Effect of Mono-, Di-, and Triethylene Glycol on the Activity of Phosphate-Doped Nimo/Al2o3 Hydrotreating Catalysts

catalysts

Article Effect of Mono-, Di-, and Triethylene Glycol on the Activity of Phosphate-Doped NiMo/Al2O3 Hydrotreating Catalysts

Alexey L. Nuzhdin 1,*, Galina A. Bukhtiyarova 1,2, Aleksander A. Porsin 1, Igor P. Prosvirin 1, Irina V. Deliy 1,2 , Vladimir A. Volodin 2,3, Evgeny Yu. Gerasimov 1,2 , Evgeniya N. Vlasova 1,2 and Valerii I. Bukhtiyarov 1,2 1 Boreskov Institute of Catalysis SB RAS, 630090 Novosibirsk, Russia; [email protected] (G.A.B.); [email protected] (A.A.P.); [email protected] (I.P.P.); [email protected] (I.V.D.); [email protected] (E.Y.G.); [email protected] (E.N.V.); [email protected] (V.I.B.) 2 Novosibirsk National Research University, 630090 Novosibirsk, Russia; [email protected] 3 Rzhanov Institute of Semiconductor Physics SB RAS, 630090 Novosibirsk, Russia * Correspondence: [email protected]; Tel.: +7-383-3269-410

Received: 19 November 2018; Accepted: 13 January 2019; Published: 17 January 2019

Abstract: The effect of glycols on the catalytic properties of phosphate-doped NiMo/Al2O3 catalysts in the hydrotreating of straight-run gas oil (SRGO) was studied. The NiMo(P)/Al2O3 catalysts were prepared using ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG) as additives. The organic agent was introduced into the aqueous impregnation solution obtained by the dissolving of MoO3 in H3PO4 solution, followed by Ni(OH)2 addition. The Raman and UV–Vis (6−x)− studies show that the impregnation solution contains diphosphopentamolybdate HxP2Mo5O23 2+ and Ni(H2O)6 , and that these ions are not affected by the presence of glycols. When the (6−x)− impregnation solution comes in contact with the γ-Al2O3 surface, HxP2Mo5O23 is decomposed completely. The catalysts were characterized by Raman spectroscopy, low-temperature N2 adsorption, X-ray photoelectron spectroscopy, and transmission electron microscopy. It is shown that the sulﬁde catalysts prepared with glycols display higher activity in the hydrotreating of straight-run gas oil than the NiMoP/Al2O3 catalyst prepared without the additive. The hydrodesulfurization and hydrodenitrogenation activities depend on the glycol type and are decreased in the following order: NiMoP-DEG/Al2O3 > NiMoP-EG/Al2O3 > NiMoP-TEG/Al2O3 > NiMoP/Al2O3. The higher activity of NiMoP-DEG/Al2O3 can be explained with the higher dispersion of molybdenum on the surface of the catalyst in the sulﬁde state.

Keywords: hydrodesulfurization; phosphate-doped NiMo/Al2O3 catalyst; straight-run gas oil; XPS; ethylene glycol; diethylene glycol; triethylene glycol; hydrodenitrogenation

1. Introduction Alumina-supported cobalt and nickel-promoted molybdenum sulfides are generally used in the refineries as hydrodesulfurization (HDS) catalysts to remove sulfur from petroleum fractions [1,2]. Increasingly stringent transportation fuel specifications and the necessity of refining of the new stocks, such as heavier crudes, shell oil, cracked stocks, and renewables, have stimulated the development of more active hydrotreating catalysts [3,4]. It is generally accepted that the active phase of hydrotreating catalysts is presented by layered nanoparticles of molybdenum disulfide decorated with cobalt or nickel atoms on the edge faces [2]. The dispersion of active metal, sulfidation degree, and amount of Co or Ni atoms on the edges of MoS2 nanoparticles are the essential factors in the preparation of the so-called CoMoS or NiMoS active phase [1,2].

Catalysts 2019, 9, 96; doi:10.3390/catal9010096 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 96 2 of 11

