Porosity and accessibility of acid sites in desilicated ZSM-5 zeolites studied using adsorption of probe molecules

Mlekodaj, Kinga; Tarach, Karolina A.; Dátka, Jerzy; Góra‐Marek, Kinga; Makowski, Wacław

doi:10.1016/j.micromeso.2013.08.051

Cited by 78 publications

(68 citation statements)

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“…For the calculations, a value of 0.62 cm mmol -1 was used for ε. [47] The data in table 5 clearly show that in the hierarchical materials (80H and 30H) the number of the accessible Brønsted sites accessible by molecules as bulky as 2,4,6-TMP is greatly increased with respect to the parent zeolites. Furthermore, it is particularly noteworthy that in the hierarchical materials, the 80H catalyst possesses a significantly greater fraction of accessible Brønsted sites.…”

Section: Resultsmentioning

confidence: 95%

“…Substituted pyridines have been widely used to study and quantify the enhanced accessibility of acid sites in hierarchical zeolites, given their impediment to diffuse through small micropores. [26,[45][46][47] . In particular, 2,4,6-trimethilpyridine (2,4,6-TMP) with a kinetic diameter ca.…”

Section: Resultsmentioning

confidence: 99%

“…In addition, due to its basicity (pKa = 7.59), when the molecule interacts with the Brønsted acid sites in zeolites, a proton transfer occurs and a protonated species (2,4,6-TMPH + ) is formed. In Scheme 1, the IR active modes of the ring both for liquid and adsorbed 2,4,6-TMP on different OH sites and on Lewis acid centres are highlighted, [46][47][48][49] In particular, the ν8a mode, which in the liquid phase appears at 1611 cm -1 , is very sensitive to the acidity of the OH sites present on the catalysts and its position changes with respect to the strength of the acid centres. In fact, when the ν8a appears at wavenumber higher than 1630 cm -1 , this can be associated with the formation of a protonated species (2,4,6-TMPH + ), whereas lower wavenumbers correspond to H-bonded adducts.…”

Section: Resultsmentioning

confidence: 99%

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Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

et al. 2016

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A versatile desilication design strategy for the creation of hierarchical H-ZSM-5 catalysts with different Si/Al ratio has been demonstrated. The nature, strength and the accessibility of the acid sites after the alkaline treatment was elucidated by employing a range of physico-chemical characterization tools; notably probebased FTIR spectroscopy along with SS MAS-NMR. In addition, structural and textural properties of the hierarchical zeolites were also explored and compared to their corresponding microporous analogues. CO was used to probe the acidic properties of the hierarchical zeolites with the concomitant deployment of a bulky molecular probe, 2,4,6-trimethylpyridine (collidine), which is too large to access the micropores, to specifically investigate the enhanced accessibility of the active sites. The hierarchical zeolites were evaluated in the industrially relevant, acid-catalyzed Beckmann rearrangement of cyclohexanone oxime to -caprolactam, the precursor for Nylon-6, in liquid phase and at low-temperatures. The catalytic findings with the hierarchical catalysts reveal a significant enhancement in the production of -caprolactam, compared with the parent microporous H-ZSM-5 zeolite, thereby highlighting the merits of our design approach in facilitating enhanced diffusion and masstransfer.

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Section: Resultsmentioning

confidence: 95%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

et al. 2016

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“…by elemental analysis. Mlekodaj et al 36 dened an accessibility factor (AF). The AF for collidine interacting with Brønsted sites was dened by the number of collidine molecules adsorbed to Brønsted sites (external surface and pore mouth) divided by the total number probed by pyridine adsorption.…”

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confidence: 99%

Effect of Al content on number and location of hydroxyl acid species in zeolites: a DRIFTS quantitative protocol without the need for molar extinction coefficients

Bräuer

Situmorang

et al. 2017

RSC Adv.

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In this work, we have systematically investigated the effect of the Al content on the number and location of bridging hydroxyls (Brønsted acid sites) and terminal hydroxyls (terminal silanol sites) in ZSM-5 zeolites with varying silica-alumina ratio (SAR ¼ SiO 2 /Al 2 O 3 ) using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) without molar extinction coefficients. Two base probe molecules with different kinetic diameters, pyridine (5.7Å) and collidine (7.4Å), were used. The total content of Brønsted acid sites is obtained by elemental analysis of the proton form and sodium exchanged form of ZSM-5. To quantify the number of internal and external Brønsted acid sites the change in area of the Brønsted acid site peak in the O-H stretching region is then used; thus, the need for molar extinction coefficients to quantify Brønsted acid sites is thereby eliminated. The results reported here show that for ZSM-5 the number of external Brønsted sites, in absolute terms, increases with increasing total Al content, as evidenced by collidine adsorption. The number of terminal silanol acid sites was found to be proportional to the external surface area probed by argon sorption experiments. In summary, this work shows that the use of DRIFTS in conjunction with elemental analysis is a valid alternative to qualitatively and quantitatively probe the number and location of hydroxyl acid sites without the use of molar extinction coefficients and gives a comprehensive picture of the effect of Al content on number and location of hydroxyl acid species in ZSM-5 zeolites with varying SAR. The work contributes to advancing the range of characterisation protocols for solid acid catalysts.

