Stability of continuous phenol hydroxylation over ultrafine TS-1 in side-stream membrane reactor

The stability of continuous phenol hydroxylation over ultrafine TS-1 in a side-stream membrane reactor was investigated. The membrane reactor system could be continuously operated over 20 h. The phenol conversion and dihydroxybenzene selectivity remained stable at about 11% and 95%, respectively. The filtration resistance increased through the operation process. The fresh and used TS-1 catalysts and ceramic membranes were characterized by different methods. The results of field emission scanning electron microscopy (FESEM) show more aggregation of used TS-1 catalysts than that of fresh ones. It demonstrates that some organic matters exist on the surface and in the channels of used TS-1 catalysts by the results of ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), N₂ adsorption-desorption and thermogravimetry (TG). The results of X-ray diffraction (XRD) indicate that the content of titanium in the framework of used TS-1 catalysts is partly lost. Both of them together cause the deactivation of TS-1 catalysts, resulting in a decrease of phenol conversion in the initial reaction stage. The used TS-1 catalysts were regenerated by calcination for 6 h at 600°C, and the regenerated catalysts were evaluated in the phenol hydroxylation experiment compared with the results of fresh and used ones. It shows that the catalytic activity of regenerated TS-1 is higher than that of used ones but lower than that of fresh ones. The FESEM pictures show that the filter cake composed by the combination of TS-1 catalysts and organic matter adsorbed on the membrane surface leads to an increase of filtration resistance. The membrane flux can be recovered effectively by acid and alkali cleaning, and the ceramic membrane used in the experiments has excellent structure stability.