Desulfurization of 2-phenylcyclohexanethiol over transition-metal phosphides

Journal of Catalysis 383 (2020) 331–342

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Desulfurization of 2-phenylcyclohexanethiol over transition-metal phosphides Xuerong Zhou a, Xiang Li b,⇑, Roel Prins c, Anjie Wang a,d, Lin Wang a, Shengnan Liu a, Qiang Sheng b a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of China Tianjin Key Laboratory of Brine Chemical Engineering and Resource Eco-utilization, College of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China c Institute of Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland d Penn State and Dalian University of Technology Joint Center for Energy Research (JCER), Dalian University of Technology, Dalian 116024, People’s Republic of China b

a r t i c l e

i n f o

Article history: Received 27 October 2019 Revised 19 January 2020 Accepted 20 January 2020

Keywords: 2-Phenylcyclohexanethiol Cycloalkyl C–S bond Desulfurization Ni2P MoP WP Transition-metal phosphosulfide Piperidine

a b s t r a c t The desulfurization of 2-phenylcyclohexanethiol (2-PCHT) was studied over Ni2P, MoP, and WP under 4.0 MPa H2 or Ar at 240 °C. The hydrodesulfurization of 2-PCHT proceeded through three parallel pathways: b-elimination, hydrogenolysis, and dehydrogenation. Under Ar, the parallel pathways were belimination, hydrogenolysis or homolytic C–S bond cleavage, and dehydrogenation. MoP and WP were more active than Ni2P. b-Elimination dominated the hydrodesulfurization of 2-PCHT over Ni2P, while hydrogenolysis was as fast as b-elimination over MoP and WP. Under Ar, b-elimination and dehydrogenation pathways were about equal over Ni2P, whereas b-elimination was the major pathway over MoP and WP. The inhibiting effects of piperidine depended on the reaction and catalyst. Phosphosulfide phases were formed under both H2 and Ar, but the sulfidation behavior of Ni2P was different from that of MoP and WP. Ni2P was more difficult to sulfide than MoP and WP under H2. Ó 2020 Elsevier Inc. All rights reserved.

1. Introduction Hydrodesulfurization (HDS) of petroleum fractions is one of the major catalytic operations in the petroleum industry [1]. Driven by the growing demand for ultraclean fuels and the increasing use of low-quality crude oil feed stocks, deep HDS has received great attention. To achieve deep HDS, the most refractory sulfurcontaining compounds, dibenzothiophene (DBT) and its alkylated derivatives (DBTs), must be removed from transportation fuels [2–4]. Hence, an in-depth understanding of the HDS mechanisms of these molecules is prerequisite for the development of efficient HDS catalysts. The two most commonly employed mechanisms for the cleavage of C–S bonds are hydrogenolysis and b-elimination. In hydrogenolysis, a C–S bond is broken by reaction with H atoms, while C–H and S–H bonds form simultaneously [5–7]. For instance, hydrogenolysis of the C–S bond of thiophene yields 1,3-butadiene [6], while biphenyl (BP) is the product of the hydrogenolysis of DBT [8]. b-Elimination involves the breaking of the a-C–S bond and a b-C–H bond, leading to the formation of H2S and a C=C double ⇑ Corresponding author. E-mail address: [email protected] (X. Li). https://doi.org/10.1016/j.jcat.2020.01.023 0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

bond. Hydrogenolysis is catalyzed by metal atoms [9,10], whereas both basic sites (to abstract the b-hydrogen atom) and acid or vacancy sites (to bind the sulfur of the reactant) are involved in the b-elimination mechanism [11]. In addition, hydrogen is a reactant in hydrogenolysis, but is not required for b-elimination. DBT and DBTs undergo HDS by two parallel pathways: direct desulfurization (DDS) and hydrogenation (HYD) [12,13]. DDS leads to the formation of biphenyls, while HYD yields not only the final desulfurized products, cyclohexylbenzenes and bicyclohexyls, but also the tetrahydro, hexahydro, and dodecahydro hydrogenated sulfur-containing intermediates (Scheme S1 in the Supplementary Material). There are three types of C–S bonds (the aryl C–S bond, the vinyl C–S bond, and the cycloalkyl C–S bond) in the HDS networks of DBT and DBTs. We demonstrated that over transitionmetal phosphides [14–16], the cleavage of the aryl and vinyl C–S bonds in DBT and 1,2,3,4-tetrahydro-dibenzothiophene (TH-DBT) occurs mainly through hydrogenolysis, while in the initial cycloalkyl C–S bond-breaking in 1,2,3,4,4a,9b-hexahydro-dibenzo thiophene (HH-DBT), both b-elimination and hydrogenolysis can be involved. Transition-metal phosphides (e.g., Ni2P, MoP, WP, CoP, and FeP) are a novel class of HDS catalysts. They have high activity and

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X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

stability under hydrotreating conditions [17,18]. Among the phosphides investigated, Ni2P, MoP, and WP are the three most active catalysts for HDS and HDN of petroleum feedstocks [17,18]. Their initial activity increases gradually with time on stream during HDS reactions [19]. It is suggested that sulfur is incorporated into the surface of these phosphide catalysts, leading to the formation of a more active surface phosphosulfide phase [20,21]. The formation, the preparation, and the nature and the catalytic performances of these novel sulfur-containing phases still need to be studied. Our previous results suggested that the formation of the molybdenum phosphosulfide phase was due to the direct reaction of the phosphide catalyst with sulfur-containing molecules during HDS, rather than to the sulfidation of the MoP surface by H2S/H2 mixtures with low H2S partial pressures (5 or 35 kPa H2S in 4.0 MPa H2) [22]. For an in-depth understanding of the cleavage of the cycloalkyl C–S bonds in the hydrogenated intermediates of DBT and DBTs, the catalytic properties of transition-metal phosphides, and the formation of the phosphosulfide phase, we studied the desulfurization of 2-phenylcyclohexanethiol (2-PCHT) over bulk Ni2P, MoP, and WP catalysts, under both a hydrogen atmosphere and an inert gas atmosphere (Ar). The molecular structure of 2-PCHT is similar to that of HH-DBT, but only possesses a cycloalkyl C–S bond. It is a chemically reasonable intermediate in the HDS of DBT, although it has never been detected so far. As proposed by Wang and Prins [23], it can be formed by the hydrogenolysis of the aryl C–S bond in HH-DBT. 2. Experimental methods 2.1. The preparation of catalyst precursors The phosphate precursors of Ni2P, MoP, and WP were prepared by adding an aqueous solution of 3.0 g (NH4)2HPO4 in 10 mL deionized water dropwise to a solution of 6.6 g Ni(NO3)2
6H2O, 4.0 g (NH4)6Mo7O24
4H2O, or 5.6 g (NH4)6W12O40
xH2O in 15 mL deionized water while stirring. The resulting mixtures were stirred while the water was evaporated to obtain the solid products, which were dried at 120 °C for 12 h and calcined at 500 °C for 3 h. The molar compositions of the precursors were NiO∙0.5P2O5, MoO3∙0.5P2O5, and WO3∙0.5P2O5.

pelleted, crushed, and sieved to 20–40 mesh. A total of 0.10 g precursor was used for each run. Prior to the reaction, the precursor was transformed into the phosphide phase by in situ H2 temperature-programmed reduction. It was reduced in a 100 mL/ min H2 flow at 1.0 MPa by heating it quickly from room temperature to 120 °C and maintaining this temperature for 1 h, then heated to 400 °C at 5 °C/min, and finally heated at 1 °C/min to 500 °C (for the preparation of Ni2P) or 650 °C (for the preparation of MoP and WP) and holding at that temperature for 2 h. Because the reaction of 2-PCHT is quite fast, it was studied at a temperature as low as 240 °C. After the precursor had been converted into the phosphide phase, the reactor was cooled to the reaction temperature, and the gas-phase feed was introduced. It consisted of 1.5 kPa 2-PCHT, 164 kPa decalin (as solvent), and about 3.8 MPa H2 or Ar both in the absence or presence of 0.25 kPa piperidine. The space–time (or the contact time) calculated based on the catalyst weight was defined as s = wcat/nfeed, where wcat denotes the catalyst weight (g) and nfeed the total molar flow to the reactor (mol/min) [25]. The space–time was changed by varying the flow rates of the liquid (from 0.1 to 0.3 mL/min) and the gas, while keeping their ratio constant (750 v/v). The reaction products were analyzed off line by an Agilent-6890N gas chromatograph equipped with an HP-5 column and a flame ionization detector. Because 3-phenyl-1-cyclohexene (3-PCHE) and phenylcyclohexane (or cyclohexylbenzene, CHB) cannot be separated by the nonpolar HP-5 column, an HP-INNOWax polyethylene glycol column was used to quantify the 1-PCHE, 3-PCHE, and 4-phenyl-1cyclohexene (4-PCHE) present in the desulfurization products [14]. The structures and the acronyms of the main compounds involved in the present work are listed in Table 1. Table 1 Structures and acronyms of the main compounds involved in the present work. Name

