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CoMoNi catalyst texture and surface properties in heavy oil processing. Part I: hierarchical macro/mesoporous alumina support Victoria S. Semeykina, Ekaterina Vasilievna Parkhomchuk, Alexander Polukhin, Pavel Parunin, Anton Igorevitch Lysikov, Artem B. Ayupov, Svetlana Cherepanova, Vladislav V. Kanazhevskiy, Vasily V Kaichev, Tatyana S. Glazneva, and Valentina Zvereva Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04730 • Publication Date (Web): 29 Feb 2016 Downloaded from http://pubs.acs.org on February 29, 2016

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Industrial & Engineering Chemistry Research

CoMoNi catalyst texture and surface properties in heavy oil processing. Part I: hierarchical macro/mesoporous alumina support Victoria S. Semeykina a,b,c*, Ekaterina V. Parkhomchuka,b,c, Alexander V. Polukhina,b, Pavel D. Parunina,b,c, Anton I. Lysikova,b, Artem B. Ayupova, Svetlana V. Cherepanovaa,b,Vladislav V. Kanazhevskiya, Vasily V. Kaicheva, Tatyana S. Glaznevaa, Valentina V. Zvereva d. a

– Boreskov Institute of Catalysis SB RAS, Lavrentieva ave. 5, Novosibirsk 630090, Russia

b

– Novosibirsk State University, 2 Pirogova st., Novosibirsk 630090, Russia

c

– Research and Education Center, NSU, 2 Pirogova st., Novosibirsk 630090, Russia

d

– Nikolaev Institute of Inorganic Chemistry SB RAS, Lavrentieva Ave. 3, Novosibirsk,

630090, Russia KEYWORDS Meso/macroporous alumina; Polystyrene beads; CoMoNi catalysts; Hydroprocessing; Heavy oil

ACS Paragon Plus Environment

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ABSTRACT

A pair of mesoporous and hierarchical macro/mesoporous alumina-supported catalysts having distinct textural parameters have been chosen to elucidate the effect of texture on activity in hydrodesulfurization (HDS) and hydrodemetallization (HDM) of heavy tatar oil possessing extremely high viscosity and sulfur content. For monitoring catalyst properties, the samples have been investigated by XRD, XFS, XPS, EXAFS, SEM, TEM, FTIR, TPD-NH3, mercury porosimetry and N2 adsorption methods. Among different factors such as support acidity, active component dispersion and texture, the last one has been found to play the most significant role in this process. The hierarchical macro/mesoporous catalyst shows lower coking rate of the hydrotreated products, as well as higher HDS and HDM conversions despite its lower active component dispersion and decreased support acidity.

1.

INTRODUCTION

Among approaches of motor oil production, crude oil upgrading still remains the one of the paramount practical importance on a global scale1. The most common industrial processes for heavy oil treating are usually divided into carbon-rejection (delayed/fluid coking) and hydrogenaddition (H-Oil, LC-Fining, HYVAHL) technologies2. In this paper we will take into consideration only the last approach providing valuable products without large amounts of coke. In this technology, special attention should be given to removal of the impurities because of an environmental hazard and a failure of the refining equipment. The typical catalyst employed for crude oil hydrotreatment conventionally includes aluminasupported Mo sulfide promoted by Co or Ni3,4,5. In heavy oil hydroprocessing, it is a running practice to use mixed or multiple beds of catalysts to achieve better quality of the product6,7,8.


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The front bed or catalyst layer is supposed to remove the major portion of asphaltenes and metals without considerable HDS, HDN or hydrocracking activity. The subsequent layers are responsible for higher HDS, HDN, hydrocracking and hydrogenation activity, more than that, catalyst performance increases as far as the feed is being supplied through the multiple layer (or the multiple bed). In the presented paper we consider catalysts intended for the protective and middle layer. Three main parameters should be taken into account in formulation of such type of catalysts. The first one is the optimal acidic properties of the support ensuring the high hydrocracking and isomerization activity but low coke deposition rate9. The second point is sufficient hydrogenation activity preventing the formation of big amounts of coke and providing the conversion of heavy fractions into middle distillates10. The third point consists in enhanced texture of the support, assuming the elimination of diffusion restrictions for high molecular reagents11. It is stressed that higher HDS activity is observed over mesoporous catalysts with high surface area, whereas large asphaltene molecules tend to convert in macropores and big mesopores. Hence, it is desirable to have maximum possible surface area compatible with the absence of diffusional limitations. Number of reports concerning improved texture properties of the catalyst has been increased for the last decades12,13,14, however, there is only scarce information about the role of intrinsic diffusion limitations in apparent catalytic activity that may be attributed to the complex influence of the texture on morphology and dispersion of the active phase. According to the theoretical and experimental study of well-defined and uniform pore-structure catalysts, the effective diffusion coefficient De for the model porphyrin-like complex depended mostly on the molecule size/pore size ratio and could be increased by 30% with the pore size extending from 10 to 70 nm and pore volume being constant15. It was also proposed that De value was higher by 60-80% for catalysts


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with double porosity and approximately the same pore size that assumed the pore volume to be the one of the most important characteristic for heavy oil hydrotreating. Mathematical modeling of Co(Ni)Mo/γ-Al2O3 catalyst deactivation during heavy oil hydroprocessing revealed that hierarchical meso/macroporous catalysts with the macropore size 215 nm and the macropore volume fraction 30 % have twofold lifetime in comparison with the conventional mesoporous analogue16. One should also mention some works on the role of diffusion limitations on the intrinsic and apparent HDS activity of thiophene over CoMoNi catalysts supported onto hierarchical meso/macroporous oxides17,18. It has been evidently shown that remarkable diffusion limitations took place in the process and their contribution to reaction rates and activation energies was especially big in the apparent kinetic region close to the real industrial conditions. Promising efforts have been made to increase mesopore size and pore volume of support using mesoporous SBA-15 materials13,19,20,21, ammonium aluminum carbonate hydroxide as a precursor21 and urea, ammonium carbonate and ammonia as hydrolysis agents22. Hierarchical oxides with a network of macroporous channels have been produced using surfactants that allowed one to preserve the mesoporous structure and create the macroporous one23. For the preparation of macroporous catalysts for heavy oil treating, a “hard” template method has been also developed recently, as it allows one to precisely control a pore size distribution of the support24. In this technique a number of various materials were used as a “hard” template: polymeric beads25, vacuum residues26, carbon black powder27. By employing the polystyrene templates mentioned in work25, it was possible to achieve the pore volume of 1.0-1.5 cm3/g with a bimodal pore size distribution in the range of 3-10 nm for mesopores and 20-100 nm for macropores.


