Reactions Of N-butane Research Articles

Structure-dependence and metal-dependence on atomically dispersed Ir catalysts for efficient n-butane dehydrogenation

Single-site pincer-ligated iridium complexes exhibit the ability for C-H activation in homogeneous catalysis. However, instability and difficulty in catalyst recycling are inherent disadvantages of the homogeneous catalyst, limiting its development. Here, we report an atomically dispersed Ir catalyst as the bridge between homogeneous and heterogeneous catalysis, which displays an outstanding catalytic performance for n-butane dehydrogenation, with a remarkable n-butane reaction rate (8.8 mol·gIr−1·h−1) and high butene selectivity (95.6%) at low temperature (450 °C). Significantly, we correlate the BDH activity with the Ir species from nanoscale to sub-nanoscale, to reveal the nature of structure-dependence of catalyst. Moreover, we compare Ir single atoms with Pt single atoms and Pd single atoms for in-depth understanding the nature of metal-dependence at the atomic level. From experimental and theoretical calculations results, the isolated Ir site is suitable for both reactant adsorption/activation and product desorption. Its remarkable dehydrogenation capacity and moderate adsorption behavior are the key to the outstanding catalytic activity and selectivity.

Sorption of n-butane on sulfated zirconia investigated using a combination of in-situ DRIFT spectroscopy, XPS and TAP

The sorption of n-butane on sulfated zirconia (SZ) was investigated using in-situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS), Photoelectron spectroscopy (XPS) and Temporal Analysis of Products (TAP) measurements. The combination of different spectroscopic techniques like XPS and DRIFTS with transient TAP experiments offer the chance to comprehensively investigate and characterize catalytic sorption sites. Using XPS, DRIFTS, and TAP it was shown that both Brønsted centers (Zr-OH) and the very reactive disulfate groups are involved in the adsorption of n-butane and thus are mainly responsible for the isomerization reaction of n-butane to iso-butane over SZ. Moreover, all methods enable to bridge the pressure gap ranging from UHV to ambient pressure. Especially, adsorption of alkanes with XPS in-situ for different adsorption times has been done for the first time.

A theoretical study on the simultaneous vapor-liquid and chemical equilibria in a highly restricted system

In this work, the isomerization reaction of n-butane (C4(2)) to isobutane (iC4(1)) in the absence of other components is studied with the help of a model, in a relatively wide pressure range, both, under single fluid phase conditions and under vapor-liquid equilibrium conditions. In this last case the phase and chemical equilibrium are solved simultaneously. This binary system was chosen, among other considerations, because of the low number of degrees of freedom that it has, according to the phase rule for reactive systems. Such high level of restriction leads to a peculiar behavior that provides interesting insights, in particular on the simultaneous chemical and phase equilibrium. The C4(2) + iC4(1) system is represented in this work by the Soave-Redlich-Kwong equation of state coupled to quadratic mixing rules with binary interaction parameters set to zero. The required computation algorithms were developed in this work. The computed fluid phase equilibrium of this reactive system is a single univariant vapor-liquid equilibrium line. Such line ends at the only reactive vapor-liquid critical point that the system has. Some reactive isochores (or isotherms) were also computed. The obtained computation results show that for this system the conversion can be changed, under certain conditions, just by modifying the overall density, while keeping the temperature and the pressure at constant values.

Optical absorption spectroscopy of reactive oxygen and nitrogen species in a surface dielectric barrier discharge

