Acidic OH Groups Research Articles

A comprehensive study on butanolysis of furfuryl alcohol to butyl levulinate using tungstated zirconia and sulfonated carbon catalysts

This work presents the formation of butyl levulinate, a potential fuel additive, and an excellent renewable chemical obtained by the butanolysis of furfuryl alcohol (FAL) over a solid acid catalyst. The butanolysis of furfuryl alcohol reaction is a strong function of acidity for which tungstated zirconia (WO3-ZrO2), a robust solid acid catalyst, and a sulfonated carbon catalyst were employed to produce high yields of butyl levulinate targeting a lower initial molar ratio of butanol to FAL. A maximum of 28 mol% yield of butyl levulinate was obtained with tungstated zirconia catalyst. Easily prepared sulfonated carbon catalyst at high reaction temperatures facilitated the complete conversion of reaction intermediate, 2-butoxymethylfuran (2-BMF) through which butyl levulinate was formed, and as high as 80 mol% of butyl levulinate yield was produced at an initial mole ratio of 8.5:1 of butanol to FAL. The better results of sulfonated carbon catalyst could be attributed to the presence of -SO3H, carboxylic acid, and phenolic OH groups on the carbon surface.

Defect-Engineered Metal–Organic Frameworks: A Thorough Characterization of Active Sites Using CO as a Probe Molecule

Defect engineering (DE) has been recognized as a powerful approach to tune the structural, optical, and chemical properties of metal–organic framework (MOF) materials. Here, a detailed characterization using ultrahigh-vacuum Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy was performed for defect-engineered NOTT-100 (Cu2(BPTC), BPTC = biphenyl-3,3′,5,5′-tetracarboxylates). Defects were introduced either via thermal defect engineering (TDE) or the defective linker approach (synthetic defect engineering, SDE). A quantitative analysis of the spectroscopic results revealed the formation of reduced, undercoordinated Cu+/Cu2+ dimer defects via both TDE and SDE approaches in a controlled manner. Exposure of the MOFs to CO led to various (CO)aCu+ and (CO)aCu2+ (a = 1,2) species. The binding energies of these species as determined by temperature-dependent experiments showed strong variations. The type and doping concentration of defective linkers as well as the annealing temperatures played a crucial role in tuning the structural and electronic properties of DE-NOTT-100 MOFs. The Brønsted acid sites exposed by protonated carboxylic acids were unambiguously identified by both the characteristic vibrational frequency of adsorbed CO and the corresponding red shift of the acidic OH group.

Strong and Weakly Acidic OH Groups of HY Zeolite into the Different Routes of Cyclohexene Reaction: An IR Operando Study

IR operando study of cyclohexene reaction on HY zeolite (Si/Al = 15) was performed to determine the role of the OH groups located in the different zeolitic cages and the silanol groups (SiOH). A parallel between the product yield and type and amount of OH groups poisoned by coking and pyridine pulses was established. The role of silanol and zeolitic OH groups during cyclohexene conversion and the coverage rate of surface groups of the dealuminated HY zeolite, were evaluated by FT-IR operando study. At the beginning of the reaction, zeolite exhibited a very intense deactivation due to coke formation. Coke is initially deposited on strong acid sites, inhibiting the H-transfer activity. Further, coke formation decreased. At this stage of the reaction, pyridine addition markedly decreased the isomerization activity. In parallel, it is noted that mainly SiOH, OH groups at hexagonal prisms, and perturbed OH groups in sodalite cages were mainly affected, indicating that these groups are responsible for the isomerization steps. These results showed that even weakly acidic sites are active sites for isomerization reaction. In cyclohexene conversion, hydrogen transfer occurs on supercage OH groups of HY zeolite. In contrast, perturbed sodalite OH and hexagonal prisms OH, as well as SiOH groups, are responsible for isomerization steps, as revealed by pyridine pulse addition during the reaction.

Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-15 materials

The interaction and nature of surface sites for water and methanol sorption on MFI-type zeolites and mesoporous SBA-15 were investigated by solid-state NMR spectroscopy and correlated with the desorption enthalpies determined via TGA/DSC. For siliceous Silicalite-1, 29Si CPMAS NMR studies support stronger methanol than water interactions with SiOH groups of Q3-type. On siliceous SBA-15, SiOH groups of Q2-type are accompanied by an enhanced hydrophilicity. In aluminum-containing Na-ZSM-5, Na+ cations are strong adsorption sites for water and methanol as evidenced by 23Na MAS NMR in agreement with high desorption enthalpies of ΔH = 66–74 kJ/mol. Solid-state NMR of aluminum-containing Na-[Al]SBA-15, in contrast, has shown negligible water and methanol interactions with sodium and aluminum. Desorption enthalpies of ΔH = 44–60 kJ/mol hint at adsorption sites consisting of SiOH groups influenced by distant framework aluminum. On H-ZSM-5, Brønsted acidic OH groups are strong adsorption sites as indicated by partial protonation of water and methanol causing low-field shifts of their 1H MAS NMR signals and enhanced desorption enthalpies. Due to the small number of Brønsted acid sites in aluminum-containing H-[Al]SBA-15, water and methanol adsorption on this material is suggested to mainly occur at SiOH groups with distant framework aluminum species, as in the case of Na-[Al]SBA-15.

Disentangling Brønsted Acid Sites and Hydrogen-Bonded Silanol Groups in High-Silica Zeolite H-ZSM-5

Hydrogen-bonded SiOH defects and Bronsted acidic bridging OH groups, defined as Bronsted acid sites (BAS), yield overlapping signals in 1H MAS NMR and FTIR spectroscopies. Here, a clear distinction...

Hydrogenation of levulinic acid to γ-valerolactone over Fe-Re/TiO2 catalysts

Hydrogenation of levulinic acid to γ-valerolactone is a key reaction in the valorization of carbohydrates to renewable fuels and chemicals. State-of-the-art catalysts are based on supported noble metal nanoparticle catalysts. We report the utility of a bimetallic Fe-Re supported on TiO2 for this reaction. A strong synergy was observed between Fe and Re for the hydrogenation of levulinic acid in water under mild conditions. Fe-Re/TiO2 shows superior catalytic performance compared to monometallic Fe and Re catalysts at similar metal content. The hydrogenation activity of the bimetallic catalysts increased with Re content. H2-TPR, XPS, XANES, EXAFS, Mössbauer spectroscopy, TEM, and low-temperature CO IR spectroscopy show that the bimetallic catalysts contain metallic Re nanoparticles covered by FeOx species and small amounts of a Fe-Re alloy. Under reaction conditions, the partially reduced surface FeOx species adsorb water and form Brønsted acidic OH groups, which are involved in dehydration of reaction intermediates. Under optimized conditions, nearly full conversion of levulinic acid with a 95 % yield of γ-valerolactone could be achieved at a temperature as low as 180 °C in water at a H2 pressure of 40 bar.

Curing acceleration of cyanate ester resin by a phenolic compound having a tertiary amino group at the ortho-position

To develop a novel organic catalyst that can effectively cure cyanate ester resins, the effects of incorporating various phenolic compounds on the curing of bisphenol A dicyanate (BADCY) was investigated. Differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy analyses revealed that o-(dimethylamino)methylphenol (o-DAMP) was a highly effective catalyst for accelerating the curing of BADCY, and the addition of o-DAMP reduced the final curing temperature of BADCY to 180 °C. The promoting effect of o-DAMP was found to be due to the presence of an acidic phenolic OH group and a basic (dimethylamino)methyl group at the ortho-positions of the molecule. The addition of o-DAMP improved the modulus, strength and fracture toughness (KIC) of the cured BADCY resins. The mechanical and thermal properties of the cured resins could be adjusted by changing the amount of o-DAMP. The investigation of the effects of incorporating various phenolic compounds on the curing of bisphenol A dicyanate (BADCY) revealed that o-(dimethylamino)methylphenol (o-DAMP) was a very effective catalyst, which reduced the final curing temperature to 180 °C. The promoting effect of o-DAMP was due to the presence of a phenolic OH and a (dimethylamino)methyl groups at the ortho-positions of the molecule. The addition of o-DAMP improved the mechanical properties of the cured BADCY resins, and the most balanced properties were obtained when the ratio of OH to OCN was 0.05.

