Reducing the Cost of Production of Bimetallic Aluminium Catalysts for the Synthesis of Cyclic Carbonates Michael North* and Carl Young[A]

DOI: 10.1002/cssc.201100239 Reducing the Cost of Production of Bimetallic Aluminium Catalysts for the Synthesis of Cyclic Carbonates Michael North* and Carl Young[a] Bimetallic aluminium complexes of general formula [(salen)- most expensive chemicals by less expensive alternatives. The Al]2O or [(acen)Al]2O catalyse the formation of cyclic carbo- largest cost saving was associated with the formation of alumi- nates from carbon dioxide and terminal epoxides under excep- nium triethoxide in situ, which reduced the cost of the chemi- tionally mild reaction conditions. To improve the potential for cals need for production of the catalysts by 49–87 %. Further industrial scale application of these catalysts, the cost of their savings were made by avoiding the use of tetrabutylammoni- production has been evaluated and reduced significantly by um bromide and acetonitrile, resulting in overall cost savings optimization of the synthesis, including replacement of the of 68–93 %. Introduction The increasing prominence of climate change related issues[1] the use of dimethyl carbonate as an oxygenating agent for and the limited nature of fossil fuel reserves[2] for use as transport fuel.[16] energy and chemical feedstocks has driven the development Current commercial processes for the production of cyclic of green chemical processes involving alternative solvents, re- carbonates employ catalysts that require the use of tempera- newable materials and minimal waste.[3] One area that is re- tures above 1808C, pressures above 50 atm (1 atm = ceiving increasing attention is the use of carbon dioxide as an 101325 Pa) and highly purified carbon dioxide.[17] The carbon inexpensive, readily available and renewable starting material dioxide emitted as a result of generating the energy required for the synthesis of commercially important chemicals.[4] Urea to achieve these reaction conditions negates any benefit from is currently manufactured from carbon dioxide on a the use of carbon dioxide in the synthesis of cyclic carbonates. 100 Mtonne per annum scale[5] and salicylic acid has been pro- The recent development of bimetallic aluminium(salen) com- duced commercially by the Kolbe–Schmidt reaction for over plexes as catalysts for cyclic carbonate synthesis has, however, 100 years.[5b,6] More recently, the synthesis and commercial ap- opened up new possibilities for the synthesis of cyclic carbo- plications of polycarbonates have attracted much attention.[7] nates from waste carbon dioxide under mild reaction condi- Carbon dioxide is, however, relatively unreactive; so the devel- tions.[18, 19] The combination of bis[(1R,2R)-N,N’-bis(3,5-di-tert- opment of processes occurring at or near room temperature butyl-salicylidene) cyclohexane-1,2-diaminoaluminium(III)] and atmospheric pressure to avoid carbon dioxide emissions oxide, complex 1, with tetrabutylammonium bromide was from energy production remains a major challenge.[8] Cyclic carbonates are another class of chemicals that can be prepared from the 100% atom economical and highly exother- mic reaction of carbon dioxide with epoxides (DHr = À140 kJmolÀ1 for ethylene carbonate),[9,10] a process that has been operated commercially for over half a century (Scheme 1).[11] Cyclic carbonates have many commercial appli- cations, for example, as solvents,[12,13] as electrolytes for lithium-ion batteries,[14] and as chemical intermediates for the synthesis of acyclic carbonates, ethylene glycol, and polymers.[11,15] The annual production of cyclic carbonates is increasing, owing to their application in lithium-ion batteries, and there is scope for further increase (by at least two orders of magnitude) if their production costs could be reduced to advocate [a] Prof. M. North, Dr. C. Young School of Chemistry and University Research Centre for Catalysis and Intensified Processing Newcastle University Bedson Building, Newcastle-upon-Tyne, NE1 7RU, UK Fax: (+ 44)191 222 6929 Scheme 1. Synthesis of cyclic carbonates. E-mail: [email protected] ChemSusChem 2011, 4, 1685 – 1693 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1685 M. North and C. Young found to catalyse the formation of cyclic carbonates from ter- known to be highly energy intensive[21] and thus expensive to minal epoxides in up to 99% yield after 24 h at 308C under operate. This class of catalyst is the only one that has demon- 1 atm pressure of carbon dioxide.[18a–d] strated activity in the removal of carbon dioxide from flue The creation of a one-component catalyst system, bis[N,N’- gas[22] and complex 3 has been shown to be tolerant of the im- bis(3-tert-butyl-5-(N-benzyl-N,N-diethylaminomethyl)salicylidene purities in both simulated[19c] and real flue gas.