Propan-2-ol dehydration on H-ZSM-5 and H-Y zeolite:a DFT study

Antonio Prestianni • Remedios Cortese • Dario Duca

Received: 14 August 2012 / Accepted: 2 November 2012 / Published online: 27 November 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract The catalytic dehydration of propan-2-ol over H-Y and H-ZMS-5 alu-

minated zeolite models, mimicking both internal cavities and external surfaces, was

studied by DFT calculations to investigate the reaction mechanism. After the

adsorption of propan-2-ol on the zeolite, the dehydration mechanism starts with

alcohol protonation, occurring by one acidic –OH group of the zeolite fragment,

followed by a concerted b-elimination to give propene. The catalytic activity is

affected by the size of the zeolite cavity, which is larger in the H-Y than in the

H-ZMS-5 zeolite. The adsorption energy of the reagent, as an example, decreases in

the order: H-Y cavity ^ H-ZMS-5 surface [ H-ZMS-5 cavity, pointing that the

adsorption process should preferentially occur either on open surface or inside

larger cavity. More interestingly, confinement effects play a twofold role in driving

the reaction pathway, resulting in two different effects on the reaction outcomes.

The thermodynamic stability, evaluated by the standard free energy difference of

the products (water and propene) with respect to the reactant (propan-2-ol), would

indeed suggest that the reaction more smoothly could occur for the systems:

H-ZMS-5 surface[non-catalyzed[H-Y cavity[H-ZMS-5 cavity. The activation

standard free energy of the process conversely decreases in the order: non-catalyzed[H-ZMS-5 surface[H-ZMS-5 cavity[H-Y cavity, suggesting that the reaction

is faster inside zeolite cavities. Experimental and computational results are in

agreement, giving confidence on the atomistic-level insights provided.

Keywords Propan-2-ol dehydration � Confinement effects � Reaction modeling �DFT and MP2 calculations

A. Prestianni � R. Cortese � D. Duca (&)

Dipartimento di Chimica dell’Universita di Palermo, Viale delle Scienze Ed. 17,

90128 Palermo, PA, Italy

e-mail: dario.duca@unipa.it

123

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DOI 10.1007/s11144-012-0522-5

Introduction

The use of acidic zeolites as solid catalysts for hydrocarbon transformation has

important technological and environmental implications [1–4]. Compared to liquid

homogeneous catalysts, zeolite materials are simpler to be handled and have larger

operating temperature ranges. Zeolite derivatives are also characterized by the

formation of less toxic by-product and waste, by higher activity and selectivity and

by catalyst separation and regeneration feasibility [5].

Morphological and chemical characteristics of the zeolites play an important role

in molecular surface adsorption and in catalytic activity and selectivity [6–10].

Acidic aluminum-substituted zeolite materials indeed provide a local protonic

environment [11–13] to the adsorbed catalytic substrates, originating carbocations

(carbonium and/or carbenium), which play the role of either intermediate or

transition state (TS) species [14–16]. In particular, the small olefin derivatives

trapped inside the zeolite cavities can lead to bulk-alkoxide and framework-

anchored carbenium species [17, 18].

Pertaining to this context, alcohol dehydration is an important reaction catalyzed

by zeolites or more generally by silica-based materials [19–21]. Differently from

silica-based processes, such reaction is usually carried out by heating the substrate

(at very high temperature) in the presence of strongly acidic mixtures, also

involving harmful environment [22]. For safety and economic reasons, it is then

important to develop efficient acidic catalysts in order to bypass drastic reaction

conditions.

Recent experimental results showed that propan-2-ol dehydration on zeolite is a

feasible process [22–24]. However, the reaction mechanism is not fully clarified

and, in addition, the nature and role of zeolite sites is still debated. As an example

van de Water et al. [21] have recently investigated the effects of the partial

substitution of Si by Ge on the acidic catalytic properties of ZSM-5 based systems,

employed as catalysts in the propan-2-ol dehydration. Interestingly, they have found

that the Ge incorporation increases the zeolite mesoporosity, having only a

negligible effect on the acid strength. The increased mesoporosity noticeably seems

to enhance the catalytic efficiency of the Ge-ZSM-5 catalyst on the propan-2-ol

dehydration. Jana et al. [25] also observed that mesoporous aluminum-substituted

MCM-41 zeolite catalysts are more active than similar microporous aluminum-

substituted zeolite materials, when employed in the same alcohol dehydration

processes. Mesopore effects were finally demonstrated by zeolite-based processes,

in which mesoporosity seemed to ensure easy reagent accessibility and selective

transport properties, inducing shape-selectivity of potential industrial interest [26].

