Diatomite Adsorbent - American Chemical Society

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Reactive Mechanism and Regeneration Performance of NiZnO/ Al2O3‑Diatomite Adsorbent by Reactive Adsorption Desulfurization Xuan Meng, Huan Huang, and Li Shi* The State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: The reactive adsorption behavior of thiophene over the reduced NiZnO/Al2O3-diatomite adsorbent was characterized by in situ Fourier transform infrared (FTIR) spectroscopy. X-ray diffraction (XRD) technology was used to investigate the phase change of sulfur species in the reactive adsorption desulfurization (RADS) process. The results indicated that S−M bonding of thiophene on the metallic Ni sites was first decomposed to form Ni3S2 while formed C4 olefins were further saturated by hydrogen to form butane which was released back into the process stream, followed by the sulfur transfer from Ni3S2 to ZnO to form ZnS in the presence of hydrogen, and then the new formed Ni sites could participate in the adsorption of thiophene once again. The muticycle fixed-bed tests showed a good prospect for adsorption desulfurization over the NiZnO/ Al2O3-diatomite adsorbents. Thermogravimetric and differential thermal analysis (TG-DTA) together with XRD was used to reveal the regeneration mechanism. The XRD results indicated that the formation of NiSO4 species led to an increase of the amount and the strength of Lewis acid sites in the regenerated adsorbents and, thus, temporarily improved the removal performance of the adsorbent for thiophene.

1. INTRODUCTION Sulfur in transportation fuels remains a major source of air pollution. Because of government mandates worldwide, refiners must produce increasingly cleaner fuels.1 The traditional hydrodesulfurization (HDS) process is currently a major process in petroleum refineries to reduce the sulfur in FCC gasoline. In such a process, organic sulfur compounds in the liquid fuels are converted to H2S through certain reactions at elevated temperatures (>300 °C) and pressures (20−100 atm H2) using Co−Mo/Al2O3 or Ni−Mo/Al2O3 catalysts that are later removed from the system. The HDS process is highly efficient for the removal of sulfur compounds such as thiols, sulfides, and disulfides but is less effective for aromatic thiophenes and thiophene derivatives (especially benzothiophene, dibenzothiophene, and their alkylated derivatives). On the other hand, HDS of gasoline can result in the saturation of olefinic compounds resulting in octane loss of about 10 numbers.2,3 Therefore, eliminating the maximum sulfur impurities without decreasing octane number is still an intrinsic issue of the HDS process. Apart from the traditional hydrodesulfurization, several nonHDS-based desulfurization technologies such as adsorptive desulfurization, charge-transfer complex formation, extraction using ionic liquids, biocatalytic treatment, etc. have been proposed recently for the desulfurization of liquid fuels.4−7 Among them, reactive adsorption desulfurization (RADS) is effective for deep desulfurization because it combines the advantages of the catalytic HDS and adsorption.8−13 Although the RADS process has the participation of H2, the reaction pathways is different from the HDS process. For HDS process, the C−S bond in thiophenic sulfur compound is broken first under the action of the catalyst. The sulfur atom is released and then reacts with H2 to form H2S. While in the RADS process, the sulfur atom is removed from the molecule and is bound by the sorbent. The hydrocarbon part is returned to the final product without any structural changes. The sulfur atom of the © 2013 American Chemical Society

sulfur-containing compounds adsorbs onto the sorbent and reacts with the sorbent. The sulfur atom is retained on the sorbent while the hydrocarbon portion of the molecule is released back into the process stream. H2S is not released into the product stream and therefore prevents recombination reactions of H2S and olefins to make mercaptans, which could otherwise increase the effluent sulfur concentration. The S Zorb process of Conoco Philips Petroleum Co. based on RADS at elevated temperatures under a low H2 pressure proved to be effective for the production of low-sulfur gasoline or diesel fuel.14,15 The study for RADS technology can be traced back to the 1920s, ZnO, MnO2, CaO, or some complex metal oxides all showed a good adsorption performance for H2S and were successfully applied to the desulfurization of natural gas, coal gas, refinery gas, and chemical feed gas.16,17 On the basis of gas desulfurization study, RADS was further applied in the desulfurization of liquid fuel. Tawara et al.8,9,18,19 first used conventional HDS catalyst in the adsorptive HDS of kerosene, the results showed that the desulfurization effect of the oxidized state Ni−Mo/Al2O3 catalyst was better than that of the sulfided Ni−Mo/Al2O3 catalyst and the sulfur capacity of the catalyst was mainly limited by the number of Ni−Mo active centers which could be sulfurized. A reduced state Ni/Al2O3 catalyst exhibited the most active performance in the adsorptive HDS process, but the sulfur capacity of the catalyst was mainly limited by the sulfurization of the Ni atom on the catalyst surface. After that, ZnO species as a kind of strong metal support interaction (SMSI) were mixed with sulfur-poisoned Ni catalyst; the results showed that the poisoned Ni catalyst Received: Revised: Accepted: Published: 6092

