Reactivation of spent S-Zorb adsorbents for gasoline desulfurization

Chemical Engineering Journal 374 (2019) 1109–1117

Contents lists available at ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Reactivation of spent S-Zorb adsorbents for gasoline desulfurization a

a

a

Yuchao Lyu , Zongwei Sun , Ying Xin , Yuxiang Liu a b

a,b

a

, Chunzheng Wang , Xinmei Liu

a,⁎

T

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266555, China College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao 266042, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

acid-base coupling method for re• An activation of the spent S-Zorb adsorbent was developed.

reactivation mechanism for the • The spent S-Zorb adsorbent was proposed. The inactive Zn SiO in the spent ad• sorbent is transformed into active ZnO 2

4

for sulfur storage.

reactivated adsorbent outper• The forms the regenerated adsorbent in terms of the desulfurization activity.

A R T I C LE I N FO

A B S T R A C T

Keywords: Reactivation Spent S-Zorb adsorbent Acid-base coupling method Gasoline desulfurization Ni/ZnO

Spent adsorbents from the S-Zorb technology are hazardous wastes. Reactivating and recycling the spent S-Zorb adsorbents is of great significance to environmental protection. However, no report concerning this subject has been found yet. Herein, we have developed an acid-base coupling method to reactivate the spent S-Zorb adsorbents. To achieve this goal, the deactivation factors of the adsorbents were studied. The results show that the formation of Zn2SiO4 phase, which results in loss of active phase (ZnO) for sulfur storage, is the main deactivation factor. The acid-base coupling method can transform Zn2SiO4 into soluble Zn2+ via nitric acid, followed by precipitation as Zn(OH)2 with sodium hydroxide addition. After calcination, the loss of ZnO can be supplemented. The newly formed ZnO bears a willow-leaf shape that is remarkably different from the original one. Also, it has smaller particle size compared to that of fresh adsorbents. Of note is that mesopores and Lewis acid sites are created over the adsorbents via the acid-base coupling method, accelerating the mass transfer and conversion of sulfur compounds. Carbon deposited over the adsorbent surface is removed and metal sulfides (ZnS and NiS) are oxidized into metal oxides (ZnO and NiO) via calcination of the spent adsorbents. The desulfurization efficiency of the reactivated adsorbent is superior to that of the regenerated adsorbent and reaches a level comparable to that of the fresh adsorbent in gasoline desulfurization.

1. Introduction Sulfur emissions from vehicle exhaust not only cause air pollution and human health problems, but also irreversibly poison the three-way catalysts in catalytic converters [1–4]. More stringent environmental regulations have been enacted to limit the sulfur content in gasoline ⁎

throughout the word [5,6]. The standard requiring sulfur content in the gasoline to be less than 10 μg/g is now imposed in many countries [7–10]. Consequently, deep removal of sulfur from the gasoline has become mandatory for current refineries. However, conventional hydrodesulfurization (HDS) cannot reduce the sulfur content of gasoline to a very low level without loss of octane number [11–14].

Corresponding author. E-mail address: [email protected] (X. Liu).

https://doi.org/10.1016/j.cej.2019.06.010 Received 5 March 2019; Received in revised form 28 May 2019; Accepted 2 June 2019 Available online 03 June 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

regenerator is operated at 530 °C in air atmosphere. The Fresh-S, Reg-S, Deac-S and Spent-S samples were supplied by the Sinopec Qingdao Refining and Chemical Co., Ltd. SiO2 and Al2O3 are the combined supports for the adsorbents. The gasoline with a sulfur content of 136.4 μg g−1 was provided by the Sinopec Qingdao Refining and Chemical Co., Ltd. Nitric acid (HNO3, analytical pure) and sodium hydroxide (NaOH, analytical pure) were purchased from XiRong Petrochemical Co., Ltd.