A twin surface dielectric barrier discharge (SDBD) ignited in a dry synthetic air gas stream is studied regarding the formation of reactive oxygen and nitrogen species (RONS) and their impact on the conversion of admixed n-butane. The discharge is driven by a damped sinusoidal voltage waveform at peak-to-peak amplitudes of 8 kVpp–13 kVpp and pulse repetition frequencies of 250 Hz–4000 Hz. Absolute densities of O3, NO2, NO3, as well as estimates of the sum of the densities of N2O4 and N2O5 are determined temporally resolved by means of optical absorption spectroscopy using a laser driven broadband light source, suitable interference filters, and a photodiode detector. The measured densities are acquired across the center of the reactor chamber as well as at the outlet of the chamber. The temporal and spatial evolution of the species’ densities is correlated to the conversion of n-butane at concentrations of 50 ppm and 400 ppm, measured by means of flame ionization detectors. The n-butane is admixed either before or after the reactor chamber, in order to separate the impact of short- and long-lived reactive species on the conversion process. It is found that, despite the stationary conversion at the selected operating points, at higher voltages and repetition frequencies the densities of the measured species are not in steady state. Based on the produced results it is presumed that the presence of n-butane modifies the formation and consumption pathways of O3. At the same time, there is no significant impact on the formation of dinitrogen oxides (N2O4 and N2O5). Furthermore, a comparatively high conversion of n-butane, when admixed at the outlet of the reactor chamber is observed. These findings are discussed together with known rate coefficients for the reactions of n-butane with selected RONS.

Effects of acidity and topology of zeolites on the n-alkane conversion at low reaction temperatures

Abstract n-Butane conversion was used as a probe reaction to investigate the influence of acidity and topology of zeolites on the reaction properties at low temperatures. The results show that the number of catalytic Bronsted acid sites determines the reaction rates. The bimolecular pathway of n-butane reaction requires two adjacent acid sites, while the monomolecular reaction occurs on the isolated Bronsted sites. The contribution of bimolecular reaction primarily affected by the number of catalytic Bronsted sites and secondly influenced by the zeolite topology. The zeolites with larger cavity provide enough space for the formation and the transformation of bulky C8 intermediates in bimolecular pathway and give rise to the higher contribution of bimolecular pathway. The distribution of bimolecular products strongly depends on the maximum free sphere diameter of zeolites, while has no direct relationship with the volume of the zeolite cavities. This can be explained by a molecular sieve effect of zeolite channels on the diffusion rates of propane, isobutane, and pentane from cavities to external surface of zeolites. This improved understanding of the molecular sieve effect allows one to design and select appropriate zeolites for a special reaction.

Thermodynamic analysis of the driving force approach: Reactive systems

A thermodynamic analysis of the driving force based approach for design of reactive systems has been performed. The thermodynamic analysis is based on the element concept for the solution of the chemical and phase equilibrium. Through the thermodynamic analysis, the fundamental relationships between the element reactive driving force and the energy involved in achieving a desired reactive separation, are highlighted. In this work, the explicit relationships of the element reactive driving force with the heat of vaporization and the reaction enthalpies of the reacting mixture are shown. Validation of the thermodynamic analysis is carried out considering three binary element reactive systems: (i) the isomerization of n-butane reaction, (ii) the dimerization of propylene reaction, both reactive systems consider an inert component, and (iii) the reactive system representing the MTBE production reaction.

Effect of O2, CO2 and N2O on Ni–Mo/Al2O3 catalyst oxygen mobility in n-butane activation and conversion to 1,3-butadiene

A commercial heterogeneous Ni–Mo/Al2O3 catalyst was tested for the oxidative dehydrogenation (ODH) reaction of n-butane with different oxidant species: O2, CO2 and N2O.

Hydrogenolysis of alkanes: reactions of n-butane on Ru/zeolite catalysts

Ru/13X zeolite catalysts containing 0.6, 2 and 5.7% Ru prepared by reduction or decomposition of the hexammine complex are active and stable forn-butane hydrogenolysis between 400 and 460 K; oxidation leads to greatly increased activity.

