Undesirable Secondary Reactions Research Articles

Production of phenolic monomers from lignin in hydrothermal medium: Effect of rapid heating and short residence time

This study investigates the influence of heating rate and short residence time (1–15 min) on the hydrothermal liquefaction (HTL) of alkali lignin, as a function of reaction temperature (280–320 °C). A novel induction-heating reactor system (IHRS) was designed and was used for rapid heating rate (∼100 °C/min), while a resistive-heating reactor system (RHRS) was used for slow heating rate (∼5.3 °C/min). The experiment using IHRS with a short residence time (1 min) resulted in the highest yield of carbon recovery in biocrude (52.2 ± 0.63 wt%) at 320 °C under autogenous pressure. In contrast, the experiments using RHRS or longer residence time (8–15 min) with IHRS showed lower biocrude yield due to the promotion of gasification and undesired secondary reactions. The detailed characterization of biocrude revealed that rapid heating rate improves the selectivity of compound in biocrude, such as phenol, while slow heating rate produced a broader distribution of compounds, including anisole and alkyl phenols at 320 °C for 1 min residence time. Furthermore, the usefulness of phenol as capping agent during IHRS of lignin was demonstrated in suppressing char formation and increasing biocrude yield.

Temperature Distribution Assessment in Gas–Solid Reactive and Nonreactive Systems Heated by Microwaves

Research studies have shown that selective heating of different phases in gas–solid systems exposed to microwave (MW) irradiation leads to a temperature gradient between gas and MW absorber solids. This can suppress undesired secondary gas-phase reactions and yield apparent kinetic improvements and energy savings. However, the effects of reaction exothermicity/endothermicity, gas velocity, temperature probe location at the microscale, and MW penetration depth in fixed beds on the temperature difference between solid and gas phases exposed to MWs and temperature distribution in the fixed beds are poorly understood. Highlighting these effects was targeted in this study and accomplished with the help of multiphysics simulations by COMSOL Multiphysics (5.6). The corresponding COMSOL model was initially verified and validated by data collected from literature studies and those obtained experimentally by the authors in this study. Simulation results from the verified and validated model indicated that the temperature gradient between the gas and the MW absorber solid increases by increasing the gas velocity or switching from an endothermic reaction to no reaction and/or an exothermic reaction. In addition, the collected results show nonuniform temperature distribution in a fixed-bed reactor made of MW absorber particles irradiated by MWs due to the limited penetration depth of MWs and hotspot formation. This operational deficiency makes the large-scale design of this type of reactor very challenging for MW heating-assisted reactions.

Promotional effect of Co cations on the catalytic activity of Co/ZSM-5 in 1,2-dichloroethane dehydrochlorination for the production of vinyl chloride monomer

In the catalytic dehydrochlorination of 1, 2-dichloroethane (DCE) for Vinyl chloride monomer (VCM) production, it is important to design catalysts with appropriate acid properties which can activate DCE, while inhibiting the further reaction of VCM. In this study, Co species were introduced into ZSM-5 through an ion-exchange method to change acid site distribution of ZSM-5. The introduction of Co cations significantly improved DCE conversion and VCM selectivity, achieving 6 and 2-fold greater than bare HZSM-5, respectively. Postmortem analyses allowed us to delineate the role of acid sites: Lewis acid sites are the major active sites, whereas Brønsted acid sites give rise to undesired secondary reactions. Thus, our results demonstrate that the introduction of Co cations increases the number of relevant active sites, and simultaneously eliminates undesirable sites. These findings help understand the effect of Brønsted and Lewis acid sites in zeolites on the DCE dehydrochlorination, providing useful information for designing rational catalysts.

The Influence of Inserted Metal Ions on Acid Strength of OH Groups in Faujasite

The number and the strength of acid sites in catalysts have paramount importance on their efficiency. In zeolites chemistry, increased content of framework Al in zeolites gives a higher number of strong acid sites. Their strength can be a disadvantage in catalytic reactions (e.g., methanol to olefins conversion) due to undesired secondary reactions of coke formation. Here, the Faujasite type of zeolite with higher content of Al has been used for investigating the role of defects in structure and inserted (wet impregnation and thermal treatment) metal cations (Mg, Co, Ni, Zn) on the strength of OH acid sites. Desorption of deuterated acetonitrile, as a probe molecule, was used for OH groups acid strength measurements at different temperatures (150, 200, and 300 °C).

