PEROSA A.

TUNDO P.

TUNDO, Pietro

PEROSA, Alvise

Archivio istituzionale della ricerca - Università degli Studi di Venezia Ca' Foscari

Reactive & Functional Polymers 54 (2003) 95–101www.elsevier.com/ locate / reactT he action of onium salts and other modifiers on Pt /C, Pd/C,and Raney–Ni catalysts in the multiphase reduction system*Pietro Tundo , Alvise Perosa`Dipartimento di Scienze Ambientali–Universita Ca’ Foscari and Consorzio Interuniversitario Nazionale ‘La Chimica per l’Ambiente’ –Dorsoduro 2137, 30123 Venezia, ItalyAbstractIn the presence of a catalyst modifier (an onium salt, an amine, or a polyethylene glycol), in a biphasic aqueous–organicmixture, the reactivity of Pt /C, Pd/C, and Raney–Ni changes significantly. Hydrodehalogenation reactions of haloaromatics, reductions of functional groups on the aromatic ring, benzyl hydrogenolysis, can be conducted under mildconditions, and with interesting rates and regio-, chemo-, and stereoselectivities. The modifier coats the catalyst, and formsan interfacial film, wherein the reactions take place. This film partitions the catalyst at the aqueous–organic interface, andinfluences the outcome of the reactions by mediating substrate–catalyst interactions, and by providing a hydrogen reservoirin the vicinity of the active sites.  2002 Elsevier Science B.V. All rights reserved.1 . Introduction amine, or a polyethylene glycol (Table 1). Itspresence, combined with the other componentsIt is possible to conduct a variety of reduction of the system, allows to achieve interestingreactions under mild conditions (T5 50 8C, yields, rates, along with chemo-, regio-, andpH 5 1 atm) in a multiphase catalytic reduction stereoselectivities [9–12].2system. For example, the hydrodehalogenation Two examples of reactions that proceed in(HDX) of halo aromatics [1–5], the selective the multiphase system are shown in Eqs. (1) andhydrogenation of aromatic carbonyls [6], the (2). Tetrachlorbenzene can be rapidly (1 h) andhydrogenolysis of benzyl ethers [7], up to selectively (100%) reduced to benzene (Eq. (1).aromatic hydrogenation [6,8], proceed smoothly p-Chloroacetophenone can be selectively re-in a system made by five different phases: duced either to acetophenone, or to phenyl-aqueous, organic, a heterogeneous catalyst (e.g. ethanol, by an appropriate choice of catalyst,Pd/C, Pt /C, Raney–Ni), hydrogen, and an modifier and aqueous base concentration (Eq.insoluble modifier. The modifier is usually an (2).onium salt such as Aliquat 336 (A336), or an In the absence of one or more of the com-ponents, the reduction reactions are often slowand not selective. In particular, the modifier*Corresponding author. Tel.: 139-041-234-8642; fax: 139-seems to be the key ingredient of the mixture,041-234-8620.E-mail address: tundop@unive.it (P. Tundo). because of the way it interacts with the catalyst,1381-5148/02/$ – see front matter   2002 Elsevier Science B.V. All rights reserved.PI I : S1381-5148( 02 )00185-296 P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101Table 1 (Eq. (2), in a system made by iso-octane,Types of modifiers used in the multiphase catalytic reductionaqueous KOH, hydrogen, charcoal supported PtModifier Ref. or Pd catalyst, and a modifier among the ones in1 Aliquat 336 (A336) [1–11] Table 1.1 22 C H (C H ) N Br [2,4]16 33 18 37 31 23 C H (n-Bu) P Br [1–4]16 33 31 24 C H (py) Br [1]16 331 25 PhCH (C H ) N Br [2]2 2 5 3 (1)1 26 (n-Bu) N HSO [2]4 47 MeO(CH CH O) H [2,3]2 2 n n|158 PEG 6000 [2]9 PPG 2000 [2]10 Brij 35 [8]11 Brij 52 [8] (2)12 Brij 58 [8]13 Brij 56 [8]1 2 The metal catalyst is used approximately 5%14 PhCH (CH CH ) N Cl [8,9]2 3 2 31 2 molar with respect to the substrate, the modifier15 PhCH (n-Bu) N Cl [8]2 316 Et NH [9]2 in a 20% molar ratio. The following points17 Et N [9]3 describe the modifier–catalyst interactions and18 n-Bu N [9]3 how they affect reactivity, selectivity, and the19 (PhCH ) N [9]2 320 n-C H NH [9] outcome of typical reactions.8 17 221 Cinchonidine [12]22 Cinchonine [12] 2 .1. Catalyst partitioningapparently by coating it [3,9,11]. The question The readily apparent effect of the modifier isof how the modifiers act in promoting these macroscopic. By adding it to the aqueous–or-effects has been addressed periodically, and has ganic–catalyst mixture (Fig. 1a) the latter visib-allowed to collect a number of pieces of evi- ly