In Situ IR‐ATR Study of the Interaction of Nitrogen Heteroaromatic Compounds with HY Zeolites: Experimental and Theoretical Approaches

AbstractIn the present work, the liquid‐solid interaction of liquid N‐heteroaromatic compounds, commonly present in the petroleum feedstocks of the refineries, with Y zeolites used as hydrocracking catalysts was followed using IR‐ATR spectroscopy. The inhibition of the zeolitic acid sites by strongly basic pyridine and weakly basic indole was highlighted using a continuous flow IR‐ATR cell. Results were assessed by Density Functional Theory calculations to compute the vibrational frequencies of pyridine and indole according to the nature of the interaction sites: silanol groups or acidic OH groups. The study points out that IR‐ATR spectroscopy opens the way for investigating the interaction modes of low vapor pressure molecules (e. g. indole) that present an inherent difficulty to be operated in the gas phase. Moreover, the IR‐ATR makes possible the analysis of the little‐explored low wavenumber zone (<800 cm−1), that presents informative vibrational modes on the adsorption mode of N‐molecules. Hence, this work points out that for pyridine, the bands at 686 and 727 cm−1 are characteristic of pyridinium species formed over zeolitic OH groups, meanwhile, the signals at 703 and 750 cm−1, are associated to pyridine in interaction with silanol groups. The IR‐ATR study reveals that indole, a weakly basic compound, can be protonated on acidic Y zeolites as unambiguously evidenced by the formation of the bands at 1617, 1608, 1543 and 705 cm−1. Findings here exposed are crucial for studying inhibitory effects exerted by weak nitrogenated compounds on acidic materials during hydrocracking processes.

IR observation of activated ether species on acidic OH groups on H-ZSM-5 zeolites

IR spectra of ethers adsorbed on zeolite (H-ZSM-5) were analyzed in detail in comparison with those of water and alcohols. Interpretation of complicated features of IR spectra observed for adsorption of water and alcohols on acidic OH groups of zeolite is summarized in relation with the adsorption structure and the potential energy based on the accepted knowledge. Since the presence of Fermi resonance in vibrational modes between acidic OH groups and adsorbed species can be an estimate for the potential energy of the protonated transition state, IR spectra of adsorption of ethers on zeolite were interpreted by changing acidic OH groups to OD. Adsorption of ethers on silanol and acidic OH groups of zeolite resulted in markedly different IR spectra. In addition, the Fermi resonance between OH stretching of acidic OH (shifted below 2000 cm−1) and CH bending vibrations of adsorbed ethers was confirmed, and the observed adsorbed species were co-existent with protonated transition state in equilibrium at 298 K.

Probing the basicity of lattice oxygen on H-form zeolites using CO2

The basicity of lattice oxygen in H-form zeolite was characterized by the CO2-probe IR method at room temperature and in the low-pressure region. The adsorbed CO2 species on basic oxygens (>2350 cm−1) were distinguished from hydrogen-bonded (2349 cm−1) and physisorbed (2339 cm−1) species. The pairing of a basic oxygen and an acidic OH group was evidenced by quantitative ammonia adsorption, where the amount of basic oxygen was decreased linearly with the amount of acidic OH consumed by ammonia adsorption. This clearly indicates the presence of basicity on H-form acidic zeolites. The number of lattice oxygens of H-form zeolites located around Al in (AlO4)− units was affected by the environment of the central Al atoms in the framework of the zeolites. Consequently, the number of base sites was dependent on zeolite topology. Zeolites diverse in the location of T sites represented by MFI showed plural IR absorption bands of adsorbed molecular CO2 (2370–2350 cm−1) due to the presence of various types of base sites, in contrast to zeolites with fewer types of T sites, such as CHA. While CO2 adsorption onto basic metal oxides results in forming carbonate species through chemical reactions, IR observation of molecularly adsorbed CO2 could be an appropriate method to detect weakly basic sites on acid catalysts.

