Al2O3 Thioetherification Catalysts with Enhanced

Article pubs.acs.org/IECR

Fe-Promoted Ni/Al2O3 Thioetherification Catalysts with Enhanced Low-Temperature Activity for Removing Mercaptans from Liquefied Petroleum Gas Deqi Huang,† Ming Ke,† Xiaojun Bao,‡ and Haiyan Liu*,† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China State Key Laboratory of Photocatalysis on Energy & Environment, Fuzhou University, Fuzhou 350116, P. R. China

‡

ABSTRACT: To enhance the low-temperature activity of monometallic Ni/Al2O3 catalyst for thioetherification, Fe-promoted Ni/Al2O3 catalysts were prepared. The activity of the catalysts was evaluated in a fixed-bed reactor, and their physicochemical properties were characterized. The results showed that the bimetallic NiFe system had significantly improved activity; in particular sample 14Ni8Fe/Al2O3 exhibited outstanding activity and stability, with conversions of CH3SH and C2H5SH reaching up to 99.5% and 97.4%, respectively, at 75 °C. The promoting effects of Fe include two aspects: (i) Partial incorporated Fe species act as textural promoters, preventing the formation of NiAl2O4, increasing the amount of nickel oxides (NiO and NiFe2O4) weakly interacting with Al2O3, and thereby generating more active sulfides [NiSx and NiS(FeS)] after sulfidation, and (ii) Fe species in a NiS(FeS) phase serving as electron donors supply electron density to the intimately contacted NiS species and weaken the Ni−S bonds, promoting the formation of a NiS(FeS) phase with superior activity.

1. INTRODUCTION Liquefied petroleum gas (LPG), mainly produced from thermal and catalytic cracking processes in the petroleum refining industry, is a flammable mixture of gaseous hydrocarbons that can be used as a fuel in heating appliances, cooking equipment, and transportation vehicles or as an important source of lowcarbon olefinic hydrocarbons especially propylene. However, it also contains a small amount of unwanted sulfur contaminants, mainly methyl and ethyl mercaptans (CH3SH and C2H5SH, respectively). These mercaptans cause hazardous SOx emissions during combustion and give rise to downstream catalyst deactivation or product pollution; therefore, their removal is an indispensable process in the petroleum refining industry. Currently, the most widely used technique for mercaptan removal is the well-known Merox sweetening process.1 In this process, the LPG stream to be treated comes into contact with a catalyst in the presence of an oxidizing reagent, on which light mercaptans such CH3SH and C2H5SH are converted to disulfides through oxidative reactions. Because the Merox process and its later modifications must be performed in an alkaline environment, their common deficiency is the need for the disposal of the caustic dross. Mercaptans in an olefinic LPG stream can also be eliminated by the hydrodesulfurization (HDS) process; however, HDS usually involves a high reaction temperature, which can cause excessive saturation of valuable olefins in LPG and degrade the value of the resultant LPG product.2 In 1988, Imai and Bricker3 patented a thioetherification process for removing mercaptans from hydrocarbon streams including LPG, in which lighter mercaptans react with diolefins in hydrocarbon streams through thioetherification reactions to form heavier thioethers over a catalyst. Taking CH3SH in LPG as an example, the thioetherification reaction can be illustrated as follows3 © 2016 American Chemical Society

CH3SH + CH 2CHCHCH 2 catalyst

⎯⎯⎯⎯⎯⎯→ CH3SCH(CH3)CHCH 2

In this reaction, the generated thioether has a much higher boiling point than LPG and can thus be separated by distillation from the treated LPG as the bottom product. Compared with the various Merox processes, no alkaline agent is involved in the thioetherification process, and therefore, the problem of the spent alkaline disposal is eliminated, demonstrating a significant industrial improvement. Some industrial processes based on thioetherification have been widely used in refineries for fluidcatalytic-cracking (FCC) naphtha sweetening such as the Prime-G+ process developed by Axens and the CDhydro/ CDHDS process developed by CDTECH. As early as 1988, Imai and Bricker3 reported an acid-type catalyst (polymeric sulfonic acid resin) for removing mercaptans from a sour hydrocarbon stream by thioetherification; however, this acidic catalyst system was rapidly deactivated when the feed stream contained active diolefins. Later, supported group VIII metals (e.g., Ni, Pd, and Ag) were explored as thioetherification catalysts,4−6 with Ni-based catalysts gaining wider application because of their good sulfur tolerance and low cost.5 Nevertheless, Ni catalysts usually have relatively low activities at low temperature, so high-temperature operation is needed, which gives rise to the polymerization of diolefins and thereby catalyst deactivation when highly reactive olefinic hydrocarbon streams are involved.7 For this reason, Bouchy et al.8 developed a NiMo catalyst system and found that the bimetallic NiMo catalyst had much higher activity for Received: Revised: Accepted: Published: 1192

October 10, 2015 January 3, 2016 January 12, 2016 January 12, 2016 DOI: 10.1021/acs.iecr.5b03797 Ind. Eng. Chem. Res. 2016, 55, 1192−1201