changes its partitioning. Generally, either thedence. The scope of this paper is to describe catalyst distributes itself preferentially in thesome conclusions on the study of the mecha- organic phase (Fig. 1b), or, at higher concen-nism for the catalytic multiphase reduction trations of modifier, this tends to separate outsystem, and to propose a mode of action for the between the aqueous–organic phases, to form acatalyst modifier. third liquid phase [13], and to incorporate thecatalyst (Fig. 1c). Either way, the modifier has ahigh affinity for the catalyst; and, in both cases2 . Results and discussion the modifier–catalyst assembly is in bettercontact with the organic than it is with theThere is plenty of flexibility in the choice of aqueous phase, hereby allowing efficient diffu-the modifier. Essentially it has to be a phase sion of the organic substrate to the reactive sitetransfer agent (PTA), i.e. a molecule with a of the catalyst.relatively polar head group, and one or morelipophylic chains (Table 1). One constraint is 2 .2. Catalyst coating by the modifierthat it must be insoluble both in the organic andin the aqueous phases. There is a high affinity of the modifier for theFor simplicity, as model reactions one can catalyst particles, which are coated by a thinconsider indifferently either the hydrodechlori- film, a kind of membrane, of modifier. Onenation (HDCl) reaction of 1,2,4,5-tetrachloro- proposed mechanism by which this film isbenzene to benzene (Eq. (1), or the HDCl and formed may be analogous to the one sketchedcarbonyl reduction of p-chloroacetophenone out in Scheme 1, where the carboxy groupsP. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101 97Fig. 1. Partitioning of the catalyst between the phases as a function of the modifier: a: organic phase1aqueous phase1supported catalyst; b:organic phase1aqueous phase1supported catalyst1modifier; c: organic phase1aqueous phase1supported catalyst1excess modifier.Scheme 1. Catalyst particle in the multiphase system: magnified view of a possible coating arrangement by an onium salt.present on the charcoal support act as anchoring with increasing concentration of modifier, untilpoints for the modifier (in this case an onium they reach a plateau above which no furtherion) [9]. increase was detected (Fig. 2) [3]. A similarThe catalyst is made more lypophilic by this behavior was observed using Raney–Ni as thesurface membrane of modifier, which explains catalyst [11].why it resides preferentially in the organic (Fig. The concentration threshold, above which1b) or modifier phase (Fig. 1c). On the contrary, additional modifier is non-influential, may haveif ethanol is used as the solvent, where the a two-fold explanation: (a) it constitutes ‘mono-modifier dissolves, and its affinity for the sup- layer’ formation, such as in Langmuir typeported catalyst fails, its effect is not felt any adsorption; and (b) when the modifier reacheslonger. the amount on the catalyst surface where aThe hydrodehalogenation rate constants, constant concentration of reactants is main-plotted versus the modifier concentration, follow tained in proximity of the catalyst, the mass-Langmuir adsorption type curves. For example, transfer limit is effectively eliminated, and theit was shown that the rate constants for hydro- intrinsic reaction rate is attained. This observa-dehalogenation of chloroethylbenzenes increase tion is substantiated by the order of the reaction,98 P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101Fig. 3. Effect of varying amounts of CD on the conversion (A)Fig. 2. HDX rate constants as a function of A336 concentration,and ee (B) of the reduction of acetophenone.for o-, m-, p-chloroethylbenzene.3). The right tradeoff is the concentration wherezero in the substrate, which indicates that when tight chiral pockets are formed on the catalyst.a steady-state concentration of substrate is ob- Other similar observations [15,16], and calcula-tained in proximity of the catalyst, then bulk tions [17], support this (Fig. 4).concentration becomes non-influential. On the It has been demonstrated that the modifierother hand the reaction rate is first order in the does not act as a surfactant, i.e. it does notcatalyst [10].Some support for the latter observation isgiven by the fact that, while in the case of Pd/Cthe modifier has scarce effect on