Cooperative catalysis of metal and acid functions in Re-HZSM-5 catalysts for ethane dehydroaromatization

A vapor-phase exchange method was used for the encapsulation of Re metal clusters in dehydrated HZSM-5 zeolites with Brønsted acid OH groups. The reaction pathway of ethane non-oxidative dehydroaromatization on Re-HZSM-5 catalysts was revealed by an experimental observation of the contact time effects on hydrocarbon product selectivity. The reaction pathway typically includes the dehydrogenation of ethane to form olefin reactant pool via ethylene formation and immediate dimerization to butene, further dehydrogenation of ethylene to acetylene, and eventual formation of benzene products. The possible elementary step of ethane non-oxidative dehydroaromatization was revealed by kinetic measurements of the dependence of ethane and hydrogen pressures on the turnover rates of benzene formation. The kinetic observation indicates that the dehydrogenation of ethylene to acetylene is the rate-determining step that mediates the whole reaction event. Cooperative catalysis of metal and acid functions is observed on Re-HZSM-5 catalysts. Re metal clusters contribute to ethane activation and formation of initial alkene intermediates. The acid sites would further promote the oligomerization and cyclization reaction and generate benzene and toluene. The bifunctional pathways involving metal and acid roles in ethane non-oxidative dehydroaromatization were further studied on Re-HZSM-5, HZSM-5, and Re-SIL-1/HZSM-5 catalysts. The site proximity between Re metal clusters and acid sites plays a critical role in benzene formation, exemplified by initial rates of benzene formation of 0.03 and 0.43 μmol(C) gcat−1 s−1 on Re-SIL-1/HZSM-5 and Re-HZSM-5 catalysts, respectively. These two catalysts contain long and short distances of approximately 75 and 1.3 nm between Re metals and acid domains.

Mechanochemical-assisted hydrolysis of pretreated rice straw into glucose and xylose in water by weakly acidic solid catalyst

In this work, carbonaceous materials bearing only weakly acidic COOH and phenolic OH groups were directly prepared by the pyrolysis of lignin and KOH in black liquor generating from rice straw that was pretreated with KOH aqueous solution. The synthesized carbon materials were used for the hydrolysis of cellulose or the alkali pretreated rice straw in water, after mixed ball-milling pretreatment, and provided a high glucose yield of 76.3% for cellulose, high yields of 52.1% glucose and 66.5% xylose for alkali pretreated rice straw, respectively, in 0.015 wt% HCl aqueous solution at 200 °C for 60 min. The weakly acidic catalyst showed good stability and recyclability in the aqueous reaction system. This work provides an efficient process for the hydrolysis of lignocellulose by biomass-derived weakly acidic catalysts in water and should have wide applications in biomass utilization.

Characterization of fractional cuts of co-solvent enhanced lignocellulosic fractionation lignin isolated by sequential precipitation

Lignin valorization is significantly hindered by the intrinsic heterogeneity of its complex structures and variability of biomass feedstocks. Fractionation of lignin can overcome these challenges by producing functionally distinct lignin cuts that can be further tailored to end products. Herein, lignin was extracted and depolymerized from poplar by the co-solvent enhanced lignocellulosic fractionation method with renewable THF to obtain CELF lignin. Several solvents were screened to separate soluble and insoluble fractions from the parent CELF lignin. The ethanol soluble portion was then fractionated into different molecular weight cuts via sequential precipitation of the lignin by reducing the concentration of THF. The physicochemical structures of different CELF lignin cuts were elucidated by GPC and NMR techniques. These results suggest that CELF lignin cuts with lower molecular weight contain progressively higher phenolic and carboxylic acid OH groups, which can be more suitable as green antioxidants than the parent lignin.

Effects of aromatic diboronic acid on thermal charateristics of polybenzoxazines based on phenol and aniline

In this work, polybenzoxazines based on phenol and aniline were prepared by curing as-synthesized and purified monomers in the presence of aromatic diboronic acid, BDBA. Condensation reactions between BOH groups of diboronic acid and phenolic OH groups of benzoxazine took place during curing only when unreacted phenol, or low mass oligomers were present in the medium. Generation of fragments involving B-O-C groups and significant improvement in thermal characteristics were detected upon curing the as-synthesized monomer in the presence of even 5 wt %BDBA. On the other hand, enhancement in thermal characteristics was not detected for the polybenzoxazine prepared by curing the purified monomer, when the wt % of BDBA was lower than 15%.

