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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.61 no.4 Concepción dez. 2016

http://dx.doi.org/10.4067/S0717-97072016000400004 

 

SUPPORT EFFECT ON CONVERSION OF QUINOLINE OVER ReS2 CATALYST

 

R. BASSMa, M. VILLARROELb, F J. GIL-LLAMBIASb, P. BAEZAa, J. L. GARCÍA-FIERROc, N. MARTÍNEZd, P. OLIVERAd, K. LEIVAd, N. ESCALONAe,f,g*

a Pontificia Universidad Católica de Valparaíso, Facultad de Ciencias, Instituto de Química, Casilla 4059, Valparaíso, Chile.
b
Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile 
c
Instituto de Catálisis y Petroquímica, CSIC, Cantoblanco, 28049 Madrid, Spain. 
d Universidad de Concepción, Facultad de Ciencias Químicas, Casilla 160C, Concepción, Chile. 
e
Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería, Pontificia Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, Macul, Santiago, Chile.
f
Departamento de Química Física, Facultad de Químicas, Pontificia Universidad Católica de Chile. 
g
Centro de Investigación en Nanotecnología y Materiales Avanzados (CIEN-UC), Pontificia Universidad Católica de Chile, Santiago, Chile.
* e-mail: neescalona@ing.puc.cl


ABSTRACT

The conversion of quinoline over ReS2 supported on y-Al2O3, SiO2, ZrO2 and TiO2 catalysts in a batch reactor at 300oC and 5 MPa of hydrogen pressure was studied. The catalysts were prepared by wet impregnation with a loading of 1.5 atoms of Re per nm2 of support. The catalysts were characterized by N2 adsorption, X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD). The Re(x)/supports catalysts displayed high activities for the conversion of quinoline, although negligible formation of N-free compounds (hydrodenitrogenation) were observed. The intrinsic activities of ReS2 were modified by the support decreased in the order: Re/TiO2 > Re/ZrO2 > Re/SiO2 > Re/y-Al2O3. The highest activity displayed by the Re/TiO2 catalyst was correlated with the Re dispersion and formation of ReS2 species. Meanwhile, the lower conversion of quinoline over the Re/ZrO2, Re/SiO2 and Re/y-Al2O3 catalysts was related to the combined effect of the textural properties of catalysts and the formation of ReS    species on the supports.

Keywords: Hydrodenitrogenation, ReS2, quinoline, supports.


 

1. INTRODUCTION

Recently, the quality of crude oil has declined due to the contents of heavier nitrogen and sulfur organic compounds which hinder the hydrotreating processes (HT), particularly hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). Moreover, stringent environmental regulations have forced the oil industry to reduce the nitrogen and sulfur levels present in fuels. In order to meet the required standards, it is essential to find catalysts that are more active than the classical HT Ni(Co)-Mo/Al2O3 catalysts. In this context, several researches have been carried out in order to either improve the activity of classical sulfides catalysts or find alternative catalysts [1, 2, 3].

Liu et al. [1] studied the effects of the addition of fluorine and phosphorus on Ni-Mo/Al2O3 catalysts in the HDN of quinoline. They found that fluorine and phosphorus promote the activity of Ni-Mo/Al2O3 catalysts, attributing this behavior to the promotion of weak and moderate acid sites, as well as the enhancement of Mo dispersion.

Deepa et al. [4] investigated the effect of supports on the HDN of quinoline and 1,2,3,4-tetrahydroquinoline (THQ1) over Ni-Mo catalysts. The highest quinoline conversion was displayed by Ni-Mo supported on a B-zeolite dealuminated blend with Al2O3 (Al2O3-DAl BEA-Al), followed by Ni-Mo/Al2O3 catalyst. On the other hand, the highest THQ1 conversion was presented by Ni-Mo/Al2O3 (DAl BEA-AL), followed by Ni-Mo/AlMCM-41 catalyst. The trend obtained was attributed mainly to differences in the acid strength of supports used.

