DOI: 10.1002/cssc.201100239 Reducing the Cost of Production of Bimetallic 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 , 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 lithi- um-ion batteries,[14] and as chemical intermediates for the syn- thesis 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 magni- tude) 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 activi- ty.[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 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 con- ditions 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. [b] Includes the cost of the tetrabutylammonium cocatalyst.

1686 www.chemsuschem.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2011, 4, 1685 – 1693 Cost Effective Bimetallic Aluminium Catalysts for Cyclic Carbonate Synthesis

Figure 1. Contribution of each chemical to the cost of catalyst 3. associated with the aluminium triethoxide, which contributed over 50 % of the chemical cost of catalysts 1–4 and 95% of the chemical costs for catalyst 5. The next highest cost for catalysts 3–4 was associated with tetrabutylammonium bromide, which was used as part of their syntheses and contributed 21–22% Scheme 2. Original synthesis of catalyst 1. of the total cost. Tetrabutylammonium bromide was used in much smaller quantities as a cocatalyst with complexes 1 and 5 during cyclic carbonate synthesis and was responsible for ligand 6 shown in Scheme 2 is already an optimized literature just 4–6% of their total costs. No other chemicals contributed procedure, the cost reduction measures for catalyst 1 com- significantly to the overall cost of catalysts 1 and 5, whereas prised the replacement of both aluminium triethoxide and the the use of acetonitrile as a solvent in two or three of the syn- tetrabutylammonium bromide cocatalyst. thesis steps for catalysts 2–4 contributed 6–9% of their overall To avoid having to purchase expensive aluminium alkoxides cost. The only other chemical responsible for more than 4% of or trialkylaluminium derivatives[25] for the synthesis of catalyst the overall cost of catalysts 2–4 was benzyl bromide, which 1, the catalysed in situ synthesis of aluminium triethox- contributed 6–9%. ide from aluminium metal and was investigated.[26] Based on this analysis, it was clear that cost reduction of the Thus, aluminium metal (either powder or shredded foil could production of catalysts 1–5 should be attempted by replace- be used) was suspended in a 1:1 mixture of ethanol and tolu- ment of aluminium triethoxide, tetrabutylammonium bromide ene and an iodine crystal added. The reaction was refluxed for and, in the cases of catalysts 2–4, the acetonitrile solvent. one hour, resulting in the formation of a grey suspension that Benzyl bromide also makes a significant contribution to the could be used directly for the synthesis of catalyst 1 simply by cost of the one-component catalysts 2–4, however, owing to adding a solution of ligand 6 dissolved in toluene. Filtration, its use in the reactivation of catalysts 2–4, its replacement was followed by a standard aqueous workup then gave catalyst 1 not investigated. with identical physical and spectroscopic properties to a sample prepared from commercial aluminium triethoxide. An improved yield of catalyst 1 was obtained on using shredded Modification to the synthesis and use of catalyst 1 aluminium foil (98 %) rather than aluminium powder (68 %), The route used to prepare catalyst 1 is shown in Scheme 2 and probably because the higher surface area of the powder result- requires no chromatographic purifications.[18] The solvents ed in more being present. The yield obtained given in Scheme 2 include those used during the reactions using aluminium foil was also higher than that obtained in the and workup steps and all chemicals and solvents, except for original synthesis (98 versus 63 %). Table 1 shows that the re- water, are included. Although salen ligand 6 is commercially vised synthesis of catalyst 1 achieves a cost saving of 63% available, it is more cost effective to prepare it from 2,4-di-tert- compared to the original synthesis. butylphenol 7 and (R,R)-cyclohexanediamine tartrate 8.[24] The The catalytic activity of complex 1 prepared by the revised chirality of the diamine was not required but catalyst 1 derived route was checked by performing batch reactions using from 8 was found to be more active than related catalysts de- 2.5 mol% of complex 1 and 2.5 mol % of tetrabutylammonium rived from achiral diamines. The use of enantiomerically pure bromide cocatalyst with epoxides 9a–d as substrates to pro- diamine 8 also avoids complications caused by the formation duce cyclic carbonates 10a–d under solvent free conditions, as of racemic and meso forms of catalyst 1.[18] As the synthesis of shown in Scheme 3. The results are presented in Table 2 (en-

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Modification to the synthesis and use of catalyst 5

The literature synthesis of acen-based catalyst 5 is shown in Scheme 4.[23] All of the chemicals used in the synthesis of ligand 11 are very inexpensive, hence aluminium triethoxide

Scheme 3. Synthesis of cyclic carbonates using catalysts 1 and 5.