Influence of zeolite acidity on proton conductivity of FAU embedded imidazole

Abstract Imidazole entrapped in H-forms of FAU zeolites with various Si/Al ratios (1, 4, 40) exhibit high and stable proton conductivity (up to ∼10−3 S/cm) in a broad range of temperatures (293–393 K). The conductivity increases with temperature and imidazole loading. The number and strength of zeolite acid sites (resulting from various Si/Al) influence markedly the conductivity of composites. The samples based on zeolites with low Si/Al (1) show markedly higher conduction than these supported on the matrix with medium Si/Al (4), regardless of very similar number of the acid OH groups. The strong acid OH groups in zeolite with Si/Al = 4 result in protonation and immobilization of adsorbed imidazole, which reduce their mobility and subsequently the conduction. The low siliceous with weaker acid sites protonate imidazole and form a chain of H-bonds effective in proton propagation. Almost neutral high silica matrix provides a high mobility of dispersed imidazole, due to weak mutual interaction. The overloaded samples indicate a phase transition effect at elevated temperature due to melting of extra-matrix imidazole, which is reflected in dramatic increase in conductivity.

Evidence for a strong polarization of n-hexane in zeolite H-ZSM-5 by FT-IR and solid-state NMR spectroscopy

Abstract The adsorption and interaction of n-hexane in acidic zeolites were investigated via the adsorbate-induced shifts of the FT-IR bands (Δ ν ˜ ) and 1H MAS NMR signals (Δδ) of acidic bridging OH groups (Si(OH)Al) in H-mordenite, H-Beta, 10H,Na-Y, 86H,Na-Y, H-ZSM-5, and US-Y. The plot of the experimentally determined Δ ν ˜ values as a function of Δδ shows a linear correlation. In a subsequent step, the influences of the average electronegativity Sav of the individual zeolites under study and the deprotonation energy ΔpA of their acidic hydroxyl protons on Δ ν ˜ and Δδ were investigated. The individual Sav values were calculated via the geometric average of all element electronegativities of the zeolite framework, while the ΔpA values were obtained via an evaluation of the CO-induced shifts of the FT-IR bands of Si(OH)Al groups compared with that of SiOH groups and using an empirical relationship. In the plots of the Δ ν ˜ /Sav and Δ ν ˜ /ΔpA ratios as a function of Δδ/Sav and Δδ/ΔpA, respectively, the data sets of all zeolites under study are closely arranged, excluding that of H-ZSM-5. This finding indicates that n-hexane adsorbed on zeolite ZSM-5 is not exclusively involved in the interaction with acidic Si(OH)Al groups, but is also influenced by the zeolite framework. The 13C solid-state NMR shifts of n-hexane adsorbed on the zeolites hint at a polarization of this alkane especially in the 10-ring pores of zeolite H-ZSM-5. This polarization of n-hexane in H-ZSM-5 is important for the reactivity of this alkane in heterogeneously catalyzed reactions.

Zeta Potential Measurements on Solid Surfaces for in Vitro Biomaterials Testing: Surface Charge, Reactivity Upon Contact With Fluids and Protein Absorption.

Surface properties of biomaterials (e.g., roughness, chemical composition, charge, wettability, and hydroxylation degree) are key features to understand and control the complex interface phenomena that happens upon contact with physiological fluids. Numerous physico-chemical techniques can be used in order to investigate in depth these crucial material features. Among them, zeta potential measurements are widely used for the characterization of colloidal suspensions, but actually poorly explored in the study of solid surfaces, even if they can give significant information about surface charge in function of pH and indirectly about surface functional groups and reactivity. The aim of the present research is application of zeta potential measurements of solid surfaces for the in vitro testing of biomaterials. In particular, bare and surface modified Ti6Al4V samples have been compared in order to evaluate their isoelectric points (IEPs), surface charge at physiological pH, in vitro bioactivity [in simulated body fluid (SBF)] and protein absorption. Zeta potential titration was demonstrated as a suitable technique for the surface characterization of surface treated Ti6Al4V substrates. Significant shift of the isoelectric point was recorded after a chemical surface treatment (because of the exposition of hydroxyl groups), SBF soaking (because of apatite precipitation IEP moves close to apatite one) and protein absorption (IEP moves close to protein ones). Moreover, the shape of the curve gives information about exposed functional groups (e.g., a plateau in the basic range appears due to the exposition of acidic OH groups and in the acidic range due to exposition of basic NH2 groups).

