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Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Study on Hydrodesulfurization of L/W Coexistence Zeolite Modified by Magnesium for FCC Gasoline Jiyuan Fan,† Xu Yang,‡ Zhen Zhao,† Aijun Duan,*,† Chunming Xu,*,† Peng Zheng,† Xilong Wang,† Guiyuan Jiang,† Jian Liu,† and Yuechang Wei† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, P. R. China Institute Dalian Research Institute of Petroleum and Petrochemicals, SINOPEC, Dalian 116041, Liaoning, P. R. China

‡

S Supporting Information *

ABSTRACT: L/W coexistence zeolite was synthesized with low-cost silica−alumina microspheres via an in situ hydrothermal method and modified by Mg as catalyst additive for FCC gasoline hydro-upgrading. The physicochemical properties of the supports and corresponding catalysts were characterized by means of XRD, SEM, N2 adsorption−desorption, NH3-TPD, Py-IR, H2-TPR, HRTEM, and Raman spectroscopy. The characterization results demonstrated that the L/W composite possessed the typical characteristics of L and W zeolites and that the morphology of L/W coexistence zeolite was completely different from that of the mechanical mixed zeolite. NH3-TPD and Py-IR results showed that the amount of acid sites decreased with an increase in Mg incorporation. Moreover, H2-TPR profiles indicated that Mg regulated the metal−support interaction, which was conducive to the formation of type-II CoMoS phase. With the promoting effects of Mg, Cat-2Mg catalyst exhibited a good balance between hydrodesulfurization (HDS) efficiency (91.4%) and the preserving ability of the octane number (the decrease in research octane number (ΔRON) = 0.8 units), which could be attributed to the synergy effects of proper texture property, suitable acidity, and moderate MoS2 slab distribution.

1. INTRODUCTION Recently,1 hydro-upgrading of gasoline has become an important topic due to strict legislation on emissions.2,3 In 2016, China’s vehicle count reached 295 million. With the rapid increase in vehicle ownership, the air in Beijing includes soot pollution and motor vehicle exhaust, directly affecting the health of the masses.4 FCC (fluid catalytic cracking) gasoline accounts for about 80% of the gasoline pool in China. Catalytic reforming of gasoline accounts for approximately 15%, while sulfur compounds in oil are mainly concentrated in FCC gasoline.5 Therefore, to produce clean gasoline which conforms to the increasingly strict oil quality standards, the upgrading of FCC gasoline has become a focus of the petroleum industry. It is a significant challenge to achieve ultralow levels of sulfur in gasoline and also preserve the octane number under typical conditions over conventional HDS catalysts. Traditional γAl2O3 catalysts hardly meet the requirement of clean fuel production due to a serious decrease in research octane number (RON).6 It is of great theoretical and practical significance to develop new catalysts with higher desulfurization activity and octane number preservability. Gao et al.7 synthesized EMT/FAU intergrowth zeolites with different morphologies and compared the catalytic performance for FCC gasoline. The EMT/FAU-incorporated catalyst CoMo/ EMT/FAU-2-γ-Al2O3 exhibited a good balance among HDS, hydroisomerization, and aromatization, which could be ascribed to suitable Brönsted and Lewis acidity sites and excellent pore structures. Fan et al.8 synthesized ZSM-5/ SAPO-11 composites and applied them to hydro-upgrading FCC gasoline. The composites had more mesopores for the © XXXX American Chemical Society

diffusion of substances and suitable acidity distribution, which markedly improved the reaction performance. Zeolite L is widely used in selective cracking,9 aromatization,10 catalytic reforming,11 and other processes and may be an ideal candidate as a support additive for hydrotreating catalysts12−14 due to excellent isomerization and aromatization activities. In the synthesis process of zeolite L, the formation of zeolite W also often occurs. The topology of zeolite W displays the MER-type characteristic comprising three-dimensional double-eight-ring structures.15 Hasegawa et al.16 studied the dehydration performance of MER-type zeolite membranes. The results suggested that the stability, permeability, and separation performance of MER-type zeolite membranes were relatively higher than those of LTA-type and MOR-type zeolite membranes. Duan et al.17 successfully synthesized zeolite W via hydrothermal crystallization and prepared CoMo/γ-Al2O3L/W catalyst with different molar ratios of L/W zeolite for hydro-upgrading of FCC gasoline. The catalyst exhibited the best HDS efficiency and octane number preservation (ΔRON = 1.22) when the ratio of zeolite W and L was 1:1, and this result can be attributed to larger pore size, moderate acid strength, and appropriate acid distribution of the catalyst. Therefore, the synergy between zeolites L and W clearly improves the catalytic performance due to the large pore size and proper B and L acid amounts. In addition, it is reported that the less acidic support will suppress hydrogenation reactions of the olefins and thus preserve the octane number.18,19 Alkali metal ions, in particular Received: October 11, 2017 Revised: December 6, 2017 Published: December 19, 2017 A

