Hydrothermal Reaction Kinetics and Pathways of Phenylalanine Alone and in Binary Mixtures

We examined the behavior of phenylalanine in high‐temperature water (HTW) at 220, 250, 280, and 350 °C. Under these conditions, the major product is phenylethylamine. The minor products include styrene and phenylethanol (1‐phenylethanol and 2‐phenylethanol), which appear at higher temperatures and longer batch holding times. Phenylethylamine forms via decarboxylation of phenylalanine, styrene forms via deamination of phenylethylamine, and phenylethanol forms via hydration of styrene. We quantified the molar yields of each product at the four temperatures, and the carbon recovery was between 80–100 % for most cases. Phenylalanine disappearance follows first‐order kinetics with an activation energy of 144±14 kJ mol −1 and a pre‐exponential factor of 10 12.4±1.4  min −1 . A kinetics model based on the proposed pathways was consistent with the experimental data. Effects of five different salts (NaCl, NaNO 3 , Na 2 SO 4 , KCl, K 2 HPO 4 ) and boric acid (H 3 BO 3 ) on phenylalanine behavior at 250 °C have also been elucidated. These additives increase phenylalanine conversion, but decrease the yield of phenylethylamine presumably by promoting formation of high molecular weight compounds. Lastly, binary mixtures of phenylalanine and ethyl oleate have been studied at 350 °C and three different molar concentration ratios. The presence of phenylalanine enhances the conversion of ethyl oleate and molar yields of fatty acid. Higher concentration of ethyl oleate leads to increased deamination of phenylethylamine and hydration of styrene. Amides are also formed due to the interaction of oleic acid/ethyl oleate and phenylethylamine/ammonia and lead to a decrease in the fatty acid yields. Taken collectively, these results provide new insights into the reactions of algae during its hydrothermal liquefaction to produce crude bio‐oil. High Temperature Water: Several products are quantified and a reaction network is developed for phenylalanine alone and in binary mixtures. This study has several implications to bio‐oil production during hydrothermal liquefaction of algae.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/93764/1/1743_ftp.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/93764/2/cssc_201200146_sm_miscellaneous_information.pd

Pyrolysis of a binary mixture of complex hydrocarbons: Reaction modeling

A reaction model was developed for the pyrolysis of binary mixtures of compounds whose reactions can be apportioned into three or fewer parallel chains coupled by chain transfer. A representative application of this model to the pyrolysis of mixtures of the asphaltene model compounds n-pentadecylbenzene (PDB) and n-tridecylcyclohexane (TDC) illustrates its utility. The model, along with its associated rate constant estimates, quantitatively correlated the experimental temporal variations of the product yields from the individual pyrolyses of PDB and TDC. Model results for binary mixtures of PDB and TDC showed that the pyrolysis rates for both compounds were accelerated by the addition of the second compound. For instance, the pyrolysis of PDB at an initial concentration of 10-4M proceeded at a 36% higher rate in the presence of TDC at an equal concentration. The rate of TDC pyrolysis at [TDC] = 10-4M, on the other hand, increased more than nine-fold upon the addition of PDB at a concentration of 10-4M. Unlike the accelerated reaction rates, product selectivities were largely insensitive to the presence of the second compound. These results are consistent with and can be explained on the basis of the influence of concentration on the relative kinetics of bimolecular and unimolecular propagation, chain transfer, and termination steps. The model results also lead to the identification of quantitative criteria for determining when an added compound can act as a rate accelerator. Finally, this study permits speculation into the effects of interactions between alkylaromatic and alkylnaphthenic moieties in asphaltenes on the pyrolysis pathways, products, and kinetics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28925/1/0000762.pd

Detailed chemical kinetics model for supercritical water oxidation of C 1 compounds and H 2

A detailed chemical kinetics model comprising 148 reversible elementory reactions for the supercritical water oxidation (SCWO) of methane, methanol, carbon monoxide and hydrogen was developed. Rate constants were taken from previous critical evaluations. The Lindemann model, at times modified with a broadening parameter, was used to account for the effects of pressure on the kinetics of unimolecular reactions. Model predictions were compared with published experimental SCWO kinetics data for 450–650°C and 240–250 atm. The model correctly predicted global reaction orders for all four fuels to within their uncertainties. In addition, the model correctly predicted that the global reaction orders for O 2 during methanol and hydrogen oxidation were essentially zero, and that the O 2 concentration had the greatest effect on the methane oxidation rate. The pseudo-first-order rate constants predicted by the model were consistently higher than the experimental values, but the global activation energies were predicted correctly for methane oxidation and for CO and H 2 oxidation at high temperatures. The model's predictions generally became worse as the temperature decreased toward the critical point of water. A sensitivity analysis revealed that fewer than 20 elementaty reactions largely controlled the oxidation kinetics for the compounds studied. Nearly half of these reactions involved HO 2 , which is an important free radical for SCWO. Quantitative agreement with the experimental methane conversions was obtained by adjusting the preexponential factors for three elementary reactions within their uncertainties. It could also be obtained by using the JANAF value (0.5 kcal/mol) for the standard heat of formation of HO 2 , but this value is lower than other recently recommended values.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37434/1/690410806_ftp.pd

