size distribution resembling that on the surface of Ni2 P/SiO2 catalysts, obtained in our previous study from phosphide precursor [26]. The comparison of the catalyst’s behaviour in the wide temperature interval showed the superior activity of the alumina-supported catalyst in comparison with the silica-supported one. The higher activity of Ni2 P/Al2 O3 catalyst in methyl palmitate HDO along with the higher amounts of oxygen-containing intermediates in the reaction products can be explained by the acceleration of acid-catalysed hydrolysis of methyl palmitate.


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2. Results 2.1. The Effect of Preparation Conditions on the Physicochemical Properties of Nix Py /γ-Al2 O3 Catalysts Two sets of Nix Py /γ-Al2 O3 catalysts were prepared by the incipient wetness impregnation from the phosphate- or phosphite-containing precursors (NiP_A/Al2 O3 or NiP_I/Al2 O3 samples, correspondingly) with the subsequent temperature-programmed reduction at 550, 600 and 650 ◦ C. The physicochemical properties of obtained Nix Py /γ-Al2 O3 catalysts after reduction and passivation are presented in Table 1. According to the chemical analysis (ICP-AES), the contents of Ni and P in the reduced samples of Nix Py /γ-Al2 O3 catalysts were not changed remarkably in comparison with that in precursors (Table 1). The Ni/P molar ratio is varied insignificantly in the range from 0.51 to 0.55 in the reduced NiP_A/Al2 O3 samples (initial Ni/P ratio 0.5) and it is retained at the level of 0.34–0.35 in the reduced NiP_I/Al2 O3 samples (initial Ni/P ratio 0.3). The Ni/P molar ratios in reduced catalysts were slightly higher than the initial Ni/P molar ratios in oxide precursors and impregnating solutions. The increase of the Ni/P molar ratio in the catalysts after reduction is caused by the formation and elimination of volatile phosphorus-containing species (namely PH3 or P, P2 etc.) during high-temperature reduction. The retained P could enter into the composition of nickel phosphide particles, unreduced POx groups [43,45], elemental phosphorus residuals [46] and AlPO4 [43]. With the increase of the reduction temperature from 550 to 650 ◦ C, the specific surface area (ABET ) of NiP_A/Al2 O3 catalysts is expanded from 127 to 158 m2 /g along with the decrease of the average pore diameter (Dpore ) from 12.7 to 10.6 nm. The specific surface area of reduced NiP_I/Al2 O3 catalysts is decreased approximately 2-fold in comparison with γ-Al2 O3 (101–120 m2 /g vs. 205 m2 /g of γ-Al2 O3 ). The average pore diameter of reduced NiP_I/Al2 O3 catalysts is also decreased from 13.4 nm to 8.3–9.2 nm in comparison with the γ-Al2 O3 support. Figure S1 shows the curves of incremental pore volume distribution depending on the average pore diameter for NiP_A/Al2 O3 and NiP_I/Al2 O3 samples. It indicates that the mesoporous structure of the support was conserved after active component loading and reduction but the total pore volume was decreased. The textural characteristics of the catalyst could have changed because of the thermal treatment [40]. Also, the decrease of ABET , Dpore and Vpore for the catalysts in comparison with alumina support is caused with the partial pore clogging by Nix Py particles and phosphorus-containing compounds (POx groups, elemental phosphorus ) or probably with the support structure rearrangement because of AlPO4 formation as will be illustrated later by NMR data. Table 1. Physicochemical properties of NiP_A/Al2 O3 , NiP_I/Al2 O3 and NiP_I/SiO2 catalysts. Sample

Treduction , ◦ C

Elemental Content after TPR, wt% Ni

P

Molar Ni/P

ABET , m2 /g

Dpore , nm

XRD Phase

DXRD , nm

NiP_A/Al2 O3 (550)

550

7.3

7.6

0.51

127

12.7

Ni Ni3 P

5.5 -

NiP_A/Al2 O3 (600)

600

7.3

7.6

0.51

130

11.9

Ni Ni3 P Ni12 P5

5.0 4.5 12.0

NiP_A/Al2 O3 (650)

650

7.5

7.2

0.55

158

10.6

Ni3 P Ni12 P5

7.6 11.5

NiP_I/Al2 O3 (550)

550

7.4

11.6

0.34

115

8.6

Ni2 P

3.8

NiP_I/Al2 O3 (600)

600

7.4

11.3

0.35

120

8.3

Ni2 P

3.8

NiP_I/Al2 O3 (650)

650

7.6

11.5

0.35

101

9.2

Ni2 P

4.4

NiP_I/SiO2 (600)

600

7.5

7.4

0.53

180

12.7

Ni2 P

5.5

H2 -TPR profiles of NiPOx _A/Al2 O3 precursor and NiPOx _I/Al2 O3 precursor as well as the reference NiPOx _I/SiO2 precursor are shown in Figure 1. H2 -TPR profiles of the NiPOx _A/Al2 O3


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and NiPOx _I/Al2 O3 precursors are entirely different. It was indicated that the reduction of NiPOx _A/Al2 O3 precursor starts at the temperature of 525 ◦ C. It was observed two maximums of hydrogen consumption at 725 ◦ C and 840 ◦ C and the hydrogen uptake does not run out up to 900 ◦ C. According to the literature data [22,37,44], similar H2 -TPR profiles are characteristic for the reduction of nickel phosphate precursors deposited on alumina and also for the reduction of phosphate species (PO4 3− , P2 O7 4− and (PO3 − )n ) at a temperature above 720 ◦ C [47,48]. There are no low-temperature peaks corresponding to the reduction of nickel oxide particles [49]. According to the literature data [50], the reduction of commercial aluminium phosphate and alumina-supported Catalysts 2018,begins 8, x FOR PEER of 23H2 -TPR phosphates at a REVIEW temperature above 700 and 800 ◦ C, correspondingly. From 4the profile of NiPOx _I/Al2 O3 precursor it is seen that the reduction of oxide precursors starts at the reduction of commercial aluminium phosphate and alumina-supported phosphates begins at a temperature of 300 ◦ C with the formation of additional amount of hydrogen, as a result of phosphorous temperature above 700 and 800 °C, correspondingly. From the H2-TPR profile of NiPOx_I/Al2O3 acid and nickel phosphites decomposition [26,51,52]. The H2 -TPR profile contains signals in the precursor it is seen that the reduction of oxide precursors starts at the temperature of 300 °C with the ◦ directed downward. In this temperature region phosphorous acid temperature of 400–600 formation ofrange additional amount C of hydrogen, as a result of phosphorous acid and nickel phosphites and phosphite-containing areprofile disproportionate to thein formation of phosphine decomposition [26,51,52].compounds The H2-TPR contains signals the temperature rangeand of H3 PO4 2+ species or 400–600 phosphates. In agreement with the literature data phosphine could interact with Ni °C directed downward. In this temperature region phosphorous acid and ◦ C [53]. In phosphite-containing compounds are disproportionate to theasformation of phosphine and H3PO4 orPH3 can producing nickel phosphides already at the temperature low as 250 addition, phosphates. In agreement with theinto literature dataphosphorus phosphine could interact with Ni 2+ decompose at higher temperatures elemental and hydrogen. From thespecies temperature ◦ C for producing nickel phosphides already at the temperature as low as 250 °C [53]. In addition, PH 3 can of ~600 the NiPO _I/Al O precursor, hydrogen absorption is associated with the reduction x 2 3 decompose at higher temperatures into elemental phosphorus and hydrogen. From the temperature of phosphate-containing components formed during the decomposition of phosphorous acid. Thus, of ~600 °C for the NiPOx_I/Al2O3 precursor, hydrogen absorption is associated with the reduction of it can be expected that the formation of nickel phosphide phase in the case of the phosphite-containing phosphate-containing components formed during the decomposition of phosphorous acid. Thus, it precursor occurs at lower reduction temperature. The H2 -TPR curve of the reference NiPOx _I/SiO2 can be expected that the formation of nickel phosphide phase in the case of the phosphite-containing precursor shows a similar of the hydrogen from temperature, however, the amount precursor occurs at lowerdependence reduction temperature. The Huptake 2-TPR curve of the reference NiPOx_I/SiO2 ◦ C is higher for the silica-supported sample of absorbed hydrogen in the temperature range of 600–900 precursor shows a similar dependence of the hydrogen uptake from temperature, however, the in amount contrast of to absorbed the alumina-supported one. This fact was analysis hydrogen in the temperature rangealso of confirmed 600–900 °Cbyischemical higher for the data silica-supported sample contrast to the Thissample fact wasinalso confirmed by showing the Ni/P molarinratio equal to alumina-supported 0.53 in NiP_I/SiOone. comparison to 0.35 in 2 (600) chemical analysis data showing the Ni/P molar ratio equal to 0.53 in NiP_I/SiO 2(600) sample NiP_I/Al O (600) (Table 1). Such a difference between the behaviours of SiO and Al O -supported 2 3 2 2 3 in comparison to 0.35 in NiP_I/Al 2O3(600) (Table 1). Such a difference between the behaviours of SiO2catalysts is due to a stronger interaction of phosphate groups with the surface of aluminium and[44,54–56]. Al2O3-supported catalysts is due to agroups stronger interaction phosphate groups with theand surface oxide As a result, phosphate supported onofsilica are readily reduced removed in of aluminium oxide [44,54–56]. As a result, phosphate groups supported on silica are readily the form of phosphine (or volatile phosphorus species), while the interaction of phosphate groups with reduced and removed in the form of phosphine (or volatile phosphorus species), while the the surface of alumina leads to the formation of alumina phosphate and surface-bonded mono-and interaction of phosphate groups with the surface of alumina leads to the formation of alumina poly-phosphates that are stablemono-and to at leastpoly-phosphates 650 ◦ C. phosphate and surface-bonded that are stable to at least 650 °C. Based dataofofthe theoxide oxide precursors the literature datathe [42], the reduction Basedon onthe theH H22-TPR -TPR data precursors andand the literature data [42], reduction ◦ C were chosen to investigate the nickel phosphide phase formation on temperatures temperaturesofof550–650 550–650 °C were chosen to investigate the nickel phosphide phase formation on alumina.The TheXRD XRDpatterns patterns of 3 support, as well as alumina-supported nickel nickel phosphide alumina. of the theγ-Al γ-Al2O as well as alumina-supported phosphide 2O 3 support, catalysts differing in the precursor natures and the reduction temperatures, are shown in Figures 2 catalysts differing in the precursor natures and the reduction temperatures, are shown in Figures 2 and 3. and 3. Ni phosphate and POx gpoups reduction

H2 consumption, a.u.

