Abstract
Palladium catalysts supported by a mesoporous form of sulfonated poly-divinylbenzene, Pd/µS-pDVB10 (1%, w/w) and Pd/µS-pDVB35 (3.6% w/w), were applied to the direct synthesis of hydrogen peroxide from dihydrogen and dioxygen. The reaction was carried for 4 h out in a semibatch reactor with continuous feed of the gas mixture (H2/O2 = 1/24, v/v; total flow rate 25 mL·min−1), at 25 °C and 101 kPa. The catalytic performances were compared with those of a commercial egg-shell Pd/C catalyst (1%, w/w) and of a palladium catalyst supported by a macroreticular sulfonated ion-exchange resin, Pd/mS-pSDVB10 (1%, w/w). Pd/µS-pDVB10 and Pd/C showed the highest specific activity (H2 consumption rate of about 75–80 h−1), but the resin supported catalyst was much more selective (ca 50% with no promoters). The nanoparticles (NP) size was somewhat larger in Pd/µS-pDVB10, showing that either the reaction was structure insensitive or diffusion limited to some extent over Pd/C, in which the support is microporous. The open pore structure of Pd/µS-pDVB10, possibly ensuring the fast removal of H2O2 from the catalyst, could also be the cause of the relatively high selectivity of this catalyst. In summary, Pd/µS-pDVB10 was the most productive catalyst, forming ca 375 molH2O2·kgPd−1·h−1, also because it retained a constant selectivity, while the other ones underwent a more or less pronounced loss of selectivity after 80–90 min. Ageing experiments showed that for a palladium catalyst supported on sulfonated mesoporous poly-divinylbenzene storage under oxidative conditions implied some deactivation, but a lower drop in the selectivity; regeneration upon a reductive treatment or storage under strictly anaerobic conditions (dry-box) lead to an increase of the activity but to both a lower initial selectivity and a higher drop of selectivity with time.
Highlights
The direct synthesis of hydrogen peroxide (DS) has been under investigation as an alternative to the anthraquinone process [1] for more than 15 years [2,3,4]
We report for the first time on the use of μS-pDVB as a support for unpromoted palladium catalysts in the DS and on the effect of the morphology of ion-exchange resins as supports on the catalytic performance
The results collected in this work show that mesoporous poly-divinylbenzene, transformed into a sulfonated ion-exchange resin, was a good support for monometallic, unpromoted palladium catalysts for the DS of H2 O2
Summary
Conditions which ensure of palladium, much more than needed to obtain a load of the supported metal of 5% (w/w) or lower. The catalysts were generally used within 24 h from 2.0 the final step of their1.0 preparation The catalysts were generally used within 24 h from the final step of their preparation (reduction and drying). The leftovers of some of them were divided into three batches, which were stored under different conditions before use. Air (closed none a:Pd/μS-pDVB35-2A time from the end of the preparation and the use ofdesiccator) the batch in the catalytic tests. A time was stored for four weeks and regenerated from theunder end of thelaboratory preparation andconditions the use of the batch in the catalytic tests. One batch was stored for two weeks in a closed desiccator under air to protect it from environmental humidity
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