Mixed Integer Linear Program model for optimized scheduling of a vanadium redox flow battery with variable efficiencies, capacity fade, and electrolyte maintenance

Redox Flow Batteries are a versatile and durable type of electrochemical storage and a promising option for large-scale stationary energy storage. The vanadium redox flow battery represents the most mature chemistry for the technology, and it is the most widely commercialized system thanks to its high chemical stability and performance. This work aims to optimize the scheduling of a vanadium redox flow battery that stores energy produced by a renewable power plant, keeping into account a thorough characterization of the battery performance, with variable efficiencies and capacity fade effects. A detailed characterization of the battery performance improves the calculation of the optimal number of cycles and revenue associated with the battery use if compared to the results obtained using simpler models, which take into account constant efficiencies and no capacity fade effects. The presented problem is nonlinear due to the functions of the battery efficiency, which depend upon charging and discharging powers and state of charge with nonlinear, non-convex correlations. Therefore, as per the state of the art of operation research, the problem is linearized using convex hulls in three dimensions. The optimization program also calculates the progressive battery capacity fade due to undesired secondary electrochemical reactions and the economic impact of capacity restoration through periodic maintenance. The final problem is solved as a Mixed-Integer Linear Program (MILP) to guarantee the global optimality of the linearized problem. The proposed optimization model has been applied to two different case studies. The first is a case of energy arbitrage, where the electric energy produced by a renewable plant is stored when the energy price is low and sold to the grid when the energy price is high to maximize the profit. The second is a case of load-shifting, balancing electric energy generation and demand from a grid-connected renewable energy community, where storage minimizes the expenses for the energy purchased from the grid. The optimization results have been compared to those obtained with constant battery efficiency models, which do not consider the capacity fade effects. Results show that simpler models overestimate the optimal number of cycles of the battery and the revenue by up to 15% if they do not take into account the degradation model of the battery, and respectively up to 32% and 42% if they also assume constant efficiency for the battery.

Photo-tuneable protein nitration by sensitiser tris(bipyridine)-Ruthenium(II) chloride complex

Post-translational modifications (PTMs) of proteins are a diverse source of variability that impacts on their functions, localisation, regulation, and lifetime. However, one of the main pitfalls in their study is that they appear in rather low frequencies and/or are only transiently observed. To overcome this issue and ease the study in vitro of stress-related protein PTMs, several methods have been proposed to model stress conditions and chemically introduce them. These techniques employ the combination of peroxides with transition metal ions or haem-containing proteins, as well as other possibilities such as peroxy radicals or UV radiation. However, their control, reproducibility and undesired secondary reactions that reduce the process yield are often a matter of concern. Here we introduce a photo-tuneable method that selectively targets nitration of aromatic residues. We initially present the adaptation of an oxidation method based on the photosensitiser tris(2,2′-bipyridine)-Ruthenium(II) chloride complex and ammonium persulfate, in which we employ an alternative radical neutralisation/trapping pathway that uses nitrite ions for the nitration of free l-Tyrosine and L-Tryptophan amino acids. After analysing the effect of several factors, we report the application of the photo-tuneable protein nitration (PTPN) method to four different model proteins in which we evaluate the nitration and oxidation of residues in each case. A mass spectrometry label-free quantitation of Tyr and Trp nitration is also described in order to compare the degree of modification and the accessibility of these residues. The method described could be employed to scale up the production of proteins with a selected range of oxidative PTMs for their characterisation, the assessment of their pathophysiological roles, and the development of detection and quantification methods to validate these PTMs as novel biomarkers associated with oxidative stress-related pathologies, such as in cardiovascular or neurodegenerative diseases.

Development and Proof of Concept of a Compact Metallic Reactor for MIEC Ceramic Membranes.

