Diphasic aluminosilicate gels with two stage mullitization in temperature range of 1200–1300 °C

The crystallization of mullite in amorphous diphasic gel aged for 6 months has been studied using non-isothermal differential scanning calorimetry (DSC) and powder X-ray diffraction with Rietveld structure refinement analysis. The diphasic premullite gels undergo structural changes by aging even when they are calcined at 700 °C. These changes imply segregation of the sample to Al₂O₃-rich and SiO₂-rich regions. From the Al₂O₃-rich region crystallizes poorly defined AlSi spinel at 977 °C followed by two-step mullite crystallization in the temperature interval of 1200–1300 °C. Two overlapped exothermic peaks on DSC scan of aged gel were observed; the first at 1233 °C and the second at 1261 °C. The former is attributed to mullite crystallization by transformation of AlSi spinel, by which excess alumina occurs, which in the second step of mullitization reacts with amorphous SiO₂-rich phase. The activation energy for mullite crystallization in the first step was E_a=935±14 kJ mol⁻¹ and the Avrami exponent n=2.5. The values E_a=1119±25 kJ mol⁻¹ and n=1.2 were obtained for mullite formation in the second step. If amorphous SiO₂-rich phase is extracted from the sample, the value E_a=805±26 kJ mol⁻¹ is obtained. Mullite crystallizing from AlSi spinel (when SiO₂-rich phase has been extracted) differentiates compositionally from that formed by both reactions. Smaller unit cell parameters and higher amount of oxygen vacancies are incorporated into tetrahedral positions of mullite structure, as was determined by Rietveld structure refinement method.     

On the stepwise change of activation energies in the hydrodechlorination of chlorobenzene over supported nickel

Kinetics of chlorobenzene hydrodechlorination have been measured over Ni on SiO₂, Al₂O₃, MgO, activated carbon and graphite. A stepwise variation of E_a is analysed using the selective energy transfer model where E_a is identified as the vibrational energy associated with an excitation of the chlorobenzene out-of-plane C–H bending mode. Variation of E_a with vibrational quantum number yields a vibrational frequency of 749 cm⁻¹ and a value (−1.1 cm⁻¹) for the anharmonicity term, which is characteristic of bending vibrational modes. Our analysis suggests that the reacting species are weakly adsorbed on the catalyst: heat of adsorption = −0.31 kJ mol⁻¹.     

A simple kinetic analysis to determine the intrinsic reactivity of coal chars

The intrinsic oxidation reactivity in air of an activated carbon char derived from bituminous coal was investigated using a new thermogravimetric analysis (TGA) method. Applying the new method, values of the Arrhenius activation energy E and pre-exponential factor A were estimated from TGA data obtained via heating samples at different constant rates. A novel statistical criterion was subsequently used to determine the heating rate at which optimum values of E and A were obtained. This is a valuable development, for in conventional non-isothermal TGA, while it is accepted that Arrhenius parameters vary with heating rate, there is no formal method for selecting one rate (and hence one set of values of E and A) over another. Using this new method, the following optimum values were obtained for the carbon at a heating rate of 25 °C min⁻¹: E=129.4 kJ mol⁻¹ and ln(A/s⁻¹)=10.4. These results are very similar to those calculated for the same material using more time consuming and less accurate isothermal TGA methods. It is therefore proposed that this new analysis method might be an improvement on conventional techniques to determine the intrinsic oxidation reactivity in air of coal chars.     

Alcohol ethoxylation kinetics: Proton transfer influence on product distribution in microchannels

The influence of the reactor type on the product distribution of the base catalyzed ethoxylation of fatty alcohols was studied.Commonly, proton exchange equilibrium is assumed when modeling this reaction to calculate the product distribution. The model is applicable for ethoxylates produced in semibatch, but cannot explain the products obtained from a continuous microstructured reactor.In this work, a non-equilibrium model is proposed to explain the observed distributions. The model is better suited to fit the distribution curves, and was used to determine the kinetic parameters of this reaction.For the propagation reaction, an activation energy E_A,P = 74 kJ mol⁻¹ was found, which is in good agreement to literature data. For the proton transfer, activation energies in the range of 56 kJ mol⁻¹ to 68 kJ mol⁻¹ were observed.     

