Nano-Au/HZSM-5Zeolite Catalyst Preparation And Its Performance In N-butane Conversion

The application and preparation of nano-Au catalyst have been received extensive attention. Zeolite with high specific surface area and unique pore pathway was considered as an important support for Au, but the isoelectric point of zeolite is too low to obtain highly dispersed Au. So far, there were few publications about the activation of light-alkane molecules on Au catalyst. Here, systematic studies were carried out to investigate the preparation of HZSM-5supported Au catalyst and its application in aromatization and isomerization of n-butane.The results and conclusions are as following:1. Highly dispersed Au on HZSM-5was prepared by deposition precipitation method unver negative pressure, combined with plasmas treatment in N2or Ar atmosphere. And the size of Au is2-10nm. XRD, UV-vis and TEM results show that the condition (temperature and atmosphere) of calcination would influence the dispersion of Au on HZSM-5. In the condition of air atmosphere, the formation of Au crystal particles began at temperature about200℃, but the size of Au crystal particles aggregated quickly if temperature reached500℃. However, at the same condition, the calcination in N2or Ar atmosphere was better than that in air or H2atmosphere, and we also found that in the same N2atmosphere the plasma calcination was superior to the traditional calcination.2. Nano-Au/HZSM-5was bi-function catalyst. NH3-TPD, XPS, H2-TPR, FTIR with pyridine or n-butane and DFT calculations show that the acidic properties of HZSM-5with high Au loading was changed, the Br(?)nsted acid sites of HZSM-5were decreased, but the Lewis acid sites were increased. Au3+and Au+are two main species of Au in Au/HZSM-5with treatment lower200℃, which could be reduced in H2flow at128℃or585℃. But with calcination temperature increased, the state of Au was transformed from Au3+(disappearance) and Au+(decrease) into Au0, and Au+exhibits high n-butane conversion activity at relatively low temperature. In addition, the valence state of Au was influenced by the preparation method and Au loading, the deposition-precipitation method facilitates to the formation of Au+compared with the hydrosulfuryl protection method with low Au loading.3. Experiments on mini-scale pulse reactor show that the n-butane conversion activity on Au/HZSM-5was obviously enhanced compared with that on pure HZSM-5. Compared Studies about catalyst modified by K+further demonstrated that the activity of Au/HZSM-5depends on Au+. In the condition of550℃, the aromatization of n-butane occurred over1.88 wt%Au/HZSM-5calcinated in N2plasma. The conversion of n-butane was64%and the yield of aromatics was higher than that on HZSM-5.However, at lower temperature (300-400℃), the isomerization of n-butane occurred over1.88%Au/HZSM-5calcinated in air. The conversion of n-butane was7.2%and the selectivity of i-butane was84%, which are both higher than that on HZSM-5. The Au/HZSM-5catalyst exhibits the feature of bi-function metal catalyst for the isomerization of n-butane。4. Au/HZSM-5modified with Zn (0.3%Au,6.0%Au, impregnation method) could greatly improve the activity of n-butane conversion, but the selectivity of aromatics was slightly decreased. In comparison with n-butane,i-butane exhibits higher activities and aromatics selectivity on Au-Zn/HZSM-5.