UC Berkeley UC Berkeley Previously Published Works Title Understanding Brønsted-Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory. Permalink https://escholarship.org/uc/item/8bt399xt Journal Chemphyschem : a European journal of chemical physics and physical chemistry, 19(4) ISSN 1439-4235 Authors Van der Mynsbrugge, Jeroen Janda, Amber Lin, Li-Chiang et al. Publication Date 2018-02-01 DOI 10.1002/cphc.201701084 Peer reviewed eScholarship.org Powered by the California Digital Library University of California DOI:10.1002/cphc.201701084 Minireviews Very Important Paper Understanding Brønsted-Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory Jeroen Vander Mynsbrugge+,[a, d] AmberJanda+,[a, e] Li-Chiang Lin,[c] Veronique VanSpeybroeck,[d] MartinHead-Gordon,[b] and Alexis T. Bell*[a] Acidic zeolites are effective catalysts for the cracking of large trinsic rate coefficient of the reaction (kint)atreaction tempera- hydrocarbon molecules into lower molecular weight products tures, since kapp is proportionaltothe product of Kads-H + and required for transportation fuels. However,the ways in which kint.Weshow that Kads-H+ cannot be calculated from experi- the zeolite structure affects the catalytic activity at Brønsted mental adsorption data collected near ambient temperature, protons are not fully understood.One waytocharacterize the but can, however,beestimated accurately from configuration- influenceofthe zeolite structure on the catalysis is to study al-biasMonte Carlo (CBMC)simulations. Using monomolecular alkane cracking and dehydrogenation at very low conversion, cracking and dehydrogenation of C3–C6 alkanes as an example, conditions for which the kinetics are welldefined. To under- we review recent efforts aimed at elucidating the influence of stand the effects of zeolite structure on the measured rate co- the acid site location and the zeolite framework structure on efficient (kapp), it is necessary to identify the equilibrium con- the observed values of kapp and its components, Kads-H+ and stant for adsorption into the reactant state (Kads-H +)and the in- kint. 1. Introduction Zeolitesare crystalline microporous aluminosilicates that are depends on the heteroatom(e.g. Al, Ti,B)[5] but not the zeolite widely used in the petroleum industry as catalysts to promote framework type[6,7] or Al location,[5] and on the spatial confine- the cracking of high molecular weightcompounds, principally ment[8–15] of the proton. The latter propertyisdefined by the alkanes, to lower molecular weight compounds required for size and shapeofthe pores and channels in the zeolite frame- gaseous and liquid fuels.[1–4] The catalytically active sites in zeo- work, which are similar in dimensions to those of reactants lites are Brønsted acidic protons that charge compensate and products and reaction transitionstates.Spatial confine- anionic sites produced by the substitution of trivalent Al for ment is also responsible forthe shape-selective properties of tetravalent Si in the zeolite framework ( Si-(OH)-Al ). The ac- zeolites.[15,16] tivity of such sites depends on the acidity of the proton, which Studies of alkane cracking have demonstrated that while cracking occurs mainly via abimolecular mechanism at high [a] J. Vander Mynsbrugge,+ A. Janda,+ Prof.Dr. A. T. Bell conversion,amonomolecular pathway in which the alkanes Department of Chemical and Biomolecular Engineering are cracked or dehydrogenated by direct interactionwith University of California, Berkeley,CA94720(USA) Brønsted acid protons prevails at very low conversions and low E-mail:[email protected] partial pressures (Figure 1).[17,18] The latter mechanism has been [b] M. Head-Gordon Department of Chemistry showntobefirst orderinthe alkane partial pressure, and the University of California, Berkeley,CA94720(USA) reactionrate in MFI (and by extension in equally or less confin- [c] L.-C. Lin ing frameworks) has been found to be unaffected by diffusion- WilliamG. Lowrie DepartmentofChemical and Biomolecular Engineering al transport to the activesites for alkanescontainingfewer The Ohio State University than 10 carbon atoms and typical zeolite crystallite dimensions 151 W. Woodruff Ave.,Columbus, OH 43210 (USA) of 0.1–2 mm.