"Learn" is not the correct verb. "Acquire" and "gain" are probably the verbs most often used with "knowledge".

It is not technically correct. I think Chinglish is a good way to describe this phrase. Knowledge is what you recieve from learning. So instead you would say that you are learning about a subject. Or that you have gained knowledge through learning. I believe your colleague may have forgotten a comma. Without seeing the whole sentence, I can't say for certain, by I believe the sentence should read, "I remember that when learning, knowledge about the periodic table of elements in the second semester... (was useful/helpful/etc)." I hope this helps.

You usually say "I am learning". :)