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Answer:
b) It speeds up the rate of crystal formation.
Explanation:
The effects of crystallization time on the physicochemical properties and the catalytic performance in the methanol conversion reaction were investigated. The first crystals were formed after 6 days and the amorphous phase had almost disappeared after 8 days. The average diameter of the crystallites and the crystal shapes of ZSM-5 did not change during the following 12 days. The bulk silica to alumina molar ratio of the resultant ZSM-5 remained constant even though crystallization time was varied from 8 to 20 days. However, the silica to alumina molar ratio at the external surface of the zeolite crystallites decreased with the crystallization time. In the methanol conversion at 540°C, the zeolite synthesized for the shortest time but for long enough to obtain 100% crystallinity showed the longest catalytic life, which was attributed to the slowest rate of coke formation over it. It is concluded that the aluminium content at the external surface of the crystallites increases with crystallization time, and that a longer crystallization time results in a shorter catalyst life.