Tandem Aromatization of Oxygenated Furans by Framework Zinc In

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Tandem Aromatization of Oxygenated Furans by Framework Zinc in Zeolites. A Computational Study Ryan E. Patet, Stavros Caratzoulas, and Dionisios G. Vlachos J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07402 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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The Journal of Physical Chemistry

Tandem Aromatization Of Oxygenated Furans By Framework Zinc In Zeolites. A Computational Study Ryan E. Patet1,2, Stavros Caratzoulas2,*, Dionisios G. Vlachos1,2 1

Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716 2 Catalysis Center for Energy Innovation (CCEI), University of Delaware, Newark, DE 19716 ABSTRACT: We have performed electronic structure calculations to characterize the active site of the zeolite CIT-6 (isomorphically substituted Zn-Beta) and to study the Diels-Alder dehydrative aromatization of methyl-5-(methoxymethyl)-furoate (MMFC) and of the dimethyl ester of 2,5furan-dicarboxylic acid (DMFDC) with ethylene. Three types of active sites have been investigated: a site where the framework charge was balanced by two protons (Z0); a site with an H+ and a Li+ as counter-cations (Z1); and an active site with two Li+ counter-cations (Z2). Using NBO and Bader analysis, we conclude that Zn incorporation into the zeolite framework is not through covalent bonding to framework oxygen atoms but rather is through ionic bonding. Despite the ionic character of the active site and the generally strong Lewis acidic nature of Zn(II) cations, we find no catalysis of the Diels-Alder reaction for the two furans tested. On the other hand, the dehydration of the Diels-Alder cycloadduct can be either Brønsted or Lewis acid catalyzed depending on the active site type. The Z0-type sites are found to be more active than the Z1 and Z2-type sites. Further, the Z0-type sites exhibit both Lewis and Brønsted acid characters with similar catalytic activities. In full agreement with experiment, we find that the conversion of MMFC to (4-methoxymethyl) benzenecarboxylate via Diels-Alder dehydrative aromatization is easier than the conversion of DMFDC to dimethyl terephthalate as the two electron withdrawing groups in DMFDC stabilize the corresponding cycloadduct against dehydration.

CORRESPONDING AUTHOR: Stavros Caratzoulas ([email protected])

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1. INTRODUCTION As natural gas liquids (shale gas) are pushing naphtha feedstocks aside, the development of alternative routes for the production of aromatics from renewables is receiving considerable attention.1 Among the possible routes is the dehydrative aromatization of the Diels-Alder (DA) product between bio-based furans with an appropriate dienophile; a tandem scheme where the DA cycloadduct (oxanorbornene derivative) is catalytically dehydrated to an aromatic (Scheme 1). The feasibility of the strategy has been demonstrated by the recently reported synthesis of pxylene from 2,5-dimethylfuran (DMF) and ethylene over Brønsted-acidic (H-Y and H-Beta) and Lewis-acidic (Sn-Beta, Zr-Beta and Ti-Beta) zeolites,2-7 as well as by the Diels-Alder aromatization of ethylene and oxidized variants of HMF (5-hydroxymethyl furfural) to terephthalic acid over Sn-Beta.8-10 According to calculations,2, 5, 11-12 the dehydration of oxanorbornene in the absence of catalysts requires ca. 60 kcal/mol of activation energy. Quite clearly, both the Brønsted and Lewis solid acids can catalyze the dehydration step in the tandem scheme. In fact, Chang et al.7 demonstrated that the Lewis-acidic zeolites Sn-, Zr- and Ti-Beta are just as effective as H-Beta in influencing the rate of p-xylene formation – a remarkable finding in its own right considering that Lewis acids do not typically catalyze dehydration reactions. However, because of the fleeting existence of the cycloadduct in all these studies, kinetics experiments have not been able to decouple the two steps and thus ascertain whether these Brønsted- and Lewis-acidic zeotypes have any effect on the rate of the Diels-Alder step itself. With furans being among the less reactive dienes, overcoming the challenge of accelerating the cycloaddition reaction will be a pivotal step in the success of producing aromatics by Diels-Alder aromatization of biomass-derived furans and appropriate dienophiles. Brønsted and Lewis acids are known to accelerate a variety of DA reactions13 by a mechanism readily explained in terms of the frontier molecular orbital (FMO) theory.14-20 In normal electron demand, protonation or complexation with the Lewis acid of the dienophile lowers the energy of its LUMO and closes the gap to the HOMO of the diene, increasing the interaction between the two FMOs and thus the rate of the reaction. In recent years, Diels-Alder reactivity has also been rationalized in terms of the polar character of the transition state and of the concepts of electronic hardness and electrophilicity.21-29 Recent calculations 2, 5, 11-12, 30 have asserted that neither the Brønsted-acidic H-FAU and HBeta, nor the Lewis-acidic Sn-, Zr- or Ti-Beta can influence the rate of the DA reaction between DMF and ethylene. In the case of the Brønsted acids, the lack of catalysis is due to the active site binding DMF more strongly than ethylene, and to the proton having higher affinity for the αposition of the furan ring instead of the ring oxygen atom, thus breaking the requisite orbital symmetry. A similar situation has been observed in Sn-, Zr- and Ti-Beta.30 Although the preferential binding of DMF to the Lewis center of the active site does lower the LUMO of the furan, it is still not enough to activate the inverse electron demand mode and to compensate for the deactivation of the normal electron demand mode due to the concomitant lowering of the HOMO of DMF. Thus the reaction seems to follow bi-directional electron demand17, with no 1 ACS Paragon Plus Environment

