Isobutene Yield Research Articles

N‐Butane Isomerization over 0.5 Pt/HPA/MCM‐41 Catalyst: Optimization Study Using Central Composite Design

AbstractIn this research, 0.5 % platinum (Pt)/HPAs/MCM‐41 (Mobil Composition of Matter No. 41) bifunctional catalysts (with different weight percentages of phosphotungstic acid (HPW) and silicotungstic acid (HSiW)) were synthesized for use in the n‐butane isomerization. The synthesized catalysts were characterized by X‐ray diffraction, X‐ray fluorescence spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, Brunauer–Emmett–Teller, and ammonia temperature‐programmed desorption analyses. The best catalytic activity and highest initial isobutane yield at 250 °C, weight hourly space velocity of 0.15 h−1, and H2/nC4 molar ratio of 3:1 belonged to 0.5 %Pt/20 % HPW/MCM‐41 (41 %) and 0.5 % Pt/40 % HSiW/MCM‐41 (45 %) samples. Subsequently, the yield of isobutane over these catalysts was evaluated and optimized by a central composite experimental design. According to the quadratic model, the maximum isobutane yield under optimal operating conditions was 34.5 % and 41.36 %, respectively, for 0.5 % Pt/20 % HPW/MCM‐41 and 0.5 % Pt/40 % HSiW/MCM‐41 catalysts.

Boosting activity of γ-alumina-supported vanadium catalyst for isobutane non-oxidative dehydrogenation via pure V3+

The vanadium-based dehydrogenation (DH) catalyst is becoming a promise alternative to the industrial used Pt- and Cr-based catalysts, due to lower cost and less environmental threat. However, the low DH activity hampered the industrial application of vanadium-based catalysts. Herein, for the first time, we introduce a method to prepare high-efficiency vanadium-based catalyst by constructing pure V3+ species on γ-Al2O3 through treatment of as-prepared thiovanadate. The V3+ species contributes to not only enhancing the DH activity, but also fabricating the V3+-O/S acid-base pair with ideal strength and stability. The isobutene yield can reach as high as 56.9 wt%. Only Lewis acid is recognized on V3+/Al2O3 catalyst, while no Brønsted acid remains. The side-reactions are consequently inhibited, and the selectivity to isobutene is improved. Besides, with the increase of vanadium loadings, the Lewis acid content increases at first and then decreases, and the content of acid sites in middle strength keeps decreasing. Though the deposited coke on V3+/Al2O3 was just 2.5 wt% during 8.5 h consecutive DH reaction, the valence state of vanadium was still influenced and the fraction of inert V4+ species increased steadily. This study will improve the potential for industrial application of vanadium-based DH catalyst, and offer theoretical guidance for optimization of ideal DH catalysts.

Improved Catalytic Performances of the NaOH-Treated ZSM-22 Zeolite in the 1-Butene Skeletal Isomerization Reaction: Effect of External Acid Sites.

In this paper, a series of alkaline-treated ZSM-22 zeolite samples were prepared by treating the parent ZSM-22 zeolite using NaOH aqueous solution with different concentrations. By investigating the effects of alkaline treatment on the parent ZSM-22 zeolite, we discovered that the alkaline treatment contributed to the reduction of Brønsted acid sites due to the coverage of extra-framework Al on its external surface. In addition, it was found that the alkaline-treated samples were favorable to the improvement of the isobutene yield and selectivity, while these features appeared to be low for the subsequent acid-washed counterparts in the skeletal isomerization reaction of 1-butene. These results indicate that the catalytic performance of ZSM-22 zeolite is related to reduced amounts of Brønsted acid sites in it. To further reveal the reasons for the promoted catalytic performances of the alkaline-treated ZSM-22 series zeolites, we studied the properties of coke deposited on the two series of samples using Raman spectroscopy and thermogravimetric analysis and mass spectrometry (TG/MS-TPO). It was shown that the carbon deposited on the alkaline-treated series samples was mainly distributed at the outer surface, while the coke was distributed to a relatively lesser extent at the exterior surface for the acid-washed series samples. Moreover, by partially passivating outer acid sites of the parent zeolite, the selected alkaline-treated zeolite, and acid-washed zeolite, their isobutene selectivities were all improved with the decrease in outer acid sites. These phenomena confirmed that the improved catalytic performances of the alkaline-treated samples are related to their decreased external Brønsted acid site density, which further demonstrated that the high isobutene yield and selectivity in the skeletal isomerization reaction of 1-butene is realized via the monomolecular reaction pathway of 1-butene.

