Article pubs.acs.org/EF
Reactive Adsorption Desulfurization on Cu/ZnO Adsorbent: Effect of ZnO Polarity Ratio on Selective Hydrogenation Yaqing Liu, Hongying Wang, Yunqi Liu,* Jinchong Zhao, and Chenguang Liu* State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China University of Petroleum (East China), 66 West Changjiang Road, Qingdao, Shandong 266580, P. R. China S Supporting Information *
ABSTRACT: The desulfurization activity and selective hydrogenation of Cu/ZnO adsorbents on the different polarity ratios of ZnO as supports was investigated in reactive adsorption desulfurization. The ZnO particles were synthesized by the hydrothermal process, and CuO/ZnO adsorbents were synthesized by incipient impregnation method. The structure and morphology of the ZnO and CuO/ZnO were characterized by X-ray diffraction (XRD), N2 adsorption−desorption, X-ray photoelectron spectra (XPS), scanning electron microscope/selected area electron diffraction (SEM/SAED), transmission electron microscopy (TEM), and temperature-programmed reduction (TPR). The surface area and polarity ratio of ZnO supports were controlled by the calcination temperature and concentration of P123, respectively. More reactive activity sites were provided by the high surface area of ZnO supports, thus improving the desulfurization activity. The polarity ratio of ZnO may strongly influence the hydrogenation reactions of olefins. The selective hydrogenation increased with the value of polarity ratios.
1. INTRODUCTION New environmental regulations, regarding the sulfur content in products, have forced researchers to develop some efficient technologies for producing clean fuels.1,2 Ultradeep desulfurization of gasoline has become an increasingly important subject in the world. However, it is trouble for the traditional hydrodesulfurization (HDS) to remove the sulfur content of gasoline to below 10 mg/L for fuels and still keep the high octane number.3 Olefins have a high content in gasoline, which are important for the enhancement of the octane number of the product. However, olefins can be easily saturated in the traditional HDS reaction process. Therefore, the selective HDS of gasoline with the high sulfur removal level and limited olefin hydrogenation (HYD) is an optimum strategy in the deep removal S-compounds process.4 Nowadays, some new methods including adsorption,5,6 oxidation,7 extraction,8 and biodesulfurization 9 have been developed for the deep ultradeep desulfurization. One of the technologies, the reactive adsorption desulfurization process (RADS), is very a great challenge to develop adsorbents which have high desulfurization activity, high selectivity, and high sulfur capacity. In the RADS process, the S-compounds are converted to H2S and hydrocarbons, and then these H2S are reacted with ZnO to form ZnS simultaneously.10,11 Recently, the new S-Zorb process has been developed by the Conoco Phillips Petroleum Co. to produce the low sulfur gasoline; however, the common technology using Ni-based adsorbent catching the sulfur compounds will inevitably react with olefins, leading to loss of the octance number in the presence of hydrogen. Besides, there are some drawbacks in the RADS processes. For example, the adsorbent may reduce the sulfur capacity in the frequent regeneration process and so on. Therefore, introduction of a novel RADS adsorbent and technology is a new solution to solve the drawbacks of the tradition S-Zorb processes. Over the years, the high performance desulfurization technology and catalyst12 have been designed and © XXXX American Chemical Society