DOI: 10.1021/acs.energyfuels.7b03072 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

heated at 150 °C for 72 h. Finally, the K-type L/W zeolite was obtained by filtration, washing with deionized water, and drying at 100 °C for 12 h. NH4-type L/W coexistence zeolite was obtained by ion-exchange with a mass ratio of 1 L/W:3 NH4Cl:30 H2O solution mixture under agitation at 90 °C for 2 h. Then the NH4-type material was washed three times and dried at 110 °C for 4 h, followed by impregnation with different concentrations of magnesium acetate solution and calcination at 550 °C for 6 h. The modified coexistence zeolite obtained was used as a support additive mixed with a commercial γAl2O3 (Aluminum Corporation of China Limited) in a mass ratio of 2:3. 2.2. Preparation of the Catalysts. The corresponding CoMosupported catalysts were prepared by two-step incipient-wetness impregnation with ammonium molybdate tetrahydrate (≥99.0%, Tianjin Guangfu Fine Chemical Research Institute) and cobalt nitrate hexahydrate (≥99.0%, Tianjin Guangfu Fine Chemical Research Institute) solution. After each impregnation step, the materials were dried at 100 °C for 12 h and calcined at 550 °C for 6 h. The metal loading of each catalyst was 15.0 wt % MoO3 and 5.0 wt % CoO. According to different concentrations of magnesium, the resulting catalysts were denoted as Cat-xMg (x = 0, 0.5, 1, 2, and 4), where x represented the mass ratio of Mg inside the coexistence zeolite. 2.3. Characterization of the Supports and Catalysts. Wideangle X-ray diffraction (XRD) of the samples and catalysts was recorded by using a Bruker D8 advance diffractometer operated at 40 kV and 30 mA with Cu Kα radiation. Scanning electronic microscope images of the support series were captured on a Quanta 200F apparatus at 20 kV. The nitrogen adsorption−desorption test was carried out at −196 °C using a Micromeritics Tristar II 3020 instrument. The specific surface area of the sample was calculated by the Brunauer−Emmett−Teller (BET) method, and the pore size and volume were measured by the Barrett−Joyner−Halenda (BJH) method. A Magnair 560 FTIR infrared spectrometer was used to quantitatively characterize the acid amounts and types of the catalysts. The acid properties of materials were characterized with a TP-5080 type multifunctional automatic adsorption apparatus (Tianjin Xianquan Industry and Trade Development Co., Ltd.). H2 temperature-programmed reduction (H2-TPR) spectra of catalysts were recorded with Autosorb iQ multifunctional physiochemical adsorbent. Visible-Raman characterization was performed using Renishaw’s inVia Raman spectrometer, with a laser source wavelength of 325 nm and a power of 0.365 MW. The MoS2 dispersion of sulfided catalysts was analyzed with an F20 transmission electron microscope (FEI Co.). The average slab length and average stacking number (Nav) of the MoS2 slabs were obtained by statistical analyses of more than 300 slabs and calculated by the following formula:

