Dehydration Rate Constants Research Articles

Effect of electrogenerated random defects on Pt(111) surface over methanol electrooxidation: Electrochemical and spectroscopic studies

In the present paper, the influence of electrogenerated (110) defects in the methanol oxidation reaction (MOR) on a Pt(111) electrode has been studied. It is shown that methanol oxidation is favored at the defected surfaces as evidenced by the displacement of the onset potential to lower values and the increase of the peak current density. The currents obtained by transient curves at short times (0.015 – 0.50 s) at 0.50 V vs RHE demonstrate the faster formation of strongly adsorbed species on disordered surfaces when compared to Pt(111). On the other hand, for long-term transient currents (600 s), the surface with a higher surface density of (110) defects presented the best activity among all ones studied. Galvanostatic results showed that potential oscillations emerged only on disordered surfaces. The large induction time up to the beginning of the oscillations as well as the lower frequency registered on these surfaces demonstrates the enhancement of CO formation on defects. This point is confirmed by in situ FTIR experiments. From the results obtained by using electrochemical techniques, it was possible to infer that the dehydration rate constant (kdh) and the rate constant for CO oxidation (kCO) are affected by the number of (110) defects, which give rise to kCO and kdh values higher than those obtained for Pt(111).

Carbonic anhydrase activity in the frontal lobe of human brain.

Carbonic anhydrase (CA) plays an important role in many biological processes. Recent technological advances have demonstrated the feasibility of measuring CA activity in the occipital lobe of human subjects in vivo. In this work we report, for the first time, in vivo measurement of CA activity in the frontal lobe of human brain, where structural and function abnormalities are strongly associated with symptoms of major psychiatric disorders. Despite the much larger magnetic field distortion in the frontal lobe, the pseudo first-order bicarbonate dehydration rate constant was determined with high precision using in vivo 13 C magnetic resonance magnetization transfer spectroscopy following oral administration of [U-13 C6 ]glucose. Nuclear Overhauser effect pulses were used to increase the signal-to-noise ratio; no proton decoupling was applied. The unidirectional dehydration rate constant of bicarbonate was found to be 0.26 ± 0.07 s-1 , which is not statistically different from the dehydration rate constant in the occipital lobe determined in our previous study, indicating that CA activity in the two brain regions is essentially indistinguishable. These results demonstrate the feasibility of characterizing CA activity in the frontal lobe for future psychiatric studies.

Influence of confining environment polarity on ethanol dehydration catalysis by Lewis acid zeolites

Lewis acidic Sn centers isolated within Beta zeolite frameworks catalyze bimolecular ethanol dehydration to diethyl ether, yet with kinetic behavior sensitive to the hydrophobic character of their confining microporous voids. Sn sites in open ((HO)-Sn-(OSi)3) and closed (Sn-(OSi)4) configurations, quantified from infrared spectra of adsorbed CD3CN before and after reaction, convert to structurally similar intermediates during ethanol dehydration catalysis (404 K) and revert to their initial configurations after regenerative oxidation treatments (21% O2, 803 K). Dehydration rate data (404 K, 0.5–35 kPa C2H5OH, 0.1–50 kPa H2O) measured on ten low-defect (Sn-Beta-F) and high-defect (Sn-Beta-OH) zeolites were described by a rate equation that was derived from mechanisms identified previously by density functional theory calculations and simplified using microkinetic modeling to identify kinetically-relevant pathways and intermediates. Polar hydroxyl defect groups located in the microporous environments that confine Sn sites preferentially stabilize reactive (ethanol-ethanol) and inhibitory (ethanol-water) dimeric intermediates over monomeric ethanol intermediates. As a result, equilibrium constants (404 K) for ethanol-water and ethanol-ethanol dimer formation are 3–4× higher on Sn-Beta-OH than on Sn-Beta-F, consistent with insights from single-component (302 K) and two-component (303 K, 403 K) ethanol and water adsorption measurements. Intrinsic dehydration rate constants (404 K) were identical, within error, among Sn-Beta-OH and Sn-Beta-F zeolites; thus, measured differences in dehydration turnover rates solely reflect differences in prevalent surface coverages of inhibitory and reactive dimeric intermediates at active Sn sites. The confinement of Lewis acidic binding sites within secondary microporous environments of different defect density confers the ability to discriminate surface intermediates on the basis of polarity, providing a design strategy to accelerate turnover rates and suppress inhibition by water.

