Environmentally Friendly Tungsten-Based Catalysts

In order to implement the Montreal Protocol to protect the Earth's ozone layer, countries around the world have introduced zero-ODP values ​​of hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) to phase out chlorofluorocarbons (CFCs) and hydrofluorides Chlorocarbons (HCFCs). At present, HFCs and HFOs have been widely used as refrigerants, cleaning agents, foaming agents, fire extinguishing agents, etchants, and etc.

At present, industrial production of HFCs or HFOs mostly adopts a gas phase catalytic fluorine-chlorine exchange reaction method of halogenated organic substances, and the method has the advantages of simple process, safe operation, and easy to have large-scale continuous production. A fluorine-chlorine exchange catalyst plays a central role in the gas phase catalytic fluorine-chlorine exchange reaction of a halogenated organic substance. At present, a common fluorine-chlorine exchange catalyst is a chromium-based catalyst whose main active component is chromium.

Chromium-based catalysts have attracted the interest of scientists from all over the world due to their advantages of easy availability of raw materials and high activity. However, with the deepening of research, it has been found that chromium catalysts still have the defects of low use temperature, low catalytic activity, short service life, difficult to recycle and reuse, and more importantly, chromium is toxic and can cause great harm to people. In particular, high-priced chromium has strong carcinogenicity.

Therefore, people began to look for non-chromium-based catalysts as fluorine-chlorine exchange catalysts, thereby providing a safe, environmentally friendly, non-chromic catalyst with high catalytic activity and long service life. At this time, tungsten-based catalyst has come up. The gas phase catalytic fluorine-chlorine exchange reaction has a role comparable to chromium. The highly active tungsten-based catalyst is obtained by the following preparation method:

1) According to the mass percentage of tungsten ions and auxiliary agents, the precursor of the tungsten ion and the precursor of the auxiliary agent are uniformly mixed and pressed to obtain a catalyst precursor;

2) The catalyst precursor obtained in the step 1) is calcined at 300 ° C to 500 ° C for 6 to 15 hours under a nitrogen atmosphere;

3) The calcined product obtained in the step 2) is firstly evacuated at 600 ° C to 800 ° C, and then activated in a fluorine gas for 6 to 15 hours under a sealed condition to obtain a highly active tungsten-based catalyst. The total amount of the fluorine gas is equal to the theoretical amount of the fluorine gas consumed by the precursor of the tungsten ion plus the theoretical amount of the fluorine element consumed by the metal element auxiliary agent; the theoretical amount of the fluorine gas consumed by the precursor of the tungsten ion is greater than that of the tungsten ion The theoretical amount of fluorine gas required to reduce tungsten ions into tungsten trifluoride in the precursor, and less than the theoretical amount of fluorine gas required to reduce tungsten ions into tungsten pentafluoride in the precursor of tungsten ions.

Compared with chromium-based catalysts, tungsten-based catalysts are internationally recognized as safe, environmentally friendly and non-toxic. Moreover, when the tungsten-based catalyst is activated by tungsten hexafluoride or fluorine gas, the precursor of the tungsten ion can form one of tungsten trifluoride, tungsten tetrafluoride or tungsten pentafluoride with fluorine gas or tungsten hexafluoride. Or several, resulting in a stronger catalytic activity of the tungsten-based catalyst and the modification of the tungsten-based catalyst by the metal element greatly improve the stability of the tungsten-based catalyst.

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