DOI:10.1002/chem.201702376 Full Paper & Reaction Mechanisms |Hot Paper| Dissociative Ionization and Thermal Decomposition of Cyclopentanone JohanI.M.Pastoors,[a] Andras Bodi,[b] PatrickHemberger,[b] and Jordy Bouwman*[a, c] Abstract: Despitethe growing use of renewable and sus- out areversebarrieropen up to oxopropenyl and cyclopro- tainable biofuels in transportation,their combustion chemis- panonecations by ethyl-radical loss and asecond ethene- try is poorly understood, limiting our efforts to reduce harm- loss channel, respectively.Astatistical Rice–Ramsperger– ful emissions. Here we report on the (dissociative) ionization Kassel–Marcus model is employed to test the viability of this and the thermaldecomposition mechanism of cyclopenta- mechanism. Thepyrolysis of cyclopentanone is studied at none, studied using imaging photoelectron photoion coinci- temperatures ranging from about 800 to 1100 K. Closed- dence spectroscopy.The fragmentation of the ions is domi- shell pyrolysis products,namely 1,3-butadiene, ketene, pro- nated by loss of CO, C2H4,and C2H5,leadingtodaughter pyne, allene, and ethene, are identified based on their pho- + ions at m/z 56 and 55. Exploring the C5H8OC potential toion mass-selected threshold photoelectron spectrum.Fur- energy surface reveals hydrogen tunneling to play an impor- thermore, reactiveradical species such as allyl, propargyl, tant role in low-energydecarbonylation and probably also in and methyl are found. Areaction mechanism is derived in- the ethene-loss processes, yielding 1-butene and methylke- corporating both stable and reactive species, which were tene cations, respectively.Athigher energies,pathways with- not predicted in prior computational studies. Introduction tion that can be produced from furfural,[3,4] which in turn is produced industriallybyacidic hydrolysis of hemicellulose, a In general, biofuelsare renewable and often sustainable alter- major constituent of plant matter,[5] or,alternatively,through natives to fossil fuels, whichcan also mitigate harmful emis- pyrolysis of biomass.[6] Cyclopentanone also serves as aprecur- sions.[1] In particular, more and more research focusesonfuels sor for other, high-density,fuels,[7] which are suitable for use in derived from waste biomass.While fossil fuels are typically airplanes. Athorough understanding of the unimolecular dis- composed of mostlycarbon and hydrogen, biofuels also con- sociation of cyclopentanone is crucial to accurately model its tain oxygen in the form of hydroxyl (e.g.,ethanol), carboxyl, al- combustion. dehyde, or ketone functional groups.[2] Their varying composi- The pyrolysis of cyclic ketoneshas been studied in some tion leads to differing combustion chemistry,whichneeds to detail. Employing fractional product condensation and IR spec- be thoroughly understood. troscopic analysis, Johnson and Walters[8] found 2-cyclopenten- One of numerouspossible classes of biofuel molecules are 1-one,1-butene, ethene, carbon monoxide, and hydrogen as [9] ketones. Cyclopentanone is acyclic ketone of C5H8Ocomposi- cyclopentanone pyrolysis products. Delles et al. performed similar experiments and reported the formation of 4-pentenal [a] J. I. M. Pastoors, Dr.J.Bouwman in addition to products reported by Johnson and Walters.[8] Radboud University,Institute for Molecules and Materials Several low-molecular-weight species were observed in agas FELIX Laboratory,Toernooiveld 7c, 6525 ED Nijmegen (The Netherlands) E-mail:[email protected] chromatogram of the reactionmixture, but could not be as- [10] [b] Dr.A.Bodi,Dr. P. Hemberger signed. Porterfield et al. studied the pyrolysis of another cy- Laboratoryfor Synchrotron Radiation and Femtochemistry cloketone, cyclohexanone (C6H10O), using flash pyrolysis in Paul Scherrer Institute, 5232 Villigen (Switzerland) combination with photoionization mass spectrometry and [c] Dr.J.Bouwman matrixisolation Fouriertransform infrared (FTIR) spectroscopy. Presentaddress:Sackler Laboratoryfor Astrophysics They reported that methylvinyl ketone and ethene are formed Leiden Observatory,Leiden University,P.O. Box 9513 2300 RA Leiden (The Netherlands) through keto–enol tautomerization followed by aretro-Diels– Supporting information and the ORCID identification numbers for the Alder reaction. Furthermore, they argued that the other detect- authors of this article can be found under https://doi.org/10.1002/ ed products, among which ketene (C2H2O), are formed by chem.201702376. meansofa, b,org C Cbond cleavage followed by rearrange- À 2017 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA. mentsofthe thus formed diradicals. This is an open access article under the terms of Creative Commons Attri- Cyclopentanone pyrolysis has also been addressed computa- bution