Subsequent Reaction Cycles Research Articles

Reactions of ethanol on Ru(0001)

The adsorption and reactions of ethanol on Ru(0001) were studied with temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS). Ethanol was found to adsorb intact onto Ru(0001) below 100K. From 175K to 200K, ethanol is converted into ethoxy groups, which undergo further dehydrogenation, leading to the formation of hydrogen, CO and surface carbon. The absence of aldehydes and methane as desorbing product molecules, as well as the intermediate species observed in the RAIRS spectra, show that the reaction proceeds via β-hydrogen abstraction of the ethoxy groups, resulting in an oxametallacycle reaction intermediate that undergoes decarbonylation to CO, C and hydrogen. The surface carbon, formed by the reaction, decreases the reactivity of the surface in subsequent reaction cycles. The amount of CO produced from a full monolayer of ethanol is limited to about 0.1ML, because of the competition between ethanol dehydrogenation and desorption.

One‐Pot Oxidative Passerini Reaction of Alcohols Using a Magnetically Recyclable TEMPO under Metal‐ and Halogen‐Free Conditions

AbstractA novel domino oxidative Passerini three‐component reaction (OP‐3CR) has been successfully developed with either primary or secondary alcohols instead of their corresponding aldehydes or ketones by using a recyclable magnetic core‐shell nanoparticle supported TEMPO (2,2,6,6‐tetramethyl‐piperidin‐1‐oxyl) (MNST, 1). It has been shown that MNST is a highly effective and recyclable catalyst system in the metal‐ and halogen‐free aerobic, oxidative, three‐component Passerini reaction. To the best of our knowledge this method can be considered as the first example of a OP‐3CR of alcohols under metal‐ and halogen‐free reaction conditions. It should be also noted that there has been no precedent example of oxidative OP‐3CR of secondary alcohols to their corresponding α‐acyloxy carboxamides. The catalyst can be magnetically recycled and successfully reused in 14 subsequent reaction cycles with only slight decreases of its catalytic activity.

Demonstration and Characterization of Ni/Mg/K/AD90 Used for Pilot-Scale Conditioning of Biomass-Derived Syngas

A fluidizable, Ni/Mg/K/AD90(90% α-Al2O3) catalyst was evaluated in a pilot-scale study with oak-derived syngas for tar and methane reforming at 900 °C. The catalyst was operated in a semi-batch mode for ten individual reaction cycles, each separated by a regeneration protocol consisting of steaming followed by H2 reduction. During the experiment, subsequent reaction cycles showed a decrease in activity as indicated by decreased initial methane conversion. During each reaction cycle, H2S poisoning was the dominant deactivation mechanism, while coking was deemed to be minor in comparison. Catalyst characterization by XRD and TPR suggest that the loss of activity for subsequent reaction cycles coincides with the formation of a NiAl2O4 species that is not fully reduced under process conditions, which in turn results in a decreased number of potential metallic nickel (Ni0) sites available for hydrocarbon steam reforming. An increase in nickel crystallite size with time-on-stream was also observed using XRD, indicating that sintering may also play a role in loss of catalyst activity, although this is not considered the primary deactivation mechanism because activity loss was observed even when nickel crystallite sizes remained nearly constant.

Immobilization of Porcine pancreas lipase on fiber-like SBA-15 mesoporous material

An immobilized enzyme had been prepared by incorporation of Porcine pancreas lipase (PPL, 4.6 nm × 2.6 nm × 1.1 nm) in the channels of fiber-like SBA-15 by virtue of the hydrogen bonding interactions between the abundant weakly acidic hydroxyl groups of the support and the lipase. The physical adsorptions of PPL on the fiber-like SBA-15 mesoporous material in buffer solution with different pH values (pH 5–10) and times (0–36 h) had been studied. A high lipase loading (926 mg enzyme per gram silica) can be obtained, but it disagreed with the high catalytic activity. The adsorbed maximum activities were observed at pH 6.0 and 3 h. The optimal pH of the hydrolysis of triacetin for the immobilized and free PPL was at 7.0. The immobilized PPL showed much more excellent adaptability of the hydrolysis of triacetin compared to free PPL during pH 6.0–9.0. Meanwhile, the thermal stability of the catalyst and its reusability were tested by performing subsequent reaction cycles of hydrolysis of triacetin. The activity of the immobilized PPL fell off rapidly to be 40% of its original activity after five successive batch reactions, because the weakly adsorbed PPL was leached out from the channels.