The integration of mixed ionic–electronic conducting separation membranes in catalytic membrane reactors can yield more environmentally safe and economically efficient processes. Concentration polarization effects are observed in these types of membranes when O2 permeating fluxes are significantly high. These undesired effects can be overcome by the development of new membrane reactors where mass transport and heat transfer are enhanced by adopting state-of-the-art microfabrication. In addition, careful control over the fluid dynamics regime by employing compact metallic reactors equipped with microchannels could allow the rapid extraction of the products, minimizing undesired secondary reactions. Moreover, a high membrane surface area to catalyst volume ratio can be achieved. In this work, a compact metallic reactor was developed for the integration of mixed ionic–electronic conducting ceramic membranes. An asymmetric all-La0.6Sr0.4Co0.2Fe0.8O3–δ membrane was sealed to the metallic reactor by the reactive air brazing technique. O2 permeation was evaluated as a proof of concept, and the influence of different parameters, such as temperature, sweep gas flow rates and oxygen partial pressure in the feed gas, were evaluated.

The development of industrial (thermal) processes in the context of sustainability: The case for microwave heating

AbstractThe volumetric, uniform, and selective heating of microwaves is a novel technique with the potential to be established in a wide range of applications. The individual interaction of each phase/component of multiphase/multicomponent systems with microwaves creates more than a temperature in the system. Having a high local temperature on an active site that is a strong microwave receptor promotes catalytic and non‐catalytic reactions. The relatively low temperature of the bulk material(s) restricts the undesirable secondary reactions correspondingly. This finding has already led to several applications. In this work, the fundamentals of microwave heating and their prospective applications are debated. Select lab‐scale results of metal and sulphur removal from petroleum oil, pyrolysis of lignin, and dry reforming of methane are demonstrated. The expected impacts of microwaves on other multiphase reactions are proposed.

Chemoselective Methylation of Phenolic Hydroxyl Group Prevents Quinone Methide Formation and Repolymerization During Lignin Depolymerization

Chemoselective blocking of the phenolic hydroxyl (Ar–OH) group by methylation was found to suppress secondary repolymerization and charring during lignin depolymerization. Methylation of Ar–OH prevents formation of reactive quinone methide intermediates, which are partly responsible for undesirable secondary repolymerization reactions. Instead, this structurally modified lignin produces more relatively low molecular weight products from lignin depolymerization compared to unmodified lignin. This result demonstrates that structural modification of lignin is desirable for production of low molecular weight phenolic products. This approach could be directed toward alteration of natural lignification processes to produce biomass that is more amenable to chemical depolymerization.

Preparation of an engineered safer immunotoxin against colon carcinoma based on the ribotoxin hirsutellin A.

Immunotoxins are chimeric proteins composed of an antibody domain that specifically directs the action of the toxic domain, resulting in the death of the targeted cells. Over recent years, immunotoxins have been widely studied and the number of different constructions has increased exponentially. Protein engineering has allowed the design of optimized versions of immunotoxins with an improved tumor binding affinity, stability or cytotoxic efficacy, although sometimes this has compromised the safety of the patient in terms of undesirable adverse secondary reactions. A triple mutant at three Trp residues (HtA3ΔW) of the ribotoxin hirsutellin A retains its specific ribonucleolytic activity, although cell internalization capacity is lacking. This toxin variant has been fused to the single chain variable fragment A33 (scFvA33). This immunoconjugate (IMTXA33HtA3ΔW) was produced in the methylotrophic yeast Pichia pastoris and purified using nickel-nitrilotriacetic acid affinity chromatography. Both target and toxic domains were characterized. The immunotoxin showed an exquisite specific binding against GPA33-positive culture cells, which results in the death of the targeted cells because of specific ribonucleolytic activity against ribosomes of the engineered hirsutellin A variant. IMTXA33HtA3ΔW represents a promising structure in the search for an improved immunotoxin without compromising the safety of patients.