One of the recognized approaches to enhance the activity of HDS catalysts in the hydrotreating reactions is the addition of phosphoric acid to the impregnating solution [5–8]. Phosphate has a beneficial effect due to several factors, such as the increased solubility of the molybdenum salts, stability of the impregnation solution, and decreased metal-support interaction. Depending on the reaction conditions, phosphoric acid reacts with the molybdenum precursor (ammonium heptamolybdate or MoO3) and gives a variety of phosphomolybdate anions, such as diphosphopentamolybdate 6− 3− 6− (P2Mo5O23 ), Keggin (PMo12O40 ), and Dawson (P2Mo18O62 ) heteropolyanions (HPAs). Further, the use of various types of organic modifiers during catalyst preparation has resulted in materials of enhanced HDS activity; both chelating agents and glycol-type additives are used for this purpose [1,3,4,6–12]. In contrast with ligand-type modifiers, such as α-hydroxycarboxylic, thioglycolic, or aminopolycarboxylic acid, only a limited number of publications have been found concerning the effect of glycol-type additives on catalytic properties of sulfide CoMo or NiMo catalysts [3,13]. Most articles describe the effect of glycols on the activity of CoMo systems, using post-treatment of CoMo(P)/Al2O3 systems with triethylene glycol (TEG) or polyethylene glycol (PEG) [14–16], or co-impregnation with the solution of active metals and glycols [6–9,17]. Just a few reports are found concerning the effect of addition of glycols to the impregnation solution on the activity of NiMo/Al2O3 catalysts [10–12]. It was shown that the glycol-modified catalysts displayed higher activity in the HDS of model compounds [6–12,16,17] and gasoil fractions [10,12,16,17], as well as in toluene hydrogenation [14,15]. Moreover, a synergetic effect of glycol-type additives and phosphate was disclosed [6,7,12]. It was proposed that glycols hinder the interaction between the active phase precursors and alumina, promoting the formation of sulfide phase with the improved activity [6,7,9–11]. However, the results concerning the effect of glycol addition on the dispersion of sulfide phase and surface distribution of promoter (Co, Ni) and Mo species (expressed as metal/alumina ratio) are rather contradictory. According to [9], dispersions of both Co and Mo are increased in the sulfide CoMo(P)/Al2O3 catalysts when ethylene glycol (EG), diethylene glycol (DEG), and TEG are added to impregnation solution. Using the results of the XPS study, the authors [12] came to the conclusion that the addition of PEG to the solution leads to the increase of Ni dispersion in the dried NiMo(P)/Al2O3 system, but has no effect on Mo distribution. The sulfide samples were not studied in this work. Using similar precursors and the XPS method for catalyst characterization, the authors [10] observed the decrease of Mo dispersion and increase of Ni dispersion in the NiMo(P)/Al2O3 sulfide catalysts after addition of TEG to impregnation solution. Besides, there is no information concerning the comparison of EG, DEG, and TEG effect on the catalytic properties of NiMo/Al2O3 catalyst. To the best of our knowledge, this effect was only studied for CoMo(P)/Al2O3 catalysts prepared starting from Keggin-type HPA in dibenzotiophene HDS [9]. In the present paper, we study the effect of various glycols on the activity of phosphate-doped NiMo/Al2O3 catalysts in the hydrotreating of straight-run gas oil (SRGO). The influence of EG, DEG, and TEG on Ni and Mo dispersion in the oxide (after drying at 110 ◦C) and sulfide (after liquid-phase sulfidation and HDS reaction) NiMo(P)/Al2O3 catalysts is also studied using XPS method.

2. Results and Discussion

2.1. Impregnation Solutions

The dissolution of MoO3 in an aqueous solution of phosphoric acid for atomic ratio P/Mo = 0.4 (pH = 1.0) leads to a formation of Dawson-type heteropolyanion [5,18], according to Equation (1):

18MoO3 + 2H3PO4 → H6P2Mo18O62, (1)

The Raman spectrum of this solution (denoted as MoP), containing major signals at 978, 955, 715, 648, 382, 242, 223, and 180 cm−1 (Figure1), conﬁrms the presence of Dawson entities [18,19]. Catalysts 2018, 8, x FOR PEER REVIEW 3 of 11 CatalystsCatalysts2019 2018, 9, ,8 96, x FOR PEER REVIEW 3 ofof 1111

715 715 180 978 180 955 223 978 242 223 955 242 382

382 648 648

150 250 350 450 550 650 750 850 950 1050 150 250 350 450 550cm 650-1 750 850 950 1050 90 cm-1 9190 Figure 1. Raman spectrum of the MoP solution. Figure 1. Raman spectrum of the MoP solution. 91 Figure 1. Raman spectrum of the MoP solution. 92 (6−x)− The HPA HxP2Mo18O62 is decomposed upon addition of nickel hydroxide in the solution (6-x)- 9392 due toThe an HPA increase HxP of2Mo pH18 [O562,18]. Theis decomposed Raman spectrum upon ofaddition obtained of NiMoPnickel hydroxide solution (pH in the = 2.3) solution has peaks due (6-x)- 9493 atto 944,an Theincrease 892, HPA 389, of H 370, pHxP2Mo and[5,18].18 216O62 The cm −isRaman1 decomposed, characteristic spectrum upon of of the obtainedaddition presence NiMoPof ofnickel diphosphopentamolybdate solution hydroxide (pH=2.3) in the hassolution peaks anion due at 9594 944,to an 892, increase 389,(6 370, −ofx) −pH and [5,18]. 216 cmThe-1 ,Raman characteristic spectrum of theof obtained presence NiMoP of diphosphopentamolybdate solution (pH=2.3) has peaks anion at HxP2Mo5O23 (Figure2a). The introduction of diethylene glycol to the NiMoP solution does 9695 H944,xP2 Mo892,5O 389,23(6-x)- 370, (Figure and 2a).216 Thecm-1 ,introduction characteristic of of diethyle the presencene glycol of diphosphopentamolybdateto the NiMoP solution does(6 anion− x)not− not affect the bands of diphosphopentamolybdate anion. In addition to peaks of HxP2Mo5O23 , (6-x)- (6-x)- 9796 theaffectHxP spectrum2Mo the5O bands23 of (Figure NiMoP-DEGof diphosphopentamolybd 2a). The solution introduction has theate of small anion. diethyle bands In neaddition atglycol 830 and to thepeaks 1060 NiMoP cmof −H1 xassignedsolutionP2Mo5O23 does to DEG, thenot −1 (6-x)- 9897 vibrationsspectrumaffect the of (Figurebands NiMoP-DEG 2ofb,c). diphosphopentamolybd The solution Raman spectrahas the ofatesmall the anion.impregnation bands In ataddition 830 solutions and to 1060peaks containing cm of H assignedxP2Mo EG5O and 23to TEGDEG, the 99 −1 98 werevibrationsspectrum recorded (Figuresof NiMoP-DEG as well 2b,c). and The showed solution Raman the hasspectra same the results ofsmall the (Figureimpregnationbands S1).at 830 solutionsand 1060 containing cm assigned EG and to TEGDEG 10099 werevibrations recorded (Figures as well 2b,c). and The showed Raman the spectra same results of the (Figureimpregnation S1). solutions containing EG and TEG 100 were recorded as well and showed the same results (Figure S1).944 944 216 892 216 892

370 370389 389 (a) (b)(a) (c)(b) (c)

