Charge-compensating Cations Research Articles

Searching for Microporous, Strongly Basic Catalysts: Experimental and Calculated 29Si NMR Spectra of Heavily Nitrogen-Doped Y Zeolites

Nitrogen substituted zeolites with high crystallinity and microporosity are obtained by nitrogen substitution for oxygen in zeolite Y. The substitution reaction is performed under ammonia flow by varying the temperature and reaction time. We examine the effect of aluminum content and charge-compensating cation (H(+)/Na(+)/NH(4)(+)) on the degree of nitrogen substitution and on the preference for substitution of Si-O-Al vs Si-O-Si linkages in the FAU zeolite structure. Silicon-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) and (1)H/(29)Si cross-polarization MAS NMR spectroscopy have been used to probe the different local environments of the nitrogen-substituted zeolites. Experimental data are compared to simulated NMR spectra obtained by constructing a compendium (>100) of zeolite clusters with and without nitrogen, and by performing quantum calculations of chemical shifts for the NMR-active nuclei in each cluster. The simulated NMR spectra, which assume peak intensities predicted by statistical analysis, agree remarkably well with the experimental data. The results show that high levels of nitrogen substitution can be achieved while maintaining porosity, particularly for NaY and low-aluminum HY materials, without significant loss in crystallinity. Experiments performed at lower temperatures (750-800 degrees C) show a preference for substitution at Si-OH-Al sites. No preference is seen for reactions performed at higher temperatures and longer reaction times (e.g., 850 degrees C and 48 h).

Rb2Sb4O11

The title compound, dirubidium tetra­anti­monate(V), Rb2Sb4O11, has been synthesized by flux reaction. It is isotypic with known A 2Sb4O11 (A = K, Cs) structures and consists of an (Sb4O11)2− skeleton and two Rb atoms as charge-compensating cations. Distorted SbO6 octa­hedra share edges and corners, resulting in a layered assembly. Alternate stacking of the layers along the c axis leads to the formation of tunnels. The Rb+ ions, surrounded by nine and ten O atoms, respectively, are located in these tunnels. Some atoms in the structure are on special positions of m symmetry (two Sb atoms, both Rb atoms and four O atoms) and 2 symmetry (one O atom).

Correlating Acid Properties and Catalytic Function: A First-Principles Analysis of Alcohol Dehydration Pathways on Polyoxometalates

Density functional theory calculations and reactivity data were used to examine the mechanism of alcohol dehydration on Keggin-type polyoxometalate (POM) catalysts and the influence of the POM composition and the degree of substitution of the alcohol on kinetically relevant elimination steps. Dehydration was found to proceed through E1 pathways in which the alcohol C—O bond is cleaved heterolytically via a carbenium-ion transition state. Dehydration rates were found to depend on the elimination rate constant and the equilibrium constant for the formation of unreactive alcohol dimers. E2-type elimination transition states, involving concerted C—H and C—O bond cleavage, were not found. The extent of substitution at the α-carbon on the alcohol was found to lead to marked effects on elimination barriers, because substitution increases the proton affinity of the alcohol and the stability of the carbenium-ion transition state. Changes in the central and addenda atoms of the POM cluster and the presence of n-donors, a support, vicinal POM clusters, or charge-compensating cations were found to lead to changes in the deprotonation energy (DPE) of the POM cluster, activation barriers to dehydration, and the stability of the unreactive dimer. These effects are all captured in a general linear relation between activation barriers and deprotonation energy, a rigorous measure of acid strength. The explicit dependence of the E1 activation barrier on the acid deprotonation energy is much weaker than that on reactant proton affinity. This results from the more effective compensation between the acid deprotonation energy and the interaction energy between the cationic hydrocarbon fragment and the anionic POM cluster at the transition state. The direct interactions between the POM protons and the support, other POM clusters, n-donors, base probe molecules, and charge-compensating cations increased the negative charge of the oxide shell of the W12O40 conjugate base, which increased the DPE and decreased the POM acid strength. This decrease in acidity is not fully compensated by the concomitant increase in the interaction energy, leading to elimination barriers that generally increase with increasing DPE.

