Letter pubs.acs.org/journal/ascecg
Dialkyl Carbonate Synthesis via in Situ Generated Carbonyl Dibromide on Porous Glass Khuong Q. Vuong,†,§ Reinhard Effenberger,‡ Joseph Zilberman,‡ Simon Smart,† Craig M. Williams,§ and Eric W. McFarland*,†,∥ †
UQ Dow Centre for Sustainable Engineering Innovation, School of Chemical Engineering, The University of Queensland, St Lucia, Queensland 4072, Australia ‡ IMI-TAMI, Institute for Research and Development Ltd, POB 10140, Haifa Bay 2611101, Israel § School of Chemistry and Molecular Biosciences, the University of Queensland, St Lucia, Queensland 4072, Australia ∥ Department of Chemical Engineering, University of California−Santa Barbara, Santa Barbara, California 93106-5080, United States S Supporting Information *
ABSTRACT: Dialkyl carbonate was synthesized from carbon monoxide, bromine and alkanol (1-butanol or methanol) via in situ formation of carbonyl dibromide (CDB) on a silica catalyst at 5−25 °C. The CDB intermediate was verified by infrared spectroscopy. Over 80% of the bromine reacted in a single-pot synthesis resulting in dialkyl carbonate formation. Bromine electrosynthesis requires less energy than chlorine, and replacement of phosgene (carbonyl dichloride) with CDB could reduce the costs associated with products requiring these reactive carbonyl intermediates. KEYWORDS: Dialkyl carbonate, Dimethyl carbonate, Phosgene, Carbonyl dibromide, Silica, Catalysis
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INTRODUCTION Dialkyl carbonates are important organic compounds and chemical intermediates. Dimethyl carbonate (DMC) is the most relevant compound among the dialkyl carbonates and is a widely used industrial chemical. The market for DMC was estimated in 2014 to be approximately 390 million USD.1 Approximately 50% of manufactured DMC is currently used in the production of diphenyl carbonate as required for polycarbonate synthesis.2 DMC is considered a potential “green solvent” and the preferred replacement for many volatile organic compounds (VOCs), and an alternative to phosgene for use in the production of polycarbonates and isocyanates.3−7 As an additive to fuel, DMC can significantly decrease particulate emissions from diesel combustion and can be readily added to existing fuel formulations.8,9 The market for DMC is projected to reach 690 million USD in 2023, expanding at a compound annual growth rate of 6.6% from 2015 to 2023.1 Until recently, DMC was produced from methanol and phosgene (carbonyl dichloride) with hydrochloric acid as the byproduct. This method has been phased out due to the high cost of recycling hydrochloric acid back into chlorine and concerns regarding the safety of handling phosgene.2,3 Current production routes for dimethyl carbonate include oxycarbonylation of methanol,10,11 methyl nitrite carbonylation,12 urea methanolysis13 and two-step synthesis3,4 from carbon dioxide, epoxides and methanol.2−4,7,9,12−14 Each of these methods have certain limitations regarding feed-stock prices, energy cost and/ or safety profiles.2,3,7 A brief comparison of these methods, including the method reported herein, is presented in Table 1. © 2017 American Chemical Society
A lower cost process for producing DMC would enable greater use as a green solvent, and facilitate safer processes for isocyanate production (i.e., polyurethane production). Cognizant of these benefits we investigated carbonyl dibromide (CBr2O, CDB) as a possible alternative to phosgene as it has several potential advantages. First, the conversion of hydrogen bromide, HBr, to molecular bromine Br2 requires less energy than HCl to Cl2, and the exotherm of reactions involving bromine is typically less than that of chlorine. Furthermore, the separation and handling of bromine and bromine related byproducts is less expensive due to the higher mass of bromine. CDB has a boiling point of 64.7 °C, which could provide ease of handling when compared with gaseous phosgene (bp 8.3 °C).15 Currently, there is no commercial production of CDB.15 An extensive literature search revealed no activity in this area since that reported by Seddon et al.15 There have been a number of attempts at synthesizing CDB. In 1863, Schiel claimed that CDB could be formed by exposing a mixture of CO and bromine vapor to sunlight.16 However, Beeson and coworkers17 were unable to repeat the result, although they managed to prepare a reasonably pure sample of CDB from the reaction of phosgene with BBr3 at 100−150 °C.17 Additionally, a process for the coproduction of BrCN and CDB has been patented. These two products formed on heating CHBr3 and NO2 in the presence of activated charcoal.18 Unlike phosgene, CDB is prone to thermal and photochemical decomposition. Received: May 11, 2017 Revised: July 20, 2017 Published: July 25, 2017 7492
DOI: 10.1021/acssuschemeng.7b01487 ACS Sustainable Chem. Eng. 2017, 5, 7492−7495
Letter
ACS Sustainable Chemistry & Engineering Table 1. Comparative Overview of Current Production Methods for the Synthesis of Dimethyl Carbonate Method
Advantages
Disadvantages
Oxycarbonylation of methanol
Readily available feedstock.
