Applied Catalysis A: General 387 (2010) 113–118 Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Synthesis and characterization of a novel polystyrene-tethered niobium methoxo species. Its application in the CO 2 -based carboxylation of methanol to afford dimethyl carbonate Michele Aresta a , Angela Dibenedetto a,∗ , Francesco Nocito a , Antonella Angelini a , Bartolo Gabriele b , Serena De Negri c a Department of Chemistry and CIRCC, University of Bari, Campus Universitario, Via Orabona 4, 70126 Bari (BA), Italy b Department of Pharmaceutical Sciences, University of Calabria, 87036 Arcavacata di Rende (CS), Italy c Department of Chemistry and Industrial Chemistry, University of Genova, 16146 Genova (GE), Italy article info Article history: Received 13 May 2010 Received in revised form 6 August 2010 Accepted 10 August 2010 Available online 17 August 2010 Keywords: Polystyrene-tethered niobium methoxo species Methanol carboxylation Dimethyl carbonate Heterogeneous catalysis abstract In this paper we describe the synthesis and characterization of a polystyrene-tethered Nb-complex, namely P-C 6 H 4 -p-CH 2 CH 2 –NbCl 4 , 1 which reacts with CH 3 OH to afford P-C 6 H 4 -p-CH 2 CH 2 –NbCl(OCH 3 ) 3 2. The latter inserts CO 2 to afford P-C 6 H 4 -p-CH 2 CH 2 –NbCl[O–C(O)OCH 3 ](OCH 3 ) 2 , 3, active in the catalytic carboxylation of methanol. The insertion of only one mol CO 2 per Nb has been determined both by measuring the CO 2 released upon treatment with HCl and by solid state 13 C NMR. 3 is easily recovered from the reaction medium and re-used; it can also be used in a flow reactor without decomposition after days. It shows good recycling properties and is much easier to use than the relevant homogeneous catalyst Nb(OCH 3 ) 4 [O–C(O)OCH 3 ]. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The large use of dimethyl carbonate (DMC) in different industrial applications for the production of chemicals [1], pharmaceuticals [2] and polymers [3] has driven to search new routes for its pro- duction which may avoid the use of toxic phosgene (that has the advantage of being quite reactive) and the consequent formation of chlorinated by-products. The direct carboxylation of methanol (Eq. (1)) represents an interesting alternative synthetic route from the point of view of both atom economy [4] and risk reduction: 2CH 3 OH + CO 2 = (CH 3 O) 2 CO + H 2 O (1a) 2CH 3 OH + 2B : + COCl 2 → (CH 3 O) 2 CO + 2BH + Cl - (1b) Nevertheless, carbon dioxide is less reactive than phosgene, due to the higher energy of the C O with respect to C–Cl bond. Also, the overall G of reaction (1a) is more positive than that of reaction (1b), as inferred from the G 0 f of CO 2 and COCl 2 species (Table 1). As a matter of fact, the G 0 f of acyclic carbonates for the direct car- boxylation of aliphatic alcohols (Eq. (1a)) is only slightly lower than ∗ Corresponding author. Tel.: +39 080 544 3606; fax: +39 080 544 3606. E-mail address: a.dibenedetto@chimica.uniba.it (A. Dibenedetto). zero [for (RO) 2 CO: if R = Me, -4.0, R = Et, -3.8, R = Pr, -3.9 kcal/mol] and is positive when phenol is used (R = Ph, +12 kcal/mol). With respect to reaction (1b) that occurs at room temperature in pres- ence of a base, the use of CO 2 requires, thus, higher temperatures and a catalyst that may improve the reaction rate. Several catalytic systems have been used in the reaction reported in Eq. (1a), either homogeneous [5–8] or heterogeneous [9–11] or else heterogenized [12]. Niobium alkoxides have been used in the carboxylation of methanol in homogeneous phase [7] in which the Nb-hemicarbonate reacts with two molecules of alcohol through an acid-plus-base activation of the latter species (Scheme 1). The mechanism based on two molecules of methanol implied in the formation of the carbonate allows the formation of water as co-product out of the co-ordination sphere of Nb [7]. Although such mechanism prevents the direct formation of Nb-hydroxo species, nevertheless the accumulation of water in the reaction medium makes that the catalyst may be destroyed. In fact, the conversion of [Nb(OMe) 5 ] 2 4 into [NbO(OMe) 3 ] x totally deactivates the catalyst [12]. So, it is necessary to recover the catalyst at the end of each equilibrium step (cycle) and re-use it in freshly dried methanol. Such procedure is not straightforward with homogeneous species. Heterogenized catalysts bearing the catalytic centre tethered to a solid matrix would be interesting because they would potentially 0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.08.013