Hierarchical ZSM-5 Materials for an Enhanced Formation of Gasoline-Range Hydrocarbons and Light Olefins in Catalytic Cracking of Triglyceride-Rich Biomass

A hierarchical ZSM-5 material with a high fraction of mesoporosity coupled to well-preserved intrinsic zeolite characteristics has been successfully prepared by postsynthesis modifications involving optimization of base treatment and subsequent strong acid washing of commercial Al-rich ZSM-5 (parent ZSM-5). The resulting hierarchical ZSM-5 material was thoroughly characterized before being tested in the cracking of triglyceride-rich biomass, i.e., model feedstock triolein and real feedstock waste cooking oil under fluid catalytic cracking conditions. The results show that the introduction of intracrystalline mesoporosity enhances the utilization of zeolite acid sites by the enlarged external surface, leading to an increased conversion. At the same time, it partially suppresses the undesired secondary reactions by shortening micropore diffusion path lengths. With such a hierarchical ZSM-5 material, higher selectivities toward the desired products, i.e., gasoline-range hydrocarbons and light olefins, than w...

Selective production of xylenes from alkyl-aromatics and heavy reformates over dual-zeolite catalyst

Disproportionation of 1,2,4-trimethylbenzene (1,2,4-TMB), its transalkylation with toluene as well as dealkylation of ethyltoluenes (ETs) over MOR and MFI zeolites were investigated in a fluidized-bed reactor at 300–400°C. MFI exhibited good ET conversion (60wt.%), high dealkylation selectivity (60%), but low disproportionation and transalkylation activities. MOR showed high ET conversion (80wt.%) with low dealkylation selectivity (33%), and high disproportionation and transalkylation activities. Based on these results, a dual-zeolite catalyst consisting of MOR and MFI was proposed to promote the diverse reactions leading to selective production of xylenes via dealkylation–transalkylation of alkylaromatics. Single-zeolite (MOR-based) and dual-zeolite (MOR- and MFI-based) catalysts were prepared by addition of binder, characterized and evaluated for the conversion of C8+ and C9+ heavy reformates (either alone or with the addition of various amounts of toluene). The catalysts exhibited little differences in disproportionation and transalkylation activities. However, the dealkylation of ETs was enhanced over dual-zeolite catalyst, which can be attributed to its higher acid site strength. The reduction in microporous volume in dual-zeolite catalysts prevented some undesired secondary reactions, which require formation of bimolecular reaction intermediates. Compared with the single-zeolite catalyst, the dual-zeolite catalyst exhibited an increase in the xylenes yield from 16.6 to 19.8wt.% and an enhancement in xylene selectively from 37.1 to 43.3% in the conversion of C9+ heavy reformate at 400°C. Addition of about 20wt.% toluene to heavy reformate feed resulted in further increase in xylenes yield to 21.2wt.% over the dual-zeolite catalyst.

Effects of Colour-filtering Solar Beam on Reaction Product Fromd-group Transition Metals in N<SUB>2</SUB>Gas Environment under Heating with Concentrated Solar Beam

Recently, Noda and collaborators at Kyoto University reported possible enhancement of photo-voltaic cell (PVC) energy conversion efficiency by allowing penetration of preferential range of wavelength components alone to PVC by filtering the solar beam through a special semiconductor thin film developed at their laboratory. During the course of our recent experimental attempts ofnitriding<i>d</i>-group transition metals in N<SUB>2</SUB> gas environment, we detected intriguing effects of colour-filtering on reaction product. In the present report, these effects of colour-filtering are reviewed under new light of recently reported evidence by Noda and co-workers. It appears that, by colour-filtering the selected wavelength range in solar beam, undesirable secondary reactions might be suppressed to result in promotion of a target reaction under heating using concentrated solar beam as the source of reaction heat.

Study on peroxide vulcanization thermodynamics of ethylene–vinyl acetate copolymer rubber using 2,2,6,6,‐tetramethylpiperidinyloxyl nitroxide