Computational chemistry has an enormous impact on the development of zeolite

catalytic materials and processes. Focusing on the ZSM-5 systems, few literature

examples of interest for the present work were reported. They namely concern

(i) reactivity [16, 27–29], (ii) adsorption–desorption processes [8, 13], (iii) reverse

hydrogen spillover in the presence of metal centers [30] as well as (iv) Si/Al molar

ratio effects on the location and characterization of the acidic sites [31–34]. With

respect to the latter point, the oxygens chosen for mimicking the title reaction were

those corresponding to the most stable sites of the considered fragments, both for

566 Reac Kinet Mech Cat (2013) 108:565–582

123

the H-ZSM-5 [34] and H-Y models [35]. In this context, we already reported the

adsorption and cis/trans isomerization of but-2-ene within a 22T1 H-ZSM-5 zeolite

cavity [36], using a DFT cluster-approach. The results pointed out that DFT

approaches, although suffering known biases [12, 13], can be employed to analyze

local effects of zeolite-based processes [27, 36, 37] and that zeolite cavities can play

a complementary role in driving reactivity.

In order to elucidate local/confinement effects of the zeolite cavities on the

propan-2-ol dehydration mechanism, in the present work, we have investigated the

interaction of the reactant, transition states and products, with the acidic site of

several H-ZSM-5 and H-Y zeolite models by DFT calculations employing a cluster-

approach [34, 36, 38, 39]. In particular, we have simulated both the external surface

and the internal pore structure of the H-ZSM-5 zeolite, with the goal to compare the

contributions of the catalyzed reaction to both internal and external acidic zeolite

sites. Conclusively, in the aim to evaluate electron correlation effects, perhaps

present in the zeolite models as a results of dispersion effects existing inside

the zeolite cavities [13, 40], the DFT optimized geometries corresponding to the

H-ZSM-5 model derivatives were also treated at the MP2 level [41]. In the

‘‘Computational details’’ section, the properties of the models are given with

the basis features, characterizing the computational approaches and methods used in

the work while in the ‘‘Results and discussion’’ section, details on the structural and

mechanistic findings are presented, deepening structural, adsorption and reaction

characteristics of the title systems.

Computational details

DFT calculations were carried out using the Gaussian 03 program package [42]

(G03) while the hybrid—Becke three parameters exchange and Lee–Yang–Parr

correlation—B3LYP functional was used throughout [43, 44]. The starting

geometries of the non-aluminated ZSM-5 and Y models were derived by the

zeolite atlas [45–47]. Cavity (22T H-ZSM-5 and 48T H-Y) and surface (26T

H-ZSM-5) aluminated models were considered. The 22T H-ZSM-5 model, see

Fig. 1a, consisted of 81 atoms of which 28 capping (cut-off) hydrogens replaced the

original oxygen atoms to truncate the periodic zeolite structure [34, 36]. The larger

48T H-Y model, see Fig. 1b consisted of 182 atoms of which 48 capping hydrogen

atoms. Both the zeolite models above represent the largest cavities for both H-ZSM-5

and H-Y. In contrast with these, the 26T H-ZSM-5 model, see Fig. 1c, was arranged

to mimic a zeolite surface edge, consisting of 95 atoms of which 30 cut-off

hydrogens.

The Brønsted acidic site concentration in the different zeolite models was chosen

to range between 500 and 1,000 lmol g-1, representing reasonable experimental

conditions [48]. So, in order to mimic aluminated zeolite systems, two aluminum

atoms substituted two silicon atoms, both in the ZSM-5 and Y models, while two

protons were singly added to them. In the following, these are collectively indicated

1 T represents one tetrahedral unit, singly including Si or Al and four O atoms.

Reac Kinet Mech Cat (2013) 108:565–582 567

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as bi-protonated systems. The selected acidic sites corresponded to the most stable

positions within the zeolite cavities considered [34, 35, 49, 50]. Hydrogens bounded

oxygens vicinal to the introduced aluminum atoms and, in the case of confined

rooms, they pointed within the zeolite cavity. In the 26T surface model, the acidic

sites were chosen to have the same topology of those considered for the 22T cavity

model [36].

