Investigation of the characteristics and deactivation of catalytic active center of Cr-Al_2O_3 catalysts for isobutane dehydrogenation

Deactivation mechanism of Cr-Al2O3catalyst and the interaction of Cr-A1 in the dehydrogenation of isobutane, as well as the nature of the catalytic active center, were studied using XRD, SEM, XPS, H2-TPR, isobutane-TPR and TPO techniques. The results revealed that the deactivation of Cr-Al2O3 catalyst was mainly caused by carbon deposition on its surface. The Cr3＋ ion could not be reduced by hydrogen but could be reduced to Cr2＋ by hydrocarbons and monoxide carbon. The active center for isobutane dehydrogenation could be Cr2＋/Cr3＋ produced from Cr6＋ by the on line reduction of hydrocarbon and carbon monoxide. The binding energy of Al3＋ was strongly affected by the state of chromium cations in the catalysts.

Synergistic effect between Sn and K promoters on supported platinum catalyst for isobutane dehydrogenation

Catalytic dehydrogenation of isobutane has recently received considerable attention because of the increasing demand for isobutene.In this study,the synergistic effect between Sn and K on PtSnK/γ-Al2O3 catalysts has been investigated by changing the content of Sn.It was found that with the presence of potassium,suitable addition of Sn could not only increase the metal dispersion,but also reduce the catalyst acidity.In these cases,the synergistic effect could also strengthen the interactions between the metal and support,which resulted in an increase in both catalytic activity and stability.In our experiments,Pt-0.6SnK/Al catalyst exhibited the lowest deactivation rate （12.4%） and showed a selectivity to isobutene higher than 94% at the isobutane conversion of about 45.3% after running the reaction for 6 h.However,with the excessive loading of Sn,surface property of active sites and the interactions between metal and support were changed.As a result,the initial optimal ratio between the metallic function and acid function would be destroyed,which was disadvantageous to the reaction.

MCM-36 zeolites tailored with acidic ionic liquid to regulate adsorption properties of isobutane and 1-butene

Adsorption properties of an adsorbent or a catalyst towards adsorbates are crucial in the process of adsorption separation or catalytic reaction. Surface morphology and structure of adsorbents have a significant impact on the adsorption properties. In this study, a novel acidic ionic liquid, 1-butyl-3-(triethoxysilylpropyl)imidazolium hydrogen sulfate(i.e., [BTPIm][HSO_4]), was synthesized and subsequently grafted onto the MCM-36 zeolite for the regulation of its adsorption properties towards isobutane and 1-butene. The resultant [BTPIm][HSO_4]-immobilized MCM-36(i.e., MCM-36-IL) was characterized by FT-IR, XPS, XRD, SEM, TG/DTG and N_2 adsorption–desorption measurement. It was found that the specific surface area, micropore volume and mesopore volume of the MCM-36 support underwent a reduction upon the immobilization of ionic liquid,while the surface density of acid increased from 0.0014 to 0.0035 mmol·m^(-2). The adsorption capacity of isobutane and 1-butene on the MCM-36-IL was determined by a static volumetric method. Results demonstrated that the interaction between isobutane and MCM-36-IL was enhanced and the interaction between 1-butene and MCM-36-IL was reduced. As a result, a tunable adsorption ratio of isobutane/1-butene on MCM-36 was achieved.With the increase in surface density of acid and the tunable adsorption ratio of isobutane and 1-butene on the functionalized MCM-36, the acidic ionic liquid-immobilized zeolites are beneficial to obtain an improved reaction yield and a prolonged catalyst life in the reactions catalyzed by solid acid.

Modeling of Isobutane/Butene Alkylation Using Solid Acid Catalysts in a Fixed Bed Reactor

A dynamic mass transfer model of isobutane/butene alkylation over solid acid catalysts in a fixed bed reactor was established. In the model, a modified equation for the relationship between point activity and effective diffusion coefficient was proposed. It is found that the simulation results fit the experimental data well and the breakthrough time of the bed layer is predicted accurately. By modeling the alkylation process, the time-space distribution of butene and point activity profiles of catalysts can be obtained. Furthermore, the reasons for the deactivation of solid acid catalysts were investigated. It indicates that the main reason for the deactivation of catalysts is the site coverage near the inlet of the reactor, while it is ascribed to the steric effect in the region far away from the inlet.

