Kinetic Evidence of an Apparent Negative Activation Enthalpy in an Organocatalytic Process

Xiao, Han; Lee, Richmond; Chen, Tao; Luo, Jie; Lü, Yixin; Huang, Kuo‐Wei

doi:10.1038/srep02557

Cited by 37 publications

(24 citation statements)

References 58 publications

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“…It is not common for the enthalpy of activation to become negative. This unusual type of kinetics has been observed in materials characterized by complex energy landscapes (50)(51)(52)(53), and negative activation parameters are observed in zeolite materials where confinement is very strong (54,55). The enthalpy-entropy compensation temperature, T comp (the slope of the linear relation) for the nanocomposites and thin films, is close to the estimated VFT temperature T 0 of their pure systems, as found in previous experimental studies of β-relaxation in GF liquids (56)(57)(58).…”

Section: Resultssupporting

confidence: 85%

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Betancourt

Hanakata

Starr

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

191

306

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The study of glass formation is largely framed by semiempirical models that emphasize the importance of progressively growing cooperative motion accompanying the drop in fluid configurational entropy, emergent elasticity, or the vanishing of accessible free volume available for molecular motion in cooled liquids. We investigate the extent to which these descriptions are related through computations on a model coarse-grained polymer melt, with and without nanoparticle additives, and for supported polymer films with smooth or rough surfaces, allowing for substantial variation of the glass transition temperature and the fragility of glass formation. We find quantitative relations between emergent elasticity, the average local volume accessible for particle motion, and the growth of collective motion in cooled liquids. Surprisingly, we find that each of these models of glass formation can equally well describe the relaxation data for all of the systems that we simulate. In this way, we uncover some unity in our understanding of glass-forming materials from perspectives formerly considered as distinct.glass formation | elasticity | cooperativity | free volume | strings T here are numerous theoretical approaches aiming to describe the universal liquid dynamics approaching the glass transition. One class of theories emphasizes the importance of the congested nature of the local atomic environment in cooled liquids, focusing on the amount of "free volume" available to facilitate molecular rearrangement (1). This free-volume approach is also linked to the more modern jamming model of glass formation (2). Older treatments of glass formation based on this perspective can be traced back to Batchinski (3), Doolittle (4), and Hildebrand (5) for small liquids, and to Williams and coworkers (6) and Duda and Vrentas (7, 8) for polymer materials. There is also more recent work based on the free-volume perspective, for example, positron lifetime measurements (9) that probe the cavity structure of glass-forming (GF) liquids. DebyeWaller measurements (9, 10), based on neutron, X-ray, or other scattering measurements, emphasize another type of free volume that is associated with the volume explored by particles as they rattle about their mean positions in a condensed material. This type of free-volume modeling has also been refined to take into account the shape of these "rattle" volumes (11,12).Another family of glass-formation models emphasizes the emergent elasticity in glassy materials (13). These approaches build on the idea that the solid-like nature of glasses is one of their most conspicuous, and perhaps defining, properties. Dyre (13) and Nemilov (14) have argued that the activation energy for transport should grow in proportion to the shear modulus. The models of Hall and Wolynes (15) and Leporini and coworkers (10, 16) can also be included in this class if the Debye-Waller factor is taken as a measure of local material stiffness.Approaches emphasizing the underlying complex potential energy surface have also found consider...

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Section: Resultssupporting

confidence: 85%

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Betancourt

Hanakata

Starr

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

191

306

View full text Add to dashboard Cite

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“…Most of these reactions, as well as the reaction of a polyvalent substrate without a catalyst, slowed down at increased temperatures, as previously observed by Bane and co‐workers, and ascribed by them to the more favorable formation of imine intermediates at lower temperature . [A similar inverse temperature dependence and formal negative enthalpy of activation have been reported for a different organocatalytic reaction (thiourea‐catalyzed Mannich alkylation of imines) in association with the formation of key hydrogen bonds in the activated complex …”

Section: Figuresupporting

confidence: 70%

Polyvalent Catalysts Operating on Polyvalent Substrates: A Model for Surface‐Controlled Reactivity

McKay

Finn

2016

Angewandte Chemie

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Unusually fast rates of nucleophilic catalysis of hydrazone ligation were observed when polyvalent anthranilic acid catalysts operating on polyvalent aldehyde substrates were used with PAMAM dendrimers as the common platform. When presented in this way, the catalyst has a strong accelerating effect at concentrations 40-400 times lower than those required for similar monovalent catalysts and displays unique kinetic parameters. We attribute these properties to polyvalent engagement between the dendrimer surface groups, and a potential "rolling" effect leading to fast interparticle kinetic turnover. The phenomenon is sensitive to the density of functional groups on each dendrimer, and insensitive to factors that promote or inhibit nonspecific particle aggregation. These findings constitute a rare experimental example of an underappreciated phenomenon in biological and chemical systems that are organized on interacting surfaces.

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“…It is interesting to observe that the latter reaction tends to disappear at high temperatures, so the lipase effectively does not act as a good catalyst at high temperatures for this reaction in particular. It is unusual to observe negative activation energies, a phenomenon that can be related to a notably negative enthalpy change with temperature [43], to protein folding dynamics [44] or to deactivation or denaturation of the enzyme, which can happen at least partially [45]. In this latter case, the effect could affect only some of the reactions catalyzed by the enzyme.…”

Section: Kinetic Modeling Of Enzymatic Transesterification Of Glyceromentioning

confidence: 99%

Effect of Operating Variables and Kinetics of the Lipase Catalyzed Transesterification of Ethylene Carbonate and Glycerol

et al. 2018

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Glycerol carbonate (GC) is a value-added product originating from the valorization of widely available glycerol (Gly), a side stream from the production of biodiesel. Here we approach the production of this chemical comparing two reactions based on the transesterification of Gly with dimethyl carbonate (DMC) and ethylene carbonate (EC). When using DMC, it was observed that the free enzyme CALB (lipase B from Candida antarctica) gave the best results, whereas Eversa Transform (a genetic modification of Thermomyces lanuginosus lipase) performed better than the rest if EC was the reagent. With the selected catalysts, their immobilized analogous enzymes Novozym 435 and Lypozyme TL IM, respectively, were also tested. Observing that the yields for the reaction with EC were significantly faster, other operating variables were evaluated, resulting the best performance using a closed system, tert-butanol as solvent, a concentration of enzyme Eversa Transform of 3% w/w, a molar excess of EC:Gly of 9:1 and a temperature of 60 °C. Finally, several runs were conducted at different temperatures and molar ratios of EC:Gly, fitting a kinetic model to all experimental data for the reaction catalyzed with Eversa Transform. This model included the bimolecular transesterification reaction of Gly and EC catalyzed by the lipase and a reversible ring-opening polymerization of EC.

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Kinetic Evidence of an Apparent Negative Activation Enthalpy in an Organocatalytic Process

Cited by 37 publications

References 58 publications

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Polyvalent Catalysts Operating on Polyvalent Substrates: A Model for Surface‐Controlled Reactivity

Effect of Operating Variables and Kinetics of the Lipase Catalyzed Transesterification of Ethylene Carbonate and Glycerol

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