Thermochemical Pathways

Ami, S. 2004. Hydrogen-rich gas production from biomass via thermochemical pathways. Energy Edu Sei Teehnol 13 47-54. 

In photochemical reactions, the population of excited states of different orbital origins can result in quite different reactivity patterns. Therefore, reaction products may occur, which are not accessible at all in thermochemical pathways. Especially in organometallic and coordination compounds, the primary photoproducts obtained are not always resulting from the lowest-lying excited state levels. Wavelength-selective excitation may then be exploited to channel the product formation process and to control a possible branching between different reactivity patterns. 

Dutta, A., Takaadge, M., Hensley, J. et al. (2011) Process Design and Economics for Conversion of Lignocellulosic Biomass to Ethanol Thermochemical Pathway by Indirect Gasification and Mixed Alcohol Synthesis, NREL/TP-5100-51400. Available at http //www.nrel.gov/biomass/pdfs/51400. pdf (accessed on 29 August 2015). 

From Figure 11.8, it can be seen that Alkane, is produced from biomass in the conversion pathway sequence of pyrolysis and Fischer-Tropsch processes 1 and 2 followed by fractional distillation of alkanes, which are all thermochemical pathways. It is worth pointing out that specific separation processes that suit the identified product can be chosen and included in the integrated biorefinery to refine and separate the final product from by-products. Hence, separation processes for alkanes are chosen based on the results of the product design identified in stage 1 of the methodology. The performance of the separation processes is then taken into consideration in identifying the product yield and economic potential of the overall conversion pathway. 

From Figure 11.9, it can be seen that alcohols are produced from biomass in the conversion pathway sequence of ammonia explosion, organosolv separation, dehydration of sugars, hydrogenation of furfural and hydrogenation of TUFA 1. Alkane,- is produced from fractional distillation of alkanes, which are produced from pyrolysis of biomass, followed by Fischer-Tropsch process 2 together with dehydration of alcohols 2. The selected conversion pathways consist of both biochemical and thermochemical pathways. The comparison of the results generated for scenario 1 and 2 is summarised in Table 11.13. 

Direct observation of the electrocyclic cleavage of the cyclopropyl anion has been reported for one case, (Equation 6.8) the observed conrotatory mode is the predicted thermochemical pathway. An isoelectronic system has provided... 

The alternative + 4a] thermochemical pathway has been invoked tentatively in a few cases, as have the [,2 + 4 ] and [ 2a +, f4s] interactions in certain intramolecular photochemical isomerizations. None of these interpretations are without some form of ambiguity, but usually because of incomplete experimental data. 

Polarizabilities and hyperpolarizabUities Thermochemical properties Reaction pathways... 

Thermodynamically-Controlled Reaction. A reaction the product ratio for which is determined solely by the relative thermochemical stabilities of the different products (product formation must be reversible, or separate low-energy pathways interconnecting the products must exist). 

The possibility of predicting solid state reactivity from calculated thermochemical data was first addressed with ketodiesters 65a-e, which were substituted with methyl groups to vary the extent of the RSE in the radicals 65-BRl - 65-BR3 involved along the photodecarbonylation pathway (Scheme 7.19). " All ketones reacted in solution to give complex product mixtures from radical combination (66a-e) and disproportionation processes. Calculations revealed RSEs of 8.9 kcal/mol, 15.1 kcal/mol, and 19.8 kcal/mol for radicals 65-BRl (primary enol radical), 65-BR2 (secondary enol radical), and 65-BR3 (tertiary enol radical), respectively. In the... 

The situation is somewhat better for the gas-phase chemistry of isolated transition-metal ions or complexes, and this area of research has received a lot of attention in the past. On the experimental side, comprehensive mass-spectrometric techniques allow for an explicit measurement of thermochemical and kinetic parameters of reactants, intermediates, and products occurring along the reaction pathways. These data can be obtained without the influence of ligands, counter ions, solvents etc. which would be a highly complicated enter-... 

