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Relationship between acidity and catalytic reactivity of faujasite zeolite: A periodic DFT study
Journal of Catalysis 344 (2016) 570–577
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Relationship between acidity and catalytic reactivity of faujasite zeolite: A periodic DFT study q Chong Liu a, Guanna Li a, Emiel J.M. Hensen a,⇑, Evgeny A. Pidko a,b,c,⇑ a
Inorganic Materials Chemistry Group, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands c ITMO University, St. Petersburg, Russia b
a r t i c l e
i n f o
Article history: Received 29 July 2016 Revised 24 October 2016 Accepted 27 October 2016
Keywords: Extra framework aluminum Acidity Zeolite H/D exchange Alkane cracking DFT calculations Reaction mechanisms Structure-activity relations
a b s t r a c t The fundamental aspects of Brønsted acidity and catalytic reactivity of faujasite-type zeolites were investigated by periodic DFT calculations. The adsorption energies of ammonia and pyridine on the Brønsted acid site (BAS) were used to determine the acidity. It is demonstrated that the acid strength of zeolite materials increases with rising Si/Al ratio (low-silica faujasite), and then levels off at high Si/Al ratio (high-silica faujasite). The presence of multinuclear extra framework Al (EFAl) in the sodalite cages substantially enhances the Brønsted acidity. The catalytic reactivity of faujasite toward protolytic propane cracking correlates well with the characterized acidity by base adsorption. However, for H/D exchange reaction of benzene the presence of EFAl species can induce deviations between the measured acidity and the reactivity of faujasite catalysts, indicating that acidity and reactivity are not always directly correlated. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Zeolites are important catalysts in the oil refining and petrochemical industry [1,2]. These microporous materials are composed of corner-sharing SiO4 tetrahedra. The negative framework charge resulting from isomorphous substitution of lattice Si4+ with Al3+ is compensated by extra framework cations such as protons which give zeolites their pronounced Brønsted acidic properties. Most industrial applications of zeolite catalysts rely on these Brønsted acid sites (BASs) embedded in micropores, giving rise to shape-selective hydrocarbon conversion [3]. The largest-scale application of zeolite as catalyst is in the fluid catalytic cracking (FCC) process that converts heavy crude oil fractions into more valuable products [4]. Two main mechanisms have been implicated in the cracking of hydrocarbons on BAS in zeolites, namely protolytic cracking and bimolecular cracking [5–7]. The latter mechanism involves a chain q This contribution is part of the virtual issue ‘‘30 years of the International Conferences on Theoretical Aspects of Catalysis (ICTAC)”. ⇑ Corresponding authors at: Inorganic Materials Chemistry Group, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands (E.A. Pidko). E-mail addresses: [email protected] (E.J.M. Hensen), [email protected] (E.A. Pidko).
http://dx.doi.org/10.1016/j.jcat.2016.10.027 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.
reaction in which carbenium ions abstract a hydride from an alkane followed by b-scission of the resulting carbenium ion into an alkene and a smaller carbenium ion. Carbenium ions that initiate the chain reaction may be formed by protonation of alkenes present in the feed as impurities. The alternative mononuclear mechanism was proposed by Haag and Dessau and involves direct activation of alkanes by acid sites [8]. According to this mechanism [9], the bridging hydroxyl groups in zeolites protonate alkanes to form a penta-coordinated carbonium ion transition state, which collapses into a smaller carbenium ion and an alkane or a hydrogen molecule. The b-elimination of a proton of the carbenium ion will then result in an alkene and regenerate the BAS. It is generally recognized that the monomolecular mechanism predominates at low alkane conversion, whereas the bimolecular mechanism prevails at high alkane conversion [10]. Furthermore, monomolecular cracking requires higher reaction temperatures, as barriers for formation of the carbonium ion transition states are considerably higher than carbenium ion transition states associated with the bimolecular cracking mechanism [11]. Cracking activity of zeolites is strongly influenced by the acidic properties of zeolites, most prominently by the strength and number of the BAS [12]. Accordingly, correlations of catalytic activity with zeolite acidity have been thoroughly investigated [13–16]. Common approaches to investigate zeolite acidity include IR and
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NMR spectroscopy as well as adsorption-desorption methods such as calorimetry and temperature-programmed desorption (TPD) [3,17–19]. These techniques rely on the interaction of the BAS with basic probe molecules such as carbon monoxide, ammonia, or pyridine. Spectroscopic methods probe the frequency shift of characteristic vibrations of the probe molecule or the perturbed O–H stretching vibration, while calorimetry and TPD methods measure the heat of adsorption. Despite many investigations, direct correlation between zeolite acidity characterized by such methods and catalytic activity remains problematic and is part of a longstanding debate on the nature of acidity in zeolites [3,18]. Haag and co-workers showed clear correlation between n-hexane cracking and framework Al content for HZSM-5 [20]. Yet, in practice it is seldom possible to construct clear structure-performance relations which is due to the heterogeneity of acid sites in nature and strength [21]. One important aspect is the influence of extra framework Al species [22]. It is important to mention that the adsorption of hydrocarbons will also greatly depend on the micropore structure [23] and entropic factors in the constrained zeolite environment also appear to play a role in zeolite-catalyzed hydrocarbon conversion [24,25]. Zeolite Y (faujasite, FAU) is synthesized in Al-rich form (Si/ Al 2.5) and displays weak Brønsted acidity and low hydrothermal stability. Therefore, for industrial application as in hydrocracking, as-synthesized zeolite Y will undergo various treatments aimed at reducing the framework Al content to improve these properties [26]. For example, high-temperature steaming of zeolite Y produces crystalline materials commonly denoted as the ultrastable Y (USY) zeolites with improved acidity and hydrothermal stability. This treatment leads to the migration of part of framework aluminum (AlF) from lattice sites to extra framework positions confined inside the zeolite pores [27]. The presence of such extra framework aluminum (EFAl or AlEF) species in zeolites enhances their acidity and catalytic performance [28–31]. Enhanced acidity has been suggested to be due to EFAl in close proximity to the BAS [32]. These EFAl species may act as Lewis acid sites and promote the intrinsic acid strength of vicinal BAS via polarization effect [33–35]. The group of Niwa proposed that the presence of the asymmetric tetrahedral EFAl inside USY zeolite enhances the Brønsted acidity [36]. The same authors reported that steaming and formation of EFAl species increase the heat of adsorption of ammonia and catalytic cracking activity [37]. The measured activation energies for alkane cracking showed strong dependence on the acidity as measured by ammonia adsorption [38]. An alternative proposal emphasizes the role of zeolite pore structure and confinement on the cracking reactivity rather than variation in intrinsic strength of the acid sites [39,40]. Van Bokhoven and co-workers showed that the presence of EFAl species does not affect the intrinsic cracking activity but increases the adsorption heat of hydrocarbon reactants [41–45]. They contended that the intrinsic acid strength of BAS in zeolites is similar and apparent cracking rate differences arise from different heats of adsorption. Gounder et al. proposed that the presence of EFAl species can decrease the zeolite pore size, resulting in more efficient stabilization of transition states by dispersive interactions, and therefore enhance the cracking activity [24]. The experimental study by Schallmoser et al. suggested that changes in the local environment of BAS associated with proximate tetrahedral EFAl species can increase the cracking rate [25]. The presence of EFAl species in zeolites complicates understanding structure-activity relationships. Their influence on acidity may also depend on the nature of the protonated complex in zeolite [22]. Experimentally, it has been demonstrated that the acidity probed by COads IR and the H/D exchange reaction of benzene only correlates well with catalytic cracking activity for zeolite Y free from EFAl [22].