NiPOx_A/Al2O3 NiPOx_I/Al2O3

NiPOx_I/SiO2

  - H3PO3 + H2O  H3PO4 + H2  - 4H3PO3  3H3PO4 + PH3

 

0

150

300 450 600 o Temperature, C

750

900

Figure 1. H1.2 -TPR profiles _A/Al andxNiPO x _I/Al 2 precursors. 2 3O 3 , NiPO 2 O3NiPO Figure H2-TPR profilesofofNiPO NiPOxx_A/Al 2O , NiPO x_I/Al 2O3 and _I/SiO2x _I/SiO precursors.

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NiP_A/Al O (550)

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Difference curve 0 I - Ni PDF #04-0850 I - Ni3P JCPDS #34-501

Difference curve I - Ni3P JCPDS #34-501 0

I - Ni PDF #04-0850

NiP_A/Al2O3(550)

O (600) I - NiNiP_A/Al P JCPDS #22-1190 2 3 12 5

2a)

-Al2O3

2b)

-Al2O3

Difference curve 0 I - Ni PDF #04-0850 I - Ni3P JCPDS #34-501

Difference curve I - Ni3P JCPDS #34-501 0

I - Ni PDF #04-0850

I - Ni12P5 JCPDS #22-1190

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Difference curve NiP_A/Al#34-501 O 650 I - Ni3P JCPDS 2 3

60

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-Al2O3 #22-1190 I - Ni12P5JCPDS Difference curve I - Ni3P JCPDS #34-501 I - Ni12P5 JCPDS #22-1190

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Figure 2. XRD patterns of NiP_A/Al2O3 catalyst prepared from phosphate precursor and reduced at Figure 2. XRD patterns of NiP_A/Al2O3 catalyst prepared from phosphate precursor and reduced at Figure 2. XRD of 600 NiP_A/Al from phosphate precursor curve. and reduced at temperature of patterns 550 °С (a), °С (b) or °С (c), prepared γ-Al2O3 support and the difference 2 O650 3 catalyst temperature of 550 °С (a), 600◦ °С (b) or 650 ◦°С (c), γ-Al2O3 support and the difference curve. ◦ temperature of 550 C (a), 600 C (b) or 650 C (c), γ-Al2 O3 support and the difference curve. NiP_I/Al O (600)

3a) 3a)

NiP_I/Al O (550)

2 3 NiP_I/Al O (550) 2 3 -Al2O3-Al O 2 3

Difference curve Difference curve - Ni P JCPDS #03-0953 I -I Ni P2JCPDS #03-0953

Difference curvecurve Difference I - Ni2PI JCPDS #03-0953 - Ni2P JCPDS #03-0953

20

20

30

30

3b) 3b)

3 NiP_I/Al2O32(600) -Al O3

-Al2O32

2

40 2(0) 50

60

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60

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NiP_I/Al2O3(650)

0 50 2 ( ) 0 50 2 ( )

40

60

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3c)

-Al2O3 NiP_I/Al O (650) 2 3

3c)

Difference curve -Al O3 I - Ni P2JCPDS #03-0953 2

Difference curve I - Ni2P JCPDS #03-0953

20

20

30

30

40

40

0

50

0

50

2 ( )

2 ( )

60

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Figure 3. XRD patterns of NiP_I/Al2 O3 catalyst prepared from phosphite precursor and reduced at temperature of 550 ◦ C (a), 600 ◦ C (b) or 650 ◦ C (c), γ-Al2 O3 support and the difference curve.


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The XRD patterns of NiP_A/Al2 O3 and NiP_I/Al2 O3 samples reduced at 550–650◦ C contain characteristic diffraction peaks from γ-Al2 O3 (PDF No. 29-0063) and nickel-containing phases. The difference intensity curves obtained by the subtraction of the XRD curve of the alumina from the catalyst ones help us to identify the Ni-containing phases. For NiP_A/Al2 O3 (550) sample the predominant phase was metallic Ni0 (PDF No. 04-0850) with average crystallite size equalled to 5.5 nm (Figure 2a, Table 1). Weak intensity peaks characteristic of Ni3 P phase (JCPDS No. 34-501) were also observed. With the reduction temperature increase to 600 ◦ C the predominant phase in NiP_A/Al2 O3 (600) sample has remained the metallic Ni0 (DXRD = 5.0 nm). Also, the presence of Ni3 P and Ni12 P5 (JCPDS No. 22-1190) phases was indicated (Figure 2b). The estimated Ni3 P and Ni12 P5 crystallite sizes were 4.0–4.5 nm and 12 nm, respectively. The further reduction of temperature increase to 650 ◦ C led to the disappearance of metallic nickel and the predominant Ni3 P phase (DXRD = 7.6 nm) formation with an additional amount of Ni12 P5 phase (DXRD = 11.5 nm) (Figure 2c). Thus, it was shown that for the set of NiP_A/Al2 O3 catalysts obtained from phosphate precursor increase in the reduction temperature from 550 to 650 ◦ C promotes the formation of the Ni3 P and Ni12 P5 phases and the average crystallite sizes growth. Nevertheless, the desired Ni2 P phase has not been obtained. The XRD patterns of NiP_I/Al2 O3 catalysts obtained from phosphite precursor are shown in Figure 3. Only the characteristic diffraction peaks corresponding to Ni2 P phase (JCPDS No. 03-0953) are found in the difference curve. With the growth of the reduction temperature from 550 to 650 ◦ C the estimated Ni2 P crystallites sizes are increased slightly from 3.8 nm to 4.4 nm. So, it is shown that using the phosphite-containing precursor makes it possible to produce highly dispersed Ni2 P crystallites on γ-Al2 O3 after reduction at 550–650 ◦ C. According to XRD analysis, the NiP_I/SiO2 (600) catalyst also contains the characteristic peaks of the Ni2 P phase with DXRD = 5.5 nm (Figure S2) [26]. Figure 4 shows the TEM images of NiP_A/Al2 O3 (650) and NiP_I/Al2 O3 (600) catalysts prepared from different precursors. The nickel phosphide particles with different size ranged in the region from 4.0 to 50 nm were observed in the NiP_A/Al2 O3 (650) sample (Figure 4a). Also, individual particles with the size up to 100 nm were found. On the contrary, NiP_I/Al2 O3 (550–650) samples demonstrate a uniform distribution of the nickel phosphide particles on the alumina surface (Figure 4b). The mean Ni2 P particle diameters of the NiP_I/Al2 O3 (550), NiP_I/Al2 O3 (600) and NiP_I/Al2 O3 (650) samples were estimated, equalling 5.1 nm, 5.6 nm and 6.0 nm, correspondingly. Thus, the mean Ni2 P particle diameters of NiP_I/Al2 O3 catalysts slowly increased with the reduction temperature growth from 550 to 650 ◦ C. The TEM image of NiP_A/Al2 O3 (650) catalyst (Figure S3a) reveals crystal lattice fringes with the d-spacing value of 2.16 Å, 2.47 Å and 2.83 Å corresponding to the (231), (031) and (130) reflections of the Ni3 P phase (PDF No. 34-501), respectively. Also, it was found the crystal lattice fringes with the d-spacing value of 2.12 Å and 2.55 Å corresponding to the (121) and (202) reflections of Ni3 (PO4 )2 (PDF No. 70-1796) that proved the formation of a thin layer of nickel phosphate after passivation covered the nickel phosphide particles (Figure S3a). The TEM images of NiP_I/Al2 O3 (600) catalyst in Figure S3b exhibit crystal lattice fringes with the d-spacing value of 2.03 Å and 2.22 Å corresponding to the (021) and (111) reflections of the Ni2 P crystalline phase (PDF No. 63-3444). The nature of the precursor of the active component affects not only the morphological features of the formed nickel phosphide particles on the alumina surface but also the number of acid sites on the surface. The NH3 -TPD technique was employed to explore the acidic properties of samples. Before NH3 -TPD experiments, the nickel phosphide catalyst precursors were reduced in situ in Autosorb-1 apparatus. Figure 5a,b shows the NH3 -TPD curves of NiP_A/Al2 O3 and NiP_I/Al2 O3 catalysts, as well as the γ-Al2 O3 support. The quantities of acid sites estimated by integration and further deconvolution of NH3 desorption peaks of applied materials are listed in Table 2.

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b) b)

Relative Relative frequency, frequency, % %

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TCDTCD signal, signal, a.u. a.u.

a) a)

750

Al2O3

750 600

NiP_A/Al  Al2O3 2O3(550) NiP_A/Al2O3(600) (550) 2 3 NiP_A/Al2O3(650) (600)

600 450

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200 200

300 400 300 400oC Temperature, o Temperature, C

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b) b)

750

TCDTCD signal, signal, a.u. a.u.

Figure 4. TEM images of NixPy/γ-Al2O3 catalysts prepared from different precursors: (a) Figure 4. TEM images NiNiP_I/Al Py /γ-Al O prepared from different precursors: (a) NiP_A/ Figure 4.2OTEM images NixP2yO 2O3 catalysts prepared from different precursors: (a) xof NiP_A/Al 3(650) andof(b) 3(600). 2/γ-Al 3 catalysts AlNiP_A/Al (b) NiP_I/Al 2O3(650) and (b) NiP_I/Al 2 O3 (650) and 2 O3 (600).2O3(600).