Yang and Satterfield [5] studied the effect of hydrogen sulfide on the HDN of quinoline using NiMo/Al2O3 catalyst. They found that the hydrogen sulfide inhibits hydrogenation and dehydrogenation reactions but markedly accelerates hydrogenolysis reactions.

Moreau et al. [6] studied HDN of quinoline using mechanical mixtures of industrial pre-sulfided alumina-supported Ni-Mo and Co-Mo catalysts. They found a substantial promotion effect, due mainly to the better hydrogenolysis properties of the catalysts mixtures. The authors indicated that this effect could be due to a novel active phase or due to a bifunctional mechanism.

The reactivity of non-classical catalysts has also been studied for HDN and HDS reactions. Ni-rich bimetallic phosphides incorporating different metals displayed high activity in the simultaneous HDN of quinoline and HDS of dibenzothiophene [2]. However, the HDS conversion and product selectivities were dependent on the adsorption of N-containing compounds on the active sites of catalysts [7, 8]. These results reiterated that the removal of nitrogen from heterocyclic compounds is a more difficult process than the removal of sulfur, and generally leads to deactivation of catalysts.

Chouzier et al. [9] studied (Ni, Co)-Mo binary and ternary nitrides in the HDN of quinoline. They found that the active phase for monometallic nitrides was MoS2 formed on the surface of nitride particles. This was confirmed by the poor activity of the nitride in the absence of sulfur. The authors reported synergism for these bimetallic nitride systems, and indicated that the active sites are probably some ensembles of metallic atoms on the surface of mixed phases Co3Mo3N and Ni2Mo3N.

On the other hand, Eijsbouts et al. [10] studied the reactivities of first-, second-, and third-row transition metal sulfide (TMS) supported on activated carbon in the HDN of quinoline. They found that the first-row transition metal sulfides displayed low quinoline conversion to hydrocarbons. On the other hand, the second- and third-row TMS formed a volcanic curve with maxima at Rh/C and Ir/C catalysts, respectively. On this curve, Re/C presented intermediate activity but with a high selectivity for propylbenzene. The ReScatalyst has been rarely studied for HDN reactions although a ReS2/Al2Ocatalyst exhibited a higher activity (1.6 times) than the Ni-Mo/Al2O3 catalyst in the HDS of thiophene [11].

Escalona et al. [12] studied the simultaneous HDS and HDN reaction over ReS2/Al2O3 catalyst. They found that the HDN/HDS selectivity increases with increasing Re loading, which was attributed to changes in the acid sites of the catalysts. A similar behavior was also observed in the simultaneous HDS and HDN over ReS2/C catalysts [13].

On the other hand, Laurenti et al. [14] studied the HDS of thiophene over ReS2 supported on Al2O3, ZrO2 and TiO2. They found that the ReS2/ZrO2 catalyst displayed the highest activity followed by ReS2/Al2O3 and ReS2/TiOcatalysts, and that catalytic activity was strongly dependent on the sulfidation mixture used, similar to previous findings by Escalona et al. [11].

It is well known that the activity of transition metal sulfides strongly depends on the type of support used. The support can influence the interaction between its surface and the active phase, the dispersion, degree of sulfurization, stability, and structure of the active phase, etc. [15, 16, 17, 18]. Therefore, the aim of this work is to study the effect of support (TiO2, Al2O3, SiO2 and ZrO2) on conversion of quinoline using Re sulfide as active phase. The preparation of Re sulfide catalysts was carried out with N2/H2S mixture, following a procedure previously reported by Escalona et al. [14].

2.- EXPERIMENTAL

2.1 Catalyst preparation

The supported rhenium-based catalysts were prepared by incipient wetness impregnation of an aqueous solution of NH4ReO4 (Aldrich, 99%) on SiO2 (BASF D10-11), y-Al2O3 (BASF D10-10), TiO2 (p25 Degussa) and ZrO2 The ZrO2 support was obtained by direct calcination of a commercial hydrous zirconium Zr(OH)4 (MEL Chemicals) at 573 K for 3 h in a muffle furnace. The catalysts were prepared with a loading 1.5 atoms of Re per nm2 of support. The impregnated catalysts were left for maturation at room temperature for 24 h, dried at 393 K for 12 h, and then calcined at 573 K for 0.5 h. The calcined samples were pre-sulfided at 673 K for 4 h under a flow of 10 % H2S/N2 mixture. The Re content was determined by ICP-OES using the Re emission line of 221.426 nm.

