Hemicellulose Hydrolysis in the Presence of Heterogeneous Catalysts

A prominent autocatalytic effect in the hydrolysis of hemicelluloses was observed in the presence of heterogeneous cation-exchange catalysts, Amberlyst 15 and Smopex 101. The kinetic models proposed were based on the reactivities of the non-hydrolysed sugar monomer units and the increase of the rate constant as the reaction progresses and the degree of polymerization decreases. General kinetic models were derived and the kinetic parameters, describing the autocatalytic effect, were determined by non-linear regression analysis. The kinetic model explained very well the overall kinetics, as well as the product distribution in the hydrolysis of hemicelluloses.     

Kinetic modeling of hemicellulose hydrolysis in the presence of homogeneous and heterogeneous catalysts

Kinetic models were developed for the hydrolysis of O‐acetyl‐galactoglucomannan (GGM), a hemicellulose appearing in coniferous trees. Homogeneous and heterogeneous acid catalysts hydrolyze GGM at about 90°C to the monomeric sugars galactose, glucose, and mannose. In the presence of homogeneous catalysts, such as HCl, H₂SO₄, oxalic acid, and trifluoroacetic acid, the hydrolysis process shows a regular kinetic behavior, while a prominent autocatalytic effect was observed in the presence of heterogeneous cation‐exchange catalysts, Amberlyst 15 and Smopex 101. The kinetic models proposed were based on the reactivities of the nonhydrolyzed sugar units and the increase of the rate constant (for heterogeneous catalysts) as the reaction progresses and the degree of polymerization decreases. General kinetic models were derived and special cases of them were considered in detail, by deriving analytical solutions for product distributions. The kinetic parameters, describing the autocatalytic effect were determined by nonlinear regression analysis. The kinetic model described very well the overall kinetics, as well as the product distribution in the hydrolysis of water soluble GGM by homogeneous and heterogeneous catalysts. The modelling principles developed in the work can be in principle applied to hydrolysis of similar hemicelluloses as well as starch and cellulose. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1066–1077, 2014     

Hemicellulose Removal from Hardwood Chips in the Pre-Hydrolysis Step of the Kraft-Based Dissolving Pulp Production Process

Abstract

A pre-hydrolysis step to remove hemicelluloses from mixed hardwood chips consisting of maple, aspen, and birch with a ratio of 7:2:1 has been carried out. The effects of parameters on the pre-hydrolysis such as time, temperature, acetic acid addition, and raw material species, were determined. Different sugars, acetic acid, and furfural formation in the pre-hydrolysis liquor were quantified. The results showed that the pre-hydrolysis is a dynamic process, in which the removal of hemicelluloses increased with time while the conversion of extracted hemicelluloses to monosaccharides due to acid hydrolysis increased and part of the xylose was converted to furfural. The maximum temperature was the most critical parameter for hemicelluloses extraction and conversion, and a temperature of 170°C was the optimum for hemicelluloses extraction with relatively low conversion of xylose to furfural. About 11% of the xylan (in both monomeric and oligomeric forms) was removed at 170°C. Due to the presence of a high amount of xylan, birch produced the highest amount of xylose, followed by maple, and then aspen.     

头孢地嗪钠溶剂化物结晶过程自催化动力学

采用在线拉曼光谱在不同结晶温度和溶液初始浓度的条件下,研究了头孢地嗪钠溶剂化物结晶过程的动力学行为,从热力学和成核机理的角度分析了温度和初始浓度对动力学的影响。根据动力学曲线的特征,将头孢地嗪钠溶剂化物的结晶过程假定为"自催化"过程。针对头孢地嗪钠溶剂化物结晶过程的特殊性,修正了Prout-Tompkins模型;并通过不同温度下的动力学数据,采用多元线性回归拟合了头孢地嗪钠溶剂化物结晶过程的自催化动力学模型参数E和k。     

Fractionnement de deux bois tropicaux (eucalyptus et wapa) par traitement thermomécanique en phase aqueuse. Partie II: Caractéristiques chimiques des résidus et considérations cinétiques sur la solubilisation des hémicelluloses

We have characterized the residual solids derived from the aqueous phase liquefaction of two tropical wood species. The liquefaction process comprises the following steps: preparation of an aqueous suspension of milled wood, preheating at 135°C, sudden decompression through an orifice (ΔP = 17.2 MPa), reaction at 80–230°C and rapid quenching. The residues have been analyzed by two methods: acid hydrolysis and elemental analysis. The quantification of the monosaccharides present in the residues allows us to follow the solubilization profiles of both hemicelluloses and cellulose. The elemental composition of the residues is related to the severity of the process and it becomes a solubilization index for the hemicelluloses and the lignin. The chromatographic analysis of the gaseous phase after liquefaction has resulted in the determination of CO₂ as the only gas produced up to 230°C. Kinetic first order models represent well the solubilization of the hemicelluloses and the CO₂ formation.     

