Understanding the Deactivation of Ag−ZrO₂/SiO₂ Catalysts for the Single‐step Conversion of Ethanol to Butenes

Abstract

Ag−ZrO2/SBA‐16 has recently been found to be efficient for catalyzing the single‐step conversion of ethanol to butene (1‐ and 2‐butene mixtures) in the presence of H2. The reaction proceeds via a cascading sequence of reactions over mixed metal and Lewis sites, with the catalyst composition tuned to selectively favor butene formation. However, the catalyst slowly deactivates when evaluated over long reaction times. In this work, we evaluated the lifetime of the Ag−ZrO2/SBA‐16 catalyst system for ethanol‐to‐butene conversion at 325 °C for up to 800 hours on stream. Several characterization techniques were used to elucidate the mechanism(s) by which catalyst deactivation occurs. Coke deposition, Ag particle sintering, and Ag0‐to‐Ag+ oxidation state change were identified to be the major causes of catalyst deactivation. Coke deposits cover primarily Lewis acid sites which are responsible for aldol condensation, Meerwein‐Ponndorf‐Verley (MPV) reduction, and dehydration reactions. Ag particle sintering and Ag oxidation state change leads to a reduction in the number of metallic Ag sites responsible for the dehydrogenation/hydrogenation steps. The fresh catalyst likely experiences hydrothermal sintering in the early stage of reaction and permanently loses some active Lewis acid sites before reaching a new structural steady state. The deactivation of Lewis acid sites leads to a decrease in overall ethanol conversion, whereas the deactivation of the metallic Ag sites decreases the butene selectivity. For catalyst regeneration, oxidative calcination (at 500 °C) followed by reduction (at 325 °C) successfully removes all the coke species on the catalyst surface and restores the metallic Ag particles of the 4Ag−4ZrO2/SBA‐16 catalysts.

Is enzyme immobilization a mature discipline? Some critical considerations to capitalize on the benefits of immobilization

“…The metal in the support may be in one stage and not be catalytically active, but it is not unlikely that during operation, it may be released (and this may already alter its catalytic potential) or it can suffer from oxidations or reductions, giving completely new catalytic potential to the metal. [484][485][486] Hence, the use of metals in enzyme immobilization supports should be performed only after careful evaluation of its inertness in the target reaction under operation conditions. In fact, even if the activity follows the direction of interest, they could have other capabilities or selectivities, driving the production of some unexpected by-products.…”

One-pot synthesis of 2-alkyl cycloketones on bifunctional Pd/ZrO2 catalyst

“…The deactivated 0.2Pd/ZrO 2 catalyst was then regenerated at 450 • C in air for 4 h. After the removal of coke, the activity of Pd/ZrO 2 was completely recovered [37], showing an excellent regenerability of this heterogeneous catalyst. The good stability and excellent regenerability of Pd/ZrO 2 catalyst would lower the cost of one-pot cascade process, even though more reaction-regeneration cycles are still needed to be considered.…”

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

“…14−20 However, because of the health hazards of C1−C2 alcohols, 4 more attention should be paid toward the dehydration process of higher alcohols such as biobutanol. 3,5,21−23 The efficient catalysts for the dehydration of butanol to butene are usually zeolites, 24,25 silica−aluminas, 7 and supported oxides. 26 In particular, zeolite-based selective catalytic dehydration of biobutanol offers a promising perspective for the production of butene because of the shape selectivity, acidity, and high surface areas of the zeolites.…”

“…Butenes are one of the most important raw chemicals in the production of fuel, fuel additives, commodity polymers, and so forth. − Typically, butenes can be obtained from petroleum-based feedstocks, renewable biomass-derived γ-valerolactone, the selective hydrogenation of 1,3-butadiene, the ethanol-to-butene process, − and the butanol-to-butene process. ,− Among them, renewable biomass-derived alcohols have emerged as one ideal candidate because they can be produced from biomass fermentation on a large scale. ,− The conversion of lower alcohols such as methanol and ethanol to olefin has been investigated recently. − However, because of the health hazards of C1–C2 alcohols, more attention should be paid toward the dehydration process of higher alcohols such as biobutanol. ,,− …”

“…The efficient catalysts for the dehydration of butanol to butene are usually zeolites, , silica–aluminas, and supported oxides . In particular, zeolite-based selective catalytic dehydration of biobutanol offers a promising perspective for the production of butene because of the shape selectivity, acidity, and high surface areas of the zeolites. ,, Both Brønsted acid sites (BASs) and Lewis acid sites (LASs) on zeolites can act as the catalytically active alcohol dehydration sites. ,, The extraframework Al species (EFALs) associated with LASs such as Al 3+ , AlO + , AlOH 2+ , Al­(OH) 2 + , Al­(OH) 3 , and AlOOH have been proposed. , Various EFALs with varied Lewis acidic strengths can be determined by 31 P solid-state nuclear magnetic resonance (NMR) of adsorbed trimethyl­phosphine (TMP) probe molecules .…”