magnesium, are known to influence the acidity of catalysts and the catalytic performance.20−23 Yang et al.21 synthesized a series of Pd catalysts supported on Mg-modified alumina and tested their activity on the catalytic combustion of methane. The catalytic performance for methane combustion was enhanced due to the promoted dispersion of PdO by the “template effect” of MgAl2O4 spinel. Hoang et al.22 studied the catalytic performance of lanthana−zirconia-supported chromium oxide (CrOx/La2O3/ZrO2 denoted as Cr/LZ) and magnesium chromium mixed oxide ((x)Mg−Cr/LZ) catalysts for the dehydrocyclization of n-octane. Results showed that Mg neutralized the strongly acidic sites of the catalyst and then decreased the initial activity of Cr/LZ but prevented its deactivation. This effect increased with the increasing content of Mg. Dieuzeide et al.24 observed that the optimum catalytic performance was obtained with 3 wt % Mg(II)-modified support over Ni/γ-Al2O3 catalyst in the steam reforming of glycerol. The results are attributed to the different contents of Mg having a simultaneous effect on Ni(0) crystallite size, acid−base character, and interactions between NiO and ́ et al.25 reported that Mg-containing CuNi/ support. Vizcaino SBA-15 mesoporous catalyst shows better catalytic behavior and inhibits carbon formation in ethanol steam reforming. This indicates that the addition of Mg increases the support alkalinity and strengthens the metal−support interaction, which favors reduction in metallic phase particle size and prevents metal sintering during the thermal process. It is reasonable to conceive that the synthesis of alkalimodified L/W coexistence zeolites and the study of the corresponding catalytic performance on FCC gasoline hydroupgrading are of great significance. Thus, this study focuses on the synthesis of L/W coexistence zeolite via feasible one-step crystallization and provides a new catalyst additive for FCC gasoline hydro-upgrading. In this research, the L/W coexistence zeolite with unique morphology was obtained via an in situ hydrothermal method by using low-cost silica−alumina microspheres as a raw material. Then the composite material was modified with magnesium before incorporation of the active phase and used as a support component of the catalysts for FCC gasoline hydro-upgrading. The physicochemical properties of the supports and catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption−desorption, infrared spectroscopy of absorbed pyridine (Py-IR), H2 temperature-programmed reduction (H2-TPR), high resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. Then the impact of magnesium on the performance of the corresponding catalysts was evaluated, and the relationship between catalyst property and catalytic performance was discussed.

n n ji zy Lav = jjjj∑ xilizzzz/∑ xi j z k i=1 { i=1

n n ji zy Nav = jjjj∑ xiNizzzz/∑ xi j z k i=1 { i=1

where xi is the number of slabs, li is the slab length of the stacked MoS2 layer, and Ni is the stacking layer number. f Mo represents the ratio of edge active Mo atoms to all Mo atoms and is calculated according to the following equation, and ni is determined from the slab length (L = 3.2(2ni − 1)Å) obtained from the HRTEM image of sulfided catalyst.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Supports. Pure L/W coexistence zeolite was prepared by hydrothermal treatment. First, the zeolite L precursor solution was synthesized with a gel composition of 0.15 K2O:0.1 Al2O3:1 SiO2:25 H2O. A typical procedure was as follows: a certain amount of potassium hydroxide (≥82.0%, Sinopharm Chemical Reagent Co., Ltd.) was dissolved in deionized water. Silica−alumina microspheres (SiO2 69.4 wt %, Al2O3 11.8 wt %, Fushun Catalyst Company) were added into the above solution under stirring, which was used as a silica and alumina resource. Then an amount of zeolite W crystal (the mass fraction of zeolite W in the composite was 8%) seeds prepared according to the literature17 was added into the homogeneous solution. The mixture was vigorously stirred for another 1 h and then transferred to a Teflon-sealed autoclave and

fMo =

Moedge Mototal

i

=

∑i = 1 (6ni − 6) i

∑i = 1 3ni2 − 3ni + 1

2.4. Catalytic Performance of Catalysts. The catalytic activity of catalysts was evaluated in a high-pressure fixed bed microreactor by using FCC gasoline produced by Shijiazhuang petrochemical industries as feedstock. The loading amount of the catalyst was 2 g, and the catalyst needed to be presulfided by cyclohexane solution B