Reactive adsorption desulfurization (RADS) is one of the most effective methods to supplement the drawbacks of the HDS [15]. The RADS exhibits various advantages including lower hydrogen consumption, ultra-deep desulfurization ability and negligible loss of octane number [16,17]. S-Zorb is a representative technology of the RADS [18] and Ni/ZnO based adsorbents are widely used in the S-Zorb technology [1,5,19]. Sulfur-containing molecules adsorb on the active Ni atom via Ni-S bond and then sulfur is removed by direct hydrogenolysis of C-S bond, resulting in the formation of NiSx and hydrocarbons [18,20–22]. The hydrocarbons are returned as the final products, while the NiSx is converted into metallic nickel with ZnO acting as sulfur acceptor by forming ZnS in a hydrogen atmosphere. The desulfurization behavior of the adsorbents is mainly determined by features of the active Ni and ZnO phases. The high dispersion of Ni provides more hydrogenolysis centers to catalytically break the C–S bond [23,24]. The smaller particle size of ZnO can not only increase the Ni dispersion, but also reduce the resistance to mass transfer to accelerate the S transfer from NiSx to ZnO, resulting in a higher ZnO conversion and sulfur capacity [2]. Besides, the porosities and surface acidity also influence the desulfurization behavior [2,25]. The high specific surface area and large pore volume are favorable for accommodating more sulfur. A wide and open mesoporous structure accelerates the mass transfer of reactants and products, increasing reactivity of the adsorbents. The organosulfur compounds such as thiophene tend to adsorb on the Lewis acid sites due to the basicity of their lone pair electrons [26,27]. Thus, high Lewis acid sites concentration favors the adsorption and conversion of these organosulfur compounds. Most of the previous works mainly paid attention to develop novel adsorbents with a high performance or improve the regeneration conditions for the temporarily deactivated adsorbents [15,28–31]. However, deactivation of the adsorbents occurs inevitably along with the RADS evolution due to sintering and sulfidation of active sites [18,32], carbon deposition, surface acidity loss and formation of nickel or zinc spinel [33]. Current industrial regeneration process can recover reversible deactivation by burning out carbon deposits and converting the inactive ZnS into active ZnO [31]. However, the zinc (nickel) spinel is unrecoverable. It accumulates in the adsorbents and consumes the active phases (Ni and ZnO). Thus the desulfurization activity of the regenerated adsorbents decreases gradually and finally, a large quantity of spent adsorbents are generated. Normally, the spent adsorbents are buried directly, but it causes soil and water pollution. If the spent adsorbents are reactivated and recycled into the desulfurization-regeneration system, the use of fresh adsorbents will be reduced, cutting production costs of the low-sulfur gasoline. Besides, the solid waste emissions can also be reduced to relieve the environmental problems. However, no work concerning the reactivation of the spent adsorbents has been reported yet. In this work, an acid-base coupling method for reactivation of the spent S-Zorb adsorbents was developed for the first time after identifying the specific deactivation factors. The physicochemical properties and desulfurization activity of the reactivated adsorbent were studied. The mechanism for the reactivation of the spent adsorbents was proposed. It is worth noting that the sulfur removal efficiency of the spent S-Zorb adsorbent is recovered to a level which is higher than that of the regenerated adsorbent and comparable to that of the fresh S-Zorb adsorbent.

2.2. Reactivation of the spent S-Zorb adsorbent The industrial spent S-Zorb adsorbent (Spent-S) was calcined in a muffle oven at 600 °C for 2 h. 20.0 g of the calcined adsorbent was mixed with 140 ml of HNO3 aqueous solution (2.0 M) and stirred at 85 °C for 2 h. Then, 12.0 g of NaOH was added into the suspension and stirred for another 2 h. The solid product was filtered and washed with deionized water until pH of the filtrate reaches ca. 7 followed by drying at 100 °C for 12 h and calcination at 500 °C for 2 h. The resulting product was termed as Reac-S. 2.3. Acid treatment of the spent adsorbent with a nitric acid solution The acid treatment of the spent adsorbent was carried out in the same procedure as that of the reactivation but without NaOH addition. The filtrate in the filtration process was collected and named as Filtrate1. Part of the Filtrate-1 was heated for water evaporation at 85 °C and the resulting solid product was used for phase structure analysis. 2.4. Characterizations Powder X-ray diffraction (XRD) patterns were performed on an X’ Pert PRO MPD diffraction meter (PANalytical B. V. Netherlands) with Cu Kα radiation (λ = 0.15418 nm). The Beam voltage was operated at 40 kV and 40 mA. Chemical compositions of the adsorbent were measured by X-ray fluorescence (XRF, ZSX-100 e using Rh and Au excitation tubes). X-ray photoelectron spectroscopy (XPS) was measured using a PHI 500 spectrometer with Al Kα radiation. Nitrogen adsorption and desorption isotherms were measured on a Tristar-3000 analyzer (Micromeritics, USA) at −196 °C. Before the adsorption experiment, samples were degassed at 300 °C under vacuum for 6 h. The specific surface area (SBET) was determined by the Brunauer-Emmett-Teller (BET) method, and the total volume (Vtotal) was estimated at P/ P0 = 0.99. The pore size distribution was calculated from adsorption isotherms using the Barrett-Joyner-Halenda (BJH) approach. Temperature programmed desorption of ammonia (NH3-TPD) was carried out on a dynamic chemisorption analyzer (Micromeritics AutoChem 2920). Prior to the adsorption of NH3, samples were preheated at 550 °C under helium (He) stream for 0.5 h. After that, physically adsorbed NH3 was removed under He stream for 0.5 h. The samples were heated to 800 °C with a rate of 10 °C min−1 and signals of NH3 desorption were recorded with a thermal conductivity detector (TCD). FT-IR spectra of pyridine (Py-IR) adsorbed on samples were recorded on a Nicolet-6700 FT-IR spectrometer. The samples were evacuated at 300 °C for 3 h, followed by adsorption of pyridine at room temperature. Then they were evacuated at pyridine desorption equilibrium level at 150 °C. The spectra were recorded at room temperature. The concentration of Lewis acid sites was estimated based on the integral areas of bands at 1450 cm−1 using the extinction of coefficients (ɛL = 1.42 cm μmol−1) [34]. Scanning electron microscope (SEM) images and surface element distributions of samples were obtained on a JEOL JSM-7900F with an EDXA energy-dispersive spectrometer. Prior to the measurement, samples were treated by gold spaying. Transmission electron microscopy (TEM) micrographs were obtained using a JEOL JEM-2100. The atomic absorption spectrum of the sample was recorded on a CONTR AA 700 atomic absorption spectrometer.

