Al2O3 catalysts ex-situ presulfided with

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Highly active CoMoS/AlO catalysts ex-situ presulfided with ammonium sulphide for selective hydrodesulfurization of FCC gasoline Bin Liu, Lei Liu, Yongming Chai, Jinchong Zhao, Yanpeng Li, Yunqi Liu, and Chenguang Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04929 • Publication Date (Web): 26 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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Industrial & Engineering Chemistry Research

Highly active CoMoS/Al2O3 catalysts ex-situ presulfided with ammonium sulphide for selective hydrodesulfurization of FCC gasoline Bin Liu*, Lei Liu, Yongming Chai, Jinchong Zhao, Yanpeng Li, Yunqi Liu, Chenguang Liu State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis of China National Petroleum Corporation (CNPC), China University of Petroleum (East China), Qingdao 266555, PR China

Abstract An improved ex-situ presulfidation method for the preparation of the CoMoS/γ-Al2O3 catalyst was developed with ammonium sulphide as the sulfiding agent and the prepared catalysts were evaluated in selective hydrodesulfurization (HDS) of FCC gasoline. The selectivity of the ex-situ presulfided catalysts was more than 4 times of that of the in-situ presulfided catalysts. The characterization by XRD, HRTEM, XPS, TPR and FT-IR indicated that ammonium sulphide effectively reacted with the supported Mo oxide to form ammonium tetrathiomolybdate as intermediate, thus realizing the more complete sulfidation of Mo oxide. However, the supported Co oxide could not be sulfided by ammonium sulphide, and the delayed sulfidation would not hinder the easy growth of MoS2 particles, subsequently lead to the significantly longer slab lengths of MoS2 particles than that of the in-situ presulfided catalyst, which effectively decreased the number of active sites for olefins, thus inducing the much higher HDS selectivity.

Key words: Ex-situ presulfidation; CoMoS/γ-Al2O3; Selective hydrodesulfurization; MoS2 morphology.

*

Corresponding Author. Tel.:+86 532 8698 4686; fax: +86 532 8698 1787 E-mail address: [email protected] 1

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1. Introduction The increasingly more stringent environment rules and people’s growing environmental protection awareness are quickening the sustained improvements in ultraclean gasoline production. Transition metal sulfides have been widely used in hydrodesulfurization (HDS) of FCC gasoline. The HDS catalysts with improved activity, selectivity, and stability are needed to minimize the hydrogenation of the octane-boosting olefins in the HDS process.1,

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It is generally known that the catalytic performance is

closely connected to the final morphology of the (Co)MoS2 active phase.3, 4 Until now, many researchers have been continuously focused on the study of structure - activity relationship in the HDS reaction. The commercial catalysts are usually obtainable in the form of oxidation, which must be presulfided in the reactor by introducing a sulfiding agent to form H2S to transform inactive oxides to sulfides. This conventional process, normally named in-situ presulfidation, correspondingly produce the sulfidation atmosphere of H2S/H2. The sulfidation of transition metal Mo oxides in H2S/H2 followed the exchange reaction between oxygen and sulfur to form MoO3-xSy at low temperature, subsequent by reduction in H2 to form MoS2-xOy at 200 ºC and finally sulfided to (>230 ºC).5-7 Due to the existence of strong reducing gas of H2, the unavoidable formation of MoO2 would affect its subsequent sulfidation to MoS2,8 and thus greatly affect the sulfidation degree of active components and cut down the catalytic properties of the catalysts. Therefore, enormous efforts are devoted to the sulfidation process regulation of the active metals, mainly including the addition of chelating agents,9, 10 the use of new supports11, 12 and incorporation of various additives13, 14 to improve the sulfidation degree of active components, as well as the modification of the sulfidation procedure.15, 16 Although the performance of the catalyst has been improved to a certain extent, the in-situ presulfidation process with the necessary existence of H2S/H2 could not realize the maximum sulfidation degree of active components. Besides, the in-situ presulfidation process also requires specific sulfiding equipment, longer start-up procedure, and use of poisonous sulfiding agent,17 which would certainly limit the large-scale applications in the future. The ex-situ presulfidation process, in which the oxidation state catalyst is firstly sulfided before loading into the reactor, with the advantages of less pollution, shorter activation time, and high sulfidation degree of active components, has been preferred over the in-situ presulfidation in the late years.18-21 The most representative technologies are the EasyActive of EURACAT,22, 23 the actiCAT of CRITERION,24 the Xpress of TRICAT25 and the EPRES of SINOPRC.17, 26 Most of the ex-situ presulfidation were performed 2


