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Selective hydrogenation of citronellal to citronellol over polymer-stabilized noble metal colloids
Reactive & Functional Polymers 44 (2000) 21–29 www.elsevier.com / locate / react
Selective hydrogenation of citronellal to citronellol over polymer-stabilized noble metal colloids Weiyong Yu, Hanfan Liu*, Manhong Liu, Zhijie Liu Polymer Chemistry Laboratory, Chinese Academy of Sciences and China PetroChemical Corporation, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 23 February 1999; accepted 30 July 1999
Abstract Citronellal was hydrogenated to citronellol by polymer-stabilized Pt and Ru colloids. The metal cations increased both the activity and the selectivity of the colloids. The modification was assumed to be due to the adsorbed metal cations activating the C=O double bonds, thus accelerating the reaction rate and increasing the selectivity to unsaturated alcohols. 2000 Elsevier Science B.V. All rights reserved. Keywords: Selective hydrogenation; Colloid; Platinum; Ruthenium; Metal cation
1. Introduction Materials with dimensions of 1–100 nm display novel properties in electronics, magnetics, optics and chemistry. Many researchers pay a lot of attention to these themes [1–3]. In the field of catalysis, nanosized metal colloids or clusters, being looked at as bridges between heterogeneous catalysts and homogeneous catalysts, are expected to show singular catalytic properties [1–4]. Metal colloids dispersed in liquid medium can be divided into four categories, that is: solvent-stabilized, surfactantstabilized, ligand-stabilized and polymer-stabilized metal colloids. Metal colloids of the first *Corresponding author. Tel.: 1 86-10-6625-54487; fax: 1 8610-6625-69564. E-mail address: [email protected] (H. Liu)
three categories are usually unstable when they are used as catalysts even when the reaction is conducted under ambient conditions. Thus, the polymer-stabilized metal colloids are extremely important for their catalytic use. Some researchers have reported novel performances of nanoscale monometallic and bimetallic colloids in many reactions [5–12]. To get unsaturated alcohols by selective hydrogenation of unsaturated aldehydes is a crucial step in the preparation of many fine chemicals. It is difficult to hydrogenate the carbonyl group and reserve the olefinic function, since the C=C double bond is easily hydrogenated over most conventional catalysts to give saturated aldehydes as the primary products [13]. Boudart [14] has pointed out that it is important to achieve this result by employing metal colloids or clusters.
1381-5148 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00073-5
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W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Citronellal (3,7-dimethyl-6-octenal, 1) is a non-conjugated unsaturated aldehyde, its corresponding unsaturated alcohol is citronellol (3,7dimethyl-6-octene-1-ol, 2). They are two essential components in many kinds of perfumes and the manufacture of them is profitable. Some papers have been published on the selective hydrogenation of citral to 2 over conventional supported catalysts [15–19], but the selective hydrogenation of 1 to 2 is seldom described. We found that by using polymer-stabilized platinum and ruthenium colloids as catalysts, 1 can be selectively hydrogenated to 2 with high selectivity; the incorporation of some metal cations can increase both the reactivity and the selectivity of the colloid catalysts. Here, we report the research results.
2. Experimental
the form of dark-brown homogeneous dispersion. Polymer-stabilized ruthenium colloid (PVP–Ru) was prepared by reducing RuCl 3 ? 3H 2 O with NaBH 4 to give a dark-brown homogeneous dispersion [21]. Hydrogenation of 1 was carried out in a 100-ml stainless steel autoclave. The reaction solution contained 40 ml EtOH, 1 mg NaOH and 0.50 g of 1 (3.24 3 10 23 mol), 1.00 g ethylene glycol (as an internal standard for gas chromatography), 20 ml PVP–Pt or PVP-Ru dispersion hcontaining Pt or Ru (2.340 3 10 25 mol), PVP [1.170 3 10 23 mol (monomeric unit)], 10 ml H 2 0 and 10 ml EtOHj and metal salt added. H 2 was charged several times to replace air and the final pressure of H 2 was 6.0 MPa. The hydrogenation reaction was performed at 333 K for 2 h. The reaction products were periodically analyzed by gas chromatography.
2.1. Materials and instruments 3. Results and discussion Poly(N-vinyl-2-pyrrolidone) (PVP, average molecular weight 40 000) was supplied by BASF. Compounds 1 and 2 were purchased from Acros and were redistilled under reduced pressure before use. Other reagents were purchased from Beijing Chemicals and were of analytical grade. Hydrogen (H 2 ) with a purity of 99.999% was supplied by Beijing Gases Factory. Transmission electron microscopy (TEM) photographs were taken by using a Hitachi 9000NAR instrument. Specimens were prepared by placing a drop of the colloid dispersion upon a copper grid covered with a perforated carbon film and then evaporating the solvent. The particle diameters were measured from the enlarged photographs. The particle size distribution histogram was obtained on the basis of the measurements of about 300 particles.