Studies on formation and siliconization of carbon template of coir fibreboard precursor to SiC ceramics

Formation reaction of carbon template of coir fibreboard samples via pyrolytic decomposition was studied by methods of thermogravimetric, derivative thermogravimetric and differential thermal analysis using different heating rates (5–20 K min⁻¹). The major decomposition reaction at 250–500 °C had activation energy and frequency parameter of 121.84 ± 8.78 kJ mol⁻¹ and 3.2 × 10⁹–2.1 × 10¹¹ min⁻¹, respectively, supporting a free radical mechanism. Siliconization reaction of the carbon template was also studied by non-isothermal DTA technique using varying heating rates (5–20 K min⁻¹); exothermic C-Si reaction was preceded by endothermic melting of Si and it had activation energy of 2793.09 ± 187.65 kJ mol⁻¹; diffusion through solid reaction product was found to control the heterogeneous chemical reaction process. On the basis of these experimental results separate experiments were conducted for preparation of carbon templates and their subsequent siliconization. The correlations between the properties of the coir fibreboard precursor, the carbon template and the siliconized product (SiC) were explained.     

Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin

Series of resin selection experiments were carried out and the KIP210 strong base anion exchange resin was confirmed to have the maximum equilibrium adsorption capacity to remove Cr(VI) from wastewater. The adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin were investigated completely and systematically. The static experiments were performed to study the effects of various parameters, such as shaking speed, resin dosage and pH during the adsorption process. The results indicate that the effect of external diffusion is eliminated at 160 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the resin dosage. The adsorption of Cr(VI) on KIP210 agrees well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH = 26.5 kJ mol⁻¹, ΔS = 126.7 J mol⁻¹ K⁻¹ and ΔG < 0. It demonstrates that the adsorption of Cr(VI) on KIP210 is a spontaneously endothermic physisorption process. Moreover, the adsorption process can be described well by a pseudo-second-order kinetic model and the activation energy is 30.9 kJ mol⁻¹. The kinetic analysis showed that the adsorption rate is controlled by intraparticle diffusion. The resin is successfully regenerated using the NaOH solutions.     

Kinetic model of CaO/fly ash sorbent for flue gas desulphurization at moderate temperatures

Characteristics of the sulphation reaction between SO₂ and CaO/fly ash sorbent were analyzed based on TGA results to develop a kinetic model for a dry moderate temperature (400–800 °C) FGD process. It was found that SO₂ diffusion within sorbent particles involved three sub-processes: inter-particle diffusion, inter-grain diffusion and diffusion through product layers and the diffusion dominated the whole sulphation reaction process. The activation energy for product layer diffusion E_diff of 49.3 kJ mol⁻¹ being greater than the chemical reaction activation energy E_a of 13.9 kJ mol⁻¹ verified the importance of the diffusion. Predictions using the kinetic model in which k₀ varies with temperature agree well with the experimental data.     

A self-healing polymer network based on reversible covalent bonding

A self-healing polymer network for potential coating applications was designed based on the concept of the reversible Diels–Alder (DA) reaction between a furan functionalized compound and a bismaleimide. The network allows local mobility in a temperature window from ca. 80 °C to 120 °C by shifting the DA equilibrium towards the initial building blocks. Changing the spacer length in the furan functionalized compound leads to tailor-made properties. Elastomeric model systems were chosen to evaluate the kinetic parameters by Fourier transform infrared spectroscopy. For the DA reaction a pre-exponential factor ln(A_DA in kg mol^?1 s^?1) equal to 13.1 ± 0.8 and an activation energy (E_DA) of 55.7 ± 2.3 kJ mol^?1 are found. For the retro-DA reaction, ln(A_rDA) and E_rDA are 25.8 ± 1.8 s^?1 and 94.2 ± 4.8 kJ mol^?1, respectively. The enthalpy and entropy of reaction are calculated as ?38.6 kJ mol^?1 and ?105.3 J mol^?1 K^?1. The kinetic results are validated by micro-calorimetry. Non-isothermal dynamic rheometry provides the gel-point temperature of the reversible network. The sealing capacity is evaluated by atomic force microscopy for micro-meter sized defects. Repeatability of the non-autonomous healing is checked by micro-calorimetry, ruling out side-reactions below 120 °C.     