All the geometries were partially optimized using the 6-31G�� basis-set, by

keeping the coordinates of the external hydrogen atoms frozen, for which the 3-21G

basis-set was used. In details, the coordinates of the cut-off hydrogen atoms of the

different fragments were initially optimized at the B3LYP/3-21G level, fixing

the positions of the remaining atoms. After, the full structures were re-optimized at

Fig. 1 Optimized zeolite models: fragments a 22T H-ZSM-5, b 48T H-Y, and c 26T H-ZSM-5. Si atomsare in light-gray, Al (close to the pointed H atoms) in dark-gray while O and H are mid-sized and small-sized; a and b were used to mimic cavity processes at variance with c, which was used for simulatingsurface processes. For the fragments a, b and c the DFT energy (EDFT) values, reported as references, inthe order were -8552.2661 au. -19254.5962 au. and -10238.7476 au

568 Reac Kinet Mech Cat (2013) 108:565–582

123

the B3LYP/6-31G�� level, fixing the positions of the external H atoms. The H-ZSM-

5 and H-Y fragments so obtained were used for the following optimizations. Alcohol

adsorption on mono-protonated H-ZSM-5 models was also investigated.

Energies were calculated under different paradigms and, in the adsorption steps,

basis-set superposition error (BSSE) was always estimated, by using the counter-

poise method [51]. The transition states were conversely localized by the QST3

method [52]. The nature of each optimized structure was defined by a subsequent

frequency and thermochemical data calculations, employing the vibration analysis

within the harmonic approximation, and all the reported energies were corrected for

the ZPE [53]. Since the geometries of the investigated models were partially

optimized (see above), spurious imaginary frequencies were observed. In our

evaluation, however, we have supposed that error cancellations occurred, consid-

ering relative values either of the calculated standard free energy or also of other

thermo-chemical parameters. In any case, by the vibration analysis of the TSs, it

was always possible to find imaginary frequencies attributable to the involved bond

transformation [53], occurring along the alcohol dehydration. As a consequence, we

have been consistently able to discriminate among energy minimum and saddle

point geometries.

Finally, to take electron correlation effects into account, in particular those

concerning the TS structures, second-order Møller–Plesset (MP2) [41] perturbation

theory calculations—as implemented in G03 [54–57]—were performed on the

H-ZSM-5 fragment derivatives, using either the basis-set scheme employed for the

geometry optimizations at the DFT level or a new scheme in which the atoms

already treated by 6-31G � � were treated by the 6-31++G � � [58], including diffuse

functions.

Results and discussion

Table 1 shows relevant structural and energetic parameters determined for the

zeolite models. Results suggest that the structural features are almost independent

on the size and shape of the zeolite frameworks considered. As an example, in every

case the O–H1 and O–H2 distances (dOH1 and dOH2) are ca. 0.97 A and also the

Al–O–H angles (aAlOHn, n = 1, 2) are similar. However, a slight decrease (ca.

20 kJ mol-1) in the deprotonation energy, evaluated as the energy difference

occurring between the mono- and the bi-protonated zeolite, was observed for the

26T model.

The title reaction was mimicked inside the cavities of the 22T H-ZSM-5 and 48T

H-Y fragments having diameter of ca. 9 and 12 A, respectively. The reaction was

also studied in gas phase (non-catalyzed) and on a portion of the external surface of

the H-ZSM-5 framework, i.e. on the 26T H-ZSM-5 model. The comparison between

open and closed cavities was aimed at characterizing the catalytic function of two

different site sets of the same catalyst, namely H-ZSM-5. With respect to this, the

role of the external surface activity in the effectiveness of zeolites in heterogeneous

catalytic applications was characterized both experimentally [59] and computa-

tionally [9] showing that the outside sites are less reactive than the inside ones both

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in H-ZSM-5 [9, 59] and H-Y [59] zeolite. In Figs. 2 and 3 the optimized geometries

of reactant (R) and products (P1 and P2) adsorbed over the three zeolite models are

reported. Interestingly, in the case of the larger 48T H-Y zeolite cavity, an

additional and more stable structure was found, when water and propene are at a

larger intermolecular distance, fragment 2c. The latter can be considered as the

result of the mutual departure of the two produced molecules (propene and water)

within the larger zeolite cavity and can be represented by the process P1/48T(H-Y)

! P2/48T(H-Y). This feature was not observed in the H-ZSM-5 models, where just

one acidic site was involved in the alcohol dehydration, and the second one behaved

as a spectator species.