Catalytic Performance and Kinetic Modeling of n-Butane Isomerization over Metal Bearing Silicotungstic Acid Supported on Mesoporous KIT-6

In this study, mesoporous KIT-6 support was synthesized hydrothermally. Its structure was improved by impregnation with phosphotungstic acid (HPW), silicotungstic acid (HSiW), and some metals such as Pt, Pd, and Ce. This work converts mesoporous KIT-6 into a bifunctional catalyst containing acid and metal sites. The structures of the synthesized catalysts were characterized by various analyses such as XRD, XRF, FTIR, SEM, EDS, BET, NH3-TPD, and TGA. Furthermore, the catalytic performances of heteropoly acids (HPW and HSiW) and KIT-6 modified samples were investigated in an n-butane isomerization process. Impregnation of HSiW on mesoporous support resulted in higher initial n-butane conversion and isobutane selectivity and improved catalyst stability compared to the unsupported sample. The introduction of 0.5 wt % platinum on the 60%HSiW/KIT-6 led to the highest n-butane conversion (72%) and isobutane yield (56.8%) at 250 °C and weight hourly space velocity (WHSV) of 0.15 h–1, at the beginning of the reaction. The 0.5%Pt/60%HSiW/KIT-6 sample was the most stable catalyst against deactivation compared to other modified samples. Its conversion and its selectivity to isobutane reached 51.8 and 73.8%, respectively, after 7 h of reaction. Also, the influence of different process variables such as reaction temperature, WHSV, and carrier gas on the catalyst activity (n-butane conversion, isobutane selectivity, and yield) was investigated. In addition, the classical bifunctional model was used to describe the kinetic behavior of 0.5%Pt/60%HSiW/KIT-6 in the isomerization reaction. This model inhibits the hydrogen and n-butane pressure effects on the reaction rate and agrees with the ideal bifunctional metal–acid mechanism. The activation energy for this model was found to be 29.64 kJ mol–1. The classical bifunctional model provided a reasonable fit for the experimental data and allowed to specify the kinetic parameters.

Hydrothermal acid etching of alumina beads and its effect on isobutane dehydrogenation performance over Pt/γ-Al2O3 catalyst

Millimeter-scale spherical alumina used as the support of Pt catalyst for isobutane dehydrogenation was prepared by alginate assisted sol-gel method, and then hydrothermal treated in 0.5 M HNO3 to improve its porous structure and surface properties. The effects of acid etching time on the composition, structure, surface acidity, redox property, catalytic dehydrogenation performance and carbon deposition of catalyst were investigated by ICP-OES, SEM, TEM, XRD, N2 adsorption-desorption, 27Al MAS NMR spectra, NH3-TPD, H2-TPR, isobutene transient response experiment and TGA. The results show that the acid treatment can effectively remove zinc species and partial aluminum from the surface of alumina beads, which can create defect sites and increase the specific surface area of alumina, thus improving the dispersion of Pt particles. In addition, acid etching significantly increases the acidity of alumina and enhances the interaction between Pt particles and alumina through the formation of defect sites. The Pt/Al2O3-NA-3 possesses the smallest size of Pt particles, the strongest Pt-alumina interaction, maximum number of acid sites and the highest isobutene yield of 33% even after 270 min reaction.

Isobutane dehydrogenation over high-performanced sulfide V-K/γ-Al2O3 catalyst: Modulation of vanadium species and intrinsic effect of potassium