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by first impregnation of the catalyst with organic polysulfides, elemental S, and water soluble sulfides,27-29 followed by a subsequent H2 activation inside the reactor. Therefore, in such ex-situ presulfidation process, the sulfiding agent are either just depositing in the catalyst pore or partially interacting with the active components through O-S exchange to form the corresponding metal oxy-sulfides.29, 30 In the unavoidable activation process, the concentrative release of heat and sulfur loss could not be effectively avoided.31 Besides, in the activation process, under hydrogen the sulfiding agent would be decomposed to the concomitant formation of H2S, which is further used for the sulfidation of the metal ions. Therefore, the inevitably produced sulfidation atmosphere of H2S/H2 would certainly reduce the sulfidation degree of active components to some extent. The Xpress of TRICAT technology, using H2S as the sulfiding agent, was performed in a specific reactor, followed by passivation with oxygen. Although the oxidation active components could be completely transformed to the sulfides, the process was rather complicated and the use of H2S was also very dangerous. For the most existing ex-situ presulfidation technologies, due to the low sulfidation activity of the sulfiding agent, the complete sulfidation of the active components could not be achieved. Moreover, it was also found that the presulfurized CoMoS/Al2O3 catalyst and NiMoS/Al2O3 catalyst prepared with ammonium tetrathiomolybdate (ATTM) as precursor, also showed relatively high activity and selectivity than those of the catalyst prepared with the traditional method in the HDS reaction.32, 33 The ATTM was normally prepared by reaction between aqueous solution of ammonium molybdate and ammonium sulphide ((NH4)2S).34 So, it could be reasonably speculated that due to the high sulfidation activity of S2-, the transition metals would react with (NH4)2S, and thus realizing the fully sulfidation of the active components. However, to our knowledge, few researchers have prepared the ex-situ presulfided CoMoS/Al2O3 catalyst with (NH4)2S as sulfiding agent. In this study, the sulfiding agent (NH4)2S was used to realize the maximum sulfidation of the CoMo/Al2O3 catalyst. Besides, the sulfidation mechanism of the transition metals Co and Mo was also delineated. The catalysts prepared by ex-situ presulfidation and in-situ presulfidation were both evaluated in selective HDS of FCC gasoline in a fixed-bed high-pressure micro-reactor, to further reveal the morphology-activity relationship of the (Co)MoS2 active phase in the HDS process.

2. Experimental section 2.1. Catalyst preparation 3



The trilobe γ-Al2O3 support was prepared in laboratory with properties of an external diameter of 1.2 mm, an average length of 2 mm, a BET surface area of 269 m2·g-1, pore volume of 0.74 cm3·g-1 and a BJH average pore diameter of 7.2 nm. The oxidized CoMo/γ-Al2O3 catalyst was obtained by incipient wetness impregnation of γ-Al2O3 support with an aqueous solution of (NH4)6Mo7O24·4H2O (AHM) and Co(NO3)2·6H2O, next by calcination at 500 ºC for 3 h in air. Finally, the as-prepared catalyst with 2.7wt% CoO and 9.3% MoO3 was obtained. The ex-situ presulfided CoMoS/γ-Al2O3 catalyst was obtained by impregnating the oxidized CoMo/γ-Al2O3 catalyst into the sulfiding agent aqueous solution of (NH4)2S (~20 wt%) with an S/Mo molar ratio of 5, next by thermal treating at 90 ºC for 1 h in N2 to perform the sulfidation of the active components, then dried at 100 ºC for 2 h in N2. The obtained CoMoS/γ-Al2O3 catalyst was named as CM(ex). The in-situ presulfided CoMoS/γ-Al2O3 catalyst was conducted using CS2 as sulfiding agent mixed in the start-up oil feed with the same oxidized CoMo/γ-Al2O3 catalyst, and thus the obtained CoMoS/γ-Al2O3 catalyst was named as CM(in). 2.2. Start-up procedure and catalytic performance tests The catalytic performance assessments were carried out in a high pressure fixed-bed continuous flow micro-reactor. The FCC gasoline with sulfur content of 1260 µg/g and olefins content of 25.8 vol.% was used as the feedstock in selective HDS of FCC gasoline The CM(ex) catalyst (5 ml) was firstly loaded in the reactor. Before starting the experiments the catalyst was stabilized by treating a light gasoline at hydrogen partial pressure of 2 MPa, liquid hourly space velocity of 2 h-1, hydrogen/feed volumetric ratio of 400 NmL/mL. During stabilization the temperature was linearly increased to 280ºC at a rate of 20 °C/h and then was kept constant for a period of 20 h. Finally, the FCC gasoline was pumped into the reactor to evaluate the HDS performance at the same conditions as the stabilization process. The CM(in) catalyst (5 ml) was firstly loaded in the reactor. Before starting the experiments the catalyst was sulfided by treating a light gasoline with 2wt% CS2 at hydrogen partial pressure of 2 MPa, liquid hourly space velocity of 2 h-1, hydrogen/feed volumetric ratio of 400 NmL/mL. During sulfidation the temperature was linearly increased to 280ºC at a rate of 20 °C/h and then was kept constant for a period of 8 h. Then, the catalyst was stabilized by treating a light gasoline at 280ºC for 20 h at the same conditions 4