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This paper aims at revealing some crucial factors affecting the activity of the catalyst in heavy oil hydroprocessing. Low accessibility of active phase and diffusion restrictions have been already demonstrated by the authors in the previous work by in-situ sulfidation of CoMo/Al2O3 catalyst during catalytic tests on heavy feed28. A pair of the CoMoNi alumina-supported catalysts with extremely distinct texture parameters – hierarchical macro/mesoporous and mesoporous, has

been

chosen

for

investigation

in

hydrocracking,

hydrodesulfurization

and

hydrodemetallization of heavy tatar oil. Phase composition, textural and acidic properties of the supports and catalysts are described in details in this paper. 2.

EXPERIMENTAL SECTION

2.1

Materials. The following chemicals were used: stabilized styrene (pure grade, Angara

reaktiv), sodium hydroxide NaOH (analytical grade, Reakhim), potassium persulfate K2S2O8 (98%, Aldrich), pseudoboehmite AlOOH (Promyshlennye katalizatory), nitric acid HNO3 (reagent grade, Reakhim), ammonium heptamolybdate tetrahydrate (NH4)6(Mo7O24) ·4H2O (Alfa Aesar), nickel(II) sulfate heptahydrate NiSO4·7H2O (analytical grade, Reakhim), cobalt (II) nitrate hexahydrate Co(NO3)2·6H2O (analytical grade, Reakhim),

citric acid monohydrate

C6Н8О7·H2O (reagent grade) and distilled H2O. Crude Tatar Oil, having extremely high viscosity and sulfur content, has been chosen as a heavy feedstock for hydrotreating experiments. Properties of the feed are listed in Table 1. Table 1. Characteristics of heavy tatar oil Viscosity, sSt 25 °C

360

80 °C

160

Density at 15 °C


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kg⋅m-3

967.5

API

14.8

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Fractional composition, wt. % Nafta 360 °C

78.8

Elemental composition*, wt. % C

81.2

H

11.0

S

3.4

V

0.033

Ni

0.067

Asphaltenes, wt.%

10.2

Conradson carbon residue, 9.3 wt. % * According to XFS. 2.2 Hierarchical and conventional Al2O3 preparation. For the preparation of macroporous alumina, 80 g of dry AlOOH powder was mixed with 20 g of dried polymeric template synthesized by the method described earlier29. The template consisted of monodisperse orderly packed polystyrene (PS) microbeads with the diameter of 250±20 nm. Then distilled water (40 mL) acidified with HNO3 was added to the mixture to produce a paste, which was kneaded for 30 min and then extruded. Cylindrical pellets (3×5 mm) were air dried for 1 day and heat treated for 4 h at 900 °C, with the heating rate being 100 °C h-1. The sample obtained was referred to as Al2O3-T. A reference alumina sample designated as Al2O3 was prepared with the same technique in absence of the PS template.


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2.3 Catalyst preparation. All the catalysts were prepared by an incipient wetness impregnation technique under conditions described in the previous paper28. The first step consisted in impregnation with NiSO4 precursor, which was chosen on availability and cost grounds, as well as due to the literature data30 that give evidence for better stability of such catalytic systems. The impregnating solution for the next step was comprised of a citric complex of molybdenum and cobalt with the Co:Mo:citric acid (CA) molar ratio being of

1:4:1.2

according to the study31. The appropriate concentration of the complex was taken to achieve the Mo content of approximately 10 wt.% in the final catalyst. The catalyst was dried at room temperature for 24 hours and calcined in air at 450 °C for 4 hours. Two samples prepared with this method were referred to as CoMoNi/Al2O3, CoMoNi/Al2O3-T. Preparing the desired active component composition (presumably, CoMoNiS phase) requires a pre-sulfidation procedure. For this purpose, the catalysts were loaded into the flow reactor and exposed to a mixture of 3% H2S, 1% H2 and inert gas at the gas flow rate 100-125 l⋅h-1 and the temperature 350 °C during 5 hours. The catalysts obtained were referred to as CoMoNiS/Al2O3 and CoMoNiS/Al2O3-T. 2.4 Catalyst characterization. All the catalysts were characterized by a set of physicochemical methods for monitoring surface and textural properties. Isotherms of N2 adsorption at 77 K were measured after degassing the samples in a vacuum of 6 mTorr at 200 °C for 4 h with instrument Autosorb-6B-Kr (Quantachrome Instruments, USA) for monitoring micro/mesoporous texture. Mercury porosimetry was carried out on AutoPore IV 9500 porosimeter (Micromeritics) for investigating the meso/macroporous structure of the samples. Scanning electron microscopy (SEM) images were taken with a JSM_6460LV microscope at the