Dehydrogenation of isobutane to isobutene over a Pt-Cu bimetallic catalyst in the presence of LaAlO_(3) perovskite

In this study,isobutane dehydrogenation to isobutene reaction was carried out in a series of PtCu bimetallic catalysts prepared by coimpregnation method.The catalysts were characterized by means of several techniques,including XRD,N_(2) adsorptiondesorption,TEM,XPS,H2TPR and TG.The results show that the existence of LaAlO_(3) perovskite can enhance the dispersion and sintering resistance of metal nanoparticles and facilitate the transfer of carbon deposits from active sites to the support.Interestingly,the perovskite nanoparticles can also inhibit the reduction of CuOx and the formation of PtCu alloys,resulting in the suitable interaction between Pt and Cu.The PtCu/LaAlO_(3)/SiO_(2)catalyst exhibits the optimal dehydrogenation performance with an isobutane conversion of 47%and isobutene selectivity of 92%after 310 min reaction,which was ascribed to the unique role of LaAlO_(3) perovskite as well as the appropriate PtCu interaction.

Comparative catalytic study on butene/isobutane alkylation over LaX and CeX zeolites: The influence of calcination atmosphere

Lanthanum-containing(LaX)and cerium-containing X zeolites(CeX)were prepared by a doubleexchange,double-calcination method.By changing the calcination atmospheres between nitrogen and air,the Ce^(IV) contents in CeX zeolites were adjusted and their impacts on physicochemical properties and catalytic performance in isobutane alkylation were established.The crystallinity of CeX zeolite was found to be negatively correlated with the Ce^(IV) content.This i s believed to be due to the water formed during the oxidation of Ce^(III),which facilitates the framework dealumination.As a consequence,calcining in air resulted in a great elimination of strong Brønsted acid sites while under nitrogen protection,this phenomenon was mostly hindered and the sample’s acidity was preserved.When tested in a continuously flowed slurry reactor,the catalyst lifetime for isobutane alkylation was found to be linearly related to the strong Brønsted acid concentration.In addition,Ce^(3+)was found more benefit for the hydride transfer compared with La^(3+),which is ascribed to the stronger polarization effect on the CH bond of isobutane.Moreover,the decline of hydride transfer activity can be slowed down by the catalytic cracking of the bulky molecules.Based on the product distribution,a new catalytic cycle of dimethylhexanes(DMHs)involving a direct formation of isobutene rather than tert-butyl carbocation was proposed in isobutane alkylation.

Isobutane dehydrogenation over chromia alumina catalysts prepared from MIL-101: Insight into chromium species on activity and selectivity

Various mesoporous chromia alumina catalysts were prepared by five different methods based on a metal-organic framework MIL-101 and their catalytic performances over isobutane dehydrogenation were investigated. The highly dispersed chromium species were produced on catalyst KCrAI-I1 with largest specific surface area of 198 m2-g-1 prepared with aluminium isopropoxide （Al（i-OC3HT）3） by ultrasonic im- pregnation method. However, the catalyst KCrAI-I2 synthesized by stirring impregnation possessed crystalline a-Cr203 phase, which was poorly dispersed. Two types of Cr-rich and Al-rich CrzA12_zO3 solid solutions, designated as CrAI-I and CrAI-II phase, were formed over the catalysts KCrAI-I3 （prepared by Al（i-OC3HT）3 with nitric acid regulation）, KCrA1-C4 （prepared by aluminium chloride hexahydrate） and KCrA1-N5 （prepared by aluminium nitrate nonahydrate）. Catalytic evaluation results revealed that KCrAI-I1 exhibited the high isobutane con- version due to its highly dispersed chromium species. However, KCrAI-I3, KCrA1-C4 and KCrA1-N5 showed the higher isobutene selectivity （95.2%-96.4%） on account of the formation of chromia alumina solid solutions in the catalysts. Moreover, the solid solution over the chromia alumina catalysts could greatly suppress the coke formation.