Compared with industrial used Pt- and Cr-based catalyst in dehydrogenation (DH) of light alkanes, the sulfide V-K/γ-Al2O3 catalyst reported in this study shows lower cost and toxicity, and significant DH performance. The yield to isobutene reached as high as 52.9%, which is among the highest reported to date. We attribute such high isobutene yield to the precise modulation of polymerization degree for vanadium species via doping of potassium and indicating that the synergy between vanadium species and acid sites is critical to enhance the DH performance. Our previous work showed sulfidation promoted the increase of DH performance for vanadium-based catalyst, and we go further in this study to explore the correlation between increased range of DH performance and the added potassium. The different loaded potassium leads to variation in sulfidation degree, affecting the properties of vanadium species and acid properties consequently. The potassium was distributed uniformly on surface of the sulfide vanadium-based catalyst and was predominantly bonded with the vanadium species rather than with the γ-Al2O3 support. With increasing the potassium amount from 0 to 3 wt%, the acid amount kept decreasing, and some specific strong acid sites appeared once adequate sulfur was introduced in the V-K/γ-Al2O3 catalyst. The characterization and DFT results both revealed that the doped potassium contributes to regulating the vanadium species in the oligomeric state. The synergy between vanadium species and acid properties was regulated by the added potassium simultaneously, and thus the DH performance was enhanced. This study provides promising strategy for preparation of environment-friendly model industrial DH catalyst.

Enhancement of the Catalytic Activity Associated with Carbon Deposition Formed on NiO/γ-Al<sub>2</sub>O<sub>3</sub> Catalysts during the Direct Dehydrogenation of Isobutane

The dehydrogenation of isobutane in the presence of CO2 over NiO supported on γ-Al2O3 was examined. For comparison, Cr2O3 supported on γ-Al2O3 was also used. It is generally accepted that a catalyst used for the dehydrogenation of various alkanes will suffer catalyst deactivation due to the formation of carbon deposits. In the present study, the yield of isobutene was significantly decreased with time-on-stream due to carbon deposition when using Cr2O3(x)/γ-Al2O3, in which x indicates the loading of a corresponding oxide by weight %. However, carbon deposits also were evident on NiO(x)/γ-Al2O3, but the yield of isobutene was enhanced with time-on-stream depending on the loading (x). This indicates that the contribution of the carbon deposition in the dehydrogenation on NiO(x)/γ-Al2O3 definitely differed from that on an ordinary catalyst system such as Cr2O3(x)/γ-Al2O3. In order to confirm the advantageous effect that carbon deposition exerted on the yield of isobutene, NiO(x)/γ-Al2O3 was first treated with isobutane and then the catalytic activity was examined. As expected, it became clear that the carbon deposits formed during the pretreatment contributed to the enhancement of the yield of isobutene. The presence of a Ni-carbide species together with the metallic Ni that was converted from NiO during dehydrogenation definitely enhanced of the yield of isobutene. Although carbon deposition is generally recognized as the main cause of catalyst deactivation, the results of the present study reveal that carbon deposition is not necessarily the cause of this phenomenon.

Fabrication of AEI-type aluminosilicate catalyst with sheet-like morphology for direct conversion of propene to butenes

A new class of AEI-type aluminosilicates with sheet-like morphology was synthesized by the CGI-assisted method, which showed a higher iso-butene yield than the cubic particle during the PTB reaction.

Promoted mesoporous Fe-alumina catalysts for the non-oxidative dehydrogenation of isobutane

Production of isobutene is commercially consequential and highly demanding from the end-use industries being a key platform molecule as well as an intermediate for a variety of value-added chemicals. Traditionally, isobutene is prepared via steam cracking and fluid catalytic cracking methods. However, the catalysts used in these conventional methods have disadvantages like coke formation, sintering, etc. In this study, the catalytic non-oxidative dehydrogenation of isobutane over acidic, alkaline, and noble metal promoted mesoporous iron-doped catalysts was investigated. Iron doping has a significant function in controlling isobutene selectivity. The synthesis method is crucial to achieve successful metal doping in the mesoporous alumina matrix. Promoted catalysts exhibited a notable difference in isobutane conversion with a marginal change in dehydrogenation selectivity. Silver promoted catalyst showed slightly higher isobutene yield due to the optimal catalytic properties. Thiscatalyst was stable for a considerable duration, and coke deposition, as well as particle agglomeration, were observed to faintly inhibit the catalytic activity.

