Al2O3 Poisoned by Thiophene during the

Regeneration of Poisoned Nickel Catalyst by Supercritical CO2 Extraction. L. Vradman, M. Herskowitz, E. Korin, and J. Wisniak. Industrial & Engineerin...
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Znd. Eng. Chem. Res. 1996,34, 3713-3717

3713

Regeneration of Ni/Al2O3 Poisoned by Thiophene during the Selective Hydrogenation of Styrene Pablo C. L’Argentiere, Doming0 A. LiprandiJ and Nora S. Figoli’ INCAPE-Institute de Investigaciones en Catcilisis y Petroquimica (FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina

The poisoning by thiophene and the regeneration of a NUN203 catalyst during the selective hydrogenation of styrene a t 353 K was studied. It was determined by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy that the sulfur species adsorbed during the catalyst poisoning is thiophene, which is adsorbed coplanarly to the surface. The catalyst was regenerated by H2 treatments for different times and at several pressures. Part of the sulfur remained irreversibly adsorbed after regeneration, and hence the activity and selectivity values corresponding to the nonpoisoned catalyst were never recovered. After the regeneration treatments at 473 K, a modification on the adsorbed sulfur electronic state was detected, which can be ascribed to thiophene hydrogenolysis, thus producing superficial sulfide adsorbed species. 1. Introduction

NUN203 catalysts are used for selective hydrogenation of dialkenes to monoalkenes in hydrocarbon mixtures. A major limitation on the useful lifetime of such catalysts is that they are severely poisoned by sulfurcontaining compounds. The interaction between these poisons and the metal has been studied by several authors (Ahmed et al., 1987; Barbier et al., 1990; Marecot et al., 1992). Despite its industrial and scientific importance, there are only a few publications about the regeneration of sulfur-poisoned nickel catalysts. Their main objective is the elimination of sulfur from the metal by reduction with hydrogen (Oliphant et al., 1978; Bartholomew et al., 19821, by oxidation with water or oxygen (RostrupNielsen, 19681, or by successive oxidation-reduction cycles (Aguinaga and Montes, 1992), in catalysts used in the hydrogenation of aromatics, methanation or hydrotreatment, reactions carried out at temperatures above 473 K. When thiophene is used as the poison under these conditions, it hydrogenolizes producing H2S, which is the actual poison. Hence, superficial metallic sulfides, such as Ni3S2, are formed, and they are more stable than bulk sulfides (Rostrup-Nielsen, 1968; Bartholomew et al., 1982). Several authors (Ahmed et al., 1987; Mirodatos et al., 1987) have pointed out that, in reactions carried out at low temperatures, thiophene is not hydrogenolized but is adsorbed coplanar to the surface, poisoning five Ni atoms per poison molecule (stoichiometry C4H4S/Ni = 0.2) o r in a perpendicular way through the sulfur atom, thus poisoning two Ni atoms per thiophene molecule (stoichiometry C4H4S/Ni = 0.5). Other authors (Blyholder and Bowen, 1962) have found, by IR spectroscopy, a C4H4S/Ni ratio approximate to 0.33. This ratio is indicative that although in the experimental conditions used thiophene is mainly adsorbed coplanar, a part of it may also be adsorbed perpendicular to the surface. The present paper refers to the deactivation by thiophene and t o the influence of pressure and time in the regeneration under reducing atmosphere of a Ni/ A1203 catalyst, taking the selective hydrogenation of styrene to ethylbenzene carried out at low temperature + Permanent address: Asignatura Quimica Inorganica, Departamento de Quimica General e Inorganica (FIQ-UNL), Santiago del Estero 2829, 3000 Santa Fe, Argentina.

(353 K) as a test reaction. Styrene is a useful model compound for investigating the catalytic performance of metal catalysts at a laboratory scale due to the presence of two types of unsaturated groups (Anderson et al., 1994).