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Quantum chemical calculations are instrumental in constructing a molecular picture of zeolite reactivity that can be used to formulate acidity-reactivity relationships in zeolite catalysis [21]. Zeolite acidity represented one of the central problems of the first computational catalysis studies in the 1980s and since then it remained a source of intense debate [46–56]. Theoretically, the intrinsic strength of the acid sites in zeolites can be described in terms of deprotonation energies (DPEs), which is the energy required to dissociate the proton from the BAS to infinite distance [52]. Sauer and co-workers investigated the acid strength of highsilica frameworks FAU, CHA, MOR, and MFI by periodic QM/MM calculations and found that the calculated DPEs do not correlate well with the computed adsorption heats of ammonia [57]. On the contrary, a periodic DFT study from the Miyamoto group on the acidity of isomorphously substituted CHA zeolites demonstrated a good correlation between these two parameters [58]. However, a recent comprehensive DFT analysis on an extended library of periodic zeolite models carried out by Iglesia and coworkers suggests that such correlations could be biased by the specific T atom arrangement and thus location of BAS in the zeolite micropores [59]. It was shown that the ensemble-averaged DPEs were nearly insensitive to the location of isolated Al and Brønsted proton as well as to the topology of zeolite framework. As such, these authors contend that the DPEs do not systematically trend with the heat of adsorption of ammonia. Computational studies on alkane cracking by zeolites revealed a direct dependence of the intrinsic activation barriers of the cracking reaction and the strength of the zeolite BAS. Bell and coworkers carried out complementary Monte Carlo simulations and DFT calculations, and demonstrated that the intrinsic and apparent rate coefficients for alkane cracking are higher in MFI than in the less acidic FAU [6]. Niwa and co-workers employed periodic DFT calculations on a series of HY and cation-exchanged Y-zeolites and found a strong dependence of alkane cracking activity on the intrinsic strength of zeolite BAS quantified by the DPE descriptor [14]. Similarly, a cluster DFT study on the acidity of HZSM-5 zeolites by Deng et al. also revealed a direct relationship between the intrinsic acidity and the barrier of propane cracking [60]. Other theoretical studies showed that the introduction of extra framework species such as Al(OH)2+ EFAl species or metal cations such as Ba2+ and Ca2+ in HY zeolites can polarize the proximate BAS and enhance their acidity [32,61–63]. In spite of the significant number of studies reported on this topic so far, the effect of zeolite topology and composition on intrinsic acidity and reactivity remain unclear. Herein, we present the results of a systematic periodic DFT study on the acidity and reactivity of faujasite-type zeolite catalysts. The enhanced acidity of BAS after dealumination is associated with two main factors. One is the increased Si/AlF ratio, and the other is the presence of EFAl species. Accordingly, we investigated the effect of these two factors on the correlation between the acidity probed by the adsorption energy of ammonia and pyridine and H/D exchange in benzene and catalytic activity in protolytic cracking of propane.
2. Computational methods Density functional theory (DFT) calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional [64] as implemented in the Vienna Ab Initio Simulation Package (VASP) [65–68]. The projected augmented waves (PAW) method was used to describe the electron-ion interactions [69,70]. The cut-off energy of the plane waves was set to 500 eV. Brillouin zone sampling was restricted to the C point [71]. Convergence was assumed to be reached when the forces on each atom were below 0.05 eV/Å. A
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modest Gaussian smearing of 0.05 eV was applied to band occupations around the Fermi level, and the total energies were extrapolated to r ? 0. Van der Waals interactions were described by the dispersion-corrected DFT-D2 method [72]. The climbing image nudged elastic band (CI-NEB) method was used to determine the minimum energy path and to locate the transition state structures [73]. The maximum energy geometries along the reaction path obtained with the CI-NEB method were further optimized using a quasi-Newton algorithm. The nature of transition states was confirmed by determining the vibrational frequencies using the finite difference method. Small displacements of 0.02 Å were used to determine the numerical Hessian matrix. Similar to our previous work [74–77], a low-symmetry rhombohedral unit cell was used to model faujasite [78]. Periodic boundary conditions were employed. The location of lattice Al was chosen according to the stability analysis, and the negative charge on the lattice was compensated by protons at O1 positions. The most stable configurations selected as the zeolite models for this study are illustrated in Fig. S1. Further details on the structural and stability parameters of the employed models can be found in Ref. [76]. The cell parameters were optimized for the defect-free faujasite models with Si/AlF (AlF = 14, 6 and 1) ratios of 2.4, 7, and 47. The optimized lattice parameters are as follows: Si/AlF = 2.4: a = b = c = 17.65 Å, a = b = c = 60°; Si/AlF = 7: a = b = c = 17.44 Å, a = b = c = 60°; Si/AlF = 47: a = b = c = 17.29 Å, a = b = c = 60°. The decrease of the unit cell dimensions with increasing Si/Al ratio was also observed in experiment [22,29]. Full geometry optimizations with the extra framework Al species (EFAl) and adsorbates were performed with fixed cell parameters. EFAl-free models with Si/AlF = 95 and 191 were based on a model with 1 Al center per unit cell (Si/AlF = 47) by creating supercells of 1 1 2 and 1 2 2 unit cells, respectively. The EFAl-containing models were all made from the unit cell with Si/AlF = 7. We studied the effect of different bi-, tri-, and tetranuclear aluminohydroxy clusters [AlxOyHz]n+ (2 6 x 6 4) stabilized
Fig. 1. (a) Rhombohedral faujasite unit cell. (b) Schematic presentation of the interaction of BAS with EFAl [AlxOyHz]n+ (2 6 x 6 4) clusters. (c) Structure of the trinuclear [Al3O4H3]4+ stabilized in the sodalite cage of faujasite.
inside the sodalite cages on the acidity of vicinal supercage BAS (Figs. 1 and S2). The positive charge of the extra framework complex was compensated by substituting an appropriate number of zeolite BAS vicinal to the EFAl resulting in a net 0 charge of the periodic EFAl-containing faujasite model. The choice for the structure and location of the EFAl species in the sodalite cage was based on another computational study [76]. The overall Si/Al ratio for the investigated EFAl/H-FAU models were in the 4.2–5.3 range, which allows comparison to our recent experimental study of zeolite Y acidity and the influence of EFAl on propane cracking [22].
3. Results and discussion To investigate the influence of framework Al density and the presence of EFAl species on acid activity, we used five defect-free faujasite models (H-FAU) with Si/AlF ratios of 2.4, 7, 47, 95, and 191 and a partially dealuminated H-FAU with Si/Al = 7 as a base for eight EFAl-containing faujasite models (EFAl/H-FAU). As EFAl species, we investigated bi-, tri-, and tetra-nuclear aluminohydroxo cationic clusters stabilized inside the sodalite cages, adjacent to a supercage BAS (Figs. 1 and S2). These multinuclear cationic complexes have been identified as candidate EFAl complexes in activated faujasite zeolite using an ab initio thermodynamics approach [76]. The acidity of the zeolites was investigated by determining the adsorption energies of ammonia and pyridine. The interaction of ammonia (NH3) and pyridine (C5H5N) with BAS results in proton transfer from the zeolite framework to the probe molecule (Scheme 1a and b, and Fig. S3). To investigate acid activity, two model reactions were used, namely H/D exchange of benzene [79,80] and monomolecular propane cracking [44,45] (Scheme 1c and d). During the H/D exchange reaction (Scheme 1c), the acidic OH group reacts with C6H6 to give an acidic OD group and C6H5D. For propane cracking (Scheme 1d), the rate-determining step is the protonation of C3H8 by a BAS resulting in a separated ion pair consisting of a carbonium [C3H9]+ and the deprotonated anionic zeolite [6]. The reaction products ethylene and methane are then obtained by rearrangement of the [C3H9]+ ion. In this study, we only considered the initial protonation step of propane activation. First, we investigated the influence of the framework Si/Al ratio on the acidity and activity. The calculated adsorption energies of ammonia [DEads(NH3)] and pyridine [DEads(C5H5N)] are listed in Table 1. Experimental studies reported DEads(NH3) of 110 kJ/mol for low-silica HY [37], and DEads(NH3) and DEads(C5H5N) of 150 kJ/mol and 180 kJ/mol, respectively, for high-silica HY [81]. The adsorption energies computed for the faujasite models (Table 1) are in good agreement with these experimental values. Increasing Si/AlF ratio results in increasing adsorption energy of ammonia and pyridine. This trend is in keeping with experimental observation [82] and well known in zeolite chemistry. Nextnearest neighbor (NNN) Al substitutions in the framework decrease the acidity of the protons, as acid strength of a bridging OH group depends strongly on the electronegativity of the cations, Si4+ or Al3+, present in the second coordination sphere around BAS [1]. When the dilution of Al centers is sufficient, the intrinsic acidity of the bridging hydroxyl groups is not affected anymore [83]. This is consistent with the finding that the adsorption energies do not increase for Si/AlF > 47. Likely, this critical limit lies much lower as usually values between 5 and 10 are reported [84,85]. Next, H/D exchange and protolytic cracking of propane were investigated on these H-FAU models. Table 1 lists the activation energies determined for these reactions. The activation barriers – for H/D exchange (DE– H/D) and propane cracking (DEcracking) initially decrease with increasing Si/AlF ratio (Si/AlF = 2.4, 7, and 47) and do not depend on the framework Al content for Si/AlF > 47. The activa-
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Scheme 1. (a) Ammonia adsorption, (b) pyridine adsorption, (c) H/D exchange of benzene, and (d) propane cracking with formation of carbonium intermediate in faujasite.