Fractional Separation of Hemicelluloses and Lignin in High Yield and Purity from Mild Ball-Milled Periploca sepium

Abstract

A novel three-step procedure for separation of hemicelluloses and lignin with high yield and purity was proposed in this study, where wood is mildly milled and successively extracted to produce three hemicellulosic and lignin fractions representing the total hemicelluloses and lignin in wood. The sequential treatments of the mild ball-milled Periploca sepium with 80% aqueous dioxane containing 0.05 M HCl at 85°C for 4 h, DMSO at 85°C for 4 h, and 8% NaOH at 50°C for 3 h resulted in a total release of over 85% of the original hemicelluloses and 86% of the original lignin. In particular, approximately 36% of the original hemicelluloses and 50% of the original lignin were separated during the first mild acidolytic hydrolysis process after low intensity milling. The structure of the acidic dioxane-, DMSO-, and alkali-soluble hemicellulosic and lignin fractions were elucidated using wet chemical analysis, FT-IR, and solution-state ¹H, ¹³C, and ³¹P NMR techniques. Results showed that both the mild ball milling and the mild acidolysis under the conditions given did not affect the separated lignin macromolecular structure. On the other hand, the mild acidolytic hydrolysis condition did cause substantial hemicellulosic depolymerization exception for a significant cleavage the ether linkages between lignin and hemicelluloses. The acidic dioxane-soluble lignin fraction was structurally different from the DMSO- and alkali-soluble lignin preparations and may originate mainly from the primary wall, while the alkali-soluble lignin preparation was mainly released from the secondary wall of Periploca sepium. Furthermore, it was found that the acidic dioxane-soluble hemicelluloses mainly contained more branched and less acidic arabinoxylans, and the 8% NaOH-soluble hemicellulosic fraction H₃ was both less branched and less acidic in structure, whereas the DMSO-soluble hemicelluloses were more acidic but less branched and consisted mainly of 4-O-methylglucuronoarabinoxylan.     

Tetrafunctional epoxy resins: Modeling the curing kinetics based on FTIR spectroscopy data

The curing kinetics of two thermosetting systems based on a tetrafunctional epoxy resin was investigated by Fourier transform infrared spectroscopy. Two formulations were studied, in which the hardener was an aromatic diamine and a carboxylic dianhydride, respectively. The quantitative evaluation of the epoxy conversion was based on spectra collected in the near‐infrared range (8000–4000 cm⁻¹) as well as in the medium infrared range (4000–400 cm⁻¹). The kinetic parameters evaluated in the above frequency intervals were significantly different. The reasons for such a discrepancy are discussed critically. Several kinetic models, based on the widely employed Kamal approach, were applied to verify their predictive capability. Satisfactory results were obtained for the amine‐cured system, particularly with a modified equation taking into account the autocatalytic nature of the process as well as a limiting diffusional effect. Less accurate results were achieved for the anhydride‐cured system. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 532–540, 1999     

Cure kinetics of aqueous phenol–formaldehyde resins used for oriented strandboard manufacturing: Analytical technique

The cure kinetics of commercial phenol–formaldehyde (PF), used as oriented strandboard face and core resins, were studied using isothermal and dynamic differential scanning calorimetry (DSC). The cure of the face resin completely followed an nth‐order reaction mechanism. The reaction order was nearly 1 with activation energy of 79.29 kJ mol^?1. The core resin showed a more complicated cure mechanism, including both nth‐order and autocatalytic reactions. The nth‐order part, with reaction order of 2.38, began at lower temperatures, but the reaction rate of the autocatalytic part increased much faster with increase in curing temperature. The total reaction order for the autocatalytic part was about 5. Cure kinetic models, for both face and core resins, were developed. It is shown that the models fitted experimental data well, and that the isothermal DSC was much more reliable than the dynamic DSC in studying the cure kinetics. Furthermore, the relationships among cure reaction conversion (curing degree), cure temperature, and cure time were predicted for both resin systems. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1642–1650, 2006     

Cross Polarization/Magic Angle Spinning 13C-NMR Characterization of Steam Exploded Poplar Hood

Abstract

High resolution solid state ¹³ C CP/MAS NMR has been used to study poplar wood samples before and after steam treatment. The principal structural components of wood have been characterized and the main effects of the rapid steam hydrolysis process (hydrolysis of the hemicelluloses, depolymerization of the lignin) were discussed. Evidence of molecular-level interactions between the principal components have been obtained from T ₁ ^Hδ relaxation times of protons belonging to the corresponding species.     