The non-catalyzed gas-phase propan-2-ol dehydration was considered as a reference

process. The reaction mechanism involves one concerted E2 b-elimination character-

ized by a TS, showing an activation free energy ðDG#Þ of ca. 264 kJ mol-1, see Fig. 4.

In contrast with the non-catalyzed process, the catalytic mechanism over the

three zeolite models, at first, involves the propan-2-ol non-activated adsorption,

illustrated by the fragments R/48T(H-Y), R/22T(H-ZSM-5) and R/26T(H-ZSM-5)

of Figs. 2 and 3. The adsorption, irrespective of the considered model, is always

determined by an interaction taking place between the oxygen of the alcoholic

group and the hydrogen of one acidic site of the zeolite derivatives. The

corresponding distance ðdH��O) values along with other relevant structural and

energetic adsorption parameters are reported in Table 2.

It is interesting to note that the O–H distances in the acidic sites of the adsorbed

zeolites are longer than that observed for the not adsorbed ones, see Fig. 1 and

Table 1. In particular, the H � �O distances of the adsorbed derivatives are clearly

indicative of the formation of hydrogen bonds, arising between one acidic site

and one alcohol molecule. Moreover, dOH values increase by decreasing dH��O and

increasing dCO distances. This shows the relation among these parameters and

suggests that the curvature of the inner surface, allowing a different distance of the

molecule from the H-Y and H-ZSM-5 inner walls, affects the interaction mode

hence the reactivity of the adsorbed molecules.

In fact, after the reactant adsorption and regardless of the involved fragments,

one b methyl hydrogen, as shown in Fig. 5, begins to interact with the protonated –

Table 1 Selected distances, angles, average deprotonation energies and protonation affinity, calculated

for different bi-protonated zeolite models employed in studying the title reaction

zeolite

model

cavity

£a (A)

dOH1b

(A)

dOH2b

(A)

aAlOH1b

(�)

aAlOH2b

(�)

\DE [ c

(kJ mol-1)

\DG�[ c

(kJ mol-1)

\PA [ c

(kJ mol-1)

48T H-Y 11.(9) 0.97 0.97 117.8 117.9 1268.(7) 1244.(7) -1237.(6)

22T H-ZSM-5 8.(8) 0.97 0.97 108.7 105.5 1267.(3) 1240.(1) -1233.(3)

26T H-ZSM-5 – 0.97 0.97 110.4 110.5 1245.(4) 1206.(9) -1214.(1)

a The cavity diameter ð£Þ values estimated on the optimized models are in close agreement with those

already reported by Milas and Nascimento [16]b In order to individuate the position of the almost equivalent acidic sites (OH1 and OH2), refer to Fig. 1c Deprotonation energy ðDEÞ, standard free energy ðDG�Þ and protonation affinity (PA) are averaged,

considering both the acidic sites present in each model

570 Reac Kinet Mech Cat (2013) 108:565–582

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OH group of the alcohol, causing the protonated-water elimination and the

contemporaneous proton displacement from the water molecule to restore the –OH

moiety on the zeolite surface. The protonated-water elimination is favored by the

persistent interaction of the acidic hydrogen, which started the dehydration process,

with the oxygen of the zeolite surface. This step consists of a concerted E2

b-elimination, after which the formed water molecule is adsorbed on the acidic site

that was previously acting as catalytic center.

In the fragments P1/22T(H-ZSM-5) and P1/26T(H-ZSM-5), the produced water

molecule behaves as bridge between the acidic site and the propene molecule that

Fig. 2 Optimized 48T H-Y zeolite derivatives; fragments a, b and c: reactant R/48T(H-Y) (a) andproducts P1/48T(H-Y) (b) and P2/48T(H-Y) (c); in fragment c, water and propene are at larger distanceand separately interact with two acid sites

Reac Kinet Mech Cat (2013) 108:565–582 571

123

also weakly interacts with the second acidic site present in the zeolite models. In the