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accelerating voltage of 15–20 kV to visualize textural differences between the templated and reference alumina. Phase composition of the supports and catalysts was studied with X-ray diffraction recorded on Bruker D8 Advanced diffractometer (2011, Germany) using CuKα monochromatic radiation (λ = 1.5418 Å) with a step of 2θ = 0.05° and a storage time of 1-2 s. Investigation of surface acidic properties was performed by low-temperature FTIR spectroscopy of adsorbed CO on Shimadzu FTIR-8300 spectrometer in the range of 400-6000 cm-1 with resolution 4 cm-1 and accumulation 100 scans. CO coverage was increased with varying of CO pressure from 1 to 10 torr to detect low-intensity IR absorbance peaks. TPD-NH3 study was also carried out using a quadrupole mass spectrometer HiCube RGA100. Temperature control was operated using a temperature controller Termodat 13KT2/5T supplying continuous heat rate of the sample. Elemental composition of catalysts was determined by inductive coupled plasma optical emission spectroscopy (ICP OES) on Spectrometer Optima 4300DV Perkin Elmerand and X-ray fluorescent analysis with synchrotron radiation (SRXRF), which was carried out on the station of SRXRF elemental analysis (storage ring VEPP-3, Siberian Synchrotron and Terahertz Radiation Center, Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia). The parameters of VEPP-3 were as follows: Eex = 2 GeV, B =2T, and Ie=100 mA. An active component was characterized by means of X-ray photoelectron spectroscopy (XPS), Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM). XFS spectra were recorded by an instrument SPECS Surface Nano Analysis GmbH supplied with semi-spherical analyzer PHOIBOS-150, 9-channel electron detector and XR-50 emitter


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with double Al/Mg anode. Experiments were carried out with Al Kα (hν = 1486.61 eV) irradiation; energy calibration was performed by Al2p signal with a method of internal standard. EXAFS spectra were recorded in transmission mode with increment 2.0 eV at Mo, Co and Ni K-edge at the EXAFS Station of the Siberian Synchrotron and Terahertz Radiation Center with the spectrometer equipped by channel cut double crystal Si (111) monochromator and two ionization chambers as detectors. A thickness of the sample was selected to achieve adsorption coefficient jump ∆µx = 0.8. For each sample the oscillating piece of EXAFS spectra (χ (k)) was treated in the form of k3χ(k) at the wave number interval of 3-14 Å-1. Radial distribution curves were constructed without Fourier filtration. Relative error of atomic distance determination does not exceed 1 %. TEM micrographs were obtained with JEM-2010 instrument at a lattice resolution of 1.4 Å and an acceleration voltage of 200 kV and Jeol Jem-2200FS instrument at a lattice resolution of 1.9 Å and the same acceleration voltage. Analysis of the local elemental composition (atomic %) was carried out using an energy-dispersive EDX spectrometer equipped with Si(Li) detector with energy resolution 130 eV. 2.5. Hydrotreating tests. 2.5.1. Reaction setup. Heavy oil hydrotreating experiments were carried out using lab scale Berty reactor as described in the previous paper28. Continuous stirring with a recirculation pump impeller provided non-gradient conditions in the reactor interior. Hydroprocessing parameters were as follows: temperature 420 °C, pressure 15 MPa, feed to H2 volume ratio 1000, liquid hourly space velocity LHSV 1 h-1, time for which the catalyst was kept in the reactor under the same conditions – 120 h. 2.5.2. Reaction product and spent catalyst analyses. After the hydrotreating tests, the catalysts were subjected to extraction by benzene in Soxhlet’s apparatus for 48 hours to remove reaction


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products, after that one part of the catalysts was characterized by XRD and elemental analysis (XFS). The other part was calcined in air at 450 °C for 4 hours and studied by N2/77 K adsorption and mercury porosimetry. Sulfur content in hydrotreated products was determined by using an X-ray fluorescence analyzer HORIBA SLFA 2100 in accordance with the GOST R 50442-92 (ASTM 4294) method. Elemental composition of the spent catalysts and coke was studied by SRXRF analysis. The Conradson carbon residue was measured under conditions specified in ASTM D 453028. For well-mixed flow continuous reactor under steady state conditions, the reaction rate was determined according to the following equation (1):

=

W+i −Wi

=

C0i τ0

−

Ci

(1)

τ

where W+i , Wi , Wri are the input, output and reaction rates, Ci0 and Ci are the initial and final concentrations of the i component, τ0 and τ are the initial and final contact times. With measuring the content of the i component, the concentrations could be written as (2) Ci =

ωi ρ Mri

, Ci0 =

ωi,0 ρ0 Mri

(2)

where ωi ,0 and ωi are input and output weight content of i component, ρ 0 and ρ are input and output density of the feed, Mri is the molecular weight. Taking into consideration the equation (3) U τ 0 V ρ0 = = = U 0 τ V0 ρ

(3)

where U and U0 are the volume input and output velocities, V0 and V are the inout and output volumes of the feed, we obtain the equation (4)


10

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ρ ρ0 = τ τ0

(4)

Therefore, the finite expression (5) for the reaction rate will be as follows:

Wri =

ω0 -ω ·ρ0 ·U0 Mri ·V

(5)

The following experimental parameters were used for calculation: Vcat = 3.42 cm3,U0 = 3.42 cm3⋅h-1, ρ0 = 0.968 g⋅cm-3. It must be noted that applying formal kinetics might be inadequate for such a complex catalytic system since it provides only apparent parameters without relation to real mechanisms. For this reason, it seems more correct to operate with HDS or HDM conversion values X =

Ci0 − Ci ⋅100% , where Ci0 and Ci are the initial and final concentrations 0 Ci

of sulfur or vanadium in feed.

3.

RESULTS AND DISCUSSION

3.1. Support and catalyst properties. 3.1.1.

Phase composition. XRD analysis of the

supports revealed that the composition of both conventional and meso/macroporous Al2O3 samples prepared with the PS template was identical and represented by a mixture of γ- and δAl2O3 phases (Fig.1, a). It is worth noting that the diffraction patterns of these modifications are exceedingly close despite some signal splitting at 2θ = 33° and 46° assigned to 220 and 400 interatomic planes. As high crystallite dispersion causes pattern broadening, it makes the quantitative determination of the phase composition difficult.


