Hydrodeoxygenation–Isomerization of Methyl Palmitate over SAPO-11-Supported Ni-Phosphide Catalysts

Abstract

Ni-phosphide catalysts on SAPO-11 were studied in the hydrodeoxygenation–isomerization of methyl palmitate (C₁₅H₃₁COOCH₃—MP). The catalysts were synthesized using temperature-programmed reduction (TPR) of a phosphate precursor ((NH₄)₂HPO₄ and Ni(CH₃CH₂COO)₂), TPR of a phosphite precursor (H₃PO₃ and Ni(OH)₂), and using phosphidation of Ni/SAPO-11 by PPh₃ in the liquid phase. The samples were characterized by ICP-AES chemical analysis, N₂ physisorption, NH₃-TPD, XRD, and TEM. First, the screening of the catalysts prepared by the TPR method was carried out in a semi-batch autoclave to determine the influence of the preparation method and conditions on one-pot HDO–isomerization (290–380 °C, 2–3 MPa). The precursor’s nature and the amount of phosphorus strongly influenced the activity of the catalysts and their surface area and acidity. Isomerization occurred only at a low P content (Ni/P = 2/1) and blocking of the SAPO-11 channels by unreduced phosphates at higher P contents did not allow us to obtain iso-alkanes. Experiments with liquid phosphidation samples in a continuous-flow reactor also showed the strong dependence of activity on phosphidation duration as well as on Ni content. The highest yield of isomerized products (66% iso-C_15–16 hydrocarbons, at complete conversion of O-containing compounds, 340 °C, 2 MPa, and LHSV = 5.3 h⁻¹) was obtained over 7% Ni₂P/SAPO-11 prepared by the liquid phosphidation method.

1. Introduction

Renewable sources have attracted much attention in the production of fuels and chemical products. Hydrodeoxygenation (HDO) of fatty-acid-based feedstocks (vegetable oils, animal fats, tall oils) leads to the formation of normal alkanes [1]. HDO is carried out over Ni(Co)Mo/Al₂O₃ catalysts, resulting in products with a high cetane index but poor low-temperature properties (high cloud and pour points) [2,3,4]. Isomerization of these products is needed to meet the requirements of commercial fuels. Noble metal catalysts on silicoaluminophosphates (SAPOs) or zeolites are used in these processes [5]. Overall, the process is two-step [6], and there is an attractive approach to combine HDO and isomerization into one step over bifunctional catalysts [7].

The most attractive SAPOs for different applications are SAPO-34 [8,9,10], SAPO-11 [11,12,13,14,15], SAPO-5 [16,17,18], SAPO-31 [19,20,21], and SAPO-41 [22,23,24]. SAPO-11 with the AEL topology is a member of the class of one-dimensional molecular sieves applied in hydroisomerization [25,26]. It possesses a medium-sized (0.4 × 0.65 nm) pore structure and mild acidity, which provides high selectivity in the hydroisomerization of n-alkanes [27,28].

New classes of catalysts based on transition metal carbides, nitrides, and phosphides are attracting a considerable amount of interest for application in hydroconversion reactions [29,30,31,32]. Due to their high activity, phosphides are particularly interesting for HDO [33,34,35]. Ni-phosphide catalysts are among the most active [36,37,38,39]. There are several works on bifunctional Ni-phosphide SAPO-11 catalysts for fatty acid ester HDO–isomerization and n-alkane isomerization.

Heng Shi et al. [40] prepared 15 wt.% Ni₂P/SAPO-11 with an initial Ni/P ratio of 1/1 by temperature-programmed reduction (TPR) of the phosphate precursor and tested it in methyl laurate (C₁₁H₂₃COOCH₃) hydroconversion. At 340 °C, 2 MPa, and WHSV = 10 h⁻¹, the methyl laurate conversion was 78% and the selectivity to C_11–12 was 92%, but only a small amount of hydroisomerization was achieved because a high Ni loading suppresses medium-strength acid sites [40,41].

Sha Zhao et al. [41] investigated the performance of 3 wt.% Ni₂P/SAPO-11 catalysts in HDO–isomerization of methyl laurate. The catalyst was prepared by TPR of the phosphate precursor (NiP = 1/1). At 360 °C, 3 MPa, and WHSV = 2 h⁻¹, the methyl laurate conversion was close to 100% and the selectivity to iso-alkanes (iso-C_11–12) decreased from ~37 to ~29% during 101 h of experimentation. The selectivity to C₆–₁₀ cracked products was ~10%.