December March 12, March 26, March 26,

18, 2012 2013 2013 2013

dx.doi.org/10.1021/ie303514y | Ind. Eng. Chem. Res. 2013, 52, 6092−6100

Industrial & Engineering Chemistry Research

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ground, and the particles of 20−40 mesh were collected through sieving. The mole ratio of Zn to Ni was 0.4 and the amounts of diatomite and pseudoboehmite were 25 wt %, respectively. 2.2. Adsorption and Regeneration. The reactive adsorption desulfurization (RADS) experiments were performed at 400 °C under 1.0 MPa with a pure H2 flow (40 mL/min). About 1.0 g of the adsorbent was performed in a stainless steel column having a bed dimension of 6 mm i.d. and 250 mm length. The packed column was placed in a multichannel convection oven designed in our laboratory for the adsorption experiments. In order to ensure that the Ni in the NiZnO/Al2O3-diatomite adsorbent is in the reduced form, the adsorbent bed was pretreated with H2 gas at a flow rate of 30 mL/min under 0.5 MPa at 370 °C, and kept at this temperature for about 1 h. After the pretreatment, the oven temperature and pressure was increased to the desired adsorption temperature and pressure in the H2 stream. In the adsorption experiments, the model fuel was sent into the adsorbent column by a microinjection pump, flowed down through the adsorbent bed at a weight hourly space velocity (WHSV) of 4 h−1, H2/oil volume ratio of 400. The liquid products were collected in a cryogenic trap with ice water bath and subjected to analysis periodically. The treated-fuel samples were analyzed, quantitatively using a Varian 3800 Gas Chromatograph and qualitatively using a Finnigan SSQ710GC-MS. The distributions of sulfur compounds in the products were analyzed in the Agilent 6890A Gas Chromatograph coupled with a 355SCD detector. Following the RADS experiments, the adsorbent was regenerated on line using air to generate gaseous sulfur oxygen-containing compounds such as sulfur dioxide, as well as to burn off any remaining hydrocarbon deposited that might be present. The regenerated experiments were performed at a temperature range of 480−550 °C under ambient pressure with air at a flow rate of 300 mL·min−1 for 1−2 h and repetitious reduction of adsorbent before RADS reaction should be necessary. 2.3. Characterization of Adsorbents. 2.3.1. In situ FTIR Studies. The RADS of thiophene on the reduced NiZnO/ Al2O3-diatomite adsorbent was studied using an FTIR spectrometer. The reduced adsorbent sample was pressed into a self-supporting wafer (10 mg·cm−2) and put into a quartz IR cell with KBr windows (see Figure 1). Then, the sample was evacuated at 380 °C under high vacuum for 120 min, followed

would release a few parts per billion of H2S through H2 and the released H2S would be accepted on neighboring ZnO particles. Therefore, an autoregenerative Ni/ZnO system as a best combination was realized for the reactive adsorption desulfurization. On the basis of the data of Tawara et al., Babich and Moulijin speculated out the RADS mechanism. They discovered that nickel reacted with sulfur under H2 to form NiS, which consequently reacted with ZnO to form ZnS and regenerated nickel.4 Huang et al. investigated the transfer of sulfur species in RADS on Ni/ZnO by the sulfur K-edge X-ray adsorption near-edge structure (XANES) and XRD. They found that the intermediate Ni sulfide during the RADS process is Ni3S2 rather than NiS.20 The further investigation is necessary to clarify it. Bezverkhyy et al. studied the kinetics of thiophene reactive adsorption on Ni/ZnO by thermal gravimetric analysis, and the results indicated that the reaction between Ni/ZnO and thiophene consists of three steps. The first step is a rapid surface reaction between thiophene sulfur and the surface Ni atom to form Ni3S2. The next step is that sulfur species, formed though thiophene decomposition, react preferably with ZnO which can be characteristic of a nucleation-controlled sulfidation of ZnO surface. The third step is thiophene molecules reacting with bulk Ni atoms, meanwhile H2S which is diffusion through surface ZnS layer reacts with bulk ZnO sulfidation and the kinetics of this step are strongly dependent on the reaction conditions.11,12 Studies are underway to verify this assumption. To meet more stringent recent environment regulations, it is essential to understand the adsorption mechanism, so as to develop Ni/ZnO-based RADS adsorbents with high selectivity and capacity. The present study employed in situ Fourier transform infrared (FTIR) examination of adsorption and reaction of thiophene over reduced NiZnO/Al2O3-diatomite adsorbent to probe the bonding mode of thiophene and products formed during RADS over a range of temperatures. Moreover, the X-ray powder diffraction (XRD) method was used to investigate the phase change of sulfur species in the RADS process. A good regeneration property is of vital importance to adsorbents. In this work, the NiZnO/Al2O3diatomite adsorbent regeneration after desulfurizing a model fuel using a two-step method was studied and thermogravimetric and differential thermal analysis (TG-DTA) together with XRD was used to reveal the regeneration mechanism.