the rates; in thecase of Raney–Ni the modifier is indispensablefor the reaction to proceed at all [11]. Theremust therefore be some relation between themodifier on the catalyst and substrate adsorp-tion.Further support for the formation of a layer ofmodifier comes by using a chiral modifier, andby observing that, under appropriate conditions,stereospecific reactions can be conducted usingthis system [12,14]. By covering the supportedcatalyst and forming chiral pockets, whichstereo-recognize the substrate, the modifier pro-motes enantioselectivity. The achievable ee’sdepend on the concentration of modifier used:Fig. 4. Representation of how cinchonidine may form prochiraltoo little gives high rates but poor ee’s, while cavities on a Pt surface (reproduced by permission of Wiley–too much means slow rates and poor ee’s (Fig. VCH) [17].P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101 99improve dispersion of the reactants or catalysts,and does not act via micellar catalysis [18].2 .3. HCl removal from the catalystAnother role of the modifier is due to itsphase transfer nature: it prevents catalystpoisoning, by transporting the HCl produced bythe reaction into the aqueous phase, where it isneutralized by the KOH. The mechanism is theclassical phase-transfer one (Scheme 2). Thepoisoning effect of HCl on the catalyst [19] Fig. 5. Hydrogen uptake by Pt /C in solution and in the presencebecomes readily obvious either with no PT of A336.agent or when insufficient base is present: thereaction becomes inhibited. modifier can perhaps be ascribed to increasedand constant concentration of hydrogen, a reser-voir, held in proximity of the catalyst.2 .4. Uptake of hydrogenThe modifier plays a subtle role on the uptake 2 .5. Chemoselectivityof hydrogen as well. In fact, the amount ofhydrogen adsorbed by the system appears larger It can be tuned by varying a number ofwhen the modifier is present. This was observed reaction parameters. Mainly modifier type andby bubbling hydrogen, and measuring its uptake amount [3,4,6], and base concentration [8–10].using a graduated burette, first in mixtures made The major effect of the modifier is to induceby isooctane–KOH(aq)–catalyst, and then in chemoselectivity in many of the reactions.identical mixtures where A336 was added. The Significantly, the reaction of p-chloro-latter mixture was able to adsorb a larger propiophenone goes all the way to ethyl ben-amount of hydrogen than the former. Analo- zene with Pt /C and no modifier; while in thegously, the fact that an additional quantity of H presence of A336 100% selectivity towards2was adsorbed, when A336 was added to a hydrodechlorination (Pd/C) or the benzyl al-mixture of isooctane–KOH(aq)–catalyst, after it cohol (Pt /C) can be obtained [8]. The modifierhad been already saturated with H , confirms is therefore somehow mediating between the2this effect (Fig. 5) [18]. substrate and the catalytic sites. The question isThis observation is significant because it how the modifier does that.implies that the rate acceleration of the hydro- Under our conditions the reduction mecha-dehalogenation reaction in the presence of the nism presumably does not involve radicalScheme 2. Phase-transfer mechanism.100 P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101species, but rather oxidative addition of the tivities. Part of its role may be in the salificationsubstrate to the catalyst, followed by hydrogen of the COOH groups present on the support,1insertion, and elimination of the products. The thereby allowing substitution of H by themodifier membrane must therefore come into onium cation (Schemes 1–2) [9]. In addition,play by mediating adsorption of the substrate on KOH may interact (as inhibitor /promoter) withthe catalyst surface, in a way that favors, for the catalytic metal sites [20]. It was also notedexample C–Cl, bond cleavage over C=O hydro- that an excess of KOH is detrimental to thegenation, to give hydrodechlorination rather reaction selectivity, perhaps too much KOHthan carbonyl reduction. wipes the catalyst clean and decreasesThere is a dependence of chemoselectivity on chemoselectivity.the hydrophilicity of the modifier. Higher hy-drophilicity, causes the reaction of Eq. (2) toyield chemoselectively acetophenone, the hy- 3 . Conclusionsdrodechlorination product. In this case perhaps,the modifier–catalyst ensemble, by hindering At this stage the conclusions that can beapproach causes a less positive interaction be- drawn on the mode of action of the modifier,tween substrate and the catalyst. This, in turn, based on experimental evidence, are the follow-could favor reaction of the more polarized C–Cl ing.bond with respect to the C=O one, i.e. higherchemoselectivity. 