Liu Chun-ya et al. [42] carried out HDO–isomerization of a methyl oleate (C₁₇H₃₃COOCH₃) and methyl stearate (C₁₇H₃₅COOCH₃) mixture over NiP/SAPO-11 with different Ni contents (1–13 wt.%), prepared by TPR of the phosphate precursor with Ni/P = 1/1. The carrier was prepared by the extrusion method with pseudo-boehmite (SAPO-11: pseudo-boehmite = 7:3). The highest isomerization rate was observed for the catalyst with 3 wt.% of Ni. At 340 °C, 2 MPa, and WHSV = 2.5 h⁻¹, the methyl ester conversion was ~98% and the isomerization selectivity was 14% (iso-C_15–18).

Shasha Tian and Jixiang Chen [43] studied Ni₂P/SAPO-11 catalysts in n-dodecane isomerization. The catalysts were prepared by TPR of the phosphate precursor with different Ni/P ratios and Ni contents. The highest isododecane selectivity of 72% was achieved at 90% conversion over the 3% Ni₂P/SAPO-11 catalyst with Ni/P = 1/1 (at 350 °C, 2 MPa, and WHSV = 2 h⁻¹).

Siyang Liu et al. [44] studied HDO–hydrocracking of castor oil over SAPO-11-supported Ni, NiAg, and Ni₂P catalysts. The 25% Ni₂P/SAPO-11 was prepared by the hypophosphite decomposition method with an initial Ni/P ratio of 1/1.5. This catalyst did not show significant isomerization and hydrocracking activity. At 300 °C, 3 MPa, and WHSV = 2 h⁻¹, the iso/normal alkanes ratio (C_8–15) was 0.1, the yield of C_5–7 hydrocarbons was 5%, the yield of C_8–15 was 2%, and the yield of C_16–19 was 92% (at 99% conversion). Similar results were obtained for 25% Ni/SAPO-11. The iso/normal alkanes ratio was 0.1, and the yields were 1% for C_5–7, 3% for C_8–15, and 95% for C_16–19.

Hongbiao Tang et al. [45] used SAPO-11, Zr-SBA-15, and ZSM-5 for NiMoP catalyst synthesis by TPR of phosphate precursors. The Ni/Mo ratio was 1/1, the total metal content was 5 wt.%, and the metal-to-P ratio was set to 1/2, 1/1, 2/1, and 3/1. The catalysts were tested in jatropha oil conversion in an autoclave at 360 °C and 3 MPa for 4 h. The products were divided into four groups: naphthenes, aromatics, oxygenates, and C_8–16 alkanes. For SAPO-11-supported catalysts, the P content did not significantly influence the selectivity of the C_8–16 alkanes (47–50%). The highest content of naphthenes was achieved for the sample with Ni/P = 1/2 (21%). The lowest content of oxygenates was achieved for the sample with Ni/P = 3/1 (5%).

Yang Zhang et al. [46] prepared a 4 wt.% Ni₂P/SAPO-31 catalyst by TPR of the phosphate precursor for isomerization of n-hexadecane. The selectivity to isohexadecane was ~82% at 80% conversion (at 380 °C, 2 MPa, and WHSV = 3.7 h⁻¹).

Previous works on SAPO-supported Ni-phosphide catalysts have mainly focused on phosphate precursors. No direct comparison of the influence of the precursor’s nature on the catalytic activity has been carried out. No one to the best of our knowledge has studied catalysts prepared by the liquid phosphidation method. However, this method may be advantageous for preventing the interaction of the P precursor with the support, which is well-known for Al₂O₃-supported catalysts [37,47,48,49,50].

The aim of this work is to study the influence of the preparation method on the catalytic activity of SAPO-11-supported Ni-phosphide catalysts in HDO–isomerization of methyl palmitate (C₁₅H₃₁COOCH₃) (MP). Our study involved the screening of the catalysts using an autoclave reactor to verify the activity of TPR catalysts and determine the conditions for optimal HDO–isomerization, the preparation and testing of liquid phosphidation catalysts in a continuous-flow reactor and a comparison with TPR catalysts, and physicochemical analysis of the most promising catalysts.