Color and Volume Change in PPy(DBS)

The optical−mechanical−electrochemical coupling in polypyrrole doped with dodecylbenzenesulfonate, PPy(DBS), is studied during electrochemical switching. Cations enter this material during reduction, leading to an increase in volume. A special experimental geometry is used in which the top surface of the PPy film is covered by a transparent ion barrier, which constrains the charge-compensating cations to enter the film only at the edges. The cation concentration determines the volume of the polymer and is also directly related to the oxidation level of the polymer, which in turn determines its color. During electrochemical reduction, the edges of the film lighten, and this light color travels as a front to the center of the film. At the same time, the height of the film increases when and where the color lightens, and the increase in height is directly proportional to the change in color intensity. The height and the intensity attain their maximum values as the front passes and do not increase further the...

Synthesis and adsorption investigations of zeolites MCM-22 and MCM-49 modified by alkali metal cations

Ion exchange was made on MCM-22 and MCM-49 zeolites with different Si/Al molar ratios, with Li+, Na+, K+, and Cs+ ions and the study of the influence of alkali metal cations on CO2 adsorption properties was performed. The degree of ion-exchange decreased for larger cations (Cs+) apparently due to steric hindrances. The exchange with different cations led to a decrease in the surface area and the micropore volume. Our study shows that the adsorption capacity of the tested zeolites depends significantly on the nature and the concentration of the charge-compensating cations. The highest CO2 adsorption capacity was obtained on the MWW zeolites with the lowest Si/Al molar ratio and the Li+ or K+ cations.

Ionothermal synthesis, crystal structure and solid-state NMR spectroscopy of a new organically templated gallium oxalatophosphate: (H2TMPD)0.5[Ga3(C2O4)0.5(PO4)3] (TMPD = N,N,N′,N′-tetramethyl-1,3-propanediamine)

A new organically templated gallium oxalatophosphate, (C7H20N2)0.5[Ga3(C2O4)0.5(PO4)3], has been synthesized by using a low-melting-point eutectic mixture of choline chloride and oxalic acid as a solvent and characterized by single-crystal X-ray diffraction, thermogravimetric analysis and solid-state NMR spectroscopy. It is the first example of ionothermal synthesis of organically templated metal oxalatophosphate. The structure contains double 6-ring units of the composition Ga6(PO4)6 which are connected by oxalate ligands and P-O-Ga bonds to form a 3-D framework. The charge-compensating organic ammonium cations which are disordered over two positions are located at the intersections of two types of 8-ring channels. 1H MAS and 13C CPMAS NMR studies confirm the presence of N,N,N',N'-tetramethyl-1,3-propanediammonium cation. The 71Ga and 31P MAS NMR spectra are also consistent with the crystal structure analysis results.

Synthesis and Characterization of a Novel Germanate Material Containing 16-Ring Channels and Templated by a Simple Primary Amine

The new framework germanate (H3NC3H7)5(H3O)[Ge18O36(OH)6]·3H2O (tetragonal, I4cm, a = 24.9728(6) Å, c = 9.8957(2) Å, Z = 4) has been synthesized by solvothermal methods and the crystal structure determined using single-crystal X-ray diffraction data. The framework of this novel material contains a three-dimensional intersecting channel system, the two largest channels of which are encompassed by large circular 16-rings and elliptical 14-rings, respectively. The material is synthesized in the presence of the relatively small structure-directing agent isopropylamine. (H3NC3H7)5(H3O)[Ge18O36(OH)6]·3H2O is only the second material to be constructed from the secondary building unit [Ge9(O,OH)25] cluster, and the frameworks of these two materials are shown to differ only by the number of chains of [Ge9(O,OH)25] clusters that are used to construct the respective frameworks. Thermal stability studies indicate that the framework of the material is stable with respect to loss of the extraframework water molecules but is unstable with respect to the removal of the extraframework charge-compensating cations.

Band Gap Variation in Prussian Blue via Cation-Induced Structural Distortion

The charge-transfer band gap of the iron cyanide framework material Prussian Blue and its dependence on the type and location of the charge-compensating interstitial cations (K(+), Rb(+), Cs(+)) are investigated via periodic density functional (DF) calculations. The calculated variation in the band gap magnitude with respect to cation type confirms recent experimental results on cation-induced spectral shifts. The role of both the cation interaction with the framework and the cation-induced lattice expansion are examined with respect to their influence on the band gap. The gap magnitude is related to the cation type but is found to be more strongly affected by cation-induced lattice distortion as the cation passes through the material. Our results support the possibility of engineering the electronic structure of Prussian Blue type materials through guest-induced host-framework distortion.

