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A Detailed Study on the Negative Effect of Residual Sodium on the Performance of Ni/ZnO Adsorbent for Diesel Fuel Desulfurization Lichun Huang,†,‡ Zhangfeng Qin,† Guofu Wang,† Mingxian Du,† Hui Ge,† Xuekuan Li,† Zhiwei Wu,†,‡ and Jianguo Wang*,† State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan, Shanxi 030001, PR China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China
Desulfurization of diesel fuel was conducted via reactive adsorption over a coprecipitated Ni/ZnO adsorbent. A negative effect of the residual sodium in Ni/ZnO adsorbent on its adsorption performance was observed. The desulfurization ability of Ni/ZnO adsorbent is markedly weakened with the increase in the residual sodium content. This negative effect can be attributed to the fact that the residual sodium decreases the adsorbent surface area and pore volume, suppresses the interaction between Ni and ZnO, and leads to an increase in the crystallite size of the active species. Moreover, the residual sodium is enriched on the adsorbent surface upon calcination and reduction treatment, which may promote the formation of the catalytically inactive Ni-Zn and NaZn(OH)3 species. 1. Introduction New environmental regulations regarding the sulfur content in fuel products have forced researchers and refineries to develop efficient processes for producing cleaner fuels. Hydrodesulfurization (HDS) using a sulfided Co/Mo or Ni/Mo catalyst at high temperatures and high pressures is widely employed by the refineries to produce low-sulfur gasoline and diesel fuel.1-3 However, other approaches based either on adsorption or on oxidation of sulfur-containing species may be more effective for the production of ultra-low-sulfur transportation fuels in some cases, especially when faced with the alkylated derivatives of benzothiophene and dibenzothiophene.4-10 Among them, reactive adsorption desulfurization (RADS) is effective for deep desulfuration because it combines the advantages of the catalytic HDS and adsorption.11-16 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.17,18 In RADS, the sulfurcontaining compounds are first decomposed through catalytic hydrogenation, and the H2S-like species formed is then adsorbed in the adsorbent.12,19 Ni/ZnO adsorbent is ideal for this purpose, in which the ZnO component acts not only as an acceptor of sulfur released during the regeneration of sulfided Ni species but also as a cocatalyst for the hydrogenation of sulfurcontaining compounds over the surface Ni species. Coprecipitation is widely used to prepare the mixed oxide catalysts/adsorbents, in which the aqueous solution of metal salts is precipitated with an alkali.11-23 The alkali usually used for precipitation includes ammonia, ammonium carbonate,11,12,19 urea,13 sodium hydroxide, and sodium carbonate.14-16 Ammonia, ammonium carbonate, or urea may be preferred because each leaves little residue in the resultant catalysts and/or adsorbents upon later calcination treatments; however, they are less stable and may interact with certain metal ions to form soluble complexes during precipitation, which may lead to some problems with respect to the control of precipitation efficiency * To whom correspondence should be addressed. Tel.: +86-3514046092. Fax: +86-351-4041153. E-mail:
[email protected]. † Institute of Coal Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
and actual precipitate composition, especially for the coprecipitation of the aqueous solution containing multi-metal salts. On the other side, the sodium-containing alkali like sodium carbonate and sodium hydroxide is more stable and effective for the precipitation; however, it is generally difficult to remove the residual sodium completely from the precipitates by washing when the sodium-containing alkali is used as the precipitating agent. The residual sodium in the catalyst may influence the catalyst structure and performance.20-23 For the synthesis of methanol from CO2 hydrogenation over Cu/Zn/Al2O3, Jun et al.21 reported that the residual sodium in the catalyst inhibited the interaction of CuO with ZnO and Al2O3, which resulted in an increase in the crystallinity of CuO and ZnO and a decrease in the catalytic activity as a result of poor Cu dispersion. For Fischer-Tropsch synthesis over coprecipitated Fe/Cu/K/SiO2 