2. Results and Discussion

2.1. Catalyst Screening

2.1.1. TPR Catalyst Screening and Search for Optimal Conditions in an Autoclave

The catalysts were prepared by incipient wetness impregnation of the support with two different precursors (phosphate and phosphite) with subsequent TPR. The samples were labeled NiP_A (phosphate) and NiP_I (phosphite).

SiO₂-supported NiP_I samples show higher activity in MP HDO than NiP_A samples [36]. Therefore, we started screening in an autoclave using NiP_I/SAPO-11 samples (7 wt.% of Ni). Figure 1 shows MP conversion (X_MP) and conversion of O-containing compounds (X_O) vs. time for NiP_I/SiO₂ and for three independently synthesized NiP_I/SAPO-11 catalysts. Surprisingly, the SAPO-11-supported samples did not have any HDO–isomerization activity. X_MP over NiP_I/SiO₂ (Ni₂P/SiO₂) reached 89% in 5 h. This demonstrates that the conditions are good for HDO.

SAPO-11 is a microporous material (Table S1) and micropores as well as mesopores can be blocked by excess P. For example, it was shown that Ni/zeolite catalysts have a larger surface area than Ni(H₂PO₄)₂/zeolite catalysts [51]. Indeed, S_BET significantly decreased after phosphide preparation (29 m²/g, Table S1) and after reaction (8 m²/g, Table S1). The initial Ni/P ratio was 1/2. During TPR at high temperatures, P can form volatile P-containing compounds (PH₃ and products of its interaction with H₂O) [52,53]. However, excess P can also remain on the support surface after TPR in the form of unreduced phosphates (PO_x) (H_nPO₄^{(3 − n)−}, P₂O₇⁴⁻, and (PO³⁻)_n) [36,54,55].

To avoid blocking the SAPO-11 pores, we tested 3 wt.% NiP_I/SAPO-11 (TPR at 450 °C). To verify whether SAPO-11 is a reliable support, we tested 3 wt.% Ni/SAPO-11. Conversion values of X_MP = 75% and X_O = 58% were reached over Ni/SAPO-11 after 5 h of reaction (Figure 2). The NiP_I/SAPO-11 remained inactive. XRD analysis showed the presence of metallic Ni in Ni/SAPO-11 and, due to the small particle sizes, NiP_I/SAPO-11 did not have any reflexes of Ni-phosphide phases (Figure S1). This experiment showed that excess P blocks the active component in the support. As it is impossible to decrease the P content in NiP_I samples without additives (due to the poor solubility of phosphites), we used phosphate NiP_A precursors to prepare samples with different Ni/P ratios.

Depending on the Ni/P ratio, the conversion values of MP were in the range of 45–49% and the conversion values of O-containing compounds were 33–45% over the NiP_A/SAPO-11 catalyst after 5 h of reaction (Figure 3).

The Ni/P ratio also influenced the selectivity (

Table 1. C₁₆/C₁₅ molar ratios and product selectivities and yields over NiP_A catalysts with different Ni/P ratios.

Ni/P Ratio	C16/C15 Molar Ratio	Selectivity to n-C15, %	Selectivity to n-C16, %	Yield of n-C15, %	Yield of n-C16, %
1/2	0.365	66	24	32	12
1/1	0.168	76	13	34	6
2/1	0.137	60	8	27	4

). The main products of the reaction were n-alkanes. The C₁₆/C₁₅ molar ratio for NiP_A 1/2 was 0.365, for NiP_A 1/1 it was 0.168, and for NiP_A 2/1 it was 0.137. These numbers are close to the results in the literature for Ni₂P/SAPO-11 [40,41,42]. Figure 4 shows product distributions vs. time. MP, n-pentadecane (n-C₁₅), and n-hexadecane (n-C₁₆) concentrations vs. time are shown in Figure 4a–c, while concentrations of intermediate compounds (palmitic acid (PA), palmityl palmitate (PP), and hexadecanol (C₁₆OH)) vs. time are shown in Figure 4d–f. The selectivity to n-C₁₅ and n-C₁₆ changed as follows: for Ni/P = 1/2, S_C15 = 66% and S_C16 = 24%; for Ni/P = 1/1, S_C15 = 76% and S_C16 = 13%; and for Ni/P = 2/1, S_C15 = 60% and S_C16 = 8% (Table 1). The overall selectivity and yield to alkanes decreased as the P content decreased, confirming the similar trend and activity dependence for SiO₂-supported Ni-phosphides [53,56]. As the P content decreased, the selectivity to O-containing intermediates and side products increased from 10% (for Ni/P = 1/2) to 31% (for Ni/P = 2/1).