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noticeable acceleration. Indeed, it has recently been shown computationally that there is only minor catalysis of the DA in Sn- and Zr-Beta, solely attributable to confinement phenomena (i.e., entropic contributions), which also vary with the translational freedom allowed to the species inside the zeolite. Despite the beneficial influence of confinement, microkinetic modelling has shown that the heterogeneous DA pathway does not contribute to the overall rate of p-xylene formation, and that the DA cycloadduct forms almost entirely in the homogeneous solution phase.30 Although inverse electron demand cycloadditions have been reported, strong Lewis acids are required.24, 31-32 Zn(II) coordination complexes are strong Lewis acids, and Eschenbrenner-Lux et al.32 recently reported for the first time that Zn/binol complexes catalyze the inverse electron demand DA reaction between electron-poor chromone dienes and cyclic imines. Thus, the recent discovery by Orazov and Davis33 that CIT-6 (Zn-Beta), an easily synthesized zincosilicate analogue of zeolite Beta, can catalyze the DA aromatization of ethylene and methyl 5-(methoxymethyl)-furoate (DMFDC) to form methyl 4-(methoxymethyl)-benzoate, and that of ethylene and the dimethyl ester of 2,5-furandicarboxylic acid (MMFC) to form dimethyl terephthalate, raises the question of whether Zn-Beta is capable of catalyzing the inverse electron demand DA between these oxygenated furans and ethylene. Over Zn-Beta, selectivities as high as 62% for methyl (4-methoxymethyl) benzenecarboxylate (MMBC) and 36% for dimethyl terephthalate (DMT) from DMFDC were achieved. Interestingly, differences in selectivities by as much as 30% were seen depending on the preparation methods of the catalyst. Orazov and Davis proposed a possible explanation for these differences based on active site characterization by IR spectroscopy of adsorbed pyridine and deuterated acetonitrile (CD3CN). Adsorbed pyridine showed no peak at ca. 1550 cm-1, from which they inferred the absence of strong Brønsted acid sites. Deuterated acetonitrile showed two peaks, at 2311 and 2290 cm-1, hypothesized to be representative of two distinct Lewis acid sites. The peak at 2311 cm-1 is similar to adsorption peaks found in Sn and Zr-Beta (ca. 23092315 cm-1), and the peak at 2290 cm-1 is similar to Li-exchanged Sn-Beta (ca. 2292 cm-1).34-37 For CIT-6, changes in the synthesis resulted in changes in the site distribution, as measured by the ratio of these two peaks in the CD3CN spectra. Li+ exchange in a moderately basic solvent generated a material possessing primarily the 2290 cm-1 band, while the neutral exchange of N(CH3)4Cl and calcination generated a material possessing primarily the 2312 cm-1 band. Thus, the observed differences in selectivity were attributed to differences in the relative abundance of these two hypothesized active sites. In Scheme 2, we show the possible active site structures proposed by Orazov and Davis.33 In the Z0-type site, the Zn atom participates in two covalent bonds with the framework and accepts a dative bond from one of the two vicinal SiOH groups. In the Z1-type site, three Zn-O covalent bonds are proposed, while one SiOH group (i.e., one proton) and a monovalent metal cation (e.g., Li+) maintain charge neutrality. In Z2, the Zn atom is fully incorporated in the framework (four Zn-O covalent bonds) and there are two metal counterions for charge neutrality (no SiOH groups, i.e., no protons).

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Considering the d10s0 electronic configuration of Zn2+, we think it unlikely that Zn can participate in more than one covalent bond, unless its valence orbitals undergo spd hybridization. In order to elucidate the electronic and bonding structure of the active site of Zn-Beta and thus understand its catalytic activity, in this article we electronically characterize the three plausible active site types, Z0, Z1 and Z2, using density-functional theory calculations in conjunction with NBO and Bader analyses. To our knowledge, no computational studies have examined the active site of framework-substituted Zn zeolites. We also make contact with experiment by investigating the binding of deuterated acetonitrile and pyridine to the three types of active site. Finally, we examine the reaction profiles for the aromatization of MMFC and DMFDC on the Z0 and Z2 active sites. We conclude that differences in adsorption and reactivity arise from blocking of the Zn atom by the Li+ cations in the Z2-type structure, rather than from differences in the nature of the bonds of the framework Zn. Qualitative experimental observations of decreased rates for DMFDC at Z2-type sites, as compared to the Z0-type and MMFC, respectively, are supported by differences in the reaction barriers of the calculated profiles. 2. COMPUTATIONAL METHODS CIT-6 is a zeolitic material with Beta morphology, containing 9 unique tetrahedral substitution positions, which can be grouped into three distinct classes (T1-T4, T5-T6, and T7T9).38 Because we have no experimental evidence as to which of the nine Zn substitution sites is thermodynamically more stable, we have selected to study the sites T2, T5 and T7 as representatives of each class. The T5 position is only accessible through one pore of the Beta zeolite, while the T2 and T7 positions are located at the intersection of two pores. We, therefore, constructed two models for each of the T2 and T7 substituent positions: one with the “top” pore defined (corresponding to the same pore with access to the T5 site) and a second with the “side” pore defined. In the following, we shall be referring to these five models as “T2-top” “T2-side”, “T5-top”, “T7-top” and “T7-side”. The pore models were constructed following the procedure outlined by Migues et al.39 In brief, a portion of the zeolite active site and surrounding pore was cut out from the extended zeolite framework and saturated with hydrogen atoms. Inside this pore, geometry optimizations of co-adsorbed DMFDC and ethylene in 2-4 different orientations were run at a theory level with a minimal basis set (M06-2X/3-21G). Upon completion of the optimization, only the framework atoms within 5 Å of the Zn substituent atom, the counter-cations (H+ or Li+), or the atoms of the co-adsorbed DMFDC and ethylene were retained for the final model. Hydrogen atoms were added to any unsaturated silicon or oxygen atoms, with all saturating hydrogens and oxygens bonded to a saturating hydrogen frozen geometrically. The resulting T2-top model has a stoichiometry of Zn[H2/Li2]Si44O85H55; the T2-side model has a stoichiometry of Zn[H2/Li2]Si41O81H52; The T5-top model has a stoichiometry of Zn[H2/Li2]Si44O88H54; the T7top model has a stoichiometry of Zn[H2/Li2]Si47O88H60; and theT7-side model has a stoichiometry of Zn[H2/Li2]Si39O81H54, as shown in Figure 1. 3 ACS Paragon Plus Environment