catalyst, An et al.22 found that the residual sodium increased the catalyst particle sizes, restrained the reduction and carburization of active species, and thus reduced the level of CO conversion markedly and shifted the product distribution to lighter hydrocarbons slightly. The Ni/ZnO adsorbent used in RADS is usually obtained through the coprecipitation of an aqueous solution of nickel and zinc nitrates. A mixture of ammonium and ammonium carbonate, ammonium carbonate, or even urea has been successfully used as an alkaline precipitating agent to prepare the Ni/ZnO adsorbent by carefully controlling the pH value and temperature of the mixed solution for the coprecipitation,11-13,19 which does not leave residual sodium in the resultant adsorbent. On the other hand, a sodium alkali is also effective for obtaining the precipitate with the desired Ni/Zn ratio,14-16 while a negative effect of the residual sodium on the performance of the resultant absorbent is also expected. The residual sodium may be removed by extensive washing; however, a detailed understanding of the effect of residual sodium in Ni/ZnO on its performance for the RADS of diesel fuel needs to be clarified. In this work, the Ni/ZnO adsorbent with different contents of residual sodium was prepared via coprecipitation with sodium carbonate and by controlling the washing procedures. The Ni/ ZnO adsorbent was used in RADS of diesel fuel, and the effect of residual sodium on its structure and desulfurization performance was then investigated.
10.1021/ie100293h 2010 American Chemical Society Published on Web 04/23/2010
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2. Experimental Section 2.1. Adsorbent Preparation. To prepare the Ni/ZnO adsorbent, 120 mL of a mixed solution of nickel nitrate (0.25 M) and zinc nitrate (0.25 M) and 100 mL of a sodium carbonate solution (1 M) were dropped simultaneously into a beaker under vigorous stirring at 90 °C. The suspension obtained was stirred for an additional 2 h and aged for 12 h at room temperature, followed by filtration and washing with distilled water. Approximately 4.6 g of the adsorbent in the oxidized NiO/ZnO form (Ni/Zn atomic ratio of 1) was obtained after the precipitate was dried at 120 °C overnight and calcined at 400 °C for 3 h in air. To obtain a series of adsorbents with different residual sodium contents, we washed the precipitate or the calcined adsorbent samples for different periods of time, each time with 400 mL of deionized water under stirring for 30 min. The adsorbent in the reduced form (Ni/ZnO) was obtained by reducing NiO/ZnO in a stream of H2 (15 mL/min) at 400 °C and 6.0 MPa for 6 h. The prepared adsorbents are denoted as Cx-y, where x is the washing times of the precipitate before drying and y is the washing times of the sample after calcination. The content of residual sodium in the adsorbent was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). 2.2. Adsorbent Characterization. The BET surface area of the reduced adsorbents was measured by nitrogen sorption at -195.8 °C with a TriStar 3000 gas absorption analyzer (Micromeritics Instrument Co.). The samples were degassed at 200 °C and 6.7 Pa for 2 h prior to the measurement. X-ray diffraction (XRD) was performed on a Bruker AXS D8 advanced X-ray diffractometer (Cu KR, λ ) 0.15406 nm, 40 kV, 40 mA) in the step scanning mode (0.02° and 0.24 s per step) with a 2θ range from 20° to 80°. The average crystallite size was estimated from the line broadening of the most intense XRD reflections by the Scherrer formula. Thermogravimetric analysis (TGA) of the dried precipitate sample was performed on a Netzsch STA 409 C thermobalance. The sample was placed in a platinum crucible, and the measurement was taken in the range of 30-1000 °C at a heating rate of 10 °C/min, with air as the purge gas (30 mL/min). X-ray photoelectron spectroscopy (XPS) was performed on a VG Multilab 2000 electron spectrograph with Mg KR radiation and a multichannel detector. Approximately 100 mg of the sample powder was compressed into a wafer for analysis. The survey spectra were recorded in the binding energy (BE) range of 0-1100 eV; the detail spectra of C 1s, O 1s, Ni 2p, Zn 2p, and Na 1s were recorded in the ranges of 280-294, 525-535, 847-886, 1014-1050, and 1065-1077 eV, respectively. BE values were calibrated with the signal of contaminated carbon C 1s at 284.6 eV. The spectra were decomposed by using XPSPEAK (version 4.1) after application of a Shirley background substraction and Gaussian (80%)-Lorentzian (20%) decomposition parameters. Atomic ratios were calculated from the peak areas calibrated by the sensitive factors provided by the manufacturer of the equipment. 