Electrochemistry-assisted selective butadiene hydrogenation with water

Alkene feedstocks are used to produce polymers with a market expected to reach 128.4 million metric tons by 2027. Butadiene is one of the impurities poisoning alkene polymerization catalysts and is usually removed by thermocatalytic selective hydrogenation. Excessive use of H2, poor alkene selectivity and high operating temperature (e.g. up to 350 °C) remain the most significant drawbacks of the thermocatalytic process, calling for innovative alternatives. Here we report a room-temperature (25~30 °C) electrochemistry-assisted selective hydrogenation process in a gas-fed fixed bed reactor, using water as the hydrogen source. Using a palladium membrane as the catalyst, this process offers a robust catalytic performance for selective butadiene hydrogenation, with alkene selectivity staying around 92% at a butadiene conversion above 97% for over 360 h of time on stream. The overall energy consumption of this process is 0.003 Wh/mLbutadiene, which is thousands of times lower than that of the thermocatalytic route. This study proposes an alternative electrochemical technology for industrial hydrogenation without the need for elevated temperature and hydrogen gas.

The cyclic voltammetry scan in 0.1M H2SO4 electrolyte, with a rate of 10 mA s -1 ; (b) 65 The linear sweep voltammetry scan in 0.1M H2SO4 electrolyte, with a rate of 5 mA s -1 ; 66 (c) The cyclic voltammetry scan in 0.2M KOH electrolyte, with a rate of 10 mA s -1 ; (d) 67 The linear sweep voltammetry scan in 0.2M KOH electrolyte, with a rate of 5 mA s -1 ;     The hydrogen atom penetration in Pd for hydrogenation 159 In this electrochemical-assisted selective hydrogenation process, Ha crossing over with a decrease from 32% to 15% and then to below 5% ( Supplementary Fig. 14d). It  Fig. 6b).

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During the exploration on the relationship between hydrogen atom coverage with 244 the system energy, a critical hydrogen coverage (i.e., 0.33 monolayer, ML) was found.

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Hydrogen atom tends to stay at sub-surface in palladium lattice once all surficial fcc  Supplementary Fig. 6d). It can be found that, as for hydrogen atom, horizontal hoping 251 on the surface is more energy favorable than that of vertical penetration into palladium 252 lattice.

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Under very low Di (i.e., below -0.15mA cm -2 ), Ha was slowly produced by the  Fig. 16a). Supplementary Fig. 16b shows that the increases from 0.217 Å to 0.315 Å, which is larger than the diameter of hydrogen atom 295 (0.25 Å). It creates a connected path for Ha diffusion (schemed in Supplementary Fig.   296 16d), and offers an enhanced Ha diffusion for catalytic reaction, i.e., the critical point 297 of TOF and FE at -5mA cm -2 (Fig. 2b).  Supplementary Fig. 7b), a swelled palladium lattice was observed 309 due to the dissolution of hydrogen atoms. Moreover, we cleave the (111) crystal surface 310 (in Supplementary Fig. 7b), the distribution of hydrogen atom in the cross section of Pd 311 (111) highly agrees with the hydrogen atom diffusion route predicted by DFT 312 calculation in Supplementary Fig. 16d. alkenes feedstocks in palladium membrane reactor by using H2 gas for hydrogen 335 permeation 10 , however, it was almost fully abandoned in the past years due to its inferior 336 catalytic performance with high reaction temperature 11-13 , e.g., ~80% of alkadienes 337 conversion in palladium membrane reactor at temperature above 130 o C by feeding H2 14 .

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However, it was surprised to observed that, with water electrolysis for hydrogen atom 339 generation and penetration, the butadiene conversion increase from below 5% at Di<5 340 mA cm -2 to above 90% at Di > 15mA cm -2 at room temperature ( Supplementary Fig.   341 7c). The further DFT calculation in Supplementary Fig. 9 indicates that the hydrogen 342 atoms at hollow site need much larger activation energy (Ea, ~1.09 eV) for the 343 combination to H2 than that of penetration (Ea, ~0.31 eV in Supplementary Fig. 6d), 344 thus the formed hydrogen atom prefer to penetration into palladium membrane.

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Importantly, the catalytic selectivity to butenes was almost independent from the 346 butadiene conversion increasing and kept at above >90% in electrochemical-assisted 347 selective hydrogenation process.