1. The modifier coats the catalyst particles, andThis explains, for example, why hydrodech- this interaction is largely responsible for thelorination is generally the fastest reaction: ap- selectivity features of the system.proach and reaction of the aromatic chlorine 2. The modifier membrane forms the nanoen-would be favored initially with respect to car- vironment wherein the reaction takes place.bonyl. 3. The modifier is adsorbed on the surface ofthe catalyst and behaves like a hydrogenreservoir for the reaction.2 .6. Kinetics4. The reaction itself occurs at the interfaceKinetic studies show that certain reaction between modified-catalyst, aqueous and or-parameters (KOH concentration, nature of the ganic phases, in a tight nanostructured en-metal, nature of the modifier) play a role on the vironment where transfers between thereaction outcome. As far as KOH is concerned, phases are favored. The substrate, H , KOH,2for example, it is apparent that, not only does it HCl, the product, and the modified-catalystneutralize HX, it also acts synergistically with are in close contact with one another, allow-the modifier in determining kinetics and selec- ing efficient transfer of the reactants betweenFig. 6. Equilibria of the reactants at the catalyst–modifier–aqueous /organic interfaces.P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101 101[7] A. Perosa, P. Tundo, S. Zinovyev, Green Chem. 4 (2002)the phases. Reagent, and product selectivity492–494.are determined by the equilibria which are [8] M. Selva, P. Tundo, A. Perosa, J. Org. Chem. 63 (1998)established by these species between the 3266–3271.[9] A. Perosa, M. Selva, P. Tundo, J. Org. Chem. 64 (1999)phases. Fig. 6 shows the membrane formed3934–3939.by the modifier on the catalyst, the liquid [10] P. Tundo, S. Zinovyev, A. Perosa, J. Catal. 196 (2000)phase made by the aqueous and organic 330–338.[11] S. Zinovyev, A. Perosa, S. Yufit, P. Tundo, J. Catal. 211solvents, and the equilibria of some of the(2002) 347–354.involved species. [12] A. Perosa, P. Tundo, M. Selva, J. Mol. Catal. A 180 (2002)169–175.[13] G.D. Yadav, C.A. Reddy, Ind. Eng. Chem. Res. 38 (1999)A cknowledgements 2245–2253;T. Ido, T. Yamamoto, G. Jin, S. Goto, Chem. Eng. Sci. 52(1997) 3511–3520;The Italian Interuniversity ConsortiumD.-H. Wang, H.-S. Weng, Chem. Eng. Sci. 50 (1995) 3477–‘Chemistry for the Environment’ (INCA), Ca’ 3486;Foscari University, INTAS grant no. 2000- D. Mason, S. Magdassi, Y. Sasson, J. Org. Chem. 56 (1991)7229–7232.00710, NATO grant no. EST.CLG.977159. are[14] see also for example: A. Baiker, J. Mol. Catal. A 163 (2000)gratefully acknowledged for funding. 205–220;This work was in part financed by ‘Piano’ ¨H.-U. Blaser, M. Muller, in: Heterogeneous Catalysis andFine Chemicals II, M. Guisnet et al. (Eds.), Elsevier Science‘Ambiente Terrestre: Chimica per l’Ambiente’,Publishers B.V., Amsterdam, 1991, pp. 73–91, and referencesLegge 488/92, MIUR.therein.[15] C. LeBlond, J. Wang, J. Liu, A.T. Andrews, Y.-K. Sun, J.Am. Chem. Soc. 121 (1999) 4920–4921.[16] J. Kubota, F. Zaera, J. Am. Chem. Soc. 123 (2001) 11115–R eferences11116.[17] J.M. Thomas, Angew. Chem. Int. Ed. Engl. Adv. Mater. 28[1] C.A. Marques, M. Selva, P. Tundo, J. Chem. Soc. Perkin (1989) 1079–1088.Trans. 1 (1993) 529–533. [18] Unpublished results.[2] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 58 (1993)´ ´ ´ ´[19] M.A. Aramendıa, V. Borau, I.M. Garcıa, C. Jimenez, F.5256–5260. Lafont, A. Marinas, J.M. Marinas, F.J. Urbano, J. Mol. Catal.[3] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 59 (1994) A 184 (2002) 237–245, and references therein.3830–3837. [20] R.L. Augustine (Ed.), Heterogeneous Catalysis for the[4] C.A. Marques, O. Rogozhnikova, M. Selva, P. Tundo, J. Synthetic Chemist, Marcel Dekker Inc, New York, 1996, p.Mol. Catal. A 96 (1995) 301–309. 220.[5] A. Perosa, M. Selva, P. Tundo, S.S. Zinovyev, Appl. Catal.B: Environ. 32 (2001) L1–L7.[6] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 60 (1995)2430–2435.