Effects of Si/Al Ratio on the Distribution of Framework Al and on the Rates of Alkane Monomolecular Cracking and Dehydrogenation in H-MFI

The aim of this study was to investigate the influence of Si/Al ratio on the locations of exchangeable cations in H-MFI and on the monomolecular cracking and dehydrogenation reactions of n-butane. On the basis of UV-visible spectroscopic analysis of Co(II) exchanged into MFI, it was inferred that the fraction of Co(II) (and, by extension, Brønsted protons) located at channel intersections relative to straight and sinusoidal channels increases with increasing Al content. Concurrently, turnover frequencies for all monomolecular reactions, and the selectivities to dehydrogenation versus cracking and to terminal cracking versus central cracking, generally increased. The changes in selectivity with Al content are consistent with the finding that the transition-state geometry for dehydrogenation is bulky and resembles a product state, and should therefore exhibit a stronger preference to occur at channel intersections relative to cracking. Increases in turnover frequencies are attributed partly to increases in intrinsic activation entropies that compensate for concurrent increases in intrinsic activation energies, most strongly for dehydrogenation and terminal cracking, resulting in increased selectivity to these reactions at higher Al content. This interpretation contrasts with the view that intrinsic activation barriers are constant. It is also observed that isobutene inhibits the rate of n-butane dehydrogenation. Theoretical calculations indicate that this effect originates from adsorption of isobutene at the channel intersections. Because cracking reaction rates are not affected by the presence of isobutene, this result suggests that the preference of dehydrogenation to occur at channel intersections is much stronger than the preference for cracking to occur at these locations.

Synchrotron Photoionization Mass Spectrometry Measurements of Product Formation in Low-Temperature n-Butane Oxidation: Toward a Fundamental Understanding of Autoignition Chemistry and n-C4H9 + O2/s-C4H9 + O2 Reactions

Product formation in the laser-initiated low-temperature (575-700 K) oxidation of n-butane was investigated by using tunable synchrotron photoionization time-of-flight mass spectrometry at low pressure (∼4 Torr). Oxidation was triggered either by 351 nm photolysis of Cl2 and subsequent fast Cl + n-butane reaction or by 248 nm photolysis of 1-iodobutane or 2-iodobutane. Iodobutane photolysis allowed isomer-specific preparation of either n-C4H9 or s-C4H9 radicals. Experiments probed the time-resolved formation of products and identified isomeric species by their photoionization spectra. For stable primary products of butyl + O2 reactions (e.g., butene or oxygen heterocycles) bimodal time behavior is observed; the initial prompt formation, primarily due to chemical activation, is followed by a slower component arising from the dissociation of thermalized butylperoxy or hydroperoxybutyl radicals. In addition, time-resolved formation of C4-ketohydroperoxides (C4H8O3, m/z = 104) was observed in the n-C4H9 + O2 and Cl-initiated oxidation experiments but not in the s-C4H9 + O2 measurements, suggesting isomeric selectivity in the combined process of the "second" oxygen addition to hydroperoxybutyl radicals and subsequent internal H-abstraction/dissociation leading to ketohydroperoxide + OH. To further constrain product isomer identification, Cl-initiated oxidation experiments were also performed with partially deuterated n-butanes (CD3CH2CH2CD3 and CH3CD2CD2CH3). From these experiments, the relative yields of butene product isomers (cis-2-butene, trans-2-butene, and 1-butene) from C4H8 + HO2 reaction channels and oxygenated product isomers (2,3-dimethyloxirane, 2-methyloxetane, tetrahydrofuran, ethyloxirane, butanal, and butanone) associated with OH formation were determined. The current measurements show substantially different isomeric selectivity for oxygenated products than do recent jet-stirred reactor studies but are in reasonable agreement with measurements from butane addition to reacting H2/O2 mixtures at 753 K.

Effect of cofeeding n-butane with methanol on aromatization performance and coke formation over a Zn loaded ZSM-5/ZSM-11 zeolite

