IMHO, the process of calcination may not be complete and some carbon particles may be present or the heating rate of the calcination process and time of calcination may not be optimal. I would suggest a repeat of the calcination process with a proper check of the heating rate and calcination time.

Soheil, the temperature of calcination for MCM should be not less than 550 °C. And better calcine it under stable flow of synthetic air (or pure O2), which avoids oxygenless static atmosphere and assures repeatability of synthesis

thanks for your kind respond.

Actually, I agree that carbon left behind (because of a non-complete burning of the organic template) would bring about a darkish-grey colour; whlch often can be removed by heating (at about 300 to 400 degrees) under an oxygen flow for about 1 hour.