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The time dependence of the gaseous unimolecular decomposition of the jet-cooled adduct ion, Ni+-OC(CH3)2, was monitored through selective detection of the Ni+CO fragment ion. Various resolved amounts of energy in the range 15600-18800 cm(-1) were supplied to initiate the dissociation reaction through absorption of laser photons by the title molecular complex. First-order rate constants, k(E), ranged from 113000 to 55000 s(-1) and decreased with decreasing amounts of internal excitation. The energy used to initiate the reaction is well below that required to fragment C-C sigma bonds and indicates the necessity of the Ni+ cation to induce bond activation and fragmentation. These measurements are carried out in a unique apparatus and represent the first direct kinetic study of such catalytic type reactions.

Rate constants for the low-energy Ni(+)-assisted C-C bond cleavage reaction of deuterium-labeled acetone have been acquired under jet-cooled conditions in the gas phase. The energies used to initiate the dissociative reactions of the precursor complex ion Ni(+)(d(6)-Ac) are well below that required to cleave C-C sigma-bonds in isolated organic molecules. The rate constants are compared to those acquired previously for the lighter Ni(+)(h(6)-Ac) isotope and result in a substantial kinetic isotope effect (k(H)/k(D) approximately 5.5). Arguments are made that implicate isomerization leading to C-C bond coupling as the rate-limiting step (not C-C sigma-bond activation) in the dissociative reaction.

Rate constants for the low-energy Ni+-assisted dissociative reaction of acetaldehyde have been measured under jet-cooled conditions in the gas phase. The rate constants are acquired through monitoring the time dependence of fragment Ni+CO formation. The decomposition of the precursor Ni+−acetaldehyde cluster ion proceeds via consecutive, parallel reaction coordinates that originate with the Ni+-assisted cleavage of either a C−C or an aldehyde C−H bond. The energies used to initiate these reactions are well below that required to cleave σ-bonds in the isolated acetaldehyde molecule. Direct measurement of the reaction kinetics over a range of energies indicates that the rate-limiting step in the dissociative mechanism changes at cluster ion internal energies = 17 200 ± 400 cm−1. Arguments are presented that this energy marks the closure of the dissociative coordinate that initiates with C−H σ-bond activation and thus provides a measure of the activation energy of this dissociative pathway.

The unimolecular decomposition kinetics of the jet-cooled Ni(+)-butanone cluster ion has been monitored over a range of internal energies (16000-18800 cm⁻¹). First-order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored in a custom instrument and yields similar valued rate constants at a common ion internal energy. The decomposition reaction is proposed to proceed along two parallel reaction coordinates. Each dissociative pathway is rate-limited by the initial Ni(+) oxidative addition into either the C-CH₃ or C-C₂H₅ σ-bond in the butanone molecule. Ratios of integrated product ion intensities as well as the measured rate constants are used to determine values for each σ-bond activation rate constant. The lowest energy measurement presented in this study occurs when the binary complex ion possesses an internal energy of 16000 cm⁻¹. Under this condition, the Ni(+) assisted decomposition of the butanone molecule is rate limited by k(act)(C-C₂H₅) = (0.92 ± 0.08) × 10⁵ s⁻¹ and k(act)(C-CH₃) = (0.37 ± 0.03) × 10⁵ s⁻¹. The relative magnitudes of the two rate constants reflect the greater probability for reaction to occur along the C-C₂H₅ σ-bond insertion pathway, consistent with thermodynamic arguments. DFT calculations at the B3LYP/6-311++G(d,p) level of theory suggest the most likely geometries and relative energies of the reactants, intermediates, and products.

EXECUTIVE SUMMARYThis project sought to address six objectives, outlined below. The objectives were met through the completion of ten tasks. 1) Solidify the theoretical understanding of the binary CO2/H2O system at reaction temperatures and pressures. The thermodynamics of pH prediction have been improved to include a more rigorous treatment of non-ideal gas phases. However it was found that experimental attempts to confirm theoretical pH predictions were still off by a factor of about 1.8 pH units. Arrhenius experiments were carried out and the activation energy for carbonic acid appears to be substantially similar to sulfuric acid. Titration experiments have not yet confirmed or quantified the buffering or acid suppression effects of carbonic acid on biomass.2) Modify the carbonic acid pretreatment severity function to include the effect of endogenous acid formation and carbonate buffering, if necessary. It was found that the existing severity functions serve adequately to account for endogenous acid production and carbonate effects.3) Quantify the production of soluble carbohydrates at different reaction conditions and severity. Results show that carbonic acid has little effect on increasing soluble carbohydrate concentrations for pretreated aspen wood, compared to pretreatment with water alone. This appears to be connected to the release of endogenous acids by the substrate. A less acidic substrate such as corn stover would derive benefit from the use of carbonic acid. 4) Quantify the production of microbial inhibitors at selected reaction conditions and severity. It was found that the release of inhibitors was correlated to reaction severity and that carbonic acid did not appear to increase or decrease inhibition compared to pretreatment with water alone. 5) Assess the reactivity to enzymatic hydrolysis of material pretreated at selected reaction conditions and severity. Enzymatic hydrolysis rates increased with severity, but no advantage was detected for the use of carbonic acid compared to water alone. 6) Determine optimal conditions for carbonic acid pretreatment of aspen wood. Optimal severities appeared to be in the mid range tested. ASPEN-Plus modeling and economic analysis of the process indicate that the process could be cost competitive with sulfuric acid if the concentration of solids in the pretreatment is maintained very high (~50%). Lower solids concentrations result in larger reactors that become expensive to construct for high pressure applications.i TABLE OF CONTENTS Project Summary 1Task 1 ii PROJECT SUMMARY Task 1a: Xylan and Xylose were hydrolysed in 1% H 2 SO 4 at 121 C for varying reaction times. Samples were analyzed with high performance anion exchange (HPAE) and an ultra-violet spectrophotometer. Peak areas for xylan oligomers were integrated by completing a mass balance on samples of varying degrees of hydrolysis. This yielded an appropriate calibration for peaks representing oligomer concentrations, and confirmed theoretical expectations that the area of oligomer peaks was propor...

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