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73 Sonocatalytic desulfurization
343
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
73 Sonocatalytic Desulfurization
Carlos E. SCOTT, Jonathan R. ALTAFULI~, U R B I N A de N A V A R R O '
Carmelo BOLIVAR
and Caribay
Universidad Central de Venezuela, Centro de C a t . i s i s Petr61eo y Petroquimica, Apartado Postal 47102, Los Chaguaramos, Caracas 1041A, Venezuela. e.mail: cscott@strLx, ciens.ucv.ve 1 Centro de Microscopia Electrfnica.
Abstract Ultrasonic (US) irradiation of thiophene ( T ) , benzothiophene (BT) or dibenzothiophene (DBT) dissolved in a water.methanol mixture, and in the presence of Ni/AI or Ni/Zn catalysts at 333 K and autogenous pressure, was used to produce desufurization of the aforementioned compounds. The system could be effective for T desufurization, and BT and DBT desufurization can also be produced, but to a lesser extent. The role of ultrasonic (US) irradiation is to increase reaction rate, and to change the catalysts' morphology producing a cleaner surface and smaller particle size, which increases catalyst activity and effectiveness. 1. INTRODUCTION Due to tougher environmental legislation, and the need for processing heavier petroleum fractions, development of new processes to improve refining technologies is desirable. For example, hydrogen generated in situ via water gas shift reaction (613 K, 4.14 MPa of CO) from water can be effective for BT and T desufurization[1], and is several times more active than externally supplied hydrogen. On the other hand, US has been used to improve reaction rates for the hydrogenation of olefinic bonds in an aqueous medium using Zn dust and Ni chloride[2], where hydrogen is produced from water. However, it has been reported that Ni on A1 is more effective to hydrogenate aromatic hydrocarbons (US not used) using hydrogen obtained from water[3]. In the present work we have explored the use of ultrasonic (US) energy to carry out T, BT and DBT desufurization, in liquid phase. Ni or Zn on A1 catalysts were used. These conditions represent a major change in the field of catalytic desufurization. 2. EXPERIMENTAL Reactions were carried out in a specially designed reactor which consisted of a cylindrical glass test tube fitted with a gas tight vacuum valve. The reactor was fed
344 C.E.Scottet
al.
witll the reaction mixture and inserted in a Branson ultrasonic bath (1210 model working at 40 kHz and 400W). The reaction mixture consisted of the substrate (T, BT or DBT), ethanol, water, decaline (internal standard) and the catalysts (Ni/AI or Ni/Zn), prepared according to ref. 2. Reaction ~me and temperature were 6 h and 333 K, respectively. After catalyst separation, liquid reaction products were analyzed by gas chromatography (Perkin Elmer Autosystem XL). Conversions were worked out from the disappearance of the substrate. Gas samples were also analyzed by GC. Catalyst samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD was carried out in a Philips PW 1830 diffractometer equipped with a Co anode (k= 1.78897). SEM was carried out in a HITACHI S-500 microscope operated at 20 KeV and 50 mA. 3. RESULTS AND DISCUSSION Several preliminary experiments were carried out in order to ensure that any observed effect was due to the combination of US, reaction mixture and the catalyst. Thus, it was found that heating the reaction mixture up to 333 K, with mechanical stirring, produced no conversion at all for T hydrodesulphurization (HDS). Also, ultrasonic irradiation of T, water, and ethanol (with no catalysts) produced no conversion either. However, when all the components were present, and the reaction mixture irradiated with US, an important conversion for the desufurization of T is obtained (Table 1). Then, both US irradiation and the catalysts are definitively playing a role in the reaction. According to previous reports[ 1,2], one can expect that one role of the catalysts is to produce hydrogen from the water/alcohol mixture. In fact, GC analysis of the gaseous products using a thermal conductivity detector showed the presence of hydrogen. In this case we used a Ni/A1 catalyst, since it is reported to be more effective for aromatic hydrocarbon hydrogenation[3]. When a Ni/Zn catalyst was used, T conversion was much lower than the one obtained for a Ni/A1 catalyst (Table 1), which is in agreement with previous reports about the hydrogenation of aromatic hydrocarbon in conventional systems[3]. Selectivity obtained for gaseous reaction products is presented in Table 1. High selectivity towards production of butane is observed. This is quite different from the result obtained in conventional catalytic HDS experiments. Thus, for T HDS, at atmospheric pressure, on cobalt molybdenum catalysts, product selectiviW is[4]: cis-but-2-ene>trans-but-2-ene>l-butene>butane. In fact, the selectivity order for US assisted desulphurization is similar to that found for HDS of T on bulk MoS2 catalysts at 573 K and 3.3 MPa of hydrogen[5]. Then, for US assisted HDS of T, more amount of hydrogenated product (n.butane) is obtained, which may be due to the fact that hydrogen is generated in situ, and the in situ generated hydrogen is more effective than externally supplied hydrogen, and/or to the high temperatures and pressures generated inside the cavitating bubble during US irradiation[6]. For desufurization of BT and DBT in water/ethanol and irradiated with US, using Ni/A1 and Ni]Zn catalysts, low conversions, for both substrates and catalysts, were observed (Table 2), and Ni/A1 was more active than Ni/Zn. Main products obtained are ethylbenzene and biphenyl for desufurization of BT and DBT, respectively. Then, for the US assisted desufurization of DBT and BT in a water-methanol solvent, the preferred reaction pathway is the hydrogenolysis of the C-S bond with little or non hydrogenation of the aromatic reaction product.
