Desilicated ZSM‐5 Catalysts: Properties and Ethanol to Aromatics (ETA) Performance

Abstract

Herein, desilication in increasingly harsh conditions was used to introduce mesopores into two different industrial ZSM‐5 catalysts (Si/Al ratio 11 or 29). For desilicated samples, increasing BET surface areas, mesopore volumes, and Si(OH) densities were noted. Brønsted acid site (BAS) densities increased upon desilication, as formerly inaccessible BAS in blocked pores became available, while the strength of the BAS was maintained upon desilication. Using KOH instead of NaOH as desilication agent can increase the mesopore volume generated per mass loss. The correlations between desilication strength and properties were largely determined by the parent Si/Al ratio. In general the introduced mesopores increased lifetimes in the ETA conversion, while additional Si(OH) groups introduced by desilication reduce the lifetime again. The lifetime is thus determined by a complex interplay of BAS density, improved reactant transport by introduced mesopores and Si(OH) density. There were no additional aromatics formed in desilicated samples during the conversion of ethanol and the samples were, in terms of aromatic yield, outperformed by a microporous parent. However, as result of longer lifetimes less ethanol was lost due to coke formation. It is concluded that desilication should be combined with other post‐modifications to increase aromatic production and lifetime.

Influence of ZSM-5 Crystal Size on Methanol-to-Olefin (MTO) vs. Ethanol-to-Aromatics (ETA) Conversion

“…The results presented therein can, for example, be understood if we remember that the authors found mesopores on their catalysts. It was in that recently shown respect that mesoporosity can lead to an increased selectivity to aromatics in the ETA conversion [14]. When it comes to S60/60 in the ETA conversion, herein, the ratio between the different products is not significantly changed, the final coke contents are similar, and the calculated mechanistic descriptors, C 3= /C 3 ratio and HTI, of S60/2 and S60/60 are almost identical.…”

“…This leads to a longer residence time inside larger particles and a changed selectivity during MTO conversion. This explains why post-modifications that enhance the coke resistance of the ZSM-5 catalyst, like mesopore formation, are very promising strategies to increase the lifetime of ETA conversion catalysts [14,18]. Thus, it was herein clarified how one parameter, ZSM-5 crystal size, impacts the ETA product distribution.…”

“…This renders it a potential future platform chemical for the chemical industry [11][12][13]. In particular, ethanol is a promising feed for hydrocarbon fuel and aromatic formation via the ethanol-to-aromatics (ETA) conversion route [14][15][16][17][18]. The fundamental difference between methanol and ethanol feed is the present C-C bond in the latter case.…”

Perspectives on Recent Advances in Hierarchical Zeolites for Bioethanol Conversion to Chemicals, Jet Fuels, and Carbon Nanotubes

“…They suggested that the introduction of mesoporosity could promote the accessibility of reactants to both Brønsted and Lewis acid sites within the zeolite. Moreover, Dittmann and co-workers revealed that the modified bulk HZSM-5 through combined desilication and dealumination could enhance the accessibility of Brønsted acid sites and increase the density of Si­(OH) species. Although the influence of these Si­(OH) species on deactivation behavior remains unclear, they observed that the rate of coke formation increased as a function of improved Si­(OH) species concentrations.…”

Solid-State NMR Probe Molecules for Catalysts and Adsorbents: Concepts, Quantification, Accessibility, and Spatial Distribution

“…Comparing the BAS density from TPP loading with the total BAS density of a sample (determined by loading with a smaller probe molecule) enables the quantification of the share of external sites and the quantification of BAS that were introduced by desilication (see Figure ). ,, Also a larger homologue of TPP, tri-( p -methoxyphenyl)­phosphine (TMPP), is unable to access micropores and suited to quantify external BAS on zeolites . Noteworthy, the long T 1 times of free TPP (7.7 ± 0.4 s) and other pure phenylphosphines make far longer delay times in between scans necessary, if all peaks in the spectrum, not only those of TPP on BAS, shall be quantified. , …”