Activity of a Sulfated Zirconia Catalyst in Isomerization of n-Butane Fractions

Palladium-containing catalyst based on binder-free granular sulfated zirconium oxide for n-butane isomerization has been investigated. It has been found that Pd content of 0.2–1.0 wt % slightly influences textural characteristics and other physicochemical properties of bifunctional catalysts; however, it determines their activity and selectivity in the reaction studied, with the optimal palladium content being 0.5 wt %. Parameters of the isomerization process have been studied depending on the composition of industrial n-butane fractions. It has been shown that impurities of isobutane, propane, neopentane, isopentane and pentane in an amount of no more than 2% do not exert a effect on isobutane production; nonetheless, the conversion of n-butane and selectivity for isobutane both increase when more pure n-butane fractions are used. It has been found that the process for isobutane production by isomerization of the n-butane fraction under the optimal conditions at H2 /n-C4 = 0.1 and 140–150°C makes it possible to obtain a high isobutane yield (up to 52 wt %) and avoid the undue formation of С1–С3 alkanes.

CATALYTIC ACTIVITY AND COKING RESISTANCE ON HYDROXYAPATITE FOR THE OXIDATIVE DEHYDROGENATION OF ISOBUTANE

Hydroxyapatite (HAp) samples were prepared from precursor solutions with different Ca/P molar ratios. The HAp samples were utilized as catalysts for the oxidative dehydrogenation (ODH) of isobutane. Also, the HAp samples were characterized by N2 and H2O adsorption-desorption measurements, X-ray diffraction, inductively coupled plasma optical emission spectrometry, thermogravimetry-differential thermal analyses, and temperature-programmed oxidation (TPO). The Ca/P molar ratio of the precursor solution had a major influence on the catalytic activity and coking resistance ability. The HAp catalyst prepared from the stoichiometric precursor solution [HAp (1.67)] showed the highest isobutene yield. Furthermore, the white color of HAp (1.67) remained unchanged during the ODH of isobutene, indicating the absence of coke on the surface of HAp (1.67) after the reaction and its higher coking resistance ability. The results of the TPO measurements showed that the HAp (1.67) catalyst possesses the strongest oxidation ability which is likely related to its higher coking resistance ability.

Formation and Regeneration of Shape-Selective ZSM-35 Catalysts for n-Butene Skeletal Isomerization to Isobutene

Skeletal isomerization of n-butene to isobutene was performed over formed ZSM-35 zeolites in a lab-scale, fixed-bed reactor. The formation conditions to produce isobutene with zeolites were varied to determine the most advantageous binding agent (pseudo-boehmite) amount and steam dealumination conditions. The optimal binding agent amount was 20 wt %. Steam dealumination stabilized the catalysts and enhanced the catalytic performance because of the stable Si/Al framework and suitable acidity. The optimized process conditions involved a reaction temperature of 410 °C, a weight hourly space velocity of 5 h–1, and an n-butene concentration in feedstock of 50%. After being on-stream for 296 h, the catalysts were stable and were able to be regenerated with a comparable catalytic performance. An isobutene yield of 33–43 wt % was achieved, and the selectivity of isobutene was higher than 90% after the reaction was carried out for longer than 15 h. Carbon deposition modified the pore structure to enhance the selectivity of isobutene because of the selective shape effect. This study shows promising results for future industrialization of the skeletal isomerization of n-butene to isobutene in the presence of optimized ZSM-35 catalysts.

Introduction of a Small Amount of Chromium to Enhance the Catalytic Performance of SBA-15 for the Oxidative Dehydrogenation of Isobutane to Isobutene

Various solid oxide catalysts are active for the oxidative dehydrogenation of propane, but achieve yields of less than 2% when these were used for the oxidative dehydrogenation of isobutane to isobutene. An improved isobutene yield of 8% was obtained in the oxidative dehydrogenation of isobutane by using a folded-sheet mesoporous material (FSM-16) and mesoporous molecular sieve (MCM-41), with or without chromium via impregnation or template-ion exchange methods. Further activity enhancement, however, could not be attained. In the present study, chromium cations were introduced into the framework of SBA-15 using a direct synthesis method by mixing chromium nitrate with tetraethylorthosilicate in an aqueous solution with an initial pH of 1.5 and subsequent hydrothermal treatment. Loading the resultant catalyst, SBA-15, with a small amount of chromium (1.84 wt% Cr-SBA-15) increased the isobutene yield (15.4%) at 723 K, while a previous report showed that chromium oxide supported on SBA-15 (CrOx/SBA-15) prepared via incipient wetness impregnation had a lower isobutene yield (11%) at 813 K. To explain this improvement, the catalysts were characterized using X-ray diffraction, transmission electron microscopy, N2 adsorption-desorption, and NH3 temperature-programmed desorption. Cr-SBA-15 was found to have a large specific surface area (1,610 m2/g), although the structural and acidic nature of the catalyst was similar to the general properties of other mesoporous silicas. The conversion of Cr3+ species into Cr6+ on Cr-SBA-15 with the large specific surface area could affect the improvement of the oxidative dehydrogenation of isobutane to isobutene.