Table 2. The effect of aluminium source and cocatalyst structure on the catalytic activity of complex 1.[a]

Entry Catalyst precursor Epoxide Cocatalyst Conversion [%] 3h 6h 24h

[b] 1 Al(OEt)3 9a Bu4NBr 62 – 98

2 Aluminium powder 9a Bu4NBr 54 68 93

3 Aluminium foil 9a Bu4NBr 57 72 94 Scheme 4. Original synthesis of catalyst 5. [b] [c] 4 Al(OEt)3 9b Bu4NBr 77 – – [c] 5 Aluminium foil 9b Bu4NBr 73 96 100 [b] 6 Al(OEt)3 9c Bu4NBr 85 – –

7 Aluminium foil 9c Bu4NBr 86 100 100 accounts for 95 % of the chemical cost associated with this cat- [b] [d] 8 Al(OEt)3 9d Bu4NBr – – 76 alyst. Therefore, this synthesis was modified exactly as de- [d] 9 Aluminium foil 9d Bu4NBr – – 100 scribed above for catalyst 1, with aluminium triethoxide being 10 Aluminium foil 9a Et NBr – – 75 4 generated in situ from aluminium foil and ethanol. As with cat- 11 Aluminium foil 9a NH4Br – – 60 alyst 1 this modified synthesis was successful and gave com- [a] Reactions performed at 268C unless stated otherwise. [b] Data taken plex 5 in an improved yield of 84% compared with the 78% from Ref. 18a, d. [c] Reaction performed at 08C. [d] Reaction performed yield obtained using commercial aluminium triethoxide. Since in a sealed reactor at <3 atm pressure of CO2. the cost of the chemicals required for the synthesis of complex 5 is dominated by the cost of the aluminium triethoxide, the replacement of this chemical reduces the cost of the catalyst tries 1–9) and confirm that catalyst 1 prepared by the revised dramatically to just 13 % of the original cost (Table 1). synthesis has equal activity to that prepared from commercial- Table 3 compares the activity of catalyst 5 prepared by the ly sourced aluminium triethoxide. The use of aluminium trieth- modified synthesis with that prepared by the original synthetic oxide in the synthesis of catalyst 1 can therefore be avoided. route for the conversion of epoxides 9a–d into cyclic carbo- Cost effective replacements for the tetrabutylammonium nates 10a–d in the presence of an ammonium bromide coca- bromide cocatalyst were next investigated. Mechanistic studies talyst. Entries 1–8 detail how complex 5 prepared from alumi- have shown this cocatalyst to have two roles in the reaction nium foil is at least as active as that prepared by the original mechanism: as a source of bromide, which carries out the ini- route. Changing the cocatalyst from tetrabutylammonium bro- tial epoxide ring opening and as a source of tertiary amine, mide to tetraethylammonium bromide or ammonium bromide which activates the carbon dioxide.[18b,d] Therefore, the use of had a much less detrimental effect on the synthesis of styrene less expensive ammonium bromides was investigated, specifi- cally tetraethylammonium bromide and ammonium bromide. These two cocatalysts were used in reactions with epoxide 9a Table 3. The effect of aluminium source and cocatalyst structure on the as substrate under the standard conditions detailed in catalytic activity of complex 5.[a] Scheme 3; the results are shown in Table 2 (entries 10 and 11). Entry Catalyst precursor Epoxide Cocatalyst Conversion [%] Although both of these cocatalysts provide an active catalytic 3 h 6 h 24 h system, they have a 20–30% reduced catalyst activity com- 1 Al(OEt) [b] 9a Bu NBr 33 52 85 pared to that of tetrabutylammonium bromide (Table 2 en- 3 4 2 Aluminium foil 9a Bu4NBr 35 58 89 [b] [c] tries 3, 10 and 11), owing probably to the lower of 3 Al(OEt)3 9b Bu4NBr 42 52 73 [c] tetraethylammonium bromide or ammonium bromide in sty- 4 Aluminium foil 9b Bu4NBr 40 55 77 5 Al(OEt) [b] 9c Bu NBr 97 97 100 rene oxide, which acted as both substrate and solvent. The 3 4 6 Aluminium foil 9c Bu4NBr 93 100 100 12% cost saving produced by the use of these cocatalysts [b] [d] 7 Al(OEt)3 9d Bu4NBr – – 58 [d] (Table 1) is more than offset by the reduction in catalyst activi- 8 Aluminium foil 9d Bu4NBr – – 60 ty. None of the other chemicals associated with the synthesis 9 Aluminium foil 9a Et4NBr 30 54 80 or use of catalyst 1 were particularly expensive, therefore the 10 Aluminium foil 9a NH4Br 26 49 75 synthesis of the catalyst from aluminium foil and the use of [a] Reactions performed at 268C unless stated otherwise. [b] Data taken tetrabutylammonium bromide as cocatalyst were taken as the from Ref. 23. [c] Reaction performed at 08C. [d] Reaction performed in a most cost effective conditions. sealed reactor at <3 atm pressure of CO2.