2. EXPERIMENTAL SECTION 2.1. Feedstocks and Adsorbents. In this study, a model gasoline was prepared by adding thiophene as a sulfur model compound with a sulfur concentration of 2000 μg·g−1 and 10 wt % of 1-octene as olefin to sulfur-free n-octane as a solvent. All the sulfur compounds and hydrocarbon compounds used for preparing the model fuels were purchased from Aldrich and used as such without further purification. NiZnO/Al2O3-diatomite adsorbents used in this study were prepared by kneading method. First, zinc oxide and nickel sesquioxide were sufficiently mixed, then pseudoboehmite and diatomite were added separately to the mixture in order to achieve the desirable reactivity and attrition resistance (all chemicals were purchased from Sinopharm Chemical Reagent Co.,Ltd.). Next, a liquid binder, dilute nitric acid, was added to the mixture in order to make a slurry. An extruder was used to formulate pellets to an outer diameter of 1 mm from the slurry, followed by drying at 120 °C for 5 h. Finally, the resulting materials were calcined at 600 °C for 1 h. They were then

Figure 1. In situ FTIR reactor system. (1) Rotary vane vacuum pumps. (2) High vacuum oil diffuse pump. (3) Antihunting device. (4) Buffer unit. (5) Vacuum gauge. (6) Thermocouple well. (7) Pyridine entrance. (8) Infrared transmission windows. (9) Heater coil. (10) Temperature controller and solid state relays. (11) Composite vacuum table. 6093



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Figure 2. In situ FTIR spectra recorded from the adsorption and thermal desorption of thiophene on reduced NiZnO/Al2O3-diatomite adsorbent. Adsorption: (a) RT. Desorption: (b) 100, (c) 200, (d) 300, (e) 400 °C.

observed to decrease with increasing temperature and this was maybe due to the desorption of the C4 species. The right panel highlights the ν(cc)sym region and δCH region of the spectra. The main spectral features were located at 1654, 1637, 1617, 1611, 1580, 1570, 1560, 1508, 1491, 1472, 1464, 1459, 1451, 1437, 1375. It is recognized that the ν(cc)sym absorbances of thiophene are sensitive to the bonding mode of thiophene adsorbed on the catalyst surface.21 The appearance of the IR band at 1437 cm−1 can be used to determine the adsorption mode of thiophene on reduced NiZnO/Al2O3diatomite adsorbent. Previous vibrational spectroscopic studies24,25 provided strong evidence that the coordination of thiophene on surface metal sites via the sulfur atom (SM mode) caused a shift of the ν(cc)sym band to higher wavenumbers while to lower wavenumbers for π-bonded thiophene (π-complexation mode). Oyama et al.21 studied thiophene adsorption on Ni2P/MCM-41 using FTIR and temperature-programed desorption. Absorbance features at 1437 cm−1 were observed and assigned to S−M bonded thiophene with nickel. Similar conclusions were made for thiophene adsorbed on Mo2N/γ-Al2O3 by Li et al.26 Therefore, it is reasonable to assign the IR band at 1437 cm−1 to thiophene via sulfur−metal mode (S−M) to metallic Ni sites of the reduced NiZnO/Al-diatomite adsorbent. Since the amounts of the residual thiophene adsorbed chemically on the adsorbents were very few, we did not observe discernible bands at 3200− 3000 cm−1. The features at 1654, 1636, 1611, 1580, 1560, 1570, and 1491 cm−1 were assigned to the symmetric stretching modes of the CC bonds of adsorbed C4 hydrocarbons, and 1464, 1459, 1452, and 1375 cm−1 could be assigned to the deformation vibration modes of CH.24−26 In the δCH regions, the intensities of the spectral features especially at 1452 and 1375 cm−1 were observed to decrease obviously with increasing temperature, indicating that the formed C4 species were gradually desorbed at higher temperatures. Many studies have been focused on the hydrogenation products of thiophene on various catalysts by in situ FTIR spectroscopy. The products of thiophene HDS are typically 1,3butadiene, 1-butene, 2-butene (cis- and trans-), and butane. Li et al.27 studied the adsorption behaviors of 1-butene, cis-2butene, trans-2-butene, and 1,3-butadiene species over a reduced Mo2N/γ-Al2O3 catalyst. They found that the IR spectras of the three butenes were quite similar to each other. The IR bands at 3010, 2965, 2922, 2860, 1619, 1612, 1452, 1408, and 1381 cm−1 were attributed to π and σ-bonded 2butenes and the IR bands at 3010, 3005, 2920, 2850, 1616,