11


γ -Al2 O 3

600

600

δ -Al2 O 3

500

500

the samples studied

400

arbitrary units

arbitrary units

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400

220 222 311

300

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200 100

Co(Ni)MoO4

CoMoNi/Al2O3-T CoMoNi/Al2O3

400 300 200 100

20

30

40 50 2θ , degrees

60

70

0

20

30

(a)

40 50 2θ, degrees (b)

60

70

Fig. 1. XRD patterns of the supports (a) and the supported CoMoNi catalysts (b). An impregnating procedure induced the growth of intensity for the reflexes at 2θ = 37° and 46° assigned to 311 and 400 interatomic planes. Precursor deposition also caused an increase in the lattice parameter from 7.887 Å to 7.937 Å for mesoporous CoMoNi/Al2O3 and 7.945 Å for macro/mesoporous CoMoNi/Al2O3-T. This could be evidence for the introduction of Ni and Co into the lattice of alumina. Both γ-Al2O3 and NiAl2O4 or CoAl2O4 have a spinel structure with spacing parameters 7.90 Å32 and 8.05-8.10 Å33,34 respectively, that means the close location of the diffraction reflexes, with the intensities being slightly changed. Therefore, one cannot exclude a possibility of the presence of spinel Ni(Co)Al2O4 species after the calcination step. As concerns an active component, a distinct signal at 2θ = 26° assigned to Co(Ni)MoO4 species was found in the templated catalyst, while only small traces were observed in case of the mesoporous catalyst (Fig.1, b). It suggests an idea that the catalyst with the higher pore volume has the lower dispersion of active component particles. Such behavior is usually explained by the lower surface area, however, this assumption is not confirmed by both the N2 adsorption and mercury porosimetry data demonstrating the similar or even higher specific surface area for the templated


12

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sample. So, the most plausible explanation for this phenomenon consists in distinct surface properties, namely – lower acidity of the support, which will be discussed further in the FTIR and TPD-NH3 study. 3.1.2. Texture. Texture properties of the support are believed to be the determining factor of catalyst stability and performance in heavy oil hydrotreating. For tailoring texture, a template method using polymeric beads was employed to prepare the hierarchical macro/mesoporous alumina. For the proper control of the texture parameters in a wide range of pore sizes, all the supports and catalysts were investigated by both nitrogen adsorption and mercury porosimetry. Introducing the PS template allowed the pore volume to increase by 2.5 times and the specific surface area – by 20% according to mercury porosimetry, with BET characteristics being almost unchanged (Table 2). In fact, both mesoporous and meso/macroporous supports must have similar SBET values, because the contribution of macropores to the total specific surface area is small in comparison with that of mesopores. One can make a crude estimate of the macropore ∙

contribution by the formula =

∙

m2/g, where k is the coefficient related to the

density of packing (k=0.74 for closely packed spheres), ρ(Al2O3) – a density of alumina (3.43 g/cm3), D – spherical pore diameter, nm. A simple estimation of macropore specific surface area gives ca. 5 m2/g for closely packed spherical pores 250 nm in size, whereas for the mesopores the value exceeds 100 m2/g. Therefore, if other parameters are constant during the support preparation step, difference in SBET value should be negligible for the mesoporous and templated sample. Table 2. Texture parameters of the supports and catalysts according to low-temperature N2 adsorption and mercury porosimetry*


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Sample

SBET, m2/g

VN2, cm3/g

SHg, m2/g VHg, cm3/g

Al2O3

139

0.29

168

0.34

Al2O3-Т

138

0.36

194

0.84

CoMoNiS/Al2O3 fresh

160

0.50

193

0.53

CoMoNiS/Al2O3 spent

127

0.35

73

0.18

CoMoNiS/Al2O3-Т fresh

164

0.94

251

1.37

CoMoNiS/Al2O3-Т spent

118

0.30

124

0.47

* All the values are recalculated to the weight of the support, for the initial textural data see Table S1. The template removal produced both spherical pores (from single PS beads) and a significant amount of macroscopic defects of the bigger size (from aggregates of PS beads), which were presented on the Hg intrusion curve as a prolonged “tale” in the range of 100-10000 nm (Fig.2, 3). An interesting phenomenon has been noticed: single spherical pores 250 nm in size were not detected by the method, while the pore mouths in the interconnected macropore system were clearly visible at R ≈ 100 nm. It could be accounted for by the fact that a significant portion of macropores produced by PS removal is interconnected and protected against Hg filling by pore mouths of lesser diameter. When the external pressure reaches the value sufficient for Hg penetrating into pore mouths, all the macropore network is filled. It must be pointed out that the macropore volume, calculated from this region of the intrusion curve, actually refers to the spherical macropores of ca.250 nm in diameter rather than macropore mouths 100 nm in size.


14

-dV/dD

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3 Cumulative Hg volume, cm /g

Page 15 of 35

0,9 (1) 0,8 0,7 0,6 0,5 0,4

(2)

0,14 0,6

0,12

(2)

0,5

0,10

(1)

0,4

0,08

0,3

0,06

0,2

0,04

0,1

0,02 0,00

0,0

10

0,3

100 Pore diameter, nm

0,2 0,1 0,0 10

100

1000

10000

100000 1000000

Pore diameter, nm Fig. 2. Mercury porosimetry data for the templated (1) and conventional (2) alumina supports. SEM images also confirmed a strong macroporosity of the templated supports and catalysts. A template powder, which is produced by milling of dried PS monoliths in a mortar, should be composed of both aggregates (with characteristic dimension of 0.5-10 µm), single beads (d≈250±20 nm) and a certain portion of densely packed particles, all of them producing pores of different morphology and size (Fig. S1). Since we add acidified solution to the paste dropwise during the support preparation, inhomogeneous distribution of peptizing agent could occur, even despite intensive kneading. Areas with augmented HNO3 concentration may undergo partial dissolution of AlOOH precursor and its penetration into voids of assembled PS blocks. Such regions are clearly seen in Fig.3 a, c. Remarkable are hollow spaces 3-5 µm in size as their walls have 3D ordered porous structure in Fig.3 c. These huge pores could arise from penetrating of a subsurface layer of the assembled PS block by the dissolved precursor with subsequent burning out. More extended orderly packed structures resembling inverse opals could


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Page 16 of 35

also be observed (Fig.3 a). Nevertheless, the major part of the samples seems to have disordered porous structure.