Influence of different precursors on isobutene production from bio-ethanol over bifunctional Zn1Zr10Ox catalysts

The effects of zinc precursors on the performance of bifunctinal Zn1Zr10Ox catalysts were investigated for direct conversion of bio-ethanol into isobutene (ETIB). The catalyst derived from zinc acetate precursor exhibited full ethanol conversion and 50% isobutene yield at a WHSV of 0.38 gethanol gcat−1 h−1 at 450 °C. Characterization results based on XRD, FT-IR, UV–vis, Raman, BET, CO2-TPD, NH3-TPD, H2-TPR, and XPS revealed that Zn1Zr10Ox catalyst prepared by zinc acetate precursor with large anion size possessed the highest zinc dispersion and the strong ZnO-ZrO2 interaction, and presented the highest ethanol reaction rate and isobutene yield under kinetic control regime. The beneficial effect of larger anion size was rationalized by the shielding effect in the incorporation model dominated by the size of capping anion in zinc precursors. Further durability test showed that the catalyst exhibited slow deactivation against coking formation while isobutene yield was maintained above 45% for 18 h.

Isomerization of n-Butane over Cost-Effective Mordenite Catalysts Fabricated via Recrystallization of Natural Zeolites

The as-received natural zeolites showed a mordenite (MOR) dominant phase with impurity phases and low total surface area (ca. 133 m2/g) as indicated by X-ray diffraction and nitrogen physisorption analysis, respectively. High-quality MOR, i.e., high crystallinity and large surface area (ca. 327 m2/g), was obtained through hydrothermal recrystallization of the low-grade natural mordenite followed by a microwave-assisted acidic ion-exchange. The time of hydrothermal recrystallization affected the physical and chemical properties of the recrystallized natural mordenite. The n-butane isomerization in a fixed bed reactor was selected as a model reaction to evaluate the samples. The n-butane conversion was boosted from 3.5% over the parent natural MOR (H-P-H) to 25% over the recrystallized natural MOR (H-R26-H). The yield of isobutane over the recrystallized natural MOR was 8%, which was far higher than the one over parent natural MOR with only 1%. The recrystallized natural MOR also exhibited a comparable isob...

Direct synthesis of iso-butane from synthesis gas or CO2 over CuZnZrAl/Pd-β hybrid catalyst

The effect of various factors on the catalytic performance of iso-butane formation over CuZnZrAl/Pd-β hybrid catalyst via synthesis gas or CO2 hydrogenation has been deeply investigated in this work. It was interesting to note that the iso-butane/n-butane ratio value was much higher than that of thermodynamic equilibrium (about 1/1), whose value was directly related to the reaction condition using this hybrid catalyst. In order to further clearly clarify this finding, various experimental reaction factors were selected to investigate the formation of iso-butane. The results revealed that increasing temperature, H2/COx, CO2/COx, and/or Pd loading possessed an inhibiting effect on the iso-butane yield. High selectivity of iso-butane could be achieved by increasing the reaction pressure, W/F and the weight ratio of CuZnZrAl methanol catalyst to Pd-β catalyst. It is also noted that the addition of water seriously suppressed the reaction activity, resulting in the low ratio of iso-butane/n-butane. A possible reaction route was elucidated based on the latest results. This might shed light on the development of a high efficient catalyst for iso-butane production from synthesis gas or CO2 hydrogenation.