2. Experimental Section 2.1. Catalyst Preparation. Alumina CK 300, cylinders of 1.5-mm diameter, having a 180 m2 g-l BET surface area and a pore volume of 0.52 mL g-l, was used as support. The support was calcined in air a t 773 K for 3 h prior to catalyst preparation. An aqueous solution of Ni(N03)~was used for impregnation in a suitable concentration to obtain a catalyst containing 18%Ni. The impregnation was carried out by means of the incipient wetness technique. The catalyst was then dried, reduced a t 773 K for 5 h under a Ha flow rate of 480 mL h-l g,,t.-l and passivated a t 373 K for 0.5 h under a 2%0 2 in N2 flow rate of 480 mL h-l gCat,-l. 2.2. Catalyst Characterization. 2.2.1. Dispersion. Dispersion was measured by hydrogen chemisorption following the method of Benson et al. (1973). For the fresh catalyst it was 43%. 2.2.2. X-ray Photoelectron Spectroscopy. The superficial electronic state of sulfur and the atomic ratios S/Ni were followed by X-ray photoelectron spectroscopy (XPS).Determinations were carried out on a Shimadzu ESCA 750 electron spectrometer coupled to a Shimadzu ESCAPAC 760 data system. The C 1s line was taken as an internal standard at 285.0 eV, and it was used to correct possible deviations caused by electric charge on the samples, as previously described (Seoane et al., 1992). 2.2.3. Infrared Spectroscopy. Poison adsorption on the catalysts was studied by infrared spectroscopy (FTIR) (Bell, 1980). Group frequency analysis was used t o verify the existence of the most characteristic absorption frequencies of sulfur in the infrared region (Blyholder and Bowen, 1962). Analyses were carried out on a Shimadzu FTIR 8101/8101 M single beam spectrometer with Fourier transform. The equipment has a Michelson type optical interferometer and a source of light covering the range from 4600 to 400 cm-l. Two chambers are available to improve the quality of the spectra. The first one has a pyroelectric detector made of a high sensitive LiTaO element, and the other has

0888-5885/95/2634-3713$09.00/00 1995 American Chemical Society

3714 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 an MCT detector and the possibility t o create a controlled N2 (or dry air) atmosphere. 2.3. Activity and Selectivity Determinations. Catalytic activity and selectivity were determined in a stirred tank reactor operated at 353 K, 22 kg cmT2 hydrogen pressure, and stirring velocity of 700 rpm, as previously described (L'ArgentiBre et al., 1993). The catalyst weight was 2 g. No diffusional limitations were observed under these conditions (Holland and Chapman, 1976). The selective hydrogenation of styrene to ethylbenzene was used as a test reaction. As previously reported (L'ArgentiBre and Figoli, 19901, ethylbenzene can be further hydrogenated to ethylcyclohexane, and polymerization of styrene can also occur as a side reaction. A 5% amount of styrene in toluene solution (feed I) or the same solution with the addition of 1000 ppm thiophene (feed 11) or 5000 ppm thiophene (feed 111) was used as feed. Reactants and products were chromatographically analyzed by means of an ionization detector and a DC 200 column. The catalytic activity was calculated from the conversion versus time plots as the number of styrene mol6cules reacted per second and per exposed metal atom (NT). Selectivity was calculated from the chromatographic data; to follow the polymerization consecutive reaction, benzene was added to the reaction mixture as an internal standard. 2.4. Procedure. The following steps were performed in order to study the sulfur poisoning and the regeneration of the NUN203 catalyst: (Step 1) The catalyst was reduced in situ a t 393 K for 10 h. Catalytic activity and selectivity were evaluated under the previously mentioned operational conditions, using the poison-free feed I. (Step 2) Fractions of the catalyst were evaluated changing feed I for feed I1 and other fractions, changing for feed 111. The operational conditions used were the same as in the previous step. (Step 3) The influence of time and hydrogen pressure in the catalyst regeneration was studied at constant temperature (473 K). Feeds I1 or I11 were drained from the reactor, and the poisoned catalysts were regenerated at 10 and 20 kg cm-l of hydrogen pressure, during 3, 10, and 20 h. (Step 4)All the catalyst samples obtained in step 3 were evaluated using the poison-free feed I, under the same operational conditions used in step 1. In order to determine if the regenerating procedures produce nickel sinterization, dispersion measurements were made over samples of fresh catalyst. These samples were treated under the same operational conditions used in the regeneration procedures (step 31, but in the absence of poison. The adsorbed sulfur superficial electronic state and the superficial SMi atomic ratio in the different steps were followed by XPS. The samples were introduced into the preparation chamber of the XPS equipment following the operation procedure described by Mallat et al. (19851, in order t o protect them from exposure to air. Determinations of the surface atomic S/Ni ratios were made by comparing the areas under the peaks after background subtraction and corrections due t o differencesin escape depths and in photoionization cross sections (Borade et al., 1990). Samples analyzed by FTIR were as follows: pure alumina, fresh Ni(18%)/A1203,catalyst poisoned with feed I1 or 111, and regenerated catalysts.