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Subsequent calculations were run in these zeolite models at the M06-2X/6-31G(d,p) theory level using the Gaussian 09 program (Rev. D.1).40 Adsorption energies have been corrected for the basis set superposition error (BSSE) using the counterpoise method. Thermal corrections to the energies of adsorbed states have been computed within the quasi-rigid rotor harmonic approximation (q-RRHO), 41-42 and assuming mobile adsorbates in two-dimensional space with a characteristic area of 800 × 800 pm2.43 Ground and transition states were characterized by frequency analysis and all transition states were further validated by intrinsic reaction coordinate (IRC) calculations. Bader analysis was performed using the AIM2000 software44 and NBO analysis was performed using the NBO 6.0 software.45 3. RESULTS AND DISCUSSION 3.1 Active Site Characterization. The images in Figure 2 depict bonding structures obtained from NBO analysis of the Z0, Z1, and Z2 active sites with Zn in the T2, T5 or T7 position. For none of these do we find spd hybridization of the Zn valence atomic orbitals. For the Z0-type sites (Figures 2(a)-(e)), the 4s orbital of Zn is the acceptor of a single, dative covalent bond from a neighboring framework O atom, irrespective of the substitution position (T2, T5 or T7). The two vicinal SiOH groups and a fourth framework O are coordinated to the Zn atom via electrostatic interactions. For the T2-top, T2-side, T7-top, and T7-side models of the Z1-type site (Figure 2, panels (f), (g), (i), and (j)) and for the T7-side Z2-type site (Figure 2(o)), the bonding structure is the same as in the Z0-type sites. For the remaining sites, however, we find no covalent bonding, at all, and the Zn atom is ionically bonded to all four nearest-neighbor O atoms. The nature of these bonds has been further investigated by Bader analysis (Table 1). For all three site types, Z0, Z1, and Z2, the negative of the Laplacian of the electron density takes on negative values at the bond critical points (BCP) of all the Zn-O bonds, indicating ionic-type bonding between the participating atoms. The electron density around all Zn-O bonds is also very small, in the 0.07-0.11 a.u. range. In large measure, the NBO and Bader analyses provide a consistent story on the nature of bonding at the active site: in a sense, the framework Zn behaves like an “extra-framework” cation. To eliminate basis set effects, we picked the T5-top model and repeated the NBO and Bader analyses using structures optimized with the 6-311G(2df,p) and 6311++G(3df,3pd) basis sets for all atoms shown in Figure 2. We observed no differences upon comparison with the results obtained at the M06-2X/6-31G(d,p) level (Table S1 in Supporting Information). Thus, the electronic structure analysis does not support the active site bonding structure suggested by Orazov and Davis (Scheme 2).33 In fact, we believe that the ionic nature of the Zn atom in the framework is very much consistent with the experimental observation that CIT-6 was ineffective at isomerizing glucose to fructose when the reaction was run in water. Irrecoverable loss in activity and decrease in Zn content were reported, probably due to leaching of the Zn cations;33 a Zn(II) atom with ionic bonds to the zeolite framework could be expected to easily leach from the zeolite into water. 4 ACS Paragon Plus Environment