Structure

Acronym

2-Phenylcyclohexanethiol

2-PCHT

1,2,3,4-Tetrahydro-dibenzothiophene

TH-DBT

1,2,3,4,4a,9b-Hexahydrodibenzothiophene

HH-DBT

1-Phenyl-1-cyclohexene

1-PCHE

3-Phenyl-1-cyclohexene

3-PCHE

4-Phenyl-1-cyclohexene

4-PCHE

Cyclohexylbenzene

CHB

Biphenyl

BP

(Cyclopentenylmethyl)benzene

CPEMB

Cyclopentylmethylbenzene

CPMB

2-Pheny-l-cyclohexenethiol

2-PCHET

2.2. The synthesis of 2-PCHT 2-PCHT was synthesized by free radical addition of 1-phenyl-1cyclohexene (1-PCHE) and thioacetic acid, followed by hydrolysis with 10% potassium hydroxide in an alcohol–water mixture (1:1, v/v) and extraction with ethyl acetate [24]. The extracted oily layer was washed successively with saturated NaHCO3 and NaCl solutions and then dried by anhydrous MgSO4. The obtained yellow oil was further purified by column chromatography on silica gel with a mixture of petroleum ether (b.p. 60–90 °C) and ethyl acetate (20:1, v/v). The synthesized 2-PCHT possessed the cis configuration. 1H NMR (400 MHz, CDCl3): 7.28 (t, J = 7.5 Hz, 2H), 7.18 (t, J = 8.4 Hz, 3H), 3.57 (s, 1H), 2.95 (d, J = 12.4 Hz, 1H), 2.13 (m, 6.4 Hz, 1H), 2.00–1.86 (m, 3H), 1.74–1.62 (m, 2H), 1.52 (d, J = 13.6 Hz, 1H), 1.41–1.30 (m, 1H), 0.94 (d, J = 5.3 Hz, 1H). 13C NMR (100 MHz, CDCl3): 144.36, 128.15, 127.69, 126.36, 47.21, 44.17, 34.28, 26.50, 24.07, 19.99. EI-MS m/z (%): 192 (90, M + ), 158 (18), 143 (13), 129 (26), 117 (48), 101 (99), 91 (1 0 0), 77 (17), 65 (10), 51 (9), 39 (9). 2.3. The desulfurization reactions The desulfurization reactions were carried out in a stainless steel tubular reactor (8.0 mm i.d.). The catalyst precursors were

X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

We found that independent of the catalyst (Ni2P, MoP, or WP) and reaction conditions (under H2 or Ar, in the presence or absence of piperidine), all reactions showed substantial deactivation (25– 30%) at the beginning of the reaction. For instance, the conversion of 2-PCHT over MoP under 4.0 MPa Ar at 240 °C, 0 kPa piperidine, and s = 0.74 g min/mol decreased from ca. 75% to 50% in the first 6 h and then became constant (Fig. S1 in the Supplementary Material). To stabilize the catalysts, each reaction was started at high space–time (s = 0.74 g min/mol) and then the space–time was decreased in three steps to s = 0.25 g min/mol, after which it was increased again to s = 0.74 g min/mol. After this procedure, the performance of all phosphide catalysts became stable under all conditions. The product distribution over Ni2P hardly changed during this run-in period, while over MoP and WP the selectivity for CHB decreased, whereas the selectivities for phenylcyclohexenes (PCHEs) increased. In the following, we will show only the results obtained with run-in catalysts. 2.4. Characterization Before characterization, the fresh catalyst samples were prepared under the same reduction conditions as used in the in situ reduction, followed by passivation with 0.5% (vol.) O2 in Ar. The XRD patterns of the catalysts were measured on a Rigaku D/Max 2400 diffractometer. Nitrogen physisorption was performed on a Quantachrome Autosorb-1-MP analyzer. The specific surface areas of the bulk Ni2P (4.3 m2/g), MoP (3.8 m2/g), and WP (4.1 m2/g) catalysts were about equally low. The compositions of the three fresh catalysts were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Optima 2000DV). The P/metal ratios of Ni2P, MoP, and WP were found to be 0.84, 0.97, and 0.91, respectively, close to those of the corresponding precursors. CO adsorption was carried out using pulsed chemisorption as described elsewhere [26]. About 0.2 g of passivated, air-exposed catalyst sample was re-reduced in a H2 flow to remove the passivation layer (80 mL/min flow rate, heating from 25 to 500 °C at a rate of 10 °C/min, and then holding at the final temperature for 1 h). The reactor was then cooled to 30 °C in a flow of H2. An Ar flow at 80 mL/min was used to flush the catalyst for 30 min to achieve an adsorbate-free reduced catalyst surface. After pretreatment, 1.25 mL pulses of 1% CO in Ar were injected into a flow of Ar (80 mL/min), and the CO uptake was measured using a TCD detector. CO pulses were repeatedly injected until the response from the detector showed no further CO uptake. The CO uptakes of the Ni2P, MoP, and WP catalysts were measured to be 4.7, 2.2, and 2.4 lmol/ g, respectively. X-ray photoelectron spectroscopy (XPS) spectra of the spent catalysts were obtained with a Thermo ESCALAB 250 X-ray photoelectron spectrometer equipped with a high-resolution doublefocusing hemispherical analyzer, using an AlKa (1486.6 eV) source. The base pressure before the analysis was better than 5  106 Pa. All the spectra were obtained in a constant analyzer energy mode. A constant pass energy of 20 eV was used for the narrowscan; the widescan spectra were acquired with a pass energy of 100 eV. The instrument was operated in Large Area XL mode and the aperture diameter was 0.5 mm. The accuracy of the binding energy values was ±0.05 eV. All binding energies were referenced to the C1s peak at 284.6 eV. The atomic concentrations of different chemical elements were extracted from XPS intensity ratios after proper normalization using atomic sensitivity factors [27]. The spent catalysts were used directly for XPS measurements after being flushed with n-heptane and dried in an Ar flow. The samples were transferred to the XPS spectrometer through air. The other basic characterizations of the transition-metal phosphide catalysts used in the present work can be found in our previous publications [15,16,28-30].