600 450

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500

600

Figure 5. NH 3-TPD proﬁles NixP2 O y/Alcatalysts prepared from different precursors: (a) Figure 5. NH profiles of NixofPy /Al prepared from different precursors: (a) NiP_A/ 3 -TPD 3 2O3 catalysts Figure 5. NH 3 -TPD proﬁles of Ni x P y /Al 2 O 3 catalysts prepared from different precursors: (a) NiP_A/Al 2 O 3 and (b) NiP_I/Al 2 O 3 and -Al 2 O 3 sample. Al2 O3 and (b) NiP_I/Al2 O3 and γ-Al2 O3 sample. NiP_A/Al2O3 and (b) NiP_I/Al2O3 and -Al2O3 sample. Table 2. Summary data of NH and MAS NMR methods for NiP_A/Al2 O32O and NiP_I/Al2 O23O3 3 -TPD Table 2. Summary data of NH 3-TPD and MAS NMR methods for NiP_A/Al 3 and NiP_I/Al catalysts, γ-Al O support as well as SiO and NiP_I/SiO catalyst. 2 3 2 2 Table 2. Summary data of MAS NMR 2methods catalysts, -Al2O3 support as NH well3-TPD as SiOand 2 and NiP_I/SiO catalyst. for NiP_A/Al2O3 and NiP_I/Al2O3 catalysts, -Al2O3 support as well as SiO2 and NH NiP_I/SiO 2 catalyst. -TPD MAS NMR Sample

Treduction , o C Sample

Sample γ-Al2 O3 γ-Al2O3 γ-Al2550 O3 NiP_A/Al2 O3 (550) NiP_A/Al2O3(550) NiP_A/Al2O3(550) NiP_A/Al2 O3 (600) 600 NiP_A/Al2O3(600) NiP_A/Al2O3(600) NiP_A/Al2 O3 (650) NiP_A/Al2650 O3(650) NiP_A/Al2O3(650) NiP_I/Al2 O3NiP_I/Al (550) 2550 O3(550) NiP_I/Al2O3(550) NiP_I/Al2 O3NiP_I/Al (600) 2600 O3(600) NiP_I/Al2O3(600) 2650 O3(650) NiP_I/Al2 O3NiP_I/Al (650) NiP_I/Al2O3(650) SiO2 SiO2 SiO2 NiP_I/SiO2(600) NiP_I/SiO2 (600) 600 NiP_I/SiO2(600)

3 3-TPD NH MAS NMR Treduction, o Overall xPMAS yx,Py , at.% AlPO 4, Quantity, Quantity, NiNi AlPO NH3-TPDOverall NMR 4 , at.% Quantity, T , C 31 NMR at.%27 Al NMR oCmax µmol/g Quantity, at.% Treduction , Tmax, oC µmol/g Overall Ni xPyP , AlPO4, μmol/g Quantity, 31P NMR 27Al NMR oC μmol/g Quantity, at.% at.% 237 Tmax, oC 106μmol/g - 27 421 31 237 106 μmol/g P NMR Al NMR 335 315 421 335 315 237 298 106 - 249 421 624 249 335 315 326 298 550 311 624 311 326 249 298 287 550 245 624 245 287 510 311 326 334 222 600 510 334 222 245 287 600 245 510 246 246 245 334 222 35 ± 10 25 ± 2 25 ± 2 440 650 306 440 35 ± 10 194 306 194 245 246 650 244 440 35 ± 10 25 ± 2 322 322 244 306 194 39 ± 10 22 ± 2 22 ± 2 477 550 308 477 39 ± 10 155 308 155 244 322 550 244 477 39 ± 10 22 ± 2 223 223 244 308 155 39 ± 10 25 ± 2 25 ± 2 354 600 285 354 39 ± 10 130 130 285 244 223 600 354 39 ± 10 25 ± 2 238 285 130 238 193 193 650 326 60 ± 10 60 ± 10 30 ± 2 30 ± 2 326 278 278 133 133 238 193 650 326 60 ± 10 30 ± 2 - 231 231 84 84 278 84 133 84 241 116 231 84 84 600 241 163 92 116 299 47 163 92 241 47 116 600 299 163 92 299 47


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NH3 -TPD curve of γ-Al2 O3 has two desorption peaks of ammonia centred at 237 ◦ C and 335 ◦ C. The first desorption peak around 237 ◦ C is usually attributed to the sites with the weakest acidity on the boundary of physiosorbed and chemisorbed ammonia, the second desorption peak at the temperature of 337 ◦ C was assigned to the moderate strength acid sites [57]. The total acidity of γ-Al2 O3 support is equal to 421 µmol/g. On the NH3 -TPD curves of NiP_A/Al2 O3 and NiP_I/Al2 O3 catalysts reduced at 550, 600 and 650 ◦ C, two desorption peaks of ammonia centred at the temperatures of 238–249 ◦ C and 278–334 ◦ C were also observed. According to the literature data, the acid sites of nickel phosphides are ascribed to weak Brønsted and Lewis acid sites specified by the presence of unreduced P-OH species and nickel sites with small positive charge (Niδ+ ) due to the electron transfer from the metal to the P atoms, respectively [44,58,59]. With the increase of the reduction temperature from 550 to 650 ◦ C the overall acidity of catalysts is decreased from 624 to 440 µmol/g and from 477 to 326 µmol/g for the samples obtained from the phosphate and phosphite precursor, correspondingly. The number of the weakest acid sites with Tmax = 238–249 ◦ C in all catalysts is increased in comparing with the alumina support. Whereas the amount of the moderate strength acid sites with Tmax = 278–334 ◦ C is retained only in NiP_A/Al2 O3 550 sample and decreased in the other investigated catalysts. The decrease of the amount of the moderate strength acid sites is caused by the shielding of the part of the alumina surface by the surplus of phosphorus-containing species especially in the samples prepared from the phosphite precursor. Earlier it was reported that the alumina-supported catalysts prepared from phosphate precursors also contain the increased number of Brønsted acid sites referring to O=P(OH)2 -O-Al species formed after interaction of phosphate precursor and alumina surface OH-groups or to free O=P(OH)3 entities interacting with the alumina surface via H-bonding [54,56]. The total acidity of silica-supported NiP_I/SiO2 (600) catalyst is lower than the acidity of alumina-supported samples (Table 2). On the NH3 -TPD curve of NiP_I/SiO2 (600) catalyst, the main desorption peak of ammonia centred at the temperature of 241 ◦ C corresponding to weak acidic sites and the minor peak corresponding to moderate strength acid sites at 299 ◦ C were observed. Figure 6 displays the 31 P 14 kHz MAS NMR spectrum of NiP_A/Al2 O3 catalyst after reduction at ◦ 650 C, which can be divided into two main regions. The narrow region around 0 ppm corresponds to compounds comprising different POx groups, including phosphorus oxides, phosphates, phosphites and surface POx groups [26,46,60–62]. The broad part of the spectrum stretching from ~500 to ~5000 ppm contains resonances of different metallic-like compounds of phosphorus, namely, nickel phosphides [26,46,63]. The most prominent signal in this part of the spectrum is located at ~1800 ppm. According to literature data, such shift corresponds to Ni3 P phase [46]. This line lies upon a wide resonance covering the area from ~1000 to ~2500 ppm, which can contain signals from several other nickel phosphides. The downfield shoulder of this resonance indicates the presence of Ni12 P5 phase that gives rise to two signals at ~2250 and ~1950 ppm. The upfield shoulder may correspond to NiP, Ni5 P4 and Ni2 P phases, however, the latter has two inequivalent sites with chemical shifts of ~1500 and ~4000 ppm and there is no signal with 4000 ppm shift on the spectrum, so it is unlikely that the sample contains Ni2 P particles. Since the line of Ni3 P is relatively narrow, we can assume that the particles of this phase are regular to some extent; moreover, this assumption is confirmed by the fact that this phase was observed earlier in the XRD pattern. On the contrary, the lines corresponding to other phases are too broad and shapeless, which can mean that their particles are small or inhomogeneous. Nonetheless, we can estimate the relative amount of phosphorus in nickel phosphides (roughly 35%, Table 2) in this sample from the ratio of spectral intensities integrated over the “phosphide” region (500–4500 ppm) and over the entire spectrum. The remaining phosphorus is present in the sample as different POx groups including AlPO4 . Figure 7 shows the experimental 14 kHz MAS 27 Al NMR spectrum of NiP_A/Al2 O3 (650) catalyst and its decomposition into three separate lines. Two of them belong to γ-Al2 O3 (octahedral site of Al2 O3 (O) at ~10 ppm and tetrahedral site of Al2 O3 (T) at ~70 ppm) and the third corresponds to the


9 of 23

AlO4 tetrahedra of AlPO4 [54,64]. All of the lines possess an irregular shape well described by the Simple Czjzek Model of Gaussian distribution of quadrupolar parameters [65]. Using this model, we deconvoluted the spectrum into the signals from γ-Al2 O3 and AlPO4 and determined the relative amount2018, of the equal to ~25 at% (Table 2). Catalysts 8, x latter FOR PEER REVIEW 9 of 23

31P Figure spectrum of of NiP_A/Al NiP_A/Al22O Figure 6. 14 14 kHz kHz MAS 31 NMR spectrum O33(650) (650) catalyst. catalyst. The The inset inset shows shows the the “phosphide” region multiplied by 16 for clarity. clarity.

Catalysts 2018, 8, x FOR PEER REVIEW

10 of 23

Since the line of Ni3P is relatively narrow, we can assume that the particles of this phase are regular to some extent; moreover, this assumption is confirmed by the fact that this phase was observed earlier in the XRD pattern. On the contrary, the lines corresponding to other phases are too broad and shapeless, which can mean that their particles are small or inhomogeneous. Nonetheless, we can estimate the relative amount of phosphorus in nickel phosphides (roughly 35%, Table 2) in this sample from the ratio of spectral intensities integrated over the “phosphide” region (500–4500 ppm) and over the entire spectrum. The remaining phosphorus is present in the sample as different POx groups including AlPO4. Figure 7 shows the experimental 14 kHz MAS 27Al NMR spectrum of NiP_A/Al2O3(650) catalyst and its decomposition into three separate lines. Two of them belong to γ-Al2O3 (octahedral site of Al2O3 (O) at ~10 ppm and tetrahedral site of Al2O3 (T) at ~70 ppm) and the third corresponds to the AlO4 tetrahedra of AlPO4 [54,64]. All of the lines possess an irregular shape well described by the Simple Czjzek Model of Gaussian distribution of quadrupolar parameters [65]. Using this model, we deconvoluted the spectrum into the signals from γ-Al2O3 and AlPO4 and determined the relative amount of the latter equal to ~25 at% (Table 2).

