Fixation of carbon dioxide into dimethyl carbonate over titanium-based zeolitic thiophene-benzimidazolate framework

A titanium-based zeolitic thiophene-benzimidazolate framework has been designed for the direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide. The developed catalyst activates carbon dioxide and delivers over 16% yield of DMC without the use of any dehydrating agent or requirement for azeotropic distillation.

. Earlier, scientists have used phosgene as starting material in DMC synthesis 13 . In order to eliminate the toxic feedstocks required for DMC synthesis transition metal catalyzed reactions have been developed 14,15 . The reported methods, however, often require longer reaction time, high temperature and pressure to accomplish the synthesis. Alternatively, oxidative carbonylation of methanol 16,17 , transesterification of cyclic carbonates 12 , methanolysis of urea 12 have been deployed but these methods suffer from low yield and diminished atom efficiency resulting in the generation of large amount of chemical waste along with compromised yield.
Carbon dioxide is one of the most basic oxygenated carbon which can be converted into corresponding DMC 15 ; several reports document transforming carbon dioxide into DMC [18][19][20] . However, they invariably suffer from reversibility of the reaction, because water is generated as a byproduct in the reaction mixture which reacts with the product DMC thus reverting it back into starting materials 21 . To carry this reaction forward, an azeotropic distillation is mandatory which is not a trivial proposition to setup under high pressure conditions. Alternatively, water trapping reagents such as molecular sieves need to be added to the mix 22,23 . Our efforts have focused on finding simple alternative protocols for the synthesis of industrially important products [24][25][26][27] . To accomplish a simple synthesis of DMC, we have now designed a titanium-based zeolitic thiophene-benzimidazolate framework (Ti-ZTBF) and demonstrated its application in the chemical fixation of carbon dioxide into DMC without the use of water trapping agents.

Results and Discussion
We planned to synthesize zeolitic framework and explore its application in DMC synthesis using carbon dioxide and methanol with a keen eye to address the main challenge that has appropriate control over the reversible nature of reaction. We envisioned that metal zeolitic framework would address the problem as the porous nature and high surface area of the material would keep water molecules away from the DMC product thus promoting the equilibrium shift in right direction. We embarked on our study to search for and synthesize a suitable ligand for the creation of zeolitic type metal framework. In our quest, we synthesized four different ligands namely, 2- imidazole. These ligands were than treated with titanium iso-butoxide in a pressure reactor at 140 °C using DMF as solvent for 24 hours; the formation of crystalline solid ensued in reaction vessel (see Supplementary). It was then centrifuged, washed with methanol and vacuum dried to form Ti-ZFF (BET surface area = 369.01 m 2 g −1 ), Ti-ZMF (BET surface area = 316.30 m 2 g −1 ), Ti-ZTF (BET surface area = 140.10 m 2 g −1 ) and Ti-ZTBF (BET surface area = 811.36 m 2 g −1 ), respectively (Fig. 1).
After securing the catalyst, we screened these zeolitic framework entities for the conversion of CO 2 to DMC. Initially, we performed the reactions at different pressure of CO 2 at room temperature; at 10 and 20 psi none of these catalysts gave any DMC (Table 1, entries 1-8). However, at 30 psi, we observed the trace amounts of DMC in presence of Ti-ZTBF (Table 1, entry 12). Porosity and high surface area of Ti-ZTBF catalyst made it more effective towards the synthesis of DMC. It became apparent that Ti-ZTBF is the effective catalyst for this transformation and we continued forward with our optimization explorations using Ti-ZTBF as a catalyst with appropriate variation in temperature in addition of increasing CO 2 pressure. The reaction at 50 °C gave 1.5% of DMC which increased to 9% and 16% isolated yield at 75 °C and 100 °C, respectively (Table 1, entries 13-15). A further increase in the temperature to 120 °C by keeping pressure at 30 psi reaction did not improve the output thus affirming that 100 °C appears to be the optimum temperature for this reaction. The effect of increased pressure was also examined at 40 and 50 psi at 100 °C but it did not improve the outcome of these control experiments as the isolated yield of DMC remained stubbornly at 16% (Table 1, entries 17-18). It seems that 30 psi pressure and 100 °C temperature are the ideal conditions for this transformation. Further probing experiments were conducted by varying the catalytic amount of Ti-ZTBF which confirmed a sharp drop in the product yield at 50 mg level. Increasing the concentration of catalyst from 100 mg to 150 mg did not show any significant improvement in the product yield thus establishing that 100 mg is the optimum amount of catalyst required for the successful synthesis of DMC (Table 1, entries [19][20].

Conclusions
Titanium-based zeolitic thiophene-benzimidazolate framework (Ti-ZTBF) has been designed and synthesized. Its application has been demonstrated in the activation of carbon dioxide and direct synthesis of dimethyl carbonate from methanol. The porous morphology and high surface area of Ti-ZTBF plays a crucial role in high activity enabling the reaction to process at lower temperature and CO 2 pressure. The Ti-ZTBF is a unique catalyst developed to date which takes the reaction in forward direction by circumventing the use of dehydrating agent and avoiding elaborative azeotropic distillation set-ups. The developed catalyst shows good recyclability and could be used up to five times without losing its activity.