333

3. Results 3.1. Characterization Since the structures of Ni2P, MoP, and WP are stable under HDS conditions (in other words, they will not transfer to other phases), we only measured the XRD patterns of the spent catalysts after the desulfurization of 2-PCHT under Ar, which were denoted as Ni2PAr, MoP-Ar, and WP-Ar, respectively. Only diffraction peaks due to the phosphide phases were observed (Fig. S2 in the Supplementary Material), indicating that the structures of these phosphide catalysts were well preserved. This is confirmed by the XPS results (Fig. S3 in the Supplementary Material). The major peaks or doublets (due to spin–orbit interaction) observed in the Ni2p, Mo3d, and W4f spectra of the spent catalysts after the reactions under H2 (denoted as Ni2P-H, MoP-H, and WP-H) and Ar can be assigned to the reduced metal atoms in the phosphide phases. These peaks are at 853.0 eV for Nid+ (0 < d < 2) in Ni2P [31], 231.6 and 228.4 eV for Mod+ (0 < d < 4) in MoP [32], and 33.6 and 31.4 eV for Wd+ (0 < d < 4) in WP [33]. In the P2p region, the predominant doublets at ca. 129.5 and 130.3 eV (Fig. S3 in the Supplementary Material) are related to the P atoms bonded to Ni, Mo, and W phosphides [31– 33]. The minor peaks or doublets at higher binding energies are due to the oxidized metal atoms and PO3 4 species generated by air exposure of the phosphide catalysts before XPS measurement [32]. Considering the measurement errors, the binding energies (and, thus, the electronic states of these elements) in the spent catalysts under H2 and Ar were equal. The surface compositions of the spent catalysts determined by XPS are given in Table S1 in the Supplementary Material, and the surface elemental ratios of phosphorus/metal (P/M) and sulfur/metal (S/M) are summarized in Table 2. The XPS P/Ni ratios of the spent Ni2P catalysts (ca. 2.2) were higher than the P/Ni ratios of the nickel phosphate precursor (1.0) and the fresh catalyst (0.84), whereas the XPS P/M ratios of the spent MoP and WP catalysts were close to those of their precursors (1.0) and the fresh catalysts (0.97 for MoP and 0.91 for WP). It seems that phosphorus is enriched in the surface of Ni2P. Sulfur was detected in the surface of the spent catalysts. The S2p spectra of all the catalysts were similar (Fig. 1). They were composed of a major peak at ca. 161.3 eV and a shoulder at higher binding energies (ca. 162.5 eV), which are in agreement with those of the sulfur species in the transition-metal phosphosulfide phases [31–33]. The S2p binding energies of the spent Ni2P catalysts were about 0.5 eV lower than those of the spent MoP and WP catalysts. The S/M ratios of WP-Ar and MoP-Ar were lower than those of WPH and MoP-H, but the S/Ni ratio of Ni2P after the reaction in Ar was more than five times as high as that after the reaction in H2. The S/ M ratio of Ni2P-Ar was the highest among the catalysts studied, whereas that of Ni2P-H was the second lowest. WP-Ar possessed the lowest S/M ratio, only half of the value of WP-H. The situation is different for MoP. The S/M ratios of Mo-H and MoP-Ar were close to each other.

3.2. Desulfurization of 2-PCHT over Ni2P Because the isomerization of cis-2-PCHT to trans-2-PCHT was fast under our conditions, trans-2-PCHT and cis-2-PCHT were considered a single compound. The conversion of 2-PCHT over Ni2P reached 44% at s = 0.74 g min/mol under H2 (Fig. 2A). The main four products were the desulfurized products 1-PCHE, 3-PCHE, CHB, and BP (Fig. 2A). Their yields at the highest space–time (0.74 g min/mol) are summarized in Table 3. 1-PCHE and 3PCHE, the two isomers of PCHE, were the two most abundant products (Fig. 2A and Table 3). While the selectivity of 1-PCHE decreased and that of 3-PCHE increased with space–time

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X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

Table 2 XPS phosphorus/metal (P/M) and sulfur/metal (S/M) ratios of the spent phosphide catalysts. Ni2P-H

Ni2P-Ar

MoP-H

MoP-Ar

WP-H

WP-Ar

P/M S/M

2.27 0.19

2.11 0.96

1.10 0.31

1.18 0.28

1.27 0.39

1.32 0.17

Intensity (a.u.)

Catalyst

Ni2P-H

MoP-H

WP-H

Ni2P-Ar

MoP-Ar

WP-Ar

166

164

162

160 166

164

162

160 166

164

ing to 5.2 at s = 0.74 g min/mol), while under H2 it was almost lower than one. The total selectivity of PCHEs was almost constant (Fig. S4 in the Supplementary Material). TH-DBT was detected as the fifth product (Fig. 3A and Table 3), but 4-PCHE and CPEMB could hardly be detected. Piperidine showed a negative influence on the conversion of 2-PCHT over Ni2P under Ar, and only 1PCHE, 3-PCHE, CHB, and BP were observed (Fig. 3C). Their selectivities were similar to those in the absence of piperidine (cf. Fig. 3B and D); only the total selectivity of PCHEs was a little bit higher in the presence of piperidine (Fig. S4 in the Supplementary Material).

162

160

3.3. Desulfurization of 2-PCHT over MoP

Binding energy (eV) Fig. 1. XPS spectra of the spent Ni2P, MoP, and WP samples in the S2p region.

(Fig. 2B), the total selectivity of the three PCHE isomers (1-PCHE, 3-PCHE, and 4-PCHE) was nearly constant (ca. 78%; Fig. S4 in the Supplementary Material). CHB and BP were the third and fourth major products. Their BP/CHB ratio varied from 1.2 at low space– time (0.25 g min/mol) to 0.5 at high space–time (0.74 g min/mol). BP behaved like a primary product, as its selectivity decreased with space–time (Fig. 2B). Four other compounds were observed at low concentrations: 4-PCHE > TH-DBT ~ cyclopentenylmethylbenzene (CPEMB) > cyclopentylmethylbenzene (CPMB). CPMB and CPEMB are isomers of CHB and PCHEs, respectively. Piperidine strongly inhibited the HDS of 2-PCHT. The 2-PCHT conversion at s = 0.74 g min/mol decreased significantly from 44% at 0 kPa piperidine (Fig. 2A) to 14% at 0.25 kPa piperidine (Fig. 2C). 1-PCHE and BP were the two most abundant compounds at s < 0.5 g min/mol, with about equal yields (Fig. 2C) and selectivities (Fig. 2D). At s > 0.5 g min/mol, the BP yield leveled off and CHB became the second most abundant product, just behind 1-PCHE. The yield of BP in the presence of piperidine was even higher than in the absence of piperidine (Fig. 2A and C and Table 3). This led to a higher BP/CHB ratio (changing from 2.65 to 0.94 with increasing space–time) in the presence of piperidine. The selectivities of 1PCHE and BP decreased with space–time (Fig. 2D), suggesting that 1-PCHE and BP are primary products of the HDS of 2-PCHT. The total selectivity of PCHEs was ca. 45%, lower than in the absence of piperidine (ca. 78%), and was nearly constant with space–time (Fig. S4 in the Supplementary Material). Only trace amounts of TH-DBT and 4-PCHE were detected, and the formation of CPMB and CPEMB was completely inhibited by piperidine. Desulfurization of 2-PCHT also occurred under Ar (Fig. 3A), but the reactivity of 2-PCHT was lower than under H2 (cf. Figs. 2A and 3A). 1-PCHE and BP were the two most abundant compounds, with about equal yields (Fig. 3A) and selectivities (Fig. 3B). The selectivity of 1-PCHE was independent of space–time, whereas that of BP decreased with space–time (Fig. 3B). The yields of 3-PCHE and CHB were about equal (Fig. 3A). Whereas under H2 the main products were 1-PCHE and 3-PCHE, under Ar their yields had decreased considerably (Fig. 3A and Table 3). Under Ar the yield of CHB plus BP was about equal to that under H2, but the ratio BP/CHB was greater than one (as high as 10 at s = 0.25 g min/mol and decreas-