developed to avoid the loss of octane number by researchers. Zhang13 reported that ZnO-active carbon adsorbents were modified with Cu, which the sulfur mass fraction of product was lower than 10 μg/g and the octane number lose only 0.3 unit. Wang et al.14 reported that the Cu-based catalysts make it possible to be the second-generation catalysts for desulfurization, which was calculated by the density function theory (DFT). At present, the Cu/ZnO catalysts were used in many fields, such as methanol steam reforming15 and so on. Currently, the metal oxides were good catalyst support for various reactions, which may influence the activity and the selectivity. For instance, Jung et al. investigated that the Cu loaded the different ZrO2 phase influence the activity and selectivity for methanol synthesis from CO and H2.16 In addition Boucher et al. observed that the gold supported on the oxygenrich ZnO (001) surfaces shows higher activity for the steam reforming of methanol.17 Nowadays, several adsorbents such as Cu-ZnO, Ni−Al2O3, Ni-SiO2, Ni-ZnO, and Ni-SBA-15 have been used for removing the S-compounds.18 Compared with other metal oxides, ZnO is the best sulfur absorptive component and supported in the RADS.19 The role of ZnO, as a very important semiconductor, has been widely used in the catalytic process. The particle sizes, quantum confinement,20 and morphology of ZnO may influence the catalytic performances.21 The wurtzite crystalline structure of ZnO has unbalanced charges either terminated in Zn2+ or O2−, according to Zn-ZnO(0001) and O-ZnO(000−1) polar planes, respectively.22 The intensities of the polar and nonpolar planes of ZnO (I(002)/I(100)) are called the polarity ratio.23 Many studies have reported on the special catalytic performance of the ZnO on polar and nonpolar facets.24 Received: July 5, 2017 Revised: August 23, 2017 Published: August 23, 2017 A
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels The oxygen vacancies on both polar and nonpolar facets play a key role in the ethanol reactions.25 In this paper, we used copper-based adsorbents instead of the nickel-based adsorbents in order to solve the selective hydrogenation and regeneration problems. We report that the Cu/ZnO adsorbents have high activity, good stability, and good selective hydrogenation. Moreover, the role of ZnO supports surface area and polarity ratio in Cu/ZnO adsorbents has been studied by reaction rate constants on the reactive adsorption desulfurization. The different polarity ratios of ZnO supports were prepared and used as supports of Cu/ZnO adsorbents. The structure and morphology of the ZnO and CuO/ZnO were characterized by XRD, SEM, XPS, TEM/SAED, N2 adsorption−desorption, and TPR. The desulfurization activity and selectivity of the Cu/ZnO adsorbents was studied. It was found that ZnO particles with high surface area could have a good desulfurization activity. Moreover, the polarity ratio of ZnO supports may influence the reducibility of copper, and the oxygen vacancies were exposed on the (001) polar facet in the reduction process, so that the high polarity ratio of ZnO shows the higher selective hydrogenation.
xH =
gasoline (wt %) and CS and CH are the thiophene and cyclohexene contents in the products (wt %). The breakthrough sulfur capacity was determined as follows:
qbreakthrough =
calcined temperature
375
ZnO375−10 ZnO375−0 ZnO350−10 ZnO350−0
350
∫0
t
(cs0 − cs) dt
(4)
H kHYD =−
FH ln(1 − xH ) W
kTRADS
(5)
kHHYD
and are the pseudo-first-order reaction constants of where thiophene HDS and cyclohexene HYD (mol g−1 h−1), respectively, xT and xH are the thiophene and cyclohexene conversions (%), respectively, FT(H) is the reactant molar flow (mol h−1), and W is the weight of the adsorbent (g). The selectivity factor was calculated using the following equation:28 seclectivity factor =
kRADS kHDY
(6)
2.5. Materials Characterizations. Samples were characterized by XRD technique to get information about the structural properties of the ZnO and CuO/ZnO, using the Pannlytical with Cu Kα at a scan rate of 2°/min. Diffraction lines of 2θ between 5° and 75° were taken to identify the crystallite phase. N2 adsorption−desorption experiments were performed on a chemBET 2000 instrument (Quantachrome, U.S.A.). Thermogravimetry (TG) was used to investigate the thermal behaviors of the as-synthesized ZnO precursors. H2-TPR was carried out to assess the reducibility of the copper precursor on the different polarity ratio ZnO supports. The number of surface metallic copper atoms was determined by N2O oxidation and followed H2 titration. The dispersion and the area of surface Cu were calculated according to equations reported by Van Der Grift et al.,29 which are shown as follows: Reduction of all copper atoms: CuO + H 2 → Cu + H 2O,
hydrogen consumption = X1
