Feng Jua,
Miao Wanga,
Hui Luanab,
Pengyu Duc,
Zhihe Tangb and
Hao Ling*a
aState Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China. E-mail: linghao@ecust.edu.cn
bResearch Institute of Safty & Environment Technology, China National Petroleum Corporation, Beijing 102206, China
cPetrochina Karamay Petrochemical Co. Ltd., Karamay, Xinjiang 834000, China
First published on 27th September 2018
Reactive adsorption desulfurization (RADS) of Fluidized Catalytically Cracked (FCC) gasoline on reduced and unreduced NiO/ZnO–Al2O3–SiO2 adsorbents was studied. Various characterizations such as powder X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), the H2/O2 pulse titration (HOPT), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) are used to evaluate the effects of hydrogen pretreatment of the adsorbents. XRD and HOPT results indicate that NiO is hard to be reduced to Ni0 under the conditions of RADS. H2-TPR shows that NiO might be reduced to Ni0 at the temperature of 598 °C, much higher than the temperature of RADS. The Ni 2p3/2 spectrum of Ni0 is not observed for the reduced adsorbent, but the main peak of Ni 2p3/2 of NiS is found for the spent adsorbent. The unreduced NiO/ZnO–Al2O3–SiO2 adsorbent performs a better desulfurization than the reduced adsorbent at the beginning of desulfurization process. NiO and Ni0 are assumed as the main active components and present a good desulfurization ability in RADS. Finally, a change in the RADS mechanism is presented and discussed.
Tawara et al. first investigated conventional HDS catalyst in the adsorptive HDS of kerosene, and found that the desulfurization performance of the oxidized Ni–Mo/Al2O3 catalyst was better than that of the sulfide Ni–Mo/Al2O3 catalyst.8 They used ZnO species as the catalyst supporter and found that the ZnO particles could adsorb the H2S which released from NiS by hydrogen. After that, Babich and Moulijin presented a RADS mechanism.9 They discovered that nickel selected and reacted with sulfur atom under hydrogen to form NiS, which consequently reacted with neighbouring ZnO to form ZnS and regenerated nickel.
The mechanism and operating conditions of RADS over Ni/ZnO-based adsorbent has been attracted more attention in recent studies.10–17 Bezverkhyy et al. studied the kinetics of thiophene reactive adsorption on Ni/ZnO.18,19 Their results indicated that the reaction between Ni/ZnO and thiophene consists of three steps. The first step is a rapid surface reaction between thiophene and the surface Ni atom to form Ni3S2. The next step is sulfur species reacting preferably with ZnO. The third step is thiophene molecules reacting with bulk Ni atoms, meanwhile H2S diffuses through ZnS layer to react with bulk ZnO. Zhang et al.20 reported the effect of ZnO particle size on the adsorptive desulfurization performance for Ni/ZnO adsorbent.20 The desulfurization activity and sulfur capacity of Ni/ZnO adsorbent with smaller ZnO particle sizes are much higher than that with larger sizes.
Normally, hydrogen pretreatment has been taken to reduce the NiO/ZnO-based sorbent to Ni/ZnO-based sorbent before the desulfurization process. Many studies confirmed that reduced Ni is the main active component interacting with sulfur at-om.21,22 However, there are few reports investigating the effect of hydrogen pretreatment on the RADS adsorbent, nor whether NiO could be the main active component for desulfurization. Bezverkhyy et al. compared the desulfurization performance of reduced Ni/ZnO adsorbent and unreduced NiO/ZnO.19 They found that NiO/ZnO adsorbents can be used in the reactive ad-sorption of thiophene without any reductive pretreatment. Moreover, the reduction of NiO/ZnO resulted in the formation of Ni–Zn alloyed particles and led to a decrease of the sulfidation rate in comparison with the unreduced sample. In our previous works the effects of dispersion of Ni species and acidity of adsorbent surface on the desulfurization of NiO/ZnO–Al2O3–SiO2 sorbents were discussed.23,24 It was noticed that hydrogen pre-treatment had little effect on desulfurization. In this work, reduced and unreduced NiO/ZnO–Al2O3–SiO2 adsorbents have been prepared to verify whether hydrogen pretreatment contributes to the desulfurization process. At last, some change of the RADS mechanism on NiO/ZnO–Al2O3–SiO2 adsorbent is discussed.
Density (20 °C) g cm−3 | Sulfur content μg g−1 | Nitrogen content μg g−1 |
---|---|---|
0.723 | 243.48 | 28.22 |
The supporter ZnO–Al2O3–SiO2 of adsorbent was prepared by coprecipitation method, and then Ni component was loaded on the supporter by impregnation method, shown in Fig. 1. A mixed aqueous solution of Al(NO3)3 and Zn(NO3)2 was dropwise added into a mixed solution of Na2CO3(0.2 mol L−1) and Na2SiO3(0.2 mol L−1) at a rate of 15 mL min−1 at a precipitation temperature of 20 °C, followed by aging at the same temperature for 2 h. Then, the precipitation was filtrated out and washed with a large quantity of deionized water to remove the residue sodium until the pH of the suspension is below 6.5.25–28 After that, the filter cake was dried at 120 °C in air for 12 h, and then calcinated at 500 °C in a muffle furnace in dry air for 4 h. The supporter ZnO–Al2O3–SiO2 was obtained.