Article

Industrial & Engineering Chemistry Research

prepared by impregnating the support using the aqueous solutions of Ni(NO3)2 and Fe(NO3)3, respectively, through the same procedure as described above. The loadings of NiO and Fe2O3 in the two monometallic catalysts were 14 and 8 wt %, respectively. 2.2. Characterizations. Nitrogen adsorption−desorption measurements were performed on a Micromeritics ASAP 2020 instrument (Norcross, GA) at −196 °C. The surface areas and pore volumes of the catalysts were calculated by the Brunauer− Emmett−Teller (BET) method and the t-plot method, respectively. The average pore sizes were determined according to the Barrett−Joyner−Halenda (BJH) method. The samples were degassed at 250 °C for at least 5 h under a vacuum of 1.33 × 10−3 Pa prior to measurements. The crystalline structures of the samples were determined by XRD on a Bruker AXS D8 ADVANCE diffractometer (Karlsruhe, Germany) using Cu Kα radiation. The instrument was operated at 40 kV and 40 mA with a 2θ scanning speed at 1°/min and diffraction lines between 10° and 80° (2θ). The acidity properties of the catalysts were determined by temperature-programmed desorption of ammonia (NH3 TPD) on a home-built apparatus. Initially, 100 mg of each calcined sample (20−40 mesh) was placed in a quartz reactor and heated in an Ar flow (99.999%, 30 mL/min) at 600 °C for 30 min. After the sample had been cooled to 100 °C, the Ar flow was switched to an ammonia flow (99.999%, 30 mL/min) for 20 min. Subsequently, the sample was purged with flowing Ar for 1 h at the same temperature to remove free and weakly adsorbed NH3 species. Finally, NH3 TPD was conducted from 100 to 600 °C at a heating rate of 10 °C/min in an Ar flow (30 mL/min). The concentration of NH3 in the effluent was monitored continuously with a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). The reducibility of the oxidic catalysts was examined by H2 TPR on the same apparatus as used for the NH3 TPD characterization. A sample (100 mg, 20−40 mesh) to be measured was treated in the quartz reactor at 450 °C for 2 h in a pure Ar flow (30 mL/min). After the sample had been cooled to room temperature, a 10 vol % H2/Ar mixture (30 mL/min) was introduced, and the catalyst sample was heated at a rate of 10 °C/min to 1000 °C and maintained at this temperature for 30 min. The H2 consumption for reducing the corresponding metal oxides in the catalyst was detected by TCD. The metal coordination environment of the active species over the support was analyzed by UV−vis DRS on a Hitachi U4100 UV−visible spectrophotometer (Tokyo, Japan) equipped with an integration sphere diffuse reflectance attachment. The results are expressed in the form of the Schuster−Kubelka− Munk function in the wavelength range of 200−800 nm using BaSO4 as the reference. UV-Raman spectroscopy characterization was conducted on a Horiba Jobin Yvon LabRAM HR800 laser micro-Raman spectrometer (Villeneuve d'Ascq, France) equipped with a CCD detector. The laser line at 244 nm of a He−Cd laser was used as the excitation source with an output power of 25 mW. The spectra were recorded at room temperature with a spectral resolution of 2 cm−1 in the range of 200−4000 cm−1. XPS measurements of the catalysts were performed on a Thermo Fisher ESCALAB 250Xi spectrometer (Loughborough, UK) with monochromatic Al Kα radiation (hν = 1486.74 eV). Before testing, the catalyst samples were ground to powders and then pressed into thin self-supporting wafers with a diameter of 9 mm and a thickness of 1 mm. A small piece of

both the transformation of mercaptans and the selective hydrogenation of diolefins at 160 °C. Shen et al.9 reported that Mo-modified Ni/Al2O3 catalysts also resulted in improved selectivity for diolefin hydrogenation and reduced polymerization of diolefins to form coke in the FCC naphtha thioetherification process, but the reaction temperature was also as high as 160 °C. In view of the fact that diolefin polymerization can be suppressed by lowering the reaction temperature, it is desirable to develop a novel thioetherification catalyst system with enhanced low-temperature activity and higher sulfur tolerance. Among the group VIII metals, Fe has been widely employed as a main active element or a promoting element in various commercial catalysts, such as those used in Fischer−Tropsch synthesis10,11 and CO hydrogenation, because of its high activity and selectivity and low cost. Tian et al.12 reported that the bimetallic NiFe/γ-Al2O3 catalyst exhibited better catalytic activity than the monometallic Ni/γ-Al2O3 catalyst in CO methanation as a result of the formation of NiFe alloy; moreover, NiFe alloy was also found to have a high activity for benzene hydrogenation13 and CO2 methanation. Hwang et al.14 investigated CO2 methanation over Ni catalysts incorporated with different second metals and concluded that the bimetallic NiFe catalyst exhibited the best catalytic performance in the CO2 methanation reaction because of the weaker metal− support interaction of the Fe-doped Ni catalyst. Oyama et al.15 studied the deep HDS of 4,6-dimethyldibenzothiophene over highly dispersed bimetallic NiFeP catalysts and ascribed the excellent selectivity to the direct desulfurization pathway of the highly active NiFeP/SiO2 catalyst to the ligand effect of Fe on active Ni. To date, however, there has been no report on bimetallic NiFe catalyst for mercaptan removal by the thioetherification process. In this work, a series of bimetallic NiFe/Al2O3 catalysts were prepared, and their catalytic performances for mercaptan removal at low temperature (75 °C) were compared with those of monometallic Ni and Fe catalysts. Moreover, the structure of the NiFe catalysts was characterized by X-ray powder diffraction (XRD), temperature-programmed reduction of hydrogen (H2 TPR), UV−visible diffuse reflectance spectroscopy (UV−vis DRS), UV-Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) with the aim of establishing the relationship between the surface properties and catalytic performance of the catalysts.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Al2O3 catalyst support used was sticks with a size of 1.5 mm × 3−4 mm made by first extruding pseudoboehmite powders (Shandong Alumina Plant, China) and then drying and calcining the resultant extrudates at 120 °C for 5 h and at 520 °C for 4 h, respectively. The oxidic NiFe catalysts were prepared by coimpregnating the Al2O3 sticks with a series of aqueous solutions with different concentrations of Ni(NO3)2 (≥98.0 wt %; Beijing Chemical Reagent Co., China) and Fe(NO3)3 (≥98.5 wt %; Beijing Chemical Reagent Co., China). The resultant sticks were then dried at 120 °C for 5 h and calcined at 480 °C for 4 h to yield a series of oxidic NiFe/Al2O3 catalysts containing 4, 8, and 12 wt % Fe2O3, with a fixed NiO loading of 14 wt %. These catalysts are denoted as 14Ni4Fe/Al 2 O 3 , 14Ni8Fe/Al 2 O 3 , and 14Ni12Fe/Al2O3. For comparison purpose, two monometallic Ni and Fe catalysts, denoted as 14Ni/Al2O3 and 8Fe/Al2O3, were also 1193

