It is actually a bit more involved than simply passing parameters when you are using the function with any of the ODE solvers (ode45 specifically here).
The function declaration must be:
function f = simul_dif(t,c,Q)
and the call to it in ode45 must be:
[t,c] = ode45(@(t,c)simul_dif(t,c,Q), trange, Cin);
With both of those changes, it will work.
There are additional considerations if you are defining ‘Q’ as individual elements of a vector in a loop.