2. Experimental 2.1. Materials and chemical agents The fresh and spent S-Zorb adsorbents were labeled as Fresh-S and Spent-S, respectively. The temporarily deactivated adsorbent after several desulfurization-regeneration cycles was extracted from the desulfurization reaction unit and labeled as Deac-S. The regenerated adsorbent extracted from regeneration setting was labeled as Reg-S. The 1110

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

2.5. RADS evaluation

Table 1 The surface element distributions of the Fresh-S and Spent-S adsorbents (wt. %).

The industrial gasoline was used to evaluate the desulfurization ability of the S-Zorb adsorbent. The RADS experiments were carried out in a continuous fixed-bed stainless-steel micro-reactor with an internal column diameter of 8 mm and a length of 250 mm. A total volume of 3 ml adsorbent was loaded into the reactor per run. Prior to the reaction, the adsorbent was reduced in-situ at a hydrogen atmosphere. After that, the gasoline was pumped into the reactor and the reaction was conducted under the conditions of 350 °C, hydrogen/oil feed volume ratio of 200, volumetric space velocity (VHSV) of 2 h−1 and 0.5 MPa. The liquid product was collected periodically and the sulfur content was analyzed on a Multi EA3100 sulfur analyzer. The sulfur removal efficiency of each adsorbent was determined according to the following formula:

R (%) =

C0 − Ct × 100 % C0

Elements

Fresh-S

Spent-S

Zn Ni Si Al O S C

33.7 33.2 2.9 6.5 23.7 – –

30.1 16.9 4.8 9.9 22.3 8.6 7.4

adsorbent also exhibits diffraction peaks of hexagonal NiS phase (JCPDS file no. 00-050-1791). The sulfidation of active components (Ni and ZnO) hinders their contacts with sulfur compounds, decreasing the desulfurization activity. Of note is that the ZnS phase is also observed in the Reg-S adsorbent. It means the incomplete oxidation of ZnS during the industrial regeneration process. In addition, intensities of the NiO peaks for the spent adsorbent increase and widths of these peaks narrow down indicating sintering of the NiO phase. The surface element distributions of the Fresh-S and Spent-S adsorbents are listed in Table 1. Both the Fresh-S and Spent-S adsorbents primarily contain elements of Zn, Ni, Al, Si, and O as expected. The Spent-S adsorbent contains additional S element which is from NiS, ZnS and adsorbed organosulfur compounds. Carbon is also detected whose content reaches 7.4 wt%. Carbon deposition can cover the active sites and then reduce the sulfur removal activity of the adsorbents. Fig. 2 shows the NH3-TPD profiles of the Fresh-S and Spent-S adsorbents. The fresh adsorbent has three peaks located at 145 °C, 260 °C and 643 °C, corresponding to weak acidity, mild acidity, and strong acidity, respectively. The peak area attributed to strong acid sites is not as large as that of weak and mild acid sites, which is expected because the strong acidity enhances carbon deposition during the RADS. By contrast, the NH3-TPD profile of the Spent-S adsorbent only has an intense peak at 685 °C. It does not result from NH3 desorption but decomposition of the carbon deposits [32], which is confirmed by the extremely weak signal in the pyridine adsorption FT-IR spectrum of the Spent-S adsorbent (Fig. S1). Thus there are almost no acid sites over the Spent-S adsorbent and the decrease of acid sites is also responsible for the deactivation of the adsorbents. The N2-adsorption isotherms of the Fresh-S, Deac-S and Spent-S adsorbents are shown in Fig. 3. All samples exhibit typical type-V adsorption isotherms according to the IUPAC. Hysteresis loops indicate the presence of mesoporous structure in the adsorbents. Table 2 shows that specific surface area (SBET) and total pore volume (Vtotal) of the adsorbents decrease in the order of Fresh-S > Deac-S > Spent-S. Compared with the Fresh-S adsorbent, the SBET and Vtotal of the spent adsorbent decrease by 65.4% and 59.6%, respectively. Besides the pore blockage by carbon deposition, the formation of metal sulfides is another key reason. The lattice parameters of the ZnS crystal (a = 0.3823 nm, c = 1.8744 nm) are much bigger than that of ZnO (a = 0.3250 nm, c = 0.5207 nm) [35]. A cell volume expansion occurs along with the transformation of ZnO into ZnS during the RADS, which may also cause the pore blockage and reduce the specific surface area and pore volume of the Spent-S adsorbent. The decrease of the surface area and pore volume reduces the sulfur capacity and hinders diffusion of the reactants and products, leading to deactivation of the adsorbents. TEM images of the Fresh-S and Spent-S adsorbents are compared in Fig. 4. The Fresh-S adsorbent exhibits a random stacking of nanoparticles with a loose structure (Fig. 4(a)). In comparison, the spent adsorbent reveals an agglomeration of the nanoparticles to form bigger and more compact particles with less wide opening pores. This is in good agreement with the XRD and N2 adsorption results. In brief, the main reasons for deactivation of the industrial S-Zorb adsorbents include the sulfidation and agglomeration of the active