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3.2 Adsorption Characterization. Pyridine, commonly used to probe for Brønsted acidity in zeolites, and deuterated acetonitrile used to probe Lewis acidity, offer insight into the nature the active sites.33 Binding energies of these molecules have been collected in Table 2. For the Z0type sites, both pyridine and CD3CN adsorb more strongly to the T7-side site than to either of the “top” sites (T2, T5 or T7), by ca. 10 kcal/mol. Binding to the T2-side is slightly weaker than to the T7-side, by ca. 3 kcal/mol. The adsorption geometries of CD3CN, in Figures 3(a)–3(e), help to illustrate the reason for this significant difference. When CD3CN adsorbs from the “side” pore, the N atom interacts directly with the Zn atom, while in all “top” structures, the N atom preferentially coordinates to the vicinal silanol groups. The silanols in the T2-side and T7-side models are not accessible by either probe molecule. Even if we had considered a single T2 site or T7 site model, with both intersecting channels defined, then binding to the Zn atom would still have been the optimal geometry, even if we started the optimization from a configuration with the probe molecule coordinated to the silanol; coordination to SiOH is a local minimum, accessible only through adsorption from the top channel. The same coordination geometries are also observed in the case of pyridine. Considering that 7 of the 9 possible Zn substituent positions in Beta can be found at the intersection of two pores, it is reasonable to think that framework-substituted Zn would, on average, be accessible to incoming adsorbates. The preferential coordination of pyridine to the Zn atom of the Z0-type site is consistent with the absence of the characteristic pyridinium IR peak for binding to a Brønsted site. Calculated adsorption strengths onto the Z2 sites do not follow the trend observed for the Z0 sites. Their adsorption strengths seem rather uniform, irrespective of the site, with CD3CN binding energies in the 20-22 kcal/mol range and pyridine binding energies in the 24-27 kcal/mol range. These values can easily be explained from examination of the adsorption geometries (Figure 3, panels (k)–(o) for CD3CN)). In the Z2-type structures, both CD3CN and pyridine preferentially coordinate to Li+ for all active site models—even for adsorption from the “side” pore—because the Li+ cations blocks access to Zn. Preferential coordination of CD3CN and pyridine to Li+ is also observed at the Z1-type sites for adsorption from the top-site pores (T2top, T5-top and T7-top models; Figures 3(f), 3(h) and 3(j)). Interestingly, while pyridine binds to the Z1 and Z2-type sites with equal strength, CD3CN binds less weakly to Z1 than to Z2-type sites, by ca. 2-8 kcal/mol. Although the reasons for that are not entirely clear to us, we should note that the N and C atoms of CD3CN appear less polarized at the Z1 sites (Figure 3, panels (f)–(j)) than at the Z2 ones (Figure 3, panels (k)–(o)), correlating to the adsorption strength. Curiously, although the Zn atom of the Z1-type sites in the T2-side and T7-side models (Figures 3(g) and 3(j)) is accessible, we see significantly weaker binding energies by ca. 9 and 25 kcal/mol, respectively, for pyridine, and by 9 and 9 kcal/mol, respectively, for CD3CN. A possible explanation is the lower lying LUMO of the Z0-type site (shown in Figure 4(a) for the T7-side model), which accepts electron density from the lone pairs of CD3CN and pyridine. Moreover, the LUMO of the Z0 site has significant amplitude in the “side” pore. Incorporation of a Li+ cation in the Z1 site (Figure 4(b)) causes an increase in the LUMO energy by ca. 6 5 ACS Paragon Plus Environment

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kcal/mol, and somewhat depresses the orbital’s spatial extension into the pore. A similar effect is seen with two Li+ cations in the Z2-type T7 site, as the LUMO energy is increased by an additional ca. 18 kcal/mol (Figure 4(c)). Overall, we see stronger binding when the guest molecule coordinates to the Zn atom and that Li+ cations in Z1 and Z2-type sites have an adverse effect, as they block access to the Zn atom and also raise the energy of the electron accepting orbital. Further, the sites T2 and T7, which are located at the intersection of channels, behave quite similarly and thus they are expected to exhibit similar Lewis acidity. 3.3 Furan Aromatization. Unexpectedly, and despite the ionic character of the active site, we find that the Lewis metal center has no effect on the cycloaddition of either MMFC or DMFDC with ethylene. In all cases examined, the active site preferentially binds the furan via its polar alkanoyl moiety, and despite the presence of electron withdrawing groups on the furan ring, the Lewis acid center seems unable to induce sufficient charge transfer to promote inverse electron demand cycloaddition. In the gas phase, the DA free energy barrier, ∆G‡, is 39 kcal/mol for MMFC and 38 kcal/mol for DMFDC. For MMFC at a Z0-type site, ∆G‡ is in the 38-40 kcal/mol range depending on the Zn site (namely, T5-top, T7-top or T7-side); at a Z2-type site, ∆G‡ is in the 37-42 kcal/mol range, again depending on the Zn site (see Figure 5(a) and Figure 5(b)). For DMFDC, ∆G‡ is in the 36-40 kcal/mol range at a Z0-type site and in the 34-36 kcal/mol range at a Z2-type site (see Figure 5(c) and Figure 5(d)). Inspection of the enthalpic contributions (see Figures S1 and S2 of the Supporting Information) reveals, however, that the active site has no electronic effect on the cycloaddition, even in the case of DMFDC at the T5top site of the Z2 type, where ∆G‡ is 4 kcal/mol lower than the uncatalyzed reaction; the corresponding ∆H‡ is, in fact, 1 kcal/mol higher than in the thermal reaction. In other words, even in this, most favorable, case, the very modest DA acceleration should be ascribed to confinement effects. In all the cases, there is strong binding between the active site and the furans, but the DA transition state is stabilized roughly by the same amount of energy and as a result the energy span remains comparable to that of the thermal reaction. In the dehydration of the cycloadduct, the most critical step is the breaking of the C-O-C bridge, which unassisted requires 60 kcal/mol of activation. In this step, we see catalysis to varying degrees, depending on the substrate (namely, cycloadduct of MMFC or of DMFDC), and on the active site. Invariably, we see that it takes less energy to cleave the bridge of the MMFC cycloadduct, because the methoxymethyl group on one of the bridgeheads is electron donating (EDG) and thus it stabilizes the resulting intermediate through resonance (Scheme 3). In the case of the MMFC cycloadduct, the Z0-type sites are more effective than the Z2-type sites. In the former, cleaving of the C-O-C bridge at the bridgehead with the EDG requires only 28-37 kcal/mol, while in the latter case ∆G‡ is in the 38-44 kcal/mol (Figures 5(a) and 5(b)), depending on the Zn substitution site. Interestingly, this step is Lewis acid catalyzed only in the T7-side active site model, where we see coordination of the cycloadduct bridge-O to the Zn atom (Figure 6(c)). In the other two models (T5-top and T7-top) we see Brønsted acid catalysis from the proton of a proximal SiOH (Figures 6(a) and 6(b)). Worth noting is also that in the case of 6 ACS Paragon Plus Environment