Reduction of surface copper atoms only: Cu 2O + H 2 → 2Cu + H 2O,
2.3. Preparation of CuO/ZnO Adsorbents. The CuO/ZnO adsorbents were used by impregnation with Cu(NO3)2·3H2O, and the amount of copper content was calculated to reach the metal loading of 2%. The products were dried at 100 °C for 12 h and calcined at 350 °C. 2.4. Reactive Adsorption Desulfurization Tests. A mixture of thiophene (107.3 μg/g), cyclohexene (20 wt %), and n-heptane was used as a model gasoline. The catalytic performances of Cu/ZnO adsorbents were evaluated in a fixed bed reactor. The CuO/ZnO adsorbent was loaded into the constant-temperature zone of the reactor. The CuO/ZnO adsorbents were in situ reduced in the reactor before the reaction. The reaction conditions are H2/feed rate of 200, press of 1 MPa, and LHSV of 2 h−1. The activity of the adsorbents was evaluated by the RADS (removal thiophene) and HYD conversions (removal cyclohexene): xT =
CS0 − CS CS0
× 100
(3)
FT ln(1 − x T) W
T kRADS =−
Table 1. Properties of ZnO Samples, (ZnCT‑py, CT Calcined Temperature, and Py Molar Ratio (Zn2+:P123) sample name
v 1000m
where qbreakthrough is the breakthrough sulfur capacity of the adsorbent (mg/g), v is the feed volumetric flow rate (mL/min), c0s is the initial sulfur content in the feed (mg/L), cs is the sulfur content in the product, and m is the weight of the adsorbents (g). The breakthrough time was defined as the RADS time when the sulfur concentration of effluent desulfurized feed exceeded 20 μg/g. The rate constants of the pseudo-first-order reaction of thiophene RADS (kTRADS) and cyclohexene HYD (kHHYD) were determined using the following equations:27
2.1. Chemicals. Zinc acetate dehydrate (Zn(CH3COO)2·2H2O), urea (CO(NH2)2), glacial acetic acid (CH3COOH), copper nitrate (Cu(NO3)2·3H2O), and block copolymer poly(ethylene glycol)-blockpoly(propylene glycol)-block−poly(ethylene glycol) (Pluronic P123, PEG-20-PPG-70-PEG-20) were used without further purification. 2.2. Preparation of ZnO Supports. The hydrothermal process was carried out by a modified method as detailed elsewhere.26 A total of 1.097 g of zinc acetate dehydrate, 6.006 g of urea, and 3 g of P123 were mixed in deionized water. The mixture was stirred under ambient conditions, and the pH of the solution was mixed to 5.0 ± 0.2 with glacial acetic. Then the solution was transferred into Teflon lined stainless steel autoclaves at 90 °C for 24 h. The white solid products were centrifugalized, washed with deionized water and ethanol more than three times, and finally dried in air. The products were calcined in a muffle furnace at 375 and 350 °C for 30 min. Table 1 shows the ZnO samples prepared. The TG curves show the complete removal of ZnO precursors at calcined temperature and are shown in Figure S1.
range
(2)
where C0S and C0H are the thiophene and cyclohexene contents in the model
2. EXPERIMENTAL SECTION
parameter studied
C H0 − C H × 100 C H0
hydrogen consumption = X 2
The dispersion and the area of surface Cu was calculated as
⎛ X ⎞ D = ⎜2 2 ⎟ × 100% ⎝ X1 ⎠
(7)
S = 2X 2Nav /(X1MCu1.4 × 1019)
(8)
Nav is Avogadro’s constant, MCu is the relative atomic mass of copper (63.46 g/mol), and 1.4 × 1019 is the number of copper atom of per square meter, because the average surface area of copper atom is assigned as 7.11 × 10−2 nm2. The average volume-surface diameter can be calculated as follow:
d = 6/(SρCu ) ≈ 0.5X1/X 2(nm)
(9)
Scanning electron microscope (SEM) analysis was carried out on a FEI Quanta 200 instrument. The TEM images of the samples were taken
(1) B
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels using a JEOL JEM-2100 UHR microscope. X-ray photoelectron spectroscopy (XPS) was performed on a VGESCALAB II spectrometer using an Al Kα (1486.6) photo source. The binding energies of the elements on the surface of the catalyst were corrected by using the carbon C 1s value, 285 eV, as an internal standard. Hydrocarbon group compositions of the model gasoline were determined by an Agilent 7890N gas chromatograph with a PONA capillary column. Quantitative analysis of the total sulfur concentration in model gasoline was determined by Multi EA 3100 S/N trace analyzer. The detection range is 0.1−1000 mg L−1.
