On the Role of Acidity in Bulk and Nanosheet [T]MFI (T=Al³⁺_, Ga³⁺, Fe³⁺, B³⁺) Zeolites in the Methanol‐to‐Hydrocarbons Reaction

Abstract

The influence of framework substituents (Al3+, Ga3+, Fe3+ and B3+) and morphology (bulk vs. nanometer‐sized sheets) of MFI zeolites on the acidity and catalytic performance in the methanol‐to‐hydrocarbons (MTH) reaction was investigated. The Brønsted acid density and strength decreased in the order Al(OH)Si>Ga(OH)Si>Fe(OH)Si≫B(OH)Si. Pyridine 15N NMR spectra confirmed the differences in the Brønsted and Lewis acid strengths but also provided evidence for site heterogeneity in the Brønsted acid sites. Owing to the lower efficiency with which tervalent ions can be inserted into the zeolite framework, sheet‐like zeolites exhibited lower acidity than bulk zeolites. The sheet‐like Al‐containing MFI zeolite exhibited the greatest longevity as a MTH catalyst, outperforming its bulk [Al]MFI counterpart. Although the lower acidity of bulk [Ga]MFI led to a better catalytic performance than bulk [Al]MFI, the sheet‐like [Ga]MFI sample was found to be nearly inactive owing to lower and heterogeneous Brønsted acidity. All Fe‐ and B‐substituted zeolite samples displayed very low catalytic performance owing to their weak acidity. Based on the product distribution, the MTH reaction was found to be dominated by the olefins‐based catalytic cycle. The small contribution of the aromatics‐based catalytic cycle was larger for bulk zeolite than for sheet‐like zeolite, indicating that shorter residence time of aromatics can explain the lower tendency toward coking and enhanced catalyst longevity.

Two-Dimensional Zeolite Materials: Structural and Acidity Properties

“…The quantitative analysis for fractions of extra-framework Al in these 2D MFI zeolites has been done by comparing the peak area of each species visible in the 27 Al MAS NMR spectra. As shown in Table 11, multilamellar MFI contains more extra-framework Al than pillared MFI, while the 3D MFI has much less extra-framework Al compared to these two derivatives [245,249]. Overall, the fraction of extra-framework Al is indicative of the defective nature of 2D MFI zeolites, which is expected to follow the order of unilamellar MFI > multilamellar MFI > pillared MFI > 3D MFI.…”

“…The results have shown that, in randomly stacked (i.e., unilamellar) MFI nanosheets, the Al element is found not only in tetrahedral environments but also in octahedral and penta-coordinated/distorted four-coordinated positions [246]. Multilamellar MFI and its pillared form contain tetrahedral framework Al and a small amount of extra-framework Al species [48,245,248,249]. The quantitative analysis for fractions of extra-framework Al in these 2D MFI zeolites has been done by comparing the peak area of each species visible in the 27 Al MAS NMR spectra.…”

“…Acidity measurements have been done on the unilamellar, multilamellar, and pillared MFI structures and have been compared to the acidity properties of conventional 3D MFI zeolite (Table 11). As seen in the characterizations for other 2D zeolites discussed above, the 27 Al MAS NMR technique has been used to examine the coordination environment of the Al element in 2D MFI zeolites [245][246][247][248][249]. The results have shown that, in randomly stacked (i.e., unilamellar) MFI nanosheets, the Al element is found not only in tetrahedral environments but also in octahedral and penta-coordinated/distorted four-coordinated positions [246].…”

“…The acid site type, quantity, and accessibility in 2D MFI zeolites have been measured by different techniques, including the FTIR of adsorbed small and bulky organic base molecules, organic base uptake measurement by GC/MS instruments, and 31 P MAS NMR spectra (Table 11). For example, the FTIR spectra of adsorbed pyridine, collidine, and DTBP molecules have shown that the 2D unilamellar and multilamellar MFI zeolites have a higher fraction of Lewis acidity than that of 3D MFI, while the accessibility of acid sites is improved due to the open structure [247][248][249][250]. The acid site accessibility of pillared MFI and SPP zeolites has been measured by direct DTBP uptake measurements, which maintain the characteristics of the high acid site accessibility in 2D MFI zeolites [173,245,251,252].…”

“…The acid site strength in 2D MFI has been measured by FTIR spectra of pyridine adsorption after desorption at different temperatures [247,249,253]. In comparison to 3D MFI zeolites, the unilamellar 2D MFI nanosheets have weaker acid strength.…”