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as the sulfidation process. Finally, the FCC gasoline was pumped into the reactor to evaluate the HDS performance at the same conditions as the stabilization process. The determination of the hydrocarbon compositions of the liquid feeds and products were carried out with a PONA-GC (Agilent 7890N), while the total sulfur content was determined by the Analytikjena’s elemental analysis (Multi EA 3100). The research octane number (RON) was calculated as described in the literature.35 The HDS activity (HDS%), hydrogenation activity of olefins (HYD%) and HDS selectivity factor (S) of the catalysts were calculated according to reference.36 2.3. Characterization X-ray powder diffraction (XRD) characterization was performed on a PANalytical’s X’Pert PRO diffractometer using graphite-filtered Cu-Kα radiation. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEM-2100UHR microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was collected by a PHI Quantera SXMTM spectrometer with a monochromatized Al Kα radiation (14 kV, 15 mA and 1486.6 eV). Temperature-programmed reduction (TPR) was performed on a Micromeritics AutoChem2950HP instrument. The H2 consumption was recorded through a thermal conductivity detector (TCD). Fourier transform infrared spectroscopy of pyridine adsorption (Py-IR) was performed on a Nicolet-58SXC IR spectrometer to evaluate the surface acid amount and type. 3. Results and discussion 3.1. Catalytic performance evaluation The effect of ex-situ and in-situ presulfidation processes on the catalytic performance of the CoMo/γ-Al2O3 catalysts in selective HDS of FCC gasoline is shown in Table 1. In order to clearly compare the selectivity of different catalysts, the catalyst selectivity factor S was defined as a ratio of the HDS rate constant of total sulfur and the HYD rate constant of olefins.37 The larger S value of the catalyst indicated its higher HDS selectivity. It could be seen that for the CM(ex) catalyst, the HDS conversion was 78.1%, which was slightly lower than the 85.7% of the CM(in) catalyst. However, the olefins content in the product of the CM(ex) catalyst was 23.4 vol.%, which was extremely higher than the 15.5 vol.% of the CM(in) catalyst. The HYD conversion of olefins was only 9.4% for the CM(ex) catalyst, which was significantly lower than the 39.9% of the CM(in) catalyst. Therefore, the S of the CM(ex) catalyst was more than 4 times of that of the CM(in) catalyst, inducing that the RON loss of the CM(ex) catalyst was only 0.8, while the RON loss of the CM(in) catalyst was 4.2. It could be obtained that the ex-situ 5



presulfidation process could effectively decreased the HYD activity of olefins, but slightly reduced the HDS activity, thus significantly improving the HDS selectivity and reducing the loss of RON. The catalytic performance of the ex-situ and in-situ presulfided CoMoS/γ-Al2O3 catalysts at different reaction temperatures was showed in Table S1 in supporting information. The consistent HDS performances and RON loss were always found for the catalysts at reaction temperatures of 270 ºC and 290 ºC, respectively. Besides, the CM(ex) catalyst also showed excellent HDS performances while another FCC gasoline with sulfur content of 323 µg/g and olefins content of 28.6 vol.% was used as the feedstock (as seen in Table S2 in supporting information). For the production of the ultraclean gasoline (S

Al2O3 catalysts ex-situ presulfided with

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