Control of the Al Distribution in the Framework of ZSM-5 Zeolite and Its Evaluation by Solid-State NMR Technique and Catalytic Properties

Abstract

13The effects of the organic structure-directing agents (OSDAs) and Na cations for the synthesis of 14 ZSM-5 on the location of Al atom in the framework as well as the acidic and catalytic properties were 15 investigated. To achieve these purposes, ZSM-5 zeolites were synthesized by using four kinds of 16 organic structure-directing agents (OSDAs) including tetrapropylammonium hydroxide cations, 17 dipropylamine, cyclohexylamine and hexamethyleneimine with or without Na cations. In-situ FT-IR 18 spectroscopy using CO as probe molecule was applied to the evaluation of the acid property of the 19 ZSM-5 zeolites. The location of Al atoms was examined by high resolution 27 Al MAS and MQMAS 20 NMR techniques. The constraint index (CI) has been also used to estimate the distribution of acid 21 sites in the micropores. The location of acid sites was investigated based on the difference in the 22 transition-state shape-selectivity through the cracking of n-hexane and 3-methylpentane. Furthermore, 23 the cracking of various types of paraffins and the conversion of aromatic compounds were conducted to 24 clarify the acid site distributions. 25 26 KEYWORDS: MFI-type zeolite, organic structure-directing agents, Al distribution in the framework, 27 constraint index, 27 Al MAS and MQMAS NMR 28 29 30

Supramolecular Organization in Confined Nanospaces

“…For zeolites, one prototypical case is 29 Si, sensitive to bond distances, angles, and next‐nearest‐neighbor environment: its usefulness in probing the Si/Al ordering in zeolites has been long recognized . Besides 29 Si, also 27 Al plays a key role in topical issues, like characterization of zeolitic acid sites, or nucleation and growth processes, while 33 P is especially relevant for aluminophosphates. Many other nuclei ( 1 H, 2 H, 13 C, 15 N, etc.)…”

Directing Aluminum Atoms into Energetically Favorable Tetrahedral Sites in a Zeolite Framework by Using Organic Structure‐Directing Agents

“…As depicted in the MD snapshots (Figure c), nevertheless, the structural matching did not allow for the free movement and rotation of OSDAs, providing the chance of OSDAs to control the Al locations. Another structural reason for selecting IFR zeolite is that its crystal structure has four distinct crystallographic T sites (Figure d), making quantitative determination of Al occupancy possible by the 27 Al MQMAS NMR technique as this technique is not able to distinguish Al sites in zeolite structures containing too many T sites such as *BEA and MFI zeolites …”

“…As a result, the interactions between OSDAs and anionic charges near Al sites are inconstant, leading to broad distributions of Al over T sites. Yet, the Al locations can be controlled in zeolite framework positions between different cavities (e.g., large vs. small cages) but not crystallographic site specific ,. In the latter situation, the T site‐specific Al can be achieved due to the low mobility of OSDAs, providing the specificity of the structure‐directing ability …”

Location of Framework Al Atoms in the Channels of ZSM‐5: Effect of the (Hydrothermal) Synthesis

“…Thus, knowledge of the effect of Na + ions on the location of Al atoms in the zeolite channel system is of crucial importance. Recently Tatsumi and co‐workers reported a comparison regarding the constraint indices for cracking of n ‐hexane and 3‐methyl pentane over ZSM‐5 zeolites synthesized either by employing exclusively TPA + or TPA + and Na + as SDAs . They suggested that the application of only TPA + led to Al atoms located at channel intersections.…”