27 Figure Blue–experimental 14 14 kHz O33(650) (650) sample, sample, Figure 7. 7. Blue–experimental kHz MAS MAS 27Al Al NMR NMR spectrum spectrum of of NiP_A/Al NiP_A/Al22O black–theoretic calculated lines corresponding to different Al O and AlPO sites, red–sum of the black–theoretic calculated lines corresponding to different Al22O33 and AlPO44 sites, red–sum of the calculated lines. calculated lines.

Figure 8 displays the “phosphide” regions of 14 kHz MAS 31P NMR spectra of NiP_I/Al2O3(550–650) catalysts reduced from phosphite precursor at the temperatures of 550, 600 and 650 °С. These spectra contain two very broad resonances centred at ~1500 and ~4000 ppm characteristic of Ni2P phase, though, very disordered. Another signal with very large FWHM is located around 2500 ppm and probably connected with Ni 12P5 phase. Quantitative analysis of spectra of samples with different reduction temperatures gives the following result: relative content


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Figure 8 displays the “phosphide” regions of 14 kHz MAS 31 P NMR spectra of NiP_I/ Al2 O3 (550–650) catalysts reduced from phosphite precursor at the temperatures of 550, 600 and 650 ◦ C. These spectra contain two very broad resonances centred at ~1500 and ~4000 ppm characteristic of Ni2 P phase, though, very disordered. Another signal with very large FWHM is located around 2500 ppm and probably connected with Ni12 P5 phase. Quantitative analysis of spectra of samples with different reduction temperatures gives the following result: relative content of phosphorus included in phosphides in the sample reduced at 650 ◦ C is much higher (~60 at% vs. ~40 at%) than in the samples reduced at lower temperatures (Table 2), while the absolute intensity of lines in the “phosphide” region of these samples is close (Figure S4). It means that some POx groups are reduced only at temperatures higher than 600 ◦ C probably resulting in the formation of phosphine or some other volatile phosphorus compound, while no additional nickel phosphides are formed. 14 kHz MAS 27 Al NMR spectra of NiP_I/Al2 O3 samples reduced at 550, 600 and 650 ◦ C display the same three lines belonging to γ-Al2 O3 (~10 and ~70 ppm) and AlPO4 (~40 ppm) as the spectrum of NiP_A/Al2 O3 sample (Figure 9). From deconvolution of these spectra into separate lines, we calculated the relative content of Al in the form of AlPO4 that was equal to 22–30 at% for all samples (Table 2), while the support maintained the near to perfect ratio Al2 O3 (O)/Al2 O3 (T) = 1/2. From the results of 27 Al and 31 P NMR, it is likely that while the amount of AlPO slightly grows with increasing reduction 4 temperature, the overall phosphorus load of samples decreases due to the reduction of excess supported POx groups with the formation of phosphine or some other volatile phosphorus compound. Thus, 31 P and 27 Al MAS NMR of NiP_A/Al2 O3 (650) sample confirm the formation of aluminium phosphates in catalysts obtained by TPR of phosphate precursor, which hinders the formation of the desired Ni2 P phase and leads to an irreversible change in composition and textural characteristics of γ-Al2 O3 support according to literature data [37,40]. Neither the nature of the precursor nor the temperature of reduction can help to prevent the formation of aluminium phosphate on the surface of the catalyst. Summarizing the XRD, TEM and NMR results of catalyst characterization it could be concluded that Ni3 P and Ni12 P5 particles supported on the surface of γ-Al2 O3 are formed after reduction of phosphate precursor at the temperature of 650 ◦ C, while Ni2 P particles are produced from phosphite after reduction in the temperature range of 550–650 ◦ C. Catalysts 2018,precursor 8, x FOR PEER REVIEW 11 of 23

◦ C. 14kHz kHzMAS MAS3131 NMRspectra spectra NiP_I/Al O3 samples reduced at 550, 600 and 650The Figure 8. 14 P PNMR ofof NiP_I/Al 2O32 samples reduced at 550, 600 and 650 °C. The region not shown. region withwith POx PO groups is notisshown. x groups

Figure Catalysts 2018,8. 8, 14 515kHz MAS

P NMR spectra of NiP_I/Al2O3 samples reduced at 550, 600 and 650 °C. The 11 of 23 region with POx groups is not shown. 31

27Al NMR spectra of NiP_I/Al2O3 samples reduced at 550, 600 and 650 °C. Figure9. 9. 14 14 kHz kHz MAS MAS 27 Figure Al NMR spectra of NiP_I/Al2 O3 samples reduced at 550, 600 and 650 ◦ C.

2.2. Catalytic Catalytic Properties Properties of of Ni NixxPPyy/γ-Al /γ-Al2O MethylPalmitate PalmitateHDO HDO 2O 3 Catalysts 2.2. 3 Catalysts ininMethyl 2.2.1. The The Effect Effect of of Preparation Preparation Conditions Conditions on on the the Catalytic Catalytic Properties Properties of /γ-Al 2O 3 Catalysts 2.2.1. of Ni NixxPPyy/γ-Al 2O 3 Catalysts inin Methyl Palmitate HDO Methyl Palmitate HDO To evaluate impact of the conditions on the catalytic the Nix Py /Al O3 To evaluatethe the impact ofpreparation the preparation conditions on theproperties catalytic of properties of 2the systems in the HDO of methyl palmitate several alumina-supported nickel phosphide catalysts NixPy/Al2O3 systems in the HDO of methyl palmitate several alumina-supported nickel phosphide differingdiffering in the precursor nature and reduction temperature were studied. It was shown in the catalysts in the precursor nature and reduction temperature were studied. It was shown in previous section that the reduction of the samples prepared from phosphite precursor at 550, 600 the previous section that the reduction of the samples prepared from phosphite precursor at 550, and 600 650 ◦650 C gives rise to Nito phase supported on alumina (NiP_I/Al2 O3 catalysts). The Ni3 PNi phase with 2 P Ni and C gives rise 2P phase supported on alumina (NiP_I/Al 2O3 catalysts). The 3P phase the admixture of Ni12ofP5Ni and some unreduced species have been observed in NiP_A/Al2 O3 catalyst with the admixture 12P5 and some unreduced species have been observed in NiP_A/Al 2O3 prepared from phosphate precursor, even after reduction at 650 ◦ C.atTherefore, NiP_A/Al 2 O3 catalyst catalyst prepared from phosphate precursor, even after reduction 650 C. Therefore, NiP_A/Al 2O3 ◦ C) and series of NiP_I/Al O samples, prepared by reduction at 550, 600 and 650 ◦ C, (reduced at 650 2 3 catalyst (reduced at 650 C) and series of NiP_I/Al2O3 samples, prepared by reduction at 550, 600 and wereC, chosen the comparative study in the HDO methyl reaction The was reaction carried out at 650 werefor chosen for the comparative study inofthe HDOpalmitate. of methylThe palmitate. was ◦ C, hydrogen pressure — 3.0 MPa, H/C ratio — 600 Nm3 /m3 , methyl palmitate the temperature of 290 carried out at the temperature of 290 C, hydrogen pressure — 3.0 MPa, H/C ratio — 600 Nm3/m3, LHSV — 2.4-12 h−1 for 8 h, during this time the concentrations of the reagent and the products have reached a constant value. The catalytic properties of Nix Py /Al2 O3 catalysts are shown in Figure 10. The presented data indicate that NiP_A/Al2 O3 catalyst is significantly inferior in activity compared to the NiP_I/Al2 O3 samples and this tendency is conserved within the temperature region of 250–330 ◦ C (Figure S5). The higher activity of NiP_I/Al2 O3 catalysts points out that Ni3 P/Ni12 P5 phases display lower activity in the HDO of methyl palmitate then the Ni2 P one. These results agree with the data obtained earlier for silica-supported systems, demonstrating the higher activity of Ni2 P/SiO2 catalyst in comparison with the Ni12 P5 /SiO2 system [17,20,27]. NiP_I/Al2 O3 catalysts also revealed higher selectivity to C18 in comparison with NiP_A/Al2 O3 , the same tendency was observed in the HDO of methyl laurate [17], methyl oleate [20] and methyl palmitate [27] over Ni2 P/SiO2 and Ni12 P5 /SiO2 catalysts. According to the proposed explanation, the enhancing of the positive charge of metal Ni sites upon transition from Ni3 P and Ni12 P5 to Ni2 P favours the activation of C=O groups and promotes the direct HDO reactions [17]. The activity of NiP_I/Al2 O3 catalysts goes through the maximum with the increase of reduction temperature from 550 to 650 ◦ C the optimal temperature being 600 ◦ C (Figure 10). Consequently, this sample, designated further as NiP_I/Al2 O3 was chosen for detailed study in the HDO of methyl palmitate.

reactions [17]. The activity of NiP_I/Al2O3 catalysts goes through the maximum with the increase of reduction temperature from 550 to 650 C the optimal temperature being 600 C (Figure 10). Consequently, this sample, designated further as NiP_I/Al 2O3 was chosen for detailed study in the HDO of methyl palmitate. Catalysts 2018, 8, 515

12 of 23

80

a)

25

b)

TTPR

TTPR

TTPR o 550 C 600oC TTPR o 650 C

o

600 C

40

20

TTPR

o

550 C

o

650 C

SC16, %

MP conversion, %

TTPR

60

TTPR o

650 C

15 10

TTPR o

650 C

20

5 0

0

NiP_A/Al2O3

NIP_I/Al2O3

NiP_A/Al2O3

NiP_I/Al2O3

Figure 10. Conversion (a) and selectivity (b) of NiP_A/Al2 O3 and NiP_I/Al2 O3 catalysts in HDO of Figure 10. Conversion andTselectivity of NiP_A/Al2O 3 3 and 3 NiP_I/Al2O3 catalysts in HDO −1 of methyl palmitate (PH2 = 3.0(a) MPa, = 290 ◦ C, H(b) 2 /feed = 600 Nm /m , methyl palmitate LHSV = 9 h ). methyl palmitate (PH2 = 3.0 MPa, T = 290 °C, H2/feed = 600 Nm3/m3, methyl palmitate LHSV = 9 h−1).