2.2. Hydrogenation reaction Polymer-stabilized platinum colloid (PVP– Pt) was prepared by a reported method [20] in
3.1. Characterization of PVP–Pt colloid The TEM photograph and the corresponding particle size distribution histogram of the platinum colloid are shown in Fig. 1. From Fig. 1, it can be seen that the average diameter of PVP–Pt colloid was 1.1 nm and the size distribution was narrow within the range of 0.6 to 1.8 nm with s 5 0.30 nm).
3.2. Characterization of PVP–Ru colloid The TEM photograph and the corresponding particle size distribution histogram of the ruthenium colloid are also obtained (Fig. 2). It can be seen from Fig. 2 that the average diameter of PVP–Ru colloid was 1.4 nm and the size distribution was narrow in the range of 0.44 to 2.94 nm with s 5 0.51 nm.
3.3. Selective hydrogenation of 1 to 2 over PVP–Pt colloid In the preparation of fine chemicals, it is an
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Fig. 1. Electromicroscopic photograph (left) and the corresponding particle size histogram (right) of PVP–Pt colloid.
Fig. 2. Electromicroscopic photograph (left) and the corresponding particle size histogram (right) of PVP–Ru colloid.
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W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Scheme 1.
important step to obtain unsaturated alcohols by selective hydrogenation of the corresponding unsaturated aldehydes. Compounds 1 and 2 are typical non-conjugated unsaturated aldehyde and unsaturated alcohol. The reaction pathway of the hydrogenation of 1 can be schematically shown as Scheme 1. It is very easy for ordinary heterogeneous and homogeneous catalysts to reduce the C=C double bond to yield 3,7-dimethyloctanal (3) as the main product [Scheme 1, reaction (2)]; however, it is comparatively difficult for them to reduce the CO double bond only to get 2 with high selectivity at high conversion of 1 [Scheme 1, reaction (1)] [13].
Under the reaction conditions described above, 70.6% conversion of 1 with 44.5% selectivity for 2 was obtained when the neat PVP–Pt colloid was employed alone as a catalyst; nonetheless, the activity was increased by 40% (conversion of 1 reached 98.3%) and the selectivity for 2 increased to 98.5% when Co 21 was introduced into the catalytic system (the time courses of them were recorded in Figs. 3 and 4, respectively). From previous work [22,23], we have known that some cations (Fe 21 , Co 21 , Ni 21 etc.) can markedly increase both the activity and the selectivity in the selective hydrogenation of cinnamaldehyde (3-phenyl-2-propenal) to cinnamyl alcohol (3-phenyl-2-propene-1-ol) catalyzed by PVP–Pt colloid. And it has been verified that non-strongly coordinating anions, 2 22 2 2 such as Cl , SO 4 , NO 3 , OAc etc. act merely as spectator ions [22]. The modification of various metal cations to this reaction system was also investigated. It can 1 1 1 be seen from Table 1 that Li , Na and K did not affect the catalytic properties of PVP–Pt; 21 21 21 Mg , Ca and Ba increased a little on the 21 selectivity and the activity; Mn enhanced the conversion to 97.8% and the selectivity to
Fig. 3. Hydrogenation of 1 over PVP–Pt colloid.
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
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Fig. 4. Hydrogenation of 1 over PVP–Pt colloid with modification of Co 21 (molar ratio of Co 21 : Pt 5 1:1).