Analysis of long term catalytic performance for isobutane alkylation catalyzed by NMA–AlCl3 based ionic liquid analog

Isobutane alkylation with 2-butene to produce high-quality gasoline was catalyzed by Nmethylacetamide(NMA)-AlCl3 based ionic liquid(IL) analog with a NMA/AlCl3 molar ratio of 0.75 and CuCl modification,which was marked as CuCl-modified 0.75 NMA-1.0 AlCl3.The long-term experiment was carried out in the autoclave operated in continuous mode to investigate the distribution of alkylate under different experimental nodes.The result indicated that the long-term alkylation was divided into three stages:rising,stable,and descending regions.C8 selectivity and molar ratio of trimethylpentanes(TMPs) to dimethylhexanes(DMHs) reached the highest level in the stable region,and research octane number(RON) of alkylate was as high as 97.Anionic Al species([AlbCl7]^-,[A1 CuC15]^-) and cationic Al species([AlCl2 L]^+) from IL analog as two active Lewis acidic species played a catalytic role in the long-term alkylation,whereas the neutral Al species did not participate into the alkylation.Moreover,the structure of CuCl-modified 0.75 NMA-1.0 AlCl3 was destroyed after the deactivation,and CuCl was enriched in the CD2 Cl2-insoluble substance,resulting in a decreasing TMP/DMH ratio.The catalytic lifetime of IL analog was similar with CuCl-modified 0.55 Et3 NHCl-1.0 AlCl3 IL,but IL analog had a lower cost.

Simple Preparation of Spherical Activated Carbon with Mesoporous Structure from Phenolic Resol and Associated Catalytic Performance in Isobutane Dehydrogenation

This study provides a detailed report on the synthesis of spherical activated carbon with mesoporous structure using a soluble low molecular weight phenolic resol precursor through an ammonium alginate assisted sol–gel method. The effects of calcinating temperature and the addition of CaCO_3 as a pore-enlarging agent on texture structure and catalytic performance in isobutane dehydrogenation to isobutene were investigated. Characterization of N_2 sorption,mechanical strength tests,and optical photographs confirmed that the obtained carbon materials had high mechanical strength,a good degree of sphericity,and a large surface area. Introducing CaCO_3 as a pore-enlarging agent during the preparation process promoted the formation of a mesoporous structure of carbon spheres and evidently increased the surface area and oxygen content,which can improve isobutane conversion and isobutene selectivity of these carbon spheres. The conversion of isobutane reached up to 28% for this spherical activated carbon,and the selectivity of isobutene reached up to 96%. Isobutane conversion increased with an increase in calcination temperature due to an increase in the oxygen content,whereas the selectivity of isobutene decreased due to the slight decrease in the specific surface area.

Effects of Cs-substitution and partial reduction on catalytic performance of Keggin-type phosphomolybdic polyoxometalates for selective oxidation of isobutane

The catalytic performance of Cs-substituted phosphomolybdic salts was studied for selective oxidation of isobutane. The results of activity tests revealed that 360 °C was the optimal reaction temperature. It was demonstrated that oxidizing sites not only took dominating part in the activation of isobutane, but also influenced the product distribution. Besides, appropriate Cs addition led to moderate acidity of catalysts, favoring the selectivity to desired products. Furthermore, to obtain partially reduced catalysts, different calcination atmospheres were investigated and certain proportion of Mo^（5＋） produced during calcination was crucial for the redox reaction. The catalyst calcined in N2 showed the highest yield of MAA（7.0%）. Fe-substitution enhanced the activity of catalysts by rapid reoxidation of Mo^（5＋）.

Optimal operating conditions of radial flow moving-bed reactors for isobutane dehydrogenation

In this study, radial flow moving bed reactors for isobutane dehydrogenation have been modeled and simulated heterogeneously based on mass and energy conservation laws. The considered reaction networks in the model are isobutene dehydrogenation as main reaction, and hydrogenolysis, propane dehydrogenation as well as coke formation as side reactions that all occur on the catalyst surface. Then, the process condition has been optimized to produce more isobutene under steady state condition. To prove the accuracy of the considered mathematical model and assumptions, simulation results are compared with the plant data. As a powerful method in the global optimization, the genetic algorithm has been used to optimize the considered objective function. The isobutane conversion and isobutene selectivity under optimal conditions are about 40.1% and 91%, respectively.

