Ethylbenzene to chemicals: Catalytic conversion of ethylbenzene into styrene over metal-containing MCM-41

Isomorphously substituted (MeDM) and impregnated metal-containing MCM-41 (MeOx/IM) catalysts, in which Me=Co, Cu, Cr, Fe or Ni, have been prepared. Structural and textural characterizations of the catalysts were performed by means of X-ray diffraction (XRD), chemical analysis, Raman spectroscopy, electron paramagnetic resonance (EPR), N-2 adsorption isotherms and temperature programmed reduction (TPR). Cu2+, Co2+ and Cr4+/Cr3+ species were found over the catalysts as cations incorporated in the MCM-41 structure (MeDM) or highly dispersed oxides on the surface (MeOx/IM). The MeDM catalysts exhibited a good performance in the dehydrogenation of ethylbenzene with CO2. However, MeOx/IM catalysts had a low performance in styrene production (activity less than 15 x 10(-3) mmol h(-1) and selectivity for styrene less than 80%) due to the high reducibility of the metals species. However, Ni2+ or Fe3+ coordinated with the MCM-41 framework, as well as NiOx and Fe2O3 extra-framework species, is continuously oxidized by the CO2 to maintain the active sites for dehydrogenating ethylbenzene. Deactivation studies on the FeDM sample showed that Fe3+ species produced active sp(2) carbon compounds. which are removed by CO2; the referred sample is catalytically selective for styrene and stable over 24 h of reaction. In contrast, highly active Ni2+ and Nil species produced a large amount of polyaromatic carbonaceous deposits from styrene, as identified by TPO, TG and Raman spectroscopy. An acid-base mechanism is proposed to operate to adsorb ethylbenzene and abstract the beta-hydrogen. CO2 plays a role in furnishing the lattice oxygen to maintain the Fe3+ active sites in the dehydrogenation of ethylbenzene to form styrene. (C) 2009 Elsevier B.V. All rights reserved.