Angewandte Research Articles Chemie How to cite: Angew.Chem. Int. Ed. 2021, 60,2304–2311 Heterogeneous Catalysis International Edition: doi.org/10.1002/anie.202009835 German Edition: doi.org/10.1002/ange.202009835 Directing the Rate-Enhancement for Hydronium Ion Catalyzed Dehydrationvia Organization of Alkanols in Nanoscopic Confinements Manish Shetty,Huamin Wang,* Feng Chen, Nicholas Jaegers,Yue Liu, Donald M. Camaioni, Oliver Y. GutiØrrez, and Johannes A. Lercher* Abstract: Alkanol dehydration rates catalyzed by hydronium tivity in these environments,asboththe adsorbed intermedi- ions are enhanced by the dimensions of steric confinements of ates and transitions states are influenced by the confinement, zeolite pores as well as by intraporous intermolecular inter- often having convoluted effects.[3] actions with other alkanols.The higher rates with zeolite MFI Theeffects of confinements on adsorption and stabiliza- having pores smaller than those of zeolite BEA for dehydra- tion of intermediates at gas-solid interfaces have been studied tion of secondary alkanols,3-heptanol and 2-methyl-3-hex- in detail.[1a,b, 3,4] Extrapolation of understanding from the gas anol, is caused by the lower activation enthalpyinthe tighter phase to the condensed phase is challenging,because the confinements of MFI that offsets aless positive activation organization of the condensed phase in the pores modifies the entropy. The higher activity in BEA than in MFI for nature of active sites,the interaction of reaction intermediates dehydration of atertiary alkanol, 2-methyl-2-hexanol, is with the active sites,and the reaction pathway.[5] In the primarily attributed to the reduction of the activation enthalpy presence of water, hydrated hydronium ions form. These ions by stabilizing intraporous interactions of the Cb-H transition induce alower standard free energy of activation for cyclo- state with surrounding alcohol molecules.Overall, we show hexanol dehydration than unconfined hydronium ions.[5c] that the positive impact of zeolite confinements results from the Confinements like those in MFI stabilize the transition state stabilization of transition state provided by the confinement (TS) by van der Waals contacts between the TS and the and intermolecular interaction of alkanols with the transition zeolite pores,which in turn leads to alow activation enthalpy state,which is impacted by both the size of confinements and that compensates for the lower activation entropy,than wider the structure of alkanols in the E1 pathway of dehydration. BEA pores.[5d] Stabilization of the acidic proton relative to positively charged TSs dramatically impacts the rates of acid- Introduction catalyzed reactions of hydroxy-containing compounds.[6] Theorganization of the solvent and reacting substrate Molecular-sized zeolite confinements enhance reaction greatly influence the activity of active sites in nanoscopic rates for awide range of organic transformations.[1] These rate confinements through enthalpic and entropic stabilization of enhancements for active sites within these microenviron- intermediates and TSs.[7] Theintraporous intermolecular ments are accompanied by increased complexity at the interactions including hydrogen bonding of the ground states, molecular level that makes understanding the kinetics reactive intermediates,TSs and surface functionalities includ- difficult.[2] Multiple factors influence the activity and selec- ing defects (silanol nests) inside zeolite pores create extended structures around active sites and reactive species.[6,7] Thefree energies of the reactive species and elementary reaction steps [*] Dr.M.Shetty,Dr. H. Wang, Dr.F.Chen, N. Jaegers, are affected by the reorganization of the intraporous environ- Dr.D.M.Camaioni, Dr.O.Y.GutiØrrez, Prof. J. A. Lercher ment to accommodate reactive intermediates and TSs.[6–8] Institute of Integrated Catalysis, Pacific Northwest National Labo- Thus,understanding the influence of molecular structure on ratory (PNNL) the organization of substrates,pore environments,and kinetic P.O. Box 999, Richland,WA99352 (USA) E-mail:[email protected] parameters is crucial for advancing catalyst design and [email protected] discovery. Dr.Y.Liu, Prof. J. A. Lercher Alcohols with asubstantial diversity of molecular struc- Department of Chemistry and Catalysis Research Center,Technical tures are key intermediates in the valorization of biomass to UniversityMünchen fuels and chemicals.Therefore,the question arises,how Lichtenbergstrasse 4, 85747 München (Germany) catalysts need to be designed to convert the complex feed- Supportinginformation and the ORCID identification number(s) for stock with balanced rates.Ontungstated zirconia, substituted the author(s) of this article can be found under: hexanols (secondary and tertiary) and alcohols with methyl https://doi.org/10.1002/anie.202009835. branching on the b-carbon exhibit higher reaction rates and 2020 The Authors. Published by Wiley-VCH GmbH. This is an lower activation energies than linear alcohols.