| Electrochemical hydrogen pump (EHP) is a novel technique derived from proton exchange membrane fuel cell (PEMFC). It can be used for hydrogen purification and demonstrates merits of high purity products, high energy efficiency and simple operation. As the EHP forms in-situ hydrogen on its cathode, it can also be used as a hydrogenation reactor to fundamentally eliminate hydrogen transport resistance between different phases of conventional heterogeneous hydrogenation which incurs high pressure, large energy consumption and complexity of equipment. Using EHP instead makes the hydrogenation process feasible under ambient pressure.Firstly, in-situ hydrogenation of butanone was implemented using an electrochemical EHP with 100% selectivity, and the membrane electrode assembly (MEA) was optimized subsequently. The hydrogenation activity of Pt is 2.4 times higher than Pd, and the maximum value of hydrogenation rate is up to 15.5 mmol gPt-1 min-1, which is much higher than that of traditional high pressure hydrogenation (1.2 mmol gPt-1 min-1). Besides, the hydrogenation performance with gas diffusion electrode (GDE)-MEA is superior to that of catalyst-coated membrane (CCM)-MEA. Although Nafion/PTFE based-EHP reactor exhibits higher hydrogenation rate than Nafion 115 based-EHP reactor, its stability is poor.As Nafion suffers from high swelling ratio and is severely permeable for organic solutions, the sulfonated poly (phthalazinone ether sulfone ketone (SPPESK) membrane was prepared to substitute Nafion in a butanone hydrogenation EHP reactor. Regardless of much lower proton conductivity of SPPESK membrane than Nafion115, butanone hydrogenation rates for both Nafion115 and SPPESK-based EHP reactors are comparable, indicating that ohmic resistance caused by PEM is not the rate limiting step of butanone hydrogenation. SPPESK membrane exhibits much lower butanone and 2-butyl alcohol swelling ratios and hydrogen crossover current (0.38 mA cm-2) than Nafion membrane, which leads to higher hydrogenation stability for the SPPESK-based EHP reactor than Nafion-based EHP reactor at higher temperature (>40 ℃) and butanone concentration (>1.0 M). Meanwhile, SPPESK membrane shows excellent 2-butyl alcohol resistance (permeability of 2-butyl alcohol across SPPESK membrane is 1/8 of that of Nafion115).The hydrogen source of the hydrogenation reaction is mainly pure hydrogen gas or water, while the hydrogen production has the problem of high cost and complex process. Meanwhile, electrolysis of water consumes extensive energy and is not economical nowadays. Therefore, hydrogen mixtures (H2/CO2 and H2/N2) were used as anode feedstock to provide hydrogen and realized the coupling of hydrogen separation and butanone hydrogenation in an EHP which is more economical. With the increase of CO2 content, the hydrogenation rates of Nafion based-EHP reactors all decrease. It is probably because CO2 permeates across the membrane to the cathode and results in dilution and poisoning effects, which is further verified by H2/N2 mixture experiment. The hydrogenation rate of SPPESK-based EHP reactor keeps constant with the increasing CO2 content due to excellent CO2 resistance of SPPESK membrane.To further investigate the universality of hydrogenation on biomass-derived ketones, high-boiling 2-heptanone was hydrogenated in a SPPESK based-EHP reactor apart from low-boiling butanone. The results show that 2-heptanol is the only product with a resultant selectivity up to 100%. Unlike the way of butanone transport (gas type),2-heptanone transports across the diffusion layer by liquid type and the hydrogenation rate with n-decane as solvent is 6.9 times than that with cyclohexane (40 ℃). The SPPESK based-EHP reactor exhibits better hydrogenation performance in other PEM based-EHP reactors. Lower temperature, higher current density and higher 2-heptanone concentration are favored by hydrogenation process. |
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