Template-free synthesis of N-doped porous carbons and their gas sorption properties

Nitrogen-rich carbon precursors are prepared and subsequently used for the preparation of N-doped porous carbons (NPCs). Two carbon precursors (CPs), CP-60 and CP-40, are prepared at different temperatures, 60 °C and 40 °C, respectively. The obtained CP materials are almost nonporous based on the N₂ adsorption/desorption analysis. These nonporous CP materials are converted into NPC materials through a carbonization at 800 °C. Porous NPC-60-800 and NPC-40-800 are prepared from CP-60 and CP-40, respectively. In contrast to the CP materials, the NPC materials exhibit higher surface areas. The surface areas of NPC-60-800 and NPC-40-800 are 422.9 m² g⁻¹ and 520.8 m² g⁻¹, respectively. The respective N-contents of NPC-60-800 and NPC-40-800 materials are 4.30 and 3.54 wt.% based on the elemental analysis. The types of N-containing groups of NPC materials are investigated by X-ray photoelectron spectroscopy (XPS) analysis. The NPC materials are good sorbents for CO₂ storage. The heats of CO₂ adsorption (Q_st) are 48.4 kJ mol⁻¹ for NPC-60-800 and 47.0 kJ mol⁻¹ for NPC-40-800. Both materials are also good H₂ sorbents at 77 K: 1.23 wt.% (NPC-60-800) and 1.22 wt.% (NPC-40-800).     

Characterization of catalysts in their active state by adsorption microcalorimetry: Experimental design and application to sulfated zirconia

A method to characterize the surface sites of catalysts in their active state by adsorption microcalorimetry was developed. A calorimeter cell was used as a flow-type reactor, and the skeletal isomerization of n-butane (1 kPa) at 378 K and atmospheric pressure proceeded at comparable rates and with the same states of induction period, maximum and deactivation phase as in a tubular reactor. The reaction was run for selected times on stream and after the removal of weakly adsorbed species, n-butane or isobutane were adsorbed at 313 K. The surface of activated sulfated zirconia was characterized by at least two different sites for n-butane adsorption, a small group of sites (about 20 μmol g^?1) that yielded heats of 50–60 kJ mol^?1 and sites that were populated at higher pressures (above about 5 hPa n-butane) and yielded heats of about 40 kJ mol^?1. The strongly interacting sites disappear during the induction period and are proposed to be the sites where the isomerization reaction is initiated.     

Master sintering curves of a nanoscale 3Y-TZP powder compacts

The sintering behavior of commercially available granulated ZrO₂–3 mol% Y₂O₃ (3Y-TZP) powder compacts with an aggregate size of 75 nm was studied. The shrinkage response of the powder compacts during non-isothermal sintering was measured in a sensitive dilatometer at different heating rates. Densification and grain growth were also studied after isothermal firing in air according to different sintering cycles. The sintering and grain growth activation energy was estimated to be Q_S = 485 ± 12 kJ mol^?1 and Q_G = 546 ± 23 kJ mol^?1, respectively. Using the estimated Q-values, the master curves for sintering and grain growth were established and used for prediction of the densification and microstructural development under different thermal histories. A good agreement between the model predictions and experimental result was obtained.     

Nonisothermal sintering of Mn doped ZnO

The sintering of pure ZnO and ZnO doped with Mn, obtained by addition of Mn(NO₃)₂. 4H₂O in the concentration from 0·1 to 1·2 mol%, was investigated by dilatometry at constant heating rates, from 1 to 15 °C min⁻¹. Mn shifts the onset of the sintering towards higher temperatures, but no significant effect of the Mn doping level on the shrinkage was observed. Accordingly, the calculated activation energy for the first stage, changed from ∼320 kJ mol⁻¹ for pure ZnO to ∼440 kJ mol⁻¹ for Mn doped ZnO. Using classical sintering models to analyse the initial stage sintering of all the compositions, two sintering mechanisms were found to control the initial stage sintering. The first region is identified with a grain boundary sliding mechanism, while volume diffusion is the controlling mechanism in the second region. With the increase of the Mn content, the grain boundary sliding rate remains constant, but the volume diffusion rate is reduced.     