345 Table 1.- US assisted T desuiurization. -Catalyst Conversion Product Selecfi.vity/% ..... /% Methane l"Butene Butane trans-But-2-ene cis-But-2-ene Ni/Al
44
< 1
3
55
29
12
ZnJA1 15 < 1 4 40 39 16 "Feed: T:Water:Ethanoi'i~'a~alYst'(0.08:0.4:l.0:0.14)'Temperature 333 K. Reaction time 6 l~'.Autogeneous pressure.
(a)
(b)
Figure 1. SEM micrographs showing the effect of US on the surface morphology of ]qi/A1 catalyst. (a) Conventional preparation; (b) US preparation.
346 C.E,Scott et
al.
'['able 2.- Sonocatalytic de_suiphurization of BT and DBT. Compound Catalyst l~roducts . . . . Conv./%_ BT Ni/A1 Ethy_!benzene and traces of ethylcycl0hexane 5 BT Ni/Zn Ethy!benzene and traces of ethylcyclohexane 3 _ DBT Ni/AI ......... Biphenyl 4 4 DBT Ni/Zn Biphenyl 1 -Feed" Substrate:Water:Ethanoi:Catalyst (0.08:0.4:1'.0:0.i4). Temperature 333 K. Reaction time 6 h.Autogeneous pressure. Scanning electron micrographs (SEM) of Ni/A1 catalysts, conventionally and ultrasonically prepared, are shown in Figures 1. It can be observed that US has two main effects on catalyst morphology. The first one is to produce a macroscopic :smoothing of the surface. This effect is the result of particle collision at a glancing angle produced by bubble collapse at the solid surfaces during the cavitation :phenomenon (which is the principal effect of US irradiation of liquids), thus removing surface oxide and other cont_Aminating coatings which maintains catalytic activity. A :second effect is to reduce particle size, which increases active surface area, and as a :~-onsequence, also augments catalyst effectiveness. XRD patterns for the freshly prepared catalysts show lines that are assigned to metallic A1 and Ni. After reaction AI lines disappeared and Ni lines greatly decreased :in intensity. The decrease in A1 diffraction lines after Ni deposition or reaction (assisted by US) could be due to the formation of amorphous A1 oxychloride (since the catalyts were prepared from NiC12) and oxide, as proposed for Zn in Ni/Zn catalysts[3], and/or to a decrease in crystal size. 4. CONCLUSIONS The effectiveness of ultrasonic irradiation to assist T HDS, in the presence of Ni/A1 catalysts in water.ethanol solutions, has been shown. Desufurization of BT and DBT is also effected, but to a lesser extent. Ultrasound has three main effects on the :reaction system: a) Increases the rate of reaction, b) changes catalyst morphology (cleaner surface) and produces smaller particle sizes, which increases catalyst activity and effectiveness, and c) dissociates water and ethanol (in presence of the catalysts) to produce the hydrogen needed for the HDS reactions. 5. ACKNOWLEDGEMENT The authors are gratefltl to FONACIT (G-97000658) for its financial support. 6. REFERENCES.
[.1] F.T.T. Ng, I.K. Milad, Applied Catal., 200(2000)243. [2] C. Petrier, J.L. Luche, Tetrahedron Letters, 28(1987)2347. [3] K. Sakai, M. Ishige, H. Kono, I. Motoyama, K Watanabe, K Hata, Bull. Chem. Soc. Japan. 41(1968)1902. [4] C.E. Scott, PhD Thesis, Reading University, U.K., 1984. [:5] C.E. Scott, B.P. Embaid, M.A. Luis, F. Gonzalez-Jimenez, L. Gengembre, R. Hubaut, J. Grimblot, Bull. Soc. Chim. Belg., 104(1995)331. ',:6] K. S. Suslick, G.J. Price, Ann. Rev. Mater. Sci., 29(1999)295.