DOI: 10.1021/acs.iecr.5b03797 Ind. Eng. Chem. Res. 2016, 55, 1192−1201

Article

Industrial & Engineering Chemistry Research each sample was introduced into the vacuum chamber for analysis. The spectra were collected with a pass energy of 30 eV under ultrahigh-vacuum conditions. The binding energies were calibrated by the internal standard method using the C 1s peak at 284.8 eV. To analyze the chemical environments of the elements in each sample, the obtained XPS spectrum was fitted using XPSPEAK software (version 4.1). The Ni 2p3/2 and Fe 2p3/2 spectra were deconvoluted into several peaks after background subtraction by the Shirley method. The line shapes were approximated by a combination of Gaussian and Lorentz functions.16 In addition to the oxidic catalysts, the corresponding sulfided catalysts were also characterized by XPS. The sulfided catalysts were obtained by sulfurizing the oxidic samples using a 3 wt % CS2/cyclohexane mixture at 300 °C and 2.5 MPa for 4 h. After sulfurization, the catalysts were cooled to room temperature and kept in cyclohexane to prevent oxidation. Before measurements, the samples were ground to powders and pressed into thin self-supporting wafers, and then they were evacuated and transferred to the analysis chamber without contact with air. 2.3. Catalytic Tests. The mercaptan transformation performance of each catalyst was tested in a continuously flowing fixed-bed microreactor, using a CH3SH and C2H5SH containing FCC LPG sampled from PetroChina Hohhot Petrochemical Company, Ltd., as the feedstock. The feed LPG was a C3−C4 stream containing propene (0.68 wt %), 1butene (12.7 wt %), trans-2-butene (12.5 wt %), cis-2-butene (8.32 wt %), isobutene (0.87 wt %), and 1,3-butadiene (0.65 wt %). For each test, 3.5 g of oxidic catalyst was loaded into the reactor, which had an internal diameter of 10.0 mm and a length of 500 mm, and was fixed in the constant-temperature zone using 1.5-mm spherical ceramic particles. Prior to testing, the catalysts were presulfided in situ with a mixture of 3 wt % CS2/cyclohexane as a vulcanizing agent for 2 h at 230, 270, 300, and 320 °C, respectively, and a hydrogen pressure of 2.5 MPa. Thereafter, the LPG feedstock and H2 were introduced into the reactor with a plunger pump and a mass flowmeter, respectively. The thioetherification reaction was carried out under the following conditions: temperature of 75 °C, pressure of 2.5 MPa, weight hourly space velocity (WHSV) of 3.0 h−1, and H2/LPG volumetric ratio of 6.0. After a stabilization period of 12 h, the reaction products were collected and analyzed. The contents of methyl and ethyl mercaptans in the feed and products were determined using an Agilent 7890A GC (Palo Alto, CA) equipped with a 30-m capillary column and a sulfur chemiluminescence detector (SCD).

Figure 1. Mercaptan removal from LPG over 8Fe/Al2O3 (N1), 14Ni/ Al2O3 (N2), 14Ni4Fe/Al2O3 (N3), 14Ni8Fe/Al2O3 (N4), and 14Ni12Fe/Al2O3 (N5). The LPG feedstock contained 120.9 μg of CH3SH and 232.4 μg of C2H5SH per gram.

enhanced activity. As seen in Figure 1, among the three bimetallic NiFe/Al2O3 catalysts, even though the Fe2O3 loading is only 4 wt %, the resulting bimetallic catalyst 14Ni4Fe/Al2O3 showed an obviously enhanced activity, with CH3SH and C2H5SH conversion ratios as high as 97.6% and 93.4%, respectively. When the Fe2O3 loading was increased to 8 wt %, the CH3SH and C2H5SH conversion ratios increased by only 1.9% and 4.0%, respectively, and when the Fe2O3 loading was further increased to 12 wt %, there was no further increase in catalytic activity. It can be reasonably inferred that Fe has an obvious promoting effect in the Ni/Al2O3 system, as confirmed by the characterization results presented in the next section. 3.2. Characterization Results. The textural properties of the Al2O3 support and its supported oxidic catalysts are listed in Table 1. Among all of the samples, the Al2O3 support has the Table 1. Textural Properties and Surface Acid Amounts of Al2O3 and Its Supported Oxidic Catalysts

a

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. The transformation of methyl and ethyl mercaptans in the feed LPG by thioetherification over the different catalysts was assessed at the relatively low temperature of 75 °C, and the results are shown in Figure 1. It can be seen that the conversion ratios of CH3SH and C2H5SH over the monometallic Ni catalyst 14Ni/Al2O3 were 72.0% and 60.1%, and those over the monometallic Fe catalyst 8Fe/Al 2 O 3 were even lower. In our previous study, unsupported Al2O3 was also tested and found to have negligible activity for thioetherification, which indicates that Ni and Fe are active for thioetherification but have unsatisfactory activities when individually supported on Al2O3. However, when Fe was introduced into the monometallic Ni catalyst, the resulting bimetallic NiFe/Al2O3 catalyst system showed dramatically

sample

BET surface area (m2/g)

pore volume (cm3/g)

acid amounta (mmol of NH3/g)

Al2O3 8Fe/Al2O3 14Ni/Al2O3 14Ni4Fe/Al2O3 14Ni8Fe/Al2O3 14Ni12Fe/Al2O3

321.6 282.6 279.5 274.9 264.6 242.2

0.65 0.55 0.54 0.51 0.45 0.41

0.650 − 0.547 0.581 0.540 0.498

Determined by NH3 TPD.

largest BET surface area and pore volume. The introduction of 14 wt % NiO into the Al2O3 support resulted in significant decreases in the surface area (from 321.6 to 279.5 m2/g) and pore volume (from 0.65 to 0.54 cm3/g) as a result of the blockage of pores in the support by the incorporated NiO. The incorporation of 4 wt % Fe2O3 into the monometallic 14Ni/ Al2O3 catalyst by coimpregnation led to only slight decreases in the BET surface area and pore volume of the resulting catalyst 14Ni4Fe/Al2O3. However, further increasing the Fe2O3 loading to 8 and 12 wt % gave rise to substantial decreases in specific surface area and total pore volume, possibly because of an inhomogeneous distribution of metal precursors at higher metal loadings causing the complete blockage of pores in the support that were only partially blocked at lower loading. 1194


