Abstract
Background Bio-acids such as acetic acid (AcOH) and formic acid (FA) are typical bio-oil compounds and platform chemicals that sourced from biomass pyrolysis. They are attracting global research attention due to their low-cost and safety merits with additional potentials as alternative in-situ hydrogen donors for bio-oil upgrading. However, the hydrogen donation performance of bio-acids have not been sufficiently evaluated, especially, investigation on high efficient catalysts to promote the process is lacking. In this study, novel catalysts of metal supported on nitrogen doped carbon nanotubes (CNTs) were thoroughly evaluated in facilitating the decomposition of both FA and AcOH for hydrogen donation by comparing ten different metal loadings and six types of CNT based substrates. Results It was found that Mo loading enabled the strongest binding energy to the bio-acid molecule among the ten evaluated transition metals, and Np (pyridinic nitrogen doped)-CNT led to bigger adsorption energy of AcOH than other substrates, e.g. Ng (graphitic nitrogen doped)-CNT or non-doped CNT. The new designed catalyst, Mo/N-CNTs, considerably facilitated the bio-acids decomposition by lowering the energy barriers, compared to the non-catalytic scenario. The favourable hydrogen donation pathways for AcOH are: CH3COOH→CH3CO→CH3→CH2→CH→C over Mo/Np-CNT, and CH3COOH→CH3CO→CH3C→CH3→CH2→CH1→C over Mo/Ng-CNT. The pathways for FA are: HCOOH→H+CO+OH over Mo/Np-CNT and HCOOH→HCO+OH over Mo/Ng-CNT. FA has showed the superiority for hydrogen donation than AcOH over Mo/N-CNT catalysts since it can be cleaved into hydroxyl group and hydrogen without an energy barrier, which will facilitate the following hydrogen donation from hydroxyl. Conclusions It was concluded that the new explored catalyst, Mo/N-CNTs, significantly lowered the decomposition energy barriers for both AcOH and FA thus promoting the hydrogen donation performance of both bio-acids. Additionally, over the designed catalyst, FA is a preferred hydrogen donor than AcOH due to the barrier-free adsorption step while the energy barriers for AcOH decompositions are relatively high.
Mini-Review on the Synthesis of Furfural and Levulinic Acid from Lignocellulosic Biomass