Solutions with Ni(NO3)2 were mixed with the above supporter and stirred for 2 h. After that, a solution of Na2CO3(0.2 mol L−1) was added dropwise into the mixed solution. The precipitation is the precursor of the sorbent. Large amount of deionized water was used to wash away the residue sodium until the pH of the solution is below 6.5. The obtained filter cake was dried at 120 °C in a vacuum oven for 12 h, and then calcinated at 500 °C in a muffle furnace for 2 h. Finally, the adsorbent was screened to 120 mesh and kept in a sealed bag before usage.
Reduction | Temp./°C | 440 |
Hydrogen pressure/MPa | 2.0 | |
Reduction time/h | 2 | |
Adsorption, desulfurization | Temp./°C | 419 |
Hydrogen pressure/MPa | 2.9 | |
Weight hourly space velocity (MHSV)/h−1 | 10.84 | |
Mole ratio (H2/oil) | 0.3 | |
H2 volume/gasoline weight (mL g−1) | 90 | |
Purge | Temp./°C | 360 |
H2 flow (mL min−1) | 200 | |
Purge time/min | 20 | |
Regeneration | Temp./°C | 360![]() |
Regeneration time/min | 30 60 | |
Total pressure/MPa | 0.15 | |
Oxygen pressure/KPa | 3.0 | |
Flow (mL min−1) | 200 |
The sulfur removal efficiency of adsorbent is defined according to the following equation:29
![]() | (1) |
The breakthrough sulfur capacity is determined as follows.
![]() | (2) |
The crystalline structures of the adsorbents were character-ized through X-ray diffraction (XRD) by using a Bruker D8 Advance X-ray diffractometer with a Cu Kα = 0.154 nm monochromatized radiation source, operating at 40 kV and 100 mA.
The crystal lattice of the adsorbents was surveyed by JEM-2100 transmission electron microscope (TEM).
The dispersity and reducibility of Nickel were undertaken by using the H2/O2 pulse titration (HOPT) with a chemisorption analyzer Autochem II 2920(Micromeritics, USA).
Temperature programmed reduction (TPR) was surveyed by the analyzer Autochem II 2920(Micromeritics, USA).
X-ray photoelectron spectroscopy (XPS) was characterized by the multi-function photoelectron spectrometer (ESCALAB 250Xi).
After reduction, the height of peaks of ZnO (2θ = 36.2°, 63°) decrease and peak width broaden. However, the characteristic diffraction peak of NiO (2θ = 43.3°) become sharp and strong, which means hydrogen reduction affects the lattice of the NiO and ZnO. Hydrogen reduction makes the crystal of NiO become bigger and promotes the dispersion of Zn. The purpose of hydrogen pretreatment is to obtain reduced Ni0 before the desulfurization process. However, the reduced Ni0 cannot be detected by XRD, which means that the Ni component is difficult to be reduced in the form of NiO under 440 °C.
Shamskar et al. studied the reduction ability of NiO–Al2O3 catalyst.30 They prepared some NiO–Al2O3 catalysts with different calcination temperature (600–900 °C), and they found the reduction temperature increased with the calcination temperature increasing. The reduction temperature of NiO–Al2O3 catalyst under 600 °C calcination temperature is over 700 °C. This result is consistent with NiO/ZnO–Al2O3–SiO2 adsorbents, whose calcination temperature is 500 °C and reduction temperature is about 600 °C. According to Tang's research, the strong metal-support interactions (SMSI) between Ni and ZnO particles of Ni/ZnO could increase the reduction temperature of NiO in NiO/ZnO to 370 °C, indicating that there exists a strong interaction between NiO and the supporter making NiO much harder to be reduced.31
Fig. 4 shows the XRD patterns of adsorbents reduced in H2 for 3 h at 440 °C and 600 °C, respectively. There are two distinguished characteristic diffraction peaks at 2θ = 43.9° and 51.7°, attributed to AlNi3. The ZnO peaks of the adsorbent reduced at 600 °C become sharper and stronger than those of the adsorbent reduced at 440 °C. Under high reduction temperature, the crystal of ZnO grows bigger, but NiO is still not reduced to Ni0. Fig. 4 shows that NiO reacts with Al2O3 to form Ni–Al alloy. In NiO/ZnO adsorbents, hydrogen pretreatment leads to form the Ni–Zn allo-y, while, Ni–Zn alloy cannot be detected in NiO/ZnO–Al2O3–SiO2 adsorbent by XRD.19 Combined with H2-TPR results, it can be concluded that hydrogen reduces NiO and Al2O3 to form the Ni–Al alloy at 600 °C. The interactions between active component Ni and the supporter are strong. The existence of Al2O3 and ZnO supporter increases the reduction temperature of NiO.