3. RESULTS AND DISCUSSION 3.1. Characterization of ZnO Supports and Cu/ZnO Adsorbents. 3.1.1. X-ray Diffraction (XRD). Crystallinity and phase characteristics of the ZnO supports were concluded by XRD, and the patterns are shown in Figure 1. The ZnO samples
Figure 2. XRD patterns of the different polarity ratio of ZnO supports on CuO/ZnO.
2θ = 43.3° and 50.4° (JCPDS, No. 085-1326). The polarity ratios of ZnO have not changed in the reduced process. 3.1.2. N2 Adsorption−Desorption. The N2 adsorption/ desorption isotherms exhibit a type II isotherm with a hysteresis loop, demonstrating their mesoporous characteristics in Figure 3. Table 2 shows the surface area BET, microspore volume, pore diameter, and polarity ratio results for the ZnO with the different calcination temperature and the amount of the P123. It can be seen that the BET surface area of the ZnO increases from 48 to 61 m2 g−1 with adding P123 at the calcination temperature of 375 °C. Meanwhile, the pore volume of the ZnO also increases from 0.23 to 0.27 cm3 g−1. The same trend is seen at the calcination temperature of 350 °C, where the surface area increases from 56 to 69 m2 g−1. Adding P123, the surface area and the pore volume of ZnO samples increase at the same specific calcination temperature. 3.1.3. Morphology of the ZnO Supports. SEM was used for observing the morphology of the ZnO samples. The SEM images of the ZnO samples synthesized with different amounts of P123 molar ratios and different calcined temperatures are depicted in Figure 4. All samples have a flower-like morphology assembled from nanosheets. The single microflower was detailed observation that nanosheets are by self-assembly and the structures resemble a carnation, which grow radially from the center outward. The histograms of the thickness of nanosheet distributions and the HRTEM images of the thickness of nanosheets are shown in Figures 5 and 6. Lattice fringes of 0.26 and 0.28 nm are determined as d002 and d100, respectively. HRTEM investigates that the thicknesses of the ZnO nanosheets show the (001) facet, and the surfaces of the ZnO nanosheets show the (100) facet. The histograms were measured by taking about 100 pieces of nanosheets in TEM images. As a result, the differences on the thickness distributions of nanosheets could be influence by the polarity ratio. While the growth direction of ZnO nanoparticles is partial to the (001) polar facet, the thickness of the ZnO nanosheets slowly increased from 7.43 to 10.23 nm. In order to further research the polarity ratio, the average thickness and the standard deviation were calculated. The average thickness of the nanosheet can be calculated as follow:
Figure 1. XRD patterns for the different polarity ratio ZnO supports.
showed three diffraction peaks at 31.77°, 34.42°, and 36.26°, which are attributed to the (100), (002), and (101) planes, respectively. It is obvious that the diffraction intensity of the (002) polar plane and the (100) nonpolar plane are clearly different from each other. The intensities of the polar and nonpolar planes of ZnO (I(002)/I(100)) are called the polarity ratio. The polarity ratios of ZnO375−10, ZnO375−0, ZnO350−10, and ZnO350−0 are 1.10, 0.79, 0.93, and 0.59, respectively. The crystal diameters of (100), (002), and (101) were calculated by the Scherer equation, and the results are shown in Table S1. It is significant that the growth direction of ZnO supports changed by adjusting the P123. It is clear that the P123 strongly influence the growth habit and the polarity raito of ZnO samples. The higher polarity samples were added to the P123, which indicates that P123 enhances the polarity ratio of the ZnO materials. The samples with P123 are elongated along the [001] direction and preferentially exposure of polar facets.26 On the contrary, the samples without P123 are elongated along the nonploar direction. The XRD patterns of CuO/ZnO samples are displayed in Figure 2. The CuO are attributed to diffraction peaks at 2θ = 35.5° and 38.7° (JCPDS, No. 089-5899), and the polarity ratios of ZnO supports have not changed by impregnating copper, which the exposure planes keep the initial preferential status. All of the reduced Cu/ZnO adsorbents and spent Cu/ZnO adsorbents are shown in Figures S2 and S3, respectively. The Cu phase are attributed to diffraction peaks at
l̅ = C
(l1 + l 2 + ··· + ln) n
(10) DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. N2 adsorption−desorption isotherms (A) and pore distributions (B) for the different polarity ratios of ZnO supports.
