PROBLEM TO BE SOLVED: To provide a solid phosphoric acid catalyst exhibiting high reactivity and dimerization selectivity when an olefin is subjected to a dimerization reaction and a method for subjecting the olefin effectively to the dimerization reaction.
SOLUTION: The solid phosphoric acid catalyst is obtained by depositing phosphoric acid on a carrier and characterized in that the amount of the moisture to be reduced when the solid phosphoric acid catalyst is heated for 20 minutes at 250°C is ≥50 mass% of that of the total moisture when phosphoric acid is converted completely into phosphorus pentoxide (P
2 O
5 ). The method for subjecting the olefin to the dimerization reaction is carried out by bringing an olefin-containing raw material containing water of 10-1,000 mass ppm into contact with the solid phosphoric acid catalyst. The method comprises: a step 1 of bringing a hydrocarbon which contains water of 10-1,000 mass ppm but does not contain the olefin substantially into contact with the solid phosphoric acid catalyst; a step 2 of adjusting the pressure and temperature to reaction initiating conditions; and a step 3 of replacing the hydrocarbon with the olefin containing water of 10-1,000 mass ppm to bring the olefin into contact with the solid phosphoric acid catalyst, which are consecutively carried out.
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