The isomerization of light paraffins to branched isomers is an important process in refining industry, which has a wide range of applications in increasing gasoline octane number, reducing diesel oil solidifying point and improving low temperature property of lubricant base oil. Although the early light hydrocarbon isomerization catalyst (such as H₂SO₄ and HF) has high acid strength, it is strong corrosive and toxic. Therefore, it has been abandoned with the growing awareness of environmental protection and safety. At present, Chloride treatment of Pt-A1₂O₃ type low-temperature isomerization catalysts are widely used in the industry and usually have high activity and selectivity. The isomerization catalyst in the use process generally requires the addition of a small amount of chlorine to maintain catalytic activity. However, there are corrosion and pollution problems over the catalyst loading and handling process. It is sensitive to water, sulfur and thus raw materials are not allowed to contain water and sulfur, which limits the application of the process. In 1979, Hino and co-workers impregnated amorphous zirconium hydroxide with sul-furic acid to give upon calcination a solid superacid (Hammett acidity < −16) capable of catalyzing the isomerization of n-butane [1] [2] . The study on the /ZrO₂ isomerization reaction for for n-butane at room temperature was performed by Hino, and /ZrO₂ showed very high catalytic ctivity. Since then, /ZrO₂ solid superacid catalysts caused extensive concern of the researchers. Compared with liquid acid catalysts, solid acid catalyst has the advantages of high catalytic efficiency, simple preparation method, easy separation of products, being non-corrosive, environmentally friendly and reusable. Solid super acid not only overcomes many disadvantages of liquid acid catalyst, but also presents high catalytic activity in many reactions of isomerization, alkylation, dehydration and esterification. However, from the view point of practice, the activity of SZ needs to be improved further, at the same time the use of isomerization catalysts can reduce costs. The task of decreasing its cost remains important and challenging. Some researchers [3] suggested that supporting sulfated zirconia on a cheaper porous material may be a good way to enhance the amount of superacid site by increasing the surface area and lowering the cost of catalysts.

By using solid superacid catalyst in light hydrocarbon isomerization process, the isomerization products are environmental gasoline harmonic components that do not contain aromatics and olefins. Moreover, the process itself is environmental as well, which overcomes the corrosion problem in the traditional low-temperature isomerization process. Hence, it has attracted increasingly attention from word-wide researchers [4] .

In this paper, the Pt-/ZrO₂-A1₂O₃ catalyst was prepared by two step method. The n-pentane selected as reaction mediums for catalyst evaluation, and n-pentane isomerization performance of catalysts was evaluated in a 10ml fixed bed microreactor-chromatography unit with high pressure continuous flow. The catalyst was placed in the middle of the reactor while the rest empty volume was filled with quartz sand. The effect of different preparetion conditions on n-pentane isomerization performance of Pt-/ZrO₂-A1₂O₃ was mainly investigated.

Chemical reagents: ZrOCl₂∙8H₂O (analytical pure, Shanghai richjoint Chemical Reagent Co. Ltd.); single water of aluminum hydroxide (Fushun No.3 oil factory); n-pentane (analysis purity, Shenyang Huadong Reagent Factory); H₂PtC1₆∙6H₂O (analytical pure, Shenyang Jinke Reagent Factory); sulfuric acid (analysis purity, Kunshan Jincheng Reagent Co. company); nitric acid (analysis purity, Kunshan Jincheng Reagent Co. Ltd.); concentrated ammonia solution (concentration 28%); silver nitrate solution (0.1 mol/L⁻¹); hydrogen (purity over 99%).

Preparation of catalyst: The catalyst was prepared as follows: The required amount of ZrOCl₂・8H₂O was dissolved in deionized water. Aqueous ammonia was added dropwise to a solution of ZrOCl₂・8H₂O until pH = 9 - 10, and then the precipitation was obtained. After being stored at room temperature for 24 h, the precipitation was washed repeatly with deionized water until the washing liquid is no longer able to check out the Cl⁻¹ ion (0.1 mol/L⁻¹ AgNO₃ solution). And then dried in oven (383 K, 24 h), the obtained powder was denoted as Zr(OH)₄ and grinded into less than 100 mesh. Zr(OH)₄ was mixed with proper amount of alumina. The obtained mixture immersed in sulfuric acid solution, dried at 383 K overnight then dipped H₂PtC1₆∙6H₂O solution. After drying at 383 K overnight, suitable amount of sesbania powders and 18% dilute nitric acid solution were added. Followed by extruding forming, drying and calcining in muffle oven at given temperature is to obtain the Pt-SO₄²⁻/ZrO₂-A1₂O₃ catalysts, denoted as PtSZA.

The best conditions for preparation of Pt-/ZrO₂-A1₂O₃ solid super acid catalyst are determined by experiment. The results show that the optimal preparing conditions are 30% of Al₂O₃ mass fraction, impregnating with 0.5 mol/L H₂SO₄ solution, 0.3% of Pt mass fraction and calcining at 650˚C. The n-pentane conversion and isopentane selectivity of Pt-/ZrO₂-A1₂O₃ could reach up to 62.17% and 91.60%, respectively. The catalyst is simple for preparing environment friendly and highly potential in industrial application.

The authors gratefully acknowledge the support from the Provincial Key Laboratory of Oil & Gas Chemical Technology of Daqing. This study was supported by scientific research fund of Heilongjiang provincial education department (12521063).