Quaternary Ammonia Immobilized Polystyrenes As Reusable Heterogeneous Catalysts For Synthesis Of Cyclic Carbonate

The fixation of CO2 to produce cyclic carbonates is an effective way of CO2 comprehensive utilization since cyclic carbonates play an important role in society.However,the use of CO2 still faces a big challenge owing to its high thermodynamic stability and kinetic inertness.Catalysts are considered to be essential to activate CO2 for cycloaddition.In the past few decades,a considerable number of homogeneous catalysts have been developed for the coupling of CO2 and epoxides.But,these homogeneous catalysts inevitably possess some problems,such as,the difficulty to recover the catalysts from products and also to purify the products.The heterogeneous catalytic process has the potential to solve these problems through easy solid/liquid separations,but the poor catalytic activity is still of concern,especially under mild reaction conditions.In view of these problems,the activity of the catalyst is improved from two angles.One is to provide a larger contact area of the catalysts with reactants,the other is to provide a catalyst with more functional ions.Three effective heterogeneous catalysts were prepared for epoxides and carbon dioxide to produce cyclic carbonates without the need for any additional cocatalyst and organic solvent.Through the epoxy functional monomer and ammonia reacted,and then with chloromethylated polystyrene microspheres reaction,acquired polystyrene-supported hydroxyl functionalized long alkyl chain morpholine salt heterogeneous catalyst([AHMIC-PS]Br)for the cycloaddition of carbon dioxide.The effects of tempreture,pressure and reaction time were investigated.With the increase of reaction temperature and the reaction time,the yield of propylene carbonate(PC)gradually increased and became balanced.Under suitable conditions,the yield of cyclic carbonate was 87%,the selectivity was 99%.Besides,the yield was 86.5% after five cycle used.The morphine salt was replaced by the use of quaternary ammonium salts,acquired the polystyrene-supported hydroxyl functionalized long alkyl chain ammonium salt heterogeneous catalyst([AHTAPC-PS]X,X = Cl,Br,I).The effect of the length of linking chains between the supports and ammonium groups was investigated.A long chain can provide a larger contact area of the catalysts with reactants,thus enhancing the reaction kinetics.A polystyrene-supported ammonium without a hydroxyl group on the linking chain was also synthesized and the impact of a pendent hydroxyl group was studied.In addition,the charges of halogen anions in the different catalysts and the interaction of the hydroxyl group were studied by density functional theory(DFT)calculations.It was found that compared with a short linking chain,a long chain can provide a catalyst with a more negative halogen anion as well as a larger contact area of the catalysts with reactants,thus enhancing the reaction kinetics.The hydroxyl group can stretch the C–O bonds of epoxides,which promotes the reaction thermodynamics.As a result,the yield is much higher for the catalyst with a long linking chain than for that with a short chain,and is further enhanced by the introduction of hydroxyl groups.Contrast with [AHMIC-PS]Br,the morphological structure can hinder the contact of some reactants with functional ions,and the three methyl groups of the quaternary ammonium salt can prevent from hindering,thus improving the catalytic activity.The effect of the reaction conditions and the reusability of the catalyst were also studied.Besides,[AHTAPC-PS]Br also possessed good reusability(PC yield =96% after 10 times cycle used)and activity on the mild condition(100°C,PC yield=78.4%).The reactivity of hexamethylenetetramine with chloromethylated polystyrene microspheres was used to prepare multiple sites quaternary ammonium salt heterogeneous catalysts([HMTA-PS]Br).The hexamethylenetetramine contains multiple tertiary nitrogen atoms,can achieve a single group possessing multiple active sites,and the tertiary nitrogen N atom without quaternization can simultaneously activate the CO2,thereby improving the reactivity of the catalytic system.Under the optimal conditions,the catalyst showed good catalytic performance(PC yield =98.7%,PC selectivity =99.8%),The catalyst [HMTA-PS]Br has good stability,PC yield> 97% after 5 cycle used.