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By comparing Figures 1 and 2, one can see that, compared to 14Ni/Al2O3, 14Ni8Fe/Al2O3 exhibits a much higher lowtemperature activity, with the conversion ratios of CH3SH and C2H5SH being increased by 16.6% and 31.1%, respectively. Interestingly, all three of the NiFe/Al2O3 catalysts with Fe2O3 loadings of at least 4 wt % displayed outstanding catalytic performance, even though they differed from each other in their quantities and strengths of acid sites, suggesting that the acidity of the bimetallic NiFe system cannot be the key factor influencing the catalytic activity. The XRD patterns of the oxidic catalysts are shown in Figure 3. For all of the samples, the diffraction peaks at 2θ = 36.7°,

By referring to the results of the catalytic tests, one can see that, although the three bimetallic catalysts differed from each other in their BET surface areas and total pore volumes, they exhibited similar activities. This indicates that textural properties are not the foremost factor influencing the catalytic activity of the bimetallic NiFe catalysts. The NH3 TPD results for the bimetallic NiFe/Al2O3 catalysts with different Fe2O3 loadings are shown in Figure 2, where the

Figure 2. NH3 TPD profiles of the Al2O3 support and the supported oxidic catalysts.

NH3 TPD profiles of the Al2O3 support and the monometallic catalyst 14Ni/Al2O3 are also presented for comparison; the total surface acid amounts, defined as the amount of NH3 desorbed per gram of a sample, are also reported in Table 1. According to the desorption temperature of NH3, acid sites are classified as weak (100−250 °C), medium (250−400 °C), and strong (>400 °C).17 As shown in Figure 2, one single broad NH3 desorption peak in the range of 100−400 °C with a peak temperature (Tmax) of ca. 230 °C can be observed for the Al2O3 support, corresponding to weak and medium acid sites arising from hydrogen-bonded sites and coordinatively unsaturated Al3+ cations. Even though the NH3 TPD profiles of all of the catalysts display similar broad peaks, their acid sites and acid strength are quite different. Compared with that of the Al2O3 support (with an acid amount of 0.650 mmol of NH3/g), the NH3 TPD profile of 14Ni/Al2O3 has a desorption peak with decreased intensity (with an acid amount of 0.547 mmol of NH3/g), ascribed to the formation of some NiAl2O4 spinel due to the interaction of the incorporated Ni2+ with Al2O3 at 480 °C,18,19 which decreases the quantity of acid sites of Al2O3 and thereby the desorption temperature. Figure 2 also shows that, for the Fe-promoted 14Ni/Al2O3 catalysts, the weak acid sites changed in different manners: For the catalyst with an Fe2O3 loading of 4 wt % (with an acid amount of 0.581 mmol of NH3/g), the desorption peak further shifted toward lower temperature as a result of the generation of more weak acid sites as compared to 14Ni/Al2O3, consistent with the results of Casaletto et al.20 However, further increasing the Fe2O3 loading brought about a reduction in weak acid sites but an increase in maximum desorption temperature. The increased weak acid sites at lower Fe2O3 loading can be associated with the well dispersed Fe3+ ions having Lewis acidity, whereas the decreased acid sites at higher Fe2O3 loading (such as for 14Ni12Fe/Al2O3 with an acid amount of 0.498 mmol of NH3/g) are possibly due to the blockage of pores and the decrease of surface area, which inhibit the adsorption of NH3.

Figure 3. XRD patterns of 8Fe/Al2O3 (N1), 14Ni/Al2O3 (N2), 14Ni4Fe/Al2O3 (N3), 14Ni8Fe/Al2O3 (N4), and 14Ni12Fe/Al2O3 (N5).

39.4°, 45.7°, and 66.4° are attributed to the Al2O3 support (JCPDS No. 29-1486). For the monometallic catalysts 14Ni/ Al2O3 and 8Fe/Al2O3, no diffraction peaks corresponding to crystalline NiO and Fe2O3 phases are observed because of their high dispersion. Similarly, no obvious characteristic peaks assigned to nickel oxide and iron oxide can be found in the XRD pattern of 14Ni4Fe/Al2O3, also indicating the high dispersion of the metal oxides on the support. With increasing Fe2O3 loading, however, the characteristic peaks assigned to the metal oxides become stronger: In the XRD patterns of 14Ni8Fe/Al2O3 and 14Ni12Fe/Al2O3, the diffraction peaks at 2θ = 24.1°, 33.1°, 40.8°, 49.5°, and 64.1° are attributed to the α-Fe2O3 phase (JCPDS No. 33-0664), demonstrating the existence of aggregated α-Fe2O3 particles on the surface of Al2O3 after calcination. Meanwhile, the peaks at 2θ = 37.3°, 43.3°, 62.8°, and 75.5° are ascribed to crystalline NiO (JCPDS No. 44-1159) arising from the aggregating effect of nickel oxide species weakly interacting with the Al2O3. Importantly, the peaks appearing at 2θ = 35.7° and 54.0° can be ascribed to NiFe2O4 (JCPDS No. 54-0964) formed from intimately contacting NiO and Fe2O3 species during the catalyst preparation. From the XRD patterns of the three bimetallic NiFe/Al2O3 catalysts, especially those with higher Fe2O3 loadings (i.e., 14Ni8Fe/Al2O3 and 14Ni12Fe/Al2O3), one can see that two forms of nickel oxide species are formed: (i) isolated NiO aggregates weakly interacting with the Al2O3 and (ii) NiFe2O4 phase formed through the intimate contact between NiO and Fe2O3. These two forms of species cannot be detected at lower Fe2O3 loading because of the detection limits of XRD. 1195