150 300 450 600 750 900 1050 -1 101 150 300 450 600cm 750 900 1050 -1 102101 FigureFigure 2. Raman 2. Raman spectra spectra of aqueous of aqueous solutions: solutions:cm ( (aa)) NiMoP, NiMoP, ( b(b)) NiMoP-DEG, NiMoP-DEG, and and (c) DEG.(c) DEG. 103102 Figure 2. Raman spectra of aqueous solutions: (a) NiMoP, (b) NiMoP-DEG, and (c) DEG. 104103 InIn orderorder to to determine determine nickel nickel species species presented presente in thed in NiMoP the NiMoP solution, solution, UV–Vis absorptionUV–Vis absorption spectrum 105104 wasspectrum recordedIn order was(Figure recordedto determine3a). (Figure The nickel broad 3a). bandThespecies broad in thepresente band 550–800 ind inthe nm the 550–800 region, NiMoP nm with solution, region, two maxima with UV–Vis two at aboutmaximaabsorption 650 at 3 3 106105 andaboutspectrum 720 650 nm, andwas is assigned 720recorded nm, tois (Figure theassigned spin-allowed 3a). to The the broadspin-allowed d–d transitionband in d–dtheA 2g550–800transition→ T1g nm(F) 3A region, of2g → octahedrally 3T 1gwith(F) of two octahedrally coordinated maxima at 2+ 107106 nickelcoordinatedabout ions650 and [Ni(H nickel 7202O) nm,ions6] is[Ni(H[ 20assigned]. This2O)6] 2+ indicatesto [20]. the spin-allowedThis that indicates no nickel-diphosphopentamolybdate d–d that transition no nickel-diphosphopentamolybdate 3A2g → 3T1g(F) of octahedrally species are 108107 formedspeciescoordinated inare the formed nickel aqueous inions the solution, [Ni(Haqueous2 asO) assumed6solution,]2+ [20]. This in as [ 6assu ].indicates Therefore,med in that [6]. the noTherefore, interaction nickel-diphosphopentamolybdate the of theinteraction MoP solution of the withMoP 109108 nickelsolutionspecies hydroxide arewith formed nickel can inhydroxide be the presented aqueous can formally besolution, presented by as Equation assu formallymed (2): in by [6]. equation Therefore, (2): the interaction of the MoP 109 solution with nickel hydroxide can be presented formally by equation (2): 5H6P2Mo18O62 + 36Ni(OH)2 + 26H3PO4 +144H2O → 36[Ni(H2O)2+6]2+ + 18[H2P2Mo5O23]44-−, (2) 5H6P2Mo18O62 + 36Ni(OH)2 + 26H3PO4 +144H2O → 36[Ni(H2O)6] + 18[H2P2Mo5O23] , (2) 5H6P2Mo18O62 + 36Ni(OH)2 + 26H3PO4 +144H2O → 36[Ni(H2O)6]2+ + 18[H2P2Mo5O23]4-, (2) 110 The UV–Vis spectrum of [Ni(H2O)6]2+ cation is maintained in the presence of glycols (Figure 3b). 111110 Thus,The we UV–Viscan conclude spectrum that of the [Ni(H glycols2O)6 ]2+do cation not in isteract maintained with the in themetal presence precursors of glycols in the (Figure aqueous 3b). 112111 solutionThus, we [6,10,11]. can conclude that the glycols do not interact with the metal precursors in the aqueous 112 solution [6,10,11].