Table 1. Activity per Exposed Metal Atom (NT)and Selectivity to Ethylbenzene (SE) for the Ni(lB%)/AlzO3 Catalyst, Evaluated with Poison-Free Feed I, Feed I1 (Containing 1000 ppm Thiophene),and Feed I11 (Containing 5000 ppm Thiophene),and Superficial Sulfur Electronic State and S/Ni Atomic Ratio feed

NT (s-l)

without poison (I) 1000 ppm thiophene (11) 5000 ppm thiophene (111)

0.06 0.03 0.00

SE (%)

98.6 75.3

S2p (eV)

S/Ni (at./at.)

164.5 164.6

0.061 0.201

Table 2. Dispersion (D, %), Activity per Exposed Metal for the Atom (NT),and Selectivity to Ethylbenzene (SE) Ni(l8%)/Alz03Catalyst Regenerated during Different Times at Constant Temperature and Hz Pressure (473 K and 10 kg cm-2,Respectively), after Being Run with Feed 11, and Superficial Sulfur Electronic State and S/Ni Atomic Ratio regeneration time (h) 3 10 20

(%I

D

W1)

(%I

SE

S2p (ev)

S/Ni (at./at.)

43 42 42

0.035 0.039 0.040

82.1 85.4 86.0

161.2 161.0 161.1

0.050 0.047 0.044

NT

Table 3. Dispersion (0,%), Activity per Exposed Metal Atom (NT),and Selectivity to Ethylbenzene (Sa) for the Ni(18%)/Al~03 Catalyst Regenerated during Different Times at Constant Temperature and Hz Pressure (473 K and 20 kg cm-2,Respectively), after Being Run with Feed 11, and Superficial Sulfur Electronic State and Atomic Ratio S/Ni regeneration time (h) 3 10 20

NT

SE

(%I

(s-1)

(%I

S2p (eV)

S/Ni iatlat.1

42 40 40

0.041 0.045 0.047

88.3 89.7 91.9

161.2 161.2 161.1

0.041 0.036 0.035

D

3. Results Table 1 shows the activity (Nr) and selectivity (5') values obtained using feeds 1-111. Both activity and selectivity to ethylbenzene decrease in the presence of 1000 ppm thiophene (feed 11). However, a residual activity (number of styrene molecules reacted per second and per exposed metal atom in the presence of thiophene) remains; the catalyst poisoned with feed I11 has no residual activity. Table 1also presents the X P S values for the S 2p line and the superficial atomic ratios S/Ni for the catalyst poisoned with feeds I1 and 111. Tables 2 and 3 present the dispersion and the activity and selectivity to ethylbenzene obtained in step 4 (feed I) for the catalyst previously run with feed I1 and regenerated for different periods of time, at constant temperature (473 K) and a t 10 and 20 kg cm-2 of hydrogen pressure, respectively. These tables also present the electronic state of the superficial sulfur species and the atomic ratio S/Ni. There are no important differences in dispersion produced by the regenerating procedure. The activity and selectivity to ethylbenzene increase with regeneration time and hydrogen pressure, meanwhile the superficial atomic ratio SMi decreases. The activity and selectivity levels for the catalyst regenerated at 20 kg cm-2 (Table 3) are higher than those for the catalyst regenerated at 10 kg cmP2 of H2 pressure (Table 2), but the values corresponding to the nonpoisoned catalyst are not recovered. The S 2p peaks for every regeneration time appear shifted to lower binding energies than those for the catalyst poisoned with feed I1 or 111. The binding energies of the S 2p lines do not change with the length of regeneration under H2.

Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 3715 Table 4. Dispersion (0,%), Activity per Exposed Metal Atom (&), and Selectivity to Ethylbenzene (SE) for the Ni(18%)/&Os Catalyst Regenerated during Different Times at Constant Temperature (473 K) and at 10 and 20 kg cm-2 of H2 Pressure,after Being Run with Feed 111, and Superficial Sulfur Electronic State and S/Ni Atomic Ratio time pressure D NT SE S2p S/Ni (h) (kgcm-2) (%) (8-l) (%) (eV) (at./at.) 3 10 42 0.003 76.2 161.3 0.191 20 10 43 0.006 78.4 161.2 0.165 20 20 42 0.008 81.2 161.1 0.142

