What reducing agent are you using?

I am not using any reducing agent. I just have to form a schiff base with amine and aldehyde coupling.

What is the reaction pH?... Lyophilization may be a versatile option...

pH was around 6.

This is a kind of reversible reaction and may be controlled by certain buffers to stabilize pH and by removing the water content further. Schiff bases are very reactive, thus be sure there is not any agent that reacts with the product in the reaction pot. In reducing conditions along with acidic or alkaline conditions, it would be hard to stabilize the final derivate. Perform the reaction at gradually increased pH to observe the best condition. The reaction temperature is also a critical consideration for these fragile compounds both during synthesis and storing the final products. Therefore, freeze-drying would be a good option for the removal of water after conceivable synthesis.

Please let us informed if any improvements are recorded in the future for your research...

I have synthesized a few Schiff bases with hydrazine (PMIDs: 31370926, 31117739). We also discussed their mechanisms in these papers. My suggestions:

1) Use aprotic solvents like acetonitrile etc. Note that these are reversible carbonyl additions, and if you use protic solvents - they too can attack the carbonyl and hydrolyze the Schiff base.

2) Acids catalyze reversible carbonyl additions. So, mix the aldehyde and the acid (TFA/acetic acid should work in aprotic solvents). I recommend 1:1 or 1:1.5 mole ratio of aldehyde:acid. This creates a charge on the carbonyl compound.

3) Attack the charged aldehyde with excess of amine. Higher the amine, better should be the yield. My recommendation: start with 1:10 mole ratio of aldehyde:amine. Go higher if possible, upto 1:100. Higher temperatures also increase the rate of reaction. We performed our reactions in a sealed vial at 90 deg C.

4) Purify again in aprotic solvents to prevent hydrolysis of your Schiff base and to remove any acid and excess of amine. Preferably, do not use acids or bases as additives to your mobile phases, they too can aid in hydrolysis.