Better Performance in C₂-Conversion to Aromatics by Optimized Feed and Catalysts

Abstract

We identify favorable parameters for the formation of aromatics from ethanol-based feeds over ZSM-5. An ethanol partial pressure of 0.3 bar, a reaction temperature of ∼700 K, a low weight hourly space velocity (WHSV), and a high Brønsted acid site density increase the content of aromatics, in the latter case, on the expense of lifetime. The aromatic fraction composition changes with the WHSV and n Si/n Al ratio. Water cofeed increases the content of aromatics in the C2:H2O stoichiometry of 1:0.5 (equals diethyl ether as feed) but decreases it if this is exceeded. Diethyl ether and ethylene lead to more aromatics than ethanol feed. The lifetime until deactivation increases in the order ethanol < diethyl ether < ethylene. This points at different reaction and/or deactivation mechanisms in converting the three feeds. Thus, a water-poor feed effectively improves the catalyst lifetime and productivity in the ethanol conversion to aromatics.

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

“…With increasing BAS density we observe also a decreased lifetime of our catalysts. This is caused by the increased amount of aromatics formed that lead to deactivation and was already described for microporous ZSM‐5 catalysts [9,11e] . In desilicated samples, however, this effect is overshadowed by a longer lifetime as result of better transport properties [11f] .…”

“…[8] The immediate dehydration of ethanol feed to ethylene makes also adjustments of the feed more promising, because unneeded water can be efficiently removed initially which enables longer lifetimes and higher aromatic yields. [9] Due to the mentioned chemical and mechanistic differences between ethanol and methanol feeds it is highly interesting if post-modifications that are frequently applied for ZSM-5 catalysts in methanol conversion have also beneficial effects on the ETA catalysts. Typical post-modifications applied to zeolites are the phosphatation, [10] dealumination, [11] both applied to alter the acidity of zeolites, and finally desilication, [10e,12] whereby both diffusion and acid properties of zeolite catalyst can be varied.…”

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

“…However, an application of solvent (CH 2 Cl 2 ) enables a soft, reproducible TPP distribution also on samples with low acid site density . TPP was used for the investigation of BAS on the external surface of amorphous silica-alumina, , H-MCM-22, , and hierarchical zeolites. , TPP is, with a diameter of 1.00 to 1.08 nm, rather voluminous and thus unable to access most intact zeolite micropores. 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 ).…”

“…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. , …”

“…However, the approach results in a variety of 31 P MAS NMR resonances that are still debated ( vide supra ). A similar method to distinguish external BAS density from BAS in micropores is a combination of the probe molecules NH 3 and TPP. , The TPP cannot access 10-MR micropores and is thus limited to the detection of BAS on the external surface and within mesopores of hierarchical zeolites. Also the corresponding oxide, TPPO, was used for a selective detection of external sites on zeolite Y .…”