The action of onium salts and other modifiers on Pt/C, Pd/C, and Raney-Ni catalyst in the multiphase reduction system

Name not available

Reactive & Functional Polymers 54 (2003) 95–101
www.elsevier.com/ locate / react
T he action of onium salts and other modifiers on Pt /C, Pd/C,
and Raney–Ni catalysts in the multiphase reduction system
*Pietro Tundo , Alvise Perosa
`Dipartimento di Scienze Ambientali–Universita Ca’ Foscari and Consorzio Interuniversitario Nazionale ‘La Chimica per l’Ambiente’ –
Dorsoduro 2137, 30123 Venezia, Italy
Abstract
In the presence of a catalyst modifier (an onium salt, an amine, or a polyethylene glycol), in a biphasic aqueous–organic
mixture, the reactivity of Pt /C, Pd/C, and Raney–Ni changes significantly. Hydrodehalogenation reactions of halo
aromatics, reductions of functional groups on the aromatic ring, benzyl hydrogenolysis, can be conducted under mild
conditions, and with interesting rates and regio-, chemo-, and stereoselectivities. The modifier coats the catalyst, and forms
an interfacial film, wherein the reactions take place. This film partitions the catalyst at the aqueous–organic interface, and
influences the outcome of the reactions by mediating substrate–catalyst interactions, and by providing a hydrogen reservoir
in the vicinity of the active sites.
  2002 Elsevier Science B.V. All rights reserved.
1 . Introduction amine, or a polyethylene glycol (Table 1). Its
presence, combined with the other components
It is possible to conduct a variety of reduction of the system, allows to achieve interesting
reactions under mild conditions (T5 50 8C, yields, rates, along with chemo-, regio-, and
pH 5 1 atm) in a multiphase catalytic reduction stereoselectivities [9–12].2
system. For example, the hydrodehalogenation Two examples of reactions that proceed in
(HDX) of halo aromatics [1–5], the selective the multiphase system are shown in Eqs. (1) and
hydrogenation of aromatic carbonyls [6], the (2). Tetrachlorbenzene can be rapidly (1 h) and
hydrogenolysis of benzyl ethers [7], up to selectively (100%) reduced to benzene (Eq. (1).
aromatic hydrogenation [6,8], proceed smoothly p-Chloroacetophenone can be selectively re-
in a system made by five different phases: duced either to acetophenone, or to phenyl-
aqueous, organic, a heterogeneous catalyst (e.g. ethanol, by an appropriate choice of catalyst,
Pd/C, Pt /C, Raney–Ni), hydrogen, and an modifier and aqueous base concentration (Eq.
insoluble modifier. The modifier is usually an (2).

onium salt such as Aliquat 336 (A336), or an In the absence of one or more of the com-
ponents, the reduction reactions are often slow
and not selective. In particular, the modifier
*Corresponding author. Tel.: 139-041-234-8642; fax: 139-
seems to be the key ingredient of the mixture,041-234-8620.
E-mail address: tundop@unive.it (P. Tundo). because of the way it interacts with the catalyst,
1381-5148/02/$ – see front matter   2002 Elsevier Science B.V. All rights reserved.
PI I : S1381-5148( 02 )00185-2
96 P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101
Table 1 (Eq. (2), in a system made by iso-octane,
Types of modifiers used in the multiphase catalytic reduction
aqueous KOH, hydrogen, charcoal supported Pt
Modifier Ref. or Pd catalyst, and a modifier among the ones in
1 Aliquat 336 (A336) [1–11] Table 1.
1 22 C H (C H ) N Br [2,4]16 33 18 37 3
1 23 C H (n-Bu) P Br [1–4]16 33 3
1 24 C H (py) Br [1]16 33
1 25 PhCH (C H ) N Br [2]2 2 5 3 (1)1 26 (n-Bu) N HSO [2]4 4
7 MeO(CH CH O) H [2,3]2 2 n n|15
8 PEG 6000 [2]
9 PPG 2000 [2]
10 Brij 35 [8]
11 Brij 52 [8] (2)12 Brij 58 [8]
13 Brij 56 [8]
1 2 The metal catalyst is used approximately 5%14 PhCH (CH CH ) N Cl [8,9]2 3 2 3
1 2 molar with respect to the substrate, the modifier15 PhCH (n-Bu) N Cl [8]2 3
16 Et NH [9]2 in a 20% molar ratio. The following points
17 Et N [9]3 describe the modifier–catalyst interactions and18 n-Bu N [9]3 how they affect reactivity, selectivity, and the19 (PhCH ) N [9]2 3
20 n-C H NH [9] outcome of typical reactions.8 17 2
21 Cinchonidine [12]
22 Cinchonine [12] 2 .1. Catalyst partitioning
apparently by coating it [3,9,11]. The question The readily apparent effect of the modifier is
of how the modifiers act in promoting these macroscopic. By adding it to the aqueous–or-
effects has been addressed periodically, and has ganic–catalyst mixture (Fig. 1a) the latter visib-
allowed to collect a number of pieces of evi- ly changes its partitioning. Generally, either the
dence. The scope of this paper is to describe catalyst distributes itself preferentially in the
some conclusions on the study of the mecha- organic phase (Fig. 1b), or, at higher concen-
nism for the catalytic multiphase reduction trations of modifier, this tends to separate out
system, and to propose a mode of action for the between the aqueous–organic phases, to form a
catalyst modifier. third liquid phase [13], and to incorporate the
catalyst (Fig. 1c). Either way, the modifier has a
high affinity for the catalyst; and, in both cases
2 . Results and discussion the modifier–catalyst assembly is in better
contact with the organic than it is with the
There is plenty of flexibility in the choice of aqueous phase, hereby allowing efficient diffu-
the modifier. Essentially it has to be a phase sion of the organic substrate to the reactive site
transfer agent (PTA), i.e. a molecule with a of the catalyst.