[9] Considering open access article under the terms of the Creative Commons that zeolites provide higher rates than macroporous solid Attribution Non-Commercial NoDerivs License, which permitsuse [5c,d] and distribution in any medium, provided the original work is acids such as tungstated zirconia, we decided to address properly cited, the use is non-commercial and no modificationsor the impact of size and structure on the condensed phase adaptations are made. dehydration of alcohols. 2304 2020 The Authors. Published by Wiley-VCH GmbH Angew.Chem.Int. Ed. 2021, 60,2304 –2311 Angewandte Research Articles Chemie 1 o In this work, we used thermochemical and kinetic 1.2 to 6.2 kJmolÀ )and leads to again in entropy (DSl,a À À 1 1 measurements,and isotope labeling to quantitatively inves- between 31.6–42.7 JmolÀ KÀ ). Theentropy gain is hypothe- tigate the reaction pathway and kinetics of C7 alkanol sized to be caused by disruption of the hydrogen bonding dehydration. We investigated alkanols with alinear aliphatic network of the hydronium ion due to the association with chain (3-heptanol) and methyl substitution at a and b-carbon alcohol. (2-methyl-2-heptanol and 2-methyl-3-hexanol) that effective- Theolefin formation rates increased marginally from 3- ly mediate the organization of the substrates in the pore and heptanol to 2-methyl-3-hexanol with the branching at b- the stabilization of the elimination TS in confined pores in carbon, along with alarge increase from 2-methyl-3-hexanol zeolite.The hydronium ions in zeolites lead to lower standard to 2-methyl-2-hexanol with the origin of branching shifting free energy barriers than hydronium ions in aqueous sol- from b to a-carbon (Figure 1). Theapparent activation utions,but both the activation enthalpy and entropy and the barriers for the dehydration of 3-heptanol, 2-methyl-3- reaction pathways inside zeolite pores are affected by the hexanol, and 2-methyl-2-hexanol decreased from 162 4to 1 Æ substrate structure and the stabilization of TS within the 154 7, and 142 2kJmolÀ ,respectively (Figure 1). The Æ Æ intraporous zeolite environment. apparent activation barriers decreased with increasing stabil- ity of the carbenium ions following the C Obond cleavage À step in an E1-elimination pathway from 3-heptanol to 2- Results and Discussion methyl-2-hexanol. Dehydration of alkanols by unconfined hydronium ions in water Adsorption of alkanols inside zeolite pores We first consider the dehydration of 3-heptanol, 2-methyl- 3-hexanol, and 2-methyl-2-hexanol in aqueous solutions. Next, we determine the microenvironment in the pores of Phosphoric acid was used to provide unconfined hydronium zeolites,used for the alkanol dehydration in aqueous sol- ions in water.The initial rates of olefin formation are utions.Alkanol adsorption isotherms were first measured and normalized to the concentration of hydronium ions after alkanol to BASratios were estimated at 298 K. Then, the corrections for temperature,the dissociation equilibrium alkanol to BASratios were extrapolated to reaction temper- (pKa)ofphosphoric acid, and the association equilibrium of atures to investigate the pore environment under reaction the alcohol with the hydronium ions in solution (Sec- conditions.The water adsorption inside zeolite pores under tion S.1.3). Theolefin formation rates show fractional positive high water chemical potentials used here,are associated with orders (0.46 0.14, 0.63 0.06 and 0.75 0.03 for 3-heptanol, hydrated hydronium ions.[10] Forexample,Eckstein et al., Æ Æ Æ 2-methyl-3-hexanol and 2-methyl-2-hexanol at 443, 423 and reported hydronium ion cluster size of 8 1HO/BAS inside Æ 2 363 K, respectively) with varying concentration of hydronium MFI pores.[10] We hypothesize the size of hydronium ion ions in solution (Figure S7). We note that the association of clusters to be slightly larger in BEA pores. alcohols to hydronium ions in solutions (determined from the Adsorption isotherms of alkanols on HMFI and HBEA regression of olefin formation rates at two different concen- zeolites (henceforth referred as MFI and BEA, respectively) trations,Figure S5) is nearly thermoneutral (DH o between from binary alkanol-water mixtures ( 0.005–0.1 M) were l,a 1 1 Figure 1. Olefin formation rates (molalcohol molacid sitesÀ sÀ )and activation energies for dehydrationofalkanols;a)3-heptanol and b) 2-methyl-3- hexanol and 2-methyl-2-hexanol. Reaction conditions: Reactor was pressurized with 40 bar H2 at ambient temperature and stirred vigorously