MoP was more active than Ni2P in the desulfurization of 2-PCHT under 4.0 MPa H2 at 0 kPa piperidine (Fig. 4A). The conversion of 2PCHT was ca. 60% at s = 0.74 g min/mol (Fig. 4A). CHB was the most abundant product, and its yield and selectivity increased with space-time (Fig. 4A and B). 1-PCHE and 3-PCHE were the second and third most abundant products, and they had similar yields (Fig. 4A). The yield of 3-PCHE increased with space–time, while that of 1-PCHE passed through a maximum between 0.50 and 0.74 g min/mol (Fig. 4A), and as a result the selectivity of 1-PCHE decreased with space–time, whereas the selectivity of 3-PCHE hardly changed (Fig. 4B). The yield of 4-PCHE was very low (less than 3%). The total selectivity of PCHEs was much lower than that over Ni2P under identical conditions and decreased slightly with space–time (Fig. S4 in Supplementary Material). BP, CPMB, and CPEMB were detected in trace amounts and TH-DBT was not detected. The HDS of 2-PCHT over MoP was inhibited by piperidine. At s = 0.74 g min/mol and 0.25 kPa piperidine, the conversion of 2PCHT was 37% (Fig. 4C). Only four products, CHB, 3-PCHE, 1PCHE, and a trace amount of BP, were detected, with CHB as the most abundant product (Fig. 4C). The yield of 3-PCHE was higher than that of 1-PCHE. The selectivity of CHB increased slightly from 47% to 53% with space–time, whereas that of 1-PCHE decreased from 22% to 16% (Fig. 4D). The total selectivity of PCHEs was more than 46% (Fig. S4 in the Supplementary Material). Because of the low yields of BP (Fig. 4A and C and Table 3), the BP/CHB ratio was very low (less than 0.03) in the HDS of 2-PCHT over MoP in the absence as well as in the presence of piperidine. The reactivity of 2-PCHT over MoP under Ar in the absence of piperidine (Fig. 5A) was lower than that under H2 (Fig. 4A), but higher than over Ni2P under Ar (Fig. 3A). 1-PCHE was the major product, with a selectivity around 43% (Fig. 5B). The yields of the other products decreased in the order BP ~ 3-PCHE > TH-DBT ~ CHB > 4-PCHE ~ CPEMB (Fig. 5A). The selectivities of all products were nearly constant (Fig. 5B). The desulfurization of 2-PCHT under Ar over MoP was inhibited by piperidine as well, and the yields of the main products decreased in the order 1-PCHE > BP > CHB > 3-PCHE (Fig. 5C). With increasing space–time, the selectivities of 1-PCHE and BP decreased, whereas that of 3-PCHE remained unchanged (Fig. 5D). The total selectivity of PCHEs was about 10% lower than in the absence of piperidine (Fig. S4 in the Supplementary Material). CHB behaved like a final product, its yield and selectivity

X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

100

2-PCHT concentration (%)

80 60 40

40 1-PCHE 3-PCHE 4-PCHE CHB BP

CPEMB CPMB TH-DBT 2-PCHT

30 20

20

10

0 0.0

0.2

Product concentration (%)

50

A

0 0.8

0.4 0.6 Space-time (g·min/mol )

100

B

1-PCHE 3-PCHE 4-PCHE CHB

Selectivity (%)

80

BP CPEMB CPMB TH-DBT

60

335

increased with space-time (Fig. 5C and D). 4-PCHE and TH-DBT were detected in trace amounts. 3.4. Desulfurization of 2-PCHT over WP WP and MoP behaved similarly in the desulfurization of 2-PCHT (Figs. S5 and S6 in the Supplementary Material). Minor differences are as follows. WP was as active as MoP in the HDS of 2-PCHT, but was more active than MoP in the desulfurization of 2-PCHT under Ar. Under H2 and 0 kPa piperidine, the selectivity of CHB over WP was slightly lower than that over MoP, while the selectivities of 1PCHE and 3-PCHE were slightly higher (Fig. 4B and Fig. S5B in the Supplementary Material). In the presence of piperidine, however, the product selectivities over the two catalysts were almost equal under H2 (Fig. 4D and Fig. S5D in the Supplementary Material). Under 4.0 MPa Ar and 0 kPa piperidine, WP showed a higher selectivity to 1-PCHE but a lower selectivity to 3-PCHE than MoP (Fig. 5B and Fig. S6B in the Supplementary Material). The formation of BP under Ar was more inhibited by piperidine over WP than over MoP (Table 3). 4. Discussion

40

4.1. The cleavage of the cycloalkyl C–S bond in 2-PCHT 20 0 0.2

0.3

0.4 0.5 0.6 Space-time (g·min/mol)

0.7

10

80

C

1-PCHE 3-PCHE 4-PCHE CHB BP TH-DBT 2-PCHT

60 40

8 6 4

20

2

0 0.0

0.2

Product concentration (%)

2-PCHT concentration (%)

100

0.8

0 0.8

0.4 0.6 Space-time (g·min/mol )

100

Selectivity (%)

80

1-PCHE 3-PCHE 4-PCHE CHB BP TH-DBT

D

60 40 20 0 0.2

0.3

0.4 0.5 0.6 Space-time (g·min/mol)

0.7

0.8

Fig. 2. Concentrations of the reactant and products (A and C) as well as the product selectivities (B and D) in the desulfurization of 2-PCHT at 240 °C over Ni2P under 4.0 MPa H2 in the absence (A and B) or presence (C and D) of 0.25 kPa piperidine as a function of space–time.

It is well known that thiols undergo hydrogenolysis to alkanes and H2S elimination to alkenes [34–37]. Therefore, in our case, 1-PCHE and 3-PCHE are the products of the elimination of the SH group from 2-PCHT, and the hydrogenolysis of 2-PCHT yields CHB under hydrogen. CHB can also be formed by the hydrogenation of PCHEs. Nevertheless, the variations of the selectivities for CHB as well as those of the total selectivities for PCHEs with space–time were minor. In particular, the selectivities of CHB, 1PCHE, and 3-PCHE were almost constant in the HDS of 2-PCHT over MoP and WP at 0.25 kPa piperidine (Fig. 4D and Fig. S5D in the Supplementary Material). These observations suggest that the hydrogenation of PCHEs to CHB over the phosphide catalysts is slow at 240 °C in the presence of sulfur-containing compounds. In other words, the HDS of 2-PCHT to PCHEs and to CHB are two parallel reactions. This is confirmed by the low reactivity of 1PCHE over Ni2P in the presence of 0.5 kPa benzothiophene at 240 °C, 4.0 MPa H2, and s = 0.74 g min/mol. The conversion of 1PCHE and the yield of CHB were less than 5% and 4.3%, respectively. Hence, hydrogenation reactions are negligible over the phosphide catalysts at 240 °C. According to our previous work [14], 4-PCHE is mainly formed by the isomerization of 3-PCHE. This is probably because of the conjugation between the double bond and the aromatic ring in 1-PCHE, which makes it more stable than 3-PCHE. Over Ni2P under H2 (4.0 MPa), 1-PCHE did not undergo isomerization to 3-PCHE and 4-PCHE at a higher temperature (340 °C) [14]. In the desulfurization of 2-PCHT over the phosphide catalysts, the yields of 4-PCHE were always very low. Additionally, the selectivities of 3-PCHE were nearly constant over all three phosphide catalysts under Ar, indicating that its further reaction is negligible. These results suggest that the isomerization of 1-PCHE and 3-PCHE is slow under the present conditions, which may be due to the low acidity of these bulk phosphide catalysts [16,29,38]. The low acidity of the phosphides is further proved by the very low yields of CPEMB and CPMB, the skeleton isomers of PCHE and CHB or the products of the heterolytic cleavage of the cycloalkyl C–S bond in 2-PCHT [16]. The two compounds disappeared after the addition of a base (piperidine). It should be noted that not only acid sites but also

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Table 3 Yields of the main products in the reaction of 2-PCHT at 240 °C and s = 0.74 g min/mol over Ni2P, MoP, and WP under H2 or Ar in the absence or presence of 0.25 kPa piperidine. Yield (%) In the absence of piperidine Atmosphere

1-PCHE

3-PCHE

CHB

BP

TH-DBT

1-PCHE

3-PCHE

CHB

BP

Ni2P

H2 Ar H2 Ar H2 Ar

20.9 7.5 9.4 15.5 11.0 24.8

12.9 1.3 12.2 5.8 12.0 6.4

4.8 1.4 33.5 3.1 29.3 3.6

2.5 7.2 0.1 6.1 0.1 9.0

0.7 0.8 –a 3.4 –a 3.6

4.3 3.3 5.9 4.0 4.0 4.4

1.5 0.8 11.3 1.9 7.5 2.4

4.0 0.7 19.7 2.8 15.9 3.3

3.7 3.2 0.2 3.0 0.5 2.8

MoP WP a

In the presence of piperidine

Catalyst

Not detected.

