Abstract

A series of nickel–alumina catalysts promoted by copper containing 1, 2, and 5 wt. % Cu and 59, 58, and 55 wt. % Ni, respectively, (symbols: 59Ni1CuAl, 58Ni2CuAl, 55Ni5CuAl) and a non-promoted catalyst containing 60 wt. % Ni (symbol: 60NiAl) were prepared following a one-step co-precipitation method. They were characterized using various techniques (N2 sorption isotherms, XRD, SEM-EDX, XPS, H2-TPR, NH3-TPD) and evaluated in the selective deoxygenation of sunflower oil using a semi-batch reactor (310 °C, 40 bar of hydrogen, 96 mL/min hydrogen flow rate, and 100 mL/1 g reactant to catalyst ratio). The severe control of the co-precipitation procedure and the direct reduction (without previous calcination) of precursor samples resulted in mesoporous nano-structured catalysts (most of the pores in the range 3–5 nm) exhibiting a high surface area (192–285 m2 g−1). The promoting action of copper is demonstrated for the first time for catalysts with a very small Cu/Ni weight ratio (0.02–0.09). The effect is more pronounced in the catalyst with the medium copper content (58Ni2CuAl) where a 17.2% increase of green diesel content in the liquid products has been achieved with respect to the non-promoted catalyst. The copper promoting action was attributed to the increase in the nickel dispersion as well as to the formation of a Ni-Cu alloy being very rich in nickel. A portion of the Ni-Cu alloy nanoparticles is covered by Ni0 and Cu0 nanoparticles in the 59Ni1CuAl and 55Ni5CuAl catalysts, respectively. The maximum promoting action observed in the 58Ni2CuAl catalyst was attributed to the finding that, in this catalyst, there is no considerable masking of the Ni-Cu alloy by Ni0 or Cu0. The relatively low performance of the 55Ni5CuAl catalyst with respect to the other promoted catalysts was attributed, in addition to the partial coverage of Ni-Cu alloy by Cu0, to the remarkably low weak/moderate acidity and relatively high strong acidity exhibited by this catalyst. The former favors selective deoxygenation whereas the latter favors coke formation. Copper addition does not affect the selective-deoxygenation reactions network, which proceeds predominantly via the dehydration-decarbonylation route over all the catalysts studied.

Highlights

  • The emission of carbon dioxide from the combustion of fossil fuels is responsible for the global warming whereas the increasing demands for fossil fuels is expected to leadEnergies 2020, 13, 3707; doi:10.3390/en13143707 www.mdpi.com/journal/energiesEnergies 2020, 13, 3707 to their progressive depletion during the 21st century [1]

  • The second difference is that the evaluation of the catalysts in the present work was performed under solvent-free conditions and the sunflower oil (SFO) volume to catalyst ratio is equal to 100 mL/1 g and the reaction time is equal to 9 h

  • There is no considerable masking of this Ni-Cu alloy in the 58Ni2CuAl catalyst explaining its highest catalytic performance

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Summary

Introduction

The emission of carbon dioxide from the combustion of fossil fuels (carbon, oil, and natural gas) is responsible for the global warming whereas the increasing demands for fossil fuels is expected to lead. The copper promoting action is expressed through the increase in the nickel dispersion and the formation of nickel-copper alloy with better catalytic behavior than nickel It seems that this curbs the C-C cracking activity of nickel, which depresses the formation of cracked paraffins, methane, and carbon deposition and favors the formation of diesel range n-alkanes and catalyst stability. The second difference is that the evaluation of the catalysts in the present work was performed under solvent-free conditions and the SFO volume to catalyst ratio is equal to 100 mL/1 g and the reaction time is equal to 9 h These correspond to an LHSV value equal to 11.1 h−1 for experiments taken place in fixed bed reactors. The choice of SFO as feedstock was done by taking into account that genetically-modified sunflower grown on marginal land has been identified as sustainable biofuel source because it does not encroach upon arable lands [31]

Synthesis of the Catalysts
Catalysts Characterization
Catalytic Tests
H2‐TPR profiles the58Ni2CuAl
Catalysts Evaluation
O-deCO which indicates that SDOThe mainly proceeds deH than throughofdeH
Conclusions
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