[19d] Scaling up ethane-1,2-diaminoaluminium(III)] oxide tetrabromide, complex the gas-phase flow reactor indicates that a reactor containing 2, through incorporation of the aluminium(salen) units and 50 tonnes of catalyst could remove 92000 tonnes of carbon di- quaternary ammonium groups within the same molecule was oxide per annum from power station flue gases.[19b] Given the subsequently achieved.[19a] Complex 2 was found to be a scale of this operation, it is essential that the cost of produc- highly active catalyst for the synthesis of cyclic carbonates tion of catalysts 1–4 be as low as possible. under mild conditions, enabling, for example, the synthesis of We have reported previously that the structure of homoge- styrene carbonate from styrene oxide with 90 % conversion neous catalyst 1 could be simplified to the corresponding acen after a reaction time of 24 h. In addition, the ammonium complex 5, bis[N,N’-bis(4-oxy-pent-3-ene-2-ylidene) ethane-1,2- groups of the salen ligand provided a convenient means of at- diaminoaluminium(III)] oxide, which is cheaper to produce as taching the one-component catalysts to an insoluble support pentan-2,4-dione and ethylenediamine such as silica to give supported catalysts 3 and 4.[19b,c] One- cost less than salicylaldehyde and cy- component, heterogeneous, immobilized catalysts 3 and 4 clohexanediamine, respectively.[23] This were used in batch reactions and in a gas-phase continuous approach could not, however, pro- flow reactor for the addition of carbon dioxide to ethylene duce supported catalysts analogous to oxide or propylene oxide.[19] In batch reactions, catalyst 3 was 3 and 4, which are essential for use in found to be highly recyclable and maintained activity over a gas-phase flow reactor. We herein more than 30 experiments, although periodic catalyst reactiva- report a chemical cost analysis of the tion by treatment with benzyl bromide was required. This indi- syntheses of catalysts 1–5 and a subsequent study on minimi- cated that dequaternization of the ammonium salts was re- zation of their production costs. sponsible for the reversible decrease in catalyst activity.[18b,d,19b,c] If used in a gas-phase continuous flow reactor, complexes 3 Results and Discussion and 4 exhibited good activity for the synthesis of ethylene car- Cost analysis bonate from carbon dioxide and ethylene oxide under a range of conditions. At 150 8C, a reactor of dimensions 3 cm long by The costs of the chemicals required to produce each of cata- 1 cm diameter containing 0.55 g of catalyst 3 could convert lysts 1–5 by the routes we have previously reported[18d,19c,23] 55% of the carbon dioxide into ethylene carbonate if using a were evaluated to determine the relative importance of each gas flow rate of 5.0 mLminÀ1 and a composition of 20% chemical and solvent used in their preparation. Table 1 gives carbon dioxide, 29 % ethylene oxide, and 51 % nitrogen; how- the total cost of the chemicals required to prepare one mole ever, deactivation of the catalyst was found to occur after just of catalysts 1, 2, and 5 and one mole of supported catalyst 24 h. Under the same conditions at 1008C, only 14% of the sites of 3 and 4. These costs were evaluated from the maxi- carbon dioxide was converted into ethylene carbonate. This mum quantities of the reagents and solvents available in labo- level of activity was, however, constant over 48 h, with catalyst ratory chemical catalogues in the UK in 2010. Bulk scale prices activity decreasing by 50% over the next 96 h. Full catalytic ac- will of course be significantly lower, probably by one order of tivity could then be restored by treatment with benzyl bro- magnitude. For catalysts 1 and 5, the cost of the cocatalyst tet- mide, as with catalyst 2. rabutylammonium bromide is included in the calculations. Catalysts 1–4 could be used commercially as replacements Figure 1 illustrates the breakdown of the chemical costs for for existing catalysts in the production of cyclic carbonates in the synthesis of supported catalyst 3; the analyses for the batch mode, saving the plant operator energy costs through other catalysts gave similar results. The largest single cost was the use of the mild reaction conditions associated with these catalysts. Catalysts 3 or 4 could [a] also be employed in a gas-phase Table 1. Chemical costs for catalysts 1–5 flow reactor using either pure Catalyst Original Replacing Replacing Bu4NBr Replacing Bu4NBr Using no ammo- Replacing carbon dioxide, or the carbon di- synthesis Al(OEt)3 by Al by Et4NBr by NH4Br nium salt MeCN oxide-containing waste gases 1 £892[b] £327[b] £289 £288 – – from a power station or other in- 2 £846 £269 – – – £224 dustrial source of carbon diox- 3 £1150 £583 £416 £356 £329 £263 ide. The latter application would 4 £1139 £572 £405 £345 £318 £252 5 £664[b] £88[b] £49 £49 – – provide a cost effective and sus- tainable alternative to carbon [a] Costs to prepare one mole of catalyst (1, 2, and 5) or supported catalyst sites (3 and 4) at 2010 chemical capture and storage,[20] a process catalogue prices.

Load more