Let us now turn to the description of the reaction pathways. Figure 13-5 schematically depicts the shapes of the corresponding potential energy curves for the sextet and quartet spin-states and Table 13-11 contains the thermochemical information obtained at different levels of theory. 

However, these are highly endothermic processes and may therefore be disregarded. (These reactions are also restricted by orbital symmetry considerations.) Thus, combining electronic and thermochemical evaluations, the preferred reaction of 48 should lead to 8. The picture that emerges from this analysis is that 47 will rearrange to 8 with 48 as a possible reaction intermediate. Overall, the reaction pathway 7 - 47 - 8 is predicted. 

The enthalpy of formation of a compound is a so-called thermodynamic state function, which means that the value depends only on the initial and final states of the system. When the formation of crystalline NaCl from the elements is considered, it is possible to consider the process as if it occurred in a series of steps that can be summarized in a thermochemical cycle known as a Born-Haber cycle. In this cycle, the overall heat change is the same regardless of the pathway that is followed between the initial and final states. Although the rate of a reaction depends on the pathway, the enthalpy change is a function of initial and final states only, not the pathway between them. The Born-Haber cycle for the formation of sodium chloride is shown as follows ... 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

Perhaps the most interesting of the inner-sphere pathways are those which result in the net transfer of an oxygen atom. The factors governing the viability of this pathway are still speculative. For cobalt-02 adducts, thermochemical considerations suggest that oxygen atom transfer should be accompanied by electron transfer in the reverse direction. Similarly, this pathway should be enhanced for MO2 complexes in which the metal... 

This criterion was originally established for the fragmentation of alkanes by Stevenson [18] and was later demonstrated to be generally valid. [19,20] The rule can be rationalized on the basis of some ion thermochemical considerations (Fig. 6.4). Assuming no reverse activation barrier, the difference in thermodynamic stability as expressed in terms of the difference of heats of formation of the respective products determines the preferred dissociation pathway ... 

However, if there is no other exothermic pathway available, all the intermediate can do is revert to reactants. In such a situation, the more favorable the addition process is, the more internal energy is in the intermediate and the faster the reverse dissociation will occur. The better the addition is thermochemically, the worse it is kinetically. For the proton transfer pathway (6b), the neutral methanol product can carry off the excess energy as translational energy (and capture some of it in the newly formed OH bond) and the reaction proceeds. 

Teo ED, Brandon NP, Vos E, Kramer GJ (2005) A critical pathway energy efficiency analysis of the thermochemical UT-3 cycle, Int J Hydrogen Energy 30 559-564... 

Surface Bond Energies Thermochemical data are very scant in the area of oxygen chemisorption (57). These data would be of great value for interpreting spectroscopic and kinetic data and for the analysis of reaction mechanisms. The vast majority of the available data are for low oxidation state systems (55). Although calorimetry offers a means for direct measurements, for analysis of reaction pathways it is necessary to have detailed values for many types of species (M-OH, MO-H, M-OR, M-R, M-O, M-H), and these are usually... 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

MINDO/3 calculations of cyclobutane formation reveal a large singlet-triplet splitting of 10 kcal - mol-1 in the cisoid butyl 1,4-diradical, which might be due to a radical-radical interaction.79 The results of a more recent SINDOl calculation are also in agreement with a nonconcerted cleavage of cyclobutane via a diradical pathway.80 Additional quantitative evidence for the 1,4-diradicals has been obtained by thermochemical studies.81... 

In addition to experiments, a range of theoretical techniques are available to calculate thermochemical information and reaction rates for homogeneous gas-phase reactions. These techniques include ab initio electronic structure calculations and semi-empirical approximations, transition state theory, RRKM theory, quantum mechanical reactive scattering, and the classical trajectory approach. Although still computationally intensive, such techniques have proved themselves useful in calculating gas-phase reaction energies, pathways, and rates. Some of the same approaches have been applied to surface kinetics and thermochemistry but with necessarily much less rigor. 

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