by thiophene adsorption at RT. Then, the sample was outgassed at different temperatures (100, 200, 300, and 400 °C) for 15 min. All infrared spectra were collected at RT on a Fourier transform infrared spectrometer (Nicolet 6700) with a resolution of 4 cm−1 and 32 scans in the region 4000−400 cm−1. Spectra are shown with subtraction of the background contribution to highlight the thiophene adsorbate bands. 2.3.2. Thermal Analysis. TG-DTA were carried out by using a SDT Q600 instrument. The temperature was elevated from ambient to 800 °C at a rate of 10 °C/min while the air flow rate was held constant at 100 mL/min. 2.3.3. X-ray Diffraction. An X-ray diffraction (XRD) technique was used to characterize the crystal structure. In this work, XRD patterns were obtained with a Siemens D-500 X-ray diffractometer equipped with Ni-filtrated Cu Kα radiation (40 kV, 100 mA). The 2θ scanning angle range was 10−80° with a step of 0.02 deg/s The average crystallite size was estimated from the line broading of the most intense XRD reflections by the Scherrer formula.

3. RESULTS AND DISCUSSION 3.1. Mechanism of Thiophene Adsorption on NiZnO/ Al2O3-Diatomite Adsorbent. 3.1.1. In situ FTIR Studies. Figure 2 displays the in situ FTIR spectra recorded from the adsorption and thermal desorption of thiophene on reduced NiZnO/Al2O3-diatomite adsorbent. The left and right panels show the νC−H region (3200−2800 cm−1) and low wavenumber region, respectively. The features in the 3200−3000 cm−1 are assigned to aromatic νC−H absorbances and the features in the 3000−2800 cm−1 are attributed to aliphatic νC−H modes.21 In the νC−H region, several bands were located at 2954, 2921, 2910, 2871, and 2850 cm−1. With increasing temperature, the intensities of these bands were observed to decrease monotonically. No discernible bands were detected in the 3200−3000 cm−1. The absence of aliphatic νC−H absorbances in the 3200− 3000 cm −1 region and the presence of aromatic ν C−H absorbances in the 3000−2800 cm−1 region, as indicated that most of the molecularly adsorbed thiophene desorbed or reacted on the reduced sample after annealing in ultrahigh vacuum. The appearance of aliphatic νC−H modes was likely attributable to some decomposition of thiophene on the adsorbent surface and formation of adsorbed C4 hydrocarbons. Such decomposition under inert conditions has been observed upon adsorption of thiophene on Ni single crystals, even at low temperatures (150 K).22,23 The intensities of these bands were 6094



