Sustainable Synthesis of Dimethyl- and Diethyl Carbonate from CO₂in Batch and Continuous Flow─Lessons from Thermodynamics and the Importance of Catalyst Stability

Abstract

Equilibrium conversions for the direct condensation of MeOH and EtOH with CO 2 to give dimethyl- and diethyl carbonate, respectively, have been calculated over a range of experimentally relevant conditions. The validity of these calculations has been verified in both batch and continuous flow experiments over a heterogeneous CeO 2 catalyst. Operating under optimized conditions of 140 °C and 200 bar CO 2 , record productivities of 235 mmol/L·h DMC and 241 mmol/L·h DEC have been achieved using neat alcohol dissolved in a continuous flow of supercritical CO 2 . Using our thermodynamic model, we show that to achieve maximum product yield, both dialkyl carbonates and water should be continuously removed from the reactor instead of the conventionally used strategy of removing water alone, which is much less efficient. Catalyst stability rather than activity emerges as the prime limiting factor and should thus become the focus of future catalyst development.

Comparative Study between 2‐Furonitrile and 2‐Cyanopyridine as Dehydrants in Direct Synthesis of Dialkyl Carbonates from CO2 and Alcohols over Cerium Oxide Catalyst

“…Meanwhile, under the “stoichiometric” (alcohol/nitrile/CH 3 CN=50 : 25 : 100) and “alcohol‐excess” (100 : 25 : 100) conditions, the possible maximum amount of dialkyl carbonates is 25 mmol. Under these three conditions, we conducted the reactions using two different amounts of CeO 2 catalyst (0.17 g and 0.68 g), which and whose related materials have been reported by many research groups to exhibit good catalytic activity for non‐reductive conversion of CO 2 [4,21–27,34–57] . The results obtained with the smaller amount of CeO 2 catalyst (0.17 g) are reflected by the reactivity of alcohols and nitrile dehydrants over the catalyst surfaces, and those with the larger amount of CeO 2 (0.68 g) provide the information about the saturation level of the product formation.…”

“…Due to the unnecessity of alteration in the oxidation state of the carbon atom in CO 2 , the energy requirement for non‐reductive transformation is relatively low compared to reductive transformation. This advantage has attracted much attentions of researchers and made the synthesis of dialkyl carbonates central to the non‐reductive CO 2 transformation [1,3,4] . Organic carbonates including dialkyl carbonates have been used as electrolytes in lithium‐ion batteries and raw materials for polycarbonates, and dimethyl carbonate is also regarded as a promising fuel additive [3] .…”

“…This advantage has attracted much attentions of researchers and made the synthesis of dialkyl carbonates central to the non-reductive CO 2 transformation. [1,3,4] Organic carbonates including dialkyl carbonates have been used as electrolytes in lithium-ion batteries and raw materials for polycarbonates, and dimethyl carbonate is also regarded as a promising fuel additive. [3] Dialkyl carbonates have been synthesized conventionally by the reaction of alcohols with phosgene (COCl 2 ) or urea, (NH 2 ) 2 CO. [3] The reaction of alcohols with (NH 2 ) 2 CO to dialkyl carbonates can be interpreted as an indirect route from CO 2 because (NH 2 ) 2 CO is synthesized from CO 2 and ammonia.…”

“…Our previous results for the other nitrile dehydrants consisting of CH 3 CN [22] and benzonitrile [23] have also indicated that the formation rate of dialkyl carbonate is dependent on the combination of alcohol and nitrile. (4) Along with the equilibrium shift, the addition of 2cyanopyridine enhances the reaction rate of CO 2 and alcohols to corresponding organic carbonates. [27] This acceleration effect is due to the formation of strong base sites via the attachment of 2-cyanopyridine on the CeO 2 surface.…”

Sustainable valorisation of renewables through dialkyl carbonates and isopropenyl esters

“…They observed that performing DAC synthesis in CF conditions (scCO 2 at 200 bar, T = 140 °C, productivities of 235 mmol (L h) −1 for DMC and 241 mmol (L h) −1 for DEC) allowed for continuous removal of both water and DAC product, preventing progressive surface M–O–M hydrolysis on the heterogeneous catalyst and paving the road towards low-temperature direct carboxylation of alcohols. 58 Aricò and co-workers developed a scalable procedure for the synthesis of non-commercial symmetrical and unsymmetrical DACs incorporating alkyl, alkoxyalkyl, aminoalkyl, and thioalkyl moieties, depicted in Scheme 6. DAC synthesis occurred via TBD-promoted transcarbonylation (10% mol); interestingly, the reported procedure allowed for large-scale production of DACs (up to 100 mL) and DAC products were purified by distillation.…”