2.2.2. Comparison of Ni2 P/Al2 O3 and Ni2 P/SiO2 Catalysts in the HDO of Methyl Palmitate 2.2.2. Comparison of Ni2P/Al2O3 and Ni2P/SiO2 Catalysts in the HDO of Methyl Palmitate Figure 11 shows the dependencies of the conversion of methyl palmitate and oxygen-containing Figure shows the temperature dependencies of the conversion of 2methyl and oxygen-containing compounds on11the reaction over NiP_I/Al P/Al2 Opalmitate 2 O3 (Ni 3 ) and NiP_I/SiO 2 (Ni2 P/SiO2 ) compounds on the2 reaction temperature 3 (Ni2P/Aland 2O3) contained and NiP_I/SiO 2 (Ni2P/SiO2) catalysts. NiP_I/SiO catalyst was preparedover usingNiP_I/Al the same2Oprocedure the comparable catalysts. 2 catalyst was prepared using the same and contained comparable amount of NiP_I/SiO Ni and the mean particle diameter equalled to procedure 5.5 nm (Table 1). In all the experiments, amount of Ni and the mean particleand diameter equalled to 5.5 nm (Table 1). In allwith experiments, the the conversions of methyl palmitate O-containing compounds are increased the growth methyl palmitate and 2O-containing compounds are increased with the growth ofconversions temperature;ofwherein the NiP_I/SiO catalyst displays lower values of conversion. The minorof temperature; wherein the NiP_I/SiO 2 catalyst displays lower values of conversion. The minor difference between the dependencies of methyl palmitate and O-containing compounds conversions between dependencies of methyl2palmitate O-containing compounds conversions ondifference temperature was the observed over NiP_I/SiO catalyst, and indicating that only small amounts of on temperature was observed are overpresent NiP_I/SiO 2 catalyst, that only oxygen-containing intermediates in the reaction indicating mixture. Indeed, the small resultsamounts of liquidof oxygen-containing intermediates are present in the reaction mixture. Indeed, the results of liquid product analysis showed only negligible amounts of oxygen-containing intermediates among the productproducts analysis in showed only temperature negligible amounts of oxygen-containing intermediates among the reaction the whole range and the main products are hexadecane and reaction products in the whole temperature range the main products are hexadecane and pentadecane (Figure 12). Such behaviour is typical for alland silica-supported Nix Py systems, regardless (Figure 12). Such behaviour isconditions typical for[24–27], all silica-supported xPyin systems, regardless ofpentadecane the precursor, Ni content or preparation pointing outNi that the presence of precursor, Nithe content ormethyl preparation conditions [24–27], pointing presence of Niof rate of palmitate conversion is lower thanout thethat ratesinofthe oxygenated x Pthe y /SiO 2 catalysts NixPy/SiO2 catalysts the rate methyl palmitate is lower than the rates ofdetermines oxygenated intermediates conversion. As a of consequence, the rateconversion of methyl palmitate transformation intermediates conversion. As a consequence, rate of methyl palmitate transformation determines the overall rate of methyl palmitate HDO overthe NiP_I/SiO The conclusion was confirmed 2 catalyst. rate experiments of methyl palmitate HDO over NiP_I/SiO 2 catalyst. The waslaurate confirmed inthe theoverall additional by the comparison of HDO conversion rateconclusion of the methyl andin Catalysts 2018, 8, x FOR PEER REVIEW 13 of 23 the additional experiments bycompounds: the comparison of acid, HDOdodecanal conversionand rate of the methyl laurate and the the corresponding intermediate lauric dodecanol (Figure S6). It was corresponding compounds: lauricmethyl acid, dodecanal and dodecanol (Figure S6). It was shown, that in theintermediate presence of NiP_I/SiO laurate demonstrated the lowest conversion 2 catalyst shown, that in the presence of NiP_I/SiO 2 catalyst methyl laurate demonstrated the lowest among the listed oxygen-containing substrates. conversion among the listed oxygen-containing substrates. NiP_I/Al2O3(600)

XO

100

XMP

Conversion, %

80

XO

NiP_I/SiO2(600)

XMP

60 40 20 0 240

260

280 300 o Temperature, C

320

340

Figure 11. 11. Temperature Temperature effect effect on on the the conversion conversion of of methyl methyl palmitate palmitate (solid (solid symbols, symbols, solid solid lines) lines) and and Figure oxygen-containing compounds symbol, dashdash lines) lines) over NiP_I/Al NiP_I/SiO oxygen-containing compounds(empty (empty symbol, over NiP_I/Al 3 and NiP_I/SiO 2 2 O3 and2O 2 catalysts ◦ C, H /feed = 600 Nm3 /m3 , 3methyl (PH2 = 3.0(PMPa, T =MPa, 250–330 palmitate LHSVLHSV = 3.6–9 h−1 ).h−1). catalysts H2 = 3.0 T = 250–330°C, H2/feed = 600 Nm /m3, methyl palmitate = 3.6–9 2

a) (a) 100

NiP_I/SIO2(600)

MP n-C15H32

(b) b)

C15H31COOH 1.0

C15H31CHO

Sum of O-containing intermediates

ol.%

ol.%

n-C16H34 80

C15H31COOC16H33

0.8

C16H33OH-1 C16H33OH-2

Ni2P/SIO2 (I) 600

240

260

280 300 o Temperature, C

320

340

Figure 11. Temperature effect on the conversion of methyl palmitate (solid symbols, solid lines) and Catalystsoxygen-containing 2018, 8, x FOR PEER REVIEW 13 2of 23 compounds (empty symbol, dash lines) over NiP_I/Al2O3 and NiP_I/SiO Catalysts 2018, 8, 515 13 of 23 3 3 −1

catalysts (PH2 = 3.0 MPa, T = 250–330°C, H2/feed = 600 Nm /m , methyl palmitate LHSV = 3.6–9 h ).

shown, that in the presence of NiP_I/SiO2 catalyst methyl laurate demonstrated the lowest conversion among the listed oxygen-containing substrates.

Concentration, mol.%

100

(b) b)

MP n-C15H32

NiP_I/SIO2(600)

C15H31CHO

SumXof O-containing MP intermediates

80

XO

Conversion, %

60 40 20 0

XMP

60 40

0.5

1.0

1.5

2.0

C16H33OH-1

0.8

C H OH-2

0.6

16 33 NiP_I/SiO 2(600)

n-C15H30 n-C16H32

0.4 0.2 0.0

20 0.0

Ni2P/SIO2 (I) 600

H31COOC H NiP_I/AlC215O 3(600)16 33

n-C16XHO34

100

80

C15H31COOH 1.0


a) (a)

2.5

3.0

3.5

0.0

4.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Contact time, min

Contact time, min 0 240

260

280

300

320

340

Figure 12.12. Concentration profiles of of thethe liquid-phase components during methyl palmitate HDO Figure Concentration profiles liquid-phase during methyl palmitate HDO o components Temperature, C over NiP_I/SiO (600) catalyst: (a) MP, C and C hydrocarbons and sum of oxygen-containing over NiP_I/SiO 2 (600) catalyst: (a) MP, C 15 and C 16 hydrocarbons and sum of oxygen-containing 2 15 16 ◦ C, H /feed = 600 Nm3 /m33 , 3 intermediates; (b)(b) thethe observed intermediates (PH2 3.0 MPa, T = T290 intermediates; observed intermediates (P=H2 = 3.0 MPa, =(solid 290 °C, = 600 Nmand /m , 2 H2/feed Figure 11. Temperature effect on the conversion of methyl palmitate symbols, solid lines) contact time = 0.83–3.75 min). contact time = 0.83–3.75 min). (empty symbol, dash lines) over NiP_I/Al2O3 and NiP_I/SiO2 oxygen-containing compounds

catalysts (PH2 = 3.0 MPa, T = 250–330°C, H2/feed = 600 Nm3/m3, methyl palmitate LHSV = 3.6–9 h−1).

The comparison of NiP_I/Al2 O3 and NiP_I/SiO2 catalysts in methyl palmitate HDO shows that MP a) of γ-Al2 O3 as the support allows b) increase of the methyl palmitate conversion. the use the significant n-C H Ni2P/Al2O3 (I) 600 100 ◦ C and from 35 C Hto COOH a)conversion of methyl palmitate n-C Hwas raised from (a) (b) b) 22 to7 67% So, the 96% at Ni2CP/Al O290 (I) 600 Hat COOH MP 2 3 Ni P/SIO (I) 600 C H COOC H NiP_I/SIO (600) 6 Summ of O-containing 2 2 2 1.0 n-C H used instead of NiP_I/SiO C H COOC H 310 ◦ C when NiP_I/Al O catalyst was (Figure 11). It should be 100 80 2 3 2 C H CHOnoted, intermediates C H CHO n-C H 5 H OH-1 that both catalysts demonstrated stable time-on-stream activity, atOH-1 least for 20 hours C(Figure S7). C H 0.8 Sum of O-containing 80 60 C H OH-2 4 C H OH-2 intermediates As opposed to NiP_I/SiO2 catalyst, a significant difference was observed between the conversion of n-C H n-C H 0.6 3 40 n-C H 60 methyl palmitate and the conversion of oxygen-containing compounds n-C H in the HDO of methyl palmitate 2 ◦ C. Besides, the appreciable 0.4 over NiP_I/Al O catalyst at 250–290 amounts of oxygen-containing 20 2 3 40 1 intermediates, like palmitic acid, hexadecanal, hexadecanols and palmityl palmitate were determined 0.2 20 0 0 among the reaction products difference in the content of0.6oxygenated compounds 0.0 0.2 0.4 0.6 (Figure 0.8 1.0 13). 1.2 The 1.4 0.0 0.2 0.4 0.8 1.0 1.2 1.4 0.0 time, min 0 among the products ofContact methyl palmitate HDO over NiP_I/SiO and NiP_I/Al O catalysts 2 1.0 Contact 3 3.5 4.0 is seen 0.0 0.5 1.5 2.0 time, 2.52 min 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 explicitly in Figure 14 Contact demonstrating the content of oxygenated intermediates atmin the same conversion Contact time, time, min Figure 13. Concentration profiles of the liquid-phase components during methyl palmitate HDO of methyl palmitate (about 57%). The observed results indicate that the rate of methyl palmitate over NiP_I/Al 2O3(600) catalyst: (a) MP, C15 and C16 hydrocarbons and sum of oxygen-containing Figure 12. Concentration profiles of the liquid-phase during methyl palmitate HDO conversion over alumina-supported catalysts is highercomponents than the conversion rates of intermediate intermediates; (b) the observed intermediates (P H2 = 3.0 MPa, T = 290 °C, H2/feed = 600 Nm3/m3, over NiP_I/SiO 2 (600) catalyst: (a) MP, C 15 and C 16 hydrocarbons and sum of oxygen-containing oxygenating compounds. To explain the observed difference in the behaviour of silica-supported contact time =(b) 0.50–1.25 min). intermediates (PH2 = 3.0 MPa, T = 290 °C, H2/feed = 600 Nm3/m3, intermediates; the and alumina-supported Niobserved 2 P catalysts the possible routes of methyl palmitate transformation should contact time = 0.83–3.75 min). be analysed. 15