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1565, 1490, 1457, 1417, and 1378 cm−1 were assigned to πadsorbed, σ-bonded, and dehydrogenated 1,3-butadiene species on the catalyst surface.26 However, for the HDS products of thiophene over a reduced Mo2N/γ-Al2O3 catalyst, the IR bands of them were a little different from the three butenes over the catalyst. This is because the reduced Mo2N/γ-Al2O3 catalyst’s surface was found to be sulfide at HDS conditions; thus, the adsorption of butenes and 1,3-butadiene were formed on a nitride catalyst, while those from thiophene/H2 were actually on a sulfide surface layer on nitride catalyst. The research results of the thiophene HDS on Ni2P/MCM-41 catalyst showed that there are strong IR bands in the aliphatic νCH region (3200−2800 cm−1) and the IR bands at 1549, 1491, and 1449 cm−1 could be assigned to vibrations of CH2 and CH3 groups of adsorbed C4 species.28 Li and co-workers26 identified that the band at 1491 cm−1 was assigned to di-π-adsorbed 1,3butadiene while the bands at 1450 and 1379 cm−1 could be assigned to σ-allyl 1-butene species (δ(CH2)as) via isomerization. Different to the conclusions obtained by the preceding scholars, the bands at 3005, 3070, 1631, and 1597 cm−1 were not detected on the reduced NiZnO/Al-diatomite adsorbent, indicating that there were not π-adsorbed 1-butene and 2butene species and the main C4 products were σ-bonded 2butene, π-adsorbed 1,3-butadiene, and σ-allyl 1-butene isomerization species. 3.1.2. XRD Studies. The reactive adsorption behavior of thiophene molecules on the reduced NiZnO/Al2O3-diatomite adsorbent has been studied by in situ FTIR spectra from the perspective of the reaction products, while the internal phase change of the adsorbent has not been well reflected in the reaction process. In order to further study the reactive mechanism, the X-ray powder diffraction (XRD) method was used to investigate the phase transformation of the active ingredient Ni and ZnO in the RADS process. The XRD patterns of the calcined NiZnO/Al2O3-diatomite samples and the used samples after the RADS of model fuel for different reaction times on stream are shown in Figure 3. The diffraction lines of NiZnO/Al2O3-diatomite after calcination mainly attribute to the crystalline phases of tetragonal SiO2, cubic NiO (2θ:37.3°, 43.3°, 62.9°, 75.4°, 79.4°, JCPDF file no. 65-5745) and hexagonal ZnO (2θ:31.8°, 34.5°, 36.3°, 47.7°, 56.7°, JCPDF file no. 36-1451). No Al2O3 characteristic peak was observed, which indicated that Al2O3 exists in an amorphous state and dispersed well in adsorbent. After RADS for 3 h, the characteristic diffraction peaks of NiO disappeared, while peaks at 44.49°, 51.85°, and 76.38° that correspond to Ni metal were observed for the used adsorbent, which indicated that NiO was converted to Ni completely by H2 activation treatment. The crystallite size of metallic nickel was 18.3 nm by Scherrer formula, revealing that small-sized metallic nickel was formed in the reduced sample. Furthermore, a weak diffraction peak appeared at 2θ = 42.87° which was attributed to a metallic Ni−Zn alloy and indicated that the reduction treatment of the adsorbent may lead to a partial reduction of zinc and its alloying with Ni, which are attested by Bezverkhyy et al. on reduced Ni/ZnO.13 The diffraction peak of the Ni−Zn alloy disappeared after RADS for 24 h. The intensity of Ni and ZnO peak decreased gradually, indicating a monotonous decrease of Ni and ZnO contents. Peaks at 28.55°, 47.49°, and 56.35° that correspond to cubic ZnS were observed on the used adsorbent after RADS for 3 h, and the intensity of ZnS peak increased gradully with the time on stream. After RADS for 12 h, peaks at 21.75°, 31.10°, 37.78°, 38.27°, 44.33°,

Figure 3. XRD patterns of the calcined NiZnO/Al2O3-diatomite sample (a); used sample after the RADS of model fuel for different reaction times on stream (b, c, d, e, and f represent 3, 6, 12, 24, and 36 h on stream, respectively) (⧫, SiO2; ●, NiO; *, ZnO; Δ, Ni; +, ZnS; #, Ni3S2).

49.73°, 50.12°, 54.62°, and 55.16° that correspond to rhombohedral Ni3S2 were observed and the intensity of Ni3S2 peak increased significantly as time on stream. The crystallite sizes of Ni3S2 increased with the time on stream from 12 to 36 h. In the case of the used samples for 12, 24, and 36 h, the average crystallite size of Ni3S2 were found to be 31.3, 36.6, and 38.5 nm, respectively. During the RADS process, the formation of ZnS and Ni3S2 indicated that a synergistic catalytic effect on thiophene desulfurization might occur over Ni metal and ZnO in H2 atmosphere, resulting in a transformation from thiophene sulfur to the Ni and ZnO so as to remove thiophene. Tawara et al.9 reported that Ni/ZnO is an autoregenerative adsorptive UD-HDS catalyst, the ZnO support is regarded to regenerate sulfur-poisoned surface Ni to active surface Ni by accepting H2S. In order to verify the “sulfur transfer mechanism” in the NiZnO/Al2O3-diatomite adsorbent, we did an additional test: the used sample after RADS of model fuel for 24 h was loaded in the reactor and treated with H2 gas at a flow rate of 40 mL/min under 1 MPa at 400 °C without passing model fuel. The XRD result of this sample is given in Figure 4. Comparison with the XRD patterns of the used sample before and after H2 treatment (Figure 4e and g), the intensity of Ni3S2 and ZnO peak decreases slightly, while that of ZnS peak increases slightly, indicating that sulfur is gradually transferred from Ni3S2 to ZnO to form ZnS in the presence of hydrogen. Ryzhikov et al.13 studied the reactive adsorption of thiophene on Ni/ZnO, they found that the major hydrocarbon products are butanes (trans-2-butene, cis-2-butene, and 1butene), butane, and isobutene in minor quantities. It should be noted that butane species were detected in our liquid products according to the analysis results of mass spectrometry and the butene species were not detected, indicating that the C4 olefins have all become saturated by hydrogen. On the basis of the analysis of XRD and FTIR for the reactive adsorption of thiophene on NiZnO/Al2O3-diatomite, the final reactive mechanism proposed by Babich, Moulijin,4 6095