2.3. Desulfurization Test and Analytic Procedure. The diesel fuel feed used here is a cycling fraction after primary hydrodesulfurization obtained from a local FCC refinery. Its BP range is 200-350 °C, with a density of 0.89 g/mL. The total sulfur mass concentration in the feed fuel is 560.0 ppm. The feed was also analyzed by using a gas chromatograph (GC SP-3400, Beifen) equipped with a capillary column [DB-17, J&W Scientific, 30 m × 0.25 mm (inside diameter), 0.25 µm film thickness] and a flame photometric detector (FPD). A GCFPD chromatogram of the diesel fuel with identification of major
Figure 1. GC-FPD chromatograms of the diesel fuel feed (a) and the effluent fuels after RADS for 24 h on stream over the adsorbents with different residual sodium contents (with different washing times): (b) C0-0, (c) C10, (d) C2-0, (e) C1-4, (f) C2-4, and (g) C4-0.
peaks is shown in Figure 1; it indicates that the major sulfur compounds present are dibenzothiophene (DBT), 4-methyl DBT (4-MDBT), 4,6-dimethyl DBT (4,6-DMDBT), and 2,4,6-trimethyl DBT (2,4,6-TMDBT). The adsorption test for the RADS of diesel fuel was conducted in a stainless steel tubular flow microreactor with an internal diameter of 6.0 mm. Approximately 1.0 g of adsorbent sample in the oxidized form of NiO/ZnO (40-60 mesh) was used per run; the adsorbent bed was in the constant-temperature zone of the microreactor and embedded between glass wool plugs and quartz beads. A thermocouple was immersed in the adsorbent bed to measure the reaction temperature, and a back pressure regulator was used to regulate the reaction pressure. Prior to the RADS reaction, the adsorbent in the oxidized form (NiO/ZnO) was reduced to Ni/ZnO through an in situ activation process in a stream of H2 (15 mL/min) at 400 °C and 6.0 MPa for 6 h. After that, the adsorbent bed was cooled to 350 °C in the H2 stream, and then the diesel fuel was fed into the reactor with a liquid pump (1.8 mL/h). RADS was then performed on the Ni/ZnO adsorbent at 350 °C, 6 MPa, a H2/ liquid fuel volume ratio of 500, and a liquid hourly space velocity (LHSV) of 1.60 h-1. The liquid products were collected in a trap with an ice-water bath and subjected to analysis periodically. The total sulfur contents in the feed and liquid products were determined by a micro coulometer analyzer (Type LC-4, Luoyang-Shuangyang), and the sensitivity of analysis for the element S is ∼0.2 ppm. 3. Results and Discussion 3.1. Textural Properties of the Adsorbents with Different Residual Sodium Contents. By controlling the washing process, we can obtain a series of reduced Ni/ZnO adsorbents with different residual sodium contents; their surface areas and pore volumes are listed in Table 1. If the washing step is omitted, the resultant adsorbent (C0-0) contains 10.59 wt % sodium. The residual sodium content in the adsorbent (from C0-0 to C4-0) decreases substantially with the increase in washing time; it decreases to 0.03 wt % after the precipitate is washed four times. Meanwhile, the surface area and pore volume of the resultant Ni/ZnO adsorbent are strongly relevant to the content of residual sodium; with the decrease in the sodium content from 10.59 to 0.03 wt %, the surface area and pore volume increase from 2.0 m2/g and 0.02 cm3/g to 27.4 m2/g and 0.22 cm3/g, respectively. Even for the adsorbent samples after the calcination treatment, the content of residual
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Table 1. Textural Properties of the Reduced Ni/ZnO Adsorbents with Different Residual Sodium Contents adsorbenta
sodium content (wt %)
nickel content (wt %)
zinc content (wt %)
surface area (m2/g)
pore volume (cm3/g)
C0-0 C1-0 C2-0 C4-0 C1-4 C2-4
10.59 2.70 0.38 0.03 0.07 0.03
28.66 35.98 37.15 36.54 38.81 37.72
35.41 43.56 45.62 44.76 46.30 45.74
2.0 3.8 10.8 27.4 19.6 22.4
0.02 0.02 0.08 0.22 0.13 0.13
a For the adsorbent Cx-y, x is the washing time of the precipitate before drying and y is the washing times of the sample after calcination.