Commensurate (C6H14N2)2[Mo8O26]·4H2O and incommensurate (C6H14N2)2[Mo8O26]·4.66H2O: a structural versatility linked to solvent content

The syntheses and structure determinations by means of single-crystal X-ray diffraction of commensurate (C(6)H(14)N(2))(2)[Mo(8)O(26)] * 4H(2)O and incommensurate (C(6)H(14)N(2))(2)[Mo(8)O(26)] * 4.66H(2)O, two new organic-inorganic hybrid compounds based on polyoxomolybdates and differing in their solvent content, are reported. Given the important disorder observed in the latter compound, only a combination of non-harmonic waves, crenel functions and TLS tensors offered a good modelling of the structure. (C(6)H(14)N(2))(2)[Mo(8)O(26)] * nH(2)O results from the self-assembly of [Mo(8)O(26)](4-) anionic chains, C(6)H(14)N(2)(2+) (H(2)DABCO(2+)) cations and water molecules. The [Mo(8)O(26)](4-) chain is built from gamma-[Mo(8)O(28)](8-) octamolybdate clusters, connected to each other through corner sharing. In both compounds, the [Mo(8)O(26)](4-) chains are separated, in a similar way, by the H(2)DABCO(2+) subunits, acting as charge-compensating cations, and by the water molecules. The orientation of the H(2)DABCO(2+) cations is shown to be different from what has been observed previously in monoclinic (H(2)DABCO)(2)[Mo(8)O(26)] * 4H(2)O, and therefore to give a different network of hydrogen bonds.

Influence of the exchanged cation in Pd/BEA and Pd/FAU zeolites for catalytic oxidation of VOCs

0.5wt% palladium supported on exchanged BEA and FAU zeolites were prepared, characterized and tested in the total oxidation of volatile organic compounds (VOCs). The BEA and FAU zeolites were exchanged with different cations to study the influence of alkali metal cations (Na+, Cs+) and H+ in Pd-based catalysts on propene and toluene total oxidation. The exchange with different cations (Na+, Cs+) and H+ led to a decrease of the surface area and the micropore volume. All Pd/BEA and Pd/FAU zeolites were found to be powerful catalysts for the total oxidation of VOCs. They were active at low temperature and totally selective for CO2 and H2O. However, their activity depends significantly on the type of zeolite and on the nature of the charge-compensating cation. The activity order for propene and toluene oxidation on FAU catalysts, Pd/CsFAU>Pd/NaFAU>Pd/HFAU, is the reverse of the activity order on BEA catalysts: Pd/HBEA>Pd/NaBEA>Pd/CsBEA. The catalytic activities can be rationalized in terms of the influence of the electronegativity of the charge-compensating cation on the Pd particles, the Pd dispersion, the PdO reducibility and the adsorption energies for VOCs.

An In Situ Al K-Edge XAS Investigation of the Local Environment of H+- and Cu+-Exchanged USY and ZSM-5 Zeolites

Aluminum coordination in the framework of USY and ZSM-5 zeolites containing charge-compensating cations (NH4+, H+, or Cu+) was investigated by Al K-edge EXAFS and XANES. This work was performed using a newly developed in-situ cell designed especially for acquiring soft X-ray absorption data. Both tetrahedrally and octahedrally coordinated Al were observed for hydrated H-USY and H-ZSM-5, in good agreement with 27Al NMR analyses. Upon dehydration, water desorbed from the zeolite, and octahedrally coordinated Al was converted progressively to tetrahedrally coordinated Al. These observations confirmed the hypothesis that the interaction of water with Brønsted acid protons can lead to octahedral coordination of Al without loss of Al from the zeolite lattice. When H+ is replaced with NH4+ or Cu+, charge compensating species that absorb less water, less octahedrally coordinated Al was observed. Analysis of Al K-edge EXAFS data indicates that the Al-O bond distance for tetrahedrally coordinated Al in dehydrated USY and ZSM-5 is 1.67 angstroms. Simulation of k3chi(k) for Cu+ exchanged ZSM-5 leads to an estimated distance between Cu+ and framework Al atoms of 2.79 angstroms.