Table 4 presents the dispersion and the activity and selectivity t o ethylbenzene obtained in step 4 for the catalyst previously poisoned with feed I11 and regenerated a t 473 K during 3 and 10 h a t 10 kg cm-2 and during 20 h at 20 kg cm-2 of H2 pressure. Table 4 also shows the electronic state of the superficial sulfur species and the atomic ratio S/Ni. No significant differences in dispersion produced by the regeneration procedures were detected. The superficial atomic ratio S/Ni decreases and activity and selectivity increase with regeneration time and hydrogen pressure. As in the previous cases (Tables 2 and 3), the values corresponding to the catalyst evaluated with the poison-free feed were never recovered. The S 2p peaks, for the different times and hydrogen regeneration pressures, appear shifted to lower binding energies than those for the catalyst poisoned with feed I1 or 111. The binding energies of the S 2p lines do not change with the time or hydrogen regeneration pressure. FTIR spectra were taken by transmission and reflection spectroscopy (Blyholder, 1968). The same spectrum, corresponding to pure alumina, was obtained for the fresh, poisoned, and regenerated catalysts. 4. Discussion

The dispersion values for the fresh catalyst and of samples of fresh catalysts subjected to similar thermal treatments than the regenerated catalysts are indicative that the regeneration procedures used did not provoke metal sinterization. The catalyst evaluated with poison free feed is active and selective for the studied reaction. When the catalyst is poisoned with 1000 ppm thiophene, the activity is reduced about 50%. A residual activity remains, suggesting that thiophene does not affect all the metallic superficial sites. On the other hand, the catalyst poisoned with 5000 ppm thiophene (feed 111) has no residual activity. Table 2 shows that when the catalyst poisoned with feed I1 is regenerated at 473 K and 10 kg cm-2 of Hz pressure, a recovery in the activity occurs, which increases as the regeneration time raises. However, the activity of the fresh catalyst was not achieved. When the hydrogen pressure was raised up to 20 kg cm-2 during the regeneration step 3 (Table 31, the activity recuperation was higher than that corresponding to 10 kg cm-2 of H2 pressure; anyhow the activity recuperation was only 78%. Similar behavior was observed when the catalyst was poisoned with feed 111; Table 4 shows that after regeneration during 20 h a t 20 kg cm-2 of Hz pressure the activity only achieves 13% of the activity corresponding to the catalyst evaluated with feed I. This is indicative that not all the sulfur can be eliminated by the regenerating procedure. The activity recovery is proportional to the time and H2 pressure.

When the catalyst was poisoned with feed I1 the selectivity decreased 21% (Table 1). After regeneration, a selectivity recovery was observed (Tables 2-4), which, as the activity recovery, increases with time and the Ha pressure. However, the selectivity of the nonpoisoned catalyst was never recovered. The decrease in selectivity due to thiophene poisoning is understood considering that the poison adsorption may cause a partial electronic deficiency induced by sulfur in the metallic surface (Biloen et al., 1980; Hegedus and McCabe, 19811,which is possible considering the electron acceptor properties of sulfur. Other authors (MarBcot et al., 1993) stated that these electron transferences can modify the relative rates of superficial competitive reactions, thus modifylng the selectivity. The S 2p peak position for the catalyst poisoned with feed I1 or I11 (Table 1)shows that the poisoning species adsorbed is mainly thiophene as it has also been found by Ramachandran and Massoth, (1982). This is in agreement with the fact that n-butane and hydrogen sulfide were never detected a t the reactor outlet, suggesting that thiophene is not hydrogenolized during the poisoning step 2, under the operational conditions used in this work. The XPS results obtained after the regeneration step 3 (Tables 2-4) show a shift of approximately 3 eV t o lower binding energies(BEs1in the S 2p peak position, referred to the poisoned but not regenerated catalyst. The S 2p line BE for the catalyst regenerated for different times and at different hydrogen pressures suggests that now the poisoning species is mainly a nickel sulfide (Shalvoy et al., 1977),possibly Ni&, which is very stable, according to the literature (Bartholomew et al., 1982; Aguinaga et al., 1994). The S 2p peak position did not change when the time and hydrogen pressure were varied during step 3, indicating that the poisoning species is the same in spite of the regeneration conditions. These results suggest that, at the temperature and hydrogen pressures used during regeneration step 3, thiophene is hydrogenolized, thus producing superficial sulfide species. This phenomenon has also been observed by other authors (Ng and Martin, 1978). Nickel sulfides are difficult to eliminate (Bartholomew et al., 19821, and consequently it is not possible to recuperate the activity of the fresh catalyst. The superficial atomic ratio S/Ni obtained for the catalyst poisoned with feed I1 (Table 1) does not correspond to a surface completely covered by thiophene, which is consistent with the fact that the catalyst presents a residual activity. As previously mentioned, the catalyst poisoned with feed I11 is not active. This is in accordance with a S/Ni value of 0.201 (Table 11, which is very similar to the one corresponding to a surface completely covered by thiophene, considering that poisoning is confined t o the metallic surface (Ahmed et al., 1987). Taking into account this value, it may be stated that, under the operational conditions of this work, thiophene is mainly adsorbed coplanarly to the surface (MarBcot et al., 1992; Seoane et al., 1992). After regeneration, a decrease in the superficial atomic ratio S/Ni, proportional t o the time and the hydrogen pressure, was observed (Tables 2-41, in agreement with the recovery in activity and selectivity. However, as stated by Barbier et al. (1990), the catalysts poisoning by sulfur cannot be explained considering only the geometrical blockage of the metal. The modifications provoked by sulfur adsorption, i.e. charge transfer or interaction between the adsorbed sulfur species and the reactants, also modify the catalytic activity. Figure 1

