THIS is absolutely an XY problem. no doubt at all.
let me explain......
the question of WHY has been asked, but unanswered.
what are you trying to do that the diode is causing problems ?
air is slow and compressible. it moves at the speed of sound.
electricity moves at the speed of light.
a pneumatic cylinder has a seal and a wall. there are motive forces that must be overcome. the slang term is 'stiction' the sticky friction.
a pneumatic cylinder will jump once the forces are delivered past that point.
now. back to the solenoid. the solenoid is a coil that you are charging with a current. said current will create an electromotive force. the piston inside of the solenoid mechanical bits will move and air will pass, compressed air ?
something else, somewhere else, a sensor ???
will tell it to stop.
you eliminate the signal and the magnetic field in the coil will fail.
when it changes from a magnetic field, the energy in that field will break down from a magnetic field into a voltage on the power lines.
the resultant voltage will be a spike of a voltage that is often about 10-15% higher than the power supply. testing shows that sometimes it gets up to 20%, but, once, only once in a thousand times it might hit 50% or possibly more. and that spike will destroy your electronical bits that are connected to it.
if you have followed this up to this point. then the question is what can we do to prevent that spike from destroying our electronical bits ?
we have to add some form of protection.
the simple, cheap answer is to invert a diode and add it across the coil. when that back-EMF spike occurs on each and every collapse of that magnetic field, the diode will shunt that spike back across the coil and the problem of that spike destroying your electronical bits is averted.
back to why this is an XY problem.
you have asked us to advise you of what diode to use.
the correct answer is that it does not matter and that as long as the diode voltage is at least 4 to 10 times more than the power supply voltage the diode will not suffer from this application.
the link you provided shows a transistor, the diode is only a 10 cent safety valve to protect a 99 cent transistor.
Let me say then, that the answer to your XY question is : if your diode is rated for more than 50 volts, it will work in this application.
the engineering answer is that it will not have any effect on the transistor and therefore will have no effect on any other part of this circuit.