Determining the Rate of Carbonic Anhydrase Reaction in the Human Brain

Carbonic anhydrase plays important role in life. This study sought to demonstrate the feasibility of detecting carbonic anhydrase activity in the human brain in vivo. After oral administration of [U-13C6]glucose, 13C saturation transfer experiments were performed with interleaved control spectra and carbon dioxide saturation spectra. Proton nuclear Overhauser effect pulses were used to increase signal to noise ratio; no proton decoupling was applied. Results showed that the 13C signal of bicarbonate was reduced by 72% ± 0.03 upon saturating carbon dioxide. The unidirectional dehydration rate constant of the carbonic anhydrase reaction was found to be 0.28 ± 0.02 sec−1 in the human brain. These findings demonstrate the feasibility of measuring carbonic anhydrase activity in vivo in the human brain, which makes it possible to characterize this important enzyme in patients with brain disorders.

Binding of the five multistate species of the anthocyanin analog 7-β-D-glucopyranosyloxy-4′-hydroxyflavylium to the β-cyclodextrin derivative captisol

The host-guest chemistry of the anthocyanin analog 7-β-D-glucopyranosyloxy-4′-hydroxyflavylium (GHF) was studied in the presence of the β-cyclodextrin derivative captisol by stopped flow, UV–visible spectroscopy, flash photolysis, circular dichroism and isothermal titration calorimetry. The equilibrium and rate constants of the multistate of chemical species derived from the flavylium ion were calculated and compared with those in the absence of the host. A new procedure to obtain the host-guest association constants of the multistate (including the transient species) by superimposing the two energy level diagrams, in the presence and absence of the cyclodextrin, was developed. The results indicate that the magnitude of the association constants follows the order, trans-chalcone ≈ cis-chalcone ≈ hemiketal > quinoidal base > flavylium cation. The hydration equilibrium constant increases ca. 42 times in the presence of captisol as the hydration and dehydration rate constants respectively increases and decreases. The other equilibrium constants are modestly affected: the rate constants of ring closure and opening are significantly decreased in the complex and the isomerization rate constants increase in both directions. The quantum yield of the photochromic system in the presence of captisol is 0.3, i.e. 3 times higher than in the absence of the host.

Cryo assisted spouted bed roasting of coffee beans

Abstract A process for roasting of coffee beans involving spouted bed roasting (SBR), followed by cryogenic cooling and grinding was developed. The temperature and time of roasting was 210 °C and 22 min. It was found that coffee subjected to SBR roasting followed by cryogenic cooling had greater impact on the retention of volatile compounds especially the furans and thiols than the conventional drum roasting (CDR). The dehydration, the zero and first order color degradation rate constants for cryo treated SBR coffee beans were found to be 0.079, 1.9 and 0.04488 respectively where as for CDR coffee beans it was 0.03942, 1.398 and 0.030 correspondingly. The sensory analysis confirmed that proposed roasting method was beneficial for the retention of volatile flavor compounds and reduction of the burning of surface, and will be very useful for coffee processing in order to produce high quality coffee beans and increase its economic value. Industrial relevance Pyrolytic reactions lead to characteristic aroma and flavor during coffee roasting. Optimizing and controlling the roasting parameters is critical for coffee quality. Coffee roasting process involving spouted bed and cryogenic cooling was developed. Proposed roasting produces high quality coffee beans leading to its improved economic importance.

Kinetics of Acid-Catalyzed Dehydration of Cyclic Hemiacetals in Organic Aerosol Particles in Equilibrium with Nitric Acid Vapor.