(1) −1

where R is the sulfur removal efficiency (%), C0 (μg g ) is the sulfur concentration in the feedstock and Ct (μg g−1) is the sulfur concentration in outlet collected during the starting 1 h of the reaction. 3. Results and discussion 3.1. Deactivation reasons for the S-Zorb adsorbents XRD patterns of the Fresh-S, Deac-S and Spent-S adsorbents are shown in Fig. 1. The Fresh-S adsorbent only has typical diffraction peaks assigned to hexagonal ZnO phase (JCPDS file No. 01-089-7102) and cubic NiO phase (JCPDS file No. 03-065-2901). It indicates the predominant content of NiO and ZnO in the fresh adsorbent. In comparison, the diffraction peaks of ZnO become weak over the Deac-S adsorbent and disappear in the Spent-S adsorbent. New diffraction peaks assigned to rhombohedral Zn2SiO4 phase (JCPDS file No. 01-0792005) are observed over the Deac-S adsorbent. Intensities of these peaks increase over the Spent-S adsorbent. The above XRD findings indicate that the ZnO phase is transformed into the Zn2SiO4 phase during the RADS. As reported that the ZnO phase works for sulfur storage [33,35], the transformation of ZnO to the Zn2SiO4 reduces the sulfur storage capacity of the adsorbents. No pronounced changes for the Zn2SiO4 phase are observed between the Deac-S and Reg-S adsorbents. This implies high stability of the formed Zn2SiO4 which cannot be recovered to the ZnO phase during the regeneration process. The Spent-S and Deac-S adsorbents have XRD patterns assigned to the hexagonal ZnS phase (JCPDS file no. 01-089-2201). The Spent-S

Fig. 1. XRD patterns of the Fresh-S, Deac-S, Reg-S and Spent-S adsorbents. 1111

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

Fig. 2. NH3-TPD profiles of the Fresh-S (a) and Spent-S (b) adsorbents.

Fig. 5. The sulfur removal efficiency of the Fresh-S, Reac-S, Reg-S and Spent-S adsorbents.

Fig. 3. Nitrogen adsorption-desorption isotherms of the Fresh-S, Spent-S and Deac-S adsorbents.

components, carbon deposition and transformation of ZnO into inactive Zn2SiO4. The carbon deposits are removed, and ZnS and NiS can be recovered to ZnO and NiO respectively during the regeneration process. However, the Zn2SiO4 cannot be recovered to ZnO under the normal regeneration conditions. It consumes most of the active ZnO phases and can be identified as the key deactivation factor for the spent adsorbents. Thus, the transformation of the inactive Zn2SiO4 phase into the active ZnO phase plays a determining role in reactivating the spent adsorbents.

Table 2 Surface area and pore volume of the Fresh-S, Spent-S and Deac-S adsorbents. Samples

SBET (m2 g−1)

Vtotal (cm3 g−1)

Fresh-S Deac-S Spent-S

26 20 9

0.109 0.093 0.044

Fig. 4. TEM images of the Fresh-S (a) and Spent-S (b) adsorbents. 1112

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

Fig. 8. Magnified TEM image of the willow-leaf shaped ZnO in the Reac-S adsorbent.

system. The XRD patterns of the Reac-S and Reg-S adsorbents are compared in Fig. 6. The Reg-S adsorbent shows diffraction peaks of ZnO, ZnS, NiO, and Zn2SiO4. In comparison, only ZnO and NiO phases are observed over the Reac-S adsorbent. This can be confirmed by the XPS spectra which also indicate that the nickel and zinc species mainly exist as NiO and ZnO, respectively in the Reac-S adsorbent (Fig. S2). Besides, additional diffraction peaks (2θ = 31.7°, 59.2°, 65.1°, and 67.9°) assigned to ZnO are observed over the Reac-S adsorbent. This finding suggests that the Zn2SiO4 and ZnS phases are transformed into the ZnO phase during the reactivation process, which contributes to the increased desulfurization activity of the Reac-S adsorbent. The full width at half maximum (FWHM) of the ZnO peak at 2θ = 36.2° for the Reac-S (0.2007°) adsorbent is larger than that for the Reg-S (0.0836°) and Fresh-S (0.0836°) adsorbents. The Reac-S adsorbent has more ZnO phase with smaller grain size. It reduces the transfer limitation of sulfur to ZnO from Ni phase and results in higher desulfurization activity [2].

Fig. 6. XRD patterns of the Reac-S and Reg-S adsorbents.

3.2. Reactivation of the spent adsorbents The sulfur removal efficiency of the Reac-S, Fresh-S, Reg-S and Spent-S adsorbents is shown in Fig. 5. The sulfur removal efficiency of the Spent-S adsorbent is 11.5% while it increases to 61.8% (increase by 50.3%) after the reactivation process. Worth noting is that the Reac-S adsorbent exhibits superior sulfur removal efficiency than the Reg-S adsorbent (48.5%). This should be attributed to the additional acid-base coupling treatment on the spent adsorbent in comparison to the regeneration process. Most interestingly, the sulfur removal efficiency of the Reac-S adsorbent is only slightly lower than that of the Fresh-S adsorbent (67.0%). The results indicate that the desulfurization activity of the spent adsorbent can be restored effectively, making it possible to recycle the spent adsorbent into the desulfurization-regeneration

Fig. 7. TEM images of the Reg-S (a, b) and Reac-S (c, d) adsorbents. 1113

Chemical Engineering Journal 374 (2019) 1109–1117

0.030

(a) Samples

SBET (m2 g-1)

Reac-S

158

0.402

Reg-S

26

0.099

(b)

0.025

Vtotal (cm3 g-1)

dV/dD (cm g , nm)

300

3 -1

200

0.002

0.020

0.001

0.015

3

-1

Quantity Adsorbed (cm g STP)

Y. Lyu, et al.

100

Reg-S Reac-S

0.000

0

10

20

30

40

0.010

Reg-S Reac-S

0.005 0.000

0 0.0

0.2

0.4

0.6

0.8

1.0

0

10

20

30

40

Pore diameter (nm)

Relative Pressure (P/P0)

Fig. 9. Nitrogen adsorption-desorption isotherms (a) and PSD curves (b) of the Reac-S and Reg-S adsorbents. Inset table shows the SBET and Vtotal of the Reac-S and Reg-S adsorbents.