74.7%; Cu 21 , Pb 21 and Zn 21 acted as poisons to the activity of PVP–Pt colloid (conversions of 1 were 0, 0 and 25.0% to Cu 21 , Pb 21 and Zn 21 , respectively, but the selectivity was 99.6% to Zn 21 ). Ce 31 , Nd 31 and Al 31 perTable 1 Hydrogenation of 1 over PVP–Pt–MCl x systems a Catalytic system
Conversion of 1 (%)
Selectivity b (%) 2
4
PVP–Pt PVP–Pt–LiCl PVP–Pt–NaCl PVP–Pt–KCl PVP–Pt–MgCl 2 PVP–Pt–CaCl 2 PVP–Pt–Ba(NO 3 ) 2 PVP–Pt–MnCl 2 PVP–Pt–CuCl 2 PVP–Pt–ZnCl 2 PVP–Pt–Pb(NO 3 ) 2 PVP–Pt–FeCl 3 PVP–Pt–FeSO 4 PVP–Pt–CoCl 2 PVP–Pt–NiCl 2 PVP–Pt–CeCl 3 PVP–Pt–Nd(NO 3 ) 3 PVP–Pt–AlCl 3
70.6 65.2 72.3 68.8 75.1 73.6 73.8 97.8 0 25.0 0 97.6 97.5 98.3 97.2 68.5 69.1 73.3
44.5 41.3 46.2 45.8 55.3 50.0 57.5 74.7 – 99.6 – 97.6 97.6 98.5 88.9 47.4 45.0 42.2
46.0 58.7 53.8 54.2 44.7 50.0 42.5 25.3 – 0.4 – 2.4 2.4 1.5 11.1 52.6 55.0 57.8
a
Molar ratio of M:Pt 5 1:1. The yield of 3,7-dimethyloctanal (3) was under the detectable level of GC except the PVP–Pt catalytic system where the selectivity to 3 was 9.5%. b
formed as spectators. However, Fe 31 , Fe 21 , Co 21 and Ni 21 remarkably enhanced both the selectivity and the activity. The overall results are similar to the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol catalyzed by PVP–Pt [23]. Actually, Fe 31 was reduced to Fe 21 in the reaction, so it displayed the same result as the latter [23]. From Fig. 4, it can be seen that under the modification of 1:1 Co 21 , only 10.6% of 1 left and the yield of 2 reached 88.3% (selectivity 98.8%) in 60 min. After another 60 min, there was 2.4% of 1 and 96.0% of 2 (selectivity 98.4). There was a little of 4 (3,7-dimethyloctanol, yield 1.6%), but no 3 was found. This indicated that the hydrogenation reaction proceeded via reaction (1) and (3) (Scheme 1). In other words, only after the C=O group is hydrogenated does the C=C group begin to be hydrogenated. The modification of metal cations to PVP–Pt colloids resulted in a different reaction pathway comparing to PVP–Pt itself and promoted this process. The abovementioned reaction condition has been optimized. It has been found that NaOH and H 2 O can affect the activity and selectivity of the PVP–Pt–MCl x catalytic system. Table 2 summarized the effects of the amounts of NaOH
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
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Table 2 Optimization of reaction condition in the selective hydrogenation of 1 to 2 over the PVP–Pt–CoCl 2 NaOH (mg)
Co 21
0 1 0 1 3 5 1 1 1
0 0 5 1 1 1 1 1 1 a
a
H2O (ml)
EtOH (ml)
Conversion of 1 (%)
Selectivity to 2 (%)
10 10 10 10 10 10 0 5 15
20 20 20 20 20 20 30 25 15
62.3 70.6 98.3 98.4 98.4 98.5 67.2 82.2 98.3
37.5 44.5 91.7 98.5 90.1 89.5 99.1 98.8 85.4
The numbers refer to the molar ratio of Co 21 :Pt.
and H 2 O, and the optimum amounts of NaOH, H 2 O and EtOH are 1 mg, 10 ml and 50 ml, respectively. This is similar to those reported in the literature [23].
3.4. Selective hydrogenation of 1 to 2 over PVP–Ru colloid Ruthenium is an important noble metal in hydrogenation of benzene to cyclohexene and of
the carbonyl group to hydroxyl group. Here, we use Co 21 as a modifier to promote the selective hydrogenation of 1 to 2. From Figs. 5 and 6, it can be seen that without Co 21 , conversion of 1 was 88.4% and yield of 2 was 84.2% (selectivity 95.2%) in one hour; the yield of 2 reached 86.4% in another hour but the selectivity decreased to 90.2%. With the modification of Co 21 , the yield of 2 reached 97.8% in one hour and changed little in another hour (selectivity was 98.8% and 97.5%
Fig. 5. Hydrogenation of 1 over PVP–Ru colloid.
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
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Fig. 6. Hydrogenation of 1 over PVP–Ru colloid with modification of Co 21 (molar ratio of Co 21 : Ru 5 1:1).
respectively), and 1 was almost wholly hydrogenated in one hour (there was only 1.0% of 1 left). From Figs. 3 and 5, a conclusion can be drawn that the catalytic properties of PVP-Ru is much superior to PVP–Pt: in one hour, the conversion of 1 and selectivity for 2 were
88.4% and 95.2% catalyzed by PVP–Ru, 56.7% and 46.9% by PVP–Pt, respectively.
3.5. Modification mechanism of metal cations Varying the molar ratio of Co 21 :Pt, we got curves of the conversion of 1 and the selectivity
Fig. 7. Conversion of 1 and selectivity to 2 vs. molar percentage of Co 21 in the PVP–Pt catalytic system with CoCl 2 ? 6H 2 O (reaction time: 2 h).