metal surfaces are able to catalyze skeletal rearrangements of hydrocarbons [39,40]. Thus, because of the low acidity of these bulk transition-metal phosphide catalysts and the low yields of the isomerization products, we cannot exclude the possibility that the formation of CPEMB and CPMB is due to the metallic character of the transition-metal phosphides. BP was detected as one of the major products in the desulfurization of 2-PCHT over all three phosphide catalysts under Ar at 0 kPa piperidine. It was the most abundant product over Ni2P (Fig. 3A) and the second most abundant product over MoP (Fig. 5A) and WP (Fig. S6A in the Supplementary Material). BP can be formed by the dehydrogenation and/or the disproportionation of PCHEs. The disproportionation of PCHEs yields BP and CHB with a BP/ CHB ratio of 0.5; that is, one PCHE molecule donates four hydrogen atoms to two PCHE molecules, forming one BP and two CHB molecules [41]. However, the BP/CHB ratios were higher than 0.5 in most cases. More importantly, the selectivities of BP over the phosphide catalysts under Ar decreased with space–time, suggesting that it is unlikely to be a secondary product of the further reaction of PCHEs. To check this, we carried out the reaction of 1-PCHE at 240 °C and s = 0.74 g min/mol over Ni2P-H, Ni2P-Ar, and a fresh Ni2P catalyst under 4.0 MPa Ar. The conversion of 1-PCHE was less than 3.5%. Although BP was the most abundant product over the fresh Ni2P catalyst, its yield was less than 1.7% at s = 0.74 g min/mol. Since the formation of BP from 2-PCHT involves the cleavage of the C–S bond and the dehydrogenation of the saturated hydrocarbon ring, these results suggest that the dehydrogenation of the cyclohexane ring of 2-PCHT must occur prior to or simultaneously with the cleavage of the C–S bond (the dehydrogenation–d esulfurization pathway). This explains the low yield or absence of BP in the HDS of 2-PCHT under H2, because the dehydrogenation reaction is not favored under high hydrogen pressure. One explanation for the formation of a minor amount of TH-DBT in the desulfurization of 2-PCHT over Ni2P (Fig. 3A), MoP (Fig. 5A), and WP (Fig. S6A in the Supplementary Material) under Ar as well as in the HDS of 2-PCHT over Ni2P (Fig. 2A) could be that ring closure of 2-PCHT yields HH-DBT, which subsequently dehydrogenates to TH-DBT. However, we never observed HH-DBT. We confirmed that the conversion of HH-DBT over Ni2P was almost zero below 280 °C under 4.0 MPa Ar or H2. Therefore, TH-DBT is likely formed by a dehydrogenation-ring closure pathway; that is, 2-PCHT first dehydrogenates to 2-phenyl-1-cyclohexenethiol (2-PCHET), which then undergoes a ring closure reaction to form TH-DBT. The ring closure reaction must be a quick reaction, because TH-DBT behaved like a primary product with nonzero initial selectivity. The mechanism of the ring closure of 2-PCHET remains a subject for future study. It could be a reverse reaction of the hydrogenolysis of the aryl C–S bond or might follow a dehydrogenative coupling mechanism. The latter is usually used to describe the intermolecular and intramolecular coupling of silane Si–H bonds with aromatic and aliphatic C–H bonds catalyzed by metals [42]. This reaction can be regarded as a reverse reaction

of oxidative insertion that occurs in the HDS of thiophenic compounds catalyzed by organometallic compounds in homogeneous systems [43]. Anyway, these results suggest that the breaking of the aryl C–S bond is also a reversible reaction. Under Ar or in the absence of H2, CHB could be a product of the disproportionation of PCHE. However, CHB did not behave like a secondary product during the desulfurization of 2-PCHT under Ar. As shown in Figs. 3B and 5B as well as Fig. S6B in the Supplementary Material, the selectivities of CHB were nearly constant. Actually, not only the selectivities of CHB but also those of the other products were almost independent of space–time over MoP (Fig. 5B) and WP (Fig. S6B in the Supplementary Material). This is further confirmed by the low reactivity of 1-PCHE and the very low yield of CHB (less than 0.3%) at 240 °C and s = 0.74 g min/mol over the fresh and spent Ni2P catalysts under 4.0 MPa Ar. Hence, mechanisms other than disproportionation of PCHE must be taken into account for the formation of CHB under inert atmosphere. One possible mechanism is homolytic C–S bond cleavage via free radical intermediates, which has been demonstrated to be the desulfurization mechanism for the HDS of cyclopropylmethylthiol both in a homogeneous system [44] and in a heterogeneous system using an actual CoMo sulfide HDS catalyst [45]. The other possibility is the hydrogenolysis of 2-PCHT, because the dehydrogenation of 2-PCHT under Ar generates H2. The present data are not sufficient to verify or rule out this possibility, which will be studied in our subsequent work. Based on analysis of the product distributions, we propose the following reaction networks for the desulfurization of 2-PCHT under H2 and Ar over the transition-metal phosphides under the reaction conditions of the present work (Scheme 1). 2-PCHT undergoes HDS by three parallel pathways: b-elimination, hydrogenolysis, and dehydrogenation. The dehydrogenation pathway also includes two parallel reactions: the dehydrogenation-ring closure reaction and the dehydrogenation-desulfurization reaction with TH-DBT and BP as the products, respectively. The rate of the dehydrogenation pathway is low (over Ni2P) or almost zero (over MoP and WP) under 4.0 MPa H2. Under inert atmosphere, the three parallel pathways of 2-PCHT desulfurization are b-elimination, hydrogenolysis or homolytic cleavage of the C–S bond, and dehydrogenation. 4.2. Comparison of the catalytic performances of Ni2P, MoP, and WP The numbers of surface active sites of Ni2P, MoP, and WP were determined by CO chemisorption. The turnover frequencies (TOF) of 2-PCHT under H2 and Ar over these catalysts at 0 and 0.25 kPa piperidine were calculated based on the 2-PCHT conversions at the lowest space–time (0.25 g min/mol) and their CO uptakes,

TOF = [F 0
X 2PCHT ]/[wcat
(CO uptake)],

ð1Þ

where F0 is the initial molar flow rate of 2-PCHT, lmol/s; X2-PCHT is the conversion of 2-PCHT; and wcat represents the weight of the cat-

X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

100

18 15

80 1-PCHE 3-PCHE 4-PCHE CHB

60

BP CPEMB TH-DBT 2-PCHT

12 9

40 6 20

Product concentration (%)

2-PCHT concentration (%)

A

3

0 0.0

0.2

0 0.8

0.4 0.6 Space-time (g ·min/mol)

100

B

1-PCHE 4-PCHE BP TH-DBT

Selectivity (%)

80

3-PCHE CHB CPEMB

60 40 20 0 0.2

0.3

0.4 0.5 0.6 Space-time (g·min/mol)

0.7

10

C

80 1-PCHE 3-PCHE CHB BP 2-PCHT

60

8 6

40

4

20

2

0 0.0

0.2

Product concentration (%)

2-PCHT concentration (%)

100

0.8

0 0.8

0.4 0.6 Space-time (g ·min/mol )

100

D

1-PCHE 3-PCHE CHB BP

Selectivity (%)

80 60 40 20 0 0.2

0.3

0.4 0.5 0.6 0.7 Space-time (g·min/mol)

0.8

Fig. 3. Concentrations of the reactant and products (A and C) as well as the product selectivities (B and D) in the desulfurization of 2-PCHT at 240 °C over Ni2P under 4.0 MPa Ar in the absence (A and B) or presence (C and D) of 0.25 kPa piperidine as a function of space–time.