Synthesis and structure characterization of two new one-dimensional reduced molybdenum (V) phosphates with covalently bonded transition metal complexes

Two new one-dimensional reduced molybdenum (V) phosphates, [Zn(phen) 2][Zn(phen) 2(H 2O)][Zn(HPO 4) 8[(MoO 2) 6(OH) 3] 2]·H 2O ( 1) and [Co(phen) 3][Co(phen) 2(H 2O)(H 2PO 4)][KNa(H 2O) 2][Co(H 2O) 2][Co(phen) 2][Co(H 2PO 4) 2(HPO 4) 3(PO 4) 3[(MoO 2) 6(OH) 3] 2]·5H 2O ( 2) (phen=1,10-phenanthroline), have been synthesized and characterized by elemental analyses, IR, TG, and single crystal X-ray diffraction. Compound 1 is built up of Zn[P 4Mo 6] 2 cluster as the structural motif covalently linked by [Zn(phen) 2] complex subunits to yield unusual one-dimensional chains, which are further in close contact forming two-dimensional supramolecular layers via π–π stacking interactions. In compound 2, Co[P 4Mo 6] 2 clusters are connected by [Co(phen) 2], [Co(H 2O) 2] and [KNa(H 2O) 2] fragments through covalent bonds, forming an unusual one-dimensional chain. Interestingly, both [Co(phen) 3] complex and [Co(phen) 2(H 2O)(H 2PO 4)] cobalt phosphate are presented as charge-compensating cations and contact each other via π–π stacking interactions to form a supramolecular chain, then the supramolecular chains link two neighboring chains that contain [P 4Mo 6] via hydrogen-bonding interactions to form a two-dimensional supramolecular layer. Furthermore, the fluorescent activity of compound 1 is reported.

Study on Thermal Desorption of Aromatic Guest Molecules Adsorbed in Zeolites

Thermal desorption for an aromatic guest molecule of phenanthrene adsorbed in Y and X zeolites exchanged with cations such as Na+, Tl+ and K+ has been studied successfully by means of TG-DTA and temperature programmed desorption (TPD)-MS. The thermal desorption of phenanthrene from the zeolites took place at temperatures between 300°C and 800°C. The temperature of thermal desorption depended critically on the cations in the order of the temperature of Na+<Tl+<K+. On the contrary, the activation energy for the desorption of phenanthrene estimated in terms of the Ozawa method, was almost the same for the three cation-exchanged Y zeolites with the approximate values of 120 kJ mol-1. On the other hand, the activation energy for the desorption of phenanthrene from the cation-exchanged X zeolites was the larger than that obtained from the Y zeolites, and was estimated as approximately 140 kJ mol-1. It is concluded that the activation energy for the desorption of phenanthrene from the zeolites does not directly correlate with the charge-compensating cation in the zeolites and depends strongly on the framework structures of the zeolites.

Understanding the Crystallization of Nanosized Cobalt Aluminate Spinel from Ion-Exchanged Zeolites Using Combined in Situ QEXAFS/XRD

By using combined QEXAFS/XRD (supported by measurements obtained using DSC, XRD, and IR spectroscopy), we have studied, in situ, the evolution of nanosized cobalt aluminate spinel (CoAl2O4) particles from cobalt-ion-exchanged zeolite precursors. All of the cobalt-ion-exchanged zeolites were observed to first undergo dehydration, followed by amorphization before the CoAl2O4 phase was observed to form. For two of the zeolites studied, this phase was observed to form at temperatures as low as 450 °C. However, with the exception of zeolite Y, a minor amount of a secondary phase (Co2SiO4) was also observed to form from amorphized zeolites when the Si/Al ratio was >1.85 and the calcination temperature exceeded 850 °C. This unusual ability of zeolite Y to yield phase-pure CoAl2O4, even though it has a Si/Al ratio of ∼2.52 can be attributed to the tendency for the cobalt ion to predominantly locate within the double six rings (SI site) of the zeolite Y structure in close proximity to the Al3+ ions (as charge-compensating cations), thereby creating nucleation sites for the spinel material. In contrast, other zeolites that lose their framework structure allow random distribution of cobalt ions, away from Al3+, allowing the formation of other phases such as Co2SiO4.