Fig. 10. SEM images of the Fresh-S (a, b), Spent-S (c, d) and Reac-S (e, f) adsorbents.

magnified TEM image, attributed to Zn2SiO4, ZnO, ZnS and NiO crystals. The Zn2SiO4 phase exhibits higher content than other components, in agreement with the strongest diffraction peaks of the Zn2SiO4 phase in Fig. 6. In comparison, Only ZnO and NiO crystals are identified in the Reac-S adsorbent (Fig. 7 (d)). Besides, the Reac-S displays a looser structure with worm-like mesopores (Fig. 7 (c)) probably result from

The newly formed ZnO can also contribute more sulfur acceptors to increase the sulfur capacity and service life of the reactivated adsorbent. The TEM images of the Reac-S and Reg-S adsorbents are shown in Fig. 7. Fig. 7(a, b) shows the compact microstructure of the Reg-S adsorbent. Four lattice fringes with different spacing are identified in the 1114

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

transformation of inactive zinc species into active ZnO phase during the reactivation process. The newly formed ZnO exhibits a much smaller particle size than the ZnO phase in the Fresh-S and Reg-S adsorbents, consistent with the XRD results (Fig. 6). The desulfurization performance of the Ni/ZnO based adsorbents is closely related to the nanostructure of ZnO [3,36,37]. Both the smaller particle size and the willowleaf shape of ZnO enhance the sulfur removal efficiency due to the easy sulfidation of ZnO and fast sulfur transfer from nickel sites to ZnO. The N2 adsorption isotherms and PSD curves of the Reac-S and RegS adsorbents are shown in Fig. 9. Both adsorbents have type-V adsorption isotherms. The Reac-s adsorbent has a smaller pore size compared with the Reg-S adsorbent. The BET surface area (158 m2 g−1) and pore volume (0.402 cm3 g−1) of the Reac-S adsorbent are five and three times higher than those of the Reg-S adsorbent (26 m2 g−1 and 0.099 cm3 g−1, respectively), consistent with the looser structure of the Reac-S adsorbent observed in the TEM images (Fig. 7). Besides, the Reac-S adsorbent even exhibits much higher BET surface area and pore volume than the Fresh-S adsorbent (26 m2 g−1 and 0.109 cm3 g−1, respectively). The morphology of the Reac-S adsorbent was characterized and compared with that of the Fresh-S and Spent-S adsorbents in Fig. 10. The Fresh-S and Spent-S adsorbents exhibit a dense structure and a relatively smooth surface. While for the Reac-S adsorbent attained from the acid-base coupling treatment of the Spent-S adsorbent, the surface becomes rough and a large number of surface holes are observed. This is consistent with the looser structure revealed by the TEM images of the reactivated adsorbent (Fig. 7(c)), both of which are attributed to the corrosion of alumina species by nitric acid during the acid treatment. This result also provides a direct explanation for the increase of BET surface area and pore volume of the Reac-S adsorbent. Fig. 11 shows the FT-IR spectra of pyridine adsorbed on the Reac-S, Fresh-S, Reg-S and Spent-S adsorbents; the concentrations of the Lewis acid sites are shown in Table S1. Compared with the Spent-S adsorbent (2.6 μmol/g), the Reg-S adsorbent (41.9 μmol/g) shows more Lewis acid sites. This is attributed to the removal of carbon deposits and organosulfur compounds from the surface of the adsorbents during the regeneration process. While the Reac-S adsorbent (126.6 μmol/g) has more Lewis acid sites than the Reg-S and Fresh-S (63.0 μmol/g) adsorbents. It is probably due to the defects generated by the reaction between nitric acid and alumina, which contributes more Lewis acid sites to the Reac-S adsorbent. Besides, the increased surface area of the Reac-S adsorbent also enhances the accessibility of the Lewis acid sites. The higher Lewis acid sites concentration favors the adsorption and decomposition of sulfur compounds over the Reac-S adsorbent.

Fig. 11. FT-IR spectra of the pyridine adsorbed on the Reac-S, Fresh-S, Spent-S and Reg-S adsorbents.