2.2 Characterization of the Catalysts

The BET surface area (SBET) and total pore volume (Vp) of the support and the ReOx/support catalysts were determined from nitrogen isotherms at 77 K using a Micromeritics-TriStar II 3020 instrument.

X-ray powder diffraction (XRD) patterns were obtained by means of a Rigaku diffractometer equipped with a nickel-filtered CuK αlradiation (l = 1.5418°A).

XPS measurements were performed using a VG Escalab 200R electron spectrometer equipped with a hemispherical electron analyzer and a Mg Kα (1253.6 eV) excitation source. Energy corrections were performed using the line of each support as internal reference. The samples were treated as previously stated, cooled to room temperature, flushed with N2, stored in flasks containing n-heptane, and transferred into the pretreatment chamber of the spectrometer. The XPS analyses of these samples were categorized as pre-reaction. After the HDN reaction, the spent catalyst was transferred into flasks containing n-dodecane, and analyzed by XPS. The intensity of the peaks was estimated by calculating the integral of each peak after subtracting an S-shaped background and fitting the experimental curve to a combination of Gaussian/Lorentzian lines.

2.3. Catalytic tests

The HDN of quinoline was carried out in an autoclave reactor operating in batch mode. The liquid reactant feed, consisting of quinoline (0.232 mol L-1) in decahydronaphthalene (decaline) (80 mL) with hexadecane (0.0341 mol L-1) as internal standard, was introduced into the reactor. Then, approximately 0.200 g of the selected catalyst was added. As previously indicated, the ReS2(x)/supports were prepared from the ex situ sulfidation and quickly transferred to the reactor to prevent oxidation. The system was closed, and N2 was bubbled through the solution for 20 min with a flow of 100 mL min-1 to purge any air in the system. Still under N2 atmosphere, the reactor was heated to the reaction temperature of 573 K with stirring. The reaction timer was initiated when the reaction temperature was reached and the pressure was adjusted to 5 MPa by H2. The pressure was kept constant during the course of the experiment. Condensed samples were taken periodically during the reaction and quantified by a gas chromatograph (Perkin-Elmer - Clarus 400) equipped with a Flame Ionization Detector (FID) and a CP-Sil 5 column (Agilent, 30 m x 0.53 mm x 1.0 pm film thickness).

The specific rate for the total conversion of quinoline was deduced from the initial slope of conversion as a function of time plot according to the following equation:

where rs is the specific rate (mol g-1s-1), b represents the initial slope of the conversion vs. time plot (dimensionless), n is the initial moles of quinoline (mol), and m is the mass of catalyst (g). The intrinsic rate was calculated from the specific rate according to the following equation:

the intrinsic rate (represent the molecules of quinoline transformed per Re atom and second; and it is expressed as molec. Re at-1s-1), and r is the specific rate (expressed as moles of quinoline transformed per gram of catalyst per second [mol. gcat-1s-1]), nRe represents the number of Re atoms per gram of catalyst, and Nav is the Avogadro number.

The selectivities (%) were determined at 40% of quinoline conversion, according to Eq. (2):

where Xi is the percentage of product formation i, and XT is the quinoline conversion.

3. RESULTS AND DISCUSSION

3.1 Characterization of catalysts

Figure 1 shows the N2 adsorption-desorption isotherms of the supports. According to IUPAC classifications, all the isotherms belong to type IV which is typical of mesoporous materials [19]. Also, Fig. 1 shows that the y-Al2O3 and ZrO2 supports displayed a hysteresis loop, closely resembling a type H1, due to capillary condensation which is typically associated to non-interconnected ink-bottle pores. Meanwhile, the TiO2 and SiO2 supports displayed a hysteresis loop resembling a type H3, suggestive of slit-shaped pores. The N2 adsorption' desorption isotherms of the catalysts (not shown here) were similar to their respective supports. The textural properties from N2 adsorption'desorption isotherms of the supports and catalysts are summarized in Table 1. The y-Al2O3 support possessed the highest surface area, followed by SiO2, TiO2 and ZrO2 supports. Also, there was a slight decrease in textural properties of the supports (surface area and the total, micropore and mesopore pore volumes) after Re impregnation, suggesting that pore mouth blockage was practically absent.