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reduction of α-Fe2O3 to Fe3O4 and that in the range of 440− 580 °C is due to the reduction of Fe3O4 to α-Fe.11,21 This indicates that most iron species in the 8Fe/Al2O3 catalyst were reduced from α-Fe2O3 to Fe3O4 below 580 °C. Similarly, the H2 TPR profile of 14Ni/Al2O3 shows only a single broad H2 consumption peak in the temperature range of 400−840 °C with a maximum reduction temperature (Tmax) of ca. 618 °C, in good agreement with the literature results. 23 In the literature,19,24 the H2 consumption band located within the temperature range of 400−530 °C was attributed to the reduction of the NiO phase that has a weak interaction with Al2O3, those overlapping in the temperature range of 530−730 °C were mainly due to the reduction of nickel oxide species exhibiting stronger interactions with Al2O3, and that within the temperature range of 730−840 °C was assigned to the reduction of the stable NiAl2O4 spinel phase that is difficult to reduce. In the H2 TPR profiles of the bimetallic NiFe catalysts prepared by incorporating different amounts of Fe2O3 into 14Ni/Al2O3, the profiles display two obvious reduction peaks. As reported in the literature,11 the H2 consumption peaks at lower temperature (185−440 °C) can be attributed to the reduction of NiFe2O4 to Fe3O4, accompanied by the reduction of separated α-Fe2O3 to Fe3O4; those at higher temperature (above 440 °C) can be ascribed to the further reduction of Fe3O4 to α-Fe, of NiFe2O4 to Ni and α-Fe, and of nickel oxide species interacting with Al2O3 to different extents to metal Ni. For the three NiFe/Al2O3 catalysts, with increasing Fe2O3 loading, Tmax in the range of 190−440 °C gradually increased as a result of the formation of more α-Fe2O3 phase, as shown in Figure 4a, whereas Tmax in the temperature range above 440 °C gradually decreased as a result of the generation of more nickel oxide species weakly interacting with Al2O3, leading to improved reducibility of the resulting catalysts. The H2 TPR results indicate that the incorporation of Fe2O3 into the monometallic Ni-based catalyst significantly improves the reducibility of the nickel oxides, mainly arising from the formation of the two types of NiO species described in the XRD results. The isolated NiO aggregates weakly interacting with Al2O3 and the NiO species in the Fe2O3 environment (the NiFe2O4 phase) are more reducible, which is favorable for the sulfidation of the nickel oxides. UV−vis DRS was applied to identify the coordination and nature of nickel and iron species supported on Al2O3, and the results are presented in Figure 4b. It can be seen that there is almost no obvious signal in the spectrum of the Al2O3 support as compared to the supported catalysts. In the UV−vis DRS profile of 8Fe/Al2O3, the absorption band below 300 nm is attributed to O → Fe3+ charge-transfer transitions related to isolated tetrahedral and octahedral Fe3+ species;25,26 that at 333 nm is assigned to small oligomeric clusters of octahedral Fe3+ species;25,26 and that at around 506 nm is ascribed to some larger Fe2O3-like aggregates,26,27 consistent with the XRD results. In the UV−vis DRS profile of 14Ni/Al2O3, the absorption band at 290 nm is characteristic of O → Ni2+ charge-transfer transition in NiO lattices,28,29 but no bulk NiO was detected, as shown by the absence of the band at around 320 nm,30 in good agreement with the XRD results and thus demonstrating the outstanding dispersion of NiO on the support. The band at about 405 nm is due to the octahedral Ni2+ species in NiO lattices,30,31 and that at around 650 nm is related to tetrahedral Ni2+ species in nickel aluminate lattices.30,31

Based on a comparison of Figures 1 and 3, we speculate that the aforementioned two types of NiO species, or more exactly, the metal sulfide species existing in the three bimetallic NiFe/ Al2O3 catalysts that are derived from them, could account for the significantly improved activities of the three bimetallic NiFe/Al2O3 catalysts, because they could both act as precursors of the active phases. Although all three of the bimetallic catalysts showed greatly enhanced catalytic activity as compared with the monometallic 14Ni/Al2O3 catalyst, the increase in Fe2O3 loading from 4 to 12 wt % did not further enhance the catalytic activity, possibly because of the aggregation of the active species at higher metal loading, which counteracts the promoting effect. H2 TPR measurements were used to examine the reducibility and metal−support interaction of the catalysts, and the results are shown in Figure 4a. In the H2 TPR profile of 8Fe/Al2O3, only one broad reduction peak at about 380 °C was recorded within the whole temperature range, similar to the results of Liu et al.21 and Meng et al.22 It was reported that the peak in the temperature range of 210−440 °C can be attributed to the

Figure 4. (a) H2 TPR profiles, (b) UV−vis diffuse reflectance spectra, and (c) Raman spectra of Al2O3 (N0), 8Fe/Al2O3 (N1), 14Ni/Al2O3 (N2), 14Ni4Fe/Al2O3 (N3), 14Ni8Fe/Al2O3 (N4), and 14Ni12Fe/ Al2O3 (N5). 1196


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spectrum of 8Fe/Al2O3, the slight decreases in the intensities of the bands at 550 and 1045 cm−1 are the consequence of the reduced amount of Fe species due to formation of the NiFe2O4 phase. XPS was also used to determine the surface nickel and iron species in the five synthesized catalyst samples. The Ni 2p XPS spectra of the bimetallic NiFe samples with different Fe2O3 loadings and the reference catalyst 14Ni/Al2O3 are shown in Figure 5a. In the Ni 2p spectra, the peaks at around 855.7 and