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W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Fig. 8. Conversion of 1 and selectivity to 2 vs. molar percentage of Co 21 in the PVP–Ru catalytic system with CoCl 2 ? 6H 2 O (reaction time: 1 h).
to 2 over PVP–Pt (Fig. 7). The incorporation of 0.05:1 Co 21 made the selectivity reach 98.4% and the conversion 85.4%. When adding 0.2:1 Co 21 , the selectivity to 2 and the conversion of 1 reached their maximum as 100% and 99.6%, respectively. A similar result was also found over PVP–Ru (Fig. 8), but the promotion effect was not as prominent as that of the PVP–Pt system. As discussed in the literature [23], we propose that metal cations are adsorbed onto the Pt or Ru colloidal particles, activate the C=O groups, thus increasing the reaction rate as well as the selectivity to unsaturated alcohols. The mechanism can be explained as in Fig. 9.
4. Conclusions 1. Some metal cations (Fe 31 , Co 21 , Ni 21 and Mn 21 ) can drastically increase both the activity and selectivity of PVP-stabilized noble metal colloids in homogeneous liquidphase selective hydrogenation of 1 to 2. 2. By the modification of Co 21 (molar ratio of Co 21 to Pt or Ru was 0.2:1), the selectivity to 2 was 99.6% at 100% conversion of 1 catalyzed by PVP-Pt and the selectivity to 2 was 99.8% at 100% conversion of 1 catalyzed by PVP–Ru colloid. The reactivity of PVP–Ru is a little higher than that of PVP– Pt colloid. 3. The modification was assumed that the adsorbed metal cations activated the C=O double bonds, thus accelerated the reaction rate and increased the selectivity to unsaturated alcohols.
Acknowledgements Fig. 9. Mechanism of the modification of metal cations to platinum colloid.
Financial support for this work by the National Science Foundation of China (Contracts No.
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
29774037, 29873058) and the Fund of the Chinese Academy of Sciences (Contract No. KJ952-J1-508) is gratefully acknowledged. References [1] G. Schmid, Chem. Rev. 92 (1992) 1709–1727. [2] L.N. Lewis, Chem. Rev. 93 (1993) 2693–2730. [3] H. Bonnemann, W. Brijoux, R. Fretzen, T. Joussen, R. Kdppler, B. Korall et al., J. Mol. Catal. 86 (1994) 129–177. [4] G. Schmid, Endeavour 14 (1990) 172–178. [5] H. Hirai, H. Chawanya, N. Toshima, Makromol, Chem. Rapid Commun. 2 (1981) 99–103. [6] H. Hirai, H. Chawanya, N. Toshima, React. Polym. 3 (1985) 127–141. [7] Y. Wang, H. Liu, Polym. Bull. 25 (1991) 139–144. [8] H. Liu, G. Mao, S. Meng, J. Mol. Catal. 74 (1992) 275–284. [9] N. Toshima, Y. Wang, Langmuir 10 (1994) 4574–4580. [10] T. Nishijima, T. Nagamura, T. Matsuo, J. Polym. Sci., Polym. Lett. Ed. 19 (1981) 65–73.
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[11] M. Han, H. Liu, Macromol. Symp. 105 (1996) 179–183. [12] Q. Wang, H. Liu, M. Han, X. Li, D. Jiang, J. Mol. Catal. A 118 (1997) 145–151. [13] P.N. Rylander, in: Catalytic Hydrogenation in Organic Synthesis, Academic Press, London, 1979, pp. 74–80. [14] M. Boudart, Nature 372 (1994) 320. [15] A.A. Wismeijer, A.P.G. Kieboom, H. van Bekkum, Appl. Catal. 25 (1986) 181–189. [16] R. Adams, B.S. Garvey, J. Am. Chem. Soc. 48 (1926) 477–482. [17] G. Neri, L. Mercadante, C. Milone, R. Pietropaolo, S. Galvagno, J. Mol. Catal. A 108 (1996) 41–50. [18] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 17 (1993) 55–61. [19] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 18 (1993) 349–355. [20] H. Hirai, J. Macromol. Sci., Chem. A 13 (1979) 633–649. [21] W. Yu, M. Liu, H. Liu, X. Ma, Z. Liu, J. Colloid Interface Sci. 208 (1998) 439–444. [22] W. Yu, H. Liu, Q. Tao, Chem. Commun. (1996) 1773–1774. [23] W. Yu, H. Liu, M. Liu, Q. Tao, J. Mol. Catal. A 138 (1999) 273–286.