The next step was to determine the conditions under which isomerization takes place. Screening of the NiP_A samples with different Ni contents and Ni/P ratios was conducted at 340 and 380 °C (Figure 5). At these temperatures, X_MP was 100%, so the selectivities are equal to the yields of the products. Catalytic experiments showed that, for the NiP_A catalyst with Ni/P = 1/2, only n-alkanes were detected both at 340 and 380 °C. Decreasing the Ni content resulted in the formation of cracked products, and no isomerized alkanes were detected (3 wt.% NiP_A, Ni/P = 1/2). Decreasing the P content in the 7 wt.% samples also contributed to the cracking; however, in the case of Ni/P = 1/1, iso-C₁₅ were detected with a selectivity of 13%. For the series of 3 wt.% NiP_A catalysts with different Ni/P ratios, decreasing the P content yielded a larger number of cracked alkanes and more iso-alkanes (because of the increase in the catalyst’s acidity, see Figure S2). Thus, the 3 wt.% NiP_A catalyst with Ni/P = 2/1 has the highest selectivity to iso-alkenes (35%) (33% of iso-C₁₅ and 2% of iso-C₁₆).

At this point, 3 wt.% NiP_A/SAPO-11 (Ni/P = 2/1) showed the highest isomerization activity, but the number of cracked products was the highest among all tested catalysts. We carried out HDO–isomerization over the 3 wt.% NiP_A series at a lower temperature (340 °C) and pressure (2 MPa) with a larger catalyst mass (1.7 g). Iso-alkanes were only obtained over the sample with Ni/P = 2/1 (35%, Figure 6). However, for this catalyst, the products were not balanced with the initial amount of MP, and no cracked products were detected. Thus, we think that 15% of the MP transformed into carbon deposits.

From the experimental results, we can conclude that, to obtain active catalysts simultaneously for HDO and isomerization, one needs to avoid large amounts of P in order not to block SAPO-11 pores and the active component. However, at the same time, one needs enough P to form the Ni-phosphide and prevent cracking and cocking. The liquid phosphidation method of Ni/SAPO-11 can meet both requirements [49]. The phosphidation degree can be regulated by the temperature and duration of this procedure, and the formation of PO_x phosphate residues can be avoided [57]. This method requires a continuous-flow reactor for both phosphidation and catalytic tests.

2.1.2. Liquid-Phase Phosphidation and Experiments in a Continuous-Flow Reactor

The catalysts prepared by liquid-phase phophidation were labeled NiP_P. First, 3 wt.% Ni/SAPO-11 was phosphidized in situ by PPh₃ according to the procedure described in [49] (heating to 380 °C (1 °C/min) and holding at this temperature for 7 h (TPP380)). MP HDO–isomerization was carried out at 340 °C, 2 MPa, LHSV = 5.3 h⁻¹, and H₂/feed ratio = 600 Ncm³/cm³.

The NiP_P/SAPO-11 TPP380 catalyst did not show any isomerization activity (Figure 7), and 2% of the MP transformed into carbon deposits. Under the same conditions, 8% of the MP transformed into carbon deposits over the NiP_A/SAPO-11 catalyst, but the selectivity to iso-alkanes was 27% (25% of iso-C₁₅, 2% of iso-C₁₆). Thus, we decided to lower the phosphidation degree by decreasing the phosphidation temperature (250 °C, TPP250) and duration (2 h). This allowed us to obtain 40% of the iso-alkanes (33% of iso-C₁₅ and 7% of iso-C₁₆ over 3 wt.% NiP_P, Figure 7), but the selectivity to carbon deposits was 6%. Increasing the Ni content proved to be successful in avoiding the formation of carbon deposits and cracked products (7 wt.% NiP_P and 12 wt.% NiP_P, Figure 7) such that the mass balance was 100%. The optimal content of Ni to obtain the highest number of iso-alkanes (54% of iso-C₁₅ and 12% of iso-C₁₆) was 7 wt.%. The MP conversion and selectivities of the products vs. time on stream (TOS) are shown in Figure S3.