relatively polar head group, and one or more
lipophylic chains (Table 1). One constraint is 2 .2. Catalyst coating by the modifier
that it must be insoluble both in the organic and
in the aqueous phases. There is a high affinity of the modifier for the
For simplicity, as model reactions one can catalyst particles, which are coated by a thin
consider indifferently either the hydrodechlori- film, a kind of membrane, of modifier. One
nation (HDCl) reaction of 1,2,4,5-tetrachloro- proposed mechanism by which this film is
benzene to benzene (Eq. (1), or the HDCl and formed may be analogous to the one sketched
carbonyl reduction of p-chloroacetophenone out in Scheme 1, where the carboxy groups
P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101 97
Fig. 1. Partitioning of the catalyst between the phases as a function of the modifier: a: organic phase1aqueous phase1supported catalyst; b:
organic phase1aqueous phase1supported catalyst1modifier; c: organic phase1aqueous phase1supported catalyst1excess modifier.
Scheme 1. Catalyst particle in the multiphase system: magnified view of a possible coating arrangement by an onium salt.
present on the charcoal support act as anchoring with increasing concentration of modifier, until
points for the modifier (in this case an onium they reach a plateau above which no further
ion) [9]. increase was detected (Fig. 2) [3]. A similar
The catalyst is made more lypophilic by this behavior was observed using Raney–Ni as the
surface membrane of modifier, which explains catalyst [11].
why it resides preferentially in the organic (Fig. The concentration threshold, above which
1b) or modifier phase (Fig. 1c). On the contrary, additional modifier is non-influential, may have
if ethanol is used as the solvent, where the a two-fold explanation: (a) it constitutes ‘mono-
modifier dissolves, and its affinity for the sup- layer’ formation, such as in Langmuir type
ported catalyst fails, its effect is not felt any adsorption; and (b) when the modifier reaches
longer. the amount on the catalyst surface where a
The hydrodehalogenation rate constants, constant concentration of reactants is main-
plotted versus the modifier concentration, follow tained in proximity of the catalyst, the mass-
Langmuir adsorption type curves. For example, transfer limit is effectively eliminated, and the
it was shown that the rate constants for hydro- intrinsic reaction rate is attained. This observa-
dehalogenation of chloroethylbenzenes increase tion is substantiated by the order of the reaction,
98 P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101
Fig. 3. Effect of varying amounts of CD on the conversion (A)
Fig. 2. HDX rate constants as a function of A336 concentration,
and ee (B) of the reduction of acetophenone.
for o-, m-, p-chloroethylbenzene.
3). The right tradeoff is the concentration where
zero in the substrate, which indicates that when tight chiral pockets are formed on the catalyst.
a steady-state concentration of substrate is ob- Other similar observations [15,16], and calcula-
tained in proximity of the catalyst, then bulk tions [17], support this (Fig. 4).
concentration becomes non-influential. On the It has been demonstrated that the modifier
other hand the reaction rate is first order in the does not act as a surfactant, i.e. it does not
catalyst [10].
Some support for the latter observation is
given by the fact that, while in the case of Pd/C
the modifier has scarce effect on the rates; in the
case of Raney–Ni the modifier is indispensable
for the reaction to proceed at all [11]. There
must therefore be some relation between the
modifier on the catalyst and substrate adsorp-
tion.
