Activity and stability of propylene oxide synthesis from propene in the presence of oxygen and hydrogen
Date Issued
2015
Date
2015
Author(s)
Lin, Lian-Yi
Abstract
Following the results of last year, more study of Au/TS-1 has been carried out for catalyzing partial oxidation of propylene (in presence of hydrogen and oxygen) to produce propylene oxide (PO) in gas phase in a fixed bed reactor at 200℃. At least two kinds of catalytic active sites on Au/TS-1. Those of gold catalyze the reaction between hydrogen and oxygen to form hydrogen peroxide. Those of TS-1 catalyze the reaction between propylene and hydrogen peroxide to form PO. TS-1 is synthesized by hydrothermal method, followed by deposition-precipitation method for loading gold nano-particles to form Au/TS-1. In this research, a high initial activity has been found, which can produce PO with a yield of 11.3% (propylene conversion of 13.3%，PO selectivity of 84.6%) at GHSV = 14000 mLgcat-1h-1 with reactor inlet compositions of 70% He, 10% Propylene, 10% H2 and 10% O2 at 1 atm total pressure. There is reaction decaying problem of Au/TS-1 system. PO yield decays to 9.0% (propylene conversion 11.0%、PO selectivity 82.1%) after 2 hours reaction, after which the decay rate slows down. Several measures were adopted to try to increase more activity of Au/TS-1, including increasing the loading of gold on Au/TS-1 for H2O2 production, and mixing Au/TS-1 with TS-1 to form a co-catalyst for propylene expoxidation. It has been found that loading more gold and no more than 0.86 wt% is effective to enhance the activity for PO production. Nevertheless, TS-1 in the co-catalyst provide no positive effect. It suggests that epoxidation reaction on active site of TS-1 support of Au/TS-1 is not the reaction rate limiting step for producing PO. On the other hand, only slight increase of hydrogen concentration can slightly increase propylene conversion, PO selectivity, and PO yield. The presence of steam in the feed of the reactor can also increase PO yield. Because both are for more production of H2O2, it indicate that H2O2 generation is the rate limiting reaction for producing PO in the whole reaction system. For solving the problem of Au/TS-1 activity decay during the reaction, several attempts have been carried out. It has been found that the decrease of reactants concentrations and the reaction temperature can lower the reaction activity for producing PO; nevertheless, the decaying rate of the reaction activity can not be improved. A much more steady activity of Au/TS-1 can be achieved either by coating the catalyst with silane, or by using uncalcined TS-1 for loading gold to form Au/TS-1. However, both methods would cause the catalysts with lower activities and resulting in lower propylene conversions and PO yields.
Subjects
expoxidation
propene
propylene oxide
nano-gold
catalytic activity decay
Type
thesis
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