Journal of Molecular Catalysis A: Chemical 315 (2010) 76–81 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata Theoretical study of TBD-catalyzed carboxylation of propylene glycol with CO 2 Jun Ma a,b , Xuelan Zhang a,b , Ning Zhao a , Abdullah S.N. Al-Arifi c , Taieb Aouak c , Zeid Abdullah Al-Othman c , Fukui Xiao a , Wei Wei a,∗ , Yuhan Sun a,∗ a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, PR China c Chemistry Department, King Saud University, Riyadh 11451, Saudi Arabia article info Article history: Received 14 July 2009 Received in revised form 2 September 2009 Accepted 4 September 2009 Available online 15 September 2009 Keywords: Catalytic mechanism TBD DFT CO2 Propylene glycol abstract The mechanisms for the reaction of propylene glycol (PG) with CO 2 catalyzed by 1,5,7-triazabicyclo [4.4.0]dec-5-ene (TBD) were theoretically investigated by density functional theory (DFT) method at the B3LYP/6-311++G(d,p) level. Through analyzing the optimized structures and energy profiles along the reaction paths, the PG-activated route was identified as the most probable reaction path, in which the rate-determining step was the nucleophilic attack of one of the O atoms in CO 2 on the hydroxyl linked C atom in PG with energy barrier 56.96 kcal/mol. The catalytic role of TBD could be considered as a proton bridge activated by the synergistic action of its N atoms. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Recently, the synthesis of propylene carbonate (PC) from propylene glycol (PG) and CO 2 has drawn much interest. Both homogeneous [1–5] and heterogeneous [6–8] catalysts have been reported to be effective in this carbonylation. In our previous work [4], an experimental study of the carbonylation of PG with CO 2 was carried out with organic bases as the catalysts. And 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) showed a superior catalytic activity compared to the others. TBD is one of the non-ionic nitrogen bases which are widely used to catalyze the carbonylation of amines or alcohols with CO 2 as the carbonylation reagent [9–13]. Unfortunately, it was not clear how TBD weakened the barriers for the carbonylation. One possi- bility was the substrate activation, which made it more susceptible to the electrophilic attack of the CO 2 [10–15]. The other was the CO 2 activation, which makes it more active to attack [16–18]. How- ever, only few studies on the description of reaction mechanisms or the possible structures of the carbonylation transition state were reported [14,15]. In the present work, the mechanisms for TBD-catalyzed car- bonylation of PG with CO 2 were first proposed and then verified by quantum chemistry computations of DFT method. ∗ Corresponding authors. E-mail addresses: weiwei@sxicc.ac.cn (W. Wei), yhsun@sxicc.ac.cn (Y. Sun). 2. Mechanism proposals According to the two possibilities mentioned above, two mech- anisms were proposed. They are presented here with the name PG-activated and CO 2 -activated, respectively. The catalytic activities of the organic bases are, in many cases, associated with their proton transfer activities [19], which would make the active site of the deprotonated molecule more susceptible to the electrophilic attack of the CO 2 [10]. TBD is a widely used pro- ton transfer agent in acid–base catalyzations [20–24], especially the reactions involving proton transfer between hydroxyl com- pounds [25,26]. The consequence of the proton transfer from the hydroxyl to the TBD is a hydrogen bonded ion-pair. Half of which, the TBD protonated molecule (TBDH + ), has already been studied experimentally as well as theoretically [26–31]. Based on these considerations, a PG-activated mechanism for the TBD-catalyzed carbonylation reaction of PG with CO 2 was pro- posed in Scheme 1, which consisted of two consecutive steps. First, PG was activated via being looted one hydroxyl H by the N atom of TBD, accompanied by the electrophilic attack of CO 2 on the hydroxyl O atom giving rise to an “ion-pair” (2 or 5). This was the CO 2 electrophilic attack step. Next, with the proton migrating from TBDH + to the other hydroxyl group, a water formed and the oxy- gen of the bonded CO 2 nucleophilicly attacked the C atom of PG to afford PC, and the catalyst TBD was recovered. This was named as the dehydration step. In the CO 2 electrophilic attack step, the proton shift and the O–C bond formation occurred simultaneously. This process is similar to 1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2009.09.003