Vallace Anthony, Shah Dhrumil R, Burentugs Enerelt, Tucker Atticus J, Cavanagh Ashley E, Jones Christopher W
School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States.
ACS Mater Au. 2024 Jul 11;4(5):523-536. doi: 10.1021/acsmaterialsau.4c00030. eCollection 2024 Sep 11.
Single-walled zeolite nanotubes (ZNT) were recently synthesized in a narrow compositional window. ZNT structural features-thin zeolitic walls and large mesopores-can allow for easy access of small molecules to zeolite micropores, but they also impart processing limitations for these materials, such as challenges with conventional aqueous ion-exchange conditions. Conventional solid- and liquid-phase ion exchange of calcined NaOH-derived ZNT (NaH-ZNT) results in structural degradation to either 2D sheet-like phases, 3D nanocrystals, or amorphous phases, motivating different direct synthesis routes and unconventional ion-exchange procedures of uncalcined ZNT precursors. Here, a modified synthesis route for ZNT synthesis is introduced that facilitates facile ion exchange as well as incorporation of additional non-Al heteroatoms in the zeolite framework. Tetrabutylammonium hydroxide (TBAOH) is used as a hydroxide source and co-OSDA, enabling synthesis of new compositions of ZNT, otherwise unachievable by post-modification of previously reported NaH-ZNT. By varying the gel composition, synthesis temperature, crystallization time, hydroxide source, silicon source, and aluminum source, productive conditions for the new TBAOH synthesis are developed, leading to increased strong acid site density in the ZNT. The collected results demonstrate the sensitivity of the ZNT synthesis to many key parameters and show that the ZNT forms only when Si/(Al + T) ∼ 30 in these synthesis gels and with specific Si and Al sources, and always in the presence of trace Na. Catalytic testing, via the tandem CO hydrogenation to methanol and methanol to aromatics reaction, shows that ZNTs provide adequate catalytic activity (acidity), relative to their conventional 3D counterparts in converting methanol to aromatic compounds.
单壁沸石纳米管（ZNT）最近在一个狭窄的组成范围内合成。ZNT的结构特征——薄的沸石壁和大的中孔——可以使小分子容易进入沸石微孔，但它们也给这些材料的加工带来了限制，比如在传统水相离子交换条件下面临挑战。煅烧后的NaOH衍生ZNT（NaH-ZNT）的传统固相和液相离子交换会导致结构降解为二维片状相、三维纳米晶体或无定形相，这促使人们探索不同的直接合成路线以及未煅烧的ZNT前驱体的非常规离子交换程序。在此，介绍一种用于ZNT合成的改进合成路线，该路线便于进行离子交换以及在沸石骨架中引入额外的非铝杂原子。四丁基氢氧化铵（TBAOH）用作氢氧化物源和共导向剂，能够合成新组成的ZNT，否则通过对先前报道的NaH-ZNT进行后改性是无法实现的。通过改变凝胶组成、合成温度、结晶时间、氢氧化物源、硅源和铝源，开发出了新的TBAOH合成的有效条件，从而提高了ZNT中强酸位点的密度。收集到的结果表明ZNT合成对许多关键参数很敏感，并且表明只有当这些合成凝胶中的Si/(Al + T) ∼ 30且使用特定的Si和Al源时，并且总是在痕量Na存在的情况下，ZNT才会形成。通过串联的CO加氢制甲醇和甲醇制芳烃反应进行的催化测试表明，相对于其传统的三维对应物，ZNT在将甲醇转化为芳烃化合物方面具有足够的催化活性（酸度）。