Table 1 – Determination of acidity and reactivity of H-FAU and EFAl/H-FAU models by DEads(NH3), DEads(C5H5N), DE– H/D and DEint as indicated in Fig. 3a, and DEcracking as indicated in Fig. 3b. All energies are in kJ/mol. Faujasite models
Si/AlF
H-FAU-2.4 H-FAU-7 H-FAU-47 H-FAU-95 H-FAU-191 EFAl/H-FAU-bi-1 EFAl/H-FAU-bi-2 EFAl/H-FAU-tri-1 EFAl/H-FAU-tri-2 EFAl/H-FAU-tri-3 EFAl/H-FAU-tetra-1 EFAl/H-FAU-tetra-2 EFAl/H-FAU-tetra-3
2.4 7 47 95 191 7 7 7 7 7 7 7 7
EFAl
[Al2O4H3]+ [Al2O4H4]2+ [Al3O4H]2+ [Al3O4H2]3+ [Al3O4H3]4+ [Al4O6H2]2+ [Al4O6H4]4+ [Al4O6H5]5+
DEads(NH3) 104 125 142 139 141 152 173 153 179 182 137 153 181
tion barriers show good correlation with the adsorption energies of ammonia and pyridine. Accordingly, we conclude that adsorption energies of ammonia and pyridine are reasonable criteria to assess the catalytic activity of EFAl-free faujasite in the protolytic cracking of FAU zeolite. One also expects good correlation with H/D exchange rates, which has been used before to probe acidity of zeolite Y [22]. The presence of EFAl species in faujasite strongly affects the acidity and reactivity of BAS. The AlF concentration in the FAU models was fixed and EFAl clusters were introduced in the sodalite cages of the models to study their influence of adsorption and catalytic properties. In general, the adsorption energies of ammonia and pyridine on the BAS increase upon addition of EFAl clusters
DEads(C5H5N) 146 163 199 191 196 219 246 220 267 274 196 230 268
DE – H/D
DEint
120 95 75 81 79 45 77 59 59 93 93 53
DE– cracking 226 200 181 182
56 21 61 23 14 73 53 12
172 148 167 141 135 184 172 140
(Table 1). This shows that the presence of the EFAl species in the sodalite cages affects the intrinsic acidity of the BAS. Table 1 also shows that the acidity increased with increasing charge of the EFAl cationic complex. Previous periodic DFT studies have revealed an enhancement of the adsorption strength of ammonia when Al (OH)2+ EFAl species are introduced [62]. In practice, the acidity of steam-activated faujasites can be further modified by poststeaming treatment with such compounds as Na2H2-EDTA or ammonium salts to remove part of the EFAl debris generated during steaming [37,86]. The resulting materials contain very strong BAS as follows by the increased heat of ammonia adsorption (137–150 kJ/mol) compared with the heat of ammonia adsorption of 110 kJ/mol for the parent HY. These experimental findings indi-
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cate that the extent of acidity enhancement by EFAl species is broad, and the acid strength of BAS in faujasite is sensitive to the structure of EFAl species. Similarly, the current computational study demonstrates that faujasite zeolites containing different EFAl species can possess BAS with acidity varying in a very wide range as probed by ammonia adsorption. The mechanism of the H/D exchange and propane cracking is affected by the presence of EFAl species in different manners (Figs. 2, S4 and S5). H/D exchange in H-FAU proceeds via a concerted mechanism, whereas the presence of EFAl clusters shifts the reaction to a stepwise mechanism involving the benzenium [C6H7]+ ion as a stable intermediate (Fig. 2a). The reaction energy
(DEint) for the formation of benzenium intermediate is sensitive to the acid strength of the protons in EFAl/H-FAU. The higher the acidity is, the lower the energy of the [C6H7]+ intermediate is (Table 1). Proton transfer from the [C6H7]+ intermediate to the zeolite framework oxygen is facile and closes the catalytic cycle. The mechanism of propane cracking is similar for H-FAU and EFAl/HFAU. The reaction proceeds via a carbonium-like transition state and intermediate (Fig. 2b). The energy barriers for the formation + of the transition state (DE– cracking) and the [C3H9] intermediate (DEcracking) decrease with increasing Brønsted acidity. Below, we will further analyze the difference between the two pathways for the H/D exchange reaction.
Fig. 2. Mechanisms of (a) H/D exchange reaction of benzene and (b) propane cracking in H-FAU and EFAl/H-FAU faujasite models.
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The calculated DE– cracking are in line with the experimental study by Katada et al. [38] that reported activation energies for propane cracking of 226 kJ/mol for HY (Si/Al = 2.4) and 104–162 kJ/mol for different post-treated USY catalysts. The correlation with the base molecule adsorption energies is better for propane cracking than for H/D exchange. For example, the EFAl/H-FAU-bi-2 model [DEads(NH3) = 173 kJ/mol, DE– H/D = 45 kJ/mol] is more reactive than the EFAl/H-FAU-tri-2, tri-3, and tetra-3 models which carry stronger acid sites [DEads(NH3) = 179 kJ/mol, 182 kJ/mol, and 181 kJ/mol, DE– H/D = 59 kJ/mol, 59 kJ/mol, and 53 kJ/mol]. On the other hand, the EFAl/H-FAU-tetra-1 and tetra-2 models with significantly different acidity [DEads(NH3) = 137 kJ/mol and 153 kJ/mol] have similar activation energy for H/D exchange (DE– H/D = 93 kJ/mol). This shows that the intrinsic acidity is not the only factor influencing the activation barrier for H/D exchange. We analyzed the trends between adsorption energies of bases and the energetics of the reactions investigated in more detail. The adsorption energies for ammonia and pyridine correlate well as shown in Fig. 3a. Thus, we expect that calorimetric characterization of acidity with these bases would give comparable acidity trends when studying faujasite zeolites. IR spectroscopy with pyridine as probe base is commonly used to study the acidic properties of zeolites. In experiment, a band at 1540 cm 1 is assigned to the presence of pyridinium [C6H5NH]+ due to protonation of pyridine by BAS [18]. The band corresponds to the 19b vibration mode, in which pyridine ring stretching and CH/NH bending are featured together [87]. We calculated the vibrational frequency of adsorbed pyridine for our faujasite models. The corresponding frequencies of 19b mode are 1543–1548 cm 1 for the H-FAU models and 1529– 1539 cm 1 for the EFAl/H-FAU models (Table S1). The calculations do not reveal any notable correlation between the zeolite acidity and the frequency shifts of the 19b vibrational mode of pyridine or the alternative 8a/8b modes (Table S1), which in the case of the 2,6-dimethylpyridine probe have been proposed to be sensitive to the variations in the zeolite acidity [88]. Overall, these computational results are in a good agreement with the available experimental data [88–90] implying that the analysis of the vibrational spectra of adsorbed pyridine cannot be used to distinguish sites of different acid strength. Fig. 3b and c shows correlations between DEads(NH3) and the activity in H/D exchange and propane cracking. The correlation with the cracking activation barrier (Fig. 3c) is better than that for the H/D exchange reaction (Fig. 3b). In the latter case, the trend only holds for the H-FAU models. The presence of EFAl species in faujasite alters the mechanism such that an intermediate ionic state is involved (Fig. 2a, H-FAU vs. EFAL/H-FAU). The stability of the benzenium [C6H7]+ ion may be expected to correlate with the acidity of faujasite. Indeed, DEads(NH3) correlates very well with the stability of [C6H7]+ (DEint, Fig. 2a). In the concerted transition state identified for H-FAU and the [C6H7]+ intermediate state for EFAl/H-FAU, the Brønsted protons are strongly bonded to the benzene molecule. The [C6H7]+ ion is stabilized by hydrogen-bonding interactions with zeolite framework oxygens, much like also observed for adsorbed ammonium. This may explain why the energetics DE– H/D for H-FAU and DEint for EFAl/H-FAU correlate well with DEads(NH3). The transition state for the formation of the ionic intermediate does not involve strong bonding of the exchanged proton with the framework. This is because deprotonation of the hydroxyl group is not complete. As such, it is clear that the increased acidity due to EFAl results in formation of a stable ionic complex. In summary, these findings show that DEads(NH3) correlate well with stability of ionic intermediates such as the carbonium ion and the benzenium ion, because in all cases the bridging hydroxyl group is deprotonated. On the other hand, the rate of the H/D exchange reaction is determined by the activation barrier (DE– H/D), which
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Fig. 3. Correlations between DEads(NH3) and (a) DEads(C5H5N), (b) DE– H/D and DEint (c) DE– cracking and the respective linear data fits.