Selective hydrogenation of citronellal to citronellol over polymer-stabilized noble metal colloids Weiyong Yu, Hanfan Liu*, Manhong Liu, Zhijie Liu Polymer Chemistry Laboratory, Chinese Academy of Sciences and China PetroChemical Corporation, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received 23 February 1999; accepted 30 July 1999
Abstract Citronellal was hydrogenated to citronellol by polymer-stabilized Pt and Ru colloids. The metal cations increased both the activity and the selectivity of the colloids. The modification was assumed to be due to the adsorbed metal cations activating the C=O double bonds, thus accelerating the reaction rate and increasing the selectivity to unsaturated alcohols. 2000 Elsevier Science B.V. All rights reserved. Keywords: Selective hydrogenation; Colloid; Platinum; Ruthenium; Metal cation
1. Introduction Materials with dimensions of 1–100 nm display novel properties in electronics, magnetics, optics and chemistry. Many researchers pay a lot of attention to these themes [1–3]. In the field of catalysis, nanosized metal colloids or clusters, being looked at as bridges between heterogeneous catalysts and homogeneous catalysts, are expected to show singular catalytic properties [1–4]. Metal colloids dispersed in liquid medium can be divided into four categories, that is: solvent-stabilized, surfactantstabilized, ligand-stabilized and polymer-stabilized metal colloids. Metal colloids of the first *Corresponding author. Tel.: 1 86-10-6625-54487; fax: 1 8610-6625-69564. E-mail address: [email protected] (H. Liu)
three categories are usually unstable when they are used as catalysts even when the reaction is conducted under ambient conditions. Thus, the polymer-stabilized metal colloids are extremely important for their catalytic use. Some researchers have reported novel performances of nanoscale monometallic and bimetallic colloids in many reactions [5–12]. To get unsaturated alcohols by selective hydrogenation of unsaturated aldehydes is a crucial step in the preparation of many fine chemicals. It is difficult to hydrogenate the carbonyl group and reserve the olefinic function, since the C=C double bond is easily hydrogenated over most conventional catalysts to give saturated aldehydes as the primary products [13]. Boudart [14] has pointed out that it is important to achieve this result by employing metal colloids or clusters.
1381-5148 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S1381-5148( 99 )00073-5
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W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Citronellal (3,7-dimethyl-6-octenal, 1) is a non-conjugated unsaturated aldehyde, its corresponding unsaturated alcohol is citronellol (3,7dimethyl-6-octene-1-ol, 2). They are two essential components in many kinds of perfumes and the manufacture of them is profitable. Some papers have been published on the selective hydrogenation of citral to 2 over conventional supported catalysts [15–19], but the selective hydrogenation of 1 to 2 is seldom described. We found that by using polymer-stabilized platinum and ruthenium colloids as catalysts, 1 can be selectively hydrogenated to 2 with high selectivity; the incorporation of some metal cations can increase both the reactivity and the selectivity of the colloid catalysts. Here, we report the research results.
2. Experimental
the form of dark-brown homogeneous dispersion. Polymer-stabilized ruthenium colloid (PVP–Ru) was prepared by reducing RuCl 3 ? 3H 2 O with NaBH 4 to give a dark-brown homogeneous dispersion [21]. Hydrogenation of 1 was carried out in a 100-ml stainless steel autoclave. The reaction solution contained 40 ml EtOH, 1 mg NaOH and 0.50 g of 1 (3.24 3 10 23 mol), 1.00 g ethylene glycol (as an internal standard for gas chromatography), 20 ml PVP–Pt or PVP-Ru dispersion hcontaining Pt or Ru (2.340 3 10 25 mol), PVP [1.170 3 10 23 mol (monomeric unit)], 10 ml H 2 0 and 10 ml EtOHj and metal salt added. H 2 was charged several times to replace air and the final pressure of H 2 was 6.0 MPa. The hydrogenation reaction was performed at 333 K for 2 h. The reaction products were periodically analyzed by gas chromatography.
2.1. Materials and instruments 3. Results and discussion Poly(N-vinyl-2-pyrrolidone) (PVP, average molecular weight 40 000) was supplied by BASF. Compounds 1 and 2 were purchased from Acros and were redistilled under reduced pressure before use. Other reagents were purchased from Beijing Chemicals and were of analytical grade. Hydrogen (H 2 ) with a purity of 99.999% was supplied by Beijing Gases Factory. Transmission electron microscopy (TEM) photographs were taken by using a Hitachi 9000NAR instrument. Specimens were prepared by placing a drop of the colloid dispersion upon a copper grid covered with a perforated carbon film and then evaporating the solvent. The particle diameters were measured from the enlarged photographs. The particle size distribution histogram was obtained on the basis of the measurements of about 300 particles.