A predictive kinetic study of lipase-catalyzed ethanolysis reactions for the optimal reutilization of the biocatalyst

A new application of the kinetics modeling for the optimal reutilization of an immobilized lipase from Pseudomonas cepacia in the ethanolysis of vegetable oils is presented. Two different rate expressions were explored to take into account the lipase inactivation. The methodology developed is based on the utilization of the pseudo reaction time that indicates how much longer the reaction mixture must remain in the reactor (actual reaction time) to achieve the conversion that would have been achieved if the enzyme had not been partially deactivated (pseudo reaction time). An initial batch of lipase was employed in 15 consecutive trials in order to quantitatively characterize the process over a range of lipase activity and to validate the ability of the methodology utilized to describe the kinetics of both ethanolysis and deactivation. Then, the model developed was employed to predict the time necessary to attain a desired conversion in subsequent reaction cycles in both, trials with the same batch of lipase with different degree of inactivation, and trials with a new batch of immobilized lipase with different specific activity. The reaction times predicted to attain a 38% disappearance of glyceryl ester bonds were experimentally verified by carrying out the corresponding ethanolysis reactions of 100 g of sunflower oil. The agreement between the desired and experimentally attained conversions achieved validates the methodology developed to estimate reaction time in lipase-catalyzed ethanolysis reactions.

Lipase Immobilization on Oleic Acid−Pluronic (L-64) Block Copolymer Coated Magnetic Nanoparticles, for Hydrolysis at the Oil/Water Interface

Here, we have reported a new approach for utilizing oleic acid-Pluronic L-64 block copolymer coated iron oxide nanoparticles as supports for enzyme immobilization. Iron oxide nanoparticles were prepared by a coprecipitation method and were coated with oleic acid and Pluronic to achieve higher stability and dispersibility. The surface morphology and size of the particle, as determined by transmission electron microscopy (TEM), was +/- 10 nm. X-ray diffraction (XRD) patterns were taken over a range from 10 degrees to 90 degrees 20, using Cu K alpha radiation. Saturation magnetization values, measured at 300 K, varied from 34.6 emu/g to 60.8 emu/g. The possible interaction behavior of oleic acid and Pluronic was observed by Fourier transform infrared (FTIR) analysis and nuclear magnetic resonance (NMR) studies. Further potential of this material as a support for lipase immobilization and enzymatic hydrolysis at the oil/water interface was also investigated. The features of the surface-coated magnetic particles enable the adsorption of lipase from Candida cylindraces via strong hydrophobic interactions, which enhances the stability of the adsorbed enzyme molecules. The stability of the catalyst and, hence, its industrial applicability was tested by performing subsequent reaction cycles for the hydrolysis of olive oil. The activity of the immobilized lipase, pretreated with its substrate, was 510 U/g-matrix and was observed to be maintained at levels as high as 90% of its original activity for up to at least seven reuses.

Stability of sulfated zirconia and the nature of the catalytically active species in the transesterification of triglycerides

Sulfated zirconia (SZ) exhibits remarkable activity for various hydrocarbon reactions under mild conditions and is of interest for biodiesel synthesis. Nevertheless, to date no detailed study has addressed its activity and stability in liquid polar media such as alcohols, although a number of papers have suggested the possibility for some sulfur leaching. This paper presents an investigation into the activity and stability of a commercial SZ catalyst for the liquid-phase transesterification of triglycerides at 120 °C. The kinetics of tricaprylin (TCP) transesterification with a series of aliphatic alcohols (methanol, ethanol, and n-butanol) were investigated at 120 °C and 6.8 atm in a Parr batch reactor. It was found that the catalytic activity for TCP conversion decreased as the number of carbons in the alkyl chain of alcohol increased, most likely as a result of increased steric hindrance. The SZ catalyst exhibited significant activity loss with subsequent reaction cycles. The characterization of used catalysts after their exposure to various alcohols at 120 °C showed that the SO 2− 4 moieties in SZ were permanently removed. The SO 2− 4 species were leached out, most likely as sulfuric acid, which further reacted with alcohols to form monoalkyl and dialkyl sulfate species, as demonstrated by 1H NMR studies. This was in essence the main route for catalyst deactivation. Our findings conclusively demonstrate for the first time that in alcoholic-liquid media at higher temperatures, SZ deactivates by leaching of its active sites, most likely leading to significant homogeneous rather than heterogeneous catalysis.