(а)

(b)

(c)

(d)

Fig. 3. SEM images of the templated (a) and conventional (b) alumina supports and templated (c) and conventional (d) CoMoNi oxide catalysts. The impregnation and calcination procedure led to the remarkable decrease in specific surface area and volume (Table S1), as it was observed by other researchers as well. However, one


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cannot ignore that Ni, Co and Mo loadings are rather high in such type of catalysts, so the overall active component (CoMoNiSxOy) weight has a great contribution (up to 20-40%) to the mass of the catalyst, which, in turn, reduces specific surface area (m2 per 1 g of catalyst) by this factor even if the textural properties have not been changed. So, it seems incorrect to compare textural characteristics of the supports and catalysts without normalizing to the support weight. Of course, it is quite difficult to determine quantitatively the contribution of different factors to the specific surface area without thorough investigation. There might be many ways in which impregnation with active component could affect the texture of support. On the one hand, CoMoNi oxide nanoparticles should contribute to total surface area. On the other hand, active component species are known to block pore mouths and thereby decrease this value. Eventually, the impregnation proceeds in acidic media (pH 2-3) and at increased temperature (80 °C) that may cause some transformations or local dissolution leading to the alteration of textural parameters. In the present paper, after normalizing to the support weight, a certain increase in specific surface area and total pore volume was observed, with the macroporous structure changed to a greater extent (Table 2). The authors suggest the impregnation procedure be responsible for the increase in SBET and SHg values due to the contribution of active component species and textural transformations during the impregnation and calcination step. Texture characterization of the catalyst after heavy oil hydrotreating showed stronger pore plugging in the hierarchical macro/mesoporous catalyst. Mesopore volume of the templated alumina-based catalyst decreased by 68 %, while that one of the non-templated catalyst was reduced only by 30 % (Table 2). BET specific surface area for CoMoNiS/Al2O3-T was shown to decrease to a greater extent as well, suggesting that HDM, HDA and HDS proceeded more


17


intensively (see the part 3.1 Hydrotreating tests) over the macroporous catalyst, which had higher accessibility of pores for heavy molecules as compared to the mesoporous catalyst. It must be emphasized that VHg and SHg are still higher for the templated sample after 120 h of the hydroprocessing of extra heavy feed despite a significant drop in textural characteristics. 3.1.3. FTIR and TPD-NH3 study. As heavy oil refining relates to the treatment of asphaltenes and large aromatic molecules, a thorough control over acidity is required to achieve a compromise between isomerization/cracking efficiency and coke deposition. According to FTIR spectra of CO adsorbed onto the alumina, acidic properties of the templated sample weakened in comparison with the conventional Al2O3 (Fig. 4). The band at 2155 cm-1 assigned to CO adsorbed on hydroxyl groups represented Brønsted acid sites. The amount of these sites was 450 µmol/g for the conventional alumina versus 260 µmol/g for the templated alumina.

10

30 20 10

2195

20

2195

2175 2187

30

40

2177 2190

40

50 2155

50

1 torr CO 4 torr CO 9 torr CO 10 torr CO

60 Absorbance, arb. un.

1 torr CO 4 torr CO 9 torr CO 10 torr CO

2155

60 Absorbance, arb. un.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 35

0

0 2100

2150 2200 Wavenumber, cm -1

2250

2100

(a)

2150 2200 Wavenumber, cm -1

2250

(b)

Fig. 4. Differential FTIR spectra of the conventional (a) and templated (b) alumina at various CO pressures. This must be related to the fact that the templated alumina had a certain amount of K2SO4 onto the surface (no larger than 0.5 wt. %) remained as an inorganic residue after removal of the


18

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polystyrene template. There are a number of contributions giving evidence that small quantities of alkali metals could significantly affect surface properties35,36. According to these papers, K+ is also shown to affect active component state by altering the dispersion of particles, increasing electron density on sulfide phase, lowering the number of coordinatively unsaturated sites. In our work we observed a remarkable augmentation in Co(Ni)MoO4 particle size for the templated catalyst (according to XRD), which was probably related to the presence of K2SO4 additives. The bands at 2175-2177, 2187-2190 and 2195 cm-1 were attributed to complexes of CO with various coordinatively unsaturated sites (Lewis acid sites). Their intensities were shown to be lower in case of the templated alumina that is likely to result from K2SO4 impurities as well. Table 3. FTIR-CO and TPD-NH3 quantitative characteristics of the supports and CoMoNi oxide catalysts νCO, cm1

N, µmole/g

QCO, kJ/mole

2175

76

26.5

2187

271

32.5

2195

24

36.5

2177

56

27.5

2190

92

33.5

2195

29

35.5

CoMoNi/Al2O3

-

-

-

0.25

CoMoNi/Al2O3-Т

-

-

-

0.13

Sample

Al2O3

Al2O3-T

Total acidity, mmol NH3/g

0.15

0.11

Thereby, the alumina supports possessed average Brønsted and Lewis acidity (Table 3). The templated alumina exhibited twofold decrease in the concentration of acid sites compared to the conventional alumina that was apparently explained by the presence of alkali additives, which came from the PS template. This fact suggests that the mesoporous catalyst with higher acidity