2.2. Hydrogenation reaction Polymer-stabilized platinum colloid (PVP– Pt) was prepared by a reported method [20] in
3.1. Characterization of PVP–Pt colloid The TEM photograph and the corresponding particle size distribution histogram of the platinum colloid are shown in Fig. 1. From Fig. 1, it can be seen that the average diameter of PVP–Pt colloid was 1.1 nm and the size distribution was narrow within the range of 0.6 to 1.8 nm with s 5 0.30 nm).
3.2. Characterization of PVP–Ru colloid The TEM photograph and the corresponding particle size distribution histogram of the ruthenium colloid are also obtained (Fig. 2). It can be seen from Fig. 2 that the average diameter of PVP–Ru colloid was 1.4 nm and the size distribution was narrow in the range of 0.44 to 2.94 nm with s 5 0.51 nm.
3.3. Selective hydrogenation of 1 to 2 over PVP–Pt colloid In the preparation of fine chemicals, it is an
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Fig. 1. Electromicroscopic photograph (left) and the corresponding particle size histogram (right) of PVP–Pt colloid.
Fig. 2. Electromicroscopic photograph (left) and the corresponding particle size histogram (right) of PVP–Ru colloid.
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W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Scheme 1.
important step to obtain unsaturated alcohols by selective hydrogenation of the corresponding unsaturated aldehydes. Compounds 1 and 2 are typical non-conjugated unsaturated aldehyde and unsaturated alcohol. The reaction pathway of the hydrogenation of 1 can be schematically shown as Scheme 1. It is very easy for ordinary heterogeneous and homogeneous catalysts to reduce the C=C double bond to yield 3,7-dimethyloctanal (3) as the main product [Scheme 1, reaction (2)]; however, it is comparatively difficult for them to reduce the CO double bond only to get 2 with high selectivity at high conversion of 1 [Scheme 1, reaction (1)] [13].
Under the reaction conditions described above, 70.6% conversion of 1 with 44.5% selectivity for 2 was obtained when the neat PVP–Pt colloid was employed alone as a catalyst; nonetheless, the activity was increased by 40% (conversion of 1 reached 98.3%) and the selectivity for 2 increased to 98.5% when Co 21 was introduced into the catalytic system (the time courses of them were recorded in Figs. 3 and 4, respectively). From previous work [22,23], we have known that some cations (Fe 21 , Co 21 , Ni 21 etc.) can markedly increase both the activity and the selectivity in the selective hydrogenation of cinnamaldehyde (3-phenyl-2-propenal) to cinnamyl alcohol (3-phenyl-2-propene-1-ol) catalyzed by PVP–Pt colloid. And it has been verified that non-strongly coordinating anions, 2 22 2 2 such as Cl , SO 4 , NO 3 , OAc etc. act merely as spectator ions [22]. The modification of various metal cations to this reaction system was also investigated. It can 1 1 1 be seen from Table 1 that Li , Na and K did not affect the catalytic properties of PVP–Pt; 21 21 21 Mg , Ca and Ba increased a little on the 21 selectivity and the activity; Mn enhanced the conversion to 97.8% and the selectivity to
Fig. 3. Hydrogenation of 1 over PVP–Pt colloid.
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
25
Fig. 4. Hydrogenation of 1 over PVP–Pt colloid with modification of Co 21 (molar ratio of Co 21 : Pt 5 1:1).
