Studies On Preparation And Properties Of Spherical Alumina And Catalyst For Selective Hydrogenation Of Acetylene

Ethylene is a monomer that is used in preparing a number of olefin polymers. Ethylene is generally made by the pyrolysis or catalytic cracking of refinery gas, ethane, propane, butane, and the like. Ethylene so produced usually contains small proportions of acetylene. In polymer grade ethylene, it is generally preferred that the acetylene content be less than about 10 ppm, most preferably less than about 5 ppm. One of the techniques that has been used in the past for reducing the amount of acetylene in an ethylene stream has involved selective hydrogenation using a catalyst.When used in fixed beds, spherical alumina facilitates uniform packing of the bed, whereby variations in the pressure drop through the bed are minimized and channeling of the feed stream of reaction components is substantially reduced. Spherical alumina prepared by oil-drop method can be employed in the selective hydrogenation reaction on account of high activity, high specific surface area, high intensity, fine pore volume and pore distribution maintaining the advantage of spherical form. The said support could not perform both said excellent performance and high purity, pore properties and high thermalstability together. In the thesis, preparing spherical alumina with controlling of structure and properties has been studied.The alumina hydrosol was firstly synthesized using aluminum powder and hydrochloric acid. The amount of impurity in alumina hydrosol was decreased during the synthesis of alumina hydrosol using reduction-magnetic separation process. Spherical alumina granules are prepared by the oil-drop method using the purified alumina hydrosol as starting material. Spherical alumina granules are modified by hot water, hot ammonia and calcining at different temperature. The analysis results show that the amount of Cl of spherical alumina modified by hot water and ammonia was highly reduced. The hydrothermal stability has been improved and the surface acidity has been decreased. The crystal types of alumina calcined at 600℃, 960℃, 1050℃and 1200℃areγ-Al₂O₃,δ-Al₂O₃,θ-Al₂O₃ andα-Al₂O₃ respectively. The specific surface area, pore volume, and surface acidity are decreased and average pore diameter is enlarged with the increase of calcining temperature.Spherical alumina modified by La, Si, and Ti was prepared. The sample prepared by La modified with 0.02 or higher of La/Al mole ratio shows LaCO₃OH in precursor and LaAlO₃ after calcined at 600℃or higher. With the increase of La/Al mole ratio, the La modified alumina's acid sites increase at the beginning, reach maximum value with 0.01 of La/Al mole ratio, and then decrease rapidly and lower than normal alumina. SiO₂ can improve the thermalstability of the spherical alumina, specific surface area, pore volume and surface acidity increase with the increase of SiO₂ content. TiO₂ can weaken the thermalstability of the spherical alumina, when calcined at 960℃or higher, there is rutile TiO₂ crystal in spherical alumina, specific surface area, pore volume and average pore diameter decrease with the increase of TiO₂ content.The catalyst prepared by spherical alumina calcining at 1150℃and loaded with active ingredient shows good activity and selectivity and a low production level of green oil in selective hydrogenation for acetylene action test. The result is better than that of exiting products. There's good prospect of the catalyst being employed in the ethylene industry.