Directing the Rate‐Enhancement for Hydronium Ion Catalyzed Dehydration Via Organization of Alkanols in Nanoscopic Confinements

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 dehydration 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 stability 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 at 700 r.p.m. Concentration of around 0.2–0.3 Mbased on density of water at room temperature. The rates were determined from the formation of 1 1 olefin after the set temperature was reached. Turnover frequencies are determined as olefin formation rates (molLÀ sÀ )normalizedtothe concentration of hydronium ions (H3PO4)ortotal BAS (zeolite). The concentration of hydronium ions in water dependsontemperature and alcohol concentration. Activation energiesare determined from Arrhenius plots (Supporting Information,Table S2).

Angew.Chem. Int.Ed. 2021, 60,2304 –2311 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 2305 Angewandte Research Articles Chemie

Table 1: Adsorption parameters for alcohol uptake into the two zeolites denoted by BEA and MFI at 298 Kand reaction conditions(433 Kfor 3- heptanol and 2-methyl-3-hexanol and 373 Kfor 2-methyl-2-hexanol).[a]

o o 1 o 1 [c] [d] Substrate Kads DHads [kJmolÀ ] DSads qmax [mmolgÀ ] V Alkanol/BAS 1 1 [J molÀ KÀ ] 298 KReaction conditions[b] MFI BEA MFI BEA MFI BEA MFI BEA MFI BEA MFI BEA MFI BEA 3-heptanol 833 (28) 233 (17) 27 21 36 24 1.08 1.51 0.62 0.90 0.74–0.87 0.62–0.80 1.47 4.31 À À À À 2-methyl-3-hexanol 331 (16) 221 (20) 24 19 31 19 1.02 1.64 0.57 1.06 0.62–0.80 0.67–0.83 1.24 5.27 À À À À 2-methyl-2-hexanol 246 (41) 199 (43) 22 19 28 21 0.95 1.77 0.66 1.15 0.80–0.91 0.81–0.91 1.64 6.27 À À À À [a] Adsorptionconstantsdetermined from the slope of the linearized Langmuir isotherm. Adsorption constantsreported at 298 K. The numbers in

brackets correspond to adsorption constantsextrapolated to reaction conditionsfrom 298 K. [b] qmax extrapolated from thermal expansionreaction o conditionsfrom 298 K. [c] V refers to fraction of the total uptake at concentrationrange of 0.10–0.25 Matthe same temperature for Kads and qmax for the alkanols. [d] Alkanol/BAS estimated at 0.25 Mconcentration.

