On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Ravi, Manoj; Sushkevich, Vitaly L.; Bokhoven, Jeroen A. van

doi:10.1039/d0sc06130a

Cited by 85 publications

(74 citation statements)

References 68 publications

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“…Moreover, EFAl species should not be present in typical commercial H-ZSM-5 catalysts prior to any hydrothermal treatments based on the results above. Of course, partially coordinated framework Al(IV)-2 can serve as precursors to hydrothermal evolution of EFAl species under high-temperature reaction conditions, as discussed recently by Bokhoven, but examination of past literature in view of our detailed results suggests that Al(IV)-2 and its OH groups can contribute to catalysis through increased rates for probe reactions . Further, it appears likely that Al(IV)-2 species can play a role as sites for phosphate tethering to the catalyst in phosphorus-based modifications of H-ZSM-5 catalysts, which are commercially practiced and designed to increase selectivity and hydrothermal stability. , The detailed experiments described here show that AHFS washing can remove Al(IV)-2 species while leaving Al(IV)-1 species intact, that mild steaming can reintroduce Al(IV)-2 without generating significant amounts of EFAls, and that severe steaming leads to both Al(IV)-2 and EFAl species like Al(V) and Al(VI).…”

Section: Resultsmentioning

confidence: 55%

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

et al. 2021

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The structure of aluminum-containing moieties in and within zeolite H-ZSM-5 catalysts is a complex function of the elemental composition of the catalyst, synthesis conditions, exposure to moisture, and thermal history. 27Al NMR data collected at field strengths ranging from 7.05 to 35.2 T, i.e., 1H Larmor frequencies from 300 to 1500 MHz, reveal that Al primarily exists as framework or partially coordinated framework species in commercially available dehydrated H-ZSM-5 catalysts with Si/Al ranging from 11.5 to 40. Quantitative direct-excitation and sensitivity-enhanced 27Al NMR techniques applied over the wide range of magnetic field strengths used in this study show that prior to significant hydrothermal exposure, detectable amounts of nonframework Al species do not exist. Two-dimensional 27Al multiple-quantum magic-angle spinning (MQMAS) along with 1H–27Al and 29Si–27Al dipolar correlation (D-HMQC) NMR experiments confirm this conclusion and show that generation of nonframework species following varying severities of hydrothermal exposure are clearly resolved from partially coordinated framework sites. The impact of hydration on the appearance and interpretation of conventional direct-excitation 27Al spectra, commonly used to assess framework and nonframework Al, is discussed. Aluminum sites in dehydrated catalysts, which are representative of typical operating conditions, are characterized by large quadrupole interactions and are best assigned by obtaining data at multiple field strengths. On the basis of the results here, an accurate initial assessment of Al sites in high-Al content MFI catalysts prior to any hydrothermal treatment can be used to guide reaction conditions, anticipate potential water impacts, and identify contributions from hydroxyl groups other than those associated with the framework bridging acid site.

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Section: Resultsmentioning

confidence: 55%

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

et al. 2021

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“…Finally, the chemical shift at 0 ppm was ascribed to octahedrally coordinated extra‐framework Al [79] . The latter have Lewis‐type acidity [80] and can be associated to the amorphous agglomerates observed during SEM examination, Figure 3 [78] . One possible explanation for the presence of this species could be that the aluminum and silicon precursors did not reacted completely within the conditions of the sampling space studied herein [48] …”

Section: Resultsmentioning

confidence: 93%

Key Factors During the Milling Stage of the Seed‐assisted and Solvent‐free Synthesis of MFI and Catalytic Behavior in the Alkylation of Phenol with Tert‐butyl Alcohol

et al. 2021

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The so-called mechanochemical method for the synthesis of zeolites reduces the generation of liquid residues and gaseous pollutants as compared to the conventional solvothermal method. Different types of zeolites have been synthesized at the laboratory scale with this method using mostly pestle and mortar. However, such an approach hinders both the systematic comprehension of the effects of the input variables of the milling process and its further scale-up towards the synthesis of the zeolites and their catalytic application. This work investigates the influence of key factors involved in the ball milling stage of the mechanochemical route for the synthesis of MFI done with the assistance of a commercial MFI seed and in the absence of solvents over the most salient physicochemical properties of this type of materials, i.e. the recovery percentage, production cost, morphology, surface area and porosity, crystallinity, acidity of the protonated MFI and catalytic performance. The synthesis of the materials was planned and executed following a full 2 4 factorial experiment whose input variables were the Na2O/SiO2 and H2O/SiO2 molar ratios and the milling time and speed. The effects of both main and interaction factors over key physicochemical properties, and catalytic behavior of the synthesized materials on the alkylation of phenol with tert-butyl alcohol were established within the explored sampling space. Results showed that the Na2O/SiO2 molar ratio plays a key role for the mechanochemical synthesis of MFI, since this variable may direct the synthesis to the preferential production of MOR instead of MFI. On the other hand, it was found that the milling time and speed and their interactions markedly impact the textural properties of MFI. Furthermore, the triple interaction between the input variables affected the concentration of Lewis acid sites of the produced materials. These effects were rationalized by considering that sodium can act as a structure directing agent during the mechanochemical synthesis of MFI and also can promote the incorporation of aluminum to its structure. On the other hand, the milling time and speed are non-linearly correlated to the milling energy required for forming the aluminosilicate precursor that crystallizes during the hydrothermal stage of the process. Overall, all the zeolites synthesized by the mechanochemical route were less crystalline than both the MFI used as seed and an MFI synthesized by solgel. This was associated to the formation of amorphous agglomerates around the zeolitic crystals. Finally, the catalytic behavior of the mechanochemical MFI zeolites in the studied reaction was found to be linearly and positively correlated with both the concentration of BrØnsted of sites and with the density of acid sites. The catalytic tendencies were consistent with the proposal of a stepwise Langmuir-Hinshelwood mechanism for the alkylation of phenol with tert-butyl alcohol.

