Typically, molecular sieves are dried under vacuum at elevated temperatures (about 200 to 300 °C depending on pore size) for several hours. Alternatively, microwave heating has been used for preparation of mol sieves at least since 1980s and takes minutes instead of hours, but a standard protocol for this procedure appears to be something left to be desired.
Early technical report Microwave regeneration of molecular sieves [1] illustrates the efficiency of the method: over 90 % of water was removed under 10 min mark. However, as convenient as it sounds, there is an issue of the microwave radiation being equally readily absorbed by both water and the sieves. This causes overheating the sieves (up to 500 °C and higher) and, arguably more importantly, temperature non-uniformity causing sintering, change in porosity and cracking of the sieves. The general recommendation was to use short regeneration cycles at full power and apply purge gas (air) flow with temperature control. The report, however, only relies on the data obtained for microwave source operating at 2450 MHz and one of the recommendation for future studies was to test other frequencies.
Kappe's work from 2009 [1] draws a similar conclusion and calls for further optimization:
However, the use of these materials in conjunction with microwave heating is further complicated because molecular sieves are known to be very strong microwave absorbers.[10] In fact, one of the best ways to dry/activate molecular sieves is to irradiate them with microwaves.[11]
[…]
[11] Although many chemists apparently use this method in the laboratory, we were not able to find an appropriate reference in the chemical literature. According to our experience, the procedure given in the Experimental Section provides reproducible results but no efforts were made to quantitatively assess the water content of the sieves obtained using this protocol. We believe that main reason to use microwaves is convenience due to rapid heating. However, care must be taken not to overheat the sieves by prolonged microwave irradiation. This will result in material with decreased efficiency for water adsorbance.
There are numerous websites with various recommendations for using microwave radiation for mol sieves activation (Reddit, Chemtips, Sciencemadness) and papers targeting narrowed-down applications [3], but it looks like a standardized procedure with recommended microwave frequency range and heating time strategy from an authoritative source is missing, and the recommendations are sometimes contradicting.
If the answer turns out to be dependent on morphology of the sieves, for the sake of simplicity it probably would make sense to post one for the most commonly used ones in the lab setting (3 Å beads, 8–12 mesh, I presume).
References
Singh, V. P. Microwave Regeneration of Molecular Sieves; F84023; Canada, 1984. (PDF)
Baghbanzadeh, M.; Kappe, C. O. Can Molecular Sieves Be Used as Water Scavengers in Microwave Chemistry? Aust. J. Chem. 2009, 62 (3), 244. DOI: 10.1071/CH08450.
Sârbu, E.; Călinescu, I. Microwave Assisted Regeneration of 3Å Molecular Sieves Used for Ethanol Dehydration. U.P.B. Sci. Bull., Series B 2014, 76 (1).