Kinetics of the Oxidative Dehydrogenation of Isobutane over Cr_2O_3/La_2(CO_3)_3

The oxidative dehydrogenation (ODH) of isobutane over Cr_2O_3/La_2(CO_3)_3 has been investigated in a low-pressure Knudsen cell reactor, under conditions where the kinetics of the primary reaction steps can be accurately determined. By heating the catalyst at a constant rate from 150-300℃, temperature fluctuations due to non-equilibrium adsorption are minimized. The evolved gas profiles show that ODH to isobutene and water is a primary reaction pathway, while carbon dioxide, which forms from the catalyst during reaction, is the only other product. This CO2 evolution may enhance the activity of the catalyst. Isobutene formation proceeds with the participation of lattice oxygen from the Cr2O3/La2(CO3)3 catalyst. The intrinsic Arrhenius rate constant for the ODH of isobutane isk(s-1) = 1011.5±2.2exp{-((55±5) -ΔHads kJmol-1)/RT}The small pre-exponential factor is expected for a concerted mechanism and for such a catalyst with a small surface area and limited porosity.

Relationship between Cr-Al Interaction and the Performance of Cr-Al₂O₃Catalysts for Isobutane Dehydrogenation

A series of catalysts were prepared using the kneading molding method and the impregnation method as well as the dry mix method by using different raw materials. By using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR) techniques, we studied the relationship between the catalyst performance and the Cr-Al interaction in the catalytic dehydrogenation of isobutane. The results demonstrated that the Cr-Al interaction in the catalyst had a direct influence on the catalytic activity and the selectivity of isobutene. The catalysts prepared using the kneading molding method had higher catalytic activity and isobutene selectivity than those prepared using the dry mix method. By comparison, XRD, XPS, and TPR results showed that the greater the Cr-Al interaction in the catalyst was, the higher the catalytic activity was. Here, we propose a mechanism of isobutane dehydrogenation.

Novel Pt-Ni Bimetallic Catalysts Pt(Ni)-LaFeO_3/SiO_2 via Lattice Atomic-Confined Reduction for Highly Efficient Isobutane Dehydrogenation

In this study, a series of novel Pt-Ni bimetallic catalysts supported on LaFeO_3/SiO_2 with different amounts of Ni were prepared by the lattice atomic-confined reduction of LaFe_(1-x)(Ni, Pt)_xO_3/SiO_2 perovskite precursors and applied in isobutane dehydrogenation to isobutene reaction. The catalysts were characterized by X-ray diffraction, H_2-temperature-programmed reduction, Brunauer-Emmett-Teller analysis, transmission electron microscopy, energy dispersive X-ray, CO chemisorption, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The as-synthesized Pt-Ni bimetallic catalysts possessed smaller most probable particle size with tunable Pt-Ni interaction, depending on the Ni content. The catalyst with Ni content of 3.0 wt% showed excellent activity and stability(the isobutane conversion and isobutene selectivity remained at about 38% and 92%, respectively, after 310 min) for the isobutane dehydrogenation reaction. It also provided approximately six times turnover frequency of the catalyst without Ni. The excellent activity and stability of the 3.0 wt% Ni-containing catalyst can be attributed to its small metal nanoparticles with high dispersion and suitable Pt-Ni interaction. Moreover, the Pt(Ni)-LaFeO_3/SiO_2 catalyst with Ni content of 3.0 wt% had been run for more than 35 h without obvious loss of activity,indicating its long-term stability, and the decrease in the Pt-Ni interaction that accompanied the formation of the FeNi alloy phase was thought to be responsible for the slight decrease in activity.