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Energy & Fuels containing 2 wt % CS2 under 340 °C, a H2/oil ratio (mL/mL) of 300, a pressure of 2.5 MPa, and WHSV of 1 h−1. The evaluation of hydroupgrading of FCC gasoline was carried out at 270 °C, a H2/oil ratio of 200, a pressure of 2.0 MPa, and WHSV 2 h−1. After the reaction was stable for 9 h, a sample was collected every 2 h. The total sulfur content of feedstock and products was determined with an RPP2000SN sulfur nitrogen analyzer (Taizhou Central Instruments). The desulfurization rate of the products was calculated by the following equation: HDS% = (Sf − Sp)/Sf × 100%, where Sf represents the sulfur content of the feedstock and Sp represents the sulfur content in the product. The group composition of the oil product was determined by chromatography (50 m × 0.2 mm × 0.5 μm OV-1 capillary column) on a 3420A gas chromatograph (Beifen Co.). The research octane number (RON) was analyzed with GC99 software developed by Beijing Petroleum Science Research Institute.

3. RESULTS 3.1. XRD Results for Mg-Modified Zeolite and Oxidized Catalysts. The wide-angle X-ray diffraction (XRD) pattern of the synthesized zeolite is shown in Figure S1. As shown in Figure S1, the synthesized microporous L molecular sieve has characteristic diffraction peaks belonging to the typical LTL topology of the L molecular sieve at 2θ = 5.5°, 19.4°, 22.7°, 28.0°, 29.1°, and 30.7°.26 The peaks that appeared at 2θ = 12.5°, 27.4°, and 28.3° can be well indexed to zeolite W.17 The characteristic peaks of zeolite L and zeolite W were found in the coexistence L/W zeolite and mechanical mixture, which indicates that the composite materials contain L and W zeolites. Figure 1 shows the XRD patterns of the series of Mg-modified zeolites. There is no reflection peak that appeared at 2θ = 43° and 62° attributed to MgO crystallites. The weakly characteristic peaks of Mg−Al spinel structural compounds appeared at 2θ = 56° and 78° with a Mg content increase, which were formed during the calcination of the support. Figure 1B shows the XRD pattern of different contents of Mg-modified catalysts. All the catalysts show intense signals at 2θ = 36.8° and 46.2°, which are attributed to γ-Al2O3. The typical characteristic peak at 2θ = 10−30° ascribed to L/W coexistence can be clearly visualized in the diffraction pattern of the catalyst, although the characteristic diffraction peaks are relatively weak due to the dilution of Al2O3, demonstrating that both L and W phases are present in the obtained catalyst. Simultaneously, the XRD patterns of all catalysts show no obvious MoO3 diffraction peaks, demonstrating that Mo species are well dispersed on the support, which is consistent with the results of Raman spectroscopy and HRTEM. The addition of Mg does not destroy the crystalline form of the catalyst and the dispersion of the active metals. 3.2. SEM Results of the Synthesized Zeolite. Figure 2 shows the scanning electron microscopic images of the assynthesized different zeolites. SEM images (Figure 2A) clearly display the perfect tablet shape of zeolite L. Zeolite W exhibits a dumbbell shaped structure (Figure 2B) with a particle size of 5−10 μm. As can be seen from the image of L/W coexistence (Figure 2C), zeolite L with smaller size is partially grown on a larger grain W zeolite. The morphology of W zeolite in the coexistence material changes from dumbbell structure into a small cluster, which may be derived from the dissolution of zeolite W seed in alkaline solution, and the growth of W molecular sieves were inhibited under the L precursor environment. The image of the mechanical mixture (Figure 2D) clearly shows that zeolite L and zeolite W exist in independent phases. Scanning electron microscopic images

Figure 1. (A) XRD patterns of Mg-modified zeolites. (a) L/W-4Mg; (b) L/W-2Mg; (c) L/W-1Mg; (d) L/W-0.5Mg; (e) L/W-0Mg. (B) XRD patterns of the series of oxidized catalysts. (a) Cat-4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat-0.5Mg; (e) Cat-0Mg.

