Steering palm oil hydrodeoxygenation towards biofuel production: An experimental and theoretical approach to unveil periodic trends

Abstract

The study presented herein examines the role of a series of alkaline earth metals (i.e. Mg, Ca, Sr, Ba) as promoters for Ni-supported zeolite beta catalysts to produce hydrocarbon fuels through hydrodeoxygenation (HDO) of palm oil. The properties of the catalytic systems were explored using XRD, HRTEM, N adsorption, H-TPR, CO-, NH-, and H-TPDs. The surface chemistry and coordination environment were analyzed using synchrotron XAFS and XPS. It is found that the chemical and structural composition of the support, as well as the use of alkaline earth promoters, significantly altered the interfacial charge distribution and consequently the electronic structure of the Ni-support interface and Ni surface sites. HDO of palm oil was conducted at 350 and 400 °C, at 30 bar; the highest conversion was attained over 10Ni/5Sr-Beta catalyst (44 %), which can be attributed to its features, such as low acidity (NH-TPD/DRIFTS), high Ni dispersion (H-TPD) and high amount of accessible Ni sites at reaction temperature (H-TPR). Product analysis, in general, demonstrated higher selectivity towards bio-gasoline (C–C) as a result of high acidity of zeolite beta (Si/Al = 12.5) that promoted cracking activity. While the effect of alkaline earth metal cation promoters was investigated experimentally, DFT was utilized to investigate the trend on the deoxygenation of palmitic acid (predominant component of palm oil) over Ni(111) surface. Results revealed that although the elongated COH bond lengths, adsorption energies, and enhanced charge transfer between the OH group and surface promoters can be linked to the periodicity of the adatom types, the thermodynamic –OH cleavage reaction energies well reflect the varying experimental deoxygenation performance of the Ni-promoted catalyst, ascribed to their varying C−O/promoter interactions subsequent to OH cleavage.