Methyl nitrite carbonylation
Avoids methanol−water and DMC separation.
Two-step synthesis from carbon dioxide, epoxide and methanol
Utilization of CO2. No water is produced. CBr2O is generated and consumed in situ. Mild conditions Silica catalyst is stable. Recycling of HBr is less energy intensive than that of HCl. No water is produced.
Method reported herein via CBr2O (from CO and Br2)
Although noted to be kinetically stable up to 200 °C, it also decomposes to CO and Br2 at wavelengths below 320 nm.15,19 Finally, CDB has been prepared on laboratory scale in moderate yield (∼50%) by the oxidation of carbon tetrabromide (CBr4) using concentrated sulfuric acid; however, the purification, although highly refined, is tedious.20 Overall, these reports suggested that the formation, and use, of CDB would be best undertaken in situ. In fact, Delledonne21 reported that, at high pressure (10−20 bar) and temperature (70−100 °C), the reaction between CO, bromine and methanol afforded dimethyl carbonate with yields as high as 90%. However, the energy required to facilitate this process was not convincing for phosgene replacement. In this Letter, we report on our results examining the in situ synthesis of CDB under mild conditions and the application of this compound to produce dialkyl carbonates (Scheme 1).
Rapid catalyst (CuCl) deactivation. Explosive risk from oxygen feed. High temperature (120−140 °C) and pressure (20−30 bar). DMC−methanol−water separation is expensive. CO2 side product. Palladium catalyst is expensive. High temperature (100−120 °C) and pressure (5−10 bar) in 2nd step. Water byproduct. High cost and safety concerns in the production of epoxides. Toxic CBr2O still has to be handled and requires careful management.
containing 1-butanol to form dibutyl carbonate (DBC) in a similar fashion to the reported reaction between phosgene and 1-butanol.22 Initially, activated carbon was evaluated as a potential catalyst for the formation of CDB since this is used in the commercial production of phosgene from carbon monoxide and chlorine.15 Activated charcoals (BDH 33033 carbon, Norit Row 0.8 Supra) did indeed give the desired dibutyl carbonate, but significant amounts of side-products were observed. We then investigated a number of silica surfaces. Different forms of silica were found to catalyze the reaction leading to dibutyl carbonate with a much higher selectivity compared to the activated charcoals. The best yield, approximately 18% DBC based on injected bromine, was achieved using silica quartz granules (0.5 mm) as the catalyst (Table S1). When smaller particle sized crystalline silica or mesoporous silica (silica gel 60) were used, the yields did not improve. The use of LED lights in combination with silica catalysis gave ambiguous results, and the reaction seemed to be independent of light promotion. In the view that the formation of CDB is equilibrium limiting with a K(293 K) = 0.13,23 we explored a one-pot process in order to overcome this limitation, that is, where CDB would react with the alcohol immediately on formation (Scheme 2).
Scheme 1. (a) Synthesis of CDB, (b) Dialkylcarbonates (R = Me or Bu) via CDB
Scheme 2. One-Pot Synthesis of Dialkyl Carbonates Light and silica surfaces were evaluated to facilitate the CDB formation reaction. 1-Butanol was initially used in place of methanol (Scheme 1, R = Bu) to assist with reaction understanding and developing workup conditions to separate products (i.e., dibutyl carbonate, bp 207 °C;22 DMC, bp 90 °C) from starting materials (1-butanol, bp 118 °C; methanol, bp 64.7 °C).