Fig. 12. XRD pattern of the sample obtained from the recrystallization of Filtrate-1.

the corrosion of alumina species by nitric acid. Such a porous structure accelerates the diffusion of sulfur compounds and sulfur-free products, leading to an increased desulfurization activity. A large number of nanoparticles in willow-leaf shape are observed in the Reac-S adsorbent, as labeled in dotted ellipses in Fig. 7(c). Fig. 8 reveals that the willow-leaf shaped nanoparticles have typical lattice fringes of ZnO crystals. These ZnO crystals are absent in the Fresh-S or Reg-S adsorbent. The disappearance of Zn2SiO4 and ZnS, as well as the newly formed ZnO crystals over the Reac-S adsorbent confirm the

3.3. Reactivation mechanism for the spent adsorbents In order to well understand the mechanism for the evolution of zinc species during the reactivation procedure, contents of nickel and zinc in the Filtrate-1 were measured by the atomic absorption spectroscopy (AAS). There were 136.8 mg zinc and 1.7 mg nickel in the filtrate when

Fig. 13. The reactivation mechanism of the spent S-Zorb adsorbents. 1115

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

1.0 g spent adsorbent was treated. Based on the element composition of the fresh adsorbent (Table S2), the quantity losses of nickel and zinc elements are 1.0 wt% and 35.0 wt%, respectively. This indicates the nitric acid mainly reacts with the zinc species and transform them into soluble zinc ions. The Filtrate-1 was recrystallized and XRD pattern of the attained solid is shown in Fig. 12. The diffraction peaks indicate the presence of ZnNO3(OH)·H2O, Zn3(OH)4(NO3)2 and ZnSO4·H2O. The nitrates of zinc are formed by the reaction of nitric acid and Zn2SiO4, as described in Eq. (2) [38]:

Acknowledgements

Zn2 SiO4 + 4H+ ⇆ Zn2 + + Si(OH)4

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.06.010.

This work was supported by the Key Technology Research and Development Program of Shandong Province (Grant No. 2018GSF117009), the Shandong Province Natural Science Foundation of China (Grant No. ZR2019MB029), and the National Natural Science Foundation of China (Grant No. 21376267). Appendix A. Supplementary data

(2)

The formed monomeric silica (Si(OH)4) may form gel or even amorphous silica precipitate via polymerization under strong acid condition [39]. The zinc sulfate is formed by oxidation of the ZnS with nitric acid. This point can be confirmed by the presence of ZnS in the Reg-S adsorbent and absence of the ZnS in the Reac-S adsorbent (Fig. 6). Compared with the regeneration process, the reactivation has an additional acid-base treatment. Thus, the disappearance of ZnS in the Reac-S adsorbent is attributed to the acid treatment on the spent adsorbent. The Zn2+ formed by the acid treatment react with OH– and precipitate as Zn(OH)2 subsequently which is converted into ZnO after calcination. The diffraction peaks of Ni2(NO3)2(OH)2·2H2O are observed in Fig. 12. The weak intensity of these peaks implies the low quantity of Ni2+ in the filtrate, consistent with the AAS result. Thus, there is almost no loss of the active components during the reactivation process, which can be confirmed by the similar compositions between the Fresh-S and Reac-S adsorbents (Table S2). Therefore, a mechanism for the reactivation of the spent adsorbents is disclosed, as shown in Fig. 13. Most of the carbon deposits and organosulfur compounds on the surface of the spent adsorbents are removed during the first calcination step. Meanwhile, the ZnS and NiS phases are oxidized and recovered into ZnO and NiO, respectively during this calcination process. When the calcined spent adsorbent is added into the aqueous solution of nitric acid, the nitric acid reacts with the Zn2SiO4 and residual ZnS on the surface of the spent adsorbent, transforming these solids into soluble Zn2+ in the aqueous solution. The Zn2+ are then precipitated in the form of Zn(OH)2 which deposit on the surface of the adsorbent when OH– are introduced into the solution. Finally, the Zn(OH)2 are converted into the new and willow-leaf shaped ZnO phase during the second calcination step. During the reactivation process, nitric acid also reacts upon Al2O3 which creates more mesopores in the Reac-S adsorbent. The wide and open pore structure accelerates the mass transfer of reactants and products, enhancing the sulfur removal rate. Defects generated by the acid treatment on the surface of adsorbents provide more adsorption sites for organosulfur compounds, increasing their decomposition efficiency.