As compared to 14Ni/Al2O3, the Fe2O3-incorporated bimetallic catalysts have distinctly different UV−vis DRS profiles. As seen in Figure 4b, the bimetallic NiFe catalysts clearly exhibit red shifts of absorption edges and show enhanced light absorption in the visible region (>400 nm) with increasing Fe2O3 loading, indicating the strong interaction between nickel and iron species and the variation in the coordination states of Ni2+ and Fe3+ ions. In the synthesized NiFe catalysts, the absorption bands in the range of 260−340 nm can be attributed to O → Fe3+ and O → Ni2+ chargetransfer transitions and some octahedral Fe3+ species with low oligomeric clusters.25,26,28 The band at around 405 nm still remained, demonstrating that the formation of octahedral Ni2+ species was not affected by the incorporation of iron, and the octahedral Ni2+ ions could be related to NiFe2O4 phase formation.32 In the wavelength range from 445 to 600 nm, the absorption bands of the three NiFe/Al2O3 catalysts gradually shifted to higher wavelength with increasing Fe2O3 loading, indicating that the aggregation of α-Fe2O3 particles became more significant, in agreement with the XRD results. Additionally, compared with 14Ni/Al2O3, every bimetallic NiFe catalyst shows an extra weak absorption band at around 769 nm due to the generation of octahedral Ni2+ species in NiO lattices.33,34 Nevertheless, no band at around 650 nm was observed for any of the bimetallic NiFe catalysts, possibly because the added Fe3+ ions partially interacted with the surface Al2O3 to form a stable AlFeO3 phase35 and thereby decreased the Ni species to form inactive NiAl2O4 phase, in accordance with the phenomenon that a Tmax value in the high temperature range gradually shifts to lower temperature in the H2 TPR profiles. Therefore, it can be seen that the two types of nickel oxides generated exhibit octahedral characteristics. Raman spectroscopy as a complementary tool to XRD was also utilized to identify the oxide phases, especially to determine the formation of the NiFe2O4 spinel phase in the NiFe system. The Raman spectra of selected oxidic samples and the Al2O3 support are presented in Figure 4c. It can be seen that, whereas the spectra of the three catalysts are similar except for that of Al2O3 having no signals, there are obvious differences in the intensities of some Raman bands: For 8Fe/Al2O3, the bands at around 425, 494, and 609 cm−1 are attributed to Fe− O vibrations of FeO6 octahedra in α-Fe2O3 phase;36,37 the weak bands centered at 315, 550 and 660−720 cm−1 might be originate from Fe−O vibrations of FeO4 tetrahedra in the Fe3O4 phase;37−39 and that near 1045 cm−1 could be due to Fe−O−Al stretching vibrations.40 For the monometallic 14Ni/ Al2O3 sample, the characteristic bands at 410, 492, 892, and 1094 cm−1 are assigned to Ni−O vibrations of the NiO phase on the surface of Al2O3,41,42 whereas that located at 566 cm−1 can be attributed to the vibration of the surface spinel NiAl2O4 phase,43 none of which could be detected by XRD because of its detection limit. For 14Ni8Fe/Al2O3, an additional weak band at around 700 cm−1 was detected, corresponding to a new phase NiFe2O4,38,42 and the relative intensities of the bands centered at 566, 892 and 1020−1101 cm−1 were weakened as compared to those of 14Ni/Al2O3 on account of the transformation of the coordinative environment of the NiO phase caused by the formation of NiFe2O4. In addition, the formation of the NiAl2O4 (566 cm−1) phase was inhibited by the incorporation of iron into the monometallic nickel catalyst, in agreement with the DRS results. In 14Ni8Fe/Al2O3, the NiFe2O4 phase was also observed at the overlapping bands of 329, 446, 486, and 577 cm−1,41,42,44 and compared with the

Figure 5. (a) Ni 2p and (b) Fe 2p XPS spectra of oxidic 8Fe/Al2O3 (N1), 14Ni/Al2O3 (N2), 14Ni4Fe/Al2O3 (N3), 14Ni8Fe/Al2O3 (N4), and 14Ni12Fe/Al2O3 (N5).

873.1 eV are due to the spin−orbit split lines of Ni 2p3/2 and Ni 2p1/2.45 The characteristic peaks of Ni 2p3/2 and Ni 2p1/2 are accompanied by shakeup satellites at binding energies of about 861.7 and 880 eV, corresponding to charge-transfer transitions.46 As shown in Figure 5a, the XPS spectrum in the Ni 2p3/2 region of the 14Ni/Al2O3 sample can be fitted to two peaks centered at 855.2 and 856.5 eV, attributed to Ni2+ present in the octahedral positions of the supported NiO phase and Ni2+ in the tetrahedral sites of NiAl2O4 phase formed by the reaction of NiO and Al2O3, respectively.9,47 With increasing Fe2O3 loading, the Ni 2p3/2 peak of the bimetallic NiFe catalysts exhibits a slight blue shift compared with that of 14Ni/Al2O3, and the Ni 2p3/2 binding energy is decreased to 855.4 eV for 14Ni8Fe/Al2O3, indicating the enhanced electron density of the nickel oxide species due to the addition of Fe2O3 to 14Ni/Al2O3. Some researchers have observed an additional fitting peak at around 855.7 eV corresponding to Ni2+ in the NiFe2O4 phase48,49 that coexists with the isolated NiO phase in NiFe catalyst systems. Moreover, for Fe-promoted Ni-based catalysts, because of the change in the chemical states of Ni2+, the deconvoluted peaks of Ni2+ in the NiO and NiAl2O4 phases shifted to 854.9 and 856.7 eV, respectively. The relative contents of the different nickel oxide species were also 1197


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Industrial & Engineering Chemistry Research calculated from the fitted peak area proportions of the corresponding nickel oxide species, and the results are summarized in Table 2. It is clear that a large quantity of Table 2. Relative Contents of the Different Nickel and Iron Oxide Species Obtained from the Deconvolution of Ni 2p3/2 and Fe 2p3/2 Spectra of the Selected Oxidic Catalysts Ni 2p3/2 (%) 2+