48T H-Y model, the produced propene initially interacts with the water molecule

and, through this, with the acidic site closest to the latter, fragment P1/48T(H-Y),

then diffuses away from the water molecule to the other acidic site, fragment P2/

48T(H-Y). In all the product fragments above, as already illustrated for the reactant

fragments, both the O–H and H���O distances, characterizing the zeolite –OH moiety

interacting with the formed water molecule and the corresponding �OH���OH2

interaction—are consistently indicative for a hydrogen-bond formation. The

optimized TS fragments, TS/48T(H-Y), TS/22T(H-ZSM-5) and TS/26T(H-ZSM-5),

are shown in Fig. 6, while a simplified representation of the dehydration pathways

over the three zeolites models and relevant calculated results, concerning the same

reaction kinetic are reported in Fig. 7 and Table 3.

The analysis of Fig. 6 confirms that the TSs in the different models are very

similar, as always the same atoms and bonds are involved. Furthermore, the

Fig. 3 Optimized 22T and 26T H-ZSM-5 zeolite derivatives; fragments a, b, c and d: reactants R/22T(H-ZSM-5) (a) and R/26T(H-ZSM-5) (c) and products P1/22T(H-ZSM-5) (b) and P1/26T(H-ZSM-5) (d);the first-row and second-row systems mimic cavity and surface events, respectively

572 Reac Kinet Mech Cat (2013) 108:565–582

123

highlighted (in black) moieties support the schematic representation of Fig. 5 hence

support the concerted E2 b-elimination mechanism, which indeed resulted the only

possible.2 The results, summarized in Table 3, point out that the standard free

energy of propan-2-ol dehydration inside the 22T H-ZSM-5 cavity is ca. 53 kJ mol-1,

with a calculated activation free energy of ca. 169 kJ mol-1, see also Fig. 7.

Fig. 4 Non-catalyzed propan-2-ol dehydration pathway: DG� vs. n. The reaction arises by ab-elimination mechanism. The propan-2-ol DFT energy value resulted -194.3686 au

Table 2 Thermodynamic and structural propan-2-ol/zeolite adsorption parameters and steric hindrance

energy (shepf) parameter, characterizing different fragment models

reaction

model

DEaads

(kJ mol-1)

DH�adsa

(kJ mol-1)

DG�adsa

(kJ mol-1)

dCOb

(A)

dOHb

(A)db

H��O(A)

shepfc

(kJ mol-1)

48T H-Y d -126.1 -125.6 -68.3 1.47 1.11 1.35 41.4 | 9.1

22T H-ZSM-5 e -68.8 | -109.5 -63.1 | -106.5 -4.8 1.45 1.03 1.52 56.7

26T H-ZSM-5 e -83.4 | -189.9 -75.5 | -191.0 -32.3 1.44 1.02 1.54 13.7

a Thermodynamic parameters were determined considering BSSE corrections [51]b Propan-2-ol intra-molecular C–O distance, zeolite acidic site O–H distance and alcohol-zeolite H���Ointer-molecular distance: the calculated C-O distance in the propan-2-ol molecule resulted 1.43 A; the

O–H distance—to be compared with the dOHn (n = 1,2) reported in Table 1—is that characterizing the

–OH acidic group interacting with the alcohol molecule; the H���O distance is determined by considering

the interaction occurring between the alcoholic oxygen and one zeolite acidic site, see fragments

R/48T(H-Y), R/22T(H-ZSM-5) and R/26T(H-ZSM-5)c Steric hindrance energy (shepf) parameters were determined by using Eq. 1, see textd The adsorption parameters are determined on the bi-protonated fragment while the different shepf

values refer to the different 48T H-Y product-fragments: P1/48T(H-Y) and P2/48T(H-Y), on the left and

on the right of the verti-bar, respectivelye Verti-bar separates, on left and right, adsorption energies and enthalpies calculated, respectively, on bi-

protonated and mono-protonated fragments

2 Non-concerted mechanisms were also attempted, without being successful.

Reac Kinet Mech Cat (2013) 108:565–582 573

123

Fig. 5 Schematicrepresentation of the TS,occurring by the E2b-elimination step on thedifferent models used to test thetitle reaction; the shadowedbasis represents the zeolitesurface while the dotted lines theinteractions/bonds involved inthe single-step transformationmechanism

Fig. 6 Optimized TS species occurring on the different zeolite models: fragments a TS/48T(H-Y), b TS/22T(H-ZSM-5), and c TS/26T(H-ZSM-5); in black are emphasized atoms (C, O and H) and bonds (C–C,C–O, C–H and O–H) involved in the imaginary vibrations; in the black moiety, oxygen can be easilyindividuated, being always linked to the acid hydrogen bound to the zeolite surface

574 Reac Kinet Mech Cat (2013) 108:565–582

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These findings, compared with those of the gas-phase reaction contribute to outline

the catalytic effect of the acidic sites.