337

alyst charged in g. The results are shown in Fig. 6. MoP and WP were much more active than Ni2P in the desulfurization of 2-PCHT both under H2 and Ar. This seems to contradict previous reports that Ni2P is more active than MoP and WP in the HDS of DBT [17]. There are two possibilities to explain this contradiction. (1) Compared with MoP and WP, Ni2P is less active in the desulfurization of 2PCHT but more active in the HDS of DBT. (2) The activity of Ni2P is strongly inhibited by excess phosphorus present in the catalyst. The P/Ni ratios of the Ni2P catalyst determined by ICP-AES and XPS were 0.84 and ca. 2.2 (Table 2), respectively. They were both higher than the stoichiometric value (0.5). Excess phosphorus has to be used for the preparation of Ni2P by temperatureprogrammed reduction of nickel phosphate precursors; otherwise metal-rich nickel phosphides like Ni12P5 with low HDS activities will be formed [46]. The large excess of phosphorus is known to have a strong negative influence on the dispersion of Ni2P and its HDS performance [47]. On the other hand, the surface P/metal ratios of WP and MoP determined by XPS (Table 2) were close to their stoichiometric values. To clarify this question, it is necessary to prepare Ni2P with stoichiometric P/Ni ratio by conventional TPR, which is challenging. Under H2, the activity of the phosphides decreased in the order MoP > WP > Ni2P, while under Ar the order was WP > MoP > Ni2P. At 0 kPa piperidine and s = 0.25 g min/mol, the turnover frequencies of 2-PCHT over Ni2P and MoP under H2 were higher than under Ar, whereas that over WP under Ar was higher than under H2. Piperidine strongly inhibited the desulfurization of 2-PCHT over all three phosphide catalysts, and WP seemed to be more sensitive to piperidine inhibition than MoP. The TOF of WP under Ar was higher than that of MoP at 0 kPa piperidine, but the turnover frequencies of the two catalysts were about equal in the presence of 0.25 kPa piperidine. This is consistent with our previous finding that WP was more sensitive to piperidine inhibition than MoP in the HDS of DBT [16]. The desulfurization of 2-PCHT over Ni2P under H2 mainly took place by b-elimination to 1-PCHE and 3-PCHE (Fig. 2A). The total selectivity of PCHEs was around 80% (Fig. S4 in the Supplementary Material). Piperidine strongly inhibited the HDS of 2-PCHT over Ni2P mainly by inhibiting the b-elimination pathway. At s = 0.74 g min/mol, the total yield of 1-PCHE and 3-PCHE decreased significantly from 33.8% in the absence of piperidine to 5.8% in the presence of piperidine (Table 3). In contrast, the addition of piperidine hardly inhibited the formation of CHB and BP or the hydrogenolysis and dehydrogenation pathways. The yield of BP in the presence of piperidine was even slightly higher than in the absence of piperidine (Fig. 2A and C, as well as Table 3). Consequently, the total selectivity of PCHEs decreased from ca. 80% at 0 kPa piperidine to ca. 45% at 0.25 kPa piperidine (Fig. S4 in the Supplementary Material). Over Ni2P under 4.0 MPa Ar, the selectivity for BP was very close to that of 1-PCHE (Fig. 3B), indicating that the dehydrogenation pathway is as fast as the b-elimination pathway. These two pathways were equally inhibited by piperidine, because no significant change in product selectivities was observed before and after the addition of piperidine (Fig. 3D), except for the disappearance of TH-DBT (the dehydrogenation-ring closure product) as well as 4-PCHE and CPEMB. These compounds were detected in trace amounts in the desulfurization of 2-PCHT over Ni2P under Ar in the absence of piperidine (Fig. 3A). The product distributions of 2-PCHT desulfurization over MoP and WP (the two group VIB transition-metal phosphides) were similar, and very different from that over Ni2P. MoP and WP exhibited much higher hydrogenolysis activities than Ni2P. The selectivities of CHB and the total selectivities of PCHEs were both around

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X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

100

100

A

B

60 40 20 0 0.0

1-PCHE 3-PCHE 4-PCHE CHB BP CPEMB CPMB 2-PCHT

60 40

0.2

0.4 0.6 Space-time (g·min/mol)

0 0.2

0.8

0.7

0.8

1-PCHE 3-PCHE CHB BP

80 Selectivity (%)

Concentration (%)

0.4 0.5 0.6 Space-time (g·min/mol)

D

C 1-PCHE 3-PCHE CHB BP 2-PCHT

60 40

20 0 0.0

0.3

100

80

40

3-PCHE CHB CPEMB

20

100

60

1-PCHE 4-PCHE BP CPMB

80 Selectivity (%)

Concentration (%)

80

20

0.2 0.4 0.6 Space-time (g·min/mol)

0 0.2

0.8

0.3

0.4 0.5 0.6 Space-time (g·min/mol)

0.7

0.8

Fig. 4. Concentrations of the reactant and products (A and C) as well as the product selectivities (B and D) in the desulfurization of 2-PCHT at 240 °C over MoP under 4.0 MPa H2 in the absence (A and B) or presence (C and D) of 0.25 kPa piperidine as a function of space–time.

100

100

A

B

60 40

1-PCHE 3-PCHE 4-PCHE CHB

BP CPEMB 2-PCHT TH-DBT

20

2-PCHT concentration (%)

60 40

40

1-PCHE 3-PCHE 4-PCHE CHB BP TH-DBT 2-PCHT

C

8 6 4 2

0.2

0.4 0.6 Space-time (g·min/mol)

0 0.8

0.3

0.4 0.5 0.6 Space-time (g·min/mol)

0.7

0.8

100

10

20 0 0.0

60

0 0.2

0.8

D

1-PCHE 3-PCHE 4-PCHE CHB BP TH-DBT

80 Selectivity (%)

0.2 0.4 0.6 Space-time (g·min/mol)

100 80

3-PCHE CHB CPEMB

20

Product concentration (%)

0 0.0

1-PCHE 4-PCHE BP TH-DBT

80 Selectivity (%)

Concentration (%)

80

60 40 20 0 0.2

0.3

0.4 0.5 0.6 Space-time (g·min/mol)

0.7

0.8

Fig. 5. Concentrations of the reactant and products (A and C) as well as the product selectivities (B and D) in the desulfurization of 2-PCHT at 240 °C over MoP under 4.0 MPa Ar in the absence (A and B) or presence (C and D) of 0.25 kPa piperidine as a function of space–time.

X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

339

Scheme 1. Reaction networks of 2-PCHT desulfurization at 240 °C over the transition-metal phosphide catalysts under 4.0 MPa H2 and Ar. The notations of the rate constants of some steps in the network under H2 are given.

0.5

0 kPa piperidine 0.25 kPa piperidine

TOF (S-1)

0.4 0.3 0.2 0.1 0.0

H2 Ar Ni2P

H2

Ar

MoP

H2 Ar WP

Fig. 6. Turnover frequencies of 2-PCHT desulfurization at 240 °C and s = 0.25 g min/mol over Ni2P, MoP, and WP under 4.0 MPa H2 or Ar in the absence or presence of 0.25 kPa piperidine.

50% (Fig. 4B, as well as Figs. S4 and S5B in the Supplementary Material), demonstrating that the hydrogenolysis and belimination pathways are equally fast. b-Elimination dominates the desulfurization of 2-PCHT under Ar over MoP and WP, and the dehydrogenation pathway is faster than the hydrogenolysis or homolytic cleavage pathway (Fig. 5 and Fig. S6 in the Supplementary Material). The b-elimination pathway was more inhibited by piperidine than the latter two pathways. Under Ar, the total selectivity of PCHEs decreased from more than 61% over MoP or 64% over WP at 0 kPa piperidine to about 52% or 54% at 0.25 kPa piperidine, respectively (Fig. S4 in the Supplementary Material). The CO uptake of MoP (2.2 lmol/g) and WP (2.4 lmol/g) was about equal, which allowed direct comparison of their catalytic performance based on product distributions. Under Ar, WP showed higher b-elimination activity than MoP. The total yield of 1-PCHE and 3-PCHE over WP was about 10% higher than that over MoP (Fig. 5A and Fig. S6A in the Supplementary Material, as well as Table 3). The dehydrogenation activity of WP was also slightly higher than that of MoP (Fig. 5A and Fig. S6A in the Supplementary Material). Hence, WP exhibited the highest desulfurization activity under Ar (Fig. 6). Under 4.0 MPa H2, MoP was the most active of the three phosphide catalysts in the hydrogenolysis of 2-PCHT to CHB (Fig. 6). Additionally, the b-elimination of 2-PCHT to PCHEs over MoP under H2 was about as fast as that under Ar, whereas the belimination of 2-PCHT over WP under H2 was slower than that

