Aqueous phase hydrogenation of maleic acid to succinic acid mediated by formic acid: the robustness of the Pd/C catalytic system

Abstract

Long-term liquid phase studies of the activity in the formic acid-mediated hydrogenation of MAc to SAc were conducted in a fixed bed continuous reactor for two relevant situations: neutralising maleic and formic acids with NaOH and at the pH determined by both acids. Deactivation could only be observed when the catalyst was subjected to a WHSV of MAc above 13 g(MAc) g(cat)(-1) h(-1). Despite this deactivation, the reaction conditions can be adjusted (lowering the reaction temperature and/or decreasing the WHSV) to compensate for the loss of activity. Thus, for the neutralisation case, the catalyst could present a yield of SAc close to 100% for at least 400 h at a WHSV-MAc of 13 g(MAc) g(cat)(-1) h(-1) by setting the temperature at 180 degrees C, corresponding to an overall productivity above 5300 g of SAc per g(cat), equivalent to more than 107 kg of SAc per g(Pd). An in-depth study of the catalyst used under these severe conditions was also conducted using a number of physico-chemical techniques. The sintering of Pd particles, Pd leaching, reduction-carbidization of Pd2+ and/or Pd into Pd carbides (PdCx) and deposition of organic compounds were identified in the spent catalysts for both the neutralised and acid runs. The presence of Na+ and of CO chemisorbed at the surface of the catalyst were also detected in the neutralised and acid cases, respectively. The rinsing of the catalyst with aqueous H2SO4 or treating the catalyst at 200 degrees C with streams containing O-2 or H-2 did not reactivate the catalyst. We concluded that the relevant causes of deactivation are Pd leaching and deposits of organic compounds; CO poisoning is also likely in the acid case. This is in agreement with the fact that deactivation is detected under high MAc-WHSV: the rate of leaching, CO chemisorption and deposition accelerates upon increasing it.