Synthesis, optimization and characterization of biochar based catalyst from sawdust for simultaneous esterification and transesterification

Abstract The development of a benign environmental catalyst for the generation of biodiesel is an area of importance to reduce the overall usage of fossil fuels. In the current work, biochar was produced by slow pyrolysis of Irul wood sawdust. The optimization for biochar generation was carried out for different reaction temperatures and heating durations. The biochar was used to prepare solid acid catalysts by sulfonation process. The characterization of biochar and the sulfonated catalyst were carried out using Elemental analysis, Fourier Transform Infrared spectroscopy (FTIR), Powder X-ray Diffraction (XRD), Thermo Gravimetric Analysis (TGA), Scanning Electron Microscopy (SEM) and Surface area analyzer (BET). The characterization results showed that sulfonation of biochar resulted in biochar based solid acid catalyst containing various functional acidic groups like weak acidic OH groups, strong acidic COOH and SO3H groups. The total acid density and sulfonic acid group density of catalyst were estimated and showed excellent acidic sites concentration which gives a good catalytic activity for biodiesel production through simultaneous esterification and transesterification. The enhanced catalytic activity is due to the high acid density of SO3H groups and the reactant accessibility towards acidic sites as well as the strong affinity between the hydrophilic reactants and the neutral OH groups which are bonded with the polycyclic aromatic carbon rings. The performance of biochar catalyst for the production of biodiesel was evaluated by comparing the yield obtained. The FTIR and Gas Chromatography–Mass Spectroscopy (GC–MS) were also carried out for the analysis of biodiesel produced.

Probing Surface Chemistry Changes Using LiCoO2-only Electrodes in Li-Ion Batteries

Fundamental understanding of the reactivity between electrode and electrolyte is key to design the safety and life of Li-ion batteries. Herein X-ray photoelectron spectroscopy was used to examine the electrode/electrolyte interface (EEI) on carbon-free, binder-free LiCoO2 powder and thin-film electrodes in LP57 electrolyte as function of potential. Upon charging of LiCoO2 a marked growth of oxygenated and carbonated species was observed on the surface, consistent with electrolyte oxidation at high potentials. We also demonstrated that LiCoO2 oxide surface was prone to decompose the salt starting at 4.1 VLi, as evidenced by the increase of LiF and LixPFyOz species upon charging. By DFT calculations we proposed a correlation between the interface composition and the thermodynamic tendency of the EC solvent for dissociative adsorption on the LixCoO2 surface, through the generation of reactive acidic OH groups on the oxide surface, which can have a role in the observed salt decomposition. This is consistent with the evidence of HF and PF2O2− species at 4.6 VLi observed by solution 19F-NMR measurements. Finally we compared EEI composition between composite and model electrodes and discussed the changes and mechanisms induced by the electrode composition or the use of electrolyte additives. We showed that the addition of diphenyl carbonate (DPC) in the electrolyte has a strong impact on the formation of solvent and salt decomposition products at the EEI layer.

Sequential microhydration of cationic 5-hydroxyindole (5HI+): infrared photodissociation spectra of 5HI+-Wn clusters (W = H2O, n ≤ 4).

Most biochemical processes occur in aqueous solution. Here, we characterize the initial microhydration steps of the 5-hydroxyindole cation (5HI+) in its 2A'' ground electronic state by infrared photodissociation (IRPD) spectroscopy of 5HI+-Wn-Lm clusters (W = H2O, L = Ar and N2, n ≤ 4, m ≤ 2) in a molecular beam and dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ). Characteristic size- and isomer-dependent XH stretch frequencies (X = O, N) of 5HI+-Wn reveal information about the preferred cluster growth and solvation energies. The IRPD spectrum of 5HI+-W is a superposition of the spectra of two isomers, in which W is H-bonded to the acidic NH or OH group, whereby OHW hydrogen-bonds (H-bonds) are stronger than NHW H-bonds. Spectra of larger 5HI+-Wn clusters (n ≥ 2) elucidate the competition between interior ion solvation and the formation of H-bonded water networks. The nature and strengths of the competing H-bonds are quantified by the noncovalent interaction approach. Comparison to results for neutral 5HI-W and 5HI+-Ln clusters with nonpolar ligands reveals the effects of ionization and ligand type on the intermolecular interaction potential and cluster growth. Comparison to corresponding microhydrated clusters of the phenol, indole, and pyrrole cations illustrates the effects of substitution of functional groups and addition of aromatic rings on the hydration process.