confirm that the coexistence material is composed of L zeolite and W simultaneously, which is consistent with the results of XRD. The surface element of the series of Mg-modified supports was analyzed by SEM-EDS, and the spectra are presented in Figure S2. The presence of Mg species can be clearly distinguished in the spectra, and the result is approximately the same as with the corresponding original system. This indicates that the Mg species is well dispersed in the framework of L/W coexistence zeolite. 3.3. BET of the Oxidized Catalysts. The N2 adsorption− desorption isotherms of the series of Mg-modified catalysts are shown in Figure 3. According to the IUPAC classification, the series of catalysts exhibits a type IV isotherm with a hysteresis loop, demonstrating mesoporous characteristics. It is shown that the structures of L/W coexistence and Al2O3 are preserved in the parent zeolites, indicating that the addition of Mg has no great influence on the structure of the catalyst. The surface areas and pore volumes of the series of catalysts and the pore size distributions are listed in Table 1, Table S1, and Figure 4, respectively. As can be seen from the data in Table 1 and Table S1, the specific surface area, pore volumes, and pore sizes are gradually reduced as the Mg content increases. When the amount of Mg is 0.5−2%, the textural properties decrease gradually (from 110.3 m2·g−1 to 95.3 m2·g−1), while the surface area (82.1 m2·g−1) and pore volume (11.7 nm) are greatly C


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Figure 2. SEM images of the as-synthesized zeolites. (A) zeolite L; (B) zeolite W; (C) L/W coexistence; (D) mechanical mixture.

interaction of Co with Mo.28 In addition, the CoMo catalyst does not show significant absorption peaks at 750 nm, indicating that no separated Co3O4 species are formed.29 It can be seen from Figure 5 that the addition of Mg increases the peak width and intensity of the active Mo species but reduces the CoMo interaction to some extent. 3.5. Acidity Results of Mg-Modified Zeolite and the Oxidized Catalysts. Temperature-programmed desorption of ammonia is used to characterize the acidity of the series of supports. NH3-TPD spectra of the Mg-modified zeolites are shown in Figure S3. As can be seen from the results, the high temperature desorption peak of the Mg-modified zeolite decreases and the low desorption temperature peak shifts to low temperature slightly with the increase in Mg incorporation. This indicates that the total amount of acid gradually decreases

reduced as the Mg addition reaches 4%. The results show that the catalyst has a more concentrated pore size distribution with the Mg addition, which favors catalytic performance. 3.4. UV−vis DRS Results of the Oxidized Catalysts. UV−vis spectra of the series of oxidized catalysts modified by Mg are shown in Figure 5. The absorption peaks between 200 and 400 nm are attributed to Mo oxides.27 Among them, the characteristic absorption peak at 220−255 nm is ascribed to the tetrahedral Mo species, while the absorption peak at around 250−350 nm is assigned to the octahedral coordinated Mo species. These species can form coordinated unsaturated active sites or sulfur vacancies, which are beneficial to the HDS reaction. The characteristic peaks between 500 and 650 nm are attributed to the absorption peak of the octahedral coordination Co species in CoMoO4 generated by the D


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Figure 5. UV−vis DRS patterns of oxidized catalysts. (a) Cat-4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat-0.5Mg; (e) Cat-0Mg.

Figure 3. N2 adsorption−desorption isotherms of the series of oxidized catalysts. (a) Cat-4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat0.5Mg; (e) Cat-0Mg.

by pyridine desorption at 350 °C. The data of the acidity distribution calculated from Py-FTIR spectra of the Mgmodified series of catalysts are listed in Table S2. As shown in Table S2, the total amounts of acid and medium strong acid of the catalyst decrease with the increase in Mg content, which is consistent with the spectra results. The reason is that Al3+ is gradually replaced by the less electronegative Mg2+, which modifies the carrier surface acid center and enhances the basicity. Therefore, the active components are well distributed over the surface which then improves the activity of the catalyst.33 3.6. H2-TPR Results of the Oxidized Catalysts. H2-TPR is usually performed to measure the redox properties CoMo species on the surface of the catalyst, which reflects the interaction between CoMo species and the support.34 H2-TPR spectra of the oxidized catalysts are shown in Figure 7. There are three strong H2 reduction peaks in the range of 400−500 °C, 500−600 °C, and 770−900 °C. The first low temperature reduction peak at about 470 °C can be attributed to the reduction of octahedrally coordinated Mo species on the alumina support. The characteristic peak in the temperature range 500−600 °C can be attributed to the reduction of polymeric Mo species.35 The third high temperature reduction peak at 800 °C is ascribed to the deep reduction of tetrahedrally coordinated Mo species in strong interaction with the support.36,37 From the results, it can be seen that with the increase in Mg content the zone of low reduction temperature begins to grow and the hydrogen consumption increases. In addition, the high temperature reduction peak areas decrease along with the Mg addition, demonstrating that the tetrahedral Mo species decrease while more octahedral Mo species are produced.35 This indicates that the weaker metal− support interaction (MSI) and the amount of reducible species increases, which will improve the HDS activity. 3.7. Raman Results of Oxidized Catalysts. Figure 8 shows the Raman spectra of the series of oxidized catalysts. As can be seen from the spectra, both Co and Mo are well dispersed on the Mg-modified composites, which is indicated by no characteristic peaks of Co compounds and no obvious peak at 990 cm−1 attributed to MoO3.38,39 These results are consistent with the XRD results. The characteristic peaks at 325 and 846 cm−1 are attributed to the bending vibration of