Selective Oxidation of Isobutane over NH_4CsFePVMoAs Heteropolycompound Catalysts

A series of NH_4Cs_(1.5)Fe_(0.08)H_xPVMo_(11)As_aO_y heteropolycompoundcatalysts for the selective oxidation of isobutane, having Keggin structure, were synthesized byusing co-precipitation method. The catalysts were characterized by FT-IR, H_2-TPR, TG-DTA, SEM andXRD. Effects of the As content, reaction time, reaction temperature and molar ratio of isobutane tooxygen in feedstock on the activity and selectivity of the catalyst were investigated. Theactivation energy of the catalysts was measured by kinetics researches. Results showed theintroduction of Cs^+ into the catalysts shortened the stable period of them and enhanced theircatalytic activity for the selective oxidation of isobutane. The highest conversion of isobutane andthe total selectivity to liquid products were 18.6% and 81.2%, respectively, which were obtained at380℃ with a space velocity of 975 h^(-1) over the NH_4Cs_(1.5)Fe_(0.08)H_xPVMo_(11) As_(0.3)O_yheteropolycompound catalyst. It is confirmed that completely oxidized products were controlled well.

Catalytic Oxidative Dehydrogenation of Isobutane over KF-Zn3（PO4）2

KF Zn 3(PO 4) 2 catalyst was found to be effective for isobutane oxidative dehydrogenation into isobutene. The effects of O 2 concentration in feed gas on isobutene yield and selectivity were discussed. The modification of zinc phosphate by potassium fluoride leads to the formation of a new compound KF Zn 3(PO 4) 2 and surface basic sites, the catalytic performance of which is thus improved.

ESR Characterization and Activity of V_2O_5/γ-Al_2O_3 Catalysts for Dehydrogenation of Isobutane

The ESR spectra of catalyst V_2O_5/γ-Al_2O_3 and its activity for the dehydrogenation of isobutane were investigated. On the basis of the results of the ESR spectroscopy associated with the TPR results, it is suggested that there is a strong interaction between V_2O_5 and γ-Al_2O_3 and the V 4+ species on the surface is the active center of catalyst V_2O_5/γ-Al_2O_3 for this reaction.

Dehydrogenation of Isobutane to Isobutene with Carbon Dioxide over SBA-15-Supported Chromia-Ceria Catalysts

A series of SBA-15-supported chromia-ceria catalysts with 3% Cr and 1%--5% Ce （3Cr-Ce/SBA） were pre- pared using an incipient wetness impregnation method. The catalysts were characterized by XRD, N2 adsorption, SEM, TEM-EDX, Raman spectroscopy, UV-vis spectroscopy, XPS and H2-TPR, and their catalytic performance for isobutane dehydrogenation with CO2 was tested. The addition of ceria to SBA-15-supported chromia improves the dispersion of chromium species. 3Cr-Ce/SBA catalysts are more active than SBA-15-supported chromia （3Cr/SBA）, which is due to a higher concentration of Cr^6＋ species present on the former catalysts. The 3Cr-3Ce/SBA catalyst shows the highest activity, which gives 35.4% isobutane conversion and 89.6% isobutene selectivity at 570℃ after 10 min of the reaction.

Keggin-type H_4PVMo_(11)O_(40)-based catalysts for the isobutane selective oxidation

Cesium heteropolysalts Cs3PMo12O40 and HCs3PVMo11O40 were synthesized by modifying the preparation conditions in order to get materials with a much higher surface area than the original Keggin-type heteropolyacids(H3PMo12O40 and H4PVMo11O40).These solids were used as carriers for the dispersion of H4PVMo11O40 heteropolyacid by the incipient wetness impregnation technique.The textural and structural properties of supports and catalysts were examined by scanning electron microscopy,N2 adsorption-desorption isotherms and Raman spectroscopy.The supported catalysts were studied before and after red/ox pretreatments by X-ray photoelectron spectroscopy,which showed that both the surface composition and oxidized to reduced species ratio depend on the used carrier.The catalytic performances of these novel supported catalysts in the selective oxidation of isobutane to methacrylic acid and methacrolein were studied.The best catalytic properties were obtained when H4PVMo11O40 was supported on HCs3PVMo11O40.The isobutane conversion and yield of the desired oxygenates increased along the unsupported H4PVMo11O40<H4PVMo11O40/Cs3PMo12O40<H4PVMo11O40/HCs3PVMo11O40 series.