32

16

a)

7

n-C16H34 Summ of O-containing intermediates

80

15

31

15

31

15

31

16

33

16

33

16

15

31

15

31

15

31

16

33

16

15

30

16

32

16

33

33

15

30

16

32

b)

MP n-C15H32

Ni2P/Al2O3 (I) 600

60 40 20 0

6


100


34


16

34

32




15

C15H31COOH

Ni2P/Al2O3 (I) 600

C15H31COOC16H33 C15H31CHO

5

C16H33OH-1 C16H33OH-2

4

n-C15H30

3

n-C16H32

2 1 0

0.0

0.2

0.4

0.6

0.8

1.0

Contact time, min

1.2

1.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Contact time, min

Figure13. 13.Concentration Concentration profiles profiles of of the theliquid-phase liquid-phasecomponents componentsduring duringmethyl methylpalmitate palmitateHDO HDO Figure overNiP_I/Al NiP_I/Al22O33(600) catalyst: (a) MP, MP, CC1515 and and CC1616 hydrocarbons hydrocarbons and andsum sumofofoxygen-containing oxygen-containing over ◦ C,°C, 3 /m 3 ,3, 3/m intermediates;(b) (b)the theobserved observedintermediates intermediates(P(P 3.0MPa, MPa, T 290 = 290 2/feed 600 Nm intermediates; T= H2H /feed = =600 Nm H2H2= =3.0 contacttime time==0.50–1.25 0.50–1.25min). min). contact

33

Catalysts 2018, 8, 515 Catalysts 2018, 8, x FOR PEER REVIEW

1414 of of 2323

Content of O-containing compounds, mol.%

8 7 6

XMP=57% LHSV=9h

XMP=57% LHSV=2h

-1

PA C16O C16OH

-1

PP

5 4 3 2 1

XMP=56% -1

LHSV=2.4h

0 Ni2P/Al2O3 Ni12P /Al2O3 Ni2P/SiO2 5 (I) 600 (I) 600 (A) 650

Figure of oxygen-containing compounds depending on the catalyst: Figure14. 14.Composition Composition of oxygen-containing compounds depending on the NiP_I/ catalyst: ◦ C, P AlNiP_I/Al O (600), NiP_A/Al O (650) and NiP_I/SiO (600) catalysts at T = 290 = 3.0 2 3 2 3 2 2 /feed 2O3(600), NiP_A/Al 2O3(650) and NiP_I/SiO 2(600) catalysts at T = H2 290 °C, MPa, PH2 =H3.0 MPa, 3 = 600 Nm3=/m palmitate LHSV wasLHSV adjusted obtain nearly the same methyl palmitate 3/m3 and H2/feed 600and Nmmethyl methyl palmitate wastoadjusted to obtain nearly the same methyl conversion h–1 ). Notation: acid, CPA–palmitic C16 OH–hexadecanol, 16 O–hexadecanal, palmitate (2.4–12 conversion (2.4–12 PA–palmitic h–1). Notation: acid, C16O–hexadecanal, PP–palmityl palmitate. C16OH–hexadecanol, PP–palmityl palmitate.

2.2.3. Reaction Scheme The comparison of NiP_I/Al2O3 and NiP_I/SiO2 catalysts in methyl palmitate HDO shows that The of methylallows palmitate transformation overof silica-supported nickel phosphide the use reaction of γ-Al2networks O3 as the support the significant increase the methyl palmitate conversion. catalysts were widely discussed in the literature [17,19,22–25,27]. Scheme 1 represents So, the conversion of methyl palmitate was raised from 22 to 67% at 290 C and fromthe 35 reaction to 96% at network methyl palmitate HDO, including intermediate and final product 310 C for when NiP_I/Al 2O3 catalyst was usedthe instead of NiP_I/SiO 2 (Figure 11).ofItmethyl shouldpalmitate be noted, conversion. Palmitic demonstrated acid and hexadecanal are usually considered primary products of methyl that both catalysts stable time-on-stream activity, as at the least for 20 hours (Figure S7). As palmitate Palmitic acid can bedifference producedwas through the hydrogenolysis of the C-Oof opposedhydroconversion. to NiP_I/SiO2 catalyst, a significant observed between the conversion bond in the methoxy group metal sites or via hydrolysis compounds over acid sites, giving methane or methyl palmitate and the over conversion of oxygen-containing in the HDO of methyl methanol, correspondingly [17,25,27,31]. Hydrogenolysis of the ester C-O bond over metal sites leads palmitate over NiP_I/Al2O3 catalyst at 250–290 C. Besides, the appreciable amounts of tooxygen-containing hexadecanal and methanol. Furtherlike conversion of palmitic acid was proposed to give pentadecane intermediates, palmitic acid, hexadecanal, hexadecanols and palmityl through the decarboxylation reaction or hexadecanal as a result of hydrogenation. Decarboxylation of palmitate were determined among the reaction products (Figure 13). The difference in the content palmitic acid can proceed to some extent over NiP_I/SiO , NiP_I/Al O and NiP_A/Al O catalysts 2 2 3 HDO over NiP_I/SiO 2 3 of oxygenated compounds among the products of methyl palmitate 2 and because the2Osmall amounts of COexplicitly among the gas productsthe butcontent the primary route of 2 were observed NiP_I/Al 3 catalysts is seen in Figure 14 demonstrating of oxygenated pentadecane formation decarbonylation of hexadecanal. The sum of CO and The CO2 observed correlates results with intermediates at the is same conversion of methyl palmitate (about 57%). pentadecane content (Figure S8). In addition, hexadecanol-1 via hydrogenation that indicate that the rate of methyl palmitatehexadecanal conversion gives over alumina-supported catalysts is higher inthan turn is transformed hexadecane through theoxygenating subsequent reactions of dehydration hexadecene the conversiontorates of intermediate compounds. To explainto the observed and hydrogenation to hexadecane. difference in the behaviour of silica-supported and alumina-supported Ni2P catalysts the possible The of scheme of palmitate methyl palmitate conversion includes some parallel and sequential transformations routes methyl transformation should be analysed. that can take place with the participation of acidic (ether hydrolysis, alcohol dehydration) and metal (hydrogenation and hydrogenolysis reactions). It is known that nickel phosphides are 2.2.3.sites Reaction Scheme characterized by the presence of metallic and acidic centres, caused by an ensemble (geometric) and/or The reaction networks of methyl palmitate transformation over silica-supported nickel ligand (electronic) effect of phosphorus [66,67]. Taking in mind the inertness of SiO2 , the mechanism of phosphide catalysts were widely discussed in the literature [17,19,22–25,27]. Scheme 1 represents ester conversion over NiP_I/SiO2 catalyst is determined by the electronic properties of the metal sites the reaction network for methyl palmitate HDO, including the intermediate and final product of and the acidity of the nickel phosphide phase. It was shown, that nickel phosphide has both Lewis and methyl palmitate conversion. Palmitic acid and hexadecanal are usually considered as the primary Brønsted acid sites, that are usually ascribed to the positively charged metal cations (Niδ+ ) and surface products of methyl palmitate hydroconversion. Palmitic acid can be produced through the P-OH groups, respectively [17,58]. The correlation was also found between the acidity of Ni2 P/SiO2 hydrogenolysis of the C‒O bond in the methoxy group over metal sites or via hydrolysis over acid catalysts, differing in precursor and preparation conditions and the content of phosphate groups sites, giving methane or methanol, correspondingly [17,25,27,31]. Hydrogenolysis of the ester C‒O determined by means of 31 P NMR [26,27]. It was proposed, that P-OH groups of silica-supported bond over metal sites leads to hexadecanal and methanol. Further conversion of palmitic acid was Ni2 P participate in the hydrolysis of aliphatic ester (methyl laurate) as well as assist the conversion of proposed to give pentadecane through the decarboxylation reaction or hexadecanal as a result of methyl laurate adsorbed on the metal sites via decarbonylation or hydrogenation reactions [17]. hydrogenation. Decarboxylation of palmitic acid can proceed to some extent over NiP_I/SiO2, NiP_I/Al2O3 and NiP_A/Al2O3 catalysts because the small amounts of CO2 were observed among the gas products but the primary route of pentadecane formation is decarbonylation of hexadecanal. The sum of CO and CO2 correlates with pentadecane content (Figure S8). In addition, hexadecanal gives hexadecanol-1 via hydrogenation that in turn is transformed to hexadecane through the subsequent reactions of dehydration to hexadecene and hydrogenation to hexadecane.

metal cations (Niδ+) and surface P-OH groups, respectively [17,58]. The correlation was also found between the acidity of Ni2P/SiO2 catalysts, differing in precursor and preparation conditions and the content of phosphate groups determined by means of 31P NMR [26,27]. It was proposed, that P-OH groups of silica-supported Ni2P participate in the hydrolysis of aliphatic ester (methyl laurate) as well as 2018, assist8, 515 the conversion of methyl laurate adsorbed on the metal sites via decarbonylation Catalysts 15 ofor 23 hydrogenation reactions [17].