does not correlate well with DEads(NH3). Thus, care must be taken in correlating H/D exchange rates to acidity. Next, we analyzed the influence of the finite temperature entropic effects on the reactivity relationships established above for the catalytic cracking. In line with the commonly employed experimental procedures, Gibbs free energies of propane adsorption and activation were estimated at the temperature of 800 K (DGads(C3H8)800K and DG– cracking,800K, Table S1), while the Gibbs free energies of ammonia adsorption were computed at both 300 and 800 K (DGads(NH3)300K, DGads(NH3)800K, Table S1). The free energy correlations are summarized in Fig. S6 of the supporting information. The variations in the zeolite structure and acidity do not affect the energetics of propane physisorption (DEads(C3H8) = 50 to 42 kJ/mol, Table S1). The intrinsic free energy barriers of propane cracking (DG– cracking,800K) follow the same linear relationships with the energies (DEads(NH3)) and Gibbs free energies (DGads(NH3)300K and DGads(NH3)800K) of adsorption of ammonia as the correlations established based on the intrinsic energetics (DE– cracking, DEads(NH3), Fig. 3c).
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The presented data show that cationic EFAl located in the sodalite cages of faujasite enhance the Brønsted acidity of protons in adjacent supercages. This weakening of the O–H bond is evident from strong adsorption of bases and lower barriers for H/D exchange and propane cracking. Importantly, no acidity enhancement was observed for the FAU model, containing a neutral EFAl and a reactive BAS at the vicinal positions inside the supercage (Fig. S7). The values of DEads(NH3) and DE– cracking in this case were identical within 1 kJ/mol to those computed for the original EFAlfree model (cf. Table 1 and Fig. S7). To gain better understanding of the influence of EFAl on acidity and reactivity, electron density difference analysis was performed (Figs. 4 and S4). Fig. 4 shows that less electrons are accumulated between the [C6H7]+ fragment and zeolite framework in EFAl/HFAU compared with the concerted transition state in H-FAU. On this basis, we conclude that the stabilization of the [C6H7]+ ionic intermediate is due to the presence of EFAl species in the sodalite cages, which help to compensate the negative charge of the deprotonated BAS after [C6H7]+ formation. Effectively, this increases the acidity of the bridging hydroxyl group. We suspect this EFAl effect to be a generic phenomenon during the zeolite-catalyzed reactions. We compare these computational results with our previous experimental observations [22]. In this work, we prepared defect-free high-silica FAU zeolite by (NH4)2SiF6 treatment of assynthesized FAU, followed by modification by EFAl species. We found that the rate of benzene H/D exchange and propane cracking increased with rising Si/Al ratio from 4.2 to 13.4 for EFAl-free zeolites. This trend is well reproduced by the current computational results, which show good correlation between framework Al density, intrinsic acidity and rate parameters for the two probe reactions. Upon introduction of EFAl, the correlation between benzene H/D exchange rate and propane cracking was much less pronounced. Some of the EFAl-modified materials performed less effectively in H/D exchange but showed comparable activity in propane cracking to the parent zeolite. Our computational findings provide a rationale for these experimental differences. The correlation between the intrinsic acidity and the rate of the H/D exchange is affected by the change in the reaction mechanism when enhanced sites are generated by the presence of EFAl. The concerted transition state in EFAl-free catalysts becomes a benzenium cation intermediate due to the additional stabilization within the EFAl/H-FAU model making the H/D exchange reaction a two-step process. Such a change in the reaction mechanism breaks the linear correlation with the intrinsic acidity established for the defect-free
models. For example, whereas EFAl/H-FAU-tetra-2 shows enhanced cracking activity compared to the parent H-FAU-7 material (DE– cracking = 200 kJ/mol vs. 172 kJ/mol), their reactivity toward the H/D exchange reaction is comparable (DE– H/D = 95 kJ/mol vs. 93 kJ/mol). For all models considered here, we find a very good correlation between the catalytic reactivity of faujasites and their acidity reflected in the adsorption energy of ammonia as acidity probe, which has also been noted experimentally for alkane cracking over various zeolites [13,14,38,91]. Our results support the notion that the acid strength is the main factor controlling the activation energy of protolytic cracking [13,14,86], rather than the (enhanced) heat of physical adsorption of alkane [40,41]. 4. Conclusion We have used periodic DFT to investigate the Brønsted acidity and catalytic reactivity of faujasite zeolite. The acidity characterization by ammonia and pyridine adsorption energies gives consistent trends for the acid strength. Increasing Si/Al ratio of low-silica faujasites promotes the acid strength of zeolite, and then the acidity levels off for high-silica faujasite. The presence of multinuclear EFAl located inside faujasite’s sodalite cages substantially enhances the acidity of vicinal supercage BAS. Mechanistic study on H/D exchange reaction of benzene and protolytic propane cracking demonstrates that the presence of cationic EFAl species can promote both reactions but affect the catalytic mechanisms in different manners. For the propane cracking, the characterized acidity by adsorption energies of basic probe molecules is appropriate to be correlated with the catalytic reactivity of all the faujasite catalysts. However, for H/D exchange reaction of benzene, the presence of EFAl species can induce deviations between the measured acidity and the reactivity of faujasite, indicating that acidity and reactivity are not always directly correlated. Acknowledgments C.L. thanks China Scholarship Council (CSC) for financial support. The Netherlands Organization for Scientific Research (NWO) is acknowledged for providing access to the supercomputer facilities. E.A.P. thanks the Government of the Russian Federation (Grant 074-U01) for his personal fellowship in the framework of the ITMO Fellowship and Professorship Program. The authors acknowledge support from the Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation programme funded by the Ministry of Education, Culture and Science of the government of the Netherlands. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2016.10.027. References [1] [2] [3] [4] [5] [6]
Fig. 4. Electron density difference upon the interaction between the [C6H7]+ with zeolite framework: (a) transition state in H-FAU-7 and (b) ionic intermediate in EFAl/H-FAU-tri-3. Yellow and blue colors represent increasing and decreasing electron densities, respectively. All the isosurface levels were set at 0.005 a0 3 (a0: Bohr radius). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Relationship between acidity and catalytic reactivity of faujasite zeolite: A periodic DFT study q Chong Liu a, Guanna Li a, Emiel J.M. Hensen a,⇑, Evgeny A. Pidko a,b,c,⇑ a
Inorganic Materials Chemistry Group, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands c ITMO University, St. Petersburg, Russia b
a r t i c l e
i n f o
Article history: Received 29 July 2016 Revised 24 October 2016 Accepted 27 October 2016
Keywords: Extra framework aluminum Acidity Zeolite H/D exchange Alkane cracking DFT calculations Reaction mechanisms Structure-activity relations