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formed Ni sites can participate in the adsorption of thiophene once again. The possible reaction paths are shown in Figure 5. 3.2. Regeneration of NiZnO/Al2O3-Diatomite Adsorbent. 3.2.1. Multicycle Desulfurization of Model Gasoline. The regeneration performance is one of the most important aspects for its successful application on FCC gasoline cleanup. Therefore, a multicycle desulfurization experiment has been conducted to gather preliminary data on the performance of the NiZnO/Al2O3-diatomite adsorbent. The adsorbent (1 g) was tested for generation after deactivation with fresh untreated model fuels. In these set of experiments, for each cycle, the adsorption run was performed at a fixed set of conditions of 400 °C, 1 MPa, H2/oil volume ratio of 400, and weight hourly space velocity (WHSV) of 4 h−1. The regeneration was achieved in a two-step fashion: (a oxidation) nickel sulfide and zinc sulfide species oxidation to nickel oxide and zinc oxide species using dry air at 480−550 °C for 1−2 h; (b reduction) nickel oxide species reduction to Ni0 using hydrogen at 370 °C for 1 h. Afterward, the fresh untreated model fuel was allowed to contact the adsorbent. Yang29 and Khare15,30 used similar approaches to regenerate desulfurization adsorbents. The results of five desulfurization cycles are shown in Figure 6. The effect of oxidative regeneration temperature on RADS performance has been plotted in cycle1−cycle2. It is evident that the oxidative regeneration temperature has a significant effect on the performance of the regenerated adsorbent. The desulfurization rate of the regenerated NiZnO/Al2O3-diatomite adsorbent is found to increase with increasing regeneration

Figure 4. XRD patterns of the used sample after the RADS of model fuel for 24 on the stream(e) and the used sample for 24 h in posttreatment with H2(g) (*, ZnO; +, ZnS; #, Ni3S2).

and Huang et al.20 can be further verified and improved as follows: First, thiophene is adsorbed onto the metallic Ni sites of the reduced adsorbents via sulfur−metal (S−M). Then the C−S bond in the adsorbed thiophene ruptures to form Ni3S2 and C4 olefins, the C4 olefins are further saturated by hydrogen to form butane, which is released back into the process stream. Lastly, the sulfur is gradually transferred from Ni3S2 to ZnO to form ZnS in the presence of hydrogen. After that, the new

Figure 5. Mechanism of reactive adsorption desulfurization process of thiophene over NiZnO/Al2O3-diatomite adsorbents. 6096



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Figure 7. TG-DTA curves of the used NiZnO/Al2O3-diatomite sample for 36 h.

Figure 6. Adsorptive desulfurization curves in successive RADS cycles of NiZnO/Al2O3-diatomite adsorbent using model fuel.

carbon over the surface and the pore canal of the adsorbent. The two consecutive steep increases in weight at 250−450 and 450−650 °C appear, which can be attributed to the formation of some intermediates in oxidation state, corresponding to the two exothermic peaks recorded in DTA, respectively. Finally, loss in weight occurs between 650 and 800 °C, which is probably caused by the decomposition of the intermediates formed in the air at high temperature. To be able to fully understand this phenomenon, further XRD study is needed. 3.2.3. XRD Studies of The Used Adsorbents. Figure 8 displays the powder XRD patterns of regenerated samples after

temperature from 480 to 550 °C. Table 1 shows the adsorptive capacity and the olefin saturation rate of the five times Table 1. Sulfur Capacities and Olefin Saturation Rates of Adsorbents cycle fresh cycle-1 cycle-2 cycle-3 cycle-4 cycle-5 a

temperature

time

sulfur capacitya

T (°C)

t (h)

(mg S/g adsorbent)

(%)

1 1 2 2 2

9.41 8.63 9.02 10.49 9.32 9.13

35.83 30.60 37.44 37.99 38.66 39.14

480 550 550 550 550

olefin saturation rateb

Sulfur adsorption capacity for 3 h. bOlefin saturation rate for 3 h.