Ni/SAPO-11 was also tested in continuous-flow mode in order to compare it with NiP_P/SAPO-11 (Figure 7). Despite the high selectivity to iso-alkanes (59% total), Ni/SAPO-11 had very high cracking and cocking activity (37% selectivity). Unfortunately, it was not possible to determine exactly the number of cracked products because the solvent (n-dodecane) also cracked and contributed to these products (Figure S4).

Due to the presence of metallic Ni in Ni/SAPO-11, it has high activity in methanation. There is no CO in gas phase products of Ni/SAPO-11, and high concentrations of CH₄, C₂H₆, and C₃H₈ are observed compared with Ni₂P/SAPO-11 catalysts (Figure 8). C₂H₆, C₃H₈, and additional amounts of CH₄ are produced by cracking reactions of MP, its intermediate and final products, and the n-dodecane solvent.

The NiP_A and NiP_P catalysts had quite similar amounts of CO and CH₄, and the most active 7 wt.% NiP_P catalyst had the lowest concentrations (Figure 8). Interestingly, there was no C₂H₆, C₃H₈, CH₃OCH₃, and CH₃OH over the NiP_P TPP380 sample.

Scheme 1 shows the possible reactions of MP HDO–isomerization. HDO has two main routes:

(1)

decarbonylation: MP → PA → C₁₆O → C₁₅ hydrocarbons;

(2)

direct HDO: MP → PA → C₁₆O → C₁₆OH → C₁₆ hydrocarbons

where MP, methyl palmitate; PA, palmitic acid; C₁₆O, hexadecanal; and C₁₆OH, hexadecanol.

HDO intermediates as well as palmityl palmitate are detectable at low reaction temperatures (290 °C, Figure 4). At higher temperatures (340 and 380 °C), the HDO reactions are quite fast and the conversion of O-containing compounds is complete (Figure 5, Figure 6 and Figure 7).

Isomerization of n-alkanes (C₁₅H₃₂ and C₁₆H₃₄) occurs through dehydration to alkenes with the subsequent formation of n-alkylcarbenium ions (C₁₅H₃₁⁺ and C₁₆H₃₃⁺), which form in protonation over acid sites of SAPO-11. An alternative route of n-alkylcarbenium ion formation is protonation of n-alkanes to n-alkylcarbonium ions (C₁₅H₃₃⁺ and C₁₆H₃₅⁺), even though this route is significantly slower [58]. The subsequent transformation of n-alkylcarbenium ions leads to monobranched alkanes, dibranched alkanes, and cracked products. According to GC-MS analysis, only trace amounts of dibranched alkanes were detected over Ni₂P/SAPO-11 catalysts. Almost all iso-alkanes are monobranched ones.

As phosphidized catalysts (TPP) are more promising and active than TPR samples, we characterized NiP_P catalysts with different Ni contents by physicochemical methods.

2.2. Physicochemical Analysis of the TPP Catalysts

2.2.1. Chemical Analysis, Textural Properties, NH₃-TPD, and XRD

The initial SAPO-11 contained 24.1 wt.% P (

Table 2. Physicochemical properties of NiP_P samples.

	Ni, wt.%	P, wt.%	Vmicro, cm3/g	Vmeso, cm3/g	Smicro, m2/g	SBET, m2/g	NH3-TPD, μmol/g	XRD Phase	DXRD, nm	DTEM, nm
SAPO-11	–	24.1	0.064	0.08	166	188	161	–	–	–
3% NiP_P	3.4	21.3	0	0.12	0	19.5	122	Ni2P	30	6.5
7% NiP_P	7.3	20.9	0	0.10	0	16.0	110	Ni2P	59	9.2
12% NiP_P	12.0	20.3	0	0.10	0	17.6	145	Ni2P	39	9.9

). Increasing the relative Ni content results in a decrease in the relative P content because P is lighter than Ni. Phosphidation increases the amount of P, but its effect was negligible compared with the overall P content in SAPO-11 and changed due to the Ni loading.