Theoretical ONIOM2 Study on Pyridine Adsorption in the Channels and Intersection of ZSM-5

The structures of the acid sites in the channels and intersections of H-, Li-, and Na-ZSM-5 (ZSM = zeolite socony mobil) and their interactions with pyridine molecule have been computed by using three corresponding models containing 22 tetrahedral sites. The calculated adsorption energies of pyridine in the intersection regions of H-, Li-, and Na-ZSM-5 are 197.0, 172.5, and 122.3 kJ/mol, respectively, in good agreement with the respective experimental values of 200 +/- 5, 155-195, and 120 kJ/mol, while those in the straight and sinusoidal channels are much smaller (157.9 and 127.6, 152.2 and 149.4, and 150.4 and 109.9 kJ/mol, respectively). These indicate that the most probable adsorption site for pyridine in ZSM-5 is the acidic site located in the intersection region. The structural parameters of the adsorption complexes show that the acidic proton in the three models of H-ZSM-5 has been transferred to the nitrogen of pyridine, while in alkali cation-exchanged ZSM-5, the coordination of the alkali cation to the nitrogen atom of pyridine dominates the overall interaction. In addition, the adsorption complexes were further stabilized by the long-range electrostatic interaction between the positively charged pyridine hydrogen atoms and the negatively charged lattice oxygen atoms of the zeolite framework. In the intersection regions of H-, Li-, and Na-ZSM-5, the coordination energy of the charge-compensating cation to the pyridine nitrogen amounts to 58, 60, and 68% of the total adsorption energy, respectively, while another 42, 40, and 32%, respectively, is due to long-range electrostatic interactions. This indicates that the zeolite lattice framework surrounding the adsorption site has important contributions to the adsorption energy of the pyridine molecule.

Ion-exchanged ETS-10 catalysts for the cycloaddition of carbon dioxide to propylene oxide

Alkali- and alkaline-earth-exchanged ETS-10 molecular sieve catalysts were investigated for the cycloaddition of carbon dioxide to propylene oxide to produce propylene carbonate. Adsorption microcalorimetry of adsorbed carbon dioxide was used to correlate reactivity to the strength and number of CO 2 adsorption sites. While carbon dioxide uptake for the ETS-10 and K-ETS-10 catalysts was the greatest for the catalysts investigated, these two catalysts were not as productive as Cs-ETS-10. The adducts formed between carbon dioxide and the charge-compensating cations appear to be prevalent in these materials and do not provide an appropriate CO 2 adsorption site for the reaction to occur. Yield of propylene carbonate was found to have an inverse relationship with the Sanderson electronegativity associated with the charge-compensating cation, indicating that surface basicity associated with the partial charge of the framework oxygen plays an important role in the catalyst effectiveness for the reaction. The alkali-exchanged samples were significantly more active than the alkaline-earth-exchanged catalysts on a per gram basis due to their higher strength CO 2 adsorption sites as indicated by adsorption microcalorimetry. The Cs-exchanged ETS-10 molecular sieve gave conversions similar to those observed for basic NaX zeolites containing occluded alkali oxide species.

Synthesis and characterization of the first organically templated metal oxalatophosphonate: (C3H12N2)0.5[Ga3(C2O4)(CH3PO3)4].0.5H2O.

The first organically templated gallium oxalatophosphonate has been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. The structure consists of GaO(6) octahedra and GaO(4) tetrahedra connected by methylphosphonate and bis-bidentate oxalate anions to form anionic sheets in the ab plane with the charge-compensating propylenediammonium cations and water molecules between the sheets. The methyl groups of the phosphonate ligands extend from both sides of the layers into the interlamellar region. Crystal data: monoclinic, space group P2(1)/n (No. 14), a = 8.8514(4) A, b = 16.3030(7) A, c = 15.0816(7) A, beta = 97.539(1) degrees, and Z = 4.

