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Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt

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Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt reactions Article Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt Kevin Parker 1 , Victoria Pho 1, Richard A. J. O’Hair 2 and Victor Ryzhov 1,* 1 Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA; [email protected] (K.P.); [email protected] (V.P.) 2 School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Melbourne, VIC 3010, Australia; [email protected] * Correspondence: [email protected]; Tel.: +1-(815)-753-6871 Abstract: Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSn) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal + complexes [(phen)M(O2CR)] ], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms Citation: Parker, K.; Pho, V.; O’Hair, involving group 10 metals differ from long-known charge-remote fragmentation reactions. R.A.J.; Ryzhov, V. Mechanism of Deoxygenation and Cracking of Fatty Keywords: biomass; gas phase; mass spectrometry; cracking; hydrocarbons Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt. Reactions 2021, 2, 102–114. https:// doi.org/10.3390/reactions2020009 1. Introduction Interest in biomass-derived hydrocarbons has grown due to socio-economic and Academic Editor: Dmitry Yu. Murzin environmental pressures to transition from fossil fuel to renewable resources [1–3]. Pro- duction of paraffinic fuel and biodiesel have drawbacks, with poor storage stability of the Received: 20 April 2021 fuels and unfavorable cold-flow properties that can be overcome with the new-generation Accepted: 11 May 2021 Published: 15 May 2021 methods of biomass conversion [1]. One of the main components of biomass suitable for biodiesel production is triglycerides containing fatty acids of varying lengths and degrees Publisher’s Note: MDPI stays neutral of unsaturation [4]. While thermal cracking of fatty acids was shown to produce hydro- with regard to jurisdictional claims in carbons as early as the beginning of the 20th century by Sabatier [2], oxygen-containing published maps and institutional affil- impurities pose a problem of compatibility with modern diesel engines [3]. Through de- iations. oxygenation, access to the pure hydrocarbon chain is granted and, by cracking chemistry, diesel-like hydrocarbons are produced [5]. These “drop-in” fuels can be used with little to no modification to combustion engines on the road today. Characterization of the hydrocarbons released by deoxygenation and cracking of fatty acids has been studied extensively in condensed-phase experiments [5–9]. Metal Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. catalysts greatly reduce the energy (or temperature) requirements needed to afford the This article is an open access article desired hydrocarbons, with Ni, Pd, and Pt showing the greatest potential [5]. The methods distributed under the terms and of deoxygenation have been studied in multiple experiments with saturated fatty acids conditions of the Creative Commons such as stearic acid [5,6,10]. In these studies, three deoxygenation pathways have been Attribution (CC BY) license (https:// observed: (i) decarbonylation paired with dehydration, (ii) decarboxylation, or (iii) a creativecommons.org/licenses/by/ combination of both. While saturated fatty acids only exhibit cracking of the hydrocarbon 4.0/). chain after deoxygenation [10], it has been shown that, for unsaturated hydrocarbons like Reactions 2021, 2, 102–114. https://doi.org/10.3390/reactions2020009 https://www.mdpi.com/journal/reactions Reactions 2021,, 2,, FORFOR PEERPEER REVIEWREVIEW 2 Reactions 2021, 2 103 chain after deoxygenation [10], it has been shown that, for unsaturated hydrocarbons like oleicoleic acid,acid, deoxygenation deoxygenation is is not not required required for for cracking cracking [11–13]. [11–13 Variou]. Various-lengths-length alkanes, alkanes, ter- terminalminal alkenes, alkenes, and and internal internal alkenes alkenes were were alll observedobserved all observed throughthrough through Gas-chromatographyGas-chromatography Gas-chromatography (GC)(GC) (GC)after aftervarying varying temperature temperature and time and timein a heated in a heated reaction reaction chamber chamber [4]. [4]. Gas-phaseGas-phase studies, studies, often