A reaction network model for phenol oxidation in supercritical water

Dilute aqueous solutions of phenol were oxidized in a flow reactor at 420, 440, 460 and 480°C at 250 atm. Phenol disappearance kinetics followed the trends exhibited by previously published data obtained at T < 420°C. By merging the two sets of data, a global rate low for phenol disappearance kinetics valid between 380 and 480°C was determined to be rate = 10 2.34 exp( −12.4/RT) [φOH] 0.85 [O 2 ] 0.50 [H 2 O] 0.42 . Undesired multiring products, whose formation was reported previously at the lower temperatures, continued to form in high selectivities at these higher temperatures. Reaction products were classified into three categories: dimers, gases, and a remainder that included products from ring-opening reactions. A global reaction network that describes the transformation of phenol into these product groups was developed. Steps in the network are: parallel oxidation paths for phenol that from dimers and ring-opening and other products, secondary decomposition of dimers of ring-opening and other products, and oxidation of the ring-opening and other products to carbon oxides. The experimental products yields were used to determine optimal values for the reaction orders and rate constants for each step in the network. This quantitative reaction model shows that dimerization is the dominant primary path for phenol consumption. High temperatures and long residence times reduce the concentration of dimers in the reactor effluent and maximize the gas yield. High oxygen concentrations also increase the gas yield. The quantitative reaction network model is consistent with previously published product yields for T = 380 – 420°C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37433/1/690410805_ftp.pd

Reactions of polycyclic alkylaromatics--VI. Detailed chemical kinetic modeling

We developed a detailed chemical kinetics model for the pyrolysis of long-chain polycyclic n-alkylarenes based on a general free-radical mechanism. The model accounts for the two major primary pathways in the pyrolysis network of polycyclic alkylaromatics. Using 1-dodecylpyrene (DDP) as an example, we show that the model qualitatively predicted the effects of time, temperature, and concentration on the product molar yields and the reaction kinetics. The model also predicted the autocatalytic kinetics associated with the cleavage of the aryl---alkyl bond. The model results showed that radical hydrogen transfer was the dominant hydrogenolysis mechanism during all but the very initial stages of the reaction when reverse radical disproportionation dominated. A sensitivity analysis revealed that reactions involving [alpha]-DDP radicals where the most important in determining the reaction kinetics and the product selectivities.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31912/1/0000865.pd

Reactions of polycyclic alkylaromatics: Structure and reactivity

A family of alkyl-substituted polycyclic aromatic hydrocarbons was pyrolyzed in microbatch reactors at temperatures between 350°C and 425°C. A general pyrolysis network was deduced for these compounds, and it comprised two major and one minor parallel pathways. The first major pathway resulted in products analogous to the major products observed from alkylbenzene pyrolysis. The second major pathway led to products via the cleavage of the strong aryl-alkyl C[bond]C bond. The third pathway led to small amounts of products, presumably through cyclization and condensation reactions. The relative importance of the two major pathways varied for the different compounds. The rates of aryl-alkyl bond cleavage differed for the different compounds, and these rates were quantitatively related to the compounds' localization energies through Dewar reactivity numbers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37418/1/690371104_ftp.pd

Kinetics of phenol oxidation in supercritical water

Aqueous solutions of phenol were oxidized in a flow reactor at temperatures between 300 and 420°C (0.89 < T r < 1.07) and pressures from 188 to 278 atm (0.86 < P r < 1.27). These conditions included oxidations in both near-critical and supercritical water. Reactor residence times ranged from 1.2 to 111 s. The initial phenol concentrations were between 50 and 330 ppm by mass, and the initial oxygen concentrations ranged from 0 to 1,100% excess. The oxidation experiments covered essentially the entire range of phenol conversions. Analysis of the kinetics data for phenol disappearance using a combination of the integral method and the method of excess revealed that the reaction was first order in phenol and 1/2 order in oxygen, and influenced by pressure. The global reaction order for water was taken to be nonzero, and the global rate constant was assumed to be independent of pressure so that the only effect of pressure was to alter the water concentration and hence the reaction rate. This approach led to a global reaction rate law that was 0.7 order in water and had a rate constant with an activation energy of 12.4 kcal/mol. The implications of these rate laws to the design of a commercial supercritical water oxidation reactor are also explored.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37422/1/690380302_ftp.pd