Further support for the formation of a layer of
modifier comes by using a chiral modifier, and
by observing that, under appropriate conditions,
stereospecific reactions can be conducted using
this system [12,14]. By covering the supported
catalyst and forming chiral pockets, which
stereo-recognize the substrate, the modifier pro-
motes enantioselectivity. The achievable ee’s
depend on the concentration of modifier used:
Fig. 4. Representation of how cinchonidine may form prochiral
too little gives high rates but poor ee’s, while cavities on a Pt surface (reproduced by permission of Wiley–
too much means slow rates and poor ee’s (Fig. VCH) [17].
P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101 99
improve dispersion of the reactants or catalysts,
and does not act via micellar catalysis [18].
2 .3. HCl removal from the catalyst
Another role of the modifier is due to its
phase transfer nature: it prevents catalyst
poisoning, by transporting the HCl produced by
the reaction into the aqueous phase, where it is
neutralized by the KOH. The mechanism is the
classical phase-transfer one (Scheme 2). The
poisoning effect of HCl on the catalyst [19] Fig. 5. Hydrogen uptake by Pt /C in solution and in the presence
becomes readily obvious either with no PT of A336.
agent or when insufficient base is present: the
reaction becomes inhibited. modifier can perhaps be ascribed to increased
and constant concentration of hydrogen, a reser-
voir, held in proximity of the catalyst.2 .4. Uptake of hydrogen
The modifier plays a subtle role on the uptake 2 .5. Chemoselectivity
of hydrogen as well. In fact, the amount of
hydrogen adsorbed by the system appears larger It can be tuned by varying a number of
when the modifier is present. This was observed reaction parameters. Mainly modifier type and
by bubbling hydrogen, and measuring its uptake amount [3,4,6], and base concentration [8–10].
using a graduated burette, first in mixtures made The major effect of the modifier is to induce
by isooctane–KOH(aq)–catalyst, and then in chemoselectivity in many of the reactions.
identical mixtures where A336 was added. The Significantly, the reaction of p-chloro-
latter mixture was able to adsorb a larger propiophenone goes all the way to ethyl ben-
amount of hydrogen than the former. Analo- zene with Pt /C and no modifier; while in the
gously, the fact that an additional quantity of H presence of A336 100% selectivity towards2
was adsorbed, when A336 was added to a hydrodechlorination (Pd/C) or the benzyl al-
mixture of isooctane–KOH(aq)–catalyst, after it cohol (Pt /C) can be obtained [8]. The modifier
had been already saturated with H , confirms is therefore somehow mediating between the2
this effect (Fig. 5) [18]. substrate and the catalytic sites. The question is
This observation is significant because it how the modifier does that.
implies that the rate acceleration of the hydro- Under our conditions the reduction mecha-
dehalogenation reaction in the presence of the nism presumably does not involve radical
Scheme 2. Phase-transfer mechanism.
100 P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101
species, but rather oxidative addition of the tivities. Part of its role may be in the salification
substrate to the catalyst, followed by hydrogen of the COOH groups present on the support,
1insertion, and elimination of the products. The thereby allowing substitution of H by the
modifier membrane must therefore come into onium cation (Schemes 1–2) [9]. In addition,
play by mediating adsorption of the substrate on KOH may interact (as inhibitor /promoter) with
the catalyst surface, in a way that favors, for the catalytic metal sites [20]. It was also noted
example C–Cl, bond cleavage over C=O hydro- that an excess of KOH is detrimental to the
genation, to give hydrodechlorination rather reaction selectivity, perhaps too much KOH
than carbonyl reduction. wipes the catalyst clean and decreases
There is a dependence of chemoselectivity on chemoselectivity.
the hydrophilicity of the modifier. Higher hy-
drophilicity, causes the reaction of Eq. (2) to
yield chemoselectively acetophenone, the hy- 3 . Conclusions
drodechlorination product. In this case perhaps,
the modifier–catalyst ensemble, by hindering At this stage the conclusions that can be
approach causes a less positive interaction be- drawn on the mode of action of the modifier,
tween substrate and the catalyst. This, in turn, based on experimental evidence, are the follow-
could favor reaction of the more polarized C–Cl ing.
bond with respect to the C=O one, i.e. higher
chemoselectivity. 1. The modifier coats the catalyst particles, and
This explains, for example, why hydrodech- this interaction is largely responsible for the
lorination is generally the fastest reaction: ap- selectivity features of the system.
proach and reaction of the aromatic chlorine 2. The modifier membrane forms the nanoen-
would be favored initially with respect to car- vironment wherein the reaction takes place.
bonyl. 3. The modifier is adsorbed on the surface of
the catalyst and behaves like a hydrogen
reservoir for the reaction.2 .6. Kinetics
4. The reaction itself occurs at the interface
Kinetic studies show that certain reaction between modified-catalyst, aqueous and or-
parameters (KOH concentration, nature of the ganic phases, in a tight nanostructured en-
metal, nature of the modifier) play a role on the vironment where transfers between the
reaction outcome. As far as KOH is concerned, phases are favored. The substrate, H , KOH,2
for example, it is apparent that, not only does it HCl, the product, and the modified-catalyst
neutralize HX, it also acts synergistically with are in close contact with one another, allow-
the modifier in determining kinetics and selec- ing efficient transfer of the reactants between
Fig. 6. Equilibria of the reactants at the catalyst–modifier–aqueous /organic interfaces.