Figure 3. XRD patterns of the calcined NiO/ZnO adsorbents from the precipitates washed for different times: (a) C0-0, (b) C1-0, (c) C2-0, and (d) C4-0.
Figure 2. TGA and DTG profiles of the dried precipitates with different washing times for the preparation of the Ni/ZnO adsorbents: (a) C0-0, (b) C1-0, (c) C2-0, and (d) C4-0.
sodium can be decreased by further washing the calcined samples (C1-4 and C2-4), and the surface area and pore volume are also improved accordingly, although the washing efficiency for removal of the residual sodium from the calcined adsorbent is lower than that from the wet precipitate. The residual sodium present on the adsorbent surface may interfere with the reconstruction of adsorbent structure and promote the formation of large particle sizes of NiO (Ni) and ZnO phases with less surface area. Moreover, the Ni-Zn and NaZn(OH)3 species formed during the reduction treatment may also block the pores in the adsorbent. The Ni/ZnO adsorbent with a higher surface area and pore volume can be obtained by decreasing the residual sodium content in it through intensive washing. Similar results have also been reported for Cu/ZnO/Al2O3 and Fe/Cu/K/SiO2 catalysts.20-22 These strongly suggest a negative effect of the residual sodium on the adsorption performance of Ni/ZnO absorbent for the RADS of diesel fuel, since the desulfurization capability is related to its surface area. 3.2. TGA Results. The TGA profiles of the dried precipitates with different washing times for the preparation of Ni/ZnO adsorbents are shown in Figure 2. The precipitate that has not undergone the washing process (C0-0) exhibits four stages of weight loss during the TGA measurement. The first stage below 180 °C accounts for a weight loss of 6.3% due to the release of physically adsorbed water. The second stage between 200 and 380 °C is attributed to the release of hydration water to form nickel and zinc oxides that accounts for a weight loss of 16.4%.
The third stage started at ∼550 °C with a weight loss of 11.4% that is attributed to the decomposition of NaNO3 preserved in the precipitate.24 The fourth stage starts at ∼920 °C and can be ascribed to the decomposition of Na2CO3.25 For the precipitate washed once, the weight loss in the first and second stages is almost equivalent to that without washing, while the weight loss in the last two stages (550-1000 °C) due to the decomposition of NaNO3 and Na2CO3 is decreased substantially. The weight loss in the last two stages becomes negligible after the precipitate is washed two to four times, suggesting that NaNO3 and Na2CO3 are almost thoroughly removed; these are consistent with the ICP results listed in Table 1. As also shown in Figure 2, the peak temperatures in the second stage of weight loss for the C0-0, C1-0, C2-0, and C4-0 samples are 228.5, 279.3, 321.0, and 323.0 °C, respectively; the temperature for the release of hydration water shifts to a higher value with the decrease in the residual sodium content. This may suggest that the residual sodium suppresses the interaction between NiO and ZnO, which is favorable for the decomposition of the hydrated nickel and zinc oxides. The temperature for the formation of NiO/ZnO increases with the decrease in the residual sodium content in the precipitate; in any case, this stage can be accomplished via calcination at