NonCommercialLicense, whichpermitsuse, distributionand repro- [11] duction in any medium, provided the original work is properlycited and is tionally.Zaras et al. characterized the energetics of the uni- not used for commercial purposes. molecular dissociation of cyclopentanone using the composite Chem. Eur.J.2017, 23,13131 –13140 13131 2017 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper G3B3 method. They considered C Cscissionreactions, among Threshold photoelectron spectrum À other possible channels,and only found products formed from reactions initiatedbya-cleavage, despite the fact that b-cleav- The cyclopentanone cation has C2 symmetry and the highest age was determinedtobelower in energy.The computational occupied molecular orbital(HOMO), shown togetherwith the results on the neutralcyclopentanone potential energy surface mass-selected threshold photoelectronspectrum in Figure 1, (PES) were employed to calculate rate constants of the initial isomerization steps over the 800–2000 Ktemperature range by using Rice–Ramsperger–Kassel–Marcus(RRKM) modeling. They concludedthat the main products are C2H4 +CO and the rate constantssuggest that keto–enol isomerization plays apivotal role in the formation of other products, namely cyclopente- none, 1,3-butadiene, and acetylene. Wang et al.[12] performedacombined theoretical and experi- mental study on the dissociative photoionization of cyclopen- + tanone, providing adetailed C5H8OC PES. Experiments were performed using afemtosecond laser to induce dissociative ionization and time-of-flight(TOF) mass spectrometry for de- tection of fragments. Based on quantumchemical computa- tions, rates and branching fractions were derived by RRKM modeling. Figure 1. Threshold photoelectron spectrum of cyclopentanone (connected Literature data to validate the theoretical computations on blue circles)displayed togetherwith aFranck–Condon simulation of the cyclopentanone dissociation are scarce. In prior experimental spectrum (green). The inset showsthe highest occupied molecular orbital on cyclopentanone. pyrolysis studies, product specieswere assigned offline after condensation, introducing the possibility of secondary reac- tions and inherently imposing uncertainties on the actual un- has antibondingcharacter along the C=Obond and bonding derlyingformation mechanism of the detected products.More- character along the! two (O=)C Cbonds. Upon ionization À over,thesestudies were not sensitive to reactive intermediates, through the X˜ + 2B X˜ 1Atransition, electron densityisremoved which could provide important clues to the overall chemistry. from the HOMO,leadingtoa0.02 contraction of the C=O Furthermore, single-photon dissociative ionization data em- bond and a0.03 elongation of the two (O=)C Cbonds, as À ploying parent-ion internal energy selection are not available. predicted by B3LYP/6–311 ++G(d,p) geometry optimizationof However,such data can be used to quantify bond dissociation the neutraland ionic ground state. The measured spectrum re- energiesonthe cationic surface accurately,and provide details veals astrong resonance at (9.27 0.02) eV,whichisinagree- Æ on the underlying dissociation mechanism,[13] relevant for un- ment with the ionization potentialfor cyclopentanone of derstanding and modeling, for example, mass spectra.[14] (9.25 0.02) eV reportedbyChadwick et al.[16] The simulated Æ Here we report on an experimentalstudy of the (dissocia- spectrum matches the resonances in the measured spectrum tive) ionization and pyrolysis of cyclopentanone, using thresh- very well. Three major features at 9.30, 9.36 and 9.43 eV can be 1 old photoelectron photoion coincidence spectroscopy (TPEPI- assigned to aCH2 rocking motion (260 cmÀ ), aCC(=O) C [15] 1 À À CO). The pyrolysis products are sampled directly from the stretching vibration (776 cmÀ )and acombination of C=Oand 1 pyrolysis reactor and are identified by comparing their thresh- aC C(=O) Cstretch (1268 cmÀ ). À À old photoelectron spectra to data availablefrom the literature. Apyrolysis mechanism is proposed and discussed in light of Dissociative ionization the recent findings on cyclohexanonepyrolysis. The experi- mental findings on the dissociativeionization are rationalized The dissociative photoionization of cyclopentanone has been by hydrogen tunneling and amodel is constructed to test the studied to distinguish dissociative ionization products from viability of this fragmentation mechanism. thermaldecomposition products.[17] Furthermore, it is interest- ing to compare and contrastthe fragmentation processes in the neutralmolecule, induced by heat in the microreactor,and in the ion, broughtabout by the excess energyofanionizing Results vacuum ultraviolet (VUV) photon.Inthe breakdowndiagram in Figure