1688 www.chemsuschem.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2011, 4, 1685 – 1693 Cost Effective Bimetallic Aluminium Catalysts for Cyclic Carbonate Synthesis carbonate 10a using catalyst 5 (Table 3, compare entries 2, 9 theses of complexes 3–4, in which it acts as a bromide source and 10) than had been seen for reactions catalysed by com- in the exchange of chloride in complexes 14a,b for bromide. plex 1 (Table 2). The conversion was only reduced by 9–14%, As the use of tetrabutylammonium bromide accounts for 21– whereas use of either of these cocatalysts reduced the cost of 22% of the cost of these syntheses, its replacement with less the catalyst system by a further 40 % (Table 1). The preparation expensive bromide sources was investigated. As shown in of complex 5 from aluminium foil and by use of tetraethylam- Table 1, the tetrabutylammonium bromide could be replaced monium bromide is the most cost effective catalytic system, by tetraethylammonium bromide or ammonium bromide, re- providing a 92% reduction in cost compared to the original sulting in a reduction in the cost of the catalyst to 30–31 % of system. the original cost. Moreover, it was found that the tetraalkylam- monium salt could be completely omitted from these reac- tions, resulting in a cost saving of 72–73 %. During the original Modification to the synthesis and use of catalysts 2–4 synthesis of supported catalysts 3 and 4, the tetrabutylammo- Catalysts 2–4 were prepared by the route shown in nium bromide was added in excess to ensure complete ex- Scheme 5.[19] Once again, the key cost reduction was in the change of the chloride counterion in complexes 14a,b. Excess conversion of ligands 12 a,b into aluminium complexes 13a,b, benzyl bromide is used in the alkylation reaction, however, performed originally with aluminium triethoxide. Complexes providing an alternative mechanism for counterion exchange. 13a,b are key intermediates in the synthesis of catalysts 2–4: Notably, mechanistic studies showed cyclic carbonate synthesis direct quaternization of complex 13 a gives catalyst 2 and reac- catalysed by complex 1 to be second order in tetrabutylammo- tion of complexes 13 a,b with chloropropyl-functionalized nium bromide,[18b,d] and even if no counterion exchange oc- silica gives immobilized complexes 14 a,b, which can be qua- curred, there would still be three tetraalkylammonium bromide ternized to catalysts 3 and 4, respectively. The synthesis of units within the supported catalysts. complexes 13a,b was modified by generating the aluminium The catalytic activity of complexes 2–4 prepared by the triethoxide in situ from aluminium foil as described above for modified synthesis was investigated for the synthesis of sty- catalysts 1 and 5. Using this method, complexes 13a,b were rene carbonate 10 a from epoxide 9a (Table 4). These reactions formed with yields of 63% and 59%, respectively, which were were performed in propylene carbonate solution to provide comparable to the 55 % and 65% yields obtained in the origi- sufficient liquid volume for efficient stirring of the reaction. En- nal synthesis.[19] As shown in Table 1, this reduced the cost of tries 1 and 2 of Table 4 show that changing the aluminium the chemicals required to produce the one-component cata- source to aluminium foil increased the activity of catalyst 3.If lysts 2–4 by 49–52 % compared to the original synthetic route. the ammonium salt was changed or omitted (entries 3–5), the Catalysts 2–4 do not require the use of tetrabutylammonium catalytic activity was reduced slightly but was still higher than bromide as a cocatalyst. As shown in Scheme 5, tetrabutylam- that reported for catalyst 3 prepared by the original route. En- monium bromide is, however, used in the final step of the syn- tries 7 and 8 of Table 4 show that production of catalyst 2 from aluminium foil still forms a highly active catalyst, and en- tries 10 and 11 show the same to be true of catalyst 4. The activity of catalysts 3 and 4 prepared from aluminium metal was also checked in the gas-phase flow reactor, previous- ly reported for the synthesis of ethylene carbonate from ethyl- ene oxide.[19b] Under standard conditions [reactor temperature 1008C; input gas composition 16% carbon dioxide, 18 % ethyl-

ene oxide, and 66 % N2 at a total flow rate of 4.3 mLminÀ1 (mea- sured at 208C) for 24 h], catalyst 3 exhibited turnover frequencies (TOFs) of 1.38 and 1.31 hÀ1 when prepared from aluminium trieth- oxide and aluminium foil, re- spectively, whereas the corre- sponding TOFs for catalyst 4 were 0.65 and 0.68 hÀ1, respec- Scheme 5. Original synthesis of catalysts 2–4. tively, thus confirming that the

ChemSusChem 2011, 4, 1685 – 1693 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 1689 M. North and C. Young

14a,b, 3, and 4, which allowed the propylene carbonate sol- Table 4. The effect of synthesis route on the catalytic activity of com- plexes 2–4 in the synthesis of styrene carbonate 10a.[a] vent to be removed by filtration rather than by evaporation (the boiling point of propylene carbonate is 2428C). Similarly, Entry Catalyst Aluminium Ammonium Solvent Conversion [%] catalyst 2 was also prepared by the quaternization of complex source salt 13a in propylene carbonate and obtained in 60 % yield after [b] 1 3 Al(OEt)3 Bu4NBr acetonitrile 69 removal of propylene carbonate and excess benzyl bromide by 2 3 Al foil Bu4NBr acetonitrile 78 vacuum distillation. Table 1 shows the combined effect of re- 3 3 Al foil Et NBr acetonitrile 75 4 placing the aluminium triethoxide, tetrabutylammonium bro- 4 3 Al foil NH4Br acetonitrile 72 5 3 Al foil – acetonitrile 73 mide, and acetonitrile in the syntheses of catalysts 2–4, which 6 3 Al foil – propylene 75 has reduced the synthesis cost of these catalysts to just 21– carbonate [b] 23% of their original values. The catalytic activity of complexes 7 2 Al(OEt)3 – acetonitrile 99 8 2 Al foil – acetonitrile 86 2–4 prepared using aluminium foil, without any quaternary 9 2 Al foil – propylene 79 ammonium bromide catalyst, and using propylene carbonate carbonate as a replacement for acetonitrile was investigated for the syn- 10 4 Al(OEt) [b] Bu NBr acetonitrile 78 3 4 thesis of styrene carbonate 10a from epoxide 9a and the re- 11 4 Al foil Bu4NBr acetonitrile 80 12 4 Al foil – propylene 76 sults are included in Table 4. Entries 6, 9 and 12 show that carbonate there is no significant loss of catalytic activity associated with