The aromatization performance of cofeeding n-butane with methanol was investigated in a fixed bed reactor over a Zn loaded ZSM-5/ZSM-11 zeolite. Coke deposited on the catalyst was characterized by TG, TPO, FTIR and Raman technique. During the 48h life-test under the conditions of 510°C, 0.4MPa, WHSV (CH2)=2h−1, pure n-butane reaction performance of the catalyst changes little, but high LPG and low aromatics yields are obtained. Only under the suitable n-butane/methanol ratio, the relatively good aromatics selectivity can be obtained and maintained during the reaction process. Compared with methanol aromatization, the integrated process exhibits a much slower rate of deactivation. Addition of methanol in n-butane inhibits the dry gas production while promotes the LPG production. Water generated from methanol dehydration may occupy the catalytic active sites for olefins aromatization over the catalyst and thus suppress the production of total aromatics. In fact, water mainly suppresses benzene formation and this inhibiting effect increases with n-butane content in the feed, in contrast to its promoting effect on formation of C8 and C9 aromatics. Increasing methanol content in the feed, a simultaneous increase in coke content of the spent catalyst and preference for coke deposition in micropore is observed. Furthermore, the fraction of aliphatics in coke decreases, while those of less-condensed aromatics and polyaromatics incease, which is associated with the increased deactivation of catalyst.

Insights into the Kinetics of Cracking and Dehydrogenation Reactions of Light Alkanes in H-MFI

Monomolecular reactions of alkanes in H-MFI were investigated by means of a dispersion-corrected density functional, ωB97X-D, combined with a hybrid quantum mechanics/molecular mechanics (QM/MM) method applied to a cluster model of the zeolite. The cluster contains 437 tetrahedral (T) atoms, within which a T5 region containing the acid site along with the representative alkane is treated quantum mechanically. The influence of active site location on reaction energetics was examined by studying cracking and dehydrogenation reactions of n-butane at two regions in H-MFI–T12, where the proton is at the intersection of straight and sinusoidal channels, and T10, where the proton is within the sinusoidal channel. Two transition states were observed for cracking: one where the proton attacks the C–C bond and another where it attacks a C atom. Dehydrogenation proceeds via a concerted mechanism, where the transition state indicates simultaneous H2 formation and proton migration to the framework. Intrinsic activatio...

Immobilizing Carbon Nanotubes on SiC Foam as a Monolith Catalyst for Oxidative Dehydrogenation Reactions

Carbon nanotubes (CNTs) have received widespread attention in the fields of emission displays, high-strength composites, fuel cells, and lithium-ions batteries owing to their unique chemical and physical properties. Recent reports have demonstrated that CNTs are also promising heterogeneous catalysts in the oxidative dehydrogenation (ODH) reactions of n-butane, 1-butene, ethylbenzene, propane, and ethane. Their outstanding catalytic performance makes CNTs candidates for replacing traditional-metal/metal-oxide catalysts in sustainable chemistry. However, commercial CNTs are typically produced in their powdered form by using the chemical vapor decomposition (CVD) method. These CNT powders tend to agglomerate and are difficult to filter in slurry-phase operation. In addition, owing to a high pressure drop, CNT powders cannot be directly used in fixed-bed reactors. Therefore, it is necessary to form CNTs into larger objects. The method of the compaction of powdered CNTs often inhibits the access of reactants to its active sites and may destroy individual CNTs during the compacting process. The immobilization of CNTs onto certain supports is a more practical approach to solving these problems. Some efforts have been made to grow CNTs or carbon nanofibers (CNFs) on carbon felt, activated carbon, and Lava supports for ODH reactions and CNFs have been formed on porous supports (carbon foams, metal foams, and porous ceramic materials) for catalytic applications. Silicon carbide (SiC) foam possesses high thermal conductivity and mechanical stability and it can be used as a support to immobilize catalysts, which could overcome the disadvantage of the pressure drop in fix-bed reactors. Recently, zeolites have been coated onto SiC substrates, thereby forming zeolite/SiC catalysts. Nitrogen-doped CNTs that are grown on SiC foam show high catalytic activity in the selective oxidation of H2S. [13] For the industrial production of alkenes with CNT catalysts, the growth of CNTs on SiC foam as a monolith catalyst is desirable and the strength of the interactions between the CNTs and the support also needs to be considered. Herein, we report a new method for immobilizing CNTs onto SiC foam as a CNT/SiC monolith and its application in ODH reactions. Structural characterization of the as-prepared monolith shows that the CNTs completely and firmly cover the surface of the SiC foam. Moreover, the CNT/SiC monolith exhibits remarkable catalytic performance and stability in the ODH reaction of 1-butene into butadiene. SiC foam was prepared by a combination of macromolecule pyrogenation with chemical bonding. The foam plastic was soaked in a mixed slurry of SiC powder, resin, EtOH, and silicon powder. After drying and solidification, the foam composite was pyrolyzed and sintered under an argon atmosphere, thereby affording the SiC foam. The preparation process of the CNT/SiC monolith is shown in Scheme 1. Firstly, the SiC foam was immersed in an aqueous solution of Fe(NO3)3, Mg(NO3)2,

