In this paper, an optimization method is used for solving a fractional optimal control problem with significant applications in chemical engineering. The considered optimal control is the control system of the isothermal continuous stirred tank reactors. The Riemann-Liouville fractional derivative is used to describe the mathematical model of control system.  For solving the fractional optimal control problem, at first by using different order of moments, we achieve an optimal control problem in space of moments, and then by using the discretization technique of variables, we will get a positive semidefinite optimization problem. Finally, by solving the equivalent optimization problem, we obtain the solution of the considered fractional optimal control problem.