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RESULTS AND DISCUSSION First, a two-pot reaction (Supporting Information, Section SI.2 and Figure S1) was investigated where carbon monoxide was used both as a carrier gas and as reactant. The gas was passed through a bubbler containing bromine, and the combined vapor reacts to form CDB in the first reaction vessel, in the presence of light and/or a catalyst. Then the resulting reaction mixture was transferred via carrier gas (CO) into a subsequent vessel
As can be seen in Table 2, the best yield of dibutyl carbonate obtained was 25%, which was a moderate improvement on the best yield when the reaction was conducted in a two-pot fashion, albeit much more practical. When the reactions were conducted at 50 °C, the yield of dibutyl carbonate was higher than that when the reaction was conducted at 5 °C to room temperature (Table 2, entries 5 and 6); however, a significant amount of 1-bromobutane was formed as a byproduct, in 7493
DOI: 10.1021/acssuschemeng.7b01487 ACS Sustainable Chem. Eng. 2017, 5, 7492−7495
Letter
ACS Sustainable Chemistry & Engineering
catalytic for selective generation of CDB in situ under mild conditions in the absence of light. The CDB formed immediately reacted with the alkanol producing dialkyl carbonates. Further process gains could be possible if the reaction were to be conducted as a reactive distillation24,25 to achieve high effective bromine conversions in a single unit operation. Further investigations in this area are underway.
Table 2. Summary of Results for the One-Pot Synthesis of Dibutyl Carbonate Using Different Forms of Silica Entry 1 2 3 4 5 6
DBC Yields (%)b
Bromine conversion (%)c
Promoters and Amounts Used
Time (h)
Glasswool (2.0 g) High surface silica pellets (2.0 g) Small frit (0.8 cm diameter) Large frit (2 cm diameter)d Large frit (2 cm diameter) Large frit (2 cm diameter), 50 °C
67 42
11 (72) 7 (NA)
15 NA
46
19 ± 2 (85)
22 ± 2
42
25 (76)
33
22 24
12 (NA) 18 (NA)
NA NA
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01487. Experimental procedures, reaction setup images, yield calculation methods and NMR spectra (PDF)
a Temperature 5°C−RT. bYields were estimated using NMR spectroscopy based on injected bromine; the yield in parentheses is based on the amount of converted bromine. cBromine conversion (%): ((total amount of bromine converted to CDB (then DBC), other side products and small losses due to the gas phase volatility)/amount of bromine injected) × 100. dAs in Figure S2.
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AUTHOR INFORMATION
Corresponding Author
*E. W. McFarland. Tel: +1 805 8934343. Email: ewmcfar@ engineering.ucsb.edu.
comparison to when the reaction was performed between 5 °C and room temperature. With substantial understanding gained through the dibutyl carbonate optimization process, we moved to the synthesis of dimethyl carbonate using the one-pot approach and the same experimental setup (i.e., using a jacketed vessel with a large glass frit as the catalytic platform in the absence of light) afforded the best yield of DMC in the order of 25%. No byproduct was observed in the 1H NMR spectra; however, it was likely that a small amount of bromomethane formed during the reaction and was not observed due to its high volatility (bp = 3.6 °C). Although, the yield obtained was lower than those reported previously by Delledonne21 in a similar synthesis of DMC, there was no need for aggressive conditions, i.e., higher pressures and temperatures used by Delledonne and coworkers. In our system, further process gain could be achieved in a reactive distillation setup,24,25 results of which will be reported in due course. We postulate that the formation of CDB on the silica surface begins with the adsorption and polarization of a molecule of bromine. Considering that CO is polarized with the negative charge on the carbon atom,26 it is suitably poised to engage the δ+ Br on the silica surface. The formation of CBr2O could then proceed by the reaction of CBrO with bromine or with adjacent adsorbed bromine, potentially similar to the mechanism reported for the synthesis of phosgene.27 To validate that CDB was in fact the intermediate in the reaction pathway to dialkyl carbonate, we undertook an experiment to detect CDB by passing CO through a pad of anhydrous (oven-dried) silica gel 60 (Merck, 40−63 μm) containing preadsorbed bromine. The outgas was collected in a flask containing anhydrous chloroform. CDB was detected using FTIR (as a solution in chloroform). The FTIR spectrum (Figure S3) of the solution mixture in chloroform showed a band at 1807 cm−1, which is consistent with the literature values at 1828 cm−1 (gas phase) and 1827 cm−1 (neat) for CDB.15
ORCID
Khuong Q. Vuong: 0000-0001-9558-9273 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Israel Chemical Limited (ICL), UQ Dow Centre, and The University of Queensland for supporting this work. REFERENCES
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CONCLUSION In conclusion, the synthesis of dialkyl carbonate via a one-pot reaction using bromine, carbon monoxide and alkanol was demonstrated. The silica surface in a glass frit was found to be 7494
DOI: 10.1021/acssuschemeng.7b01487 ACS Sustainable Chem. Eng. 2017, 5, 7492−7495
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DOI: 10.1021/acssuschemeng.7b01487 ACS Sustainable Chem. Eng. 2017, 5, 7492−7495