References [1] R. Ullah, P. Bai, P. Wu, U.J. Etim, Z. Zhang, D. Han, F. Subhan, S. Ullah, M.J. Rood, Z. Yan, Superior performance of freeze-dried Ni/ZnO-Al2O3 adsorbent in the ultradeep desulfurization of high sulfur model gasoline, Fuel Process. Technol. 156 (2017) 505–514. [2] Y. Zhang, Y. Yang, H. Han, M. Yang, L. Wang, Y. Zhang, Z. Jiang, C. Li, Ultra-deep desulfurization via reactive adsorption on Ni/ZnO: the effect of ZnO particle size on the adsorption performance, Appl. Catal. B: Environ. 119–120 (2012) 13–19. [3] M. Gupta, J. He, T. Nguyen, F. Petzold, D. Fonseca, J.B. Jasinski, M.K. Sunkara, Nanowire catalysts for ultra-deep hydro-desulfurization and aromatic hydrogenation, Appl. Catal. B: Environ. 180 (2016) 246–254. [4] Q. Gu, G. Wen, Y. Ding, K.-H. Wu, C. Chen, D. Su, Reduced graphene oxide: a metalfree catalyst for aerobic oxidative desulfurization, Green Chem. 19 (2017) 1175–1181. [5] F. Ju, M. Wang, H. Luan, P. Du, Z. Tang, H. Ling, Reactive adsorption desulfurization of NiO and Ni0 over NiO/ZnO–Al2O3–SiO2 adsorbents: role of hydrogen pretreatment, RSC Adv. 8 (2018) 33354–33360. [6] X. Zeng, X. Xiao, Y. Li, J. Chen, H. Wang, Deep desulfurization of liquid fuels with molecular oxygen through graphene photocatalytic oxidation, Appl. Catal. B: Environ. 209 (2017) 98–109. [7] N.A. Khan, C.M. Kim, S.H. Jhung, Adsorptive desulfurization using Cu–Ce/metalorganic framework: improved performance based on synergy between Cu and Ce, Chem. Eng. J. 311 (2017) 20–27. [8] L. Wang, Q. Liu, C. Jing, J. Yin, N. Mominou, S. Li, Simultaneous removal of sulfides and benzene in FCC gasoline by in situ hydrogenation over NiLaIn/ZrO2-r-Al2O3, J. Hazard. Mater. 342 (2018) 758–769. [9] A. Fihri, R. Mahfouz, A. Shahrani, I. Taie, G. Alabedi, Pervaporative desulfurization of gasoline: a review, Chem. Eng. Process. 107 (2016) 94–105. [10] P. Tan, Y. Jiang, L.-B. Sun, X.-Q. Liu, K. AlBahily, U. Ravon, A. Vinu, Design and fabrication of nanoporous adsorbents for the removal of aromatic sulfur compounds, J. Mater. Chem. A 6 (2018) 23978–24012. [11] G.I. Danmaliki, T.A. Saleh, Effects of bimetallic Ce/Fe nanoparticles on the desulfurization of thiophenes using activated carbon, Chem. Eng. J. 307 (2017) 914–927. [12] G. Wang, Y. Wen, J. Fan, C. Xu, J. Gao, Reactive characteristics and adsorption heat of Ni/ZnO–SiO2–Al2O3 adsorbent by reactive adsorption desulfurization, Ind. Eng. Chem. Res. 50 (2011) 12449–12459. [13] A. Bazyari, Y. Mortazavi, A.A. Khodadadi, L.T. Thompson, R. Tafreshi, A. Zaker, O.T. Ajenifujah, Effects of alumina phases as nickel supports on deep reactive adsorption of (4,6-dimethyl) dibenzothiophene: comparison between γ, δ, and θ-alumina, Appl. Catal. B: Environ. 180 (2016) 312–323. [14] F. Subhan, S. Aslam, Z. Yan, L. Zhen, M. Ikram, R. Ullah, U.J. Etim, A. Ahmad, Ammonia assisted functionalization of cuprous oxide within confined spaces of SBA-15 for adsorptive desulfurization, Chem. Eng. J. 339 (2018) 557–565. [15] L. Huang, L. Yan, M. Tang, G. Wang, Z. Qin, H. Ge, Effect of pretreatment on the adsorption performance of Ni/ZnO adsorbent for dibenzothiophene desulfurization, ACS Omega 3 (2018) 18967–18975. [16] H.-J. Jeon, C.H. Ko, S.H. Kim, J.-N. Kim, Removal of refractory sulfur compounds in diesel using activated carbon with controlled porosity, Energy Fuel. 23 (2009) 2537–2543. [17] Y. Liu, Y. Pan, H. Wang, Y. Liu, C. Liu, Ordered mesoporous Cu-ZnO-Al2O3 adsorbents for reactive adsorption desulfurization with enhanced sulfur saturation capacity, Chinese J. Catal. 39 (2018) 1543–1551. [18] L. Huang, G. Wang, Z. Qin, M. Dong, M. Du, H. Ge, X. Li, Y. Zhao, J. Zhang, T. Hu, J. Wang, In situ XAS study on the mechanism of reactive adsorption desulfurization of oil product over Ni/ZnO, Appl. Catal. B: Environ. 106 (2011) 26–38. [19] R. Ullah, Z. Zhang, P. Bai, P. Wu, D. Han, U.J. Etim, Z. Yan, One-pot cation-anion double hydrolysis derived Ni/ZnO–Al2O3 absorbent for reactive adsorption desulfurization, Ind. Eng. Chem. Res. 55 (2016) 3751–3758. [20] J. Fan, G. Wang, Y. Sun, C. Xu, H. Zhou, G. Zhou, J. Gao, Research on reactive adsorption desulfurization over Ni/ZnO−SiO2−Al2O3 adsorbent in a fixed-fluidized bed reactor, Ind. Eng. Chem. Res. 49 (2010) 8450–8460. [21] I.V. Babich, J.A. Moulijn, Science and technology of novel processes for deep desulfurization of oil refinery streams: a review, Fuel 82 (2003) 607–631. [22] L. Huang, G. Wang, Z. Qin, M. Du, M. Dong, H. Ge, Z. Wu, Y. Zhao, C. Ma, T. Hu, J. Wang, A sulfur K-edge XANES study on the transfer of sulfur species in the reactive adsorption desulfurization of diesel oil over Ni/ZnO, Catal. Commun. 11

4. Conclusions An acid-base coupling method for reactivation of the industrial spent S-Zorb adsorbents was successfully developed. The formation of Zn2SiO4, carbon deposition as well as sintering and sulfidation of active sites cause decrease of desulfurization capacity of the adsorbents. The reactivation by the acid-base coupling method can result in the transformation of the inactive Zn2SiO4 into the active ZnO phase. The newly formed ZnO in the reactivated samples bears a new morphology and smaller particle size. The carbon deposits are removed, and the ZnS and NiS are converted to ZnO and NiO respectively during the calcination. In addition, a porous structure is reconstructed and more Lewis acid sites are created over the reactivated adsorbent. The reactivated adsorbent has a higher desulfurization efficiency than the regenerated adsorbent and exhibits a comparable activity to the fresh adsorbent in the gasoline desulfurization.

