Hydrogenolysis of cellulose to valuable chemicals over activated carbon supported mono- and bimetallic nickel/tungsten catalysts

Abstract

The hydrogenolysis of cellulose was systematically investigated at 488 K and under 65 bar H₂ in the absence of a catalyst and over six different catalytic systems containing nickel and/or tungsten on activated carbon (AC) in order to understand the role of individual active components (AC, W/AC, Ni/AC, a physical mixture of Ni/AC + W/AC, and two differently prepared Ni/W/AC catalysts) with respect to the product distribution wherein polyols (e.g. ethylene glycol (EG), propylene glycol, and sorbitol) are highly valuable chemicals. Without a catalyst and when using only AC, a hydrochar, due to hydrothermal carbonization of cellulose, was obtained. Although the catalyst W/AC was effective for the degradation of cellulose (high conversion of 90%) and facilitates C–C bond cleavage, selective production of any product was not possible, and the carbon efficiency (CEL) is the lowest (9.1%). Also, with highly dispersed Ni on AC the polyol yield was only 5.3%. The desired behavior showed Ni/W/AC provided its preparation occurs by a two-step incipient wetness (IW) technique. Starting with a remarkably high cellulose/catalyst ratio of 10, a cellulose conversion of 88.4%, CEL of 78.4% and EG yield of 43.7% were achieved (overall polyol yield = 62.1%). Drastically lower yields towards EG by an order of magnitude and decreased CEL were obtained by a co-impregnated Ni/W/AC catalyst and the Ni/AC + W/AC mixture. By the detailed analysis via XRD, TPR and CO chemisorption, it can be concluded that in the Ni/W/AC catalyst, after the first IW step of the activated carbon with ammonium metatungstate hydrate and the following reduction in H₂ up to 1128 K, metallic tungsten was formed. This leads, in combination with the hydrogenation properties of nickel introduced in the second IW step, to a virgin bimetallic catalyst, i.e. before the hydrogenolysis starts, in which both components must be metallic. This is a prerequisite for high polyol production. Finally, varying the AC type, high space–time-yields up to 2.5 g polyols (g_catalyst h)⁻¹ were obtained. A slight deactivation after two runs followed by a strong decrease of polyol yield in the next two runs was observed. Leaching and structural changes on the catalyst surface (formation of NiWO₄) are mainly responsible for deactivation.