The S_BET of the NiP_P samples was significantly lower than the S_BET of the original SAPO-11 (Table 2). This may have been due to blocking of the SAPO-11 pores by the active component and excess P. It resulted in the absence of micropores, as V_micro = 0 cm³/g and S_micro = 0 m²/g. The pore size distributions of the support and the catalysts are shown in Figure S5.

The acidity of the catalysts was lower than for SAPO-11 (Table 2, Figure 9). In the NH₃-TPD curve of SAPO-11, there are two main signals with maximums at ~180 °C and ~300 °C. The first signal corresponds to weak acid sites, and the second signal corresponds to medium-strength acid sites [26,43]. These signals remain in the NiP_P samples, but an additional broad signal appears after phosphidation at ~500 °C. This may correspond to the formation of volatile P-containing compounds at high temperatures of NH₃-TPD. The acidity of the 12% sample is the highest among the NiP_P catalysts (145 μmol/g), and the acidity of the 3 and 7% samples is similar (122 and 110 μmol/g, respectively).

Figure 10 shows XRD patterns of NiP_P samples with different Ni contents. All catalysts contain the Ni₂P phase. Table 1 lists the D_XRD of Ni₂P particles, and it is quite large (30–59 nm). It should be noted that small particles (<3 nm) could not be detected by XRD; therefore, the D_XRD is larger than the D_TEM.

2.2.2. TEM and SEM Analysis

The initial SAPO-11 support consisted of agglomerates of elongated particles with sizes of ~1 μm (Figure S6). TEM images of the NiP_P catalysts show that the SAPO-11 particles are evenly covered with Ni₂P nanoparticles (Figure 11).

Figure 11a–c show TEM images and particle size distributions of NiP_P samples with different Ni contents. All the distributions have a maximum at 4.5 nm. As the Ni content increased, the distributions became broader and bimodal for 7% NiP_P and 12% NiP_P. The mean values (D_TEM) of particle sizes are listed in Table 1. The D_TEM increased from 6.5 nm to 9.9 nm as the Ni content increased. EDX mapping showed the presence of Ni-phosphide particles on SAPO-11 (Figure 11d–f). All particles were covered with an oxidized layer, which mainly consisted of Ni₃(PO)₄ according to the EDX analysis (Figure 11g–i). Interplanar distances were identified for several particles (Figure 11g–i): 0.192, 0.202, 0.203, and 0.507 nm. These distances correspond to the Ni₂P crystal structure (Ni₂P JCPDS # 03-0953). The distances are related to the (210), (201), and (100) XRD reflexes, respectively.

3. Materials and Methods

3.1. Materials

For SAPO-11 synthesis, H₃PO₄ (85 wt.%, analytical grade) from «Spektrchem» (Saint Petersburg, Russia), SiO₂ Rosil-175 powder (90%, grade B) from JSC «BSK» (Sterlitamak, Russia), and hydrated alumina/oxide–hydroxide alumina were used (JSC «Promcatalys», Ryazan, Russia). Dipropylamine (DPA, 99%) from «Acros Organics» (Newark, NJ, USA) was used as a molecular template. SiO₂ was purchased from «KhromAnalit» LTd. (KSKG-type, Moscow, Russia). Ni(CH₃CH₂COO)₂·4H₂O was purchased from «Reachim» (GOST 11125-84, ≥98%, Moscow, Russia). (NH₄)₂HPO₄ was purchased from «Alfa Aesar» (Technical Grade, Kandel, Germany). Ni(OH)₂ was purchased from «Acros Organics» (≥98%, NJ, USA). H₃PO₃ was purchased from «Sigma-Aldrich» (≥97%, Burlington, MA, USA).

Methyl palmitate (C₁₅H₃₁COOCH₃) was purchased from «Sigma-Aldrich» (≥97%, Burlington, USA), n-dodecane was purchased from «Acros Organics» (≥98%, NJ, USA), and PPh₃ was purchased from «Sigma-Aldrich» (99%, Burlington, USA).

3.2. SAPO-11 Synthesis

SAPO-11 was synthesized via conventional hydrothermal synthesis (HTS) in an autoclave with a volume of 10 L. Aluminum and phosphorus precursors were mixed together with the subsequent addition of DPA and a silica source. All precursors were mixed until the reaction mixture became homogeneous. The final reaction mixture underwent hydrothermal synthesis at 200 °C for 24 h under stirring during HTS.