1116

Chemical Engineering Journal 374 (2019) 1109–1117

Y. Lyu, et al.

(2010) 592–596. [23] I. Bezverkhyy, O.V. Safonova, P. Afanasiev, J.P. Bellat, Reaction between thiophene and Ni nanoparticles supported on SiO2 or ZnO. In situ synchrotron X-ray diffraction study, J. Phys. Chem. C 113 (2009) 17064–17069. [24] A. Ryzhikov, I. Bezverkhyy, J.-P. Bellat, Reactive adsorption of thiophene on Ni/ ZnO: role of hydrogen pretreatment and nature of the rate determining step, Appl. Catal. B: Environ. 84 (2008) 766–772. [25] F. Ju, C. Liu, K. Li, C. Meng, S. Gao, H. Ling, Reactive adsorption desulfurization of fluidized catalytically cracked (FCC) gasoline over a Ca-doped Ni-ZnO/Al2O3–SiO2 adsorbent, Energy Fuel 30 (2016) 6688–6697. [26] M.A. Larrubia, A.D. Gutièrrez-Alejandre, J. Ramı ̀rez, G. Busca, A FT-IR study of the adsorption of indole, carbazole, benzothiophene, dibenzothiophene and 4,6-dibenzothiophene over solid adsorbents and catalysts, Appl. Catal. A: Gen. 224 (2002) 167–178. [27] D. Yi, H. Huang, X. Meng, L. Shi, Adsorption–desorption behavior and mechanism of dimethyl disulfide in liquid hydrocarbon streams on modified Y zeolites, Appl. Catal. B: Environ. 148–149 (2014) 377–386. [28] W.-D. Oh, J. Lei, A. Veksha, A. Giannis, W.-P. Chan, G. Lisak, T.-T. Lim, Ni-Zn-based nanocomposite loaded on cordierite mullite ceramic for syngas desulfurization: performance evaluation and regeneration studies, Chem. Eng. J. 351 (2018) 230–239. [29] A.M. Tsybulevski, O.P. Tkachenko, E.J. Rode, K.C. Weston, L.M. Kustov, E.M. Sulman, V.Y. Doluda, A.A. Greish, Reactive adsorption of sulfur compounds on transition metal polycation-exchanged zeolites for desulfurization of hydrocarbon streams, Energy Technol. 5 (2017) 1627–1637. [30] Y. Liu, H. Wang, Y. Liu, J. Zhao, C. Liu, Reactive adsorption desulfurization on Cu/

[31]

[32]

[33]

[34]

[35]

[36]

[37] [38] [39]

1117

ZnO adsorbent: effect of ZnO polarity ratio on selective hydrogenation, Energy Fuel 31 (2017) 9930–9938. X. Meng, H. Huang, L. Shi, Reactive mechanism and regeneration performance of NiZnO/Al2O3-diatomite adsorbent by reactive adsorption desulfurization, Ind. Eng. Chem. Res. 52 (2013) 6092–6100. F. Ju, C. Liu, C. Meng, S. Gao, H. Ling, Reactive adsorption desulfurization of hydrotreated diesel over a Ni/ZnO–Al2O3–SiO2 adsorbent, Energy Fuel 29 (2015) 6057–6067. L. Qiu, K. Zou, G. Xu, Investigation on the sulfur state and phase transformation of spent and regenerated S Zorb sorbents using XPS and XRD, Appl. Surf. Sci. 266 (2013) 230–234. C.A. Emeis, Determination of integrated molar extinction coefficients for infrared absorption bands of pyridine adsorbed on solid acid catalysts, J. Catal. 141 (1993) 347–354. R. Ullah, P. Bai, P. Wu, B. Liu, F. Subhan, Z. Yan, Cation–anion double hydrolysis derived mesoporous mixed oxides for reactive adsorption desulfurization, Micropor. Mesopor. Mater. 238 (2017) 36–45. Y. Liu, N. She, J. Zhao, T. Peng, C. Liu, Fabrication of hierarchical porous ZnO and its performance in Ni/ZnO reactive-adsorption desulfurization, Petrol. Sci. 10 (2013) 589–595. P. Bai, P. Wu, Z. Yan, J. Zhou, X.S. Zhao, Self-assembly of clewlike ZnO superstructures in the presence of copolymer, J. Phys. Chem. C 111 (2007) 9729–9733. B. Terry, A.J. Monhemius, Acid dissolution of willemite (Zn, Mn)2 SiO4) and hemimorphite (Zn4Si2O7(OH)2 H2O), Metall. Tran. B 14 (1983) 335–346. B. Terry, The acid decomposition of silicate minerals part II. Hydrometallurgical applications, Hydrometallurgy 10 (1983) 151–171.