 

Figure 1: Nitrogen adsorption – desorption isotherms of supports.

 

Table 1. Composition, textural properties and crystal size obtained from X-ray diffraction
of ReOx(x)/support catalysts.

a Determined from BET equation.
b Calculated from the amount adsorbed at a relative pressure of 0.96.
c
Determined from Dubinin-Radushkevich (D-R) equation. 
d Difference between Vp and Vp.
e ReO4- crystal size determined from Scherrer equation using 2q = 25.4°
* ReO4- crystal size was calculated using 2q = 16.7°

 

Figure 2 shows the XRD patterns of the calcined Re/support catalysts. The patterns obtained were compared to JCPDS data files for phase identification (ReO4-: 010-0252, ReO3: 041-0967, TiO2: 01-086-1157; SiO2: 01-087-0710, ZrO2: 037-1484, Al2O3: 010-0425). Figure 2a and 2b shows the presence of ReO4- (main peaks 2q = 16.7; 25.4; 27.6; 30.4; 40.6; 41.3; 43.48; 47.2; 49.2; 50.9; 52.2) and ReO3 (main peaks 2q = 34.8; 37.9; 59.0; 82.4; 85.5) species over SiO2 support. Also, it can be observed in Figs. 2a and 2b that the intensity of the ReO4- peaks was higher than the intensity of the ReO3 peaks, suggesting that ReO4' species is the main crystalline phase on this supports, in agreement with Escalona et al. [12]. On the contrary, Fig. 2c shows no detected diffraction peaks attributed to ReO4- and ReO3 species for the ZrO2-supported catalysts, suggesting that either ReOx was highly dispersed on this support or that some of the ZrO2 peaks may have masked the ReOx peaks. Meanwhile, Fig. 2d shows that ReO4- species was detected on TiO2 support. Table 1 summarized the crystal size calculated by Scherrer equation. The ReO4- crystallite size was similar on all supports, around 50 nm. However, this result contradicts a previously reported study by Arnoldy et al. [20]; they found by XRD that the ReOx particles size supported on Re(2.5)/SiO2 and Re(2.43)/y-Al2O3 catalysts was around 200 nm. This difference in ReOx particles size can be attributed to the impregnation method used by Arnoldy et al. [20]. They used several pore volume impregnation steps to reach high Re content, suggesting that this procedure favor the formation of larger ReOx particles size.

 

Figure 2: X-ray diffraction of a) Re/SiO2, b) Re/y-Al2O3, c) Re/ZrO2 and d) Re/TiO2 catalysts.

 

Table 2. XPS binding energies (eV) and surface atomic ratios of sulfided catalysts.

 

Figure 3 shows the XPS spectra of Re/support catalysts with a surface density of 1.5 atoms of Re per nm2 of supports. Curve fitting of spectra in Figures 3a and 3d revealed one doublet, each one containing the Re 4f7/2 and Re 4f5/2 peaks, indicating the presence of one Re species. Meanwhile, curve fitting of the spectra in Fig. 3b and 3c shows two partially overlapping doublets containing the Re 4f and Re 4f5/2 peaks, indicating the presence of two Re species. Table 2 summarizes the binding energies (BE) of the most intense Re 4f7/2 component of each doublet, its relative proportions (between parentheses) and surface atomic ratio for Re/supports catalysts. Table 3 shows that the Re 4f7/2 components of the most intense doublet remained constant in all the catalysts, at about 41.4 eV ± 0.3, and corresponds closely to the values reported for ReS2[12, 21, 22]. A single peak for S 2p peaks at 162 eV ± 0.4 indicates the presence of S2- for all the sulfided catalysts. The BE of the Re 4f7/2 peaks of the less intense doublet displayed by Re/y-Al2O3 and Re/ZrO2 catalysts was around 43.9 eV ± 0.3, which can be assigned to Re oxysulfide species [12, 13, 21, 22]. These results indicate that Re sulfidation was slightly incomplete, and that the procedure used led to an estimated 90 % sulfidation for y-Al2O3 and ZrO2-supported catalysts. This result suggests a strong interaction between ReOx species and these supports. On the other hand, the Re/TiO2, and Re/SiOcatalysts displayed only a component of the Re 4f7/2 peak, indicating complete Re sulfidation.