Previous studies have shown that 1,4-hydroxycarbonyls, which are often major products of the atmospheric oxidation of hydrocarbons, can undergo acid-catalyzed cyclization and dehydration in aerosol particles to form highly reactive unsaturated dihydrofurans. In this study the kinetics of dehydration of cyclic hemiacetals, the rate-limiting step in this process, was investigated in a series of environmental chamber experiments in which secondary organic aerosol (SOA) containing cyclic hemiacetals was formed from the reaction of n-pentadecane with OH radicals in dry air in the presence of HNO3. A particle beam mass spectrometer was used to monitor the formation and dehydration of cyclic hemiacetals in real time, and SOA and HNO3 were quantified in filter samples by gravimetric analysis and ion chromatography. Measured dehydration rate constants increased linearly with increasing concentration of HNO3 in the gas phase and in SOA, corresponding to catalytic rate constants of 0.27 h(-1) ppmv(-1) and 7.0 h(-1) M(-1), respectively. The measured Henry's law constant for partitioning of HNO3 into SOA was 3.7 × 10(4) M atm(-1), ∼25% of the value for dissolution into water, and the acid dissociation constant was estimated to be <8 × 10(-4), at least a factor of 10(4) less than that for HNO3 in water. The results indicate that HNO3 was only weakly dissociated in the SOA and that dehydration of cyclic hemiacetals was catalyzed by molecular HNO3 rather than by H(+). The Henry's law constant and kinetics relationships measured here can be used to improve mechanisms and models of SOA formation from the oxidation of hydrocarbons in dry air in the presence of NOx, which are conditions commonly used in laboratory studies. The fate of cyclic hemiacetals in the atmosphere, where the effects of higher relative humidity, organic/aqueous phase separation, and acid catalysis by molecular H2SO4 and/or H(+) are likely to be important, is discussed.

Kinetics of xylose dehydration into furfural in acetic acid

In this paper kinetics of xylose dehydration into furfural using acetic acid as catalyst was studied comprehensively and systematically. The reaction order of both furfural and xylose dehydration was determined and the reaction activation energy was obtained by nonlinear regression. The effect of acetic acid concentration was also investigated. Reaction rate constants were gained. Reaction rate constant of xylose dehydration is k1=4.189×1010A0.1676exp−108.6×1000RT, reaction rate constant of furfural degradation is k2=1.271×104A0.1375exp−63.413×1000RT and reaction rate constant of condensation reaction is k3=3.4051×1010A0.1676exp−104.99×1000RT. Based on this, the kinetics equation of xylose dehydration into furfural in acetic acid was set up according to theory of Dunlop and Furfural generating rate equation is dFdt=k1X0e−k1t−k2F−k3X0e−k1tF.

Unimolecular decomposition of formic and acetic acids: A shock tube/laser absorption study

The thermal decomposition of formic acid (HCOOH) and acetic acid (CH3COOH), two carboxylic acids which play an important role in oxygenate combustion chemistry, were investigated behind reflected shock waves using laser absorption. The rate constants of the primary decomposition pathways of these acids:HCOOH→CO+H2O(R1)HCOOH→CO2+H2(R2)CH3COOH→CH4+CO2(R3)CH3COOH→CH2CO+H2O(R4)were measured using simultaneous infrared laser absorption of CO, CO2 and H2O at wavelengths of 4.56, 4.18 and 2.93 microns, respectively. Reaction test conditions covered temperatures from 1230 to 1821K and pressures from 1.0 to 6.5atm for dilute mixtures of acids (0.25–0.6%) in argon. The rate constants of dehydration (R1) and decarboxylation (R2) reactions of formic acid were calculated by fitting exponential functions to the measured CO, CO2 and H2O time-history profiles. These two decomposition channels were found to be in the fall-off region and have a branching ratio, k1/k2, of approximately 20 over the range of pressures studied here. The best-fit Arrhenius expressions of the first-order rates of R1 and R2 were found to be:k1(1atm)=1.03×1011exp(-25651/T)s-1(±37%)k1(6.5atm)=9.12×1012exp(-30275/T)s-1(±32%)k2(1atm)=1.79×108exp(-21133/T)s-1(±41%)k2(6.5atm)=2.73×108exp(-20074/T)s-1(±37%)The rate constants for acetic acid decomposition were obtained by fitting simulated profiles, using an acetic acid pyrolysis mechanism, to the measured species time-histories. The branching ratio, k4/k3, was found to be approximately 2. The decarboxylation and dehydration reactions of acetic acid appear to be in the falloff region over the tested pressure range:k3(1atm)=3.18×1011exp(-28679/T)s-1(±30%)k3(6atm)=3.51×1012exp(-31330/T)s-1(±26%)k4(1atm)=7.9×1011exp(-29056/T)s-1(±34%)k4(6atm)=6.34×1012exp(-31330/T)s-1(±31%).