Table 1. Textural Properties of the Mg-Modified Catalysts catalyst

surface area (m2·g−1)

pore volume (cm3·g−1)

Cat-0Mg Cat-0.5Mg Cat-1Mg Cat-2Mg Cat-4Mg

110 99 99 95 82

0.38 0.33 0.32 0.31 0.29

Figure 4. BJH pore diameter distribution of the series of oxidized catalysts.

as the content of Mg increases. This is consistent with the results provided by Zhang et al.30 FT-IR measurements of adsorbed pyridine were carried out to further study the effect of Mg on the series of oxidized catalysts, and the results are shown in Figure 6. Generally, the adsorption peak at 1540 cm−1 is attributed to Brönsted (B) acid sites, while the vibration adsorption peak at wavenumber 1450 cm−1 is attributed to Lewis acid. The band at 1490 cm−1 is ascribed to the characteristic peak produced by the interaction of B acid and L acid centers.31,32 The total amount of Brönsted and Lewis acidity can be calculated from the desorption of pyridine collected at 200 °C, while the amount of medium and strong Brönsted and Lewis acidity is obtained E


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Figure 6. Py-FTIR results of the oxidized catalysts modified by different Mg contents. (a) Cat-4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat-0.5Mg; (e) Cat-0Mg.

reduced in the presulfidation process.40 With the increase in Mg content, the peak intensity at 932 cm−1 increases and reaches the maximum when the addition amount of Mg is 2%. The presence of more reducible Mo species on the Cat-2Mg catalyst surface is confirmed, resulting in better HDS activity. This indicates a good agreement with the H2-TPR results. 3.8. HRTEM Results of the Sulfided Catalysts. HRTEM images of the series of sulfided catalysts were studied to investigate the influence of incorporation of Mg on the morphology and the dispersion degree of active MoS2. The corresponding representative micrographs and the stacking number of MoS2 crystallites of the sulfided Cat-4Mg, Cat-2Mg, Cat-1Mg, Cat-0.5Mg, and Cat-0Mg catalysts is shown in Figure 9(a−e). Compared with Cat-0Mg sulfided catalyst, the catalysts modified by Mg exhibit more concentrated MoS2 stacking layer distribution and the relative proportion of the three layers gradually increases with the addition of Mg. The average length and average stacking number (Nav) statistical results of the sulfided slabs are listed in Table 2. As can be seen from the results, the Nav of the MoS2 crystallites over the series of catalysts increases in the following order: Cat-0Mg (2.68) < Cat-0.5Mg (2.71) < Cat-1Mg (2.74) < Cat-2Mg (2.78) < Cat-4Mg (2.92). The average slab length behaves differently; Lav decreases to 3.17 nm when Mg added reaches 2.0 wt %. It can be concluded that a limited addition of Mg blocked the growth of MoS2 slabs. The increase in the stacking number suggests that the interaction between the support and the Mo sulfide phase weakened with the incorporation of Mg.20 The value of f Mo follows the order of Cat-2Mg (0.40) > Cat-1Mg (0.34) > Cat-0.5Mg (0.32) > Cat0Mg (0.31) > Cat-4Mg (0.29), which is consistent with the Raman results. This indicates that Cat-2Mg catalyst has the best MoS2 dispersion and possesses more active edge sites, which favors HDS activity of the catalyst.13,41 3.9. Catalytic Performance. The series of L/W coexistence zeolites modified by different Mg contents and the separated zeolite were used as carrier additives, and the hydro-upgrading performance of FCC gasoline was investigated. The results are given in Table 3 and Table S3, respectively. From the results of Table S3, the catalyst with coexistence zeolite as the support additive exhibits better catalytic performance than those based on separated or mechanical mixture zeolite. As shown in Table 3, the HDS efficiencies increase at first and decrease subsequently with the