AbstractA step forward in the understanding of rubber vulcanization with organic peroxides is provided by combining a proper arrangement between polymer, vulcanizing agent and cure conditions. For this purpose, an ethylene–vinyl acetate copolymer with a high content of vinyl acetate (70 mol%) was used since a fully saturated polymer backbone allows its vulcanization via peroxide. For the range of conditions analysed here, it is shown that the predominant process taking place is crosslinking via radical recombination, minimizing or even avoiding undesirable secondary reactions such us polymer degradation. Once conditions had been optimized, peroxide vulcanization was analysed in more depth in the presence of 2,2,6,6‐tetramethylpiperidinyloxyl, which is a mediating stable free radical commonly used in controlled radical polymerization. Consequently, it was possible to differentiate the termination reaction from the initiation and propagation steps, allowing the determination of the enthalpy of formed C–C crosslinks as measured using calorimetry. It was possible to isolate and determine the contribution of the crosslinking pathway from the global vulcanization reaction by means of calorimetric methods at optimum conditions. In fact, this simple methodology could be an important tool for understanding in detail the complex peroxide vulcanization of elastomers since reactions involved in this process determine the final network structure, and thus the final elastic properties of these compounds.

Post-deposition laser treatment of plasma sprayed titania-hydroxyapatite functionally graded coatings

The viability of a high power diode laser source as effective post-deposition treatment technique of functionally graded titania-HA coatings was checked. In particular, several laser treatments were performed on various coatings plasma-sprayed under different conditions to verify the presence of an operative window large enough for practical purposes and, subsequently, to identify the most promising settings of the laser parameters. Laser power as low as 80–100 W and focus distance as high as −4 mm were found to be the most feasible choice to improve the overall coating properties as well as to inhibit undesired secondary reactions between calcium phosphates and titania. Finally, the best set of the laser parameters were applied to a pure HA coating and to a titania-HA graded one, plasma-sprayed under the same conditions, to perform a comparative evaluation. The microstructural characterization by scanning electron microscopy, X-ray diffraction and the local mechanical investigation by Vickers micro-indentations proved that the degree of crystallinity of HA at the outermost layers of the graded coating could be improved without significantly altering the compositional and functional gradient. Furthermore, the properties of the pure HA coating, as-sprayed and laser treated, were found to be substantially less advantageous than those of the titania-HA functionally graded coating (higher microstructural defectiveness; inferior degree of crystallinity of HA at the working surface; lower Vickers hardness), thus confirming the beneficial effect of the compositional gradient.

Effect of steam on heavy oil catalytic pyrolysis

This paper studied the influence of steam on heavy oil catalytic pyrolysis. Experiments of Chinese Daqing atmospheric residue catalytic pyrolysis on CEP-1 were conducted in a confined fluidized bed reactor, with steam and nitrogen being used as the fluidizing gas, respectively. The effects of steam on the volume of contents of the components in pyrolysis gas, product yields, and carbon dioxide formation were discussed. The results of contrast tests showed that steam not only had the action of the fluidizing gas and the dilution gas, but also participated in steam gasification and water-gas shift reactions. In catalytic pyrolysis processes, steam could maintain the activity of catalysts, accelerate the reactions of carbonium ions, inhibit undesired secondary reactions, and enhance the yields and selectivity of the aimed products.

Development of a new “flow-through” sample cup for the vertical micro-furnace pyrolyzer to reduce undesirable secondary reactions and band broadening of resulting pyrolyzates

When a vertical micro-furnace pyrolyzer is adopted for pyrolysis-gas chromatography (Py-GC), the sample is usually introduced into the center of the preheated pyrolysis chamber by mounting it in a small cylindrical sample cup with a dead-end bottom having a volume of a few dozens micro liters which in some case might be responsible for a stagnant flow of carrier gas in the pyrolysis chamber to cause undesirable secondary reactions of the resulting pyrolyzates and/or peak broadening phenomenon. In order to quench the dead-end flow, a new “flow-through” sample cup in which a small hole (ca. 1.0 mm i.d.) was drilled through its bottom was developed, and its basic characteristics were first examined by measuring the pyrogram of polycarbonate (PC) at 600 °C under different carrier gas flow rates. Various secondary products such as phenol and p-isopropenyl phenol derived from bisphenol A, the main pyrolyzatate of PC were observed when the conventional sample cup without the hole was used under a relatively lower flow rate as 25 ml/min. However, they were markedly decreased when the “flow-through” sample cup was used. Furthermore, the resolution of the lower boiling point pyrolyzates (C 1–C 8) from high density polyethylene (HDPE) was also much improved by use of the “flow-through” sample cup.

