Ammonia Titration Methods To Quantify Brønsted Acid Sites in Zeolites Substituted with Aluminum and Boron Heteroatoms

Abstract

Ammonia titration methods were developed to discriminate and quantify Brønsted acid sites of different strength that compensate aluminum and boron heteroatoms incorporated within zeolite frameworks. Borosilicate and boroaluminosilicate MFI zeolites (B-Al-MFI) were synthesized with different Al contents and crystallite sizes, which are typically correlated structural properties in aluminosilicates synthesized hydrothermally, but independently varied here by incorporating boron as a second framework heteroatom and using ethylenediamine as a structure directing agent. Temperature-programmed desorption (TPD) of ammonia from B-Al-MFI samples saturated via liquid-phase NH4NO3 ion exchange resulted in quantifying the total number of Al and B heteroatoms. In contrast, TPD performed after NH4-form B-Al-MFI samples were purged in flowing helium (433 K), or after gas-phase NH3 adsorption (433 K) onto H-form B-Al-MFI samples, quantified only protons charge-compensating framework Al heteroatoms. Turnover rates for methanol dehydration to dimethyl ether, when measured in zero-order kinetic regimes that are sensitive predominantly to Brønsted acid strength, are dependent only on the number of protons compensating framework Al atoms in B-Al-MFI zeolites. The NH3 titration methods developed here are useful in rigorously normalizing turnover rates of Brønsted acid-catalyzed reactions in boroaluminosilicate zeolites, which have been recognized previously to be dependent solely on Al content. The incorporation of B heteroatoms into zeolite frameworks, which generate protons that are essentially unreactive, provides a strategy to influence crystallite sizes independently of Al content, especially relevant in cases where catalytic behavior is influenced by intracrystalline transport phenomena.

On the Acid Sites of Phosphorous Modified Zeosils

“…Alcohol dehydration has been extensively used as a probe reaction to characterize acidity in zeotype materials, 6,[35][36][37][38] and recent computational as well as experimental studies have led to detailed understanding of underlying reaction mechanisms and pathways of light (C1-C4) alcohol dehydration on solid acid catalysts. 37,[39][40][41][42][43][44][45][46][47] The ubiquitous themes tying these studies together are: (i) the evidence for the coupling of unimolecular and bimolecular dehydration pathways, where adsorbed dimers that produce di-alkyl ethers (bimolecular dehydration product) can also contribute to olefin formation (unimolecular dehydration product), 43,44 (ii) inhibition of olefin formation rates at high alcohol partial pressures (> 50 torr) due to the higher stability of adsorbed alcohol dimers relative to monomers, 37,39 and (iii) an increased preference to unimolecular dehydration with increasing temperatures.…”

On the Acid Sites of Phosphorous Modified Zeosils

“…Alcohol dehydration has been extensively used as a probe reaction to characterize acidity in zeotype materials, 6,[35][36][37][38] and recent computational as well as experimental studies have led to detailed understanding of underlying reaction mechanisms and pathways of light (C1-C4) alcohol dehydration on solid acid catalysts. 37,[39][40][41][42][43][44][45][46][47] The ubiquitous themes tying these studies together are: (i) the evidence for the coupling of unimolecular and bimolecular dehydration pathways, where adsorbed dimers that produce di-alkyl ethers (bimolecular dehydration product) can also contribute to olefin formation (unimolecular dehydration product), 43,44 (ii) inhibition of olefin formation rates at high alcohol partial pressures (> 50 torr) due to the higher stability of adsorbed alcohol dimers relative to monomers, 37,39 and (iii) an increased preference to unimolecular dehydration with increasing temperatures.…”

Synthesis and Characterization of ZSM-5 Zeolite Using Ethelinediammine as Organic Template: Via Hydrothermal Process

“…Synthesis of ZSM-5 can be carried out through single template methods using tetrapropylammoniumhidroxyde (TPAOH) as structure directing agent (SDA) (Lupulescu, et al, 2012). Moreover, in the some researches Ethylene diamine was used as structure directing additive while it was used also by some researchers as a template for borosilicate and boroaluminosilicate (Perego et al, 2003;Kester et al, 2018).…”