often carried carried out out via via mass mass spectrometry spectrometry (MS) (MS) experiments, experiments, provide pro- avide different a different angle forangle looking for looking at deoxygenation/cracking at deoxygenation/cracking chemistry. chemistry. For example, For example, charge- remotecharge-remote fragmentation fragmentation of unsaturated of unsaturated fatty acids fatty has acids been shownhas been to produceshown to hydrocarbons produce hy- viadrocarbons fatty acid via chain fatty cracking acid chain without cracking deoxygenation without deoxygenation [14,15]. These experiments [14,15]. These have experi- long beenments used have to long determine been used the number to determine and position the number of double and bondsposition in theof double fatty acid bonds chain in [the5]. Thefatty main acid advantage chain [5]. ofThe studying main advantage these reactions of studying in the gas these phase reactions is that, ininconjunction the gas phase with is theoreticalthat,that, inin conjunctionconjunction calculations, withwith they theoreticaltheoretical can be used calculatiocalculatio to determinens, they detailed can be used mechanisms to determine and pathway detailed energetics,mechanisms thus and allowing pathway for energetics, intelligent thus catalyst allowi designng for intelligent [16,17]. Given catalyst the design prevalence [16,17]. of unsaturatedGiven the prevalence fatty acids of found unsaturated in biomass, fatty it isacids important found toin understand biomass, it theis important mechanism to and un- chemistryderstand the involved mechanism in production and chemistry of hydrocarbons involved fromin production their cracking of hydrocarbons reactions. Recent from worktheirtheir fromcrackingcracking our reactions. groupreactions. has RecentRecent shown workwork such details fromfrom our for group the catalytic has shown deoxygenation such details and for cracking the cat- ofalytic stearic deoxygenation acid by Ni and and Pd cracking complexes of stearic studied acid by MS.by Ni The and deoxygenation Pd complexes mechanism studied by was MS. calculatedThe deoxygenation for propionic mechanism acid used was as calculated the simple for gas-phase propionic model acid for used larger as the fatty simple acids, gas- as shownphase model in Scheme for larger1[10]. fatty acids, as shown in Scheme 1 [10]. SchemeScheme 1.1. CatalyticCatalytic deoxygenation deoxygenation and and dehydrogenation dehydrogenation of propionic of propionic acid by acid ternary by ternary 1,10-phe- 1,10- phenanthrolinenanthroline and and metal metal (Ni (Ni and and Pd) Pd) complexes. complexes. OnceOnce deoxygenationdeoxygenation has has occurred, occurred, the the resulting resulting organometallic organometallic ion ion can can undergo undergo facile fac- “chainileile “chain“chain walking” walking”walking” (when (when(when the the metalthe metalmetal is Ni isis or NiNi Pd). oror Pd).Pd). The TheThe isomerization isomerizationisomerization barriers barriersbarriers were werewere shown shownshown to betoto be lowbe lowlow enough enoughenough to toto produce produceproduce a mixtureaa mixturemixture of ofof organometallic organometallicorganometallic isomers isomersisomers prior priorprior to toto fragmentation fragmentationfragmentation (Scheme(Scheme2 )[2) 18[18].]. SchemeScheme 2.2. IsomerizationIsomerization ofof of alkanealkane alkane fromfrom from alphaalpha alpha toto to betabeta beta andand and betabeta beta toto togammagamma gamma isis reversiblereversible is reversible andand and lowlow low enough in energy to be observed in simple gas-phase experiments. enoughenough inin energyenergy toto bebe observedobserved inin simplesimplegas-phase gas-phaseexperiments. experiments. TheThe subsequentsubsequent “cracking”“cracking” allowedallowed forfor anyany lengthlength ofof neutralneutral alkenealkene toto bebe extruded extruded fromfrom thethe hydrocarbonhydrocarbon chain chain [ [19–24].19–24]. ThisThis mechanismmechanism was was modeled modeled with with a a hexyl hexyl chain chain in in ourour recentrecent workwork andand cancan bebe usedused toto describedescribe thethethecracking crackingcrackingchemistry chemistrychemistry observed observedobserved in inin the thethefatty fattyfatty acidacid chemistrychemistry describeddescribed
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