An explanation of the increasing energy content inside the zeolite cavity,

following the reaction, can be provided by considering hindrance effects, here

accounted for by the steric hindrance energy parameter associated with the product

formation (shepf), defined as in the following:

shepf ¼ ðEizeoDFT þ E

rpzeoDFT Þ � ðEwzeo

DFT þ EpzeoDFTÞ ð1Þ

In shepf, whose values for the different systems are reported in Table 2, EDFT

terms represent DFT minima, concerning the isolated zeolite fragment (izeo) and the

zeolite fragment containing the (i) reaction products (rpzeo) as well as either just (ii)

water (wzeo) or (iii) propene (pzeo). In the latter fragments, water and propene

molecules were placed at the same location they had in the corresponding rpzeofragment. In Eq. 1, all the terms that included adsorbed molecules, namely rpzeo,

wzeo, and pzeo, were corrected for the BSSE [51]. Therefore, shepf arises from

the confined water-propene inter-molecular attraction–repulsion effects within the

zeolite cavity, produced by the occurrence of the title reaction. Of course, also the

local interactions between the molecules and the inner zeolite surface will contribute

to the shepf values. The latter computed for the reaction within the H-ZSM-5 cavity

corresponds to 56.7 kJ mol-1, see Table 2, suggesting that the diffusion of propene

and water outside the H-ZSM-5 cavity would further provide energy stabilization.

Propan-2-ol dehydration inside the H-Y cavity occurs through a mechanism

similar to that observed within the H-ZSM-5 cavity. The standard free energy

Fig. 7 Catalyzed propan-2-ol dehydration energetic pathways, occurring inside the zeolite cavity (22TH-ZSM-5, in dark-gray and 48T H-Y, in light-gray) and outside (26T H-ZSM-5, in mid-gray): DG�vs. n.After the fragment acronyms of Figs. 2, 3 and 6, dark-gray (3a, 6b, 3b), ight-gray (2a, 6a, 2b, 2c) andmid-gray (3c, 6c, 3d) pathways, represent the following transformations: dark-gray R/22T(H-ZSM-5)!TS/22T(H-ZSM-5) ! P1/22T(H-ZSM-5); light-gray R/48T(H-Y) ! TS/48T(H-Y) ! P1/48T(H-Y) !P2/48T(H-Y); mid-gray R/26T(H-ZSM-5) ! TS/26T(H-ZSM-5) ! P1/26T(H-ZSM-5). For R/22T(H-ZSM-5), R/48T(H-Y) and R/26T(H-ZSM-5)—fragments 3a, 2a and 3c—the DFT energy values, reportedas references, in the order resulted -8746.6678, -19448.9995 and -10433.1474 au

Reac Kinet Mech Cat (2013) 108:565–582 575

123

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576 Reac Kinet Mech Cat (2013) 108:565–582

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difference between the product and the reactant is equal to 44.0 kJ mol-1 while the

activation free energy corresponds to ca. 148 kJ mol-1. After diffusion of the

products within the zeolite cavity—with the consequent formation of the fragment

P2/48T(H-Y)—a further decrease of ca. 4 kJ mol-1 is observed, see Fig. 7 and

Table 3. The shepf value within the H-Y cavity is ca. 41.4 kJ mol-1 and

after diffusing water and propene, faraway to one another on different acidic sites,

9.1 kJ mol-1. This reveals that in H-Y zeolite there is less steric hindrance than in

H-ZSM-5 zeolite.

The free energy needed for the title reaction on the H-ZSM-5 external surface,

compared to that arising on closed cavities, decreases by ca. 22 kJ mol-1, see Fig. 7

and Table 3. At the same time, the activation free energy increases by ca. 31 kJ

mol-1, proving that the reaction is not favorite on external surface [9, 59]. However,

the produced species are thermodynamically more stable on the surface than within

the cavity, as confirmed by the corresponding shepf difference value: ca. 43 kJ mol-1.

