Effects of Brønsted acid site proximity in chabazite zeolites on OH infrared spectra and protolytic propane cracking kinetics

Design and Synthesis of the Active Site Environment in Zeolite Catalysts for Selectively Manipulating Mechanistic Pathways

“…As shown in Figure a, this ratio is clearly the highest in the case of IWV-M and decreases in the order: IWV-M > IWV-D > FAU-CBV720 > FAU-CBV760 > MOR. Since the reaction mechanism in IWV-M and IWV-D is not different, and the acid sites are equally active, we hypothesized that the difference between IWV-M and IWV-D in DEB-Bz transalkylation should be caused by the accessibility of the bulky diaryl intermediates to the active sites, following recent work relating enhanced catalytic activity for some reactions with particular arrangements of Al atoms in the CHA framework. , …”

“…Since the reaction mechanism in IWV-M and IWV-D is not different, and the acid sites are equally active, we hypothesized that the difference between IWV-M and IWV-D in DEB-Bz transalkylation should be caused by the accessibility of the bulky diaryl intermediates to the active sites, following recent work relating enhanced catalytic activity for some reactions with particular arrangements of Al atoms in the CHA framework. 37,38 To clarify this point, new DFT calculations were performed in which the channels of the IWV structure were completely filled with the I1 + diaryl intermediate (see Figure 6b). In this situation, the central C atoms of two diaryl intermediates are separated by 6.9 Å, and the shortest C−C distance between them is 3.3 Å. Interestingly, the same spatial distribution is found for the mimic OSDA used in the synthesis of IWV-M, with the P atoms being separated by 8.9 Å and the shortest C− C distance between two OSDA molecules being 4.1 Å (Figure 6c).…”

Influence of Substrate Concentration on Kinetic Parameters of Ethanol Dehydration in MFI and CHA Zeolites and Relation of These Kinetic Parameters to Acid–Base Properties

“…Since the introduction of zeolites as acid catalysts, great efforts have been made to characterize the number, strength and availability of potentially active acid sites and to correlate these values with their catalytic activity. Many characterization methods have been developed to determine acid-base characteristics, and these methods are based on various theoretical methods (DFT [9,[11][12][13][14][15][16]) or experimental techniques (MAS NMR [13,14], Raman spectroscopy [17], FTIR spectroscopy [10,11,14,[17][18][19][20][21] or temperature programmed desorption (TPD) of probe molecules [15,16,20]). All of these methods share one common disadvantage: the assumption that the acid-base properties of the zeolite catalyst do not change under actual reaction conditions.…”

“…An appropriately selected probe reaction can then be used as a "pragmatic scale" of the acid-base properties of the zeolitic catalyst. Unfortunately, there is no one universal or generally accepted probe reaction and many reactions have been tested for this purpose-e.g., cracking of alkanes [11,13,22,23], dehydration of alcohols (methanol [4,9,24,25] or ethanol [14,[26][27][28][29]), and isotopic exchange reaction [30][31][32].…”

Altering the Arrangement of Framework Al Atoms in MEL Zeolites Using Mixtures of Tetrabutylammonium and Sodium Structure-Directing Agents

“…3,6 Kester et al reported first-order rate constants for monomolecular propane cracking (per H + , 748 K) that were higher (12×) at 6-MR paired Al sites than at 6-MR isolated sites in CHA. 2 The number of possible Al−Al site pair arrangements increases drastically with decreasing symmetry of the zeolite topology; however, DFT estimates of Co 2+ binding energies of different Al−Al site pairs can be used to identify likely binding sites for Co 2+ in lower symmetry frameworks. DFT estimates of Co 2+ binding energies for 202 of the 612 total possible Al−Al arrangements in a 96 T-atom MFI unit cell showed that arrangements of two Al in the 5-and 6-MRs of MFI zeolites are likely quantified by Co 2+ titration.…”