measured between 280 and 313 Ktoinvestigate the environ- Dehydration activity of hydronium ions inside zeolite pores ment inside the zeolite pores (Figure S9 and Section S.1.6). Thephysicochemical properties are compiled in Supplemen- Thereactivity of the alkanols in MFI and BEA shows that tary Table S1 and Figures S1–S4. By fitting Langmuir adsorp- hydrated hydronium ions in the constrained environment tion isotherms to the uptakes,the equilibrium constants were more than an order of magnitude higher than uncon- o (Kads )and saturation uptakes (qmax)were determined (Ta- fined hydronium ions in H3PO4 solution (Figure 1). While the ble 1). At 298 K, the ratio of saturation uptake of alcohol to olefin formation rates were dependent on alcohol concen- the BAS(Alkanol/BAS) were determined to be 2.9, 2.7 and trations in H PO solution (Figure S6 and S7), the zeolites 3 4 2.5 in MFI, and 8.9, 9.6 and 10.4 on BEA, for 3-heptanol, 2- exhibit anear zero-order rate dependence in the concen- methyl-3-hexanol and 2-methyl-2-hexanol, respectively (Ta- tration range of 0.10–0.50 M, (Figure S8) driven by the o ble 1). Adsorption enthalpies (DHads )were determined from complete association of hydronium ions with alkanols inside o o the VantHoff plot of Kads .The Kads of alcohol is lower with zeolite pores (Table 1). Mass transfer limitations can be ruled o o BEA than with MFI pores.The DHads and DSads were lower out based on the previous work on the zeolites for similarly- with MFI than with BEA because of the stronger dispersive sized substrates.[5c,d] Therole of pore diffusion on the kinetic interactions of the alcohols with the narrower pore of MFI. In measurements was further ruled out by the near-zero-order o both zeolites,the Kads of the alcohol inside the pores dependence of dehydration rates on alcohol concentration decreased as its branching increased, but to alarger extent (Section S.2.3). in MFI than in BEA. This is attributed to the less ideal fit and Forthe dehydration of 3-heptanol and 2-methyl-3-hex- the beginning of repulsive interactions of the branched anol, the activation barriers on BEA resemble those in alkanes analogous to similar observations with alkanes.[11] aqueous solution (Figure 1) with values 163 4and 156 o 1 Æ Æ This phenomenon is well documented by the higher DHads 2kJmolÀ ,respectively.The activation barriers on MFI are 1 from 3-heptanol to 2-methyl-2-hexanol. In the larger BEA lower with values 143 6and 141 4kJmolÀ ,respectively. Æ Æ pore,the lower interaction of alcohol molecules with the Forboth substrates,hydronium ions in the smaller pores of larger BEA pore leads to minimal changes with the alcohol MFI were more reactive than hydronium ions in BEA. MFI o structure.Any variations in DHads were,however, compen- was marginally more reactive than BEA for 2-methyl-3- sated for by avariation in DS o showing aperfect compen- hexanol, within afactor of 2. For2-methyl-2-hexanol, the ads sation behavior for all alcohols and zeolites studied. The apparent activation barriers were similar for both BEA negative DS o contributes to the lowering of K o at higher (131 5) and MFI (128 4). Notably,the wider pore BEA ads ads Æ Æ temperatures. was more reactive than MFI for 2-methyl-2-hexanol, which

Themaximum uptake of alcohols (qmax)did not vary contrasts the behavior of 2-methyl-3-hexanol with avery markedly for the same zeolite.Its decrease with increasing similar molecular size.Itsuggests that additional factors temperatures is attributed to the thermal expansion of the related to the alcohol structure govern the reactivity of adsorbed phase (Supplementary Table S17). Themaximum confined hydronium ions (vide infra). uptakes at reaction conditions (433 Kfor 3-heptanol and 2- methyl-3-hexanol and 373 Kfor 2-methyl-2-hexanol) corre- 1 spond to 0.6, 0.6, and 0.7 mmolgÀ in MFI and 0.9, 1.1, Dehydration reaction mechanism of different alkanols 1 and 1.2 mmolgÀ in BEA (Table 1). Theratio of alcohol molecule to hydronium ion (Alkanol/BAS) is 1.2–1.6 with We investigated the influence of the alcohol structure on MFI and 4.3–6.3 with BEA at 0.25 M(Table 1) under reaction the mechanism by measuring kinetic H/D isotope effects conditions.The ratio of uptake of alcohol is higher in both (KIE) along with 16O-18Oexchange experiments (Table 2). zeolites than the hydronium ion concentration. Therefore,we KIE values between 2.1 and 2.4, 2.4 and 3.0, and 2.6 and 3.2 determined first, that the hydronium ions are fully associated were observed for 3-heptanol, 2-methyl-3-hexanol, and 2- with alcohol molecules and second, that pores of BEA are methyl-2-hexanol, respectively,for hydronium ions in solu- enriched with more alkanol molecules relative to hydronium tions and inside zeolite pores.AKIE of such magnitude

ions as compared to MFI pores. implies that aCb-H(D) bond cleavage is involved in the

2306 www.angewandte.org 2020 The Authors. Published by Wiley-VCH GmbH Angew.Chem.Int. Ed. 2021, 60,2304 –2311 Angewandte Research Articles Chemie

