Heartfelt adds salts to their beads. Kitty litter has no salts added. These salts allow the silica to regulate humidity to a certain point. Without the salts, it doesn't. From the HF web site:

"DO NOT immerse you[r] beads in distilled water or dip the beads in distilled water. Trying to add distilled water this way will ultimately rinse out the salts that the beads are impregnated with. This rinsing off of the salts will ultimately make the beads not be able to hold the given rh level that they are set for."

Only HF will regulate to a certain RH. Kitty litter will not, because it's not impregnated with the necessary salts, because to absorb cat pee it doesn't need to be.