H2 -CH4

H2 -CH3OH

H2O -CH3OH

H2

H2

-H2O

-H2

-CO2

H2

-CO

-H2O -CO2 -H2

H2

H2 -CO -H2

-H2O

Scheme 1. 1. Probable Probable reaction reaction network network for for HDO HDO of of methyl methyl palmitate. palmitate. Red Red lines–the lines–the decarboxylation decarboxylation Scheme and decarbonylation routes, green lines–the hydrodeoxygenation route. and decarbonylation routes, green lines–the hydrodeoxygenation route.

The higher activity of NiP_I/Al2 O3 catalyst in methyl palmitate HDO along with the higher The higher activity of NiP_I/Al2O 3 catalyst in methyl palmitate HDO along with the higher production of palmitic acid in the reaction products can be explained by the participation of production of palmitic acid in the reaction products can be explained by the participation of acid acid sites on the alumina support in the conversion of methyl palmitate through hydrolysis sites on the alumina support in the conversion of methyl palmitate through hydrolysis route. Indeed, route. Indeed, the alumina-supported Ni2 P catalyst is characterized by the higher amount of the alumina-supported Ni2P catalyst is characterized by the higher amount of acid sites in acid sites in comparison with NiP_I/SiO2 sample according to NH3 -TPD data (Table 2). It is well comparison with NiP_I/SiO2 sample according to NH3-TPD data (Table 2). It is well established, that established, that alumina can provide the acid-catalysed reactions, such as hydrolysis, dehydration alumina can provide the acid-catalysed reactions, such as hydrolysis, dehydration and esterification and esterification [32,35,40,68]. In our case the exact nature of acid sites on the surface of NiP_I/Al O [32,35,40,68]. In our case the exact nature of acid sites on the surface of NiP_I/Al 2O3 catalyst2 is3 catalyst is undisclosed. NiP_I/Al2 O3 catalyst produces the increasing amounts of the product of undisclosed. NiP_I/Al2O3 catalyst produces the increasing amounts of the product of acid-catalysed acid-catalysed esterification, namely palmityl palmitate (Figure 14), thereby indirectly confirming esterification, namely palmityl palmitate (Figure 14), thereby indirectly confirming our assumption our assumption about the acceleration of acid-catalysed reactions over alumina-supported Ni2 P in about the acceleration of acid-catalysed reactions over alumina-supported Ni2P in comparison with comparison with silica-supported one. Despite on the higher acidity of NiP_A/Al2 O3 catalyst in silica-supported one. Despite on the higher acidity of NiP_A/Al2O3 catalyst in comparison with comparison with NiP_I/Al2 O3 sample, NiP_A/Al2 O3 catalyst displays lower activity in methyl NiP_I/Al2O3 sample, NiP_A/Al 2O3 catalyst displays lower activity in methyl palmitate palmitate hydrodeoxygenation. This observation can be explained by the assumption that lower hydrodeoxygenation. This observation can be explained by the assumption that lower activity of activity of Ni P/Ni12 P5 in metal-catalysed reactions led to the accumulation of oxygen-containing Ni3P/Ni12P5 in 3metal-catalysed reactions led to the accumulation of oxygen-containing intermediates intermediates that retard the reversible hydrolysis reaction. Therefore, the balance between active that retard the reversible hydrolysis reaction. Therefore, the balance between active metal sites of metal sites of Ni2 P and acid sites of support is necessary for the design of the efficient catalyst for the Ni2P and acid sites of support is necessary for the design of the efficient catalyst for the fatty acid fatty acid esters hydrodeoxygenation. The obtained results prove our assumption that the activity of esters hydrodeoxygenation. The obtained results prove our assumption that the activity of the the phosphide catalysts in the HDO of aliphatic esters can be improved by the employment of the support with the acidic properties instead of silica [31]. Detailed kinetic experiments may subsequently throw light on the mechanism of methyl palmitate hydrodeoxygenation and help to evaluate the contribution of metal- and acid-catalysed reactions that take place over multifunctional catalysts. 3. Materials and Methods 3.1. Materials Commercial γ-Al2 O3 of “VGO” brand (ABET – 205 m2 /g, pore volume – 0.70 cm3 /g, average pore diameter – 13.6 nm) and SiO2 of “KSKG” brand (ABET – 300 m2 /g, pore volume – 0.80 cm3 /g, average pore diameter – 10.6 nm) were purchased from “Promkataliz” Ltd. (Ryazan, Russia) and «ChromAnalit» Ltd. (Salavat, Russia), respectively. Prior to use for catalyst preparation, granules of support were dried at 110 ◦ C for 4 h and calcined at 500 ◦ C for 4 h. Afterward, it was crushed and sieved to a fraction


16 of 23

of 0.25–0.50 mm. Ni(NO3 )2 ·6H2 O (“Reakhim,” Samara , Russia, ≥99%), Ni(OH)2 (“Acros Organics,” New Jersey, NJ, USA, ≥98%), (NH4 )2 HPO4 (“Alfa Aesar,” Ward Hill, MA, USA, technical grade, P2 O5 ≤ 53 wt%), H3 PO3 (“Sigma-Aldrich,” Saint Louis, MO, USA, ≥97%) and HNO3 (“Reakhim,” Samara, Russia, ≥70%) were used for the catalyst preparation. The chemical products utilized in the reactivity investigation were methyl palmitate (C15 H31 COOCH3 , “Sigma-Aldrich,” Saint Louis, MO, USA, ≥97%), n-octane (“Kriokhrom,” Saint Petersburg, Russia, ≥99.5%) and n-dodecane (C12 H26 , “Acros Organics,” New Jersey, NJ, USA, ≥99%). 3.2. Catalyst Preparation Two sets of alumina-supported nickel phosphide catalysts with the nickel content about 7.5 wt% were prepared by the incipient wetness impregnation method from the phosphate- or phosphite-containing precursors with the subsequent temperature-programmed reduction. Catalyst preparation from the phosphate-containing precursor (A). Support (γ-Al2 O3 ) was incipiently impregnated with an aqueous solution of (NH4 )2 HPO4 and Ni(NO3 )2 , followed by drying in air at room temperature overnight and at 110 ◦ C for 3 h and then calcination at 500 ◦ C in air for 3 h. The initial Ni/P molar ratio in precursor was 0.5. Then the oxide precursor was reduced with H2 flow (100 mL/(min·g)) using the temperature program: heating to 370 ◦ C at a ramp rate of 3 ◦ C/min and then to 550, 600 or 650 ◦ C at a ramp rate of 1 ◦ C/min and keeping at the reduction temperature for 1 h. After reduction, the samples were passivated at room temperature for 2 h in 1 vol% O2 /He flow (40 mL/(min·g)) before taking out into the air for investigation by physicochemical methods. For the reactivity test, the catalysts were reduced in the catalytic reactor before experiments. The catalysts prepared from phosphate-containing precursor are denoted on the base of the precursor type (A) and the TPR temperature, for example, NiP_A/Al2 O3 (550) (shown in Table 1). Catalyst preparation from the phosphite-containing precursor (I). The support (γ-Al2 O3 , SiO2 ) was incipiently impregnated with an aqueous solution of Ni(OH)2 and H3 PO3 with consequent drying in air at room temperature overnight and at 80 ◦ C for 24 h. The initial Ni/P molar ratio in precursor was 0.3. Dried precursors were reduced directly without calcination by TPR in H2 flow (100 mL/(min·g)) using the following temperature program: heating to 550, 600 or 650 ◦ C at a ramp rate of 1 ◦ C/min and keeping at the reduction temperature for 1 h. The catalysts prepared from phosphite precursor are denoted on the base of the precursor type (I) and the TPR temperature, for example, NiP_I/Al2 O3 (550) (Table 1). For comparison, the reference silica-supported Ni2 P/SiO2 catalyst (NiP_I/SiO2 (600)) was prepared by the same procedure. 3.3. Catalyst Characterization The elemental analysis of the samples was conducted using inductively coupled plasma atomic emission spectroscopy (ICP-AES) on Optima 4300 DV (Perkin Elmer, France). The textural properties of the catalysts were characterized using N2 physisorption at 77 K with an ASAP 2400 instrument (Micromeritics, Norcross, GA, USA). The specific surface area (ABET ) was determined using a multipoint Brunauer-Emmett-Teller (BET) model. The average pore diameter (Dpore ) and the pore size distribution were determined by the Barret-Joyner-Halenda (BJH) method using the desorption branch of the isotherm. The total pore volume (Vpore ) was estimated at a relative pressure of 0.99. The reduction peculiarities of the catalyst precursors were determined by the H2 temperatureprogrammed reduction method (H2 -TPR). The calcined phosphate type precursor (or dried sample in the case of phosphite type precursor) in the amount of 0.10 g was loaded into a quartz U-tube reactor (5 mm in inner diameter) and pre-treated in argon atmosphere at 200 ◦ C for 1 h. The experiments were carried out in 10 vol.% H2 /Ar flow (flow rate = 60 mL/min) at a heating rate of 10 ◦ C/min in a temperature range from 80 to 900 ◦ C. The H2 consumption was evaluated using a thermal conductivity detector (TCD). A cold trap with a temperature of −60 ◦ C mounted before the TCD was used to remove water from the exhaust gas.