Table 2: H/D KIE and 18Oexchange experiments.[a] hydrolysis of formed ether as alike- 18 Substrate H/D KIE 18Oexchange/Olefin formation ly pathway of Oexchange.The 18 H3PO4 MFI BEA H3PO4 MFI BEA SN2pathway for Oexchange between the alkanols (secondary/ter- 3-heptanol 2.1 2.3 2.1 0.5 0.3 0.6 2-methyl-3-hexanol 3.0 2.4 2.8 0.9 1.0 1.0 tiary) and water can be ruled out for 2-methyl-2-hexanol 3.2 2.4 2.6 4.5 3.6 3.5 the alkanols due to steric reasons

18 18 and the reduction in local activity of [a] Extent of Oexchange from H2 O(97%isotopic purity) into unlabeled alkanol and its conversion 18 water molecules in the vicinity of during dehydration (concentration:0.25 MinH2 O). H/D KIE calculated from comparison of alkanol dehydration at aconcentrationof0.05–0.10 M. Details given in the Supporting Information (Tables S12– the protonated alcohol, involved in [5c,d,14] S16). the SN2pathway. Theanalysis of product selectivities further point towards an E1 kinetically relevant step (Section S.1.5). Forbrevity,inanE1 elimination pathway (Section S.2.1). Forexample,the dehy- mechanism, as shown in Scheme 1, an alcohol coordinates dration of 2-methyl-3-hexanol forms 2-methyl-1-hexene with hydronium ions.The subsequent protonation leads to an (formed from the Cb-H cleavage of the 2-methyl-2-hexyl increase in standard free energy and passes through aTS carbenium ion from the 1,2 hydride shift of the 2-methyl-3- (TS °). Next, astepwise cleavage of C-O (TS °)and C H hexyl carbenium ion following C Obond cleavage) with 1 2 À À (TS °)occurs through astable carbenium ion intermediate. aselectivity of 15–18%onall catalysts.The largely 3 TheKIE values are inconsistent with C Obond cleavage step invariant product selectivities for the alkanols in awide range À in E1 mechanism being rate limiting.KIEs for rehybridization of conversion of alkanol, from 2–45%(Figure S10) points of a-C from sp3 to sp2 is estimated to be < 1.35 at temper- to the dominant coverage of alkanol-derived species on the atures greater than 808Cfor all alcohols.Inturn, either an E1 acid site and negligible olefin hydration rates under differ- mechanism with akinetically relevant C Hbond cleavage ential conversions.The extent of 18Oincorporation is similar À step or an E2 mechanism with concerted C-O and C Hbond in water and in zeolite pores for all substrates,suggesting the À cleavage is consistent with the KIE values. substrate rather than the zeolite pores affect the relative relevance of C-O and C Hbond cleavage steps.The larger À KIE value for 2-methyl-2-hexanol as compared to 3-heptanol is accompanied by the increased 18Oincorporation in the recovered alcohol. This establishes the increased relevance of C Hbond cleavage in the reaction energetics for 2-methyl-2- À hexanol that follows the increased stability of carbenium ion following C Obond cleavage step.Taken together,we À conclude that all three alkanols follow E1 mechanism for the hydronium ion catalyzed dehydration on all three catalysts.

Apparent activation enthalpies (DHo°)and entropies (DSo°)

Theactivation enthalpies (DHo°)and entropic contributions (TDSo°)are compiled in Figure 2. It should be emphasized that both DHo° and DSo° include all the changes

in enthalpy and entropy from the ground state to the Cb H ° À bond cleavage transition state (TS3 ,Scheme 1), including protonation, C Obond cleavage,and C Hbond cleavage Scheme 1. Elementary steps for an E1 mechanism for alcohol dehydra- À À tion to olefin in water. steps.The intrinsic catalytic activity of hydronium ions is the lowest in unconstrained environments as seen by the highest DGo° values (Supplementary Table S11). Thehigh DSo° on all 18 Theresults with unlabeled alkanol and H2 Oassolvent catalysts point to asignificant product-like TS with significant (Table 2) at 0.25 Mand the negligible hydration of olefin C Hbond breakage in the kinetically relevant TS for all bÀ allows us to conclude that only the E1 mechanism is substrates and the existence of E1 mechanism. Thevalues of consistent with the large amount of 18Oincorporation in the DSo° are dependent on the microenvironment (either in recovered alcohol, whereby the 18Oincorporation occurs unconstrained water or in zeolite pores). Notably,inside both 18 o° from the recombination between H2 Oand carbenium ion BEA and MFI pores,the entropic contributions (TDS )at 1 intermediate that follows the C Obond cleavage (Scheme 1). 400 Kare within 4kJmolÀ for all alkanols.Ingeneral, the À Thepotential role of framework oxygen exchanged with 18O gain in entropy going from ground state to the TS is can be ruled out,[12] as the alkanol is associated with ahydrated significantly greater in BEA compared to MFI for all the hydronium cluster rather than with the framework.[13] The three substrates (Figure 2), attributed to the higher accessible negligible ether formation allows to rule out also the volume inside the larger void space inside BEA pores.For an