Reactions of polycyclic alkylaromatics: 5. pyrolysis of methylanthracenes

1-, 2- and 9-methylanthracene were pyrolyzed neat at temperatures between 350 and 450°C for batch holding times up to 300 min. The pyrolysis proceeded through three parallel primary reaction pathways: one led to anthracene via demethylation; the second to dimethylanthracenes through methyl addition; and the third to methyl-9,10-dihydroanthracenes through hydrogenation. The relative importance of these three paths varied for different methylanthracene isomers. The presence of these primary pathways can be rationalized in terms of recently elucidated hydrogentransfer mechanisms and other aspects of the developing free-radical chemistry of polycyclic alkylarenes. The demethylation rate at 400°C for the methylanthracenes and seven other methylarenes was correlated with Dewar reactivity numbers, which provide a measure of the localization energy, as ln rate (arene yield/min) = 3.7– 7.1 N ts , where N ts is the Dewar reactivity number for the perpheral aromatic carbon atom bearing the methyl substituent. This correlation may be useful in molecular-based reaction models for the conversion of heavy hydrocarbon resources such as coals and heavy oils.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37427/1/690390812_ftp.pd

Biodiversity improves the ecological design of sustainable biofuel systems

For algal biofuels to become a commercially viable and sustainable means of decreasing greenhouse gas emissions, growers are going to need to design feedstocks that achieve at least three characteristics simultaneously as follows: attain high yields; produce high quality biomass; and remain stable through time. These three qualities have proven difficult to achieve simultaneously under the ideal conditions of the laboratory, much less under field conditions (e.g., outdoor culture ponds) where feedstocks are exposed to highly variable conditions and the crop is vulnerable to invasive species, disease, and grazers. Here, we show that principles from ecology can be used to improve the design of feedstocks and to optimize their potential for “multifunctionality.” We performed a replicated experiment to test these predictions under outdoor conditions. Using 80 ponds of 1,100 L each, we tested the hypotheses that polycultures would outperform monocultures in terms of the following functions: biomass production, yield of biocrude from biomass, temporal stability, resisting population crashes, and resisting invasions by unwanted species. Overall, species richness improved stability, biocrude yield, and resistance to invasion. While this suggests that polycultures could outperform monocultures on average, invasion resistance was the only function where polycultures outperformed the best single species in the experiment. Due to tradeoffs among different functions that we measured, no species or polyculture was able to maximize all functions simultaneously. However, diversity did enhance the potential for multifunctionality—the most diverse polyculture performed more functions at higher levels than could any of the monocultures. These results are a key finding for ecological design of sustainable biofuel systems because they show that while a monoculture may be the optimal choice for maximizing short‐term biomass production, polycultures can offer a more stable crop of the desired species over longer periods of time.We tested the hypothesis that multi‐species polycultures of algae can be designed to improve performance in biofuel cultivation and outperform the best single species. Our experiment of 80 open ponds (1,100 L each) showed that polycultures can simultaneously improve crop stability, bio‐crude yield, and resistance to invasive algae ‐ three characteristics that have been difficult to attain under field conditions yet are essential for biofuels to become part of the renewable energy portfolio.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146341/1/gcbb12524_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146341/2/gcbb12524-sup-0001-Supinfo.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146341/3/gcbb12524.pd

Fugacity coefficients for free radicals in dense fluids: HO 2 in supercritical water

The fugacity coefficients of the hydroperoxyl radical in supercritical water are estimated through molecular simulations. A potential function for the radical is first derived from ab initio self-consistent field molecular orbital calculations at the unrestricted Hartree-Fock level and from data in the literature. Molecular dynamics simulations of the hydroperoxyl radical are then performed in supercritical water and the fugacity coefficient of the radical is calculated by the free-energy perturbation method using the dynamic coupling parameter window-modification technique. The values of the fugacity coefficients at 773 K differ from unity. This methodology facilitates the incorporation of thermodynamic nonidealities in mechanism-based kinetic models for free-radical reactions in supercritical water.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/37444/1/690430517_ftp.pd