The average thickness and standard deviation are calculated by eqs 10 and 11. The average thicknesses of the ZnO375−10, ZnO375−0, ZnO350−10, and ZnO350−0 nanosheets are 10.56, 9.87, 10.35, and 7.35 nm, respectively. The standard deviations are 59%, 47%, 51%, and 48%, respectively. The crystal growth habit of ZnO was modified by adding P123 to selective adsorption on the polar planes,30 which the ZnO always preferred to be hexagonal with the crystals elongated along the c-axis. The major diffraction rings match well with the SAED patterns (Figure S4) of ZnO370−10, ZnO370−0, ZnO350−10, and ZnO350−0, respectively. 3.1.4. XPS Analysis of ZnO Supports. The surface properties of ZnO supports were characterized by the XPS analysis in Figure 7. The O 1s peaks centered at 530.1 and 531.5 eV and can be fitted by the Gaussian equation. The peak of 530.1 ± 0.3 eV is described as O2− ions on the wurtzite structure of a hexagonal
Table 2. BET Surface Area, Pore Volume, BJH Pore Diameter, and Polarity Ratio of ZnO Supports sample
surface area (m2/g)
pore volume (cm3/g)
pore diameter (nm)
polarity ratio (I(002)/I(100))
ZnO375−10 ZnO375−0 ZnO350−10 ZnO350−0
61 48 69 56
0.27 0.23 0.28 0.23
15.50 16.71 14.00 14.75
1.10 0.79 0.93 0.59
where l ̅ is the average thickness of the nanosheet and n is the number of the nanosheets. The standard deviation can be calculated as follow: n
S=
∑i = 1 (li − l ̅ )2 n
(11)
where S is the standard deviation of the samples.
Figure 4. SEM of the different polarity ratios ZnO, (A) ZnO375−10, (B) ZnO375−0, (C) ZnO350−10, and (D) ZnO350−0. D
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 5. Histograms of thickness of the nanosheets distributions (A) ZnO375−10, (B) ZnO375−0, (C) ZnO350−10 and (D) ZnO350−0.
Figure 6. HRTEM images of thickness of the nanosheets distributions (A) ZnO375−10, (B) ZnO375−0, (C) ZnO350−10, and (D) ZnO350−0.
Zn2− ion array. The oxygen defect regions with the matrix of ZnO are centered at 531.5 ± 0.3 eV. The results of O 1s peaks are
shown in Table 3. The concentrations of oxygen defect are 32%, 25%, 28%, and 23%, respectively. It is clearly shown that the E
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. O 1s spectra of the ZnO supports.
Table 3. XPS of the O 1s Peak for the ZnO Supports with the Different Polarity Ratios samples
peak position
area (%)
peak position
area (%)
ZnO375−10 ZnO375−0 ZnO350−10 ZnO350−0
530.1 ± 0.3 530.2 ± 0.3 530.0 ± 0.3 530.2 ± 0.3
68 75 72 77
531.5 ± 0.3 531.3 ± 0.3 531.4 ± 0.3 531.6 ± 0.3
32 25 28 23
oxygen defect regions increased with the polarity ratio, indicating increasing the polarity ratio to enhance the number of the oxygen defects. The different polarity ratio of ZnO shows the different surface properties to influence the reaction activities. 3.1.5. Temperature-Programmed Reduction. The reducibility of the copper species is always an indication to measure the interaction between copper and ZnO. So, any changes of the TPR profiles were interpreted as the interaction of copper and ZnO supports. TPR profiles of Cu/ZnO adsorbents with different polarity ratios of ZnO supports and the adsorbents calcined at 350 °C are shown in Figure 8. The broad reduction profiles with shoulders were found at the temperature around of 200−350 °C. The peaks α and β can be attributed to a stepwise reduction of copper species from Cu2+ to Cu+, closely followed by the reduction from Cu+ to Cu0.31−34 The Gaussian fitting was used for further investigating the properties of samples and the
Figure 8. TPR profiles of CuO/ZnO samples of different polartity ratios of ZnO as supports.