The initial molar composition contained Al₂O₃/P₂O₅/SiO₂ = 1/1/0.1. The amount of molecular template used was in the range of 0.9–1.5 moles to 1 mol of Al₂O₃.

After synthesis, the product was collected, washed with distilled water from the mother liquor, dried, and calcined at 650 °C with a heating rate of 3 °C/min to remove the organic template.

3.3. NiP/SAPO-11 and NiP/SiO₂ Synthesis

SAPO-11 powders or SiO₂ granules (0.25–0.5 mm) were incipiently impregnated with aqueous solutions of precursors.

NiP_A (phosphate samples). To prepare phosphate precursors, (NH₄)₂HPO₄ was dissolved in distilled water. Then, Ni(CH₃CH₂COO)₂·4H₂O was added. A green-yellow precipitate of Ni-phosphate was formed, which was then dissolved by the dropwise addition of concentrated HNO₃. The supports were impregnated by this solution. The samples were dried overnight at room temperature, then at 110 °C for 3 h. Afterwards, the samples were calcined at 500 °C for 3 h. To form phosphides, the precursors were reduced in a hydrogen flow (250 mL/min) using the following temperature program: heating to 380 °C on a 3 °C/min ramp, then heating to 600 °C on a 1 °C/min ramp. The content of Ni in the reduced catalysts was ~3 or ~7 wt.%. The content of P was varied using different initial Ni/P molar ratios (2/1, 1/1, and 1/2) in the precursors.

NiP_I (phosphite samples). To prepare phosphite samples, the precursor Ni(OH)₂ was dissolved in a H₃PO₃ water solution. The initial Ni/P molar ratio was 1/2. The supports were impregnated by this solution. The samples were dried overnight at room temperature, then at 80 °C for 24 h. Then, the precursors were reduced in a hydrogen flow (250 mL/min) using the following temperature program: heating to 600 °C on a 1 °C/min ramp. The content of Ni in the reduced catalysts was ~3 or ~7 wt.%.

NiP_P (liquid phosphidation samples). To prepare metallic samples for liquid phosphidation, Ni(CH₃CH₂COO)₂·4H₂O was dissolved in distilled water. The green solution was used to impregnate the supports. The samples were dried overnight at room temperature, then at 110 °C for 3 h. Then, the precursors were calcined at 400 °C for 3 h. The content of Ni after calcination was ~3, ~7, or ~12 wt.%. The oxide precursors were reduced in situ in a continuous-flow fixed-bed reactor at 400 °C for 2 h (on a 1 °C/min ramp). After cooling, phosphidation was carried out using 2 wt.% PPh₃ in dodecane (LHSV = 5.3 h⁻¹, P_H2 = 0.5 MPa, H₂/feed = 600 Ncm³/cm³) with heating to 250 °C at 1 °C/min and holding at 250 °C for 2 h.

3.4. Catalyst Characterization

Elemental analysis was performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on an Optima 4300 DV («Perkin Elmer», Waltham, MA, USA).

Textural properties were determined by N₂ physisorption on an ASAP 2400 («Micromeritics», Norcross, GA, USA).

NH₃-TPD analysis was carried out using a Chemosorb («Neosib» LTd., Novosibirsk, Russia). A total of 200 mg of sample was preheated for 1 h in He flow (60 mL/min) at 500 °C. The adsorption of NH₃ was carried out at 100 °C for 1 h. Then, to remove physically adsorbed ammonia, the sample was kept at 100 °C for 1 h in He flow (60 mL/min). Desorption was carried out at a heating rate of 10 °C/min in He flow (60 mL/min). Desorbed NH₃ was determined by a thermal conductivity detector.

X-ray diffraction analysis (XRD) was performed on an XTRA diffractometer («Thermo Fisher Scientific», Waltham, MA, USA) using CuKα radiation (wavelength λ = 1.5418 Å).

TEM microphotographs were obtained on a JEM-2010 («JEOL», Tokyo, Japan) transmission electron microscope with an accelerating voltage of 200 kV and a resolution of 0.14 nm.