Transesterification of poultry fat with methanol using Mg–Al hydrotalcite derived catalysts

The synthesis of biodiesel from poultry fats provides a way to convert the by-product of a renewable resource to a very important value-added biofuel. In this work, the use of heterogeneous base catalysts derived from Mg–Al hydrotalcite was investigated for the conversion of poultry lipids to biodiesel. This solid base showed high activity for triglyceride (TG) transesterification with methanol without signs of catalyst leaching. Catalytic performance was significantly affected by pretreatment and operating conditions. Calcination at optimum temperature was key in obtaining the highest catalyst activities. Rehydration of the calcined catalyst before reaction using wet nitrogen decreased catalytic activity for the transesterification of poultry fat, opposite to what has been reported for condensation reactions. Also, methanol had to be contacted with the catalyst before reaction; otherwise, catalyst activity was seriously impaired by strong adsorption of triglycerides on the active sites. Both temperature (60–120 °C) and methanol-to-lipid molar ratio (6:1–60:1) affected the reaction rate in a positive manner. The use of a co-solvent (hexane, toluene, THF), however, gave rise to a change in TG conversion profile which cannot be explained solely by a dilution effect. The catalyst underwent significant deactivation during the first reaction cycle probably due to deactivation of the strongest base sites. Subsequent reaction cycles showed stable activity. By re-calcination in air, complete catalyst regeneration was achieved.

Effect of carbon chain length on esterification of carboxylic acids with methanol using acid catalysis

This paper reports on an investigation into the impact of carboxylic acid chain length on the kinetics of liquid-phase acid-catalyzed esterification. Using sulfuric acid and a commercial Nafion/silica composite solid acid catalyst (SAC-13), initial kinetics were measured for the reactions of a series of linear chain carboxylic acids (acetic, propionic, butyric, hexanoic, and caprylic acid) with methanol at 60 °C. It was found that reaction rate decreased as the number of carbons in the linear alkyl chain increased for both H 2SO 4 and SAC-13. This trend is discussed in terms of the polar and steric effects of the alpha-substituent to the carboxylic group and evaluated by a Taft-type correlation. Using a mechanistically based kinetic model, the reaction kinetic parameters of SAC-13 catalysis were determined and compared for different carboxylic acids. Moreover, important parameters, such as water deactivation, catalyst reusability, and regeneration, were also affected by the size of the carboxylic acid used. SAC-13 underwent significantly more activity loss with subsequent reaction cycles as the size of the alkyl tail on the carboxylic acid increased. Characterization of the catalyst after reaction suggested that the deactivation of SAC-13 is likely caused by the entrapment of bulky reaction intermediates in or on Nafion polymeric nanodomains blocking catalytic acid sites.

Functionalization of mesoporous silica for lipase immobilization: Characterization of the support and the catalysts

A support for enzyme immobilization was prepared by functionalization of mesoporous silica with octyltriethoxysilane. The features of the surface enables the adsorption of lipase from Candida antarctica B via strong hydrophobic interactions, enhancing the stability of the adsorbed enzyme molecules. Derivatives with a high enzyme loading (200 mg protein/g of silica) can be obtained due to the high porosity and surface properties of the support while the immobilization occurs in a monolayer fashion. The lack of inactive enzyme aggregates, together with the high enzyme loading, are responsible for the high catalytic activity achieved by these species. Derivatives were prepared with different lipase loading, and the activities were tested and compared to the commercial derivative Novozym 435. The stability of the catalyst and hence its industrial applicability were tested by performing subsequent reaction cycles of acylation of ethanolamine with lauric acid in acetonitrile. Conversion was quantitative even after 15 reaction cycles.

Kinetics of K(+)-stimulated dephosphorylation and simultaneous K+ occlusion by Na,K-ATPase, studied with the K+ congener Tl+. The possibility of differences between the first turnover and steady state

Using the K+ congener Tl+ and rapid mixing combined with special quench techniques, we have investigated (at 20 degrees C) what is usually assumed to be the enzymatic correlate of active transport of K+ by Na,K-ATPase: the Tl(+)-catalyzed dephosphorylation of the K(+)-sensitive phosphointermediate(s), EP, and the resulting occlusion of Tl+ in the enzyme protein. We measured [EP] and [occluded Tl] as a function of time in phosphorylation, as well as dephosphorylation experiments with the following results. First, we found that with 150 mM Na+ and 600 mM Na(+)--NO3- was the anion--[Tl+] = 0.1-1 mM was without influence on the phosphorylation rate. Tl(+)-catalyzed dephosphorylation and Tl+ occlusion appeared to be simultaneous, and the stoichiometry was always 2 Tl+ occluded/EP dephosphorylated. Second, we tried computer simulations of the transient kinetic experiments, using an Albers-Post-type reaction scheme. This produced satisfactory curve-fits only in the case of 150 mM Na+, and although we could arrange that calculated [EP]steady-state was equal to that measured, the calculated steady-state Na,Tl-ATPase hydrolysis rates were always two to four times the rates measured directly. Third, we propose, as one (possibly of several) solution to these discrepancies between model and data, an expanded kinetic model consisting of an initiation reaction sequence followed by a propagation (or steady-state) reaction cycle. In this alternative model the first turnover of the enzyme is kinetically different from subsequent reaction cycles, and this allowed us to obtain both satisfactory curve-fits and accordance between calculated and measured values of hydrolysis-rate and [EP]steady-state.