Evaluation of the pyrolytic and kinetic characteristics of Enteromorpha prolifera as a source of renewable bio-fuel from the Yellow Sea of China

The pyrolytic and kinetic characteristics of Enteromorpha prolifera from the Yellow Sea were evaluated at heating rates of 10, 20 and 50 °C min^?1, respectively. The results indicated that three stages appeared during pyrolysis; dehydration, primary devolatilization and residual decomposition. Differences in the heating rates resulted in considerable differences in the pyrolysis of E. prolifera. Specifically, the increase of heating rates resulted in shifting of the initial temperature, peak temperature and the maximum weight loss to a higher value. The average activation energy of E. prolifera was 228.1 kJ mol^?1, the pre-exponential factors ranged from 49.93 to 63.29 and the reaction orders ranged from 2.2 to 3.7. In addition, there were kinetic compensation effects between the pre-exponential factors and the activation energy. Finally, the minimum activation energy was obtained when a heating rate of 20 °C min^?1 was used.     

Comparison of the rate of CO2 absorption into aqueous ammonia and monoethanolamine

Aqueous ammonia has been proposed as an absorbent for use in CO₂ post combustion capture applications. It has a number of advantages over MEA such as high absorption capacity, low energy requirements for CO₂ regeneration and resistance to oxidative and thermal degradation. However, due to its small molecular weight and large vapour pressure absorption must be carried at low temperature to minimise ammonia loss. In this work the rate of CO₂ absorption into a falling thin film has been measured using a wetted-wall column for aqueous ammonia between 0.6 and 6 mol L^?1, 278–293 K and 0–0.8 liquid CO₂ loading. The results were compared to 5 mol L^?1 MEA at 303 and 313 K. It was found that the overall mass transfer coefficient for aqueous ammonia was at least 1.5–2 times smaller than MEA at the measured temperatures. From determination of the second-order reaction rate constant k₂ (915 L mol^?1 s^?1 at 283 K) and activation energy E_a (61 kJ mol^?1) it was shown that the difference in mass transfer rate is likely due to both the reduced temperature and differences in reactivity between ammonia and MEA with CO₂.     

Tuning the selectivity for ring-opening reactions of methylcyclopentane over Pt catalysts: A mechanistic study from first-principles calculations

Using density functional calculations, we studied the conversion of methylcyclopentane to its ring-opening products: branched hexanes [2-methylpentane (2MP), 3-methylpentane (3MP)], as well as unbranched n-hexane (nHx). We employed flat Pt(1 1 1) and stepped Pt(2 1 1) to describe terrace-rich large and defect-rich small Pt particles, respectively. On Pt(1 1 1), the barriers of all elementary steps for the paths leading to branched hexanes lie below 90 kJ mol^?1, while the formation of nHx features a barrier of 116 kJ mol^?1 in its C–C bond scission step. This higher barrier impedes the formation of nHx on Pt(1 1 1) and thus rationalizes the experimental observations that terrace-rich large Pt particles selectively produce branched hexanes. However, on Pt(2 1 1), the barrier of C–C scission for the formation of nHx decreases to 94 kJ mol^?1, thus implying enhanced formation of nHx over the defects, in agreement with the essentially statistical product distribution observed with defect-rich small Pt particles.     

Densification and grain growth during sintering of porous Ce0.9Gd0.1O1.95 tape cast layers: A comprehensive study on heuristic methods

The sintering behavior of porous Ce_0.9Gd_0.1O_1.95 (CGO10) tape cast layers was systematically investigated to establish fundamental kinetic parameters associated to densification and grain growth. Densification and grain growth were characterized by a set of different methods to determine the dominant sintering mechanisms and kinetics, both in isothermal and at constant heating rate (iso-rate) conditions. Densification of porous CGO10 tape is thermally activated with typical activation energy which was estimated around 440–470 kJ mol^?1. Grain growth showed similar thermal activation energy of ～427 ± 22 kJ mol^?1 in the temperature range of 1100–1250 °C. Grain-boundary diffusion was identified to be the dominant mechanism in porous CGO10 tapes. Grain growth and densification mechanism were found strictly related in the investigated temperature range. Porosity acts as a grain growth inhibitor and grain boundary mobility in the porous body was estimated around 10^?18–10^?16 m³ N^?1 s^?1 at the investigated temperature range.     