Facile preparation of PtSn-La/Al2O3 catalyst with large pore size and its improved catalytic performance for isobutane dehydrogenation

Al2O3 beads with various pore sizes were successfully synthesized by combining ammonium alginate assisted sol-gel and emulsion template methods using liquid paraffin as pore-enlarging agent and polysorbate 80 as dispersant. The great influences of textural structure of Al2O3 beads on the activity and stability of PtSn-La/Al2O3 catalyst for isobutane dehydrogenation were investigated by N2 adsorption-desorption, XRD, NH3-TPD, SEM, HRTEM, H2-TPR, XPS, isobutene transient response experiment and TG analysis. The results showed that Al2O3 beads with tunable pore sizes (7.4–13.3nm), surface areas (294-322m2/g) and pore volumes (0.56–1.1cm3/g) could be obtained by changing the addition amount of liquid oil. Moreover, the transient response experiment stated that the PtSn-La/Al2O3 catalysts with large pore sizes could enhance the diffusion speed of the gas molecule in the channels. With increasing the amount of liquid oil from 0 to 20%wt, the particle sizes of platinum decreased from 2.86nm to 1.45nm, the amount of carbon deposit declined by 30.6% and the final yield of isobutene increased from 42% to 48%. The enhancement of catalytic performance of PtSn-La/Al catalyst with larger pore size was related to its smaller Pt particles, lower surface acidity and diffusion resistance of reaction products in the pore.

Effects of Acidic-Basic Properties on Catalytic Activity for the Oxidative Dehydrogenation of Isobutane on Calcium Phosphates, Doped and Undoped with Chromium

Catalytic activities of calcium hydroxyapatite (HAp) and β-type tricalcium phosphate (β-TCP) were examined for use in the oxidative dehydrogenation (ODH) of isobutane. β-TCP was catalytically inactive for the ODH of isobutane, but stoichiometric HAp afforded a high isobutene yield (5.6%). The isobutane conversion and isobutene selectivity of HAp depended on the atomic ratio of Ca/P. HAp with Ca/P = 1.67 showed the highest isobutene selectivity and isobutene yield among the HAp catalysts with different Ca/P ratios. The characterization of the acidic-basic properties showed that these properties affect the catalytic performance of HAp, and that its basicity is necessary for high catalytic activity. To improve the catalytic activities of calcium phosphates, they were impregnated with Cr. Despite a much lower surface area for β-TCP, Cr-impregnated β-TCP showed a higher isobutene yield (up to 8.4%) than that of Cr-impregnated HAp. The results of the XPS measurement showed that the Cr3+ species on calcium phosphates, owing to basicity, worked as active sites in the ODH of isobutane.

VOx–K2O/γ-Al2O3 catalyst for nonoxidative dehydrogenation of isobutane

V–K/γ-Al2O3 catalysts were prepared using an incipient impregnation method and characterized by N2 adsorption, MAS-NMR, TPR, FT-IR, XPS and Py-FT-IR techniques. The catalytic activity of V–K/γ-Al2O3 in non-oxidative dehydrogenation of isobutane was investigated in a continuous flow packed bed micro reactor. The results indicate that V–K/γ-Al2O3 catalysts display a bifunctional effect. Characterization study suggests that the distribution of various VOx species were influenced by vanadium loadings to a great extent. The catalytic activity of vanadium-based catalysts was consequently promoted to different degrees by diverse VOx species and the highest dehydrogenation activity was obtained for the sample with the highest dispersion of the oligomeric VOx species. The reduced state of vanadium oxide i.e. V3+ and V4+ sites were proved to be the catalytic active sites for dehydrogenation reaction. Moreover, the activity and selectivity of catalysts were strongly related with the acidity of the catalysts. The ratio of Lewis acid sites to Brønsted acid sites (NLS/NBS) was negatively correlated to the isobutene selectivity. Among the tested catalysts, V–K/γ-Al2O3 with an optimum vanadium loading of 10% showed the highest isobutene yield of 42.5%, with ideal stability of presenting yield up to 37.8% after 10h continuous dehydrogenation reaction.

Acid site density effects in zeolite-catalyzed 1-butene skeletal isomerization

We have investigated the effect of acid site density on the catalytic performance of high-silica (50⩽Si/Al⩽380) H-HPM-1 zeolite in the 1-butene skeletal isomerization. Changes (⩽0.5) in the number of framework Al atoms, and thus of Brønsted acid sites, per unit cell (60 tetrahedral atoms) led to striking variations in both 1-butene conversion and isobutene selectivity. Combining isobutene yield with catalyst durability, H-HPM-1 with a framework Si/Al ratio of 350 was found to perform better than any of the catalysts studied for this reaction thus far.