2+

Fe 2p3/2 (%) 2+

sample

Ni in NiO

Ni in NiFe2O4

Ni in NiAl2O4

Fe3+ in Fe−O−Al

14Ni/Al2O3 8Fe/Al2O3 14Ni4Fe/Al2O3 14Ni8Fe/Al2O3 14Ni12Fe/Al2O3

43.3 − 36.9 37.4 39.3

− − 33.0 35.6 36.1

56.7 − 30.1 27.0 24.7

− 26 15.2 15.8 16.2

Ni2+ species participated in the formation of the NiO and NiFe2O4 phases when Fe2O3 was incorporated into 14Ni/ Al2O3. With increasing Fe2O3 loading, the total percentage of Ni2+ species in the NiO and NiFe2O4 phases gradually increased from 69.9% to 75.4%, 26.6% higher than that in 14Ni/Al2O3. Meanwhile, the relative content of the Ni2+ species corresponding to the NiAl2O4 phase decreased gradually from 30.1% to 24.7%, whereas that of the NiAl2O4 species was as high as 56.7% for the monometallic 14Ni/Al2O3 catalyst. Apparently, Fe incorporation substantially inhibits the formation of the inactive NiAl2O4 phase and greatly increases the amounts of NiO and NiFe2O4 phases on the catalyst surface, both of which act as precursors of the active phases. The Fe 2p XPS spectra for oxidic 8Fe/Al2O3, 14Ni4Fe/ Al2O3, 14Ni8Fe/Al2O3, and 14Ni12Fe/Al2O3 are presented in Figure 5b. The characteristic peak of Fe 2p3/2 is centered at about 710.7 eV, and that of Fe 2p1/2 is at around 724.2 eV (not shown in the spectra), both corresponding to the spin−orbit splitting of iron oxide species.50 For the monometallic 8Fe/ Al2O3 sample, the Fe 2p3/2 spectrum is composed of three distinct peaks located at 709.8, 711.2, and 713.2 eV: The peak at about 709.8 eV is associated with the presence of Fe2+ and Fe3+ species in the form of Fe3O4,51 whereas that at around 711.2 eV is assigned to Fe3+ in the Fe2O3 phase,51 and that centered at 713.2 eV is attributed to the incorporation of Fe3+ into Al2O3 lattice and, thereby, the formation of Fe−O−Al bonds.48,52 In the bimetallic NiFe systems, the above-mentioned binding energies of the different Fe species exhibited slight changes due to the effects of Ni species. Furthermore, an extra peak at about 710.5 eV was observed by deconvoluting the Fe 2p3/2 peak that corresponds to Fe3+ in NiFe2O4.48,53 Undoubtedly, the amount of Fe3+ in the NiFe2O4 phase increased with increasing Fe2O3 loading in the bimetallic NiFe samples. In addition, as seen in Table 2, the increase in the Fe2O3 loading from 4 to 12 wt % did not substantially increase the proportion of Fe3+ species in Fe−O−Al (from 15.2% to 16.2%), much lower than that (26.0%) in 8Fe/Al2O3, thus, indicating that more Fe3+ ions are involved in the formation of the NiFe2O4 species. The sulfided monometallic Ni catalyst and the bimetallic NiFe catalysts were also characterized by XPS, and the spectra of the Ni 2p3/2 region were deconvoluted into the contribution of the different Ni species; the results are presented in Figure 6a. For the sulfided monometallic Ni/Al2O3 catalyst, a distinct peak with a binding energy of 852.7 eV was observed as compared to its oxidic precursor, corresponding to NiSx species

Figure 6. (a) Ni 2p and (b) S 2p and Fe 2p XPS spectra of sulfided 8Fe/Al2O3 (N1), 14Ni/Al2O3 (N2), 14Ni4Fe/Al2O3 (N3), 14Ni8Fe/ Al2O3 (N4), and 14Ni12Fe/Al2O3 (N5).

generated from the NiO phase,54 whereas fitted peaks corresponding to NiO (∼855.1 eV) and NiAl2O4 (∼856.5 eV)9,47 coexist for the sulfided sample. The respective proportions of the different Ni species in the sulfided 14Ni/ Al2O3 catalyst are listed in Table 3. Owing to the strong Table 3. Relative Contents of the Different Ni Species Obtained from the Deconvolution of Ni 2p3/2 Spectra of Selected Sulfided Catalysts Ni 2p3/2 (%) sample

NiSx

NiS(FeS)

NiO

Ni2+ in NiFe2O4

14Ni/Al2O3 14Ni4Fe/Al2O3 14Ni8Fe/Al2O3 14Ni12Fe/Al2O3

16.2 13.0 16.3 18.2

− 5.92 11.2 13.8

45.8 27.9 26.9 25.1

− 25.3 21.3 19.3

interaction of Ni2+ ions with the Al2O3 support, only a relatively lower percentage of Ni species were sulfided (∼16.2%); additionally, the lower sulfiding temperature could also result in a lower sulfidation degree.55 For the deconvolution of the Fe-decorated Ni catalysts, the NiSx phase is still observed due to the sulfidation of the isolated NiO weakly interacting with Al2O3 in the three NiFe catalysts; furthermore, the extra decomposed peak at a binding energy of 853.5 eV is associated with the nickel sulfide phase55 in intimate contact with the iron sulfide phase due to the sulfidation of the NiFe2O4, which could be denoted as NiS(FeS), coexisting with the NiSx phase on the sulfided catalyst surface. As shown in Figure 6a, the intensities of the decomposed peaks of the NiSx and NiS(FeS) species were both enhanced. The fitted peaks located at 855.2 and 855.9 eV are assigned to the coexisting 1198