Probing the dynamics of instability in zeolitic materials

Zeolites collapse under modest pressure or temperature, their microporous structurestransforming into glasses of conventional density. Using in situ synchrotron radiationdiffraction methods we show how pressure and temperature-induced amorphization areequivalent processes and that these are mirrored by changes in the local structure of chargecompensating cations. Evidence for a low density amorphous phase and a high densityamorphous phase present during zeolite collapse emerges from small angle scatteringexperiments. Combining powder diffraction with increasing temperature or pressure,we find that the thermobaric characteristics for zeolite collapse have negatived T/d P slopes, consistent with increasing density during amorphization. However, this is not confinedto a single melting curve but, instead, the regime extends over a significant region ofT–P space. Moreover, zeolite amorphization involves depressurization and cavitation effectswhich can be used to set empirical boundaries for the stability of the low densityamorphous phase. Within the region of zeolite instability the pressure or temperature ofamorphization is found to be governed by the rate at which the stress is introduced—themore rapid this is, the higher the pressure or temperature the zeolite structure survives to.The temperature dependence of the rate of collapse is Arrhenian, suggesting that the initiallow density amorphous phase has the characteristics of a superstrong liquid incontrast to the fragility of a conventionally melt quenched glass. Possibilities forcreating ‘perfect glasses’ from the collapse of microporous crystals are discussed.

Electronic and vibrational properties of fluorenone in the channels of zeolite L.

Fluorenone (C13H8O) was inserted into the channels of zeolite L by using gas-phase adsorption. The size, structure, and stability of fluorenone are well suited for studying host-guest interactions. The Fourier transform IR, Raman, luminescence, and excitation spectra, in addition to thermal analysis data, of fluorenone in solution and fluorenone/zeolite L are reported. Normal coordinate analysis of fluorenone was performed, based on which IR and Raman bands were assigned, and an experimental force field was determined. The vibrational spectra can be used for nondestructive quantitative analysis by comparing a characteristic dye band with a zeolite band that has been chosen as the internal standard. Molecular orbital calculations were performed to gain a better understanding of the electronic structure of the system and to support the interpretation of the electronic absorption and luminescence spectra. Fluorenone shows unusual luminescence behavior in that it emits from two states. The relative intensity of these two bands depends strongly on the environment and changes unexpectedly in response to temperature. In fluorenone/zeolite L, the intensity of the 300 nm band (lifetime 9 micros) increases with decreasing temperature, while the opposite is true for the 400 nm band (lifetime 115 micros). A model of the host-guest interaction is derived from the experimental results and calculations: the dye molecule sits close to the channel walls with the carbonyl group pointing to an Al3+ site of the zeolite framework. A secondary interaction was observed between the fluorenone's aromatic ring and the zeolite's charge-compensating cations.

Electronic Structure and Stability of Double Six-Membered Rings of Oxygen-Bridged Silicon and Aluminum Atoms Related to Cation Site Occupancy in FAU Zeolites: a DFT Study

The electronic structure and relative stability of clusters with general composition (Mn+)x/nH12Si12-xAlxO18 with x = 0, 2, 4, M = H+, Li+, Na+, K+, Ca2+ forming double-six-membered rings (D6R), are studied by the B3LYP method. In Ca2H12Si8Al4O18 clusters, maximum separation of the Al atoms is favored, which implies a minimum number of Al−O−Si−O−Al linkages. A large number of isomers with Si,Al distributions strongly related to the type and location of the charge-compensating cations were determined for the ratio Si/Al = 5. All cations favor maximum separation of Al atoms within the T6O6 rings. Concurrent occupation of extraframework cation sites in the center of the D6R and above the T6O6 window is energetically unfavorable for Li+ and Na+ cations, whereas this is achieved with K+, in agreement with experimental data. The Si,Al ordering with one Al atom per T6O6 ring and at the largest distance within the D6R is the most stable one with Li+ and K+ as extraframework cations, whereas Na+ and Ca2+ favor the configuration with two Al atoms in one T6O6 ring. The small and medium size cations Li+, Na+, and Ca2+ approach the regions of high electron density in the D6R. The HOMOs of both the siliceous fragments and the aluminum-containing fragments with the extraframework cations located near the four-membered ring walls consist mainly of O 2p nonbonding AOs. When the extraframework cations take up positions at the D6R center, or near the T6O6 rings, the HOMOs O 2p nonbonding character is reduced: Al 3s and Al 3p AOs are admixed to the O 2p AOs. The maximum electron density is concentrated at the Al atoms and the Al-bonded oxygen atoms. When the Al atoms are in one T6O6 ring of a D6R, the oxygen atoms of this ring bear a higher electron density. Lewis basic sites appear in proximity to the Brønsted acid sites in proton balanced clusters.