desulfurization cycles. For the adsorbent regenerated at 550 °C for 1 h, the adsorptive capacity of the regenerated adsorbent has recovered to 90% of that of the fresh adsorbent, and the olefin saturation rate decreased slightly. The effect of oxidative regeneration time on RADS performance has been plotted in cycle2−cycle3. As the regeneration time extends to 2 h, the regeneration performance has recovered further. In fact, the desulfurization rate in cycle-3 is slightly higher than that in the fresh process. This may indicate that some rearrangement of the inner structure of the adsorbent, which enhances diffusion and adsorption properties, occurs. A similar observation is also made by Sánchez and Ruiz.31 As shown in cycle 3−cycle 5, the adsorbent demonstrates good, reproducible performance. This is a clear indication of complete regeneration and the olefin saturation rate increased slightly. 3.2.2. TG-DTA Analyses of the Used Adsorbent. In order to investigate the oxidative regeneration mechanism of the used adsorbents, the structure changes of the used adsorbent during the oxidative regeneration process were studied by thermogravimetric analyses. Figure 7 shows the relative thermogravimetric curve (TG), differential thermogravimetric curve (DTG), and differential thermal analysis (DTA) curves corresponding to the used adsorbent for 36 h. The DTG curve indicates the presence of two primary weight loss regions and two weight gain regions. The first weight loss process in the temperature range 30−250 °C has been typically attributed to the removal of the surface adsorbed species such as surface water, interlayer water, hydrocarbon feedstock, thiophene, and a small amount of product molecules, as well as the deposited

Figure 8. XRD patterns of NiZnO/Al2O3-diatomite adsorbents with different regeneration temperature.

oxidation treatment in air at 400, 600, and 800 °C for 2 h. The diffraction lines of fresh sample mainly attribute to the crystalline phases of tetragonal SiO2, cubic NiO, and hexagonal ZnO. After RADS for 36 h, the characteristic diffraction peaks of ZnO and NiO in the used sample tend to disappear. In addition, new crystallite phases of ZnS and Ni3S2 were observed and the most intense signals for ZnS and Ni3S2 locate at 2θ of 28.6°and 32.1°, respectively. This suggests that most of Ni and ZnO in the used sample have been transformed to Ni3S2 and ZnS after RADS for 36 h. And what’s more, the Al2ZnO4, Al2NiO4, and Zn2SiO4 phases were detected although the peak intensities were very weak. It indicated that Ni and ZnO active components interacted strongly with the Al2O3 and SiO2 supports and formed nonactive componentsAl2ZnO4, Al2NiO4, and Zn2SiO4during the RADS process. 6097



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However, after several cycles, the loss of the NiO activity component in the regenerated adsorbent will inevitably cause the reducing of the RADS performance of the adsorbent. Corresponding to the TG curve of the used sample in Figure 7, the weight increase can be attributed to the formation of NiSO4 at 450−650 °C as shown in the following reaction:

Curve c, d and e represent the X-ray diffraction patterns of the regenerated samples after oxidation treatment in air at 400, 600, and 800 °C for 2 h, respectively. It can be noted that the compositions of the regenerate samples have evidently changed with increasing oxidative regeneration temperature. For the sample after regeneration at 400 °C, except a sharp SiO2 diffraction peak (2θ = 21.98°), the intensities of the other diffraction peaks appeared more diffuse. The detected diffraction peaks are mainly assigned to various nickel sulfides (NiS, Ni3S2, Ni9S8, Ni0.96S, Ni4S3, and Ni7S6), zinc sulfides (ZnS, Al2ZnS4), zinc oxide (ZnO), and nickel oxide (NiO) as well as a small amount of carbonaceous species such as SiC. However, the formation of these oxidation regeneration intermediates could not result in the increase of the weight of the regenerated samples. It is notable that a series of diffuse characteristic diffraction peaks at 2θ lower than 50° which is assigned to Zn3O(SO4)2 were detected. The formation of Zn3O(SO4)2 was also observed in Wang et al.’s study on the adsorption desulfurization process conditions of S-Zorb unit.32 Zn3O(SO4)2, which was generated from the reaction of ZnS and excessive O2, could decompose into ZnO, ZnS, and H2O in the next reduction step. This would lead to the fact that there is always a part of nonactive ZnS, which cannot be transformed to active ZnO remaining in the regenerated samples. Corresponding to TG curve of the used sample in Figure 7, the weight increase can be attributed to the formation of Zn3O(SO4)2 at 250−450 °C. The XRD peaks of nickel sulfides and zinc sulfides implied that these sulfides were not completely restored to their initial oxidation states after the regeneration process at 400 °C as a result of the insufficient regeneration temperature. It was predicted that the following reaction occurred under this condition: Ni3S2 + O2 → NixSy + SO2

(1)

NixSy + O2 → NiO + SO2

(2)