[a] Reactions were run over 24 h using 2.5 mol% of catalyst (2) or catalyst these changes in the preparation of the catalysts. active sites (3 and 4) in propylene carbonate solvent at 268C under 1 atm pressure of CO . [b] Data taken from Ref. 19a, c. 2 Conclusions Figure 2 shows the cost reductions that have been achieved catalysts prepared by the modified synthesis retained full cata- for the syntheses of catalysts 1–5 in the course of this work. lytic activity if used in the gas-phase flow reactor. Catalysts 1, 2, and 5 are intended for use in homogeneous The remaining aspect of the syntheses of catalysts 2–4 re- batch reactions and the cost of the chemicals required to syn- quiring optimization was the use of acetonitrile as a solvent thesize these catalysts has been reduced by 68, 74, and 93 %, for all steps from intermediates 13a,b onwards. Acetonitrile is a polar aprotic solvent used widely as a solvent for amine alky- lations and quaternizations. The cost of this solvent has, how- ever, fluctuated widely between 2008 and 2011, with the price peaking at £60 LÀ1 before returning to a cost of £7 LÀ1 (£1 = $1.56, E1.15; October 2011). The contribution of acetonitrile to the cost of catalysts 2–4 was more than 1.5 times that of all the other solvents combined (Figure 1). In addition, acetonitrile is an environmentally unfriendly solvent as it can generate NOx on incineration. Therefore, replacement of this solvent was de- sirable for both economic and environmental reasons. We looked specifically to use a polar aprotic solvent for the amine alkylation steps, considering the removal of the tetrabutylam- monium bromide, to ensure that reactions went to comple- tion. Other conventional polar aprotic solvents offer little envi- ronmental advantage over acetonitrile. Propylene carbonate 10b, however, is a liquid at room temperature and is being in- [12,13] creasingly used as a polar aprotic solvent. Since propylene Figure 2. Costs for preparing catalysts 1–5 by various routes. carbonate can be prepared by a 100% atom economical reac- tion from propylene oxide and carbon dioxide (Scheme 3), [12] generates no NOx or SOx on incineration, and is nontoxic, it respectively. The costs of the chemicals needed for the produc- was a suitable candidate for the replacement of acetonitrile, tion of catalysts 3 and 4, which can be used as heterogeneous particularly as it was available at one third of the cost. catalysts in batch reactions or in a gas-phase flow reactor, have The immobilization of complexes 13 a,b was performed as been reduced by 77 and 78 %, respectively. It is apparent from shown in Scheme 5, replacing acetonitrile with propylene car- Figure 2 that catalyst 5 is available at a far lower cost than cat- bonate, to give immobilized complexes 14 a,b in 45–52% alysts 1–4 and is the most cost effective homogeneous cata- yield. The subsequent quaternization of complexes 14 a,b was lyst. also performed in propylene carbonate using benzyl bromide There is no significant price difference (per mole of catalyti- in the absence of any ammonium salt cocatalyst and gave cat- cally active sites) between silica-supported catalysts 3 and 4. alysts 3 and 4 in 89 and 84% yield, respectively. These synthe- Catalyst 3, however, has double the intrinsic catalytic activity ses were facilitated by the insoluble nature of complexes of catalyst 4, as indicated by the TOFs, therefore only half as