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Catalysts 2018, 8, x FOR PEER REVIEW 720 4 of 11 3 650 4 (b) 2 720 (a) 3 650 1 Absorbance, Absorbance, a.u. (b) 2 (a) 0 1 200 300 400 500 600 700 800 Absorbance, Absorbance, a.u. Wavelength, nm 113 Figure 3. UV–Vis0 spectra of the solutions: (a) NiMoP and (b) NiMoP-DEG. 114 Figure 3. UV–Vis200 spectra 300 of the 400 solutions: 500 (a 600) NiMoP 700 and (b 800) NiMoP-DEG. 2+ The UV–Vis spectrum of [Ni(H2O)6] cationWavelength, is maintained nm in the presence of glycols (Figure3b). 113115 2.2. Catalyst Characterization Thus, we can conclude that the glycols do not interact with the metal precursors in the aqueous 116 2 3 114solution [6The,10 ,11phosphate-doped]. Figure 3. UV–Vis NiMo/Al spectraO of thecatalysts solutions: were (a) NiMoPprepared and (byb) NiMoP-DEG.impregnation of γ-alumina 117 granules with a subsequent drying at 110 °C for 4 h. The list and chemical composition of the 115 1182.2. Catalyst2.2.prepared Catalyst Characterization catalysts Characterization are presented in Table 1. The catalysts contained approximately the same amount 119 of Mo with the Ni/Mo molar ratios of 0.4. The Raman spectra of dried samples are shown in Figure 4. 116 The phosphate-doped NiMo/Al2O3 catalysts were prepared by impregnation of γ-alumina The phosphate-doped NiMo/Al O catalysts were prepared by impregnation of γ-alumina granules 117120 granulesAll samples with showed a subsequent broad bands, drying2 which 3at 110 can °C be for a ttributed4 h. The tolist a mixedand chemical NiMo oxyhydroxidecomposition ofphase the ◦ (6-x)- 118121with prepared a[18,21]. subsequent Meanwhile, catalysts drying are the presented at 110spectraC in fordo Table 4not h. 1. Theco Thentain list catalysts andthe bands chemical contained associated composition approximately with HPA of the the H preparedsamexP2Mo amount5O23 catalysts. 119122are presentedofTherefore, Mo with in thewhen Table Ni/Mo 1.the The molar impregnation catalysts ratios containedof 0.4. so lutionThe Rama approximately comesn spectra in contact of the dried same with samples amount the areγ-Al ofshown2 MoO3 withsurface, in Figure the the Ni/Mo 4. 120123molar Alldiphosphopentamolybdate ratios samples of 0.4. showed The Ramanbroad bands,anion spectra iswhich decomposed of dried can be samples a completely.ttributed are to shown Thisa mixed is in in FigureNiMo accordance oxyhydroxide4. All with samples literature phase showed 124 data, in which it is stated that interaction of HxP2Mo5O23(6-x)- with the hydroxyl groups of the alumina 121broad [18,21]. bands, Meanwhile, which can the be spectra attributed do not to acomixedntain the NiMo bands oxyhydroxide associated with phase HPA [ 18Hx,21P2Mo].5 Meanwhile,O23(6-x)-. 125 results in the disintegration of the complex with the formation of amorphous(6−x)− AlPO4 layer and 122the spectraTherefore, do notwhen contain the impregnation the bands associated solution comes with HPA in contact HxP2Mo with5O 23the γ-Al. Therefore,2O3 surface, when the the 123126 diphosphopentamolybdatemolybdates [22,23]. anion is decomposed completely. This is in accordance with literature impregnation solution comes in contact with the γ-Al2O3 surface, the diphosphopentamolybdate 124 data, in which it is stated that interaction of HxP2Mo5O23(6-x)- with the hydroxyl groups of the alumina 127anion is decomposed completely.Table 1. ThisList and is inchemical accordance composition with of literature the catalysts data, 1. in which it is stated that 125 results in the disintegration(6−x) −of the complex with the formation of amorphous AlPO4 layer and interaction of HxP2Mo5O23 with the hydroxyl groups of the alumina results in the disintegration 126 molybdates [22,23]. Catalyst Mo, wt% Ni, wt% P, wt% of the complex with the formation of amorphous AlPO4 layer and molybdates [22,23]. NiMoP/Al2O3 13.4 3.4 2.0 1 127 TableNiMoP-EG/Al 1. List and2O chemical3 13.1composition of 3.4 the catalysts 1.9 . Table 1. List and chemical composition of the catalysts 1. NiMoP-DEG/AlCatalyst 2O3 Mo,13.2 wt% Ni, 3.5wt% P, wt% 1.6 NiMoP-TEG/Al2O3 13.3 3.3 1.8 CatalystNiMoP/Al2O3 Mo,13.4 wt% Ni,3.4 wt% 2.0 P, wt% 128 1 The content of elements was determined by atomic absorption spectroscopy after calcination of the NiMoP/AlNiMoP-EG/AlO 2O3 13.413.1 3.4 3.4 1.9 2.0 129 samples at 550 °C for 4 h. 2 3 NiMoP-EG/AlNiMoP-DEG/Al2O3 2O3 13.113.2 3.5 3.4 1.6 1.9 NiMoP-DEG/AlNiMoP-TEG/Al2O32O3 13.213.3 3.3 3.5 1.8 1.6 128 1 The content of elementsNiMoP-TEG/Al was dete2rminedO3 by atomic13.3 absorption 3.3 spectroscopy 1.8after calcination of the 129 1 Thesamples content ofat elements550 °C for was 4 h. determined by atomic absorption spectroscopy after calcination of the samples at 550 ◦C for 4 h.

(c)

(d)

(c)(b) (a) (d) 100 200 300 400 500 600 700 800 900 1000 1100 cm-1 (b) 130 (a)

100 200 300 400 500 600 700 800 900 1000 1100 -1 cm 130 Figure 4. Raman spectra of dried solids: (a) NiMoP/Al2O3,(b) NiMoP-EG/Al2O3,(c) NiMoP-DEG/Al2O3, and (d) NiMoP-TEG/Al2O3.

Catalysts 2019, 9, 96 5 of 11

The chemical surface composition of the catalysts in the oxide form was evaluated by XPS. The Mo3d spectrum of dried NiMoP/Al2O3 catalyst contains Mo3d5/2 and Mo3d3/2 doublet with a binding energy of 232.3 and 235.3 eV (Figure S2), which can be attributed to Mo6+ in oxide compound [9]. The spectra of other samples show slightly lower binding energies (231.9 and 234.9 eV) 6+ 5+ for Mo3d5/2 and Mo3d3/2 peaks. This can be explained by partial reduction of Mo to Mo during drying in the presence of glycol. The binding energy for Ni2p is 856.7 eV (Figure S3), which is characteristic of Ni2+ in nickel hydroxide [24,25]. Analysis of the recorded XPS spectra allowed estimating the Ni/Al and Mo/Al ratios. It is seen that addition of glycols to the impregnation solution has a minor effect on the Mo/Al ratio, but the Ni/Al ratio increases noticeably, from 0.044 to 0.051–0.065 (Table2). Thus, the glycols provide the increase of Ni dispersion in the dried samples coinciding with the results obtained in [12].

Table 2. XPS data for phosphate-doped NiMo/Al2O3 catalysts. Oxide Form Sulﬁde Form Catalyst Ni/Al Mo/Al Ni/Al Mo/Al NiMoP/Al2O3 0.044 0.113 0.051 0.074 NiMoP-EG/Al2O3 0.051 0.112 0.041 0.141 NiMoP-DEG/Al2O3 0.065 0.123 0.056 0.151 NiMoP-TEG/Al2O3 0.063 0.120 0.040 0.116