Figure 7. H2-TPR spectra of the series of oxidized catalysts. (a) Cat4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat-0.5Mg; (e) Cat-0Mg.

Figure 8. Raman spectra of the series of oxidized catalysts. (a) Cat4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat-0.5Mg; (e) Cat-0Mg.

MoO in the tetrahedral coordination MoO42−. The peak at 932 cm−1 is assigned to the stretching vibration of the MoO bonds of Mo7O246− octahedral species, which are more easily F


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Figure 9. HRTEM images of the sulfided catalysts. (a) Cat-4Mg; (b) Cat-2Mg; (c) Cat-1Mg; (d) Cat-0.5Mg; (e) Cat-0Mg; (f) distribution of the length of the MoS2 particles dispersed on the prepared catalysts.

isoparaffin. Cat-1Mg shows a superior HDS efficiency (90.7%) compared with other catalysts and also has the smallest RON decrease (0.71 units). Therefore, Cat-2Mg catalyst exhibits the best overall catalytic performance.

Table 2. Average Length and Average Layer Number (Nav) of MoS2 Crystallites catalysts

Lav (nm)

Nav

f Mo

Cat-0Mg Cat-0.5Mg Cat-1Mg Cat-2Mg Cat-4Mg

3.29 3.14 3.34 3.17 3.39

2.68 2.71 2.74 2.78 2.92

0.31 0.32 0.34 0.40 0.29

4. DISCUSSION Hydro-upgrading performance over a series of catalysts of FCC gasoline is closely related to the physicochemical properties. As shown in the BET results, when the Mg content reached 4%, the pores may be blocked, resulting in greater loss of surface area, which falls from 110.3 m2/g to 82.1 m2/g, and a pore size decrease from 12.9 to 11.7 nm for Cat-4Mg catalyst. Cat-2Mg catalyst possesses a more concentrated pore size distribution

increase in Mg content. Compared with other catalysts, Cat0Mg gives the lowest HDS efficiency (86.5%) and the greater decrease in octane number (1.11 units). Cat-2Mg exhibits the highest HDS efficiency (91.4%) and with a lower decrease in octane number (0.8 units) due to the higher content of G


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Energy & Fuels Table 3. Hydro-upgrading Results of FCC Gasoline over Mg-Modified Catalystsa cat.

P

i-P

O

N

A

RON

S, mg·L−1

HDS%

ΔRON

feed Cat-0Mg Cat-0.5Mg Cat-1Mg Cat-2Mg Cat-4Mg

5.71 9.94 10.10 9.80 10.12 10.12

29.69 36.39 38.39 37.71 38.14 37.98

34.25 21.67 19.31 20.36 18.50 18.04

9.17 10.87 13.71 11.08 11.10 11.88

21.18 21.13 18.50 21.04 22.14 21.99

91.62 90.51 90.74 90.91 90.82 90.32

668.2 90.4 81.1 62.1 57.2 75.5

− 86.5 87.7 90.7 91.4 88.7

− −1.11 −0.88 −0.71 −0.80 −1.30

a

P: paraffin; i-P: isoparaffin; O: olefin; N: naphthene; A: aromatics.

dispersion degree (0.40). These observations are well in line with the Raman-characterized result. Cat-2Mg catalyst showed the strongest characteristic peak at 932 cm−1 attributed to Mo7O246− octahedral species. The peak area of the octahedral species is closely related to the HDS catalyst activity.48 In summary, the highly concentrated pore size, moderate specific surface area, suitable acidity, moderate metal support interaction, and high edge active dispersion degree give Cat2Mg catalyst superior catalytic performance for hydroupgrading of FCC gasoline.

