Substitution of Cetyltrimethylammonium for OSDA Cations During B-SSZ-70 Zeotype Synthesis and Its Influence on Delamination

Abstract

We investigated the substitution of structure-directing agent diisobutylimidazolium hydroxide with cetyltrimethylammonium hydroxide (CTA⁺OH⁻) surfactant in the synthesis gel of layered-zeotype-precursor B-SSZ-70(P). At low substitution levels, CTA⁺ interacts with the external-zeotype surface, where it assembles. At higher surfactant amounts, CTA⁺ intercalation occurs, leading to swelling of the precursor material, as evidenced by powder X-ray diffraction. Characterization of adsorbed CTA⁺ cations by ¹³C MAS NMR spectroscopy demonstrates unique broad resonances in the intercalated layers, which represent a more rigid environment than in the external-surface assemblies. At still higher CTA⁺ substitution levels in the synthesis gel, we observe the formation of amorphous mesoporous silica. We attempted to delaminate materials synthesized with varying CTA⁺ substitution levels using mild conditions, in which the conventional sonication step is replaced with an easy-to-scale high-shear mixing, and in which the use of halide reagents, which are corrosive and have been required in the past, is also avoided. Our data demonstrate that synthesis of a swollen material per se is not essential as an intermediate for delamination, and that layered zeolite precursor swelling can actually undermine zeolite delamination. Our results instead point to the CTA⁺ external-surface assemblies as facilitating ease of SSZ-70 delamination under these mild conditions. When calcined and functionalized with grafted titanium centers, the delaminated materials are highly active and selective olefin epoxidation catalysts with organic hydroperoxides, showing a 40% higher epoxide conversion than a catalyst prepared from conventional B-SSZ-70.

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