Kinetic and thermodynamic investigations of non-isothermal decomposition process of a commercial silver nitrate in an argon atmosphere used as the precursors for ultrasonic spray pyrolysis (USP): The mechanistic approach

The non-isothermal decomposition process of commercial silver nitrate used as the precursor for the USP procedure was investigated by simultaneous TGA–DTA measurements at different heating rates, in an argon atmosphere. Detailed kinetic and thermodynamic analyses, with special emphasis on the formation of a complete mechanistic scheme of the process were performed. It was found that the process under study can be described by the acceleratory power law kinetic model (P2), in the range of the extent of conversion (α) values (0.15 ≤ α ≤ 0.85), where the value of the apparent activation energy (E_a) can be considered as the constant (141.3 kJ mol⁻¹). The kinetic prediction analysis was shown that only the power law kinetic model (f(α) = 2α^1/2) gives the value of E_a which is consistent with the value obtained from the isothermal conditions. The critical temperature (T_c) of decomposition process was determined. The resulting value of T_c was in fairly good agreement with the starting temperature of thermal decomposition of silver oxide (Ag₂O). The thermodynamic functions of decomposition process are calculated by the activated complex theory and showed that the silver–oxygen bond secession can be interpreted as a “slow” stage of the decomposition process.     

Thermochemistry of onion-like carbons

The standard enthalpies of formation at 25 °C of onion-like carbons (OLC) with different structural ordering have been investigated by high-temperature oxidation calorimetry. In terms of enthalpy and depending on the degree of structural ordering, OLC can be up to 16 kJ mol⁻¹ less stable than graphite but up to 27 kJ mol⁻¹ more stable than their fullerene allotropes. Furthermore, OLC are approximately 5–9 kJ mol⁻¹ less stable than single-wall carbon nanotubes (SWCNTs). The samples prepared at 1800 °C are energetically less stable than samples made at 1300 and 1500 °C. These changes in energetics may stem from oxygen-containing functional groups bonded to the structure or from the creation of topological defects (polygonization and pentagon formation) whose concentration increases with increasing temperature and whose higher energy is balanced by configurational entropy.     

Octanol ethoxylation in microchannels

The propagation kinetics of the ring opening polymerization of ethylene oxide with octanol (“octanol ethoxylation”) was studied in a continuous microstructured reactor. Temperature was varied from 130 °C to 240 °C, and application of elevated pressure ensured a homogeneous liquid phase process. The activation energy was found to be E_A = 72.2 ± 0.9 kJ mol⁻¹. A numerical model was developed which was able to predict ethylene oxide consumption as function of residence time.The new kinetic parameters were used to determine the process intensification potential of this reaction and to study the limits of stable reactor operation.     

Kinetic modelling of the liquid-phase dimerization of isoamylenes on Amberlyst 35

Selectivity, conversion, yield and kinetics of the liquid-phase dimerization of 2-methyl-1-butene and 2-methyl-2-butene mixture have been studied in a batch stirred tank reactor in the temperature range 60–100 °C catalysed by the acid resin Amberlyst 35 and using ethanol as a selectivity enhancer. Selectivity to dimers showed a maximum at R_IA/EtOH = 20. Obtained diisoamylenes consisted mainly of 3,4,4,5-tetramethyl-2-hexene, 2,3,4,4-tetramethyl-1-hexene and 3,4,5,5-tetramethyl-2-hexene. LHHW type kinetic model was derived for the dimerization reaction as a whole. The kinetic model assumes that three active sites take part in the rate-limiting step of dimerization; the apparent activation energy for the dimerization reaction being 65 kJ mol⁻¹. A pseudohomogeneous kinetic model was used to describe the dimerization reaction network. The apparent activation energy for each dimerization reaction was found to be in the range 61–81 kJ mol⁻¹.