a b s t r a c t The fundamental aspects of Brønsted acidity and catalytic reactivity of faujasite-type zeolites were investigated by periodic DFT calculations. The adsorption energies of ammonia and pyridine on the Brønsted acid site (BAS) were used to determine the acidity. It is demonstrated that the acid strength of zeolite materials increases with rising Si/Al ratio (low-silica faujasite), and then levels off at high Si/Al ratio (high-silica faujasite). The presence of multinuclear extra framework Al (EFAl) in the sodalite cages substantially enhances the Brønsted acidity. The catalytic reactivity of faujasite toward protolytic propane cracking correlates well with the characterized acidity by base adsorption. However, for H/D exchange reaction of benzene the presence of EFAl species can induce deviations between the measured acidity and the reactivity of faujasite catalysts, indicating that acidity and reactivity are not always directly correlated. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Zeolites are important catalysts in the oil refining and petrochemical industry [1,2]. These microporous materials are composed of corner-sharing SiO4 tetrahedra. The negative framework charge resulting from isomorphous substitution of lattice Si4+ with Al3+ is compensated by extra framework cations such as protons which give zeolites their pronounced Brønsted acidic properties. Most industrial applications of zeolite catalysts rely on these Brønsted acid sites (BASs) embedded in micropores, giving rise to shape-selective hydrocarbon conversion [3]. The largest-scale application of zeolite as catalyst is in the fluid catalytic cracking (FCC) process that converts heavy crude oil fractions into more valuable products [4]. Two main mechanisms have been implicated in the cracking of hydrocarbons on BAS in zeolites, namely protolytic cracking and bimolecular cracking [5–7]. The latter mechanism involves a chain q This contribution is part of the virtual issue ‘‘30 years of the International Conferences on Theoretical Aspects of Catalysis (ICTAC)”. ⇑ Corresponding authors at: Inorganic Materials Chemistry Group, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands (E.A. Pidko). E-mail addresses: [email protected] (E.J.M. Hensen), [email protected] (E.A. Pidko).
http://dx.doi.org/10.1016/j.jcat.2016.10.027 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.
reaction in which carbenium ions abstract a hydride from an alkane followed by b-scission of the resulting carbenium ion into an alkene and a smaller carbenium ion. Carbenium ions that initiate the chain reaction may be formed by protonation of alkenes present in the feed as impurities. The alternative mononuclear mechanism was proposed by Haag and Dessau and involves direct activation of alkanes by acid sites [8]. According to this mechanism [9], the bridging hydroxyl groups in zeolites protonate alkanes to form a penta-coordinated carbonium ion transition state, which collapses into a smaller carbenium ion and an alkane or a hydrogen molecule. The b-elimination of a proton of the carbenium ion will then result in an alkene and regenerate the BAS. It is generally recognized that the monomolecular mechanism predominates at low alkane conversion, whereas the bimolecular mechanism prevails at high alkane conversion [10]. Furthermore, monomolecular cracking requires higher reaction temperatures, as barriers for formation of the carbonium ion transition states are considerably higher than carbenium ion transition states associated with the bimolecular cracking mechanism [11]. Cracking activity of zeolites is strongly influenced by the acidic properties of zeolites, most prominently by the strength and number of the BAS [12]. Accordingly, correlations of catalytic activity with zeolite acidity have been thoroughly investigated [13–16]. Common approaches to investigate zeolite acidity include IR and
C. Liu et al. / Journal of Catalysis 344 (2016) 570–577
NMR spectroscopy as well as adsorption-desorption methods such as calorimetry and temperature-programmed desorption (TPD) [3,17–19]. These techniques rely on the interaction of the BAS with basic probe molecules such as carbon monoxide, ammonia, or pyridine. Spectroscopic methods probe the frequency shift of characteristic vibrations of the probe molecule or the perturbed O–H stretching vibration, while calorimetry and TPD methods measure the heat of adsorption. Despite many investigations, direct correlation between zeolite acidity characterized by such methods and catalytic activity remains problematic and is part of a longstanding debate on the nature of acidity in zeolites [3,18]. Haag and co-workers showed clear correlation between n-hexane cracking and framework Al content for HZSM-5 [20]. Yet, in practice it is seldom possible to construct clear structure-performance relations which is due to the heterogeneity of acid sites in nature and strength [21]. One important aspect is the influence of extra framework Al species [22]. It is important to mention that the adsorption of hydrocarbons will also greatly depend on the micropore structure [23] and entropic factors in the constrained zeolite environment also appear to play a role in zeolite-catalyzed hydrocarbon conversion [24,25]. Zeolite Y (faujasite, FAU) is synthesized in Al-rich form (Si/ Al 2.5) and displays weak Brønsted acidity and low hydrothermal stability. Therefore, for industrial application as in hydrocracking, as-synthesized zeolite Y will undergo various treatments aimed at reducing the framework Al content to improve these properties [26]. For example, high-temperature steaming of zeolite Y produces crystalline materials commonly denoted as the ultrastable Y (USY) zeolites with improved acidity and hydrothermal stability. This treatment leads to the migration of part of framework aluminum (AlF) from lattice sites to extra framework positions confined inside the zeolite pores [27]. The presence of such extra framework aluminum (EFAl or AlEF) species in zeolites enhances their acidity and catalytic performance [28–31]. Enhanced acidity has been suggested to be due to EFAl in close proximity to the BAS [32]. These EFAl species may act as Lewis acid sites and promote the intrinsic acid strength of vicinal BAS via polarization effect [33–35]. The group of Niwa proposed that the presence of the asymmetric tetrahedral EFAl inside USY zeolite enhances the Brønsted acidity [36]. The same authors reported that steaming and formation of EFAl species increase the heat of adsorption of ammonia and catalytic cracking activity [37]. The measured activation energies for alkane cracking showed strong dependence on the acidity as measured by ammonia adsorption [38]. An alternative proposal emphasizes the role of zeolite pore structure and confinement on the cracking reactivity rather than variation in intrinsic strength of the acid sites [39,40]. Van Bokhoven and co-workers showed that the presence of EFAl species does not affect the intrinsic cracking activity but increases the adsorption heat of hydrocarbon reactants [41–45]. They contended that the intrinsic acid strength of BAS in zeolites is similar and apparent cracking rate differences arise from different heats of adsorption. Gounder et al. proposed that the presence of EFAl species can decrease the zeolite pore size, resulting in more efficient stabilization of transition states by dispersive interactions, and therefore enhance the cracking activity [24]. The experimental study by Schallmoser et al. suggested that changes in the local environment of BAS associated with proximate tetrahedral EFAl species can increase the cracking rate [25]. The presence of EFAl species in zeolites complicates understanding structure-activity relationships. Their influence on acidity may also depend on the nature of the protonated complex in zeolite [22]. Experimentally, it has been demonstrated that the acidity probed by COads IR and the H/D exchange reaction of benzene only correlates well with catalytic cracking activity for zeolite Y free from EFAl [22].
