Effect of the niobium state on the properties of NbSiBEA as bifunctional catalysts for gas- and liquid-phase tandem processes

Abstract

International audienceNbSiBEA zeolites contained isolated framework mononuclear Nb(V) (Nb0.7SiBEA) and a mixture of framework mononuclear and extra-framework polynuclear Nb(V) (Nb2.0SiBEA) were prepared by two-step postsynthesis method as evidenced by XRD, NMR, DR UV–vis and FTIR. DR UV–vis showed that two types of framework mononuclear Nb(V) are present in Nb0.7SiBEA, while Nb2.0SiBEA mainly contained isolated mononuclear Nb(V) in the framework of zeolite and polynuclear Nb(V) in the extra-framework position. FTIR with pyridine and 2,6-di-tert-butylpyridine as probe molecules showed that major amount Lewis and weak Brønsted acidic sites are formed by incorporation of niobium in the framework of zeolites as mononuclear Nb(V). The catalytic properties of Nb-containing zeolites were investigated in tandem processes of ethanol conversion into 1,3-butadiene (gas-phase) and synthesis of unsymmetrical ethers from aromatic aldehyde and aliphatic alcohol (liquid-phase). It has been found that Nb0.7SiBEA catalyst, containing only isolated framework mononuclear Nb(V) is more active than Nb2.0SiBEA in the conversion of ethanol and ethanol/acetaldehyde mixture into 1,3-butadiene, MPV reduction of crotonaldehyde with ethanol and etherification of 4-methoxybenzyl alcohol with 2-butanol. The higher specific activity (turnover number/frequency) of Nb0.7SiBEA than Nb2.0SiBEA catalyst has been revealed for gas- and liquid-phase tandem processes

Sn-BEA zeolites prepared by two-step postsynthesis method: Physicochemical properties and catalytic activity in processes based on MPV reduction

“…4), due to the protons of H-bonded Si-OH groups present at vacant T-atom sites and isolated Si-OH groups respectively, as earlier shown for different zeolites [20,21]. In addition, two small peaks at 7.7 and 3.2 ppm appeared probably due to protons of water molecules and of H-bonded Si-OH groups located in a second type of crystallographic site, in line with earlier work [19].All above-mentioned peaks are also well visible in 1 H MAS NMR spectra of the Sn 0.5 SiBEA and Sn 2.0 SiBEA samples (Fig. 4).…”

“…2) are type I according to IUPAC. BET surface area in the range of 430-467 m 2 g −1 and micropore volume of 0.18-0.20 cm 3 g −1 (Table 1) are typical values reported for dealuminated BEA zeolites [19]. Higher surface area obtained by Langmuir and NLDFT models (e.g., 707 and 728 m 2 g −1 for Sn 0.5 SiBEA) may indicate that BET method applied on isotherms of microporous Sn x SiBEA zeolites in "standard" relative pressure range underestimates the true internal surface area.…”

“…The similar specific surface areas and crystallinity for SiBEA [19] and Sn x SiBEA as well as absence of extra-framework crystalline compounds or long-range amorphization indicate that tin is incorporated into the zeolite matrix as well dispersed species.…”

Recent Breakthroughs in the Conversion of Ethanol to Butadiene

“…However, several alternatives have been recently investigated. Originally evaluated together with zirconium decades ago, tantalum and niobium oxide have been revisited by Kyriienko et al, this time supported on zeolite BEA [38,39,48,67]. Both were found to lack the redox properties required to be active in the one-step process, but were reasonably active in the two-step process.…”

“…Ivanova et al have reiterated this claim, adding that niobium-amongst other metals-could be used to produce butadiene from ethanol when properly promoted [53]. Kyriienko et al have investigated niobium-modified zeolite BEA for the ETB reaction [39]. This was done in the context of a study on the effects of the metal's incorporation with the zeolite framework state on a gas-and liquid-phase tandem process.…”