74.7%; Cu 21 , Pb 21 and Zn 21 acted as poisons to the activity of PVP–Pt colloid (conversions of 1 were 0, 0 and 25.0% to Cu 21 , Pb 21 and Zn 21 , respectively, but the selectivity was 99.6% to Zn 21 ). Ce 31 , Nd 31 and Al 31 perTable 1 Hydrogenation of 1 over PVP–Pt–MCl x systems a Catalytic system
Conversion of 1 (%)
Selectivity b (%) 2
4
PVP–Pt PVP–Pt–LiCl PVP–Pt–NaCl PVP–Pt–KCl PVP–Pt–MgCl 2 PVP–Pt–CaCl 2 PVP–Pt–Ba(NO 3 ) 2 PVP–Pt–MnCl 2 PVP–Pt–CuCl 2 PVP–Pt–ZnCl 2 PVP–Pt–Pb(NO 3 ) 2 PVP–Pt–FeCl 3 PVP–Pt–FeSO 4 PVP–Pt–CoCl 2 PVP–Pt–NiCl 2 PVP–Pt–CeCl 3 PVP–Pt–Nd(NO 3 ) 3 PVP–Pt–AlCl 3
70.6 65.2 72.3 68.8 75.1 73.6 73.8 97.8 0 25.0 0 97.6 97.5 98.3 97.2 68.5 69.1 73.3
44.5 41.3 46.2 45.8 55.3 50.0 57.5 74.7 – 99.6 – 97.6 97.6 98.5 88.9 47.4 45.0 42.2
46.0 58.7 53.8 54.2 44.7 50.0 42.5 25.3 – 0.4 – 2.4 2.4 1.5 11.1 52.6 55.0 57.8
a
Molar ratio of M:Pt 5 1:1. The yield of 3,7-dimethyloctanal (3) was under the detectable level of GC except the PVP–Pt catalytic system where the selectivity to 3 was 9.5%. b
formed as spectators. However, Fe 31 , Fe 21 , Co 21 and Ni 21 remarkably enhanced both the selectivity and the activity. The overall results are similar to the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol catalyzed by PVP–Pt [23]. Actually, Fe 31 was reduced to Fe 21 in the reaction, so it displayed the same result as the latter [23]. From Fig. 4, it can be seen that under the modification of 1:1 Co 21 , only 10.6% of 1 left and the yield of 2 reached 88.3% (selectivity 98.8%) in 60 min. After another 60 min, there was 2.4% of 1 and 96.0% of 2 (selectivity 98.4). There was a little of 4 (3,7-dimethyloctanol, yield 1.6%), but no 3 was found. This indicated that the hydrogenation reaction proceeded via reaction (1) and (3) (Scheme 1). In other words, only after the C=O group is hydrogenated does the C=C group begin to be hydrogenated. The modification of metal cations to PVP–Pt colloids resulted in a different reaction pathway comparing to PVP–Pt itself and promoted this process. The abovementioned reaction condition has been optimized. It has been found that NaOH and H 2 O can affect the activity and selectivity of the PVP–Pt–MCl x catalytic system. Table 2 summarized the effects of the amounts of NaOH
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
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Table 2 Optimization of reaction condition in the selective hydrogenation of 1 to 2 over the PVP–Pt–CoCl 2 NaOH (mg)
Co 21
0 1 0 1 3 5 1 1 1
0 0 5 1 1 1 1 1 1 a
a
H2O (ml)
EtOH (ml)
Conversion of 1 (%)
Selectivity to 2 (%)
10 10 10 10 10 10 0 5 15
20 20 20 20 20 20 30 25 15
62.3 70.6 98.3 98.4 98.4 98.5 67.2 82.2 98.3
37.5 44.5 91.7 98.5 90.1 89.5 99.1 98.8 85.4
The numbers refer to the molar ratio of Co 21 :Pt.
and H 2 O, and the optimum amounts of NaOH, H 2 O and EtOH are 1 mg, 10 ml and 50 ml, respectively. This is similar to those reported in the literature [23].
3.4. Selective hydrogenation of 1 to 2 over PVP–Ru colloid Ruthenium is an important noble metal in hydrogenation of benzene to cyclohexene and of
the carbonyl group to hydroxyl group. Here, we use Co 21 as a modifier to promote the selective hydrogenation of 1 to 2. From Figs. 5 and 6, it can be seen that without Co 21 , conversion of 1 was 88.4% and yield of 2 was 84.2% (selectivity 95.2%) in one hour; the yield of 2 reached 86.4% in another hour but the selectivity decreased to 90.2%. With the modification of Co 21 , the yield of 2 reached 97.8% in one hour and changed little in another hour (selectivity was 98.8% and 97.5%
Fig. 5. Hydrogenation of 1 over PVP–Ru colloid.
W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
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Fig. 6. Hydrogenation of 1 over PVP–Ru colloid with modification of Co 21 (molar ratio of Co 21 : Ru 5 1:1).
respectively), and 1 was almost wholly hydrogenated in one hour (there was only 1.0% of 1 left). From Figs. 3 and 5, a conclusion can be drawn that the catalytic properties of PVP-Ru is much superior to PVP–Pt: in one hour, the conversion of 1 and selectivity for 2 were
88.4% and 95.2% catalyzed by PVP–Ru, 56.7% and 46.9% by PVP–Pt, respectively.
3.5. Modification mechanism of metal cations Varying the molar ratio of Co 21 :Pt, we got curves of the conversion of 1 and the selectivity
Fig. 7. Conversion of 1 and selectivity to 2 vs. molar percentage of Co 21 in the PVP–Pt catalytic system with CoCl 2 ? 6H 2 O (reaction time: 2 h).