The Mo3d spectra of the sulfide catalysts after testing in the hydrotreating of SRGO contain the Mo 3d5/2 and 3d3/2 doublet with binding energy at 228.9 and 232.1 eV (Figure5 and Figure S4). The peak, with a binding energy of 226.3 eV, is assigned to S2s [9,10]. The data obtained from deconvolution of Mo3d and Ni2p spectra [26] are sufficiently similar for all catalysts (Table S1). The percentage of Mo atoms in different states is approximately the same in the different catalysts, being within the following limits of %: Mo4+—75.8 ± 1.4; Mo5+—14.2 ± 0.7; Mo6+—10.0 ± 1.4. Consequently, the portion of Mo atoms in the MoS2, as well as sulfidation degree, are similar for the studied catalysts. The evolution of Mo and Ni dispersion in terms of Mo/Al and Ni/Al ratios on the surface of the catalysts upon sulfidation is rather different. The Ni/Al ratio decreases in the EG, DEG, and TEG-modified catalysts after sulfidation, while this value increases for the NiMoP/Al2O3 sample (Table2). The Mo/Al atomic ratio decreases on the surface of NiMoP/Al2O3 and NiMoP-TEG/Al2O3 catalysts, while sulfidation of NiMoP-EG/Al2O3 and NiMoP-DEG/Al2O3 leads to an increase in this ratio. Thus, the comparison of Ni/Al and Mo/Al ratios on the surface of NiMo(P)/Al2O3 catalysts in the oxide and sulfide state allows us to come to the conclusion that considerable redistribution of active metals occurs during sulfidation, and the nature of organic additive has a noticeable effect on this process. The Mo dispersion in the sulfide catalysts in the term of Mo/Al ratio is decreased in the following order: NiMoP-DEG/Al2O3 > NiMoP-EG/Al2O3 > NiMoP-TEG/Al2O3 > NiMoP/Al2O3. The XRD patterns of the NiMo(P)/Al2O3 sulfide catalysts contain the characteristic diffraction peaks from γ-Al2O3 (PDF No. 29-0063) and MoS2 (JCPDS No. 017-0744) phases (Figure S5). In all samples, the MoS2 phase parameters slightly exceed the standard values (a = b = 3.162–3.163 Å, c = 18.371 Å, α = β = 90◦, γ = 120◦), apparently due to the incorporation of Ni atoms into molybdenum disulfide structure. The formation of nickel sulfide crystallites was not detected. The difference curves obtained by the subtraction of the XRD curve of the alumina from the catalyst showed two low-intensive broad diffraction peaks at 2θ = 33◦–35◦ and 58◦–60◦, characteristic for the 100 and 110 planes of the MoS2 hexagonal phase. The coherently scattering domains (DXRD) were 4.0–4.5 nm (Table3). The TEM images of the NiMoP/Al2O3 and NiMoP-DEG/Al2O3 catalysts are presented in Figure6; they are typical of NiMo/Al 2O3 sulfide catalysts. The observed black thread-like fringes correspond to the structure of molybdenum disulfide MoS2. The TEM images of the NiMoP-EG/Al2O3 and NiMoP-TEG/Al2O3 catalysts are very similar to the picture of NiMoP-DEG/Al2O3 sample. The average particle sizes determined by statistical processing of the TEM images are in the range Catalysts 2018, 8, x FOR PEER REVIEW 6 of 11 Catalysts 2019, 9, 96 6 of 11 175 NiMoP-EG/Al2O3 and NiMoP-TEG/Al2O3 catalysts are very similar to the picture of 176 NiMoP-DEG/Al2O3 sample. The average particle sizes determined by statistical processing of the 177 TEMof 4.5–5.2 images nm are (Table in the3 ).range The of average 4.5–5.2 nm number (Table of 3). layers The average per particle number is 1.9of layers for NiMoP/Al per particle2 Ois3 1.9; a 178 forslightly NiMoP/Al smaller2O3 value; a slightly of stacking smaller number value of in stacking the sulfide number nanoparticles in the sulfide (1.4–1.6) nanoparticles are obtained (1.4–1.6) for the 179 areNiMoP-EG/Al obtained for2O the3, NiMoP-DEG/Al NiMoP-EG/Al2O2O3,3 NiMoP-DEG/Al, and NiMoP-TEG/Al2O3, and2O3 NiMoP-TEG/Alcatalysts. These2O data3 catalysts. correlate These with 180 datathe XPS correlate results, with which the giveXPS higherresults, Mo/Al which ratiosgive hi forgher glycol-modified Mo/Al ratios for catalysts. glycol-modified Thus, the TEMcatalysts. and 181 Thus,XRDCatalysts datathe 2018 TEM give, 8, x andFOR the evidencePEERXRD REVIEWdata that give the the procedures evidence that used the in procedures catalyst preparation used in catalyst and sulfidation preparation ensure6 andof 11 182 sulfidationthe formation ensure of dispersed the formation NiMoS of nanoparticles. dispersed NiMo TheS textural nanoparticles. properties The of textural all catalysts properties in the sulfide of all 175 NiMoP-EG/Al2O3 and NiMoP-TEG/Al2O3 catalysts are very similar to the picture of 183 catalystsstate did in not the differ sulfide significantly state did not (Table differ3). significantly (Table 3). 176 NiMoP-DEG/Al2O3 sample. The average particle sizes determined by statistical processing of the 177 TEM images are in the range of 4.5–5.2 nm (Table 3). The average number of layers per particle is 1.9 178 for NiMoP/Al2O3; a slightly smaller value of stacking number in the sulfide nanoparticles (1.4–1.6) 179 are obtained for the NiMoP-EG/Al2O3, NiMoP-DEG/Al2O3, and NiMoP-TEG/Al2O3 catalysts. These 180 data correlate with the XPS results, which give higher Mo/Al ratios for glycol-modified catalysts. 181 Thus, the TEM and XRD data give the evidence that the procedures used in catalyst preparation and 182 sulfidation ensure the formation of dispersed NiMoS nanoparticles. The textural properties of all 183 catalysts in the sulfide state did not differ significantly (Table 3).

