SusChemE 2015
International Conference on Sustainable Chemistry & Engineering
October 8-9, 2015, Hotel Lalit, Mumbai
Catalytic Conversion of CO2 into Biodegradable Polycarbonates
Joby Sebastian, Darbha Srinivas*
Catalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Pune-411 008, India
E-mail: d.srinivas@ncl.res.in
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1. Introduction
Copolymerization of epoxides and CO2 is a sustainable chemical process in the atom efficient fixation of greenhouse gas CO2 in biodegradable polymers. Solid Co-Zn double-metal cyanide (DMC) complexes have shown encouraging
performance for this reaction, but need a deep insight to the structural aspects of the catalysts to control their
activity/selectivity especially when different epoxides are employed [1]. We report here the factors influencing the catalytic
activity of Co-Zn DMC catalysts for the copolymerization and terpolymerization of propylene oxide (PO), cyclohexene
oxide (CHO) and CO2.
2. Materials and Methods
Co-Zn DMC catalysts having varying crystal structure (cubic, monoclinic and rhombohedral designated as Co-Zn-C, CoZn-C/M and Co-Zn-M/R, respectively) and compositions were prepared by complexation of K3[Co(CN)6] with ZnCl2 by
different procedures involving the selective/optional use of complexing (tert.-butanol) and co-compelxing agents
(polyethylene glycol, Mn = 4000) and studied for the polymerizations (Scheme 1). Percentage incorporation of CO2 (FCO2)
and the amount of cyclic propylene carbonate (W PC) impurity were determined from 1H-NMR studies of the product
polymers [2, 3].
Scheme 1: Copolymerization (1 and 2) and terpolymerization (3) of PO, CHO and CO2.
3. Results and Discussion:
Co-Zn-C and Co-Zn-C/M contains the basic structural unit Zn3[Co(CN)6].xH2O with negligible amounts of K+ and Cl- ions.
Co-Zn-M/R is a mixture of KZn[Co(CN)6] and Zn3[Co(CN)6].mZnCl2.xH2O with significant amounts of Zn2+, K+ and Clions. Co-Zn-C and Co-Zn-C/M were highly crystalline as compared to Co-Zn-M/R. Introduction of complexing agent in
Co-Zn-C/M and Co-Zn-M/R has reduced the crystalline symmetry from cubic to monoclinic and rhombohedral. Lewis
acidity of the catalysts decreased in the order: Co-Zn-M/R (2.27 mmol NH3/g) > Co-Zn-C (2.16 mmol NH3/g) > Co-ZnC/M (1.79 mmol NH3/g). Method of preparation of DMC complexes has a great influence on its physicochemical properties
[2]. Co-Zn-C was inactive for the polymerizations. In contrary, catalysts prepared with complexing agent showed complete
conversion of epoxides in PO/CO2 and CHO/CO2 polymerizations (Co-Zn-C/M: productivities 37 and 50 g/gcat with 54 and
80 mol% FCO2 respectively, WPC = 18 wt%, Co-Zn-M/R: productivities 37 and 53 g/gcat with 31 and 86 mol% FCO2
respectively, WPC = 27 wt%). Activity behaviours indicated Co-Zn-C/M is highly selective in PO/CO2 copolymerization and
less selective in CHO/CO2 copolymerization compared to Co-Zn-M/R. In terpolymerization, Co-Zn-M/R showed good
compositional control of propylene and cyclohexene carbonate units (53: 47) with respect to the molar ratio of epoxide
taken (1:1) than Co-Zn-C/M (59:41) [3]. Co-Zn-M/R was an instant active catalyst in all polymerizations whereas Co-ZnC/M showed an induction period (~ 2 h) (Figure 1).
Co-Zn-M/R
Co-Zn-C/M
500
Reactor pressure (psi)
PO/CHO/CO2
450
400
400
450
350
350
400
300
300
450
PO/CHO/CO2
CHO/CO2
PO/CO2
Reactor pressure (psi)
CHO/CO2
PO/CO2
480
360
400
440
350
400
300
300
360
0
4
8
12
0
4
8
12
0
4
8
12
0
4
Reaction time (h)
8
12
0
4
8
12
0
4
8
12
Reaction time (h)
Figure 1: Induction period of Co-Zn DMCs during copolymerization and terpolymerization.
4. Conclusions:
Structure- activity correlations revealed that coordinated complexing agent (activity), acid density (productivity), alkali
content (selectivity), strength and dispersed ZnCl2 (induction period) are the critical features of DMC catalysts to be
controlled/ tuned for polycarbonate synthesis.
References
[1]
M.R. Kember, A. Buchard, C.K. Williams, Chem. Commun. 47 (2011) 141.
[2]
J. Sebastian, D. Srinivas, Appl Catal A: Gen. 482 (2014) 308.
[3]
J. Sebastian, D. Srinivas, RSC Adv. 5 (2015) 18203.