results of peak position and relative concentration are shown in Table 3. Moreover, CuO/ZnO with the different polarity ratios of ZnO supports show the different fitting peaks areas in the reduction process, which copper loaded on the high polarity ratio F
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels of ZnO supports might be difficult reduced at the same temperature.35 The N2O oxidation and following H2 titration were used for detecting the dispersion and average diameter of copper species in Table 4.The dispersion of copper supporting on the different Table 4. Results of TPR and Cu Dispersion of Cu/ZnO Samples with Different Polarity Ratios of ZnO Supports TPR peak position (°C) and concentration (%)
a
Cu dispersion
sample
peak α
peak β
dispersion (%)a
Cu particle (nm)b
Cu/ZnO375−10 Cu/ZnO375−0 Cu/ZnO350−10 Cu/ZnO350−0
260 (23.1) 255 (45.9) 262 (35.4) 257 (79.4)
293 (76.9) 287 (54.1) 293 (64.6) 287 (20.6)
11.9 ± 6 11.2 ± 6 11.9 ± 6 11.8 ± 6
8.4 8.9 8.4 8.4
Figure 9. RADS performance of Cu/ZnO with the different polarity ratios of ZnO particles; obtained for a hydrogen pressure of 1 MPa, a gas to liquid ratio of 200, LHSV of 2 h−1, and temperature of 300 °C.
Calculated by eq 7. bCalculated by eqs 8 and 9.
polarity ratio of ZnO375−10, ZnO375−0, ZnO350−10, and ZnO350−0 are 11.9%, 11.2%, 11.9%, and 11.8%, respectively. The average diameters of copper supporting on the different polarity ratios of ZnO375−10, ZnO375−0, ZnO350−10, and ZnO350−0 are 8.4, 8.9, 8.4, and 8.4 nm, respectively. It demonstrates that the dispersion and average diameter have not changed with the polarity ratio of ZnO particles. The dispersion and average diameter of copper in catalysts might be attributed to their surface area. In theory, the same weight loadings and the similar dispersion have approximately the same hydrogen consumption. However, the observed hydrogen consumption stoichiometries indicate the different hydrogen consumption at the same reduced temperature in each of the adsorbents. The TPR results evidence that the interaction of copper and ZnO in the Cu/ZnO adsorbents is obviously influenced by the polarity ratio of ZnO supports. 3.2. Reactive Adsorption Desulfurization Results. Before the kinetic experiment, the fresh adsorbent was stabilized by treating the FCC gasoline for about 800 h under conditions similar to these used in the experiments: hydrogen pressure of 1 MPa and a constant temperature of 300 °C. The properties of FCC gasoline and the product are list in Table S2. The breakthrough curve is shown in Figure S5. The breakthrough time was 720 h, and the adsorption amount of sulfur was 68.4 mg/g by eq 3. The RADS of performance of Cu/ZnO is good stability and activity. To compare the RADS performance of copper loaded on the different polarity ratio of ZnO supports, the RADS testing was carried out in a fix-bed micro reactor with model gasoline and the results are shown in Figure 9. The sulfur content in the liquid increase from 103 mg/L to less than 20 mg/L, which prove that Cu/ZnO have high desulfurization activities. The ZnO particles with similar surface areas reveal similar RADS activities. It is implied that the RADS activities are related to the particles size of the ZnO3 and the dispersion and average diameter of copper. The olefins inevitably are in saturation with hydrogen. The olefins contents maintain about 17.3, 16.4, 16.7, and 15.2 wt %, respectively. It indicates that the copper loaded the different polarity ratios ZnO show the difference of the olefins saturation. The kinetic activity of RADS depending on the ZnO surface area was investigated on the Cu/ZnO in RADS. The RADS process was treated as a pseudo-first reaction in RADS. Table 5 shows the Cu/ZnO adsorbents on the desulfurization performance. The rate constants of thiophene decrease in the following order: Cu/ZnO350−10 > Cu/ZnO375−10 > Cu/ZnO350−0 > Cu/ZnO375−0.
