Cis-​trans isomerism and the role of dilution in organic structure direction to relevant zeolite catalysts AEI (SSZ-​39) and GME (CIT-​9)

Abstract

Zeolites are thriving as functional and stable materials in catalysis, sorption and even specialty applications. Besides their dominant presence in petrochem., their potential in the conversion of renewables and gaseous feedstock is huge (SCR, MTO, methane-​partial oxidn.)​, and often a single framework excels at a specific task. Although novel structures are being reported frequently, for some structurally-​relevant topologies, no effective synthetic aluminosilicate recipes are known. Natural or synthetic gmelinite (GME topol.) for example, is nearly always encountered with stacking faults (intergrowths) blocking the main channel and thus limiting porosity for application in catalysis. The search for fault-​free synthetic GME has been going on for ± 40 years, because the framework is a 3-​dimensional large pore zeolite, with a promising 12x8x8 channel system for relevant hydrocarbon chemistries, e.g. hexane isomerization or reactions where mol. traffic control is vital (compare to LTL)​. Interestingly, the framework can be entirely constructed by linking d6r composite building units. Here, we present a new route for making GME zeolites, labeled CIT-​9, based on the use of a simple org. structure directing agent (OSDA)​, with a remarkable cis-​trans isomeric specificity. Only cis-​N,​N,​3,​5-​tetramethylpiperidinium hydroxide leads to CIT-​9, while the trans form leads to SSZ-​39 of the AEI topol. AEI is an interesting cage-​contg. zeolite with 8MR small-​pore windows, and a documented excellent activity in SCR (Cu ions) and MTO (H+)​. The selectivity of synthesis also depends on the diln. of the synthesis suspension and thus the concn. of OSDA (selectivity to GME vs AEI in Figure 1) . The origin of both cis/trans and cage vs-​channel selectivity is investigated and confirmed from mol. modeling. Finally, in the light of catalysis, the novel GME is studied with rotation electron diffraction and variable temp. XRD, to monitor its faulting and thermal stability. In short, the material is non-​faulted, porous, and stable in its K-​form. In a side note, the first methanol-​to-​olefin study with the other produced zeolite, SSZ-​39 (AEI) is introduced as well.