Angew.Chem. Int.Ed. 2021, 60,2304 –2311 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 2307 Angewandte Research Articles Chemie

cannot be simply explained by the stability of the carbenium ion, because,ifitwere the dominant factor governing its activity inside the zeolite pores,the relative reactivity trends across the alkanols on zeolites should be unaffected. Next, we explore the underlying reasons for this observation and

consider the different enthalpic stabilization of the Cb-H cleavage TS on BEA and MFI across the alkanols due to intraporous alkanol concentration, structural factors of the alkanols,and confinement size dimensions.

Influence of confinement dimension, intraporous alkanol concentration, and alkanol structure on enthalpic stabilization

of Cb-H transition state

Theactivation enthalpy determines the reactivity difference among the alkanols and also their different responses to the changing confinement dimensions (provided by BEA and MFI). We first rule out the role of the nature of the ground state for the alkanols (alcohol adsorbed inside zeolite pores with hydronium ions) and the size of hydronium ion clusters. Figure 2. Activation enthalpies (DHo°)and entropic components Theformation of ground-state from the alkanol in the o° (TDS )atT = 400 Kfor the hydronium ion catalyzed dehydration of 3- aqueous phase involves the adsorption of alkanol inside the heptanol, 2-methyl-3-hexanol, and 2-methyl-2-hexanol in water (H3PO4) zeolite pores and subsequent association with the hydronium and MFI and BEA zeolites. Data for H PO ,MFI, and BEA are shown 3 4 ions.Asnoted earlier,wehypothesize the size of hydronium in black, blue, and red, respectively.The activation enthalpies and entropies are derived from kinetic measurements and the TS formal- ion clusters inside BEA to be larger than 8 1H2O/BAS [10] Æ ism (Section S.1.4). reported inside MFI pores. Thefavorable enthalpic or entropic contributions towards stabilization of the ground- state (inside MFI pores than BEA and across all substrates) E1 mechanism, the similar compiled activation entropies can be ruled out via adsorption isotherms (Table 1and (DSo°)for the TS of the three substrates in individual Supplementary Table S22). First, only minor changes are microenvironment (inside the confinements of MFI and observed in the ground-state enthalpies relative to alcohol BEA, and in solution), implies asimilar activation volume molecules in the aqueous phase (Table 1);the differences are 1 1 compared to the ground state for the substrates in the within 5kJmolÀ on MFI and 2kJmolÀ on BEA for all three individual microenvironments.Asanote in passing,wewould substrates.Therefore,the changes in the DHo° values cannot like to point to the two apparent compensation correlations be attributed to the changes in ground-state energies relative (the two secondary alkanols and 2-methyl-2-hexanol) be- to the TS (Supplementary Table S22). Thehydronium ion tween standard transition enthalpies and entropies in Fig- protonates the associated alkanol (Scheme 1). Therole of the ure S13 with alkanol molecules in the liquid state as hypothesized size difference of hydronium ion towards the areference. stabilization of kinetically relevant C Hbond cleavage (and bÀ We next explore the DHo° values across the alkanols to the formation of hydronium ions) inside MFI and BEA pores interrogate the reactivity differences inside zeolites.There is can be ruled out as the proton affinities of the hydronium ion adecrease in DHo° from 3-heptanol to 2-methyl-2-hexanol for cluster above five water molecules inside zeolite pores has all catalysts,ascarbenium ions after C Obond cleavage in an been suggested to be negligible.[5c,d] À E1-type pathway become more stable going from 3-heptanol DHo° and 2-methyl-3-hexanol to 2-methyl-2-hexanol, in accordance Zeolite ¼ with the expected Polanyi relation for this step.With the increasing stability of the carbenium ion following C Obond DHo DHo DHo À Des alcohol Deprotonation H H2 O n Des H2 O n cleavage,the C Hbond cleavage step has ahigher degree of þ þðÞþ ðÞþ bÀ 1 ð Þ rate control in the reaction kinetics (Table 2). o o o° o° DHDPE DHProtonation H O n DHHomogenous However,the impact of carbenium ion stability on DH is þ ðÞ2 þ À much different over the zeolites,with the alkanol structure having agreater effect on DHo° inside the BEA pore DHo DHo DHo Es À vdW pore À IntermolecularAlcohol compared to MFI. Forinstance,from 2-methyl-3-hexanol to 2-methyl-2-hexanol, the activation enthalpies (DHo°)de- We consider next the differences in enthalpic stabilization 1 1 creased by 25 kJmolÀ on BEA as compared to 12 kJmolÀ on of the C Hbond cleavage due to differences in the bÀ MFI (Figure 2). In other words,2-methyl-2-hexanol behaves intraporous alkanol concentration, structural factors of the differently in response the change of confinement dimension alkanols,and confinement size dimensions that govern the as compared to the two secondary alkanols.Such difference difference in the observed trends in reactivity for MFI and