The results till now reported indicate that the catalytic role of the zeolite cavity

size is mainly related to the drop of the TS energy gaps. To this effect should

contribute a destabilization of the propan-2-ol molecule inside the cavities. Indeed,

isolating the alcohol molecules in the fragments R/22T(H-ZSM-5), R/48T(H-Y) and

R/26T(H-ZSM-5) and calculating the corresponding energies, their relative values

with respect to that of the gas-phase species resulted 19.2, 14.7 and 13.5 kJ mol-1

for the 22T H-ZSM-5, the 48T H-Y and the 26T H-ZSM-5 systems. This behavior is

connected to the zeolite room sizes, hence to confinement effects. The hypothesis

above is confirmed by the analysis of the distance parameter values reported in

Table 2, which suggests that the local interaction modes of the acidic –OH groups

with the alcohol molecules ðdH���OÞ—and the consequent structural modifications on

the molecular substrates (dCO) and on the catalyst sites (dOH)—are peculiar to the

zeolite frameworks while they are not correlated to the energies of the zeolite

fragments.

Concerning this, it can be noticed that inside the largest 48T H-Y cavity both the

values of reaction free energy (DG�p�r, see Table 3) and steric hindrance energy

(shepf, see Table 2), after the product diffusion, P1/48T(H-Y) ! P2/48T(H-Y),

closely resemble those found for the ‘‘open’’ 26T H-ZSM-5 zeolite surface model.

Furthermore, considering that the adsorption energies of the reactant on the three

zeolite models decrease, see Table 2, in the order 48T H-Y[26T H-ZMS-5[22T

H-ZMS-5, it is possible to state that it is easier for the reagent to be adsorbed on

larger than on smaller cavities and for the same zeolite on open surfaces than inside

cavities. Moreover, the thermodynamic stability of the reactant with respect to that

of the products is decreased by larger reaction environments, i.e. larger cavities and

open surfaces, while TS energy is increased by smaller cavities. The absence of

cavities, corresponding to processes occurring on open surfaces, however increases

to a larger extent the transition energy gap. Adsorption and reaction results hence

confirm the role of the curvature of the inner surface and the role of the steric

hindrance in ruling physicochemical processes on zeolite materials.

It is worth commenting on the difference between the calculated values of enthalpy

and free energy reported in Tables 2 and 3. Such comparison shows a noticeable

Reac Kinet Mech Cat (2013) 108:565–582 577

123

difference for all the reactant adsorption over the zeolite, where as an example, the

entropic contribution reduces the total free energy of ca. 60 kJ mol-1 on the H-ZSM-5

and on the H-Y cavities and of ca. 40 kJ mol-1 on the H-ZSM-5 surface. The

similitude between the values of DH�p�r and DG�p�r as well as of DH# and DG#

indicates that the large entropic contribution has to be extended to all the reaction

steps occurring over zeolite. The adsorption enthalpies calculated for the bi-

protonated H-ZSM-5 systems might seem underestimated with respect of those

determined calorimetrically3 [48]. However, it has to be noticed that the H-ZSM-5

systems studied using differential calorimetry by Lee et al. [48] showed an amount

larger than 25 % of unsaturated acidic sites. Thus, in order to simulate this

experimental zeolite characteristic, we mimicked the adsorption of the alcohol

molecule on mono-protonated 22T and 26T H-ZSM-5 systems. Table 2 shows that

the adsorption enthalpies in the partially unsaturated H-ZSM-5 models increase,

reaching values in good agreement with the experimental ones. Moreover, in the same

paper, it is stated that the adsorption enthalpy increases as the proton affinity does.

Also this aspect is captured by our simulations, as it is shown by Tables 1 and 2.

