mbarazeen implied that the supply voltage to the ICs would be 3.3 Volt. So I also made that assumption. Thus a 3V3 Zener should be adequate. However, you need to check the Zener spec as low voltage Zeners don't have a very sharp "knee".
The limit to the input voltage is set by the current source voltage limit. Check its spec.
The 22 k resistor is necessary to limit the current into the IC input protection diodes.
There is also a trade off between it and the 100 k. The 100 k is necessary as a "pull down resistor since:-
1. the Zeners will have some leakage current
2. the CMOS logic has a very high input resistance (around 10^12 Ohm) so their inputs must not be allowed to "float".
As I said above, the low voltage Zeners don't have a sharp knee, so the current through them at voltages less than the specified Zener voltage must be checked.
The higher voltage Zeners ie. > 6 Volt have a sharp knee and small leakage current below the knee. So you could probably use a higher than 100 k resistor for those cases.
I did not check the Zener specs, the 100 k is an educated guess.