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the T7-top model, the proton catalyzed bridge breaking is slower than in the T5-top model by ca. 9 kcal/mol. The Z2-type sites are less effective, to the tune of 5-15 kcal/mol compared to the Z0sites, because the two Li cations block access to the Zn atom. Thus, the cycloadduct of MMFC binds to Li+ (Figure 6, panels (e)–(g)), which seems to be a less effective Lewis acid. The weaker activity of Li+ as a dehydration catalyst is consistent with calculated C-O cleavage barriers for the dehydration of the cycloadduct of DMF and ethylene in ion-exchanged Y zeolites.11-12 In the case of the DMFDC cycloadduct, we observe the same trends, albeit with higher free energies of activation for reasons we described earlier, namely, the two electron withdrawing alkanoyl groups on both bridgehead, which stabilize the cycloaddut against dehydration; see Figures 5(c) and 5(d) for the corresponding free energy profiles and Figures 6(d) and 6(h) for representative transitions state structures at a T7-side active site. We have not explicitly tested the reactivity of Z1-type sites since the weak binding of both CH3CN and pyridine (Section 3.2), the higher lying LUMO orbital and the depressed LUMO amplitude in the direction of the probe molecules strongly suggest that these sites are less active than the Z0-type and no more active than the Z2-type sites. Further, as we have not explicitly tested the T2 site since, as we noted earlier, binding to T2 is similar to T7 and thus the two sites are expected to exhibit the same Lewis acidity. Our results rationalize two experimental observations made by Orazov and Davis:33 first, materials containing a higher percentage of Z0-type sites were more active for aromatization than those containing sites of the Z2-type; and second, MMFC was more active towards aromatization than DMFDC. 4. CONCLUSIONS We have performed electronic structure calculations to characterize the active site of the Znsubstituted Beta zeolite and to study the dehydrative aromatization of methyl-5(methoxymethyl)-furoate (MMFC) and the dimethyl ester of 2,5-furandicarboxylic acid with ethylene (DMFDC). Three types of active sites have been studied—a Z0 site, where the framework charge was balanced by two pore protons; a Z1 site, with an H+ and a Li+ as counter-cations; and a Z2 active site with two Li+ counter-cations. NBO and Bader analyses showed Zn incorporation into the zeolite framework through weak ionic bonding to the neighboring framework O atoms. Differences in the active sites were found through adsorption studies, where the strongest adsorption was observed for coordination to the Zn atom at the Z0-type sites. Incorporation of Li+ cations caused this interaction to decrease significantly, primarily due to blocking of the Zn atoms. Despite the ionic character of the active site and the generally strong Lewis acidic nature of Zn(II) cations, we found no catalysis of the Diels-Alder reaction of the tested furans with ethylene. The cleavage of the C-O-C bridge of the Diels-Alder cycloadduct can be either Brønsted or Lewis acid catalysed depending on the active site type. The Z0-type sites are invariably more active than the Z2-type sites. At the Z0-type sites we may have both Lewis and 7 ACS Paragon Plus Environment

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Brønsted acid catalysis, depending on whether the cycloadduct coordinates to the Lewis metal center or to a SiOH group proximal to the Zn center; both seem to exhibit similar activities in terms of the required activation energy. The dehydration of the MMFC cycloadduct is more facile than that of DMFDC, on account of the electron donating group that stabilizes the bridge opening. Our results and conclusions are consistent with experimental observations made by Orazov and Davis.33 SUPPORTING INFORMATION Full enthalpy and Gibbs free energy profiles for MMFC and DMFDC dehydrative aromatization. Bader analysis data. ACKNOWLEDGEMENTS We acknowledge support from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award number DE-SC0001004. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DEAC02-05CH11231. We are also thankful to Dr. Marat Orazov for numerous helpful discussions.



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TABLES AND FIGURES:

Scheme 1. Dehydrative aromatization of furans.


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Scheme 2. Proposed active site structures of framework-substituted Zn-Beta.33



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Figure 1. Zn-Beta models used in this study: (a) Zn atom in T2 site with the “top” pore defined; (b) Zn atom in T2 site with the “side” pore defined; (c) Zn atom in T5 site with the “top” pore defined; (d) Zn atom in T7 position with the “top” pore defined; and (e) Zn atom in T7 position with the “side” pore defined. Atoms located above the substituted Zn atom (from shown perspective) are shown as wireframes for clarity. Color code: Zn, orange; Si, grey; O, red; H, white.