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“…The addition of TPAOH or TBAOH to NaOH solution protects the framework of high Silica ZSM-5 against excessive desilication. The role of amines is to bind to the silica surface controlling the extension of the desilication and also the size of mesopores [68,96,[99][100][101][102]]. …”

Section: 53hierarchical Zeolitesmentioning

confidence: 99%

New micro and mesoporous materials for the reaction of methanol to olefins

Li¹

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i ResumenLa transformación de metanol en olefinas empleando catalizadores ha atraí do el interés de la comunidad cientí fica debido a las amplias reservas de gas natural. El metanol se obtiene a partir del gas natural a través del gas de sí ntesis y también a partir de biomasa mediante gasificación, por lo que representa una alternativa frente a otras fuentes fósiles de energí a como el petróleo y el carbón. En esta tesis se han estudiado nuevos materiales catalí ticos para la reacción de metanol en olefinas optimizando las propiedades de los que actualmente se emplean de manera comercial y proponiendo nuevos materiales alternativos. Los catalizadores empleados actualmente están basados en zeolitas que son materiales microporosos de alta área especí fica pero presentan como limitaciones una vida corta antes de la desactivación por coque, o una baja estabilidad frente a la presencia de agua en el medio de reacción. El aumento del área especí fica a través de la disminución del tamaño de cristal o de la mesoporosidad mejorará las propiedades de difusión del catalizador.El silicoaluminofosfato SAPO-34 es empleado comercialmente en la actualidad.En esta tesis se ha optimizado la sí ntesis de este material disminuyendo drásticamente el tamaño de cristal y mejorando la estabilidad frente a la humedad ambiente. La disminución del tamaño de cristal ha aumentado la vida del catalizador frente a la deposición de coque y ha permitido relacionar la selectividad a productos con la distribución de silicio en la partí cula, distribución que es variable al contacto con la humedad ambiente. La estabilidad frente a la humedad ha sido mejorada mediante el tratamiento con vapor post-sí ntesis y también se ha estudiado el efecto de la eliminación del agente orgánico director de estructura en atmósfera reductora de hidrógeno y agua. De esta manera se han sintetizado muestras con mayor tiempo de vida en la reacción de metanol. El efecto de la reducción del tamaño de cristal se ha estudiado también para la zeolita SSZ-13, isomorfa con SAPO-34, mediante la adición de agentes surfactantes durante la sí ntesis y también se ha probado cómo esta reducción de tamaño produce el aumento el tiempo de vida del catalizador.ii Por último se ha optimizado el tiempo de vida y la estabilidad hidrotérmica de la zeolita ZSM-5, también usada comercialmente como componente activo del catalizador del proceso de metanol a propileno. Se ha estudiado el efecto del aumento de la mesoporosidad mediante tratamientos básicos/ácidos en las propiedades catalí ticas y se ha comprobado cómo la mesoporosidad aumenta el tiempo de vida pero afecta a la estabilidad hidrotérmica del catalizador. En la parte final se ha optimizado la estabilidad hidrotérmica de ZSM-5 mesoporosa mediante la adición de fósforo. El estudio ha permitido comprobar que la actividad del catalizador basado el ZSM-5 mesoporosa se puede preservar mediante la mejora de la estabilidad hidrotérmica, incluyendo fósforo en la composición del catalizador.iii Resum La transformació de metanol en olefines ...

show abstract

Porosity and accessibility of acid sites in desilicated ZSM-5 zeolites studied using adsorption of probe molecules

Cited by 78 publications

References 36 publications

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

Creating Accessible Active Sites in Hierarchical MFI Zeolites for Low‐Temperature Acid Catalysis

Effect of Al content on number and location of hydroxyl acid species in zeolites: a DRIFTS quantitative protocol without the need for molar extinction coefficients

New micro and mesoporous materials for the reaction of methanol to olefins

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