Acid strength and solvation in catalysis by MFI zeolites and effects of the identity, concentration and location of framework heteroatoms

The effects of heteroatom identity (Al3+, Ga3+, Fe3+, or B3+), concentration and location on catalysis by MFI zeolites are examined and interpreted mechanistically using methanol dehydration rate constants and density functional theory estimates of acid strength (deprotonation energies, DPE). In doing so, we shed light on the concomitant effects of confinement and acid strength on catalytic reactivity. Rate constants (per H+ from pyridine titrations during catalysis) in the first-order and zero-order kinetic regimes decreased exponentially as the DPE of MFI with different heteroatoms increased. These trends reflect a decrease in the stability of ion-pair transition states relative to the relevant precursors (H-bonded methanol and methanol dimers, respectively, for these two regimes) with decreasing acid strength and resemble those in mesoporous solid acids (e.g., polyoxometalates). Confinement effects, weaker in mesoporous solids, give larger rate constants on MFI than on POM clusters with similar DPE. Such reactivity enhancements reflect the effects of MFI voids that solvate transition states preferentially over smaller precursors via van der Waals interactions with the confining voids. Both dehydration rate constants on MFI with 0.7–2.4 H+ per unit cell volume (5.2nm3) are independent of Al or H+ densities, indicating that neither H+ location nor acid strength depend on acid site concentration. Higher site densities (3.6 H+ per unit cell) lead to larger first-order rate constants, but do not influence their zero-order analogs. These data reflect, and in turn provide evidence for, the initial siting of H+ in less constrained channel intersections and their ultimate placement within the more solvating environments of the channels themselves. Thus, the higher reactivity of Al-rich samples, often attributed to the stronger acid sites, arises instead from the ubiquitous role of zeolites as inorganic solvents for the relevant transition state, taken together with H+ siting that depends on Al density. We find that heteroatom composition, but not Al density, influences acid strength in MFI, consistent with experiment and theoretical estimates of DPE, and that methanol dehydration rate constants, properly interpreted, provide relevant insights into the combined effects of acid strength and confinement on the reactivity of solid Brønsted acids.

Substituent effects on the dehydration of arene hydrates in aqueous solution

Rate constants have been determined by UV spectrophotometry at 25 °C for the acid‐catalyzed dehydration of different types of monocyclic arene hydrates including those substituted at the 1‐, 2‐ or 3‐positions. General acid catalysis was not observed, and linear plots of pseudo‐first‐order rate constants for dehydration against hydronium concentration were obtained. A Hammett plot of the second‐order rate constants for acid‐catalyzed dehydration, kH (M−1s−1), of unsubstituted‐ (8a), 3‐substituted (8b, 8c, 8d, 8e) and 1‐substituted‐benzene hydrates (14f and 14h) shows an excellent correlation with σ+ values and yields a large negative ρ‐value of –6.5. The results are consistent with rate‐determining formation of a benzenium ion in which direct mesomeric interaction with the substituent occurs, presumably permitted by the coplanar arrangement of the diene and carbocation centre in the intermediate. Data points for 2‐substituted arene hydrates (13f, 13g, 13h, 13i) deviate negatively from the Hammett plot as direct mesomeric interaction with the substituent is not possible in the corresponding benzenium intermediates. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Dehydration of Isobutanol and the Elimination of Water from Fuel Alcohols

Rate coefficients for the dehydration of isobutanol have been determined experimentally from comparative rate single pulse shock tube measurements and calculated via multistructural transition state theory (MS-TST). They are represented by the Arrhenius expression, k(isobutanol → isobutene + H2O)(experimental) = 7.2 × 10(13) exp(-35300 K/T) s(-1). The theoretical work leads to the high pressure rate expression, k(isobutanol → isobutene + H2O)(theory) = 3.5 × 10(13) exp(-35400 K/T) s(-1). Results are thus within a factor of 2 of each other. The experimental results cover the temperature range 1090-1240 K and pressure range 1.5-6 atm, with no discernible pressure effects. Analysis of these results, in combination with earlier single pulse shock tube work, made it possible to derive the governing factors that control the rate coefficients for alcohol dehydration in general. Alcohol dehydration rate constants depend on the location of the hydroxyl group (primary, secondary, and tertiary) and the number of available H-atoms adjacent to the OH group for water elimination. The position of the H-atoms in the hydrocarbon backbone appears to be unimportant except for highly substituted molecules. From these correlations, we have derived k(isopropanol → propene + H2O) = 7.2 × 10(13) exp(-33000 K/T) s(-1). Comparison of experimental determination with theoretical calculations for this dehydration, and those for ethanol show deviations of the same magnitude as for isobutanol. Systematic differences between experiments and theoretical calculations are common.

