Suk Bong Hong

Synthesis, characterization, and catalytic properties of zeolites IM-5 and NU-88

Abstract The synthesis, characterization, and catalytic properties of two high-silica zeolites IM-5 and NU-88, whose structures still remain unresolved, are presented. When the 1,5-bis( N -methylpyrrolidinium)pentane and 1,6-bis( N -methylpyrrolidinium)hexane cations are used as organic structure-directing agents, respectively, crystallization of pure IM-5 and NU-88 was possible only from synthesis mixtures with a narrow range of SiO 2 /Al 2 O 3 and NaOH/SiO 2 ratios. The overall characterization results of this study strongly suggest that IM-5 is a new multidimensional large-pore zeolite, whereas NU-88 is a nanocrystalline material that could be an intergrowth of several hypothetical polymorphs in the beta family of zeolites. Despite its nanocrystalline nature, however, no detectable extraframework Al species were found to exist in H–NU-88, revealing excellent thermal stability. H–IM-5 and H–NU-88 both exhibit a very high 1-butene conversion compared to H–ZSM-35 over the period of time studied here, whereas the opposite holds for the formation of isobutene. This reveals that their pore topologies are large enough to allow undesired side reactions such as 1-butene dimerization followed by cracking to light olefins. They also show the initial n -octane cracking activity comparable to that on H–ZSM-5. However, a notable decrease in n -octane conversion on these two zeolites with increasing time on stream is observed. When the isomerization and cracking activities of all zeolites employed in this study are correlated with their coke-forming propensities, it can be concluded that both materials present a shape-selective character falling within the category of multidimensional, large-pore zeolites.

Influence of framework silicon to aluminium ratio on aluminium coordination and distribution in zeolite Beta investigated by 27Al MAS and 27Al MQ MAS NMR

27Al magic-angle spinning (MAS) and triple quantum (3Q) MAS NMR spectroscopic techniques were used to characterise zeolite Beta samples with framework Si/Al ratios between 9 and 215, obtained by synthesis in fluoride medium. A carefully controlled stepwise calcination procedure was adopted to obtain H-Beta. A partial resolution of the T-sites was observed in the 27Al MAS NMR spectra, the resolution increasing with increasing the Si/Al ratios. The relative intensity of these peaks varied gradually, with Si/Al ratio showing that the relative occupancy of the crystallographic T-sites changes with Si/Al ratio. The tetraethylammonium cation, used as an organic structure-directing agent in Beta synthesis, affects the average chemical shift of aluminium atoms in different T-sites. In H-Beta, octahedrally coordinated framework-associated aluminium atoms that could be quantitatively reverted into tetrahedral coordination were observed. The amount of this octahedral aluminium species decreases with increasing Si/Al ratio and it was absent for the two high-silica H-Beta samples with Si/Al = 110 and 215. Specific framework tetrahedral T-sites tend to convert to framework-associated octahedral sites during calcination. It is suggested that two aluminium T-sites, which are adjacent or close to each other, obeying the Loewenstein’s rule (i.e., no Al–O–Al linkage), are required for the hydrolysis of a Si–O–Al bond for the formation of octahedrally coordinated aluminium. The distribution of aluminium in zeolite Beta is a function of the Si/Al ratio and is non-uniformly distributed over the crystallographic T-sites.

Zeolite synthesis in the presence of flexible diquaternary alkylammonium ions (C2H5)3N+(CH2)nN+(C2H5)3 with n = 3-10 as structure-directing agents

Abstract The use of flexible diquaternary alkylammonium ions (C2H5)3N+(CH2)nN+(C2H5)3 (Et6-diquat-n with n=3–10) as structure-directing agents for zeolite synthesis in the presence of alkali metal cation is described. Among the organic structure-directing agents studied here, a considerable diversity in the phase selectivity was observed only for the Et6-diquat-5 ion: this cation can produce five different zeolite structures (i.e., P1, SSZ-16, SUZ-4, ZSM-57, and mordenite), depending on the oxide composition of synthesis mixtures. Analysis of the variable-temperature 1H CRAMPS NMR spectra obtained from the Et6-diquat-5 molecules in these five zeolites reveals that the host–guest interactions occurring within the respective materials maintain in a manner different from one another even at 160 °C at which the zeolite hosts crystallize.

