Kinetics of Carboxylic Acid Enolization on Metal Oxides in Vapor Phase

Kinetics of Carboxylic Acid Enolization on Metal Oxides in Vapor Phase | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Kinetics of Carboxylic Acid Enolization on Metal Oxides in Vapor Phase Alexey V. Ignatchenko, Reem Ibrahim, Jordan R. Kostera, Jordan D. Walker, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8081029/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract An invisible, reversible, previously overlooked catalytic reaction — enolization — occurs consistently when carboxylic acid vapors contact metal oxide surfaces. While the CO₂H group responsible for adsorption readily forms surface carboxylates, the weaker α-C–H acidity becomes evident only through reversible H/D exchange. The enolized carboxylate condensation with another surface carboxylate becomes important in the mechanism of decarboxylative formation of ketones, as part of oxygen removal during biofuels upgrading. In our kinetic study, the rate of approaching equilibrium was measured for H/D isotopic exchange on alpha-carbon of isobutyric acid used in various concentrations in a vapor phase mixture with D 2 O as well as for reversed D/H exchange between alpha-deuterated isobutyric acid and H 2 O upon contact with monoclinic zirconia and anatase titania catalysts. Faster rate for H/D vs. D/H exchange points to alpha-deprotonation, i.e., enolization, as the rate determining step of the exchange mechanism. The intrinsic rate of enolization was deduced using McKay equation for equilibrium reactions. Kinetic activation parameters were obtained through temperature dependence of the rate constant for both exchange directions, H/D and D/H. KOH doping on ZrO 2 changes the geometry of the transition state leading to higher rates of enolization and increasing H/D kinetic isotope effect from 1.4 to 5.8. The opposite effect of KOH doping is observed on anatase TiO 2 – enolization rates are slightly decreased, k H /k D remains relatively constant at 2.6-2.8 indicating that the nature of basic centers on TiO 2 is unaffected. These results confirm C-C coupling, not enolization, being the rate limiting step of the decarboxylative ketonization mechanism. isobutyric acid isotopic exchange McKay method monoclinic zirconia anatase titania Full Text Additional Declarations No competing interests reported. Supplementary Files Supportinginfoenolizationvaporphase.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 18 Nov, 2025 Reviews received at journal 17 Nov, 2025 Reviewers agreed at journal 12 Nov, 2025 Reviewers invited by journal 11 Nov, 2025 Editor assigned by journal 11 Nov, 2025 Submission checks completed at journal 11 Nov, 2025 First submitted to journal 10 Nov, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8081029","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":547048665,"identity":"4f97fd55-3640-463d-856e-edfd1563b04f","order_by":0,"name":"Alexey V. 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Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"isobutyric acid, isotopic exchange, McKay method, monoclinic zirconia, anatase titania","lastPublishedDoi":"10.21203/rs.3.rs-8081029/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8081029/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAn invisible, reversible, previously overlooked catalytic reaction — enolization — occurs consistently when carboxylic acid vapors contact metal oxide surfaces. While the CO₂H group responsible for adsorption readily forms surface carboxylates, the weaker α-C–H acidity becomes evident only through reversible H/D exchange. The enolized carboxylate condensation with another surface carboxylate becomes important in the mechanism of decarboxylative formation of ketones, as part of oxygen removal during biofuels upgrading. In our kinetic study, the rate of approaching equilibrium was measured for H/D isotopic exchange on alpha-carbon of isobutyric acid used in various concentrations in a vapor phase mixture with D\u003csub\u003e2\u003c/sub\u003eO as well as for reversed D/H exchange between alpha-deuterated isobutyric acid and H\u003csub\u003e2\u003c/sub\u003eO upon contact with monoclinic zirconia and anatase titania catalysts. Faster rate for H/D vs. D/H exchange points to alpha-deprotonation, i.e., enolization, as the rate determining step of the exchange mechanism. The intrinsic rate of enolization was deduced using McKay equation for equilibrium reactions. Kinetic activation parameters were obtained through temperature dependence of the rate constant for both exchange directions, H/D and D/H. KOH doping on ZrO\u003csub\u003e2\u003c/sub\u003e changes the geometry of the transition state leading to higher rates of enolization and increasing H/D kinetic isotope effect from 1.4 to 5.8. The opposite effect of KOH doping is observed on anatase TiO\u003csub\u003e2\u003c/sub\u003e – enolization rates are slightly decreased, k\u003csub\u003eH\u003c/sub\u003e/k\u003csub\u003eD\u003c/sub\u003e remains relatively constant at 2.6-2.8 indicating that the nature of basic centers on TiO\u003csub\u003e2\u003c/sub\u003e is unaffected. These results confirm C-C coupling, not enolization, being the rate limiting step of the decarboxylative ketonization mechanism.\u003c/p\u003e","manuscriptTitle":"Kinetics of Carboxylic Acid Enolization on Metal Oxides in Vapor Phase","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-21 05:48:33","doi":"10.21203/rs.3.rs-8081029/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-18T15:46:29+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-17T09:43:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"300264208829144837784590272936111082073","date":"2025-11-12T09:47:59+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-11-11T16:56:52+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-11T05:13:48+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-11-11T05:12:56+00:00","index":"","fulltext":""},{"type":"submitted","content":"Catalysis Letters","date":"2025-11-11T00:27:26+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"catalysis-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Catalysis Letters](https://link.springer.com/journal/10562)","snPcode":"10562","submissionUrl":"https://submission.springernature.com/new-submission/10562/3","title":"Catalysis Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"bc52c35b-60a7-4b9c-99ed-adabeda6d88e","owner":[],"postedDate":"November 21st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2025-12-03T15:23:19+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-21 05:48:33","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8081029","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8081029","identity":"rs-8081029","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}