19


should demonstrate better isomerization and cracking activity as well as stronger coke deposition, however, it was not observed in real catalytic experiments (see the part 3.1 Hydrotreating tests). To extend the data on acidic properties, the authors also carried out TPD-NH3 experiments. The information obtained by this technique is in line with that of the FTIR study: the templated alumina shows the lower concentration (by ca. 30%) and strength of acidic sites (Table 3). After the impregnation with CoMoNi precursors and subsequent calcination stronger acidic sites appear, just as expected (Fig. 5). The most interesting fact is that the total amount of accumulated NH3 decreased twice as much for the macro/mesoporous templated catalyst. Taking into consideration slightly higher CoMoNi content in this sample, we could suggest that the dispersion of oxide CoMoNi species is much lower than that in the mesoporous analogue, which is in accordance with XRD data as well. Despite this fact, HDS and hydrocracking activity were higher for the macro/mesoporous catalyst that allows the authors to propose the determining role of diffusion limitations in this process (see the part 3.1 Hydrotreating tests).

1,4

NH3 desorption, µmol/K/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 35

Al2O3

165 137

Al2O3 -T

1,2

CoMoNi/Al2O3

1,0

CoMoNi/Al2O3-T

0,8 0,6 0,4 0,2

146 135

0,0 200

400

Temperature (oC)

Fig. 5. TPD-NH3 profiles of the supports and CoMoNi oxide catalysts


20

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3.1.4. EXAFS study of the catalysts. The active component precursor was prepared using citric acid as it is a widespread reagent forming the complexes with Mo and Co easily. In studies31,37 citric ligands are stated to effectively screen the metals from the strong interaction with support and to significantly enhance the simultaneous sulfidation of active phase. The catalysts prepared with citric acid were reported to have superior HDS activity in vacuum gas oil hydroprocessing. To elucidate the structure of Co,Mo-containing citric complex used as an active component precursor, a water solution of CoMo citric complexes, as well as dried and calcined CoMo/Al2O3 catalysts were investigated by EXAFS spectroscopy method. Polynuclear Mo complexes with citric ligands have attracted attention of many researches since 1990 and, therefore, their structures have been studied sufficiently by a set of methods. Xray single-crystal structural analysis data reported in the study38 allow us to adequately match the signals obtained by EXAFS study to the particular bonds. Thereby, Mo-edge spectra are constituted of four well-defined peaks: the first one, apparently, corresponds to the superposition of terminal Mo=O (1.7 Å) and bridge Mo-O-Mo (1.9 Å) bonds; the second one (2.3 Å) is referred to the bond between Mo and oxygen of α-carboxylic/αhydroxyl group; the signal at 3.2 Å is likely to attribute to the bond between Mo and oxygen of β-carboxylic group. The intensity and position of the last peak (3.6 Å) enable us to consider it belonging to the Mo-metal bond, most probably, Mo-Mo distance (Fig. S2). When being impregnated, the complex undergoes some transformation noticeable in both the external and internal Mo coordination sphere. Prolonged calcination at 200°C results in decreasing of Mo-O peak at 2.3-2.4 Å (metal-ligand bonds) with simultaneous increasing the fraction of bridge Mo-O bonds and shrinkage of Mo-Mo bond (Fig. S3, a). This fact supports the idea of only partial complex decomposition and sintering during the calcination stage.


21


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Page 22 of 35

Co undergoes more substantial transformation according to EXAFS spectra on Co-edge (Fig. S3, b). It suggests an idea that promoter atoms might not interact with the internal sphere of the complex upon deposition and form separate compounds that could experience partial or full decomposition during the calcination more easily. However, low signal-to-noise ratio does not allow the EXAFS method to get detailed information about its structure. EXAFS investigation also confirmed that Mo is subjected to additional sulfidation during the hydroprocessing test (signals at 2.4 and 3.1-3.2 Å ascribed to Mo-S and Mo-Mo(Co,Ni) respectively). A small peak at 1.8 Å referred to Mo-O bond became more pronounced after hydrotreating experiments, indicating a certain part of Mo still remains unreacted (Fig. S4, a). Co in spent catalyst shows an intensive broad peak with the maximum at 2.1 Å referred to CoS bond in cobalt sulfide and small peak at 3.6 Å that could be related to Co-Mo or Co-Co distances (Fig. S4, b). Ni is mostly present in the form of NiAl2O4 that could be accounted for by strong signals at 2.0, 3.0 and 3.6 Å ascribed to Ni-O, Ni-Ni and Ni-Al respectively (Fig. S4, c). More detailed information on the samples studied is presented in Table S2 (see Supporting Information). 3.1.5. XPS and XFS study. XPS spectra of Mo for presulfided catalysts are presented by two doublets Mo3d5/2-Mo3d3/2 at 227.8-228.8 and 231.8-232.4 eV, which correspond to Mo4+ and Mo6+ oxidation states, respectively, and could be ascribed to molybdenum sulfide and oxide species in different surrounding (Table S3, Fig. S5). The sulfidation degree of the pre-sulfided catalysts, defined as Mo4+/(Mo4++Mo6+) ratio, is shown to be about 60%.

Two-week

hydroprocessing tests augment this ratio up to 86-91%. It must be noted that catalysts may experience partial oxidation on air before XPS measurement, so the quantitative data on sulfide content should be considered carefully.


