Acidity as Descriptor for Methanol Desorption in B-, Ga- and Ti-MFI Zeotypes

Abstract

The isomorphous substitution of Si with metals other than Al in zeotype frameworks allows for tuning the acidity of the zeotype and, therefore, to tailor the catalyst’s properties as a function of the desired catalytic reaction. In this study, B, Ga, and Ti are incorporated in the MFI framework of silicalite samples and the following series of increasing acidity is observed: Ti-silicalite < B-silicalite < Ga-silicalite. It is also observed that the lower the acidity of the sample, the easier the methanol desorption from the zeotype surface. In the target reaction, namely the direct conversion of methane to methanol, methanol extraction is affected by the zeotype acidity. Therefore, the results shown in this study contribute to a more enriched knowledge of this reaction.

Isomorphous Substitution of Gallium into MFI-Framework Zeolite Increases 2,5-Dimethylfuran to Aromatics Selectivity and Suppresses Catalyst Deactivation

“…Ga-silicate crystals are cubic-shaped and have formed larger, connected intergrowths, suggesting the presence of mesopores [24]. In all samples, irregular structures were identified on the external facets, suggesting the presence of impurities or the potential migration of Al or Ga species during calcination [24]. In previous work, it was concluded that Ga was still inside the framework of Gasilicate using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [24].…”

“…Silicalite-1 crystals and ZSM-5(41) crystals are coffinshaped whereas ZSM-5 (11) contains both coffin-shaped and cubic-shaped crystals. Ga-silicate crystals are cubic-shaped and have formed larger, connected intergrowths, suggesting the presence of mesopores [24]. In all samples, irregular structures were identified on the external facets, suggesting the presence of impurities or the potential migration of Al or Ga species during calcination [24].…”

“…In all samples, irregular structures were identified on the external facets, suggesting the presence of impurities or the potential migration of Al or Ga species during calcination [24]. In previous work, it was concluded that Ga was still inside the framework of Gasilicate using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [24]. The vibration at 3616 cm −1 confirms the presence of Brønsted acid sites caused by the OH stretch vibration in the -Ga-O(H)-Si-species.…”

“…The following materials were used as catalysts: commercially available H-ZSM-5 (AkzoNobel, Si/Al=11, further noted as ZSM-5 (11) ), H-ZSM-5 (Si/Al=41, further noted as ZSM-5(41)), Silicalite-1 and Ga-silicate. The latter three were prepared as described elsewhere [24] following the method by Szotak et al [25]. In short, tetraethyl orthosilicate was hydrolyzed in water and oxalic acid (pH 2), followed by the addition of gallium nitrate hydrate or sodium aluminate to synthesize Ga-silicate or H-ZSM-5 respectively.…”

“…An accelerating voltage of 1.5 kV was used and backscattered electrons were measured using an SE2 detector. Additional in-depth characterization of the materials is available elsewhere for Silicalite-1 [26] and Ga-silicate [21,24]. apparent surface area by BET method, p∕p 0 = 0.002-0.03 for Si-1 and p∕p 0 = 0.002-0.05 for H-ZSM-5 and Ga-silicate following consistency criteria [27] b by t-plot method (Harkins and Jura equation, t = 5.0-8.0 Å).…”