2308 www.angewandte.org 2020 The Authors. Published by Wiley-VCH GmbH Angew.Chem.Int. Ed. 2021, 60,2304 –2311 Angewandte Research Articles Chemie

BEA across the alkanols.Besides alarger confinement provided by BEA, the ratio of alcohol molecule to hydronium ion is 4.3–6.3 inside BEA pores,which is much larger than 1.2–1.6 with MFI at 0.25 Munder reaction conditions (Table 1). It suggests there exists at most one alcohol molecule between hydronium clusters on MFI and at least five for 2-methyl-2-hexanol vs.three and four for 3-heptanol and 2-methyl-3-hexanol, respectively,assuming association of one alcohol molecule with each hydronium ion. We note that the BEA pores were enriched with 2-methyl-2-hexanol as compared to other alkanols (Table 1), which is hypothesized to be caused either by the steric factors or by more favorable interactions of 2-methyl-2-hexanol with defect sites on BEA (internal silanol groups,Figure S1) as compared to water. Such enrichment of alkanols in the pore is likely to play an important role for stabilizing TS via intraporous intermolecular interactions. We now first examine the difference between BEA and

MFI in the enthalpic stabilization of the Cb-H TS over agiven alkanol by analyzing aBorn-Haber thermochemical cycle and then elucidate how structural differences of these alkanols further impact the reactivity differences between the two zeolites.Asshown in Scheme 2, analysis of such athermo- chemical cycle helps in the investigation of how zeolite pores and alcohol properties individually influence the observed Scheme 2. Thermochemical cycle for the elimination transitionstate of activation enthalpy.[3] Theconsidered steps are arbitrary,but alkanols inside zeolite pores. The thermochemical cycle involvesthe the state function nature of thermodynamics enables the formation of the C -H TS DHo° inside zeolite pores from the [3] b Zeolite decoupling the catalyst and molecular effects. ground state (alcohol adsorbed inside zeolite pores). The TS formation ÀÁ Thethermochemical cycle includes steps such as 1) has contributionsfrom the desorption of reacting alcohol o desorption of an alcohol molecule to the liquid (water) (DHDes Alcohol), deprotonation of the water cluster (DHo ), desorption of the water cluster (DHo ), phase;2)deprotonation of the hydrated hydronium ion Deprotonation H H2 O n Des H2 O n and deprotonationþðenthalpyÞ (DHo )tonon-interacting distancesðÞin leading to a“dry” proton on the framework Al and water DPE the aqueous phase. This is followed by protonation of water cluster to cluster and 3) desorption of water cluster into the liquid form hydronium ions (DHo )and subsequent formation of ProtonationH2 O n phase;4)deprotonation of the “dry” proton and moving the o° ðÞ Cb-H TS (DHHomogeneous)inaqueous phase. Finally,the Cb-H TS is proton to non-interacting distances (deprotonation energy); adsorbed back inside the zeolite pores with contributions from electro- o 5) protonation of water cluster to form hydrated hydronium static interactionofthe ion-pair (DHEs), the van der Waals stabilization o ion in aqueous phase;6)protonation of alcohol, C Obond from the zeolite pore walls (DHvdW Pore)and the intermolecular À D o cleavage of the alcohol in the liquid phase leading to the interactions with intraporousalkanolmolecules ( HIntermolecular Alcohol). relevant late C Hbond cleavage TS (as shown in Scheme 1). bÀ Thefinal step for placing the TS inside the zeolite pore can be further considered to be composed of 7) an electrostatic component given by the interaction of the TS with the In contrast, the deprotonation (step 2) and protonation negative charge on the framework, 8) the van der Waals energies (step 5) may be considered to be slightly different (vdW) interactions of the TS with the pore walls,and 9) across zeolites due to varying hydronium ion cluster size. intraporous intermolecular interactions considering there However,proton affinities of hydronium-ion size clusters exists more than one alkanols near TS in the zeolite pores. start approaching aconstant value as the cluster size starts Thethermochemical cycle enables representation of the approaching asize of n = 5, as compared to asize larger than activation enthalpy of late C Hbond cleavage TS with 8 1inside MFI and BEA pores.[5c] Therefore,the enthalpic bÀ Æ respect to the steps (1) through (9). values of steps 2and 5are not expected to vary appreciably. We first consider that step 2, 4, 5, 6, and 7are expected to Step 6, hydronium-ion catalyzed dehydration in water with- be similar for MFI and BEA. Thedeprotonation energy of out zeolite confinement, leading up to the TS is also dry proton (step 4), which is independent of the confinement independent of the zeolites. as interaction of zeolite pores and protons are not appreciably Therefore,the activation enthalpy difference between [15] o° affected by the confinement. Theelectrostatic component MFI and BEA for agiven alkanol, DDH MFI-BEA is given by: (step 7) would depend on the charge distribution in the TS and their interaction with the anion. Such charge distributions o° o o DDHMFI BEA DDHDes Alcohol; MFI BEA DHDes H2O n; MFI BEA can be considered to be similar across all zeolite frameworks À ¼ À þ ðÞ À À 2 ð Þ because of similar acid strengths and similar stabilities of o o [15] DDHvdW;MFI BEA DDHIntermolecularAlcohol; MFI BEA conjugate anions at all framework locations. À þ À

Angew.Chem. Int.Ed. 2021, 60,2304 –2311 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 2309 Angewandte Research Articles Chemie