2Ni + O2 → 2NiO

(3)

2ZnS + 3O2 → 2ZnO + 2SO2

(4)

ZnS + 5.5O2 → Zn3O(SO4 )2 + SO2

(5)

2NiO + 2SO2 + O2 → 2NiSO4

(6)

As can be seen from the XRD patterns of the sample regenerated at 800 °C, the characteristic diffraction peaks of NiSO4 and Zn3O(SO4)2 almost disappeared. Corresponding to the weight loss at 650−800 °C in TG curves of the used sample in Figure 7, which can be attributed to the decomposition of NiSO4 and Zn3O(SO4)2 species. The XRD pattern also reflects that the peak intensities of NiO in the sample regenerated at 800 °C increased further and have almost recovered to the level in the fresh sample, while the peaks of ZnO gradually disappeared. Moreover, a series of significant spinel structures were detected at 2θ = 31.26°, 55.63°, 59.33°, and 65.21° by XRD, and possible spinel structures are Al2ZnO4 and Al2NiO4. However, most of the spinel structures were due to Al2ZnO4, because ZnO was not observed in the sample regenerated at 800 °C. In addition, the Zn2SiO4 phases were detected although the peak intensity was very diffuse. The main reactions under this condition are represented by the following equations: Zn3O(SO4 )2 → 3ZnO + 2SO3

(7)

NiSO4 → NiO + SO3

(8)

ZnO + Al 2O3 → Al 2ZnO4

(9)

2ZnO + SiO2 → Zn2SiO4

(10)

Ni + Al 2O3 → Al 2NiO4

(11)

This result indicates that higher temperature will enhance the interactions of ZnO and the support components (Al2O3 and SiO2), which make active ZnO component convert into nonactive spinel structures and willemite structures, leading to permanent deactivation of partial active ZnO component. Therefore, we conclude that a range of 550−600 °C can be recommended as the optimum oxidation regeneration temperature. The changes of nickel-phase and zinc-phase in the adsorbents during the oxidative regeneration process can be well reflected in Figure 9.

In the case of the sample after regeneration at 600 °C, the XRD results of regenerated samples showed nickel sulfides and zinc sulfides almost disappeared, while NiO appeared as major peaks along with minor peaks such as the ZnO peak. This indicates that the used sample could be oxidized more sufficiently at 600 °C than at 400 °C. In addition, the appearance of NiSO4 characteristic diffraction peaks in XRD patterns can be clearly noted. Wang and his group33 have reported that the nanosized HZSM-5 catalyst modified by NiSO4 showed more stable HDS activity than that modified by NiO, which is due to a strong interaction between SO42− and Al2O3 in the catalyst modified by NiSO4. And this strong interaction greatly increased both the amount and the strength of acid sites by creating a kind of acid, being like super acid, in which the Lewis acid increased mainly. It is reported that thiophenic compounds with lone pair electrons present Lewis basicity which are apt to adsorb on the Lewis acid site.34 Therefore, the reason why the desulfurization rate in cycle-3 is slightly higher than that in the fresh process in the multicycle desulfurization experiments is most probably because NiSO4 gradually formed in the used sample regenerated at 550 °C for 2 h, and the regenerated adsorbent is stronger in Lewis acidity.

4. CONCLUSIONS The in situ FTIR and XRD were effective to investigate the thiophene reaction products over the reduced NiZnO/Al2O3diatomite adsorbent and the phase change of sulfur species in the RADS process. The results indicated that S−M bonding of thiophene on the metallic Ni sites was first decomposed to form Ni3S2 while formed C4 olefins were further saturated by hydrogen to form butane which was released back into the process stream, followed by the sulfur transfer from Ni3S2 to ZnO to form ZnS in the presence of hydrogen, and then, the newly formed Ni sites could participate in the adsorption of thiophene once again. A two-step regeneration scheme was deemed to be appropriate to recover the adsorbent to its original state. The comparatively suitable oxidative regeneration conditions were as follows: regeneration temperature 550 °C, regeneration time 6098



Article

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Figure 9. Changes in Ni-phase and Zn-phase during the oxidative regeneration of the used adsorbents.

2 h. The XRD results indicated that a new NiSO4 species was detected in the regenerated adsorbent at around 600 °C, while a strong interaction between SO42− and Al2O3 led to the increase of the amount and the strength of Lewis acid sites in the regenerated adsorbents and, thus, temporarily improved the removal performance of the adsorbent for thiophene.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 021-64252274. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National Natural Science Foundation of China (No. 21276086) and Opening Project of State Key Laboratory of Chemical Engineering of East China University of Science and Technology (No. SKL-ChE-11C04).

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