SBET (m2 g−1) | Vtotal (cm3 g−1) | DA (nm) |
---|---|---|
149.01 | 0.33 | 5.90 |
Table 3 presents that the specific surface area is about 149 m2 g−1, and pore diameter is about 5.90 nm. It means the sorbents have a big specific surface area, and the pore diameter distribution is large enough for the adsorption of sulfur components, even refractory sulfur compounds like benzothiophene (BT).
Sample | D (%) | SANi (m2 g−1) | MCS (nm) |
---|---|---|---|
Adsorbent | 0.0236 | 0.1568 | 3582.4852 |
XPS spectra of Ni and Zn are presented in Fig. 7. Normally, Ni XPS spectra of Ni-bearing compounds consist of a main photo-peak and an associated satellite peak located at 6 to 8 eV higher binding energy than the main peak. Fig. 7(a) presents Ni 2p3/2 photo-peaks of the Ni compounds. In the figure, the Ni 2p3/2 spectrum 852.5 (±0.2) eV of Ni metal was not observed for the three adsorbents.32 This indicates that NiO cannot be reduced by hydrogen pretreatment. Peaks of 855.1 eV and 861.5 eV of the fresh adsorbents are attributed to the main peak and the satellite peak of the Ni 2p3/2 spectra of NiO, respectively. The peaks centered at bind energy of 873.0 eV and 879.7 eV are attributed to Ni 2p1/2 spectra of NiO.33 In the XPS spectra of the spent adsorbent, a peak at bind energy 853.3 eV appears after desulfurization. This is the main peak of Ni 2p3/2 in NiS. Besides, small increments of binding energy of Ni 2p3/2 photo-peaks were observed for the reduced adsorbent and the spent adsorbent. This suggests that the chemical interaction between NiO and the support ZnO–Al2O3–SiO2 is enhanced.
Fig. 7(b) shows the Zn 2p spectra of various adsorbents. The peak centered at bind energy of 1022.1 eV in fresh adsorb-ent is assigned to Zn 2p3/2, and the peak at bind energy of 1045.25 eV is attributed to Zn 2p1/2. After desulfurization, the bind energy of Zn 2p increases a little, indicating that sulfur atom reacts with ZnO to form ZnS. Furthermore, for S 2p spectra (Fig. 6), the peak at 162.4 eV is attributed to ZnS group, also indic-ating sulfur atom transfers to ZnS from ZnO.
The above analysis demonstrates that NiO is hard to be reduced to Ni0 by hydrogen pretreatment. Also, NiO/ZnO–Al2O3–SiO2 adsorbent without hydrogen pretreatment performs a better desulfurization than the adsorbent after hydrogen pretreatment. It means hydrogen pretreatment is not necessary for RADS process. NiO and Ni0 show a good synergistic desulfurization ability as the active components in the desulfurization process. Apparently, the mechanism of desulfurization on NiO is different from that on Ni. The mechanism of desulfurization on NiO/ZnO–Al2O3–SiO2 adsorbent needs to be further discussed.
Based on the above mechanism, reduced Ni is considered as the main active component, but it probably ignores the fact that NiO could directly react with thiophene compound. On the basis of the testing results in this work, a small change of the RADS mechanism is proposed and shown in Fig. 10. In this mechanism, firstly, the sulfur atoms from thiophene molecules is attached to the surface NiO molecules through a reaction which leads to the formation of NiS. This new reaction is proved by the H2-TPR results and the desulfurization performance shown in Fig. 9. The next step will form Ni0 in that a certain proportion of NiS could be reduced. Besides, a direct transportation of sulfur from NiS to ZnO probably happened, which directly form NiO and ZnS. The conversion of ZnO to ZnS is progress-ed by both the latter reaction and the immobilization of H2S releas-ed by the reduction of NiS. After that, with the existence of metallic nickel, along with nickel oxide, further feed of thiophene is treated not only in the way described in step 1 but also through the reaction of Ni0.
Theoretically, desulfurization will continue till all ZnO species contained in adsorbents have been converted to ZnS. If the actual situation is taken into account, it should not be ignored that carbon deposits accumulate on the surface as the reaction goes on, which may strongly prevent the progressing of the significant step 2. After being placed in the mixed gas of 1% oxygen and 99% nitrogen at a certain temperature for a few hours listed in Table 2, carbon deposits on used adsorbents can be burned out.23 Usually those regenerated sorbents can serve as relatively fresh adsorbents to participate in desulfurization continuously.
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