Effect of Pt Promotion on the Ni-Catalyzed Deoxygenation of Tristearin to Fuel-Like Hydrocarbons

Ryan Loe,Robert Pace,Mark Isaacs,Yang Song,Kelsey Huff,Tonya Morgan,Morgan Walli,Eduardo Santillan-Jimenez,Dali Qian,Mark Crocker

doi:10.3390/catal9020200

Abstract

Pt represents an effective promoter of supported Ni catalysts in the transformation of tristearin to green diesel via decarbonylation/decarboxylation (deCOx), conversion increasing from 2% over 20% Ni/Al2O3 to 100% over 20% Ni-0.5% Pt/Al2O3 at 260 °C. Catalyst characterization reveals that the superior activity of Ni-Pt relative to Ni-only catalysts is not a result of Ni particle size effects or surface area differences, but rather stems from several other phenomena, including the improved reducibility of NiO when Pt is present. Indeed, the addition of a small amount of Pt to the supported Ni catalyst dramatically increases the amount of reduced surface metal sites, which are believed to be the active sites for deCOx reactions. Further, Pt addition curbs the adsorption of CO on the catalyst surface, which decreases catalyst poisoning by any CO evolved via decarbonylation, making additional active sites available for deoxygenation reactions and/or preventing catalyst coking. Specifically, Pt addition weakens the Ni-CO bond, lowering the binding strength of CO on surface Ni sites. Finally, analysis of the spent catalysts recovered from deCOx experiments confirms that the beneficial effect of Pt on catalyst performance can be partially explained by decreased coking and fouling.

Highlights

The limited availability and uneven geographical distribution of fossil resources, as well as environmental concerns associated with their use, demand the development of renewable and carbon neutral alternatives
Catalytic deoxygenation processes are being developed to convert the same feedstocks to fuel-like hydrocarbons that, in addition to being renewable, are chemically identical to fossil fuels. Investigations into such processes have revealed two dominant reaction pathways: (1) hydrodeoxygenation (HDO), where oxygen is eliminated as water; and (2) decarbonylation/decarboxylation, in which oxygen is removed as CO or CO2, respectively
Results of catalyst screening tests in a semi-batch reactor show that Pt is an effective promoter of supported Ni catalysts in the deCOx of tristearin at 260 ◦ C, with the conversion increasing from 2%

Summary

Introduction

The limited availability and uneven geographical distribution of fossil resources, as well as environmental concerns associated with their use, demand the development of renewable and carbon neutral alternatives. Catalytic deoxygenation processes are being developed to convert the same feedstocks to fuel-like hydrocarbons that, in addition to being renewable, are chemically identical (and a drop-in alternative) to fossil fuels Investigations into such processes have revealed two dominant reaction pathways: (1) hydrodeoxygenation (HDO), where oxygen is eliminated as water; and (2) decarbonylation/decarboxylation (deCOx ), in which oxygen is removed as CO or CO2 , respectively. The HDO reaction pathway is typically very selective, yielding the desired hydrocarbon products with a good carbon efficiency and saturating the C=C double bonds that are abundant in many feed sources This approach requires high H2 pressures and problematic sulfided catalysts that deactivate in the presence of water and risk contaminating the products with sulfur. The deCOx pathway yields hydrocarbons containing one less carbon than the original feed, but does so avoiding the use of sulfided catalysts and requiring little to no H2 for oxygen removal [7]

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