[19–21] For these reasons, monomolecular cracking [d] J. Vander Mynsbrugge,+ V. VanSpeybroeck Centerfor Molecular Modeling, Ghent University and dehydrogenation of alkanes are ideally suited for probing Tech LaneGhent Science Park CampusA the influence of zeolite structure (pore size and topology) on Technologiepark 903, 9052 Zwijnaarde (Belgium) the intrinsic activity and selectivity of Brønsted acid sites. [e] A. Janda+ The mechanisms by which alkane cracking and dehydrogen- Present address:Department of ChemicalEngineering ation occur are showninFigure 1. At lowconversion,both re- StanfordUniversity,Stanford, CA 94305 (USA) actions begin with adsorption of the alkane at aBrønsted acid [+] J.V.d.M. and A.J. contributed equally to this work site. Monomolecular cracking or dehydrogenation then occurs The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/cphc.201701084. from this reactant state and, as illustrated, can lead to avariety An invited contribution to aSpecialIssue on Reactions in Confined of products via secondary reactions at higher conversion. Spaces Figure2illustrates the processes of alkane adsorption into the ChemPhysChem 2018, 19,341 –358 341 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireviews Figure 1. Reactionmechanisms for alkane cracking over Brønstedacid zeolites at low conversion (monomolecular reactions; blue) and high conversion (bimolecular reactions; red). Figure 2. Schematic enthalpyand entropy landscapes illustrating the various steps in monomolecular reactions of alkanesinaBrønstedacid zeolite: the alkane adsorbs onto an active site, is converted into an alkene and asmaller alkane(cracking, m <n)orH2 (dehydrogenation, m =n), and the cracking or de- ° ° hydrogenation products desorb from the active site.Apparentactivationparameters (DH app and DS app)extracted directly from experimentalmeasurements are determined by both the adsorption enthalpy (DHads-H+) and adsorption entropy (DSads-H +)ofthe reactantsatthe Brønstedacid sites and the intrinsic ° ° activationparameters (DH int and DS int)associated with the chemicaltransformation at the active site. reactantstate and the subsequent reaction in terms of the rel- of alkane within the zeolite pores is small due to the weak in- evant changes in the enthalpy and entropy of the reactantrel- teraction of the alkane with the Brønsted acid site and the low ative to its presence in the gas phase.Since the concentration concentration of alkane within the zeolite pores,[8,22] the occu- ChemPhysChem 2018, 19,341 –358 www.chemphyschem.org 342 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireviews pancy of these sites (qads-H +)can be described by Henry’s law: and entropy changes shown in Figure 2. To do so, experimen- tal rate data are used to determine the apparent activationen- ° ° qads-H KH-H Pa 1 thalpy and entropy(DH and DS )from the temperature þ ¼ þ ð Þ app app dependence of the measured first-order rate coefficients,kapp, [8–12,22,24–26] In [Eq. (1)], KH-H + is the Henry’s law constant for alkane ad- (e.g. an Arrhenius plot) for each reactionpathway. ° ° sorptiontothe reactant state, which is defined as any configu- Values of DH int and DS int can then be determined from ration in which an alkane C Cbond is located within 5 of an [Eq. (4) and (5)],asfollows: [23] À Al T-atom, and Pa is the alkane partial pressure. The relation- ° ° ship of K to the dimensionless thermodynamic equilibrium DH app DHads-H DH int 4 H-H + ¼ þ þ ð Þ constantfor adsorption to the reactantstate, Kads-H +,isgiven ° ° [24] DS app DSads-H DS int 5 by [Eq. (2)] ¼ þ þ ð Þ RT It is evident from these equationsthat to determine DH° K H-H K ads-H 2 int þ VH nH þ ð Þ ° þ þ and DS int at reactiontemperatures (>673 K) it is necessary to first determine DH and DS at reactiontemperatures. where V is thevolumecontainedwithinone mole of reactant ads-H+ ads-H H+ Until recently,[27] this has not been attempted using experimen- statespheres of radius 5 and n is themoles of protonsper H+ tal adsorption measurements because of the tendency of the unit mass of zeolite. Undersuchcircumstances,the apparent alkane to relocatetosiliceous parts of the framework at high rate coefficient(k )ofanelementaryreactiondepends on app temperature, and because of the occurrence of chemical reac- both the thermodynamics of the adsorption at the active sites tions at temperaturesabove 600 K.[28–30] Moreover, several (K )and the intrinsic rate coefficient (k )[Eq. (3)]:[9,24] ads-H+ int theoretical studies have shownthat the distribution of ad- kapp K ads-H kint 3 sorbed alkanes within the pores of azeolite changes with the þ ð Þ adsorption temperature, and consequently any insights To improveoverall performance, and select or design the gleaned from low temperature adsorption experiments are un- optimal catalystfor aspecific chemical transformation, it is im- likely to be transferrable to reactiontemperatures.[24,28–31] To portanttounderstand how the environment enclosing the overcome these concerns, Janda et al. have developed atheo- acid sites affects the enthalpy (DH)and entropy(DS)changes retical approach based on configurational-bias Monte Carlo