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
73 Sonocatalytic Desulfurization
Carlos E. SCOTT, Jonathan R. ALTAFULI~, U R B I N A de N A V A R R O '
Carmelo BOLIVAR
and Caribay
Universidad Central de Venezuela, Centro de C a t . i s i s Petr61eo y Petroquimica, Apartado Postal 47102, Los Chaguaramos, Caracas 1041A, Venezuela. e.mail: cscott@strLx, ciens.ucv.ve 1 Centro de Microscopia Electrfnica.
Abstract Ultrasonic (US) irradiation of thiophene ( T ) , benzothiophene (BT) or dibenzothiophene (DBT) dissolved in a water.methanol mixture, and in the presence of Ni/AI or Ni/Zn catalysts at 333 K and autogenous pressure, was used to produce desufurization of the aforementioned compounds. The system could be effective for T desufurization, and BT and DBT desufurization can also be produced, but to a lesser extent. The role of ultrasonic (US) irradiation is to increase reaction rate, and to change the catalysts' morphology producing a cleaner surface and smaller particle size, which increases catalyst activity and effectiveness. 1. INTRODUCTION Due to tougher environmental legislation, and the need for processing heavier petroleum fractions, development of new processes to improve refining technologies is desirable. For example, hydrogen generated in situ via water gas shift reaction (613 K, 4.14 MPa of CO) from water can be effective for BT and T desufurization[1], and is several times more active than externally supplied hydrogen. On the other hand, US has been used to improve reaction rates for the hydrogenation of olefinic bonds in an aqueous medium using Zn dust and Ni chloride[2], where hydrogen is produced from water. However, it has been reported that Ni on A1 is more effective to hydrogenate aromatic hydrocarbons (US not used) using hydrogen obtained from water[3]. In the present work we have explored the use of ultrasonic (US) energy to carry out T, BT and DBT desufurization, in liquid phase. Ni or Zn on A1 catalysts were used. These conditions represent a major change in the field of catalytic desufurization. 2. EXPERIMENTAL Reactions were carried out in a specially designed reactor which consisted of a cylindrical glass test tube fitted with a gas tight vacuum valve. The reactor was fed
344 C.E.Scottet
al.
witll the reaction mixture and inserted in a Branson ultrasonic bath (1210 model working at 40 kHz and 400W). The reaction mixture consisted of the substrate (T, BT or DBT), ethanol, water, decaline (internal standard) and the catalysts (Ni/AI or Ni/Zn), prepared according to ref. 2. Reaction ~me and temperature were 6 h and 333 K, respectively. After catalyst separation, liquid reaction products were analyzed by gas chromatography (Perkin Elmer Autosystem XL). Conversions were worked out from the disappearance of the substrate. Gas samples were also analyzed by GC. Catalyst samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD was carried out in a Philips PW 1830 diffractometer equipped with a Co anode (k= 1.78897). SEM was carried out in a HITACHI S-500 microscope operated at 20 KeV and 50 mA. 3. RESULTS AND DISCUSSION Several preliminary experiments were carried out in order to ensure that any observed effect was due to the combination of US, reaction mixture and the catalyst. Thus, it was found that heating the reaction mixture up to 333 K, with mechanical stirring, produced no conversion at all for T hydrodesulphurization (HDS). Also, ultrasonic irradiation of T, water, and ethanol (with no catalysts) produced no conversion either. However, when all the components were present, and the reaction mixture irradiated with US, an important conversion for the desufurization of T is obtained (Table 1). Then, both US irradiation and the catalysts are definitively playing a role in the reaction. According to previous reports[ 1,2], one can expect that one role of the catalysts is to produce hydrogen from the water/alcohol mixture. In fact, GC analysis of the gaseous products using a thermal conductivity detector showed the presence of hydrogen. In this case we used a Ni/A1 catalyst, since it is reported to be more effective for aromatic hydrocarbon hydrogenation[3]. When a Ni/Zn catalyst was used, T conversion was much lower than the one obtained for a Ni/A1 catalyst (Table 1), which is in agreement with previous reports about the hydrogenation of aromatic hydrocarbon in conventional systems[3]. Selectivity obtained for gaseous reaction products is presented in Table 1. High selectivity towards production of butane is observed. This is quite different from the result obtained in conventional catalytic HDS experiments. Thus, for T HDS, at atmospheric pressure, on cobalt molybdenum catalysts, product selectiviW is[4]: cis-but-2-ene>trans-but-2-ene>l-butene>butane. In fact, the selectivity order for US assisted desulphurization is similar to that found for HDS of T on bulk MoS2 catalysts at 573 K and 3.3 MPa of hydrogen[5]. Then, for US assisted HDS of T, more amount of hydrogenated product (n.butane) is obtained, which may be due to the fact that hydrogen is generated in situ, and the in situ generated hydrogen is more effective than externally supplied hydrogen, and/or to the high temperatures and pressures generated inside the cavitating bubble during US irradiation[6]. For desufurization of BT and DBT in water/ethanol and irradiated with US, using Ni/A1 and Ni]Zn catalysts, low conversions, for both substrates and catalysts, were observed (Table 2), and Ni/A1 was more active than Ni/Zn. Main products obtained are ethylbenzene and biphenyl for desufurization of BT and DBT, respectively. Then, for the US assisted desufurization of DBT and BT in a water-methanol solvent, the preferred reaction pathway is the hydrogenolysis of the C-S bond with little or non hydrogenation of the aromatic reaction product.