1690 www.chemsuschem.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2011, 4, 1685 – 1693 Cost Effective Bimetallic Aluminium Catalysts for Cyclic Carbonate Synthesis many moles of supported catalyst 3 are required to achieve a and evaporated in vacuo to give complex 1 (4.16 g, 98% if using given conversion of carbon dioxide to ethylene carbonate in aluminium foil, 2.88 g, 68% if using aluminium powder) as a 20 [18d] 20 the flow reactor, making it twice as cost effective as catalyst 4. yellow powder. [a]D =À640 (c=0.09, toluene) [lit. [a]D = Interestingly, the catalyst loadings obtainable with catalyst 4 À653 (c=0.11, toluene)]. Spectroscopic data were identical to those reported previously.[18a,d] are reproducibly double those obtained with catalyst 3, owing possibly to the tert-butyl groups in catalyst 3 sterically hinder- ing the attachment of the catalyst to the support. Thus, the Synthesis of 5 from aluminium metal same mass of supported catalysts 3 and 4 is required to ach- Shredded aluminium foil (ca. 0.5 cm by 1 mm; 0.7 g, 26.8 mmol) ieve a given conversion of carbon dioxide to ethylene carbon- was added to dry ethanol (50 mL) and dry toluene (50 mL). A crys- ate. Catalysts 3 and 5 are, overall, the most cost effective cata- tal of iodine (ca. 90 mg) was added and the reaction heated to lysts for use in continuous flow and batch reactions, respec- reflux for one hour, during which time a grey suspension formed. tively. Ligand 11 (3.0 g, 13.4 mmol) in dry toluene (40 mL) was then All of the prices quoted in this paper are based on the pur- added and the reaction refluxed for a further 3 h. After cooling to chase of kilogram-scale quantities of chemicals from chemical ambient temperature, the reaction mixture was filtered through a catalogues. The bulk cost of the chemicals is usually one order Celite pad and the filtrate evaporated in vacuo. The residue was of magnitude lower than this, suggesting that the cost of the taken up in CH2Cl2 (80 mL) and washed with water (350 mL) and saturated brine (50 mL), dried (Na SO ) and evaporated in vacuo to chemicals required to produce catalysts 1–5 on a multitonne 2 4 À1 give complex 5 (2.9 g, 84%) as a yellow powder. Spectroscopic scale could be as low as £5–29 mol . Such a synthesis would, data were identical to those reported previously.[23] however, have to be undertaken by a commercial chemical manufacturer, incurring labour, energy, and overhead costs. Therefore, a cost of £15–£90 molÀ1 for the production of bulk Syntheses of 13 a,b from aluminium metal quantities of the catalysts would be a more realistic evaluation. Aluminium foil (ca. 0.5 cm by 1 mm) (0.4–0.5 g, 15.2–18.2 mmol, 2 equivalents with respect to ligand 12a,b) was added to a 1:1 mix- Experimental Section ture of dry ethanol (50 mL) and dry toluene (50 mL). A crystal of iodine (ca. 90 mg) was added and the reaction heated to reflux for All commercially available chemicals were used as received except one hour, during which time a grey suspension formed. Ligand for toluene, which was distilled over sodium before use. 2-Tert-bu- 12a[19c] (4.0 g, 7.6 mmol) or 12b[19c] (4.0 g, 9.1 mmol) dissolved in tylphenol, THF, Et3N, EtOAc, hydrochloric acid, MeCN, EtOH, petro- dry toluene (50 mL) was then added and the reaction refluxed for leum ether, toluene and CH2Cl2 were obtained from Fisher Scientif- a further 3 h. After cooling to ambient temperature, the reaction ic. Salicylaldehyde, Na2SO4, Celite, Bu4NBr, aluminium powder, mixture was filtered through a Celite pad and the filtrate evaporat-

K2CO3 and acetylacetone were obtained from Alfa Aesar. (+)-Tarta- ed in vacuo. The residue was taken up in CH2Cl2 (100 mL) and ric acid was obtained from Sigma-Aldrich, silica from Fluorochem, washed with water (350 mL) and saturated brine (50 mL), dried and all other chemicals from Acros. GC–MS was performed on a (Na2SO4) and evaporated in vacuo to give complex 13a (2.8 g, Varian CP-800-SATURN 2200 GC–MS ion-trap mass spectrometer 63%) or 13b (2.5 g, 59%) as a yellow solid. Analytical data were using a FactorFour (VF-5 ms) capillary column (30 m0.25 mm) identical to those reported previously.[19a–c] with helium as the carrier gas. The initial temperature of 608C was maintained for 3 min, followed by a ramp rate of 158C minÀ1 to a final temperature of 2708C, which was maintained for 5 min. For Syntheses of 14 a,b in propylene carbonate the first 3.50 min, the eluent was routed away from the mass de- Chloropropyl-functionalized amorphous silica (1.5 g) was added to tector, which was operated subsequently in full EI scan mode. a solution of aluminium complex 13a (2.0 g, 1.7 mmol) or 13b t (styrene oxide) 7.33 min, t (styrene carbonate) 12.09 min. R R (1.6 g, 1.7 mmol) in propylene carbonate (20 mL) and the mixture 1H and 13C NMR spectra were recorded on a Bruker Avance 300 refluxed overnight. The mixture was then filtered and washed with spectrometer at 300 and 75 MHz, respectively. All spectra were re- EtOAc (450 mL) to give the silica-supported complex 14a (1.40 g, corded at ambient temperature and referenced to the residual sol- 52%) or 14b (2.0 g, 45%) as a yellow powder. Analytical data were vent peak. Inductively coupled plasma optical emission spectrosco- identical to those reported previously.[19c] py (ICP–OES) data was obtained from a UNICAM 701 emission spectrometer. Syntheses of 3 and 4 in propylene carbonate