Thecontribution from desorption of alcohol (step 1) can substrate molecules.The higher dehydration rates of secon- be estimated by adsorption enthalpies calculated from the dary alkanols,3-heptanol and 2-methyl-3-hexanol, in MFI isotherms (Table S21). Together with the measured activation zeolite with pores smaller than those of zeolite BEA, is enthalpy,the combined vdW (step 8) and intermolecular caused by alower activation enthalpy in the tighter confines interactions (step 9) along with the desorption of water cluster of MFI. It offsets aless positive activation entropy.With the (step 3) contributing towards the difference in stabilization of increasing ease in the formation of C HTSfor 2-methyl-2- bÀ TS in BEA pores compared to MFI is estimated to be 25, 20, hexanol, the stabilization provided by the confinement 1 and 5kJmolÀ for 3-heptanol, 2-methyl-3-hexanol, and 2- assumes lesser significance and an additional enthalpic methyl-2-hexanol, respectively.The desorption enthalpies of stabilization of the TS due to dispersive interactions with water cluster are not expected to be alkanol-structure other alcohol molecules become important. This makes the dependent, but rather dependent on the zeolite pore.We larger-pore BEA zeolite more reactive than the smaller-pore infer, therefore,the pore environment inside the BEA pore, MFI zeolite for dehydration of 2-methyl-2-hexanol. Our which enables the combined vdW and intermolecular inter- results demonstrate additional avenues for tuning the micro- actions accounts for the higher enthalpic penalties in the environment inside the zeolites to enhance rate kinetics inside formation of the TS going from 2-methyl-2-hexanol to 3- nanoscopic confinements. heptanol. Thequestion arises now as to how structural differences of the alkanols,especially between the tertiary 2-methyl-2- Acknowledgements hexanol and the secondary 2-methyl-3 hexanol and 3- heptanol, impact the vdW interaction of TS with zeolite pore This work was part of the Chemical Transformations Initiative and the intraporous intermolecular interactions differently in at the Pacific Northwest National Laboratory (PNNL), MFI and BEA. On one hand, with the decreasingly enthalpi- conducted under the Laboratory Directed Research and cally demanding TS formation from 3-heptanol to 2-methyl-2- Development Program at PNNL. D.M.C.and J.A.L. were hexanol (Figure 2), the vdW stabilization provided by the supported by the U.S. Department of Energy (DOE), Office narrower MFI pore over BEA assumes reduced significance of Science,Office of Basic Energy Sciences,Division of from the secondary to tertiary alkanols.Onthe other hand, Chemical Sciences,Geosciences and Biosciences (Transdisci- with the increasing ease of TS formation, the intraporous plinary Approaches to Realize Novel Catalytic Pathways to stabilization provided by the co-adsorbed alkanol molecules Energy Carriers,FWP 47319) for contributing to the dis- inside BEA assumes greater significance than the weaker