under Ar (Table 3). As a consequence, MoP performed best in the HDS of 2-PCHT (Fig. 6). The addition of piperidine hardly affected the selectivity of the three reaction pathways over MoP under H2 (Fig. 4), except for the increase in the 3-PCHE/1-PCHE ratio and the disappearance of 4-PCHE, CPMB, and CPEMB. As a base, piperidine is expected to inhibit isomerization reactions. For WP under H2, the b-elimination pathway was more inhibited by piperidine than the hydrogenolysis pathway (Fig. S5 in the Supplementary Material). The total selectivity for PCHEs at 0.25 kPa piperidine was about 4–8% lower than that at 0 kPa piperidine (Fig. S4 in the Supplementary Material). It is worth noting that the ratios of the PCHE isomers over the phosphide catalysts under Ar differed from those under H2. Because of the conjugation between the double bond and the aromatic ring in 1-PCHE, it is more energetically stable than 3-PCHE. However, the b-elimination of the H atom attached to carbon atom C(2) of 2-PCHT to form 1-PCHE (the b1 pathway) is more sterically hindered than the b-elimination of the H atom attached to carbon atom C(6) to yield 3-PCHE (the b2 pathway). For example, trans elimination of H2S from trans-2-PCHT can only occur by b-H elimination from C(6), giving 3-PCHE. Therefore, the ratio of the PCHE isomers is affected by more than one factor. Over Ni2P, the belimination reaction preferred to occur through the b1 pathway, both under H2 and Ar. The 3-PCHE/1-PCHE ratio under H2 (ca. 0.5) was much higher than that under Ar (ca. 0.15), indicating that the b2 pathway was favored under H2 (Figs. 2A and 3A). The situation is quite different over MoP and WP. The 3-PCHE/1-PCHE ratios were close to 1 under H2 (Fig. 4A and Fig. S5A in the Supplementary Material). At 0.25 kPa piperidine the ratios were even higher than one (Fig. 4C and Fig. S5C in the Supplementary Material). Under Ar, however, the 3-PCHE/1-PCHE ratios were much lower than one (Fig. 5 and Fig. S6 in the Supplementary Material). This suggests that the surface active sites or reactions under H2 and Ar are different. To fully understand these important questions, further experimental and theoretical work is needed. We studied the kinetics of the desulfurization of 2-PCHT by a pseudo-first-order model, which is usually applied to describe the HDS of DBT and its derivatives [12,48]. The rate equation of the pseudo-first-order model for a plug-flow reactor is

-ln(1 - X 2PCHT ) = k2PCHT
s,

ð2Þ

where k2-PCHT is the rate constant, mol g1 min1. ln(1  X2-PCHT) is then plotted against s, and the rate constant k2-PCHT can be determined from the slope of each fitted line. The regression analysis (Figs. S7 and S8 in the Supplementary Material) demonstrated that

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X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342

the HDS of 2-PCHT over the phosphide catalysts can be fitted by a first-order rate model, but the desulfurization of 2-PCHT under Ar cannot be described by the power-law rate expression. The calculated concentration-s curves of 2-PCHT desulfurization under H2 and Ar from the obtained pseudo-first-order rate constants are shown in Figs. S9 and S10 in the Supplementary Material, respectively. Consistent with regression analysis, the experimental data of 2-PCHT concentrations under H2 agreed with the calculated concentration-s curves, whereas those under Ar deviated largely from the curves predicted by the first-order model. This is reasonable. As shown in Scheme 1, the reaction network of 2-PCHT desulfurization involves several kinds of reactions that proceed on different active sites. In the present work, we only calculated the rate constants of the HDS of 2-PCHT by assuming pseudo-firstorder kinetics. The constants of the three primary parallel steps and some of the following steps were obtained from these rate constants and the initial product selectivities estimated by extrapolating the selectivity-s curves to s = 0. A detailed study of the kinetics of 2-PCHT desulfurization will be an important subject of our future research. These rate constants are helpful for a semiquantitative comparison of the HDS performances of the phosphide catalysts. As shown in Table 4, the hydrogenolysis activity of Ni2P was very low. The rate of the hydrogenolysis pathway (khyg) at 0 kPa piperidine was about one order of magnitude lower than that of the belimination pathway (kb). It was even lower than the rate constant of the dehydrogenation pathway (kd). MoP and WP all exhibited high hydrogenolysis activity. Their hydrogenolysis pathway rate constants were 8.0 and 6.3 times as high as that of Ni2P, respectively. The rate constants of the b-elimination pathway of all the three phosphide catalysts were comparable to each other. Ni2P was more active than MoP and WP in the dehydrogenation of 2PCHT. As far as the b-elimination pathway is concerned, the ratio of the rate constant of the b1 pathway (kb1) to that of the b2 pathway (kb2) in the absence of piperidine decreased in the order Ni2P (2.8) > WP (1.6) > MoP (1.3). The b1 pathway is favored over Ni2P. Both the hydrogenolysis pathway and the b-elimination pathway were inhibited by piperidine, whereas the dehydrogenation pathway was hardly affected (Ni2P) or even improved (MoP and WP) after the addition of piperidine. In summary, piperidine showed different effects on different reaction pathways in the desulfurization of 2-PCHT over different phosphide catalysts, which can be an indication of the complexity of the cleavage of the cycloalkyl C–S bond in 2-PCHT and the active sites in the surfaces of the transition-metal phosphide catalysts. 4.3. The phosphosulfide phases of the transition-metal catalysts It has been established that the real active phases for the working Ni2P, MoP, and WP catalysts during HDS reactions are phosphosulfide phases. These phases can be obtained during the course of HDS reactions, or by the treatment of the transition-metal phosphide with sulfidation agents (H2S or organic sulfur-containing

compounds) under H2, or by the reduction of a sulfur-containing precursor (Ni2P2S6) [21,22,31,32]. All these methods require hydrogen. As revealed by the S2p XPS spectra (Fig. 1), sulfur species assigned to those in phosphosulfide phases were detected in the surfaces of the spent phosphide catalysts after the desulfurization of 2-PCHT both under H2 and under Ar. This not only confirms the formation of transition-metal phosphosulfide phases, but also indicates that the phosphosulfide phases can be formed by direct reaction of the phosphide catalysts with the sulfur-containing compounds, even in the absence of hydrogen, which provides a more practical and safer method for their preparation. According to the covalent nature of transition-metal phosphides and the XPS analysis, which demonstrated that the metal, phosphorus, and sulfur species in the phosphosulfide phases were not in their ionic and elemental states (Fig. 1 and Fig. S3 in the Supplementary Material), the phosphosulfide phases might be covalent in nature. A typical S2p XPS spectrum is composed of a doublet structure due to the presence of S2p1/2 and 2p3/2 levels with spin–orbit splitting of 1.18 eV and area ratio of 1:2. The S2p XPS spectra of all the spent catalysts featured a broad asymmetric peak with a shoulder at higher binding energy. These peaks cannot be fitted well by considering single sulfur species (Fig. S11 in the Supplementary Material), suggesting that several sulfur species may be present in the phosphosulfide phases. A DFT calculation revealed that two kinds of sulfur species may be present in the phosphosulfide phase, the sulfur species that replaces the phosphorus species in the phosphide phase and atomic sulfur deposited on the threefold hollow sites [49]. Therefore, we fitted the S2p spectra by assuming the coexistence of two sulfur species (the S-I and S-II species) in the phosphosulfide phases. The following assumptions were made in the curve fitting. (1) A Shirley background was used and the peaks were all fitted with Gaussian functions. (2) The energy difference between the S2p1/2 and 2p3/2 states, as well as their area ratio, were fixed to the theoretical values, 1.18 eV and 1:2, respectively. (3) The full width at half-maximum (FWHM) values of the S2p1/2 and 2p3/2 doublet peaks of each sulfur species were equal and were fixed for all the catalysts (Table 5). That is, the FWHM of the doublet peaks of the S-I species was fixed to 1.5, while that of the S-II species was fixed to 1.0. According to the fitting results (Fig. 1 and Table 5), the S-II species has a lower oxidation state than the S-I species. The binding energies of the doublets of the S-I and S-II species were close to those of the sulfur species in a thiolate-type environment and a sulfide species (S2), respectively [32]. No sulfur oxide was detected. In the fields of optoelectronic devices and photovoltaics, sulfur passivation by (NH4)2S or H2S is used to improve electronic properties of group III–V semiconductors, such as InP and GaP [50]. We found in our previous work [30] that a H2S-passivated Ni2P/MCM-41 catalyst showed higher HDS activity than its O2-passivated counterpart. The H2S-passivated Ni2P/MCM41 catalyst was stable up to 150 days (kept in air). Moreover, it was not necessary to re-reduce the H2S-passivated Ni2P/MCM-41 catalyst prior to the HDS reaction. This suggests that transition-metal