Table 5. Rate Constants in the RADS of Thiophene and in the HYD of Cyclohexenea k × 105 (mol h−1 g−1) samples
xT
xH
kRADSb
kHYDc
selectivity index (S)d
Cu/ZnO375−10 Cu/ZnO375−0 Cu/ZnO350−10 Cu/ZnO350−0
9.3% 9.1% 9.5% 9.2%
4.3% 19.2% 14.7% 23.1%
24.80 23.17 25.35 23.89
35.36 153.20 121.52 184.20
0.70 0.15 0.21 0.13
a
Obtained for hydrogen pressure: 1 MPa; gas to liquid ratio: 200; LHSV: 2 h−1; temperature: 300 °C. bCalculated by eq 4. cCalculated by eq 5. dCalculated by eq 6.
Comparison of the rate constants of the RADS, the higher surface area of ZnO particles clearly increases RADS activity. The textural structures of ZnO samples may influence the results of desulfurization activity.36 In the RADS process, the role of ZnO could be as S-acceptor and transform into the ZnS.10 It is the main reason for the desulfurization activity to depend on the surface area of ZnO; the larger surface area and smaller particles of ZnO can provide the lower resistance in a nucleationcontrolled sulfidation on the ZnO surface.37 Moreover, the more active sites for thiophene decomposition in hydrogen were provided by the higher dispersion and smaller particles of copper. The polarity ratio of ZnO how to influence the hydrogenation reaction of olefins was further discussed. The RADS properties of all of the Cu/ZnO adsorbents were evaluated with reactive adsorption experiments and the kinetic results are given in Table 4. It can be seen that the rate constant of cyclohexene on these adsorbents decreases in the following order: ZnO350−0 > ZnO375−0 > ZnO350−10 > ZnO375−10. The adsorbents with different polarity ratios show the largest difference in the HYD of cyclohexene. The polarity ratio of ZnO influences the hydrogenation reactions of olefins. In the literature,38 the strong metal support interaction (SMSI) between copper and ZnO is known to play a key role. The different strong interactions between copper and the different polarity ratio ZnO supports were evidenced by the TPR results, which facilitates the different ability on reducibility of copper oxide leading to the different hydrogen reactions of olefins. The adsorbent selectivity index S was an important index to evaluate the selectivity of the reaction. The larger S value showed G
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 10. Structure of ZnO on a polar facet (100) and nonpolar facet (001).
the Fundamental Research Funds for the Central Universities (Grant No. 15CX06051A).
higher selectivity, which is shown in Table 5. The results imply that the selective hydrogenation was related to the polarity ratio of ZnO samples, while the ZnO supports have similar surface areas. The structure of ZnO was described with the software of the Diamond on the polar (001) facet and nonpolar (100) facet in Figure 10. Compared with the nonpolar (100) facet, the polar (001) facet exposes more oxygen atoms. The results agreed with the XPS analysis. The copper and ZnO supports have different interactions in the reduction process. The more polar à dsorbent Cu/ZnOAC‑375 has a higher selectivity, which can supply more sites of reactivity at the Cu/ZnO polar interface.
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4. CONCLUSIONS The properties of ZnO, such as the surface area and polarity were studied on the desulfurization activity and hydrogenation activity in Cu/ZnO adsorbents for reactive adsorption desulfurization. The ZnO surface area was adjusted by the calcination temperature of ZnO precursors. Meanwhile, the polarity ratio was controlled by the concentration of P123. The good dispersion and small copper particles depend on the higher surface area of ZnO, which have a higher desulfurization activity of Cu/ZnO adsorbents. The high dispersion of copper particles provides more reactive activity sites and in favor of transforming the ZnO to ZnS. Moreover, increasing the polarity ratio of ZnO supports could offer more selective sites at the polar facet in RADS.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01935. Additional figures (Figures S1−S5) and tables (Tables S1 and S2) as discussed in the text. (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 532 8698 1861. Fax: +86 832 8698 1861. E-mail:
[email protected]. *Tel.: +86 532 8698 1861. Fax: +86 832 8698 1861. E-mail:
[email protected]. ORCID
Yaqing Liu: 0000-0003-4838-2610 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been financially support by the National Natural Science Foundation (Grant Nos. 21176258 and 21676300) and H
DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.7b01935 Energy Fuels XXXX, XXX, XXX−XXX