P. Tundo, A. Perosa / Reactive & Functional Polymers 54 (2003) 95–101 101
[7] A. Perosa, P. Tundo, S. Zinovyev, Green Chem. 4 (2002)the phases. Reagent, and product selectivity
492–494.
are determined by the equilibria which are [8] M. Selva, P. Tundo, A. Perosa, J. Org. Chem. 63 (1998)
established by these species between the 3266–3271.
[9] A. Perosa, M. Selva, P. Tundo, J. Org. Chem. 64 (1999)phases. Fig. 6 shows the membrane formed
3934–3939.by the modifier on the catalyst, the liquid [10] P. Tundo, S. Zinovyev, A. Perosa, J. Catal. 196 (2000)
phase made by the aqueous and organic 330–338.
[11] S. Zinovyev, A. Perosa, S. Yufit, P. Tundo, J. Catal. 211solvents, and the equilibria of some of the
(2002) 347–354.involved species. [12] A. Perosa, P. Tundo, M. Selva, J. Mol. Catal. A 180 (2002)
169–175.
[13] G.D. Yadav, C.A. Reddy, Ind. Eng. Chem. Res. 38 (1999)
A cknowledgements 2245–2253;
T. Ido, T. Yamamoto, G. Jin, S. Goto, Chem. Eng. Sci. 52
(1997) 3511–3520;The Italian Interuniversity Consortium
D.-H. Wang, H.-S. Weng, Chem. Eng. Sci. 50 (1995) 3477–
‘Chemistry for the Environment’ (INCA), Ca’ 3486;
Foscari University, INTAS grant no. 2000- D. Mason, S. Magdassi, Y. Sasson, J. Org. Chem. 56 (1991)
7229–7232.00710, NATO grant no. EST.CLG.977159. are
[14] see also for example: A. Baiker, J. Mol. Catal. A 163 (2000)gratefully acknowledged for funding. 205–220;
This work was in part financed by ‘Piano’ ¨H.-U. Blaser, M. Muller, in: Heterogeneous Catalysis and
Fine Chemicals II, M. Guisnet et al. (Eds.), Elsevier Science‘Ambiente Terrestre: Chimica per l’Ambiente’,
Publishers B.V., Amsterdam, 1991, pp. 73–91, and referencesLegge 488/92, MIUR.
therein.
[15] C. LeBlond, J. Wang, J. Liu, A.T. Andrews, Y.-K. Sun, J.
Am. Chem. Soc. 121 (1999) 4920–4921.
[16] J. Kubota, F. Zaera, J. Am. Chem. Soc. 123 (2001) 11115–R eferences
11116.
[17] J.M. Thomas, Angew. Chem. Int. Ed. Engl. Adv. Mater. 28[1] C.A. Marques, M. Selva, P. Tundo, J. Chem. Soc. Perkin (1989) 1079–1088.Trans. 1 (1993) 529–533. [18] Unpublished results.[2] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 58 (1993)
´ ´ ´ ´[19] M.A. Aramendıa, V. Borau, I.M. Garcıa, C. Jimenez, F.5256–5260. Lafont, A. Marinas, J.M. Marinas, F.J. Urbano, J. Mol. Catal.[3] C.A. Marques, M. Selva, P. Tundo, J. Org. Chem. 59 (1994) A 184 (2002) 237–245, and references therein.3830–3837. [20] R.L. Augustine (Ed.), Heterogeneous Catalysis for the[4] C.A. Marques, O. Rogozhnikova, M. Selva, P. Tundo, J. Synthetic Chemist, Marcel Dekker Inc, New York, 1996, p.Mol. Catal. A 96 (1995) 301–309. 220.[5] A. Perosa, M. Selva, P. Tundo, S.S. Zinovyev, Appl. Catal.
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The action of onium salts and other modifiers on Pt/C, Pd/C, and Raney-Ni catalyst in the multiphase reduction system

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