This is not my area of specialization.

NaH or NaBH4 produce highest yields (close to 100%) of BH3 in THF from BF3-Et2O. LiBH4 is also excellent but is costly. Examples for external and in situ (consumed) preparations are in attachment

Greetings:

I never did, but it's a suggestion: 

1- Calculate the molarity need from the from your THF-BH3 then calculate the mass of BH3

2-Add THF needed to make this molarity to the BH3-Et2O volume that contains the required mass

3- Evaporate at r.t. under reduced pressure the ether from your mix. this means that you will reduce the total volume to, almost, it's half.

This suggestion depends on diff. of boiling points of Et2O= 35 C and THF= 66

I hope it works

I made diborane many times from BF3.Et2O and NaBH4. The gas so obtained can be bubbled into THF, and so get the solution.

Add an excess of dried THF and heat at 45 degrees (open flask under argon) over night. It should work.

good luck !

Thank You All for your answers and suggestions.

Hi Sai Siva,

I am also interested in knowing whether we can produce BH3.THF from BF3.Et2O. Did you find the suggestions helpful. Could you please share with me your process description.