Encapsulated Cobalt Oxide on Carbon Nanotube Support as Catalyst for Selective Continuous Hydrogenation of the Showcase Substrate 1‐Iodo‐4‐nitrobenzene

Abstract

Acarbon nanotube supported catalyst containing cobalt/cobalto xide (Co/Co 3 O 4 )n anoparticles encapsulatedw ithin as hell of nitrogen-doped graphene layers (Co 3 O 4 /NGr@CNT) was prepared. It shows excellent chemoselectivity in the hydrogenation of 1-iodo-4-nitrobenzene,w hich contains an iodine substituenth ighly sensitive against hydrodehalogenation. In contrast to traditional activated charcoal-supported catalysts such as Pt-V/C or the closely relatedV ulcan carbon black supported Co 3 O 4 /NGr@C, the advantageous morphological properties of the CNT support allow for the applica-tion of the new Co 3 O 4 /NGr@CNT as af ixed bed catalyst in ac ontinuous flow reactor. Under optimized conditions,n od ehalogenation side productsc ould be detected. This remarkable selectivity in combination with its mechanical stability under operation conditions renderC o 3 O 4 /NGr@CNT ac atalyst particularly relevant for applicationi nc ontinuousp rocesses based on apacked bed reactor.

Polymer Encapsulated Cobalt‐Based Catalysts (Co EnCatTM) for Selective Continuous Hydrogenation of 1‐Iodo‐4‐nitrobenzene

“…Increasing the temperature to 120 °C resulted in a higher yield of 4‐iodoaniline 2 (91 %). Detrimentally, dehalogenation increased in line with previously observed temperature–dehalogenation relationships . At 120 °C, deactivation of Co EnCat TM 01 was not detected; reverting to the initial temperature (100 °C) resulted in a slight decrease of iodoaniline 2 yield (Figure ).…”

“…The hydroxylamine reduction is known to be the rate‐limiting step in the complex mechanism of nitroarene reduction . As previously discussed, the accumulation of hydroxylamine in the reaction mixture can result in formation of high molecular weight side products, which are not detectable by GC . The latter combined with some analytical error could explain the observed deviations in the carbon mass balance.…”

“…These conditions generated 91 % of 2 at complete conversion of substrate 1 , with low dehalogenation to aniline 3 (1 % yield). Compared with flow experiments performed with analogous Co‐based catalysts, Co EnCat TM 01 is more active than Raney‐Co and less active than Co 3 O 4 /NGr@CNT, which showed the highest productivity (Table ). However, Co EnCat TM 01 was about ten times more active than Raney‐Co with respect to cobalt amount.…”

“…Another prominent example is the recently introduced Co/Co 3 O 4 /NGr@C, which consists of cobalt oxide nanoparticles encased in N‐doped graphene . This catalyst exhibited excellent results in the hydrogenation of halogenated nitroaromatics and inspired the development of two novel, highly selective Co‐based catalysts for the hydrogenation of 1 : the cobalt–organic framework derived Co/C‐N‐600 and the carbon nanotube supported Co 3 O 4 /NGr@CNT . The latter was designed as a flow compatible catalyst and as an alternative to commercially available Raney‐Co, which was previously found to perform very well in the continuous selective hydrogenation of model compound 1 .…”

“…[10] This catalyst exhibited excellentr esults in the hydrogenation of halogenatedn itroaromatics [10] and inspiredt he development of two novel, highly selectiveC o-based catalysts for the hydrogenation of 1:t he cobalt-organic framework derived Co/C-N-600 [11] and the carbon nanotube supported Co 3 O 4 /NGr@CNT. [12] The latter was designed as af low compatible catalysta nd as an alternative to commercially available Raney-Co, which was previously found to performv ery welli nt he continuous selective hydrogenation of model compound 1. [13] Continuous-flow New cobalt catalysts confined in an EnCat TM polyurea matrix by micro-encapsulation of Al 5 Co 2 or Al 5 Co 2 /Al 3 Ni 2 phase containing alloys are disclosed for the reductive hydrogenation of 1-iodo-4-nitrobenzene 1.T he product 4-iodo-aniline 2 readily undergoes hydro-desiodination andw as used to demonstrate the extraordinary selectivity.C oE nCat TM significantly suppressed this follow-up reactionu nder both batch and flow modes.T he correlation between performance, elementalc om-position,a nd alloy content is presented for these novel catalysts.…”