Direct Synthesis Of Hydrogen Peroxide Via Hydrogen-Oxygen Plasma And Its Use In The Gas-Phase Epoxidation Of Propylene

Hydrogen peroxide is an important green oxidizing agent. The conventional process for hydrogen peroxide production (the anthraquinone) employs multiple unit operations, generates considerable waste and requires significant energy input, raising the production cost and limiting the wider applications of H₂O₂. Hence, the development of a simple and highly efficient process for the synthesis of H₂O₂ is of significant commercial interest. Since the beginning of the last century, extensive studies have been done on the direct synthesis of H₂O₂ from H₂ and O₂ with supported palladium or palladium alloy. Notwithstanding, this catalytic route has not yet been industrialized mainly because that the catalysts for H₂O₂ synthesis are also active for the combustion of H₂, the decomposition and the hydrogenation of H₂O₂. By this catalytic route it is very difficult to achieve high H₂O₂ concentration and high H₂O₂ selectivity at the same time.According to the earlier literatures, H₂/O₂ can be converted into H₂O₂ with high selectivity under atmospheric pressure if they are activated into non-equilibrium plasma by silent electric discharge. However, the plasma method has not received recognition because of low H₂O₂ yield (Ca. less than 5%), high energy cost and safety anxiety.In this paper, the direct synthesis of H₂O₂ from the gaseous non-equilibrium plasma of H₂O₂ is systematically studied with self-designed dielectric-barrier discharge (DBD) reactors in order to seek breakthrough to the state of the art of the plasma method. On the basis of the direct synthesis of H₂O₂ by the plasma method, the TS-1 catalyzed gas-phase epoxidation of propylene with in-site H₂O₂ is also attempted. The following results have been obtained:1. The configuration of the DBD reactor is the most important factor to the efficiency of H₂O₂ formation and the discharge safety of H₂/O₂ mixture. It is observed that, with the self-designed self-cooling double dielectric-barrier discharge reactor (self-cooling DDBD reactor), no explosion or ignition occur during the continuous discharge in the H₂/O₂ mixture that contains O₂ up to 25 mol% (the explosion and ignition limit of H₂/O₂ mixture is 4-94 mol % O₂). However, when the H₂/O₂ mixtures that have explosive composition are fed into the common single dielectric-barrier discharge reactor (SDBD reactor), explosions would take place immediately after the start of the discharge. Using the self-cooling DDBD reactor, the direct synthesis of H₂O₂ has achieved 97.9 % O₂ conversion, 64.9% H₂O₂ selectivity, and 63.5 % H₂O₂ yield under the conditions of the HV electrode diameter 2.8 mm, the space between electrodes 4.1 mm, the reactor length 250 mm, the discharge frequency 12 kHz, the reactor input power 2.6 W, the H₂/O₂ mixture flow rate 2.4 L/h, the O₂ concentration in the H₂/O₂ mixture 25 % and the reactor wall temperature 5℃. The yield of H₂O₂ is an order of magnitude higher than that reported, and the energy efficiency of H₂O₂ formation has reached 222.6 g·kWh⁺¹. These mean that a big breakthrough has been made in the direct synthesis of H₂O₂ by the plasma method.2. In-situ plasma diagnosis by optical emission spectroscopy (OES) discloses that, in the self-cooling DDBD reactor, the electrons have lower energy, the non-equilibrium plasma of the H₂/O₂ mixture gives more intensive emission in 280-530 nm which is attributed to the transition of H₂ (a³âˆ‘_g⁺→b³âˆ‘_u⁺) , but does not show any emission related to the excited molecules and atoms of oxygen, In contrast, in the common SDBD reactor, the electrons have higher energy, the non-equilibrium plasma of the H₂/O₂ mixture produces weaker emission in 280-530 nm, but stronger emissions around 323 nm, 369 nm and 454 nm that relate to the transitions of O₂ Herzberg states (c¹âˆ‘_u^-, A³âˆ‘_u⁺, A′△u). Based on the electron excitation cross sections of H₂ and O₂, excitation thresholds of H₂ and O₂ and the ¹⁶O₂/¹⁸O₂ isotope tracing experiments, it is concluded that, in the self-cooling DDBD reactor, the activation of the H₂/O₂ mixture by the inelastic impact of the energetic electrons in the non-equilibrium plasma mainly leads to the dissociation of H₂ molecules into H atoms. Because the electrons have lower energy in this reactor, the dissociation of O₂ by electron impact is suppressed, whereas the electron impact dissociation of H₂ could take place easily through the time-after-time inelastic impact with the low energy electrons. The direct synthesis of H₂O₂ is proceeded via the reaction pathway of H+O₂→HO₂, HO₂+HO₂→H₂O₂+O₂. This formation pathway of H₂O₂ is completely different from that reported in earlier literatures (OH+OH→H₂O₂). The suppression of the O₂ dissociation is the key to achieving high H₂O₂ selectivity and to restraining OH chain reaction which is apt to incur explosion.3. H₂O₂ decomposition experiments indicate that, in the presence of O₂, the decomposition of H₂O₂ by the dielectric barrier discharge is neglectable. However, when O₂ in the H₂/O₂ mixture is completely consumed, the decomposition of H₂O₂ by the dielectric barrier discharge is very fast, most probably via the reaction of H+H₂O₂→H₂O+OH. In addition, it is found that the thermal decomposition of H₂O₂ takes place remarkably when the temperature of the discharge reactor is beyond 30℃. Hence, in order to avoid the decomposition of H₂O₂ during the direct Synthesis of H₂O₂ in the discharge reactor, it is very important to prevent O₂ in the H₂/O₂ mixture from being completely converted and to keep the temperature of the discharge reactor not higher than 30℃. 4. A specially designed reactor has been used to exploit the reactions of the H₂O₂ non-equilibrium plasma as the novel in-site H₂O₂ production mode for the gas-phase propylene epoxidation catalyzed by titanium silicalite (TS-1). Results show that, under the conditions of the HV electrode diameter 2.9 mm, the space between electrodes 4.5 mm, the reactor length 250 mm, the discharge frequency 12 kHz, the reactor input power 2.6 W, the H₂O₂ mixture flow rate 10 L/h, the O₂ concentration in the H₂/O₂ mixture 5.1%, the reactor wall temperature 5℃, and the conditions of the epoxidation temperature 90℃, the propylene feeding rate 1.1 L/h and propylene WHSV 2.53h^-1, the epoxidation of propylene reaches 7.2% propylene conversion, 93.1% propylene oxide (PO) selectivity and 240 mgPOgTS-1^-1h^-1 PO yield over a Na₂SO₃ modified large crystal TS-1 (TS-1-0.5 %Na₂SO₃-690). The PO yield is much higher than the best result reported by literature (134 mgPOgTS-1^-1h^-1. It is observed that, the modification of the large crystal TS-1 (hydrothermally synthesized with cheap template TPABr) by suitable amount of Na₂SO₃ could dramatically enhance the conversion of propylene, the selectivity and yield of PO. In addition, in the absence of methanol, the direct gas-phase epoxidation of propylene with H₂O₂ can take place effectively, which gives a PO selectivity of more than 93%. However, the presence of small amount of methanol (propylene/methanol=10/1, mol) obviously favors the conversion of propylene.5. The role of modification of TS-1 by Na₂SO₃ was investigated by XRF, XRD, framework FT-IR, Pyridine-IR and UV-Vis. Results show that, the modification of TS-1 by Na₂SO₃ could selectively disable the catalytic ability of the extraframework titanium species in the decomposition of H₂O₂ and in the deep oxidations of propylene and PO.