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3.3.1 Offer of a hypothesis of the mechanism of hydrogenolysis of cellulose

Studying of kinetics and the reaction mechanism is of great importance, both from the point of view of the theory, and from the point of view of practice since allows to present in the best way a pattern of interaction of reacting molecules and, finally, to choose the optimal composition of the catalyst and a mode of conducting process.

At the same time it is necessary to note complexity of the decision of the yielded question. If the reaction kinetics can be described by means of the experimental curves showing changes of concentration of reactants in time under various conditions of experiment,
Representations about the reaction mechanism can be positioned only after the careful analysis of the big experimental material.

Determinative for chemical behaviour of a molecule of polyol (as well as for carbohydrates as a whole) is presence of the big number of the electronegative groups causing corresponding induction effects. Presence of the big number of hydroxyl groups impoverishes electronic density of carbons-carbon bonds of a molecule of polyalcohol, directs fractional positive charge at carbon atoms, result of that is the simplification of nucleophilic attack of a molecule and comparative ease of bond breaking S-S (fig. 3.13).

Drawing 3.13 - the Schema of bond breaking S-S

Considering questions of the mechanism of hydrogenolysis of polyalcohol it is necessary to consider necessarily the yielded simplification of bond breaking S-S from - for presence of the big number of hydroxyl groups. The central communication of a molecule of polyoxycompound appears the most weakened thus. Taking into consideration this circumstance, and also results of experimental researches on hydrogenolysis of cellulose and pure substances (glucose, sorbitol, xylitol, erythritol, glitserola, ethylene - and propylene glycol), it is possible to assume the schema of hydrogenolysis of the cellulose, presented on drawing 3.14. Process of hydrogenolysis of cellulose consists of two basic stages: hydrolysis of cellulose to glucose and its hydrogenolysis to poliolov. At temperatures 463 - 493 To hydrogenolysis S-S of communications starts to go intensively
Glucoses with formation of appreciable quantities glitserola and glycols. However in the conditions of process (478 To, 6 MPa H2) the basic part of formed glucose is hydrogenated to sorbitol that can be caused high enough value of fractional pressure of hydrogen at which the significant part of a surface of a composite is occupied by molecules H2. It is not excluded, that under such circumstances glucose molecules have possibility to contact to a composite surface only one end, possibly carbonyl group. As acknowledgement of the yielded assumption the selectivity increase on the lowest polyalcohol serves at temperature increase - at smaller fillings of a surface of a composite with hydrogen the glucose molecule can settle down along a composite surface that promotes passing of reaction of hydrogenolysis.

Drawing 3.14 - the Schema of hydrogenolysis of glucose on rutenijsoderzhashchih composites

Where k1 - a specific reaction rate gljukozasorbit;

Reaction rate k2-constant глюкоза→пропиленгликоль+глицерин;

Reaction rate k3-constant sorbsh^ksilit;

Reaction rate k4-constant сорбш^пропиленгликоль+глицерин;

Reaction rate k5-constant sorbsh^eritrit;

Reaction rate k6-constant sorbsh^etilenglikol;

Reaction rate k7-constant ksilsh^etilenglikol.

Sorbitol in the yielded conditions (as well as other polyalcohol) is steadier against hydrogenolysis in comparison with glucose that can speak, including, its chemical structure (absence weakening communications S-S σ, π - the linking resulting enolizatsii glucose). Therefore in the specified conditions sorbitol is a process main product. Nevertheless, the sorbitol part (especially with growth of temperature of process) is exposed to hydrogenolysis with formation of xylitol, erythritol, glitserola propylene - and ethylen glycol. The selectivity increase on the lowest polyalcohol is characteristic at temperatures and pressure more low and above optimum values. In case of the former it can be explained increase in intensity of hydrogenolysis of glucose, in the second case - sorbitol hydrogenolysis.

Process of hydrogenolysis of cellulose consists of two basic stages: hydrolysis of cellulose to glucose and its hydrogenolysis to poliolov (fig. 3.14). Pre-award studying has shown, that stages of all process the stage of hydrogenolysis of glucose is limiting. In this connection, for promotion of a hypothesis of the mechanism of hydrogenolysis, made kinetic experiments with pure glucose at a variation of time of reaction: 0, 10, 30, 60 and 90 minutes.

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A source: Filatova Anastasia Evgenevna. Physical and chemical bases of process of hydrogenolysis of cellulose. The dissertation on competition of a scientific degree of a Cand.Chem.Sci. Tver - 2016. 2016

More on topic 3.3.1 Offer of a hypothesis of the mechanism of hydrogenolysis of cellulose:

  1. Influence of a parity Ru/cellulose on conversion and selectivity of process of hydrogenolysis of cellulose
  2. 1.2.3 Hydrogenolysis of cellulose in polyalcohol
  3. 1.4 Kinetics of process of hydrogenolysis of cellulose
  4. 3.1.2 Temperature effect on conversion and selectivity of process of hydrogenolysis of cellulose
  5. Time effect of reaction to conversion and selectivity of process of hydrogenolysis of cellulose
  6. Influence of the maintenance of ruthenium in a composite on conversion and selectivity of process of hydrogenolysis of cellulose
  7. Research of process of hydrogenolysis of cellulose in the environment of subcritical water
  8. Influence of fractional pressure of hydrogen on conversion and selectivity of process of hydrogenolysis of cellulose
  9. 1.3 Composites cellulose used in the course of hydrogenolysis to polyalcohol
  10. Research of process of hydrogenolysis of cellulose
  11. Influence of type of the carrier of an active phase of a composite on conversion and selectivity of process of hydrogenolysis of cellulose
  12. Filatova Anastas Evgenevna. Physical and chemical bases of process of hydrogenolysis of cellulose. The dissertation on competition of a scientific degree of a Cand.Chem.Sci. Tver - 2016, 2016
  13. FILATOVA Anastas Evgenevna. PHYSICAL AND CHEMICAL BASES of PROCESS of HYDROGENOLYSIS of CELLULOSE. The dissertation AUTOABSTRACT on competition of a scientific degree of a Cand.Chem.Sci. Tver - 2016, 2016
  14. 4.2. A working hypothesis of the mechanism of increase of collective ability of reagent OPSK
  15. 3.3 Hypotheses of the mechanism and mathematical modelling of process of hydrogenolysis
  16. a research hypothesis.
  17. 1.2.4 Process of hydrogenolysis in ionic liquids
  18. 2.2.5 Promotion of a hypothesis
  19. a hypothesis ranzhirovannoj salientnosti R.Giory
  20. the Working hypothesis of research