cussion of the results,planning, and writing of the manuscript. vdW stabilization, making BEA more reactive.AsBEA is Some of the experiments were performed at the William R. entrained with more alkanol molecules than MFI, these Environmental Molecular Science Laboratory,anational intermolecular interactions assume greater significance.Bre- scientific user facility sponsored by the DOE Office of gante et al. have reported enthalpically unfavorable interac- Biological and Environmental Research located at Pacific tions between long aliphatic chain of olefin epoxidation TS Northwest National Laboratory.Open access funding ena- with water clusters.[7] Therefore,the higher entrainment of bled and organized by Projekt DEAL. alkanol molecules in BEA pores increases the relative importance of TS-alkanol interactions than TS-water interactions.The increased stabilization of TS of 2-methyl-2- Conflict of interest hexanol as compared to secondary alcohols inside BEA pores relative to MFI may be attributed to this larger enthalpic Theauthors declare no conflict of interest. stabilization driven by favorable TS-intraporous alkanol interactions.While the role of alkanol structure on the Keywords: aliphatic alcohols ·confinement effect ·

enthalpic stabilization of Cb-H TS is clear from our data, the enthalpy-entropy compensation ·hydronium ion ·zeolites defect sites and pore hydrophilicity can also contribute to the differences in alkanol uptakes inside zeolite pores and the [7,16] stabilization of Cb-H TS. This can be further explored with [1] a) S. H. Chai, H. P. Wang,Y.Liang,B.Q.Xu, Green Chem. 2007, zeolites with different Si:Al ratio and different synthesis 9,1130;b)R.Weingarten,J.Cho,W.C.Conner,G.W.Huber, methods.[7,16] Despite the difficulty in deconvolution of the Green Chem. 2010, 12,1423;c)P.A.Zapata,J.Faria, M. P. Ruiz, R. E. Jentoft, D. E. Resasco, J. Am. Chem. Soc. 2012, 134,8570; number of such complex interactions that affect the reactivity d) C. Zhao,J.A.Lercher, Angew.Chem. Int. Ed. 2012, 51,5935; on asolid–liquid interface,our study provides an important Angew.Chem. 2012, 124,6037. step in furthering the understanding of the catalysis at solid– [2] a) P. Deshlahra,R.T.Carr,E.Iglesia, J. Am. Chem. Soc. 2014, liquid interfaces. 136,15229;b)P.Deshlahra, E. Iglesia, J. Phys.Chem. C 2016, 120,16741;c)L.Annamalai, Y. L. Liu, S. Ezenwa, Y. L. Dang, S. L. Suib,P.Deshlahra, ACSCatal. 2018, 8,7051. Conclusion [3] G. Noh, Z. C. Shi, S. I. Zones,E.Iglesia, J. Catal. 2018, 368,389. [4] P. Deshlahra, E. Iglesia, ACSCatal. 2016, 6,5386. [5] a) Y. S. Liu, E. Barath, H. Shi, J. Z. Hu, D. M. Camaioni, J. A. We show here how the steric constraints of zeolite pores Lercher, Nat. Catal. 2018, 1,141;b)S.Prodinger, H. Shi, H. M. influence the catalytic activity of hydronium ions and how the Wang,M.A.Derewinski, J. A. Lercher, Appl. Catal. B 2018, 237, environment influences the local organization of solvents and 996;c)Y.S.Liu, A. Vjunov,H.Shi, S. Eckstein,D.M.Camaioni,