184 Figure 5. XPS Mo3d spectrum of the NiMoP-DEG/Al O sulﬁde catalyst. 185 Figure 5. XPS Mo3d spectrum of the NiMoP-DEG/Al22O33 sulfide catalyst. Table 3. Morphological and textural characteristics of the sulﬁde catalysts.

Average Slab Stacking S , m2 V , Catalyst D nm BET pore D , nm XRD, Length, nm Number g−1 cm3 g−1 pore

NiMoP/Al2O3 4.5 4.9 1.9 114 0.34 11.8 NiMoP-EG/Al2O3 4.0 4.7 1.4 120 0.36 11.9 184 NiMoP-DEG/Al2O3 4.4 5.2 1.6 129 0.34 10.7 NiMoP-TEG/Al O 4.1 4.5 1.5 127 0.35 10.9 185 2 Figure3 5. XPS Mo3d spectrum of the NiMoP-DEG/Al2O3 sulfide catalyst.

(a) (b)

186 Figure 6. TEM image of sulfide catalyst: (a) NiMoP/Al2O3 , (b) NiMoP-DEG/Al2O3.

187 Table 3. Morphological and textural characteristics of the sulfide catalysts.

2 -1 3 DXRD, nm Average slab Stacking SBET, m g Vpore, cm Dpore, Catalyst -1 length, nm number g nm

NiMoP/Al2O3 (a) 4.5 4.9 1.9 114 (b) 0.34 11.8 NiMoP-EG/Al2O3 4.0 4.7 1.4 120 0.36 11.9 186 FigureFigure 6. 6.TEM TEM image image of of sulﬁde sulfide catalyst: catalyst: ( a()a) NiMoP/Al NiMoP/Al22OO33,( , (bb)) NiMoP-DEG/AlNiMoP-DEG/Al22OO33. . NiMoP-DEG/Al2O3 4.4 5.2 1.6 129 0.34 10.7 NiMoP-TEG/Al2.3. Catalytic Activity2O3 4.1 4.5 1.5 127 0.35 10.9 187 Table 3. Morphological and textural characteristics of the sulfide catalysts. The catalytic properties of the NiMoP/Al O , NiMoP-EG/Al O , NiMoP-DEG/Al O , and 2 3 2 23 -1 3 2 3 188 2.3. Catalytic Activity DXRD, nm Average slab Stacking SBET, m g Vpore, cm Dpore, NiMoP-TEG/AlCatalyst 2O3 catalysts were investigated in the hydrotreating of straight-run gas oil at length, nm number g-1 nm NiMoP/Al2O3 4.5 4.9 1.9 114 0.34 11.8 NiMoP-EG/Al2O3 4.0 4.7 1.4 120 0.36 11.9 NiMoP-DEG/Al2O3 4.4 5.2 1.6 129 0.34 10.7 NiMoP-TEG/Al2O3 4.1 4.5 1.5 127 0.35 10.9

188 2.3. Catalytic Activity

Catalysts 2019, 9, 96 7 of 11

◦ temperature of 330–340 C, hydrogen pressure of 3.5 MPa, H2/feedstock ratio of 300, and LHSV of 2 h−1. The properties of the feedstock are listed in Table4. The content of residual sulfur and nitrogen was determined for three samples taken 10, 11, and 12 h after the start of the experiment. According to the results obtained, the catalytic activity are not decreased during the reaction under the conditions used (Figures S6 and S7).

Table 4. Characteristics of the feedstock.

Property Method IBP 1:0.5%V, ◦C 214.3 ASTM D2887 5%V, ◦C 243.5 10%V, ◦C 250.7 20%V, ◦C 259.3 30%V, ◦C 266.8 50%V, ◦C 282.9 70%V, ◦C 303.4 80%V, ◦C 315.5 90%V, ◦C 330.9 95%V, ◦C 343.2 FBP 2: 99.5%V, ◦C 349.4 Sulfur content, ppm 8870 ASTM D 4294 Nitrogen content, ppm 110 ASTM D 4629 Total aromatics, wt% 28.0 IP 391 Monoaromatics, wt% 19.2 Diaromatics, wt% 7.1 Polyaromatics, wt% 1.7 Density, g cm−3 0.847 ASTM D 4052 1 Initial boiling point; 2 Final boiling point.

The catalysts prepared using glycols show a higher HDS activity than the catalyst prepared without the additive (Table5). This is consistent with the results obtained in [ 6–12]. The HDS activity decreased in the following order: NiMoP-DEG/Al2O3 > NiMoP-EG/Al2O3 > NiMoP-TEG/Al2O3 > NiMoP/Al2O3. The same effect is observed for the hydrodenitrogenation (HDN) of SRGO. ◦ A comparison of the hydrodesulfurization rate constants (k) calculated at 330 C for NiMoP/Al2O3, NiMoP-EG/Al2O3, NiMoP-DEG/Al2O3, and NiMoP-TEG/Al2O3 catalysts demonstrates that the addition of EG, DEG, and TEG in the impregnation solution increases the catalytic activity by a factor of 2.2, 2.8, and 1.3, respectively. These results can be explained by the inﬂuence of organic additives on the dispersion of Mo in the sulﬁde catalyst (Table2). Besides, the activity of NiMoP-DEG/Al 2O3 exceeding that of NiMoP-EG/Al2O3 may be associated with a higher Ni dispersion (in terms of Ni/Al ratio).