3716 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 0050 I

I

1

+

e

0 n

0

0.125

I 0 250

S/Ni (at/at 1 Figure 1. Activity per exposed metal atom versus superficial S/Ni

atomic ratio. Feed 11: +, poisoned. (A) regeneration at 20 kg cm-2, filled box, 20 h; open box, 10 h; half-filled box, 3 h. (B) regeneration a t 10 kg cm-2, filled diamond, 20 h, open diamond, 10 h; halffilled diamond, 3 h. Feed 111: filled triangle, poisoned; filled circle, regenerated 20 h at 20 kg cm-2; open circle, regenerated 10 h at 10 kg cm-2; half-filled circle, regenerated 3 h a t 10 kg cm-2.

shows the activity versus the superficial atomic ratios SNi, for the different poisoning and regeneration steps. The modifications in activity caused by the sulfur adsorption are higher at low surface coverage. The shape of the curve is similar to that found by Barbier et al. (1990) when fresh and poisoned catalysts are compared, although in that case no regenerated catalysts were included. This means that the regenerating procedure restores a surface with similar catalytic properties to that partially poisoned, although in the former case the adsorbed species was thiophene and in the other a nickel sulfide was formed. These results indicate that sulfide species have the same poisoning effect as that of adsorbed thiophene. FTIR results of the poisoned samples show the total absence of thiophene characteristic group frequencies, suggesting the possibility of one of the following alternatives: (a) Presence of thiophene adsorbed coplanar to the catalyst surface, thus producing the disappearance of the vibrational bands characteristic of thiophene, as also found by Rochester et al., (1978) using IR techniques. The presence of thiophene perpendicularly adsorbed on the surface may be neglected, otherwise frequencies corresponding to the ring vibrations should be observed. The possible hydrogenation of thiophene t o thiophane may also be neglected, as its characteristic frequencies were not detected, in agreement with other authors (Ahmed et al., 1987). (b) Presence of a metallic sulfide on the surface generated by thiophene hydrogenolysis. Alternative a is in accordance with X P S results: The S 2p BE for the poisoned catalysts indicates the presence of thiophene. Moreover, the superficial atomic ratio S/Ni for the catalyst poisoned with feed I11 indicates that thiophene is mainly adsorbed coplanarly. In b nickel sulfide would appear (Hair, 1967) with low values of vibration frequencies, values not detected in the range available in our equipment. The FTIR results of the regenerated catalysts also show the total absence of thiophene characteristic frequencies. The presence of a nickel sulfide on the surface of these catalysts may be accepted from XPS results which show the presence of a sulfide species.

6. Conclusion

The poisoning of NVAl.203 by thiophene during the selective hydrogenation of styrene a t 353 K causes a decrease in activity and selectivity. Results from X P S and FTIR suggest that the poisoning species is thiophene, which is mainly adsorbed coplanar to the catalyst surface. The activity and selectivity to ethylbenzene increases after regeneration under hydrogen pressure, while the superficial atomic ratio S/Ni decreases; however, the values corresponding to the nonpoisoned catalyst are never recovered. After the regeneration treatments, a modification on the adsorbed sulfur electronic state was detected due to thiophene hydrogenolysis, thus producing superficial sulfide species. These species have the same poisoning effect as that of adsorbed thiophene. Dispersion is not modified during regeneration at 473 K, this being an important advantage of the proposed regenerating procedure.

Acknowledgment The authors are indebted t o the Japan International Cooperation Agency (JICA) for the donation of an ESCA 750 Shimadzu spectrometer and a Shimadzu FTIR8101/8101M. The experimental assistance of F. Pucheta and C. MBzzaro is greatly acknowledged. D.A.L. is also indebt to M. A. Ulla for her cooperation.

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Receiued for review January 4 , 1995 Accepted J u n e 20, 1995@ IE9500228

Abstract published in Advance ACS Abstracts, September 15, 1995. @