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Figure 2. Local bonding structures for active site models, obtained from NBO analysis: (a) T2, “top”, Z0; (b) T2, “side”, Z0; (c) T5, “top”, Z0; (d) T7, “top”, Z0; (e) T7, “side”, Z0; (f) T2, “top”, Z1; (g) T2, “side”, Z1; (h) T5, “top”, Z1; (i) T7, “top”, Z1; (j) T7, “side”, Z1; (k) T2, “top”, Z2; (l) T2, “side”, Z2; (m) T5, “top”, Z2; (n) T7, “top”, Z2; (o) T7, “side”, Z2. Atoms beyond the Si-atoms immediately neighboring the Zn atom have been removed from the images for clarity. Color code: Zn, orange; Si, grey; O, red; Li, purple; H, white.



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Figure 3. Adsorption geometries of deuterated acetonitrile: (a) T2, “top”, Z0; (b) T2, “side”, Z0; (c) T5, “top”, Z0; (d) T7, “top”, Z0; (e) T7, “side”, Z0; (f) T2, “top”, Z1; (g) T2, “side”, Z1; (h) T5, “top”, Z1; (i) T7, “top”, Z1; (j) T7, “side”, Z1; (k) T2, “top”, Z2; (l) T2, “side”, Z2; (m) T5, “top”, Z2; (n) T7, “top”, Z2; (o) T7, “side”, Z2. In panels (b), (e), (g) and (j) we have coordination to the Zn atom; in panels (a), (c) and (d) coordination to a silanol group; and in the rest coordination to a Li+ cation. Atoms beyond the Si-atoms immediately neighboring the Zn atom have been removed from the images for clarity. Values shown next to N and C atoms of CD3CN indicate NBO charges. Color code: Zn, orange; Si, light grey; C, grey (small balls); N, blue; O, red; Li, purple; H, white.


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Figure 4. Lowest unoccupied molecular orbitals (LUMO) for the (a) Z0, (b) Z1, and (c) Z2-type active stite, with Zn substituted in the T7 position the “side” pore defined. All orbitals are illustrated for a surface isovalue of 0.02 a.u. Color code: Zn, green; Si, grey; O, red; Li, purple; H, white.



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Figure 5. Reaction profiles of the first two elementary steps of the reaction of ethylene with (a) MMFC at Z0type active sites, (b) MMFC at Z2-type active sites, (c) DMFDC at Z0-type active sites, and (d) DMFDC at Z2type active sites.


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Scheme 3. Resonance structure of protonated cycloadduct of ethylene with DMFDC.



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Figure 6. Transition state geometries of the C-O cleavage for (a) MMFC on T5, “top”, Z0; (b) MMFC on T7, “top”, Z0; (c) MMFC on T7, “side”, Z0; (d) DMFDC on T7, “side”, Z0; (e) MMFC on T5, “top”, Z2; (f) MMFC on T7, “top”, Z2; (g) MMFC on T7, “side”, Z2; and (h) DMFDC on T7, “side”, Z2 active site models. Atoms beyond the Si-atoms immediately neighboring the Zn atom have been hidden for clarity. Color code: Zn, orange; Si, light grey; C, grey (small balls); O, red; Li, purple; H, white.


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Table 1. Bader analysis of bond critical points of the active site. Atom numbering scheme corresponds to Figure 2. ૉ (a.u.) // ૚ - ൗ૝ સ ૛ ሺૉሻ (a.u.)

Z0

Z1

Z2

Zn-O4

Zn-O5

Zn-O6

Zn-O7

O5-H2/Li2

O6-H3/Li3

T5 "Top"

0.11 // -0.14

0.06 // -0.07

0.07 // -0.08

0.11 // -0.13

0.34 // 0.51

0.35 // 0.51

T7 "Top"

0.11 // -0.13

0.07 // -0.08

0.07 // -0.08

0.11 // -0.14

0.33 // 0.51

0.35 // 0.53

T7 "Side"

0.11 // -0.13

0.06 // -0.07

0.07 // -0.08

0.12 // -0.15

0.34 // 0.51

0.35 // 0.52

T5 "Top"

0.09 // -0.11

0.09 // -0.10

0.07 // -0.08

0.11 // -0.12

0.34 // 0.52

0.03 // -0.06

T7 "Top"

0.10 // -0.11

0.09 // -0.11

0.07 // -0.08

0.11 // -0.13

0.35 // 0.53

0.03 // -0.06

T7 "Side"

0.09 // -0.09

0.09 // -0.11

0.07 // -0.08

0.12 // -0.14

0.35 // 0.52

0.03 // -0.05

T5 "Top"

0.09 // -0.11

0.06 // -0.07

0.09 // -0.10

0.11 // -0.13

0.03 // -0.06

0.04 // -0.09

T7 "Top"

0.08 // -0.08

0.09 // -0.11

0.09 // -0.10

0.11 // -0.13

0.04 // -0.07

0.04 // -0.09

T7 "Side"

0.07 // -0.08

0.09 // -0.11

0.09 // -0.10

0.11 // -0.14

0.04 // -0.08

0.04 // -0.09



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Table 2. Adsorption enthalpies for deuterated acetonitrile and pyridine (kcal/mol).