In order to compare the obtained computational findings with available

experimental data, it could be interesting, at first, to notice the high operative

experimental temperatures, which characterize the alcohol dehydration processes

[21, 25]. These are clearly compatible with the relatively high Ea (and DG#) values,

calculated in this work for the different fragments. Furthermore, as already reported,

larger alcohols due to steric hindrance effects should react on the zeolite outer

surface [19] and, as a consequence, the calculated smaller catalytic activity of the

Brønsted acidic sites placed outside the zeolite cavities could be compatible with the

smaller reactivity of the larger alcohols generally observed. Moreover, the steric

hindrance effects characterizing differently sized alcohol molecules inside given

zeolite cavities [23] could also drive the catalytic deactivation of the same zeolite

materials. In fact, smaller molecules of the produced unsaturated hydrocarbons [21]

could be more easily and strongly adsorbed on the inner zeolite surface. Hence, the

smaller the produced hydrocarbons are, the more easily they undergo oligomeriza-

tion and consequently degradation to carbonaceous species [60]. Therefore, as

experimentally shown, smaller alcohol molecules should reduce more rapidly the

diffusion inside the zeolite inner rooms [21] and as a result the catalytic activity [23].

A complementary aspect related to the importance of the steric effects even

concerns the formation of 2-propan-2-yloxypropane molecules inside the zeolite

cavities. We indeed tried to allocate that ‘‘cumbersome’’ ether molecule using the

cavity and the surface models, without being successful. This show that the by-side

ether formation is, as experimentally demonstrated, a minor process that should

occur just on very flat outer surface [21].

The tightest fit between experimental and calculated findings, however, concerns

the effects of the zeolite room size on the catalytic activity of the same zeolite

employed in the alcohol dehydration. With respect of this, both van de Water et al.

3 The enthalpy value is ca. 125 kJ mol-1 at concentrations of propan-2-ol per H-ZSM-5 grams larger

than 600 lmol g-1. The latter correspond to the concentrations mimicked by the 22T and 26T H-ZSM-5

systems.

578 Reac Kinet Mech Cat (2013) 108:565–582

123

[21] and Jana et al. [25] reported experimental evidences on H-ZSM-5 and H-Y

systems, showing how an increase in the zeolite room size produces an increment in

the catalytic dehydration activity. The latter is conversely lowered by the use of

amorphous silica–alumina catalysts. The explanation of these evidences—clearly

reproduced by the present computational study—were related [21, 25] to the

changes in the acidity of the materials and to the reduction of the negative effects of

the coke formation (on the diffusion of the reactants), caused by the introduction of

additional mesopores. As illustrated above, we have been able to add inferences on

the origin of the poisoning carbonaceous species to this interpretation and on the

modification of the catalyst activity that, from our findings, seems to be mostly

attributable to the environment in which the reaction occurs. This clearly affects

local properties of both catalyst and catalytic substrate.

In order to get confidence on the effectiveness of the method suggested and

namely to account for the dispersion effects present inside the zeolite cavities [13]

we tested by MP2 methods [41, 54–57] the energetic results concerning both the

H-ZSM-5 fragments. Table 3 shows that, irrespective of the considered system and

basis-set scheme, the agreement between DFT and MP2 is quite good, being the

average percentage difference between DFT and MP2 results ca. 2 and 6 % for

structures corresponding to transition states and minima, respectively. It is finally

worth noticing that the energy differences discussed in this paper are in well

agreement with data reported on different H-ZSM-5 catalytic applications by Silva

and Nascimento [61].

Conclusions

The dehydration of propan-2-ol over H-Y and H-ZSM-5 models, mimicking both

inner and outer surfaces, was studied by DFT and MP2 calculations. The first

reaction step consists of the protonation of the alcoholic group by one acidic zeolite

site, which starts a concerted E2 b-elimination process to give propene and water.

Although characterized by comparable values of Brønsted acidity, the different

zeolite models crucially influence (i) the adsorption energy of propan-2-ol, (ii) the

relative stability of reactant and products and (iii) the energy of the transition states.

These effects could be attributable to the sizes of the zeolite cavities, which are able

to affect also the chemical properties of the reacting systems. Indeed, the

confinement effects in the alcohol dehydration process appear to play a twofold role

leading to two different results, which specifically concern both the thermodynamic

stability of the products with respect to the reactant and the activation energy

characterizing the TS. In particular, for the title reaction the best compromise

between these effects seems to be reached when the alcohol dehydration occurs over

the H-Y zeolite. The computational results are in good agreement with several

experimental findings hence furnish a molecular level explanation of the reaction

mechanism related to the alcohol dehydration over zeolites.

Acknowledgments This work was supported by the University of Palermo and by the Italian Ministero

dell’Istruzione, dell’Universitaa e della Ricerca.

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