2310 www.angewandte.org 2020 The Authors. Published by Wiley-VCH GmbH Angew.Chem.Int. Ed. 2021, 60,2304 –2311 Angewandte Research Articles Chemie

D. H. Mei, E. Barath, J. A. Lercher, Nat. Commun. 2017, 8, [12] S. M. Pugh, P. A. Wright, D. J. Law,N.Thompson, S. E. 14113;d)H.Shi, S. Eckstein, A. Vjunov,D.M.Camaioni,J.A. Ashbrook, J. Am. Chem. Soc. 2020, 142,900. Lercher, Nat. Commun. 2017, 8,15442. [13] D. H. Mei, O. A. Lercher, J. Phys.Chem. C 2019, 123,25255. [6] M. A. Mellmer,C.Sener,J.M.R.Gallo,J.S.Luterbacher, D. M. [14] C. A. Bunton, A. Konasiewicz, D. R. Llewellyn, J. Chem. Soc. Alonso,J.A.Dumesic, Angew.Chem. Int. Ed. 2014, 53,11872; 1955,604. Angew.Chem. 2014, 126,12066. [15] L. Qi, R. Alamillo,W.A.Elliott, A. Andersen,D.W.Hoyt, E. D. [7] D. T. Bregante,A.M.Johnson, A. Y. Patel, E. Z. Ayla, M. J. Walter,K.S.Han, N. M. Washton, R. M. Rioux, J. A. Dumesic, Cordon, B. C. Bukowski, J. Greeley,R.Gounder,D.W.Flaherty, S. L. Scott, ACSCatal. 2017, 7,3489. J. Am. Chem. Soc. 2019, 141,7302. [16] M. J. Cordon, J. W. Harris,J.C.Vega-Vila, J. S. Bates,S.Kaur,M. [8] R. Gounder, Catal. Sci. Technol. 2014, 4,2877. Gupta, M. E. Witzke,E.C.Wegener,J.T.Miller,D.W.Flaherty, [9] J. Rorrer,S.Pindi, F. D. Toste,A.T.Bell, ChemSusChem 2018, D. D. Hibbitts,R.Gounder, J. Am. Chem. Soc. 2018, 140,14244. 11,3104. [10] S. Eckstein,P.H.Hintermeier, R. X. Zhao,E.Barath, H. Shi, Y. Liu, J. A. Lercher, Angew.Chem. Int. Ed. 2019, 58,3450; Angew. Chem. 2019, 131,3488. Manuscript received:July 17, 2020 [11] a) F. Eder,J.A.Lercher, J. Phys.Chem. B 1997, 101,1273;b)F. Acceptedmanuscript online: October 2, 2020 Eder,J.A.Lercher, Zeolites 1997, 18,75. Version of record online: November 27, 2020

Angew.Chem. Int.Ed. 2021, 60,2304 –2311 2020 The Authors. Published by Wiley-VCH GmbH www.angewandte.org 2311