Second Youth of a Metal-Free Dehydrogenation Catalyst: When γ-Al<inf>2</inf>O<inf>3</inf> Meets Coke under Oxygen- And Steam-Free Conditions

Abstract

© 2019 American Chemical Society. The role of carbonaceous deposits (coke) formed in dehydrogenation catalysis has been extensively investigated over the last few decades mainly with respect to the deactivation of metal-based and metal-free heterogeneous catalysts. Although much less emphasized, coke deposits grown on selected metal oxides have also been described as active and selective phases for alkene dehydrogenation under an oxidative or non-oxidative atmosphere. This work describes the straightforward preparation of "coked" γ-Al2O3 composites and their catalytic performance in the ethylbenzene (EB) direct dehydrogenation (DDH) to styrene (ST) under steam- and oxygen-free conditions. The study unveils the effective potentiality of a catalytic system already known to the scientific community but never employed for EB DDH under severe conditions, close to those commonly used in industrial plants (600 °C, 10 vol % EB/He, GHSV = 3000 h-1). Such a simple catalytic system has revealed a significant stability on long-term trials (≥150 h) and markedly high ST selectivity (≥97%) along with process rates (λ up to 16.3 mmolST gcat-1 h-1) that are the highest claimed so far for related carbon systems at work in the process. Furthermore, the outlined performance of our composites in DDH is close to that claimed for classical iron-based industrial catalysts operating in the presence of a large amount of steam. γ-Al2O3 precoking with an aliphatic C-source has shown additional beneficial effects on the ultimate γ-Al2O3@C performance in DDH. These findings pave the way for the development of cheap and durable dehydrogenation catalysts. They rewrite (in part at least) the role of coke in a challenging heterogeneous process while offering important hints to the comprehension of the reaction mechanism promoted by plain C-sites.