Synthesis of 1 from aluminium metal BnBr (0.63 mL, 5.5 mmol) was added to a suspension of silica-sup- ported complex 14a or b (1.6 g, 0.7 mmol or 2.0 g, 2.1 mmol) in Shredded aluminium foil (ca. 0.5 cm by 1 mm) or aluminium propylene carbonate (25 mL) and the resulting mixture refluxed powder (0.20 g, 7.32 mmol) was added to dry ethanol (50 mL) and overnight. The mixture was then filtered and washed with EtOAc dry toluene (50 mL). A crystal of iodine (ca. 90 mg) was added and (450 mL) to give silica-supported catalysts 3 or 4 (2.2 g or 3.0 g, the reaction heated to reflux for one hour, during which time a 84–89%) as a yellow powder. ICP–OES showed catalyst 3 to con- grey suspension formed. Ligand 6 (2.00 g, 3.66 mmol) in dry tolu- tain 51.44 ppm aluminium (catalyst loading 0.50 mmolgÀ1) if pre- ene (20 mL) was then added and the reaction refluxed for a further pared using aluminium metal and acetonitrile, and 46.76 ppm alu- 3 h. After cooling to ambient temperature, the reaction mixture minium (catalyst loading 0.45 mmolgÀ1) if prepared using alumini- was filtered through a Celite pad and the filtrate evaporated in va- um metal and propylene carbonate, whereas the catalyst prepared cuo. The residue was taken up in CH2Cl2 (50 mL), washed with by the original synthesis contained 48.48 ppm aluminium (catalyst À1 [19c] water (320 mL) and saturated brine (20 mL), dried over Na2SO4, loading 0.47 mmolg ). Catalyst 4 contained 97.21 ppm alumini-

ChemSusChem 2011, 4, 1685 – 1693 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemsuschem.org 1691 M. North and C. Young

À1 um (catalyst loading 0.87 mmolg ) if prepared using aluminium washed thoroughly with CH2Cl2 (20 mL). The filtrate and washings metal and acetonitrile, and 89.22 ppm aluminium (catalyst loading were combined and evaporated in vacuo and the residue analysed 0.80 mmolgÀ1) if prepared using aluminium metal and propy- by 1H NMR spectroscopy to determine the conversion of styrene lene carbonate, whereas the catalyst prepared by the original syn- oxide to styrene carbonate. For reactions catalysed by complex 2, thesis contained 115.1 ppm aluminium (catalyst loading EtOAc (10 mL) was added to the reaction mixture, which was then 1.03 mmolgÀ1).[19c] Other analytical data were identical to those re- passed through a silica pad. The eluent was evaporated in vacuo ported previously.[19c] and analysed as above; conversions are given in Table 4.

Synthesis of 2 in propylene carbonate Synthesis of ethylene carbonate 10d using catalyst 3 or 4 BnBr (1.2 mL, 10.2 mmol) was added to a solution of bimetallic Ethylene oxide was collected as a liquid from a commercially sup- complex 13a (1.5 g, 1.3 mmol) in propylene carbonate (10 mL) and plied cylinder (BOC) into a beaker cooled to À788C and placed, the reaction stirred at 80–858C for 24 h. The solvent and excess with a magnetic stirrer bar, into a 360 mL stainless steel pressure BnBr were evaporated in vacuo using a kugelrohr distillation appa- vessel, which had been precooled to À308C in a cryostatic bath. ratus to give compound 2 as a pale orange powder (1.6 g, 66%). The vessel was then sealed. N2 and CO2 were taken from gas cylin- [19a] Analytical data were identical to those reported previously. ders by using mass flow controllers and their respective lines merged to the inlet of the pressure vessel. All tubing used in the system was made of stainless steel with an internal diameter of ap- Syntheses of cyclic carbonates 10 a-c using catalyst 1 or proximately 1.6 mm. The CO and N flow rates were 0.7 mLminÀ1 [18,23] 2 2 5 and 2.8 mLminÀ1, respectively, resulting in an ethylene oxide evap- À1 Epoxide 9a–c (1.7 mmol), catalyst 1 or 5 (41.5 mmol) and an ammo- oration rate of 0.09 mLh . The vessel outlet line was connected to nium bromide (41.5 mmol) were placed in a sample vial fitted with a stainless steel tubular reactor (3 cm long 1 cm internal diame- a magnetic stirrer bar and placed in a large conical flask. The coni- ter) packed with silica-supported catalyst 3 or 4 (0.55 g) and plug- cal flask was placed in an oil bath thermostatted at 268C (or an ged at both ends with a small amount of cotton wool. The tubular ice/water bath at 08C if propylene oxide 9b was the substrate). reactor was heated to 1008C using a thermostatically controlled Cardice pellets were added to the conical flask, which was fitted oven. The mixture of CO2, ethylene oxide, and N2 was passed with a rubber stopper pierced by a needle attached to a deflated through the reactor column at a steady flow rate. The reactor balloon. The reaction was stirred for 24 h with samples being re- outlet was connected to a glass vial by a needle to collect ethylene moved after 3, 6, and 24 h for analysis by 1H NMR spectroscopy to carbonate. After running for 24 h, the flow reactor was dismantled determine the conversion of epoxide to cyclic carbonate. Cyclic and the catalyst washed with EtOAc (20 mL). The washings were carbonates 10a–c had spectroscopic data consistent with those re- combined with the ethylene carbonate that had collected in the 1 ported previously.[18a,d] glass vial, evaporated in vacuo, and analysed by H NMR spectros- copy and GC–MS.