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Quantum chemical calculations are instrumental in constructing a molecular picture of zeolite reactivity that can be used to formulate acidity-reactivity relationships in zeolite catalysis [21]. Zeolite acidity represented one of the central problems of the first computational catalysis studies in the 1980s and since then it remained a source of intense debate [46–56]. Theoretically, the intrinsic strength of the acid sites in zeolites can be described in terms of deprotonation energies (DPEs), which is the energy required to dissociate the proton from the BAS to infinite distance [52]. Sauer and co-workers investigated the acid strength of highsilica frameworks FAU, CHA, MOR, and MFI by periodic QM/MM calculations and found that the calculated DPEs do not correlate well with the computed adsorption heats of ammonia [57]. On the contrary, a periodic DFT study from the Miyamoto group on the acidity of isomorphously substituted CHA zeolites demonstrated a good correlation between these two parameters [58]. However, a recent comprehensive DFT analysis on an extended library of periodic zeolite models carried out by Iglesia and coworkers suggests that such correlations could be biased by the specific T atom arrangement and thus location of BAS in the zeolite micropores [59]. It was shown that the ensemble-averaged DPEs were nearly insensitive to the location of isolated Al and Brønsted proton as well as to the topology of zeolite framework. As such, these authors contend that the DPEs do not systematically trend with the heat of adsorption of ammonia. Computational studies on alkane cracking by zeolites revealed a direct dependence of the intrinsic activation barriers of the cracking reaction and the strength of the zeolite BAS. Bell and coworkers carried out complementary Monte Carlo simulations and DFT calculations, and demonstrated that the intrinsic and apparent rate coefficients for alkane cracking are higher in MFI than in the less acidic FAU [6]. Niwa and co-workers employed periodic DFT calculations on a series of HY and cation-exchanged Y-zeolites and found a strong dependence of alkane cracking activity on the intrinsic strength of zeolite BAS quantified by the DPE descriptor [14]. Similarly, a cluster DFT study on the acidity of HZSM-5 zeolites by Deng et al. also revealed a direct relationship between the intrinsic acidity and the barrier of propane cracking [60]. Other theoretical studies showed that the introduction of extra framework species such as Al(OH)2+ EFAl species or metal cations such as Ba2+ and Ca2+ in HY zeolites can polarize the proximate BAS and enhance their acidity [32,61–63]. In spite of the significant number of studies reported on this topic so far, the effect of zeolite topology and composition on intrinsic acidity and reactivity remain unclear. Herein, we present the results of a systematic periodic DFT study on the acidity and reactivity of faujasite-type zeolite catalysts. The enhanced acidity of BAS after dealumination is associated with two main factors. One is the increased Si/AlF ratio, and the other is the presence of EFAl species. Accordingly, we investigated the effect of these two factors on the correlation between the acidity probed by the adsorption energy of ammonia and pyridine and H/D exchange in benzene and catalytic activity in protolytic cracking of propane.
2. Computational methods Density functional theory (DFT) calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional [64] as implemented in the Vienna Ab Initio Simulation Package (VASP) [65–68]. The projected augmented waves (PAW) method was used to describe the electron-ion interactions [69,70]. The cut-off energy of the plane waves was set to 500 eV. Brillouin zone sampling was restricted to the C point [71]. Convergence was assumed to be reached when the forces on each atom were below 0.05 eV/Å. A
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modest Gaussian smearing of 0.05 eV was applied to band occupations around the Fermi level, and the total energies were extrapolated to r ? 0. Van der Waals interactions were described by the dispersion-corrected DFT-D2 method [72]. The climbing image nudged elastic band (CI-NEB) method was used to determine the minimum energy path and to locate the transition state structures [73]. The maximum energy geometries along the reaction path obtained with the CI-NEB method were further optimized using a quasi-Newton algorithm. The nature of transition states was confirmed by determining the vibrational frequencies using the finite difference method. Small displacements of 0.02 Å were used to determine the numerical Hessian matrix. Similar to our previous work [74–77], a low-symmetry rhombohedral unit cell was used to model faujasite [78]. Periodic boundary conditions were employed. The location of lattice Al was chosen according to the stability analysis, and the negative charge on the lattice was compensated by protons at O1 positions. The most stable configurations selected as the zeolite models for this study are illustrated in Fig. S1. Further details on the structural and stability parameters of the employed models can be found in Ref. [76]. The cell parameters were optimized for the defect-free faujasite models with Si/AlF (AlF = 14, 6 and 1) ratios of 2.4, 7, and 47. The optimized lattice parameters are as follows: Si/AlF = 2.4: a = b = c = 17.65 Å, a = b = c = 60°; Si/AlF = 7: a = b = c = 17.44 Å, a = b = c = 60°; Si/AlF = 47: a = b = c = 17.29 Å, a = b = c = 60°. The decrease of the unit cell dimensions with increasing Si/Al ratio was also observed in experiment [22,29]. Full geometry optimizations with the extra framework Al species (EFAl) and adsorbates were performed with fixed cell parameters. EFAl-free models with Si/AlF = 95 and 191 were based on a model with 1 Al center per unit cell (Si/AlF = 47) by creating supercells of 1 1 2 and 1 2 2 unit cells, respectively. The EFAl-containing models were all made from the unit cell with Si/AlF = 7. We studied the effect of different bi-, tri-, and tetranuclear aluminohydroxy clusters [AlxOyHz]n+ (2 6 x 6 4) stabilized
Fig. 1. (a) Rhombohedral faujasite unit cell. (b) Schematic presentation of the interaction of BAS with EFAl [AlxOyHz]n+ (2 6 x 6 4) clusters. (c) Structure of the trinuclear [Al3O4H3]4+ stabilized in the sodalite cage of faujasite.
inside the sodalite cages on the acidity of vicinal supercage BAS (Figs. 1 and S2). The positive charge of the extra framework complex was compensated by substituting an appropriate number of zeolite BAS vicinal to the EFAl resulting in a net 0 charge of the periodic EFAl-containing faujasite model. The choice for the structure and location of the EFAl species in the sodalite cage was based on another computational study [76]. The overall Si/Al ratio for the investigated EFAl/H-FAU models were in the 4.2–5.3 range, which allows comparison to our recent experimental study of zeolite Y acidity and the influence of EFAl on propane cracking [22].
3. Results and discussion To investigate the influence of framework Al density and the presence of EFAl species on acid activity, we used five defect-free faujasite models (H-FAU) with Si/AlF ratios of 2.4, 7, 47, 95, and 191 and a partially dealuminated H-FAU with Si/Al = 7 as a base for eight EFAl-containing faujasite models (EFAl/H-FAU). As EFAl species, we investigated bi-, tri-, and tetra-nuclear aluminohydroxo cationic clusters stabilized inside the sodalite cages, adjacent to a supercage BAS (Figs. 1 and S2). These multinuclear cationic complexes have been identified as candidate EFAl complexes in activated faujasite zeolite using an ab initio thermodynamics approach [76]. The acidity of the zeolites was investigated by determining the adsorption energies of ammonia and pyridine. The interaction of ammonia (NH3) and pyridine (C5H5N) with BAS results in proton transfer from the zeolite framework to the probe molecule (Scheme 1a and b, and Fig. S3). To investigate acid activity, two model reactions were used, namely H/D exchange of benzene [79,80] and monomolecular propane cracking [44,45] (Scheme 1c and d). During the H/D exchange reaction (Scheme 1c), the acidic OH group reacts with C6H6 to give an acidic OD group and C6H5D. For propane cracking (Scheme 1d), the rate-determining step is the protonation of C3H8 by a BAS resulting in a separated ion pair consisting of a carbonium [C3H9]+ and the deprotonated anionic zeolite [6]. The reaction products ethylene and methane are then obtained by rearrangement of the [C3H9]+ ion. In this study, we only considered the initial protonation step of propane activation. First, we investigated the influence of the framework Si/Al ratio on the acidity and activity. The calculated adsorption energies of ammonia [DEads(NH3)] and pyridine [DEads(C5H5N)] are listed in Table 1. Experimental studies reported DEads(NH3) of 110 kJ/mol for low-silica HY [37], and DEads(NH3) and DEads(C5H5N) of 150 kJ/mol and 180 kJ/mol, respectively, for high-silica HY [81]. The adsorption energies computed for the faujasite models (Table 1) are in good agreement with these experimental values. Increasing Si/AlF ratio results in increasing adsorption energy of ammonia and pyridine. This trend is in keeping with experimental observation [82] and well known in zeolite chemistry. Nextnearest neighbor (NNN) Al substitutions in the framework decrease the acidity of the protons, as acid strength of a bridging OH group depends strongly on the electronegativity of the cations, Si4+ or Al3+, present in the second coordination sphere around BAS [1]. When the dilution of Al centers is sufficient, the intrinsic acidity of the bridging hydroxyl groups is not affected anymore [83]. This is consistent with the finding that the adsorption energies do not increase for Si/AlF > 47. Likely, this critical limit lies much lower as usually values between 5 and 10 are reported [84,85]. Next, H/D exchange and protolytic cracking of propane were investigated on these H-FAU models. Table 1 lists the activation energies determined for these reactions. The activation barriers – for H/D exchange (DE– H/D) and propane cracking (DEcracking) initially decrease with increasing Si/AlF ratio (Si/AlF = 2.4, 7, and 47) and do not depend on the framework Al content for Si/AlF > 47. The activa-
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Scheme 1. (a) Ammonia adsorption, (b) pyridine adsorption, (c) H/D exchange of benzene, and (d) propane cracking with formation of carbonium intermediate in faujasite.