Z0

Z1

Z2

Pyridine

CD3CN

T5 "Top"

-26.4

-16.9

T7 "Top"

-24.3

-14.1

T7 "Side"

-35.3

-24.7

T5 "Top"

-24.7

-12.0

T7 "Top"

-27.7

-12.7

T7 "Side"

-10.3

-6.1

T5 "Top"

-26.9

-21.5

T7 "Top"

-27.4

-20.2

T7 "Side"

-25.3

-20.4


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REFERENCES 1. Dauenhauer, P. J.; Huber, G. W., Biomass at the shale gas crossroads. Green Chem. 2014, 16, 382-383. 2. Williams, C. L.; Chang, C.-C.; Do, P.; Nikbin, N.; Caratzoulas, S.; Vlachos, D. G.; Lobo, R. F.; Fan, W.; Dauenhauer, P. J., Cycloaddition of biomass-derived furans for catalytic production of renewable p-xylene. ACS Catal. 2012, 2, 935-939. 3. Do, P. T. M.; McAtee, J. R.; Watson, D. A.; Lobo, R. F., Elucidation of Diels–Alder reaction network of 2,5-dimethylfuran and ethylene on HY zeolite catalyst. ACS Catal. 2013, 3, 41-46. 4. Chang, C.-C.; Green, S. K.; Williams, C. L.; Dauenhauer, P. J.; Fan, W., Ultra-selective cycloaddition of dimethylfuran for renewable p-xylene with H-BEA. Green Chem. 2014, 16, 585-588. 5. Patet, R. E.; Nikbin, N.; Williams, C. L.; Green, S. K.; Chang, C.-C.; Fan, W.; Caratzoulas, S.; Dauenhauer, P. J.; Vlachos, D. G., Kinetic regime change in the tandem dehydrative aromatization of furan Diels-Alder products. ACS Catal. 2015, 5, 2367-2375. 6. Green, S. K.; Patet, R. E.; Nikbin, N.; Williams, C. L.; Chang, C.-C.; Yu, J.; Gorte, R. J.; Caratzoulas, S.; Fan, W.; Vlachos, D. G., et al., Diels-Alder cycloaddition of 2-methylfuran and ethylene for renewable toluene. Appl. Catal., B 2016, 180, 487-496. 7. Chang, C.-C.; Je Cho, H.; Yu, J.; Gorte, R. J.; Gulbinski, J.; Dauenhauer, P.; Fan, W., Lewis acid zeolites for tandem Diels-Alder cycloaddition and dehydration of biomass-derived dimethylfuran and ethylene to renewable p-xylene. Green Chemistry 2016, 18, 1368-1376. 8. Pacheco, J. J.; Davis, M. E., Synthesis of terephthalic acid via Diels-Alder reactions with ethylene and oxidized variants of 5-hydroxymethylfurfural. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 8363-8367. 9. Pacheco, J. J.; Labinger, J. A.; Sessions, A. L.; Davis, M. E., Route to renewable PET: Reaction pathways and energetics of Diels-Alder and dehydrative aromatization reactions between ethylene and biomass-derived furans catalyzed by Lewis acid molecular sieves. ACS Catal. 2015, 5, 5904-5913. 10. Tomás, R. A. F.; Bordado, J. C. M.; Gomes, J. F. P., p-Xylene oxidation to terephthalic acid: A literature review oriented toward process optimization and development. Chem. Rev. 2013, 113, 7421-7469. 11. Nikbin, N.; Do, P. T.; Caratzoulas, S.; Lobo, R. F.; Dauenhauer, P. J.; Vlachos, D. G., A DFT study of the acid-catalyzed conversion of 2,5-dimethylfuran and ethylene to p-xylene. J. Catal. 2013, 297, 35-43. 12. Nikbin, N.; Feng, S.; Caratzoulas, S.; Vlachos, D. G., p-Xylene formation by dehydrative aromatization of a Diels-Alder product in Lewis and Brønsted acidic zeolites. J. Phys. Chem. C 2014, 118, 24415-24424. 13. Pindur, U.; Lutz, G.; Otto, C., Acceleration and selectivity enhancement of Diels-Alder reactions by special and catalytic methods. Chem. Rev. 1993, 93, 741-761. 14. Woodward, R. B.; Hoffmann, R., The conservation of orbital symmetry. Angew. Chem. Int. Ed. 1969, 8, 781-932. 15. Fukui, K., Recognition of stereochemical paths by orbital interaction. Acc. Chem. Res. 1971, 4, 57-64. 16. Fukui, K., Role of frontier orbitals in chemical reactions. Science 1982, 218, 747-754.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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17. Gilchrist, T. L.; Storr, R. C., Organic reactions and orbital symmetry, Second ed.; Cambridge University Press: New York, 1979. 18. Houk, K. N., Generalized frontier orbitals of alkenes and dienes. Regioselectivity in Diels-Alder reactions. J. Am. Chem. Soc. 1973, 95, 4092-4094. 19. Houk, K. N.; Strozier, R. W., Lewis acid catalysis of Diels-Alder reactions. J. Am. Chem. Soc. 1973, 95, 4094-4096. 20. Houk, K. N., Frontier molecular orbital theory of cycloaddition reactions. Acc. Chem. Res. 1975, 8, 361-369. 21. Geerlings, P.; De Proft, F.; Langenaeker, W., Conceptual density functional theory. Chem. Rev. 2003, 103, 1793-1874. 22. Domingo, L. R.; Arnó, M.; Andrés, J., Influence of reactant polarity on the course of the inverse-electron-demand Diels-Alder reaction. A DFT study of regio- and stereoselectivity, presence of Lewis acid catalyst, and inclusion of solvent effects in the reaction between nitroethene and substituted ethenes. J. Org. Chem. 1999, 64, 5867-5875. 23. Domingo, L. R., A density functional theory study for the Diels-Alder reaction between n-acyl-1-aza-1,3-butadienes and vinylamines. Lewis acid catalyst and solvent effects. Tetrahedron 2002, 58, 3765-3774. 24. Domingo, L. R.; Asensio, A.; Arroyo, P., Density functional theory study of the Lewis acid-catalyzed Diels-Alder reaction of nitroalkenes with vinyl ethers using aluminum derivatives. J. Phys. Org. Chem. 2002, 15, 660-666. 25. Domingo, L. R.; Aurell, M. J.; Perez, P.; Contreras, R., Quantitative characterization of the global electrophilicity power of common diene/dienophile pairs in Diels-Alder reactions. Tetrahedron 2002, 58, 4417-4423. 26. Domingo, L. R.; Aurell, M. J.; Perez, P.; Contreras, R., Quantitative characterization of the local electrophilicity of organic molecules. Understanding the regioselectivity on Diels-Alder reactions. J. Phys. Chem. A 2002, 106, 6871-6875. 27. Domingo, L. R., Why do electron-deficient dienes react rapidly in Diels-Alder reactions with electron-deficient ethylenes? A density functional theory analysis. Eur. J. Org. Chem. 2004, 4788-4793. 28. Domingo, L. R.; Perez, P.; Contreras, R., Reactivity of the carbon-carbon double bond towards nucleophilic additions. A DFT analysis. Tetrahedron 2004, 60, 6585-6591. 29. Domingo, L. R.; Sáez, J. A., Understanding the mechanism of polar diels–alder reactions. Org. Biomol. Chem. 2009, 7, 3576-3583. 30. Patet, R.; Fan, W.; Vlachos, D. G.; Caratzoulas, S., Tandem Diels-Alder of dimethylfuran and ethylene and dehydration to para-xylene catalyzed by zeotypic Lewis acids. ChemCatChem 2017. 31. Chattaraj, P. K.; Sarkar, U.; Roy, D. R., Electrophilicity index. Chem. Rev. 2006, 106, 2065-2091. 32. Eschenbrenner-Lux, V.; Kuchler, P. Z., S.; Kumar, K.; Waldmann, H., An enantioselective inverse-electron-demand imino Diels-Alder reaction. Angew. Chem. Int. Ed. 2014, 53, 2134−2137. 33. Orazov, M.; Davis, M. E., Catalysis by framework zinc in silica-based molecular sieves. Chem. Sci. 2016, 7, 2264-2274. 34. Pelmenschikov, A. G.; van Santen, R. A.; Jänchen, J.; Meijer, E., CD3CN as a probe of Lewis and Bronsted acidity of zeolites. J. Phys. Chem. 1993, 97, 11071-11074.


Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35. Lavalley, J. C., Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules. Catal. Today 1996, 27, 377-401. 36. Bermejo-Deval, R.; Orazov, M.; Gounder, R.; Hwang, S.-J.; Davis, M. E., Active sites in Sn-Beta for glucose isomerization to fructose and epimerization to mannose. ACS Catal. 2014, 4, 2288-2297. 37. Ren, L.; Guo, Q.; Kumar, P.; Orazov, M.; Xu, D.; Alhassan, S. M.; Mkhoyan, K. A.; Davis, M. E.; Tsapatsis, M., Self-pillared, single-unit-cell Sn-MFI zeolite nanosheets and their use for glucose and lactose isomerization. Angew. Chem. Int. Ed. 2015, 54, 10848-10851. 38. Baerlocher, C.; McCusker, L. Database of zeolite structures. http://www.izastructure.org/databases (accessed March 9, 2017). 39. Migues, A. N.; Muskat, A.; Auerbach, S. M.; Sherman, W.; Vaitheeswaran, S., On the rational design of zeolite clusters. ACS Catal. 2015, 5, 2859-2865. 40. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Gaussian, Inc.: Wallingford, CT, 2009. 41. Grimme, S., Supramolecular binding thermodynamics by dispersion-corrected density functional theory. Chem. Eur. J. 2012, 18, 9955-9964. 42. Li, Y.-P.; Gomes, J.; Mallikarjun Sharada, S.; Bell, A. T.; Head-Gordon, M., Improved force-field parameters for QM/MM simulations of the energies of adsorption for molecules in zeolites and a free rotor correction to the rigid rotor harmonic oscillator model for adsorption enthalpies. J. Phys. Chem. C 2015, 119, 1840-1850. 43. De Moor, B. A.; Reyniers, M.-F.; Gobin, O. C.; Lercher, J. A.; Marin, G. B., Adsorption of C2−C8 n-alkanes in zeolites. J. Phys. Chem. C 2011, 115, 1204-1219. 44. Biegler-König, F.; Schönbohm, J., Update of the AIM2000-program for atoms in molecules. J. Comput. Chem. 2002, 23, 1489-1494. 45. Glendening, E. D.; Landis, C. R.; Weinhold, F., NBO 6.0: Natural bond orbital analysis program. J. Comput. Chem. 2013, 34, 1429-1437.



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TOC Graphic


Tandem Aromatization of Oxygenated Furans by Framework Zinc In

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