1 Table 5. Catalytic properties of NiMo(P)/Al2O3 catalysts in the hydrotreating of SRGO .

Entry Catalyst T, ◦C S, ppm N, ppm k, h−1 ppm−0.6

1 NiMoP/Al2O3 330 340 19 0.031 2 NiMoP/Al2O3 340 166 15 – 3 NiMoP-EG/Al2O3 330 109 11 0.067 4 NiMoP-EG/Al2O3 340 60 6 – 5 NiMoP-DEG/Al2O3 330 74 4 0.086 6 NiMoP-DEG/Al2O3 340 34 3 – 7 NiMoP-TEG/Al2O3 330 246 16 0.039 8 NiMoP-TEG/Al2O3 340 142 12 – 1 3 −3 −1 H2 pressure of 3.5 MPa, H2/feedstock of 300 Nm m and LHSV of 2 h . Catalysts 2019, 9, 96 8 of 11

3. Materials and Methods

3.1. Catalyst Preparation

The support was γ-Al2O3 in the form of granules, with a trefoil-shaped cross section and a size of 1.2 mm (Promyshlennye Katalizatory Co., Ryazan, Russia). The textural properties of the carrier were BET surface area 235 m2 g−1, pore volume 0.79 cm3 g−1, average pore diameter 13.4 nm. Previously to impregnation, the support was dried at 110 ◦C for 2 h. The catalysts were prepared by incipient wetness impregnation of γ-alumina granules with an aqueous solution containing the required amounts of precursors of active metals [6,10]. Chemicals were purchased from Acros Organics (Geel, Belgium) and Vekton (St. Petersburg, Russia) with ≥99% purity grade. Impregnation solutions were prepared as follows: MoO3 (5.4 g, Vekton) and 85 wt% H3PO4 (1.0 mL, Vekton) were dissolved in water under ◦ stirring and reﬂuxed at 80 C. When the MoO3 was dissolved, 1.39 g of Ni(OH)2 (Acros Organics) was added. At the next step, we added 1.6 mL of EG (Acros Organics), 2.75 mL of DEG (Acros Organics), and 3.9 mL of TEG (Acros Organics) for the preparation of the NiMoP-EG, NiMoP-DEG, and NiMoP-TEG solution, respectively. Molar concentrations of Mo, Ni, P, and glycol in impregnation solutions corresponded to 2.08 M, 0.83 M, 0.83 M, and 1.56 M, respectively. After impregnation, the samples were dried in nitrogen ﬂow at room temperature and then at 110 ◦C for 4 h. Reference material without an organic additive was also prepared.

3.2. Characterization Techniques The Mo, Ni, and P content was measured by atomic absorption spectroscopy on an Optima 4300 DV instrument (Perkin Elmer, Waltham, MA, USA). The transmission electron microscopy (TEM) studies were performed by using a JEM-2010 (JEOL, Tokyo, Japan) electron microscope. The average size of the NiMoS nanoparticles was determined by counting over 350 particles in TEM images taken with medium magnification. Textural characteristics were determined from low temperature nitrogen adsorption isotherms obtained on a Micromeritics ASAP® 2400 analyzer (Micromeritics, Norcross, GA, USA). UV–Vis spectra were recorded in the wavelength range of 200–800 nm using a Cary 60 UV-Vis Agilent spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). Powder XRD patterns were obtained on a ARL X’TRA diffractometer (Thermo Fisher Scientific, Waltham, MA, USA) using CuKα radiation. The measurements were performed in a range of 2θ from 5◦ to 70◦ with a scanning step of 0.05◦ and an accumulation time of 3 s per point. The JCPDS database was used in the phase analysis. The average crystallite sizes were defined using the Scherrer equation. The quantitative phase analysis and refining of the unit cell parameters were carried out by Rietveld analysis of a diffraction pattern using X’Pert High Score Plus software. Raman measurements were carried out on a Raman spectrometer T64000 (Horiba Jobin Yvon, Kyoto, Japan). The spectra were recorded in the backscattering geometry at room temperature using Ar+ laser for excitation; wavelength is 514.5 nm. Incident light was linearly polarized. The spectral resolution was not worse than 2 cm−1. The silicon CCD matrix was used as photodetector; it was cooled with liquid nitrogen. An attachment based on an Olympus microscope was used for microscopic analysis of the Raman spectra. The power of the laser beam at the sample was about 2 mW. To avoid heating the structures by laser beam, the sample was placed slightly further than the focus, so the spot size was 20 µm. X-ray photoelectron spectra (XPS) were recorded on a SPECS (SPECS GmbH, Berlin, Germany) photoelectron spectrometer using a hemispherical PHOIBOS-150-MCD-9 analyzer (non-monochromatic Al Kα radiation, hν = 1486.6 eV, 200 W). The binding energy (BE) scale was pre-calibrated using the positions of the peaks of Au4f7/2 (BE = 84.0 eV) and Cu2p3/2 (BE = 932.67 eV) core levels. The samples in the form of a powder were loaded onto a conducting double-sided copper scotch. The Al2p peak 3+ (BE = 74.5 eV) corresponding to Al from Al2O3 support was used as an internal reference [27]. The atomic concentration ratios of elements on the catalyst surface were calculated from the integral photoelectron peak intensities (Al2p, Mo3d and Ni2p), which were corrected with theoretical sensitivity factors based Catalysts 2019, 9, 96 9 of 11 on the Scofield’s photo-ionization cross sections [28]. The differences in the binding energies for each catalyst did not exceed 0.1 eV.