Kinetic model for the hydrolysis of sterilized palm press fibre

In this study, a kinetic model for enzymatic hydrolysis of oil palm residues considering the effect of sterilization was proposed and validated. Experiments were performed in batch reactor using palm oil mill effluent (POME) supplemented with sterilized and non-sterilized pressed pericarp fibers (PPF) as substrates. Kinetic parameters were estimated by fitting the experimental data to the models. It was found that the sterilization process as well as the variety in substrate particle size exerted an effect on the apparent rate constant (k), but no effect on the apparent Michaelis constant (K_M) and apparent competitive inhibition constant (K_I). When compared with the experimental data, the kinetic model provided good prediction to the oil palm residues hydrolysis with mean square error less than 10%.     

Heterogeneous hydrolytic degradation of poly(lactic‐co‐glycolic acid) microspheres: Mathematical modeling

A new mathematical model for the prediction of the heterogeneous hydrolytic degradation of poly(D,L‐lactide‐co‐glycolide) (PLGA)‐based microspheres was developed. The model takes into account the autocatalytic effect of carboxylic groups and polymer composition on the degradation rate. It is based on mass balances for the different species, considering the kinetic and mass transport phenomena involved. The model estimates the evolution of average molecular weight, mass loss, and morphological change of the particles during degradation, and it was validated with novel experimental data. Theoretical predictions are in agreement with the hydrolysis data of PLGA microspheres (error values less than 5%). The model is able to predict the effect of particle size and molecular weight on the degradation of PLGA‐based microspheres and estimates the morphological changes of the particles due to the autocatalytic effect. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45464.     

Comparative DSC kinetics of the reaction of DGEBA with aromatic diamines. II. Isothermal kinetic study of the reaction of DGEBA with m-phenylene diamine

In the first part of the present series the non-isothermal kinetics of the reaction of an epoxy resin based on diglycidyl ether of bis-phenol A (DGEBA) with m-phenylene diamine (mPDA) was studied. A four step kinetic analysis was applied using differential scanning calorimetry (DSC) data. It allowed us to confirm the validity of the three molecular autocatalytic model of this reaction, as well as to obtain reliable kinetic data in programmed temperature mode.The isothermal study of the DGEBA-mPDA reaction was performed applying a similar kinetic approach: (i) analysis at the peak maximum of the DSC curves; (ii) apparent activation energy analysis of the isothermal DSC data; (iii) integral and differential curve fitting methods; and (iv) modeling of the reaction and comparison of the model with the experiment.It was established that the overall kinetic parameters measured under programmed temperature conditions sufficiently well described the isothermal shift of the DSC curves along the logarithmic time scale, especially the initial stage of the reaction. A more precise analysis of the data showed that the isothermal DSC kinetics obeyed a formal model whose power exponent was approximately 2.5, or it was not well represented by the mechanistic-like three molecular autocatalytic velocity equation. Nevertheless, the activation energy of the autocatalytic rate constant determined at constant temperature mode, i.e. was found out in close agreement with the one obtained previously in programmed temperature mode, On the contrary, the ratio of the impurity catalytic to autocatalytic rate constant was slightly temperature dependent.     

Interaction of Hemicelluloses with Monolignols

Abstract

Interactions of hemicelluloses, xylan and glucomannan, with monolignols, coniferyl alcohol and sinapyl alcohol, and their glucosides, coniferin and syringin, were investigated by surface plasmon resonance analysis. Hemicelluloses with different aldehyde contents, prepared by sodium periodate oxidation, were immobilized onto carboxymethyl–dextran–coated gold plates by reductive amination. The interaction of the hemicelluloses with monolignols was evaluated as weight gain of the hemicelluloses at neutral pH. Both hemicelluloses adsorbed the monolignols in much larger amounts than the corresponding glucosides. The adsorption tendency was independent of aldehyde content. These results suggest the hemicelluloses have higher affinities to the aglycons than the glucosides. Thus, the interactions of the hemicelluloses with monolignols are primarily the result of hydrophobic interactions. However, binding constants were not estimated because of the low solubility of the monolignols in the aqueous buffered solution.     

Comparison of model‐fitting kinetics for predicting the cure behavior of commercial phenol–formaldehyde resins

Phenol–formaldehyde (PF) resins have been the subject of many model‐fitting cure kinetic studies, yet the best model for predicting PF dynamic and isothermal cure has not been established. The objective of this research is to compare and contrast several commonly used kinetic models for predicting degree of cure and cure rate of PF resins. Toward this objective, the nth‐order Borchardt–Daniels (nth‐BD), ASTM E698 (E698), autocatalytic Borchardt–Daniels (Auto‐BD), and modified autocatalytic methods (M‐Auto) are evaluated on two commercial PF resins containing different molecular weight distributions and thus cure behaviors. The nth‐BD, E698, and M‐Auto methods all produce comparable values of activation energies, while Auto‐BD method yields aberrant values. For dynamic cure prediction, all models fail to predict reaction rate, while degree of cure is reasonably well predicted with all three methods. As a whole, the nth‐BD method best predicts degree of cure for both resins as assessed by mean squared error of prediction. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Esterification of wheat straw hemicelluloses in the N,N‐dimethylformamide/lithium chloride homogeneous system