Application of a Simple Lumped Kinetic Model for the Catalytic Cracking Reaction of n-Butane Over the HZSM-5 Zeolite

A simple lumped kinetic model appliedton-butane catalytic cracking over synthesized HZSM-5 zeolite, with SiO2/Al2O3=484 in a stainless steel plug flow reactor under various operating conditions developed. To suitably estimate the kinetic parameters of catalytic cracking reactions of n-butane, a lumped kinetic modelconsisted of 6 reaction steps and 5 lumped compoundsconsidered. The Levenberg-Marquardt algorithm utilized toestimate kinetic parameters using MATLAB R.7.10 software.In order to confirm the significance of optimized parameters, the statistical F-test and variance analysis indicatingthe accuracy of the calculated data were employed. Furthermore, the validity of the model was confirmed. Ultimately, results from the proposed kinetic model showed excellent agreement with experimental data.

Towards understanding sodium effect on USY zeolite

Samples of a commercial USY zeolite were exchanged with NaCl solution and then thermally treated under dry atmosphere. Resulting samples were evaluated by n-butane cracking, isopropylamine ( i-pam) decomposition, TG–DSC, nitrogen adsorption, and powder XRD. Activation barriers for n-butane reactions were determined as a function of sodium load in the samples. The addition of sodium chloride has a marked negative effect upon material crystallinity, but dealumination rate does not seem to be affected, which suggests framework destruction mediated by sodium does not involve aluminum atoms. As sodium was loaded, samples were less able to retain water because of loss in crystallinity as shown by XRD and volume of micropore measurements. Activity to n-butane cracking decreases monotonically with sodium load, but selectivity to cracking, dehydrogenation, and hydride transfer products remained unaltered; that shows sodium chloride effect is not selective to a particular kind of acid site. The Brønsted acid site (BAS) density measured by i-pam decomposition exhibits a volcano-like profile with sodium load; at low sodium contents there is a gain in i-pam accessibility due to structural collapse but at higher loads BAS disappearance overcomes accessibility and density of acid sites decreases. The average pore size increases with sodium load. After thermal treatment in presence of sodium chloride, gates are opened in zeolite structure, allowing n-butane and isopropylamine to reach formerly inaccessible sites; hydrocarbon conversion can occur in spatially constrained environments where stabilization effects lead to low-energy/low-entropy activated complexes and apparent activation energy shows a minimum with sodium chloride load. Sodium chloride has an effect on Y zeolite even in dry conditions at temperatures similar to those predominant in FCC regenerator.

The Study Of Deactivation And Regeneration Of A Fluid Cracking Zeolite Catalysts

The catalytic activities of modified and unmodified sodium Y-Zeolites catalysts was investigated. Modification by framework dealumination and gallium oxide incorporation led to an increase in catalytic activity. The catalytic activities of both parent and the modified samples were reduced in a time dependent manner due to coke deposition. Incorporation of gallium oxide altered the reaction pathways of C8 intermediates from n-butane reactions, thus producing more aromatic products. However, Ga2O3 in excess of 1% resulted in a large decrease of catalytic activity due to loss in Bronsted acidity. As Bronsted sites are removed, direct protonation of the alkane became difficult and catalytic activity was lowered at high Ga2O3 loading. The oxygen adsorption isotherms of the various coked catalysts showed that the deactivation process involved blockage of the zeolite pore sites by the deposited coke. Interestingly, regeneration of the coked sample was seen to completely restore the catalytic activity of both samples.