Synthesis of ethylene carbonate 10d using catalyst 1 or 5[18,23] Acknowledgements

Catalyst 1 or 5 (41.5 mmol) and Bu4NBr (13.4 mg, 41.5 mmol) were The authors thank the Technology Support Board (TSB) for finan- added to a reaction vial, to which precooled ethylene oxide 9d cial support (grant number BR032G). (88.4 mg, 1.66 mmol) was added. The reaction vial was fitted with a magnetic stirrer and placed inside a stainless steel reaction vessel with sufficient cardice pellets to pressurize the system to approxi- Keywords: aluminium · industrial chemistry · heterogeneous mately 3 atm. The stainless steel reactor was sealed and the reac- catalysis · supported catalysts · green chemistry tion left to stir at 268C for 24 h, after which the remaining ethylene oxide was allowed to evaporate and Et O (ca. 20 mL) was added to 2 [1] Climate Change 2007 –Synthesis Report, Cambridge University Press, the residue. The resulting mixture was filtered to remove the cata- Cambridge, 2007. lyst and Bu4NBr and the solution evaporated in vacuo to give ethyl- [2] a) R. W. Bentley, Energy Policy 2002, 30, 189– 205; b) D. L. Greene, J. L. ene carbonate 9g (147 mg, 100% yield if using catalyst 1; 88 mg, Hopson, J. Li, Energy Policy 2006, 34, 515 –531; c) BP Statistical Review of 60% yield if using catalyst 5), which had spectroscopic data consis- World Energy June 2010, available from www.bp.com/statisticalreview. tent with those reported previously.[27] [3] a) Sustainable Industrial Chemistry, F. Cavani, G. Centi, S. Perathoner, F. Trifir, Wiley-VCH, Weinheim, 2009; b) M. Lancaster, Green Chemistry: An Introductory Text, 2nd Edition, RSC Publishing, Cambridge, 2010;c)P.J. Synthesis of cyclic carbonate 10 a using one-component cat- Dunn, A. S. Wells, M. T. Williams, Green Chemistry in the Pharmaceutical alysts 2–4[19c] Industry, Wiley-VCH, Weinheim, 2010; d) C. Jimnez-Gonzlez, D. J. C. Constable, Green Chemistry and Engineering, Wiley, New Jersey, 2011; Catalyst 2–4 (41.5 mmol or 41.5 mmol of active sites) was suspend- e) P. Anastas, N. Eghbali, Chem. Soc. Rev. 2010, 39, 301– 312. ed in propylene carbonate (0.85 g) in a sample vial fitted with a [4] M. Aresta, Carbon Dioxide as Chemical Feedstock, Wiley-VCH, Weinheim, magnetic stirrer bar. Styrene oxide 9a (200 mg, 1.66 mmol) was 2010. then added and the sample vial was placed in a large conical flask, [5] a) I. Omae, Catal. Today 2006, 115, 33–52; b) T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107, 2365 –2387. which was held in an oil bath thermostatted at 268C. Cardice pel- [6] a) A. S. Lindsey, H. Jeskey, Chem. Rev. 1957, 57, 583– 620; b) M. Aresta, lets were added to the conical flask, which was fitted with a A. Dibenedetto, Catal. Today 2004, 98, 455 –462; c) M. Aresta, A. Dibe- rubber stopper pierced by needle attached to a deflated balloon, nedetto, Dalton Trans. 2007, 28, 2975 –2992. and the reaction stirred for 24 h. Reactions involving supported [7] a) D. J. Darensbourg, Chem. Rev. 2007, 107, 2388 –2410; b) S. S, J. K. catalysts 3–4 were then filtered to remove the catalyst, which was Min, J. E. Seong, S. J. Na, B. Y. Lee, Angew. Chem. 2008, 120, 7416– 7419;

1692 www.chemsuschem.org 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2011, 4, 1685 – 1693 Cost Effective Bimetallic Aluminium Catalysts for Cyclic Carbonate Synthesis

