Influence of Tetrapropylammonium and Ethylenediamine Structure-Directing Agents on the Framework Al Distribution in B–Al–MFI Zeolites

Abstract

We report synthetic protocols to independently influence composition and crystallite sizes of MFI zeolites, properties that contribute to Thiele moduli, and the proximity of Al heteroatoms at fixed composition (Si/Al ∼ 50). Crystallite sizes decrease with the addition of non-catalytic B heteroatoms in B–Al–MFI zeolites. Using only tetra-n-propylammonium (TPA+) as a structure-directing agent (SDA) leads to occlusion of one TPA+ per channel intersection and a finite percentage of framework Al atoms (20–40%) in proximal configurations. In contrast, using mixtures of ethylenediamine (EDA) and TPA+ (EDA/TPA+ = 15) leads to incorporation of EDA that displaces some TPA+ and to nearly all (>95%) Al in isolated configurations. These findings indicate how adding B and EDA to zeolite synthesis solutions provides a route to crystallizing MFI zeolites of similar composition but with framework Al distributed at varying relative proximity, or with similar Al distributions but different diffusion path lengths.

Analysis and control of acid sites in zeolites

“…In methanol to propene process, this Al-B-ZSM-5 sample showed higher propene selectivity, lower aromatics, and alkanes selectivity and longer catalyst lifetime than the materials prepared without the assistance of boron [304]. In the case of MFI synthesis systems where TPA and Si/Al ratio were kept constant (≈ 50) and B content varied, 20-40% of Al sites were paired, independently on the boron quantity added [305]. On the other hand, in analogous experiments where a mixture of tetrapropylammonium cations and ethylenediamine (EDA) is used as OSDA, the obtained B-Al-MFI zeolites contain less than 5% of Al in paired configurations, irrespectively of the Si/B ratio.…”

“…First-order rate constants of methanol dehydration to dimethylether over B-Al-MFI indicate acid sites are confined within smaller voids. Given this, the Al distribution within the zeolite framework is directed by the widespread impact of the organic molecules over the boron atoms present within the synthesis mixture [305]. The boron-containing zeolite Beta materials were prepared in the presence of seeds (Si/B = 4-38; Si/Al = 170-90).…”

Interzeolite Conversion and the Role of Aluminum: Toward Generic Principles of Acid Site Genesis and Distributions in ZSM-5 and SSZ-13

“…Most recent studies on short-range Al distributions have deduced 'thermodynamic' synthesis outcomes based on localized charge balances at the cage level between a positively charged (organic) structure-directing agent (O)SDA and the negatively charged framework from either aluminum or defect sites. [8][9][10][11][12][13][14][15] Furthermore, Al separation rules such as Löwenstein rule 16 or Dempsey's rule 17 and the intrinsic thermodynamic preference of Al for particular T-sites (framework dependent) 18 also seem to influence the short-range Al distribution in synthesized zeolites, despite the strong influence of kinetic processes during zeolite crystallization. 19,20 The use of alternative Si and Al sources −and other recipe alterations− may provoke Al distribution outcomes unexpected from a charge-balancing perspective.…”

Seed-Assisted Synthesis of ZSM-5 Aggregates Assembled from Regularly Stacked Nanosheets and Their Performance in n-Hexane Aromatization

“…Over the past few years, ZSM-5-based catalysts have been paid considerable attention in the petrochemical industry due to their excellent selectivity properties. , However, conventional ZSM-5 zeolite still suffers from the problem of diffusion limitation because of its sole and narrow pore structure. The diffusion limitation can be partially solved by reducing the path length through zeolite crystals .…”