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Industrial & Engineering Chemistry Research NiO and NiFe2O4 phases, respectively,9,49 due to the incomplete sulfidation at lower sulfiding temperature.55 Table 3 also lists the respective contributions of the different Ni species in the three sulfided NiFe/Al2O3 catalysts, and as compared with the results for 14Ni/Al2O3, increasing the Fe2O3 loading from 4 to 12 wt % gave rise to an increase in the overall percentage of the sulfided Ni species from 18.9% to 32.0%, indicating that the presence of Fe2O3 has a dramatic promoting effect on the sulfidation degree of Ni in the NiFe catalyst system. The percentages of NiSx and NiS(FeS) species increased by 5.2% and 7.9%, respectively, illustrating that the NiFe2O4 phase can be more easily sulfided, consistent with the results of H2 TPR. The S 2p XPS spectra of sulfided 8Fe/Al2O3, 14Ni/Al2O3, and 14Ni8Fe/Al2O3 are presented in Figure 6b. For 14Ni8Fe/ Al2O3, the S 2p3/2 peak is centered at 161.7 eV, slightly lower than that for 14Ni/Al2O3 (∼162.1 eV), thus indicating that the S species in the sulfided 14Ni8Fe/Al2O3 have higher electron densities as a result of the incorporation of Fe into 14Ni/Al2O3. The spectra of the S 2p3/2 region were deconvoluted into three distinct peaks located at 161.6, 162.3, and 163.5 eV, assigned to the monosulfide (S2−), disulfide (S22−), and polysulfide (Sn2−), respectively,56,57 bonding with the metal atoms. Among the three sulfided catalysts, the NiSx species as the active phases in 14Ni/Al2O3 have moderate contents of S2− and S22− species bonded with Ni atoms according to the fitted peak area proportions of the sulfide species, whereas the sulfided Fe species (denoted as FeS) as the active sites56 in 8Fe/Al2O3 have the lowest contents of S2− and S22− species but the highest amount of Sn2− species bonded to Fe atoms. In contrast, for the active sites of 14Ni8Fe/Al2O3, the NiSx and NiS(FeS) species have the highest amounts of S2− and S22− species and the lowest amounts of Sn2− species, which contribute to the lowest binding energy of the S 2p3/2 peak as compared with those of monometallic 14Ni/Al2O3 and 8Fe/Al2O3. Figure 6b compares the Fe 2p XPS spectra of the sulfided 8Fe/Al2O3 and 14Ni8Fe/Al2O3 catalysts. In the XPS spectrum of the sulfided 8Fe/Al2O3 catalyst, the Fe 2p3/2 peak is centered at 711.0 eV, whereas in the spectrum of the sulfided 14Ni8Fe/ Al2O3 catalyst, the binding energy is increased to 711.8 eV, indicating that Fe species also has the ability to donate electron density to the closely contacted NiS species in the sulfiding process. This is also confirmed by the decrease of the binding energy of the S 2p3/2 peak in the XPS spectrum of the sulfided 14Ni/Al2O3 catalyst after Fe incorporation (shown in Figure 6b), which demonstrates that the weakened Ni−S bond can be correlated with the higher activity.58 According to the above characterization results, it can be reasonably concluded that Fe species in the bimetallic NiFe system play two important roles in promoting the catalytic activity: (i) Partial incorporated Fe species acting as a textural promoter occupy part of the surface of Al2O3 and strongly interact with the support in the preparation step, increasing the amount of nickel oxides weakly interacting with the support and thus forming more active nickel sulfides after sulfidation [the XPS characterization results show that nickel sulfides are present in two forms, i.e., NiSx and NiS(FeS) phases, whose amounts increase with increasing Fe2O3 loading], and (ii) Fe species in the NiS(FeS) phases acting as electron donors transfer electron density to the closely contacted NiS species, weakening the Ni−S bonds and contributing to the higher activity of NiS(FeS). These two promoting effects of Fe endow

the resulting bimetallic catalysts with higher mercaptan removal activities at low temperature. 3.3. Stability Testing Results of 14Ni8Fe/Al2O3. The catalytic stability of the bimetallic 14Ni8Fe/Al2O3 catalyst, which exhibited the highest activity for the removal of CH3SH and C2H5SH from LPG, was tested, and the results are shown in Figure 7a.

Figure 7. (a) Stability of 14Ni8Fe/Al2O3 for mercaptan removal. (The LPG feedstock contained 141.7 μg of CH3SH and 219.7 μg of C2H5SH per gram.) (b) GC-SCD spectra of the products and feed obtained in the stability assessment tests.

It can be seen that, during a period of over 100 h, the catalyst exhibited a stable activity for the removals of both CH3SH and C2H5SH, giving CH3SH and C2H5SH conversion ratios as high as >98.0%. By analyzing the sulfur-containing compounds in the feed and products using a GC equipped with an SCD (GCSCD), we found that 14Ni8Fe/Al2O3 exhibited an outstanding activity and stability for the removals of both CH3SH and C2H5SH, as shown by the nearly invisible signals for CH3SH (with a retention time of ∼1.87 min) and C2H5SH (with a retention time of ∼3.34 min) in the product in Figure 7b. Correspondingly, several strong signals with retention times greater than 10 min were detected, demonstrating the formation of sulfides with much higher boiling points. According to the qualitative analysis results by GC-SCD, only a small amount of dimethyl disulfide (with a retention time of ∼11.10 min) was detected in the products, which originated from the reaction of two CH3SH molecules; the most abundant sulfur products were butyl methyl sulfide (with a retention time of ∼13.03 min) and butyl ethyl sulfide (with a retention time of ∼14.99 min), mainly generated from the reaction of reactive 1,3-butadiene with CH3SH and C2H5SH,59 which indicates that 1199


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14Ni8Fe/Al2O3 provides good performance in the thioetherification of light mercaptans and reactive diolefin.

4. CONCLUSIONS In conclusion, the introduction of Fe into monometallic Ni/ Al2O3 catalyst led to a bimetallic NiFe/Al2O3 system with dramatically enhanced activity for mercaptan transformation through thioetherification reactions. The promoting effects of Fe species include two vital aspects: (i) Partial incorporated Fe species acting as a textural promoter strongly interact with the surface of Al2O3, which prevents the formation of the NiAl2O4 phase and thus increases the amount of nickel oxides weakly interacting with Al2O3 [nickel oxide species are mainly present in the form of isolated NiO weakly interacting with Al2O3 and NiO in close contact with Fe2O3 (NiFe2O4), which can be easily converted into catalytically active NiSx and NiS(FeS), respectively, after sulfidation], and (ii) partially incorporated Fe species serving as electron donors transfer electron density to the closely contacted NiS in NiS(FeS) species, leading to weakened Ni−S bonds and thereby contributing to the higher activity of NiS(FeS). These two promoting effects of Fe endow the resulting bimetallic catalysts with significantly enhanced low-temperature activities. Among the bimetallic catalysts with different Fe2O3 loadings, 14Ni8Fe/Al2O3 showed superior catalytic activity and stability, giving CH3SH and C2H5SH conversions as high as 99.5% and 97.4%, respectively, at 75 °C.

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-89734979. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grants U1462203 and 21276276) and the Ministry of Science and Technology of China through the National Basic Research Program (Grant 2010CB226905).

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REFERENCES

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Al2O3 Thioetherification Catalysts with Enhanced

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