Table 1 – Determination of acidity and reactivity of H-FAU and EFAl/H-FAU models by DEads(NH3), DEads(C5H5N), DE– H/D and DEint as indicated in Fig. 3a, and DEcracking as indicated in Fig. 3b. All energies are in kJ/mol. Faujasite models
Si/AlF
H-FAU-2.4 H-FAU-7 H-FAU-47 H-FAU-95 H-FAU-191 EFAl/H-FAU-bi-1 EFAl/H-FAU-bi-2 EFAl/H-FAU-tri-1 EFAl/H-FAU-tri-2 EFAl/H-FAU-tri-3 EFAl/H-FAU-tetra-1 EFAl/H-FAU-tetra-2 EFAl/H-FAU-tetra-3
2.4 7 47 95 191 7 7 7 7 7 7 7 7
EFAl
[Al2O4H3]+ [Al2O4H4]2+ [Al3O4H]2+ [Al3O4H2]3+ [Al3O4H3]4+ [Al4O6H2]2+ [Al4O6H4]4+ [Al4O6H5]5+
DEads(NH3) 104 125 142 139 141 152 173 153 179 182 137 153 181
tion barriers show good correlation with the adsorption energies of ammonia and pyridine. Accordingly, we conclude that adsorption energies of ammonia and pyridine are reasonable criteria to assess the catalytic activity of EFAl-free faujasite in the protolytic cracking of FAU zeolite. One also expects good correlation with H/D exchange rates, which has been used before to probe acidity of zeolite Y [22]. The presence of EFAl species in faujasite strongly affects the acidity and reactivity of BAS. The AlF concentration in the FAU models was fixed and EFAl clusters were introduced in the sodalite cages of the models to study their influence of adsorption and catalytic properties. In general, the adsorption energies of ammonia and pyridine on the BAS increase upon addition of EFAl clusters
DEads(C5H5N) 146 163 199 191 196 219 246 220 267 274 196 230 268
DE – H/D
DEint
120 95 75 81 79 45 77 59 59 93 93 53
DE– cracking 226 200 181 182
56 21 61 23 14 73 53 12
172 148 167 141 135 184 172 140
(Table 1). This shows that the presence of the EFAl species in the sodalite cages affects the intrinsic acidity of the BAS. Table 1 also shows that the acidity increased with increasing charge of the EFAl cationic complex. Previous periodic DFT studies have revealed an enhancement of the adsorption strength of ammonia when Al (OH)2+ EFAl species are introduced [62]. In practice, the acidity of steam-activated faujasites can be further modified by poststeaming treatment with such compounds as Na2H2-EDTA or ammonium salts to remove part of the EFAl debris generated during steaming [37,86]. The resulting materials contain very strong BAS as follows by the increased heat of ammonia adsorption (137–150 kJ/mol) compared with the heat of ammonia adsorption of 110 kJ/mol for the parent HY. These experimental findings indi-
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cate that the extent of acidity enhancement by EFAl species is broad, and the acid strength of BAS in faujasite is sensitive to the structure of EFAl species. Similarly, the current computational study demonstrates that faujasite zeolites containing different EFAl species can possess BAS with acidity varying in a very wide range as probed by ammonia adsorption. The mechanism of the H/D exchange and propane cracking is affected by the presence of EFAl species in different manners (Figs. 2, S4 and S5). H/D exchange in H-FAU proceeds via a concerted mechanism, whereas the presence of EFAl clusters shifts the reaction to a stepwise mechanism involving the benzenium [C6H7]+ ion as a stable intermediate (Fig. 2a). The reaction energy
(DEint) for the formation of benzenium intermediate is sensitive to the acid strength of the protons in EFAl/H-FAU. The higher the acidity is, the lower the energy of the [C6H7]+ intermediate is (Table 1). Proton transfer from the [C6H7]+ intermediate to the zeolite framework oxygen is facile and closes the catalytic cycle. The mechanism of propane cracking is similar for H-FAU and EFAl/HFAU. The reaction proceeds via a carbonium-like transition state and intermediate (Fig. 2b). The energy barriers for the formation + of the transition state (DE– cracking) and the [C3H9] intermediate (DEcracking) decrease with increasing Brønsted acidity. Below, we will further analyze the difference between the two pathways for the H/D exchange reaction.
Fig. 2. Mechanisms of (a) H/D exchange reaction of benzene and (b) propane cracking in H-FAU and EFAl/H-FAU faujasite models.
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The calculated DE– cracking are in line with the experimental study by Katada et al. [38] that reported activation energies for propane cracking of 226 kJ/mol for HY (Si/Al = 2.4) and 104–162 kJ/mol for different post-treated USY catalysts. The correlation with the base molecule adsorption energies is better for propane cracking than for H/D exchange. For example, the EFAl/H-FAU-bi-2 model [DEads(NH3) = 173 kJ/mol, DE– H/D = 45 kJ/mol] is more reactive than the EFAl/H-FAU-tri-2, tri-3, and tetra-3 models which carry stronger acid sites [DEads(NH3) = 179 kJ/mol, 182 kJ/mol, and 181 kJ/mol, DE– H/D = 59 kJ/mol, 59 kJ/mol, and 53 kJ/mol]. On the other hand, the EFAl/H-FAU-tetra-1 and tetra-2 models with significantly different acidity [DEads(NH3) = 137 kJ/mol and 153 kJ/mol] have similar activation energy for H/D exchange (DE– H/D = 93 kJ/mol). This shows that the intrinsic acidity is not the only factor influencing the activation barrier for H/D exchange. We analyzed the trends between adsorption energies of bases and the energetics of the reactions investigated in more detail. The adsorption energies for ammonia and pyridine correlate well as shown in Fig. 3a. Thus, we expect that calorimetric characterization of acidity with these bases would give comparable acidity trends when studying faujasite zeolites. IR spectroscopy with pyridine as probe base is commonly used to study the acidic properties of zeolites. In experiment, a band at 1540 cm 1 is assigned to the presence of pyridinium [C6H5NH]+ due to protonation of pyridine by BAS [18]. The band corresponds to the 19b vibration mode, in which pyridine ring stretching and CH/NH bending are featured together [87]. We calculated the vibrational frequency of adsorbed pyridine for our faujasite models. The corresponding frequencies of 19b mode are 1543–1548 cm 1 for the H-FAU models and 1529– 1539 cm 1 for the EFAl/H-FAU models (Table S1). The calculations do not reveal any notable correlation between the zeolite acidity and the frequency shifts of the 19b vibrational mode of pyridine or the alternative 8a/8b modes (Table S1), which in the case of the 2,6-dimethylpyridine probe have been proposed to be sensitive to the variations in the zeolite acidity [88]. Overall, these computational results are in a good agreement with the available experimental data [88–90] implying that the analysis of the vibrational spectra of adsorbed pyridine cannot be used to distinguish sites of different acid strength. Fig. 3b and c shows correlations between DEads(NH3) and the activity in H/D exchange and propane cracking. The correlation with the cracking activation barrier (Fig. 3c) is better than that for the H/D exchange reaction (Fig. 3b). In the latter case, the trend only holds for the H-FAU models. The presence of EFAl species in faujasite alters the mechanism such that an intermediate ionic state is involved (Fig. 2a, H-FAU vs. EFAL/H-FAU). The stability of the benzenium [C6H7]+ ion may be expected to correlate with the acidity of faujasite. Indeed, DEads(NH3) correlates very well with the stability of [C6H7]+ (DEint, Fig. 2a). In the concerted transition state identified for H-FAU and the [C6H7]+ intermediate state for EFAl/H-FAU, the Brønsted protons are strongly bonded to the benzene molecule. The [C6H7]+ ion is stabilized by hydrogen-bonding interactions with zeolite framework oxygens, much like also observed for adsorbed ammonium. This may explain why the energetics DE– H/D for H-FAU and DEint for EFAl/H-FAU correlate well with DEads(NH3). The transition state for the formation of the ionic intermediate does not involve strong bonding of the exchanged proton with the framework. This is because deprotonation of the hydroxyl group is not complete. As such, it is clear that the increased acidity due to EFAl results in formation of a stable ionic complex. In summary, these findings show that DEads(NH3) correlate well with stability of ionic intermediates such as the carbonium ion and the benzenium ion, because in all cases the bridging hydroxyl group is deprotonated. On the other hand, the rate of the H/D exchange reaction is determined by the activation barrier (DE– H/D), which
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Fig. 3. Correlations between DEads(NH3) and (a) DEads(C5H5N), (b) DE– H/D and DEint (c) DE– cracking and the respective linear data fits.
