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 °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 (PdC_x) 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 °C with streams containing O₂ or H₂ 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.