The esterified hemicelluloses were prepared under homogeneous reaction conditions in the system N,N ‐dimethylformamide/lithium chloride by reacting the native hemicelluloses with various acyl chlorides (C₃—C₁₈) in the presence of 4‐dimethylaminopyridine as a catalyst and triethylamine as a base within 30 min at 70–75°C. The products obtained were characterized by means of Fourier transform IR chromatography, gel permeation chromatography, thermal analysis, and solubility. The degree of substitution of esterified hemicelluloses was controlled between 0.38 and 1.75 as a function of experimental conditions. Under an optimum reaction condition xylose unit/acyl chloride (molar ratio 1 : 3, tetraethylammonium % 160, 75°C, 30 min), about 95% hydroxyl groups in native hemicelluloses were esterified. The molecular weight measurements showed a minimal degradation and hydrolysis of the products. The thermal stability of the products was also increased by modification. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2301–2311, 1999     

Kinetic studies of cationic photopolymerizations of cycloaliphatic epoxide,triethyleneglycol methyl vinyl ether,and cyclohexene oxide

The cationic photopolymerizations of monofunctional monomers and cycloaliphatic diepoxide monomer were examined in detail. Phenomenological kinetic models were developed for photopolymerizations of the monofunctional and difunctional monomers. For monofunctional monomers, the nonlinear models gave an excellent fit to the experimental data. Although a second‐order autocatalytic model was shown to be invalid above 10% conversion for the polymerization of a difunctional monomer (3,4‐epoxycyclohexylmethyl‐3′,4′‐epoxycyclohexane carboxylate, ECH), an autocatalytic model using diffusion‐controlled reaction with a Williams‐Landel‐Ferry (WLF) equation fits the experimental data well over the entire conversion range. A mechanistic model based on the free volume theory was also developed for cationic polymerization of the cycloaliphatic di‐epoxide monomer by using the method of moments. The model gives a good fit for the conversion and M_w development, but the calculated M_n results are lower than the experimental values. POLYM. ENG. SCI., 45:1546–1555, 2005. © 2005 Society of Plastics Engineers     

Thermal Decomposition of Energetic Materials 69. Analysis of the kinetics of nitrocellulose at 50 °C–500 °C

Kinetic constants for decomposition of nitrocellulose in the 50 °C to 500°C range are analyzed. At T < 100°C, three processes (depolymerization, peroxide formation, and hydrolysis) are consistent with the reported kinetics. For T = 100°C–200°C, 28 of 30 previously reported kinetic measurements can be organized clearly into two categories by the use of the kinetic compensation effect. These two groups fit the first-order and autocatalytic processes. Conflicting interpretations are reconciled by this approach. At T > 200°C, the kinetics are consistent with the existence of the first-order step and desorption of the products as two parallel processes which, together, control the rate. Time-to-exotherm and mass burning rate kinetics are compared as temperature-dependent reaction-desorption events.     

A kinetic study of the activated anionic polymerization of ε-caprolactam

The diversities existing among published kinetic studies on activated anionic polymerization of ε-caprolactam are closely examined. A kinetic model derived from a regular, linear reversible reaction mechanism is employed to explain the experimentally observed autocatalytic character of the polymerization system and to examine the dependence of the apparent activation energy on the experimental method. Several existing kinetic models tested with our experimental data show that the autocatalytic type rate equation best describes the polymerization process.     

Enzymatic hydrolysis of dissolved corn stalk hemicelluloses: reaction kinetics and modeling

The enzymatic hydrolysis of hemicelluloses as a filtrate originating from aqueous/steam pretreated corn stalks was carried out using commercial enzyme systems of several activities composed of cellulases and β‐D ‐xylosidases from Aspergillus niger. The hydrolysis was conducted using free enzymes in aqueous substrate solution at a temperature of 30 °C and a pH of 5. Saccharification corresponding to 90% of potential simple sugar release was obtained after 10 h using 0.12 activity units (U) of Cellulases_1 complex per mg of dissolved solids present in the filtrate. Synergetic action of Cellulases_1 enzyme complex and β‐D ‐xylosidases proved to be effective for the hydrolysis of plant hemicelluloses. A lumped model based on the Michaelis–Menten approach successfully described the fate of the lumped variables describing the hydrolysis of the overall kinetics of corn stalk hemicelluloses. The maximum saccharification rate evolved with the cellulases concentration as . This overall and pseudokinetic tendency was comparable to those reported in the literature for more simple systems employing a defined substrate and a pure hydrolytic enzyme. The n‐value was found to be in the range of 0.1–0.9 depending on the substrate lump involved in the reaction system. Copyright © 2003 Society of Chemical Industry