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

Abstract

The performance of zeolite catalysts depends not only on the strength and number of Brønsted acid (or exchange) sites but also on synergistic effects derived from their proximity, in particular, and their distribution, in general. Little is known on the genesis of acid sites and site distributions in hydrothermal zeolite synthesis. By an extensive study of five crystallization systems yielding ZSM-5 (MFI) and SSZ-13 (CHA), with a focus on interzeolite conversion (IZC) methods, several synthesis factors and mechanisms that are key in determining the output acid site distribution have been identified. Key in this study were temporal synthesis profiles while probing the distribution and evolution of proximal acid sites with divalent cation capacity measurements. Over the course of different crystallizations, changing local charge distributions are detected, notably within crystalline materials upon prolonged exposure (maturation). Aluminum is clearly the key driver in IZC syntheses, from charge, dissolution, concentration, and mobility points of view. Quasigeneric principles for IZC syntheses are proposed, distinguishing between Al-loving and Al-averse systems, enabling a new degree of control over the acidity and ion-exchange properties of zeolites, of use to tailoring catalytic activity.

On the key role of aluminium and other heteroatoms during interzeolite conversion synthesis

“…127 A good argument in favour of non-selective heterogeneous nucleation can be found from our kinetic study of FAU-to-MFI and FAU-to-CHA (Si/Al P ¼ 40). 47 In this work, it was demonstrated that nucleation occurred swily around one hour of FAU-to-MFI vs. 16 hours for amorphous (non-IZC) counterparts. The same time is required for FAU-to-CHA in equimolar compositions, which is a more structuralsimilar transformation.…”

“…Some interesting studies are those reporting (changes in) element yields throughout the crystallization process. 47,65,93,107 Nishitoba et al present exemplary temporal results in their LTLto-CHA and FAU-to-CHA IZC (Si/Al P ¼ 3.0; Si/Al B ¼ 15, Fig. 6).…”

“…11, purple). 47 A gel-containing phase was encountered in both the IZC and the non-IZC syntheses before successfully achieving MFI (Si/Al ¼ 40, alkali-free) crystals, seemingly indicating that a (slower) gel-mediated mechanism is necessary to assemble MFI with a relatively high content of Al in alkali-free mixtures. The later ndings on Al-averse systems (alkalifree MFI) add additional arguments to the theory that different mechanisms prevail at different Al contents, 123 and support growth theories in IZC mixtures based on charge-balancing.…”

“…106 Very recently, some interesting high Si IZC studies have investigated the full course of crystallization, also encompassing the crucial dissolution step. 47,65,107,108 In 2019, our group published work on FAU-to-CHA (Si/Al P ¼ 40) using simply commercial US-Y (CBV780; Zeolyst) and N,N,N-trimethyl-1adamantammonium hydroxide (TMAda-OH) at alkali-free molar batch compositions. 65 Most recently, the investigation was broadened to study FAU-to-MFI under the same conditions, using tetrapropylammonium (TPA) as OSDA.…”

“…From the combined ndings of these four kinetically studied IZC systems, a kinetic growth model for IZC is proposed with some generic features. 47 In the latter it was motivated that IZC synthesis mixtures prosper particularly well in assembly conditions with preferential Al uptake (Al-loving), rather than in Al-averse ones (rather scarce). Aer longer synthesis times, the peculiar Al-averse system (FAUto-MFI, Si/Al P ¼ 40; OH/Si ¼ 0.35; no Na) reaches a maximum (100%) crystallinity and yield (48 h) (Fig.…”