Characterization of Gallosilicate MFI-Type Zeolites by IR Spectroscopy of Adsorbed Probe Molecules

Abstract

Three MFI-type, H-GaZSM-5 gallosilicates were synthesized with Si/Ga ratios of 25, 50, and 75, respectively. Incorporation of gallium into the zeolite framework was checked by powder X-ray diffraction, which showed an approximately linear variation of d spacings with Si/Ga ratio. Fourier transform IR spectroscopy of CO and N 2 , adsorbed at liquid nitrogen temperature, was used to characterize these gallosilicates after activation in vacuo at temperatures from 673 to 973 K. It was found that both probe molecules formed stable adducts with polarizing centers having either Brönsted or Lewis acid character. On samples activated at a temperature smaller than 700 K Brönsted sites were found to be predominant; apart from regular Si(OH)Ga sites, adsorbed carbon monoxide also revealed the presence of a small fraction of Brönsted acid groups associated with nonregular sites. These could be located either at the mouth of small cavities (presumably associated with defective regions of the zeolite framework where gallium was segregated) or on amorphous regions. On the sample with Si/Ga ) 25 activated at temperatures higher than 700 K, gallium showed a marked tendency to go into either partial or total extraframework positions, thus generating coordinatively unsaturated Ga 3+ centers with strong Lewis acid character. These centers were revealed by formation of CO and N 2 adducts where stretching frequencies of the adsorbed molecules showed a strong hypsochromic shift, ∆ν. Values of ∆ν as high as 82 and 25 cm -1 were found for CO and N 2 , respectively. In the case of dinitrogen, interaction with Lewis acid centers also brings about a pronounced increase of IR band intensity, and this effect can advantageously be used to monitor extraframework species.

Catalytic conversion of furanic compounds over Ga-modified ZSM-5 zeolites as a route to biomass-derived aromatics

“…Thus, adsorption of CO on re-oxidized catalysts leads to formation of two bands at 2174 and 2140 cm -1 corresponding to CO complexes with framework hydroxyls and physically adsorbed CO, respectively. [33][34][35] Pyridine as a probe was used to quantitatively determine the amount of BAS and LAS (spectra in Figure S1a.). Table 1 shows that the amount of BAS decreases with the Ga content, implying that ion-exchange/calcination/reduction treatment leads to replacement of protons by cationic Ga species.…”

“…Driven by societal and economic concerns related to the use of fossil resources, there is growing attention for their replacement by renewable carbon sources. 12 The conversion of whole lignocellulosic biomass or its constituents -cellulose, hemi-cellulose 34 , and lignin 5 -into sustainable fuels and chemicals is expected to become increasingly important for the chemical industry. Common approaches involve upgrading of oils derived from biomass for instance by pyrolysis 6 or of synthesis gas obtained from biomass gasification.…”

Supramolecular Organization in Confined Nanospaces

“…Zeolites where atoms other than Si and Al occupy tetrahedral sites (zeotypes) are important for catalysis: [58] for example, the acid strength of proton-containing zeolites [155,[225][226][227][228][229][230] can be tuned by replacing Al with Ga [231][232][233][234] or B. [235] For instance, boron zeolites are less acidic than Al-ones due to the tendency of boron to be trigonal.…”

Evidence for controlled insertion of Fe ions in the framework of clinoptilolite natural zeolites

“…The measured shifts (297 cm À1 for Fe 2+ -OPAZ and 281 cm À1 for Fe 3+ -OPAZ) testify of the Brønsted acidity of the involved OH groups, in agreement with those observed for bridging hydroxyl groups Si(OH)T (T = Al, Fe, Ga, etc.) in synthetic zeolites or aluminophosphates [43][44][45][46][47]. This evidence demonstrates the reinsertion of Fe 3+ ions in zeolite framework positions as a consequence of the hydrothermal treatments, irrespective of the initial oxidation state of the iron salts.…”

“…More interesting is the appearance, in Fe 2+ -and Fe 3+ -OPAZ samples, of a well-defined band at 3619 cm À1 , which can be assigned to bridging hydroxyl groups Si(OH)Fe, related to framework iron sites [21,44,45]. Upon CO dosage at 110 K (dashed lines and difference curves in Fig.…”