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Ind. Eng. Chem. Res. 2008, 47, 6904–6916
Ultradeep Adsorption-Desulfurization of Gasoline with Ni/Al-SiO2 Material Catalytically Facilitated by Ethanol Miron V. Landau,*,† Moti Herskowitz,† Rajeev Agnihotri,‡ and James E. Kegerreis‡ Blechner Center for Applied Catalysis and Process DeVelopment, Chemical Engineering Department, Ben-Gurion UniVersity of the NegeV, Beer-SheVa, Ben-Gurion aVenue 1, 84105, Israel, and ExxonMobil Research and Engineering Company, Fairfax, Virginia 22037
Adsorptive desulfurization of low-sulfur (22 ppmw) gasoline on solids in a liquid-full (no hydrogen) fixed bed was conducted at 503 K. Addition of 3 wt % ethanol to gasoline increased significantly the adsorption rate of sulfur-containing hydrocarbons on Ni/Al-SiO2 and its sulfur capacity, leading to 2 wt %. 3.2. Blocking the Catalyst Surface with Coke Deposits. Heavy products of oligomerization of olefins present in gasoline or produced from EtOH decomposition could block the active metallic nickel surface, decreasing its ability for adsorption of sulfur compounds. This was tested with Ni/Al-SiO2 at 230 °C
Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 6907 Table 1. Texture Characteristics and Carbon/Sulfur Content of Reduced Ni/Al-SiO2 Sorbent after Desulfurization of Gasoline at Different Setup Conditions (T ) 503 K, LHSV ) 3 h-1) entry no.
sorbent history
1 2
reduction, 723 K, H2 reduction, running EtOHsn-decane mixture: 220 h reduction, running gasoline: 140 h 210 h reduction, running gasoline-ethanol mixture: 110 h 300 h reduction, running gasoline in presence of gas produced from EtOH decomposition: 110 h 210 h reduction, running gasoline in presence of hydrogen: 235 h
3 4
5
6
surface area, m2/g
pore volume, cm3/g
average pore diameter, nm
265 232
0.28 0.26
4.2 4.4
46 43
0.05 0.04
4.3 3.7
103 70
0.12 0.09
4.8 6.5
110 66 115
0.13 0.10 0.18
4.7 6.3 4.5
and 3 h-1 running 8 wt % n-hexene-1 in n-decane for 140 h and then switching to 3 wt % EtOH in gasoline for 100 h. No effect compared with the performance of freshly reduced Ni/ Al-SiO2 was detected. A similar result was recorded in a test conducted with 3 wt % EtOH in decane for 220 h. It indicates that olefins from various sources have a negligible effect on the activity of the sorbent. The characteristics of spent Ni/ Al-SiO2 after running 3 wt % EtOH in n-decane for 220 h were compared with that of the freshly reduced material (Table 1, entries 1 and 2). The change in the sorbent texture was negligible (reducing the surface area by 10% and the total pore volume by 7%). The slight increase of the average pore diameter corresponded to blocking of micropores with coke deposits (2.8 wt % accumulated carbon) decreasing the micropore surface area by 27%. The sorbent texture changed significantly after running gasoline (Table 1, entries 1, 3, and 4). With pure gasoline, the sorbent surface area and pore volume decreased from 265 to 46 m2/g and from 0.28 to 0.05 cm3/g, respectively, after 140 h on stream, blocking micropores by coke accumulation (5.6 wt % carbon). This reflects a strong pore-blocking effect. An additional 70 h on stream changed little the texture characteristics, increasing the carbon content by 673 K ethanol disproportionation produces carbon deposition manifested as carbon nanofibers or nanotubes.34 The contribution of the dehydration route (C2H5OH f C2H4 + H2O) to the total ethanol conversion with Ni/ Al-SiO2 at selected conditions was negligible due to the absence of C2 products. 3.4. Effect of Ethanol on the Sorbent Phase Composition. The addition of ethanol to gasoline changed significantly the phase composition of spent sorbent. An XRD diffractogram of Ni/Al-SiO2, before reduction, displayed an amorphous halo centered at 23° related to the silica gel and reflections characteristic of NiO phase (peaks at 2θ ) 37.25, 43.28, and 62.88°; bunsenite, cubic structure; ICGDD Card 43-1469) with a crystal domain size of 3 nm (Figure 3, curve 1). After
6908 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008
Figure 3. XRD patterns of Ni/Al-SiO2 sorbent recorded after air calcination at 773 K (1), H2 reduction at 723 K (2), running gasoline for 140 h (3), and running 3% EtOH-gasoline for 300 h (4) (conditions of Figure 1).