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W. Yu et al. / Reactive & Functional Polymers 44 (2000) 21 – 29
Fig. 8. Conversion of 1 and selectivity to 2 vs. molar percentage of Co 21 in the PVP–Ru catalytic system with CoCl 2 ? 6H 2 O (reaction time: 1 h).
to 2 over PVP–Pt (Fig. 7). The incorporation of 0.05:1 Co 21 made the selectivity reach 98.4% and the conversion 85.4%. When adding 0.2:1 Co 21 , the selectivity to 2 and the conversion of 1 reached their maximum as 100% and 99.6%, respectively. A similar result was also found over PVP–Ru (Fig. 8), but the promotion effect was not as prominent as that of the PVP–Pt system. As discussed in the literature [23], we propose that metal cations are adsorbed onto the Pt or Ru colloidal particles, activate the C=O groups, thus increasing the reaction rate as well as the selectivity to unsaturated alcohols. The mechanism can be explained as in Fig. 9.
4. Conclusions 1. Some metal cations (Fe 31 , Co 21 , Ni 21 and Mn 21 ) can drastically increase both the activity and selectivity of PVP-stabilized noble metal colloids in homogeneous liquidphase selective hydrogenation of 1 to 2. 2. By the modification of Co 21 (molar ratio of Co 21 to Pt or Ru was 0.2:1), the selectivity to 2 was 99.6% at 100% conversion of 1 catalyzed by PVP-Pt and the selectivity to 2 was 99.8% at 100% conversion of 1 catalyzed by PVP–Ru colloid. The reactivity of PVP–Ru is a little higher than that of PVP– Pt colloid. 3. The modification was assumed that the adsorbed metal cations activated the C=O double bonds, thus accelerated the reaction rate and increased the selectivity to unsaturated alcohols.
Acknowledgements Fig. 9. Mechanism of the modification of metal cations to platinum colloid.
Financial support for this work by the National Science Foundation of China (Contracts No.
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29774037, 29873058) and the Fund of the Chinese Academy of Sciences (Contract No. KJ952-J1-508) is gratefully acknowledged. References [1] G. Schmid, Chem. Rev. 92 (1992) 1709–1727. [2] L.N. Lewis, Chem. Rev. 93 (1993) 2693–2730. [3] H. Bonnemann, W. Brijoux, R. Fretzen, T. Joussen, R. Kdppler, B. Korall et al., J. Mol. Catal. 86 (1994) 129–177. [4] G. Schmid, Endeavour 14 (1990) 172–178. [5] H. Hirai, H. Chawanya, N. Toshima, Makromol, Chem. Rapid Commun. 2 (1981) 99–103. [6] H. Hirai, H. Chawanya, N. Toshima, React. Polym. 3 (1985) 127–141. [7] Y. Wang, H. Liu, Polym. Bull. 25 (1991) 139–144. [8] H. Liu, G. Mao, S. Meng, J. Mol. Catal. 74 (1992) 275–284. [9] N. Toshima, Y. Wang, Langmuir 10 (1994) 4574–4580. [10] T. Nishijima, T. Nagamura, T. Matsuo, J. Polym. Sci., Polym. Lett. Ed. 19 (1981) 65–73.
29
[11] M. Han, H. Liu, Macromol. Symp. 105 (1996) 179–183. [12] Q. Wang, H. Liu, M. Han, X. Li, D. Jiang, J. Mol. Catal. A 118 (1997) 145–151. [13] P.N. Rylander, in: Catalytic Hydrogenation in Organic Synthesis, Academic Press, London, 1979, pp. 74–80. [14] M. Boudart, Nature 372 (1994) 320. [15] A.A. Wismeijer, A.P.G. Kieboom, H. van Bekkum, Appl. Catal. 25 (1986) 181–189. [16] R. Adams, B.S. Garvey, J. Am. Chem. Soc. 48 (1926) 477–482. [17] G. Neri, L. Mercadante, C. Milone, R. Pietropaolo, S. Galvagno, J. Mol. Catal. A 108 (1996) 41–50. [18] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 17 (1993) 55–61. [19] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 18 (1993) 349–355. [20] H. Hirai, J. Macromol. Sci., Chem. A 13 (1979) 633–649. [21] W. Yu, M. Liu, H. Liu, X. Ma, Z. Liu, J. Colloid Interface Sci. 208 (1998) 439–444. [22] W. Yu, H. Liu, Q. Tao, Chem. Commun. (1996) 1773–1774. [23] W. Yu, H. Liu, M. Liu, Q. Tao, J. Mol. Catal. A 138 (1999) 273–286.