Rehydration properties of precooked whole beans (Phaseolus vulgaris) dehydrated at room temperature

The aim of this research was to study the rehydration behavior of precooked whole beans (Phaseolus vulgaris) dehydrated at room temperature (25°C) and soaked in water at three temperatures (40, 60, and 80°C). The water absorption of the beans was determined by the gain in weight. The dehydration kinetic at room temperature of the beans was adequately described by the Logarithmic model (R 2 = 0.9989) with a dehydration rate constant (k d) value of 1.96 × 10−3 min−1. A semi-empirical first-order differential equation adequately described the rehydration characteristics of precooked whole bean under the experimental conditions (R 2 = 0.9934–0.9976). The rehydration rate constant (k r) increased from 6.10 × 10−2 to 12.14 × 10−2 min−1 with an increase in the temperature from 40 to 80°C. The temperature dependence of k r was well described by Arrhenius-type equation (R 2 = 0.8785), with estimated activation energy of 16.03 kJ mol−1 K−1.

Influence of Air Temperature and Humidity on Dehydration Equilibria and Kinetics of Theophylline.

The effect of hygrothermal conditions (air temperature and relative humidity) on the dehydration of theophylline monohydrate was investigated. Firstly, the equilibrium states of theophylline were investigated. The data from gravimetric analysis at constant temperature and humidity were reported as desorption isotherms. The PXRD analysis was used to identify the different polymorphic forms of theophylline: the monohydrate, the metastable anhydrate, and the stable anhydrate. Solid-solid phase diagrams for two processing times were proposed. Secondly, the dehydration kinetics were studied. The water content evolutions with time were recorded at several temperatures from 20°C to 80°C and several relative humidities from 4% to 50%. Different mathematical models were used to fit the experimental data. The spatially averaged solution of 2D Fickian transient diffusion equation best represented the water mass loss versus time experimental relationship. The dehydration rate constant was found to increase exponentially with air temperature and to decrease exponentially with air relative humidity.

Chemodynamics of Soft Nanoparticulate Complexes: Cu(II) and Ni(II) Complexes with Fulvic Acids and Aquatic Humic Acids

The dynamics of metal complexation by small humic substances (fulvic acid and aquatic humic acid, collectively denoted as “fulvic-like substance”, FS) are explored within the framework of concepts recently developed for soft nanoparticulate complexants. From a comprehensive collection of published equilibrium and dissociation rate constants for CuFS and NiFS complexes, the association rate constant, ka, is determined as a function of the degree of complexing site occupation, θ. From this large data set, it is shown for the first time that ka is independent of θ. This result has important consequences for finding the nature of the rate limiting step in the association process. The influence of electric effects on the rate of the association process is described, namely (i) the accelerating effect of the negatively charged electrostatic field of FS on the diffusion of metal ions toward it, and (ii) the extent to which metal ions electrostatically accumulate in the counterionic atmosphere of FS. These processes are discussed qualitatively in relation to the derived values of ka. For slowly dehydrating metal ions such as Ni(H2O)6 2+ (dehydration rate constant, kw), ka is expected to derive straight from kw. In contrast, for rapidly dehydrating metal ions such as Cu(H2O)6 2+, transport limitations and electric effects involved in the formation of the precursor outer-sphere associate appear to be important overall rate-limiting factors. This is of great significance for understanding the chemodynamics of humic complexes in the sense that inner-sphere complex formation would not always be the (sole) rate limiting step.