Table 4 Pseudo-first-order rate constants of some reactions in the network of the desulfurization of 2-PCHT at 240 °C over Ni2P, MoP, and WP under 4.0 MPa H2 in the absence or presence of piperidine. Catalyst

Piperidine partial pressure (kPa)

Pseudo-first-order rate constant (mol g1 min1) kba

kb1

kb2

khyg

kdb

kdd

kdrc

Ni2P

0 0.25 0 0.25 0 0.25

0.57 0.097 0.63 0.39 0.66 0.24

0.42 0.091 0.35 0.19 0.41 0.11

0.15 0.006 0.28 0.20 0.25 0.13

0.08 0.022 0.64 0.27 0.51 0.23

0.11 0.10 0.007 0.01 0.01 0.02

0.10 0.10 0.007 0.01 0.01 0.02

0.01 0.004 0 0 0 0

MoP WP a b

kb is the sum of kb1 and kb2. kd is the sum of kdd and kdrc.

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X. Zhou et al. / Journal of Catalysis 383 (2020) 331–342 Table 5 Deconvolution results of the S2p XPS spectra of the spent Ni2P, MoP, and WP catalysts. Catalyst

S-I

S-II

2p3/2

Ni2P-H Ni2P-Ar MoP-H MoP-Ar WP-H WP-Ar

2p1/2

S-II/S-I

2p3/2

2p1/2

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

162.2 162.2 162.7 162.6 162.5 162.0

1.5 1.5 1.5 1.5 1.5 1.5

163.4 163.4 163.9 163.8 163.7 163.2

1.5 1.5 1.5 1.5 1.5 1.5

161.4 161.1 161.8 161.6 161.6 161.2

1.0 1.0 0.9 1.0 1.0 0.9

162.6 162.3 163.0 162.8 162.8 162.4

1.0 1.0 0.9 1.0 1.0 0.9

phosphosulfide phases could be more stable under air or less affected by air than their corresponding sulfur-free phases. It must be noted that since the nature of the phosphosulfide phase is still not well understood. Our fitting results cannot be used to interpret the real structures of the phosphosulfide phases. They are only helpful for a quantitative comparison of the phosphosulfide phases formed under H2 and Ar. The different binding energies and the S-II/S-I ratios of the sulfur species suggest different interactions between sulfur and the phosphide catalysts (Table 5). The S-II/S-I ratios of the spent MoP and WP catalysts under H2 were higher than those under Ar. Moreover, the S/M ratios of MoP-H and WP-H were also higher than those of MoPAr and WP-Ar, respectively (Table 2), which suggests that the sulfidation degree of the spent MoP and WP is higher under H2 than under Ar. Although Ni2P-Ar possessed the highest S/M ratio among all the spent catalysts, the S/M ratio of Ni2P-H was low (Table 2). It was only 1/5 of the value of Ni2P-Ar, and was very close to the S/M ratio of WP-Ar, the lowest one (Table 2). The S-II/S-I ratio of Ni2P-H was the lowest among all the spent catalysts (Table 5). Thus, the formation of nickel phosphosulfide, particularly the formation of the sulfur-containing species with sulfur in a lower oxidation state (the S-II species), is difficult under H2. This suggests that the nickel phosphosulfide is more easily reduced under H2 than the molybdenum and tungsten phosphosulfides. This may explain why the formation of the nickel phosphosulfide is more difficult than that of the molybdenum phosphosulfide [51]. By means of CO-IR, Sun et al. reported that a MoP/SiO2 catalyst can be sulfided by a mixture of thiophene/H2 (10/100 Torr), but more severe sulfiding conditions were required to transform a Ni2P/SiO2 catalyst to its phosphosulfide counterpart. They obtained the nickel phosphosulfide phase using a high concentration H2S/H2 mixture (10 vol%) as the sulfidation agent [51]. The different sulfidation behavior of Ni2P, MoP, and WP might be due to their different nature. The sulfidation behavior of the two group VIB transition-metal phosphides (MoP and WP) was similar, and different from that of Ni2P, a group VIII transition-metal phosphide. At present we cannot give a definite answer to this important question, which will be studied in our subsequent work.

5. Conclusions The desulfurization of 2-PCHT over the transition metal phosphides at 240 °C under 4.0 MPa H2 proceeded mainly through three parallel pathways: b-elimination, hydrogenolysis, and dehydrogenation. The dehydrogenation pathway consisted of two parallel reactions: the dehydrogenation-ring closure reaction with THDBT as the product and the dehydrogenation-desulfurization reaction with BP as the product. Under 4.0 MPa Ar, the three parallel pathways are b-elimination, homolytic C–S bond cleavage or hydrogenolysis, and dehydrogenation. MoP and WP exhibited much higher activity than Ni2P in the desulfurization of 2-PCHT. They were the most active catalysts under H2 and Ar, respectively.

1.72 3.24 2.50 1.74 2.91 2.03

2-PCHT underwent hydrodesulfurization over Ni2P mainly via the b-elimination pathway. Under Ar, the b-elimination and dehydrogenation pathways were about equal. Over MoP and WP, the hydrogenolysis pathway was as fast as the b-elimination pathway under H2, while b-elimination dominated the desulfurization of 2PCHT under Ar. Piperidine inhibited different reaction pathways to different extents, which may indicate the complexity of the cleavage of the cycloalkyl C–S bond in 2-PCHT and the active sites in the surfaces of the transition-metal phosphides. Phosphosulfide phases were formed under both H2 and Ar, demonstrating that their formation may be due to the reaction of the phosphide catalysts with the sulfur-containing compounds and that H2 is not necessary for the reaction. The sulfidation degree of spent MoP and WP under H2 was higher than under Ar, whereas the formation of the nickel phosphosulfide phase was difficult under H2. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research was financially supported by the Natural Science Foundation of China (21673029) and the Natural Science Foundation of Tianjin (19JCZDJC31700). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2020.01.023. References [1] R.V. Mom, J.N. Louwen, J.W.M. Frenken, I.M.N. Groot, In situ observations of an active MoS2 model hydrodesulfurization catalyst, Nat. Commun. 10 (2019) 1– 8. [2] F. van Looij, P. van der Laan, W.H.J. Stork, D.J. DiCamillo, J. Swain, Key parameters in deep hydrodesulfurization of diesel fuel, Appl. Catal. A 170 (1998) 1–12. [3] E.M. Morales-Valencia, C.O. Castillo-Araiza, S.A. Giraldo, V.G. BaldovinoMedrano, Kinetic assessment of the simultaneous hydrodesulfurization of dibenzothiophene and the hydrogenation of diverse polyaromatic structures, ACS Catal. 8 (2018) 3926–3942. [4] L. Zhang, W. Fu, Q. Yu, T. Tang, Y. Zhao, H. Zhao, Y. Li, Ni2P clusters on zeolite nanosheet assemblies with high activity and good stability in the hydrodesulfurization of 4,6-dimethyldibenzothiophene, J. Catal. 338 (2016) 210–221. [5] J.H. Sinfelt, Catalytic hydrogenolysis on metals, Catal. Lett. 9 (1991) 159–171. [6] J.M.J.G. Lipsch, G.C.A. Schuit, The CoO-MoO3-Al2O3 catalyst: III. Catalytic properties, J. Catal. 15 (1969) 179–189. [7] E.J.M. Hensen, M.J. Vissenberg, V.H.J. de Beer, J.A.R. van Veen, R.A. van Santen, Kinetics and mechanism of thiophene hydrodesulfurization over carbonsupported transition metal sulfides, J. Catal. 163 (1996) 429–435. [8] Th. Weber, J.A.R. van Veen, A density functional theory study of the hydrodesulfurization reaction of dibenzothiophene to biphenyl on a singlelayer NiMoS cluster, Catal. Today 130 (2008) 170–177.

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