Hydrogen peroxide (H2O2) is a highly effective, green oxidant that has found application in sectors ranging from the synthesis of fine chemicals and waste stream treatment to the extraction of precious metals and the bleaching of paper pulp and textiles. The growing demand for H2O2 has seen it become one of the 100 most important chemicals in the world. The direct synthesis of H2O2 from H2 and O2 has been a challenge for the scientific community for over 100 years and represents an attractive alternative to the current means of production. Herein we discuss the historical perspective of the direct synthesis process, the recent literature regarding catalyst design and the role of additives as well as the application of H2O2 as an in situ oxidant. We discuss the key problems that remain and conclude that although there has been progress with respect to the selectivity of hydrogen utilisation, there is a need to now concentrate on catalyst activity as the key remaining problem requiring a solution is the concentration of H2O2 that can be achieved especially in flow reactors.

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Recent Advances in the Direct Synthesis of H2O2

With the growing demand for propylene and the rapid depletion of petroleum resources, the methanol-to-propylene (MTP) reaction has aroused much interest in the industrial and academic research fields as an alternative route to produce propylene. Developing a highly efficient MTP catalyst by adopting a facile, efficient route is currently an important research goal. It is clear that the synthesis of a hierarchical micro-meso-macroporous ZSM-5 zeolite is regarded as the ideal strategy owing to its ability to minimize coke formation and to produce maximal catalytic efficiency in the MTP reaction. In this study, a superior MTP catalyst with a hierarchical micro-meso-macroporous structure (denoted as M-ZSM-5) has been successfully developed based on the passivation effect and the strong self-condensation of the short-chain organosilane 3-aminopropyltrimethoxy-silane (APTES) on the surfaces of nanocrystals in a quasi-solid-state system. This environmentally friendly synthetic strategy is operationally simple, with a high yield (above 80%) and low cost and thus has considerable potential for industrial applications. In the quasi-solid-state system, APTES not only induced the formation of ultrafine nanocrystals but also ensured that these ultrafine nanocrystals were assembled in situ into hierarchical micro-meso-macroporous ZSM-5 zeolites. When applied to the methanol-to-propylene (MTP) reaction, the synthesized MZ-x (x = 1–5) zeolites exhibited a longer catalytic lifetime (31 h–89 h) (at a high weight hourly space velocity of 8 h-1) than that of a commercial ZSM-5 catalyst (denoted as C1Z-1)(11 h), which are nearly three times and eight times longer than that of the C1Z-1 sample. In addition, the MZ-x (x = 1–5) catalysts also present a slightly higher propylene selectivity (39.92%–41.89%), butylene selectivity (22.29%–23.65%) and light olefins (C2= − C4 = ) selectivity (69.19%–71.43%) than that of the C1Z-1 (39.86%, 21.70%, 68.46%). These results were attributed to the abundant mesopores and macropores of M-ZSM-5 built by the ultrafine nanocrystals, offering a shorter diffusion path and more-accessible active sites for reaction.

Facile synthesis of a superior MTP catalyst: Hierarchical micro-meso-macroporous ZSM-5 zeolites

The direct synthesis of hydrogen peroxide (H2O2) from molecular H2 and O2 over Au-Pd / TiO2 catalysts prepared using a modified impregnation methodology is studied. Initially we investigated the role of Au: Pd ratio on the catalytic activity towards H2O2 formation and its subsequent degradation under conditions that have previously been found to be optimal for the formation of H2O2 using high pressure reactants and low temperatures. Subsequently, we have conducted a detailed study of H2O2 synthesis under ambient conditions which are both more challenging for achieving high yields of H2O2 and are more likely to be used for future industrial applications.

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Direct Synthesis of Hydrogen Peroxide over Au–Pd Supported Nanoparticles under Ambient Conditions

In this study we show that using gold palladium nanoparticles supported on a commercial titanium silicate (TS‐1) prepared using a wet co‐impregnation method it is possible to produce hydrogen peroxide from molecular H2 and O2 via the direct synthesis reaction. The effect of Au: Pd ratio and calcination temperature is evaluated as well as the role of platinum addition to the AuPd supported catalysts. The effect of platinum addition to gold‐palladium nanoparticles is observed to result in a significant improvement in catalytic activity and selectivity to hydrogen peroxide with detailed characterisation indicating this is a result of selectively tuning the ratio of palladium oxidation states.

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The Direct Synthesis of H2O2 Using TS-1 Supported Catalysts

The oxidation products of cyclohexene find several applications as intermediates for the manufacture of useful chemicals like cyclohexanol, cyclohexenol/cyclohexenone, cyclohexadiene, etc. The uncatalysed oxidation of cyclohexene with molecular oxygen was studied in a batch reactor. The products, cyclohexene hydroperoxide, cyclohexenol, cyclohexenone and cyclohexene oxide were realised in the temperature range of 343-393 K. The effect of various kinetic parameters such as temperature, speed of agitation and wall material was studied. The kinetic data were generated at different temperatures and fitted successfully with a rate equation based on autocatalysis by total products. The values of rate constants and activation energy were determined. Recognising the fact that the industrial route to cyclohexene from benzene hydrogenation yields a mixture of cyclohexene and cyclohexane which are difficult to separate, some experiments on oxidation of the mixture of cyclohexene and cyclohexane were also conducted. The values of rate constants remain unchanged in the presence of cyclohexane. The reaction was also studied in an aluminium lined reactor to examine the wall catalytic effect, if any, of the stainless steel wall. The reaction was found to be significantly slower in the aluminium reactor while the selectivity towards the hydroperoxide remained unchanged. The oxidation of cyclohexene represents a unique example of an uncatalysed liquid-phase organic oxidation which can be conducted up to relatively high conversions.