Conversion of carbohydrates and lignocellulosic biomass into 5-hydroxymethylfurfural using AlCl3·6H2O catalyst in a biphasic solvent system

AlCl3·6H2O in a biphasic medium of water/tetrahydrofuran (THF) is effective for the synthesis of 5-hydroxymethylfurfural (HMF) from glucose-based carbohydrates. For glucose, an HMF yield of 61% was achieved in 10 min at 160 °C under microwave heating. The reaction time profile revealed the intermediacy of fructose en route to HMF with a dehydration rate constant that is approximately 4 times that of glucose isomerization to fructose. Addition of NaCl did not increase HMF yields significantly but it diminished lactic acid formation. Disaccharides (maltose and cellobiose) and starch gave good yields of HMF. However, cellulose required a higher temperature (180 °C) and longer reaction time (30 min) to give a modest yield of 37%. Several lignocellulosic biomass variants (corn stover, pine wood, grass, and poplar) were investigated with the AlCl3·6H2O biphasic system. The yields of HMF were modest (20–35%) but high concurrent yields of furfural were observed (51–66%). The described AlCl3·6H2O–NaCl–H2O/THF biphasic medium has potential because it is economic, nontoxic, and it exhibits fast kinetics (10 min) under microwave heating.

Electric Relaxation Processes in Chemodynamics of Aqueous Metal Complexes: From Simple Ligands to Soft Nanoparticulate Complexants

The chemodynamics of metal complexes with nanoparticulate complexants can differ significantly from that for simple ligands. The spatial confinement of charged sites and binding sites to the nanoparticulate body impacts on the time scales of various steps in the overall complex formation process. The greater the charge carried by the nanoparticle, the longer it takes to set up the counterion distribution equilibrium with the medium. A z+ metal ion (z > 1) in a 1:1 background electrolyte will accumulate in the counterionic atmosphere around negatively charged simple ions, as well as within/around the body of a soft nanoparticle with negative structural charge. The rate of accumulation is often governed by diffusion and proceeds until Boltzmann partition equilibrium between the charged entity and the ions in the medium is attained. The electrostatic accumulation proceeds simultaneously with outer-sphere and inner-sphere complex formation. The rate of the eventual inner-sphere complex formation is generally controlled by the rate constant of dehydration of the metal ion, k(w). For common transition metal ions with moderate to fast dehydration rates, e.g., Cu(2+), Pb(2+), and Cd(2+), it is shown that the ionic equilibration with the medium may be the slower step and thus rate-limiting in their overall complexation with nanoparticles.

Hyperaromatic Stabilization of Arenium Ions: Acid-Catalyzed Dehydration of 2-Substituted 1,2-Dihydro-1-naphthols

Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = -0.49 - 8.8σ(I), with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σ(I) and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C-H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that k(cis)/k(trans) rate ratios for substituents (Me, 8.4; Bu(t), 12.7; Ph, 3.8; NH(3)(+), 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N(3)) or suspected (EtSO, EtSO(2)) of stabilizing the cation by a π or σ neighboring group effect. The good Taft correlation is taken to indicate that cis substuents are reacting normally, differentiated only by their inductive effects. The slower reactions of the trans isomers are the judged to be "abnormal". This is confirmed by comparing effects of cis and trans β-OH substituents on the reactivities of dihydro phenols, naphthols, and phenanthrols. Whereas k(H)/k(OH) for cis substituents varies by less than 8-fold and is consistent with the influence of an inductive effect of the OH group (k(H)/k(OH) ≈ 2000), k(H)/k(OH) for the trans substituents varies by 3 orders of magnitude, reflecting the additional influence of the lesser hyperconjugating ability of a C-OH bond compared to a C-H bond. The magnitude and variation of this difference is consistent with C-H hyperconjugation conferring aromatic character on the arenium ions.

Catalytic consequences of acid strength in the conversion of methanol to dimethyl ether

