Effects of Sodium-to-OSDA Ratio in the Synthesis Gel on SSZ-39 Formation and Material Properties

This work quantifies how varying the Na/OSDA ratio in the synthesis gel (at fixed total [OH] content) affects the formation of SSZ-39, its growth kinetics, and the composition of the products obtained. It was found that it is possible to make phase-pure SSZ-39 with Si/Al ratios varying from 6.3 to 10.7 with Na/OSDA ratios from 9.1 to 1.7 in the synthesis gel. Higher Na/OSDA ratios lead to faster crystallization, supporting the hypothesis that FAU dissolution is the rate-limiting step in SSZ-39 synthesis when FAU serves as the aluminum source. DFT modeling suggests that, in the presence of OSDA molecules, increased Na content lowers the energy penalty for placing Al atoms in close proximity, which may explain why higher NaOH/OSDA ratios experimentally yield lower Si:Al ratios. This work offers another way to control the framework composition and potentially impact the local structure of the SSZ-39 that is obtained. Cobalt titration was performed to probe the presence of so-called aluminum pairs in samples made with different Na/OSDA ratios. The cobalt uptake in the H-form products is consistently low and suggests that factors other than aluminum pairing, such as solution pH, could be important in influencing the cobalt uptake.