Effect of Stannic Species Modification on the Acidity of Silicalite-1 and Its Enhancement in Transforming Ethylenediamine to Heterocyclic Amines

Abstract

In this study, a series of SnO 2 modified zeolite catalysts (Snx-S-1; x is the weight percentage of Sn) were prepared with SnCl 2 and a defective Silicalite-1 (S-1) zeolite via facile deposition-precipitation method. It was found that the stannic species modified all-silica zeolite catalysts were active for the intermolecular condensation of ethylenediamine (EDA) to 1, 2-Diazabicyclo [2, 2, 2] octane (TEDA) and piperazine (PIP). The best catalyst Sn6-S-1 (6 wt.% Sn loading) showed 86% EDA conversion and 93% total selectivity to TEDA and PIP. By contrast, the defective S-1 zeolite parent showed only approximately 9% EDA conversion under the same conditions. With the help of catalyst characterization techniques including hydroxyl vibration and pyridine adsorption FT-IR spectroscopy (transmission mode), the enhancement of the catalytic activity of the SnO 2 modified zeolite catalysts (Snx-S-1) was mainly attributed to the formation of mild Lewis acid sites in the siliceous zeolite. Both the hydroxyl nests of the defective S-1 zeolite and the dispersed SnO 2 clusters should be the important factors for the formation of mild Lewis acid sites on the modified zeolite. Based on the catalytic performance of the modified zeolite in the conversion of EDA to PIP and TEDA, it is inferred that the mildly acidified defective S-1 zeolite by the SnO 2 deposition modification might become a very active and durable catalyst for reactions involving strongly alkaline reactants and products.Catalysts 2020, 10, 211 2 of 16 hydroxyl groups) of H-ZSM-5 zeolite. For this reason, some modification methods such as alkali metal ion modification [7] and dealumination [9], etc. have been proposed to weaken the strength of acidic sites in H-ZSM-5. However, such efforts often cause negative effects such as worse diffusivity due to pore channel blocking. In 2003, titanium silicalite-1 (TS-1) was patented for the reaction of EDA to TEDA and PIP [10]. Later, people conducted a detailed study on the intermolecular condensation of EDA to make PIP and TEDA over TS-1 zeolite [11]. Silicalite-1 (S-1) zeolite is an aluminum-free crystalline silicate with the same MFI topological framework as the Al-containing ZSM-5 zeolite and Ti-containing silicate zeolite (TS-1). It is well known that the presence of defective sites in S-1 lattice can form mild acidity (silanol nests). The defective S-1 zeolite has been reported as an active Beckmann rearrangement catalyst [12][13][14][15]. However, previous study indicated that the defective S-1 zeolite itself was not very active and selective for the reaction of EDA to TEDA and PIP, giving only 19.5% EDA conversion and 7.5% TEDA selectivity [11]. This implies that the acidity of defective S-1 is too weak to be an efficient catalyst for the conversion of EDA.In recent years, Sn-β zeolite has attracted increasing attention because its isolated framework Sn is an active Lewis acid site for many reactions, including Baeyer-Villiger oxidation, sugar isomerization, and Meerwein-Ponndorf-Verley reaction [16,17]. Sn-MFI zeol...

Silanol defect engineering and healing in zeolites: opportunities to fine-tune their properties and performances

“…The silanol nests present in a defective silicalite-1 were found to provide acidity for ethylenediamine (EDA) conversion, meanwhile this acidity could not ensure alone a high conversion. 175,176 In the case of the MFI zeolite containing Ti (TS-1), silanol groups in the vicinity of Ti sites were pointed as the ones responsible by the EDA condensation, rather than the Lewis acid sites from Ti(V). The higher acidity of silanols in TS-1 compared to silicalite-1 was related to an electronic effect or structural deformation generated by the Ti atom, as in Fig.…”

Understanding the Role of Fe Doping in Tuning the Size and Dispersion of GaN Nanocrystallites for CO2-Assisted Oxidative Dehydrogenation of Propane

“…51−55 Additionally, a weak band at 3675 cm −1 corresponding to weakly hydrogen-bonded vicinal silanols was also discernible. 56,57 After the doping of Fe atoms, the broad band with the maximum at 3438 cm −1 increased prominently, whereas the weak band at 3675 cm −1 was almost unaltered. The absorption band at 3726 cm −1 was also enhanced for 500-Fe-silicalite-1 and 100-Fe-silicalite-1, but it was inconspicuous for 2000-Fesilicalite-1 and 1000-Fe-silicalite-1.…”

“…The IR spectra under a vacuum via outgassing (Figure B) showed that there were two characteristic absorption bands with maxima at 3726 and 3438 cm –1 assigned to the terminal silanols located within the interior surface of zeolite and hydrogen-bonded groups of silanol nests, respectively. − Additionally, a weak band at 3675 cm –1 corresponding to weakly hydrogen-bonded vicinal silanols was also discernible. , After the doping of Fe atoms, the broad band with the maximum at 3438 cm –1 increased prominently, whereas the weak band at 3675 cm –1 was almost unaltered. The absorption band at 3726 cm –1 was also enhanced for 500-Fe-silicalite-1 and 100-Fe-silicalite-1, but it was inconspicuous for 2000-Fe-silicalite-1 and 1000-Fe-silicalite-1.…”