Probing the non-random aluminum distribution in zeolite merlinoite with ultra-high-field (18.8 T) 27Al and 29Si MAS NMR

Abstract Two crystallographically distinct tetrahedral Al sites in zeolite merlinoite are directly identified by ultra-high-field (18.8 T) 27 Al MAS NMR spectroscopy. The experimental chemical shifts and peak areas indicate a preferential siting of Al in site TII. This non-random aluminum distribution is independently confirmed by ultra-high-field (18.8 T) 29 Si MAS NMR spectroscopy.

Stability of zeolite MCM-22 with varying Si al ratios; A solid-state NMR investigation

Determination of the local environment of aluminum in zeolites after various post-synthesis treatments is important for understanding structure-performance relationships. MCM-22 zeolites, synthesized with different silicon-to-aluminum ratios, were treated under various conditions: direct calcination in air, calcination in ammonia and oxygen, and ammonia treatment after calcination. The aluminum co-ordination in a zeolitic framework is highly sensitive to the treatments. The aluminum distribution over crystallographic T-sites in the zeolitic framework was followed by 27 Al MAS and 27 Al MQ-MAS NMR experiments. Loss of tetrahedrally co-ordinated aluminum on several T-sites and appearance of octahedrally coordinated aluminum was observed after calcination. Higher Si/Al ratios result in less octahedrally co-ordinated aluminum. This partial structural collapse can be restored by treatment with a base such as ammonia. An increased disorder in the framework is found after the conversion of octahedrally co-ordinated aluminum to tetrahedrally co-ordinated aluminum compared to the starting material.

Diquaternary (CH3)2(C2H5)N+ (CH2)nN+ (C2H5)(CH3)2 and (C2H5)2-(CH3)N+ (CH2)nN+ (CH3)(C2H5)2 ions with n=4-6 as structure-directing agents in zeolite synthesis

The use of fexible, linear diquaternary alkylammonium ions (CH3)2(C2H5)N+ (CH2)nN+-(C2H5) (CH3)2 (Me4Et2-diquat-n) and (C2H5)2(CH3)N+ (CH2)nN+(CH3)(C2H5)2 (Et4 Me2-diquat-n) with n=4–6 as structure-directing agents in zeolite syntheses is described. Among the ammonium ions studied here, the Et4Me2-diquat-4 and Et4Me2-diquat-6 ions were found to be new structure-directing agents for the crystallization of MCM-47 SSZ-16, respectively.

Structure of the novel large pore gallosilicate TNU-7

The structure of a novel large pore gallosilicate zeolite, TNU-7, has been refined from X-ray powder diffraction data. The silicate framework is built up from two different types of layer which are shown by X-ray and electron diffraction to be arranged in a fully ordered way. This gives rise to 12-MR pores and asymmetric cation distributions. TNU-7 is prepared hydrothermally over a very narrow compositional range and only in the presence of Ga. Other zeolitic phases crystallise at lower and higher Ga contents confirming the unique structure-directing ability of this element. The location of ion exchanged Cs+ and Sr2+ cations in TNU-7 has been studied by high resolution X-ray powder diffraction.

Physicochemical characteristics of Ti-PILC as a catalyst support for the reduction of NO by NH3

Abstract A pillared interlayered clay (PILC) intercalated by titania has been prepared as a catalyst support in an attempt to overcome the drawbacks of titania. The morphological, thermal, and surface properties of Ti-PILC have been particularly examined to use as a support for a NO SCR catalyst. The Ti-PILC prepared here was found to exhibit higher surface area and stronger acidity and thermal stability than the titania used as a common catalyst support.