 

Figure 3: XPS of sulfides: a) Re/SiO2, b) Re/y-Al2O3 c) Re/ZrO2 and
d) Re/TiO2 catalysts.

 

Figure 4: Quinoline conversion and yield of products over a) Re/SiO2, b) Re/y-Al2O3,
c) Re/ZrO2 and d) Re/TiO2 catalysts.

 

Table 2 also shows that Re/M atomic ratios for the ReS2/supports catalysts. This shows that Re/TiO2 catalysts displayed the highest Re/M atomic ratios, followed by Re/SiO2, Re/y-Al2O3 and Re/ZrO2 catalysts. This trend suggests that ReS2 was most highly dispersed on the TiO2 support, while it was poorly dispersed on the ZrO2 support, as previously observed by Laurenti et al. [14]. Unfortunately, HRTEM cannot be used to estimate the size of sulfide because the lamellar structure decomposes into spherical metallic particles under electron beam [14].

 

Figure 5: Reaction network for the
conversion of quinoline.

 

1.2 Catalytic activity of catalysts

Figure 4 illustrates the conversion of quinoline and yield of products over ReS2/support catalysts as a function of time. It can be observed that the main reaction product was 1,2,3,4 tetrahydroquinoline (THQ1), and that decahydroquinoline (DHQ) was observed in minor amounts for all the catalysts. The conversion of quinoline by the ReS2(x)/supports catalyst can be depicted in the reaction scheme shown in Figure 5, in agreement with previously reported studies by several authors [4, 5, 6, 9, 10]. Figure 5 shows that the formation of DHQ compound can be produced through hydrogenation of quinoline (either via THQ1 or THQ5 intermediate compounds). However, THQ5 was not detected under the experimental conditions used. Figure 6 illustrates the products distribution calculated at 40 % of quinoline conversion.

 

Figure 6: Products distribution at 40% conversion of quinoline
for ReS2(x)/supports catalysts.

 

The Re/SiO2 and Re/y-Al2O3 catalysts displayed a higher yield to DHQ compound than the Re/ZrO2 and Re/TiO2 catalysts, suggesting that SiO2 and y-Al2O3 supports slightly modified the active sites favouring the hydrogenation route. In this sense, Laurenti et al. [14] studied the HDS of thiophene over Re/ZrO2, Re/TiO2 and Re/y-Al2O3 catalysts, and suggested the formation of ReS(2-x) under the reaction condition used. In other words, the authors proposed the formation of a metallic character in ReS2. In fact, C. Sepulveda et al. [23] demonstrated by kinetic approach of competitive hydrogenation the presence of a metallic character on ReS2 supported on SiO2 and y-Al2O3. Therefore, the higher yield to DHQ displayed by Re/SiO2 and Re/y-Al2O3 catalysts could be attributed to higher ReS2 metallic character on these supports.