does not correlate well with DEads(NH3). Thus, care must be taken in correlating H/D exchange rates to acidity. Next, we analyzed the influence of the finite temperature entropic effects on the reactivity relationships established above for the catalytic cracking. In line with the commonly employed experimental procedures, Gibbs free energies of propane adsorption and activation were estimated at the temperature of 800 K (DGads(C3H8)800K and DG– cracking,800K, Table S1), while the Gibbs free energies of ammonia adsorption were computed at both 300 and 800 K (DGads(NH3)300K, DGads(NH3)800K, Table S1). The free energy correlations are summarized in Fig. S6 of the supporting information. The variations in the zeolite structure and acidity do not affect the energetics of propane physisorption (DEads(C3H8) = 50 to 42 kJ/mol, Table S1). The intrinsic free energy barriers of propane cracking (DG– cracking,800K) follow the same linear relationships with the energies (DEads(NH3)) and Gibbs free energies (DGads(NH3)300K and DGads(NH3)800K) of adsorption of ammonia as the correlations established based on the intrinsic energetics (DE– cracking, DEads(NH3), Fig. 3c).
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The presented data show that cationic EFAl located in the sodalite cages of faujasite enhance the Brønsted acidity of protons in adjacent supercages. This weakening of the O–H bond is evident from strong adsorption of bases and lower barriers for H/D exchange and propane cracking. Importantly, no acidity enhancement was observed for the FAU model, containing a neutral EFAl and a reactive BAS at the vicinal positions inside the supercage (Fig. S7). The values of DEads(NH3) and DE– cracking in this case were identical within 1 kJ/mol to those computed for the original EFAlfree model (cf. Table 1 and Fig. S7). To gain better understanding of the influence of EFAl on acidity and reactivity, electron density difference analysis was performed (Figs. 4 and S4). Fig. 4 shows that less electrons are accumulated between the [C6H7]+ fragment and zeolite framework in EFAl/HFAU compared with the concerted transition state in H-FAU. On this basis, we conclude that the stabilization of the [C6H7]+ ionic intermediate is due to the presence of EFAl species in the sodalite cages, which help to compensate the negative charge of the deprotonated BAS after [C6H7]+ formation. Effectively, this increases the acidity of the bridging hydroxyl group. We suspect this EFAl effect to be a generic phenomenon during the zeolite-catalyzed reactions. We compare these computational results with our previous experimental observations [22]. In this work, we prepared defect-free high-silica FAU zeolite by (NH4)2SiF6 treatment of assynthesized FAU, followed by modification by EFAl species. We found that the rate of benzene H/D exchange and propane cracking increased with rising Si/Al ratio from 4.2 to 13.4 for EFAl-free zeolites. This trend is well reproduced by the current computational results, which show good correlation between framework Al density, intrinsic acidity and rate parameters for the two probe reactions. Upon introduction of EFAl, the correlation between benzene H/D exchange rate and propane cracking was much less pronounced. Some of the EFAl-modified materials performed less effectively in H/D exchange but showed comparable activity in propane cracking to the parent zeolite. Our computational findings provide a rationale for these experimental differences. The correlation between the intrinsic acidity and the rate of the H/D exchange is affected by the change in the reaction mechanism when enhanced sites are generated by the presence of EFAl. The concerted transition state in EFAl-free catalysts becomes a benzenium cation intermediate due to the additional stabilization within the EFAl/H-FAU model making the H/D exchange reaction a two-step process. Such a change in the reaction mechanism breaks the linear correlation with the intrinsic acidity established for the defect-free
models. For example, whereas EFAl/H-FAU-tetra-2 shows enhanced cracking activity compared to the parent H-FAU-7 material (DE– cracking = 200 kJ/mol vs. 172 kJ/mol), their reactivity toward the H/D exchange reaction is comparable (DE– H/D = 95 kJ/mol vs. 93 kJ/mol). For all models considered here, we find a very good correlation between the catalytic reactivity of faujasites and their acidity reflected in the adsorption energy of ammonia as acidity probe, which has also been noted experimentally for alkane cracking over various zeolites [13,14,38,91]. Our results support the notion that the acid strength is the main factor controlling the activation energy of protolytic cracking [13,14,86], rather than the (enhanced) heat of physical adsorption of alkane [40,41]. 4. Conclusion We have used periodic DFT to investigate the Brønsted acidity and catalytic reactivity of faujasite zeolite. The acidity characterization by ammonia and pyridine adsorption energies gives consistent trends for the acid strength. Increasing Si/Al ratio of low-silica faujasites promotes the acid strength of zeolite, and then the acidity levels off for high-silica faujasite. The presence of multinuclear EFAl located inside faujasite’s sodalite cages substantially enhances the acidity of vicinal supercage BAS. Mechanistic study on H/D exchange reaction of benzene and protolytic propane cracking demonstrates that the presence of cationic EFAl species can promote both reactions but affect the catalytic mechanisms in different manners. For the propane cracking, the characterized acidity by adsorption energies of basic probe molecules is appropriate to be correlated with the catalytic reactivity of all the faujasite catalysts. However, for H/D exchange reaction of benzene, the presence of EFAl species can induce deviations between the measured acidity and the reactivity of faujasite, indicating that acidity and reactivity are not always directly correlated. Acknowledgments C.L. thanks China Scholarship Council (CSC) for financial support. The Netherlands Organization for Scientific Research (NWO) is acknowledged for providing access to the supercomputer facilities. E.A.P. thanks the Government of the Russian Federation (Grant 074-U01) for his personal fellowship in the framework of the ITMO Fellowship and Professorship Program. The authors acknowledge support from the Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation programme funded by the Ministry of Education, Culture and Science of the government of the Netherlands. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2016.10.027. References [1] [2] [3] [4] [5] [6]
Fig. 4. Electron density difference upon the interaction between the [C6H7]+ with zeolite framework: (a) transition state in H-FAU-7 and (b) ionic intermediate in EFAl/H-FAU-tri-3. Yellow and blue colors represent increasing and decreasing electron densities, respectively. All the isosurface levels were set at 0.005 a0 3 (a0: Bohr radius). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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