22

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Co2p3/2 spectra for fresh and spent catalysts show a signal at 778.5-778.7 eV with a notable shake-up satellite that could unequivocally attribute this peak to cobalt in sulfide species (Table S3, Fig. S6). Besides, a signal at 781.7 eV, which is referred to Co2+ in oxygen surrounding, is observed for both fresh catalysts and spent CoMoNiS/Al2O3. There is a sharp contrast in the spectrum of the spent templated sample – a peak at 778.5-778.7 eV is totally substituted by the peak at 780.2 eV corresponding to Сo3O4. The appearance of more oxidized state of cobalt may be resulted from the more dispersed form of the active component in the spent catalyst after hydroprocessing that is logically followed from the higher molar volume of cobalt sulfides compared with cobalt oxides. The conventional sample probably underwent such transformation to a lesser extent due to diffusion restrictions and lower catalytic performance, that is why its spectra before and after tests were quite similar. Another feature of the templated catalyst is a faster oxidation of the active component during the sample preparation for XPS analysis that is likely resulted from the enhanced diffusion of gases into the templated sample. Ni2p3/2 spectra indicate the signal at 856.5-856.6 eV suggesting the presence of Ni in oxygen surrounding (NiO and NiAl2O4 species). S2p peaks are presented by the energies of 161.2-161.6 and 168.4-168.9 eV ascribed to the sulfides and sulfates respectively (Table S3, Fig. S6). Both the templated and mesoporous catalysts have similar active component composition before hydrotreating tests according to the quantitative XPS data (Table S4). No significant leaching was observed for the spent catalysts according to this method; however, XFS analysis showed almost double-fold decrease in active component content (Table 4). It means that XPS spectra should be used for qualitative estimations. Table 4. XFS analysis of the catalysts before and after heavy oil hydrotreating tests


23


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Page 24 of 35

Content, wt. %

Mo

Co

Ni

V

CoMoNiS/Al2O3 fresh

12.4

0.3

0.12

0.0

CoMoNiS/Al2O3 spent

5.7

0.1

0.8

0.6

CoMoNiS/Al2O3-T fresh

11.1

0.2

0.11

0.0

CoMoNiS/Al2O3-T spent

5.7

0.1

0.8

1.2

Having regard to this fact, one can conclude that carbon deposition after two weeks on stream is twice as much more intensive for the catalyst with macro-mesoporous texture (Table S5). Surface sulfur content is shown to increase for the templated sample as well, suggesting higher catalytic performance of the macro-mesoporous CoMoNiS/Al2O3-T. It could argue again for enhanced surface availability and reduced diffusion constraints for the templated catalyst. XFS analysis seems to be more precise for metal content estimations as opposed to XPS measurements (Table 4, Table S5). It confirmed that the templated catalyst performed 85% higher V removal in comparison with the conventional one. 3.1.6. TEM study. TEM investigation of the mesoporous and templated samples (both nonsulfided and pre-sulfided) did not reveal any difference in active component morphology, in spite of the fact that the XRD data indicated crystalline CoMoO4 phase about 10-15 nm in size present in the macro-mesoporous catalyst before the presulfidation. Typical images of the fresh and spent catalysts are shown in Fig. 6. The active phase (presumably, Mo, Co and Ni sulfides) has fairly uniform distribution and is presented by stacked slabs with the interplanar spacing of 5-6 Å. Slabs are estimated to have a high stacking rate (3-5 layers per particle on average) in the fresh pre-sulfided catalysts (Fig. 6, a), however, hydroprocessing conditions resulted in forming monolayer MoS2 phase as well (Fig. 6, b). A big proportion of highly stacked layers ( > 3) could be attributed to using the bimetallic organic complex as a precursor providing lower active


24

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component-support interaction and, therefore, more intensive aggregation of active phase. Highly stacked slabs are likely to result from relatively large precursor particles weakly connected to the support; this process could take place during the pre-sulfidation. Monolayer slabs seem to generate from small species strongly connected to the support, which are quite resistant to sulfidation, so the transformation could proceed under hydroprocessing conditions only. The XPS study also confirmed further catalyst sulfidation at reaction conditions, indicating the increase in Mo4+/Mo4++Mo6+ ratio from 60 to 85-90%.

(a)

(b)

Fig. 6. Typical TEM images of the fresh (a) and spent (b) CoMoNiS/Al2O3 catalyst. 3.2. Hydrotreating tests. All the catalysts prepared have been tested in hydroprocessing of heavy tatar oil (Table 1). Rather harsh operating conditions have been selected in terms of the literature data39,40 and experimental tests. As reported above, the feed employed for hydroprocessing tests had extremely high viscosity (360 sSt at 25 °C) and density (967.5 kg m-3, 14.8 API). For this reason, the authors were confronted with excessive coke formation that made catalytic tests very difficult to be conducted. Brief screening of operation conditions over meso/macroporous CoMo/Al2O3 demonstrated that the most appropriate range of parameters includes the temperature of 420 °C, pressure of 15 MPa and high H2/feed ratio of 1000.


25


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Page 26 of 35

The mesoporous and templated catalyst are shown to decrease the Conradson carbon residue of the hydrotreated products from 8.0 to 6.7 and 5.9 wt. %, respectively (Table 5). It could be expected that twice higher acidity of the mesoporous alumina would provide higher isomerization

and

hydrogenation

rate;

nevertheless,

the

catalyst

with

enhanced

meso/macroporous structure had better performance. Putting it another way, the accessibility of the internal catalyst surface for asphaltenes and large aromatic molecules lifted diffusion constrains and, therefore, stimulated their transformation to less resistant compounds. Table 5. Catalytic performance of the samples in heavy tatar oil hydrotreating tests Carbon residue, wt. %