The negligible formation of N-free compounds observed for the Re/support catalysts contrasts with the results reported recently by Fagar et al. [24] in the HDN of quinoline over MoS2 catalyst. The MoS2 displayed high selectivity to THQ1 and C9 products. This behaviour can be due to an inhibitive effect similar to those observed in the HDN of indole [25] and carbazol [26]. Laredo et al. [25] studied, by kinetic approach mechanism, the inhibition of indole and o-ethylaniline in the HDS of difenzothiophene (DBT) over commercial Co-Mo/y-Al2O3 catalyst at conditions commonly used in the hydrotreatment of diesel feedstock. They observed an unexpected self-inhibiting effect of indole and o-ethylaniline in their hydrogenation process during DBT HDS. Similar self-inhibitive effect was observed in the HDN of carbazol and the HDS of DBT over Ni-MoP/y-Al2O3 commercial catalyst [26]. These results suggest a possible retardation in the formation of N-free compounds in the batch reactor. In fact, reaction of these same catalysts in continuous flow reactor (elimination of self-inhibiting effect) showed high formation of N-free compounds (data not show here).

Figure 7 shows the activity expressed as the initial rate (per gram of catalysts) and intrinsic initial rate (by metal atom) of all the catalysts evaluated in this study. Fig. 7a shows that the Re/SiO2 catalyst displayed the highest initial rate followed by Re/Al2O3, Re/ZrO2 and Re/TiO2 catalysts. This behaviour can be a function of Re atoms surface loading due to differences in the surface area of supports. Regarding the intrinsic activity, Fig. 7b shows that the ReS2/TiOcatalyst displayed the highest intrinsic activity, followed by Re/ZrO2, Re/SiOand Re/y-Al2O3 catalysts. Similar trend was observed previously by Laurenti et al. [14] in the conversion of thiophene. The highest intrinsic activity observed over the Re/TiO2 catalyst can be attributed to higher ReS2 dispersion on the TiO2 support, as suggested by XPS. On the contrary, the lower intrinsic activity displayed by Re/y-Al2O3 catalysts cannot be correlated with XPS results. This catalyst presented a similar ReS2 dispersion to the Re/ZrO2 catalyst; however, the Re/ZrO2 catalyst displayed an intrinsic activity about 4.5 times higher than the Re/y-Al2O3 catalyst. These results did not show a clear correlation between the catalytic activity and the structure ofthe y-Al2O3-, ZrO2- and SiO2-supported ReS2 catalysts, suggesting that other factors are involved in the conversion of quinoline, such as the existence of an electronic effect induced on the active phase by the support. Laurenti et al. [14] and Sepulveda et al. [23] reported the formation of ReS species under reaction condition over SiO2, y-Al2O3- and ZrO2-, while over TiO2 catalyst apparently this specie was not formed. Therefore, the formation of ReS over y-Al2O3, ZrO2 and SiO2 supports could favor the HYD pathway but decrease the quinoline conversion. This behavior contrasts results previously observed for the conversion of thiophene, suggesting that this species disfavor the adsorption of quinoline over the active site. However, in the case of ZrO2 support the intrinsic rate was higher than Re/SiO2 and Re/y-Al2O3 catalysts, this behavior is not clear. Future research should be aimed at clarifying this possible electronic effect (exact ReOx nature) versus Re dispersion on the HDN reaction over this active phase.

 

Figure 7: a) Specific rate and b) intrinsic rate of ReS2(x)/supports catalysts.

 

4. CONCLUSIONS

The ReS2(1.5)/supported catalysts are highly active in the conversion of quinoline but exhibit negligible activity on the formation of N-compounds in a batch reactor. This behavior was attributed mainly to a self-inhibitive effect by some of the products from quinoline conversion in the batch reactor. The highest activity presented by the ReS2/TiO2 catalyst was attributed to the combined effect of Re dispersion and the formation of ReS2 species. Meanwhile, the activity trends displayed over the Re/γ-Al2O3, Re/SiO2 and Re/ZrO2 catalysts cannot be related to any structural and textural properties of the catalysts, suggesting that the support can be involved in modifying the active site by electronic effect. The Re/SiO2 and Re/γ-Al2O3 catalysts displayed a higher yield to DHQ compound than the Re/ZrO2 and Re/TiO2 catalysts. The results were attributed to higher ReS2 metallic character over Re/SiO2 and Re/γ-Al2O3 catalysts favouring of yield to DHQ compound but decreasing the quinoline conversion.

ACKNOWLEDGMENTS

The authors thank CONICYT-Chile for FONDECYT N° 1130749 grant.

 

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