Angew. Chem. Int. Ed. 2008, 47, 7306 –7309; c) S. J. Na, S. S.A. Cyriac, [19] a) J. Melndez, M. North, P. Villuendas, Chem. Commun. 2009, 2577 – B. E. Kim, J. Yoo, Y. K. Kang, S. J. Han, C. Lee, B. Y. Lee, Inorg. Chem. 2009, 2579; b) M. North, P. Villuendas, C. Young, Chem. Eur. J. 2009, 15, 48, 10455 –10465; d) J. Yoo, S. J. Na, H. C. Park, A. Cyriac, B. Y. Lee, 11454 –11457; c) M. North, P. Villuendas, C. Young, Dalton Trans. 2011, Dalton Trans. 2010, 39, 2622 –2630; e) M. R. Kember, A. Buchard, C. K. 40, 3885 –3902. Williams, Chem. Commun. 2011, 47, 141 –163. [20] a) R. Steeneveldt, B. Berger, T. A. Torp, Chem. Eng. Res. Des. 2006, 84, [8] a) S. Enthaler, ChemSusChem 2008, 1, 801–804; b) S. N. Riduan, Y. 739– 763; b) D. P. Schrag, Science 2007, 315, 812– 813; c) H. Yang, Z. Xu, Zhang, Dalton Trans. 2010, 39, 3347 –3357. M. Fan, R. Gupta, R. B. Slimane, A. E. Bland, I. Wright, J. Environ. Sci. [9] Calculated using standard heats of formation given in Ref. 6c and 2008, 20, 14–27. Matheson Gas Data Book, C. L. Yawes, 7th Edition, McGraw-Hill, 2001,p. [21] X. Xiaoding, J. A. Moulijin, Energy Fuels 1996, 10, 305– 325. 374. [22] The intellectual property associated with catalysts 1–5 is protected by [10] M. North, R. Pasquale, C. Young, Green Chem. 2010, 12, 1514 –1539. three patent families and the technology is licensed to Dymeryx Ltd. to [11] a) W. J. Peppel, Ind. Eng. Chem. 1958, 50, 767– 770; b) J. H. Clements, facilitate its commercialization; a) Synthesis of cyclic carbonates in the Ind. Eng. Chem. Res. 2003, 42, 663 –674; c) M. Yoshida, M. Ihara, Chem. presence of dimeric aluminium(salen)catalysts, WO/2008/132474 A1; Eur. J. 2004, 10, 2886 – 2893. b) Synthesis of cyclic carbonates, WO/2009/109765 A1; c) Synthesis of [12] For a review of the use of organic carbonates as solvents see: B. Schff- cyclic carbonates, WO/2010/106324 A1. ner, F. Schffner, S. P. Verevkin, A. Bçrner, Chem. Rev. 2010, 110, 4554 – [23] M. North, C. Young, Catal. Sci. Technol. 2011, 1, 93–99. 4581. [24] a) T. A. Whitney, J. Org. Chem. 1980, 45, 4214 –4216; b) T. V. Hansen, L. [13] For recent examples see: a) M. North, F. Pizzato, P. Villuendas, ChemSu- Skattebøl, Tetrahedron Lett. 2005, 46, 3829 –3830. sChem 2009, 2, 862– 865; b) M. North, M. Omedes-Pujol, Tetrahedron [25] Catalyst 1 has previously been prepared from trimethylaluminium, see: Lett. 2009, 50, 4452 –4454; c) M. North, P. Villuendas, Org. Lett. 2010, 12, a) M. S. Taylor, E. N. Jacobsen, J. Am. Chem. Soc. 2003, 125, 11204– 2378 –2381; d) W. Clegg, R. W. Harrington, M. North, F. Pizzato, P. Vil- 11205; b) M. S. Taylor, D. N. Zaltan, A. M. Lerchner, E. N. Jacobsen, J. Am. luendas, Tetrahedron: Asymmetry 2010, 21, 1262– 1271; e) M. North, M. Chem. Soc. 2005, 127, 1313 – 1317; c) G. M. Sammis, H. Danjo, E. N. Ja- Omedes-Pujol, Beilstein J. Org. Chem. 2010, 6, 1043 –1055. cobsen, J. Am. Chem. Soc. 2004, 126, 9928 –9929. [14] W. H. Meyer, Adv. Mater. 1998, 10, 439 –448. [26] a) J. H. Gladstone, A. Tribe, J. Chem. Soc. 1876, 29, 158 –162; b) N. Y. [15] a) Y. Ono, Catal. Today 1997, 35, 15–25; b) P. Tundo, M. Selva, Acc. Turova, E. P. Turevskaya, V. G. Kessler, M. I. Yanovskaya, The chemistry of Chem. Res. 2002, 35, 706 –716. metal alkoxides, Kluwer Academic Publishers, Dordrecht, 2002, Chapter [16] a) M. A. Pacheco, C. L. Marshall, Energy Fuels 1997, 11, 2–29; b) D. Li, W. 12.5. Fang, Y. Xing, Y. Guo, R. Lin, J. Hazard. Mater. 2009, 161, 1193– 1201. [27] a) J. Katzhendler, I. Ringel, S. Sarel, J. Chem. Soc. Perkin Trans. 2 1972, [17] R. Srivastava, T. H. Bennur, D. Srinivas, J. Mol. Catal. A-Chem. 2005, 226, 2019 –2025; b) A. G. M. Barrett, P. A. Prokopiou, D. H. R. Barton, J. Chem. 199– 205. Soc. Perkin Trans. 1 1981, 1510 – 1516; c) X.-B. Lu, Y.-J. Zhang, K. Jin, L.- [18] a) J. Melndez, M. North, R. Pasquale, Eur. J. Inorg. Chem. 2007, 3323 – M. Luo, H. Wang, J. Catal. 2004, 227, 537 –541. 3326; b) M. North, R. Pasquale, Angew. Chem. 2009, 121, 2990– 2992; Angew. Chem. Int. Ed. 2009, 48, 2946 –2948; c) I. S. Metcalfe, M. North, R. Pasquale, A. Thursfield, Energy Environ. Sci. 2010, 3, 212– 215; d) W. Clegg, R. W. Harrington, M. North, R. Pasquale, Chem. Eur. J. 2010, 16, Received: May 9, 2011 6828 –6843. Published online on November 2, 2011

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