and moderate surface area, which favors reactant molecular transfer and is beneficial to HDS reaction. As can be seen from the results of Figure 1A, the characteristic peaks attributed to zeolites L and W are easily observed at 2θ = 5−30°. This indicates that the addition of Mg did not destroy the crystal form of the coexistence zeolite. Moreover, there are no obvious MgO and MoO3 diffraction peaks in Figure 1B, demonstrating that Mg and the active species are well dispersed on the catalyst. This is also supported by the results from the N2 adsorption−desorption isotherm of the series of catalysts. Furthermore, the appropriate acidity distribution is a vital factor for FCC gasoline hydro-upgrading.42,43 The L acid sites can adsorb and transform the sulfide molecules with L alkaline, while the B acid sites can promote the direct rupture of C−S between sulfide molecules.42,44 The synergy effect from suitable amounts of B acid and L acid is helpful for the aromatization and isomerization reactions in the hydroupgrading process for FCC gasoline, which contributes to the preservation of octane number.7,45 The traditional aluminasupported catalyst usually exhibits a large decrease in octane number due to a single large amount of L acids.12 Therefore, many researchers have introduced B acid into the catalyst to improve activity for hydro-upgrading.46 In this study, catalysts with suitable acidity are obtained by introducing Mg-modified L/W coexistence zeolites. Chen et al.20 studied the effect of Mg addition to alumina on the properties of sulfided Mo/ Al2O3Mg(x) catalysts. It was observed that Mg addition decreases the weak Brönsted acidity and increases the basicity of the alumina support, resulting in an increase in the hydrogenation activity of the Mo catalysts for HDS and HDN. From the Py-IR results, the total acid amount and strong L acid content of the catalyst decreases with the increase in Mg content, which were favorable to the surface distribution of the active species. Among the catalysts, Cat-2Mg catalyst shows a suitable B/L value, which can improve the catalytic activity for hydro-upgrading of FCC gasoline. Caloch et al.47 observed that the incorporation of MgO plays a structure-promoting role and alters the interaction between the active phases and support surface. The H2-TPR results show that the area of high temperature reduction weakened while the area attributed to low and medium temperature strengthened with the addition of Mg, which indicated that the metal support interaction weakened. The dispersion degree of MoS2 and slab distribution is closely related to the MSI and the specific area. Cat-0Mg catalyst shows the lowest slab stacking number (2.68) and lower dispersion degree (0.31) due to the strong metal−support interaction, which contributes to form type-I CoMoS species. The moderate MSI and suitable slab stacking number (2.78) on Cat-2Mg catalyst is conducive to the formation of type-II CoMoS active species, which exposes more edge active sites and shows the highest

5. CONCLUSION L/W coexistence zeolite was successfully synthesized in situ using low-cost solid silica−alumina microspheres by a hydrothermal crystallization method. The coexistence zeolite was modified by different contents of magnesium acetate and used as a catalyst additive for hydro-upgrading of FCC gasoline. The characterization results showed that L/W coexistence zeolite possessed both zeolite L and W characteristic structure and unique morphology. It was found that the incorporation of Mg regulated the acidity of the support, which gave the catalyst a suitable B/L value. In addition, the addition of Mg regulated metal−support interaction (MSI), which was conducive to the formation of type-II CoMoS species, thus contributing to isomerization and preserving the octane number. Among the series of CoMo catalysts, Cat-2Mg catalyst exhibited a good balance between HDS efficiency (91.4%) and RON preservation ability (ΔRON = 0.8 units) which can be ascribed to a desirable textural property, suitable acidity, suitable stacking number, and the highest dispersion degree of active Mo species.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b03072. Additional figures and tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 10 89732290. *E-mail: [email protected]. Tel: +86 10 89733392. ORCID

Aijun Duan: 0000-0001-5964-7544 Jian Liu: 0000-0003-3392-9812 Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (nos. 21676298, U1463207, and 21503152), CNOOC project (CNOOC-KJ 135 FZDXM 00 LH 003 LH-2016), Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2015K003), and CNPC Key Research Project and KLGCP (GCP201401).

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