Global S&T Development Trend Analysis Platform of Resources and Environment
| DOI | 10.1126/science.abb8061 |
| Electrification of the chemical industry | |
| John L. Barton | |
| 2020-06-12 | |
| 发表期刊 | Science
 |
| 出版年 | 2020 |
| 英文摘要 | Curbing carbon emissions while maintaining quality of life is a global challenge for manufacturing processes that will require process innovation. One approach is replacing energy from the burning of carbon-based fuels with energy supplied by “green” electrons. This goal can be achieved in some cases by simply replacing heat supplied by combustion with electrical heating ([ 1 ][1]). In chemical synthesis, it can also more elegantly supply reaction energy through electrochemistry. On page 1228 of this issue, Leow et al. ([ 2 ][2]) propose an electrochemical route to ethylene oxide (EO) and propylene oxide (PO) that promises cleaner, more efficient, and more selective processing. Ethylene and propylene were epoxidized electrochemically to EO and PO, respectively, at industrially relevant current densities with Faradaic (electron-specific) selectivities ∼70% to the target epoxide ([ 2 ][2]).
Leow et al. coupled an electrochemical flow cell to homogeneous reactions for an overall reaction, C2H2 + H2O → C2H2O + H2, for EO synthesis (see the figure). Two electrochemical reactions drive this reaction. Chlorine evolution occurs at the anode, 2Cl− → 2 e − + Cl2, and hydrogen evolution occurs at the cathode, 2H2O + 2 e − → H2 + 2OH−, where e is the charge on the electron. These reactions are not particularly interesting; what is innovative is coupling these two simple reactions with three subsequent, homogeneous chemical reactions. Dissolved chlorine in the anodic solution dissociates into hydrochloric and hypochlorous acid (HCl and HOCl, respectively). The latter reacts with ethylene to make 1-chloro-2-hydroxyethane (HOCH2CH2Cl), or ethylene chlorohydrin. The cathodic solution is enriched in hydroxide from the H2 evolution. The EO and Cl− are produced when the anodic aqueous ethylene chlorohydrin solution is mixed with the basic cathodic solution. A similar route can produce PO from propylene. The authors also develop a process to produce ethylene from CO2 and subsequently upgrade it to EO.
Technoeconomic analysis by Leow et al. suggests that this process could scale to produce EO at a cost comparable with current industrial practices with a lower carbon footprint when supplied with renewable energy ([ 2 ][2]). Such a process would be a carbon-negative path to an important, large-scale commodity chemical. Improvements are still possible, particularly in product selectivity and catalyst selection. Nonetheless, the electrochemical productivity of EO reported in this study is a factor of 10 higher than that of the electrochemical process of Simmrock and Hellemanns ([ 3 ][3]).
![Figure][4]
Electrochemical production of epoxides
An electrochemical flow process developed by Leow et al. can produce ethylene or propylene oxide from water and the corresponding olefin.
GRAPHIC: A. KITTERMAN/ SCIENCE
Current routes to EO and PO operate at capacities for each greater than 8 million tonnes per year ([ 4 ][5], [ 5 ][6]). Historic production of EO was through the chlorohydrin process, which is based on chlorine produced electrochemically. This process was replaced by the direct partial oxidation process with silver catalysts, and currently, all EO is produced this way ([ 5 ][6]). More than one-third of global PO production still uses the chlorohydrin process, and no viable direct oxidation process with molecular oxygen is available. Most PO is now produced by using a peroxide as the oxidant ([ 4 ][5]). The chlorohydrin process produces immense quantities of wastewater. As new plants are built, the use of a peroxide-based process is generally more favorable financially and environmentally. The chlorohydrin process uses Cl2 produced through the electrochemical chloro-alkali process ([ 6 ][7]) that operated at a lower current density than the process presented by Leow et al. This new process option raises the possibility that chloro-alkali assets could be repurposed to produce PO with greater efficiency and with a lower barrier to implementation.
Energy intensity, which is the total energy required to produce a product from raw materials (embodied energy), is a primary metric for comparison of process sustainability ([ 7 ][8]). Leow et al. produce EO or PO with an electrical energy demand of 0.83 MJ/mol (19 MJ/kg EO or 14 MJ/kg PO) from the corresponding olefin and water, which compares poorly with process energy estimates for direct oxidation to EO (4.0 MJ/kg EO) or a peroxidation route to PO (6.0 MJ/kg PO) ([ 8 ][9]). The direct oxidation route for EO will be substantially less energy intensive even if renewable electricity supplies the electrochemical process. In the case of PO, this process could be expected to have an energy input similar to that of the current chlorohydrin process, which is estimated to be more energy intensive than the hydrogen peroxide route by ∼50% ([ 4 ][5]).
The energy intensity can be reduced through cell design (for example, changing the flow rate or electrode thickness), but the pairing of electrochemical reactions (Cl2 and H2 evolution) is unlikely to allow for the energy intensity to be reduced below the chloro-alkali process because it is directly proportional to the cell voltage. To make a marked improvement on the energy demand, the electrochemistry would need to be altered to reduce the cell voltage, but there could be more subtle advantages outside of the analysis here, such as a reduced water demand. Although this particular process reported by Leow et al. is unlikely to be the next major route to PO or EO, the development of alternative processes to commodity chemicals, especially those with clear options for incorporation of renewable energy, is necessary to realize greener processes.
1. [↵][10]1. H. Thunman et al
., Sustainable Mater. Technol. 22, e00124 (2019).
[OpenUrl][11]
2. [↵][12]1. W. R. Leow et al
., Science 368, 1228 (2020).
[OpenUrl][13][Abstract/FREE Full Text][14]
3. [↵][15]1. K. H. Simmrock,
2. G. Hellemann
, U.S. Patent 4,119,507 (1978).
4. [↵][16]1. H. Baer,
2. M. Bergamo,
3. A. Forlin,
4. L. H. Pottenger,
5. J. Lindner
, in Ullmann's Encyclopedia of Industrial Chemistry (2012); |
| 领域 | 气候变化 ; 资源环境 |
| URL | 查看原文 |
| 引用统计 | |
| 文献类型 | 期刊论文 |
| 条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/274456 |
| 专题 | 气候变化 资源环境科学 |
| 推荐引用方式 GB/T 7714 | John L. Barton. Electrification of the chemical industry[J]. Science,2020. |
| APA | John L. Barton.(2020).Electrification of the chemical industry.Science. |
| MLA | John L. Barton."Electrification of the chemical industry".Science (2020). |
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