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“…After absorbing TMPO molecules,the invisible coordinated Al 3+ cations become detectable Al IV species. This is because the interaction between TMPO and Al 3+ cation results in a reduced C QC C, which is originated from the formation of flexible coordination and the increasing in local symmetry (Figure d) . For Al/h-ZSM-5(50), three different species can be observed in the 27 Al MQ MAS NMR spectra (Figure b).…”

Section: Resultsmentioning

confidence: 97%

“…This is because the interaction between TMPO and Al 3+ cation results in a reduced C QC C, which is originated from the formation of flexible coordination and the increasing in local symmetry (Figure 3d). 42 For Al/h-ZSM-5 (50), three different species can be observed in the 27 Al MQ MAS NMR spectra (Figure 4b). The distortion of the signal contours along the F2 dimension is attributed to the second-order quadrupolar interaction.…”

Section: Resultsmentioning

confidence: 99%

Synergy of Extraframework Al³⁺ Cations and Brønsted Acid Sites on Hierarchical ZSM-5 Zeolites for Butanol-to-Olefin Conversion

et al. 2021

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ZSM-5 zeolite catalyzed dehydration of biobutanol has been widely used for the sustainable production of butene. Brønsted acid sites (BASs) in zeolites usually act as catalytically active sites. Lewis acid sites (LASs) are also active for alcohol dehydration. Therefore, extraframework aluminum species have been introduced to zeolites as LASs. Tricoordinated Al species are the strongest LASs, which can enhance the acidity of adjacent surface BASs. Here, we combined solid-state nuclear magnetic resonance (NMR) spectroscopy and in situ diffuse reflectance infrared spectroscopy (DRIFTS) to investigate the local structures and acidity of hierarchical ZSM-5 zeolites with/without the introduction of Al3+ cations as well as their correlations with the catalytic performance in biobutanol dehydration. 27Al magic angle spinning (MAS) NMR, 27Al multiple-quantum (MQ) MAS NMR, and 31P MAS NMR after loading trimethylphosphine oxide (TMPO) probe molecules showed that the tricoordinated Al species are dominant acid sites after introducing Al3+ cations into pure silica hierarchical ZSM-5 zeolites. For BASs-rich hierarchical ZSM-5 zeolites (SiO2/Al2O3 = 50), the Al3+ cations tend to stay in proximity to the bridging Si–OH–Al and thereby strongly enhance the acidic strength of BASs via the synergy of LASs and BASs. Contributing from the synergy effect, the ultrastrong BAS has been formed, which gives an obvious improvement in the biobutanol conversion and butene selectivity. In situ DRIFTS showed that tricoordinated Al3+ cations cooperating with BASs can promote the formation of butoxy intermediates at a low reaction temperature, which further improves the butanol dehydration.

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On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Abstract: Framework-associated aluminum is demonstrated to facilitate a reversible switch between Lewis and Brønsted acidity in zeolites with the Lewis acid sites preferentially populating the side-pockets in the case of mordenite.

Cited by 85 publications

References 68 publications

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Key Factors During the Milling Stage of the Seed‐assisted and Solvent‐free Synthesis of MFI and Catalytic Behavior in the Alkylation of Phenol with Tert‐butyl Alcohol

Synergy of Extraframework Al³⁺ Cations and Brønsted Acid Sites on Hierarchical ZSM-5 Zeolites for Butanol-to-Olefin Conversion

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On the location of Lewis acidic aluminum in zeolite mordenite and the role of framework-associated aluminum in mediating the switch between Brønsted and Lewis acidity

Abstract: Framework-associated aluminum is demonstrated to facilitate a reversible switch between Lewis and Brønsted acidity in zeolites with the Lewis acid sites preferentially populating the side-pockets in the case of mordenite.

Cited by 85 publications

References 68 publications

Distribution of Aluminum Species in Zeolite Catalysts: 27Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Distribution of Aluminum Species in Zeolite Catalysts: 27Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Key Factors During the Milling Stage of the Seed‐assisted and Solvent‐free Synthesis of MFI and Catalytic Behavior in the Alkylation of Phenol with Tert‐butyl Alcohol

Synergy of Extraframework Al3+ Cations and Brønsted Acid Sites on Hierarchical ZSM-5 Zeolites for Butanol-to-Olefin Conversion

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Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Distribution of Aluminum Species in Zeolite Catalysts: ²⁷Al NMR of Framework, Partially-Coordinated Framework, and Non-Framework Moieties

Synergy of Extraframework Al³⁺ Cations and Brønsted Acid Sites on Hierarchical ZSM-5 Zeolites for Butanol-to-Olefin Conversion