High Surface Area Vanadium Phosphate Catalysts for n-Butane Oxidation

Vanadium phosphorus oxide (VPO) was prepared using the precipitation procedure and tested for potential use in the partial oxidation reaction of n-butane to maleic anhydride. In particular, the effect of reducing agents such as the isobutanol, 1-butanol, and glycol, subsequent water treatment, and microwave heating were investigated in detail. The optimum synthesis conditions were identified with respect to catalyst activity for the oxidation of n-butane. The activity and selectivity of VPO prepared catalysts have been evaluated in a fixed bed microreactor and in situ gas chromatography (GC) was used to evaluate the system efficiency and analyze the product effluent stream. The different catalysts exhibited a range of activities and selectivities under the same reaction conditions. The range in catalyst performance may be attributed to the crystal size as well as particle size of catalyst. The results were interpreted in terms of surface area and catalyst nanostructure, and it has been generally concluded...

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H2-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO4 · 1/2 H2O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H2-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V4+–O− pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.

Investigation of the thermal stability of the chromia-alumina catalysts for the process of the one-stage dehydrogenation of n-butane

The effect of the hugh-temperature (800–1000°C) treatment in air of the Cr2O3/Al2O3 catalyst, which contained 18Cr2O3 + 0.4Na+, wt % and was prepared under laboratory conditions with the use of Pural SB1 grade high-purity pseudoboehmite, on the variation in the phase composition of the catalyst, specific surface, and catalytic characteristics in the dehydrogenation reaction of n-butane (yield and selectivity by ΣC4 olefins and 1,3-butadiene, conversion of n-butane) was investigated depending on the calcination temperature of the catalyst. It is shown that thermal stability depends on the following main factors: the method preparation of catalysts, the phase composition of the starting aluminum hydroxide, carrier texture, and the presence of modifying additions and impurities of other metals. In the case of the same chemical composition of the catalyst, the samples obtained by the wet mixing of pseudoboehmite with an aqueous solution of chromic anhydride are most thermally stable compared with the impregnation samples. It is established that the addition of cerium improves the thermal stability and activity of the impregnation Al-Cr catalyst, while the impurity of the Fe3+ ions (up to 0.1 wt %) does not worsen these characteristics. The investigated samples of the catalyst are more thermally stable than the imported industrial catalyst, which loses activity and specific surface after calcination at 900–1000°C. The determination of the thermal stability of fresh catalysts and the factors affecting it can be used as the preliminary evaluation of the future lifetime of catalysts.

Oxidation of organic compounds in a microstructured catalytic reactor

A microstructured catalytic reactor for the oxidation of organic compounds has been fabricated from aluminum alloy AlMgSiCu1 (6082 series, Al51st). The catalyst section was assembled of 63 microstructured plates with catalytic coating. In each plate of 416m thickness, 45 semicylindrical microchannels of 208m in radius with a distance in between of 150m were electrodischarge machined. A porous alumina layer of 29 ± 1m thickness was produced on the plates by anodic oxidation. The resulting coatings were impregnated with an aqueous solution of copper dichromate followed by drying and calcination at 450 ◦ C to produce active catalysts. Kinetics of deep oxidation of organic compounds n-butane, ethanol, and isopropanol was studied in the reactor at 150–360 ◦ C and of 1,1-dimethylhydrazine (unsymmetrical dimethylhydrazine, UDMH) at 200–375 ◦ C. Intermediate reaction products in the reactions of alcohols and UDMH oxidation were identified. For UDMH, these are methane, dimethylamine, formaldehyde 1,1-dimethylhydrazone, and 1,2-dimethyldiazene. Nitrogen atoms from the UDMH and N-containing intermediates were shown to convert mainly to N2. Kinetic parameters of the reactions of n-butane and alcohols (rate constants and apparent activation energies) were calculated using kinetic modeling based on a modified method of quickest descent. © 2007 Elsevier B.V. All rights reserved.