reduction, the intensity of these peaks decreased (Figure 3, curve 2), while reflections at 2θ ) 44.50, 51.84, and 76.37° related to metallic nickel with a cubic structure (ICGDD Card 04-0850) and crystal domain size of 4.5 nm appeared. After running gasoline for 140 h, the XRD patterns (Figure 3, curve 3) did not change relative to that recorded with the freshly reduced material. Adding EtOH to gasoline and running for 300 h generated major changes in the phase composition. Reflections characteristic of NiO phase disappeared, and peaks characteristic for nickel carbide (Ni3C; 2θ ) 45.00, 41.92, 39.55, and 59.00°; hexagonal structure; ICGDD Card 72-1467) and nickel sulfide phases Ni3S2 (2θ ) 21.7, 31.10, 55.16, and 49.73°; ICGDD Card 44-1418) and Ni3S4 (2θ ) 26.59, 31.33, 37.98, 54.79, and 49.99°; ICGDD Card 43-1469) (Figure 3, curve 4) were detected. The same XRD patterns with lower intensities of the peaks corresponding to nickel sulfides were detected in the spent sorbent after 110 h of running the EtOH-modified gasoline (not shown). A metallic nickel phase with hexagonal structure and XRD patterns close to that of Ni3C carbide (2θ ) 44.52, 41.53, and 78.00°; ICGDD Card 45-1027) is known.36 Therefore the formation of nickel carbide phase was confirmed by XPS spectra. The main contribution to the C 1s XPS spectra recorded after running gasoline was a peak centered at 284.8 eV with a wide component of low intensity at 285-290 eV (Figure 4a). After running the EtOH-gasoline mixture, the peak of C 1s core consisted of three contributions centered at 286.5, 284.8, and 283.5 eV (Figure 4b). The main peak in Figure 4a and contribution to the peak at 284.6 eV in Figure 4b are attributed to amorphous (graphitic) carbon.37,38 The wide peak at 285-290 eV is attributed to surface contamination, i.e., with carbonyl compounds.39 The main contribution at 283.5 eV (Figure 4b) is due to carbon in the Ni3C phase,40 confirming its formation in the presence of EtOH in agreement with XRD data. The peak at 286.5 eV is ascribed to the C-OH bond41 and appears in the carbon deposits accumulated by the sorbent at an amount higher than that of graphitic carbon only after running of EtOHmodified gasoline. This can be a result of the participation of ethanol in the condensation reactions leading to the accumulation of coke.
Figure 4. X-ray photoelectron spectra of C 1s core level recorded with Ni/Al-SiO2 after running at conditions of Figure 1: gasoline for 140 h (a); 3% EtOH-gasoline mixture for 110 h (b).
The nickel carbide phase can be formed by the chemical interaction of nickel-containing phases in reduced Ni/Al-SiO2 (NiO and/or metallic nickel) with the gaseous products of ethanol decomposition. Adsorption of the carbon-containing molecule (CH4 or CO) on metallic nickel nanoparticles, their decomposition, and diffusion of carbon into the nickel metal forms nickel carbide.42 This process is slow and at 473-673 K leads to formation of thin carbide films on the surface of metallic nickel.42,43 The formation of bulk Ni3C according to the Boudouart reaction between nickel nanoparticles and carbon oxide44 requires higher temperatures than that used in our runs. It is unlikely that the appearance of Ni3C phase with the diffracting crystallite size of 25 nm at the amount of 40 wt % (XRD data) is due to reaction of reduced Ni0 nanoparticles in the sorbent material with the products of ethanol decomposition. This is consistent with the constant content of cubic Ni0 phase during the run, detected by quantitative XRD phase analysis in the sorbent. The NiO phase in the fresh sorbent after its partial reduction disappeared in the spent sorbent (Figure 2, curve 4). The formation of nickel carbide Ni3C according to consecutive reactions of NiO with CH4 and CO is favored thermodynamically at relatively low temperatures of 1 ppmw) was observed after 25 h on stream. Changing the feedstock after 50 h on stream for 3 wt % EtOH in gasoline dropped the outlet sulfur to 1 ppmw after 20 h. Gas feeding at this point returned the residual sulfur content in the gasoline to