345 Table 1.- US assisted T desuiurization. -Catalyst Conversion Product Selecfi.vity/% ..... /% Methane l"Butene Butane trans-But-2-ene cis-But-2-ene Ni/Al
44
< 1
3
55
29
12
ZnJA1 15 < 1 4 40 39 16 "Feed: T:Water:Ethanoi'i~'a~alYst'(0.08:0.4:l.0:0.14)'Temperature 333 K. Reaction time 6 l~'.Autogeneous pressure.
(a)
(b)
Figure 1. SEM micrographs showing the effect of US on the surface morphology of ]qi/A1 catalyst. (a) Conventional preparation; (b) US preparation.
346 C.E,Scott et
al.
'['able 2.- Sonocatalytic de_suiphurization of BT and DBT. Compound Catalyst l~roducts . . . . Conv./%_ BT Ni/A1 Ethy_!benzene and traces of ethylcycl0hexane 5 BT Ni/Zn Ethy!benzene and traces of ethylcyclohexane 3 _ DBT Ni/AI ......... Biphenyl 4 4 DBT Ni/Zn Biphenyl 1 -Feed" Substrate:Water:Ethanoi:Catalyst (0.08:0.4:1'.0:0.i4). Temperature 333 K. Reaction time 6 h.Autogeneous pressure. Scanning electron micrographs (SEM) of Ni/A1 catalysts, conventionally and ultrasonically prepared, are shown in Figures 1. It can be observed that US has two main effects on catalyst morphology. The first one is to produce a macroscopic :smoothing of the surface. This effect is the result of particle collision at a glancing angle produced by bubble collapse at the solid surfaces during the cavitation :phenomenon (which is the principal effect of US irradiation of liquids), thus removing surface oxide and other cont_Aminating coatings which maintains catalytic activity. A :second effect is to reduce particle size, which increases active surface area, and as a :~-onsequence, also augments catalyst effectiveness. XRD patterns for the freshly prepared catalysts show lines that are assigned to metallic A1 and Ni. After reaction AI lines disappeared and Ni lines greatly decreased :in intensity. The decrease in A1 diffraction lines after Ni deposition or reaction (assisted by US) could be due to the formation of amorphous A1 oxychloride (since the catalyts were prepared from NiC12) and oxide, as proposed for Zn in Ni/Zn catalysts[3], and/or to a decrease in crystal size. 4. CONCLUSIONS The effectiveness of ultrasonic irradiation to assist T HDS, in the presence of Ni/A1 catalysts in water.ethanol solutions, has been shown. Desufurization of BT and DBT is also effected, but to a lesser extent. Ultrasound has three main effects on the :reaction system: a) Increases the rate of reaction, b) changes catalyst morphology (cleaner surface) and produces smaller particle sizes, which increases catalyst activity and effectiveness, and c) dissociates water and ethanol (in presence of the catalysts) to produce the hydrogen needed for the HDS reactions. 5. ACKNOWLEDGEMENT The authors are gratefltl to FONACIT (G-97000658) for its financial support. 6. REFERENCES.
[.1] F.T.T. Ng, I.K. Milad, Applied Catal., 200(2000)243. [2] C. Petrier, J.L. Luche, Tetrahedron Letters, 28(1987)2347. [3] K. Sakai, M. Ishige, H. Kono, I. Motoyama, K Watanabe, K Hata, Bull. Chem. Soc. Japan. 41(1968)1902. [4] C.E. Scott, PhD Thesis, Reading University, U.K., 1984. [:5] C.E. Scott, B.P. Embaid, M.A. Luis, F. Gonzalez-Jimenez, L. Gengembre, R. Hubaut, J. Grimblot, Bull. Soc. Chim. Belg., 104(1995)331. ',:6] K. S. Suslick, G.J. Price, Ann. Rev. Mater. Sci., 29(1999)295.