Uncatalysed oxidation of cyclohexene

Mesoporous titanium silicalite-1(TS-1)was hydrothermally synthesized with the addition of triethanolamine( TEA)in the conventional process, and used in the cyclohexanone ammoximation in a continuous slurry reactor. The as-prepared TS-1 was characterized with X-ray diffraction(XRD), scanning electron microcopy( SEM), N2 adsorption-desorption, Fourier transform infrared (FT-IR)spectroscopy, UV-Visible(UV-Vis) diffuse reflectance spectra and UV Raman spectroscopy. The results indicated that the addition of TEA resulted in the formation of mesopores and the slight increase of framework titanium in TS-1. TS-1 synthesized with the addition of TEA exhibited a higher stability in the cyclohexanone ammoximation than that without the addition of TEA, attributing to the increase of mesopore volumes and the slight increase of the framework titanium in TS-1. However, when the addition of TEA was up to TEA/SiO2 ratio of 0.24, the crystallinity and framework titanium of TS-1 decreased markedly, and the average crystal sizes of TS-1 increased, with the catalyst stability becoming poor.

Synthesis of mesoporous titanium silicalite-1 with high stability in cyclohexanone ammoximation

Titanium silicate-1 (TS-1) was treated with a mixed alkaline of tetrapropyl ammonium hydroxide (TPAOH) and NaOH. It was characterized by XRD, nitrogen physical adsorption, SEM, FT-IR, UV–Vis and ICP-OES, and studied in propylene epoxidation. The mixed alkaline treatment with TPAOH/NaOH solution did not destroy the MFI structure of TS-1. With increasing NaOH concentrations, the relative crystallinity and the framework titanium decreased to some extent while the mesopore volume, mesopore diameter, and extra-framework titanium increased appreciably. When NaOH concentration was 0.0333 mol L−1, the best catalytic performance was obtained.

Effect of TS-1 Treatment by Mixed Alkaline on Propylene Epoxidation

ZSM-5 zeolite prepared by a hydrothermal method with the addition of seeds was treated with different concentrations of NaOH. The obtained samples were characterized by XRD, N2 adsorption, NH3-TPD, FT-IR, SEM, and studied in the catalytic performance of the hydration of cyclohexene to cyclohexanol. The characterization results showed that with the increase of NaOH concentration, the crystallinity of the treated samples decreased monotonously, and the acid sites of ZSM-5 zeolites first increased and then decreased, while more mesopores formed inside the ZSM-5 zeolites. The experimental results of catalytic performance showed that cyclohexene conversion can be improved through introducing the mesopores and enhancing the acidity of ZSM-5 with the NaOH treatment at a low concentration. The highest cyclohexene conversion of 12.8% was obtained when the concentrations of NaOH solution were in the range of 0.2–0.6 mol/L. The selectivity of cyclohexanol on all samples was higher than 99.6%.

Effect of NaOH Treatment on Catalytic Performance of ZSM-5 in Cyclohexene Hydration

Titanium silicalite-1(TS-1)was synthesized by a hydrothermal synthesis method with different amounts of tetrapropyl ammonium hydroxide(TPAOH)as template. The as-prepared TS-1 was characterized by scanning electron microscopy, X-ray powder diffraction, Fourier-transform infrared resonance spectroscopy, ultravioletvisible diffuse reflectance spectroscopy and nitrogen physical adsorption and desorption, and studied in the propylene epoxidation with hydrogen peroxide in a fixed bed reactor. The results showed that the amount of TPAOH had a strong influence on the grain morphology, the amount of framework Ti, and the average particle sizes of TS-1. With the increase of TPAOH amount in the synthesis(the molar ratio of TPAOH/SiO2 increasing from 0.25 to 0.45), the morphology changed gradually from ellipsoids to cubes, the particle sizes of TS-1 decreased slightly, the amount of the framework Ti increased appreciably, and the catalytic stability in the propylene epoxidation increased markedly. Moreover, all the catalysts had the same selectivity to propylene epoxide. However, when the molar ratio of TPAOH/SiO2 was further increased to 0.55, the particles became large hexagons with the size distribution in a wide range, and the catalytic stability decreased sharply although the amount of the framework Ti increased further, which can be attributed to the long diffusion paths of the reactants in the zeolite.

Yongjie Lin

Papers

Synthesis of mesoporous titanium silicalite-1 with high stability in cyclohexanone ammoximation

Effect of TS-1 Treatment by Mixed Alkaline on Propylene Epoxidation

Effect of NaOH Treatment on Catalytic Performance of ZSM-5 in Cyclohexene Hydration

Effects of the amount of tetrapropyl ammonium hydroxide in synthesis on TS-1 properties and catalytic performance in epoxidation of propylene