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X-ray diffraction (XRD) patterns were acquired on an X-ray diffractometer Bruker D8 Advance (Bruker, Germany) using CuKα radiation (wavelength λ = 1.5418 Å) in 2θ scanning range from 10 to 70◦ . The qualitative phase analysis was performed using the JCPDS database [69]. The quantitative phase analysis and refining of the unit cell parameters were carried out by Rietveld analysis of a diffraction pattern using X’Pert High Score Plus software. The average crystallite size (DXRD ) was estimated using the Scherrer equation. Transmission electron microscopy (TEM) images were obtained on a JEM-2010 instrument (JEOL, Tokyo, Japan) with an accelerating voltage of 200 kV and a resolution of 0.14 nm. To obtain statistical information, the structural parameters of ca. 250 particles were measured. The Sauter mean diameter was used to determine the mean particles diameter (Ds ) by the Equation: DS =

∑ ni d3i , ∑ ni d2i

(1)

where ni is the number of particles, di is the diameter of the ith particle. NMR experiments were carried out using a Bruker Avance-400 spectrometer (Bruker, Germany) at the resonance frequencies of 104.31 and 161.923 MHz for 27 Al and 31 P nuclei respectively. Magic Angle Spinning (MAS) spectra were measured using a Bruker MAS NMR probe with 4-mm (outer diameter) ZrO2 rotors at the spinning frequency of 14 kHz. The detailed procedure of 31 P MAS NMR measurements can be found in reference [27]. 27 Al MAS NMR spectra were measured at the same spinning frequency with a simple one-pulse sequence. We used a short non-selective π/8 pulse of 0.5 µs with a 500 ms second repetition delay; 512 transients were recorded for each sample. The spectra were referenced to 0.1 mM Al[H2 O]6 3+ solution. Deconvolution of 27 Al NMR spectra was carried out in Dmfit program [70] using Simple Czjzek model that accounts for Gaussian quadrupole parameters distribution in disordered solids [65]. To avoid oxidation of reduced catalysts by air, all samples were sealed in glass ampoules without oxygen access. Prior to NMR experiments, the catalysts were transferred from ampoule to NMR rotor in a glove box under argon atmosphere. This technique helps to minimize possible oxidation during NMR spectra acquisition. The acidic properties of the catalysts were characterized by temperature-programmed desorption of ammonia (NH3 -TPD) using an Autosorb-1 apparatus (Quantachrome Instruments, Boynton Beach, FL, USA). Prior to ammonia adsorption, the precursor sample (0.25 g) was reduced at the given reduction temperature (550–650 ◦ C) for 1 hour in hydrogen flow (25 mL/min) and then cooled to 120 ◦ C. Subsequently, the sample was saturated with NH3 for 30 min. The physically adsorbed ammonia was desorbed from the sample with He flow (25 mL/min) at 120 ◦ C for 30 min. Desorption of ammonia was started by increasing the temperature from 120 ◦ C up to 600 ◦ C at a heating rate of 10 ◦ C/min. The desorbed NH3 was detected by a TCD. 3.4. Catalytic Activity Measurements The reaction of methyl palmitate HDO was performed in a high-pressure fixed-bed reactor (inner diameter 9 mm and total length 265 mm) at the temperature range of 250–330 ◦ C, hydrogen pressure 3.0 MPa, H2 /feed volume ratio 600 Nm3 /m3 and methyl palmitate LHSV in the range of 2.4–12 h−1 ((cm3 of MP)/(cm3 of catalysts) per hour). The composition of the reaction mixture amounted to methyl palmitate – 10 wt% (oxygen content – 1.183 wt%), n-octane (used as internal standard for quantification of liquid product) – 0.5 wt% and the rest – n-dodecane. For catalytic activity measurements, 0.5 cm3 of the catalyst precursor was diluted with silicon carbide (0.2–0.3 mm) in a ratio of 1:9 and placed in the reactor between two layers of SiC. Then the sample of the precursor was reduced as described in Section 3.2. The liquid reaction products were collected every hour till steady-state condition and time on stream was not less than 8 h for each stage.


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The conversion of methyl palmitate (XMP ), conversion of oxygen-containing compounds (XO ) and the selectivity to C16 hydrocarbons (SC16 ) were calculated according to Equations: X MP = XO = SC16 =

n 1 − oMP n MP n 1− O o nO

× 100, %

(2)

× 100, %

nC16 o n MP − n MP

(3)

× 100, %

(4)

where no MP and nMP are the initial and the current methyl palmitate content (mol/L), no O and nO are the initial and current oxygen content (mol/L), nC16 is the concentration of C16 hydrocarbons (mol/L) in the reaction mixture at XMP = 100%. The relative error in determining of the methyl palmitate (from GC data) and oxygen-containing compounds (from elemental analysis by Vario EL Cube) conversions was no more than 1%. 3.5. Product Analysis The reaction products were identified using a GC-MS technique (Agilent Technologies 6890N with MSD 5973, Santa Clara, CA, USA) with a VF-5MS quartz capillary column (30 m × 0.25 mm × 0.25 µm). The liquid samples were analysed with a gas chromatography system (Agilent 6890N, Santa Clara, CA, USA) equipped with HP-1MS column (30 m × 0.32 mm × 1.0 µm) and flame ionization detector (FID). Gaseous phase products were analysed online with a gas chromatograph (Chromos 1000, Dzerzhinsk, Russia) equipped with a column packed with 80/100 mesh HayeSep®(Sigma-Aldrich, Saint Louis, MO, USA) and FID. The concentrations of CO and CO2 were analysed in the form of methane after methanation over reduced Pd catalyst at 340 ◦ C. The carbon balance across the reactor for all experiments was not less than 95%. The total oxygen content in the reaction mixtures was evaluated using CHNSO elemental analyser Vario EL Cube (Elementar Analysensysteme GmbH, Hanau, Germany). The method is based on high-temperature pyrolysis of a liquid sample of the reaction mixture in a reducing atmosphere at 1170 ◦ C and the resulting oxygen-containing radicals interact quantitatively with the carbon filler to form carbon monoxide (Boudouard equilibrium). An IR detector was used to quantify the CO. 4. Conclusions Effect of precursors on the physicochemical and catalytic properties of alumina-supported nickel phosphide catalysts differing in precursors was studied. Ni(NO3 )2 and (NH4 )2 HPO4 (phosphate precursor, NiP_A/Al2 O3 catalyst) or Ni(OH)2 and H3 PO3 (phosphite precursor, NiP_I/Al2 O3 catalyst) pairs of nickel and phosphorus compounds were used for catalyst preparation by temperature-programmed reduction. It was found that NiP_I precursor is reduced at lower temperatures apparently due to nickel phosphite and H3 PO3 disproportionation with the formation of PH3 interacting readily with nickel species and producing Ni2 P nanoparticles in the temperature range of 550–650 ◦ C. On the other hand, starting from phosphate precursor, we did not succeed in the production of Ni2 P and the reduction at the temperature as high as 650 ◦ C gives the mixture of Ni3 P and Ni12 P5 phases. Using 31 P and 27 Al MAS NMR, the formation of aluminium phosphates in all catalysts was revealed. Interaction of phosphate groups with alumina surface hinders the formation of the desired Ni2 P phase and leads to an irreversible change in textural characteristics and surface properties of γ-Al2 O3 . Indeed, the increase in the number of weak acid sites, which are usually associated with Brønsted acidity, has been observed by means of NH3 -TPD in the supported catalysts in comparison with the alumina support. NiP_I/Al2 O3 catalyst displays significantly higher activity in methyl palmitate HDO compared to the NiP_A/Al2 O3 sample within the temperature region of


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250–330 ◦ C that is in agreement with the literature data reported the superiority of Ni2 P phase against Ni3 P/Ni12 P5 systems in HDO of aliphatic esters over silica-supported nickel phosphide catalysts. The performances of alumina- and silica-supported Ni2 P catalysts were compared in methyl palmitate HDO using NiP_I/Al2 O3 and NiP_I/SiO2 catalysts with nearly the same nickel content and mean particle size. The employment of alumina instead of silica for Ni2 P supporting allows a significant increase of the methyl palmitate conversion. Considering the tentative scheme of methyl palmitate transformation and reaction products distribution over NiP_I/Al2 O3 and NiP_I/SiO2 catalysts, we conclude that the acceleration of methyl palmitate hydrolysis over acid sites of the alumina-supported catalyst is the reason of increased activity of NiP_I/Al2 O3 catalyst. In general, the proper balance between metal and acid sites provides the superior performance of the NiP_I/Al2 O3 catalyst in the complicated scheme of methyl palmitate transformation, which includes the framework of acid- and metal-catalysed reactions. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/8/11/515/s1, Figure S1: Pore size distributions determined from the desorption branch of N2 isotherm for (a) NiP_A/Al2 O3 and (b) NiP_I/Al2 O3 catalysts reduced at 550, 600 and 650 ◦ C as well as for Al2 O3 support. Figure S2: XRD patterns of NiP_I/SiO2 (600) catalyst prepared from phosphite precursor and reduced at temperature of 600 ◦ C and SiO2 support. Figure S3: TEM images of Nix Py /γ-Al2 O3 catalysts prepared from different precursors: (a) NiP_A/Al2 O3 650 and (b) NiP_I/Al2 O3 600. Figure S4: Full-scale mass-normalized 14 kHz MAS 31 P spectra of NiP_I/Al2 O3 reduced at 550, 600 and 650 ◦ C. A significant decrease in intensity of the line corresponding to POx groups can be observed for the sample reduced at 650 ◦ C. Figure S5: Temperature effect on the conversion of methyl palmitate and oxygen-containing compounds over NiP_A/Al2 O3 650 and NiP_I/Al2 O3 600 catalysts. Figure S6: Conversions of methyl laurate, lauric acid, dodecanal and dodecanol over Ni2 P/SiO2 catalyst. Figure S7: MP conversion and total oxygen-containing compounds conversion as a function of time on stream for Ni2 P/Al2 O3 and Ni2 P/SiO2 catalysts. Figure S8: Dependence of the CO and CO2 sum from pentadecane content in methyl palmitate hydrodeoxygenation over NiP_A/Al2 O3 650 catalysts. Author Contributions: G.A.B. conceived and designed the experiments and supervised the work; I.V.D. and I.V.S. performed the catalyst synthesis; I.V.D., I.V.S. and P.V.A. performed the catalytic activity tests; E.Y.G. performed the catalyst characterization by TEM; V.P.P. performed the catalyst characterization by XRD; E.G.K. performed the catalyst characterization by NH3 -TPD; I.V.Y. and O.B.L. performed the catalyst characterization by MAS NMR; I.V.D., P.V.A. and G.A.B. analysed the experimental data and wrote the paper. Funding: This work was conducted within the framework of the budget project No. AAAA-A17-117041710075-0 for Boreskov Institute of Catalysis. Acknowledgments: The authors are grateful to V.A. Rogov for catalysts characterization by H2 -TPR, A.V. Ishchenko for catalysts characterization by TEM and S.A. Prikhod’ko for reaction products identification using GC/MS technique. Conflicts of Interest: The authors declare no conflict of interest.

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