S in product s, wt. %

S in coke, wt. %

V in coke, wtppm

HDS1, %

HDS*2, %

HDM, %

CoMoNiS /Al2O3

6.7

2.3

0.50

168

32

7

50

2,7

33

3.2⋅10-3

CoMoNiS /Al2O3-T

5.9

1.7

0.37

106

50

33

68

3,3

51

4.3⋅10-3

Sample

, mmol"# V%&' ∙ h

)* , mmol+ V%&' ∙ h

), , mmolV%&' ∙ h

1

HDS was calculated from total S content in hydrotreated products

2

additional parameter HDS* was calculated from S content in coke by SRXRF analysis, i.e. it

shows how much sulfur from extra heavy fraction has been transformed into less resistant compounds In accordance with XPS analysis of the products, 50 % HDS conversion was performed by CoMoNiS/Al2O3-T (Table 5), while for the mesoporous sample the HDS activity did not exceed 35%. Different factors may contribute to the total catalytic activity, including active component state and dispersion, acidic and textural properties of the support. To elucidate this complex behavior we took the following issues into close consideration. According to the XPS and TEM study of the fresh catalysts, no significant difference was found in active component structure and morphology. However, the precursor appeared to have


26

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the lower dispersion in the templated catalyst according to the XRD and TPD-NH3 study. This fact cannot be explained by slight difference in Mo content but could be ascribed to the decreased surface acidity or higher pore volume, affecting the precursor deposition during the impregnation. The size of CoMoO4 species was estimated by Scherrer equation to be of 14 nm, suggesting that the lower catalyst activity should be expected. On the other hand, this sample performed higher sulfur removal that was probably due to the optimized texture properties. Sufficiently high concentration of acid sites was supposed to promote isomerization and hydrogenation, reducing the Conradson carbon residue of the products. Nevertheless, the more active catalyst was that one having larger pore size and higher pore volume despite the reduced acidic properties. According to our previous results, concerning the investigation of CoMo catalysts supported onto the templated and conventional alumina in the same process, both CoMo-catalysts had a similar moderate activity in hydrogenation of the hydrocarbon macromolecules28. This fact indicates a significant role of Ni in conversion of heavy fractions despite we did not observe any nickel sulfide species by XPS. As provided by XFS analysis of the coke, sulfur removal from extra-heavy oil fractions (HDS*) correlated well with the pore volume of the catalyst. So, macroporous CoMoNiS/Al2O3T showed the sulfur conversion HDS* of 33 %, while the conventional CoMoNiS/Al2O3 demonstrated only 7 % removal. The metal removal efficiency went in accordance with the pore volume as well: the templated CoMoNiS/Al2O3-T catalyst showed higher efficiency (68 %) than CoMoNiS/Al2O3 (49 %) (Table 5). XFS analysis of the spent catalyst also demonstrated the same correlation for the V capacity (see Table 4). The above totally confirmed the importance of the macroporous texture for sulfur and metal removal from heavy feed and corroborated the ideas developed in works17,18 on the model


27


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Page 28 of 35

compounds. A similar picture was also observed by the authors earlier28, when the meso/macroporous non-sulfided CoMo/Al2O3 catalyst had the increased activity in HDM and HDS reactions as compared to the mesoporous analogue. 4.

SUMMARY AND CONCLUSIONS

A hard-template method using polystyrene microbeads was shown to be a prospective technique for the preparation of hierarchical macro/mesoporous alumina support. The alumina produced with this approach exhibited a bimodal pore size distribution with two maximums referred to mesopores (9 nm) and macropores (ca. 250 nm spherical pores with 100 nm mouths), while the reference sample represented purely mesoporous structure (8 nm). Also, introducing the PS template allowed increasing the pore volume by 2.5 times from 0.34 to 0.84 cm3/g and the specific surface area by 20 % from 168 to 194 m2/g, with BET characteristics being almost unchanged. During the calcination step, the active component precursor supported on alumina underwent a partial transformation into Co(Ni)MoO4 species, with the crystallite size being much larger for the templated sample. These crystallites did not disappear entirely under the hydroprocessing conditions that assumed large Co(Ni)MoO4 particles to be rather resistant to sulfidation. Higher (twice as much) concentration of moderate acid sites in the reference mesoporous sample was expected to promote the isomerization and cracking activity, providing the oil products with low carbon residue. However, the opposite behavior was revealed in the experiment ‒ the templated macro/mesoporous catalyst showed the lower Conradson carbon residue of the hydrotreated products (by 14 %) that implies the textural parameters to be of greater importance. Strong macroporous texture was also shown to reduce the catalyst plugging after catalytic tests on real heavy feed.


28

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High pore size and volume of the templated catalyst resulted in higher (by 56 %) HDS conversion that could be accounted for by reducing diffusion limitations and increasing the availability of active sites. HDM activity of the catalysts corresponded well with the pore volume: the templated CoMoNiS/Al2O3-T catalyst showed higher efficiency (68 %) as compared to CoMoNiS/Al2O3 (49 %).

Supporting Information Textural data for the supports and catalysts EXAFS data for the active component precursors and catalysts XPS data for the catalysts SEM image of the polystyrene template powder

AUTHOR INFORMATION Corresponding Author * – (Viktoriya S. Semeykina) Tel.: (383)330-49-82; e-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


29


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The

work was performed in the

Page 30 of 35

framework of the joint Research and Educational

Center for Energy Efficient Catalysis (Novosibirsk State University, Boreskov Institute of Catalysis). Also authors would like to thank Bulavchenko О.А., Тrunova V.А., Rudina N.А. for the invaluable contribution to research. ABBREVIATIONS HDS, hydrodesulfurization; HDM, hydrodemetallization; XRD, X-ray diffraction; XFS, X-ray fluorescent spectroscopy; XPS X-ray photoelectron spectroscopy; EXAFS, Extended X-ray absorption fine structure; SEM, Scanning electron microscopy; TEM, Transmission electron microscopy; ICP OES, inductive coupled plasma optical emission spectroscopy; SRXRF, X-ray fluorescent analysis with synchrotron radiation.


30

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86x43mm (300 x 300 DPI)


Mesoporous Alumina Support - ACS Publications

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