Oxidative pyrolysis of natural gas in a spouted-bed reactor: reaction stoichiometry and experimental reactor design

A simple reaction network is developed to understand methane oxidative pyrolysis in a spouted-bed reactor. Oxidative pyrolysis for converting natural gas is attractive because it gives high methane conversions (80–90% per pass) at high selectivities to C 2+ (40%) and carbon monoxide (40%). Based on results reported in a British Petroleum patent, our analysis shows that oxidative pyrolysis is a combination of methane partial oxidation to carbon monoxide which produces heat and of methane pyrolysis to C 2+ (mostly ethylene and acetylene) which consumes heat. The methane pyrolysis reaction is thermodynamically limited and requires temperatures above 1000°C for significant conversion. A pilot-plant oxidative pyrolysis reactor was designed from cold flow studies to verify the BP results obtained at atmospheric pressure and to establish process feasibility at higher pressures. Scoping economic studies show that higher pressure operation is needed to avoid costly product compression in downstream upgrading processes. The pilot-plant spouted-bed reactor evolved from a fragile quartz tube to a more robust one of stainless steel. The stainless steel performs similarly to the quartz reactor when the walls are coated with an aluminum oxide layer to passivate inherent catalytic activity. Although a number of experimental challenges remain, initial runs have been successful in demonstrating that our method of preheating the feed successfully ignites the reaction and that our analytical technique gives excellent closure in detailed material balances. Based on results presented in this paper ways are needed to improve on BP C 2+ yields, such as by process variable optimization and rapid thermal quench to minimize undesirable secondary pyrolysis reactions.

Peroxy-initiated chain degradation, crosslinking, and grafting in PP-PE blends

Polyolefines are frequently functionalized with polar monomers via peroxide-initiated grafting that starts at macroradicals. However, polyolefine macroradicals also undergo undesired secondary reactions. Polypropylene (PP) is degraded, while polyethylene (PE) is crosslinked. Mechanistically, PP radicals are split by β scission while PE radicals recombine to clusters. If these opposed tendencies can be balanced in PP–PE blends was investigated in this study. In principle, coupling of PP and PE radicals can lead to graft copolymers PE-g-PP. But the chances for graft reactions are good only in homogenous PP–PE blend melts. It is difficult to decide whether these blends are, in fact, in the melt one-phase or two-phase systems because molten PP and PE are too similar in all respects. PP–PE blends were processed with peroxide in the melt and, for comparison, also in solution. According to viscosity and gel permeation chromatography and differential scanning calorimetry results, graft reactions occurred only in the (presumably homogeneous) blend solutions in which degradation was subdued and crosslinking prevented. In PP–PE blend melts, on the contrary, the two polymers reacted fairly independently of each other. PP was degraded, and PE crosslinked. Apparently, these blend melts, although transparent, are two-phase systems. © 1998 John Wiley & Sons, Inc. J Appl Polm Sci 68: 2019–2028, 1998

Molecular and mechanistic aspects of the functionalization of polyolefins with ester groups

AbstractIn the present paper the functionalization reaction of polyolefins in the melt with diethylmaleate (DEM) in the presence of peroxide is revisited. The process was analyzed on the basis of several experiments carried out in the Brabender mixer with 30 min residence time and at constant temperature, 190° C for EPR and 200° C for LLDPE samples, while varying the number of branching in the starting polyolefin, the amount of DEM and of dicumyl peroxide (DCP). For reference purposes similar experiments were also examined as performed with EPDM and isotactic polypropylene (PP).At the end of the reaction, the polymeric product was treated with boiling acetone to remove low molecular weight products. In general, the acetone insoluble polymer was soluble in heptane and the functionalization degree (FD) referred to this fraction was in the range 0.2–2.0% mol of DES groups per 100 moles of monomelic units.The contemporary occurrence of the degradation and chain extension, including crosslinking reactions, provides a reasonable explanation of the results obtained. The analysis of the process performed in the present work helps to select proper conditions for controlling the characteristics and FD of the polyolefin and keep undesired secondary reactions under control.