3.5. Catalytic Experiments

Screening of the HDO–isomerization activity of the NiP_A and NiP_I catalysts was carried out in a 300 mL «Autoclave Engineers» (Erie, PA, USA) Bolted Pressure Vessel Closure (Autoclave reactor) in semi-batch mode (H₂ continuous flow). For a typical experiment, the catalyst was reduced ex situ, then transferred under Ar to the reactor and heated to 180 °C in H₂ flow (100 mL/min). Subsequently, 100 mL of 10% MP in n-dodecane was added to the reactor, the pressure was set to 3 MPa, and the temperature was set to 290–380 °C. When the temperature reached the reaction temperature, mixing was started (500 RPM).

NiP_P catalysts were prepared and tested in a continuous-flow fixed-bed reactor (i.d. = 12 mm) with a coaxial thermocouple with a diameter of 3 mm. The precursors were reduced and phosphidized; then, after the catalyst was cooled and washed with n-dodecane, HDO–isomerization of MP (10 wt.% in n-dodecane) was carried out at 250–340 °C, 2 MPa, LHSV = 5.3 h⁻¹, and H₂/feed = 600 Ncm³/cm³.

Liquid products were analyzed by a gas chromatograph («Agilent» N6890, Santa Clara, CA, USA) with an HP-1 MS column (30 m × 0.32 mm × 1 μm) and a flame ionization detector (FID). O content was determined by a Vario EL Cube elemental analyzer (Elementar Analysensysteme GmbH, Langenselbold, Germany). Gas-phase products were analyzed by a gas chromatograph («Chromos 1000», Dzerzhinsk, Russia) with a packed column (with HayeSep, «Sigma-Aldrich», Burlington, USA), methanator with a Pd catalyst, and an FID.

The conversion of MP was calculated as follows:

$$X_{MP}=\left(\frac{C_{MP}^0-C_{MP}}{C_{MP}^0}\right)\cdot 100\%,$$

(1)

where $C_{MP}^0$ is the initial concentration of MP and $C_{MP}$ is the MP concentration in the products.

The conversion of O-containing compounds was calculated as follows:

$$X_O=\left(\frac{C_O^0-C_O}{C_O^0}\right)\cdot 100\%,$$

(2)

where $C_O^0$ is the initial concentration of oxygen and $C_O$ is the concentration of oxygen in the products.

The selectivity was calculated as follows:

$$S_i=\left(\frac{C_i}{C_{MP}^0-C_{MP}}\right)\cdot 100\%,$$

(3)

where $C_i$ is the concentration of the i-th compound in the products. For cracked products and carbon deposits, selectivity was estimated based on mass balance.

The yield was calculated as follows:

$$Y_i=\left(\frac{C_i}{C_{MP}^0}\right)\cdot 100\%.$$

(4)

4. Conclusions

The activity in HDO–isomerization of MP over SAPO-11-supported Ni-phosphide catalysts proved to depend on the preparation method and the phosphidation degree of the catalysts. NiP_I/SAPO-11 (the phosphite precursor) did not show activity in both HDO and isomerization. This was attributed to unreduced PO_x residues that blocked the surface of the catalyst and the active component. NiP_A/SAPO-11 (the phosphate precursor) activity correlated with the Ni content and Ni/P molar ratio. A low Ni loading (3 wt.%) and low P content (Ni/P = 2/1) resulted in the formation of iso-C₁₅ and iso-C₁₆ hydrocarbons. The selectivity to iso-alkanes at 340 °C, 2 MPa, and at full conversion in a semi-batch autoclave was 35%. At the same time, the selectivity to carbon deposits was 15%. In the continuous-flow reactor, the same catalyst produced 27% of the iso-alkanes and 8% of the carbon deposits (at T = 340 °C, 2 MPa, LHSV = 5.3 h⁻¹, and H₂/feed = 600 Ncm³/cm³). This catalyst has a metallic character that is too strong, which results in a high degree of cracking activity. In situ liquid-phase phosphidation by PPh₃ allowed us to finely tune the phosphidation degree of the Ni₂P/SAPO-11 catalysts (NiP_P) in order to make them suitable for simultaneous HDO–isomerization. The highest selectivity to iso-alkanes was 66% (54% of iso-C₁₅ and 12% of iso-C₁₆) for 7 wt.% NiP_P/SAPO-11 phosphidized at 250 °C for 2 h. No carbon deposits formed over this catalyst.

The evidence from this study suggests that liquid-phase phosphidation is an effective way to prepare Ni₂P/SAPO-11 catalysts for one-pot HDO–isomerization of fatty esters.