The effects of acid identity on CH 3OH dehydration are examined here using density functional theory (DFT) estimates of acid strength (as deprotonation energies, DPE) and reaction energies, combined with rate data on Keggin polyoxometalate (POM) clusters and zeolite H-BEA. Measured first-order ( k mono) and zero-order ( k dimer) CH 3OH dehydration rate constants depend exponentially on DPE for POM clusters; the value of k mono depends more strongly on DPE than k dimer does. The chemical significance of these rate parameters and the basis for their dependences on acid strength were established by using DFT to estimate the energies of intermediates and transition states involved in elementary steps that are consistent with measured rate equations. We conclude from this treatment that CH 3OH dehydration proceeds via direct reactions of co-adsorbed CH 3OH molecules for relevant solid acids and reaction conditions. Methyl cations formed at ion-pair transition states in these direct routes are solvated by H 2O and CH 3OH more effectively than those in alternate sequential routes involving methoxide formation and subsequent reaction with CH 3OH. The stability of ion-pairs, prevalent as intermediates and transition states on solid acids, depends sensitively on DPE because of concomitant correlations between the stability of the conjugate anionic cluster and DPE. The chemical interpretation of k mono and k dimer from mechanism-based rate equations, together with thermochemical cycles of their respective transition state formations, show that similar charge distributions in the intermediate and transition state involved in k dimer cause its weaker dependence on DPE. Values of k mono involve uncharged reactants and the same ion-pair transition state as k dimer; these species sense acid strength differently and cause the larger effects of DPE on k mono. Confinement effects in H-BEA affect the value of k mono because the different sizes and number of molecules in reactants and transition states selectively stabilize the latter; however, they do not influence k dimer, for which reactants and transition states of similar size sense spatial constraints to the same extent. This combination of theory and experiment for solid acids of known structure sheds considerable light on the relative contributions from solvation, electrostatic and van der Waals interactions in stabilizing cationic transition states and provides predictive insights into the relative contributions of parallel routes based on the size and charge distributions of their relevant intermediates and transition states. These findings also demonstrate how the consequences of acid strength on measured turnover rates depend on reaction conditions and their concomitant changes in the chemical significance of the rate parameters measured. Moreover, the complementary use of experiment and theory in resolving mechanistic controversies has given predictive guidance about how rate and equilibrium constants, often inextricably combined as measured rate parameters, individually depend on acid strength based on the magnitude and spatial distributions of charges in reactants, products and transition states involved in relevant elementary steps. The unique relations between k mono, k dimer and DPE developed here for CH 3OH dehydration can be applied in practice to assess the acid strength of any solid acid, many of which have unknown structures, preventing reliable calculations of their DPE by theory.

Functional assessment of the strength of solid acid catalysts

Abstract We describe here a rigorous method to estimate the deprotonation energy (DPE) and acid strength for solid Bronsted acids with uncertain structure using rate constants for reactions involving cationic transition states. The approach exploits relations between turnover rates for dehydration and isomerization reactions and DPE values on Keggin polyoxometalates and H-BEA solids with known structures. These relations are used to estimate the strength of acid sites in SO 4 –ZrO 2 (SZr), WO x –ZrO 2 (WZr), and perfluorosulfonic resins (SAR) from their alkanol dehydration and alkane isomerization rate constants. Alkanol dehydration and alkane isomerization proceed via pathways independent of acid identity and are limited by steps involving late transition states. Turnover rates (per accessible acid sites measured by titration during catalysis) are related to the relevant rate constants and are used to estimate DPE values for SZr, WZr, and SAR. Isomerization data estimate DPE values of 1110 kJ mol −1 and 1120 kJ mol −1 for SZr and WZr, respectively, while dehydration rate data lead to slightly higher values (1165 kJ mol −1 and 1185 kJ mol −1 ). The DPE value for SAR was 1154 kJ mol −1 from dehydration reactions, but diffusional constraints during reactions of non-polar alkanes precluded isomerization rate measurements. SZr and SAR contain stronger acid sites than zeolites (1185 kJ mol −1 ), but weaker than those in H 3 PW 12 O 40 and H 4 SiW 12 O 40 (1087 kJ mol −1 and 1105 kJ mol −1 ). Acid sites present in WZr during alkane isomerization are stronger than those present in zeolites, but these become similar in strength in the polar environment prevalent during dehydration catalysis. These effects of reaction media (and treatment protocols) reflect differences in the extent of dehydroxylation of catalytic surfaces. OH groups remaining after dehydroxylation are stronger acid sites because of a concomitant decrease in electron density in the conjugate anion and the formation of Bronsted–Lewis acid conjugate pairs. The method proposed and used here probes acid strength (as DPE) on sites of uncertain structure and within solvating media inherent in their use as catalysts. It can be used for any Bronsted acid or reaction, but requires reactivity-DPE relations for acids of known structure, the mechanistic interpretations of rates, and the measurement of accessible protons during catalysis. The resulting DPE values provide a rigorous benchmark for the structural fidelity of sites proposed for acids with uncertain structure, a method to assess the consequences of the dynamic nature of active sites in acid catalysis, and a connection between theory and experiment previously unavailable.