3.3. Catalytic Activity Test The catalysts were tested in a flow reactor with an inner diameter of 16 mm and length of 570 mm, as described in [29]. The reactor was loaded with catalyst granules (granule length of 4–5 mm, total volume of 10 mL) diluted with an inert material, carborundum (grain size of 0.10–0.25 mm), in a volume ratio of 1:2. The catalyst was sulfided in situ prior to the experiment with straight-run gas oil, additionally containing 0.6 wt% sulfur as dimethyl disulfide (H2 pressure of 3.5 MPa; H2/feedstock of 300 Nm3/m3; liquid hourly space velocity of 2 h−1). The sulfidation was performed in two steps, the first one at 240 ◦C for 8 h and the second at 340 ◦C for 6 h. The straight-run gas oil (from Urals crude oil) was used as the feedstock (Table4). The catalytic experiments were performed at temperature of ◦ 3 −3 330–340 C, hydrogen pressure of 3.5 MPa, H2/feedstock ratio of 300 Nm hydrogen m feedstock, and liquid hourly space velocity of 2 h−1. The duration of each stage differing in condition was 12 h; the residual S and N content was obtained by averaging the data for three samples taken through 10, 11, and 12 h after the beginning of the current stage. The sulfur content in the feedstock and hydrogenated products was measured on a Lab-X 3500SCl energy dispersive X-ray fluorescence analyzer (Oxford Instruments, Abingdon, UK) and on an ANTEK 9000NS analyzer (for products containing less than 100 ppm S). The nitrogen content was determined using an ANTEK 9000NS (Antek, Houston, TX, USA). To perform XPS and TEM analysis, the sulfide catalyst after testing in the hydrotreating of SRGO was dumped out from the reactor and then washed by hexane. To remove hexane, the sample was placed in the vacuum heat chamber at room temperature, evacuated to 5 mbar, and dried at this temperature for 15 h. To measure HDS activity, the reaction rate constant (k) was calculated using Equation (3):

k(n − 1) 1 1 = − n−1 n−1 , (3) W [S] [S0]

3 3 −1 −1 where W is the liquid hourly space velocity ((m feedstock) (m catalyst) h ), [S] and [S0] are the sulfur contents of the hydrogenated products and feedstock (wt%), respectively, and n = 1.6 [29].

4. Conclusions The effect of ethylene glycol (EG), diethylene glycol (DEG), and triethylene glycol (TEG) on the properties of phosphate-doped NiMo/Al2O3 hydrotreating catalysts has been investigated. The addition of glycols to the impregnation solution prepared by the dissolving of MoO3,H3PO4, and Ni(OH)2 in water leads to the increased dispersion of Mo on the surface of sulfide catalysts, in terms of Mo/Al ratio. Both Mo dispersion in the terms of Mo/Al ratio and HDS and HDN activity of catalysts in SRGO hydrotreating are decreased in the following order: NiMoP-DEG/Al2O3 > NiMoP-EG/Al2O3 > NiMoP-TEG/Al2O3 > NiMoP/Al2O3. Consequently, the beneficial effect of glycols on the HDS and HDN activity can be explained by the improved dispersion of Mo on the surface of sulfide catalysts prepared with the glycol-type additives. The highest dispersion of Mo and activity were shown by the catalyst with DEG, which is 2.8 times more active in the HDS of straight-run gas oil than a similar catalyst prepared without additive. Comparing the Ni/Al and Mo/Al ratios on the surface of NiMo(P)/Al2O3 catalysts in the oxide and sulfide state allows us to conclude that a considerable redistribution of active metals takes place during sulfidation. Moreover, the nature of organic additive has a noticeable effect on this process. To get a deeper insight into the effect of the glycol nature on the dispersion of sulfide nanoparticles and catalytic properties, a thorough study of sulfidation mechanism should be performed using H2S or dimethyldisulfide as the sulfiding agent.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/9/1/96/s1: Figure S1: Raman spectrum of NiMoP-TEG solution; Figure S2: XPS Mo3d spectra of the catalysts in oxide form: Catalysts 2019, 9, 96 10 of 11

(a) NiMoP/Al2O3, (b) NiMoP-EG/Al2O3, (c) NiMoP-DEG/Al2O3, and (d) NiMoP-TEG/Al2O3; Figure S3: XPS Ni2p spectra of the catalysts in oxide form: (a) NiMoP/Al2O3, (b) NiMoP-EG/Al2O3, (c) NiMoP-DEG/Al2O3, and (d) NiMoP-TEG/Al2O3; Figure S4: XPS Mo3d and S2s spectra of the catalysts in sulfide state: (a) NiMoP/Al2O3, (b) NiMoP-EG/Al2O3, (c) NiMoP-DEG/Al2O3, and (d) NiMoP-TEG/Al2O3; Figure S5: XRD patterns of the NiMoP-DEG/Al2O3 sulfide catalyst; Figure S6: Catalytic properties of NiMo(P)/Al2O3 catalysts ◦ ◦ in hydrodesulfurization of SRGO: (a) 330 C and (b) 340 C; Figure S7: Catalytic properties of NiMo(P)/Al2O3 catalysts in hydrodenitrogenation of SRGO: (a) 330 ◦C and (b) 340 ◦C; Figure S8: XPS Ni2p spectra of the catalysts in sulfide state: (a) NiMoP/Al2O3, (b) NiMoP-EG/Al2O3, (c) NiMoP-DEG/Al2O3, and (d) NiMoP-TEG/Al2O3; Figure S9: Deconvolution of Ni2p spectrum of the NiMoP-DEG/Al2O3 sulfide catalyst; Table S1: Molybdenum and nickel surface species of phosphate-doped NiMo/Al2O3 sulfide catalysts, as determined by XPS analysis. Author Contributions: G.A.B. and A.L.N. planned and designed the experiments. A.A.P. performed the catalyst synthesis and the characterization of impregnation solutions by UV-Vis. I.V.D., E.N.V., A.L.N., and A.A.P. performed the catalytic activity tests. I.P.P. performed the catalyst characterization by XPS. V.A.V. performed the characterization of impregnation solutions and catalysts by Raman spectroscopy. E.Y.G. performed the characterization by TEM. A.L.N. wrote the manuscript. G.A.B. revised the manuscript. V.I.B. supervised the project and revised the manuscript. All authors discussed the results and approved the final version of the manuscript. Funding: The work was supported by the Ministry of Education and Science of the Russian Federation, project No. 14.575.21.0128, unique identification number RFMEFI57517X0128. Conflicts of Interest: The authors declare no conflict of interest.

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