Deactivation Studies of Alkali-Promoted Trimetallic Co-Rh-Mo Sulfide Catalysts for Higher Alcohols Synthesis from Synthesis Gas

摘要

Multiwailed carbon nanotubes (MWCNTs) and activated carbon (AC)-supported K (9 wt %)-promoted trimetallic Co (4.5 wt%)-Rh (1.5 wt%)-Mo(l5 wt%) catalysts are used to study the long-term deactivation for continuous 720 h of higher alcohols synthesis (HAS). The fresh catalysts are extensively characterized in both oxide and sulfide phases, together with the spent catalysts. The catalysts are tested for the synthesis of higher alcohols from synthesis gas under similar conditions of 330 °C, 9.1 MPa (1320 psig). and 3.8 nr (STP)/((kg of cat.)/hi, using a H2:CO molar ratio of 1.25. The alkali-promoted trimetallic Co-Rh-Mo catalyst supported on MWCNTs has shown two different deactivation steps: loss of sulfur from the surface and sintering of the catalyst species located on the outer surfaces of the carbon nanotubes. After regeneration, the total activity recovery (～10%) over this catalyst is close to the total activity loss during the first deactivation step. The percentage CO conversion decreased from 58% to 51% and from 43% to 30% over the catalysts supported on MWCNTs and AC. respectively, over 720 h of time-on-stream; indicating a loss of activity of 12% and 30% over the catalysts supported on MWCNT and AC. respectively. Characterizations of the spent catalyst supported on activated carbon revealed that deactivation occurs because of the sintering of metal sulfides, which causes low metal dispersions and high pore blockage of the support. The total hydrocarbon formation decreased rapidly during the first deactivation step and then slowly leveled off. The initial decrease in the total hydrocarbons space time yield (STY) may be explained from the loss of sulfur from unstable metal sulfide crystallites on the surface of the catalyst to the products and sintering of the catalyst species is responsible for the later steps. The total alcohols STY of 0.281 and 0.202 g/(g of cat., h) are observed during the reaction period of 24 h over the catalysts supported on MWCNTs and activated carbon, respectively. The sulfided MWCNT-supported catalyst suffers a 1% decrease while the AC-supported catalysts exhibits a 4% decrease in normalized higher alcohols yield between 24 and 720 h on-stream.